2.1. Cellular Resources Constrain Phenotype
The availability of nutrients,
machinery, and bioenergy define the cellular resource budget, and consumption of these resources defines the resource costs of biological activity (
Figure 1). How does the cell manage its resource budget and how do these constraints affect phenotype?
2.1.1. Nutrients: resources informing allocation
Supplies and signals. Extracellular nutrients contribute to the total resource budget as substrates for machinery synthesis and bioenergetic pathways (Appendix A, Appendix D). For example, amino acids can be incorporated as building blocks for proteins, whereas glucose and glutamine can be catabolized for energy or nucleotide synthesis (e.g., for oligonucleotides)(Fan et al., 2013; Hosios et al., 2016). However, in managing resource allocation, nutrients play a particularly important role as extracellular cues. Their presence informs the cells of which metabolic pathways may be utilized and ultimately which objectives may be achieved, guiding allocation across the metabolic network. In this sense, through regulatory programs(Efeyan et al., 2015), nutrients will induce activation of specific metabolic pathways and express the associated machinery that catalyze those pathways.
Signals of scarcity. Nutrient allocation has evolved to cope with nutrient scarcity(Efeyan et al., 2015). Mammalian cells have many strategies to handle nutrient scarcity; the global metabolic network is robust to nutrient inputs, capable of utilizing distinct nutrient-pathway combinations to meet cellular demands(Bennett et al., 2020; Edelman and Gally, 2001; Jeong et al., 2000). For example, cells will shift glucose usage from energy metabolism to de novo serine synthesis when serine is scarce(Chaneton et al., 2012) and use fatty acid oxidation to generate energy when glucose is scarce(Cantó et al., 2010). Additionally, cells may degrade intracellular components through autophagy and divert the resultant substrates to the most necessary pathways(Efeyan et al., 2015). Finally, cells may scavenge for more complex extracellular resources, such as proteins and lysophospholipids, to catabolize through mechanisms such as macropinocytosis(Zhu and Thompson, 2019). These resources are often produced by other cells in a multicellular system.
Multicellularity decreases the likelihood of individual cells facing nutrient scarcity. This is because mammalian cells have multiple subcompartments and specialized cell types that improve nutrient storage and delivery systems to create nutrient-rich microenvironments (Chantranupong et al., 2015; Palm and Thompson, 2017). Storage is a multicellular example of the hedging strategies discussed later, diverting resources away from current objectives in anticipation of future context-dependent fluctuations in nutrient levels(Fischer et al., 2011).
Allocation in abundance. Resource allocation in nutrient-rich environments becomes a decision-making problem. Cell activity still comes at a resource cost. Thus, the cell must choose which nutrients to shuttle to which metabolic pathways to most efficiently achieve its objective (Appendix D). Sometimes, it will even disregard available extracellular nutrients, such as non-essential amino acids, choosing instead to produce them intracellularly(Palm and Thompson, 2017).
Since cell activity evolved under nutrient scarcity, mammalian cells must tightly control nutrient uptake to prevent irregular phenotypes such as uncontrolled growth (see Appendix C for consequences of overabundance). Unlike prokaryotes, which typically uptake nutrients upon sensing them, mammalian cells tend to couple nutrient sensors with signaling proteins such as growth factors for an additional layer of regulation (
Figure 3c)(Palm and Thompson, 2017; Rathmell et al., 2000). This additional regulatory layer serves a dual purpose of allowing cells and tissues across the organism to coordinate via combined nutrient- and signaling-protein- circuits, maintaining steady-state circulating nutrient levels and mobilizing nutrient stores when necessary(Itoh et al., 2003; Zisman et al., 2000). Thus, nutrient scarcity tends to be a local and context-specific constraint in multicellular systems, such as in wound-healing and poorly vascularized regions(Zhu and Thompson, 2019). The cell’s microenvironment (e.g., nutrient concentration, interactions with other cell types, and presence of other extracellular nutrients) affect metabolic activity. Thus, it is important to appropriately account for a cell’s microenvironment when studying resource allocation, which may not always be accurately represented
in vitro. For example,
in vivo early-activated CD8+ T-cells utilize glucose differently than those under the super-physiological conditions of cell culture(Ma et al., 2019), shifting flux from aerobic glycolysis to oxidative phosphorylation to create a larger bioenergy budget.
Distribution drives function. Ultimately, nutrient availability and allocation affect cell phenotypes, such as growth(Chen et al., 2019; Son et al., 2015; Zhu and Thompson, 2019) and secretion(da Silva Novaes et al., 2019), as well as organismal phenotypes, such as development(Hu and Yu, 2017) and immunity. In immunity, nutrient availability affects cell fate, function, and composition. For example, under both amino acid and glucose deprivation, mTORC1 activation and CD4+ T-regulatory cell proliferation decreases(Long et al., 2021). Additionally, effector T-cells rely on both glucose(Cham et al., 2008; Cham and Gajewski, 2005) and glutamine(Carr et al., 2010) for cytokine secretion whereas invariant Natural Killer T cell cytotoxicity is independent of these nutrients(Khurana et al., 2021). In early development, glucose is shuttled to anabolic pathways that trigger synthesis of key machinery controlling blastocyst formation, with cells using other carbon sources such as pyruvate and lactate to generate bioenergy. In the absence of glucose prior to compaction, zygotes do not invest biosynthetic resources into expressing the glucose transporter SLC2A3, resulting in reduced cell growth and degenerated morulae(Pantaleon et al., 2008).
We note that there is an interplay between the three resource classes: nutrients, machinery, and bioenergetics. So far, this is largely presented as nutrients used to synthesize the cell’s machinery, and machinery and metabolic substrates together dictating the bioenergetic pathways that the cell utilizes. However, this crosstalk is not unidirectional, with each resource class depending on the others (
Figure 1a). Building machinery requires energy(Lynch and Marinov, 2015), and without the right machinery, nutrients can't be used effectively. Transporters, for example, are machinery that deliver extracellular nutrients to the cell. In CD8+ T-cells, knocking-out amino acid transporters limits nutrient intake, altering the ratio of terminal effector to memory precursor subpopulations(Huang et al., 2021).
2.1.2. Machinery: resources actuating allocation
As actuators of biological function, the machinery budget contributes to pathway activity and cell phenotype. The machinery component of the resource budget depends on two factors: machinery activity and machinery abundance (Appendix E). Higher activity increases the amount of substrate a single unit of machinery is able to convert into a product. There are innumerable examples in which altered catalytic efficiency, e.g. via point mutations, has disrupted homeostasis and led to disease states (reviewed here(Stefl et al., 2013)). On the other hand, higher abundance increases the proportion of machinery to its substrate. This is evident for individual machinery. For example, CARKL expression levels change depending on activation signals to alter energy metabolism and ultimately dictate macrophage fate.(Haschemi et al., 2012) Similarly, pyruvate kinase isoforms interact based on expression levels to affect nucleotide metabolism and dictate proliferation(Lunt et al., 2015). Abundance is also coordinated across multiple components. CRISPR screens have tested thousands of genes and found that machinery groups by shared effects across multiple phenotypes;(Funk et al., 2022) meanwhile, the abundance of hundreds of secretory pathway machinery together coordinate a cell’s capacity for secreting specific proteins(Kuo et al., 2021). Unlike the cell’s nutrient budget, which is largely limited by extracellular availability, machinery abundance is constrained by the resource costs of synthesis. Beyond diverting nutrients to anabolism, cells must invest bioenergy and biosynthetic machinery for gene expression.
Machinery Pose Non-negligible Auto-catalytic Costs. The machinery facilitating anabolism and gene expression are auto-catalytic, meaning they contribute to their own synthesis. As a result, machinery synthesis costs include the use of biosynthetic machinery(Reuveni et al., 2017). For example, in eukaryotes, individual pre-mRNAs compete for the shared pool of splicing machinery to be properly processed(Munding et al., 2013). Since ribosomal genes represent a substantial mass fraction of the proteome, global splicing efficiency is associated with ribosomal expression. Given that ribosome expression increases with growth rate to accommodate increasing biomass production demand, and that nutrient signaling via TOR influences ribosome gene expression, these observations couple cell growth with nutrient and machinery constraints of macromolecular synthesis (Appendix A). In fact, under nutrient scarcity, reducing splicing efficiency through intron-mediated regulation improves cell survival by decreasing ribosomal expression(Parenteau et al., 2019).
Various gene expression modeling approaches account for this concept of resource loading (Appendix E) to study the limiting biosynthetic machinery (Frei et al., 2020; Jones et al., 2020; Qin et al., 2023; Rondelez, 2012). Studies in prokaryotes have quantified costs of translation and ribosome biogenesis, showing them as major constraints on gene synthesis. In mammalian cells, rough estimates for HeLa cells show that all ribosomes must be constantly active to maintain global protein levels(Yewdell, 2001). However, quantification of exogenous gene circuit costs suggests transcriptional machinery loading as a substantial component of resource burden. Frei et al. demonstrate in different human cell lines that a combination of both transcriptional and translational resources can limit gene expression(Frei et al., 2020). This is in line with a study in yeast that found the relative impact of transcriptional and translational costs change across nutrient-limiting conditions, pointing to the crosstalk between machinery and nutrients(Kafri et al., 2016). In exploring the contribution of various intrinsic gene features, a separate study on mammalian cell lines found that those related to transcriptional resources have a larger effect on resource loading(Di Blasi et al., 2023). This is supported by two studies that introduced exogeneous circuits to mammalian cell lines, wherein one studied how RNA polymerase is limiting depending on the competing genes’ promoter strength(Qin et al., 2023), and another considered all gene expression resources to identify the Mediator complex(Allen and Taatjes, 2015) as the primary limiting resource(Jones et al., 2020). Overall, these results suggest that, in mammalian cells, transcriptional machinery also impose non-negligible resource limitations on gene expression.
Machinery Biosynthesis has Substantial Energetic Costs. The energetic costs of machinery synthesis can be quantified across genomic, transcriptional, and translational processes as a function of gene features. Protein synthesis costs are highly sensitive to the energy budget(Buttgereit and Brand, 1995) and are a large resource burden: ribosomal translation alone represents 30% (BioNumbers(Milo and Phillips, 2015) ID 110441) of a mammalian cell’s energy budget. Also, net protein synthesis costs represent more than 70% (BioNumbers(Milo and Phillips, 2015) ID 111918) of a generic cell’s energy budget (due to the higher contribution of amino acid synthesis relative to polymerization(Lynch and Marinov, 2015)). Lane and Martin argue that protein synthesis costs represent such a substantial portion of the total cellular energy budget that they prevent the evolution of eukaryotic genome complexity without mitochondria(Lane and Martin, 2010). In contrast, Lynch and Marinov contend that, considering the cell’s total lifetime, the energy budget scales with cell volume to more than compensate for increased costs of genome complexity, regardless of the presence of mitochondria(Lynch and Marinov, 2015).
Strategies to Minimize Machinery Costs. Due to these costs, cells employ various strategies to efficiently generate the machinery budget. For example, protein degradation is costly (Appendix A, Appendix D) because proteasomal degradation consumes ATP(Peth et al., 2013) and sequesters proteases. From a resource allocation perspective, this aligns with the fact that short-lived proteins have low abundance(Cambridge et al., 2011),(Li et al., 2021),(Schwanhäusser et al., 2011), representing just 5%(Li et al., 2021) of the human proteome. By only dedicating degradation resources to lowly expressed proteins, the cell reduces the cost of tuning protein abundance in modes beyond translational control. In contrast to proteasomal degradation, protein turnover via autophagy recovers more energy and nutrient resources than it consumes and represents a large fraction of total protein degradation in mammalian cells(Marchingo and Cantrell, 2022; Singh and Cuervo, 2011).
Short-lived proteins also constitute only a small portion of complexes(Li et al., 2021), supporting the notion that cells conserve protein degradation resources by minimizing their use in more abundant proteins. An alternative strategy proposed to reduce costs in complexes is proportional synthesis. Observed in both prokaryotes(G.-W. Li et al., 2014) and eukaryotes(Taggart and Li, 2018), cells express complex subunits in proportion to their stoichiometries by tuning synthesis rates, avoiding excess resource expenditure on synthesis. Additionally, degradation would be required to achieve proper stoichiometries and clear misfolded proteins resulting from excess expression. Furthermore, there are spatio-temporal variations in complexes’ stoichiometries, many of which are regulated beyond transcription(Ori et al., 2016). Together with proportional synthesis, this suggests context-specific tuning of translation rates to minimize synthesis costs.
These various strategies demonstrate that cells work to optimize machinery expression levels by minimizing synthesis costs while maximizing the associated fitness benefits(Dekel and Alon, 2005). This cost-benefit trade-off in gene expression was characterized in a high-throughput manner in eukaryotes, testing the impact of 81 genes’ expression levels on growth rate. Crucially, 83% of genes demonstrated distinct fitness curves correlating with differential gene expression(Keren et al., 2016). Thus, minimizing machinery costs has proven to be an accurate optimality principle to predict and understand metabolic activity(Noor et al., 2016, 2014) (Appendix B), such as in relationships between energy metabolism and growth (Appendix A).
2.1.3. Bioenergy: resources fueling allocation
Energy metabolism (e.g., glycolysis and oxidative phosphorylation) uses machinery to transfer the energy stored in extracellular nutrients to its main intermediate currency, ATP, and long-term nutrient stores. This energy budget is spent to fuel a multitude of tasks(Bennett et al., 2020; Rolfe and Brown, 1997; Yang et al., 2021) that prompts the cell to organize its activities accordingly. Several methods identify the energetic cost of executing function at varying resolutions, broadly by measuring or estimating the metabolic flux associated with bioenergy generation or consumption(Ghosh et al., 2022; Schmidt et al., 2021); these include estimates from measures such as the oxygen consumption rate and ATP equivalents, i.e. mechanistic delineation of the number of ATP molecules consumed, each of which yield ~50 kJ/mol (BioNumbers(Milo and Phillips, 2015) ID 100775, 100776) of energy from hydrolysis.
As discussed previously, a large fraction of the energy budget is spent on gene expression and protein translation(Lynch and Marinov, 2015). The remainder of the energy budget is available for other tasks. For example, migrating cells consume energy to displace the extracellular matrix(Mosier et al., 2021; van Helvert et al., 2018). The energetic cost of motility changes with physical features of the matrix such as stiffness and spatial confinement. As such, cells minimize these migratory costs by choosing context-specific migratory mechanisms(Li et al., 2019) and migrating through paths that require lower energy expenditure(Zanotelli et al., 2019). Neurons also demonstrate energy allocation. They consume energy for functions distributed across maintenance and activity, including neurotransmitter uptake and release by synaptic vesicles and action potential generation(Attwell and Laughlin, 2001; Du et al., 2008; Pulido and Ryan, 2021). Consequently, the brain has a high rate of energetic expenditure(Wang et al., 2010), accounting for 20% of the total energy costs of the human body (BioNumbers(Milo and Phillips, 2015) ID: 103264, 110878), and must use its energy budget efficiently. At the cell-scale, neurons employ combinatorial strategies to express sets of ion channels with specific kinetics that minimize energetic costs while meeting functional requirements (e.g., spiking rate)(Alle et al., 2009; Hasenstaub et al., 2010). At the tissue-scale, energetic constraints limit the total number(Herculano-Houzel, 2011) and active fraction(Lennie, 2003) of neurons in the brain. As such, the brain analogously employs combinatorial strategies to efficiently encode information into sets of neurons in such a manner that limits the number of active neurons, and thus energy demands(Levy and Baxter, 1996),(Attwell and Laughlin, 2001).
While whole-cell modeling has demonstrated that the total energy budget is nearly equivalent to the total energy cost of a synthetic minimal cell(Thornburg et al., 2022), it has also demonstrated excess production of energetic intermediates in
Mycoplasma genitalium(Karr et al., 2012). Whole-cell modeling approaches (Appendix B,
Table 1) are yet to be translated to mammalian cells, but such insights could prove invaluable to understanding energy resource allocation. An excess budget indicates either a deprioritization of evolutionary optimality for the energy budget over other objectives, cell hedging for future energy demands, or incomplete accounting of energy consumption.
2.2. Molecular Processes Coordinate Resources
Mechanistically, resource constraints are tied to the activity of molecular processes such as gene synthesis, metabolism, and molecular communication. These processes can be coupled to each other using systems approaches (Appendix B,
Table 1), providing quantitative details regarding how resources are used. Resource constraints that affect the activity of one process propagate to others and eventually affect phenotype (
Figure 1c).
2.2.1. Gene Expression: Coordinating mRNA with Protein
To understand the global relationship between mRNA and protein expression levels, proteomics is often compared with transcriptomics using correlative metrics(Buccitelli and Selbach, 2020; Vogel and Marcotte, 2012). Comparing per cell absolute protein copy numbers to transcripts-per-million mRNA levels in multiple human tissue cell lines identified an average Pearson correlation of 0.6(Edfors et al., 2016). Importantly, a gene-specific, tissue-independent protein-to-RNA (PTR)(Edfors et al., 2016; Wilhelm et al., 2014) ratio increases the correlation; similar results were also shown in single-cells, with remaining variability largely explained by other context-specific effects such as the microenvironment(Popovic et al., 2018). This indicates a gene-specific effect on resource loading, wherein intrinsic features yield a competitive advantage for shared translational resources. For example, sequence length is a positive indicator of the variation in the PTR ratio, consistent with longer genes having a larger resource cost. Furthermore, the PTR ratio increases with transcriptional abundance(Csárdi et al., 2015; Wang et al., 2019), signifying cooperation between transcription and translation in generating highly abundant proteins. Gene-specific competitive advantages also coordinate across processes within transcription. For example, mammalian cells use “economies of scale”, wherein splicing efficiency increases with transcription rate to prioritize production of strongly transcribed genes(Ding and Elowitz, 2019). Distinct intrinsic gene features related to either transcriptional or translational resources may also interact, resulting in a large combinatorial space in which these features can influence resource allocation(Di Blasi et al., 2023).
The role of gene-intrinsic features makes it apparent that abundance alone does not sufficiently explain mRNA to protein coupling. It follows that such features inform gene expression rates; broadly, there are four rates to consider: transcription, translation, mRNA decay, and protein degradation. The gene-specific combinations of these rates give rise to global steady-state levels of mRNA and protein in a context-dependent manner, especially with dynamic responses requiring rapid adaptation(Jovanovic et al., 2015; Rabani et al., 2011). Resource allocation limits the existing rate combinations, with certain combinations resulting in synthesis costs that outweigh the gene’s contribution to the cell objective. Specifically, there is an evolutionary lack of genes with high transcription rates and low translation rates, despite these rates being mechanistically feasible. This is driven by a “precision-economy trade-off” between stochastic protein abundance (precision) and resource costs of synthesis (economy), with high transcription and low translation not providing an advantage in either(Hausser et al., 2019).
The four synthesis rates are commonly modeled using a simplified set of ordinary differential equations (
Table 1). Derivation of these rates requires absolute quantification–the number of molecules per cell(Liu et al., 2016). Notably, these rates have a larger contribution to changes in the absolute level of a protein than to its relative level(Jovanovic et al., 2015). This makes sense from a resource allocation perspective since there is high inequality in the distribution of absolute protein abundances, with a small number of proteins constituting the majority of the total proteome by mass and copy number(Harper and Bennett, 2016; Li et al., 2021; Wang et al., 2019)
,(Hukelmann et al., 2016). Thus, highly abundant proteins will sequester a disproportionate fraction of cellular resources, independent of the fact that they tend to have an intrinsic competitive advantage in using cellular resources.
Translational efficiency also impacts gene-specific PTRs(Gingold and Pilpel, 2011). Gene-specific sequences regulate translational efficiency to prevent ribosomal jamming and excess translational machinery costs(Tuller et al., 2010). Genes with high translational efficiency also have high mRNA abundance, enabling them to outcompete their lower efficiency counterparts for ribosomes by both concentration and affinity. Furthermore, these genes are functionally enriched for macromolecular synthesis and energy metabolism(Al-Bassam et al., 2018), indicating the cell is optimizing for the production of its machinery and bioenergy resource budgets. From a resource loading perspective, ribosome machinery saturation imposes an upper bound on the dynamic range of protein abundance.
2.2.2. Actuation: Coordinating Protein with Metabolism
Metabolism links gene expression to cell phenotype(Basan, 2018; Hofmeyr and Cornish-Bowden, 2000). Metabolic inputs, outputs, and intermediates are incorporated into all the resource classes and molecular processes discussed, forming the basis upon which cell activity occurs(Karr et al., 2012). There are innumerable examples of how metabolic activity affects cell phenotype. For example, shifts from purine to serine synthesis promote cell motility(Kiweler et al., 2022; Soflaee et al., 2022), mitochondrial metabolism modulates stem cell fate(Carey et al., 2015; Schell et al., 2017; Vannini et al., 2016), and many pathways affect growth(Zhu and Thompson, 2019).
Gene expression produces the machinery that catalyzes metabolism. As mentioned previously, machinery activity and abundance together determine the flux that an enzyme-catalyzed biochemical reaction can carry (Appendix E)(Davidi and Milo, 2017; Nilsson et al., 2017). Assuming Michaelis-Menten kinetics, the maximum flux that a reaction can carry is the product of the catalytic rate constant and the enzyme concentration. There are several considerations in vivo that may prevent a cell from realizing these maximum rates, which tend to be reported under in vitro, nonphysiological conditions. First, the maximum rate assumes that the enzyme is fully saturated. However, cells try to minimize intermediate metabolite concentrations for homeostatic maintenance and quick adaptation to new contexts(Schuster et al., 1991). Yet, enzymes are more efficient at high (saturating) substrate concentrations. Thus, efficient enzyme usage must be balanced against minimizing metabolic intermediates(Tepper et al., 2013) and rapid substrate consumption. More generally, there is a trade-off between control over reaction fluxes and metabolic intermediate concentrations(Hofmeyr and Cornish-Bowden, 2000). Second, reaction thermodynamics affects reaction kinetics via the mass-action ratio due to such variables as intracellular pH and metabolite concentrations(Beard and Qian, 2007). The extent of backwards flux due to thermodynamics requires a higher machinery investment to maintain the same reaction rate (Appendix B)(Noor et al., 2014). Finally, regulatory effects such as allostery and post-translational modifications can alter kinetics to alter reaction rates.
Thus, the observed reaction rate will change with variables such metabolite concentration in a context-dependent manner. Combining proteomic measurements of enzyme abundance with metabolic modeling estimates of fluxomics to estimate in vivo observed catalytic rate constants demonstrated that the maximum value identified across multiple growth conditions agrees with the in vitro catalytic rate constants. Such studies decompose discrepancies between experimental and theoretical values into the underlying saturating, thermodynamic, and regulatory factors (Appendix E)(Davidi et al., 2016; Noor et al., 2016). The extent to which a reaction is active, the “capacity utilization”, can be defined as the ratio between this context-specific, observed reaction flux and the maximum reaction flux(Davidi and Milo, 2017). If this ratio is one, enzymes are being utilized at full capacity and activity is machinery-limited. However, if this ratio is less than one, one of the aforementioned factors is decreasing the reaction rate. If this is due to saturation effects, not all of the expressed enzyme is used(Xia et al., 2022) and activity is instead nutrient-limited. Unused, free enzymes may point to hedging for rapid adaptation to future conditions which require increased flux through those reactions (Appendix A).
2.2.3. Communication: Coordinating Signaling and Secretion with Gene Expression
Signaling and secretion link a cell’s extracellular environment with its intracellular activity, regulating the higher-order functions of multicellular systems. While signaling pathways sense and respond to extracellular signals, the secretory pathway produces communicatory molecules to send such information. These two molecular processes are not only complementary conceptually, but also biologically, coordinating each others’ activity(Farhan and Rabouille, 2011; Shvartsman et al., 2002). Secreted proteins are the product of gene expression. Unlike machinery, these proteins do not directly contribute to biomass production or intracellular activity. Yet, human cells allocate a massive amount of resources to protein secretion: secreted proteins represent >25% of the proteome by mass(Kuo et al., 2021), despite the fact that these proteins do not contribute to intracellular tasks such as biomass production. This speaks to the importance of secretory tasks such as cell-cell communication(Francis and Palsson, 1997; Le Bihan et al., 2012; Wegrzyn et al., 2010), cytotoxicity(Lopez et al., 2012; Reefman et al., 2010), and remodeling of the extracellular matrix(Bonnans et al., 2014) to multicellular systems. Signal transduction pathways and their downstream transcription regulatory networks use extracellular cues, i.e. nutrients and communicatory molecules, to induce gene expression. Specifically, a receptor will sense the extracellular cue, downstream machinery processes the information encoded by that signal, and transcription factors induce the synthesis of target genes. As such, signaling pathways enable context-specific cellular decision-making(Hill et al., 2017; Klumpe et al., 2022; Larson et al., 2022; Yang Shen et al., 2022; Shvartsman et al., 2002).
Signaling pathways have multiple possible objectives, including signal amplification, sensing precision (i.e., noise mitigation)(Lestas et al., 2010), information transfer, parameter robustness, and response time(Alves et al., 2021). However, the signaling pathway activities underlying these objectives are constrained by energetic and machinery resource costs that the cell minimizes(Lan et al., 2012; Mehta and Schwab, 2012; T.-L. Wang et al., 2022). For example, Goldbeter–Koshland push–pull network sensing systems–signal transduction pathways ubiquitous across prokaryotes and eukaryotes(van Albada and ten Wolde, 2007)–prioritize sensing precision. Here, receptors, downstream signaling machinery, and energy each independently constrain the extent to which noise can be mitigated. For optimality, these three constraints evolved to be equally limiting(Govern and Ten Wolde, 2014). A lack of sensing precision can propagate downstream to cause stochastic gene expression (Elowitz et al., 2002) (Appendix B). Recent studies have indicated that, when sufficiently accounting for cell phenotype and context, this stochasticity is minimal(Battich et al., 2015; Foreman and Wollman, 2020). This is because individual mammalian cells can mitigate downstream noise of the final machinery products by “smoothing” across time. For example, subcompartmentalization shows that increased nuclear retention times decrease cytoplasmic mRNA noise(Battich et al., 2015) and reduced protein degradation rates, relative to mRNA degradation rates, decrease protein noise (Raj et al., 2006). Similarly, noise can also be mitigated by the signaling network topology(Austin et al., 2006; Zhang et al., 2007).
More broadly, the efficiency by which signaling pathways and downstream transcription regulatory networks achieve their objective depends on their network topology(Alon, 2007). Consequently, evolution has converged on a small number of prevalent topologies, termed network motifs, that determine the dynamics and robustness of network input-output relationships. Since there are multiple possible signaling objectives, it remains unclear how the interplay between these various objectives may affect resource allocation. For example, only two experimentally observed network motifs demonstrate fold-change detection across a wide range of organisms because they are uniquely Pareto optimal for response time, sensing precision, and signal amplification(Adler et al., 2017).
Network motifs are building blocks for global network structures. Convex analysis(Schilling et al., 2000) of signaling networks identifies a minimal set of pathways representative of the network state from its global topology. These “extreme pathways” reveals how cells divert metabolic and machinery resources across the network, encoding for signaling crosstalk and redundancy between pathway reactions to achieve robust input-output relationships(Papin and Palsson, 2004a, 2004b). Reducing crosstalk trades-off with increasing sensing precision(Barton and Sontag, 2013) due to the energetic costs of signaling modularity(Saez-Rodriguez et al., 2005). Thus, cells employ combinatorial strategies that instead leverage crosstalk to reduce resource costs. For example, by sharing the tumor necrosis factor (TNF) ligand, the nuclear factor kappa B (NF-κB) and c-Jun N-terminal protein kinase (JNK) pathways bypass noisy signaling to increase information transfer relative to either pathway acting in isolation.
Just as multiple signaling pathways can share a ligand to increase information transfer within an individual cell, multiple cells can share a signaling pathway (and thus, a ligand) to analogously improve the average response to a ligand.(Cheong et al., 2011) In contrast to an individual cell, sharing a ligand across cells does not have an upper bound on information transfer(Cheong et al., 2011), demonstrating the utility of multicellularity. Furthermore, individual cells can couple their responses to other cells, leveraging their inherent stochasticity in combination with the underlying signaling network topology, to control the extent of heterogeneity across the tissue(Smith and Grima, 2018). This demonstrates how multicellular systems can use communication to take advantage of noise at the single-cell level to yield emergent properties at the tissue-level. In fact, when coordinating in multicellular systems (
Figure 3b), a number of strategies reduce the resource costs associated with synthesis and secretion of communicatory molecules. For example, a single shared ligand can achieve diverse population-level behaviors such as bistability, e.g. all-or-nothing responses, and bimodality, e.g. cell fate decisions, by combining autocrine and paracrine communication(Youk and Lim, 2014). Finally, when considering both multiple signaling pathways and a cell population, promiscuous ligand-receptor combinations enable more robust activation of multiple cell types as compared to one-to-one binding(Su et al., 2022). Overall, mammalian cells have developed a number of resource optimization strategies to efficiently communicate, lowering the barriers to multicellularity.
2.3. Trade-offs Occur Due to Multiple Objectives
Cells balance multiple objectives(Alon, 2019). Allocating resources towards one objective induces a trade-off because the shared and limited pool of resources must be diverted away from the pathways that enable a competing objective. A simple case is in the expression of two different genes within a cell: under a fixed resource budget, due to resource loading of biosynthetic machinery, increased expression of one gene is achieved by decreasing expression of the other(Gyorgy et al., 2015). Additionally, at the tissue-scale, different systems in the brain are each specialized for one or more tasks and trade-off against each other(Alonso et al., 2013). These tissue-scale tasks are coordinated within the multicellular system(Rueffler et al., 2012) to appropriately distribute resources and enable context-specific task prioritization.
Under resource optimality assumptions, such trade-offs are mathematically represented by a Pareto front (
Figure 3a), or the set of all optima for which performance of one objective cannot be improved without decreasing performance of another objective. Pareto analysis reduces the resource-constrained phenotype space(Szekely et al., 2013), enabling more accurate identification of the biological mechanisms evolution converged upon. Using genome-scale models (GEMs) (Appendix B,
Table 1) in Pareto analysis has demonstrated trade-offs between five objectives (e.g., synthesis of albumin, glutathione, and NADPH depending on nutrient availability) in hepatocytes(Nagrath et al., 2007) and between growth and protein secretion in CHO cells(Gutierrez et al., 2020). Pareto analysis has also been adopted to understand omics data, enabling the identification of the number and type of objectives present in the dataset, and the features that support each objective (
Table 1). For example, applying this to human breast cancer tumors revealed four distinct objectives (de-differentiation and division, differentiation in healthy tissue, signaling, and growth) that changed according to the tumor type(Hart et al., 2015). In the proceeding sections, we discuss the various manners by which a cell may encounter more than one objective.
2.3.1. Context-specific Trade-offs Underlie Cellular Decision-Making
Trade-offs between multiple objectives require a cell to choose how to allocate its resources and prioritize certain activities. Context-dependent changes in phenotype and the underlying cell state can be understood by cellular decision-making: different contexts introduce different objectives and resource budgets to the cell, imposing trade-offs along the context dimension (
Figure 3a). Context-dependent changes to the biological objective are reflected in the underlying molecular processes that drive activity, such as global changes in translational efficiency during differentiation(Ingolia et al., 2011) and spatial changes in glycolytic fluxes during development(Bulusu et al., 2017).
Multiple objectives are not always present simultaneously, but may arise in sequence according to some context variable (e.g., as a cell migrates, ages, or encounters stress). For example, trade-offs between growth and non-growth associated maintenance (NGAM) over time can explain age-related declines in biological function (Appendix D). Additionally, the extent to which eukaryotic gene expression programs optimize for growth depends on nutrient availability, indicative of context-specific tuning of machinery for non-growth objectives(Keren et al., 2016). It is worth noting that mammalian cells, particularly in homeostatic tissue, likely often prioritize for non-growth objectives; however, research in this area remains limited, partially because studies using physiological readouts beyond growth are uncommon (see Conclusion for details).
Proteome allocation demonstrates how global strategies of machinery expression are selected according to context-specific trade-offs. Proteome allocation assumes a context-independent, constant upper bound allocated to total protein abundance due to synthesis costs and spatial constraints(Elsemman et al., 2022; Hui et al., 2015; Mori et al., 2016; Nilsson et al., 2017; Xia et al., 2022; Yang et al., 2016). Due to this upper bound, increases in expression of one or a group of proteins that support a given function requires compensatory decreased expression of others. An insightful consequence of this proteome allocation perspective is in the use of respiratory or glycolytic pathways in energy metabolism (Appendix A). Proteome allocation trade-offs explain why cells employ the machinery cost minimization strategies previously discussed, as it enables them to not only efficiently perform a single task, but also to have a larger capacity to perform multiple tasks simultaneously and express some machinery in excess to hedge for future conditions. Highly abundant proteins perform “core” functions, such as gene expression and energy metabolism, that tend to be conserved across contexts. In contrast, context-specific proteins have lower abundance, suggesting proteome re-allocation across contexts minimizes machinery biosynthetic costs by unevenly distributing their abundance according to function(Beck et al., 2011).
2.3.2. Cells Hedge for Future Contexts
Cells adjust their pathway activity accordingly to adapt to the resource demands of each context. To rapidly adapt, cells employ hedging: rather than being fully optimized for one context, cells divert some of their resources in anticipation of new tasks.
For example, the E. coli proteome is not fully optimized for growth, re-allocating up to 95% of its proteome by mass fraction from growth functions such as energy production to those such as cell signaling and membrane transport in anticipation of stresses(Yang et al., 2016). Indeed, E. coli balances multiple objectives, including growth, minimizing total metabolic fluxes (Appendix B), and ATP yield. However, they do not lie exactly on the Pareto front to decrease the cost of adjustment between objectives(Schuetz et al., 2012). In S. cerevisiae, cells allocate some of their ribosomes for future growth, and under hyperosmotic stress undergo cell cycle arrest, sacrificing the speed of their adaptive response to maintain glycogen reserves in preparation for subsequent stresses(Bonny et al., 2020). Similarly, mammalian cells may allocate some of their proteome towards aerobic glycolysis to hedge for hypoxia (Appendix A).
The extent to which cells utilize hedging varies, as some cells specialize for specific contexts. For example, different microbial strains tend to be optimized for glycolytic or gluconeogenic growth, resulting in long lag times upon nutrient shifts in one direction of central carbon metabolism. Probing this further, trade-offs between lag time, growth rate, and futile cycling prevent optimality in both directions, driving these cells towards specialization(Schink et al., 2022). While these trade-offs shape the decision-making of unicellular organisms, multicellular, mammalian organisms have evolved strategies to limit trade-offs and the need for hedging.
2.4. Multicellularity: From Cells to Organisms
Multicellularity emerged to efficiently manage and mitigate resource trade-offs (
Figure 2). Tissues are multicellular organizations composed of diverse individual cells. Since tissues receive a variety of cues, individual cells display context-dependent heterogeneity. This heterogeneity, which can be quantitatively characterized using single-cell measurements, results in distinct cell types or states. From a resource allocation perspective, these distinct cell types are optimally fulfilling distinct objectives according to the context in a manner that imposes demands on distinct resources(Jerby-Arnon and Regev, 2022). Context-specific objectives and trade-offs at the cellular scale induce multicellular coordination to optimize tissue-level functions(Almet et al., 2021; Toda et al., 2019) that ultimately impact the whole-organism (Appendix C).
2.4.1. Division of Labor Distributes Resource Burdens
Individual cells represent the fundamental unit of tissues. These cells occupy a spectrum of distinct states according to their contexts, and single-cell resolution measurements can comprehensively quantify this heterogeneity(Wagner et al., 2016). For example, single-cell RNA-sequencing has helped characterize the pathogen specificity of immune responses(Blecher-Gonen et al., 2019), treatment effects in autoimmune disease(Baghdassarian et al., 2023), cell developmental trajectories across time(Fischer et al., 2019; Yeo et al., 2021), and the spatial organization of cells within tissue(Ren et al., 2020).
From a resource allocation perspective, such diverse cell states represent functional specialization(Arendt, 2008) to support a
division of labor strategy employed by multicellular organisms: rather than performing all tissue tasks, individual cells specialize to more efficiently perform a subset of these tasks. Since the context dictates the information and resources provided to a cell via extracellular cues, a cell will specialize to optimally perform particular objectives accordingly. In steady-state tissues, spatial context gradients are often paramount (
Figure 3b). Single-cell measurements characterize the context-dependent gene expression programs a cell pursues in support of its objectives. For example, single-cell analysis reveals five distinct enterocyte cell states that form longitudinally along the small intestine, each with distinct metabolic activity to support absorption of specific nutrients (e.g., fatty acids, carbohydrates, lipoproteins and amino acids, and cholesterol and steroids)(Zwick et al., 2023). Similarly, hepatocytes alter the expression patterns of ~50% of their genes in accordance with their spatial location. These patterns reflect coordination between all three resource classes–liver zones with higher oxygen availability also demonstrated machinery shifts towards oxidative phosphorylation, likely generating energy to support higher protein secretion(Halpern et al., 2017).
In parallel, Pareto analysis has been used to identify relevant tissue objectives that trade-off against each other, and how cell types are distributed across these objectives. For example, neural arbors arrange their network topology in a Pareto optimal manner to minimize wiring costs and conduction delay(Chandrasekhar and Navlakha, 2019). These two objectives support efficient communication, indicating the importance of coordination between cells, a concept that will be elaborated on in the following section. In this study, 15 distinct cell types were distributed across this Pareto front, indicating varying degrees of specialization towards either objective. Golgi and stellate cells, for example, were specialized for minimizing conduction delay and wiring costs, respectively, whereas projection cells represented generalists equally prioritizing both tasks.
Finally, single-cell measurements have been combined with the aforementioned high-dimensional Pareto analysis method(Hart et al., 2015) In line with the neural arbor cell type analysis(Chandrasekhar and Navlakha, 2019), these studies broadly demonstrate that, subject to trade-offs, single-cells represent specialists optimized for a particular task or generalists that can perform multiple tasks (
Figure 3a). For example, analysis of human colon crypt progenitor cells identified four objectives: stemness, activation of cell-type specific genes, reduction of global gene expression, and halting of division. Notably, mature cell types are also specialized in a manner corresponding to their spatial location(Korem et al., 2015). Similarly, enterocytes are distributed across three objectives–cell adhesion and lipid transport, carbohydrate and amino-acid uptake, and anti-bacterial defense–in a manner correlated to their location in the intestinal villus(Adler et al., 2019).
Both the colon progenitors and terminally differentiated enterocytes form continuums in gene expression space, bounded by their respective objectives. Single-cell continuums can be explained by external spatial gradients, are indicative of varying degrees of specialization, and are necessary to optimize tissue function (i.e., maximize net performance across all objectives)(Adler et al., 2019). Thus, division of labor does not simply lead to a random assortment of heterogeneous cells, but rather confers optimality at the tissue-scale. While each individual cell is specialized to perform some subset of tissue tasks, division of labor distributes these tasks across multiple cell types in a manner yielding synergistic effects: the cells’ combined functions amplify performance across a range of tissue-level tasks. For example, division of labor mitigates both the cost of switching between multiple objectives in a tissue(Goldsby et al., 2012) and the loss of tissue function when cells proliferate (instead of performing the objective they are specialized for) for homeostatic maintenance and turnover(Rodríguez-Caso, 2013). Furthermore, diversity in the extent of specialization (represented as continuums in single-cell gene expression space) enables robustness against perturbations(Rueffler et al., 2012). In the next section, we will discuss the coordination strategies cells employ to achieve these higher-order functions.
2.4.2. Coordination Enables Higher-Order Functions
As cells specialize, emergent tissue-level functions arise from coordination, wherein resources are distributed across systems(Alonso et al., 2013) and yield synergistic effects that enable phenotypes no individual cell type can(Rueffler et al., 2012). At the molecular scale, coordination can be seen as the joint molecular activity across distinct cell types (i.e., “multicellular programs”) (Jerby-Arnon and Regev, 2022; Mitchel et al., 2023; Ramirez Flores et al., 2023). Systems modeling approaches can probe the connection between the molecular activity of individual cells and tissue-level functions by characterizing the relationships between a cell and its environment (e.g., extracellular cues and neighboring cells)(Adler et al., 2023a; Polychronidou et al., 2023). GEMs, for example, can model intercellular resource allocation and metabolic crosstalk between cells(Martins Conde et al., 2016). Multicellular GEMs for brain astrocytes and neurons demonstrated that energy pathways are distributed across cell types to minimize protein costs(Gustafsson et al., 2022) and revealed metabolic phenotypes underlying Alzheimer’s Disease(Lewis et al., 2010b). GEMs also elucidated how fibroblasts reprogram colorectal cancer cell metabolism by stimulating pathways such as glycolysis and glutaminolysis without altering growth(J. Wang et al., 2022).
Coordination and the collective behavior of cells depends on information transfer (see
Section 2.2.3 for the role of resource allocation in sending and interpreting information), often via cell-cell communication. Within the single-cell Pareto optimality framework previously described(Korem et al., 2015), the type and range of cell-cell communication influences tissue spatial patterning to coordinate specialization, enabling cells to self-organize as efficient multicellular systems. Colon fibroblasts distributed across one of five objectives (ECM degradation, ECM production, integrin production, regulation of the immune response, and contractile functions) communicate via specific ligand-receptor pairs that correspond to their spatial location and specialization(Adler et al., 2023b; Halpern et al., 2017; Zwick et al., 2023). Due to communication costs, resource allocation also impacts how the information to organize multicellular systems is distributed (
Figure 3b). As discussed previously, signaling pathways optimize for specific objectives such as sensing precision(Govern and Ten Wolde, 2014). To manage spatial-gradient-induced noise during wound-healing, cells maximize sensing precision by coordinating within optimal local distances via paracrine growth factor communication(Handly et al., 2015). Indeed, Pareto optimal task-distribution in tissues can be separately influenced by spatial gradients and local communication(Adler et al., 2023b; Halpern et al., 2017; Zwick et al., 2023).
Ultimately, coordination leads to higher-order properties such as compositional homeostasis and organismal defense. Communication, for example, maintains steady-state cell type proportions in tissue by tuning cell proliferation and removal rates (
Figure 3c,d). Zhou
et al. identified a two-cell macrophage-fibroblast circuit that uses growth factor exchange for compositional homeostasis. In this circuit, fibroblast proliferation is limited by resource constraints whereas macrophages are limited by negative feedback of the growth factor CSF1(Zhou et al., 2018). The specific growth factors used are context-dependent across nutrient-limiting conditions, and the distinct cell types are competing for different limiting resources(Zhou et al., 2022). However, while context affects individual parameters, such as cell growth rates, the circuit’s stability is generalizable. Separately, in the rove beetle tergan gland, cell specialization resulted in the co-evolution of two specialized cell types that each produce small molecules which are innocuous in isolation, but form a defensive toxin when combined(Brückner et al., 2021). Coordination between cell types can also be advantageous for disease states. In cancer, for example, groups of circulating tumor cells demonstrate more than an order of magnitude increased metastatic potential compared to individual cells (Aceto et al., 2014). Extending the previous discussion of motility resource allocation(Zanotelli et al., 2019) to the multicellular dynamics of cancer metastasis, cells invade cooperatively to minimize energetic costs of migration. A “leader” cell disproportionately expends its resources to displace the matrix, making migration easier for “follower” cells. After it has depleted its energy budget, the leader cell is replaced by a follower cell that still has a high energy budget(Zhang et al., 2019). In a simplified model, one study demonstrated that metastatic invasion occurs in the presence of a nutrient gradient, indicating the metastatic population is willing to pay the energetic costs of migration in search of a location with more nutrient resources(Liu et al., 2013).
2.4.3. Resource Competition Maintains Homeostasis
While division of labor improves the efficiency by which cells allocate resources and allows for coordination, much like the intracellular pathways of an individual cell, cells in the same microenvironment compete for a shared pool of extracellular resources. In some cases, cells in the same microenvironment–particularly if they share the same identity–are not only using the same pool of resources but are also using those resources in the same manner because they have a shared objective. We saw this in the macrophage-fibroblast circuits, in which macrophages competed for growth factors whereas fibroblasts competed for space(Zhou et al., 2022). During development, competition actually serves as a coordinating mechanism to improve morphogenesis (i.e., growth, differentiation, and structure), with resource scarcity being the coordinating signal(Smiley and Levin, 2022). Thus, competition is an important homeostatic mechanism constraining any individual or population of cells’ objective (e.g., growth) from dominating.
Canonically, cell competition is defined as the “active elimination of intrinsically viable cells that differ in some way from their neighbors”(Baker, 2020). Within this definition, resource competition is one such mechanism for elimination, with less fit “loser” cells unable to use resources as efficiently as “winner” cells to achieve their objective, thus optimizing overall tissue fitness(Matamoro-Vidal and Levayer, 2019). For example, under conditions of nutrient abundance, lower Myc expression causes cells to have a lower anabolic capacity, ribosomal abundance, protein synthesis rates, and proliferative capacity(Clavería et al., 2013; Ellis et al., 2019). These less-fit cells are eliminated via apoptotic signaling and asymmetric cell division. In epidermal expansion, during mouse embryogenesis, for example, this elimination ensures appropriate tissue architecture.
Similarly, oncogenic epithelial cells are eliminated by wild-type cells via apical extrusion. Metabolically, the less-fit cells are inefficient, shifting towards aerobic glycolysis and exhausting glucose without substantially increasing ATP(Kon et al., 2017). In contrast to homeostatic maintenance, metabolic competition in tumors, which have high resource demands, can lead to disease progression. For example, tumor cells reduce tumor infiltrating lymphocyte (TIL) effector function by competing for glucose. Lower glucose availability decreases TILs’ oxidative phosphorylation flux and their biosynthetic capacity to secrete interferon gamma(Chang et al., 2015). The high lactic acid levels produced by tumors via aerobic glycolysis in low glucose conditions also decreases immune cell ATP levels and biosynthetic capacity, ultimately suppressing TIL infiltration and survival(Brand et al., 2016). However, the presence of other nutrient sources and cell types may mitigate the role of competition(Reinfeld et al., 2021). Recent work has leveraged the high resource demands of cancer cells by engineering adipocyte glucose and lipid metabolism to outcompete tumors(Nguyen et al., 2023; Seki et al., 2022).