1. Introduction
Traditionally, the glycolytic metabolism of glucose to lactate has been associated with anaerobic or hypoxic conditions. In the 1920s, Otto Warburg discovered that rapidly proliferating tumor cells prefer glucose uptake and fermentation even in the presence of oxygen, leading to the intracellular and extracellular accumulation of lactate [
1]. This phenomenon, known as aerobic glycolysis, results from metabolic reprogramming that suppresses mitochondrial oxidative phosphorylation (OXPHOS) [
2]. Lactate plays dual roles as an energy substrate and a signaling molecule within a shuttle system between different cell populations. Various cell types, including neurons, cardiomyocytes, and tumor cells, can uptake lactate, convert it to pyruvate under aerobic conditions, and integrate it into the tricarboxylic acid (TCA) cycle for complete oxidation, yielding ATP. Moreover, lactate functions as a signaling molecule and modulates processes such as cell proliferation, metabolism, angiogenesis, invasion, and immune responses [
3,
4,
5,
6].
In the central nervous system (CNS), lactate regulates neuronal excitability, synaptic plasticity, and learning and memory processes through specific transporters and receptors [
7]. In 2019, Zhang et al. [
8]. discovered that lactate induces a novel post-translational modification called lactylation, significantly impacting gene regulation and cellular functions. Elevated lactate levels and histone lactylation were observed in an M1 polarization model of macrophages stimulated by bacterial infection. Histone lactylation is particularly enriched at the promoters of M2-like homeostatic genes (e.g., Arginase-1 and Vegfa), thereby modulating transcription and promoting tissue repair and wound healing. Emerging evidence indicates that metabolic reprogramming leading to lactate production and subsequent lactylation occurs in various cells and diseases, including pulmonary hypertension, heart failure, renal fibrosis, atherosclerosis, neurodegenerative diseases, psychiatric disorders, trauma, and hypoxic/ischemic injuries. Lactylation is involved in numerous physiological and pathological processes, such as cell fate determination, neuronal excitability, oxidative stress response, and immune regulation [
6,
9]. Consequently, research on lactylation has gained momentum, particularly in the CNS, where it influences neural development, neuropsychiatric disorders, glioblastomas, and hypoxia-related brain injuries [
10,
11,
12,
13].
This review systematically summarizes the regulatory roles and mechanisms of lactation in CNS development and diseases, offering valuable insights and targets for future research and clinical treatment.
2. Lactate Metabolism in the Brain
Since lactate was first reported in 1780, scientific research has significantly advanced our understanding of its role in metabolism [
14]. Under normoxic conditions, cells metabolize glucose to pyruvate, which enters the TCA cycle to produce water, carbon dioxide, and ATP, meeting cellular energy demands. During hypoxia, glycolysis is activated, and pyruvate is reduced to lactate by lactate dehydrogenase (LDH) to generate ATP. Initially considered a metabolic waste product, lactate is now recognized as a vital component cleared primarily by the liver through gluconeogenesis (Cori cycle) and OXPHOS (Krebs cycle) [
15]. Notably, many tumor cells rely on aerobic glycolysis for energy production, fermenting glucose to lactate even in the presence of oxygen–a phenomenon known as the Warburg effect [
16]. Lactate, as a product of glycolysis and a substrate for mitochondrial respiration, bridges the glycolytic and aerobic pathways. The lactate shuttle hypothesis posits that lactate transcends compartmental barriers, shuttling within and between cells, tissues, and organs, playing critical roles in energy metabolism, immune responses, memory formation, wound healing, and tumorigenesis [
3,
6].
Neurons, which are characterized by high metabolic activity, exhibit significant energy demands. Although the brain constitutes only 2% of body weight, it accounts for approximately 20% of the body’s oxygen and energy consumption, primarily for maintaining membrane potentials, synaptic transmission [
17], and neurotransmitter synthesis and release. Brain activation leads to dynamic changes in energy demand, necessitating those neurons meet both sustained and transient high-energy demands [
18]. Under resting conditions (normoxia), neurons primarily derive energy from aerobic glucose oxidation, whereas astrocytes rely on aerobic glycolysis. Metabolic remodeling during neuronal activation increases glycolytic activity [
19,
20]. Activated neurons release glutamate at synapses, which is then taken up by astrocytes, promoting glucose uptake and lactate production. Neurons then utilize lactate from astrocytes as an energy substrate through the astrocyte-neuron lactate shuttle (ANLS) [
21].
Isotopic labeling of lactate and glucose has shown that lactate enters neurons and is converted to pyruvate by highly expressed LDH1, which is a key step in lactate as an energy molecule [
22,
23]. During high neuronal activity, lactate serves as a faster and more direct energy source than glucose [
24]. Studies on cultured neurons, brain slices, and in vivo models have revealed that lactate sustains neuronal vitality and function during activation more effectively than glucose. Higher lactate levels were observed in astrocytes than in neurons in the mouse brain, and a higher cytosolic NADH/NAD+ ratio was observed in astrocytes in hippocampal slices, supporting greater lactate production in astrocytes and indicating net lactate transport from astrocytes to neurons [
19]. The specific expression of glycolytic enzymes in astrocytes, such as fructose-2,6-bisphosphatase 3 (PFKFB3), enhances the activity of the glycolysis-limiting enzyme phosphofructokinase (PFK), enabling high glycolysis levels in astrocytes [
25], making astrocytes a primary lactate synthesizer converting glucose from the bloodstream or stored glycogen into pyruvate and lactate [
26,
27].
The relative contributions of glucose and lactate as primary energy substrates, along with their respective metabolic pathways (glycolysis and OXPHOS in neurons), remain subjects of ongoing debate [
18,
28]. Research has shown that substituting or supplementing glucose with pyruvate or lactate in artificial cerebrospinal fluid significantly reduces NADH overshoot and oxygen consumption during neuronal network activation. Both neurons and astrocytes consume energy during this period, with neurons utilizing glucose as a crucial energy source [
29]. These findings challenge the view that astrocyte-derived lactate is the primary energy substrate for neurons but do not directly elucidate the relative contributions of glycolysis and other metabolic pathways to energy provision. Transient changes in the NADH/NAD+ ratio of individual neurons after stimulation were measured in the brains of awake mice using genetically encoded fluorescent biosensors. These measurements revealed that the transient changes were caused by a rapid increase in cytosolic glycolysis. Blocking lactate entry into neurons via monocarboxylate transporters (MCTs) during the resting state decreases neuronal NADH levels. However, during neuronal activation, the rapid increase in the NADH/NAD+ ratio is not inhibited by MCT blockers, supporting the presence of ANLS in the resting state but not during activation [
17].
Based on a comprehensive analysis of past and recent studies, lactate has both potential and limitations as a supplementary fuel for synaptic transmission and neuronal network oscillation. During low-energy-demand network activities, such as in anesthetized neurons, lactate can effectively substitute for glucose and may even demonstrate higher efficiency. However, in high-energy-demand network activities, lactate can only serve as a supplementary fuel, partially replacing glucose [
30]. This indicates that the brain’s energy sources are diverse, including glucose and lactate, and that metabolic pathways are not singular; the utilization of specific energy substrates depends on the brain’s ongoing demands and substrate availability. In different pathophysiological contexts, neurons optimize their overall network function by utilizing the best metabolic pathways and energy substrate combinations based on their activity status. These potential mechanisms warrant further investigation.
3. Lactate as a Signaling Molecule in the Brain
In the ANLS paradigm, lactate functions not only as a neuronal energy substrate but also as a signaling molecule that regulates neural excitability, synaptic plasticity, and memory consolidation [
31,
32]. The lactate shuttle flux is modulated by lactate concentration gradients, pH gradients, and redox states. Lactate transport between neurons is facilitated by MCTs and gap junction hemichannels. MCT1, MCT2, and MCT4 are extensively expressed in the brain. MCT1 is predominantly localized in vascular endothelial cells, astrocytes, and oligodendrocytes; MCT2 is primarily found in neurons, particularly in dendritic spines; and MCT4 is almost exclusively expressed in astrocytes [
33]. The high affinity of MCT2 for lactate ensures efficient lactate uptake by neurons [
34]. CD147 functions as an essential chaperone molecule for MCT1 and MCT4, stabilizing their structure and facilitating translocation and localization to the cell membrane [
35]. Gap junction hemichannels, composed of connexins such as connexin 30 and 43, enable extensive astrocyte communication and network synchronization [
36,
37]. The G protein-coupled receptor GPR81, also known as hydroxycarboxylic acid receptor 1(HACR1), acts as an endogenous lactate receptor and is expressed in neurons of the cerebral cortex and hippocampus at excitatory postsynaptic membranes, and is enriched in the blood-brain barrier [
38]. Together, MCTs, gap junction hemichannels, and GPR81/HCAR1 form the molecular foundation of lactate signaling.
Recent research highlights the role of lactate in neuronal excitability. Lactate dehydrogenase A (LDHA) in glial cells of the dorsomedial prefrontal cortex (mPFC) is instrumental in regulating lactate homeostasis, thereby influencing neuronal excitability and depression-like behavior [
23]. Experimental evidence indicates that LDHA knockdown reduces lactate production, leading to decreased neuronal excitability and exacerbated depression-like behavior in mice subjected to social defeat. Conversely, LDHA overexpression enhances lactate production, increases neuronal excitability, and mitigates depression-like phenotypes. The administration of lactate in mouse models of depression has therapeutic effects on depressive behaviors, underscoring the pivotal role of astrocyte-derived lactate in modulating neuronal excitability. Further studies have demonstrated that restricted lactate shuttling diminishes neuronal excitability. Specifically, the inhibition of MCT2 expression in neurons prevents restoration of neuronal firing or alleviation of depressive-like phenotypes despite exogenous lactate supplementation. Mechanistic studies have revealed that lactate enhances neuronal excitability by inhibiting rapid after-hyperpolarization, an effect mediated by large-conductance Ca²⁺-activated potassium (BK) channels. Additional research has indicated that blocking gap junction hemichannel proteins significantly reduces synaptic excitability in hippocampal neurons. The astrocyte-specific knockout of connexin 43 impairs lactate shuttling, decreasing the excitability of arousal-promoting orexin neurons and causing excessive sleepiness and fragmented arousal in mice [
36]. These findings suggest that lactate inhibits function in preventing neuronal hyperactivation. Moreover, lactate modulates neuronal excitability via HCAR1 activation, reducing presynaptic spontaneous calcium-spiking activity. In epilepsy models, HCAR1 knockout mice exhibit reduced seizure thresholds, increased severity, and prolonged duration, similar to LDHA inhibition [
39,
40]. HCAR1 may regulate neural activity through functional interactions with other Gi-coupled receptors via the Giα and Giβγ pathways [
41]. Lactate has also been shown to induce glutamatergic synaptic enhancement in hippocampal CA3 pyramidal cells, an effect that is synapse-specific and dependent on N-methyl-d-aspartic acid receptor (NMDARs) activation, cholera toxin-sensitive G-protein-coupled receptors, and post-synaptic calcium accumulation [
42]. In conclusion, lactate plays a critical role in regulating neuronal excitability and synaptic activity, though further studies are needed to clarify the precise mechanisms underlying these effects.
Synaptic plasticity represents a dynamic alteration in the strength and architecture of synaptic connections that underpin learning and memory processes. During early development, the human brain exhibits high levels of aerobic glycolysis and substantial synaptic proliferation, peaking around five years of age. In adulthood, aerobic glycolysis remains active only in the regions expressing juvenile genes, with a decline in synaptic plasticity-related gene expression, linking glycolysis to synaptic plasticity [
43]. Inhibition of astrocyte glycogen phosphorylase using DAB impedes lactate production and suppresses long-term potentiation (LTP) and memory consolidation during training in the rat hippocampus. Conversely, exogenous lactate restores these processes, whereas glucose does not, suggesting that the role of lactate extends beyond merely serving as an energy substrate [
44]. In the step-through inhibitory avoidance (IA) paradigm, training mice showed upregulated expression of genes associated with astrocyte-neuron metabolism, notably MCT1 and MCT4. Mice deficient in MCT1 and with suppressed MCT2 expression exhibit compromised memory [
45,
46]. Another study revealed that diminished expression of MCT4 in dorsal hippocampal astrocytes and MCT2 in neurons impairs spatial learning and memory reversible by lactate administration if MCT2 expression remains intact [
47]. MCT-mediated lactate transport in the hippocampus is crucial for learning spatial tasks. Inhibition of MCT1, MCT2, and MCT4 expression, along with DAB injection, attenuates methamphetamine (METH)-induced conditioned place preference (CPP), plasticity-related gene expression, neuronal Ca²⁺ levels, and synaptic structure. Exogenous lactate reverses these effects except when MCT2 is inhibited [
48]. These findings indicate that astrocyte-derived lactate, transported to neurons via MCT2, regulates synaptic activity and memory consolidation [
7]. Molecular mechanisms reveal that lactate enhances synaptic remodeling and memory consolidation by increasing synaptic remodeling gene expression [
31]. NMDAR activation by glutamate and glycine synergistically increases intracellular calcium concentration, Erk1/2 phosphorylation, and immediate early gene (IEG) expression such as Arc and Zif268 [
49]. Lactate metabolism elevates the intracellular NADH/NAD ratio, enhancing neuronal redox state and potentiating NMDAR activation and subsequent Erk1/2 signaling through redox-sensitive NR1 subunits, promoting IEG expression. Specific inhibitors MK801 and U0126 abolish lactate-induced IEG expression [
50]. cAMP, a pivotal intracellular second messenger, integrates signals from multiple G protein-coupled receptors (GPCRs) [
51]. Lactate may facilitate synaptic remodeling and memory consolidation through GPCR activation and downstream cAMP/protein kinase A (PKA) signaling pathways. Activation of cAMP leads to the targeting of PKA to key proteins involved in hippocampal synaptic plasticity and memory storage [
52,
53]. In summary, lactate plays an essential role in synaptic remodeling and memory consolidation via multiple signaling pathways, including IEG expression promotion through NMDARs and Erk1/2 signaling cascades, as well as the GPCR/cAMP/PKA pathway.
5. Targeted Therapy
With the in-depth study of lactylation mechanisms and functions, targeting lactylation has emerged as a novel strategy for treating tumors, metabolic diseases, and inflammation-related conditions. The lactylation process involves lactate generation, shuttling, receptor binding, and regulation by “writers” and “erasers,” all of which represent potential therapeutic targets [
77,
120,
131].
Targeting key glycolytic enzymes such as LDHA, HK, pyruvate dehydrogenase kinase (PDK), PKM2, and PFKFB3 to modulate lactate production has shown promise in cancer treatment, with several drugs advancing to clinical trials. LDHA inhibitors such as gossypol and its isomer AT-101 have shown significant effects in Phase II clinical trials, both as monotherapy and in combination with standard chemoradiotherapy [
132]. Stiripentol, an FDA-approved antiepileptic drug for Dravet syndrome, and its analogs (e.g., isosafrole), act as non-specific LDH inhibitors, blocking the conversion of pyruvate to lactate and exerting antiepileptic effects [
133]. The glycolytic inhibitor 2-DG has demonstrated safety and efficiency in Phase I/II trials (NCT00096707 and NCT00633087) for prostate cancer and advanced solid tumors [
134,
135]. In healthy brains with an intact blood-brain barrier (BBB), 2-DG can cross the BBB via glucose transporters, suggesting its potential for treating central nervous system diseases. 2-DG reduces lactate accumulation and H3K9 Lactylation, alleviating behavioral changes in MK801-induced schizophrenia (SCZ) mice [
103]. Dichloroacetate (DCA) enhances mitochondrial glucose oxidation and reduces lactate production by inhibiting PDK. A Phase I trial (NCT01029925) indicated that DCA is feasible and well tolerated in patients with recurrent glioblastoma and brain metastases [
136]. A Phase II trial (NCT01386632) demonstrated the safety of combining DCA with chemoradiotherapy for advanced head and neck squamous cell carcinoma, without adverse effects on survival and metabolites [
137]. In AD research, disrupting the glycolysis/H4K12la/PKM2 positive feedback loop with PKM2 inhibitors, such as shikonin or compound 3 K, reduced lactate and H4K12la levels, inhibited pro-inflammatory microglial activation, and improved spatial learning and memory in AD mice [
97]. Additionally, inhibiting PFKFB3 with the small-molecule inhibitor 3PO can suppress H4K12la and reduce the excessive transcription of NF-κB-related genes involved in chronic kidney disease fibrosis [
128].
Targeting monocarboxylate transporters (MCTs) effectively blocks lactate uptake. AZD3965, a dual MCT1 and MCT2 inhibitor, showed positive therapeutic effects in late-stage solid tumors and B-cell lymphoma in a Phase I trial (NCT01791595) [
138]. The MCT1 and MCT4 molecular chaperone inhibitors, CD147, Meplazumab, demonstrated safety and tolerability in Phase I and II trials in healthy volunteers and COVID-19 patients [
139]. Additionally, two clinical trials on the GPR81 inhibitor curcumin are ongoing: one assessing its safety in children with acute lymphoblastic leukemia during chemotherapy maintenance (NCT05045443) and another exploring whether curcumin combined with piperine can delay or prevent progression in prostate cancer, monoclonal gammopathy of undetermined significance, or specific myelomas (NCT04731844). The MCT1 inhibitor CHC can alleviate lactate-induced Snail1 lactylation and activation of TGF-β/Smad2 after hypoxia, reducing endothelial-to-mesenchymal transition, myocardial fibrosis, and cardiac dysfunction post-myocardial infarction [
67].
Furthermore, “writers” and “erasers” directly influence lactylation formation and removal, presenting new therapeutic opportunities. The CBP/p300 inhibitor, C646, reduces inflammation in hepatic ischemia-reperfusion injury by inhibiting HMGB1 lactylation [
130], whereas A-485 exhibits anti-angiogenic effects on proliferative retinopathy by reducing YY1 lactylation [
140]. The HDAC1-3 inhibitor MS-275 (entinostat) significantly increased H3K14la and H3K18la levels during early development [
71]. In mouse hematopoietic stem cells, HDAC inhibitors such as apicidin and MS-275 increased H3K18ac but decreased H3K18la levels due to acetylation crosstalk, reducing fibrosis-related gene expression [
141]. HDAC inhibitors, such as Vorinostat, Romidepsin, Belinostat, and Tucidinostat have been approved for the treatment of lymphomas [
142]. SIRT3, which delactylates CCNE2, may represent a new target for liver cancer treatment [
143]. Targeted lactylation has significant potential for disease treatment. Ongoing research and clinical trials of various drugs and targets have made substantial progress. However, lactylation shares “writers” and “erasers” with other post-translational modifications, particularly acetylation, raising the risk of off-target effects. As the molecular and regulatory mechanisms of lactylation in diseases become clearer, the development of more precise targeted drugs is anticipated.
6. Conclusion and Perspectives
Lactylation, an emerging post-translational modification, offers promising avenues for investigating both physiological functions and pathological conditions in the human body, with significant clinical implications. Elucidating the role of lactylation as a linkage between NCC metabolic states, gene regulatory networks (GRN), and developmental gene expression expands our insights into neurocristopathies, such as orofacial clefts and congenital heart disease, potentially paving the way for novel metabolism-based strategies in developmental and regenerative medicine. Moreover, the identification of lactylation in CNS diseases underscores its potential as a biomarker for early diagnosis, treatment, and prognosis.
Moving forward, elucidating the precise molecular mechanisms of lactylation remains imperative. Current research indicates lactyl-CoA and LGSH as substrates for lactylation, yet exploring additional substrates could deepen our understanding of lactylation’s role in various diseases and physiological states. Regulatory factors governing lactylation likely involve numerous “writers” and “erasers.” For instance, HBO1 from the MYST family mediates H3K9la, influencing the transcription of tumor-associated genes [
144], highlighting its role as a novel histone lactylation “writer.” Recent studies have also identified a p300-dependent interaction with the histone chaperone ASF1A in regulating H3K18la, impacting vascular endothelial injury [
145]. Additionally, alanyl-tRNA synthetase (AARS1) acts as a lactate sensor and a transferase in global lysine lactylation in tumor cells [
146], paralleled by AARS2 in mitochondrial protein lactylation [
121]. In prokaryotes, YiaC and CobB serve as “writers” and “erasers” for protein lactylation [
147], expanding our understanding of lactylation regulators and suggesting new therapeutic avenues.
Considering the complexity of epigenetic modifications, interventions targeting lactylation may inadvertently affect other acylation modifications, leading to off-target effects. Therefore, further safety studies are crucial to confirm the viability of these strategies in clinical applications. This review investigates the significance of lactylation modification in lactate metabolism, its underlying mechanisms, and its roles in neurodevelopment, neuropsychiatric disorders, nervous system tumors, and cerebrovascular diseases, thereby demonstrating the diverse effects of lactylation on brain function and disease progression. By elucidating the precise molecular mechanisms of lactylation and its specific effects on various stages of CNS diseases and cell types, we lay a robust foundation for precise neurological disease treatments through lactylation targeting.