1.1. Introduction
Muscular dystrophies (MD) are hereditary or genetic (spontaneous/non-hereditary) diseases characterized by severe progressive deterioration of locomotor, respiratory, and/or cardiac muscles. While there are many types of muscular dystrophies, X-linked dystrophinopathy defined by over 70 different mutations in the dystrophin gene are the most common and occurs predominantly in males [
1,
2]. Such mutations can lead to a complete absence of dystrophin (Duchenne muscular dystrophy; DMD) or a truncated transcript (Becker muscular dystrophy; BMD) [
3,
4,
5,
6]. While DMD is generally more severe than BMD, both diseases cause muscle weakness, atrophy, and fibrosis leading to reduced mobility, respiratory and cardiac dysfunction, as well as reduced lifespans. Dystrophin mutations often result in additional systemic dysfunctions including cognitive impairment, digestive abnormalities, anxiety, depression, or obesity [
7,
8,
9].
While there is no cure, ongoing pre-clinical research seeks to restore the normal genetic sequence through emerging gene editing technologies such as CRISPR or produce truncated dystrophin transcripts with exon skipping and microdystrophin gene therapies [
10,
11,
12]. Exon skipping therapy was recently approved by the FDA in the USA for certain mutations, but the technology must be adapted for each of the numerous mutations. Their ability to partially improve muscle dysfunction underscores the importance of maintaining glucocorticoid therapy as a major standard of care given systemic inflammation is a major contributor to muscle dysfunction [
13,
14,
15] for virtually all persons with DMD. While the effectiveness of glucocorticoids in slowing the decline of muscle function demonstrates the value in targeting secondary contributors to these diseases, the eventual decline in muscle function and side effects (i.e. attenuated growth, obesity, mood disorders, other) [
16] underscores the importance of developing additional treatments that provide benefits for most patients regardless of the specific underlying mutation.
Exogenously administered agonists of adiponectin (ApN) and its downstream cell-surface receptors have emerged as an attractive candidate for pharmacological intervention given their anti-inflammatory properties in DMD [
17,
18,
19,
20,
21,
22]. Although pre-clinical data from
in vivo rodent and
in vitro human cell-based models suggests that exogenous ApN agonism may be beneficial for attenuating some of the secondary physiological stressors associated with DMD, further insight is required to elucidate the specific mechanisms. In addition to attenuating inflammation, ApN also induces considerable metabolic reprogramming in muscle [
10,
23,
24,
25,
26,
27,
28,
29]. In this regard, the purpose of this review is to provide an overview of the pathophysiology of DMD with a perspective of translating disease mechanisms to the development of ApN agonist therapies.
1.2. Muscle Damage and Inflammation in DMD
Dystrophin is an indispensable component of cellular architecture given that it is responsible for anchoring the actin cytoskeleton to the dystrophin-associated glycoprotein complex (DGC) at the cell membrane (sarcolemma) to maintain cellular stability [
30]. Dystrophin-deficient models are characterized by loss of myofiber integrity, essentially rendering muscle fibres susceptible to contraction-induced damage [
31]. In DMD, muscle fibres demonstrate chronic damage that arises from contraction-relaxation cycles. However, the regenerative capacity of muscle fibres eventually exhausts, leading to impaired homeostatic muscle repair and turnover [
32,
33,
34]. This cycle leads to muscle fibre necrosis and fibrofatty replacement of necrotic tissue, as well as atrophy, as a last-resort mechanism to uphold cyto-structure [
35].
Key to the pathology of DMD is prolonged activation of the innate immune system in response to the chronic contraction-induced damage of muscle fibres [
36]. The innate immune response is triggered when granulocytes, monocytes and monocyte-derived-macrophages, and dendritic cells are triggered by damage-associated molecular patterns (DAMPs) that leak from damaged muscle fibres [
37,
38]. DAMPs trigger the recruitment of macrophages and neutrophils to sites of damage by binding to their pathogen recognition receptors (PRRs), which include toll-like receptors (TLR2/4/7) [
33,
38,
39,
40,
41]. Interestingly, data has demonstrated that deleting TLR2 or administering a TLR7/9 antagonist in C57BL/10.
mdx mice (commonly utilized rodent model of DMD) reduces muscle inflammation and improves skeletal muscle function, thus supporting the notion that PRRs play a pivotal role in promoting muscle degeneration [
42,
43].
Following TLR activation, downstream inflammatory signaling is mediated by nuclear factor kappa B (NF-κB) [
44], c-Jun NH
2-terminal kinase (JNK) [
45], and interferon regulatory factors (IRFs), which are activated by tumour necrosis factor alpha (TNFα) [
46], interleukin (IL) 6 (IL-6) [
47], and the myeloid differentiation primary response 88 (MyD88)-dependent pathways [
34,
39]. NF-κB activation induces the expression of pro-inflammatory genes in the nucleus [
37,
39] including IL-6, which promotes inflammation. IL-6 also interferes with muscle satellite cell populations and impedes muscle regeneration [
48,
49].
The induction of pro-inflammatory signaling events occurs in M1 classically-activated macrophages [
36,
50]. Since DMD, by definition, is characterized by asynchronous cycles of muscle damage and repair, M1 macrophages must be continuously recruited to sites of damage to sustain an immune response. Consequently, a high concentration of pro-inflammatory cytokines such as TNFα, IL-6, and IL-1β perpetuates a chronic inflammatory state [
51]. Although many different chemo-attractive molecules can stimulate the recruitment of immune cells to dystrophic muscles, C-C motif chemokine receptor type 2 (CCR2) has demonstrated a significant role for recruiting inflammatory cells to sites of injury in C57BL/10.
mdx muscle [
32,
36]. During early phases of inflammation, elevated pro-inflammatory cytokine concentrations can lead to the production of inducible nitric oxide synthase (iNOS), which alongside other cytoplasmic and mitochondrial oxidizing radicals [
52,
53], can significantly damage dystrophin deficient skeletal muscle by increasing damage to surrounding tissues and causing aberrant cell lysis [
50]. While M1 pro-inflammatory macrophages generally induce damage, M2 CD206-expressing alternatively-activated anti-inflammatory macrophages release anti-inflammatory cytokines like IL-10, IL-4, and insulin-like growth factor-1 (IGF-1) which downregulate iNOS production and promote muscle repair in dystrophin deficient muscle [
50]. Among the many responsibilities of M2 macrophages, they are vital for regulating skeletal muscle regeneration by ensuring the proliferation and maturation of muscle progenitor cells, which include satellite cells and collagen-secreting fibroblasts [
54].
These steps culminate in two major mechanisms regulating muscle dysfunction. First, inflammation inhibits muscle satellite cells and regeneration [
55]. Second, continual recruitment of M2 macrophages leads to increased release of transforming growth factor beta (TGFβ) that stimulates fibroblast activity and production of extracellular matrix (ECM) proteins including excessive collagen to create a form of ‘reactive fibrosis’ [
56]. The balance between classically activated M1 populations and alternatively-activated M2 populations remains critical to consider when examining processes that maximize the reparative potential of muscle.
Neutrophils remove cellular debris that accumulates in damaged regions [
57]. Studies have shown that neutrophils are recruited to sites of injury at early stages of dystrophinopathy in C57BL/10.
mdx mice, and can be approximately 30% more numerous as macrophages in dystrophic muscle [
50,
58]. Despite the protective properties of neutrophils in healthy physiological systems, they can also impair regeneration in dystrophic muscle by stimulating the secretion of myeloperoxidase (MPO), which is predominantly involved in catalyzing the production of hypochlorous acid (HOCl) – a damaging and reactive oxidant – in the presence of hydrogen peroxide (H
2O
2) and chloride (Cl
-), at sites of inflammation [
36,
59,
60]. Data has shown that golden retriever muscular dystrophy (GRMD) muscle exhibits significantly higher levels of MPO compared to healthy WT muscles, thus suggesting that neutrophil-derived MPO might be contributing to muscle damage by inducing oxidative stress [
59]. In addition to MPO release, proteomic analyses of C57BL/10.
mdx muscle has also revealed elevated production of neutrophil elastase (NE), compared to healthy WT muscle [
61]. NE can be particularly damaging in dystrophic muscle due to its propensity to impair myoblast survival and proliferation by promoting cell adhesion molecule (CAM) degradation [
61].
Cytotoxic T-lymphocytes, or the CD4
+ T-cells have been identified as additional mediators of the dystrophic immune response. These cells, which can be subdivided into regulatory T-cells (Tregs) and conventional T helper (Th) cells, have been associated with reductions to muscle inflammation and damage in dystrophic muscle [
36]. The differentiation of Tregs from naïve T-cells is controlled by the transcription factor FoxP3, whose expression is induced by TGFβ [
36]. The presence of Tregs has generally been cited as being beneficial for dystrophic muscle given that they are immunosuppressive and express the anti-inflammatory cytokine, IL-10 [
36]. Two separate models, one employing rapamycin-treated C57BL/10.
mdx muscle to demonstrate elevated Tregs [
62] and the other depleting Tregs via antibody depletion of CD25
+ cells [
63], implicated Tregs in reducing muscle fibre damage, managing serum creatine kinase (CK) concentrations, and also reducing muscle inflammation and interferon γ (IFNγ) expression [
36,
50]. Additionally, ablation of FoxP3-expressing Tregs exacerbated
mdx muscle damage and led to elevated IFNγ expression and reductions to the expression of the M2 macrophage-specific marker, CD206 [
50]. Collectively, this data suggests that although Tregs exist at extraordinarily low frequency of occurrence in sites of damage, they still play a vital role in the transitionary stages of early C57BL/10.
mdx pathology to later-onset regenerative stages [
36].
Many of these inflammatory responses are attenuated by common glucocorticoids used as standard of care (e.g. prednisolone) for DMD. For example, a study conducted by [
64] investigated the effects of prednisolone treatment in C57BL/10.
mdx mice between 2 and 4 weeks of age on several immune-related markers, including pro-inflammatory macrophages (using an anti-F4/80 marker), CD4
+ T cells, and CD8
+ T cells in quadriceps and soleus muscle. The group determined that prednisolone treatment reduced F4/80-positive macrophages (57-59% reduction), CD4
+ T cells (50-60% reduction), and CD8
+ T cells (48-58% reduction) in both muscles [
64], suggesting that glucocorticoids may play an essential role in modulating the DMD-induced immune response. The effects of glucocorticoids on macrophage markers and T cell activation has been well characterized in several other pathological models with robust inflammation as well [
65,
66]. Interestingly, work investigating the effects of glucocorticoid treatment on human peripheral lung inflammation in asthma identified significant elevations to neutrophils following treatment, accompanied by improvements to lung function [
67], which may attest to the beneficial/pleotropic role of neutrophil recruitment at sites of injury and damage such as in DMD. However, the side effects of glucocorticoids warrant consideration of alternative therapies. As mentioned previously, ApN is an attractive candidate for consideration in DMD due to its anti-inflammatory properties – however, the mechanisms through which these anti-inflammatory effects are elicited still require elucidation as discussed in the next section. Understanding the role of ApN in various pathological models serves as a foundation for investigating how exogenous ApN administration might be efficacious in DMD for attenuating secondary physiological stressors.