Alterations in skeletal muscle repair in young adults with type 1 diabetes mellitus

Though preclinical models of type 1 diabetes (T1D) exhibit impaired muscle regeneration, this has yet to be investigated in humans with T1D. Here, we investigated the impact of damaging exercise (eccentric quadriceps contractions) in 18 physically active young adults with and without T1D. Pre- and postexercise (48 h and 96 h), the participants provided blood samples, vastus lateralis biopsies, and performed maximal voluntary quadriceps contractions (MVCs). Skeletal muscle sarcolemmal integrity, extracellular matrix (ECM) content, and satellite cell (SC) content/proliferation were assessed by immunofluorescence. Transmission electron microscopy was used to quantify ultrastructural damage. MVC was comparable between T1D and controls before exercise. Postexercise, MVC was decreased in both groups, but subjects with T1D exhibited moderately lower strength recovery at both 48 h and 96 h. Serum creatine kinase, an indicator of muscle damage, was moderately higher in participants with T1D at rest and exhibited a small elevation 96 h postexercise. Participants with T1D showed lower SC content at all timepoints and demonstrated a moderate delay in SC proliferation after exercise. A greater number of myofibers exhibited sarcolemmal damage (disrupted dystrophin) and increased ECM (laminin) content in participants with T1D despite no differences between groups in ultrastructural damage as assessed by electron microscopy. Finally, transcriptomic analyses revealed dysregulated gene networks involving RNA translation and mitochondrial respiration, providing potential explanations for previous observations of mitochondrial dysfunction in similar cohorts with T1D. Our findings indicate that skeletal muscle in young adults with moderately controlled T1D is altered after damaging exercise, suggesting that longer recovery times following intense exercise may be necessary.

Normal to enhanced intrinsic mitochondrial respiration in skeletal muscle of middle- to older-aged women and men with uncomplicated type 1 diabetes

AIMS/HYPOTHESIS

This study interrogated mitochondrial respiratory function and content in skeletal muscle biopsies of healthy adults between 30 and 72 years old with and without uncomplicated type 1 diabetes.

METHODS

Participants (12 women/nine men) with type 1 diabetes (48 ± 11 years of age), without overt complications, were matched for age, sex, BMI and level of physical activity to participants without diabetes (control participants) (49 ± 12 years of age). Participants underwent a Bergström biopsy of the vastus lateralis to assess mitochondrial respiratory function using high-resolution respirometry and citrate synthase activity. Electron microscopy was used to quantify mitochondrial content and cristae (pixel) density.

RESULTS

Mean mitochondrial area density was 27% lower (p = 0.006) in participants with type 1 diabetes compared with control participants. This was largely due to smaller mitochondrial fragments in women with type 1 diabetes (-18%, p = 0.057), as opposed to a decrease in the total number of mitochondrial fragments in men with diabetes (-28%, p = 0.130). Mitochondrial respiratory measures, whether estimated per milligram of tissue (i.e. mass-specific) or normalised to area density (i.e. intrinsic mitochondrial function), differed between cohorts, and demonstrated sexual dimorphism. Mass-specific mitochondrial oxidative phosphorylation (OXPHOS) capacity with the substrates for complex I and complex II (C_I + II) was significantly lower (-24%, p = 0.033) in women with type 1 diabetes compared with control participants, whereas mass-specific OXPHOS capacities with substrates for complex I only (pyruvate [C_{I pyr}] or glutamate [C_{I glu}]) or complex II only (succinate [C_{II succ}]) were not different (p > 0.404). No statistical differences (p > 0.397) were found in mass-specific OXPHOS capacity in men with type 1 diabetes compared with control participants despite a 42% non-significant increase in C_{I glu} OXPHOS capacity (p = 0.218). In contrast, intrinsic C_I + II OXPHOS capacity was not different in women with type 1 diabetes (+5%, p = 0.378), whereas in men with type 1 diabetes it was 25% higher (p = 0.163) compared with control participants. Men with type 1 diabetes also demonstrated higher intrinsic OXPHOS capacity for C_{I pyr} (+50%, p = 0.159), C_{I glu} (+88%, p = 0.033) and C_{II succ} (+28%, p = 0.123), as well as higher intrinsic respiratory rates with low (more physiological) concentrations of either ADP, pyruvate, glutamate or succinate (p < 0.012). Women with type 1 diabetes had higher (p < 0.003) intrinsic respiratory rates with low concentrations of succinate only. Calculated aerobic fitness (Physical Working Capacity Test [PWC₁₃₀]) showed a strong relationship with mitochondrial respiratory function and content in the type 1 diabetes cohort.

CONCLUSIONS/INTERPRETATION

In middle- to older-aged adults with uncomplicated type 1 diabetes, we conclude that skeletal muscle mitochondria differentially adapt to type 1 diabetes and demonstrate sexual dimorphism. Importantly, these cellular alterations were significantly associated with our metric of aerobic fitness (PWC₁₃₀) and preceded notable impairments in skeletal mass and strength.

Impaired Function and Altered Morphology in the Skeletal Muscles of Adult Men and Women With Type 1 Diabetes

CONTEXT

Previous investigations on skeletal muscle health in type 1 diabetes (T1D) have generally focused on later stages of disease progression where comorbidities are present and are posited as a primary mechanism of muscle dysfunction.

OBJECTIVE

To investigate skeletal muscle function and morphology across the adult lifespan in those with and without T1D.

DESIGN

Participants underwent maximal contraction (MVC) testing, resting muscle biopsy, and venous blood sampling.

SETTING

Procedures in this study were undertaken at the McMaster University Medical Centre.

PARTICIPANTS

Sixty-five healthy adult (18-78 years old) men/males and women/females (T1D = 34; control = 31) matched for age/biological sex/body mass index; self-reported physical activity levels were included.

MAIN OUTCOME MEASURES

Our primary measure in this study was MVC, with supporting histological/immunofluorescent measures.

RESULTS

After 35 years of age ("older adults"), MVC declined quicker in T1D subjects compared to controls. Loss of strength in T1D was accompanied by morphological changes associated with accelerated aging. Type 1 myofiber grouping was higher in T1D, and the groups were larger and more numerous than in controls. Older T1D females exhibited more myofibers expressing multiple myosin heavy chain isoforms (hybrid fibers) than controls, another feature of accelerated aging. Conversely, T1D males exhibited a shift toward type 2 fibers, with less evidence of myofiber grouping or hybrid fibers.

CONCLUSIONS

These data suggest impairments to skeletal muscle function and morphology exist in T1D. The decline in strength with T1D is accelerated after 35 years of age and may be responsible for the earlier onset of frailty, which characterizes those with diabetes.

Muscle and serum myostatin expression in type 1 diabetes

Type 1 diabetes (T1D) has been reported to negatively affect the health of skeletal muscle, though the underlying mechanisms are unknown. Myostatin, a myokine whose increased expression is associated with muscle‐wasting diseases, has not been reported in humans with T1D but has been demonstrated to be elevated in preclinical diabetes models. Thus, the purpose of this study was to determine if there is an elevated expression of myostatin in the serum and skeletal muscle of persons with T1D compared to controls. Secondarily, we aimed to explore relationships between myostatin expression and clinically important metrics (e.g., HbA_1c, strength, lean mass) in women and men with (N = 31)/without T1D (N = 24) between 18 and 72 years old. Body composition, baseline strength, blood sample and vastus lateralis muscle biopsy were evaluated. Serum, but not muscle, myostatin expression was significantly elevated in those with T1D versus controls, and to a greater degree in T1D women than T1D men. Serum myostatin levels were not significantly associated with HbA_1c nor disease duration. A significant correlation between serum myostatin expression and maximal voluntary contraction (MVC) and body fat mass was demonstrated in control subjects, but these correlations did not reach significance in those with T1D (MVC: R = 0.64 controls vs. R = 0.37 T1D; Body fat: R = −0.52 controls/R = −0.02 T1D). Collectively, serum myostatin was correlated with lean mass (R = 0.45), and while this trend was noted in both groups separately, neither reached statistical significance (R = 0.47 controls/R = 0.33 T1D). Overall, while those with T1D exhibited elevated serum myostatin levels (particularly females) myostatin expression was not correlated with clinically relevant metrics despite some of these relationships existing in controls (e.g., lean/fat mass). Future studies will be needed to fully understand the mechanisms underlying increased myostatin in T1D, with relationships to insulin dosing being particularly important to elucidate.

Loss of dystrophin expression in skeletal muscle is associated with senescence of macrophages and endothelial cells

Cellular senescence is the irreversible arrest of normally dividing cells and is driven by cell cycle inhibitory proteins such as p16, p21, and p53. When cells enter senescence, they secrete a host of proinflammatory factors known as the senescence-associated secretory phenotype, which has deleterious effects on surrounding cells and tissues. Little is known of the role of senescence in Duchenne muscular dystrophy (DMD), the fatal X-linked neuromuscular disorder typified by chronic inflammation, extracellular matrix remodeling, and a progressive loss in muscle mass and function. Here, we demonstrate using C57-mdx (8-wk-old) and D2-mdx (4-wk-old and 8-wk-old) mice, two mouse models of DMD, that cells displaying canonical markers of senescence are found within the skeletal muscle. Eight-week-old D2-mdx mice, which display severe muscle pathology, had greater numbers of senescent cells associated with areas of inflammation, which were mostly Cdkn1a-positive macrophages, whereas in C57-mdx muscle, senescent populations were endothelial cells and macrophages localized to newly regenerated myofibers. Interestingly, this pattern was similar to cardiotoxin (CTX)-injured wild-type (WT) muscle, which experienced a transient senescent response. Dystrophic muscle demonstrated significant upregulations in senescence pathway genes [Cdkn1a (p21), Cdkn2a (p16^INK4A), and Trp53 (p53)], which correlated with the quantity of senescence-associated β-galactosidase (SA-β-Gal)-positive cells. These results highlight an underexplored role for cellular senescence in murine dystrophic muscle.

The Role of AMPK in Neuromuscular Biology and Disease

AMP-activated protein kinase (AMPK) is a primary regulator of cellular metabolism. Recent studies have revealed that AMPK also mediates the maintenance and plasticity of α-motoneurons, the neuromuscular junction, and skeletal muscle. Furthermore, AMPK stimulation by either genetic, pharmacological, or physiological approaches elicits beneficial phenotypic remodeling in neuromuscular disorders (NMDs). Here, we review the role of AMPK as a governor of neuromuscular biology, and present evidence for AMPK as an effective molecular target for therapeutic pursuit in the context of the most prevalent NMDs, including Duchenne muscular dystrophy, spinal muscular atrophy, and myotonic dystrophy type 1. This information may be useful for engineering AMPK-targeted pharmacological- or lifestyle-based strategies to treat disorders of the neuromuscular system.