Increased mitophagy in the skeletal muscle of spinal and bulbar muscular atrophy patients

Abstract

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disorder caused by polyglutamine expansion in the androgen receptor (AR) and characterized by the loss of lower motor neurons. Here we investigated pathological processes occurring in muscle biopsy specimens derived from SBMA patients and, as controls, age-matched healthy subjects and patients suffering from amyotrophic lateral sclerosis (ALS) and neurogenic atrophy. We detected atrophic fibers in the muscle of SBMA, ALS and neurogenic atrophy patients. In addition, SBMA muscle was characterized by the presence of a large number of hypertrophic fibers, with oxidative fibers having a larger size compared with glycolytic fibers. Polyglutamine-expanded AR expression was decreased in whole muscle, yet enriched in the nucleus, and localized to mitochondria. Ultrastructural analysis revealed myofibrillar disorganization and streaming in zones lacking mitochondria and degenerating mitochondria. Using molecular (mtDNA copy number), biochemical (citrate synthase and respiratory chain enzymes) and morphological (dark blue area in nicotinamide adenine dinucleotide-stained muscle cross-sections) analyses, we found a depletion of the mitochondria associated with enhanced mitophagy. Mass spectrometry analysis revealed an increase of phosphatidylethanolamines and phosphatidylserines in mitochondria isolated from SBMA muscles, as well as a 50% depletion of cardiolipin associated with decreased expression of the cardiolipin synthase gene. These observations suggest a causative link between nuclear polyglutamine-expanded AR accumulation, depletion of mitochondrial mass, increased mitophagy and altered mitochondrial membrane composition in SBMA muscle patients. Given the central role of mitochondria in cell bioenergetics, therapeutic approaches toward improving the mitochondrial network are worth considering to support SBMA patients.

Polyglutamine-expanded androgen receptor disrupts muscle triad, calcium dynamics and the excitation-contraction coupling gene expression program

“…By immunofluorescence of transversal sections of the quadriceps muscle we found reduced RYR1 immunoreactivity in AR100Q mice (Figure 5A and Supplementary Figure 7). Notably, in some fibers we noticed areas deprived of RYR1 staining, which resemble the muscle pathology that we previously described in SBMA patients (28). These abnormalities were exclusively found in fast-fibers and were not detected at 4 weeks of age.…”

“…Mutations in CACNA1S and RYR genes lead to malignant hyperthermia and specific forms of congenital myopathy, such as central core disease (55,56), which are characterized by muscle weakness, centrally located cores, and lack of mitochondria and oxidative enzymes with disorganized contractile apparatus (57). We have previously described a pattern of muscle pathology in SBMA patients that resembles central core disease, with muscle fibers presenting central cores deprived of mitochondria and oxidative enzyme activity (28). Importantly, a similar muscle pathology was detected also in AR100Q transgenic mice.…”

“…Importantly, we found upregulation of denervation markers, including embryonic myosin heavy chain 3 (Myh3), perinatal myosin heavy chain 8 (Myh8), myogenin (MyoG), and muscle-specific kinase (Musk), in 8-and 12-week-old, but not 4-week-old AR100Q mice ( Figure 3D). By hematoxylin and eosin (H/E) staining, we found severe muscle pathology at late stage (12 weeks) of disease characterized by a high grade of fiber size variability, high number of atrophic fibers, often angulated and grouped up to [16][17][18] fibers, presence of large hypertrophic fibers with central nuclei, which is a feature of SBMA muscle (28), a modest increase of perimysium connective tissue, all together revealing a picture of denervation atrophy with myopathic signs (Figure 3E), which recapitulate the muscle pathology described in SBMA patients (27). These observations indicate that denervation is a late event in this mouse model of SBMA, as it is in knock-in SBMA mice (11,30), and imply that reduced muscle strength precedes denervation, suggesting that it results from early pathological processes mainly ascribable to muscle.…”

“…In the last decade clinical and experimental evidence have established that in SBMA skeletal muscle atrophy is not only secondary to MN dysfunction, but it also results from primary cell-autonomous toxicity of polyQ-expanded AR in this peripheral tissue (23,24). Early muscle pathology and clear signs of myopathy are present in patients, including elevated levels of serum creatine kinase (CK), myofiber degeneration and structures similar to central cores detected before clinical symptoms (25)(26)(27)(28). Likewise, signs of myopathy and reduced intrinsic muscle force precede spinal cord pathology also in transgenic and knock-in mouse models of SBMA (11,29).…”

Kennedy disease (X-linked recessive bulbospinal neuronopathy): A comprehensive review from pathophysiology to therapy

“…It is not completely clear which hypothesis is the correct one and whether protein aggregation is a toxic or a protective mechanism [53]. Together with aggregates formation, mitochondrial abnormalities and axonal transport alterations may participate to disease pathogenesis [54] [55].…”