Fiber Type Grouping Research Articles

Recovery from muscle weakness by exercise and FES: lessons from Masters, active or sedentary seniors and SCI patients

Many factors contribute to the decline of skeletal muscle that occurs as we age. This is a reality that we may combat, but not prevent because it is written into our genome. The series of records from World Master Athletes reveals that skeletal muscle power begins to decline at the age of 30years and continues, almost linearly, to zero at the age of 110years. Here we discuss evidence that denervation contributes to the atrophy and slowness of aged muscle. We compared muscle from lifelong active seniors to that of sedentary elderly people and found that the sportsmen have more muscle bulk and slow fiber type groupings, providing evidence that physical activity maintains slow motoneurons which reinnervate muscle fibers. Further, accelerated muscle atrophy/degeneration occurs with irreversible Conus and Cauda Equina syndrome, a spinal cord injury in which the human leg muscles may be permanently disconnected from the nervous system with complete loss of muscle fibers within 5-8years. We used histological morphometry and Muscle Color Computed Tomography to evaluate muscle from these peculiar persons and reveal that contraction produced by home-based Functional Electrical Stimulation (h-bFES) recovers muscle size and function which is reversed if h-bFES is discontinued. FES also reverses muscle atrophy in sedentary seniors and modulates mitochondria in horse muscles. All together these observations indicate that FES modifies muscle fibers by increasing contractions per day. Thus, FES should be considered in critical care units, rehabilitation centers and nursing facilities when patients are unable or reluctant to exercise.

Failed reinnervation in aging skeletal muscle

BackgroundSkeletal muscle displays a marked accumulation of denervated myofibers at advanced age, which coincides with an acceleration of muscle atrophy.MethodsIn this study, we evaluated the hypothesis that the accumulation of denervated myofibers in advanced age is due to failed reinnervation by examining muscle from young adult (YA) and very old (VO) rats and from a murine model of sporadic denervation secondary to neurotrypsin over-expression (Sarco mouse).ResultsBoth aging rat muscle and Sarco mouse muscle exhibited marked fiber-type grouping, consistent with repeating cycles of denervation and reinnervation, yet in VO muscle, rapsyn at the endplate increased and was associated with only a 10 % decline in acetylcholine receptor (AChR) intensity, whereas in Sarco mice, there was a decline in rapsyn and a 25 % decrease in AChR intensity. Transcripts of muscle-specific kinase (21-fold), acetylcholine receptor subunits α (68-fold), ε (threefold) and γ (47-fold), neural cell adhesion molecule (66-fold), and runt-related transcription factor 1 (33-fold) were upregulated in VO muscle of the rat, consistent with the marked persistent denervation evidenced by a large proportion of very small fibers (>20 %). In the Sarco mice, there were much smaller increases in denervation transcripts (0–3.5-fold) and accumulation of very small fibers (2–6 %) compared to the VO rat, suggesting a reduced capacity for reinnervation in aging muscle. Despite the marked persistent denervation in the VO rat muscle, transcripts of neurotrophins involved in promoting axonal sprouting following denervation exhibited no increase, and several miRNAs predicted to suppress neurotrophins were elevated in VO rat.ConclusionsOur results support the hypothesis that the accumulation of denervated fibers with aging is due to failed reinnervation and that this may be affected by a limited neurotrophin response that mediates axonal sprouting following denervation.Electronic supplementary materialThe online version of this article (doi:10.1186/s13395-016-0101-y) contains supplementary material, which is available to authorized users.

Histopathological changes in skeletal muscle associated with chronic ischaemia.

Muscle biopsy is an essential part in the diagnostic workup in patients with suspected neuromuscular disorders. It is therefore important to be aware of morphological alterations that can be caused by systemic factors or natural ageing. Chronic limb ischaemia is frequent in elderly individuals. This study was performed to examine histopathological and mitochondrial changes in muscle in patients with chronic critical limb ischaemia. Muscle biopsy of skeletal muscle of the lower limb of patients with chronic ischaemia leading to amputation was performed and compared with muscle biopsies of healthy, age-matched controls. The histopathological abnormalities included fibrosis, necrosis, atrophy, glycogen depletion, internal nuclei, rimmed vacuoles, fibre type grouping, cytochrome c oxidase deficient fibres, MHC-I upregulation, and signs of microangiopathy. The only alteration found in age-matched controls was a few cytochrome c oxidase deficient fibres. There were also increased levels of multiple mitochondrial DNA deletions in ischaemic muscles compared with controls. Critical limb ischaemia is associated with significant histopathological changes in muscle tissue and also increased levels of mitochondrial DNA deletions. Since the alterations mimic different primary myopathic changes, chronic ischaemia is important to consider as a differential diagnosis in elderly individuals, investigated with muscle biopsy for muscle disease.

Denervation-Associated Change in the Palatinus and Levator Veli Palatini Muscles of Dogs with Elongated Soft Palate

Muscle lesions and decreased numbers of peripheral nerve branches have been reported in the soft palates of dogs presenting with brachycephalic airway obstruction syndrome (BAOS). Myosin adenosine triphosphatase staining was employed to investigate whether muscle lesions in the elongated soft palate (ESP) of dogs with BAOS reflect the presence of denervation. Soft palates were collected from nine brachycephalic dogs during surgical intervention for BAOS and from five healthy beagle dogs as controls. In the control soft palates, myofibres with relatively uniform diameters and a random mosaic pattern of type I and II myofibres were observed in the palatinus muscle (PM), while almost all of the myofibres in the levator veli palatini muscle (LVPM) were of type II. In the ESPs, small group atrophy, large group atrophy and angular-shaped atrophy were observed in myofibres of the PM and rarely in the LVPM. Fibre type grouping and an increase in type IIC myofibres were found only in the PM. Morphometric analysis of ESPs revealed a significant increase in the number of type I and II myofibres in the PM showing atrophy or hypertrophy compared with controls. A significant increase in atrophic type II myofibres was found in the LVPM of affected dogs. Myopathy consistent with denervation was observed in the PM, but rarely in the LVPM, of ESP specimens. The results suggest that the myopathy seen in dogs with ESP may partly reflect atrophy of myofibres resulting from damage to peripheral nerve branches, with subsequent reinnervation of myofibres.

Polyglucosan bodies in intramuscular nerves: Association with muscle fiber denervation atrophy

Polyglucosan bodies (PB) in the intramuscular nerves have been rarely studied, and their presence particularly in subjects without neurologic disorders has been thought to be age-related. We examined, by using light and electron microscopy, 204 consecutive muscle biopsies. PB was found in 5 quadriceps intramuscular nerves (2.45% of all biopsies). All 5 quadriceps containing PB exhibited varying degrees of muscle fiber denervation atrophy with or without fiber type grouping. These quadriceps with PB, compared with the other 119 quadriceps without PB, showed a significantly greater association with muscle fiber denervation atrophy (5/5 versus 55/119; p=0.02, by two-tailed Fisher's exact test), for which aging is not confounding. Electron microscopy identified PB in intramuscular nerve myelinated fibers along with ongoing degenerative changes. Our observation suggests that PB in intramuscular nerves may be pathologic and associated with muscle fiber denervation atrophy.

G.P.315 - Coexisting TIA1 and MYH7 distal myopathy

Distal myopathies are genetically heterogeneous. TIA1 mutations cause Welander distal myopathy. MYH7 mutations result in various clinical phenotypes, including Laing distal myopathy, myosin storage myopathy and hypertrophic/dilated cardiomyopathy. We describe a family affected by distal myopathy and variable presence of cardiomyopathy due to coexisting TIA1 and MYH7 mutations. The propositus is a 67-year-old woman of Swedish and Greek descent with history of easy tripping since childhood who has experienced a stepwise progression of asymmetric distal limb weakness in the past 20 years. Neurological examination revealed mild to severe asymmetric distal lower >upper limb weakness. CK value was normal. EMG study showed myopathic changes with minimal fibrillation potentials and myotonic discharges. Biopsy of the extensor carpi radialis muscle showed: rare rimmed vacuoles, minicore-like structures and congophilic inclusions; few necrotic and regenerating fibers; fiber splitting; increased internal nuclei; and fiber type grouping. There was no cardiac involvement by ECG and echocardiogram. Her 66-year-old sister has 8 years of mild distal limb weakness, supraventricular tachycardia and hypertrophic cardiomyopathy documented by echocardiogram and MRI. Another sibling has distal weakness while parents were asymptomatic. Both sisters carry a known pathogenic heterozygous TIA1 mutation (p.Glu384Lys) and a rare heterozygous MYH7 variant (c.5459G > A; p.Arg1820Gln), which involves a highly conserved residue and is predicted to be pathogenic. Because a homozygous MYH7 mutation (p.Arg1820Trp) was reported in recessive myosin storage myopathy, aCGH deletion/duplication analysis of MYH7 was performed in the proband and it was normal. The clinical phenotype is dictated by the coexistent TIA1 and MYH7 mutations, which can result in similar pathological findings. The proband's early onset of distal leg weakness and the sister's cardiomyopathy support the pathogenicity of the MYH7.

Biology of Muscle Atrophy and of its Recovery by FES in Aging and Mobility Impairments: Roots and By-Products.

There is something in our genome that dictates life expectancy and there is nothing that can be done to avoid this; indeed, there is not yet any record of a person who has cheated death. Our physical prowess can vacillate substantially in our lifetime according to our activity levels and nutritional status and we may fight aging, but we will inevitably lose. We have presented strong evidence that the atrophy which accompanies aging is to some extent caused by loss of innervation. We compared muscle biopsies of sedentary seniors to those of life long active seniors, and show that these groups indeed have a different distribution of muscle fiber diameter and fiber type. The senior sportsmen have many more slow fiber-type groupings than the sedentary people which provides strong evidence of denervation-reinnervation events in muscle fibers. It appears that activity maintains the motoneurons and the muscle fibers. Premature or accelerated aging of muscle may occur as the result of many chronic diseases. One extreme case is provided by irreversible damage of the Conus and Cauda Equina, a spinal cord injury (SCI) sequela in which the human leg muscles may be completely and permanently disconnected from the nervous system with the almost complete disappearance of muscle fibers within 3-5 years from SCI. In cases of this extreme example of muscle degeneration, we have used 2D Muscle Color CT to gather data supporting the idea that electrical stimulation of denervated muscles can retain and even regain muscle. We show here that, if people are compliant, atrophy can be reversed. A further example of activity-related muscle adaptation is provided by the fact that mitochondrial distribution and density are significantly changed by functional electrical stimulation in horse muscle biopsies relative to those not receiving treatment. All together, the data indicate that FES is a good way to modify behaviors of muscle fibers by increasing the contraction load per day. Indeed, it should be possible to defer the muscle decline that occurs in aging people and in those who have become unable to participate in physical activities. Thus, FES should be considered for use in rehabilitation centers, nursing facilities and in critical care units when patients are completely inactive even for short periods of time.

Dramatic changes in muscle contractile and structural properties after 2 botulinum toxin injections.

Botulinum toxin is frequently administered serially to maintain therapeutic muscle paralysis, but the effect of repeated doses on muscle function are largely unknown. This study characterized the muscle response to 2 onabotulinum toxin (BoNT) injections separated by 3 months. Animal subjects received a single toxin injection (n = 8), 2 BoNT injections separated by 3 months (n = 14), or 1 BoNT and 1 saline injection separated by 3 months (n = 8). The functional effect of 2 serial injections was exponentially greater than the effect of a single injection. While both groups treated with a single BoNT injection had decreased torque in the injected leg by approximately 50% relative to contralateral legs, the double BoNT injected group had decreased torque by over 95% relative to the preinjection level. Both single and double BoNT injections produced clear signs of fiber-type grouping. These experiments demonstrate a disproportionately greater effect of repeated BoNT injections.

Superior Indices of Neuromuscular Function in Elite Octogenarian Masters Athletes

Aging is associated with a progressive deterioration of muscle mass and function that leads to greatest clinical impact in those 蠅80 y of age. Conversely, elite octogenarian masters athletes (MA) retain a remarkable degree of physical function. Because aging muscle is profoundly impacted by neuromuscular changes, we hypothesized that muscle of MAs would have attenuated changes in muscle morphology and neuromuscular function compared to non-athlete (NA) controls. Muscle biopsies of vastus lateralis muscle were obtained for analysis of fiber type grouping and fiber size in NA (n = 14, age: 81 ± 4 y) and elite MAs (n = 15, age: 80 ± 5 y). Motor unit number estimate, compound muscle action potential, and strength were conducted in the tibialis anterior muscle. Quadriceps area (QA) and strength (QS), and functional capacity (chair stand and stand-up & go) were also measured. Elite MAs exhibited greater QA (p = 0.015), QS (p = 0.013), and functional performance (p < 0.01) compared to NA. MAs had greater fiber size (p = 0.024), lower percentage of small fibers (< 2500µm; p = 0.031) and more fiber type grouping (p = 0.029) compared to NA. Furthermore, MAs had significantly greater motor unit number estimate and compound muscle action potential than NA. Consistent with our hypothesis, elite MAs exhibited not only superior indices of functional performance and muscle strength, but also better preserved muscle morphology and neuromuscular function including 1) fewer small fibers, 2) increased fiber type grouping (suggesting superior reinnervation), and 3) better retention of motor unit numbers versus NA.

Electromyographic Permutation Entropy Quantifies Diaphragmatic Denervation and Reinnervation

Spontaneous reinnervation after diaphragmatic paralysis due to trauma, surgery, tumors and spinal cord injuries is frequently observed. A possible explanation could be collateral reinnervation, since the diaphragm is commonly double-innervated by the (accessory) phrenic nerve. Permutation entropy (PeEn), a complexity measure for time series, may reflect a functional state of neuromuscular transmission by quantifying the complexity of interactions across neural and muscular networks. In an established rat model, electromyographic signals of the diaphragm after phrenicotomy were analyzed using PeEn quantifying denervation and reinnervation. Thirty-three anesthetized rats were unilaterally phrenicotomized. After 1, 3, 9, 27 and 81 days, diaphragmatic electromyographic PeEn was analyzed in vivo from sternal, mid-costal and crural areas of both hemidiaphragms. After euthanasia of the animals, both hemidiaphragms were dissected for fiber type evaluation. The electromyographic incidence of an accessory phrenic nerve was 76%. At day 1 after phrenicotomy, PeEn (normalized values) was significantly diminished in the sternal (median: 0.69; interquartile range: 0.66–0.75) and mid-costal area (0.68; 0.66–0.72) compared to the non-denervated side (0.84; 0.78–0.90) at threshold p<0.05. In the crural area, innervated by the accessory phrenic nerve, PeEn remained unchanged (0.79; 0.72–0.86). During reinnervation over 81 days, PeEn normalized in the mid-costal area (0.84; 0.77–0.86), whereas it remained reduced in the sternal area (0.77; 0.70–0.81). Fiber type grouping, a histological sign for reinnervation, was found in the mid-costal area in 20% after 27 days and in 80% after 81 days. Collateral reinnervation can restore diaphragm activity after phrenicotomy. Electromyographic PeEn represents a new, distinctive assessment characterizing intramuscular function following denervation and reinnervation.

Histopathological assessment of intrinsic laryngeal musculature in horses with dynamic laryngeal collapse

The pathogenesis of bilateral dynamic laryngeal collapse associated with poll flexion (DLC) of horses is unknown but might be associated with intrinsic laryngeal muscle weakness. To investigate histopathological characteristics of the cricoarytenoideus dorsalis, the cricothyroid (CT) and the cricoarytenoideus lateralis muscles in DLC-affected horses and compare these with unaffected controls. Our hypotheses were that evidence of neurogenic atrophy of the CT or cricoarytenoideus dorsalis muscles would be found in DLC-affected horses and that observed changes would be symmetrically (left/right) distributed, or that muscle fibre diameter would be significantly reduced in DLC-affected horses compared to unaffected controls, reflecting an underlying paresis. Case-control study. Five DLC horses and 8 controls were included. Muscle samples were harvested immediately following euthanasia. Fibre type proportions and size were evaluated by multiple immunofluorescence labelling of cryosections, and compared between sides (left/right) and groups (DLC-affected cases/ unaffected controls). Subjective and objective assessments of fibre type grouping were compared between sides and groups. Fibre type proportions, fibre size and the subjective assessment of fibre type grouping did not reveal any statistically significant differences between the groups. Objective assessment of fibre type grouping revealed significantly more large clusters of T1 fibres within the left cricoarytenoideus lateralis muscle of DLC-affected cases versus controls, and within the right CT muscle of control horses compared to the DLC-cases. The absence of bilateral symmetric fibre type grouping, fibre type loss and fibre atrophy in the DLC-affected cases do not support a neuromuscular component within the pathogenesis of DLC. The objective assessment of fibre type grouping revealed some statistical differences between the DLC-affected cases and the unaffected controls; however, these findings were inconsistent with regard to DLC. An alternative aetiology of DLC seems likely.

G.P.241: Muscle histopathology in late-onset spinal muscular atrophies differs from c9orf72-related-ALS

Muscle biopsy findings in amyotrophic lateral sclerosis (ALS) patients have been described in several publications, but the cohorts have been etiologically heterogeneous. Our aim was to assess myopathological differences between three genetically defined motor neuron disease entities. We analyzed all muscle biopsies of c9ORF72-related ALS (c9ALS) patients performed at our unit and compared them with biopsies of patients with benign hereditary lower motor neuron disorders (Late-onset spinal motor neuronopathy, LOSMoN and spinobulbar muscular atrophy, SBMA). We found the fiber type grouping of also normal sized type IIA (2A by ATPase) fibers to be very specific to the late-onset spinal muscular atrophy (LOSMoN/SBMA) cohort, whereas in c9ALS patients, type IIA fiber groups were always markedly atrophic. Severely atrophic rounded type IIA fibers, scattered or in groups, were significantly more common in LOSMoN/SBMA than c9ALS. Small group atrophy consisting of fast and slow fibers were more common in c9ALS than in LOSMoN/SBMA. According to our results, muscle biopsy may be a useful additional diagnostic tool in suspected motor neuron diseases.

Long-Term High-Level Exercise Promotes Muscle Reinnervation With Age

The histologic features of aging muscle suggest that denervation contributes to atrophy, that immobility accelerates the process, and that routine exercise may protect against loss of motor units and muscle tissue. Here, we compared muscle biopsies from sedentary and physically active seniors and found that seniors with a long history of high-level recreational activity up to the time of muscle biopsy had 1) lower loss of muscle strength versus young men (32% loss in physically active vs 51% loss in sedentary seniors); 2) fewer small angulated (denervated) myofibers; 3) a higher percentage of fiber-type groups (reinnervated muscle fibers) that were almost exclusive of the slow type; and 4) sparse normal-size muscle fibers coexpressing fast and slow myosin heavy chains, which is not compatible with exercise-driven muscle-type transformation. The biopsies from the old physically active seniors varied from sparse fiber-type groupings to almost fully transformed muscle, suggesting that coexpressing fibers appear to fill gaps. Altogether, the data show that long-term physical activity promotes reinnervation of muscle fibers and suggest that decades of high-level exercise allow the body to adapt to age-related denervation by saving otherwise lost muscle fibers through selective recruitment to slow motor units. These effects on size and structure of myofibers may delay functional decline in late aging.

Reinnervation of Vastus lateralis is increased significantly in seniors (70-years old) with a lifelong history of high-level exercise

It has long been recognized that histological changes observed in aging muscle suggest that denervation contributes to muscle deterioration and that disuse accelerates the process while running activity, sustained for decades, protects against age-related loss of motor units. Here we show at the histological level that lifelong increased physical activity promotes reinnervation of muscle fibers. In muscle biopsies from 70-year old men with a lifelong history of high-level physical activity, we observed a considerable increase in fiber-type groupings (almost exclusively of the slow type) in comparison to sedentary seniors, revealing a large population of reinnervated muscle fibers in the sportsmen. Slow-type transformation by reinnervation in senior sportsmen seems to be a clinically relevant mechanism: the muscle biopsies fluctuate from those with scarce fiber-type transformation and groupings to almost fully transformed muscle, going through a process in which isolated fibers co-expressing fast and slow MHCs seems to fill the gaps. Taken together, our results suggest that, beyond the direct effects of aging on the muscle fibers, changes occurring in skeletal muscle tissue appear to be largely, although not solely, a result of sparse denervation. Our data suggest that lifelong exercise allows the body to adapt to the consequences of the age-related denervation and to preserve muscle structure and function by saving otherwise lost muscle fibers through recruitment to different, mainly slow, motor units. These beneficial effects on motoneurons and, subsequently on muscle fibers, serve to maintain size, structure and function of muscle fibers, delaying the functional decline and loss of independence that are commonly seen in late aging.

P.10.17 Morphological changes in skeletal muscle associated with chronic ischemia

Chronic ischemia of the lower limbs is frequent in elderly individuals and is usually caused by arteriosclerosis and/or diabetes. In elderly people changes caused by chronic ischemia may influence the interpretation of a muscle biopsy analysis. It is therefore important to be aware of morphological alterations that may be caused by chronic ischemia. We investigated muscle biopsy specimens from the tibialis anterior muscle in 26 individuals (54–90 years of age, 13 with diabetes) amputated because of lower leg ischemia and compared them with 10 controls (72–78 years of age). None of the patients had a diagnosis of a primary neuromuscular disease. The specimens were investigated by routine histological techniques and by LX-PCR to identify mtDNA deletions. Morphological alterations in the 26 patients with chronic ischemia that were not present in controls included rimmed vacuoles (12 patients), interstitial fibrosis (14 patients), fiber type grouping (22 patients), group atrophy (19 patients) fibers with glycogen depletion (7 patients), necrotic fibers (6 patients), internalized nuclei (26 pateints), MHC class I upregulation (3 patients) and microangiopathy (13 patients). Cytochrome c oxidase deficient muscle fibres were identified in 25 patients (0.1–1.8% of the fibers) and in 7 controls (0.2–0.5% of the fibers). Multiple mtDNA deletions were identified in 16 patients and one control. None of the alterations were predominant in patients with diabetes. The results demonstrate that chronic ischemia is associated with marked morphological alterations in muscle that may mimic a primary myopathy. Neurogenic changes were frequent both among diabetic and non-diabetic patients.

P.12.2 Dominant distal myopathy due to slow channelopathy

Slow channel syndrome, first recognized by Engel in 1982, has distinct phenotypic features, dominant inheritance, selective weakness of cervical scapular and finger extensor weakness, ophtalmoparesis. Histochemical findings show type 1 fibers preponderance, small group of atrophic fibers of either fiber type, increased fiber size variability, tubular aggregates. We examined 3 biopsies in 3 patients (2 males – 55 years and 66 years, 1 female – 61 years) presenting a dominant distal myopathy with variable age at onset (from second to third decade). The patients had ophtalmoparesis, upper limb extensor, forearm and finger weakness and distal myopathy with a progressive slow course. The woman had also some difficulty in anterior tibial flexor muscle but also atrophy of forearms muscles. Electrophysiology on single stimulus of ulnar nerve showed repetitive CMAP. The genomic wide linkage analysis identified 3 regions co-segregating with the disease in chromosome 1, 13, 17. Next generation sequencing showed in all patients a known mutation in epsilon-subunit of AchR receptor and a mutation in phospholipase. Muscle showed mild changes consisting in atrophic fibers, mostly of type 2, some fiber type grouping, central nuclei and ring fibers. Such fibers are consistent with abnormal regeneration.We suggest that the occurrence of mutations involving both AchR receptor and phospholipase might concur in determining a new distal phenotype with permanent weakness. Our family was previously diagnosed as myotonic dystrophy or distal myopathy. The presence of the histochemical findings of both myopathy and neuropathy were present. We suggest the possibility of calcium overload due to prolonged open time of the AChR channel with a presynaptic component might determine a distal neuro-myopathy with slow evolution and muscle rearrangement due to a muscle fiber degeneration and functional denervation. The slow channel mutation should be searched in families with permanent distal weakness.

A PATIENT WITH CHOREA-ACANTHOCYTOSIS AND DILATED CARDIOMYOPATHY

We discuss the case of a 43-year-old male with a 20-year history of a slowly progressive neurodegenerative syndrome. He first presented with epilepsy at the age of 26 but over...

Altered Pharyngeal Muscles in Parkinson Disease

Dysphagia (impaired swallowing) is common in patients with Parkinson disease (PD) and is related to aspiration pneumonia, the primary cause of death in PD. Therapies that ameliorate the limb motor symptoms of PD are ineffective for dysphagia. This suggests that the pathophysiology of PD dysphagia may differ from that affecting limb muscles, but little is known about potential neuromuscular abnormalities in the swallowing muscles in PD. This study examined the fiber histochemistry of pharyngeal constrictor and cricopharyngeal sphincter muscles in postmortem specimens from 8 subjects with PD and 4 age-matched control subjects. Pharyngeal muscles in subjects with PD exhibited many atrophic fibers, fiber type grouping, and fast-to-slow myosin heavy chain transformation. These alterations indicate that the pharyngeal muscles experienced neural degeneration and regeneration over the course of PD. Notably, subjects with PD with dysphagia had a higher percentage of atrophic myofibers versus with those without dysphagia and controls. The fast-to-slow fiber-type transition is consistent with abnormalities in swallowing, slow movement of food, and increased tone in the cricopharyngeal sphincter in subjects with PD. The alterations in the pharyngeal muscles may play a pathogenic role in the development of dysphagia in subjects with PD.

Destabilization of the neuromuscular junction by proteolytic cleavage of agrin results in precocious sarcopenia

Etiology and pathogenesis of sarcopenia, the progressive decline in skeletal muscle mass and strength that occurs with aging, are still poorly understood. We recently found that overexpression of the neural serine protease neurotrypsin in motoneurons resulted in the degeneration of their neuromuscular junctions (NMJ) within days. Therefore, we wondered whether neurotrypsin-dependent NMJ degeneration also affected the structure and function of the skeletal muscles. Using histological and functional analyses of neurotrypsin-overexpressing and neurotrypsin-deficient mice, we found that overexpression of neurotrypsin in motoneurons installed the full sarcopenia phenotype in young adult mice. Characteristic muscular alterations included a reduced number of muscle fibers, increased heterogeneity of fiber thickness, more centralized nuclei, fiber-type grouping, and an increased proportion of type I fibers. As in age-dependent sarcopenia, excessive fragmentation of the NMJ accompanied the muscular alterations. These results suggested the destabilization of the NMJ through proteolytic cleavage of agrin at the onset of a pathogenic pathway ending in sarcopenia. Studies of neurotrypsin-deficient and agrin-overexpressing mice revealed that old-age sarcopenia also develops without neurotrypsin and is not prevented by elevated levels of agrin. Our results define neurotrypsin- and age-dependent sarcopenia as the common final outcome of 2 etiologically distinct entities.

Late-onset lower motor neuronopathy

Characterization of a new type of late-onset autosomal dominant lower motor neuron disease. Patients from 2 families underwent detailed neurologic, electrophysiologic, muscle biopsy, and laboratory investigations. MRI of lower limbs was performed in selected patients. DNA samples from leukocytes were used for molecular genetic linkage studies. First symptoms were muscle cramps and fasciculations after age 25-30, followed by a slowly progressive proximal and distal weakness without overt atrophy during the first decades of symptoms. Nerve conduction velocities were within normal range and EMG showed widespread neurogenic alterations. Muscle biopsy revealed characteristic neurogenic findings: fiber type grouping and group atrophy. MRI showed diffuse fatty-degenerative changes, marked in medial gastrocnemius. Exactly the same clinical phenotype has not previously been described, and linkage studies showed exclusion of known chromosomal loci for hereditary motor neuropathies, suggesting the disease we report may represent a new disorder.