Muscle Growth, Regeneration, and Injury PDF

NEUR3101 MUSCLE AND MOTOR CONTROL MUSCLE GROWTH, INJURY AND REGENERATION Dr Bart Bolsterlee Outline This lecture will cover: 1. Development of muscle cells (primary myogenesis) 2. Muscle response to injury 3. Muscle damage through eccentric exercise Four stages of muscle...

NEUR3101 MUSCLE AND MOTOR CONTROL MUSCLE GROWTH, INJURY AND REGENERATION Dr Bart Bolsterlee Outline This lecture will cover: 1. Development of muscle cells (primary myogenesis) 2. Muscle response to injury 3. Muscle damage through eccentric exercise Four stages of muscle development Four stages of neuromuscular development: 1. Axonal outgrowth (nerve meets muscle) 2. Myogenesis (birth of the muscle cell) 3. Synaptogenesis (birth of the neuromuscular junction) 4. Synapse elimination (elimination of extra neuromuscular connections) Lieber (2010) Lippincott Williams & Wilkins Axonal outgrowth  Motor nerves exit through the ventral root of the spinal cord to innervate muscles. Lieber (2010) Lippincott Williams & Wilkins Axonal outgrowth  Motor nerves exit through the ventral root of the spinal cord to innervate muscles.  Connections between muscle and nerve are specific.  How do nerves ‘find’ the appropriate muscle? ─ Cross-innervation experiments show that even when the geographical location of a muscle is altered, the nerve finds the right muscle! Lance & Landmesser, 1980  Specificity begins when nerves first grow out from the ventral horn. Lieber (2010) Lippincott Williams & Wilkins Four stages of muscle development Four stages of neuromuscular development: 1. Axonal outgrowth (nerve meets muscle) 2. Myogenesis (birth of the muscle cell) 3. Synaptogenesis (birth of the neuromuscular junction) 4. Synapse elimination (elimination of extra neuromuscular connections) Lieber (2010) Lippincott Williams & Wilkins Myogenesis – birth of the muscle cell  Myogenesis is controlled by myogenic regulatory factors (transcription factors). A. Somites differentiate into myoblasts (mono- nucleated precursor cells). B. Myoblasts fuse to form primary myotubes (100-300 μm). Lieber (2010) Lippincott Williams & Wilkins Myogenesis – birth of the muscle cell C. More myoblasts fuse at the ends of the primary myotubes. The basal lamina encapsulates the primary myotube. D. More myoblasts aggregate, using the primary myotube as a structural scaffold to form secondary myotubes. Some unfused myoblasts remain as satellite cells. Myogenesis – birth of the muscle cell E. Cross-sectional view of myogenesis. Myotubes grow by adding contractile proteins, pushing the nucleus to the boundary. F. Further development of primary and secondary myotubes within the same basal lamina. Myogenesis – birth of the muscle cell G. Primary and secondary myotubes separate to form myofibres, each containing myonuclei and satellite cells. H. The distinct polygonal pattern of muscle tissue arises with myonuclei at the periphery and satellite cells between the myotubes and basal lamina. Myogenesis – birth of the muscle cell  Different muscles originate from populations of precursor cells (somites) located at different locations along the spinal cord.  These differences in origin may explain differences in how muscle degenerate and regenerate. Four stages of muscle development Four stages of neuromuscular development: 1. Axonal outgrowth (nerve meets muscle) 2. Myogenesis (birth of the muscle cell) 3. Synaptogenesis (birth of the neuromuscular junction) 4. Synapse elimination (elimination of extra neuromuscular connections) Lieber (2010) Lippincott Williams & Wilkins Synaptogenesis – neuromuscular junction formation A. During development, acetyl choline (ACh) receptors are distributed along the muscle fibre. B. When the nerve contacts the fibre, ACh receptors aggregate around the junction. Synaptogenesis – neuromuscular junction formation C. The number of extrajunctional ACh receptors decreases during maturation. D. When nerve contact is lost, extrajunctional ACh receptors increase in number (to attract new incoming nerve?) Four stages of muscle development Four stages of neuromuscular development: 1. Axonal outgrowth (nerve meets muscle) 2. Myogenesis (birth of the muscle cell) 3. Synaptogenesis (birth of the neuromuscular junction) 4. Synapse elimination (elimination of extra neuromuscular connections) Lieber (2010) Lippincott Williams & Wilkins Synapse elimination  Two to six motoneurons may form synapses at the same neuromuscular junction during development.  Shortly after birth, extra synapses are eliminated so that a single motoneuron innervates a mature muscle fibre. Development of fibre types  Mammalian muscles are composed of different fibre types (slow-twitch, fast-twitch, and a range of types in between). Development of fibre types  Mammalian muscles are composed of different fibre types (slow-twitch, fast-twitch, and a range of types in between).  Neural innervation plays an important role in determining fibre type.  Cross-reinnervation experiments showed that a mostly fast-twitch muscle can become slow- twitch through innervation by a nerve that initially innervated a mostly slow-twitch muscle (and vice versa). Buller et al. J Physiol 1960 Development of fibre types  Myoblasts can already exist as different types.  Primary and secondary myotubes can also exist as slow- and fast-twitch types.  So when the nerve meets the muscle, fibre type is already heterogeneous. Muscle injury and regeneration  Muscles have a remarkable ability to regenerate after injury/damage.  Damage can be caused by: ─ metabolic factors ─ mechanical factors  The regenerative process largely mimics the developmental process. Muscle injury and regeneration  The major steps of muscle regeneration (secondary myogenesis) include: 1. digestion of the damaged cellular components 2. activation and proliferation of satellite cells to form new muscle-building material 3. fusion of satellite cells to form new myotubes and muscle fibres.  Satellite cell activation is normally inhibited. Injury to the muscle membrane or the cell’s basal lamina can remove these inhibitions. Muscle injury and regeneration 1. Digestion of damaged cellular components by enzymes released from macrophages. ─ If microcirculation is absent (ischaemia or circulation is traumatised), this process is delayed. Lieber (2010) Lippincott Williams & Wilkins Muscle injury and regeneration 2. Proliferation of satellite cells ─ Satellite cells are located along the periphery between the sarcolemma and basal lamina. ─ Proliferation is controlled by growth factors. ─ An intact sarcolemma (muscle cell membrane) inhibits proliferation. Lieber (2010) Lippincott Williams & Wilkins Muscle injury and regeneration 3. Fusion and maturation of myoblasts ─ The rest of the regeneration cycle is in many ways similar to normal development. ─ Satellite cells align along the basal lamina. ─ Maturation involves continued differentiation and synthesis of new myofibrillar proteins. Lieber (2010) Lippincott Williams & Wilkins Single myoblasts on a culture plate Day 2-3: myoblast fusion Day 2-3: myoblasts elongate and After day 13: The fibres have normal start to look like fibres electrical and contractile properties. Muscle injury and regeneration  After repeated cycles of regeneration: ─ fibre diameter is more variable ─ nuclei become more centralised ─ regeneration efficiency decreases (satellite cells depleted?) ─ fat infiltration ─ abnormal branching Tidball & Wehling-Henricks. (2004). Pediatr Res 56 p831-841 Regeneration of muscle function  Muscles can regenerate some, but not all, of their function when transplanted. Lieber (2010) Lippincott Williams & Wilkins Faulkner et al. (1980). Cell Physiol 238 p120-126.  Regeneration of muscle function  Muscle mass restored to 73% of the control value.  Maximum isometric tension restored to 29% of the control value. Faulkner et al. (1980). Cell Physiol 238 p120-126. Restoration of muscle function  The capacity to regenerate depends on the age of the recipient animal more than on the age of the donor tissue. Y = young O = old white bars are control values Lieber (2010) Lippincott Williams & Wilkins Carlson & Faulkner (1989). Am J Physiol 256 p1262-1266 Muscle damage induced by exercise  Eccentric contractions (active lengthening contractions) are particularly effective to induce damage through exercise. Muscle damage induced by exercise  Eccentric contractions (active lengthening contractions) are particularly effective to induce damage through exercise.  Results of 3 × 5 minutes isometric, concentric, eccentric and ‘sham’ exercise in mice.  10 minutes after eccentric exercise (dotted line), but not after isometric, concentric and sham exercise, isometric force is reduced significantly compared to before exercise (solid line). McCully & Faulkner (1985). J Appl Physiol 59 p119-126. Muscle damage induced by exercise  Eccentric exercise incurs long-lasting reduction in capacity to generate force. McCully & Faulkner (1985). J Appl Physiol 59 p119-126. Muscle damage induced by exercise  Fast glycolytic fibres (2X and 2B) are preferentially affected in eccentric exercise.  Subsequent bouts of exercise reduce the amount of damage. McCully & Faulkner (1985). J Appl Physiol 59 p119-126. Muscle damage induced by exercise After eccentric exercise, intracellular calcium concentrations are increased. This may trigger selective hydrolysis or disruption of the intermediate filament network, causing a disrupted fibre structure. Lieber et al. (1991). J Appl Physiol 70 p2498-2507. Muscle damage induced by exercise McHugh et al. (1999). Sports Med 27 p157-170. Muscle damage induced by exercise  Mechanical factors play an important role in eccentric damage.  Muscles remodel following eccentric exercise: they add sarcomeres in series (i.e. increase in fascicle length and a shift in force-length properties to the right).  Understanding mechanisms of muscle damage by eccentric-exercise is an active area of research, because it has many potential clinical applications. Summary  Muscle development follows four stages: axonal outgrowth, myogenesis, synaptogenesis, synapse elimination.  Secondary myogenesis occurs after injury: satellite cells form new myofibrils in a process much like primary myogenesis.  Eccentric exercise induces fibre disruptions, increased calcium concentrations in muscle fibres and, eventually, muscle remodelling. Further reading Chapter 1 and 6 of: Lieber, Richard L. Skeletal muscle structure, function, and plasticity. (Ed. 3) Lippincott Williams & Wilkins, 2010.