Regenerating muscle fibers induce directional sprouting from nearby nerve terminals: studies in living mice

Abstract

The principal aim of this work was to better understand how regenerating muscle fibers become innervated in adult animals. To induce muscle regeneration, individual identified muscle fibers in a mouse were damaged with a laser focused through a microscope. The muscle fiber that degenerated and the muscle fiber that was formed in its place were followed by viewing the same site repeatedly over a period of 2 d to 40 weeks. Commonly, the nerve terminal innervating the irradiated muscle fiber partially retracted during muscle fiber degeneration, and then sprouted to innervate the regenerating muscle fiber at the same site it had previously innervated the muscle fiber that was damaged. During the early phase of muscle regeneration we also observed sprouts originating from nerve terminals on adjacent muscle fibers. The new nerve growth was a response to the regenerating muscle fiber rather than to the degenerated fiber it replaced because repeated damage of the same site every 2–3 d over a 10 d period (to prevent regeneration) did not cause any sprouting. The direction of the sprouts on adjacent muscle fibers showed a bias toward the regenerating muscle fiber, although they avoided the region occupied by the original nerve terminal. Forty percent of the sprouts managed to reach the regenerated fiber. Nonetheless, by 11 d after muscle fiber damage, all sprouts had regressed, leaving the new fiber innervated by the same motor axon that innervated the fiber that was damaged. On the other hand, when the overlying nerve terminal as well as the muscle fiber was damaged, the sprouts from nearby muscle fibers were both more numerous and more stable, and in five cases we observed two or more new synaptic junctions on the regenerating fiber originating from different axons. In one case we witnessed a protracted competition between the original motor axon as it sprouted back and the sprouts from nearby junctions for sole innervation of the regenerate. Ultimately, the surviving sprouts myelinated and became the permanent and exclusive input to the new fiber. These results indicate that regenerating muscle fibers emit a signal that induces directional sprouting from nearby undamaged nerve terminals. Reinnervation of the regenerating muscle fiber by one axon apparently prevents the maintenance of such neurites. Because the process of muscle regeneration shares many features in common with myogenesis during embryonic development, it is likely that developing muscle fibers present an analogous stimulus to ingrowing motor axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Types of neural guides and using nanotechnology for peripheral nerve reconstruction

“…[7][8][9][10][11] Since then, conduits from many different biologic tissues have been used with varying success. These include the use of arteries, 10,11 veins, 12-14 muscles, [15][16][17][18] and other materials which are extensively reviewed by Doolabh et al 19 Other nerve tube conduits have been made from modified biologic tissues such as laminin 19 and collagen, 20,21 and have proved successful in specific situations. There are a number of disadvantages with the use of blood vessel, muscle, and other biologic tissues in bridging peripheral nerve defects, including tissue reaction, early fibrosis, scar infiltration, and lack of precise control of the conduit's mechanical properties.…”

The dynamics of recycled acetylcholine receptors at the neuromuscular junction in vivo

“…Adult female mice (20-27 g; NSA, Harlan Sprague Dawley, Indianapolis, IN) were anesthetized with an intraperitoneal injection of ketamine and xylazine (KX; 87 mg and 13 mg, respectively, per kg body weight). Sternomastoid muscle exposure and NMJ imaging were performed as previously described (Akaaboune et al, 1999;Lichtman et al, 1987;van Mier and Lichtman, 1994). Briefly, the anesthetized mouse was placed on its back on the stage of a customized epifluorescence microscope, and NMJs were viewed under a coverslip with water immersion objectives (20ϫ and 60ϫ UAPO 0.7 NA, Olympus BW51, Optical Analysis Corporation, NH) and a digital CCD camera (Retiga EXi, Burnaby, BC, Canada).…”

Nerve Terminal Withdrawal from Rat Neuromuscular Junctions Induced by Neuregulin and Schwann Cells

“…Changes in such signals would not have been detected in our experiments. It should be noted, however, that even in the complete absence of a postsynaptic muscle fiber, portions of nerve terminal branches are maintained at the mammalian nmj (Rich and Lichtman, 1989b;Van Mier and Lichtman, 1994) and f unctional terminals remain at the frog nmj (Dunaevsky and Connor, 1995), suggesting that in addition to the muscle fiber, the basal lamina and other cell types at the nmj influence nerve terminal stability. On the basis of our data, we argue that terminal SC s play a role in regulating the stability of neuromuscular junctions.…”

“…Evidence from the mammalian neuromuscular junction (nmj) suggests that retrograde factors supplied by postsynaptic muscle fibers are required for the maintenance of motor nerve terminals. L oss of acetylcholine receptors (AChRs) from a portion of the synapse (Rich and Lichtman, 1989a;Balice-Gordon and Lichtman, 1994) or degeneration of the muscle fiber itself (Rich and Lichtman, 1989b;Van Mier and Lichtman, 1994) precedes a rapid, partial loss of nerve terminal branches. Similar studies in frog muscles suggest that signals in the synaptic basal lamina are capable of regulating the maintenance of nerve terminal branches.…”