Neuromuscular Development in the Absence of Programmed Cell Death: Phenotypic Alteration of Motoneurons and Muscle

Abstract

The widespread, massive loss of developing neurons in the central and peripheral nervous system of birds and mammals is generally considered to be an evolutionary adaptation. However, until recently, models for testing both the immediate and long-term consequences of preventing this normal cell loss have not been available. We have taken advantage of several methods for preventing neuronal death in vivo to ask whether rescued neurons [e.g., motoneurons (MNs)] differentiate normally and become functionally incorporated into the nervous system. Although many aspects of MN differentiation occurred normally after the prevention of cell death (including the expression of several motoneuron-specific markers, axon projections into the ventral root and peripheral nerves, ultrastructure, dendritic arborization, and afferent axosomatic synapses), other features of the neuromuscular system (MNs and muscle) were abnormal. The cell bodies and axons of MNs were smaller than normal, many MN axons failed to become myelinated or to form functional synaptic contacts with target muscles, and a subpopulation of rescued cells were transformed from ␣-to ␥-like MNs. Additionally, after the rescue of MNs in myogenin glial cell line-derived neurotrophic factor (MyoGDNF) transgenic mice, myofiber differentiation of extrafusal skeletal muscle was transformed and muscle physiology and motor behaviors were abnormal. In contrast, extrafusal myofiber phenotype, muscle physiology, and (except for muscle strength tests) motor behaviors were all normal after the rescue of MNs by genetic deletion of the proapoptotic gene Bax. However, there was an increase in intrafusal muscle fibers (spindles) in Bax knock-out versus both wild-type and MyoGDNF mice. Together, these data indicate that after the prevention of MN death, the neuromuscular system becomes transformed in novel ways to compensate for the presence of the thousands of excess cells.

Misplacement of Purkinje Cells during Postnatal Development in Bax Knock-Out Mice: A Novel Role for Programmed Cell Death in the Nervous System?

“…For example, in previous studies, we found that a subset of excess motoneurons rescued from PCD cannot maintain their original phenotype and either become atrophied or differentiate abnormally (Sun et al, 2003;Buss et al, 2006b). In the present study, we found that misplaced PCs also exhibit an atrophied morphology and limited differentiation with eventual loss of a biochemical marker (CB) for PCs.…”

“…The PCD of postmitotic neurons can also occur before synaptogenesis, at which time it may function to eliminate ectopically located neurons and aberrant axon projections (Clarke et al, 1998;Thanos, 1999;Oppenheim et al, 2001). The proapoptotic gene Bax is required for the normal PCD of many populations of developing postmitotic neurons (White et al, 1998;Sun et al, 2003Sun et al, , 2004Buss et al, 2006b), whereas the PCD of proliferating neuronal progenitor cells is less affected by Bax deletion, suggesting that the molecular cell death machinery differs for mitotic versus postmitotic neurons.…”

Differential sensitivity of skeletal and fusimotor neurons to Bcl-2-mediated apoptosis during neuromuscular development

“…Interestingly, it was also demonstrated that a substantial population of unmyelinated axons was rescued by bax deletion. 34,37 While the ventral roots of adult wild-type mice normally contain very low numbers of unmyelinated axons, this is also true for our lineages of bcl-2-null mice (Supplementary Figure 1). Given the model proposed for bcl-2, and the nature of interactions between Bcl-2 and Bax, this finding is not surprising and paints a consistent picture of Bcl-2's action.…”

“…[34][35][36] That gamma motor neurons require glial cell line-derived neurotrophic factor (GDNF) for survival derived from muscle spindles is suggested from studies in which significant losses of small diameter motor axons are observed in gdnf heterozygous animals. Similarly, transgenics ectopically expressing GDNF within skeletal muscle exhibit a substantial increase in small diameter motor axons within the L4 ventral root, 34,35 although the precise nature of the motor neurons contributing these axons is unclear. Analysis of the L4 ventral root in bax null mice reveals a significant increase in total numbers of myelinated axons, which is contributed exclusively by small diameter axons believed to be derived from gamma motor neurons.…”

“…Analysis of the L4 ventral root in bax null mice reveals a significant increase in total numbers of myelinated axons, which is contributed exclusively by small diameter axons believed to be derived from gamma motor neurons. 34,36 The greater relative increase in motor axons in bax null mice (71%) compared to the reductions seen in bcl-2-null mice (24%) likely reflects the greater potential for functional compensation by Bcl-2 homologues (Bcl-x L , Mcl-1, Bcl-w, etc.) compared to Bax homologues (Bak, Bok, etc.)…”