A differential developmental pattern of spinal interneuron apoptosis during synaptogenesis: insights from genetic analyses of the protocadherin-γ gene cluster

Abstract

Although the role of developmental apoptosis in shaping the complement and connectivity of sensory and motoneurons is well documented, the extent to which cell death affects the 13 cardinal classes of spinal interneurons is unclear. Using a series of genetic manipulations in vivo, we demonstrate for the first time a differential pattern of developmental apoptosis in molecularly identified spinal interneuron populations, and implicate the adhesion molecule family encoded by the 22-member protocadherin-γ (Pcdh-γ ) gene cluster in its control. In constitutive Pcdh-γ null mouse embryos, many interneuron populations undergo increased apoptosis, but to differing extents: for example, over 80% of En1-positive V1 neurons are lost, whereas only 30% of Chx10-positive V2a neurons are lost and there is no reduction in the number of V1-derived Renshaw cells. We show that this represents an exacerbation of a normal, underlying developmental pattern: the extent of each population's decrease in Pcdh-γ mutants is precisely commensurate both with the extent of its loss during normal embryogenesis and with the extent of its increase in Bax -/-mice, in which apoptosis is genetically blocked. Interneuron apoptosis begins during the first wave of synaptogenesisis in the spinal cord, occurring first among ventral populations (primarily between E14 and E17), and only later among dorsal populations (primarily after P0). Utilizing a new, conditional Pcdh-γ mutant allele, we show that the γ-Pcdhs can promote survival non-cell-autonomously: mutant neurons can survive if they are surrounded by normal neurons, and normal neurons can undergo apoptosis if they are surrounded by mutant neurons.

Preservation of VGLUT1 synapses on ventral calbindin-immunoreactive interneurons and normal locomotor function in a mouse model of spinal muscular atrophy

“…Moreover, a recent study has shown that mice deficient for ␥-protocadherin molecules experience a loss of ventral interneurons but not of motoneurons. In these animals, there is a 70% loss in the afferent collaterals projecting to ventral interneurons and a 2.5 increase in the area of terminals in the motor nucleus (Prasad et al 2008;Prasad and Weiner 2011). Thus a loss of afferent input to ventral interneurons is accompanied by an increase in the number of inputs to motoneurons.…”

Combinatorial homophilic interaction between γ-protocadherin multimers greatly expands the molecular diversity of cell adhesion

“…This has important implications for any potential role for γ-Pcdhs in the control of synaptic specificity, which remains to be demonstrated but would be consistent with some (although not all; see ref. 11) extant functional data in vivo (9,10,12,13,46). Several reports indicate that single neurons of the same cell type can express different subsets of clustered Pcdh isoforms (4,8,(46)(47)(48).…”

“…We previously demonstrated critical roles for the 22-member γ-Pcdh family in central nervous system development through analysis of mice in which the entire gene cluster has been deleted (5). These null mutants exhibit severe neurologic defects and die shortly after birth, exhibiting severe apoptosis of spinal cord interneurons and concomitant loss of synapses (5,9,10). An increase in neuronal apoptosis is also observed in the postnatal retina (11) and hypothalamus (12) when the γ-Pcdhs are disrupted using a conditional allele.…”

Clustered protocadherins

“…The first role identified for Pcdhg family members is in neuronal survival, as evidenced by the massive apoptosis and eventual loss of spinal interneurons and retinal cells in Pcdhg-deficient mice (Wang et al, 2002b;Weiner et al, 2005;Lefebvre et al, 2008;Prasad et al, 2008). In both the spinal cord and retina, the neuronal loss is accompanied by reduced numbers of synapses, raising the possibility that the loss of Pcdhg proteins may lead to synaptic loss, which in turn compromises neuronal survival.…”