The assembly of developing motor neurons depends on an interplay between spontaneous activity, type II cadherins and gap junctions

Abstract

A core structural and functional motif of the vertebrate central nervous system is discrete clusters of neurons or 'nuclei'. Yet the developmental mechanisms underlying this fundamental mode of organisation are largely unknown. We have previously shown that the assembly of motor neurons into nuclei depends on cadherin-mediated adhesion. Here, we demonstrate that the emergence of mature topography among motor nuclei involves a novel interplay between spontaneous activity, cadherin expression and gap junction communication. We report that nuclei display spontaneous calcium transients, and that changes in the activity patterns coincide with the course of nucleogenesis. We also find that these activity patterns are disrupted by manipulating cadherin or gap junction expression. Furthermore, inhibition of activity disrupts nucleogenesis, suggesting that activity feeds back to maintain integrity among motor neurons within a nucleus. Our study suggests that a network of interactions between cadherins, gap junctions and spontaneous activity governs neuron assembly, presaging circuit formation.

Pharmacological Profiling of Purified Human Stem Cell-Derived and Primary Mouse Motor Neurons

“…To assess for contamination from interneurons, we applied bicuculline, which blocks gamma-aminobutyric acid (GABA)-A receptors and did not observe a significant effect on network bursting in either species. Finally, we looked for direct electrical coupling between motor neurons using the gap junction inhibitor carbenoxolone and found that it reduced networking, consistent with the role of gap junctions in motor neuron development 23,24 . This effect was larger in mouse cultures compared with human cultures.…”

Motor pools organization depends on the combined function of N-cadherin and type II cadherins

“…Combinatorial expression of type II cadherins defines pools at a molecular level and manipulation of classical cadherin signaling perturbs segregation and clustering of motor neurons ( Fig. S1A; Price et al, 2002;Demireva et al, 2011;Bello et al, 2012;Astick et al, 2014;Montague et al, 2017). The precise organization of motor pools in the lateral motor column (LMC) of the spinal cord is achieved by a two-step process; first, inside-out migration separates early-and lateborn neurons in a two-layered structure on the mediolateral axis, the medial and lateral divisions (Palmesino et al, 2010;Bello et al, 2012).…”

Cadherins regulate nuclear topography and function of developing ocular motor circuitry

“…In addition to controlling somal location, one further possible mechanism to mediate the effect of cadherins on larval eye movements could relate to findings that cadherin-dependent clustering of motor neurons underlies the development of gap junction coupling and spontaneous activity ( Montague et al, 2017 ), both of which are known to contribute to mature activity patterns required for motor control ( Warp et al, 2012 ; Chang et al, 1999 ; Korn and Bennett, 1975 ). Cadherins may also contribute to selective molecular recognition between neurons within sensorimotor circuits, indeed type II cadherin expression has been shown to delineate functional circuits and control diverse cellular interactions throughout the CNS (reviewed in Redies, 2000 and Basu et al, 2015 ; Friedman et al, 2015 ; Duan et al, 2018 ).…”

“…Differential expression of type II cadherins, which participate in homophilic as well as heterophilic interactions ( Patel et al, 2006 ), drives the segregation and coalescence of somatomotor and branchiomotor neurons into discrete cranial motor nuclei in the hindbrain ( Astick et al, 2014 ). Furthermore, cadherins participate in an intricate network of interactions with gap junctions and spontaneous activity, to regulate the emergence of mature topography in brainstem motor nuclei ( Montague et al, 2017 ). Although the role of cadherins in the segregation of different motor neuron subtypes has been investigated, their requirement for the development of subnuclear architecture or motor function is unclear.…”