Molecular Control of Spinal Accessory Motor Neuron/Axon Development in the Mouse Spinal Cord

Abstract

Within the developing vertebrate spinal cord, motor neuron subtypes are distinguished by the settling positions of their cell bodies, patterns of gene expression, and the paths their axons follow to exit the CNS. The inclusive set of cues required to guide a given motor axon subtype from cell body to target has yet to be identified, in any species. This is attributable, in part, to the unavailability of markers that demarcate the complete trajectory followed by a specific class of spinal motor axons. Most spinal motor neurons extend axons out of the CNS through ventral exit points. In contrast, spinal accessory motor neurons (SACMNs) project dorsally directed axons through lateral exit points (LEPs), and these axons assemble into the spinal accessory nerve (SAN). Here we show that an antibody against BEN/ALCAM/ SC1/DM-GRASP/MuSC selectively labels mouse SACMNs and can be used to trace the pathfinding of SACMN axons. We use this marker, together with a battery of transcription factor-deficient or guidance cue/receptor-deficient mice to identify molecules required for distinct stages of SACMN development. Specifically, we find that Gli2 is required for the initial extension of axons from SACMN cell bodies, and that netrin-1 and its receptor Dcc are required for the proper dorsal migration of these cells and the dorsally directed extension of SACMN axons toward the LEPs. Furthermore, in the absence of the transcription factor Nkx2.9, SACMN axons fail to exit the CNS. Together, these findings suggest molecular mechanisms that are likely to regulate key steps in SACMN development.

ALCAM Regulates Mediolateral Retinotopic Mapping in the Superior Colliculus

“…Signaling through EphB2/EphB3 in retinal axons might modulate L1 phosphorylation on Tyr1229, altering ankyrin binding, cytoskeletal engagement, and adhesion to ALCAM. Because ALCAM is prominently expressed not only in the visual system but also in other axonal projections, such as spinal cord pathways (Fashena and Westerfield, 1999;Arancio et al, 2001;Dillon et al, 2005), ALCAM may have a broader role in L1-dependent synaptic targeting, vital to establishment of connectivity in multiple regions of the developing nervous system. Future studies will be aimed at exploring ALCAM-L1 genetic and physical interactions in other axonal projections during brain development.…”

Transcription factors and effectors that regulate neuronal morphology

“…Spinal accessory motor neurons (SACMNs) are dorsally exiting neurons found at cervical levels of the spinal cord that innervate neck and back muscles (Dillon et al, 2005). SACMNs are derived from an Nkx2.9+ progenitor domain and retain Nkx2.9 expression postmitotically.…”

“…SACMNs are derived from an Nkx2.9+ progenitor domain and retain Nkx2.9 expression postmitotically. In the absence of Nkx2.9, SACMN axons fail to exit the spinal cord (Dillon et al, 2005;Pabst et al, 2003). A recent study In Drosophila embryos, motor neurons that innervate the body wall muscles required for larval crawling arise from multiple neuroblast lineages that express distinct combinations of transcription factors (Landgraf et al, 1997).…”

Motor axon exit from the mammalian spinal cord is controlled by the homeodomain protein Nkx2.9 via Robo-Slit signaling

“…Motor axons grow in a directed manner to specialized exit points through which they emerge from the CNS (Bravo-Ambrosio and Kaprielian, 2011;Jacob et al, 2001;Lieberam et al, 2005;Schneider and Granato, 2003;Sharma et al, 1998;Shirasaki and Pfaff, 2002). MN subtypes can be distinguished by the positions of their exit points: ventral MNs (vMNs) and dorsal MNs (dMNs) utilize ventral and dorsal exit points, respectively (Chandrasekhar, 2004;Cordes, 2001;Dillon et al, 2005;Guthrie, 2007;Lieberam et al, 2005;Schubert and Kaprielian, 2001;Sharma et al, 1998;Snider and Palavali, 1990). Although Cxcl12-Cxcr4 signaling regulates the growth of vMN axons to their exit points in mice (Lieberam et al, 2005) and myotomal-derived diwanka (plod3 -ZFIN) glycosyltransferase is required for motor axon growth into the periphery in zebrafish (Schneider and Granato, 2006), the molecular mechanisms that control motor axon exit from the vertebrate spinal cord are poorly understood.…”

“…Spinal accessory motor neurons (SACMNs) are branchiomotor dMNs that reside within the cervical spinal cord and project dorsally directed axons to and through a highly localized lateral exit point (LEP) situated midway along the dorsoventral axis of the spinal cord (Dillon et al, 2005;Hirsch et al, 2007;Lieberam et al, 2005). Upon exiting the CNS, SACMN axons execute a rostral turn and assemble into the longitudinally projecting spinal accessory nerve (SAN), which innervates particular neck and back muscles (Dillon et al, 2005;Dillon et al, 2007;Schubert and Kaprielian, 2001;Snider and Palavali, 1990).…”