dILA neurons in the dorsal spinal cord are the product of terminal and non-terminal asymmetric progenitor cell divisions, and require Mash1 for their development

Abstract

dILA and dILB neurons comprise the major neuronal subtypes generated in the dorsal spinal cord, and arise in a salt-and-pepper pattern from a broad progenitor domain that expresses the bHLH factor Mash1. In this domain, Mash1-positive and Mash1-negative cells intermingle. Using a Mash1GFP allele in mice, we show here that Mash1+ progenitors give rise to dILA and dILB neurons. Using retroviral tracing in the chick, we demonstrate that a single progenitor can give rise to a dILA and a dILB neuron, and that dILA neurons are the product of asymmetric progenitor cell divisions. In Mash1-null mutant mice, the development of dILA, but not of dILB neurons is impaired. We provide evidence that a dual function of Mash1 in neuronal differentiation and specification accounts for the observed changes in the mutant mice. Our data allow us to assign to Mash1 a function in asymmetric cell divisions, and indicate that the factor coordinates cell cycle exit and specification in the one daughter that gives rise to a dILA neuron.

A nonclassical bHLH–Rbpj transcription factor complex is required for specification of GABAergic neurons independent of Notch signaling

“…This mutant has no activity in inducing Ptf1a-dependent neurons when overexpressed in the chick neural tube, an activity normally detected with wild-type Ptf1a (Wildner et al 2006). Furthermore, in mice with Ptf1a…”

“…Solutions of supercoiled plasmid DNA (0.5 µg/µL) in PBS/0.02% Trypan Blue were injected into the lumen of the closed neural tube at stage HH26, and embryos were electroporated as described previously (Muramatsu et al 1997;Funahashi et al 1999;Nakada et al 2004;Wildner et al 2006). An expression vector with nuclear localized GFP (pCIG) (Megason and McMahon 2002) was coinjected as a control to monitor efficiency and extent of electroporation.…”

“…Inhibitory and excitatory interneurons in the dorsal horn of the spinal cord require the combined actions of bHLH and homeodomain (HD) transcription factors (Gowan et al 2001;Gross et al 2002;Müller et al 2002;Cheng et al 2004Cheng et al , 2005Glasgow et al 2005;Helms et al 2005; Mizuguchi et al 2006; Wildner et al 2006). Neu-rogenesis in this CNS region is characterized by two waves: an early wave (embryonic day 10-11.5 [E10-E11.5]) that generates sensory relay and local interneurons that settle in the deep dorsal horn (dI1-dI6), and a late wave (E11.5-E13) that generates interneurons in the superficial laminae (dIL A and dIL B ) (for reviews, see Caspary and Anderson 2003;Helms and Johnson 2003).…”

“…Another bHLH factor, Ascl1 (previously Mash1), has a more complex role in specifying these neurons. In early stages it is required with the HD factors Gsh1 and Gsh2 for dI5 glutamatergic neurons (Helms et al 2005;Mizuguchi et al 2006), but in the late-born dIL populations it is required for Ptf1a expression and, thus, for the dIL A GABAergic neurons (Mizuguchi et al 2006;Wildner et al 2006). The central role played by Ptf1a in guiding inhibitory versus excitatory neuronal specification makes it an attractive target for studies revealing the molecular mechanisms controlling neuronal network formation.…”

Mitotic spindle orientation distinguishes stem cell and terminal modes of neuron production in the early spinal cord

“…Retroviral labelling studies indicate that both asymmetric and symmetric modes of neuron production underlie sensory neuron generation in the latestage spinal cord (Wildner et al, 2006). However, the first neurons to be born in the chick spinal cord are reticular spinal interneurons located in dorsal and intermediate regions, followed by ventral motor neurons (Sechrist and Bronner Fraser, 1991), and the modes of division that produce these crucial components of the spinal cord have not been determined.…”