New perspectives on the mechanisms establishing the dorsal-ventral axis of the spinal cord

Abstract

Distinct classes of neurons arise at different positions along the dorsal-ventral axis of the spinal cord leading to spinal neurons being segregated along this axis according to their physiological properties and functions. Thus, the neurons associated with motor control are generally located in, or adjacent to, the ventral horn whereas the interneurons (INs) that mediate sensory activities are present within the dorsal horn. Here, we review classic and recent studies examining the developmental mechanisms that establish the dorsal-ventral axis in the embryonic spinal cord. Intriguingly, while the cellular organization of the dorsal and ventral halves of the spinal cord looks superficially similar during early development, the underlying molecular mechanisms that establish dorsal vs ventral patterning are markedly distinct. For example, the ventral spinal cord is patterned by the actions of a single growth factor, sonic hedgehog (Shh) acting as a morphogen, i.e., concentration-dependent signal. Recent studies have shed light on the mechanisms by which the spatial and temporal gradient of Shh is transduced by cells to elicit the generation of different classes of ventral INs, and motor neurons (MNs). In contrast, the dorsal spinal cord is patterned by the action of multiple factors, most notably by members of the bone morphogenetic protein (BMP) and Wnt families. While less is known about dorsal patterning, recent studies have suggested that the BMPs do not act as morphogens to specify dorsal IN identities as previously proposed, rather each BMP has signal-specific activities. Finally, we consider the promise that elucidation of these mechanisms holds for neural repair.

Keywords: Dorsal; Neural development; Neural progenitors; Neurons; Regeneration; Spinal cord; Ventral.

Figure 1:. Organization of the adult spinal cord

Schematics depicting the organization and function of the adult spinal cord. (A) Sensory information from the peripheral nervous system, is relayed into the dorsal horn of the spinal cord, processed by interneurons in the dorsal and ventral spinal cord and then relayed to higher centers in the brains, or modulate motor function. (B) The adult spinal cord is organized with a laminar architecture comprising seven distinct layers. Afferent sensory information is decoded in specific layers. Through fate mapping, genetic and connectivity studies, the general function and location of the dorsal IN populations have been identified in the adult spinal cord.

Figure 2:. Spinal cord development

Schematics describing the stages of neuronal development in the spinal cord (A) Inductive signaling centers signal t o neural tube progenitors to pattern neural identity (modified with permission from (Tanabe and Jessell, 1996)). Sonic hedgehog (Shh) is secreted from the notochord and floor plate (FP) to pattern the ventral spinal cord. BMPs and Wnts are expressed from the roof plate (RP) to pattern the dorsal spinal cord Retinoic acid (RA) is secreted from the somites in the paraxial mesoderm to specify axial level and promote neuronal differentiation. (B) During early spinal cord development, the nuclei of proliferating progenitors oscillate along the medial-lateral axis as they undergo cell division (C) In response to inductive growth factors, progenitors are patterned into distinct domains within the ventricular zone (VZ) along the dorsal-ventral axis. As progenitors differentiate they exit the cell cycle and migrate to the lateral edge of the spinal cord. The dorsal spinal cord has six distinct interneurons (IN) populations, the dI1–6, while the ventral spinal cord has five distinct neuronal populations, the V0-V3 IN populations and the spinal MNs.

Figure 3.. Role of the Shh signaling pathway

(A) Shh signal transduction (modified with permission from (Briscoe and Therond, 2013)). In the absence of Shh ligand, the patched (Ptch) accumulates at the base of, and within, the primary cilium. Through an indirect mechanism, the presence of Ptch represses the activation and movement of smoothened (Smo) into the primary cilium. In the absence of Smo, full length Gli proteins (Gli) are cleaved and processed into transcriptional repressors (GliR), which then translocate to the nucleus and repress downstream gene targets that include Gli1 and Ptch1. (B) In the presence of Shh, Shh ligand binds to Ptch, which is then is endocytosed and degraded. In the absence of Ptch, Smo is activated and translocates into the primary cilium. In the presence of Smo, Gli proteins are processed into transcriptional activators (GliA). (C) Ventral spinal cord patterning (modified with permission from (Briscoe and Novitch, 2008; Jessell, 2000; Oosterveen et al., 2012)). Shh ligand is secreted by the notochord (N) and the floor plate (FP) of the neural tube. Shh acts in a dose-dependent manner to alter the processing of Gli proteins from transcriptional repressors (GliR) to transcriptional activators (GliA). The net balance between GliA and GliR influences the expression of various homeodomain transcription factors along the dorsal-ventral axis. These transcription factors can be grouped into two classes: class I proteins (Dbx2, Dbx1, Pax6, and Irx3) that are influenced by RA signaling and present only in the absence of Shh and class II proteins (Nkx6.1, Nkx6.2, Nkx2.2, and Olig2) that require Shh for their activation. Ultimately, unique combinations of class I and class II transcription factors subdivide the ventral spinal cord into five molecularly distinct progenitor domains (p0, p1, p2, pMN, and p3). Over time, each of these progenitor domains give rise to distinct types of neurons and then glia.

Figure 4.. BMP signaling pathway

(A) Schematic of BMP signaling (modified with permission from (Balemans and Van Hul, 2002)). BMP dimers bind to a complex of type I and type II receptors which phosphorylate receptor (R) activated Smads 1, 5 and 8. R-Smads complex with Co-Smad4 and enter the nucleus to regulate transcription. Inhibitory (I) Smads 6 and 7 negatively regulate the signaling pathway by competitively binding with other components of the signaling pathway. (B-E) Model for a “mix and match” code for the specification of the RP, dI1s, dI2s and dI3s. In this model. (B) BMP6 (mouse) and BMP7 (chicken) are the most effective at directing RP identity through the BmprIa receptor (mouse). (C) Both BMP4 and BMP7 can promote dP1 patterning through BmprIa or BmprIb (chicken), but only BMP4 directs progenitors to differentiate as dI1s through BmprIb (mouse and chicken)). (D) BMP4 also specifically directs dI2 differentiation in chicken, thereby depleting the pool of dP2s. (E) All BMPs tested in both species, including BMP4, BMP5, BMP6 and BMP7, can act though either BmprIa or BmprIb to promote the dI3 fate.