Floor plate and motor neuron induction by different concentrations of the amino-terminal cleavage product of sonic hedgehog autoproteolysis

AbstractThe differentiation of floor plate cells and motor neurons can be induced by Sonic hedgehog (SHH), a secreted signaling protein that undergoes autoproteolytic cleavage to generate amino- and carboxy-terminal products. We have found that both floor plate cells and motor neurons are induced by the aminoterminal cleavage product of SHH (SHH-N). The threshold concentration of SHH-N required for motor neuron induction is about 5-fold lower than that required for floor plate induction. Higher concentrations of SHH-N can induce floor plate cells at the expense of motor neuron differentiation. Our results suggest that the induction of floor plate cells and motor neurons by the notochord in vivo is mediated by exposure of neural plate cells to different concentrations of the amino-terminal product of SHH autoproteolytic cleavage

The role of sulfoglucuronosyl glycosphingolipids in the pathogenesis of monoclonal IgM paraproteinemia and peripheral neuropathy

In IgM paraproteinemia and peripheral neuropathy, IgM M-protein secretion by B cells leads to a T helper cell response, suggesting that it is antibody-mediated autoimmune disease involving carbohydrate epitopes in myelin sheaths. An immune response against sulfoglucuronosyl glycosphingolipids (SGGLs) is presumed to participate in demyelination or axonal degeneration in the peripheral nervous system (PNS). SGGLs contain a 3-sulfoglucuronic acid residue that interacts with anti-myelin-associated glycoprotein (MAG) and the monoclonal antibody anti-HNK-1. Immunization of animals with sulfoglucuronosyl paragloboside (SGPG) induced anti-SGPG antibodies and sensory neuropathy, which closely resembles the human disease. These animal models might help to understand the disease mechanism and lead to more specific therapeutic strategies. In an in vitro study, destruction or malfunction of the blood-nerve barrier (BNB) was found, resulting in the leakage of circulating antibodies into the PNS parenchyma, which may be considered as the initial key step for development of disease

Motor neuron position and topographic order imposed by β- and γ-catenin activities

Neurons typically settle at positions that match the location of their synaptic targets, creating topographic maps. In the spinal cord, the organization of motor neurons into discrete clusters is linked to the location of their muscle targets, establishing a topographic map of punctate design. To define the significance of motor pool organization for neuromuscular map formation, we assessed the role of cadherin-catenin signaling in motor neuron positioning and limb muscle innervation. We find that joint inactivation of {beta}- and {gamma}-catenin scrambles motor neuron settling position in the spinal cord but fails to erode the predictive link between motor neuron transcriptional identity and muscle target. Inactivation of N-cadherin perturbs pool positioning in similar ways, albeit with reduced penetrance. These findings reveal that cadherin-catenin signaling directs motor pool patterning and imposes topographic order on an underlying identity-based neural map

Whirlin function in proprioceptive mechanotransduction.

Proprioceptive sensory feedback is critical for many aspects of motor control. This form of sensory feedback derives largely from specialized mechanoreceptors located within muscles: the muscle spindle (MS; responsive to changes in muscle length) and the Golgi tendon organ (GTO; responsive to changes in muscle tension) (1). Anatomical and physiological analysis has provided insight into the sensory transduction process in MS and GTO afferents and has demonstrated that the afferent stretch response is primarily carried by sodium currents (2). The stretch-evoked MS-afferent impulse frequency is significantly diminished by amiloride, implicating the Degenerin/ENaC family of sodium channels as components of the MS-afferent mechanotransduction channel (3). Glutamate, released from sensory terminals, tonically maintains afferent excitability, possibly by regulating membrane insertion of mechano-transduction channels (4). Despite these recent advances, the molecular mechanisms that underlie the transformation of proprioceptive mechanical stimuli into electrical impulses remain largely unknown. In a molecular screen for new proprioceptor specific molecules, we recently found that whirlin is selectively expressed in proprioceptive sensory neurons in dorsal root ganglia. Whirlin encodes a scaffold protein with important roles in hair cell and photoreceptor sensory transduction (5), raising the possibility that whirlin also functions in the proprioceptive mechanotransduction process. Using an in vitro muscle/nerve preparation, we find that the activation of spindle afferents by mechanical stretch is compromised in whirlin mutant mice when compared to heterozygous mice. Application of exogenous glutamate normalizes afferent stretch-sensitivity. These observations suggest that essential components of the proprioceptive transduction machinery are inefficiently ‘deployed’ in whirler mutant mice. Given that whirlin contains three PDZ-domains, and is known to recruit macromolecular complexes to specific subcellular locations, we speculate that whirlin may function to recruit and/or ensure the proper subcellular localization of a mechano-transduction complex in proprioceptive sensory terminals. Our current studies are aimed at testing this hypothesis. The identification of whirlin provides opportunities to identify additional components of the proprioceptive transduction machinery. The parallel expression of whirlin in proprioceptive muscle afferents, hair cells and photoreceptor cells raises the possibility of a central role for whirlin in diverse sensory transduction processes

Different levels of repressor activity assign redundant and specific roles to Nkx6 genes in motor neuron and interneuron specification

Specification of neuronal fate in the vertebrate central nervous system depends on the profile of transcription factor expression by neural progenitor cells, but the precise roles of such factors in neurogenesis remain poorly characterized. Two closely related transcriptional repressors, Nkx6.2 and Nkx6.1, are expressed by progenitors in overlapping domains of the ventral spinal cord. We provide genetic evidence that differences in the level of repressor activity of these homeodomain proteins underlies the diversification of interneuron subtypes, and provides a fail-safe mechanism during motor neuron generation. A reduction in Nkx6 activity further permits V0 neurons to be generated from progenitors that lack homeodomain proteins normally required for their generation, providing direct evidence for a model in which progenitor homeodomain proteins direct specific cell fates by actively suppressing the expression of transcription factors that direct alternative fates

Pax6 controls progenitor cell identity and neuronal fate in response to graded Shh signaling

Distinct classes of motor neurons and ventral interneurons are generated by the graded signaling activity of Sonic hedgehog (Shh). Shh controls neuronal fate by establishing different progenitor cell populations in the ventral neural tube that are defined by the expression of Pax6 and Nkx2.2. Pax6 establishes distinct ventral progenitor cell populations and controls the identity of motor neurons and ventral interneurons, mediating graded Shh signaling in the ventral spinal cord and hindbrain

Distinct roles for secreted semaphorin signaling in spinal motor axon guidance.

SummaryNeuropilins, secreted semaphorin coreceptors, are expressed in discrete populations of spinal motor neurons, suggesting they provide critical guidance information for the establishment of functional motor circuitry. We show here that motor axon growth and guidance are impaired in the absence of Sema3A-Npn-1 signaling. Motor axons enter the limb precociously, showing that Sema3A controls the timing of motor axon in-growth to the limb. Lateral motor column (LMC) motor axons within spinal nerves are defasciculated as they grow toward the limb and converge in the plexus region. Medial and lateral LMC motor axons show dorso-ventral guidance defects in the forelimb. In contrast, Sema3F-Npn-2 signaling guides the axons of a medial subset of LMC neurons to the ventral limb, but plays no major role in regulating their fasciculation. Thus, Sema3A-Npn-1 and Sema3F-Npn-2 signaling control distinct steps of motor axon growth and guidance during the formation of spinal motor connections