Search

Members of the bone morphogenetic protein family of secreted protein signals have been implicated as axon guidance cues for specific neurons in Caenorhabditis elegans and in mammals. We have examined axonal pathfinding in mice lacking the secreted bone morphogenetic protein antagonist Noggin. We have found defects in projection of several groups of neurons, including the initial ascending projections from the dorsal root ganglia, motor axons innervating the distal forelimb, and cranial nerve VII. The case of the dorsal root ganglion defect is especially interesting: initial projections from the dorsal root ganglion enter the dorsal root entry zone, as normal, but then project directly into the gray matter of the spinal cord, rather than turning rostrally and caudally. Explant experiments suggest that the defect lies within the spinal cord and not the dorsal root ganglion itself. However, exogenous bone morphogenetic proteins are unable to attract or repel these axons, and the spinal cord shows only very subtle alterations in dorsal-ventral pattern in Noggin mutants. We suggest that the defect in projection into the spinal cord is likely the result of bone morphogenetic proteins disrupting the transduction of some unidentified repulsive signal from the spinal cord gray matter. Key Words: Noggin; BMP; axon guidance.

Results show that axons possess complete machinery for protein synthesis and cell surface expression. Data suggest that individual isolated axons and growth cones can effect protein translation as well as export of the proteins to the cell surface, indicating that an RNA-based mechanism is localized within a specific region of the axons.

Results demonstrate that sunday driver protein transports the axonal transport kinesin driven vesicles by interacting with the kinesin light chain. Data reveal that sunday driver protein is localized to tubulovesicular structures and forms complex with kinesin-1 in in vivo.

A study was conducted to identify the molecular mechanism that guide axons along their stereotypical pathways and to determine how axons respond to bifunctional guidance cues. Experiments with the Xenopus axon turning assay revealed that transmembrane receptors determine whether the growth cone along the axons' pathway see guidance cues as attractive or repulsive. It was also found that heteromeric receptors control the response of growth cones to bifunctional guidance cues.

The effects of netrin-1 on commisural axions of chicks are mediated by the Deleted in Colorectal Cancer (DCC) gene via cellular signal transduction pathway. The DCC gene is a netrin-1 binding protein that is expressed on the axons and growth cones of spinal commissural neurons during their migration to the floor plate. The binding protein also acts as a receptor during the guidance of commissural axons from the dorsal to the ventral midline region of the spinal cord.

Research shows that neurons expressing different odorant receptors express ephrin-A protein at different concentrations on their axons. Results demonstrate that ephrin-A molecules, together with odorant receptors, are involved in the formation of a sensory map in the olfactory bulb.

Netrins and its receptors are involved in axon guidance in diverse animals from worms to mammals. The UNC-5 family of Netrin receptors predominantly mediates axon repulsion, and the UNC-40/DCC family of Netrin receptors mediates axon attraction. The signaling events downstream of these Netrin receptors are largely unknown. We report here the Caenorhabditis elegans max-1 gene that functions to modulate and transduce the UNC-5 signal. In wild-type worms, the ventral cord type D motor neurons grow circumferential commissures using the guidance signal of UNC-6/Netrin and UNC-5 receptor, max-1 loss of function mutations cause fully penetrant but variable defects in the D neuron commissural guidance. Weak alleles of unc-5 and unc-6 strongly enhance Max-1 phenotypes and null alleles of unc-5 and unc-6 act as dominant enhancers. In contrast, unc-40 mutations, which affect another receptor of UNC-6, do not dominantly enhance Max-1 phenotypes, max-1 encodes a novel conserved protein that has two PH, one Myth4 and one FERM domains. PH domains are known to interact with lipid membrane; FERM domains are also membrane-anchoring domains, originally found in Band4.1 protein and Ezrin/Radixin/Moesin (ERM) proteins and can interact with PDZ domains and the cytoplasmic tails of several transmembrane proteins. MAX-1 functions in motor neurons and is localized to neuronal processes. Yeast two-hybrid analysis supports a physical association of UNC-5 and MAX-1. We propose that MAX-1 may modulate UNC-5 activity by binding to UNC-5 and also acting as a scaffold to transduce UNC-5 signals in axon repulsion.

Fetuses that develop without both Myf5 and MyoD genes entirely lack skeletal myogenesis and as a consequent of that their spinal motor and proprioceptive neurons undergo complete apoptosis. Primary motor cortex of these fetuses completely lack giant pyramidal cells, but their absence is not correlated with increased levels of apoptosis. This suggests a failure in competence of progenitor cells. To further investigate how the peripheral axonal inputs influence neuronal precursor cell differentiation in various brain areas involved in motor control wild-type and mutant brains were assessed for structural and molecular differences. Mutant brain anatomical milestones appeared unaffected, but nestinexpressing clusters of neural precursor cells, the calretininexpressing pyramidal neurons, and the calbindin-expressing clusters of differentiating neurons appeared significantly less defined in mutant motor cortex area. Consistently, some areas of the extra-pyramidal tract (e.g., caudoputamen and globus pallidus) were also affected in mutant brains. Surprisingly, we were unable to detect any change in proliferation or cell death in the mutant neuroepithelium. Taken together, it appears that patterning mechanisms intrinsic to the areas for motor control specify the basic organization of its subdivisions. However, external axonal inputs seem to change intrinsic properties of neural progenitors and influence the ratios of the cell types they produce.

A study was conducted to determine the effect of the interaction of two families of receptors, namely, the Netrin receptors of the Deleted in Colorectal Cancer (DCC) and the UNC5 family receptors in axon guidance. Experiments with Xenopus spinal cord neurons revealed that the introduction of UNC5 proteins in cells transform the spinal axon-attraction of DCC netrin receptors to axon-repulsion. This is effected by the interaction of netrin-1 to UNC5.

A study was conducted to evaluate the function of a Drosophila homolog labeled, p21-activated kinase (Pak) in R cell axon guidance. Results suggest that the Pak regulates R cell axon guidance and targeting by colocalizing and physically interacting with the R cell axons. It was also observed that a membrane-tethered form of Pak functions as a dominant gain-of-function protein. Moreover, the Pak has been found to act as the downstream effector of the SH2/SH3 adaptor protein dock.

Axonal interactions, which are mediated by cell adhesion molecules (CAMs) as well as other types of membrane proteins, are important for sensory axon pathfinding in the developing chick hindlimb. We have previously shown that injection of antibodies that block the function of either G4/L1 or N-cadherin into the limb, starting when the first sensory axons reach the plexus, alters the segmental pattern of projections along cutaneous nerves. Specific removal of polysialic acid from NCAM using the enzyme endoneuraminidase N (Endo N) also resulted in significant changes in cutaneous projection patterns, while injection of antibodies against NCAM itself had no obvious effect (M. G. Honig and U.S. Rutishauser, 1996, Dev. Biol. 175, 325-337). To help understand the cellular basis for these findings, we developed a tissue culture system in which the axons from dorsal root ganglion explants grow within defined laminin lanes and examined whether the same treatments increased or decreased a growth cone's tendency to be closely associated with neighboring axons. After 2 days in culture, images of the cultures were recorded, antibodies or Endo N was added, and images of the same fields were recaptured an hour later. To quantify the results, growth cones located in defined regions of the laminin lanes were classified, before and after the perturbation, as 'free' (i.e., growing primarily on the laminin substratum), 'fasciculated' (i.e., growing tightly along other neurites), or 'intermediate' (i.e., growing both on the laminin substratum and in contact with other neurites). We found that anti-G4/L1 and anti-N-cadherin, but not anti-NCAM, caused an increase in defasciculated growth cones, whereas Endo N resulted in an increase in fasciculated growth cones. These changes in fasciculation are consistent with the changes in cutaneous projections seen in our previous in ovo perturbations. The results from these tissue culture experiments thus provide strong support for the idea that one mechanism by which CAMs affect sensory axon pathfinding in vivo is by regulating the affinity of sensory growth cones for neighboring axons, which in turn can modulate the growth cone's ability to navigate through the surrounding environment. Key Words: pathfinding; cell adhesion molecules; fasciculation.