Primary sensory olfactory neurons reside in a neuroepithelium lining the nasal cavity and project topographically onto the surface of the olfactory bulb, a rostral extension of the telencephalon. Galectin-1, a bivalent galactose-binding vertebrate lectin, is expressed in the developing rodent olfactory system. In the present study, the mouse olfactory neuron cell line 4.4.2 was used to examine the role of galectin-l in neurite outgrowth in vitro. Recombinant galectin-l has neurite outgrowth-promoting activity when used as a substrate for 4.4.2 cells. When either galectin-1 or lactose was added to the culture media, the neurite outgrowth-promoting activity was abolished. These results demonstrate that galectin-1 can modulate neurite growth in vitro. The in vivo role of galectin-1 was investigated by examining the topographical organization of the olfactory pathway in mice carrying a null mutation for galectin-1. Using Dolichos biflorus agglutinin as a convenient histochemical marker of a subpopulation of primary sensory olfactory neurons which project topographically to the dorsomedial olfactory bulb, we show an aberrant topography of olfactory axons in the null mutants. A subset of primary sensory olfactory axons failed to project to their correct target sites in the caudal olfactory bulb. These data indicate that galectin-1 is involved in the growth and/or guidance of primary sensory olfactory axons between the nasal cavity and the olfactory bulb. This is the first demonstration that a lectin has neurite outgrowth-promoting activity and plays a role in neuronal pathfinding in the mammalian nervous system.

Interactions between carbohydrate ligands and their receptors play an important role in cell adhesion and migration in many tissues. Cell-surface carbohydrates that contain terminal galactose have previously been implicated in primary sensory axon growth in the rodent olfactory system. The aim of the present study was to determine whether galectin-1, a galactose-binding receptor, was expressed within the rat primary olfactory pathway. Immunohistochemical and in situ hybridisation analyses revealed expression of galectin-1 by primary sensory olfactory neurons during the major embryonic period of axonogenesis as well as in maturity. In the adult olfactory bulb, galectin-1 was expressed by both second-order projection neurons and interneurons and was selectively localised to the synaptic neuropil layers. Mitral cells, the principal postsynaptic target of primary olfactory axons, began expressing this lectin soon after genesis and maintained high levels into adulthood. The expression of galectin-1 in the primary olfactory pathway and olfactory bulb neuropil suggests a role for this lectin both in the initial formation and in the subsequent maintenance of neuronal connections between the peripheral and the central olfactory neurons as well as between neurons within the bulb.

Primary sensory olfactory neurons exhibit a mosaic topographical projection from the olfactory neuroepithelium in the nasal cavity to the olfactory bulb form-ation of the telencephalon. Axons from primary neurons that are widely scattered in the epithelium terminate in discrete regions of the olfactory bulb. It has been hypothesised that carbohydrates present on the surface of primary olfactory axons mediate selective fasciculation and the formation of the topographical pathway. We examined the expression of the disaccharide N-acetyl-lactosamine in both the developing and the adult rat olfactory system. N-acetyl-lactosamine was expressed by all primary sensory olfactory neurons and by their terminations in the olfactory bulb throughout embryonic development and early postnatal life. In adults, N-acetyl-lactosamine was restricted to a subpopulation of primary sensory olfactory neurons that were dispersed throughout the neuroepithelium but that projected predominantly to the ventrolateral and ventromedial surfaces of the olfactory bulb. The axons of these neurons sort out in the outer layer of the bulb and preferentially self-fasciculate to form distinct axon bundles that terminate within select glomeruli. The role of N-acetyl-lactosamine in axon growth was tested by culturing primary sensory olfactory neurons on substrate-bound carbohydrates. Olfactory neuroepithelium cultures from both embryonic and postnatal rats revealed that substratebound N-acetyl-lactosamine was a strong and specific neurite growth-promoting agent. These data suggest that, during development of the olfactory projection, N-acetyl-lactosamine, which is present on all olfactory axons, acts as a nonselective permissive substrate for axon growth. In adults, however, the restricted distribution of N-acetyl-lactosamine on a subpopulation of axons may facilitate sorting out and self-fasciculation, which is necessary for preserving the mosaic nature of the olfactory pathway in this highly plastic region of the nervous system. These results support the hypothesis that cell surface carbohydrates are involved in axon growth in the olfactory system.

The functional activity of the neural cell adhesion molecule N‐CAM can be modulated by posttranslational modifications such as glycosylation. For instance, the long polysialic acid side chains of N‐CAM alter the adhesion properties of the protein backbone. In the present study, we identified two novel carbohydrates present on N‐CAM, NOC‐3 and NOC‐4. Both carbohydrates were detected on N‐CAM glycoforms expressed by subpopulations of primary sensory olfactory neurons in the rat olfactory system. Based on the expression of NOC‐3 and NOC‐4 and the olfactory marker protein (OMP), four independent subpopulations of primary sensory olfactory neurons were characterized. These neurons expressed: both NOC‐3 and NOC‐4 but not OMP; both NOC‐4 and OMP but not NOC‐3; NOC‐3, NOC‐4, and OMP together; and OMP alone. The NOC‐3‐ and NOC‐4‐expressing neurons were widely dispersed in the olfactory neuroepithelium lining the nasal cavity. The axons of NOC‐4 expressing neurons innervated all glomeruli in the olfactory bulb, whereas the NOC‐3 expressing axons terminated in a discrete subset of glomeruli scattered throughout the whole olfactory bulb. We propose that both NOC‐3 and NOC‐4 are part of a chemical code of olfactory neurons which is used in establishing the topography of connections between the olfactory neuroepithelium and the olfactory bulb. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 659–670, 1997

We have investigated the differentiation potential of precursor cells within the developing spinal cord of mice and have shown that spinal cord cells from embryonic day 10 specifically give rise to neurons when plated onto an astrocytic monolayer, Ast-1. These neurons had the morphology of motor neurons and >83% expressed the motor neuron markers choline acetyltransferase, peripherin, calcitonin gene-related peptide, and L-14. By comparison, <10%o of the neurons arising on monolayers ofother neural cell lines or 3T3 fibroblasts had motor neuron characteristics. Cells derived from dorsal, intermediate, and ventral regions of the spinal cord all behaved similarly and gave rise to motor neuron-like cells when plated onto Ast-1. By using cells that expressed the lacZ reporter gene, it was shown that >93% of cells present on the Ast-1 monolayers were motor neuron-like. Time-lapse analysis revealed that the precursors on the Ast-1 monolayers gave rise to neurons either directly or following a single cell division. Together, these results indicate that precursors in the murine spinal cord can be induced to differentiate into the motor neuron phenotype by factors produced by Ast-I cells, suggesting that a similar factor(s) produced by cells akin to Ast-1 may regulate motor neuron differentiation in vivo.The induction of motor neurons in the spinal cord, one of the first neurons to arise developmentally (1-3), is influenced by epigenetic factors. It has been shown in the chicken that neuronal differentiation (4, 5), and later motor neuron differentiation (6, 7) in the spinal cord, can be regulated by the notochord. Further, conditioned medium from the notochord and floor plate can stimulate motor neuron differentiation in the intermediate zone, a region that does not normally give rise to motor neurons (8). These findings suggest that soluble factors within notochord conditioned medium are responsible for the motor neuron-inducing activity. In addition, these results suggest that precursor cells within the chicken spinal cord are not committed to any particular neuronal pathway. To examine the potential of individual precursors within the spinal cord, and to determine the direct effect of factors on the precursors, we have developed an in vitro assay in which the differentiation of individual precursors from the mouse spinal cord could be monitored.MATERIALS AND METHODS Cell Culture. All mice used in these experiments were of the CBA/CaWEHI strain except for transgenic mice, which were Bl/6 x DBA/2 hybrids (F6). The transgenic mice (line H253) contained a lacZ gene insertion (9). Mice heterozygous for the transgene were mated and embryos containing the lacZ transgene were identified by the 5-bromo-4-chloro-3-indolyl P3-Dgalactoside histochemistry (10) of biopsied material. Spinal cord cells from embryonic day 10 (E10) to E16 mice, where EO was the day a vaginal plug was detected, were prepared as described (11,12).Astrocytes were derived from E18 spinal cord as described (13); in addition, the cells were passed t...