The expression of the growth-associated protein B-50/GAP43 in the olfactory system of neonatal and adult rats

B50/GAP43 is a neuron-specific phosphoprotein whose expression is associated with neural development and synaptic plasticity. Its postnatal ontogeny was investigated in the primary olfactory pathway of the rat using immunohistochemical methods. The unique ability of the olfactory neuroepithelium to generate new neurons from a population of precursor cells present in the basal cell layer of this tissue makes it a valuable model in the study of neural development. In newborn rats B50/GAP43 is present throughout the entire population of olfactory receptor neurons. These cells are stained throughout, from the ciliated dendritic knob to their axon terminals in the bulb. This appears to be the first example of unambiguous B50/GAP43 expression in dendritic processes. With increasing age the distribution of this protein becomes progressively restricted to a subpopulation of olfactory neurons. Comparison of the expression of B50/GAP43 and the olfactory marker protein (OMP), a polypeptide only present in mature olfactory neurons, revealed that during postnatal development of the olfactory system these 2 proteins are expressed in a nearly reciprocal fashion. In adult animals (3.5 months-6 months of age), B50/GAP43-positive cells are exclusively present adjacent to the basal cell layer of the neuroepithelium. Basal cells appear to be unstained. The region of the epithelium containing the B50/GAP43-positive cells is virtually devoid of OMP-positive neurons. A significant fraction of these B50/GAP43- containing cells bear dendritic and neuritic processes. However, these cells do not express olfactory cilia. It is probable that the olfactory neurons expressing the growth-associated B50/GAP43 protein may correspond to a particular subset of olfactory neurons at an intermediate state of maturation

Transgenic expression of B-50/GAP-43 expression in immature olfactory neurons

The adult mammalian olfactory neuroepithelium is an unusual neural tissue, since it maintains its capacity to form new neurons throughout life. Newly formed neurons differentiate in the basal layers of the olfactory neuroepithelium and express B-50/GAP-43, a protein implicated in neurite outgrowth. During maturation these neurons migrate into the upper portion of the epithelium, upregulate expression of olfactory marker protein (OMP) and concomitantly downregulate the expression of B-50/GAP-43. Transgenic mice that exhibit OMP-promoter directed expression of B-50/GAP-43 in mature olfactory neurons display an unexpected decrease in the complement of B-50/GAP-43-positive cells in the lower region of the olfactory epithelium [A.J.G.D. Holtmaat, P.A. Dijkhuizen, A.B. Oestreicher, H.J. Romijn, N.M.T. Van der Lugt, A. Berns, F.L. Margolis, W.H. Gispen, J. Verhaagen, Directed expression of the growth-associated protein B-50/GAP-43 to olfactory neurons in transgenic mice results in changes in axon morphology and extraglomerular growth, J. Neurosci. 15 (1995) 79537965]. We have investigated whether the decrement in B-50/GAP-43-positive cells in this region was due to a dislocation of the immature neurons to other regions of the olfactory epithelium or to a downregulation of B-50/GAP-43 synthesis in these immature neurons. In eight of nine independent transgenic mouse lines that express the transgene in different numbers of olfactory neurons, a decline in the number of B-50/GAP-43-expressing neurons in the basal portion of the olfactory neuroepithelium was observed, both at the protein level and the mRNA level. An alternative marker for immature cells, a juvenile form of tubulin, was normally expressed in this location, indicating that the olfactory epithelium of OMP-B-50/GAP-43 transgenic mice contains a normal complement of immature olfactory neurons and that most of these neurons display a downregulation of B-50/GAP-43 expression

B-50/GAP43 Expression Correlates with Process Outgrowth in the Embryonic Mouse Nervous System

The hypothesis that B-50/GAP43, a membrane-associated phosphoprotein, is involved in process outgrowth has been tested by studying the developmental pattern of expression of B-50/GAP43 mRNA and protein during mouse neuroembryogenesis. B-50/GAP43 mRNA is first detectable at embryonic day 8.5 (E8.5) in the presumptive acoustico-facialis ganglion. Subsequently, both B-50/GAP43 mRNA and protein were co-expressed in a series of neural structures: in the ventral neural tube (from E9.5) and dorsal root ganglia (from E10.5), in the marginal layer of the neuroepithelium surrounding the brain vesicles and in the cranial ganglia (from E9.5), in the autonomic nervous system (from E10.5), in the olfactory neuroepithelium and in the mesenteric nervous system (from E11.5), in a continuum of brain regions (from E12.5) and in the retina (from E13.5). Immunoreactive fibers were always seen arising from these regions when they expressed B-50/GAP43 mRNA. The spatial and temporal pattern of B-50/GAP43 expression demonstrates that this protein is absent from neuroblasts and consistently appears in neurons committed to fiber outgrowth. The expression of the protein in immature neurons is independent of their embryological origin. Our detailed study of B-50/GAP43 expression during mouse neuroembryogenesis supports the view that this protein is involved in a process common to all neurons elaborating fibers

Directed expression of the growth-associated protein B-50/GAP-43 to olfactory neurons in transgenic mice results in changes in axon morphology and extraglomerular fiber growth

B-50/GAP-43, a neural growth-associated phosphoprotein, is thought to play a role in neuronal plasticity and nerve fiber formation since it is expressed at high levels in developing and regenerating neurons and in growth cones. Using a construct containing the coding sequence of B- 50/GAP-43 under the control of regulatory elements of the olfactory marker protein (OMP) gene, transgenic mice were generated to study the effect of directed expression of B-50/GAP-43 in a class of neurons that does not normally express B-50/GAP-43, namely, mature OMP-positive olfactory neurons. Olfactory neurons have a limited lifespan and are replaced throughout adulthood by new neurons that migrate into the upper compartment of the epithelium following their formation from stem cells in the basal portion of this neuroepithelium. Thus, the primary olfactory pathway is exquisitely suited to examine a role of B-50/GAP- 43 in neuronal migration, lifespan, and nerve fiber growth. We find that B-50/GAP-43 expression in adult olfactory neurons results in numerous primary olfactory axons with enlarged endings preferentially located at the rim of individual glomeruli. Furthermore, ectopic olfactory nerve fibers in between the juxtaglomerular neurons or in close approximation to blood vessels were frequently observed. This suggests that expression of B-50/GAP-43 in mature olfactory neurons alters their response to signals in the bulb. Other parameters examined, that is, migration and lifespan of olfactory neurons are normal in B-50/GAP-43 transgenic mice. These observations provide direct in vivo evidence for a role of B-50/GAP-43 in nerve fiber formation and in the determination of the morphology of axons