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

Abstract

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.

Targeted Overexpression of the Neurite Growth-Associated Protein B-50/GAP-43 in Cerebellar Purkinje Cells Induces Sprouting after Axotomy But Not Axon Regeneration into Growth-Permissive Transplants

“…To achieve this, the L7⌬AUG vector was digested with BamHI, and the 3Ј and 5Ј overhangs were converted to blunt ends with K lenow polymerase. The B-50/GAP-43 ORF was removed from pBluescript K S by digestion with AatII and KpnI (Holtmaat et al, 1995), also end-filled with K lenow polymerase and ligated in the BamHI-digested L7⌬AUG vector. The B-50/GAP-43 junctions were sequenced to determine whether the orientation of the insert was correct.…”

“…T wo of these mice, which passed the transgene to their offspring, were used for breeding to generate transgenic lines (designated L1657 and L1658). Transgenic mice were identified by Southern blotting of genomic tail-DNA according to previously described methods (Laird et al, 1991;Holtmaat et al, 1995).…”

“…Spontaneous sprouting or altered patterns of terminal axon distribution, however, have been observed in several adult brain regions from other transgenic mice overexpressing B-50/GAP-43 (Aigner et al, 1995;Holtmaat et al, 1995). This discrepancy between transgenic Purkinje cells and other neuron types overexpressing B-50/GAP-43 is likely attributable to population-specific differences in the constitutive propensity to axon growth and plasticity.…”

“…Thus, although the precise role of B-50/GAP-43 in axon growth is still debated, its expression has been strictly associated with the plastic and regenerative potential of adult neurons. B-50/GAP-43 overexpression in transgenic mice enhances spontaneous or induced sprouting in both the peripheral nervous system and the CNS (Aigner et al, 1995;Holtmaat et al, 1995), and it potentiates the regeneration of crushed peripheral axons (Aigner et al, 1995). However, it is unknown whether B-50/GAP-43 overexpression can increase the regenerative capabilities of transsected axons in the adult brain.…”

“…Its overexpression in transgenic mice produces spontaneous axonal sprouting and enhances induced remodeling in several neuron populations (Aigner et al, 1995;Holtmaat et al, 1995). We examined the capacity of this protein to increase the regenerative potential of injured adult central axons, by inducing targeted B-50/GAP-43 overexpression in Purkinje cells, which normally show poor regenerative capabilities.…”