Growth-associated protein GAP-43 and L1 act synergistically to promote regenerative growth of Purkinje cell axons in vivo

Abstract

Neuronal expression of growth-associated protein 43 (GAP-43) and the cell adhesion molecule L1 has been correlated with CNS axonal growth and regeneration, but it is not known whether expression of these molecules is necessary for axonal regeneration to occur. We have taken advantage of the fact that Purkinje cells do not express GAP-43 or L1 in adult mammals or regenerate axons into peripheral nerve grafts to test the importance of these molecules for axonal regeneration in vivo. Transgenic mice were generated in which Purkinje cells constitutively express L1 or both L1 and GAP-43 under the Purkinje cell-specific L7 promoter, and regeneration of Purkinje cell axons into peripheral nerve grafts implanted into the cerebellum was examined. Purkinje cells expressing GAP-43 or L1 showed minor enhancement of axonal sprouting. Purkinje cells expressing both GAP-43 and L1 showed more extensive axonal sprouting and axonal growth into the proximal portion of the graft. When a predegenerated nerve graft was implanted into double-transgenic mice, penetration of the graft by Purkinje cell axonal sprouts was strongly enhanced, and some axons grew along the entire intracerebral length of the graft (2.5-3.0 mm) and persisted for several months. The results demonstrate that GAP-43 and L1 coexpressed in Purkinje cells can act synergistically to switch these regeneration-incompetent CNS neurons into a regenerationcompetent phenotype and show that coexpression of these molecules is a key regulator of the regenerative ability of intrinsic CNS neurons in vivo.axonal regeneration ͉ CNS injury͞repair ͉ nerve graft ͉ transgenic mice A lthough intrinsic CNS neurons of adult mammals are normally unable to regenerate their axons after injury (1), some can be induced to regrow axons into and along segments of peripheral nerve grafted into the brain or spinal cord (2). Retinal ganglion cells and thalamic reticular nucleus neurons regenerate well into grafts (3, 4), but neocortical and neostriatal projection neurons do so very poorly, and cerebellar Purkinje cells do not do so at all (5). Successful regeneration of axons along nerve grafts occurs only from neurons that show prolonged upregulation of growth-associated molecules, including growthassociated protein 43 (GAP-43), cell adhesion molecule L1, and close homologue of L1 CHL1 (6).GAP-43 was the first neuronal growth-associated protein to be identified (7). It is concentrated in the cortical cytoskeleton of axonal growth cones, interacts with the cell membrane and actin filaments, and overexpression results in enhanced neurite outgrowth in vitro and enhanced axonal sprouting in vivo (8,9). Although the nervous system of GAP-43 knockout mice is grossly normal, there are pathway and circuit abnormalities in such mice (10-12); the current consensus view is that GAP-43 is involved in axonal sprouting and in the response of growth cones to guidance cues.L1 is the prototype of a widely distributed family of cell adhesion molecules (CAMs). It is present at the surface of growing axons during d...

L1 cell adhesion molecule is not required for small-diameter primary afferent sprouting after deafferentation

“…Although the addition of L1 by soluble, viral, or genetic means produces conflicting evidence that L1 either can enhance or inhibit axonal outgrowth (Kobayashi et al, 1995;Webb et al, 2001;Adcock et al, 2004;Zhang et al, 2005;Chen et al, 2007), results from the adult L1 mutant model provide clear-cut negative evidence that L1 is not required for C-fiber primary afferents (present study) or corticospinal tract axons (Jakeman et al, 2006) to sprout. Furthermore, we can conclude from the present data that the absence of L1 does not permit enhanced outgrowth of peptidergic sensory neurons after an injury.…”

“…If L1 has a role in CGRP-positive afferent sprouting after injury, the present results indicate that alternate factors must compensate for its absence (Adcock et al, 2004;Jakeman et al, 2006). Although L1 promotes axonal outgrowth in some models (Kobayashi et al, 1995;Webb et al, 2001;Adcock et al, 2004;Zhang et al, 2005;Chen et al, 2007), it also can have inhibitory effects depending on its binding partners (for review see Kamiguchi and Lemmon, 1997;Castellani et al, 2000). For example, Chaudhry et al (2006) found that co-expression of L1 with nerve growth factor reduced CGRP-positive sprouting within the spinal cord, and that this reduction did not occur with other cell adhesion molecules.…”

Promoting remyelination for the treatment of multiple sclerosis: opportunities and challenges

“…it is downregulated on myelinated axons and re-expressed in regenerating axons after spinal cord injury [64,65] . Exogenous L1CAM is beneficial in promoting axon growth and functional recovery after spinal cord injury [66] and optic nerve lesion [67] and is involved in the regenerative growth of Purkinje cell axons in vivo [68] .…”

Molecular Mechanisms, Biological Actions, and Neuropharmacology of the Growth-Associated Protein GAP-43

“…With an intact optic nerve, lens puncture still allowed macrophage infiltration into the eye that caused an increase in GAP-43 expression in ganglion cells across the entire retina [52]. GAP-43 and L1 have been shown to act synergistically to promote the regenerative outgrowth of Purkinje cell axons in vivo [104]. Purkinje cells do not express GAP-43 or L1 in adult mammals or regenerate axons into peripheral nerve grafts [104].…”

“…GAP-43 and L1 have been shown to act synergistically to promote the regenerative outgrowth of Purkinje cell axons in vivo [104]. Purkinje cells do not express GAP-43 or L1 in adult mammals or regenerate axons into peripheral nerve grafts [104]. Transgenic mice were generated which expressed either L1, GAP-43, or L1 and GAP-43 [104].…”