Electron microscopic immunocytochemistry of GAP-43 within proximal and chronically denervated distal stumps of transected peripheral nerve

Use of stem cells to augment nerve injury repair

OBJECTIVE

The purpose of this review is to summarize the basic science literature related to chronic nerve injuries, and to then use this as the background to provide emerging insights into the promising role of cellular therapy for nerve injury repair.

METHODS

The literature pertinent to the experimental and clinical aspects of chronic nerve injury was reviewed, as was emerging literature and our own recent experience in using cellular therapy to repair injured nerves.

RESULTS

Peripheral nerves have the potential to regenerate axons and reinnervate end organs. Yet, outcome after peripheral nerve injury, even after nerve repair, remains relatively poor. The single most important quantitative contributor to poor motor recovery is chronic denervation of the distal nerve. Chronic denervation is common because of the often extensive injury zone that prevents any axonal outgrowth or (even if outgrowth occurs) the relatively slow rate of regeneration. As a consequence, the distal nerve remains chronically devoid of regrowing axons. In turn, prolonged denervation of Schwann cells (SCs) seems to be the critical factor that makes them unreceptive for axonal regeneration. Regenerative success was demonstrated when denervated SCs were replaced with healthy SCs cultured from a secondary nerve. This cell-replacement strategy is, however, limited in the clinical setting by the inability to obtain sufficient numbers of cells and the requirement for sacrifice of additional nerve tissue. We, along with several other groups, have therefore begun investigating stem cell therapies to improve the regenerative environment.

CONCLUSION

There are several avenues of stem cell-based approaches to peripheral nerve repair. One of these, skin-derived precursor cells, are easily accessible, autologous adult stem cells that can survive and myelinate in the peripheral nerve environment and become SC-like in their apparent differentiation.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

Regeneration into Protected and Chronically Denervated Peripheral Nerve Stumps

OBJECTIVE

Delayed repair of peripheral nerve injuries often results in poor motor functional recovery. This may be a result of the deterioration or loss of endoneurial pathways in the distal nerve stump before motor axons can regenerate into the stump.

METHODS

Using the rat femoral nerve, we protected distal endoneurial pathways of the saphenous nerve with either cross-suture of the quadriceps motor nerve (Group A) or resuture of the saphenous nerve (Group B) to compare later motor regeneration into the "protected" saphenous nerve pathway to chronic denervation and "unprotected" saphenous nerve (Group C). A total of 60 rats, 20 per group, were operated on. After this protection (or lack thereof) for 8 weeks, the motor branch of the femoral nerve was cut and sutured to the distal saphenous nerve to allow motor regeneration into protected and unprotected saphenous nerve stumps. The quantitative assessment of axonal regeneration was performed after 6 weeks by use of nerve sampling for axon counts and retrogradely labeled motor neuron counts.

RESULTS

Significantly more myelinated axons innervated the motor (A) than the sensory (B) and no-protection (C) groups. There were significantly more retrogradely labeled femoral motor neurons in Group A than in the unprotected group (C).

CONCLUSION

We conclude that even 2 months of denervation of the distal nerve pathway is deleterious to regeneration and that protection of the pathway improves subsequent reinnervation and regeneration. Moreover, if the desired regeneration is motor, protection of the distal nerve pathway by a motor nerve conditions is better than a sensory nerve.

Axonal regrowth downregulates the synthesis of glial cell line-derived neurotrophic factor in the lesioned rat sciatic nerve

The effect of axonal regeneration on de novo synthesis of glial cell line-derived neurotrophic factor (GDNF) in rat sciatic nerves was examined. Transection of the sciatic nerve caused a prominent increase in the GDNF content in the distal segments within 1 week. The high level was sustained until 4 weeks in the animal model in which the nerve ends were ligated with thread (non-regeneration group); however, it was reduced to the original level within 2 or 4 weeks after the transection only in the segments invaded by regenerating axons in the models in which the nerve ends were coaptated (regeneration group). Expression of both GDNF protein and mRNA was decreased with a reciprocal increase in the density of neurofilaments, used as a marker of axonal ingrowth in distal segments of the regeneration group, suggesting that axonal contact turned off the GDNF-mediated nerve regeneration activity.

Long-term morphometric and immunohistochemical findings in human free microvascular muscle flaps

Reinnervation, muscle regeneration, density of microvessels, and muscle-type specific atrophy were studied 3-4 years after surgery in surgically nonreinnervated free microvascular muscle flaps to 13 patients transplanted to the upper or lower extremities. Routine histology and immunohistochemistry for PGP 9.5 and S-100 (neuronal markers), Ki-67 (cell proliferation), myosin (muscle fiber types), and CD-31 (endothelium) were carried out, and results were analyzed morphometrically. Three to 4 years after surgery, severe atrophy of predominantly slow-type fibers was seen in 9 cases. In 4 cases, muscle-fiber diameter and fiber-type distribution were close to normal. Long intraoperative muscle ischemia and postoperative immobilization were associated with poor muscle bulk in flaps. The density of microvessels in flaps did not differ from control muscles. PGP 9.5 and S-100 immunopositive nerve fibers were detected in 7 patients. Reinnervation was associated with good muscle bulk. In 4 patients, activation of satellite cells was evident. The results suggest that in some cases, spontaneous reinnervation may occur in free muscle flaps, and that several years after microvascular free flap transfer, the muscle still attempts to regenerate.

FK506 enhances regeneration of axons across long peripheral nerve gaps repaired with collagen guides seeded with allogeneic Schwann cells

We assessed the effects of FK506 administration on regeneration after a 6-mm gap repair with a collagen guide seeded with allogeneic Schwann cells (SCs) in the mouse sciatic nerve. SCs were isolated from predegenerated adult sciatic nerves and expanded in culture using a defined medium, before being seeded in the collagen guide embedded in Matrigel. Functional reinnervation was evaluated by noninvasive methods to determine recovery of motor, sensory, and autonomic functions in the hindpaw over 4 months postoperation. Histological analysis of the regenerated nerves was performed at the end of the study. Using simple collagen guides for tubulization repair, treatment with an immunosuppressant dose of FK506 (5 mg/kg/day) resulted in significant improvement of the onset and the degree of reinnervation. While the introduction of allogeneic SCs did not improve regeneration versus a collagen guide filled only with Matrigel, treatment with FK506 allowed for successful regeneration in all the mice and for significant improvement in the levels of functional recovery. Compared with the untreated group, there was greater survival of transplanted pre-labeled SCs in the FK506-treated animals. Morphologically, the best nerve regeneration (in terms of nerve caliber and numbers of myelinated axons) was obtained with SC-seeded guides from FK506-treated animals. Thus, FK506 should be considered as adjunct therapy for various types of tubulization repair.

ATF3 upregulation in glia during Wallerian degeneration: differential expression in peripheral nerves and CNS white matter

BACKGROUND

Many changes in gene expression occur in distal stumps of injured nerves but the transcriptional control of these events is poorly understood. We have examined the expression of the transcription factors ATF3 and c-Jun by non-neuronal cells during Wallerian degeneration following injury to sciatic nerves, dorsal roots and optic nerves of rats and mice, using immunohistochemistry and in situ hybridization.

RESULTS

Following sciatic nerve injury--transection or transection and reanastomosis--ATF3 was strongly upregulated by endoneurial, but not perineurial cells, of the distal stumps of the nerves by 1 day post operation (dpo) and remained strongly expressed in the endoneurium at 30 dpo when axonal regeneration was prevented. Most ATF3+ cells were immunoreactive for the Schwann cell marker, S100. When the nerve was transected and reanastomosed, allowing regeneration of axons, most ATF3 expression had been downregulated by 30 dpo. ATF3 expression was weaker in the proximal stumps of the injured nerves than in the distal stumps and present in fewer cells at all times after injury. ATF3 was upregulated by endoneurial cells in the distal stumps of injured neonatal rat sciatic nerves, but more weakly than in adult animals. ATF3 expression in transected sciatic nerves of mice was similar to that in rats. Following dorsal root injury in adult rats, ATF3 was upregulated in the part of the root between the lesion and the spinal cord (containing Schwann cells), beginning at 1 dpo, but not in the dorsal root entry zone or in the degenerating dorsal column of the spinal cord. Following optic nerve crush in adult rats, ATF3 was found in some cells at the injury site and small numbers of cells within the optic nerve displayed weak immunoreactivity. The pattern of expression of c-Jun in all types of nerve injury was similar to that of ATF3.

CONCLUSION

These findings raise the possibility that ATF3/c-Jun heterodimers may play a role in regulating changes in gene expression necessary for preparing the distal segments of injured peripheral nerves for axonal regeneration. The absence of the ATF3 and c-Jun from CNS glia during Wallerian degeneration may limit their ability to support regeneration.

Satellite cell proliferation, reinnervation, and revascularization in human free microvascular muscle flaps

BACKGROUND

Satellite cell proliferation, reinnervation, and revascularization were studied in human nonreinnervated free microvascular muscle flaps to characterize mechanisms of muscle regeneration after flap surgery.

MATERIALS AND METHODS

Patient biopsies (n = 19) were taken at operation and five timepoints up to 9 months after operation, and corresponding clinical data were obtained. Immunohistochemistry for Ki-67 was used to detect proliferating satellite cells, CD-31 to identify endothelial cells, and S-100 and PGP 9.5 proteins to detect reinnervation.

RESULTS

Two weeks after operation, the expression of PGP 9.5 and S-100 had virtually disappeared in all larger nerve fibers and half of smaller nerve fibers. By 6 months, however, a strong expression of PGP 9.5 and S-100 had reappeared in larger nerve fibers in three of four flaps, suggesting that reinnervation had taken place. The number of mitotic satellite cells already peaked at 2 weeks, indicating onset of muscle regeneration. The number of intramuscular capillaries first increased but later decreased to lower than original level. Flaps with more muscle volume showed more reinnervation and satellite cell mitotic activity. In cases of a delay occurring in reconstructive surgery, a low level of reinnervation was seen.

CONCLUSION

Three patients of four showed spontaneous muscle reinnervation in microvascular free flaps with satellite cell activation followed by restored morphology. Late reconstruction and obesity lead to poor reinnervation, placing emphasis on timing of surgery and patient selection.

HISTOLOGICAL DISTRIBUTION AND ULTRASTRUCTURAL FEATURES OF IMMUNOREACTIVE TERMINALS AGAINST RT97, A MONOCLONAL ANTIBODY TO A 200 kD NEUROFILAMENT, IN THE SPINAL DORSAL HORN OF A RAT

Localization and ultrastructural features of immunoreactive fibers and terminals against RT-97, a mouse monoclonal antibody that recognizes subunit of a 200-kD neurofilament, were examined in the spinal dorsal horn of adult rats. Under a light-microscope, many RT-97 immunoreactive fibers were detected in the dorsal root, collaterals of the dorsal root in the dorsal funiculus, and laminae III and IV in the dorsal horn. Few immunoreactive fibers were found in laminae I and II. Electron microscopic observation demonstrated that almost all RT-97 immunoreactive fibers in the dorsal root were myelinated, and unmyelinated fibers immunonegative. The immunoreactive fibers entered into the dorsal horn passing through the collaterals of the dorsal root along the superficial gray lamina. In the dorsal horn, these fibers ascended into and then terminated in lamina II. RT-97 immunoreactive central terminals were semicircular or ellipsoid in appearance and contained many flat-type presynaptic vesicles. Some terminals made synaptic contact with dendritic profiles in lamina II. Our present results indicate that RT-97 is a useful marker for ultrastructural examination of terminals served by non-nociceptive A-fibers.

Age-related changes and the possible adaptability of rat jaw muscle spindles: immunohistochemical and fine structural studies

Afferent signals from jaw muscle spindles contribute to the feedback mechanism that regulates mastication. The integrity and adaptability of this proprioceptor to age-related changes of the surrounding structures are therefore essential to maintain an appropriate masticatory function throughout life. In this study, we examined muscle spindles obtained from temporal and masseter muscles of 10-week-, 12-, 18-, and 24-month-old Wistar rats, employing immunohistochemistry for protein gene product 9.5 (PGP 9.5) or growth-associated protein (GAP-43) in addition to transmission electron microscopy, in order to investigate their morphological changes in relation to the effect of aging on the adaptive potential of the receptors. Immunohistochemistry for PGP 9.5 showed virtually similar reactions at sensory nerve terminals in all age groups. On the other hand, immunoreactivity for GAP-43 in the sensory nerve ending of the muscle spindles was found 2 and 3 weeks after birth but became almost undetectable by 10 weeks. However GAP-43 immunoreactions occasionally reappeared in those of spindles in 12- and 18-month old animals, and vanished again by 24 months of age. Electron microscopic observations also revealed age-related morphological changes in the intrafusal muscle fibers of the rats in 12-month and older groups. The extent of degenerative and/or atrophic alterations of intrafusal fibers increased with age and involved the nerve elements of spindles by 24 months. These findings indicate that the adaptation potential of rat jaw muscle spindles is well preserved until middle age, but diminishes in elderly animals. Structural changes of muscle spindles in elderly animals probably contribute to the deterioration of the muscular function.

A decline in glial cell-line-derived neurotrophic factor expression is associated with impaired regeneration after long-term Schwann cell denervation

In the peripheral nervous system, regeneration of motor and sensory axons into chronically denervated distal nerve segments is impaired compared to regeneration into acutely denervated nerves. In order to find possible causes for this phenomenon we examined the changes in the expression pattern of the glial cell-line-derived neurotrophic factor (GDNF) family of growth factors and their receptors in chronically denervated rat sciatic nerves as a function of time with or without regeneration. Among the GDNF family of growth factors, only GDNF mRNA expression was rapidly upregulated in Schwann cells as early as 48 h after denervation. This upregulation peaked at 1 week and then declined to minimal levels by 6 months of denervation. The changes in the protein expression paralleled the changes in the expression of the GDNF mRNA. The mRNAs for receptors GFRalpha-1 and GFRalpha-2 were upregulated only after maximal GDNF upregulation and remained elevated as late as 6 months. There were no significant changes in the expression of GFRalpha-3 or the tyrosine kinase coreceptor, RET. When we examined the expression of GDNF in a delayed regeneration paradigm, there was no upregulation in the distal chronically denervated tibial nerve even when the freshly axotomized peroneal branch of the sciatic nerve was sutured to the distal tibial nerve. This study suggests that one of the reasons for impaired regeneration into chronically denervated peripheral nerves may be the inability of Schwann cells to maintain important trophic support for both motor and sensory neurons.

The Ruffini ending as the primary mechanoreceptor in the periodontal ligament: its morphology, cytochemical features, regeneration, and development

The periodontal ligament receives a rich sensory nerve supply and contains many nociceptors and mechanoreceptors. Although its various kinds of mechanoreceptors have been reported in the past, only recently have studies revealed that the Ruffini endings--categorized as low-threshold, slowly adapting, type II mechanoreceptors--are the primary mechanoreceptors in the periodontal ligament. The periodontal Ruffini endings display dendritic ramifications with expanded terminal buttons and, furthermore, are ultrastructurally characterized by expanded axon terminals filled with many mitochondria and by an association with terminal or lamellar Schwann cells. The axon terminals of the periodontal Ruffini endings have finger-like projections called axonal spines or microspikes, which extend into the surrounding tissue to detect the deformation of collagen fibers. The functional basis of the periodontal Ruffini endings has been analyzed by histochemical techniques. Histochemically, the axon terminals are reactive for cytochrome oxidase activity, and the terminal Schwann cells have both non-specific cholinesterase and acid phosphatase activity. On the other hand, many investigations have suggested that the Ruffini endings have a high potential for neuroplasticity. For example, immunoreactivity for p75-NGFR (low-affinity nerve growth factor receptor) and GAP-43 (growth-associated protein-43), both of which play important roles in nerve regeneration/development processes, have been reported in the periodontal Ruffini endings, even in adult animals (though these proteins are usually repressed or down-regulated in mature neurons). Furthermore, in experimental studies on nerve injury to the inferior alveolar nerve, the degeneration of Ruffini endings takes place immediately after nerve injury, with regeneration beginning from 3 to 5 days later, and the distribution and terminal morphology returning to almost normal at around 14 days. During regeneration, some regenerating Ruffini endings expressed neuropeptide Y, which is rarely observed in normal animals. On the other hand, the periodontal Ruffini endings show stage-specific configurations which are closely related to tooth eruption and the addition of occlusal forces to the tooth during postnatal development, suggesting that mechanical stimuli due to tooth eruption and occlusion are a prerequisite for the differentiation and maturation of the periodontal Ruffini endings. Further investigations are needed to clarify the involvement of growth factors in the molecular mechanisms of the development and regeneration processes of the Ruffini endings.

Growth-associated protein-43 (GAP-43) in the regenerating periodontal Ruffini endings of the rat incisor following injury to the inferior alveolar nerve

Alterations in the levels of growth-associated protein 43 (GAP-43)-like immunoreactivity (-LI) were examined in the lingual periodontal ligament of the rat incisor following two types of injury (resection and crush) to the inferior alveolar nerve (IAN). In normal animals, GAP-43-like immunoreactive (IR) structures were observed as tree-like ramifications in the alveolar half of the lingual periodontal ligament of incisors. Under immunoelectron microscopy, GAP-43-LI appeared in the Schwann sheaths associated with periodontal Ruffini endings; neither cell bodies of the terminal Schwann cells nor axonal profiles showed GAP-43-LI. During regeneration of the periodontal Ruffini endings following resection of the IAN, GAP-43-LI appeared in the cytoplasm of the terminal Schwann cell bodies and axoplasm of the terminals. The distribution of GAP-43-LI in the Ruffini endings returned to almost normal levels on days 28 and 56 following the injury. The changes in the distribution of GAP-43-LI following the crush injury were similar to those following resection; however, expression of GAP-43-LI was slightly higher for the entire experimental period compared with the resection. The transient expression of GAP-43 in the terminal Schwann cells and axonal profiles of the periodontal Ruffini endings following nerve injury suggests that GAP-43 is closely associated with axon-Schwann cells interactions during regeneration.

Alterations in ultrastructural localization of growth-associated protein-43 (GAP-43) in periodontal Ruffini endings of rat molars during experimental tooth movement

It is known that orthodontic forces induce discomfort and/or abnormal sensation after application of an orthodontic appliance in patients, suggesting the adaptation of periodontal neural elements to environmental changes. However, no morphological data have been provided. The present study investigated, by immunoelectron microscopy, the localization of growth-associated protein-43 (GAP-43) in periodontal Ruffini endings in rat molars during experimental tooth movement. In the untreated control group, immunoelectron microscopy demonstrated that GAP-43-like immunoreactivity in the Ruffini endings was confined to the Schwann sheaths around the axon terminals, and was in neither the cell bodies of terminal Schwann cells nor the axon terminals themselves. Immunoelectron microscopic observation revealed alterations in the localization of GAP-43-like immunoreactivity in the periodontal Ruffini endings during experimental tooth movement. After 1 day of treatment, the cell bodies of the terminal Schwann cells associated with Ruffini endings appeared to contain immunoreaction products for GAP-43, and retained GAP-43-like immunoreactivity during tooth movement. From 5 to 7 days, a major population of the axoplasm of the periodontal Ruffini endings, which was immunonegative in control, filled the GAP-43 immunoreactions, showing a tendency to decrease in number later, and disappeared completely at 14 days. These findings suggest that orthodontic forces easily induce the remodeling of the mechanoreceptive Ruffini endings as well as the active tissue remodeling in a close relationship. Since the ultrastructural localization of GAP-43-like immunoreactivity was drastically changed in the Ruffini endings during tooth movement, GAP-43 functions as one of the key molecules in the remodeling of mechanoreceptive Ruffini endings during tooth movement.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

Growth-associated protein 43 immunoreactivity in the superficial dorsal horn of the rat spinal cord is localized in atrophic C-fiber, and not in sprouted A-fiber, central terminals after peripheral nerve injury

Peripheral nerve injury induces the up-regulation in dorsal root ganglion cells of growth-associated protein 43 (GAP-43) and its transport to the superficial laminae of the dorsal horn of the spinal cord, where it is located primarily in unmyelinated axons and growth-cone like structures. Peripheral nerve injury also induces the central terminals of axotomized myelinated axons to sprout and form novel synaptic contacts in lamina II of the dorsal horn. To investigate whether the sprouting of A-fiber central terminals into lamina II is the consequence of GAP-43 incorporation into their terminal membranes, we have used an ultrastructural analysis with double labelling to identify the localization of GAP-43 immunoreactivity. Transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to identify C-fiber terminals. Transganglionic transport of the B fragment of cholera toxin conjugated to horseradish peroxidase (B-HRP) was used to label A-fiber sciatic nerve central terminals in combination with GAP-43 immunocytochemistry. GAP-43 was found to colocalize only with WGA-HRP- and not with B-HRP-labelled synapses or axons. In addition, many single-labelled GAP-43 synapses were observed. Many of the WGA-HRP-labelled terminals that were characterized by degenerative changes were GAP-43 immunoreactive. Our results indicate that peripheral nerve injury induces novel synapse formation of A fibers in lamina II but that up-regulated levels of GAP-43 are present mainly in other axon projections to the superficial dorsal horn.

Ultrastructural evidence for the lack of co-transport of B-50/GAP-43 and calmodulin in myelinated axons of the regenerating rat sciatic nerve

Following peripheral nerve injury, neurons respond with synthesis of proteins required for axonal regeneration. Newly synthesized membrane proteins, like B-50/GAP-43, are transported with the fast component of anterograde axonal transport. Structural proteins and calmodulin are transported by the slow component. Since B-50/GAP-43 can bind calmodulin, it has been hypothesised that B-50/GAP-43 may act as a carrier for fast anterograde transport of calmodulin, so that both proteins are delivered rapidly to the distally outgrowing axons ('the fast carrier hypothesis'). We have investigated whether this hypothesis is valid in myelinated axons of the regenerating rat sciatic nerve. Seven days after crush, the nerve was ligated to accumulate fast transported proteins. Nerve pieces were dissected proximal to the ligation and processed for immunofluorescence and quantitative electron microscopy by postembedding single and double immunogold labelling. By light microscopy, we observed a qualitative increase in B-50/GAP-43 immunofluorescence in the axonal element immediately proximal to the nerve ligation (termed 'accumulated') compared to an upstream site (termed 'regenerating') closer to the cell body. The immunofluorescence for calmodulin appeared to be the same at both sites. Using electron microscopy, we observed that organelles had collected at the 'accumulated' site, moreover the density of B-50/GAP-43 immunolabelling was significantly increased compared to the 'regenerating' site, where the axoplasmic structure was undisturbed. The increase in B-50/GAP-43 immunolabelling was largely associated with vesicles. The density of calmodulin immunolabelling was similar at both sites. Approximately 25% of the total B-50/GAP-43 was associated with vesicles of which only 15% also contained labelling for calmodulin. Thus, ligation of the nerve resulted in accumulation of vesicles, including those carrying B-50/GAP-43, largely without calmodulin. Therefore, contrary to 'the fast carrier hypothesis', the bulk of calmodulin is not co-transported with B-50/GAP-43 in myelinated axons of the sciatic nerve.

GAP-43 mRNA expression in early development of human nervous system

The temporal and spatial distribution of GAP-43 mRNA in early human development, from 6 to 23 gestational weeks (g.w.), was examined by in situ hybridization histochemistry. GAP-43 mRNA was expressed as early as 6 g.w. in all regions of developing nervous system, the spinal cord, brainstem, cerebellum, diencephalic and telencephalic regions. Although the pronounced level of expression persisted during the entire examined period, the intensity of expression varied along the spatial axis over time. Analysis at the cellular level revealed that early on in development (6 g.w.) GAP-43 mRNA was expressed in the entire neuroblast population. With the onset of differentiation, at 13-23 g.w., GAP-43 mRNA expression had switched to the neurons that are in the process outgrowth. The highest level of GAP-43 mRNA expression was localized in the regions consisting of differentiating neurons, such as the cortical plate and intermediate zone of the telencephalic wall, and several delineated subcortical and thalamic nuclei. The spatial and temporal pattern of GAP-43 mRNA expression obtained suggests a possible dual role of GAP-43 in the development of the human nervous system: in the embryonic brain it could be involved in fundamental processes underlying cell proliferation; in the fetal brain its expression is specifically correlated with differentiation and the outgrowth of axons.

TGF-beta s and cAMP regulate GAP-43 expression in Schwann cells and reveal the association of this protein with the trans-Golgi network

We have shown previously that growth-associated protein 43 (GAP-43) is expressed by rat Schwann cells and is restricted to non-myelin-forming Schwann cells in vivo. Here we examined the regulation of GAP-43 using agents that are known to control Schwann cell differentiation in vitro. GAP-43 protein and mRNA levels are decreased by forskolin and other agents that elevate intracellular cAMP (and promote expression of the myelinating Schwann cell phenotype). We also found that expression of GAP-43 protein but not mRNA is down-regulated by transforming growth factor betas (TGF-beta s). Moreover, TGF-beta treatment of Schwann cells results in cell clumping, process retraction and disappearance of GAP-43 from the plasma membrane, revealing that GAP-43 is associated with the Golgi apparatus. This association was confirmed by partial overlap of GAP-43 with the trans-Golgi network marker (23c) and the disruption of the Golgi with brefeldin A or monensin leading to altered GAP-43 distribution. Golgi-associated GAP-43 appeared to have the same molecular weight as the plasma membrane-associated GAP-43. Thus these results show that GAP-43 expression in Schwann cells is subject to regulation by both extracellular and intracellular signalling molecules and that Schwann cell GAP-43 is often associated with the Golgi apparatus.

The increase in B-50/GAP-43 in regenerating rat sciatic nerve occurs predominantly in unmyelinated axon shafts: a quantitative ultrastructural study

The growth-associated protein B-50/GAP-43 is thought to play a crucial role in axonal growth. We investigated, by quantitative immunoelectron microscopy, whether there are differences in the subcellular distribution of B-50 in unmyelinated and myelinated axons of intact and regenerating sciatic nerves. Adult rats received an unilateral sciatic nerve crush and were euthanized 8 days later. Nerve pieces proximal from the crush site were embedded, and B-50 was visualized by specific B-50 antibodies and immunogold detection in ultrathin sections. The density of B-50 at the plasma membrane of unmyelinated axon shafts was significantly increased in the ipsilateral regenerating nerve in comparison to that of the contralateral intact nerve. In contrast, there was no significant difference in the B-50 density at the axolemma of myelinated regenerating and intact axon shafts. In the contralateral intact nerve, more B-50 was associated with the axolemma of unmyelinated axons than with the plasma membrane of myelinated axons. The density of axoplasmic B-50 was similar in intact unmyelinated and myelinated axon shafts, but was higher in regenerating nerve than in intact nerve. This suggests that enhanced axonal transport of B-50 occurs during axon outgrowth. Our study demonstrates a differential subcellular distribution of B-50 in unmyelinated and myelinated axon shafts in both the intact and regenerating sciatic nerve, indicating a differential inducible capacity for remodeling of the axon shafts.

Developmental changes in GAP-43 expression in primary cultures of rat cerebellar granule cells

GAP-43 is a growth-associated protein that has been implicated in the developmental outgrowth of axons. We have examined the profile of GAP-43 levels in rat cerebellar granule cells during their development in vitro. During the first 1-2 days after plating, the majority of cells expressed neurites and after 8 days a complex neuronal network had developed. In situ hybridization studies showed that GAP-43 mRNA levels rapidly increased to peak at 1-2 days and gradually returned to initial values after 7-8 days. Analysis of GAP-43 protein levels followed a similar transient profile. Initially, granule cell perikarya and structures associated with neuritogenesis all displayed GAP-43 immunoreactivity. In older cultures, perikaryal labelling was lost after 10 days whilst process staining decreased more gradually. During the first 48 hours detailed analysis of GAP-43 mRNA revealed two populations of granule cells. It was suggested that cells with significant label originated from the external germinal layer which displays much GAP-43 mRNA in cerebellar sections. Cells with little or no GAP-43, however, probably originated from the internal granular layer since this region displayed no specific labelling. Granule cells within clumps expressed more GAP-43 mRNA compared to isolated cells perhaps indicating cell-cell regulation of expression. These results describe the transient rise in GAP-43 protein and mRNA levels expressed by developing cerebellar granule cell neurons in vitro and provide further evidence for the role GAP-43 plays during neuritogenesis.

Expression of growth-associated protein-43 kD in Schwann cells is regulated by axon-Schwann cell interactions and cAMP

We have examined the regulation of growth-associated protein 43 kD (GAP-43) in rat Schwann cells. In unlesioned adult nerves, GAP-43-immunoreactivity was restricted to non-myelinating Schwann cells and unmyelinated axons. When adult nerves were transected to cause permanent axotomy, previously myelinating Schwann cells expressed progressively more GAP-43-immunoreactivity over 3 weeks, and GAP-43 mRNA levels increased over a similar time course. The peak level of GAP-43 mRNA occurred at least 2 weeks later than that of nerve growth factor receptor, another marker of denervated Schwann cells. In contrast, after nerve-crush, which allows axonal regeneration, many fewer Schwann cells had GAP-43-immunoreactivity, and the amount of GAP-43 mRNA was markedly lower than in transected nerves. Forskolin, a drug that activates adenylate cyclase and mimics many effects of axon-Schwann cell interactions, markedly reduced GAP-43-immunoreactivity and mRNA expression in cultured Schwann cells, whereas interleukin-1 had no effect. These data demonstrate that axon-Schwann cell interactions inhibit the expression of GAP-43 in Schwann cells and that this effect is mimicked by forskolin.

GAP-43 and p75NGFR immunoreactivity in presynaptic cells following neuromuscular blockade by botulinum toxin in rat

Peripheral nerve lesion results in changes in protein expression by neurons and denervated Schwann cells. In the present study we have addressed the question whether similar changes take place following functional denervation. Using immunohistochemistry and immunoelectron microscopy we examined changes in growth-associated protein (GAP-43) and low-affinity nerve growth factor receptor (p75NGFR) in rat gastrocnemius muscle following botulinum toxin-induced paralysis. GAP-43 and p75NGFR were selected because they are not expressed by mature intact motor neurons or Schwann cells, but are expressed following nerve lesion in both motor neurons and denervated Schwann cells. In control muscle, GAP-43 and p75NGFR immunoreactivity was seen only in nerve fibres near blood vessels. Two weeks after toxin injection, GAP-43 immunoreactivity could be seen at the motor endplates and in axons. Intensity of staining increased with longer survival and reached a peak between 4 and 8 weeks post-injection. Ultrastructurally, GAP-43 immunoreactivity was confined to nerve terminals and axons, whereas Schwann cells remained negative. Immunostaining for p75NGFR also increased following toxin injection and was detected in some terminal Schwann cells and in perineurial cells of small nerve fascicles near the paralyzed target cells, but not in axons. These results show that changes in expression of GAP-43 in motor neurons following functional denervation closely resemble the changes following anatomical interruption of nerve-muscle contact. GAP-43 was not expressed in Schwann cells, indicating that its upregulation in these cells is induced by loss of axonal contact or nerve degeneration products. There is no support for a role of p75NGFR in incorporation of neurotrophins in axons. The restriction of p75NGFR expression to terminal Schwann cells and perineurial cells in close proximity to the paralyzed target suggests a role for a target-derived signal or, alternatively, macrophages in eliciting this expression.

The ontogeny of GAP-43 (neuromodulin) mRNA in postnatal rat brain: evidence for a sex dimorphism

GAP-43 is a membrane-bound protein selectively concentrated in axonal growth cones during brain development and implicated in axonal outgrowth and elongation. A sex dimorphism in the number of synapses in certain regions of the adult rat brain has been attributed to differences in gonadal steroid hormone action during early postnatal life. The results of recent studies have demonstrated that gonadal steroids modulate GAP-43 mRNA in regions of the postnatal and adult brain where steroid hormone receptors are concentrated. Since gonadal steroids influence the development of the sexually undifferentiated brain during the first few weeks of postnatal life, the present study investigated the ontogeny of GAP-43 mRNA in the male and female rat brain between postnatal days 1 and 25. On postnatal days 1, 3, 6, 12, 18, and 25, brains were collected from male and female postnates and frozen, and 16 microns cryostat sections were processed and hybridized with a 35S-labeled antisense riboprobe complementary to GAP-43 mRNA. Evaluation of film autoradiograms demonstrated a widespread distribution of GAP-43 mRNA in postnatal brain regions, including the cerebral cortex; bed nucleus of the stria terminalis; and medial preoptic area, ventromedial nucleus, and arcuate nucleus of the hypothalamus. Densitometric measurements revealed that GAP-43 mRNA was transiently elevated during early postnatal life, with a subsequent decrease during brain maturation, although the pattern of change varied among the brain regions investigated. In addition, the level of GAP-43 hybridization signal was significantly higher in the male cortex, bed nucleus, and medial preoptic nucleus, but not the ventromedial and arcuate nuclei, than in postnatal females. Analysis of slide autoradiograms demonstrated that the change in GAP-43 mRNA during postnatal development was due to changes at the cellular level. The present results indicate that expression of GAP-43 mRNA is transiently elevated and sexually dimorphic in certain regions of the early postnatal rat brain. The results further suggest that the differential expression of GAP-43 in the male and female postnatal brain may be related to sex differences in neuronal outgrowth and connectivity resulting in a dimorphism in the pattern of adult neuronal circuitry.