Expression of GAP-43 in normal and regenerating nerves in the frog

Phosphorylation of GAP-43 T172 is a molecular marker of growing axons in a wide range of mammals including primates

GAP-43 is a vertebrate neuron-specific protein and that is strongly related to axon growth and regeneration; thus, this protein has been utilized as a classical molecular marker of these events and growth cones. Although GAP-43 was biochemically characterized more than a quarter century ago, how this protein is related to these events is still not clear. Recently, we identified many phosphorylation sites in the growth cone membrane proteins of rodent brains. Two phosphorylation sites of GAP-43, S96 and T172, were found within the top 10 hit sites among all proteins. S96 has already been characterized (Kawasaki et al., 2018), and here, phosphorylation of T172 was characterized. In vitro (cultured neurons) and in vivo, an antibody specific to phosphorylated T172 (pT172 antibody) specifically recognized cultured growth cones and growing axons in developing mouse neurons, respectively. Immunoblotting showed that pT172 antigens were more rapidly downregulated throughout development than those of pS96 antibody. From the primary structure, this phosphorylation site was predicted to be conserved in a wide range of animals including primates. In the developing marmoset brainstem and in differentiated neurons derived from human induced pluripotent stem cells, immunoreactivity with pT172 antibody revealed patterns similar to those in mice. pT172 antibody also labeled regenerating axons following sciatic nerve injury. Taken together, the T172 residue is widely conserved in a wide range of mammals including primates, and pT172 is a new candidate molecular marker for growing axons.

Developmental and regional patterns of GAP-43 immunoreactivity in a metamorphosing brain

Growth-associated protein-43 is typically expressed at high levels in the nervous system during development. In adult animals, its expression is lower, but still observable in brain areas showing structural or functional plasticity. We examined patterns of GAP-43 immunoreactivity in the brain of the bullfrog, an animal whose nervous system undergoes considerable reorganization across metamorphic development and retains a strong capacity for plasticity in adulthood. Immunolabeling was mostly diffuse in hatchling tadpoles, but became progressively more discrete as larval development proceeded. In many brain areas, intensity of immunolabel peaked at metamorphic climax, the time of final transition from aquatic to semi-terrestrial life. Changes in intensity of GAP-43 expression in the medial vestibular nucleus, superior olivary nucleus, and torus semicircularis appeared correlated with stage-dependent functional changes in processing auditory stimuli. Immunolabeling in the Purkinje cell layer of the cerebellum and in the cerebellar nucleus was detectable at most developmental time points. Heavy immunolabel was present from early larval stages through the end of climax in the thalamus (ventromedial, anterior, posterior, central nuclei). Immunolabel in the tadpole telencephalon was observed around the lateral ventricles, and in the medial septum and ventral striatum. In postmetamorphic animals, immunoreactivity was confined mainly to the ventricular zones and immediately adjacent cell layers. GAP-43 expression was present in olfactory, auditory and optic cranial nerves throughout larval and postmetamorphic life. The continued expression of GAP-43 in brain nuclei and in cranial nerves throughout development and into adulthood reflects the high regenerative potential of the bullfrog's central nervous system.

Distribution of hyaluronan in the central nervous system of the frog

The qualitative and quantitative distribution pattern of hyaluronan (HA), a component of the extracellular matrix (ECM), was studied in the frog central nervous system by using a highly specific HA probe and digital image analysis. HA reaction was observed in both the white and the gray matter, showing a very intense staining around the perikarya and dendrites in the perineuronal net (PN). In the telencephalon, strong reaction was found in different parts of the olfactory system, in the pallium, and in the amygdala. In the diencephalon, intensive staining was found in the nucleus of Bellonci, the dorsal habenula, the lateral and central thalamic nuclei, and the subependymal zone of the third ventricle. In the mesencephalon, layers of optic tectum displayed different intensities, with the strongest reaction in layers B, D, F, 3, and 5. Other structures of the mesencephalon showed regional differences. The PN was especially intensively stained around the perikarya of the toral nuclei, the oculomotor and trochlear nuclei, and the basal optic nucleus. In the rhombencephalon, the granular layer of cerebellum, the vestibulocochlear nuclei, the superior olive, the spinal tract of the trigeminal nerve, and parts of the reticular formation showed the most intense reaction in the PN. In the spinal cord, considerable HA staining was found in the white matter and around the perikarya of motoneurons. The present study is the first description of the HA-positive areas of frog brain and spinal cord demonstrating the heterogeneity of HA distribution in the frog central nervous system.

FGF-2 modulates expression and distribution of GAP-43 in frog retinal ganglion cells after optic nerve injury

Basic fibroblast growth factor (bFGF or FGF-2) has been implicated as a trophic factor that promotes survival and neurite outgrowth of neurons. We found previously that application of FGF-2 to the proximal stump of the injured axon increases retinal ganglion cell (RGC) survival. We determine here the effect of FGF-2 on expression of the axonal growth-associated phosphoprotein (GAP)-43 in retinal ganglion cells and tectum of Rana pipiens during regeneration of the optic nerve. In control retinas, GAP-43 protein was found in the optic fiber layer and in optic nerve; mRNA levels were low. After axotomy, mRNA levels increased sevenfold and GAP-43 protein was significantly increased. GAP-43 was localized in retinal axons and in a subset of RGC cell bodies and dendrites. This upregulation of GAP-43 was sustained through the period in which retinal axons reconnect with their target in the tectum. FGF-2 application to the injured nerve, but not to the eyeball, increased GAP-43 mRNA in the retina but decreased GAP-43 protein levels and decreased the number of immunopositive cell bodies. In the tectum, no treatment affected GAP-43 mRNA but FGF-2 application to the axotomized optic nerve increased GAP-43 protein in regenerating retinal projections. We conclude that FGF-2 upregulates the synthesis and alters the distribution of the axonal growth-promoting protein GAP-43, suggesting that it may enhance axonal regrowth.

Expression of neuropeptides and growth-associated protein 43 (GAP-43) in cutaneous and mucosal nerve structures of the adult rat lower lip after mental nerve section

The reinnervation of the adult rat lower lip has been investigated after unilateral section of the mental nerve. Rats were sacrificed at 4, 7, 9, 14, 30, and 90 days after the operation. A further group of animals with section of the mental nerve and block of the alveolar nerve regeneration, was sacrificed at 14 days. Specimens were processed for immunocytochemistry with antibodies against PGP 9.5, GAP-43 or neuropeptides (CGRP, SP and VIP). Four days after nerve section, axonal degeneration seems evident in the mental nerve branches and inside skin and mucosa. GAP-43 immunoreactivity is intense in the mental nerve 7 days after nerve section and it reaches its maximal expression and distribution in peripheral nerve fibres at 14 days. At 30 days, the decline in its expression is associated with the increase of PGP9.5-, SP-, and CGRP immunopositivity. VIP is observed only in perivascular fibres at all times observed. Present results suggest that, after sensory denervation of the rat lip, nerve fibres in skin and mucosa remain at lower density than normal. The different time courses in the expression of neuropeptides and GAP-43 suggest a possible early involvement of GAP-43 in peripheral nerve regeneration.

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.