Overexpression of the neural growth-associated protein GAP-43 induces nerve sprouting in the adult nervous system of transgenic mice

Role of GAP-43 in mediating the responsiveness of cerebellar and precerebellar neurons to axotomy

To determine whether the competence for axonal sprouting and/or regeneration in the cerebellar system correlates with GAP-43 expression, we have studied GAP-43 mRNA and protein expression in the postlesioned cerebellum and inferior olive. Purkinje cells transiently express GAP-43 during their developmental phase (from E15 to P5 in the rat) which consists of fast axonal growth and the formation of the corticonuclear projection. Adult Purkinje cells, which in control adult rats do not express GAP-43, are extremely resistant to the effects of axotomy but cannot regenerate axons. However, a late and protracted sprouting of axotomized Purkinje cells occurs spontaneously and correlates with a mild expression of GAP-43 mRNA. In contrast, inferior olivary neurons, despite their high constitutive expression of GAP-43, do not sprout but retract their axons and die after axotomy. Furthermore, mature Purkinje cells in cerebellar explants of transgenic mice that overexpress GAP-43 do not regenerate after axotomy, even in the presence of a permissive substrate (cerebellar embryonic tissue) and, contrary to the case in wild-type mice, they do not survive in the in vitro conditions and undergo massive cell death. These results show that the expression of GAP-43 is not only associated with axonal growth, but also with neuronal death.

BDNF increases the number of axotomized rat retinal ganglion cells expressing GAP-43, L1, and TAG-1 mRNA--a supportive role for nitric oxide?

The death of neurons and the limited ability to activate growth-associated genes prevent the restoration of lesioned fiber tracts in the adult mammalian CNS. Here, we characterized the effects of the survival-promoting neurotrophin brain-derived neurotrophic factor (BDNF) on mRNA expression of GAP-43, L1, TAG-1, and SC-1 in axotomized and regenerating rat retinal ganglion cells (RGCs). BDNF led to de novo upregulation of TAG-1 mRNA in axotomized RGCs and to a threefold increase in the number of GAP-43 and L1 mRNA-expressing RGCs. SC-1 expression remained unchanged. However, BDNF did not improve long-distance axon regeneration into a peripheral nerve graft. Surprisingly, potentiating BDNF-mediated neuroprotection by simultaneous administration of a spin trap or a NOS inhibitor counteracted the BDNF-induced growth-associated gene expression. This led us to hypothesize that the BDNF effects on GAP-43, L1, and TAG-1 mRNA expression are mediated by a NO-dependent mechanism. In summary, our data support the idea that survival and axon regeneration of lesioned CNS neurons can be regulated independently.

Schwann cell dependence of regenerating rat sensory neurons is inversely related to the quality of axon growth substratum

It is still controversial to what extent elongation of regenerating sensory axons depends on proliferating Schwann cells (SCs) in an injured peripheral nerve. We hypothesized that such regeneration was independent of SC support early after nerve injury, but later became SC-dependent. The sural nerve in rats was crushed, and freezing destroyed cells but not their basal laminae (BL) in the distal nerve segment. Sensory axon elongation was assessed by the nerve pinch test and their abundance was examined immunohistochemically. Sensory axons regenerated fairly rapidly during the first week even if SC migration was prevented. Thereafter, they ceased to elongate and withdrew until their terminals contacted the SCs migrating from the proximal nerve segment. Intrinsic neuronal capacity for growth without cell support, however, had not been lost. Rather, progressive degradation of the former SC BL and loss of laminin in the acellular segment arrested axon growth. Further elongation occurred only when SC migration was possible, corroborating our hypothesis. Sensory neurons continued to elongate and maintain their axons in spite of deteriorating growth substratum if, prior to injury the axons had been allowed to sprout into the denervated skin. Previous sprouting exposed the sensory neurons to high levels of NGF.

Spinal axon regeneration evoked by replacing two growth cone proteins in adult neurons

In contrast to peripheral nerves, damaged axons in the mammalian brain and spinal cord rarely regenerate. Peripheral nerve injury stimulates neuronal expression of many genes that are not generally induced by CNS lesions, but it is not known which of these genes are required for regeneration. Here we show that co-expressing two major growth cone proteins, GAP-43 and CAP-23, can elicit long axon extension by adult dorsal root ganglion (DRG) neurons in vitro. Moreover, this expression triggers a 60-fold increase in regeneration of DRG axons in adult mice after spinal cord injury in vivo. Replacing key growth cone components, therefore, could be an effective way to stimulate regeneration of CNS axons.

bFGF stimulates GAP-43 phosphorylation at ser41 and modifies its intracellular localization in cultured hippocampal neurons

Cultured hippocampal neurons have been used to study GAP-43 phosphorylation and subcellular distribution. By immunofluorescence, GAP-43 was found associated with adherent membrane patches that remained attached to the substratum after in situ permeabilization with Nonidet-NP40. This association increases during neuronal development and is stabilized by the actin cytoskeleton. Basic fibroblast growth factor (bFGF) promotes GAP-43 translocation from the cytosol to adherent membrane patches and, at the same time, stimulates GAP-43 phosphorylation, mainly at the protein kinase C (PKC) site (Ser41). Inhibition of PKC prevented bFGF-stimulated GAP-43 phosphorylation and translocation, while activation by phorbol esters mimicked bFGF effects, suggesting that phosphorylation at Ser41 regulates GAP-43 subcellular localization. Using biochemical fractionation and phosphorylation analysis, it was found that Ser41 phosphorylation was highest in cytoskeleton-associated GAP-43 and lowest in membrane-associated GAP-43. It is proposed that GAP-43 is continuously cycling between intracellular compartments depending on its phosphorylation state and could be taking part in initial adhesive complexes assembled during growth cone advance.

Mouse models of alpha-synucleinopathy and Lewy pathology

The discovery of two missense mutations (A53T and A30P) in the gene encoding the presynaptic protein alpha-synuclein (alphaSN) that are genetically linked to rare familial forms of Parkinson's disease and its accumulation in Lewy bodies and Lewy neurites has triggered several attempts to generate transgenic mice overexpressing human alphaSN. Analogous to a successful strategy for the production of transgenic animal models for Alzheimer's disease we generated mice expressing wildtype and the A53T mutant of human alphaSN in the nervous system under control of mouse Thy1 regulatory sequences. These animals develop neuronal alpha-synucleinopathy, striking features of Lewy pathology, neuronal degeneration and motor defects. Neurons in brainstem and motor neurons appeared particularly vulnerable. Motor neuron pathology included axonal damage and denervation of neuromuscular junctions, suggesting that alphaSN may interfere with a universal mechanism of synapse maintenance. Thy1-transgene expression of wildtype human alphaSN resulted in comparable pathological changes thus supporting a central role for mutant and wildtype alphaSN in familial and idiopathic forms of diseases with neuronal alpha-synucleinopathy and Lewy pathology. The mouse models provide means to address fundamental aspects of alpha-synucleinopathy and to test therapeutic strategies.

Up-regulation of growth-associated protein 43 mRNA in rat medial septum neurons axotomized by fimbria-fornix transection

Transection of septohippocampal fibres is widely used to study the response of CNS neurons to axotomy. Septohippocampal projection neurons survive axotomy and selectively up-regulate the transcription factor c-Jun. In the present study we investigated whether these cells concomitantly up-regulate the growth-associated protein-43 (GAP-43), a potential target gene of c-Jun implicated in axonal growth and regeneration. Using in situ hybridization histochemistry (ISHH) it was demonstrated that postlesional c-jun mRNA expression is accompanied by an increased expression of GAP-43 mRNA in the medial septum 3 days following fimbria-fornix transection (FFT). The increase reached a maximum at 7 days and gradually declined thereafter (17 days, 3 weeks). Retrograde prelabeling with Fluoro-Gold followed by axotomy and ISHH revealed that GAP-43 mRNA was up-regulated in septohippocampal projection neurons. Colocalization of GAP-43 mRNA and choline acetyltransferase protein showed that GAP-43 mRNA was expressed by cholinergic medial septal neurons after axotomy. Selective immunolesioning of the cholinergic component of the septohippocampal projection with 192 IgG-saporin followed by FFT demonstrated that GAP-43 mRNA was also synthesized by axotomized GABAergic neurons. These results demonstrate an up-regulation of GAP-43 mRNA in axotomized septohippocampal projection neurons independent of their transmitter phenotype which is closely correlated with c-Jun expression. Because the GAP-43 gene contains an AP-1 site, we hypothesize a c-Jun-driven up-regulation of GAP-43 in lesioned medial septal neurons that may contribute to their survival and regenerative potential following axotomy.

Plasticity of the superior olivary complex

The superior olivary complex (SOC) is part of the auditory brainstem of the vertebrate brain. Residing ventrally in the rhombencephalon, it receives sensory signals from both cochleae through multisynaptic pathways. Neurons of the SOC are also a target of bilateral descending projections. Ascending and descending efferents of the SOC affect the processing of auditory signals on both sides of the brainstem and in both organs of Corti. The pattern of connectivity indicates that the SOC fulfills functions of binaural signal integration serving sound localization. But whereas many of these connectional features are shared with the inferior colliculus (with the important exception of a projection to the inner ear), cellular and molecular investigations have shown that cells residing in SOC are unique in several respects. Unlike those of other auditory brainstem nuclei, they specifically express molecules known to be involved in development, plasticity, and learning (e.g., GAP-43 mRNA, specific subunits of integrin). Moreover, neurons of the SOC in adult mammals respond to various kinds of hearing impairment with the expression of plasticity-related substances (e.g., GAP-43, c-Jun, c-Fos, cytoskeletal elements), indicative of a restructuring of auditory connectivity. These observations suggest that the SOC is pivotal in the developmental and adaptive tuning of binaural processing in young and adult vertebrates.

Differential regulation of primary protein kinase C substrate (MARCKS, MLP, GAP-43, RC3) mRNAs in the hippocampus during kainic acid-induced seizures and synaptic reorganization

In the mature hippocampus, kainic acid seizures lead to excitotoxic cell death and synaptic reorganization in which granule cell axons (mossy fibers) form ectopic synapses on granule cell dendrites. In the present study, we examined the expression of four major, developmentally regulated protein kinase C (PKC) substrates (MARCKS, MLP, GAP-43, RC3), which have different subcellular and regional localizations in the hippocampus at several time points (6 hr, 12 hr, 18 hr, 24 hr, 48 hr, 5 days, or 15 days) following kainic acid seizures using in situ hybridization. Consistent with previous reports, following kainate seizures, GAP-43 mRNA expression exhibited a delayed and protracted elevation in the granule cell layer, which peaked at 24 hr, whereas expression in fields CA1 and CA3 remained relatively unchanged. Conversely, RC3 mRNA expression exhibited a delayed reduction in the granule cell layer that was maximal at 18 hr, as well as a reduction CA1 at 48 hr, whereas CA3 levels did not change. MARCKS mRNA expression in the granule cell layer and CA1 remained stable following kainate, although an elevation was observed in subfield CA3c at 12 hr. Similarly, MLP mRNA expression did not change in the granule cell layer or CA1 following kainate but exhibited a protracted elevation in subfields CA3b,c beginning at 6 hr post-kainate. Collectively these data demonstrate that different PKC substrate mRNAs exhibit unique expression profiles and regulation in the different cell fields of the mature hippocampus following kainic acid seizures and during subsequent synaptic reorganization. The expression profiles following kainate seizures bear resemblance to those observed during postnatal hippocampal development, which may indicate the recruitment of common regulatory mechanisms.

Imaging neuronal subsets in transgenic mice expressing multiple spectral variants of GFP

We generated transgenic mice in which red, green, yellow, or cyan fluorescent proteins (together termed XFPs) were selectively expressed in neurons. All four XFPs labeled neurons in their entirety, including axons, nerve terminals, dendrites, and dendritic spines. Remarkably, each of 25 independently generated transgenic lines expressed XFP in a unique pattern, even though all incorporated identical regulatory elements (from the thyl gene). For example, all retinal ganglion cells or many cortical neurons were XFP positive in some lines, whereas only a few ganglion cells or only layer 5 cortical pyramids were labeled in others. In some lines, intense labeling of small neuronal subsets provided a Golgi-like vital stain. In double transgenic mice expressing two different XFPs, it was possible to differentially label 3 neuronal subsets in a single animal.

The cell recognition molecule CHL1 is strongly upregulated by injured and regenerating thalamic neurons

Close homologue of L1 (CHL1) is a cell recognition molecule known to promote axonal growth in vitro. We have investigated the expression of CHL1 mRNA by regenerating central nervous system (CNS) neurons, by using in situ hybridisation 3 days to 10 weeks following the implantation of living and freeze-killed peripheral nerve autografts into the thalamus of adult rats. At all survival times after implantation of living grafts, neurons of the thalamic reticular nucleus (TRN), close to the graft tip and up to 1 mm away from it, displayed strong signal for CHL1 mRNA, even though TRN neurons show very low levels of CHL1 mRNA expression in unoperated animals. When the cell bodies of regenerating neurons were identified by retrograde labelling from the distal portion of the grafts, 4-6 weeks after operation, most of the labelled cells were found in the TRN and could be shown to haveupregulated CHL1 mRNA. In addition, some neurons in dorsal thalamic nuclei near the graft tip transiently upregulated CHL1 mRNA during the first 3 weeks after graft implantation, and glial cells showing CHL1 mRNA expression were present at the brain/graft interface 3 days to 2 weeks after operation. Freeze-killed grafts, into which axons do not regenerate, caused a transient upregulation of CHL1 in very few TRN neurons near the graft tip and in glial cells at the brain/graft interface but did not produce prolonged CHL1 mRNA expression. CHL1 can therefore be added to the list of molecules (including GAP-43, L1, and c-jun) strongly expressed by CNS neurons that regenerate their axons into nerve grafts, but not by those neurons that fail to regenerate their axons.

Transcriptional and post-transcriptional regulation of a brain growth protein: regional differentiation and regeneration induction of GAP-43

During axonal regeneration synthesis of different growth-associated proteins is increased. As yet there is no clear picture of the specific contribution made by the transcriptional and post-transcriptional machinery that provides the gene products necessary for process outgrowth. Here we focus our study on the transcriptional processes in neurons by using intron-directed in situ hybridization to the primary transcript of a brain growth protein GAP-43. In most brain regions, levels of primary transcript expression of GAP-43 were highly correlated with levels of its mRNA. However, there were notable dissociations: in hippocampal granule cells, high levels of primary transcript were evident yet no GAP-43 mRNA was detected. In locus coeruleus the reverse was true; there were high levels of GAP-43 mRNA but no detectable primary transcript. A primary transcript antitermination mechanism is proposed to explain the first dissociation, and a post-transcriptional mRNA stabilization mechanism to explain the second. Transcriptional activation during nerve regeneration was monitored by assessing primary transcript induction of GAP-43 in mouse facial motor neurons. This induction, as well as its mRNA, was restricted to the side of the facial nerve crush. Increases were first observed at 24 h with a rapid increase in both measures up to 3 days. To our knowledge, this is the first in vivo evidence demonstrating transcriptional activation of a brain growth protein in regenerating neurons. The present study points to the GAP-43 transcriptional mechanism as a key determinant of GAP-43 synthesis. Along with the recruitment of post-transcriptional mechanisms, such synthesis occurs in response to both intrinsic developmental programs and extrinsic environmental signals.

Overexpression of HuD, but not of its truncated form HuD I+II, promotes GAP-43 gene expression and neurite outgrowth in PC12 cells in the absence of nerve growth factor

We have previously shown that the RNA-binding protein HuD binds to a regulatory element in the growth-associated protein (GAP)-43 mRNA and that this interaction involves its first two RNA recognition motifs (RRMs). In this study, we investigated the functional significance of this interaction by overexpression of human HuD protein (pcHuD) or its truncated form lacking the third RRM (pcHuD I+II) in PC12 cells. Morphological analysis revealed that pcHuD cells extended short neurites containing GAP-43-positive growth cones in the absence of nerve growth factor (NGF). These processes also contained tubulin and F-actin filaments but were not stained with antibodies against neurofilament M protein. In correlation with this phenotype, pcHuD cells contained higher levels of GAP-43 without changes in levels of other NGF-induced proteins, such as SNAP-25 and tau. In mRNA decay studies, HuD stabilized the GAP-43 mRNA, whereas HuD I+II did not have any effect either on GAP-43 mRNA stability or on the levels of GAP-43 protein. Likewise, pcHuD I+II cells showed no spontaneous neurite outgrowth and deficient outgrowth in response to NGF. Our results indicate that HuD is sufficient to increase GAP-43 gene expression and neurite outgrowth in the absence of NGF and that the third RRM in the protein is critical for this function.

Overexpression of GAP-43 in thalamic projection neurons of transgenic mice does not enable them to regenerate axons through peripheral nerve grafts

It is well established that some populations of neurons of the adult rat central nervous system (CNS) will regenerate axons into a peripheral nerve implant, but others, including most thalamocortical projection neurons, will not. The ability to regenerate axons may depend on whether neurons can express growth-related genes such as GAP-43, whose expression correlates with axon growth during development and with competence to regenerate. Thalamic projection neurons which fail to regenerate into a graft also fail to upregulate GAP-43. We have tested the hypothesis that the absence of strong GAP-43 expression by the thalamic projection neurons prevents them from regenerating their axons, using transgenic mice which overexpress GAP-43. Transgene expression was mapped by in situ hybridization with a digoxigenin-labeled RNA probe and by immunohistochemistry with a monoclonal antibody against the GAP-43 protein produced by the transgene. Many CNS neurons were found to express the mRNA and protein, including neurons of the mediodorsal and ventromedial thalamic nuclei, which rarely regenerate axons into peripheral nerve grafts. Grafts were implanted into the region of these nuclei in the brains of transgenic animals. Although these neurons strongly expressed the transgene mRNA and protein and transported the protein to their axon terminals, they did not regenerate axons into the graft, suggesting that lack of GAP-43 expression is not the only factor preventing thalamocortical neurons regenerating their axons.

Expression of GAP-43 and SCG10 mRNAs in lateral geniculate nucleus of normal and monocularly deprived macaque monkeys

We performed nonradioactive in situ hybridization histochemistry (ISH) in the lateral geniculate nucleus (LGN) of the macaque monkey to investigate the distribution of mRNA for two growth-associated proteins, GAP-43 and SCG10. GAP-43 and SCG10 mRNAs were coexpressed in most neurons of both magnocellular layers (layers I and II) and parvocellular layers (layers III-VI). Double-labeling using nonradioactive ISH and immunofluorescence revealed that both GAP-43 and SCG10 mRNAs were coexpressed with the alpha-subunit of type II calcium/calmodulin-dependent protein kinase, indicating that both mRNAs are expressed also in koniocellular neurons in the LGN. We also showed that GABA-immunoreactive neurons in the LGN did not contain GAP-43 and SCG10 mRNAs, indicating that neither GAP-43 nor SCG10 mRNAs were expressed in inhibitory interneurons in the LGN. GABA-immunoreactive neurons in the perigeniculate nucleus, however, contained both GAP-43 and SCG10 mRNAs, indicating that both mRNAs were expressed in inhibitory neurons in the perigeniculate nucleus, which project to relay neurons in the LGN. Furthermore, to determine whether the expression of GAP-43 and SCG10 mRNAs is regulated by visual input, we performed nonradioactive ISH in the LGN and the primary visual area of monkeys deprived of monocular visual input by intraocular injections of tetrodotoxin. Both mRNAs were downregulated in the LGN after monocular deprivation for 5 d or longer. From these results, we conclude that both GAP-43 and SCG10 mRNAs are expressed in the excitatory relay neurons of the monkey LGN in an activity-dependent manner.

Neuropathology in mice expressing human alpha-synuclein

The presynaptic protein alpha-synuclein is a prime suspect for contributing to Lewy pathology and clinical aspects of diseases, including Parkinson's disease, dementia with Lewy bodies, and a Lewy body variant of Alzheimer's disease. alpha-Synuclein accumulates in Lewy bodies and Lewy neurites, and two missense mutations (A53T and A30P) in the alpha-synuclein gene are genetically linked to rare familial forms of Parkinson's disease. Under control of mouse Thy1 regulatory sequences, expression of A53T mutant human alpha-synuclein in the nervous system of transgenic mice generated animals with neuronal alpha-synucleinopathy, features strikingly similar to those observed in human brains with Lewy pathology, neuronal degeneration, and motor defects, despite a lack of transgene expression in dopaminergic neurons of the substantia nigra pars compacta. Neurons in brainstem and motor neurons appeared particularly vulnerable. Motor neuron pathology included axonal damage and denervation of neuromuscular junctions in several muscles examined, suggesting that alpha-synuclein interfered with a universal mechanism of synapse maintenance. Thy1 transgene expression of wild-type human alpha-synuclein resulted in similar pathological changes, thus supporting a central role for mutant and wild-type alpha-synuclein in familial and idiotypic forms of diseases with neuronal alpha-synucleinopathy and Lewy pathology. These mouse models provide a means to address fundamental aspects of alpha-synucleinopathy and test therapeutic strategies.

Transgenic mice overexpressing truncated trkB neurotrophin receptors in neurons show increased susceptibility to cortical injury after focal cerebral ischemia

It has been suggested that the increased production of endogenous BDNF after brain insults supports the survival of injured neurons and limits the spread of the damage. In order to test this hypothesis experimentally, we have produced transgenic mouse lines that overexpress the dominant-negative truncated splice variant of BDNF receptor trkB (trkB.T1) in postnatal cortical and hippocampal neurons. When these mice were exposed to transient focal cerebral ischemia by occluding the middle cerebral artery for 45 min and the damage was assessed 24 h later, transgenic mice had a significantly larger damage than wild-type littermates in the cerebral cortex (204 +/- 32% of wild-type, P = 0.02), but not in striatum, where the transgene is not expressed. Our results support the notion that endogenously expressed BDNF is neuroprotective and that BDNF signaling may have an important role in preventing brain damage after transient ischemia.

MMP-2 and MMP-9 increase the neurite-promoting potential of schwann cell basal laminae and are upregulated in degenerated nerve

Compared to degenerated nerves, the ability of normal adult peripheral nerve to support axonal regeneration is poor and may be attributed to the inhibition of endoneurial laminin by chondroitin sulfate proteoglycan (CSPG). In cryoculture assays, neuritic growth of neonatal and adult peripheral neurons was increased on sections of normal nerve by pretreatment with CSPG-degrading enzymes, including the matrix metalloproteinases MMP-2 and MMP-9. Axonal regeneration is known to occur within the Schwann cell basal laminae of degenerated nerve. Similarly, deconvolution microscopy revealed that neuritic growth on nerve tissue sections occurred principally on the lumenal surface of enzymatically modified basal laminae. Compared to normal nerve, there was a marked increase in the neurite-promoting activity of the degenerated nerve, and this activity was not increased significantly by subsequent MMP treatment. Additionally, the expression and activation of MMP-2 and MMP-9 were elevated in degenerated nerve, suggesting that degradation of inhibitory CSPG by the MMPs contributes to the growth-promoting properties of degenerated nerve.

Striatal responses to partial dopaminergic lesion: evidence for compensatory sprouting

Dopaminergic lesions result in the acute loss of striatal dopamine content, the loss of tyrosine hydroxylase-immunoreactive fibers, upregulation of preproenkephalin mRNA expression, and compensatory changes in the synthesis and metabolism of dopamine. Despite the severe loss of fine tyrosine hydroxylase-immunoreactive fibers, larger fibers persist. We found that some tyrosine hydroxylase fiber types increase their branching and become thicker after partial lesion. To determine whether the remaining tyrosine hydroxylase fibers were degenerative or part of a compensatory response, we morphologically characterized striatal tyrosine hydroxylase fibers and compared them to silver-stained degenerative structures. Branched and large tyrosine hydroxylase fiber types were nondegenerative. Furthermore, normal preproenkephalin mRNA expression was maintained despite severe overall loss of tyrosine hydroxylase fibers in striatal regions with abundant branching, whereas preproenkephalin mRNA expression increased in severely depleted regions that lacked branched fibers, indicating that branching or sprouting was involved in the compensation for dopamine depletion and the maintenance of normal preproenkephalin expression. In support of compensatory sprouting by tyrosine hydroxylase fibers, mRNA for growth associated protein-43 was upregulated in dopaminergic midbrain cells. We conclude that an important compensatory response to partial dopaminergic depletion is the formation of new branches or sprouting.

Absence of GAP-43 can protect neurons from death

The main function of GAP-43 is thought to be regulating growth cone motility and axon guidance signals. GAP-43 is highly expressed during development and in regenerating nerves and in particular regions of the adult brain. We here present the first evidence that GAP-43 can modulate guidance signals emanating from Semaphorin III (SemaIII) in cultured NGF-dependent sensory neurons. We further show that absence of GAP-43 dramatically increases resistance of specific sensory neurons to apoptotic stimuli in vitro. NGF-dependent sensory neurons from GAP-43 (+/-) and null mutant mice are strongly protected against SemaIII-induced death. Furthermore, NGF- and BDNF-dependent neurons, but not NT-3-dependent neurons, from GAP-43 null mutant mice are much more resistant to apoptosis induced by trophic factor deprivation. We also show that early postnatal Purkinje cells from GAP-43 (+/-) mice are more resistant to cell death in organotypic cultures. We conclude that GAP-43 can influence neuronal survival and modulate repulsive axon guidance signals.

Shared and unique roles of CAP23 and GAP43 in actin regulation, neurite outgrowth, and anatomical plasticity

CAP23 is a major cortical cytoskeleton-associated and calmodulin binding protein that is widely and abundantly expressed during development, maintained in selected brain structures in the adult, and reinduced during nerve regeneration. Overexpression of CAP23 in adult neurons of transgenic mice promotes nerve sprouting, but the role of this protein in process outgrowth was not clear. Here, we show that CAP23 is functionally related to GAP43, and plays a critical role to regulate nerve sprouting and the actin cytoskeleton. Knockout mice lacking CAP23 exhibited a pronounced and complex phenotype, including a defect to produce stimulus-induced nerve sprouting at the adult neuromuscular junction. This sprouting deficit was rescued by transgenic overexpression of either CAP23 or GAP43 in adult motoneurons. Knockin mice expressing GAP43 instead of CAP23 were essentially normal, indicating that, although these proteins do not share homologous sequences, GAP43 can functionally substitute for CAP23 in vivo. Cultured sensory neurons lacking CAP23 exhibited striking alterations in neurite outgrowth that were phenocopied by low doses of cytochalasin D. A detailed analysis of such cultures revealed common and unique functions of CAP23 and GAP43 on the actin cytoskeleton and neurite outgrowth. The results provide compelling experimental evidence for the notion that CAP23 and GAP43 are functionally related intrinsic determinants of anatomical plasticity, and suggest that these proteins function by locally promoting subplasmalemmal actin cytoskeleton accumulation.

GAP43, MARCKS, and CAP23 modulate PI(4,5)P(2) at plasmalemmal rafts, and regulate cell cortex actin dynamics through a common mechanism

The dynamic properties of the cell cortex and its actin cytoskeleton determine important aspects of cell behavior and are a major target of cell regulation. GAP43, myristoylated alanine-rich C kinase substrate (MARCKS), and CAP23 (GMC) are locally abundant, plasmalemma-associated PKC substrates that affect actin cytoskeleton. Their expression correlates with morphogenic processes and cell motility, but their role in cortex regulation has been difficult to define mechanistically. We now show that the three proteins accumulate at rafts, where they codistribute with PI(4,5)P(2), and promote its retention and clustering. Binding and modulation of PI(4, 5)P(2) depended on the basic effector domain (ED) of these proteins, and constructs lacking the ED functioned as dominant inhibitors of plasmalemmal PI(4,5)P(2) modulation. In the neuron-like cell line, PC12, NGF- and substrate-induced peripheral actin structures, and neurite outgrowth were greatly augmented by any of the three proteins, and suppressed by DeltaED mutants. Agents that globally mask PI(4,5)P(2) mimicked the effects of GMC on peripheral actin recruitment and cell spreading, but interfered with polarization and process formation. Dominant negative GAP43(DeltaED) also interfered with peripheral nerve regeneration, stimulus-induced nerve sprouting and control of anatomical plasticity at the neuromuscular junction of transgenic mice. These results suggest that GMC are functionally and mechanistically related PI(4,5)P(2) modulating proteins, upstream of actin and cell cortex dynamics regulation.

Enhanced learning after genetic overexpression of a brain growth protein

Ramón y Cajal proposed 100 years ago that memory formation requires the growth of nerve cell processes. One-half century later, Hebb suggested that growth of presynaptic axons and postsynaptic dendrites consequent to coactivity in these synaptic elements was essential for such information storage. In the past 25 years, candidate growth genes have been implicated in learning processes, but it has not been demonstrated that they in fact enhance them. Here, we show that genetic overexpression of the growth-associated protein GAP-43, the axonal protein kinase C substrate, dramatically enhanced learning and long-term potentiation in transgenic mice. If the overexpressed GAP-43 was mutated by a Ser --> Ala substitution to preclude its phosphorylation by protein kinase C, then no learning enhancement was found. These findings provide evidence that a growth-related gene regulates learning and memory and suggest an unheralded target, the GAP-43 phosphorylation site, for enhancing cognitive ability.

Brain-derived neurotrophic factor (BDNF) induces dendritic targeting of BDNF and tyrosine kinase B mRNAs in hippocampal neurons through a phosphatidylinositol-3 kinase-dependent pathway

This study aims to understand the mechanisms of dendritic targeting of brain-derived neurotrophic factor (BDNF) and tyrosine kinase B (TrkB) mRNAs. We show that brief depolarizations are sufficient to induce accumulation of BDNF and TrkB mRNAs in dendrites of hippocampal neurons. Endogenous BDNF, secreted during the KCl stimulation, contributes significantly to the dendritic accumulation of BDNF-TrkB mRNAs. In the absence of depolarization, 1 min pulses of exogenous BDNF are sufficient to induce dendritic accumulation of BDNF-TrkB mRNAs. After binding to TrkB, BDNF exerts this action by activating a PI-3 kinase-dependent pathway. The accumulation of dendritic mRNA by BDNF is not mediated by BDNF-induced neurotransmitter release. Because most hippocampal neurons coexpress BDNF and TrkB receptors, these results show that the subcellular distribution of BDNF-TrkB mRNAs is under the control of an autocrine-paracrine BDNF-TrkB-dependent loop.

Syntaxin 13 is a developmentally regulated SNARE involved in neurite outgrowth and endosomal trafficking

In addition to its role in exocytosis, SNAP-25 is essential for axonal outgrowth. In order to identify SNARE proteins involved in neurite growth we have used SNAP-25 antibodies to affinity-purify protein complexes enriched in developing rat brain membrane extracts. We have identified a complex between SNAP-25 and syntaxin 13 predominantly present in brain at embryonic or early postnatal stages. We show that syntaxin 13 is developmentally regulated with a decrease in adult brain. In differentiated neuroendocrine PC12 cells as well as primary cortical neurons the protein is localized to a punctated and tubular staining in the perinuclear region and along processes with high levels in the central region of growth cones. Carboxy-terminally tagged syntaxin 13 was also detected on the plasma membrane by in vivo surface-labelling where it colocalized with SNAP-25. Syntaxin 13 has recently been shown to be implicated in early endosomal trafficking. In our study, colocalization with internalized transferrin in the cell body and along neurites confirmed endosomal location in both compartments. Finally, overexpression of full-length syntaxin 13 enhanced neurite outgrowth in NGF-stimulated PC12 cells, whilst it had no effect on regulated secretion. The data suggest that a syntaxin 13-dependent endocytic trafficking step plays a limiting role in membrane expansion during neuronal development.

A qualitative and quantitative study of grumose degeneration in progressive supranuclear palsy

Grumose degeneration (GD) of the dentate nucleus is a common feature in progressive supranuclear palsy (PSP), but its pathogenesis has not been well studied, and its clinical significance remains unknown. This report describes a quantitative study of GD in 9 cases of PSP using image analysis with single- and double-immunolabeling, as well as histochemical stains for myelin and axons. GD was associated with demyelination, axonal loss, glial tau pathology, and microgliosis in regions juxtaposed to the dentate nucleus (DN). Specifically, demyelination and microgliosis were prominent in the superior cerebellar peduncle (SCP), dentate hilus, and cerebellar hemispheric white matter. Tau pathology and microgliosis were less prominent in the DN itself. The degree of myelin loss correlated with the tau burden in the SCP. GAP-43, which is a phosphoprotein known to be involved in axonal growth and sprouting, was decreased in the DN of PSP, and the degree of GAP-43 loss correlated with severity of GD. These results suggest that GD may be related to progressive pathology in the dentatorubrothalamic tract as well as the cerebellar hemispheric white matter, and that GD may be a consequence of concurrent degeneration in both output from and input to the DN. The results further suggest a possible role for oligodendroglial and myelin pathology in the pathogenesis of PSP.

Early and selective loss of neuromuscular synapse subtypes with low sprouting competence in motoneuron diseases

The addition or loss of synapses in response to changes in activity, disease, or aging is a major aspect of nervous system plasticity in the adult. The mechanisms that affect the turnover and maintenance of synapses in the adult are poorly understood and are difficult to investigate in the brain. Here, we exploited a unique anatomical arrangement in the neuromuscular system to determine whether subtypes of synapses can differ in anatomical plasticity and vulnerability. In three genetic mouse models of motoneuron disease of diverse origin and severity, we observed a gradual and selective loss of synaptic connections that begun long before the onset of clinical deficits and correlated with the timing of disease progression. A subgroup of fast-type (fast-fatiguable) neuromuscular synapses was highly vulnerable and was lost very early on. In contrast, slow-type synapses resisted up to the terminal phase of the disease. Muscle-specific differences were also evident. Similar selective losses were detected in aged mice. These selective vulnerability properties of synapses coincided with hitherto unrecognized major differences in stimulus-induced anatomical plasticity that could also be revealed in healthy mice. Using paralysis and/or growth-associated protein 43 overexpression to induce synaptic sprouting, we found that slow-type, disease-resistant synapses were particularly plastic. In contrast, fast-type synapses with the highest vulnerability failed to exhibit any stimulus-induced change. The results reveal pronounced subtype specificity in the anatomical plasticity and susceptibility to loss of neuromuscular synapses and suggest that degenerative motoneuron diseases involve a common early pathway of selective and progressive synaptic weakening also associated with aging.

The generation of localized calcium rises mediated by cell adhesion molecules and their role in neuronal growth cone motility

Neurite growth and guidance depends on the transduction of extracellular guidance cues into motile responses by the sensory apparatus at the tip of the neurite, the growth cone. Contact of the growth cone with extracellular ligands leads to the cytoskeletal reorganisation required for changes in rate of motility and direction of outgrowth. Differential adhesion mediated by cell adhesion molecules and signal transduction pathways mediated by growth cone receptors were once seen as separate but cooperative events in controlling growth cone motility. However, recent findings suggest that cell adhesion molecules can activate novel signalling pathways in the growth cone by the recruitment of fibroblast growth factor receptors leading to neurite outgrowth. This Review focuses on work by various laboratories centering on the intracellular consequences of the cell adhesion molecule-mediated activation of the fibroblast growth factor receptor. These include activation of a lipase cascade including phospholipase C and diacylglycerol lipase and culminating in the release of arachidonic acid. This release of arachidonic acid is proposed to activate the transient opening of voltage dependent ion-channels leading to localised rises in growth Ca(2+). Recent findings demonstrating this previously undetectable rise in Ca(2+) in the growth cone are discussed in light of the proposed roles and mechanisms of Ca(2+) in controlling neurite outgrowth. The Ca(2+) rises are thought to induce the activation of GAP43 and Ca(2+)/calmodulin-dependent kinase II, molecules implicated in the modulation of cytoskeletal remodelling. The evidence that this pathway may be involved in the guidance of retinal ganglion cells is evaluated.

Protein profiling of rat cerebella during development

Protein profiles of developing rat cerebella were analyzed by means of two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS). The analysis of adult rat cerebellum gave rise to a protein map comprising approximately 3000 spots detectable by silver staining following high resolution 2-DE with a pH range of 3-10 and a mass range of 8-100 kDa. To obtain landmarks for comparison of developmental profiles of cerebellar proteins, 100 spots were subjected to peptide mass fingerprinting using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and 67 spots were assigned on the map. Analysis of profiles of the developing cerebella revealed significant changes in the expression of proteins during development. In most cases the expression levels of proteins increased as the cerebellum matured, while the expression of 42 spots appeared specific or remarkably abundant in the immature cerebellum. Peptide mass fingerprinting of these spots allowed us to identify 29 proteins, which include, in addition to proteins of unknown function, many proteins known to have roles in the development of the central nervous system. These results suggest that the proteomic approach is valuable for mass identification of proteins involved in cerebellar morphogenesis.

The SCG10-related gene family in the developing rat retina: persistent expression of SCLIP and stathmin in mature ganglion cell layer

Neuronal growth-associated proteins (GAPs), such as GAP-43 and SCG10, are thought to play crucial roles in both axonal and dendritic outgrowth during neural development and regeneration, although the underlying mechanisms remain largely unknown. The recent finding that SCG10 is a microtubule regulator and also the identification of RB3 and SCLIP as two new SCG10-related members prompted us to investigate the roles of SCG10-related family in neural development, using the retina as a model system. We determined the temporal expression and the spatial distribution of SCG10-related mRNAs in the developing rat retina. Semiquantitative analysis by RT-PCR revealed that in prenatal retina, levels of SCG10 and stathmin mRNAs were higher than those of RB3 and SCLIP. In the postnatal retina, the level of SCLIP increased, whereas the level of RB3 remained low. In situ hybridization revealed that GAP-43 and all of the SCG10-related family mRNAs were present in the retinal ganglion cells (RGCs) at all stages of retinal development, and that stathmin mRNA was present in mitotic neuroblastic cells. Differential expression of SCG10 and other members of the family became more evident as retinal development proceeded; SCG10 and RB3 expression were relatively specific in the RGCs and amacrine cells, whereas SCLIP was also evident in bipolar and horizontal cells. Stathmin mRNA was highly expressed both in the RGCs and other interneurons. These results indicate that multiple SCG10-related proteins are expressed in single neurons including RGCs, and suggest that these nGAPs play similar but distinct roles in differentiation and functional maintenance of retinal neurons.

Repeated stimuli for axonal growth causes motoneuron death in adult rats: the effect of botulinum toxin followed by partial denervation

Axons of motoneurons to tibialis anterior and extensor digitorum longus muscles of adult rats were induced to sprout by injecting botulinum toxin into them, by partial denervation or by a combination of the two procedures. Ten weeks later, the number of motoneurons innervating the control and operated tibialis anterior and extensor digitorum longus muscles was established by retrograde labelling with horseradish peroxidase. In the same preparations, the motoneurons were also stained with a Nissl stain (gallocyanin) to reveal motoneurons in the sciatic pool. Examination of the spinal cords from animals treated with botulinum toxin showed that the number of retrogradely labelled cells and those stained with gallocyanin in the ventral horn on the treated compared to the control side was unchanged. In rats that had their L4 spinal nerve sectioned on one side, the number of retrogradely labelled cells on the operated side was 48+/-3% (n = 5) of that present in the control unoperated ventral horn. Thus, just over half the innervation was removed by cutting the L4 spinal nerve. Counts made from gallocyanin-stained sections showed that 94+/-4% (n = 5) of motoneurons were present in the ventral horn on the operated side. Thus, section of the L4 spinal nerve did not lead to any death of motoneurons. In rats that had their muscles injected with botulinum toxin three weeks prior to partial denervation, the number of retrogradely labelled cells was reduced from 48+/-3% (n = 5) to 35+/-4% (n = 5). Moreover, only 67+/-5% (n = 5) of motoneurons stained with gallocyanin, suggesting that a proportion of motoneurons died after this combined procedure. This result was supported by experiments in which motor unit numbers in extensor digitorum longus muscles were determined by measurements of stepwise increments of force in response to stimulation of the motor nerve with increasing stimulus intensity. In partially denervated extensor digitorum longus muscles, 16.6+/-0.7 (n = 5) motor units could be identified, and in animals treated with botulinum toxin prior to partial denervation only 13.3+/-0.9 (n = 3) motor units were present. Taken together, these results show that treatment with botulinum toxin followed by partial denervation causes motoneuron death in adult rats.

A retrograde signal is involved in activity-dependent remodeling at a C. elegans neuromuscular junction

We have characterized how perturbations of normal synaptic activity influence the morphology of cholinergic SAB motor neurons that innervate head muscle in C. elegans. Mutations disrupting components of the presynaptic release apparatus, acetylcholine (ACh) synthesis or ACh loading into synaptic vesicles each induced sprouting of SAB axonal processes. These sprouts usually arose in the middle of the normal innervation zone and terminated with a single presynaptic varicosity. Sprouting SAB neurons with a similar morphology were also observed upon reducing activity in muscle, either by using mutants lacking a functional nicotinic ACh receptor subunit or through muscle-specific expression of a gain-of-function potassium channel. Analysis of temperature-sensitive mutants in the choline acetyltransferase gene revealed that the sprouting response to inactivity was developmentally regulated; reduction of synaptic activity in early larval stages, but not in late larval stages, induced both sprouting and addition of varicosities. Our results indicate that activity levels regulate the structure of certain synaptic connections between nerve and muscle in C. elegans. One component of this regulatory machinery is a retrograde signal from the postsynaptic cell that mediates the formation of synaptic connections.

Selection of donor nerves--an important factor in end-to-side neurorrhaphy

We have examined the effects of end-to-side neurorrhaphy on peripheral nerve regeneration using the median nerve as recipient nerve and either the antagonistic radial nerve or the agonistic ulnar nerve as donor nerves in rat upper limbs. A perineural window was created in all cases. Motor recovery up to 16 weeks postoperation was tested with the grasping test. No recovery of motor function was evident after end-to-side neurorrhaphy of the median nerve to the antagonistic radial nerve, whereas six of eight rats with end-to-side neurorrhaphy to the agonistic ulnar nerve achieved 367 g +/- 47 g grasping power as compared to 526 g +/- 6 g in end-to-end coapted control animals. No significant difference in flexor digitorum sublimus-motor nerve conduction velocity was found among all three groups. Radial nerve stimulation produced simultaneous contraction of both extensor and flexor muscles of the lower arm that disabled any coordinated movement of the paw. Histology (toluidine blue, acetylcholinesterase-stain) showed multiple regenerated (motor)-axons distal to the coaptation site in the median nerve. Reinnervation of the median nerve solely by the respective donor nerve was demonstrated by a retrograde double labelling technique. These results show that averaged 70% muscle power as compared to end-to-end neurorrhaphy with well coordinated muscle function can be achieved by axonal sprouting through end-to-side neurorrhaphy if an agonistic nerve is used as donor nerve. However, satisfying results are unpredictable. Antagonistic nerves show the ability to induce axonal regeneration, but no useful function can be expected.

Fiber types in the mouse levator auris longus muscle: a convenient preparation to study muscle and nerve plasticity

The histochemical composition of the levator auris longus (LAL) muscle has been investigated in adult NMRi mice. Histochemical reaction for myofibrillar adenosine triphosphatase (ATPase) after preincubation in alkaline and acidic media, nicotine amideadenine-dinucleotide dehidrogenase (NADH-dehydrogenase), and alpha-glycerophosphate dehydrogenase were performed on cryosections of LAL muscle. Expression of myosin heavy chain (MyHC) isoforms was detected with the immunoperoxidase method applying monoclonal antibodies against MyHC isoforms -1, -2a, -2x/d, and -2b, as well as by sodium dodecylsulfate (SDS) glycerol gel electrophoresis. The muscle was proven to be a pure fast-twitch muscle. The most numerous fibers in LAL muscles contained MyHC-2b and some MyHC-2a. Histochemically, pure IIA fibers with oxidative metabolism and pure IIB fibers with glycolytic metabolism were detected. In contrast to the majority of mature control muscles, numerous hybrid fibers coexpressing MyHC-2x/d with MyHC-2a or MyHC-2b were present. Both hybrids were oxidative-glycolytic; additionally, some hybrids containing MyHC-2a were oxidative. In one out of six muscles, traces of MyHC-1 were detected both with immunoperoxidase staining and with SDS glycerol gel electrophoresis. Rare fibers that exceptionally expressed small amounts of MyHC-1 always coexpressed MyHC-2a, which is an additional proof that pure type I fibers do not exist in LAL. Due to these histochemical characteristics and to its previously described morphological features, the use of the LAL muscle as a model for various studies, particularly muscle and nerve interactions, is emphasized.

Ectopic expression of NCAM in skeletal muscle of transgenic mice results in terminal sprouting at the neuromuscular junction and altered structure but not function

The neuromuscular system provides an excellent model for the analysis of molecular interactions involved in the development and plasticity of synaptic contacts. The neural cell adhesion molecule (NCAM) is believed to be involved in the development and plasticity of the neuromuscular junction, in particular the axonal sprouting response observed in paralyzed and denervated muscle. In order to explore the role of myofiber NCAM in modulating the differentiation of motor neurons, we generated transgenic mice expressing a GPI-anchored NCAM isoform that is normally found in developing and denervated muscle, under the control of a skeletal muscle-specific promoter. This results in the constitutive expression of NCAM at postnatal ages, a time when the endogenous mouse NCAM is absent from the myofiber. We found that a significant number of neuromuscular junctions in adult transgenic animals displayed terminal sprouting (>20%) reminiscent of that elicited in response to cessation of neuromuscular activity. Additionally, a significant increase in the size and complexity of neuromuscular synapses as a result of extensive intraterminal sprouting was detected. Electrophysiological studies, however, revealed no significant alterations of neuromuscular transmission at this highly efficient synapse. Sprouting in response to paralysis or following nerve crush was also significantly enhanced in transgenic animals. These results suggest that in this ectopic expression model NCAM can directly modulate synaptic structure and motor neuron-muscle interactions. The results contrast with knockout experiments of the NCAM gene, where very limited changes in the neuromuscular system were observed.

Alterations in heavy and light neurofilament proteins in hippocampus following chronic ECS administration

Chronic administration of electroconvulsive seizures (ECS), one of the most effective treatments for depression, induces sprouting of the mossy fibers in the hippocampus. This sprouting requires chronic ECS administration and appears to occur in the absence of hilar neuronal loss. Dynamic regulation of cytoarchitecture plays a vital role in such profound alterations of neuronal morphology. In particular, alterations in the neurofilament protein subunits have been implicated in neurite sprouting, neuronal regeneration, and growth. The present study was carried out to determine the influence of chronic ECS administration on the neurofilament subunits and other molecular markers of neuronal plasticity. Chronic ECS administration decreases the level of phosphorylated heavy neurofilament subunit (NF-H). In addition, the total level of the light neurofilament subunit (NF-L) but not the medium neurofilament subunit (NF-M) is decreased following chronic ECS treatment. Other cytoskeletal proteins, including actin, microtubule-associated protein (MAP-2), and tau, are not influenced by chronic ECS administration. Expression of the growth-associated protein (F1/GAP-43) also remains unchanged following chronic ECS treatment. The changes observed in neurofilaments may be part of the cytoskeletal remodeling that contributes to the mossy fiber sprouting induced by chronic ECS treatment.

Targeted disruption of GAP-43 in P19 embryonal carcinoma cells inhibits neuronal differentiation. As well as acquisition of the morphological phenotype

GAP-43 is expressed in proliferating neuroblasts in vivo and in vitro, but its role during early neurogenesis has not been investigated. Here we show that neuroectodermal differentiation stimulated by retinoic acid (RA) in the embryonal carcinoma (EC) line P19 is accompanied by upregulation of GAP-43 expression in neuroepithelial precursor cells. In contrast, when upregulation of GAP-43 expression was prevented in 3 independent P19 lines because of a targeted insertion into the gene, generation of neuroepithelial precursors was inhibited. Consequently, neuronal number was significantly decreased, neuronal morphology was abnormal and fewer than 20% of all neurons were able to initiate neuritogenesis. Extracellular matrix (ECM) was unable to rescue initiation of neuritogenesis in the mutant cells, however those neurites that were extended responded normally to ECM-stimulated neurite outgrowth-promoting signals. These data suggest that GAP-43 function is required for commitment to a neuronal phenotype as well as initiation of neurite extension. However, stimulation of neurite outgrowth by ECM in P19s occurs independently of GAP-43.

Micro-scale chromophore-assisted laser inactivation of nerve growth cone proteins

Directed growth cone movement is crucial for the correct wiring of the nervous system. This movement is governed by the concerted actions of cell surface receptors, signaling proteins, cytoskeleton-associated molecules, and molecular motors. In order to investigate the molecular basis of growth cone motility, we applied a new technique to functionally inactivate proteins: micro-scale Chromophore-Assisted Laser Inactivation [Diamond et al. (1993) Neuron 11:409-421]. Micro-CALI uses laser light of 620 nm, focused through microscope optics into a 10-microm spot. The laser energy is targeted via specific Malachite green-labeled, non-function-blocking antibodies, that generate short-lived protein-damaging hydroxyl radicals [Liao et al. (1994) Proc Natl Acad Sci USA 91:2659-2663]. Micro-CALI mediates specific loss of protein function with unachieved spatial and temporal resolution. Combined with time-lapse video microscopy, it offers the possibility to induce and observe changes in growth cone dynamics on a real time base. We present here the effects of the acute and localized inactivation of selected growth cone molecules on growth cone behavior and morphology. Based on our observations, we propose specific roles for these proteins in growth cone motility and neurite outgrowth.

Protein myristoylation in protein-lipid and protein-protein interactions

Various proteins in signal transduction pathways are myristoylated. Although this modification is often essential for the proper functioning of the modified protein, the mechanism by which the modification exerts its effects is still largely unknown. Here we discuss the roles played by protein myristoylation, in both protein-lipid and protein-protein interactions. Myristoylation is involved in the membrane interactions of various proteins, such as MARCKS and endothelial NO synthase. The intermediate hydrophobic nature of the modification plays an important role in the reversible membrane anchoring of these proteins. The anchoring is strengthened by a basic amphiphilic domain that works as a switch for the reversible binding. Protein myristoylation is also involved in protein-protein interactions, which are regulated by the interplay between protein phosphorylation, calmodulin binding, and membrane phospholipids.

Transgenic expression of B-50/GAP-43 in mature olfactory neurons triggers downregulation of native 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) 7953-7965]. 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.

Influence of the axotomy to cell body distance in rat rubrospinal and spinal motoneurons: differential regulation of GAP-43, tubulins, and neurofilament-M

Axotomized motoneurons regenerate their axons regardless of whether axotomy occurs proximally or distally from their cell bodies. In contrast, regeneration of rubrospinal axons into peripheral nerve grafts has been detected after cervical but not after thoracic injury of the rubrospinal tract. By using in situ hybridization (ISH) combined with reliable retrograde tracing methods, we compared regeneration-associated gene expression after proximal and distal axotomy in spinal motoneurons versus rubrospinal neurons. Regardless of whether they were axotomized at the iliac crest (proximal) or popliteal fossa (distal), sciatic motoneurons underwent highly pronounced changes in ISH signals for Growth Associated Protein 43 (GAP-43) (10-20x increase) and neurofilament M (60-85% decrease). In contrast, tubulin ISH signals substantially increased only after proximal axotomy (3-5x increase). To compare these changes in gene expression with those of axotomized rubrospinal neurons, the rubrospinal tract was transected at the cervical (proximal) or thoracic (distal) levels of the spinal cord. Cervically axotomized rubrospinal neurons showed three- to fivefold increases in ISH signals for GAP-43 and tubulins (only transient) and a 75% decrease for neurofilament-M. In sharp contrast, thoracic axotomy had only marginal effects. After implantation of peripheral nerve transplants into the spinal cord injury sites, retrograde labeling with the sensitive retrograde tracer Fluoro-Gold identified regenerating rubrospinal neurons only after cervical axotomy. Furthermore, rubrospinal neurons specifically regenerating into the transplants were hypertrophied and expressed high levels of GAP-43 and tubulins. Taken together, these data support the concept that, even if central nervous system (CNS) axons are presented with a permissive/supportive environment, appropriate cell body responses to injury are a prerequisite for CNS axonal regeneration.

Leukemia inhibitory factor, glial cell line-derived neurotrophic factor, and their receptor expressions following muscle crush injury

Using in situ hybridization histochemistry, we characterized the spatiotemporal gene expression patterns of leukemia inhibitory factor (LIF) and glial cell line-derived neurotrophic factor (GDNF), and their receptor components (LIFR, GFR-alpha1, RET) induced in muscle cells, intramuscular nerves, and motoneurons in the regeneration processes of both muscle cells and nerves following muscle contusion. Muscle contusion induced upregulation of GDNF and GFR-alpha1 mRNAs in Schwann cell-like cells in the intramuscular nerves and of LIFR mRNA in damaged muscle cells. LIFR, GFR-alpha1, and RET mRNA expressions in motoneurons were upregulated following muscle contusion. Muscle contusion also induced more rapid, prominent transactivations of GFR-alpha1 and RET genes in motoneurons than did sciatic nerve axotomy. These findings suggest that rapid and prominent upregulation of the receptor components for LIF and GDNF in motoneurons is important for the regeneration of intramuscular motor nerves damaged by muscle contusion.

Cerebral amyloid induces aberrant axonal sprouting and ectopic terminal formation in amyloid precursor protein transgenic mice

A characteristic feature of Alzheimer's disease (AD) is the formation of amyloid plaques in the brain. Although this hallmark pathology has been well described, the biological effects of plaques are poorly understood. To study the effect of amyloid plaques on axons and neuronal connectivity, we have examined the axonal projections from the entorhinal cortex in aged amyloid precursor protein (APP) transgenic mice that exhibit cerebral amyloid deposition in plaques and vessels (APP23 mice). Here we report that entorhinal axons form dystrophic boutons around amyloid plaques in the entorhinal termination zone of the hippocampus. More importantly, entorhinal boutons were found associated with amyloid in ectopic locations within the hippocampus, the thalamus, white matter tracts, as well as surrounding vascular amyloid. Many of these ectopic entorhinal boutons were immunopositive for the growth-associated protein GAP-43 and showed light and electron microscopic characteristics of axonal terminals. Our findings suggest that (1) cerebral amyloid deposition has neurotropic effects and is the main cause of aberrant sprouting in AD brain; (2) the magnitude and significance of sprouting in AD have been underestimated; and (3) cerebral amyloid leads to the disruption of neuronal connectivity which, in turn, may significantly contribute to AD dementia.

Recent advances in the genetics of epilepsy: insights from human and animal studies

Progress in understanding the genetics of epilepsy is proceeding at a dizzying pace. Due in large part to rapid progress in molecular genetics, gene defects underlying many of the inherited epilepsies have been mapped, and several more are likely to be added each year. In this review, we summarize the available information on the genetic basis of human epilepsies and epilepsy syndromes, and correlate these advances with rapidly expanding information about the mechanisms of epilepsy gained from both spontaneous and transgenic animal models. We also provide practical suggestions for clinicians confronted with families in which multiple members are afflicted with epilepsy.

Auditory brainstem: development and plasticity of GAP-43 mRNA expression in the rat

Expression of the growth and plasticity associated protein GAP-43 is closely related to synaptogenesis and synaptic remodeling in the developing as well as in the mature nervous system. We have studied the postnatal development of GAP-43 mRNA expression in the auditory brainstem and determined the time course of its reexpression following deafening through cochlear ablation using a digoxigenin-coupled mRNA probe. By the first postnatal day, GAP-43 mRNA was expressed at high levels in all auditory brainstem nuclei. But whereas GAP-43 mRNA is almost entirely lost in most of these nuclei in the adult animal, significant levels of this molecule are retained in the inferior colliculus and, most notably, in the lateral and medial superior olivary nucleus. As a consequence of unilateral cochleotomy, GAP-43 mRNA rose dramatically in some neurons of the ipsilateral lateral superior olive, whereas the hybridization signal decreased in others. Using double staining protocols, we found that those olivary neurons that increase their level of GAP-43 mRNA appear to be identical with the cells developing strong GAP-43 immunoreactivity after cochleotomy. By combining axonal tracing with in situ hybridization, we proved that at least some of the cells with increased levels of GAP-43 mRNA and protein are the cells of origin of olivocochlear projections. A substantial decrease of the level of GAP-43 mRNA took place in the inferior colliculus contralateral to the lesioned cochlea. Our results led us to suggest that neurons in the superior olivary complex may play a crucial role in orchestrating auditory brainstem plasticity.

Targeted expression of an oncogenic adaptor protein v-Crk potentiates axonal growth in dorsal root ganglia and motor neurons in vivo

The ability of neurons to survive and to target axonal growth requires a coordinated series of cell extrinsic and intrinsic events. Previously, in a cellular model for neuronal differentiation, we showed that pheochromocytoma (PC12) cells expressing v-Crk, an oncogenic form of the SH2/SH3-containing c-Crk adaptor protein, potentiates axonal growth and prolongs nerve growth factor (NGF)-independent survival. In the present study, we have generated transgenic mice that express v-Crk in sensory, motor, and enteric neurons by placing v-crk under the control of the neuron-specific peripherin promoter. In contrast to wild-type (wt) mice, dorsal root ganglia (DRG) neurons explanted from post-natal day 1 transgenic mice demonstrated a reduced dependence on trophic factors for both survival and axonogenesis. v-Crk also caused an increase in the number of surviving spinal motor neurons (SMN), and interestingly, upon staining of sternomastoid muscle fibers with rhodamine conjugated alpha-bungarotoxin, many muscle fibers displayed an apparent increase in volume of motor end plates, and an increase in complexity of neuromuscular junctions (NMJ). Our data suggest that v-Crk may be involved in transducing extracellular signals to regulate cytoskeletal organization, and may act on an intrinsic determinant for axonal growth in a variety of neural types including sensory and motor neurons during development.