The expression of GAP-43 and synaptophysin in the developing rat retina

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

Changes in expression of Class 3 Semaphorins and their receptors during development of the rat retina and superior colliculus

Background

Members of the Semaphorin 3 family (Sema3s) influence the development of the central nervous system, and some are implicated in regulating aspects of visual system development. However, we lack information about the timing of expression of the Sema3s with respect to different developmental epochs in the mammalian visual system. In this time-course study in the rat, we document for the first time changes in the expression of RNAs for the majority of Class 3 Semaphorins (Sema3s) and their receptor components during the development of the rat retina and superior colliculus (SC).

Results

During retinal development, transcript levels changed for all of the Sema3s examined, as well as Nrp2, Plxna2, Plxna3, and Plxna4a. In the SC there were also changes in transcript levels for all Sema3s tested, as well as Nrp1, Nrp2, Plxna1, Plxna2, Plxna3, and Plxna4a. These changes correlate with well-established epochs, and our data suggest that the Sema3s could influence retinal ganglion cell (RGC) apoptosis, patterning and connectivity in the maturing retina and SC, and perhaps guidance of RGC and cortical axons in the SC. Functionally we found that SEMA3A, SEMA3C, SEMA3E, and SEMA3F proteins collapsed purified postnatal day 1 RGC growth cones in vitro. Significantly this is a developmental stage when RGCs are growing into and within the SC and are exposed to Sema3 ligands.

Conclusion

These new data describing the overall temporal regulation of Sema3 expression in the rat retina and SC provide a platform for further work characterising the functional impact of these proteins on the development and maturation of mammalian visual pathways.

Fatty acid amide hydrolase expression during retinal postnatal development in rats

The endocannabinoid (eCB) system is thought to participate in developmental processes in the CNS. The rodent retina represents a valuable model to study CNS development because it contains well-identified cell types with established developmental timelines. The distribution of cannabinoid receptor type 1 (CB1R) was recently revealed in the developing retina; however, the expression patterns of other elements of this system remain unknown. In this study, we investigated the expression pattern of the degradative enzyme fatty acid amide hydrolase (FAAH), a key regulator of the eCB system, in the rat retina during postnatal development. To identify the cells expressing the enzyme, co-stainings were carried out for FAAH and retinal cell type markers. FAAH was expressed at postnatal day (P) 1 in ganglion and cholinergic amacrine cells. In the course of development, it appeared in cones, horizontal, and bipolar cells. For most cell types (horizontal, cholinergic amacrine cells, and cone bipolar cells), FAAH was transiently expressed, suggesting an important redistribution of the enzyme during postnatal development and thus a potential role of the eCB system in developmental processes. Our results also indicated that, in the adult retina, FAAH is expressed in cones, rod bipolar cells, and some retinal ganglion cells. The presence of FAAH in adult animals supports the hypothesis that the eCB system is involved in retinal functions. Overall these results indicate that, as shown in other structures of the brain, the eCB system could play an instrumental role in the development and function of the retina.

Protective effects of triamcinolone acetonide upon the upregulation and phosphorylation of GAP 43 in an animal model of retinopathy of prematurity

PURPOSE

The aim of the current study was to investigate the effects of triamcinolone acetonide (TA) upon the expression and phosphorylation of growth-associated protein 43 (GAP 43) in the retinas of oxygen-induced retinopathy (OIR) rats.

METHODS

Oxygen-induced retinopathy was induced by exposing Sprague-Dawley rats to hyperoxia (80% oxygen) from postnatal (P) days 2-14 and then returning the rats to normoxic conditions. Triamcinolone acetonide or a conditioned saline (control) was injected intravitreally into the right or left eye, respectively, of OIR rats at P15. We then assessed the molecular and histological changes in the expression of GAP 43 and phospho-GAP 43 in OIR and control rat retinas, and also after treatment with TA by RT-PCR, Western blotting and immunohistochemistry.

RESULTS

Growth-associated protein 43 mRNA levels were found to be increased by 1.6-fold (p=0.001, n=5) in the retinas of P18 OIR rats compared with the control rats. The protein levels of GAP 43 and phospho-GAP43 were found to be elevated in the retina of P18 OIR rats (2.40- and 2.39-fold greater than each control, p<0.001, n=5, respectively). Immunoreactivities of GAP 43 and phospho-GAP 43 were stronger in the inner plexiform layer in OIR rat retinas compared with the control. However, treatment with TA attenuated GAP 43 and phospho-GAP 43 upregulation in the OIR retinas.

CONCLUSION

Our results indicate that GAP 43 and phospho-GAP 43 participate in retinal (potentially pathologic) changes following oxygen-induced damage. Triamcinolone acetonide protects the retinal damage in relatively hypoxic retinas of OIR rats. Therefore, TA treatment does not induce the expression and phosphorylation of GAP 43 in OIR rat retinas.

Translin-associated factor-X (Trax) is a molecular switch of growth-associated protein (GAP)-43 that controls axonal regeneration

The ability of neurons to form axons requires the choreographed assembly of growth cones. We show that there is a time window from postnatal day 14 (P14) until P21/22 when axons of rat retinal ganglion cells will regenerate under serum-free culture conditions. In contrast, no outgrowth occurred before P13, and growth declined from P22 and ceased after P30. Using proteomics, we have identified translin-associated factor X (Trax), a DNA-binding factor that is expressed during this period of postnatal development. Trax is shown to coexpress with growth-associated protein GAP-43. Small interfering RNA-mediated inhibition of Trax expression resulted in downregulation of both Trax and GAP-43 transcripts and protein both before and during the period of regeneration (P8) and (P16). In contrast, silencing of Trax at P30 resulted in significant upregulation of the GAP-43 transcript and protein and induced outgrowth of axons. These data suggest that Trax regulates GAP-43 transcription and regeneration-promoting effects during the postnatal maturation period. Trax may represent a new potent therapeutic target gene for optic nerve and spinal cord injuries.

GAP-43 expression is upregulated in retinal ganglion cells after ischemia/reperfusion-induced damage

In response to injury, the adult mammalian retina shows signs of structural remodeling, possibly in an attempt to preserve or regain some of its functional neural connections. In order to study the mechanisms involved in injury-induced plasticity, we have studied changes in growth associated protein 43 (GAP-43) after retinal ischemia/reperfusion in the rat. GAP-43 is a marker for neuronal remodeling and is involved in synapse formation. Ischemic injury of the rat retina was induced by 60 min of ischemia followed by reperfusion times varying from 2h up to 4 weeks. GAP-43 mRNA levels were significantly increased between 12h and 72 h reperfusion with a peak around 24h. GAP-43 specific antibodies showed that the total amount of GAP-43 labeling in the inner plexiform layer was diminished after 12h of reperfusion by approximately 35% and remained at this level up to 1 week postischemia despite the reduction in thickness of this layer during this period resulting from the ischemia-induced cell loss. At 2 and 4 weeks reperfusion, the amount of labeling was reduced by 70%, simultaneously with a decrease of GAP-43 transcript level. Between 72 h up to 2 weeks postischemia, the induction of intense GAP-43 labeling was observed in NeuN- and beta-tubulin-positive ganglion cell somata and in horizontally and vertically oriented processes in the inner plexiform layer. Ischemia also induced GAP-43 expression in some GFAP-positive Müller cells. Double-labeling showed that in controls and after ischemia GAP-43 was expressed by some amacrine cells of the glycinergic (glycine transporter 1), calretinin-positive, and dopaminergic (tyrosine hydroxylase) subpopulations. No increase of GAP-43 expression levels was found in these amacrine cells. The results demonstrate that ganglion cells show an elevated expression of GAP-43 after ischemia-inflicted damage. These findings suggest a temporal window during which ganglion cells may remodel their neuronal network in the damaged retina.

Structural changes in the developing retina maintained in vitro

The present study examined the emergence of structural remodeling in explanted neonatal rat retina. Immunohistochemical analysis demonstrated signs of glial and neuronal remodeling after 11 days in vitro and included the activation of Müller cells, the formation of ectopic neuropil areas and sprouting of photoreceptor terminals. We also observed that cholinergic and GABA-ergic amacrine cells displayed signs of disorganized laminations. These results demonstrate that retinal culturing initiates structural changes that show morphological similarities to glial and neuronal remodeling identified in retinitis pigmentosa retinas and experimentally detached retinas.

Cellular remodeling in mammalian retina: results from studies of experimental retinal detachment

Retinal detachment, the separation of the neural retina from the retinal pigmented epithelium, starts a cascade of events that results in cellular changes throughout the retina. While the degeneration of the light sensitive photoreceptor outer segments is clearly an important event, there are many other cellular changes that have the potential to significantly effect the return of vision after successful reattachment. Using animal models of detachment and reattachment we have identified many cellular changes that result in significant remodeling of the retinal tissue. These changes range from the retraction of axons by rod photoreceptors to the growth of neurites into the subretinal space and vitreous by horizontal and ganglion cells. Some neurite outgrowths, as in the case of rod bipolar cells, appear to be directed towards their normal presynaptic target. Horizontal cells may produce some directed neurites as well as extensive outgrowths that have no apparent target. A subset of reactive ganglion cells all fall into the latter category. Muller cells, the radial glia of the retina, undergo numerous changes ranging from proliferation to a wholesale structural reorganization as they grow into the subretinal space (after detachment) or vitreous after reattachment. In a few cases have we been able to identify molecular changes that correlate with the structural remodeling. Similar changes to those observed in the animal models have now been observed in human tissue samples, leading us to conclude that this research may help us understand the imperfect return of vision occurring after successful reattachment surgery. The mammalian retina clearly has a vast repertoire of cellular responses to injury, understanding these may help us improve upon current therapies or devise new therapies for blinding conditions.

Cellular positioning and dendritic field size of cholinergic amacrine cells are impervious to early ablation of neighboring cells in the mouse retina

We have examined the role of neighbor relationships between cholinergic amacrine cells upon their positioning and dendritic field size by producing partial ablations of this population of cells during early development. We first determined the effectiveness of L-glutamate as an excitotoxin for ablating cholinergic amacrine cells in the developing mouse retina. Subcutaneous injections (4 mg/g) made on P-3 and thereafter were found to produce a near-complete elimination, while injections at P-2 were ineffective. Lower doses on P-3 produced only partial reductions, and were subsequently used to examine the effect of partial ablation upon mosaic organization and dendritic growth of the remaining cells. Four different Voronoi-based measures of mosaic geometry were examined in L-glutamate-treated and normal (saline-treated) retinas. Partial depletions of around 40% produced cholinergic mosaics that, when scaled for density, approximated the mosaic geometry of the normal retina. Separate comparisons simulating a 40% random deletion of the normal retina produced mosaics that were no different from those experimentally depleted retinas. Consequently, no evidence was found for positional regulation in the absence of normal neighbor relationships. Single cells in the ganglion cell layer were intracellularly filled with Lucifer Yellow to examine the morphology and dendritic field extent following partial ablation of the cholinergic amacrine cells. No discernable effect was found on their starburst morphology, and total dendritic field area, number of primary dendrites, and branch frequency were not significantly different. Cholinergic amacrine cells normally increase their dendritic field area after P-3 in excess of retinal expansion; despite this, the present results show that this growth is not controlled by the density of neighboring processes.

Growth and guidance cues for regenerating axons: where have they gone?

Both attractive and repellent cues are required to guide developing axons to their targets in the central nervous system. Critical guidance molecules in the developing brain include the semaphorins, netrins, slits, and ephrins. Current research indicates that many of these molecules and their receptors are expressed in the adult central nervous system (CNS), and that injury can alter the levels of these ligands/receptors. Recent studies have begun the process of elucidating the functions of these receptors in adult mammals, and the effects that they have on the regeneration of adult neurons. This review addresses our current knowledge with respect to the response of adult CNS neurons to axonal injury, interventions for enhancing the survival and regeneration of injured neurons, and the expression of developmental axon guidance cues in the injured mature CNS, with specific focus on the retino-tectal projection.

Developmental plasticity of photoreceptors

During development, retinal ganglion cells undergo conspicuous structural remodeling as they gradually attain their mature morphology and connectivity. Alterations in their dendritic organization and in their axonal projections can also be achieved following early insult to their targets or their afferents. Other retinal cell types are thought not to display this same degree of developmental plasticity. The present review will consider the evidence, drawn largely from recent experimental studies in the carnivore retina, that photoreceptors also undergo structural remodeling, extending their terminals transiently into inner plexiform layer before retracting to the outer plexiform layer. The determinants of this transient targeting to the inner plexiform layer are considered, and the role of cholinergic amacrine cells is discussed. The factors triggering this retraction are also considered, including the concurrent maturational changes in outer segment formation and in the differentiation of the outer plexiform layer. These results provide new insight into the life history of the photoreceptor cell and its connectivity, and suggest a transient role for the photoreceptors in the circuitry of the inner retina during early development, prior to the onset of phototransduction.

Expression of vesicular glutamate transporter 1 in the mouse retina reveals temporal ordering in development of rod vs. cone and ON vs. OFF circuits

Glutamatergic transmission is crucial to the segregation of ON and OFF pathways in the developing retina. The temporal sequence of maturation of vesicular glutamatergic transmission in rod and cone photoreceptor and ON and OFF bipolar cell terminals is currently unknown. Vesicular glutamate transporters (VGLUTs) that load glutamate into synaptic vesicles are necessary for vesicular glutamatergic transmission. To understand better the formation and maturation of glutamatergic transmission in the rod vs. cone and ON vs. OFF pathways of the retina, we examined the developmental expression of VGLUT1 and VGLUT2 immunocytochemically in the mouse retina. Photoreceptor and bipolar cell terminals showed only VGLUT1-immunoreactivity (-IR); no VGLUT2-IR was present in any synapses of the developing or adult retina. VGLUT1-IR was first detected in cone photoreceptor terminals at postnatal day 2 (P2), several days before initiation of ribbon synapse formation at P4-P5. Rod terminals showed VGLUT1-IR by P8, when they invade the outer plexiform layer (OPL) and initiate synaptogenesis. Developing OFF bipolar cell terminals showed VGLUT1-IR around P8, 2-3 days after bipolar terminals were first identified in the inner plexiform layer (IPL) by labeling for the photoreceptor and bipolar cell terminal marker, synaptic vesicle protein 2B. Although terminals of ON bipolar cells were present in the IPL by P6-P8, most did not show VGLUT1-IR until P8-P10 and increased dramatically from P12. These data suggest a hierarchical development of glutamatergic transmission in which cone circuits form prior to rod circuits in both the OPL and IPL, and OFF circuits form prior to ON circuits in the IPL.

Differentiation of ganglion cells and amacrine cells in the rat retina: correlation with expression of HuC/D and GAP-43 proteins

In order to understand the development of retinal cells, we have studied the temporal expression of HuC/D protein in embryonic, postnatal and adult rat retina. During development and in the adult retina, practically all cell somata in the ganglion cell layer and the vast majority of conventional amacrine cells in the inner nuclear layer displayed HuC/D immunoreactivity. Most but not all ganglion cells expressed HuC/D at embryonic day 15, suggesting a delay between final mitosis and the initiation of HuC/D expression. Immunoreactivity for HuC/D was also evident in developing but not mature horizontal cells. Combined immunohistochemical visualization of HuC/D protein and the growth-associated protein (GAP-43) showed a distinct localization of GAP-43 in a specific compartment close to the somato-dendritic region of developing HuC/D-positive cell somata. The localization of GAP-43 immunoreactivity to a specific soma compartment became less evident during maturation. Immunoreactivity for HuC/D and GAP-43 was also discernible in horizontal cells at postnatal day 14. In the adult retina, most GAP-43 immunoreactivity was seen in the inner plexiform layer. Detailed analysis showed that HuC/D and GAP-43 expression is restricted to subsets of retinal neurons during development and in the mature retina. Thus, GAP-43 appears to be correlated with initial steps of differentiation and outgrowth of dendritic processes in HuC/D-positive ganglion and amacrine cells.

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.

Evidence that ganglion cells react to retinal detachment

Growth associated protein 43 (GAP 43) is involved in synapse formation and it is expressed in the retina in a very specific pattern. Although GAP 43 is downregulated at the time of synapse formation, it can be re-expressed following injury such as axotomy or ischemia. Because of this we sought to characterize the expression of GAP 43 after retinal detachment (RD). Immunoblot, immunocytochemical and quantitative polymerase chain reaction (QPCR) techniques were used to assess the level of GAP 43 expression after experimental RD. GAP 43 was localized to three sublaminae of the inner plexiform layer of the normal retina. GAP 43 became upregulated in a subset of retinal ganglion cells following at least 7 days of RD. By immunoblot GAP 43 could be detected by 3 days. QPCR shows the upregulation of GAP 43 message by 6hr of detachment. To further characterize changes in ganglion cells, we used an antibody to neurofilament 70 and 200kDa (NF) proteins. Anti-NF labels horizontal cells, ganglion cell dendrites in the inner plexiform layer, and ganglion cell axons (fasicles) in the normal retina. Following detachment it is upregulated in horizontal cells and ganglion cells. When detached retina was double labelled with anti-GAP 43 and anti-NF, some cells were labelled with both markers, while others labelled with only one. We have previously shown that second order neurons respond to detachment; here we show that third order neurons are responding as well. Cellular remodelling of this type in response to detachment may explain the slow recovery of vision that often occurs after reattachment, or those changes that are often assumed to be permanent.

Induction and axonal localization of epithelial/epidermal fatty acid-binding protein in retinal ganglion cells are associated with axon development and regeneration

Epithelial/epidermal fatty acid-binding protein (E-FABP) is induced in peripheral neurons during nerve regeneration and is found at high levels in central neurons during neuronal migration and development. Furthermore, E-FABP expression is required for normal neurite outgrowth in PC12 cells treated with nerve growth factor (NGF). The present study examined whether E-FABP plays a role in retinal ganglion cell (RGC) differentiation and axon growth. Rat retinal tissues from embryonic (E) and postnatal (P) development through adulthood were examined using immunocytochemical labeling with E-FABP and growth-associated protein 43 (GAP-43) antibodies. E-FABP colocalized with GAP-43 at E14 through P10. At E14, E-FABP immunoreactivity was confined to the somas of GAP-43-positive cells in the ganglion cell layer, but it was localized to their axons by E15. The axons in the optic nerve were GAP-43-positive and E-FABP-negative on E15, but the two proteins were colocalized by E18. Retinal cultures at E15 confirmed that E-FABP and GAP-43 colocalize in RGCs. Postnatally, labeling was present between P1 and P10 but decreased at older ages and was minimally present or absent in adult animals. Western immunoblotting revealed that at E18, P1, and P10 E-FABP levels were at least fourfold greater than those in the adult. By P15, protein levels were only twofold greater, with adult levels reached by P31. Furthermore, E-FABP could be reinduced during axon regeneration. Dissociated P15 retinal cells cultured in the presence of brain-derived neurotrophic factor, ciliary neurotrophic factor, and basic fibroblast growth factor exhibited sixfold more GAP-43 and E-FABP double-positive RGCs (cell body and axons) than controls. Moreover, all GAP-43-immunoreactive RGCs were also positive for E-FABP. Taken together, these results indicate the following: 1) E-FABP is expressed in RGCs as they reached the ganglion cell layer and 2) E-FABP plays a functional role in the elaboration of RGC axons in both development and regeneration.

Differential expression of syntaxin-1 and synaptophysin in the developing and adult human retina

Synaptophysin and syntaxin-1 are membrane proteins that associate with synaptic vesicles and presynaptic active zones at nerve endings, respectively. The former is known to be a good marker of synaptogenesis; this aspect, however, is not clear with syntaxin-1. In this study, the expression of both proteins was examined in the developing human retina and compared with their distribution in postnatal to adult retinas, by immunohistochemistry. In the inner plexiform layer, both were expressed simultaneously at 11-12 weeks of gestation, when synaptogenesis reportedly begins in the central retina. In the outer plexiform layer, however, the immunoreactivities were prominent by 16 weeks of gestation. Their expression in both plexiform layers followed a centre-to-periphery gradient. The immunoreactivities for both proteins were found in the immature photoreceptor, amacrine and ganglion cells; however, synaptophysin was differentially localized in bipolar cells and their axons, and syntaxin was present in some horizontal cells. In postnatal-to-adult retinas, synaptophysin immunoreactivity was prominent in photo-receptor terminals lying in the outer plexiform layer; on the contrary, syntaxin-1 was present in a thin immunoreactive band in this layer. In the inner plexiform layer, however, both were homogeneously distributed. Our study suggests that (i) syntaxin-1 appears in parallel with synapse formation; (ii) synaptogenesis in the human retina might follow a centre-to-periphery gradient; (iii) syntaxin-1 is likely to be absent from ribbon synapses of the outer plexiform layer, but may occur at presynaptic terminals of photoreceptor and horizontal cells, as is apparent from its localization in these cells, which is hitherto unreported for any vertebrate retina.

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.

Postnatal development of synaptophysin immunoreactivity in the rat nucleus tractus solitarii and caudal ventrolateral medulla

Synaptophysin (SY) is a major integral membrane protein of small synaptic vesicles. In the present study, SY immunohistochemistry was used to investigate the postnatal development of the rat nucleus tractus solitarii (NTS) and nucleus ambiguus/ventrolateral medulla (NA/VLM). Whatever the age of the animal, SY immunoreactivity showed a typical pattern of punctate staining reminiscent of presynaptic terminal labeling. In the NTS and the NA/VLM, SY immunoreactive puncta were few at birth and increased in number during the first postnatal days. These changes were quantified by measuring the volumetric fraction occupied by SY immunoreactive puncta at various postnatal ages. Using volumetric fraction data, an index of the total volume occupied SY immunoreactivity in each region was then calculated. Between birth and adulthood, this index increased by 6-fold in the NTS and by 7-fold in the NA/VLM, suggesting that most of the synaptic development of these regions occurs postnatally.

Patterns of synaptophysin expression during development of the inner ear in the chick

The onset of active neural connections between the periphery and the central nervous system is integral to the development of sensory systems. This study presents patterns of synaptogenesis in the chick basilar papilla (i.e., cochlea) by examining the immunohistochemical expression of synaptophysin with a specific monoclonal antibody, SBI 20.10. The initial onset of synaptophysin expression occurs in nerve fibers and ganglion cell bodies at a time when neurites reach the basement membrane of the chick cochlea on embryonic day 6-7 (ED 6-7). By ED 8, synaptophysin positive fibers invade the neural side of the entire length of the cochlea, so that by ED 9-10, fibers are forming multiple terminals on the basolateral ends of retracting receptor or hair cells. In contrast, on the abneural side, immunoreactive terminals are seen first as small, punctate contacts and then as large, synaptophysin positive calyceal endings beneath short hair cells. These terminals are sparse during early development, more numerous by ED 17-19, but still incomplete after 2 weeks posthatching. In comparison, hair cells show synaptophysin immunoreactivity in both supra- and infranuclear regions by ED 11-12, a time when efferent innervation is incomplete. Thus, during development, synaptophysin is expressed at both synaptic and nonsynaptic sites, is relatively selective in its regional distribution, and is expressed in hair cells at a time when auditory function begins. Our results present a framework with which to understand the potential role of synaptophysin in early synaptogenesis of the cochlea.

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.

Developmental expression of synaptophysin, synapsin I and syntaxin in the rat retina

Expression of synaptophysin, synapsin I and syntaxin was studied immunocytochemically in the developing rat retina using indirect immunoperoxidase technique. In the inner plexiform layer (IPL), syntaxin immunoreactivity appeared at postnatal day 1 (P1) whereas synaptophysin and synapsin I staining were first observed at P2. In the outer plexiform layer (OPL), synaptophysin appeared at P4, while synapsin I and syntaxin appeared at P8. In the case of synaptophysin, a punctate pattern of staining was observed from the time of its appearance (P4) in the OPL and from P12 onwards in the IPL. Synapsin I and syntaxin immunoreactivity in the OPL were of a low intensity throughout the development and in the adult stage. These findings are discussed in relation to synaptogenesis in the rat retina.

The growth-associated protein GAP-43 is specifically expressed in tyrosine hydroxylase-positive cells of the rat retina

In the adult retina, the growth-associated protein GAP-43 is exclusively present in three distinct sublaminae of the inner plexiform layer. During postnatal development, it is transiently expressed in the optic nerve fibers. No conclusions about the GAP-43 expressing cells can be derived from immunohistochemical stainings because GAP-43 protein is rapidly transported into the distal neuronal processes. We have combined immunohistochemistry to study the protein expression of GAP-43 and non-radioactive in situ hybridization to study the cellular expression of GAP-43 in the rat retina. We have found that in the mature retina GAP-43 mRNA is present only in retinal ganglion cells and in a small subset of cells of the inner nuclear layer. During postnatal development, no cells besides retinal ganglion cells and a subpopulation of cells in the inner nuclear layer express GAP-43 mRNA. Double staining experiments with tyrosine hydroxylase (TH) immunohistochemistry and GAP-43 in situ hybridization showed that GAP-43 expressing cells in the inner nuclear layer are immunoreactive for TH. They are most probably dopaminergic amacrine cells. Our results show that GAP-43 expression in the retina is restricted to very few cell types. They suggest that TH-positive cells (probably dopaminergic amacrine cells) retain a higher degree of structural plasticity in the adult retina.

Synaptogenesis in the rat retina: subcellular localization of glycine receptors, GABA(A) receptors, and the anchoring protein gephyrin

The mechanisms by which neurotransmitter receptors are clustered at postsynaptic sites of neurons are largely unknown. The 93-kDa peripheral membrane protein gephyrin has been shown to be essential for the formation of postsynaptic glycine receptor clusters, and there is now evidence that gephyrin can also be found at gamma-aminobutyric acid (GABA)ergic synapses. In this study, we have analyzed the synaptic localization of glycine receptors, GABA(A) receptors, and the anchoring protein gephyrin in the inner plexiform layer of the developing rat retina, by using immunofluorescence with subunit specific antibodies. At early postnatal stages, the antibodies produced a diffuse staining, suggesting that early retinal neurons can express glycine and GABA(A) receptors. A clustered distribution of the subunits in "hot spots" was also observed. The number of "hot spots" increased during development and reached adult levels in about 2 weeks. Electron microscopy showed that synapses of the conventional type are present in the inner plexiform layer of the postnatal retina and that the hot spots correspond to an aggregation of receptors at postsynaptic sites. Gephyrin was also localized to "hot spots," and double immunofluorescence revealed a colocalization of gephyrin with the alpha2 subunit of the GABA(A) receptor. These results indicate that clustering of receptor subunits occurs in parallel with the formation of morphologically identifiable synaptic specializations and suggest that gephyrin may be involved in clustering of GABA(A) receptors at postsynaptic sites.

Axon-regenerating retinal ganglion cells in adult rats synthesize the cell adhesion molecule L1 but not TAG-1 or SC-1

Retinal ganglion cells (RGCs) in rats regenerate axons in the presence of a PNS nerve graft. To determine if axon-regenerating RGCs synthesize cell adhesion/recognition molecules which they possessed during development, retinae were subjected to in situ hybridization with antisense cRNA probes of L1, TAG-1, and SC-1 (and GAP-43 for comparison). L1 and TAG-1 (and GAP-43) proteins on axons were detected with antibodies. L1, TAG-1, and SC-1 (and Gap-43) mRNAs and L1 and TAG-1 (and Gap-43) proteins were expressed by RGCs in embryonic, postnatal, and adult rats. After optic nerve lesion (ONL), the surviving RGCs between 2 and 28 days after ONL continued to express L1. TAG-1 and SC-1 expression, however is lost. In grafted rats, axon-regenerating RGCs express L1 (together with GAP-43) but neither TAG-1 nor SC-1. Thus, axonal regeneration in grafted rats occurs in the presence of L1 (and GAP-43) but in the absence of TAG-1 and SC-1).

Formation of synaptic specializations in the inner plexiform layer of the developing chick retina

The development of synapse-like specializations was investigated in the inner plexiform layer of the developing chick retina by using light and electron microscopy. Six monoclonal antibodies, directed against glycine and gamma-aminobutyric acid (GABA)A receptor subunits, the intracellular receptor-associated protein gephyrin, synaptotagmin, and synaptophysin were used to determine the initial appearance and distribution of their antigens. Synaptophysin and synaptotagmin immunoreactivity was detected in the retina concurrent with the formation of the inner plexiform layer at embryonic day 7. This early appearance before synaptic differentiation, together with the transient expression of synaptotagmin immunoreactivity in the synapse-free optic fiber layer, suggests that in the developing central nervous system (CNS) these proteins are not confined to synapses. The first immunofluorescence signal detected with specific antibodies against the beta 2 and beta 3-subunits of the GABAA receptor, the glycine receptor, and gephyrin appeared at embryonic day 12. In contrast, the alpha 1-subunit of the adult-type glycine receptor heteromeric complex was detectable only at later stages of development, after embryonic day 16, suggesting a change in the subunit composition of some glycine receptor complexes. The staining was clearly punctate, indicating the clustering of the alpha 1-subunit at synapses. Electron microscopic investigation revealed the first postsynaptic densities and active zones in the inner plexiform layer of the retina at embryonic day 12. These results reveal different patterns of development for the investigated pre- and postsynaptic proteins and indicate a parallel appearance of gephyrin, glycine receptor, and the beta 2 and beta 3-subunits of the GABAA receptor with the first synaptic specializations in the inner plexiform layer of the developing chick retina.

Maturational gradients in the retina of the ferret

In the present set of studies, we have examined the site for the initiation of retinal maturation in the ferret. A variety of maturational features across the developing inner and outer retina were examined by using standard immunohistochemical, carbocyanine dye labelling, and Nissl-staining techniques, including 1) two indices of early differentiation of the first-born retinal ganglion cells, the presence of beta-tubulin and of neuron-specific enolase; 2) the receding distribution of chondroitin sulfate proteoglycans within the inner retina; 3) the distribution of the first ganglion cells to grow axons along the optic nerve; 4) the emergence of the inner plexiform layer; 5) the emergence of the outer plexiform layer and 6) the onset of synaptophysin immunoreactivity within it; 7) the differentiation of calbindin-immunoreactive horizontal cells; and 8) the cessation of proliferative activity at the ventricular surface. Although we were able to define distinct maturational gradients that are associated with many of these features of inner and outer retinal development (each considered in detail in this report), with dorsal retina maturing before ventral retina, and with peripheral retina maturing last, none showed a clear initiation in the region of the developing area centralis. Rather, maturation began in the peripapillary retina dorsal to the optic nerve head, which is consistent with previous studies on the topography of ganglion cell genesis in the ferret. These results make clear that the order of retinal maturation and the formation of the area centralis are not linked, at least not in the ferret.

Synaptic organization of an organotypic slice culture of the mammalian retina

Vertical slices of postnatal day 6 (P6) rat retina were cut and cultured using the roller-tube technique. The organotypic differentiation during a culture period of up to 30 days has been described in a previous study (Feigenspan et al., 1993a). Here we concentrated on the synaptic organization in the retinal slice culture. Electron microscopy revealed the presence of ribbon synapses in the outer plexiform layer and conventional and ribbon synapses in the inner plexiform layer. Immunofluorescence with antibodies that recognize specific subunits of GABAA or glycine receptors revealed a punctate distribution of the receptors. They were aggregated in "hot spots" that correspond to a concentration of receptors at postsynaptic sites. Different isoforms of GABAA and glycine receptors occurred in the slice cultures. The experiments show that there is a differentiation of synapses and a diversity of transmitter receptors in the slice cultures that is comparable to the in vivo retina.