Topographic maps and molecular gradients

Analysis of the retina in the zebrafish model

The zebrafish is one of the leading models for the analysis of the vertebrate visual system. A wide assortment of molecular, genetic, and cell biological approaches is available to study zebrafish visual system development and function. As new techniques become available, genetic analysis and imaging continue to be the strengths of the zebrafish model. In particular, recent developments in the use of transposons and zinc finger nucleases to produce new generations of mutant strains enhance both forward and reverse genetic analysis. Similarly, the imaging of developmental and physiological processes benefits from a wide assortment of fluorescent proteins and the ways to express them in the embryo. The zebrafish is also highly attractive for high-throughput screening of small molecules, a promising strategy to search for compounds with therapeutic potential. Here we discuss experimental approaches used in the zebrafish model to study morphogenetic transformations, cell fate decisions, and the differentiation of fine morphological features that ultimately lead to the formation of the functional vertebrate visual system.

Approaches to study neurogenesis in the zebrafish retina

Similar to other vertebrate species, the zebrafish retina is simpler than other regions of the central nervous system (CNS). Relative simplicity, rapid development, and accessibility to genetic analysis make the zebrafish retina an excellent model system for the studies of neurogenesis in the vertebrate CNS. Numerous genetic screens have led to isolation of an impressive collection of mutations affecting the retina and the retinotectal projection in zebrafish. Mutant phenotypes are being studied using a rich variety of markers: antibodies, RNA probes, retrograde and anterograde tracers, as well as transgenic lines. Particularly impressive progress has been made in the characterization of the zebrafish genome. Consequently, positional and candidate cloning of mutant genes are now fairly easy to accomplish in zebrafish. Many mutant genes have, in fact, already been cloned and their analysis has provided important insights into the gene circuitry that regulates retinal neurogenesis. Genetic screens for visual system defects will continue in the future and progressively more sophisticated screening approaches will make it possible to detect a variety of subtle mutant phenotypes in retinal development. The remarkable evolutionary conservation of the vertebrate eye provides the basis for the use of the zebrafish retina as a model of human disorders. Some of the genetic defects of the zebrafish retina indeed resemble human retinopathies. As new techniques are being introduced and improved at a rapid pace, the zebrafish will continue to be an important organism for the studies of the vertebrate visual system.

Target-specific innervation of embryonic cerebellar transplants by regenerating olivocerebellar axons in the adult rat

The reestablishment of topographically organized connections is a necessary prerequisite to obtain a full anatomical repair following brain injury. One system where such an issue can be addressed is the olivocerebellar system, where, normally, clusters of inferior olive neurons project to neurochemically heterogeneous Purkinje cell compartments defined by the expression of cell-specific markers, such as zebrin II. To assess whether adult injured olivocerebellar axons that regenerate into cerebellar transplants are able to establish target-specific innervation of grafted Purkinje cells, we made surgical transections in the white matter of adult rat cerebella and placed solid grafts from the embryonic cerebellar anlage into the lesion site. The transplanted tissue developed highly organized minicerebella, in which Purkinje cells were distributed into distinct clusters of zebrin II-immunopositive or -immunonegative neurons, mimicking the cortical compartments present in the normal adult cerebellum. Olivocerebellar axons, labeled by biotinylated dextran amine tracing, regenerated into the transplants where they formed discrete patches made of several terminal arbors impinging upon Purkinje cell dendrites. Among 401 such climbing fiber patches, 96% exclusively innervated Purkinje cells of either phenotype and stopped at the border of the zebrin II(+/-) Purkinje cell clusters, whereas only 4% were extended across this boundary and innervated both zebrin II-positive and -negative Purkinje cells. The results obtained support the view that the embryonic cerebellar tissue provides target-specific information that can be decoded by ingrowing adult olivocerebellar axons in order to establish appropriate innervation patterns with zebrin II-positive or -negative Purkinje cell compartments.

Roles for ephrins in positionally selective synaptogenesis between motor neurons and muscle fibers

Motor axons form topographic maps on muscles: rostral motor pools innervate rostral muscles, and rostral portions of motor pools innervate rostral fibers within their targets. Here, we implicate A subfamily ephrins in this topographic mapping. First, developing muscles express all five of the ephrin-A genes. Second, rostrally and caudally derived motor axons differ in sensitivity to outgrowth inhibition by ephrin-A5. Third, the topographic map of motor axons on the gluteus muscle is degraded in transgenic mice that overexpress ephrin-A5 in muscles. Fourth, topographic mapping is impaired in muscles of mutant mice lacking ephrin-A2 plus ephrin-A5. Thus, ephrins mediate or modulate positionally selective synapse formation. In addition, the rostrocaudal position of at least one motor pool is altered in ephrin-A5 mutant mice, indicating that ephrins affect nerve-muscle matching by intraspinal as well as intramuscular mechanisms.

Regulatory interrelations among topographic molecules CBF1, CBF2 and EphA3 in the developing chick retina

It has been shown that topographic expression of two winged-helix transcription factors, CBF1/c-qin and CBF2, and a receptor tyrosine kinase EphA3 (Mek4/Cek4) play important roles in establishing the topographic retinotectal projection map along the rostrocaudal axis. The interrelationship among these topographic molecules in the chick retina was studied during development. The topographic expression of CBF1 and CBF2 preluded the graded expression of EphA3, but their precise expression profiles did not exactly fit together. However, interestingly, CBF1 and CBF2 were properly expressed, together with EphA3, in immortalized cell lines derived from the quail retina, which maintained position-specific characteristics. The expression of another topographic molecule SOHo-1, the sensory organ homeobox-1 transcription factor, was separate from EphA3 expression. Ectopic expression of CBF1 using in ovo electroporation repressed the expression of CBF2, and misexpression of CBF2 influenced the graded localization of EphA3 in the retina, albeit imperfectly. Taken together, it is suggested that retinal cells first begin to express CBF1 or CBF2 according to their topographic positions, generate cellular descendants in which the expression of CBF1 and CBF2 is maintained cell-autonomously, and then establish the nasotemporal gradient of EphA3 under the control of CBF2, although indirect.

Positionally selective growth of embryonic spinal cord neurites on muscle membranes

Motor neurons from distinct positions along the rostrocaudal axis generally innervate muscles or muscle fibers from corresponding axial levels. These topographic maps of connectivity are partially restored after denervation or transplantation under conditions in which factors of timing and proximity are eliminated. It is therefore likely that motor neurons and some intramuscular structures bear cues that bias synapse formation in favor of positionally matched partners. To localize these cues, we studied outgrowth of neurites from embryonic spinal cord explants on carpets of membranes isolated from perinatal rat muscles. Neurites from rostral (cervical) and caudal (lumbar) spinal cord slices exhibit distinct growth preferences. In many instances, rostrally derived neurites grew selectively on membranes from forelimb muscles or from a single thoracic muscle (the serratus anterior) when given a choice between these membranes and membranes from hindlimb muscles or laminin. Caudally derived neurites almost never exhibited such rostral preferences, but instead preferred membranes from hindlimb muscles or a single hindlimb muscle (the gluteus) to rostral muscles or laminin. Likewise, spinal neurites exhibited distinct position-related preferences for outgrowth on membranes of clonal myogenic cell lines derived from specific rostral and caudal muscles. Taken together these results suggest that the membranes of motor axons and myotubes bear complementary labels that vary with rostrocaudal position and regulate neuromuscular connectivity.

Neuronal growth cone collapse triggers lateral extensions along trailing axons

Axonal outgrowth is generally thought to be controlled by direct interaction of the lead growth cone with guidance cues, and, in trailing axons, by fasciculation with pioneer fibers. Responses of axons and growth cones were examined as cultured retinal ganglion cell (RGC) axons encountered repellent cues. Either contact with cells expressing ephrins or mechanical probing increased the probability of lead growth cone retraction. Lateral extension of filopodia and lamellipodia hundreds of microns behind the lead growth cone was correlated with its collapse. Transmission electron microscopy showed that some of the lateral extensions originate from the pioneer axon, whereas others represent growth cones of defasciculating trailing axons.

Mechanisms involved in development of retinotectal connections: roles of Eph receptor tyrosine kinases, NMDA receptors and nitric oxide

Axons of retinal ganglion cells exhibit a specific pattern of connections with the brain. Within each visual nucleus in the brain, retinal connections are topographic such that axons from neighboring ganglion cells have neighboring synapses. Research is beginning to shed light on the mechanisms responsible for development of topographic connections in the visual system. Much of this research is focused on the axonal connections of the retina with the tectum. In vivo and in vitro experiments indicate that the pattern of retinotectal connections develops in part due to positional labels carried by the growing retinal axons and by the tectal cells. Evidence suggests that gradients of Eph receptor tyrosine kinases serve as positional labels on the growing retinal axons, and gradients of ligands for these receptors serve as positional labels in the tectum. Blocking expression of EphA3, a receptor tyrosine kinase, in the developing retina resulted in disruption of the topography of the retinotectal connections, further supporting the role of these, molecules. Although positional labels appear to be important, other mechanisms must also be involved. The initial pattern of retinotectal connections lacks the precision seen in the adult. The adult pattern of connections arises during development by activity dependent refinement of a roughly ordered prepattern. The refinement process results in elimination of projections to the wrong side of the brain, to non-visual nuclei and to inappropriate regions within a nucleus. Blocking NMDA receptors during the period of refinement preserved anomalous retinotectal projections, which suggests that elimination of these projections is mediated by NMDA receptors. Furthermore, tectal cells normally express high levels of nitric oxide synthase (NOS) during the period of refinement, and blocking nitric oxide (NO) synthesis also preserved inappropriate projections. Thus, both NMDA receptors and NO appear to be involved in refinement. Blocking NMDA receptor activation reduced NOS activity in tectal cells, which suggests the possibility that NO is the downstream mediator of NMDA function related to refinement. A quantitative comparison of blocking NMDA receptors, NO synthesis or both showed that all three treatments have comparable effects on refinement. This indicates that the role of NMDA receptor activation relative to refinement may be completely mediated through nitric oxide. Quantitative analysis also suggests that other mechanisms not involving NMDA receptors or NO must be involved in refinement. Other mechanisms appear to include cell death.

A key role for GAP-43 in the retinotectal topographic organization

To have a proper spatial visual perception, vertebrate retinal ganglion cells connect to their brain targets in a highly ordered fashion. The molecular bases for such topographic retinotectal connection in mammals still remain largely unknown. Using the gene knock-out approach in mice, we report here a key role for the GAP-43 growth cone protein in the development of the visual system. In mice bearing a targeted disruption of GAP-43 exon 1, a high proportion of retinal ganglion cell (RGC) axons was found to grow abnormally into the ipsilateral optic tract and into the hypothalamus. After leaving the optic chiasm during development, the GAP-43-deficient RGC axons generally follow the optic tracts but are unable to form proper terminal zones in the lateral geniculate nucleus. Moreover, in the superior colliculus, RGC axons lacking GAP-43 are intermingled. These results suggest an essential role for GAP-43 in development of the topographic retinotectal connection.

Development of the retina

As in other vertebrate species, the zebrafish retina is simpler than other regions of the central nervous system. This relative simplicity along with rapid development, and accessibility to genetic analysis make the zebrafish retina an excellent model system for studies of neurogenesis in the vertebrate CNS. Several genetic screens have led to the isolation of an impressive collection of mutants affecting the retina and the retinotectal projections in zebrafish. A variety of techniques and markers are available to study the isolated mutants. These include several antigen- and transcript-detection methods, retrograde and anterograde labeling of neurons, blastomere transplantations, H3 labeling, and others. As past genetic screens have achieved a rather low level of saturation, the current collection of mutants can only grow in the future. Morphological and behavioral criteria have been successfully applied in zebrafish to search for defects in spinal development. In future genetic screens, progressively more sophisticated screening approaches will make it possible to detect very subtle changes in the retinal development. The remarkable evolutionary conservation of the vertebrate eye provides the basis for using the zebrafish as a model system for the detection and analysis of genetic defects potentially related to human eye disorders. Some of the genetic defects of the zebrafish retina indeed resemble human retinopathies. As the genetic analysis of the vertebrate visual system is far from being complete and new techniques are being introduced at a rapid pace, the zebrafish embryo will become increasingly useful as a model for studies of the vertebrate retina.

Molecular regulators involved in vertebrate eye development

Development of the eye can be subdivided into three phases. The first phase is the formation of the major structures of the eye by the processes of induction and regional specification. The second is the maturation of these structures to form the functional eye, and the third phase is the formation of neuronal connections between retina and the optic tectum. These processes are tightly regulated by signalling cascades that direct axonal outgrowth, cellular proliferation and differentiation. Some members of these signalling cascades have been identified in recent studies. These include secreted factors which transmit signals extracellularly, and receptors and transcription factors which are members of intracellular signalling pathways that respond to extracellular signals. This review summarizes the recent research that has implicated these factors in playing a role in eye development on the basis of functional or expression criteria.

NADPH-d positive neurons in the developing somatosensory cortex of the rat: effects of early and late environmental enrichment

The effects of environmental enrichment upon the topographic arrangement of NADPH diaphorase-positive neurons (NADPH-d+ neurons) was studied in the somatosensory cortex of 56 Sprague-Dawley albino rats during early stages of development (18th, 24th, 30th and 60th postnatal days). This diaphorase is easily demonstrable, providing a convenient marker for quantitative studies. Environmental enrichment diminished the number of NADPH-d+ neurons and exerted its maximal influence during lactation, a time of exceptional cortical susceptibility. This implies that the magnitude of such effects on the density of NADPH-d+ neurons is age-dependent. Furthermore, it was found that the experience-dependent cortical changes persisted after a subsequent period without environmental stimulation. The effects of early environmental enrichment did not occur uniformly throughout the cerebral hemispheres but, instead, such effects were maximal in the latero-ventral sector of the cerebral cortex where a dramatic reduction in the number of NADPH-d+ neurons was observed. Particularly striking was the existence of a latero-medial sequence of NADPH-d+ neurons in the infragranular layer and a reversed distribution of labeled cells, in the supragranular layer. Both ontogenetic sequences of NADPH-d+ neurons remained unchanged during postnatal development in controls and enriched subjects (18th-60th postnatal days).

The ephrins and Eph receptors in neural development

The Eph receptors are the largest known family of receptor tyrosine kinases. Initially all of them were identified as orphan receptors without known ligands, and their specific functions were not well understood. During the past few years, a corresponding family of ligands has been identified, called the ephrins, and specific functions have now been identified in neural development. The ephrins and Eph receptors are implicated as positional labels that may guide the development of neural topographic maps. They have also been implicated in pathway selection by axons, the guidance of cell migration, and the establishment of regional pattern in the nervous system. The ligands are anchored to cell surfaces, and most of the functions so far identified can be interpreted as precise guidance of cell or axon movement. This large family of ligands and receptors may make a major contribution to the accurate spatial patterning of connections and cell position in the nervous system.

Synchronizing retinal activity in both eyes disrupts binocular map development in the optic tectum

Spatiotemporal correlations in the pattern of spontaneous and evoked retinal ganglion cell (RGC) activity are believed to influence the topographic organization of connections throughout the developing visual system. We have tested this hypothesis by examining the effects of interfering with these potential activity cues during development on the functional organization of binocular maps in the Xenopus frog optic tectum. Paired recordings combined with cross-correlation analyses demonstrated that exposing normal frogs to a continuous 1 Hz of stroboscopic illumination synchronized the firing of all three classes of RGC projecting to the tectum and induced similar patterns of temporally correlated activity across both lobes of the nucleus. Embryonic and eye-rotated larval animals were reared until early adulthood under equivalent stroboscopic conditions. The maps formed by each RGC class in the contralateral tectum showed normal topography and stratification after strobe rearing, but with consistently enlarged multiunit receptive fields. Maps of the ipsilateral eye, formed by crossed isthmotectal axons, showed significant disorder and misalignment with direct visual input from the retina, and in the eye-rotated animals complete compensatory reorientation of these maps usually induced by this procedure failed to occur. These findings suggest that refinement of retinal arbors in the tectum and the ability of crossed isthmotectal arbors to establish binocular convergence with these retinal afferents are disrupted when they all fire together. Our data thus provide direct experimental evidence that spatiotemporal activity patterns within and between the two eyes regulate the precision of their developing connections.

A compartment-based, asymmetric representation of the retina in an induced projection to the olfactory cortex

Displacing the optic nerve into the telencephalon in adult Rana pipiens induces a projection to olfactory cortex. We have examined the topographic organization of this projection anatomically by injecting a mixture of biotin dextran (BDA) with 3H-amino acids into the affected eye immediately after making cuts across defined sectors of the nerve fiber layer to trace the complementary patterns of anterograde migration of BDA and 3H label in the cut and intact retinal axons, respectively. Fibers from the temporal side of the optic disc terminated in an oblique band along the posterior two-thirds or more of the ectopic projection field. In contrast, fibers arising in the nasal retina terminated in a parallel strip occupying the anterior one-third or less of the field. Varying the location of the cuts within each hemiretina did not reveal any further organization along the nasotemporal or dorsoventral axes of the retina. The retinal location of the cells involved in this projection was further studied with injections of wheat germ agglutinin conjugated to horseradish peroxidase into the olfactory cortex. Ganglion cells labeled by retrograde transport were found throughout the retina, but they were much more numerous on the temporal side, having a mean spatial density 3.7-7.4 times greater in the temporal hemiretina, whereas the overall ganglion cell density (labeled plus unlabeled) was roughly the same in the two halves of the retina. These data provide an example of a permanent projection in which the overall representation of the retina, though nontopological, is polarized in one axis (nasotemporal) and, therefore, compartmentally organized.

A gradient of basic fibroblast growth factor in rod photoreceptors in the normal human retina

Retinitis pigmentosa (RP) is an inherited disease that causes primary degeneration of rod photoreceptors in the retina. Although the causal gene (e.g. rhodopsin) is thought to be expressed in all rods across the retina, the degeneration is typically nonuniform, with rods in the far periphery surviving significantly longer than those in the midperiphery and macula. Basic fibroblast growth factor (bFGF) is a putative survival factor for photoreceptors, and the characteristic regional pattern of rod cell survival in RP suggested that bFGF might be distributed nonuniformly in the human retina. We performed double-label immunocytochemistry on 15 normal human retinas, using anti-bFGF and other antibody markers for retinal neurons and glia. Immunoreactivity for bFGF was consistently absent from cones but was present in rods, populations of cone bipolar and amacrine cells, Müller glial cells, and astrocytes. In the macula, the percentage of bFGF-reactive rods was very low (approximately 0.5%) but it increased in a central to peripheral gradient, accounting for up to approximately 88% of the rods in the far periphery. These findings suggest that a central to peripheral gradient of rod bFGF is present in normal human retina and may influence the pattern of photoreceptor degeneration in RP. The absence of bFGF in cones and the low number of bFGF-positive rods in the macula may correlate with the vulnerability of these cells in RP, age-related macular degeneration, and other retinal diseases.

Reestablishment of the olivocerebellar projection map by compensatory transcommissural reinnervation following unilateral transection of the inferior cerebellar peduncle in the newborn rat

It is unclear whether reparative processes in the injured mammalian brain are able to restore the topographic organisation of neuronal connections. To address this question, we have investigated the plasticity of the olivocerebellar system. This pathway has a precise topographic arrangement, in which subsets of inferior olivary neurons project to parasagittally oriented Purkinje cell compartments. Following unilateral transection of the inferior cerebellar peduncle in newborn rats, axons from the contralateral projection cross the cerebellar midline and reinnervate the deafferented hemicerebellum. By this experimental approach, we first analysed the behaviour of calcitonin gene-related peptide (CGRP)-immunoreactive climbing fibres. This marker is transiently expressed by a subset of developing inferior olivary axons, which terminate in the cerebellar cortex into several parasagittal strips. We show that transcommissural axons reestablish the original pattern of climbing fibre bands within a few days after lesion. Then, in adult animals injured at birth, we assessed whether the newly formed climbing fibre bands align with zebrin II+/- Purkinje cell compartments, as in normal conditions. The newly formed projection is organised in parasagittally oriented strips which mirror the distribution of their counterparts on the intact side and are precisely aligned to the heterogeneous Purkinje cell compartments. In addition, the patchy distribution of olivo-nuclear fibres suggests that specific reinnervation is also achieved in the deep nuclei. Thus, transcommissural olivocerebellar reinnervation is not random, but it is regulated by selective interactions between distinct subsets of olivocerebellar axons and target neurons aimed at reestablishing the correct projection map.

Retroviral misexpression of engrailed genes in the chick optic tectum perturbs the topographic targeting of retinal axons

We have investigated the role of the homeodomain transcription factor genes En-1 and En-2, homologs of the Drosophila segment polarity gene engrailed, in regulating the development of the retinotopic map in the chick optic tectum. The En proteins are distributed in a gradient along the rostral-caudal axis of the developing tectum, with highest amounts found caudally. Previous evidence suggests that En-1 and En-2 may regulate the polarity of the rostral-caudal axis of the tectum and the subsequent topographic mapping of retinal axons. We have tested this hypothesis by using a recombinant replication-competent retrovirus to overexpress the En-1 or En-2 genes in the developing tectum. Anterograde labeling with the axon tracer Dil was used to analyze the topographic mapping of retinal axons after the time that the retinotectal projection is normally topographically organized. Overexpression of either En-1 or En-2 perturbed the topographic targeting of retinal axons. In En-infected tecta, nasal retinal axons form an abnormally diffuse projection with numerous aberrant axons, branches, and arbors found at topographically incorrect locations, colocalized with domains of viral infection. In contrast, temporal axons did not form a diffuse projection or discrete aberrant arbors; however, many temporal axons were stunted and ended aberrantly rostral to their appropriate TZ, or in other cases either did not enter the tectum or formed a dense termination at its extreme rostral edge. These findings indicate that En-1 and En-2 are involved in regulating the development of the retinotopic map in the tectum. Furthermore, they support the hypothesis that En genes regulate the polarity of the rostral-caudal axis of the tectum, most likely by controlling the expression of retinal axon guidance molecules.

The behavior of optic axons on substrate gradients of retinal basal lamina proteins and merosin

To study the behavior of optic axons to continuously changing concentrations of their substrate, explants from embryonic retina were placed across gradients of retinal basal lamina proteins and merosin. The following growth patterns of axons in response to the substrate gradients were found: (1) Axons that grew up gradients, i.e., from low to high substrate concentrations, became longer and less fasciculated with increasing concentration of the substrate. On shallow basal lamina gradients, the axons also showed a directional response that resulted in guidance to higher substrate concentrations. (2) Axons that grew down gradients, i.e., from high to low substrate concentrations, became shorter and more fasciculated with decreasing concentrations of the substrate. On gradients of merosin, a significant alteration in the axonal growth direction toward higher substrate concentrations was detected. Axons heading down gradients never U turned to higher substrate concentrations. (3) Axons confronted with discontinuous substrates were confined to the borders of the substrate exclusively, whereas axons confronted with substrate gradients were able to cross into the territory beyond the substrate. (4) The growth patterns of axons on substrate gradients of basal lamina proteins and merosin were similar but not identical, indicating that axons may respond to substrate gradients dependent on its chemical composition. The present results show that substrate gradients can regulate length and fasciculation of neurites and have a limited capability to direct axons to higher substrate concentrations.

Visual projection map specified by topographic expression of transcription factors in the retina

Topographical maps of neuronal connectivity occur in various brain regions. In the visual system of birds, retinal ganglion-cell axons from the anterior retina connect to a posterior part of the optic tectum, and posterior retinal axons connect to the anterior part, thereby establishing a point-to-point projection map. The chemoaffinity theory predicts that the orderly retinotectal projection is generated by a topographical arrangement of molecules. We report here that we have found several genes topographically expressed along the nasotemporal (anterior-posterior) axis in the embryonic chicken retina. Among these, two transcriptional regulators, belonging to the winged-helix family are expressed in a mutually exclusive manner in either the nasal or temporal part of the retina. Misexpression of each factor causes misprojection on the tectum along the rostrocaudal axis, showing that topographical expression of these transcription factors controls formation of the retinotectal map.

Preferential growth of neonatal rat dorsal root ganglion cells on homotypic peripheral nerve substrates in vitro

Developing sensory neurons interact with molecular signals in the local environment to generate stereotypic nerve pathways. Regenerating neurons seem to lose the ability to reinnervate their original sites in the targets, resulting in abnormal sensory input and consequent clinical pathophysiology. The specificity of reinnervation of peripheral targets by regenerating axons is thus crucial for normal recovery of function. In this study, we have examined evidence for selectivity of interactions between primary afferent neurons from identified levels of the spinal cord and different peripheral nerve environments by culturing these neurons on sections of nerves to muscle and viscera. We have compared the growth of a population of sensory afferents normally innervating somatic targets (dorsal root ganglion cells from L4 and L5) with populations containing many afferents innervating visceral targets (L6 and S1 dorsal root ganglia and nodose ganglion). These neurons, from newly born rats, were cultured on unfixed cryostat sections of normal and prelesioned gastrocnemius nerve, pelvic spinal nerve and vagus nerve from adult rats. Normal muscle nerve was seen to support the regeneration of a significantly greater proportion of somatic neurons, with longer neurites, than the visceral nerves. Similarly, much higher proportions of the 'visceral' population of afferent neurons were seen to extend neurites on the normal visceral nerve substrates, with longer neurites, than on the muscle nerve substrate. The selectivity displayed by the sensory neurons for their normal nerve substrates was abolished when they were cultured on prelesioned nerve substrates subjected to Wallerian degeneration, which was apparent from the equivalent and increased proportions of growing neurons having comparable neurite lengths, on all the nerve substrates. We conclude that sensory neurons recognize and respond to substrate-specific and substrate-bound molecules present in normal adult peripheral nerves, and that these differences are lost in prelesioned nerves following Wallerian degeneration.

Axons of Xenopus neural tube respond to reversals of neural tube orientation

Axonal trajectories of the Kolmer-Agduhr (KA) neurons of Xenopus embryos, were observed after anterior-posterior (A-P) inversions of neural tube grafts to determine whether KA axons follow cell-inherent directional cues, cues from their immediate environment, or rostrocaudal signals from the embryo. KA axons form one of the earliest ascending spinal pathways in Xenopus and are visible in the lateral marginal zone of whole mounts processed for GABA immunoreactivity. Grafts were made at trunk levels at stages 22-24, 3-5 h before the first KA neurons were detectable and prior to axonal out-growth. Embryos were fixed and immunostained 6-36 h later. KA trajectories within and adjacent to reversed grafts were compared to those of nonrotated control grafts and to neural tube lengths comparable in position and in length in unoperated embryos. Most KA axons within rotated grafts followed the graft's orientation. However, others changed direction, taking novel routes, including turning to conform to the orientation of the host embryo. Reorientations were most common near the posterior host/graft interface. Some host KA cells also reoriented, always within a few hundred microns of the graft interface. Taken together, these growth patterns show that most KA axons within the grafts grow normally with respect to the original polarity of the graft neural tube and maintain that direction even into tissue of opposite polarity, suggesting that their routes are mainly determined by cell-intrinsic and/or local tissue factors. However, the reorientation of many other axons, particularly near graft seams, implies that KA axons can respond to local fluctuations in directional or segment identity signals generated in both host and graft after this perturbation.

Eph receptor tyrosine kinases and their ligands in neural development

Receptor tyrosine kinases play important roles in many developmental phenomena, including the regulation of cell proliferation, differentiation, and survival. The largest subfamily of receptor tyrosine kinases are the Eph receptors, which are widely expressed in the nervous system. With the recent identification of several Eph ligands, it has become evident that Eph receptors and their ligands are involved in the guidance of retinal axons and in the process of hindbrain segmentation.

Pre- and postnatal expression of amino acid neurotransmitters, calcium binding proteins, and nitric oxide synthase in the developing superior colliculus

Neurons within the superior colliculus (SC) contain a variety of neurochemicals, including the amino acid neurotransmitters GABA and glutamate, the calcium binding proteins calbindin and parvalbumin, and the neuromodulator nitric oxide. We have examined the development of expression of these substances using antibody immunocytochemistry. These results are summarized in Fig. 10. GABA and calbindin are expressed very early in development, at a time when cells are still dividing and migrating from the subventricular zone. The expression of both GABA and CB is maximal at around E40-46, the age at which these cells have just established their adult lamination and extrinsic afferents have begun to grow into the tectum. GABA and CB likely play diverse roles during this stage of development, including the regulation of intracellular calcium during cell migration and neurite outgrowth. Glutamate is expressed somewhat later in development while parvalbumin immunoreactivity does not appear until shortly after birth. These two substances continue to increase in density throughout the period of postnatal growth, at a time when synapse formation and evoked electrical activity are beginning to develop. Both PV and glutamate may be involved in one or both of these activity-dependent processes. Nitric oxide synthase (NOS) is expressed at different times in different cell groups. NOS appears very early in prenatal development in cells within the SVZ and in the deep gray layer of SC. On the other hands, cells within the intermediate gray layer of SC do not express NOS until shortly before birth. The igl cells that express NOS at this age are clustered neurons similar to those that project to the CFR in the adult. NOS expression occurs in these cells at precisely the time when axons begin to form patches that innervate these clusters. Based upon this temporal correlation, we hypothesize that nitric oxide may regulate synapse formation in this cell group.

Target-independent diversification and target-specific projection of chemically defined retinal ganglion cell subsets

In diverse vertebrate species, defined subsets of retinal ganglion cells (RGCs, the neurons that project from retina to brain) are distinguishable on the basis of their dendritic morphology, physiological properties, neurotransmitter content and synaptic targets. Little is known about when this diversity arises, whether diversification requires target-derived signals, and how subtype-specific projection patterns are established. Here, we have used markers for two chemically defined RGC subsets in chick retina to address these issues. Antibodies to substance P (SP) and the nicotine acetylcholine receptor (AChR) beta 2 subunit label two small (< 10%), mutually exclusive groups of RGCs in mature retina. SP and AChRs accumulate in distinct RGCs before retinotectal synapses have formed. Moreover, both populations of RGCs form in retinae that develop following tectal ablation or transplantation to the coelomic cavity. Thus, RGC subsets acquire distinct neurotransmitter phenotypes in the absence of extraretinal cues. In the mature optic tectum, SP- and AChR-positive RGC axonal arbors are confined to distinct retinorecipient (synaptic) laminae. In the developing tectum, SP- and AChR-positive axons are initially intermingled in a superficial fiber layer, but then enter and arborize in appropriate laminae soon after those laminae form. Importantly, SP-positive axons, which synapse in a superficial lamina, never extend into the deeper, AChR-positive lamina. Tectal interneurons rich in SP receptors are concentrated in the lamina to which SP-positive RGC axons project, and a set of cholinergic (choline acetyltransferase-positive) tectal projection neurons elaborate dendrites in the lamina to which AChR-positive RGC axons project. These populations of tectal neurons, which are likely targets of the RGC subsets, form in tecta that develop following enucleation. Thus, RGCs and their targets can diversify in each others absence. Accordingly, we propose that the lamina-selective connectivity we observe reflects the presence of complementary cues on RGC subsets and their laminar targets.

Target preference of embryonic retina cells and retinal cell lines is cell-autonomous, position-specific, early determined and heritable

Retinal ganglion cells (RGCs) form the topographic connection between retina and optic tectum in the developing avian embryo. In vitro, neurons with the morphological traits and marker expression of RGCs were found both in single-cell cultures from embryonic day (E) 6 chick retina and in retinal cell lines derived from E3.5 quail retina. Rapid and substantial differentiation of RGC-like cells could be induced in the lines by addition of fibroblast growth factor aFGF or bFGF. RGC-like cells were examined with respect to their target discrimination properties as single cells in the stripe carpet assay. In this assay system, alternating stripes of membrane vesicles prepared from the anterior and posterior tectum are offered to growing axonal processes as a substrate. Temporal RGC-like cells, both primary cells prepared from the temporal retina and immortalized cells of those retinal lines derived from the temporal retina, avoid stripes of membrane vesicles from posterior tectum; they prefer to grow on membrane vesicles from the anterior tectum, which is their in vivo target. Nasal RGC-like cells did not exhibit a target preference, in accordance with previous findings. Together the experiments show that target preference of RGCs is a cell-autonomous and heritable mechanism that is determined early and is position-dependent.

Molecular cloning of TOPAP: a topographically graded protein in the developing chick visual system

Topographically graded molecules representing position-specific differences among otherwise similar cells are thought to play a role in the patterning of the developing nervous system. In the embryonic chick visual system, a 40 kDa protein, TOPAP, is expressed in a posterior > anterior gradient in the retina and in an inverted anterior > posterior gradient in the optic tectum, the major retinal projection area. Here we report the isolation and nucleotide sequencing of a complementary DNA clone encoding the chick TOPAP protein and demonstrate that the mRNA encoding this coiled-coil integral membrane protein is topographically graded within the retina and is present in a variety of chick tissues.

Expression of zfh-4, a new member of the zinc finger-homeodomain family, in developing brain and muscle

We have identified zfh-4, a new member of a recently recognized zinc finger-homeodomain (zfh) family of putative transcription factors. Zfh-4 expression is prominent in developing muscle and brain. In both tissues, zfh-4 RNA levels are highest embryonically, then decrease gradually to barely detectable levels in adults. In myogenic cell lines, far more zfh-4 is expressed in proliferating myoblasts than in myotubes, suggesting a cellular basis for the developmental regulation observed in vivo. In contrast, zfh-4 RNA in brain is more abundant in postmitotic cells of the marginal zone than in proliferating cells of the ventricular zone. Within the brain, zfh-4 RNA is regionally localized: expression is highest in midbrain, readily detectable in hindbrain, and very low in cerebral cortex. Its patterns of expression, and its homology to known DNA binding proteins, support the idea that zfh-4 may be a regulator of gene expression in developing brain and muscle.

Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes

Motor neurons located at different positions in the embryonic spinal cord innervate distinct targets in the periphery, establishing a topographic neural map. The topographic organization of motor projections depends on the generation of subclasses of motor neurons that select specific paths to their targets. We have cloned a family of LIM homeobox genes in chick and show here that the combinatorial expression of four of these genes, Islet-1, Islet-2, Lim-1, and Lim-3, defines subclasses of motor neurons that segregate into columns in the spinal cord and select distinct axonal pathways. These genes are expressed prior to the formation of distinct motor axon pathways and before motor columns appear. Our results suggest that LIM homeobox genes contribute to the generation of motor neuron diversity and may confer subclasses of motor neurons with the ability to select specific axon pathways, thereby initiating the topographic organization of motor projections.

Control of topographic retinal axon branching by inhibitory membrane-bound molecules

Retinotopic map development in nonmammalian vertebrates appears to be controlled by molecules that guide or restrict retinal axons to correct locations in their targets. However, the retinotopic map in the superior colliculus (SC) of the rat is developed instead by a topographic bias in collateral branching and arborization. Temporal retinal axons extending across alternating membranes from the topographically correct rostral SC or the incorrect caudal SC of embryonic rats preferentially branch on rostral membranes. Branching preference is due to an inhibitory phosphatidylinositol-linked molecule in the caudal SC. Thus, position-encoding membrane-bound molecules may establish retinotopic maps in mammals by regulating axon branching, not by directing axon growth.

Evidence for a mitogenic effect of Wnt-1 in the developing mammalian central nervous system

The analysis of mutant alleles at the Wnt-1 locus has demonstrated that Wnt-1-mediated cell signalling plays a critical role in development of distinct regions of the embryonic central nervous system (CNS). To determine how these signals participate in the formation of the CNS, we have ectopically expressed this factor in the spinal cord under the control of the Hoxb-4 Region A enhancer. Ectopic Wnt-1 expression causes a dramatic increase in the number of cells undergoing mitosis in the ventricular region and a concomitant ventricular expansion. Although this leads to consistent changes in the relative proportions of dorsal and ventral regions, Wnt-1 does not appear to act as a primary patterning signal. Rather, our experiments indicate that Wnt-1 can act as a mitogen in the developing CNS.

Expression and functional roles of neural cell surface molecules and extracellular matrix components during development and regeneration of peripheral nerves

By combining both immunocytochemical and functional investigations, a hypothetical framework will be developed for the molecular mechanisms underlying neuron-glia interactions during development and regeneration of peripheral nerves. In particular, the immunoglobulin-like molecules L1, N-CAM, MAG and P0, the extracellular matrix molecules laminin and tenascin, and the carbohydrates PSA and L2/HNK-1 will be considered. During early stages of limb bud innervation in embryos, L1 and N-CAM are expressed on axons and Schwann cells and are involved in axonal fasciculation, whereas tenascin is thought to be involved in forming a scaffold around the nerve possibly preventing axons and/or Schwann cells from leaving the nerve. PSA has been shown to be involved in pathway selection at initial stages of limb bud innervation. Later on, when motor axons enter muscles, the carbohydrates determine the branching pattern of the nerves. During myelination, L1 appears to play a pivotal role during the formation of the first Schwann cell loops around the prospective myelin-containing axons. MAG and P0 appear also to be functionally involved at initial stages of myelin formation. Additionally, MAG may contribute to the formation and maintenance of non-compacted myelin and axon-Schwann cell apposition whereas P0 is involved in myelin compaction. Under regenerative conditions, L1, N-CAM, laminin, and tenascin are strongly up-regulated by denervated Schwann cells. In vitro observations strongly suggest that these molecules might foster axonal regeneration. The carbohydrate PSA is confined to regrowing axons and is also a candidate to support axonal regrowth. L2/HNK-1, which is found on motor axon-associated Schwann cells, may provide regenerating motor axons with a selective advantage over others resulting in appropriate reinnervation of motor pathways. Since many of the functional studies this review refers to have been performed in vitro, some of the conclusions drawn need reexamination in vivo. Gene manipulations, such as the generation of null mutants followed by a thorough morphological and immunocytochemical investigation may be a powerful tool to resolve this problem.

The molecular basis of retinotectal topography

Over 50 years have passed since Roger Sperry formulated a simple model of how visual space, as seen by the retina, can be projected onto the brain in a two-dimensional, topographic map during development. Sperry posited a set of two orthogonal gradients in the retina that gives each cell a positional identity. He further suggested that these molecules could be used to match up with complementary gradients in the target field of the retinal projection, the tectum. While some investigators hold that the existence of such molecules may not be necessary to establish retinotectal maps, recent work has identified several cell surface proteins whose distributions are of the type predicted by Sperry. An unexpected twist comes from culture assays demonstrating that inhibitory activities on tectal membranes can guide the growth of processes from retinal neurons. Moreover, the expression patterns of several enzymes and three transcription factors suggest that these proteins are candidates for regulatory agents in the determination of cell position in the retina. In addition, results from perturbation experiments support the candidacy of two of the enzymes, and a new mutant screen has uncovered several as yet unidentified genes that are required for establishment of the proper retinotectal map. A number of these results were presented at a recent meeting on neurospecificity held in Cargese, Corsica and sponsored by NATO and NSF.

Strategies for rescue of retinal photoreceptor cells

A variety of genetic and environmental factors cause degeneration of retinal photoreceptors. This review focuses on current strategies to rescue defective, but still viable, rods and cones, including transplantation of normal retinal pigment epithelium cells, corrective gene therapy, administration of survival/growth factors, protection from the damaging effects of light, and dietary supplementation of vitamin A.

The role of spatio-temporal firing patterns in neuronal development of sensory systems

The emergence of precise and orderly sets of neuronal connections often depends upon coordinated electrical activity during the early stages of development. In recent years, an increasing number of reports have shown that neurons of immature sensory systems can spontaneously generate electrical activity that occurs synchronously amongst adjacent cells. These patterns of correlated activity seem to be well suited to the role of providing the cues that are necessary for the activity-dependent refinement of the neural connections in the developing visual, auditory and somatosensory pathways.