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ABSTRACT Background: Pyrrolizidine alkaloids (PAs) are commonly found in many plants including those used in medical therapeutics. The hepatotoxicities of PAs have been demonstrated both in vivo and in vitro; [...]

Angiotensin (Ang) II plays a critical role in cardiovascular homeostasis and neuroendocrine regulation. Little is known about whether central angiotensin-converting enzyme (ACE) is functional in the fetal brain. We investigated cardiovascular and neuroendocrinological responses to intracerebroventricular (icv) application of Ang I in the chronically prepared near-term ovine fetus in utero and examined the action sites marked by c-fos expression in the fetal hypothalamus. ACE mRNA was detected in the specific central areas. Intracerebroventricular Ang I significantly increased fetal blood pressure and c-fos expression in the supraoptic nuclei (SON) and the paraventricular nuclei (PVN) in the hypothalamus, accompanied by an increase of fetal plasma arginine vasopressin (AVP). Double labeling demonstrated that AVP neurons in the fetal SON and PVN were expressing c-fos. Captopril, an inhibitor of ACE, significantly suppressed fetal pressor responses and plasma AVP. Double labeling experiments showed colocalization of [AT.sub.1] receptor ([AT.sub.1]R) and c-fos expression in both SON and PVN following icv Ang I. The results indicate that central endogenous ACE has been functional at least at the last third of gestation and the endogenous brain renin-angiotensin system-mediated pressor responses and AVP release via [AT.sub.1]Rs by acting at the sites consistent with the cardiovascular network in the hypothalamus. arginine vasopressin; vasopressin; brain development doi: 10.1152/ajpendo.00060.2010.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2009.06.015 Byline: Camilla Larsen, Diana Shy, Shana R. Spindler, Siaumin Fung, Wayne Pereanu, Amelia Younossi-Hartenstein, Volker Hartenstein Keywords: Drosophila; Brain; Lineage; Pathfinding; Connectivity Abstract: The Drosophila central brain is composed of approximately 100 paired lineages, with most lineages comprising 100-150 neurons. Most lineages have a number of important characteristics in common. Typically, neurons of a lineage stay together as a coherent cluster and project their axons into a coherent bundle visible from late embryo to adult. Neurons born during the embryonic period form the primary axon tracts (PATs) that follow stereotyped pathways in the neuropile. Apoptotic cell death removes an average of 30-40% of primary neurons around the time of hatching. Secondary neurons generated during the larval period form secondary axon tracts (SATs) that typically fasciculate with their corresponding primary axon tract. SATs develop into the long fascicles that interconnect the different compartments of the adult brain. Structurally, we distinguish between three types of lineages: PD lineages, characterized by distinct, spatially separate proximal and distal arborizations; C lineages with arborizations distributed continuously along the entire length of their tract; D lineages that lack proximal arborizations. Arborizations of many lineages, in particular those of the PD type, are restricted to distinct neuropile compartments. We propose that compartments are 'scaffolded' by individual lineages, or small groups thereof. Thereby, the relatively small number of primary neurons of each primary lineage set up the compartment map in the late embryo. Compartments grow during the larval period simply by an increase in arbor volume of primary neurons. Arbors of secondary neurons form within or adjacent to the larval compartments, resulting in smaller compartment subdivisions and additional, adult specific compartments. Author Affiliation: Department of Molecular Cell and Developmental Biology, University of California Los Angeles, Los Angeles, CA 90095, USA Article History: Received 15 April 2009; Revised 9 June 2009; Accepted 11 June 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2009.07.035 Byline: Shana R. Spindler, Irma Ortiz, Siaumin Fung, Shigeo Takashima, Volker Hartenstein Keywords: Glia; Drosophila; Brain; Lineage Abstract: Glial cells play important roles in the developing brain during axon fasciculation, growth cone guidance, and neuron survival. In the Drosophila brain, three main classes of glia have been identified including surface, cortex, and neuropile glia. While surface glia ensheaths the brain and is involved in the formation of the blood-brain-barrier and the control of neuroblast proliferation, the range of functions for cortex and neuropile glia is less well understood. In this study, we use the nirvana2-GAL4 driver to visualize the association of cortex and neuropile glia with axon tracts formed by different brain lineages and selectively eliminate these glial populations via induced apoptosis. The larval central brain consists of approximately 100 lineages. Each lineage forms a cohesive axon bundle, the secondary axon tract (SAT). While entering and traversing the brain neuropile, SATs interact in a characteristic way with glial cells. Some SATs are completely invested with glial processes; others show no particular association with glia, and most fall somewhere in between these extremes. Our results demonstrate that the elimination of glia results in abnormalities in SAT fasciculation and trajectory. The most prevalent phenotype is truncation or misguidance of axon tracts, or abnormal fasciculation of tracts that normally form separate pathways. Importantly, the degree of glial association with a given lineage is positively correlated with the severity of the phenotype resulting from glial ablation. Previous studies have focused on the embryonic nerve cord or adult-specific compartments to establish the role of glia. Our study provides, for the first time, an analysis of glial function in the brain during axon formation and growth in larval development. Author Affiliation: Department of Molecular Cell and Developmental Biology, University of California Los Angeles, Los Angeles, CA 90095, USA Article History: Received 10 June 2009; Revised 23 July 2009; Accepted 23 July 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2008.12.029 Byline: Angel Gato (a), Mary E. Desmond (b) Keywords: Neurogenesis; Neural stem cells; Neural tube; Hydrostatic pressure; Ventricle expansion; Osmotic gradient; Proteoglycans; Growth factors; Mitosis Abstract: The key focus of this review is that both the neuroepithelium and embryonic cerebrospinal fluid (CSF) work in an integrated way to promote embryonic brain growth, morphogenesis and histiogenesis. The CSF generates pressure and also contains many biologically powerful trophic factors; both play key roles in early brain development. Accumulation of fluid via an osmotic gradient creates pressure that promotes rapid expansion of the early brain in a developmental regulated way, since the rates of growth differ between the vesicles and for different species. The neuroepithelium and ventricles both contribute to this growth but by different and coordinated mechanisms. The neuroepithelium grows primarily by cell proliferation and at the same time the ventricle expands via hydrostatic pressure generated by active transport of Na.sup.+ and transport or secretion of proteins and proteoglycans that create an osmotic gradient which contribute to the accumulation of fluid inside the sealed brain cavity. Recent evidence shows that the CSF regulates relevant aspects of neuroepithelial behavior such as cell survival, replication and neurogenesis by means of growth factors and morphogens. Here we try to highlight that early brain development requires the coordinated interplay of the CSF contained in the brain cavity with the surrounding neuroepithelium. The information presented is essential in order to understand the earliest phases of brain development and also how neuronal precursor behavior is regulated. Author Affiliation: (a) Departamento de Anatomia y Radiologia, Facultad de Medicina, Universidad de Valladolid, Avda Ramon y Cajal 7, 47005 Valladolid, EspaA[+ or -]a (b) Department of Biology, Mendel Science Center, Villanova University, Villanova, PA 19085, USA Article History: Received 15 July 2008; Revised 10 November 2008; Accepted 17 December 2008

Retinal neurogenesis ceases by the early postnatal period, although retinal progenitor cells (RPCs) persist throughout life. In this study, we show that in the mammalian eye, the function of Toll-like receptor 4 (TLR4) extends beyond regulation of the innate immune response; it restricts RPC proliferation. In TLR4-deficient mice, enhanced proliferation of cells reminiscent of RPCs is evident during the early postnatal period. In vitro experiments demonstrate that TLR4 acts as an intrinsic regulator of RPC fate decision. Increased TLR4 expression in the eye correlates with the postnatal cessation of cell proliferation. However, deficient TLR4 expression is not sufficient to extend the proliferative period but rather contributes to resumption of proliferation in combination with growth factors. Proliferation in vivo is inhibited by both MyD88-dependent and -independent pathways, similar to the mechanisms activated by TLR4 in immune cells. Thus, our study attributes a novel role to TLR4 as a negative regulator of RPC proliferation.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2008.05.524 Byline: Yoshiharu Sato, Robert O. Heuckeroth Keywords: Enteric nervous system; Vitamin A; Neurite growth; Retinoids; Ubiquitin ligase Abstract: Enteric nervous system (ENS) precursors undergo a complex process of cell migration, proliferation, and differentiation to form an integrated network of neurons and glia within the bowel wall. Although retinoids regulate ENS development, molecular and cellular mechanisms of retinoid effects on the ENS are not well understood. We hypothesized that retinoids might directly affect ENS precursor differentiation and proliferation, and tested that hypothesis using immunoselected fetal ENS precursors in primary culture. We now demonstrate that all retinoid receptors and many retinoid biosynthetic enzymes are present in the fetal bowel at about the time that migrating ENS precursors reach the distal bowel. We further demonstrate that retinoic acid (RA) enhances proliferation of subsets of ENS precursors in a time-dependent fashion and increases neuronal differentiation. Surprisingly, however, enteric neurons that develop in retinoid deficient media have dramatically longer neurites than those exposed to RA. This difference in neurite growth correlates with increased RhoA protein at the neurite tip, decreased Smurf1 (a protein that targets RhoA for degradation), and dramatically decreased Smurf1 mRNA in response to RA. Collectively these data demonstrate diverse effects of RA on ENS precursor development and suggest that altered fetal retinoid availability or metabolism could contribute to intestinal motility disorders. Author Affiliation: Department of Pediatrics, Department of Developmental Biology, Washington University School of Medicine, 660 South Euclid Avenue, Box 8208, St. Louis MO 63110, USA Article History: Received 14 January 2008; Revised 1 May 2008; Accepted 2 May 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2007.12.041 Byline: Jung-Il Moon, Susan J. Birren Keywords: Sympathetic neuron; Bone morphogenetic protein; Basic helix-loop-helix transcription factor; Matrix Gla protein; Cardiac myocyte; Target contact; Microarray Abstract: Target-derived factors modulate many aspects of peripheral neuron development including neuronal growth, survival, and maturation. Less is known about how initial target contact regulates changes in gene expression associated with these developmental processes. One early consequence of contact between growing sympathetic neurons and their cardiac myocyte targets is the inhibition of neuronal outgrowth. Analysis of neuronal gene expression following this contact revealed coordinate regulation of a bone morphogenetic protein (BMP)-dependent growth pathway in which basic helix-loop-helix transcription factors and downstream neurofilament expression contribute to the growth dynamics of developing sympathetic neurons. BMP2 had dose-dependent growth-promoting effects on sympathetic neurons cultured in the absence, but not the presence, of myocyte targets, suggesting that target contact alters neuronal responses to BMP signaling. Target contact also induced the expression of matrix Gla protein (MGP), a regulator of BMP function in the vascular system. Increased MGP expression inhibited BMP-dependent neuronal growth and MGP expression increased in sympathetic neurons during the period of target contact in vivo. These experiments establish MGP as a novel regulator of BMP function in the nervous system, and define developmental transitions in BMP responses during sympathetic development. Author Affiliation: Department of Biology, National Center for Behavioral Genomics, Brandeis University, Waltham, MA 02254-9110, USA Article History: Received 10 July 2007; Revised 19 December 2007; Accepted 29 December 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2007.08.037 Byline: Manish Rauthan (a), Catarina Morck (b), Marc Pilon (b) Keywords: C. elegans; Pharynx; Kruppel-like factor; mnm-2; Sp1 factor; Neuron differentiation; Axon guidance; Growth cone Abstract: The invariant cell-cell interactions occurring during C. elegans development offer unique opportunities to discover how growing axons may receive guidance cues from neighboring cells. The mnm-2 mutant was isolated because of its defects in the axon trajectory of the bilateral M2 pharyngeal neurons in C. elegans. We found that mnm-2 enhances the effects of many growth cone guidance mutations on these axons, suggesting that it performs a novel function during axon guidance. We cloned mnm-2 and found that it encodes a protein with three C2H2 zinc finger domains related to the Kruppel-like Factor protein family. mnm-2 is expressed only transiently in the M2 neuron, but exhibits a sustained expression in its sister cell, the M3 neuron. Strikingly, the expression of mnm-2 is not sustained in the M3 cell of the mnm-2 mutant, indicating that this gene positively regulates itself in that cell. Electropharyngeograms also indicate that the M3 cell is functionally impaired in the mnm-2 mutant. We used an M3-specific promoter to show that the M2 axon defect can be rescued by expression of mnm-2 in its sister cell M3. The same promoter was used to express the pro-apoptotic gene egl-1 to kill the M3 cell, which resulted in an M2 axon guidance defect similar to that found in the mnm-2 mutant. Our results suggest an M2 axon guidance model in which the M3 cell provides an important signal to the growth cone of its sister M2 and that this signal and the proper differentiation of M3 both depend on mnm-2 expression. This mechanism of axon guidance regulation allows fine-tuning of trajectories between sister cells. Author Affiliation: (a) Department of Chemical and Biological Engineering, Chalmers University, S-405 30, Goteborg, Sweden (b) Department of Cell Mol Biol, Goteborg University, S-405 30 Goteborg, Sweden Article History: Received 10 April 2007; Revised 14 August 2007; Accepted 20 August 2007

The fibroblast growth factor binding protein (FGF-BP; GenBank accession no. NP_005121) is a secreted protein that mobilizes FGFs from the extracellular matrix, protects them from degradation, and enhances their biological activity. Several previous studies reported that FGF-BP is an early response gene upregulated during tissue repair processes including wound healing and atherogenesis. In this study we analyzed whether FGF-BP expression was impacted by spinal cord injury and could have an effect on neuronal cell viability. Immunohistochemical and in situ hybridization studies revealed a dramatic upregulation of FGF-BP protein and mRNA levels following unilateral hemisection and contusion injury of adult rat spinal cord. In spinal cord sections of laminectomized rats, increased FGF-BP expression was observed in the fibers and cell bodies ipsilateral to the lesion site but was absent in the uninjured spinal cord tissue contralateral to the lesion. Increased expression of FGF-BP was observed at all postinjury time points, examined with peak levels occurring at day 4, a time when injury-induced increased levels of FGF2 have also been reported to be maximal. Moreover, using PC12 cells as a neuronal model, we observed that exogenous FGF-BP increased the capacity of FGF2 to stimulate neurite outgrowth and to increase cell survival. At the molecular level, FGF-BP enhanced FGF2-induced protein tyrosine phosphorylation and AKT/PKB activation. Collectively, these results suggest that FGF-BP is an early response gene after spinal cord injury and that its upregulation in regenerating spinal cord tissue may provide a molecular mechanism for enhancing the initial FGF2-mediated neurotrophic effects occurring after such tissue damage. PC12 cells; neurite outgrowth; apoptosis

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2007.01.051 Byline: Chen-bing Guan (1)(2), Hua-tai Xu (1), Ming Jin (1), Xiao-bing Yuan (1), Mu-ming Poo (1)(3) Keywords: CELLBIO; MOLNEURO; DEVBIO Abstract: Neuronal migration and growth-cone extension are both guided by extracellular factors in the developing brain, but whether these two forms of guidance are mechanistically linked is unclear. Application of a Slit-2 gradient in front of the leading process of cultured cerebellar granule cells led to the collapse of the growth cone and the reversal of neuronal migration, an event preceded by a propagating Ca.sup.2+ wave from the growth cone to the soma. The Ca.sup.2+ wave was required for the Slit-2 effect and was sufficient by itself to induce the reversal of migration. The Slit-2-induced reversal of migration required active RhoA, which was accumulated at the front of the migrating neuron, and this polarized RhoA distribution was reversed during the migration reversal induced by either the Slit-2 gradient or the Ca.sup.2+ wave. Thus, long-range Ca.sup.2+ signaling coordinates the Slit-2-induced changes in motility at two distant parts of migrating neurons by regulating RhoA distribution. Author Affiliation: (1) Institute of Neuroscience and Key Laboratory of Neurobiology, Shanghai Institute of Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China (2) Graduate School of the Chinese Academy of Sciences, Shanghai, 200031, China (3) Division of Neurobiology, Department of Molecular and Cell Biology, Helen Wills Institute of Neuroscience, University of California, Berkeley, CA 94720-3200, USA Article History: Received 19 July 2006; Revised 16 November 2006; Accepted 26 January 2007 Article Note: (miscellaneous) Published: April 19, 2007

Evidence has shown that late onset of neuronal cell loss generally occurs via autophagy, which is a process in which eukaryotic cells self-digest parts of their contents during development or to survive starvation. The analysis has suggested that overactivation of autophagy in the cells of the nervous system is the eventual cause of physiological death.

The contribution of RhoA gene in creating equilibrium between planar and apico-basal cell divisions during neurogenesis in chicks is studied.

The role of retinoid signalling and retinoic acid receptor in neurite outgrowth from neurons in hedgehog is studied.

A study examines the production of cells in the neuronal of embryo of mouse and the factors that affect its development.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2006.02.018 Byline: Jennifer M. Dmetrichuk, Robert L. Carlone, Gaynor E. Spencer Keywords: Regeneration; Electrophysiology; Lymnaea stagnalis; Invertebrate; Retinoids; Electrical excitability; Trophic factors; Chemotaxis; Neurite outgrowth; Cell culture Abstract: Identification of molecules involved in neurite outgrowth during development and/or regeneration is a major goal in the field of neuroscience. Retinoic acid (RA) is a biologically important metabolite of vitamin A that acts as a trophic factor and has been implicated in neurite outgrowth and regeneration in many vertebrate species. Although abundant in the CNS of many vertebrates, the precise role of RA in neural regeneration has yet to be determined. Moreover, very little information is available regarding the role of RA in invertebrate nervous systems. Here, we demonstrate for the first time that RA induces neurite outgrowth from invertebrate neurons. Using individually identified neurons isolated from the CNS of Lymnaea stagnalis, we demonstrated that a significantly greater proportion of cells produced neurite outgrowth in RA. RA also extended the duration of time that cells remained electrically excitable in vitro, and we showed that exogenously applied RA acted as a chemoattractive factor and induced growth cone turning toward the source of RA. This is the first demonstration that RA can induce turning of an individual growth cone. These data strongly suggest that the actions of RA on neurite outgrowth and cell survival are highly conserved across species. Author Affiliation: Department of Biology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, Canada L2S 3A1 Article History: Received 24 August 2005; Revised 22 November 2005; Accepted 14 February 2006

The role of Brg1 (brahma-related gene 1) for the sustenance of the neural stem cells and development of glial cells in mouse embryos is studied.

The contribution of vascular endothelial growth factor-B in regulating neurogenesis in the knockout mice brain is studied.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2005.05.002 Byline: Luz M. MuA[+ or -]oz, Andrew Zayachkivsky, Richard B. Kunz, James M.E. Hunt, Guoying Wang, Sheryl A. Scott Keywords: Chick; Limb; Sensory neuron; DRG; Ephrin; Eph receptor; Axon guidance; Development; Skin; Dorsal root ganglion Abstract: EphA-ephrin signaling has recently been implicated in the establishment of motor innervation patterns, in particular in determining whether motor axons project into dorsal versus ventral nerve trunks in the limb. We investigated whether sensory axons, which grow out together with and can be guided by motor axons, are also influenced by Eph-ephrin signaling. We show that multiple EphA receptors are expressed in DRGs when limb innervation is being established, and EphA receptors are present on growth cones of both NGF-dependent (predominantly cutaneous) and NT3-dependent (predominantly proprioceptive) afferents. Both soluble and membrane-attached ephrin-A5 inhibited growth of approximately half of each population of sensory axons in vitro. On average, growth cones that collapsed in response to soluble ephrin-A5 extended more slowly than those that did not, and ephrin-A5 significantly slowed the extension of NGF-dependent growth cones that did not collapse. Finally, we show that ectopic expression of ephrin-A5 in ovo reduced arborization of cutaneous axons in skin on the limb. Together these results suggest that sensory neurons respond directly to A-class ephrins in the limb. Thus, ephrins appear to pattern sensory axon growth in two ways -- both directly, and indirectly via their inhibitory effects on neighboring motor axons. Author Affiliation: Department of Neurobiology and Anatomy, University of Utah School of Medicine, 20 North 1900 East, Salt Lake City, UT 84132, USA Article History: Received 14 January 2005; Revised 3 May 2005; Accepted 4 May 2005

The regulation of tyrosine phosphorylation is recognized as an important developmental mechanism. Both addition and removal of phosphate moieties on tyrosine residues are tightly regulated during development. Originally, most attention focused on the role of tyrosine kinases during development, but more recently, the developmental importance of tyrosine phosphatases has been gaining interest. Receptor protein tyrosine phosphatases (RPTPs) are of particular interest to developmental biologists because the extracellular domains of RPTPs are similar to those of cell adhesion molecules (CAMs). This suggests that RPTPs may have functions in development similar to CAMs. This review focuses on the role of RPTPs in development of the nervous system in processes such as axon guidance, synapse formation, and neural tissue morphogenesis. Keywords: Protein tyrosine phosphatase; Axon guidance; Neural development

To investigate the role of brain-derived neurotrophic factor (BDNF) in differentiation of cranial sensory neurons in vivo, we analyzed development of nodose (NG), petrosal (PG), and vestibular (VG) ganglion cells in genetically engineered mice carrying null mutations in the genes encoding BDNF and the proapoptotic Bcl-2 homolog Bax. In [bax.sup.-/-] mutants, ganglion cell numbers were increased significantly compared to wild-type animals, indicating that naturally occurring cell death in these ganglia is regulated by Bax signaling. Analysis of [bdnf.sup.-/-] [bax.sup.-/-] mutants revealed that, although the Bax null mutation completely rescued cell loss in the absence of BDNF, it did not rescue the lethality of the BDNF null phenotype. Moreover, despite rescue of BDNF-dependent neurons by the bax null mutation, sensory target innervation was abnormal in double null mutants. Vagal sensory innervation to baroreceptor regions of the cardiac outflow tract was completely absent, and the density of vestibular sensory innervation to the cristae organs was markedly decreased, compared to wild-type controls. Moreover, vestibular afferents failed to selectively innervate their hair cell targets within the cristae organs in the double mutants. These innervation failures occurred despite successful navigation of sensory fibers to the peripheral field, demonstrating that BDNF is required locally for afferent ingrowth into target tissues. In addition, the box null mutation failed to rescue expression of the dopaminergic phenotype in a subset of NG and PG neurons. These data demonstrate that BDNF signaling is required not only to support survival of cranial sensory neurons, but also to regulate local growth of afferent fibers into target tissues and, in some cells, transmitter phenotypic expression is required. Keywords: Neurotrophin; Cell death; Differentiation; Transmitter expression; Target innervation; Vestibular; Nodose; Petrosal

Neuronal migration is required for the establishment of specific neural structures, such as layers and nuclei. Neurons migrate along specific migratory routes toward their final destinations, sometimes across long distances. However, the cellular and molecular interactions that control neuronal migration are largely unknown. Here, we examined the mechanism underlying the transmedian migration of precerebellar neurons using a flat whole-mount preparation of the rat embryo. These neurons were initially attracted by the floor plate (FP) at the ventral midline. However, after crossing the midline, they lost their responsiveness to the FP and became attracted by the alar plate (AP). Although the loss of responsiveness to FP cues was caused by an encounter of migrating cells with the FP, the gain of responsiveness to AP cues occurred irrespective of their encounter with the FP. These results identify a crucial change in the response of migrating cells to attractive guidance cues during the transmedian migration of precerebellar neurons. Key Words: tangential migration; transmedian migration; precerebellar neurons; floor plate; alar plate; netrin-1; attraction.

Previous studies in the chick embryo have shown that sensory neurons fail to innervate muscle in the absence of motor neurons. Instead, motor neuron deletion causes more sensory axons to project to the skin. We used this experimental paradigm to determine when sensory neurons are specified to become proprioceptive afferents. Experimental embryos were treated with either saline or exogenous neurotrophin-3 (NT-3) to promote the survival of proprioceptive afferents. In saline-treated embryos, motor neuron deletion caused an increase in sensory neuron apoptosis on the deleted side, an effect reversed by NT3. Motor neuron deletion also eliminated the sartorious muscle nerve, as previously reported. In NT3-treated embryos, this altered nerve pattern was accompanied by the enlargement of the adjacent cutaneous nerve. These embryos were further analyzed by using immunohistochemistry for trkC (a receptor for NT3) retrograde and transganglionic labeling. Our results show that, following motor neuron deletion, more trkC+ afferents project in cutaneous nerves on the deleted side of NT3-treated embryos. Transganglionic labeling demonstrated that at least some of these neurons made spinal projections that are typical of proprioceptive afferents. These results therefore indicate that the proprioceptive phenotype is specified prior to target innervation and that these neurons can retain their identity despite projecting to inappropriate (cutaneous) targets. Key Words: neurotrophins; neurotrophin-3; sensory neurons; cutaneous afferents; muscle afferents; proprioceptive afferents; motor neurons; spinal cord projections; fate specification; differentiation.

The cutaneous sensory neurons of the ophthalmic lobe of the trigeminal ganglion are derived from two embryonic cell populations, the neural crest and the paired ophthalmic trigeminal (opV) placodes. Pax3 is the earliest known marker of opV placode ectoderm in the chick. Pax3 is also expressed transiently by neural crest cells as they emigrate from the neural tube, and it is reexpressed in neural crest cells as they condense to form dorsal root ganglia and certain cranial ganglia, including the trigeminal ganglion. Here, we examined whether Pax3+ opV placode-derived cells behave like Pax3+ neural crest cells when they are grafted into the trunk. Pax3+ quail opV ectoderm cells associate with host neural crest migratory streams and form Pax3+ neurons that populate the dorsal root and sympathetic ganglia and several ectopic sites, including the ventral root. Pax3 expression is subsequently downregulated, and at E8, all opV ectoderm-derived neurons in all locations are large in diameter, and virtually all express TrkB. At least some of these neurons project to the lateral region of the dorsal horn, and peripheral quail neurites are seen in the dermis, suggesting that they are cutaneous sensory neurons. Hence, although they are able to incorporate into neural crest-derived ganglia in the trunk, Pax3+ opV ectoderm cells are committed to forming cutaneous sensory neurons, their normal fate in the trigeminal ganglion. In contrast, Pax3 is not expressed in neural crest-derived neurons in the dorsal root and trigeminal ganglia at any stage, suggesting either that Pax3 is expressed in glial cells or that it is completely downregulated before neuronal differentiation. Since Pax3 is maintained in opV placode-derived neurons for some considerable time after neuronal differentiation, these data suggest that Pax3 may play different roles in opV placode cells and neural crest cells.

Results reveal that endogenous progenitors activation leads to regeneration of hippocampal pyramidal neurons following ischemic brain injury. Data indicate that regenerated neurons integrate into the brain circuitry, which is augmented by infusion of growth factors. Implications of the findings in the therapies for stroke and neurological diseases are discussed.

This review discusses some new strategies for therapeutically enhancing the regenerative capacity of the central nervous system. Research shows that the adult brain can make use of stem cells to generate functional neurons. Implications of the findings for repair of the central nervous system injuries and treatment of neurodegenerative diseases are examined.

Inferior olivary neurons (ION) migrate circumferentially around the caudal rhombencephalon starting from the alar plate to locate ventrally close to the floor-plate, ipsilaterally to their site of proliferation. The floor-plate constitutes a source of diffusible factors. Among them, netrin-1 is implied in the survival and attraction of migrating ION in vivo and in vitro. We have looked for a possible involvement of slit-l/2 during ION migration. We report that: (1) slit-1 and slit-2 are coexpressed in the floor-plate of the rhombencephalon throughout ION development; (2) robo-2, a slit receptor, is expressed in migrating ION, in particular when they reach the vicinity of the floor-plate; (3) using in vitro assays in collagen matrix, netrin-1 exerts an attractive effect on ION leading processes and nuclei; (4) slit has a weak repulsive effect on ION axon outgrowth and no effect on migration by itself, but (5) when combined with netrin-1, it antagonizes part of or all of the effects of netrin-1 in a dose-dependent manner, inhibiting the attraction of axons and the migration of cell nuclei. Our results indicate that slit silences the attractive effects of netrin-1 and could participate in the correct ventral positioning of ION, stopping the migration when cell bodies reach the floor-plate. Key Words: nuclear translocation; axon outgrowth; brainstem; floor-plate; precerebellar nuclei; chemotropic molecules; robo receptors; collagen assays; in situ hybridization; mice.

Transient elevations of intracellular calcium (calcium transients) play critical roles in many developmental processes, including differentiation. Although the factors that regulate calcium transients are not clearly defined, calcium influx may be controlled by molecules interacting with calcium channels, including channel regulatory subunits. Here, we describe the chick [gamma]4 regulatory subunit (CACNG4), the first such subunit to be characterized in early development. CACNG4 is expressed early in the cranial neural plate, and later in the cranial and dorsal root ganglia; importantly, the timing of this later expression correlates precisely with the onset of neuronal differentiation. CACNG4 expression is also observed in nonneuronal tissues undergoing differentiation, specifically the myotome and a subpopulation of differentiating myoblasts in the limb bud. Finally, within the distal cranial ganglia, we show that CACNG4 is expressed in placode-derived cells (prospective neurons), but also, surprisingly, in neural crest-derived cells, previously shown to form only glia in this location; contrary to these previous results, we find that neural crest cells can form neurons in the distal ganglia. Given the proposed role of CACNG4 in modulating calcium channels and its expression in differentiating cells, we suggest that CACNG4 may promote differentiation via regulation of intracellular calcium levels. Key Words: CACNG4 ([gamma]4); calcium channel; neuronal differentiation; myoblasts; cranial ganglia; placodes.

Research has been conducted on the D-erythro-homoceramide analogues. The synthesis of these analogues has been investigated and the results indicate that due to the lack of metabolism to glucosylhomoceramide none of the tested compounds have exhibited any biological activity.

Axonal retrograde transport is essential for neuronal growth and survival. However, the nature and dynamics of the membrane compartments involved in this process are poorly characterized. To shed light on this pathway, we established an experimental system for the visualization and the quantitative study of retrograde transport in living motor neurons based on a fluorescent fragment of tetanus toxin (TENT [H.sub.c]). Morphological and kinetic analysis of TeNT [H.sub.c] retrograde carriers reveals two major groups of organelles: round vesicles and fast tubular structures. TeNT [H.sub.c] carriers lack markers of the classical endocytic pathway and are not acidified during axonal transport. Importantly, TeNT [H.sub.c] and NGF share the same retrograde transport organelles, which are characterized by the presence of the neurotrophin receptor [p7.sup.5NTR]. Our results provide the first direct visualization of retrograde transport in living motor neurons, and reveal a novel retrograde route that could be used both by physiological ligands (i.e., neurotrophins) and TeNT to enter the central nervous system.

The axon stability is mediated by p190 Rho GTpase activating protein as indicated by axon branch retraction following p190 inactivation. Data suggest that a retraction pathway from RhoA to myosin in maturing neurons is repressed by p190.

Neurotrophins play an essential role in the regulation of actin-dependent changes in growth cone shape and motility. We have studied whether neurotrophin signaling can promote the localization of beta-actin mRNA and protein within growth cones. The regulated localization of specific mRNAs within neuronal processes and growth cones could provide a mechanism to modulate cytoskeletal composition and growth cone dynamics during neuronal development. We have previously shown that beta-actin mRNA is localized in granules that were distributed throughout processes and growth cones of cultured neurons. In this study, we demonstrate that the localization of beta-actin mRNA and protein to growth cones of forebrain neurons is stimulated by neurotrophin-3 (NT-3). A similar response was observed when neurons were exposed to forskolin or db-cAMP, suggesting an involvement of a cAMP signaling pathway. NT-3 treatment resulted in a rapid and transient stimulation of PKA activity that preceded the localization of beta-actin mRNA. Localization of beta-actin mRNA was blocked by prior treatment of cells with Rp-cAMP, an inhibitor of cAMP-dependent protein kinase A. Depolymerization of microtubules, but not microfilaments, inhibited the NT-3 localization of beta-actin mRNA. These results suggest that NT-3 activates a cAMP-dependent signaling mechanism to promote the microtubule-dependent localization of beta-actin mRNA within growth cones. Key words: MRNA localization, beta-actin, neurotrophin, growth cone, neurite outgrowth