Genetic deletion of a neural cell adhesion molecule variant (N-CAM-180) produces distinct defects in the central nervous system

Expression of the Neural Cell Adhesion Molecule NCAM by Peptide- and Steroid-Producing Endocrine Cells and Tumors: Alternatively Spliced Forms and Polysialylation

The adhesive properties of neural cell adhesion molecules (NCAMs) can be modified by alternative splicing of the primary transcript or by posttranslational modifications, such as sialylation. In this article, we describe distinct forms of alternative splicing and posttranslational modification of the extracellular domain of NCAM of various endocrine tissues and derived tumor cells of the rat and of steroid- and peptide-hormone producing endocrine cells in humans. NCAM-140 is the major isoform expressed in the rat adrenal gland, adenohypophysis, and in granulosa and granulosa-lutein cells. NCAM-180 is predominant in the neurohypophysis. Polysialylated NCAM is expressed in different endocrine tissues and tumor cells of the rat. Different amounts of NCAM mRNA containing the "extra-exon" VASE at the exon 7/8 splice boundary were detected in endocrine cells of rats. Human granulosa cells in culture undergo luteinization. During this process, the VASE-containing NCAM isoform is supplemented by an alternatively spliced isoform without this insert. Thus, modifications of NCAM may be important for adhesive interactions in normal and neoplastic endocrine cells. In addition, the differential expression and the alternative splicing of NCAM during luteinization of granulosa cells raise the possibility that NCAM could be involved in folliculogenesis and the formation of the corpus luteum in humans.

Transient upregulation of NCAM mRNA in astrocytes in response to entorhinal cortex lesions and ischemia

Axonal sprouting and synaptic reorganization play an important role in the adaptation of the CNS to injury. However, the molecular mechanisms underlying this neuronal plasticity are poorly understood. In the present study we used in situ hybridization to examine the expression of NCAM mRNA in normal hippocampus, and in response to entorhinal cortex (EC) lesions and transient global ischemia. Both neurons and astrocytes were labeled by digoxygenin-tagged cRNA probes which recognize all three major NCAM isoforms of the adult CNS. In contrast, NCAM180-specific probes labeled only neurons in the hippocampus. After unilateral EC lesion, a transient and anatomically restricted upregulation of NCAM120/140 mRNA in reactive astrocytes in the denervated molecular layer of the dentate gyrus was observed. This increase was only present 2-4 days after the lesion whereas the GFAP mRNA increase was present up to 30 days postlesion. Following global ischemia a similar, transient increase of NCAM120/140 mRNA labeling of reactive astrocytes was observed; this increase was anatomically restricted to CA1, where neuronal loss occurred. Results suggest that the transient upregulation of NCAM120/140 mRNA in reactive astrocytes shortly after injury might be an important molecular mechanism in the cascade of events underlying neuronal plasticity in the adult CNS.

Telencephalin: a neuronal area code molecule?

Cell adhesion molecules (CAMs) with expression restricted to specific developmental and structural units of the brain and/or selective neuronal types would play critical roles in the formation of functional neuronal networks. In this article, we summarize recent progress in knowledge on a brain segment-specific CAM, telencephalin (TLN). TLN has the following characteristic properties. (1) TLN is a neuronal glycoprotein whose expression is restricted within telencephalon, the most rostal segment of the brain. (2) TLN is localized to the soma-dendritic membrane of subsets of telencephalic neurons, but not to the axonal membrane. (3) Abrupt appearance of TLN around birth parallels the timing of dendritic development and synapse formation in the telencephalon. (4) TLN belongs to the immunoglobulin superfamily and its structure is most closely related to intercellular adhesion molecules (ICAMs)-1 and -3. These findings suggest that TLN is the first example of dendrite-associated cell adhesion molecules (DenCAMs) and that TLN may be involved in the brain segmental organization, cell-cell interactions during dendritic development, and maintenance of functional neuronal networks. We discuss the possibility that TLN is an area code-like address signal that is displayed selectively by telencephalic neurons and is decoded by specific subsets of growing axons to make proper synaptic connections.

Interactions of the chondroitin sulfate proteoglycan phosphacan, the extracellular domain of a receptor-type protein tyrosine phosphatase, with neurons, glia, and neural cell adhesion molecules

Phosphacan is a chondroitin sulfate proteoglycan produced by glial cells in the central nervous system, and represents the extracellular domain of a receptor-type protein tyrosine phosphatase (RPTP zeta/beta). We previously demonstrated that soluble phosphacan inhibited the aggregation of microbeads coated with N-CAM or Ng-CAM, and have now found that soluble 125I-phosphacan bound reversibly to these neural cell adhesion molecules, but not to a number of other cell surface and extracellular matrix proteins. The binding was saturable, and Scatchard plots indicated a single high affinity binding site with a Kd of approximately 0.1 nM. Binding was reduced by approximately 15% after chondroitinase treatment, and free chondroitin sulfate was only moderately inhibitory, indicating that the phosphacan core glycoprotein accounts for most of the binding activity. Immunocytochemical studies of embryonic rat spinal phosphacan, Ng-CAM, and N-CAM have overlapping distributions. When dissociated neurons were incubated on dishes coated with combinations of phosphacan and Ng-CAM, neuronal adhesion and neurite growth were inhibited. 125I-phosphacan bound to neurons, and the binding was inhibited by antibodies against Ng-CAM and N-CAM, suggesting that these CAMs are major receptors for phosphacan on neurons. C6 glioma cells, which express phosphacan, adhered to dishes coated with Ng-CAM, and low concentrations of phosphacan inhibited adhesion to Ng-CAM but not to laminin and fibronectin. Our studies suggest that by binding to neural cell adhesion molecules, and possibly also by competing for ligands of the transmembrane phosphatase, phosphacan may play a major role in modulating neuronal and glial adhesion, neurite growth, and signal transduction during the development of the central nervous system.

Rapid expression and transport of embryonic N-CAM in dentate gyrus following entorhinal cortex lesion: ultrastructural analysis

Neural cell adhesion molecules are known to be important in axon guidance and synapse formation in the developing brain. The embryonic form of neural cell adhesion molecule (eN-CAM) is reexpressed in the outer molecular layer (OML) of the dentate gyrus following entorhinal cortex (ERC) lesion. Ultrastructural analysis revealed localization of eN-CAM to the membrane of granule-cell dendritic membranes and occasionally axons within the denervated zone. Because eN-CAM is expressed rapidly (within 2 days) after ERC lesion, we were interested in the temporal sequence of expression. Denervated hippocampi (12, 15, 24, and 48 hours post-ERC lesion) were stained with anti-eN-CAM and processed for immunoelectron microscopy. At 12 hours, there was no evidence of staining for eN-CAM. By 15 hours after lesion, membranes of both dendrites and axons throughout the molecular layer exhibited moderate eN-CAM staining, and dendritic cytoplasm was heavily labeled. Twenty-four hours following lesion, plasma membrane staining of eN-CAM on both axons and dendrites had increased in intensity within the OML, whereas membrane eN-CAM staining was diminished in the inner molecular layer (IML), and the intradendritic cytoplasmic staining disappeared. By 48 hours after lesion, eN-CAM staining had disappeared from the IML but remained intense and widely distributed in the OML. These findings suggest a rapid transport of de novo synthesized protein. A generalized reaction appears to occur immediately following denervation, and eN-CAM is up-regulated in the complete expanse of the dendritic membrane, despite the fact that only the OML is denervated.(ABSTRACT TRUNCATED AT 250 WORDS)

NCAM-dependent neurite outgrowth is inhibited in neurons from Fyn-minus mice

Src-related nonreceptor protein tyrosine kinases in nerve growth cones (p59fyn, pp60c-src, and pp62c-yes) are potential intracellular signaling molecules for cell adhesion molecule-directed axonal growth. To determine whether src-related tyrosine kinases mediate NCAM-dependent neurite outgrowth, cultures of cerebellar and sensory neurons from fyn-, src-, and yes- minus mice were analyzed for neurite outgrowth on monolayers of NCAM140-transfected L fibroblasts. NCAM-dependent neurite outgrowth was selectively inhibited in cultures of cerebellar and dorsal root ganglion neurons from fyn-, but not src- or yes- mice. Neurite outgrowth by fyn-, src-, or yes- neurons on untransfected fibroblast monolayers was unaffected, indicating that these kinases do not contribute significantly to axon growth on at least some integrins or other adhesive substrates present on fibroblasts. This study demonstrates that p59fyn is an essential component of the NCAM signaling pathway leading to axonal growth.

Genetic analysis of Fasciclin II in Drosophila: defasciculation, refasciculation, and altered fasciculation

The Drosophila neural cell adhesion molecule Fasciclin II (Fas II) is expressed dynamically on a subset of embryonic CNS axons, many of which selectively fasciculate in the vMP2, MP1, and FN3 pathways. Here we show complementary fasII loss-of-function and gain-of-function phenotypes. Loss-of-function fasII mutations lead to the complete or partial defasciculation of all three pathways. Gain-of-function conditions, using a specific control element to direct increased levels of Fas II on the axons in these three pathways, rescue the loss-of-function phenotype. Moreover, the gain-of-function can alter fasciculation by abnormally fusing pathways together, in one case apparently by preventing normal defasciculation. These results define an in vivo function for Fas II as a neuronal recognition molecule that controls one mechanism of growth cone guidance-selective axon fasciculation--and genetically separates this function from other aspects of outgrowth and directional guidance.

The regulation of kidney development: new insights from an old model

The embryonic kidney is an excellent model system in which to address many fundamental issues in developmental biology. Inductive interactions are required for proliferation and differentiation of the ureter epithelium and kidney mesenchyme. Recent studies implicate a receptor-type tyrosine kinase as a target of inductive signals in the developing ureter. In the mesenchyme, the early induction response requires at least two transcription factors, WT1 and Pax-2. Through the integrated application of in vitro culture models and gene targeting methods, the molecular mechanisms underlying kidney morphogenesis are becoming clearer.

Neural recognition molecules in disease and regeneration

The genetic analysis of inherited human diseases of the nervous system and the characterization of transgenic mice deficient in neural recognition molecules is opening up a new dimension in understanding the cellular and molecular mechanisms underlying neuro-developmental and -degenerative diseases, as well as in delineating the functions of recognition molecules in cell-cell interactions. Progress in identifying recognition molecules that inhibit neurite outgrowth and further characterization of the mechanisms that promote neurite outgrowth are shedding more light on the processes of regeneration in the mature nervous system. In the adult, recognition functions are fine-tuned by glycan moeities associated with neural recognition molecules, and successful neurite outgrowth is likely to depend on the delicate balance between growth-promoting and inhibitory recognition cues.

The arrest of luteinizing hormone-releasing hormone neuronal migration in the genetic arhinencephalic mouse embryo (Pdn/Pdn)

From previous observations, it was suggested that non-attachment of the olfactory nerve to the telencephalon blocked the induction of the olfactory bulbs in genetic arhinencephalic mouse embryos (Pdn/Pdn). The olfactory nerve ends in a tangle beneath the forebrain in these embryos. From these observations, we speculated that the migration of luteinizing hormone-releasing hormone (LHRH) neurons might be disturbed in the olfactory nerve. A mass of LHRH neurons was observed in the end of the olfactory nerve fibers, but LHRH neurons were found in the hypothalamus in Pdn/Pdn embryos on day 16 of gestation. Narrow by-paths were found between the olfactory nerve and the forebrain, and the migration of LHRH neurons through these by-paths was observed in Pdn/Pdn embryos on day 13 of gestation. From the reports that a gene deleted in the arhinencephalic syndrome (Kallmann's syndrome) shares homology with neural cell adhesion molecules (N-CAM), it was speculated that non-attachment of the olfactory nerve in the Pdn/Pdn embryo might be associated with abnormalities of N-CAM. The axon fibers of the olfactory nerve reacted specifically with anti-N-CAM IgG both in +/- (+/+ and/or Pdn/+) and Pdn/Pdn on day 11.5 and 12, but not on day 13 and 16 of gestation. The axon fibers of the olfactory nerve were positive to anti-N-CAM IgG specifically just during the developmental period that the olfactory nerve fibers attached to the telencephalon. It is still not clear whether non-attachment of the olfactory nerve may be associated with N-CAM or not from the present observations.

Expression of polysialylated neural cell adhesion molecule by proliferating cells in the subependymal layer of the adult rat, in its rostral extension and in the olfactory bulb

The highly sialylated isoform of the neural cell adhesion molecule is thought to be expressed predominantly in the developing nervous system, where it is implicated in a variety of dynamic events linked to neural morphogenesis. It has become increasingly evident, however, that this "embryonic" neural cell adhesion molecule isoform continues to be expressed in certain adult neuronal systems, and in particular, in those that can undergo structural plasticity. In the present study, we performed light microscopic immunocytochemistry with an antibody specific for polysialylated neural cell adhesion molecule and confirmed our earlier observations [Bonfanti L. et al. (1992) Neuroscience 49, 419-436] showing polysialylated neural cell adhesion molecule-immunoreactive cells in the subependymal layer of the lateral ventricle of the adult rat, a region where cell proliferation continues into the postnatal period. In addition, we used an antibody raised against the proliferating cell nuclear antigen and found that proliferating cells continue to be visible in this area, even in the adult. Double immunolabeling showed that many of these newly generated cells displayed high polysialylated neural cell adhesion molecule immunoreactivity. Cells from a portion of the subependymal layer migrate to the olfactory bulb and contribute to the continual replacement of its granule neurons [Luskin M. B. (1993) Neuron 11, 173-189]. We found polysialylated neural cell adhesion molecule-immunoreactive cells all along the pathway purported to be followed by the newly generated cells to their final destination and in neurons corresponding to granular and periglomerular cells in the olfactory bulb. Our present observations thus support the contention that polysialylation is a feature of neurons capable of dynamic change and may contribute to the molecular mechanisms permitting cell proliferation and migration not only during development but also in the adult.

Ectopic and increased expression of Fasciclin II alters motoneuron growth cone guidance

We used the enhancer detection/GAL4 system in Drosophila to direct increased levels of Fasciclin II (Fas II) expression on motoneuron growth cones and axons and to direct ectopic Fas II expression on other cells they encounter. Four classes of abnormal phenotypes are observed: "bypass" phenotypes, in which axons fail to defasciculate at the choice point where they would normally enter their muscle target region and instead extend past their target; "detour" phenotypes, in which these bypass growth cones enter their muscle target region at a different location; "stall" phenotypes, in which axons that enter their muscle target region fail to defasciculate from one another to probe their muscle targets; and "misroute" phenotypes, in which growth cones are diverted onto abnormal pathways by contact with Fas II-positive cells. These phenotypes show that changes in the pattern and level of Fas II expression can alter growth cone guidance, apparently in part by modulating the ability of these growth cones to respond to other guidance cues.

N-CAM mutation inhibits tangential neuronal migration and is phenocopied by enzymatic removal of polysialic acid

The mutation of N-CAM in mice produces a phenotype dominated by an undersized olfactory bulb and accumulation of precursors in the subependymal layer. We demonstrate here that this defect can be duplicated by injection of an enzyme that specifically destroys the polysialic acid (PSA) moiety associated with N-CAM. Studies of BrdU-labeled and pyknotic cells suggest that this defect reflects a decrease in the rostral migration of olfactory precursors and not a change in the proliferation or rate of death of these cells. In addition to their ectopic location, these cells had fewer growth cone-like processes oriented along the migration route. In contrast to tangential movement, radial migration of granule cells in the olfactory bulb was not affected by loss of PSA. These results support the proposed role for PSA in cell translocation, discriminate between different mechanisms of cell migration, and provide insight as to the nature of the N-CAM mutant phenotype.

Recognition, adhesion, transmembrane signaling and cell motility in guided neuronal migration

Recent studies indicate that migration of neurons from their place of origin to their final destination requires the orchestration of multiple molecular events, including the selection of a pathway by cell recognition receptors, the formation of adhesive interactions with cellular and extracellular substrates through multiple adhesion molecules and the activation of specific ion channels and receptors that provide second messenger mediated signals for the diverse cellular mechanisms involved in cell motility. New approaches allow for the examination of the role of individual molecular components that mediate these processes.