A novel action of collapsin: collapsin-1 increases antero- and retrograde axoplasmic transport independently of growth cone collapse

Repulsive axon guidance molecule Sema3A inhibits branching morphogenesis of fetal mouse lung

Semaphorin III/collapsin-1 (Sema3A) guides a specific subset of neuronal growth cones as a repulsive molecule. In this study, we have investigated a possible role of non-neuronal Sema3A in lung morphogenesis. Expression of mRNAs of Sema3A and neuropilin-1 (NP-1), a Sema3A receptor, was detected in fetal and adult lungs. Sema3A-immunoreactive cells were found in airway and alveolar epithelial cells of the fetal and adult lungs. Immunoreactivity for NP-1 was seen in fetal and adult alveolar epithelial cells as well as endothelial cells. Immunoreactivity of collapsin response mediator protein CRMP (CRMP-2), an intracellular protein mediating Sema3A signaling, was localized in alveolar epithelial cells, nerve tissue and airway neuroendocrine cells. The expression of CRMP-2 increased during the fetal, neonate and adult periods, and this pattern paralleled that of NP-1. In a two-day culture of lung explants from fetal mouse lung (E11.5), with exogenous Sema3A at a dose comparable to that which induces growth cone collapse of dorsal root ganglia neurons, the number of terminal buds was reduced in a dose-dependent manner when compared with control or untreated lung explants. This decrease was not accompanied with any alteration of the bromodeoxyuridine-positive DNA-synthesizing fraction. A soluble NP-1 lacking the transmembrane and intracellular region, neutralized the inhibitory effect of Sema3A. The fetal lung explants from neuropilin-1 homozygous null mice grew normally in vitro regardless of Sema3A treatment. These results provide evidence that Sema3A inhibits branching morphogenesis in lung bud organ cultures via NP-1 as a receptor or a component of a possible multimeric Sema3A receptor complex.

Widespread expression of rat collapsin response-mediated protein 4 in the telencephalon and other areas of the adult rat central nervous system

The rat collapsin response-mediated protein 4 (rCRMP-4) is a member of a family of proteins that are involved in axonal growth. It is found transiently in postmitotic neurons, such as those that are generated in the adult hippocampus. The authors used immunocytochemistry to investigate whether areas of the rat central nervous system (CNS) that retain postnatal neurogenesis express this protein. They found pronounced rCRMP-4 immunoreactivity in recently generated cells in the dentate granular layer, the subventricular zone, the olfactory bulbs, and the rostral migratory stream, four areas in which the production or migration of neurons occurs in adulthood. However, rCRMP-4 immunoreactivity also is expressed in many other regions of the rat brain in which there is no record of adult neurogenesis or neuronal migration, e.g., in the olfactory glomeruli and in neurons of the cerebral cortex. In the hypothalamus, intensely rCRMP-4-labeled neurons populated the supraoptic, paraventricular, and periventricular nuclei as well as the median eminence and the arcuate nucleus. Immunoreactivity for rCRMP-4 also was present in certain neurons of the interpeduncular nucleus, median raphe, superior colliculus, and scattered granule cerebellar neurons. Many of these regions are known to display axonal outgrowth and/or synaptic rearrangement in adulthood and to coexpress the polysialylated form of the neural cell adhesion molecule. Thus, the results of this study suggest that rCRMP-4 expression in the CNS is associated with cells that are migrating or are undergoing axonal growth. Nevertheless, small, rCRMP-4-immunoreactive cells were seen throughout the brain. These cells did not express neuronal, astroglial, or microglial markers, although some of them also were immunoreactive for rip antibody, suggesting an oligodendroglial lineage.

Semaphorin3A enhances endocytosis at sites of receptor-F-actin colocalization during growth cone collapse

Axonal growth cone collapse is accompanied by a reduction in filopodial F-actin. We demonstrate here that semaphorin 3A (Sema3A) induces a coordinated rearrangement of Sema3A receptors and F-actin during growth cone collapse. Differential interference contrast microscopy reveals that some sites of Sema3A-induced F-actin reorganization correlate with discrete vacuoles, structures involved in endocytosis. Endocytosis of FITC-dextran by the growth cone is enhanced during Sema3A treatment, and sites of dextran accumulation colocalize with actin-rich vacuoles and ridges of membrane. Furthermore, the Sema3A receptor proteins, neuropilin-1 and plexin, and the Sema3A signaling molecule, rac1, also reorganize to vacuoles and membrane ridges after Sema3A treatment. These data support a model whereby Sema3A stimulates endocytosis by focal and coordinated rearrangement of receptor and cytoskeletal elements. Dextran accumulation is also increased in retinal ganglion cell (RGC) growth cones, in response to ephrin A5, and in RGC and DRG growth cones, in response to myelin and phorbol-ester. Therefore, enhanced endocytosis may be a general principle of physiologic growth cone collapse. We suggest that growth cone collapse is mediated by both actin filament rearrangements and alterations in membrane dynamics.

Expression of the dihydropyrimidinase related protein 2 (DRP-2) in Down syndrome and Alzheimer's disease brain is downregulated at the mRNA and dysregulated at the protein level

Deteriorated migration, axonal pathfinding and wiring of the brain is a main neuropathological feature of Down Syndrome (DS). Information on the underlying mechanisms is still limited, although basic functions of a series of growth factors, cell adhesion molecules, guidance factors and chemoattractants for brain histogenesis have been reported. We used proteomics to detect differences in protein expression between control, DS and Alzheimer's disease brains: In five individual brain regions of 9 individuals of each group we performed two dimensional electrophoresis with MALDI--identification of proteins and determined mRNA levels of DRP-2. Significantly decreased mRNA levels of DRP-2 in four brain regions of patients with DS but not with AD as compared to controls were detected. 2D electrophoresis revealed variable expression of DRP-2 proteins, which showed a high heterogeneity per se. Dysregulation of DRP-2 was found in brains of patients with DS and AD presenting with an inconsistent pattern, which in turn may reflect the inconsistent neuropathological findings in patients with DS and AD. The decrease of mRNA DRP-2 steady state levels in DS along with deteriorated protein expression of this repulsive guidance molecule of the semaphorin/collapsin family, may help to explain deranged migration and histogenesis of DS brain and wiring of AD brain.

Functions of semaphorins in axon guidance and neuronal regeneration

The semaphorin family comprises secreted and transmembrane signaling proteins that function in the nervous, immune, respiratory and cardiovascular systems. Sema3A, a secreted type of semaphorin, is now recognized as the most potent repulsive molecule inhibiting or repelling neurite outgrowth. The biological actions of Sema3A are mediated via neuropilin (Npn)-1, a receptor or one of the components of a receptor complex for Sema3A. Although the molecular mechanisms of Sema3A-Npn-1 signaling are largely unknown, a pertussis toxin-sensitive trimeric G protein(s), Rac-1, collapsin response mediator protein (CRMP), cyclic nucleotides and tyrosine kinase(s) have been implicated as essential and/or modulatory components of these processes. As repulsive molecules could be impediments to axon outgrowth, determining how these repulsive molecules exert their actions has the potential of uncovering new therapeutic approaches to injury and/or degeneration of neuronal tissues.

Disease-specific alterations in frontal cortex brain proteins in schizophrenia, bipolar disorder, and major depressive disorder. The Stanley Neuropathology Consortium

Severe psychiatric disorders such as schizophrenia, bipolar disorder and major depressive disorder are brain diseases of unknown origin. No biological marker has been documented at the pathological, cellular, or molecular level, suggesting that a number of complex but subtle changes underlie these illnesses. We have used proteomic technology to survey postmortem tissue to identify changes linked to the various diseases. Proteomics uses two-dimensional gel electrophoresis and mass spectrometric sequencing of proteins to allow the comparison of subsets of expressed proteins among a large number of samples. This form of analysis was combined with a multivariate statistical model to study changes in protein levels in 89 frontal cortices obtained postmortem from individuals with schizophrenia, bipolar disorder, major depressive disorder, and non-psychiatric controls. We identified eight protein species that display disease-specific alterations in level in the frontal cortex. Six show decreases compared with the non-psychiatric controls for one or more diseases. Four of these are forms of glial fibrillary acidic protein (GFAP), one is dihydropyrimidinase-related protein 2, and the sixth is ubiquinone cytochrome c reductase core protein 1. Two spots, carbonic anhydrase 1 and fructose biphosphate aldolase C, show increase in one or more diseases compared to controls. Proteomic analysis may identify novel pathogenic mechanisms of human neuropsychiatric diseases.

Differential gene expression in the activation and maturation of human monocytes

Differential-display or RNA fingerprint was applied to identify genes differentially expressed in monocyte maturation induced by an immunomodulating peptide on human peripheral blood mononuclear cells. Two unknown sequences (06c22 and 06c71) and p21 protein (cyclin dependent kinase inhibitor) were repressed, and three genes activated: Cathepsin D, DRP2 (dihydropirimidinase related protein 2), and gp91phox (91-kDa subunit of citochrome b(558)). Phenotype of evolving monocytes was analyzed by flow cytometry and mRNA level of identified genes determined by reverse transcription-PCR. The expression pattern of identified genes seemed to correlate with different monocyte subsets, monocyte-derived cells, and expected functional changes. After peptide addition, immature monocytes were initially activated, increasing the expression of CD25, CD69, and HLA-DR markers. This was accompanied by repression of p21 and the two unknown sequences, along with the simultaneous activation of Cathepsin D and DRP2. Later, the differentiation marker CD16 rose, and gp91phox gene expression activated. Further maturation led certain monocytes to express marker CD23 and gp91phox expression to reach a maximum, while Cathepsin D and DRP2 dropped to preactivation levels. Results reflect part of the evolution of immature monocytes toward macrophages and monocyte-derived dendritic cell precursors.

Semaphorins as mediators of neuronal apoptosis

Shrinkage and collapse of the neuritic network are often observed during the process of neuronal apoptosis. However, the molecular and biochemical basis for the axonal damage associated with neuronal cell death is still unclear. We present evidence for the involvement of axon guidance molecules with repulsive cues in neuronal cell death. Using the differential display approach, an up-regulation of collapsin response mediator protein was detected in sympathetic neurons undergoing dopamine-induced apoptosis. A synchronized induction of mRNA of the secreted collapsin-1 and the intracellular collapsin response mediator protein that preceded commitment of neurons to apoptosis was detected. Antibodies directed against a conserved collapsin-derived peptide provided marked and prolonged protection of several neuronal cell types from dopamine-induced apoptosis. Moreover, neuronal apoptosis was inhibited by antibodies against neuropilin-1, a putative component of the semaphorin III/collapsin-1 receptor. Induction of neuronal apoptosis was also caused by exposure of neurons to semaphorin III-alkaline phosphatase secreted from 293EBNA cells. Anti-collapsin-1 antibodies were effective in blocking the semaphorin III-induced death process. We therefore suggest that, before their death, apoptosis-destined neurons may produce and secrete destructive axon guidance molecules that can affect their neighboring cells and thus transfer a "death signal" across specific and susceptible neuronal populations.

Growth cone neuropilin-1 mediates collapsin-1/Sema III facilitation of antero- and retrograde axoplasmic transport

Collapsin-1/Sema III, a member of the semaphorin family, has been implicated in axonal pathfinding as a repulsive guidance cue. Cellular and molecular mechanisms by which collapsin-1 exerts its action are not fully understood. Collapsin-1 induces growth cone collapse via a pathway which may include neuropilin-1, a cellsurface collapsin-1 binding protein, as well as intracellular CRMP-62 and heterotrimeric G proteins. We previously identified a second action of collapsin-1, the facilitation of antero- and retrograde axoplasmic transport. This response occurs via a mechanism distinct from that causing growth cone collapse. To investigate the possible involvement of neuropilin-1 in the action of collapsin-1 on axoplasmic transport, we produced a soluble neuropilin-1 (sNP-1) lacking the transmembrane and intracellular region. sNP-1 progressively displaced the dose-response curve for collapsin-1 to induce growth cone collapse to higher concentrations. sNP-1 also inhibited collapsin-1-induced augmentation of both antero- and retrograde axoplasmic transport. Furthermore, an anti-neuropilin-1 antibody blocked the collapsin-induced axoplasmic transport. These results together indicate that neuropilin-1 mediates collapsin-1 action on axoplasmic transport. To visualize collapsin-1 binding to endogenous neuropilin-1, we used a truncated collapsin-1-alkaline phosphatase fusion protein (CAP-4). CAP-4 stains the growth cone, neurite, and cell body. However, local application of collapsin-1 to growth cone but to neither neurite nor cell body promotes axoplasmic transport. Thus, growth cone NP-1 mediates the facilitatory action of collapsin-1 on antero- and retrograde axoplasmic transport.

Growth cone guidance: first steps towards a deeper understanding

Growth cones, the hand-like structures at the tip of growing neurites, possess remarkable abilities to detect directional cues. On their way to their targets they traverse a dense jungle of many different cells, expressing a variety of different molecular guidance cues. Proper reading and integration of these cues is essential for precise wiring of different parts of the peripheral and central nervous systems. Guidance cues have been classified according to the response they elicit as either attractive or repulsive. Recent work, however, suggests that this might not represent an absolute distinction and that the internal state of the growth cone can dictate whether it detects a cue as repulsive or attractive. This article reviews some new experimental approaches to understanding growth cone signal transduction mechanisms induced by extracellular guidance cues.

Myelin and collapsin-1 induce motor neuron growth cone collapse through different pathways: inhibition of collapse by opposing mutants of rac1

Precise growth cone guidance is the consequence of a continuous reorganization of actin filament structures within filopodia and lamellipodia in response to inhibitory and promoting cues. The small GTPases rac1, cdc42, and rhoA are critical for regulating distinct actin structures in non-neuronal cells and presumably in growth cones. Collapse, a retraction of filopodia and lamellipodia, is a typical growth cone behavior on contact with inhibitory cues and is associated with depolymerization and redistribution of actin filaments. We examined whether small GTPases mediate the inhibitory properties of CNS myelin or collapsin-1, a soluble semaphorin, in chick embryonic motor neuron cultures. As demonstrated for collapsin-1, CNS myelin-evoked growth cone collapse was accompanied by a reduction of rhodamine-phalloidin staining most prominent in the growth cone periphery, suggesting actin filament disassembly. Specific mutants of small GTPases were capable of desensitizing growth cones to CNS myelin or collapsin-1. Adenoviral-mediated expression of constitutively active rac1 or rhoA abolished CNS myelin-induced collapse and allowed remarkable neurite extension on a CNS myelin substrate. In contrast, expression of dominant negative rac1 or cdc42 negated collapsin-1-induced growth cone collapse and promoted neurite outgrowth on a collapsin-1 substrate. These findings suggest that small GTPases can modulate the signaling pathways of inhibitory stimuli and, consequently, allow the manipulation of growth cone behavior. However, the fact that opposite mutants of rac1 were effective against different inhibitory stimuli speaks against a universal signaling pathway underlying growth cone collapse.

Cloning and characterization of a novel class VI semaphorin, semaphorin Y

Semaphorins comprise a large family of proteins implicated in axonal guidance. We cloned a novel transmembrane semaphorin, semaphorin Y (Sema Y), which has a class VI sema domain. Sema Y shows growth cone collapsing activity on DRG neurons in vitro, and the target regions of the DRG neurons express sema Y mRNA during development. Sema Y may be a stop signal for these neurons in their target areas. Interestingly, sema Y mRNA was also detected in other neurons and their targets. Two isoforms of Sema Y derived from alternative splicing were identified and their expression was found to be regulated in a tissue- and age-dependent manner. Distribution of sema Y mRNA suggests that Sema Y might also be important during maintenance of axonal connections and/or differentiation and migration of cells. Sequence comparison among class VI semaphorins revealed two short conserved sequence stretches in their cytoplasmic domains, suggesting interaction of these semaphorins with a common intracellular component(s).

Neuronal and non-neuronal collapsin-1 binding sites in developing chick are distinct from other semaphorin binding sites

The collapsin and semaphorin family of extracellular proteins contributes to axonal path finding by repulsing axons and collapsing growth cones. To explore the mechanism of collapsin-1 action, we expressed and purified a truncated collapsin-1-alkaline phosphatase fusion protein (CAP-4). This protein retains biological activity as a DRG growth cone collapsing agent and saturably binds to DRG neurons with low nanomolar affinity. Specific CAP-4 binding sites are present on DRG neurons, sympathetic neurons, and motoneurons, but not on retinal, cortical, or brainstem neurons. Outside the nervous system, high levels of CAP-4 binding sites are present in the mesenchyme surrounding major blood vessels and developing bone and in lung. These sites provide a substrate for the collapsin-1-dependent patterning of non-neuronal tissues perturbed in sema III (-/-) mice. The staining patterns for mouse semaphorin D/III and chick collapsin-1 fusion proteins are indistinguishable from one another but quite separate from that for semaphorin B and M-semaphorin F fusion proteins. These data imply that a family of high-affinity semaphorin binding sites similar in complexity to the semaphorin ligand family exists.