The Drosophila neuregulin vein maintains glial survival during axon guidance in the CNS.

Neuron-glia interactions are necessary for the formation of the longitudinal axon trajectories in the Drosophila central nervous system. Longitudinal glial cells are required for axon guidance and fasciculation, and pioneer neurons for trophic support of the glia. Neuregulin is a neuronal molecule that controls glial survival in the vertebrate nervous system. The Drosophila protein Vein has structural similarities with Neuregulin. We show here that Vein functions like a Neuregulin to maintain glial cell survival. We present direct in vivo evidence at single-cell resolution that Vein is produced by pioneer neurons and maintains the survival of neighboring longitudinal glia. This mechanism links axon guidance to control of glial cell number and may contribute to plasticity during the establishment of normal axonal trajectories.     

Axon-glial relationships in early CNS–PNS transitional zone development: an ultrastructural study

The CNS–PNS transitional zone of rat cervical ventral rootlets develops in two stages: first, axon segregation, then transitional node formation. This ultrastructural study examines the former. Material was prepared by standard methods. Shortly after they grow out from the neural tube, ventral motoneuron axon bundles are extensively segregated by a matrix of fine processes forming a barrier across the rootlet, just distal to the cord surface. These processes arise from cell clusters on the rootlet surface. This barrier is prominent until the period around birth, when it is replaced by a second in which the axons are completely segregated from one another. The perikarya and processes forming this barrier resemble those of the first, but lie at or just below the cord surface. Thus, beginning at the earliest stage, a barrier crosses the axon bundle and segregates its axons before axon segregation is advanced either in the PNS or (especially) in the CNS. This may prevent central Schwann cell migration. Evidence is presented suggesting that the second barrier may arise through a relative proximal relocation of the first, as the cord grows radially. Near the cord surface, a complete, funnel-shaped sleeve of glial processes surrounds the axon bundle. This is continuous at the cord surface with the glia limitans. It constitutes an integral part of the transitional zone apparatus. It is also continuous centrally with the sheath which enfolds the bundle of ventral motoneuron axons as they run between the ventral horn and the transitional zone. Axon segregation at the cord surface, and therefore the formation of the definitive astrocytic CNS–PNS barrier occur relatively (and perhaps surprisingly) late at the cord surface. The definitive sharp discontinuity of central and peripheral tissue types characteristic of the transitional zone is established only after birth.     

Receptor tyrosine phosphatases regulate axon guidance across the midline of the Drosophila embryo

Neural receptor-linked protein tyrosine phosphatases (RPTPs) are required for guidance of motoneuron and photoreceptor growth cones in Drosophila. These phosphatases have not been implicated in growth cone responses to specific guidance cues, however, so it is unknown which aspects of axonal pathfinding are controlled by their activities. Three RPTPs, known as DLAR, DPTP69D, and DPTP99A, have been genetically characterized thus far. Here we report the isolation of mutations in the fourth neural RPTP, DPTP10D. The analysis of double mutant phenotypes shows that DPTP10D and DPTP69D are necessary for repulsion of growth cones from the midline of the embryonic central nervous system. Repulsion is thought to be triggered by binding of the secreted protein Slit, which is expressed by midline glia, to Roundabout (Robo) receptors on growth cones. Robo repulsion is downregulated by the Commissureless (Comm) protein, allowing axons to cross the midline. Here we show that the Rptp mutations genetically interact with robo, slit and comm. The nature of these interactions suggests that DPTP10D and DPTP69D are positive regulators of Slit/Roundabout repulsive signaling. We also show that elimination of all four neural RPTPs converts most noncrossing longitudinal pathways into commissures that cross the midline, indicating that tyrosine phosphorylation controls the manner in which growth cones respond to midline signals.     

Repellent cues in axon guidance.

There is increasing evidence that axons are guided by repulsion in several regions of the developing nervous system, although this has yet to be confirmed directly in vivo. As more candidate repulsion molecules are identified, it is becoming clear that collapse of the growth cone in vitro may be mediated by more than one intracellular mechanism. The present emphasis on molecular cloning of the ligands and their receptors should enable a proper definition of their function during development.     

Regulation of CNS and motor axon guidance in Drosophila by the receptor tyrosine phosphatase DPTP52F.

Receptor-linked protein tyrosine phosphatases (RPTPs) regulate axon guidance and synaptogenesis in Drosophila embryos and larvae. We describe DPTP52F, the sixth RPTP to be discovered in Drosophila. Our genomic analysis indicates that there are likely to be no additional RPTPs encoded in the fly genome. Five of the six Drosophila RPTPs have C. elegans counterparts, and three of the six are also orthologous to human RPTP subfamilies. DPTP52F, however, has no clear orthologs in other organisms. The DPTP52F extracellular domain contains five fibronectin type III repeats and it has a single phosphatase domain. DPTP52F is selectively expressed in the CNS of late embryos, as are DPTP10D, DLAR, DPTP69D and DPTP99A. To define developmental roles of DPTP52F, we used RNA interference (RNAi)-induced phenotypes as a guide to identify Ptp52F alleles among a collection of EMS-induced lethal mutations. Ptp52F single mutant embryos have axon guidance phenotypes that affect CNS longitudinal tracts. This phenotype is suppressed in Dlar Ptp52F double mutants, indicating that DPTP52F and DLAR interact competitively in regulating CNS axon guidance decisions. Ptp52F single mutations also cause motor axon phenotypes that selectively affect the SNa nerve. DPTP52F, DPTP10D and DPTP69D have partially redundant roles in regulation of guidance decisions made by axons within the ISN and ISNb motor nerves.     

Characterization of a plasma membrane protein present in non-myelin-forming PNS and CNS glia,a subpopulation of PNS neurons,perineurial cells and smooth muscle in adult rats

Summary A plasma membrane protein common to nonmyelin-forming peripheral glia, including non-myelin-forming Schwann cells, satellite cells and enteric glia, is recognized and defined by monoclonal antibody A5E3. It is not detectable immunohistochemically on myelin-forming Schwann cells. The antigen is also present in large amounts on smooth muscle cells and perineurial cells, on some PNS neurons, and at lower levels on astrocytes of adult rat. In neonatal but not adult animals, the antigen is present on skeletal muscle fibres and myoblasts. In immunoblots and immune precipitation experiments on smooth muscle and Schwann cell extracts the antigen is a polypeptide with an apparent molecular weight of 130 kd. In being present in some non-neural tissues, albeit very highly restricted in cell type, this antigen resembles several other cell surface glycoproteins found in large amounts in the nervous system.     

Conservation and divergence of axon guidance mechanisms.

Analysis of axon guidance mechanisms in vertebrates, Caenorhabditis elegans, and Drosophila melanogaster has led to the identification of several signaling pathways, many of which are strikingly conserved in function. Recent studies indicate that several axon guidance mechanisms are highly conserved in all animals, whereas others, though still conserved in a general sense, show strong evolutionary divergence at a detailed mechanistic level.     

Troy/Taj and its role in CNS axon regeneration

Binding of myelin inhibitors to the NgR1/p75/LINGO-1 signaling complex activates RhoA to mediate the inhibition of axonal outgrowth. The nerve growth factor receptor p75, a TNF family receptor, is absent or poorly expressed in certain types of neurons that respond to myelin inhibitors, thereby prompting speculation that other TNF family receptors are involved in the NgR1 complex. Troy/Taj is an orphan TNF family receptor that is broadly expressed in postnatal and adult neurons. Troy binds to NgR1 and can functionally replace p75 in the p75/NgR1/LINGO-1 complex to activate RhoA and block neurite outgrowth in the presence of myelin inhibitors. Neurons from Troy-deficient mice are more resistant to the suppressive action of the myelin inhibitors. The discovery of TROY function in axon growth is an important step for understanding the complex regulation of axonal regeneration by diverse members of the TNF receptor family.     

Semaphorins and their receptors in olfactory axon guidance.

The mammalian olfactory system is capable of discriminating among a large variety of odor molecules and is therefore essential for the identification of food, enemies and mating partners. The assembly and maintenance of olfactory connectivity have been shown to depend on the combinatorial actions of a variety of molecular signals, including extracellular matrix, cell adhesion and odorant receptor molecules. Recent studies have identified semaphorins and their receptors as putative molecular cues involved in olfactory pathfinding, plasticity and regeneration. The semaphorins comprise a large family of secreted and transmembrane axon guidance proteins, being either repulsive or attractive in nature. Neuropilins were shown to serve as receptors for secreted class 3 semaphorins, whereas members of the plexin family are receptors for class 1 and V (viral) semaphorins. The present review will discuss a role for semaphorins and their receptors in the establishment and maintenance of olfactory connectivity.     

Embryonic expression of Drosophila IMP in the developing CNS and PNS

Drosophila IMP (dIMP) is related to the vertebrate RNA-binding proteins IMP1-3, ZBP1, Vg1RBP and CRD-BP, which are involved in RNA regulatory processes such as translational repression, localization and stabilization. The proteins are expressed in many fetal tissues, including the developing nervous system, and IMP up-regulation in solid tumors correlates with a high metastatic potential and poor prognosis. In this study, we used immunohistochemistry and live-imaging of an endogenous promoter-driven GFP-dIMP fusion protein to reveal the expression pattern of dIMP protein throughout embryogenesis. In the cellular blastoderm, immunoreactivity was seen in the entire cell-layer, where it was localized apically to the nucleus, and in the pole cells. Later, the GFP-dIMP fusion protein appeared in the developing central nervous system, both in the brain and in the ventral nerve cord. In the peripheral nervous system, immunoreactivity was detected in both neurons and accessory cells of chordotonal and external sensory organs.     

Laminin-immunoreactive glia distinguish regenerative adult CNS systems from non-regenerative ones.

Most regions of the adult mammalian central nervous system (CNS) do not support axonal growth and regeneration. Laminin, expressed by cultured astrocytes and known to promote neurite outgrowth of cultured neurons, is normally present in brain basement membranes, and only transiently induced in adult brain astrocytes by injury. Here I provide three lines of evidence which suggest that the continued expression of laminin by astrocytes may be a prerequisite for axonal growth and regeneration in adult CNS. Firstly, laminin is continuously present in astrocytes of adult rat olfactory bulb apparently in close association with the olfactory nerve axons. Secondly, laminin is continuously expressed by astrocytes in adult frog brain, and sectioning of the optic tract further increases laminin immunoreactivity in astrocytes of the optic tectum during the period of axonal regeneration. Lastly, laminin appears normally in astrocytes of the frog and goldfish optic nerves which regenerate, but not in astrocytes of the rat or chick optic nerves which do not regenerate. The selective association of laminin with axons that undergo growth and regeneration in vivo is consistent with the possibility that astrocytic laminin provides these central nervous systems with their regenerative potential.     

Repulsive factors and axon regeneration in the CNS

During the past year, a major advance in the study of axon regeneration was the molecular cloning of Nogo. The expression of Nogo protein by CNS myelin may be a major factor in the failure of CNS axon regeneration. The effect of disrupting Nogo-dependent axon inhibition can now be studied conclusively. In related work, immunization with a Nogo-containing CNS myelin preparation was shown to promote regeneration and dramatic functional recovery after spinal cord trauma.     

Functional conservation of atonal and Math1 in the CNS and PNS

To determine the extent to which atonal and its mouse homolog Math1 exhibit functional conservation, we inserted (beta)-galactosidase (lacZ) into the Math1 locus and analyzed its expression, evaluated consequences of loss of Math1 function, and expressed Math1 in atonal mutant flies. lacZ under the control of Math1 regulatory elements duplicated the previously known expression pattern of Math1 in the CNS (i.e., the neural tube, dorsal spinal cord, brainstem, and cerebellar external granule neurons) but also revealed new sites of expression: PNS mechanoreceptors (inner ear hair cells and Merkel cells) and articular chondrocytes. Expressing Math1 induced ectopic chordotonal organs (CHOs) in wild-type flies and partially rescued CHO loss in atonal mutant embryos. These data demonstrate that both the mouse and fly homologs encode lineage identity information and, more interestingly, that some of the cells dependent on this information serve similar mechanoreceptor functions.