Zebrafish semaphorin Z1a collapses specific growth cones and alters their pathway in vivo

Semaphorins and their receptors in vertebrates and invertebrates

The semaphorins are a family of intercellular signaling proteins that has grown to include 19 identified members in higher vertebrates. Several of its members act as axonal guidance molecules. One participates in signaling in the immune system. The majority, however, do not yet have known biological functions. Recent studies have shown that neuropilins and plexins act as receptors for semaphorins. The most important challenge for the future is to define the biological roles of semaphorins in vivo.

Growth cones stall and collapse during axon outgrowth in Caenorhabditis elegans

During nervous system development, neurons form synaptic contacts with distant target cells. These connections are formed by the extension of axonal processes along predetermined pathways. Axon outgrowth is directed by growth cones located at the tips of these neuronal processes. Although the behavior of growth cones has been well-characterized in vitro, it is difficult to observe growth cones in vivo. We have observed motor neuron growth cones migrating in living Caenorhabditis elegans larvae using time-lapse confocal microscopy. Specifically, we observed the VD motor neurons extend axons from the ventral to dorsal nerve cord during the L2 stage. The growth cones of these neurons are round and migrate rapidly across the epidermis if they are unobstructed. When they contact axons of the lateral nerve fascicles, growth cones stall and spread out along the fascicle to form anvil-shaped structures. After pausing for a few minutes, they extend lamellipodia beyond the fascicle and resume migration toward the dorsal nerve cord. Growth cones stall again when they contact the body wall muscles. These muscles are tightly attached to the epidermis by narrowly spaced circumferential attachment structures. Stalled growth cones extend fingers dorsally between these hypodermal attachment structures. When a single finger has projected through the body wall muscle quadrant, the growth cone located on the ventral side of the muscle collapses and a new growth cone forms at the dorsal tip of the predominating finger. Thus, we observe that complete growth cone collapse occurs in vivo and not just in culture assays. In contrast to studies indicating that collapse occurs upon contact with repulsive substrata, collapse of the VD growth cones may result from an intrinsic signal that serves to maintain growth cone primacy and conserve cellular material.

Zebrafish semaphorin Z1b inhibits growing motor axons in vivo

Zebrafish semaphorin 1b (sema Z1b) is a new member of the semaphorin family, related to mammalian sema D/III. It is expressed in rhombomeres three and five, and in the posterior half of newly formed somites which is avoided by ventrally extending motor axons. Embryos injected at the 1-2 cell stage with synthetic sema Z1b mRNA developed normally but many (63%) showed missing or severely stunted ventral motor nerves. Other axons, somites, and hindbrain rhombomeres were not affected. No abnormalities were seen in control embryos injected with lacZ mRNA. Sema Z1b might normally influence the midsegmental pathway choice of the ventrally extending motor axons by contributing to a repulsive domain in the posterior somite.

nacre encodes a zebrafish microphthalmia-related protein that regulates neural-crest-derived pigment cell fate

We report the isolation and identification of a new mutation affecting pigment cell fate in the zebrafish neural crest. Homozygous nacre (nac(w2)) mutants lack melanophores throughout development but have increased numbers of iridophores. The non-crest-derived retinal pigment epithelium is normal, suggesting that the mutation does not affect pigment synthesis per se. Expression of early melanoblast markers is absent in nacre mutants and transplant experiments suggested a cell-autonomous function in melanophores. We show that nac(w2) is a mutation in a zebrafish gene encoding a basic helix-loop-helix/leucine zipper transcription factor related to microphthalmia (Mitf), a gene known to be required for development of eye and crest pigment cells in the mouse. Transient expression of the wild-type nacre gene restored melanophore development in nacre(-/-) embryos. Furthermore, misexpression of nacre induced the formation of ectopic melanized cells and caused defects in eye development in wild-type and mutant embryos. These results demonstrate that melanophore development in fish and mammals shares a dependence on the nacre/Mitf transcription factor, but that proper development of the retinal pigment epithelium in the fish is not nacre-dependent, suggesting an evolutionary divergence in the function of this gene.

Molecular cloning, expression, and activity of zebrafish semaphorin Z1a

Semaphorins/collapsins are a large family of secreted and cell surface molecules that are thought to guide growth cones to their targets. Although some members are clearly repulsive to specific growth cones in vitro, the in vivo role of many of these molecules in vertebrate embryos is still unclear. As a first step towards clarifying the in vivo role of semaphorins/collapsins, we analyzed semaZ1a in the simple and well-characterized zebrafish embryo. SemaZ1a is a secreted molecule that is highly homologous to Sema III/D/collapsin-1, and it can collapse chick dorsal root ganglion growth cones in vitro. It is expressed in highly specific patterns within the developing embryo, which suggests that it influences outgrowth by a variety of growth cones including those of the posterior lateral line ganglion. Consistent with this hypothesis, the peripherally extending growth cones of posterior lateral line neurons retract and partially collapse during normal outgrowth.

Cloning, expression, and genetic mapping of Sema W, a member of the semaphorin family

The semaphorins comprise a large family of membrane-bound and secreted proteins, some of which have been shown to function in axon guidance. We have cloned a transmembrane semaphorin, Sema W, that belongs to the class IV subgroup of the semaphorin family. The mouse and rat forms of Sema W show 97% amino acid sequence identity with each other, and each shows about 91% identity with the human form. The gene for Sema W is divided into 15 exons, up to 4 of which are absent in the human cDNAs that we sequenced. Unlike many other semaphorins, Sema W is expressed at low levels in the developing embryo but was found to be expressed at high levels in the adult central nervous system and lung. Functional studies with purified membrane fractions from COS7 cells transfected with a Sema W expression plasmid showed that Sema W has growth-cone collapse activity against retinal ganglion-cell axons, indicating that vertebrate transmembrane semaphorins, like secreted semaphorins, can collapse growth cones. Genetic mapping of human SEMAW with human/hamster radiation hybrids localized the gene to chromosome 2p13. Genetic mapping of mouse Semaw with mouse/hamster radiation hybrids localized the gene to chromosome 6, and physical mapping placed the gene on bacteria artificial chromosomes carrying microsatellite markers D6Mit70 and D6Mit189. This localization places Semaw within the locus for motor neuron degeneration 2, making it an attractive candidate gene for this disease.

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).

Cellular and molecular bases of axonal pathfinding during embryogenesis of the fish central nervous system

The accessibility of the zebrafish embryo offers unique possibilities to study the mechanisms that guide growing axons in the developing vertebrate central nervous system. This review examines the current understanding of the pathfinding decisions by the growing axons, their substrates, and the recognition molecules that mediate axon-substrate interactions. The detailed analysis of pathfinding at the level of individual axons demonstrates that growing axons chose their paths unerringly. To do so, they rely on cues presented by their environment, in particular by neuroepithelial cells. Our understanding of the molecular bases of axon-substrate interactions is increasing. Members of most classes of recognition molecules have been identified in fish. Experimental evidence for the functions of these molecules in the zebrafish nervous system is accumulating. In the future, this analysis is expected to profit greatly from genetic screens that have recently been initiated.

Semaphorins III and IV repel hippocampal axons via two distinct receptors

The semaphorins are the largest family of repulsive axon guidance molecules. Secreted semaphorins bind neuropilin receptors and repel sensory, sympathetic and motor axons. Here we show that CA1, CA3 and dentate gyrus axons from E15-E17 mouse embryo explants are selectively repelled by entorhinal cortex and neocortex. The secreted semaphorins Sema III and Sema IV and their receptors Neuropilin-1 and −2 are expressed in the hippocampal formation during appropriate stages. Sema III and Sema IV strongly repel CA1, CA3 and dentate gyrus axons; entorhinal axons are only repelled by Sema III. An antibody against Neuropilin-1 blocks the repulsive action of Sema III and the entorhinal cortex, but has no effect on Sema IV-induced repulsion. Thus, chemorepulsion plays a role in axon guidance in the hippocampus, secreted semaphorins are likely to be responsible for this action, and the same axons can be repelled by two distinct semaphorins via two different receptors.

Receptors for collapsin/semaphorins

Chemorepulsive signals that repel or paralyze neuronal growth cones have been found to play important roles in axon guidance in a stereotyped manner. Recent progress in the identification of neuropilins as the receptors for class III secreted collapsin/semaphorin subfamily members, which are neuronal repellents, and in the analysis of mutant mice lacking neuropilin function has confirmed the importance of these chemorepellents in axon guidance. In addition, characterization of the neuropilin protein has yielded new insights into the functions of this molecule in vascular formation and in axon guidance.