Developmental expression of sema3G, a novel zebrafish semaphorin

Plexin B3 guides axons to cross the midline in vivo

During the development of neural circuits, axons are guided by a variety of molecular cues to navigate through the brain and establish precise connections with correct partners at the right time and place. Many axon guidance cues have been identified and they play pleiotropic roles in not only axon guidance but also axon fasciculation, axon pruning, and synaptogenesis as well as cell migration, angiogenesis, and bone formation. In search of receptors for Sema3E in axon guidance, we unexpectedly found that Plexin B3 is highly expressed in retinal ganglion cells of zebrafish embryos when retinal axons are crossing the midline to form the chiasm. Plexin B3 has been characterized to be related to neurodevelopmental disorders. However, the investigation of its pathological mechanisms is hampered by the lack of appropriate animal model. We provide evidence that Plexin B3 is critical for axon guidance in vivo. Plexin B3 might function as a receptor for Sema3E while Neuropilin1 could be a co-receptor. The intracellular domain of Plexin B3 is required for Semaphorin signaling transduction. Our data suggest that zebrafish could be an ideal animal model for investigating the role and mechanisms of Sema3E and Plexin B3 in vivo.

Dietary inclusion of plant ingredients induces epigenetic changes in the intestine of zebrafish

Epigenetic modifications, such as DNA methylation, can be regulated by nutrition and dietary factors. There has been a large increase in the use of sustainable plant-based protein sources in fish feed due to limitations of fishmeal resources, which are needed to sustain a rapidly growing aquaculture industry. With this major transition from marine ingredients to plant-based diets, fish are abruptly introduced to changes in dietary composition and exposed to a variety of phytochemicals, some of which known to cause epigenetic changes in mammals. However, the effect of plant ingredients on the epigenome of fish is barely understood. In the present study, the nutriepigenomic effects of the addition of pea, soy, and wheat gluten protein concentrate to aquafeeds were investigated using zebrafish as a model. A genome-wide analysis of DNA methylation patterns was performed by reduced representation bisulphite sequencing to examine global epigenetic alterations in the mid intestine after a 42-day feeding trial. We found that inclusion of 30% of wheat gluten, pea and soy protein concentrate in the diet induced epigenetic changes in the mid intestine of zebrafish. A large number of genes and intergenic regions were differentially methylated with plant-based diets. The genes concerned were related to immunity, NF-κB system, ubiquitin-proteasome pathway, MAPK pathway, and the antioxidant defence system. Epigenetic regulation of several biological processes, including neurogenesis, cell adhesion, response to stress and immunity was also observed. Ultimately, the observed epigenetic changes may enable zebrafish to rapidly regulate inflammation and maintain intestinal homoeostasis when fed plant protein-based diets.

Attractant and repellent cues cooperate in guiding a subset of olfactory sensory axons to a well-defined protoglomerular target

Olfactory sensory axons target well-defined intermediate targets in the zebrafish olfactory bulb called protoglomeruli well before they form odorant receptor-specific glomeruli. A subset of olfactory sensory neurons are labeled by expression of the or111-7:IRES:GAL4 transgene whose axons terminate in the central zone (CZ) protoglomerulus. Previous work has shown that some of these axons misproject to the more dorsal and anterior dorsal zone (DZ) protoglomerulus in the absence of Netrin 1/Dcc signaling. In search of additional cues that guide these axons to the CZ, we found that Semaphorin 3D (Sema3D) is expressed in the anterior bulb and acts as a repellent that pushes them towards the CZ. Further analysis indicates that Sema3D signaling is mediated through Nrp1a, while Nrp2b also promotes CZ targeting but in a Sema3D-independent manner. nrp1a, nrp2b and dcc transcripts are detected in or111-7 transgene-expressing neurons early in development and both Nrp1a and Dcc act cell-autonomously in sensory neurons to promote accurate targeting to the CZ. dcc and nrp1a double mutants have significantly more DZ misprojections than either single mutant, suggesting that the two signaling systems act independently and in parallel to direct a specific subset of sensory axons to their initial protoglomerular target.

In utero and lactational dioxin exposure induces Sema3b and Sema3g gene expression in the developing mouse brain

In the developing mammalian brain, neural network formation is regulated by complex signaling cascades. In utero and lactational dioxin exposure is known to induce higher brain function abnormalities and dendritic growth disruption in rodents. However, it is unclear whether perinatal dioxin exposure affects the expression of genes involved in neural network formation. Therefore, we investigated changes in gene expression in the brain regions of developing mice born to dams administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dose: 0, 0.6, or 3.0 μg/kg) on gestational day 12.5. Quantitative RT-PCR showed that TCDD exposure induced Ahrr expression in the cerebral cortex, hippocampus, and olfactory bulb of 3-day-old mice. Gene microarray analysis indicated that the mRNA expression levels of Sema3b and Sema3g, which encode proteins that are known to control axonal projections, were elevated in the olfactory bulb of TCDD-exposed mice, and the induction of these genes was observed during a 2-week postnatal period. Increased Sema3g expression was also observed in the brain but not in the kidney, liver, lung, and spleen of TCDD-exposed neonatal mice. These results indicate that the Sema3b and Sema3g genes are sensitive to brain-specific induction by dioxin exposure, which may disrupt neural network formation in the mammalian nervous system, thereby leading to abnormal higher brain function in adulthood.

Changes in expression of Class 3 Semaphorins and their receptors during development of the rat retina and superior colliculus

Background

Members of the Semaphorin 3 family (Sema3s) influence the development of the central nervous system, and some are implicated in regulating aspects of visual system development. However, we lack information about the timing of expression of the Sema3s with respect to different developmental epochs in the mammalian visual system. In this time-course study in the rat, we document for the first time changes in the expression of RNAs for the majority of Class 3 Semaphorins (Sema3s) and their receptor components during the development of the rat retina and superior colliculus (SC).

Results

During retinal development, transcript levels changed for all of the Sema3s examined, as well as Nrp2, Plxna2, Plxna3, and Plxna4a. In the SC there were also changes in transcript levels for all Sema3s tested, as well as Nrp1, Nrp2, Plxna1, Plxna2, Plxna3, and Plxna4a. These changes correlate with well-established epochs, and our data suggest that the Sema3s could influence retinal ganglion cell (RGC) apoptosis, patterning and connectivity in the maturing retina and SC, and perhaps guidance of RGC and cortical axons in the SC. Functionally we found that SEMA3A, SEMA3C, SEMA3E, and SEMA3F proteins collapsed purified postnatal day 1 RGC growth cones in vitro. Significantly this is a developmental stage when RGCs are growing into and within the SC and are exposed to Sema3 ligands.

Conclusion

These new data describing the overall temporal regulation of Sema3 expression in the rat retina and SC provide a platform for further work characterising the functional impact of these proteins on the development and maturation of mammalian visual pathways.

Experience-dependent versus experience-independent postembryonic development of distinct groups of zebrafish olfactory glomeruli

Olfactory glomeruli are innervated with great precision by the axons of different olfactory sensory neuron types and act as functional units in odor information processing. Approximately 140 glomeruli are present in each olfactory bulb of adult zebrafish; these units consist of either highly stereotypic large glomeruli or smaller anatomically indistinguishable glomeruli. In the present study, we investigated developmental differences among these types of glomeruli. We observed that 10 large and individually identifiable glomeruli already developed before hatching, at 72 h after fertilization, in configurations that resembled their mature organization. However, the cross-sectional area of these glomeruli increased throughout larval development, and they eventually comprised the largest units in postlarval olfactory bulbs. In contrast, small and anatomically indistinguishable glomeruli formed only after hatching, apparently by segregating from five larger precursors that were identifiable during embryonic development. The differentiation of these small glomeruli proceeded with conspicuous variation in number and arrangement, both among larvae and between olfactory bulbs of the same individuals. To determine factors that might contribute to this variability, we investigated the effects of olfactory enrichment on the development of amino acid-responsive lateral glomeruli, which include both large and small units. Larvae reared in an amino acid-enriched environment had normal large lateral glomeruli, but the small lateral glomeruli were more numerous and displayed reduced cross-sectional areas compared with glomeruli in control animals. Our results suggest that large and small glomeruli mature via distinct developmental processes that may be differentially influenced by sensory experience.

Receptor complexes for each of the Class 3 Semaphorins

The Class 3 Semaphorins (Sema3s) are a sub-family of proteins whose known biological roles are varied and growing. The mechanism of action of the Sema3s requires binding to transmembrane receptors that comprise heteromeric complexes of Neuropilins, Plexins and cell adhesion molecules (CAMs). However, knowledge of the receptor components of the Sema3s remains incomplete, and there may be receptor components which are as yet undiscovered. The receptor complexes of the Sema3s share receptor components with each other, and it is the specific combination of these components within a heteromeric complex that is thought to give rise to selective binding and signalling for individual Sema3s. This crosstalk makes it experimentally difficult to define a single holoreceptor for each Sema3. Furthermore, the receptor composition for a given Sema3 may differ between cell types, and change as a function of developmental state or pathological situation. Nevertheless, there are at least some known differences in the constitutive structure of the receptors for the Sema3s. For example in neural cells, Sema3a and Sema3f signal through different Neuropilins (Nrp1 and Nrp2 respectively) and L1cam only appears important for Sema3a signaling, while Nrcam forms a complex with Nrp2. Further complexity arises from crosstalk of other families of ligands (e.g., VEGF) with Sema3 receptor components. Thus the Sema3s, which have been shown as antagonists for each other, can also act as antagonists for other families of molecules. This review compiles experimental evidence describing the receptor components for the Sema3s, detailing the current state of knowledge of which components are important for signaling of each Sema3 before going on to consider possible future directions for the field.

Position fine-tuning of caudal primary motoneurons in the zebrafish spinal cord

In zebrafish embryos, each myotome is typically innervated by three primary motoneurons (PMNs): the caudal primary (CaP), middle primary (MiP) and rostral primary (RoP). PMN axons first exit the spinal cord through a single exit point located at the midpoint of the overlying somite, which is formed beneath the CaP cell body and is pioneered by the CaP axon. However, the placement of CaP cell bodies with respect to corresponding somites is poorly understood. Here, we determined the early events in CaP cell positioning using neuropilin 1a (nrp1a):gfp transgenic embryos in which CaPs were specifically labeled with GFP. CaP cell bodies first exhibit an irregular pattern in presence of newly formed corresponding somites and then migrate to achieve their proper positions by axonogenesis stages. CaPs are generated in excess compared with the number of somites, and two CaPs often overlap at the same position through this process. Next, we showed that CaP cell bodies remain in the initial irregular positions after knockdown of Neuropilin1a, a component of the class III semaphorin receptor. Irregular CaP position frequently results in aberrant double exit points of motor axons, and secondary motor axons form aberrant exit points following CaP axons. Its expression pattern suggests that sema3ab regulates the CaP position. Indeed, irregular CaP positions and exit points are induced by Sema3ab knockdown, whose ectopic expression can alter the position of CaP cell bodies. Results suggest that Semaphorin-Neuropilin signaling plays an important role in position fine-tuning of CaP cell bodies to ensure proper exit points of motor axons.

Expression of multiple class three semaphorins in the retina and along the path of zebrafish retinal axons

Retinal ganglion cells (RGCs) extend axons that exit the eye, cross the midline at the optic chiasm, and synapse on target cells in the optic tectum. Class three semaphorins (Sema3s) are a family of molecules known to direct axon growth. We undertook an expression screen to identify sema3s expressed in the retina and/or brain close to in-growing RGC axons, which might therefore influence retinal-tectal pathfinding. We find that sema3Aa, 3Fa, 3Ga, and 3Gb are expressed in the retina, although only sema3Fa is present during the time window when the axons extend. Also, we show that sema3Aa and sema3E are present near or at the optic chiasm. Furthermore, sema3C, 3Fa, 3Ga, and 3Gb are expressed in regions of the diencephalon near the path taken by RGC axons. Finally, the optic tectum expresses sema3Aa, 3Fa, 3Fb, and 3Gb. Thus, sema3s are spatiotemporally placed to influence RGC axon growth.

Analysis of zebrafish sidetracked mutants reveals a novel role for Plexin A3 in intraspinal motor axon guidance

One of the earliest guidance decisions for spinal cord motoneurons occurs when pools of motoneurons orient their growth cones towards a common, segmental exit point. In contrast to later events, remarkably little is known about the molecular mechanisms underlying intraspinal motor axon guidance. In zebrafish sidetracked (set) mutants, motor axons exit from the spinal cord at ectopic positions. By single-cell labeling and time-lapse analysis we show that motoneurons with cell bodies adjacent to the segmental exit point properly exit from the spinal cord, whereas those farther away display pathfinding errors. Misguided growth cones either orient away from the endogenous exit point, extend towards the endogenous exit point but bypass it or exit at non-segmental, ectopic locations. Furthermore, we show that sidetracked acts cell autonomously in motoneurons. Positional cloning reveals that sidetracked encodes Plexin A3, a semaphorin guidance receptor for repulsive guidance. Finally, we show that sidetracked (plexin A3) plays an additional role in motor axonal morphogenesis. Together, our data genetically identify the first guidance receptor required for intraspinal migration of pioneering motor axons and implicate the well-described semaphorin/plexin signaling pathway in this poorly understood process. We propose that axonal repulsion via Plexin A3 is a major driving force for intraspinal motor growth cone guidance.

PlexinA3 restricts spinal exit points and branching of trunk motor nerves in embryonic zebrafish

The pioneering primary motor axons in the zebrafish trunk are guided by multiple cues along their pathways. Plexins are receptor components for semaphorins that influence motor axon growth and path finding. We cloned plexinA3 in zebrafish and localized plexinA3 mRNA in primary motor neurons during axon outgrowth. Antisense morpholino knock-down led to substantial errors in motor axon growth. Errors comprised aberrant branching of primary motor nerves as well as additional exit points of axons from the spinal cord. Excessively branched and supernumerary nerves were found in both ventral and dorsal pathways of motor axons. The trunk environment and several other types of axons, including trigeminal axons, were not detectably affected by plexinA3 knock-down. RNA overexpression rescued all morpholino effects. Synergistic effects of combined morpholino injections indicate interactions of plexinA3 with semaphorin3A homologs. Thus, plexinA3 is a crucial receptor for axon guidance cues in primary motor neurons.

Modulation of semaphorin signaling by Ig superfamily cell adhesion molecules

During axon navigation, growth cones continuously interact with molecular cues in their environment, some of which control adherence and bundle assembly, others axon elongation and direction. Growth cone responses to these different environmental cues are tightly coordinated during the development of neuronal projections. Several recent studies show that axon sensitivity to guidance cues is modulated by extracellular and intracellular signals. This regulation may enable different classes of cues to combine their effects and may also represent important means for diversifying pathway choices and for compensating for the limited number of guidance cues. This chapter focuses on the modulation exerted by Ig Super-family cell adhesion molecules (IgSFCAMs) on guidance cues of the class III secreted semaphorins.