Knockdown of Nav1.6a Na+ channels affects zebrafish motoneuron development

scn1bb, a zebrafish ortholog of SCN1B expressed in excitable and nonexcitable cells, affects motor neuron axon morphology and touch sensitivity

Voltage-gated Na(+) channels initiate and propagate action potentials in excitable cells. Mammalian Na(+) channels are composed of one pore-forming alpha-subunit and two beta-subunits. SCN1B encodes the Na(+) channel beta1-subunit that modulates channel gating and voltage dependence, regulates channel cell surface expression, and functions as a cell adhesion molecule (CAM). We recently identified scn1ba, a zebrafish ortholog of SCN1B. Here we report that zebrafish express a second beta1-like paralog, scn1bb. In contrast to the restricted expression of scn1ba mRNA in excitable cells, we detected scn1bb transcripts and protein in several ectodermal derivatives including neurons, glia, the lateral line, peripheral sensory structures, and tissues derived from other germ layers such as the pronephros. As expected for beta1-subunits, elimination of Scn1bb protein in vivo by morpholino knock-down reduced Na(+) current amplitudes in Rohon-Beard neurons of zebrafish embryos, consistent with effects observed in heterologous systems. Further, after Scn1bb knock-down, zebrafish embryos displayed defects in Rohon-Beard mediated touch sensitivity, demonstrating the significance of Scn1bb modulation of Na(+) current to organismal behavior. In addition to effects associated with Na(+) current modulation, Scn1bb knockdown produced phenotypes consistent with CAM functions. In particular, morpholino knock-down led to abnormal development of ventrally projecting spinal neuron axons, defasciculation of the olfactory nerve, and increased hair cell number in the inner ear. We propose that, in addition to modulation of electrical excitability, Scn1bb plays critical developmental roles by functioning as a CAM in the zebrafish embryonic nervous system.

Modulation of potassium channel function confers a hyperproliferative invasive phenotype on embryonic stem cells

Ion transporters, and the resulting voltage gradients and electric fields, have been implicated in embryonic development and regeneration. These biophysical signals are key physiological aspects of the microenvironment that epigenetically regulate stem and tumor cell behavior. Here, we identify a previously unrecognized function for KCNQ1, a potassium channel known to be involved in human Romano-Ward and Jervell-Lange-Nielsen syndromes when mutated. Misexpression of its modulatory wild-type beta-subunit XKCNE1 in the Xenopus embryo resulted in a striking alteration of the behavior of one type of embryonic stem cell: the pigment cell lineage of the neural crest. Depolarization of embryonic cells by misexpression of KCNE1 non-cell-autonomously induced melanocytes to overproliferate, spread out, and become highly invasive of blood vessels, liver, gut, and neural tube, leading to a deeply hyperpigmented phenotype. This effect is mediated by the up-regulation of Sox10 and Slug genes, thus linking alterations in ion channel function to the control of migration, shape, and mitosis rates during embryonic morphogenesis. Taken together, these data identify a role for the KCNQ1 channel in regulating key cell behaviors and reveal the molecular identity of a biophysical switch, by means of which neoplastic-like properties can be conferred upon a specific embryonic stem cell subpopulation.

Sensory neuron sodium current requires nongenomic actions of thyroid hormone during development

Development of the embryonic nervous system requires thyroid hormone. However, the underlying mechanisms and targets of thyroid hormone action are not well defined. To identify embryonic roles for thyroid hormone we tested for effects on a key neuronal trait, voltage-gated sodium current (I(Na)), in the zebrafish model system. We recorded from Rohon-Beard sensory neurons (RBs) using whole cell voltage-clamp methods. Here, we provide in vivo evidence for thyroid hormone regulation of I(Na). Chronic thyroid hormone application increased RB peak I(Na) density 1.4-fold. However, I(Na) density showed a similar increase within 5 min of an acute hormone application, a time course not expected for a genomic mechanism. Tetraiodothyroacetic acid (tetrac), a thyroid hormone blocker, blocked both chronic and acute effects. Further, the thyroid hormone precursor thyroxine (T4) affected I(Na), yet the traditionally active form triiodothyronine did not. Consequently, we tested for a nonconventional T4 receptor. LM609, a selective antagonist of integrin alphaVbeta3, occluded the rapid effect of T4, implicating a specific integrin dimer as a T4 receptor. Chronic application of either tetrac or LM609 significantly reduced sodium conductance, demonstrating an in vivo requirement for T4-integrin regulation of I(Na). Further, removing endogenous T4 levels via yolkectomy reduced sodium conductance, an effect that was partially rescued by T4 supplementation following surgery. Because RBs mediate the embryonic touch response, we tested for behavioral effects. Tetrac and LM609 significantly reduced the percentage of touch trials eliciting a normal touch response. T4's rapid effect on RB I(Na) highlights the importance of embryonic T4 availability and nongenomic T4 signaling.

Embryonic motor activity and implications for regulating motoneuron axonal pathfinding in zebrafish

Zebrafish embryos exhibit spontaneous contractions of the musculature as early as 18-19 h post fertilization (hpf) when removed from their protective chorion. These movements are likely initiated by early embryonic central nervous system activity. We have made the observation that narrowminded mutant embryos (hereafter, nrd(-/-)) lack normal embryonic motor output upon dechorionation. However, these mutants can swim and respond to tactile stimulation by larval stages of development. nrd(-/-) embryos exhibit defects in neural crest development, slow muscle development and also lack spinal mechanosensory neurons known as Rohon-Beard (RB) neurons. At early developmental stages (i.e. 21-22 hpf) and while still in their chorions, nrd siblings (nrd(+/?)) exhibited contractions of the musculature at a rate similar to wild-type embryos. Anatomical analysis indicated that RB neurons were present in the motile embryos, but absent in the non-motile embryos, indicating that the non-motile embryos were nrd(-/-) embryos. Further anatomical analysis of nrd(-/-) embryos revealed errors in motoneuron axonal pathfinding that persisted into the larval stage of development. These errors were reversed when nrd(-/-) embryos were raised in high [K(+)] beginning at 21 hpf, indicating that the abnormal axonal phenotypes may be related to a lack of depolarizing activity early in development. When activity was blocked with tricaine in wild-type embryos, motoneuron phenotypes were similar to the motoneuron phenotypes in nrd(-/-) embryos. These results implicate early embryonic activity in conjunction with other factors as necessary for normal motoneuron development.

UVA-induced photo recovery during early zebrafish embryogenesis

DNA photorepair has been widely studied in simple aquatic organisms that live in the marine environment, but is less understood in more complex species that live in freshwater. In the present study, we evaluated UVA-induced DNA photo recovery in embryonic stages of zebrafish, Danio rerio, a freshwater model species. Evaluation of UVB exposure and UVA photo recovery of zebrafish embryos revealed different UVB tolerances and capacities for UVA photo recovery at different stages of development. Effective UVA photo recovery was observed at 3h post-fertilization (hpf), 6-7 hpf, and 12 hpf, but not in the early cleavage stage (2-32 cells). UVA photo recovery was most effective during the gastrula stage (6-7 hpf) of development, and less effective at earlier stages (e.g., 3 hpf) or later stages (e.g., 12 hpf). Embryos at the cleavage stage of development were found to be tolerant to extreme levels of UVB exposure, and possible mechanisms were discussed. For embryos at 6-7 hpf, examination of time window (or delay of UVA exposure) that would still permit recovery from UVB exposure suggested a short time period of 2h. The transgenic fli-1 zebrafish with fluorescent vascular structure was used to show that embryos with normal morphological appearance could exhibit a disrupted vascular patterning, suggesting that this endpoint could provide a sensitive tool for detection of UV damage.

The Met receptor tyrosine kinase prevents zebrafish primary motoneurons from expressing an incorrect neurotransmitter

BACKGROUND

Expression of correct neurotransmitters is crucial for normal nervous system function. How neurotransmitter expression is regulated is not well-understood; however, previous studies provide evidence that both environmental signals and intrinsic differentiation programs are involved. One environmental signal known to regulate neurotransmitter expression in vertebrate motoneurons is Hepatocyte growth factor, which acts through the Met receptor tyrosine kinase and also affects other aspects of motoneuron differentiation, including axonal extension. Here we test the role of Met in development of motoneurons in embryonic zebrafish.

RESULTS

We found that met is expressed in all early developing, individually identified primary motoneurons and in at least some later developing secondary motoneurons. We used morpholino antisense oligonucleotides to knock down Met function and found that Met has distinct roles in primary and secondary motoneurons. Most secondary motoneurons were absent from met morpholino-injected embryos, suggesting that Met is required for their formation. We used chemical inhibitors to test several downstream pathways activated by Met and found that secondary motoneuron development may depend on the p38 and/or Akt pathways. In contrast, primary motoneurons were present in met morpholino-injected embryos. However, a significant fraction of them had truncated axons. Surprisingly, some CaPs in met morpholino antisense oligonucleotide (MO)-injected embryos developed a hybrid morphology in which they had both a peripheral axon innervating muscle and an interneuron-like axon within the spinal cord. In addition, in met MO-injected embryos primary motoneurons co-expressed mRNA encoding Choline acetyltransferase, the synthetic enzyme for their normal neurotransmitter, acetylcholine, and mRNA encoding Glutamate decarboxylase 1, the synthetic enzyme for GABA, a neurotransmitter never normally found in these motoneurons, but found in several types of interneurons. Our inhibitor studies suggest that Met function in primary motoneurons may be mediated through the MEK1/2 pathway.

CONCLUSION

We provide evidence that Met is necessary for normal development of zebrafish primary and secondary motoneurons. Despite their many similarities, our results show that these two motoneuron subtypes have different requirements for Met function during development, and raise the possibility that Met may act through different intracellular signaling cascades in primary and secondary motoneurons. Surprisingly, although met is not expressed in primary motoneurons until many hours after they have extended axons to and innervated their muscle targets, Met knockdown causes some of these cells to develop a hybrid phenotype in which they co-expressed motoneuron and interneuron neurotransmitters and have both peripheral and central axons.

Knockdown of zebrafish Nav1.6 sodium channel impairs embryonic locomotor activities

Although multiple subtypes of sodium channels are expressed in most neurons, the specific contributions of the individual sodium channels remain to be studied. The role of zebrafish Na(v)1.6 sodium channels in the embryonic locomotor movements has been investigated by the antisense morpholino (MO) knockdown. MO1 and MO2 are targeted at the regions surrounding the translation start site of zebrafish Na(v)1.6 mRNA. MO3 is targeted at the RNA splicing donor site of exon 2. The correctly spliced Na(v)1.6 mRNA of MO3 morphants is 6% relative to that of the wild-type embryos. Na(v)1.6-targeted MO1, MO2 and MO3 attenuate the spontaneous contraction, tactile sensitivity, and swimming in comparison with a scrambled morpholino and mutated MO3 morpholino. No significant defect is observed in the development of slow muscles, the axonal projection of primary motoneurons, and neuromuscular junctions. The movement impairments caused by MO1, MO2, and MO3 suggest that the function of Na(v)1.6 sodium channels is essential on the normal early embryonic locomotor activities.