Zebrafish motor neuron subtypes differ electrically prior to axonal outgrowth

Timing and precision of rattlesnake spinal motoneurons are determined by the KV72/3 potassium channel

The evolution of novel motor behaviors requires modifications in the central pattern generators (CPGs) controlling muscle activity. How such changes gradually lead to novel behaviors remains enigmatic due to the long time course of evolution. Rattlesnakes provide a unique opportunity to investigate how a locomotor CPG was evolutionarily modified to generate a novel behavior-in this case, acoustic signaling. We show that motoneurons (MNs) in the body and tail spinal cord of rattlesnakes possess fundamentally different physiological characteristics, which allow MNs in the tail to integrate and transmit CPG output for controlling superfast muscles with high temporal precision. Using patch-clamp electrophysiology, we demonstrate that these differences in locomotor and rattle MNs are mainly determined by KV72/3 potassium channels. However, although KV72/3 exerted a significantly different influence on locomotor and rattle MN physiology, single-cell RNA-seq unexpectedly did not reveal any differences in KV72/3 channels' expression. VIDEO ABSTRACT.

Mechanism of Bisphenol F Affecting Motor System and Motor Activity in Zebrafish

Bisphenol F (BPF; 4,4'-dihydroxydiphenylmethane) is one of the most frequently used compounds in the manufacture of plastics and epoxy resins. Previous studies have demonstrated that BPF affects locomotor behavior, oxidative stress, and neurodevelopment in zebrafish. However, its neurotoxic effects are controversial, and the underlying mechanisms are unclear. In order to determine whether BPF affects the motor system, we exposed zebrafish embryos to BPF and assessed behavioral, histological, and neurochemical changes. Spontaneous locomotor behavior and startle response were significantly decreased in BPF-treated zebrafish larvae compared with control larvae. BPF induced motor degeneration and myelination defects in zebrafish larvae. In addition, embryonic exposure to BPF resulted in altered metabolic profiles of neurochemicals, including neurotransmitters and neurosteroids, which may impact locomotion and motor function. In conclusion, exposure to BPF has the potential to affect survival, motor axon length, locomotor activity, myelination, and neurochemical levels of zebrafish larvae.

Developmental exposure to domoic acid targets reticulospinal neurons and leads to aberrant myelination in the spinal cord

Harmful algal blooms (HABs) produce neurotoxins that affect human health. Developmental exposure of zebrafish embryos to the HAB toxin domoic acid (DomA) causes myelin defects, loss of reticulospinal neurons, and behavioral deficits. However, it is unclear whether DomA primarily targets myelin sheaths, leading to the loss of reticulospinal neurons, or reticulospinal neurons, causing myelin defects. Here, we show that while exposure to DomA at 2 dpf did not reduce the number of oligodendrocyte precursors prior to myelination, it led to fewer myelinating oligodendrocytes that produced shorter myelin sheaths and aberrantly wrapped neuron cell bodies. DomA-exposed larvae lacked Mauthner neurons prior to the onset of myelination, suggesting that axonal loss is not secondary to myelin defects. The loss of the axonal targets may have led oligodendrocytes to inappropriately myelinate neuronal cell bodies. Consistent with this, GANT61, a GLI1/2 inhibitor that reduces oligodendrocyte number, caused a reduction in aberrantly myelinated neuron cell bodies in DomA-exposed fish. Together, these results suggest that DomA initially alters reticulospinal neurons and the loss of axons causes aberrant myelination of nearby cell bodies. The identification of initial targets and perturbed cellular processes provides a mechanistic understanding of how DomA alters neurodevelopment, leading to structural and behavioral phenotypes.

Modeling neuromuscular diseases in zebrafish

Neuromuscular diseases are a diverse group of conditions that affect the motor system and present some overlapping as well as distinct clinical manifestations. Although individually rare, the combined prevalence of NMDs is similar to Parkinson's. Over the past decade, new genetic mutations have been discovered through whole exome/genome sequencing, but the pathogenesis of most NMDs remains largely unexplored. Little information on the molecular mechanism governing the progression and development of NMDs accounts for the continual failure of therapies in clinical trials. Different aspects of the diseases are typically investigated using different models from cells to animals. Zebrafish emerges as an excellent model for studying genetics and pathogenesis and for developing therapeutic interventions for most NMDs. In this review, we describe the generation of different zebrafish genetic models mimicking NMDs and how they are used for drug discovery and therapy development.

Effects of the herbicide glyphosate on fish from embryos to adults: a review addressing behavior patterns and mechanisms behind them

The use of agrochemicals has grown in recent years following the increase in agricultural productivity, to eliminate weeds that can compromise crop yields. The intensive use of these products combined with the lack of treatment of agricultural wastewater is causing contamination of the natural environments, especially the aquatics. Glyphosate [N-(phosphonomethyl) glycine] is the most commonly used herbicide in agriculture worldwide. Studies have shown that this compound is toxic to a variety of fish species at the concentrations of environmental relevance. Glyphosate-based herbicides can affect fish biochemical, physiological, endocrine, and behavioral pathways. Changes in behaviors such as foraging, escaping from predators, and courtship can compromise the survival of species and even communities. The behavior patterns of fish has been shown to be a sensitive tool for risk assessment. In this sense, this review summarizes and discusses the toxic effects of glyphosate and its formulations on the behavior of fish in different life stages. Additionally, behavioral impairments were associated with other negative effects of glyphosate such as energy imbalance, stress responses, AChE inhibition, and physiological and endocrine disturbances, which are evidenced and described in the literature. Graphical abstract.

Nexmifa Regulates Axon Morphogenesis in Motor Neurons in Zebrafish

Nexmif is mainly expressed in the central nervous system (CNS) and plays important roles in cell migration, cell to cell and cell-matrix adhesion, and maintains normal synaptic formation and function. Nevertheless, it is unclear how nexmif is linked to motor neuron morphogenesis. Here, we provided in situ hybridization evidence that nexmifa (zebrafish paralog) was localized to the brain and spinal cord and acted as a vital regulator of motor neuron morphogenesis. Nexmifa deficiency in zebrafish larvae generated abnormal primary motor neuron (PMN) development, including truncated Cap axons and decreased branches in Cap axons. Importantly, RNA-sequencing showed that nexmifa-depleted zebrafish embryos caused considerable CNS related gene expression alterations. Differentially expressed genes (DEGs) were mainly involved in axon guidance and several synaptic pathways, including glutamatergic, GABAergic, dopaminergic, cholinergic, and serotonergic synapse pathways, according to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation. In particular, when compared with other pathways, DEGs were highest (84) in the axon guidance pathway, according to Organismal Systems. Efna5b, bmpr2b, and sema6ba were decreased markedly in nexmifa-depleted zebrafish embryos. Moreover, both overexpression of efna5b mRNA and sema6ba mRNA could partially rescued motor neurons morphogenesis. These observations supported nexmifa as regulating axon morphogenesis of motor neurons in zebrafish. Taken together, nexmifa elicited crucial roles during motor neuron development by regulating the morphology of neuronal axons.

Compromised N-Glycosylation Processing of Kv3.1b Correlates with Perturbed Motor Neuron Structure and Locomotor Activity

Neurological difficulties commonly accompany individuals suffering from congenital disorders of glycosylation, resulting from defects in the N-glycosylation pathway. Vacant N-glycosylation sites (N220 and N229) of Kv3, voltage-gated K+ channels of high-firing neurons, deeply perturb channel activity in neuroblastoma (NB) cells. Here we examined neuron development, localization, and activity of Kv3 channels in wildtype AB zebrafish and CRISPR/Cas9 engineered NB cells, due to perturbations in N-glycosylation processing of Kv3.1b. We showed that caudal primary (CaP) motor neurons of zebrafish spinal cord transiently expressing fully glycosylated (WT) Kv3.1b have stereotypical morphology, while CaP neurons expressing partially glycosylated (N220Q) Kv3.1b showed severe maldevelopment with incomplete axonal branching and extension around the ventral musculature. Consequently, larvae expressing N220Q in CaP neurons had impaired swimming locomotor activity. We showed that replacement of complex N-glycans with oligomannose attached to Kv3.1b and at cell surface lessened Kv3.1b dispersal to outgrowths by altering the number, size, and density of Kv3.1b-containing particles in membranes of rat neuroblastoma cells. Opening and closing rates were slowed in Kv3 channels containing Kv3.1b with oligomannose, instead of complex N-glycans, which suggested a reduction in the intrinsic dynamics of the Kv3.1b α-subunit. Thus, N-glycosylation processing of Kv3.1b regulates neuronal development and excitability, thereby controlling motor activity.

Graphene oxide nanosheets modulate spinal glutamatergic transmission and modify locomotor behaviour in an in vivo zebrafish model

Graphene oxide (GO), an oxidised form of graphene, is widely used for biomedical applications, due to its dispersibility in water and simple surface chemistry tunability. In particular, small (less than 500 nm in lateral dimension) and thin (1-3 carbon monolayers) graphene oxide nanosheets (s-GO) have been shown to selectively inhibit glutamatergic transmission in neuronal cultures in vitro and in brain explants obtained from animals injected with the nanomaterial. This raises the exciting prospect that s-GO can be developed as a platform for novel nervous system therapeutics. It has not yet been investigated whether the interference of the nanomaterial with neurotransmission may have a downstream outcome in modulation of behaviour depending specifically on the activation of those synapses. To address this problem we use early stage zebrafish as an in vivo model to study the impact of s-GO on nervous system function. Microinjection of s-GO into the embryonic zebrafish spinal cord selectively reduces the excitatory synaptic transmission of the spinal network, monitored in vivo through patch clamp recordings, without affecting spinal cell survival. This effect is accompanied by a perturbation in the swimming activity of larvae, which is the locomotor behaviour generated by the neuronal network of the spinal cord. Such results indicate that the impact of s-GO on glutamate based neuronal transmission is preserved in vivo and can induce changes in animal behaviour. These findings pave the way for use of s-GO as a modulator of nervous system function.

The Requirement of Sox2 for the Spinal Cord Motor Neuron Development of Zebrafish

Sex-determining region Y box 2 (Sox2), expressed in neural tissues, plays an important role as a transcription factor not only in the pluripotency and proliferation of neuronal cells but also in the opposite function of cell differentiation. Nevertheless, how Sox2 is linked to motor neuron development remains unknown. Here, we showed that Sox2 was localized in the motor neurons of spinal cord by in situ hybridization and cell separation, which acted as a positive regulator of motor neuron development. The deficiency of Sox2 in zebrafish larvae resulted in abnormal PMN development, including truncated but excessively branched CaP axons, loss of MiP, and increase of undifferentiated neuron cells. Importantly, transcriptome analysis showed that Sox2-depleted embryos caused many neurogenesis, axonogenesis, axon guidance, and differentiation-related gene expression changes, which further support the vital function of Sox2 in motor neuron development. Taken together, these data indicate that Sox2 plays a crucial role in the motor neuron development by regulating neuron differentiation and morphology of neuron axons.

Investigation of Islet2a function in zebrafish embryos: Mutants and morphants differ in morphologic phenotypes and gene expression

Zebrafish primary motor neurons differ from each other with respect to morphology, muscle targets and electrophysiological properties. For example, CaP has 2-3-fold larger densities of both inward and outward currents than do other motor neurons. We tested whether the transcription factor Islet2a, uniquely expressed in CaP, but not other primary motor neurons, plays a role in specifying its stereotypic electrophysiological properties. We used both TALEN-based gene editing and antisense morpholino approaches to disrupt Islet2a function. Our electrophysiology results do not support a specific role for Islet2a in determining CaP’s unique electrical properties. However, we also found that the morphological phenotypes of CaP and a later-born motor neuron differed between islet2a mutants and morphants. Using microarrays, we tested whether the gene expression profiles of whole embryo morphants, mutants and controls also differed. Morphants had 174 and 201 genes that were differentially expressed compared to mutants and controls, respectively. Further, islet2a was identified as a differentially expressed gene. To examine how mutation of islet2a affected islet gene expression specifically in CaPs, we performed RNA in situ hybridization. We detected no obvious differences in expression of islet1, islet2a, or islet2b in CaPs of mutant versus sibling control embryos. However, immunolabeling studies revealed that an Islet protein persisted in CaPs of mutants, albeit at a reduced level compared to controls. While we cannot exclude requirement for some Islet protein, we conclude that differentiation of the CaP’s stereotypic large inward and outward currents does not have a specific requirement for Islet2a.

Development, functional organization, and evolution of vertebrate axial motor circuits

Neuronal control of muscles associated with the central body axis is an ancient and essential function of the nervous systems of most animal species. Throughout the course of vertebrate evolution, motor circuits dedicated to control of axial muscle have undergone significant changes in their roles within the motor system. In most fish species, axial circuits are critical for coordinating muscle activation sequences essential for locomotion and play important roles in postural correction. In tetrapods, axial circuits have evolved unique functions essential to terrestrial life, including maintaining spinal alignment and breathing. Despite the diverse roles of axial neural circuits in motor behaviors, the genetic programs underlying their assembly are poorly understood. In this review, we describe recent studies that have shed light on the development of axial motor circuits and compare and contrast the strategies used to wire these neural networks in aquatic and terrestrial vertebrate species.

Zebrafish In Situ Spinal Cord Preparation for Electrophysiological Recordings from Spinal Sensory and Motor Neurons

Zebrafish, first introduced as a developmental model, have gained popularity in many other fields. The ease of rearing large numbers of rapidly developing organisms, combined with the embryonic optical clarity, served as initial compelling attributes of this model. Over the past two decades, the success of this model has been further propelled by its amenability to large-scale mutagenesis screens and by the ease of transgenesis. More recently, gene-editing approaches have extended the power of the model. For neurodevelopmental studies, the zebrafish embryo and larva provide a model to which multiple methods can be applied. Here, we focus on methods that allow the study of an essential property of neurons, electrical excitability. Our preparation for the electrophysiological study of zebrafish spinal neurons involves the use of veterinarian suture glue to secure the preparation to a recording chamber. Alternative methods for recording from zebrafish embryos and larvae involve the attachment of the preparation to the chamber using a fine tungsten pin^1,2,3,4,5. A tungsten pin is most often used to mount the preparation in a lateral orientation, although it has been used to mount larvae dorsal-side up⁴. The suture glue has been used to mount embryos and larvae in both orientations. Using the glue, a minimal dissection can be performed, allowing access to spinal neurons without the use of an enzymatic treatment, thereby avoiding any resultant damage. However, for larvae, it is necessary to apply a brief enzyme treatment to remove the muscle tissue surrounding the spinal cord. The methods described here have been used to study the intrinsic electrical properties of motor neurons, interneurons, and sensory neurons at several developmental stages^6,7,8,9.

pigk Mutation underlies macho behavior and affects Rohon-Beard cell excitability

The study of touch-evoked behavior allows investigation of both the cells and circuits that generate a response to tactile stimulation. We investigate a touch-insensitive zebrafish mutant, macho (maco), previously shown to have reduced sodium current amplitude and lack of action potential firing in sensory neurons. In the genomes of mutant but not wild-type embryos, we identify a mutation in the pigk gene. The encoded protein, PigK, functions in attachment of glycophosphatidylinositol anchors to precursor proteins. In wild-type embryos, pigk mRNA is present at times when mutant embryos display behavioral phenotypes. Consistent with the predicted loss of function induced by the mutation, knock-down of PigK phenocopies maco touch insensitivity and leads to reduced sodium current (INa) amplitudes in sensory neurons. We further test whether the genetic defect in pigk underlies the maco phenotype by overexpressing wild-type pigk in mutant embryos. We find that ubiquitous expression of wild-type pigk rescues the touch response in maco mutants. In addition, for maco mutants, expression of wild-type pigk restricted to sensory neurons rescues sodium current amplitudes and action potential firing in sensory neurons. However, expression of wild-type pigk limited to sensory cells of mutant embryos does not allow rescue of the behavioral touch response. Our results demonstrate an essential role for pigk in generation of the touch response beyond that required for maintenance of proper INa density and action potential firing in sensory neurons.

Presynaptic architecture of the larval zebrafish neuromuscular junction

This article shows the ultrastructural architecture of larval zebrafish (Danio rerio) neuromuscular junctions in three dimensions. We compare classical electron microscopy fixation techniques with high-pressure freezing followed by freeze substitution (HPF/FS) in combination with electron tomography. Furthermore, we compare the structure of neuromuscular junctions in 4- and 8-dpf zebrafish larvae with HPF/FS because this allows for close-to-native ultrastructural preservation. We discovered that synaptic vesicles of 4-dpf zebrafish larvae are larger than those of 8-dpf larvae. Furthermore, we describe two types of dense-core vesicles and quantify a filamentous network of small filaments interconnecting synaptic vesicles as well as tethers connecting synaptic vesicles to the presynaptic cell membrane. In the center of active zones, we found elaborate electron-dense projections physically connecting vesicles of the synaptic vesicle pool to the presynaptic membrane. Overall this study establishes the basis for systematic comparisons of synaptic architecture at high resolution in three dimensions of an intact vertebrate in a close-to-native state. Furthermore, we provide quantitative information that builds the basis for diverse systems biology approaches in neuroscience, from comparative anatomy to cellular simulations.

Spinal neurons require Islet1 for subtype-specific differentiation of electrical excitability

BACKGROUND

In the spinal cord, stereotypic patterns of transcription factor expression uniquely identify neuronal subtypes. These transcription factors function combinatorially to regulate gene expression. Consequently, a single transcription factor may regulate divergent development programs by participation in different combinatorial codes. One such factor, the LIM-homeodomain transcription factor Islet1, is expressed in the vertebrate spinal cord. In mouse, chick and zebrafish, motor and sensory neurons require Islet1 for specification of biochemical and morphological signatures. Little is known, however, about the role that Islet1 might play for development of electrical membrane properties in vertebrates. Here we test for a role of Islet1 in differentiation of excitable membrane properties of zebrafish spinal neurons.

RESULTS

We focus our studies on the role of Islet1 in two populations of early born zebrafish spinal neurons: ventral caudal primary motor neurons (CaPs) and dorsal sensory Rohon-Beard cells (RBs). We take advantage of transgenic lines that express green fluorescent protein (GFP) to identify CaPs, RBs and several classes of interneurons for electrophysiological study. Upon knock-down of Islet1, cells occupying CaP-like and RB-like positions continue to express GFP. With respect to voltage-dependent currents, CaP-like and RB-like neurons have novel repertoires that distinguish them from control CaPs and RBs, and, in some respects, resemble those of neighboring interneurons. The action potentials fired by CaP-like and RB-like neurons also have significantly different properties compared to those elicited from control CaPs and RBs.

CONCLUSIONS

Overall, our findings suggest that, for both ventral motor and dorsal sensory neurons, Islet1 directs differentiation programs that ultimately specify electrical membrane as well as morphological properties that act together to sculpt neuron identity.

Zebrafish models of human motor neuron diseases: advantages and limitations

Motor neuron diseases (MNDs) are an etiologically heterogeneous group of disorders of neurodegenerative origin, which result in degeneration of lower (LMNs) and/or upper motor neurons (UMNs). Neurodegenerative MNDs include pure hereditary spastic paraplegia (HSP), which involves specific degeneration of UMNs, leading to progressive spasticity of the lower limbs. In contrast, spinal muscular atrophy (SMA) involves the specific degeneration of LMNs, with symmetrical muscle weakness and atrophy. Amyotrophic lateral sclerosis (ALS), the most common adult-onset MND, is characterized by the degeneration of both UMNs and LMNs, leading to progressive muscle weakness, atrophy, and spasticity. A review of the comparative neuroanatomy of the human and zebrafish motor systems showed that, while the zebrafish was a homologous model for LMN disorders, such as SMA, it was only partially relevant in the case of UMN disorders, due to the absence of corticospinal and rubrospinal tracts in its central nervous system. Even considering the limitation of this model to fully reproduce the human UMN disorders, zebrafish offer an excellent alternative vertebrate model for the molecular and genetic dissection of MND mechanisms. Its advantages include the conservation of genome and physiological processes and applicable in vivo tools, including easy imaging, loss or gain of function methods, behavioral tests to examine changes in motor activity, and the ease of simultaneous chemical/drug testing on large numbers of animals. This facilitates the assessment of the environmental origin of MNDs, alone or in combination with genetic traits and putative modifier genes. Positive hits obtained by phenotype-based small-molecule screening using zebrafish may potentially be effective drugs for treatment of human MNDs.

Blurring the boundaries: developmental and activity-dependent determinants of neural circuits

The human brain comprises approximately 100 billion neurons that express a diverse, and often subtype-specific, set of neurotransmitters and voltage-gated ion channels. Given this enormous complexity, a fundamental question is how is this achieved? The acquisition of neurotransmitter phenotype was viewed as being set by developmental programs 'hard wired' into the genome. By contrast, the expression of neuron-specific ion channels was considered to be highly dynamic (i.e., 'soft wired') and shaped largely by activity-dependent mechanisms. Recent evidence blurs this distinction by showing that neurotransmitter phenotype can be altered by activity and that neuron type-specific ion channel expression can be set, and perhaps limited by, developmental programs. Better understanding of these early regulatory mechanisms may offer new avenues to avert the behavioral changes that are characteristic of many mental illnesses.

α3Na+/K+-ATPase deficiency causes brain ventricle dilation and abrupt embryonic motility in zebrafish

Na(+)/K(+)-ATPases are transmembrane ion pumps that maintain ion gradients across the basolateral plasma membrane in all animal cells to facilitate essential biological functions. Mutations in the Na(+)/K(+)-ATPase α3 subunit gene (ATP1A3) cause rapid-onset dystonia-parkinsonism, a rare movement disorder characterized by sudden onset of dystonic spasms and slow movements. In the brain, ATP1A3 is principally expressed in neurons. In zebrafish, the transcripts of the two ATP1A3 orthologs, Atp1a3a and Atp1a3b, show distinct expression in the brain. Surprisingly, targeted knockdown of either Atp1a3a or Atp1a3b leads to brain ventricle dilation, a likely consequence of ion imbalances across the plasma membrane that cause accumulation of cerebrospinal fluid in the ventricle. The brain ventricle dilation is accompanied by a depolarization of spinal Rohon-Beard neurons in Atp1a3a knockdown embryos, suggesting impaired neuronal excitability. This is further supported by Atp1a3a or Atp1a3b knockdown results where altered responses to tactile stimuli as well as abnormal motility were observed. Finally, proteomic analysis identified several protein candidates highlighting proteome changes associated with the knockdown of Atp1a3a or Atp1a3b. Our data thus strongly support the role of α3Na(+)/K(+)-ATPase in zebrafish motility and brain development, associating for the first time the α3Na(+)/K(+)-ATPase deficiency with brain ventricle dilation.

Activity-dependent competition regulates motor neuron axon pathfinding via PlexinA3

The role of electrical activity in axon guidance has been extensively studied in vitro. To better understand its role in the intact nervous system, we imaged intracellular Ca(2+) in zebrafish primary motor neurons (PMN) during axon pathfinding in vivo. We found that PMN generate specific patterns of Ca(2+) spikes at different developmental stages. Spikes arose in the distal axon of PMN and were propagated to the cell body. Suppression of Ca(2+) spiking activity in single PMN led to stereotyped errors, but silencing all electrical activity had no effect on axon guidance, indicating that an activity-based competition rule regulates this process. This competition was not mediated by synaptic transmission. Combination of PlexinA3 knockdown with suppression of Ca(2+) activity in single PMN produced a synergistic increase in the incidence of pathfinding errors. However, expression of PlexinA3 transcripts was not regulated by activity. Our results provide an in vivo demonstration of the intersection of spontaneous electrical activity with the PlexinA3 guidance molecule receptor in regulation of axon pathfinding.

A functional scaffold of CNS neurons for the vertebrates: the developing Xenopus laevis spinal cord

In young and developing amphibians and fish the spinal cord is functional but remarkably simple compared with the adult. Is the pattern of neurons and their connections common across at least these lower vertebrates? Does this basic pattern extend into the brainstem? Could the development of simple functioning neuronal networks depend on very basic rules of connectivity and act as pioneer networks providing a substrate for the development of more complex and subtle networks. In this review of the functional neuron classes in the Xenopus laevis tadpole spinal cord up to hatching, we will consider progress and difficulties in using anatomy, transcription factor expression, physiology, and activity to define spinal neuron types. Even here it is not straightforward and is rarely possible to bring all the different strands of evidence together. But, we think we have a rather complete picture of the hatchling tadpole spinal neuron types and can define clear roles for most of them in behavior. Our present knowledge about the hatchling Xenopus spinal cord should set up many of the problems to be unraveled in the future by more developmentally oriented research.

The LIM-homeodomain protein islet dictates motor neuron electrical properties by regulating K(+) channel expression

Neuron electrical properties are critical to function and generally subtype specific, as are patterns of axonal and dendritic projections. Specification of motoneuron morphology and axon pathfinding has been studied extensively, implicating the combinatorial action of Lim-homeodomain transcription factors. However, the specification of electrical properties is not understood. Here, we address the key issues of whether the same transcription factors that specify morphology also determine subtype specific electrical properties. We show that Drosophila motoneuron subtypes express different K⁺ currents and that these are regulated by the conserved Lim-homeodomain transcription factor Islet. Specifically, Islet is sufficient to repress a Shaker-mediated A-type K⁺ current, most likely due to a direct transcriptional effect. A reduction in Shaker increases the frequency of action potential firing. Our results demonstrate the deterministic role of Islet on the excitability patterns characteristic of motoneuron subtypes.

A gradient in endogenous rhythmicity and oscillatory drive matches recruitment order in an axial motor pool

The rhythmic firing behavior of spinal motoneurons is a function of their electrical properties and synaptic inputs. However, the relative contribution of endogenous versus network-based rhythmogenic mechanisms to locomotion is unclear. To address this issue, we have recorded from identified motoneurons and compared their current-evoked firing patterns to network-driven ones in the larval zebrafish (Danio rerio). Zebrafish axial motoneurons are recruited topographically from the bottom of the spinal cord up. Here, we have explored differences in the morphology of axial motoneurons, their electrical properties, and their synaptic drive, to reveal how they match the topographic pattern of recruitment. More ventrally located "secondary" motoneurons generate bursts of action potentials in response to constant current steps, demonstrating a strong inherent rhythmogenesis. The membrane potential oscillations underlying bursting behavior occur in the normal frequency range of swimming. In contrast, more dorsal secondaries chatter in response to current, while the most dorsally distributed "primary" motoneurons all fire tonically. We find that systematic variations in excitability and endogenous rhythmicity are inversely related to the level of oscillatory synaptic drive within the entire axial motor pool. Specifically, bursting cells exhibit the least amount of drive, while tonic cells exhibit the most. Our data suggest that increases in swimming frequency are accomplished by the recruitment of axial motoneurons that progressively rely on instructive synaptic drive to shape their oscillatory activity appropriately. Thus, within the zebrafish spinal cord, there are differences in the relative contribution of endogenous versus network-based rhythms to locomotion and these vary predictably according to order of recruitment.

Spinocerebellar ataxia type 13 mutation that is associated with disease onset in infancy disrupts axonal pathfinding during neuronal development

Spinocerebellar ataxia type 13 (SCA13) is an autosomal dominant disease caused by mutations in the Kv3.3 voltage-gated potassium (K⁺) channel. SCA13 exists in two forms: infant onset is characterized by severe cerebellar atrophy, persistent motor deficits and intellectual disability, whereas adult onset is characterized by progressive ataxia and progressive cerebellar degeneration. To test the hypothesis that infant- and adult-onset mutations have differential effects on neuronal development that contribute to the age at which SCA13 emerges, we expressed wild-type Kv3.3 or infant- or adult-onset mutant proteins in motor neurons in the zebrafish spinal cord. We characterized the development of CaP (caudal primary) motor neurons at ∼36 and ∼48 hours post-fertilization using confocal microscopy and 3D digital reconstruction. Exogenous expression of wild-type Kv3.3 had no significant effect on CaP development. In contrast, CaP neurons expressing the infant-onset mutation made frequent pathfinding errors, sending long, abnormal axon collaterals into muscle territories that are normally innervated exclusively by RoP (rostral primary) or MiP (middle primary) motor neurons. This phenotype might be directly relevant to infant-onset SCA13 because interaction with inappropriate synaptic partners might trigger cell death during brain development. Importantly, pathfinding errors were not detected in CaP neurons expressing the adult-onset mutation. However, the adult-onset mutation tended to increase the complexity of the distal axonal arbor. From these results, we speculate that infant-onset SCA13 is associated with marked changes in the development of Kv3.3-expressing cerebellar neurons, reducing their health and viability early in life and resulting in the withered cerebellum seen in affected children.

Activity-dependent neurotransmitter respecification

For many years it has been assumed that the identity of the transmitters expressed by neurons is stable and unchanging. Recent work, however, shows that electrical activity can respecify neurotransmitter expression during development and in the mature nervous system, and an understanding is emerging of the molecular mechanisms underlying activity-dependent transmitter respecification. Changes in postsynaptic neurotransmitter receptor expression accompany and match changes in transmitter specification, thus enabling synaptic transmission. The functional roles of neurotransmitter respecification are beginning to be understood and appear to involve homeostatic synaptic regulation, which in turn influences behaviour. Activation of this novel form of plasticity by sensorimotor stimuli may provide clinical benefits.

Spinocerebellar ataxia type 13 mutant potassium channel alters neuronal excitability and causes locomotor deficits in zebrafish

Whether changes in neuronal excitability can cause neurodegenerative disease in the absence of other factors such as protein aggregation is unknown. Mutations in the Kv3.3 voltage-gated K(+) channel cause spinocerebellar ataxia type 13 (SCA13), a human autosomal-dominant disease characterized by locomotor impairment and the death of cerebellar neurons. Kv3.3 channels facilitate repetitive, high-frequency firing of action potentials, suggesting that pathogenesis in SCA13 is triggered by changes in electrical activity in neurons. To investigate whether SCA13 mutations alter excitability in vivo, we expressed the human dominant-negative R420H mutant subunit in zebrafish. The disease-causing mutation specifically suppressed the excitability of Kv3.3-expressing, fast-spiking motor neurons during evoked firing and fictive swimming and, in parallel, decreased the precision and amplitude of the startle response. The dominant-negative effect of the mutant subunit on K(+) current amplitude was directly responsible for the reduced excitability and locomotor phenotype. Our data provide strong evidence that changes in excitability initiate pathogenesis in SCA13 and establish zebrafish as an excellent model system for investigating how changes in neuronal activity impair locomotor control and cause cell death.

Investigations of the in vivo requirements of transient receptor potential ion channels using frog and zebrafish model systems

Transient Receptor Potential (TRP) channels are cation channels that serve as cellular sensors on the plasma membrane, and have other less-well defined roles in intracellular compartments. The first TRP channel was identified upon molecular characterization of a fly mutant with abnormal photoreceptor function. More than 20 TRP channels have since been identified in vertebrates and invertebrate model systems, and these are divided into subfamilies based on structural similarities. The biophysical properties of TRP channels have primarily been explored in tissue culture models. The in vivo requirements for TRPs have been studied in invertebrate models like worm and flies, and also in vertebrate models, primarily mice and rats. Frog and zebrafish model systems offer certain experimental advantages relative to mammalian systems, and here a selection of papers which capitalize on these advantages to explore vertebrate TRP channel biology are reviewed. For instance, frog oocytes are useful for biochemistry and for electrophysiology, and these features were exploited in the identifcation TRPC1 as a candidate vertebrate mechanoreceptor. Also, the spinal neurons from frog embryos can be readily grown in culture. This feature was used to establish a role for TRPC1 in axon pathfinding in these neurons, and to explore how TRPC1 activity is regulated in this context. Zebrafish embryos are transparent making them well suited for in vivo imaging studies. This quality was exploited in a study in which the trpc2 gene promoter was used to label and trace the axon pathway of a subset of olfactory sensory neurons. Another experimental advantage of zebrafish is the speed and low cost of manipulating gene expression in embryos. Using these methods, it has been shown that TRPN1 is necessary for mechanosensation in zebrafish hair cells. Frogs and fish genomes have been mined to make inferences regarding evolutionary diversification of the thermosensitive TRP channels. Finally, TRPM7 is required for early morphogenesis in mice but not in fish; the reason for this difference is unclear, but it has caused zebrafish to be favored for exploration of TRPM7's role in later events in embryogenesis. The special experimental attributes of frogs and zebrafish suggest that these animals will continue to play an important role as models in future explorations of TRP channel biology.

Functional effects of spinocerebellar ataxia type 13 mutations are conserved in zebrafish Kv3.3 channels

Background

The zebrafish has been suggested as a model system for studying human diseases that affect nervous system function and motor output. However, few of the ion channels that control neuronal activity in zebrafish have been characterized. Here, we have identified zebrafish orthologs of voltage-dependent Kv3 (KCNC) K⁺ channels. Kv3 channels have specialized gating properties that facilitate high-frequency, repetitive firing in fast-spiking neurons. Mutations in human Kv3.3 cause spinocerebellar ataxia type 13 (SCA13), an autosomal dominant genetic disease that exists in distinct neurodevelopmental and neurodegenerative forms. To assess the potential usefulness of the zebrafish as a model system for SCA13, we have characterized the functional properties of zebrafish Kv3.3 channels with and without mutations analogous to those that cause SCA13.

Results

The zebrafish genome (release Zv8) contains six Kv3 family members including two Kv3.1 genes (kcnc1a and kcnc1b), one Kv3.2 gene (kcnc2), two Kv3.3 genes (kcnc3a and kcnc3b), and one Kv3.4 gene (kcnc4). Both Kv3.3 genes are expressed during early development. Zebrafish Kv3.3 channels exhibit strong functional and structural homology with mammalian Kv3.3 channels. Zebrafish Kv3.3 activates over a depolarized voltage range and deactivates rapidly. An amino-terminal extension mediates fast, N-type inactivation. The kcnc3a gene is alternatively spliced, generating variant carboxyl-terminal sequences. The R335H mutation in the S4 transmembrane segment, analogous to the SCA13 mutation R420H, eliminates functional expression. When co-expressed with wild type, R335H subunits suppress Kv3.3 activity by a dominant negative mechanism. The F363L mutation in the S5 transmembrane segment, analogous to the SCA13 mutation F448L, alters channel gating. F363L shifts the voltage range for activation in the hyperpolarized direction and dramatically slows deactivation.

Conclusions

The functional properties of zebrafish Kv3.3 channels are consistent with a role in facilitating fast, repetitive firing of action potentials in neurons. The functional effects of SCA13 mutations are well conserved between human and zebrafish Kv3.3 channels. The high degree of homology between human and zebrafish Kv3.3 channels suggests that the zebrafish will be a useful model system for studying pathogenic mechanisms in SCA13.

Developmental regulation of subtype-specific motor neuron excitability

At early embryonic stages, zebrafish spinal neuron subtypes can be distinguished and accessed for physiological studies. This provides the opportunity to determine electrophysiological properties of different spinal motor neuron subtypes. Such differences have the potential to then regulate, in a subtype-specific manner, activity-dependent developmental events such as axonal outgrowth and pathfinding. The zebrafish spinal cord contains a population of early born neurons. Our recent work has revealed that primary motor neuron (PMN) subtypes in the zebrafish spinal cord differ with respect to electrical properties during early important periods when PMNs extend axons to their specific targets. Here, we review recent findings regarding the development of electrical properties in PMN subtypes. Moreover, we consider the possibility that electrical activity in PMNs may play a cell nonautonomous role and thus influence the development of later developing motor neurons. Further, we discuss findings that support a role for a specific sodium channel isoform, Nav1.6, expressed by specific subtypes of spinal neurons in activity-dependent processes that impact axonal outgrowth and pathfinding.