Neural stem/progenitor cells from olfactory neuroepithelium collected by nasal brushing as a cell model reflecting molecular and cellular dysfunctions in schizophrenia.

OBJECTIVES: Neural stem/progenitor cells derived from olfactory neuroepithelium (hereafter olfactory neural stem/progenitor cells, ONSPCs) are emerging as a potential tool in the exploration of psychiatric disorders. The present study intended to assess whether ONSPCs could help discern individuals with schizophrenia (SZ) from non-schizophrenic (NS) subjects by exploring specific cellular and molecular features. METHODS: ONSPCs were collected from 19 in-patients diagnosed with SZ and 31 NS individuals and propagated in basal medium. Mitochondrial ATP production, expression of ß-catenin and cell proliferation, which are described to be altered in SZ, were examined in freshly isolated or newly thawed ONSPCs after a few culture passages. RESULTS: SZ-ONSPCs exhibited a lower mitochondrial ATP production and insensitivity to agents capable of positively or negatively affecting ß-catenin expression with respect to NS-ONSPCs. As to proliferation, it declined in SZ-ONSPCs as the number of culture passages increased compared to a steady level of growth shown by NS-ONSPCs. CONCLUSIONS: The ease and safety of sample collection as well as the differences observed between NS- and SZ-ONSPCs, may lay the groundwork for a new approach to obtain biological material from a large number of living individuals and gain a better understanding of the mechanisms underlying SZ pathophysiology.

Multiple isoforms of the Activin-like receptor baboon differentially regulate proliferation and conversion behaviors of neuroblasts and neuroepithelial cells in the Drosophila larval brain.

In Drosophila coordinated proliferation of two neural stem cells, neuroblasts (NB) and neuroepithelial (NE) cells, is pivotal for proper larval brain growth that ultimately determines the final size and performance of an adult brain. The larval brain growth displays two phases based on behaviors of NB and NEs: the first one in early larval stages, influenced by nutritional status and the second one in the last larval stage, promoted by ecdysone signaling after critical weight checkpoint. Mutations of the baboon (babo) gene that produces three isoforms (BaboA-C), all acting as type-I receptors of Activin-type transforming growth factor ß (TGF-ß) signaling, cause a small brain phenotype due to severely reduced proliferation of the neural stem cells. In this study we show that loss of babo function severely affects proliferation of NBs and NEs as well as conversion of NEs from both phases. By analyzing babo-null and newly generated isoform-specific mutants by CRISPR mutagenesis as well as isoform-specific RNAi knockdowns in a cell- and stage-specific manner, our data support differential contributions of the isoforms for these cellular events with BaboA playing the major role. Stage-specific expression of EcR-B1 in the brain is also regulated primarily by BaboA along with function of the other isoforms. Blocking EcR function in both neural stem cells results in a small brain phenotype that is more severe than baboA-knockdown alone. In summary, our study proposes that the Babo-mediated signaling promotes proper behaviors of the neural stem cells in both phases and achieves this by acting upstream of EcR-B1 expression in the second phase.

Gulf toadfish (Opsanus beta) gill neuroepithelial cells in response to hypoxia exposure.

Neuroepithelial cells (NECs) within the fish gill contain the monoamine neurochemical serotonin (5-HT), sense changes in the partial pressure of oxygen (PO2) in the surrounding water and blood, and initiate the cardiovascular and ventilatory responses to hypoxia. The distribution of neuroepithelial cells (NECs) within the gill is known for some fish species but not for the Gulf toadfish, Opsanus beta, a fish that has always been considered hypoxia tolerant. Furthermore, whether NEC size, number, or distribution changes after chronic exposure to hypoxia, has never been tested. We hypothesize that toadfish NECs will respond to hypoxia with an increase in NEC size, number, and a change in distribution. Juvenile toadfish (N = 24) were exposed to either normoxia (21.4 ± 0.0 kPa), mild hypoxia (10.2 ± 0.3 kPa), or severe hypoxia (3.1 ± 0.2 kPa) for 7 days and NEC size, number, and distribution for each O2 regime were measured. Under normoxic conditions, juvenile toadfish have similar NEC size, number, and distribution as other fish species with NECs along their filaments but not throughout the lamellae. The distribution of NECs did not change with hypoxia exposure. Mild hypoxia exposure had no effect on NEC size or number, but fish exposed to severe hypoxia had a higher NEC density (# per mm filament) compared to mild hypoxia-exposed fish. Fish exposed to severe hypoxia also had longer gill filament lengths that could not be explained by body weight. These results point to signs of phenotypic plasticity in these juvenile, lab-bred fish with no previous exposure to hypoxia and a strategy to deal with hypoxia exposure that differs in toadfish compared to other fish.

Loss of SHROOM3 affects neuroepithelial cell shape through regulating cytoskeleton proteins in cynomolgus monkey organoids.

Neural tube defects (NTDs) are severe congenital neurodevelopmental disorders arising from incomplete neural tube closure. Although folate supplementation has been shown to mitigate the incidence of NTDs, some cases, often attributable to genetic factors, remain unpreventable. The SHROOM3 gene has been implicated in NTD cases that are unresponsive to folate supplementation; at present, however, the underlying mechanism remains unclear. Neural tube morphogenesis is a complex process involving the folding of the planar epithelium of the neural plate. To determine the role of SHROOM3 in early developmental morphogenesis, we established a neuroepithelial organoid culture system derived from cynomolgus monkeys to closely mimic the in vivo neural plate phase. Loss of SHROOM3 resulted in shorter neuroepithelial cells and smaller nuclei. These morphological changes were attributed to the insufficient recruitment of cytoskeletal proteins, namely fibrous actin (F-actin), myosin II, and phospho-myosin light chain (PMLC), to the apical side of the neuroepithelial cells. Notably, these defects were not rescued by folate supplementation. RNA sequencing revealed that differentially expressed genes were enriched in biological processes associated with cellular and organ morphogenesis. In summary, we established an authentic in vitro system to study NTDs and identified a novel mechanism for NTDs that are unresponsive to folate supplementation.

Potential of olfactory neuroepithelial cells as a model to study schizophrenia: A focus on GPCRs (Review).

Schizophrenia (SZ) is a multifactorial disorder characterized by volume reduction in gray and white matter, oxidative stress, neuroinflammation, altered neurotransmission, as well as molecular deficiencies such as punctual mutation in Disrupted­in­Schizophrenia 1 protein. In this regard, it is essential to understand the underlying molecular disturbances to determine the pathophysiological mechanisms of the disease. The signaling pathways activated by G protein­coupled receptors (GPCRs) are key molecular signaling pathways altered in SZ. Convenient models need to be designed and validated to study these processes and mechanisms at the cellular level. Cultured olfactory stem cells are used to investigate neural molecular and cellular alterations related to the pathophysiology of SZ. Multipotent human olfactory stem cells are undifferentiated and express GPCRs involved in numerous physiological functions such as proliferation, differentiation and bioenergetics. The use of olfactory stem cells obtained from patients with SZ may identify alterations in GPCR signaling that underlie dysfunctional processes in both undifferentiated and specialized neurons or derived neuroglia. The present review aimed to analyze the role of GPCRs and their signaling in the pathophysiology of SZ. Culture of olfactory epithelial cells constitutes a suitable model to study SZ and other psychiatric disorders at the cellular level.

Homophilic interaction of cell adhesion molecule 3 coordinates retina neuroepithelial cell proliferation.

Correct cell number generation is central to tissue development. However, in vivo roles of coordinated proliferation of individual neural progenitors in regulating cell numbers of developing neural tissues and the underlying molecular mechanism remain mostly elusive. Here, we showed that wild-type (WT) donor retinal progenitor cells (RPCs) generated significantly expanded clones in host retinae with G1-lengthening by p15 (cdkn2a/b) overexpression (p15+) in zebrafish. Further analysis showed that cell adhesion molecule 3 (cadm3) was reduced in p15+ host retinae, and overexpression of either full-length or ectodomains of Cadm3 in p15+ host retinae markedly suppressed the clonal expansion of WT donor RPCs. Notably, WT donor RPCs in retinae with cadm3 disruption recapitulated expanded clones that were found in p15+ retinae. More strikingly, overexpression of Cadm3 without extracellular ig1 domain in RPCs resulted in expanded clones and increased retinal total cell number. Thus, homophilic interaction of Cadm3 provides an intercellular mechanism underlying coordinated cell proliferation to ensure cell number homeostasis of the developing neuroepithelia.

The control of breathing in fishes - historical perspectives and the path ahead.

The study of breathing in fishes has featured prominently in Journal of Experimental Biology (JEB), particularly during the latter half of the past century. Indeed, many of the seminal discoveries in this important sub-field of comparative respiratory physiology were reported first in JEB. The period spanning 1960-1990 (the 'golden age of comparative respiratory physiology') witnessed intense innovation in the development of methods to study the control of breathing. Many of the guiding principles of piscine ventilatory control originated during this period, including our understanding of the dominance of O2 as the driver of ventilation in fish. However, a critical issue - the identity of the peripheral O2 chemoreceptors - remained unanswered until methods for cell isolation, culture and patch-clamp recording established that gill neuroepithelial cells (NECs) respond to hypoxia in vitro. Yet, the role of the NECs and other putative peripheral or central chemoreceptors in the control of ventilation in vivo remains poorly understood. Further progress will be driven by the implementation of genetic tools, most of which can be used in zebrafish (Danio rerio). These tools include CRISPR/Cas9 for selective gene knockout, and Tol2 systems for transgenesis, the latter of which enables optogenetic stimulation of cellular pathways, cellular ablation and in vivo cell-specific biosensing. Using these methods, the next period of discovery will see the identification of the peripheral sensory pathways that initiate ventilatory responses, and will elucidate the nature of their integration within the central nervous system and their link to the efferent motor neurons that control breathing.

Spalt and disco define the dorsal-ventral neuroepithelial compartments of the developing Drosophila medulla.

Spatial patterning of neural stem cell populations is a powerful mechanism by which to generate neuronal diversity. In the developing Drosophila medulla, the symmetrically dividing neuroepithelial cells of the outer proliferation center crescent are spatially patterned by the nonoverlapping expression of 3 transcription factors: Vsx1 in the center, Optix in the adjacent arms, and Rx in the tips. These spatial genes compartmentalize the outer proliferation center and, together with the temporal patterning of neuroblasts, act to diversify medulla neuronal fates. The observation that the dorsal and ventral halves of the outer proliferation center also grow as distinct compartments, together with the fact that a subset of neuronal types is generated from only one half of the crescent, suggests that additional transcription factors spatially pattern the outer proliferation center along the dorsal-ventral axis. Here, we identify the spalt (salm and salr) and disco (disco and disco-r) genes as the dorsal-ventral patterning transcription factors of the outer proliferation center. Spalt and Disco are differentially expressed in the dorsal and ventral outer proliferation center from the embryo through to the third instar larva, where they cross-repress each other to form a sharp dorsal-ventral boundary. We show that hedgehog is necessary for Disco expression in the embryonic optic placode and that disco is subsequently required for the development of the ventral outer proliferation center and its neuronal progeny. We further demonstrate that this dorsal-ventral patterning axis acts independently of Vsx1-Optix-Rx and thus propose that Spalt and Disco represent a third outer proliferation center patterning axis that may act to further diversify medulla fates.

A lateral protrusion latticework connects neuroepithelial cells and is regulated during neurogenesis.

Dynamic contacts between cells within the developing neuroepithelium are poorly understood but play important roles in cell and tissue morphology and cell signalling. Here, using live-cell imaging and electron microscopy we reveal multiple protrusive structures in neuroepithelial apical endfeet of the chick embryonic spinal cord, including sub-apical protrusions that extend laterally within the tissue, and observe similar structures in human neuroepithelium. We characterise the dynamics, shape and cytoskeleton of these lateral protrusions and distinguish them from cytonemes, filopodia and tunnelling nanotubes. We demonstrate that lateral protrusions form a latticework of membrane contacts between non-adjacent cells, depend on actin but not microtubule dynamics, and provide a lamellipodial-like platform for further extending fine actin-dependent filipodia. We find that lateral protrusions depend on the actin-binding protein WAVE1 (also known as WASF1): misexpression of mutant WAVE1 attenuated protrusion and generated a round-ended apical endfoot morphology. However, this did not alter apico-basal cell polarity or tissue integrity. During normal neuronal delamination, lateral protrusions were withdrawn, but precocious protrusion loss induced by mutant WAVE1 was insufficient to trigger neurogenesis. This study uncovers a new form of cell-cell contact within the developing neuroepithelium, regulation of which prefigures neuronal delamination. This article has an associated First Person interview with the first author of the paper.

Neuroepithelial cells (NECs) and mucous cells express a variety of neurotransmitters and neurotransmitter receptors in the gill and respiratory air-sac of the catfish Heteropneustes fossilis (Siluriformes, Heteropneustidae): a possible role in local immune defence.

Heteropneustes fossilis is an air-breathing teleost inhabiting environments with very poor O2 conditions, and so it has evolved to cope with hypoxia. In the gills and respiratory air-sac, the sites for O2 sensing and the response to hypoxia rely on the expression of acetylcholine (Ach) acting via its nicotinic receptor (nAChR). This study examined the expression patterns of neuronal markers and some compounds in the NECs of the gills and respiratory air sac having an immunomodulatory function in mammalian lungs. Mucous cells, epithelial cells and neuroepithelial cells (NECs) were immunopositive to a variety of both neuronal markers (VAChT, nAChR, GABA-B-R1 receptor, GAD679) and the antimicrobial peptide piscidin, an evolutionary conserved humoral component of the mucosal immune system in fish. We speculate that Ach release via nAChR from mucous cells may be modulated by GABA production in the NECs and it is required for the induction of mucus production in both normoxic and hypoxic conditions. The presence of piscidin in mucous cells may act in synergy with the autocrine/paracrine signals of Ach and GABA binding to GABA B R1B receptor that may play a local immunomodulatory function in the mucous epithelia of the gills and the respiratory air sac. The potential role of the NECs in the immunobiological behaviour of the gill/air-sac is at moment a matter of speculation. The extent to which the NECs as such may participate is elusive at this stage and waits investigation.

Identification of disease-relevant modulators of the SHH pathway in the developing brain.

Pathogenic gene variants in humans that affect the sonic hedgehog (SHH) pathway lead to severe brain malformations with variable penetrance due to unknown modifier genes. To identify such modifiers, we established novel congenic mouse models. LRP2-deficient C57BL/6N mice suffer from heart outflow tract defects and holoprosencephaly caused by impaired SHH activity. These defects are fully rescued on a FVB/N background, indicating a strong influence of modifier genes. Applying comparative transcriptomics, we identified Pttg1 and Ulk4 as candidate modifiers upregulated in the rescue strain. Functional analyses showed that ULK4 and PTTG1, both microtubule-associated proteins, are positive regulators of SHH signaling, rendering the pathway more resilient to disturbances. In addition, we characterized ULK4 and PTTG1 as previously unidentified components of primary cilia in the neuroepithelium. The identification of genes that powerfully modulate the penetrance of genetic disturbances affecting the brain and heart is likely relevant to understanding the variability in human congenital disorders.

Replacement of mitochondrion-rich cells during regeneration of the gills and opercular epithelium in zebrafish (Danio rerio).

Transport epithelia maintain the volume, ion concentration and acid-base balance of blood and extracellular fluids. In teleost fish, mitochondrion-rich cells (MRCs) are specialized ionocytes that perform this role. These cells are found in epithelia of the gills and buccal surface of the operculum (the bony structure covering the gills). Proliferation of MRCs in response to changes in water salinity and other environmental stressors is well documented, but the cellular mechanisms underlying MRC proliferation are poorly understood. Recently, regeneration and epithelial cell replacement in the gill filaments was demonstrated in the model vertebrate, zebrafish (Danio rerio), raising the question of whether MRCs are replaced during regrowth of transport epithelia. We chose two anatomical sites where MRCs are found-the gills and the opercular epithelium-to investigate whether MRCs were replaced following surgical resection of these structures. In live imaging experiments, we observed gradual replacement of the branchiostegal valve, an extension of the operculum, in zebrafish over a period of 21 days post-resection (dpr). In regenerating epithelia of both the operculum and gills, we detected MRCs by immunohistochemical localization of the α subunit of plasma membrane Na+/K+-ATPase. In both tissues, MRCs appeared soon after resection, and as early as 1 dpr in the gill filaments. We report regeneration of the operculum and proliferation of MRCs in regenerating tissue in adult zebrafish. These studies may contribute to our understanding of how MRC populations are regulated during the regenerative process, which may occur following exposure to environmental stressors, chemical toxicity or disease.

Scribble mutation disrupts convergent extension and apical constriction during mammalian neural tube closure.

Morphogenesis of the vertebrate neural tube occurs by elongation and bending of the neural plate, tissue shape changes that are driven at the cellular level by polarized cell intercalation and cell shape changes, notably apical constriction and cell wedging. Coordinated cell intercalation, apical constriction, and wedging undoubtedly require complex underlying cytoskeletal dynamics and remodeling of adhesions. Mutations of the gene encoding Scribble result in neural tube defects in mice, however the cellular and molecular mechanisms by which Scrib regulates neural cell behavior remain unknown. Analysis of Scribble mutants revealed defects in neural tissue shape changes, and live cell imaging of mouse embryos showed that the Scrib mutation results in defects in polarized cell intercalation, particularly in rosette resolution, and failure of both cell apical constriction and cell wedging. Scrib mutant embryos displayed aberrant expression of the junctional proteins ZO-1, Par3, Par6, E- and N-cadherins, and the cytoskeletal proteins actin and myosin. These findings show that Scribble has a central role in organizing the molecular complexes regulating the morphomechanical neural cell behaviors underlying vertebrate neurulation, and they advance our understanding of the molecular mechanisms involved in mammalian neural tube closure.

Radial Glial Cells: New Views on Old Questions.

Radial glial cells (RGC) are at the center of brain development in vertebrates, acting as progenitors for neurons and macroglia (oligodendrocytes and astrocytes) and as guides for migration of neurons from the ventricular surface to their final positions in the brain. These cells originate from neuroepithelial cells (NEC) from which they inherit their epithelial features and polarized morphology, with processes extending from the ventricular to the pial surface of the embryonic cerebrum. We have learnt a great deal since the first descriptions of these cells at the end of the nineteenth century. However, there are still questions regarding how and when NEC transform into RGC or about the function of intermediate filaments such as glial fibrillary acidic protein (GFAP) in RGCs and their dynamics during neurogenesis. For example, it is not clear why RGCs in primates, including humans, express GFAP at the onset of cortical neurogenesis while in rodents it is expressed when it is essentially complete. Based on an ultrastructural analysis of GFAP expression and cell morphology of dividing progenitors in the developing neocortex of the macaque monkey, we show that RGCs become the main progenitor in the developing cerebrum by the start of neurogenesis, as all dividing cells show glial features such as GFAP expression and lack of tight junctions. Also, our data suggest that RGCs retract their apical process during mitosis. We discuss our findings in the context of the role and molecular characteristics of RGCs in the vertebrate brain, their differences with NECs and their dynamic behavior during the process of neurogenesis.

Cell non-autonomy amplifies disruption of neurulation by mosaic Vangl2 deletion in mice.

Post-zygotic mutations that generate tissue mosaicism are increasingly associated with severe congenital defects, including those arising from failed neural tube closure. Here we report that neural fold elevation during mouse spinal neurulation is vulnerable to deletion of the VANGL planar cell polarity protein 2 (Vangl2) gene in as few as 16% of neuroepithelial cells. Vangl2-deleted cells are typically dispersed throughout the neuroepithelium, and each non-autonomously prevents apical constriction by an average of five Vangl2-replete neighbours. This inhibition of apical constriction involves diminished myosin-II localisation on neighbour cell borders and shortening of basally-extending microtubule tails, which are known to facilitate apical constriction. Vangl2-deleted neuroepithelial cells themselves continue to apically constrict and preferentially recruit myosin-II to their apical cell cortex rather than to apical cap localisations. Such non-autonomous effects can explain how post-zygotic mutations affecting a minority of cells can cause catastrophic failure of morphogenesis leading to clinically important birth defects.

Mechanical Brain Injury Increases Cells' Production of Cystathionine ß-Synthase and Glutamine Synthetase, but Reduces Pax2 Expression in the Telencephalon of Juvenile Chum Salmon, Oncorhynchus keta.

The considerable post-traumatic brain recovery in fishes makes them a useful model for studying the mechanisms that provide reparative neurogenesis, which is poorly represented in mammals. After a mechanical injury to the telencephalon in adult fish, lost neurons are actively replaced due to the proliferative activity of neuroepithelial cells and radial glia in the neurogenic periventricular zone. However, it is not enough clear which signaling mechanisms are involved in the activation of adult neural stem cells (aNSC) after the injury (reactive proliferation) and in the production of new neurons (regenerative neurogenesis) from progenitor cells (NPC). In juvenile Pacific salmon, the predominant type of NSCs in the telencephalon are neuroepithelial cells corresponding to embryonic NSCs. Expression of glutamine synthetase (GS), a NSC molecular marker, was detected in the neuroepithelial cells of the pallium and subpallium of juvenile chum salmon, Oncorhynchus keta. At 3 days after a traumatic brain injury (TBI) in juvenile chum salmon, the GS expression was detected in the radial glia corresponding to aNSC in the pallium and subpallium. The maximum density of distribution of GS+ radial glia was found in the dorsal pallial region. Hydrogen sulfide (H2S) is a proneurogenic factor that reduces oxidative stress and excitotoxicity effects, along with the increased GS production in the brain cells of juvenile chum salmon. In the fish brain, H2S producing by cystathionine ß-synthase in neurogenic zones may be involved in maintaining the microenvironment that provides optimal conditions for the functioning of neurogenic niches during constitutive neurogenesis. After injury, H2S can determine cell survivability, providing a neuroprotective effect in the area of injury and reducing the process of glutamate excitotoxicity, acting as a signaling molecule involved in changing the neurogenic environment, which leads to the reactivation of neurogenic niches and cell regeneration programs. The results of studies on the control of the expression of regulatory Sonic Hedgehog genes (Shh) and the transcription factors Paired Box2 (Pax2) regulated by them are still insufficient. A comparative analysis of Pax2 expression in the telencephalon of intact chum salmon showed the presence of constitutive patterns of Pax2 expression in neurogenic areas and non-neurogenic parenchymal zones of the pallium and subpallium. After mechanical injury, the patterns of Pax2 expression changed, and the amount of Pax2+ decreased (p < 0.05) in lateral (Dl), medial (Dm) zones of the pallium, and the lateral zone (Vl) of the subpallium compared to the control. We believe that the decrease in the expression of Pax2 may be caused by the inhibitory effect of the Pax6 transcription factor, whose expression in the juvenile salmon brain increases upon injury.

Neural tube closure requires the endocytic receptor Lrp2 and its functional interaction with intracellular scaffolds.

Pathogenic mutations in the endocytic receptor LRP2 in humans are associated with severe neural tube closure defects (NTDs) such as anencephaly and spina bifida. Here, we have combined analysis of neural tube closure in mouse and in the African Clawed Frog Xenopus laevis to elucidate the etiology of Lrp2-related NTDs. Lrp2 loss of function impaired neuroepithelial morphogenesis, culminating in NTDs that impeded anterior neural plate folding and neural tube closure in both model organisms. Loss of Lrp2 severely affected apical constriction as well as proper localization of the core planar cell polarity (PCP) protein Vangl2, demonstrating a highly conserved role of the receptor in these processes, which are essential for neural tube formation. In addition, we identified a novel functional interaction of Lrp2 with the intracellular adaptor proteins Shroom3 and Gipc1 in the developing forebrain. Our data suggest that, during neurulation, motifs within the intracellular domain of Lrp2 function as a hub that orchestrates endocytic membrane removal for efficient apical constriction, as well as PCP component trafficking in a temporospatial manner.

Brain and gills as internal and external ammonia sensing organs for ventilatory control in rainbow trout, Oncorhynchus mykiss.

Ammonia is both a respiratory gas and a toxicant in teleost fish. Hyperventilation is a well-known response to elevations of both external and internal ammonia levels. Branchial neuroepithelial cells (NECs) are thought to serve as internal sensors of plasma ammonia (peripheral chemoreceptors), but little is known about other possible ammonia-sensors. Here, we investigated whether trout possess external sensors and/or internal central chemoreceptors for ammonia. For external sensors, we analyzed the time course of ventilatory changes at the start of exposure to high environmental ammonia (HEA, 1 mM). Hyperventilation developed gradually over 20 min, suggesting that it was a response to internal ammonia elevation. We also directly perfused ammonia solutions (0.01-1 mM) to the external surfaces of the first gill arches. Immediate hypoventilation occurred. For central chemoreceptors, we injected ammonia solutions (0.5-1.0 mM) directly onto the surface of the hindbrain of anesthetized trout. Immediate hyperventilation occurred. This is the first evidence of central chemoreception in teleost fish. We conclude that trout possess both external ammonia sensors, and dual internal ammonia sensors (perhaps for redundancy), but their roles differ. External sensors cause short term hypoventilation, which would help limit toxic waterborne ammonia uptake. When fish cannot avoid HEA, the diffusion of waterborne ammonia into the blood will stimulate both peripheral (NECs) and central (brain) chemoreceptors, resulting in hyperventilation. This hyperventilation will be beneficial in increasing ammonia excretion via the Rh metabolon system in the gills not only after HEA exposure, but also after endogenous ammonia loading from feeding or exercise.

Identification of oxygen-sensitive neuroepithelial cells through an endogenous reporter gene in larval and adult transgenic zebrafish.

In teleost fish, specialized oxygen (O2) chemoreceptors, called neuroepithelial cells (NECs), are found in the gill epithelium in adults. During development, NECs are present in the skin before the formation of functional gills. NECs are known for retaining the monoamine neurotransmitter, serotonin (5-HT) and are conventionally identified through immunoreactivity with antibodies against 5-HT or synaptic vesicle protein (SV2). However, identification of NECs in live tissue and isolated cell preparations has been challenging due to the lack of a specific marker. The present study explored the use of the transgenic zebrafish, ETvmat2:GFP, which expresses green fluorescent protein (GFP) under the control of the vesicular monoamine transporter 2 (vmat2) regulatory element, to identify NECs. Using immunohistochemistry and confocal microscopy, we confirmed that the endogenous GFP in ETvmat2:GFP labelled serotonergic NECs in the skin of larvae and in the gills of adults. NECs of the gill filaments expressed a higher level of endogenous GFP compared with other cells. The endogenous GFP also labelled intrabranchial neurons of the gill filaments. Flow cytometric analysis demonstrated that filamental NECs could be distinguished from other dissociated gill cells based on high GFP expression alone. Acclimation to 2 weeks of severe hypoxia (PO2 = 35 mmHg) induced an increase in filamental NEC frequency, size and GFP gene expression. Here we present for the first time a transgenic tool that labels O2 chemoreceptors in an aquatic vertebrate and its use in high-throughput experimentation.

Disruption of tph1 genes demonstrates the importance of serotonin in regulating ventilation in larval zebrafish (Danio rerio).

Serotonergic neuroepithelial cells (NECs) in larval zebrafish are believed to be O2 chemoreceptors. Serotonin (5-HT) within these NECs has been implicated as a neurotransmitter mediating the hypoxic ventilatory response (HVR). Here, we use knockout approaches to discern the role of 5-HT in regulating the HVR by targeting the rate limiting enzyme for 5-HT synthesis, tryptophan hydroxylase (Tph). Using transgenic lines, we determined that Tph1a is expressed in skin and pharyngeal arch NECs, as well as in pharyngeal arch Merkel-like cells (MLCs), whereas Tph1b is expressed predominately in MLCs. Knocking out the two tph1 paralogs resulted in similar changes in detectable serotonergic cell density between the two mutants, yet their responses to hypoxia (35 mmHg) were different. Larvae lacking Tph1a (tph1a-/- mutants) displayed a higher ventilation rate when exposed to hypoxia compared to wild-types, whereas tph1b-/- mutants exhibited a lower ventilation rate suggesting that 5-HT located in locations other than NECs, may play a dominant role in regulating the HVR.