Biology subjects

Jonz, M. G.

Publications and source records attributed to Jonz, M. G..

6 recordsLinked to original sources

bioRxiv2026

Mitochondrial ATP-dependent K+ channels downregulate ionotropic glutamate receptors during hypoxia in retinal horizontal cells of goldfish

Retinal neurons rely on continuous oxygen supply to maintain function, yet some species, like goldfish, withstand prolonged hypoxia due to adaptations that remain unclear. Horizontal cells (HCs) are inhibitory interneurons of the retina that express ionotropic glutamate receptors (iGluRs) and secrete gama-aminobutyric acid (GABA), allowing them to shape visual signalling by feeding back to photoreceptors. We investigated whether iGluRs in dissociated goldfish HCs are suppressed during hypoxia, and whether this process is mediated by oxygen-sensitive activation of mitochondrial ATP-sensitive K+ (mKATP) channels. Through perforated patch-clamp electrophysiology, we determined that 20 min of hypoxia reduced peak inward current density associated with glutamate application by ~25% (P = 0.0059). When mKATP channels were selectively blocked with 100 microM glibenclamide, hypoxic suppression of iGluRs was abolished. Inhibiting mitochondrial Ca2+ uptake via the mitochondrial Ca2+ uniporter with ruthenium red, or blocking Ca2+ release through ryanodine receptors in the endoplasmic reticulum, also abolished the hypoxia-induced suppression. Similarly, inhibition of calmodulin with calmidazolium, or of protein phosphatase 1 and 2A (PP1/PP2A) activity with calyculin A, prevented the hypoxia-induced suppression of iGluRs. Taken together, these results indicate that mKATP activation is required to mediate an intracellular Ca2+ signal that downregulates iGluR activity during hypoxia. Through the reduction of iGluR activity, HCs may lower their metabolic demand, allowing them to maintain cell function during hypoxia. Additionally, GABA release from HCs, as measured by mass spectrometry, was not affected by hypoxia, and we propose that downregulation of iGluR may account for hypoxic suppression of the retina in hypoxia-tolerant organisms.

physiology↗

bioRxiv2025

Serotonin stimulates proliferation of ionocytes via the 5-HT2A receptor in zebrafish larvae

Osmoregulation is an essential process in all living organisms. For aquatic organisms, such as freshwater fishes whose natural environment is hyperosmotic, specialized cells, called ionocytes, are present in the skin during developmental stages and contribute to the maintenance of osmotic homeostasis. Such cells are known to proliferate in response to osmotic stress, but the molecular mechanism by which that process is regulated remains poorly characterized. In this study, using immunohistochemistry and confocal microscopy, we demonstrate that cutaneous ionocytes in developing zebrafish (Danio rerio) express serotonin 2A (5-HT2A) receptors by co-labeling with other known ionocyte markers, such as the Na+/K+-ATPase, Concanavalin A and Mitotracker. Furthermore, by quantifying ionocyte number through early stages of development, we implicate 5-HT2A receptors in initiating ionocyte proliferation. Exposure of zebrafish embryos and larvae to acidic pH, or exogenous 5-HT, increased the number of cutaneous ionocytes. The effects of both stimuli were abolished in the presence of the 5-HT2A receptor-specific antagonist, ketanserin. Moreover, activation of 5-HT2A receptors led to increased detection of ionocytes with phosphorylated extracellular signal-related kinase (ERK), a key regulator of cell division and differentiation linked with 5-HT2A. We used tetrabenazine, an inhibitor of vesicular monoamine transporter 2 (vmat2) and 5-HT storage, to deplete potential sources of 5-HT. Tetrabenazine treatment in fish exposed to acidic pH reduced ionocyte proliferation, implicating an endogenous source of 5-HT in regulation of ionocyte populations.These results demonstrate the importance of a pathway initiated by 5-HT2A activation that regulates ionocyte proliferation in developing zebrafish exposed to osmotic stress. SUMMARY STATEMENTSerotonin stimulates 5-HT2A receptors on ionocytes leading to proliferation upon acclimation to acidic environments in zebrafish larvae.

cell biology↗

bioRxiv2025

Dual cholinergic and serotonergic excitatory pathways mediate oxygen sensing in the zebrafish gill

The evolution of oxygen sensing included a transition from a diffuse distribution of respiratory chemoreceptors in the gills of water-breathing vertebrates to chemoreceptor clusters confined to the pulmonary epithelium and carotid body in air-breathers. Since the excitatory neurotransmitters mediating oxygen sensing in anamniotes have never been confirmed, the origins of oxygen sensing in vertebrates have remained controversial. In gills isolated from Tg(elavl3:GCaMP6s) zebrafish expressing a genetically-encoded reporter of intracellular Ca2+ concentration ([Ca2+]i), we demonstrate that acetylcholine (ACh) and nicotine induced a dose-dependent increase in [Ca2+]i in postsynaptic sensory neurons innervating oxygen-chemoreceptive neuroepithelial cells (NECs). Hypoxic stimulation of NECs evoked a similar rise in neuronal [Ca2+]i that was abolished by nicotinic antagonist, hexamethonium. Using immunohistochemistry and RT-qPCR, we identified a novel population of ACh-containing NECs associated with sensory neurons expressing the 2 subunit of nicotinic ACh receptors. In vivo whole-larva Ca2+ imaging showed that cholinergic and hypoxic activation of the gills generated Ca2+ activity in neurons of vagal sensory ganglia with time-dependent characteristics of neurotransmission towards the hindbrain. We identified a second source of hypoxic activity in vagal sensory ganglia operating exclusively through 5-HT3 receptors and dependent upon vesicular monoamine transport (VMAT2) in the gill. We traced expression of 5-HT3 receptors to nerve terminals surrounding a separate population of serotonergic VMAT2-positive NECs. Our investigation reveals independent cholinergic and serotonergic autonomic pathways of oxygen sensing in zebrafish and provides the first physiological evidence that gill chemoreceptors may be homologues of both pulmonary and carotid body chemoreceptors in mammals. Significance statementThe excitatory neurotransmitters mediating oxygen sensing in anamniotes have never been confirmed. Thus, the origins of oxygen sensing in vertebrates have remained controversial. In transgenic zebrafish expressing a genetically-encoded reporter of intracellular Ca2+ concentration, we identified two independent pathways of oxygen sensing in the gill: one involving acetylcholine and interneurons intrinsic to the gill, and the other via serotonin acting directly upon ganglionic neurons. Both pathways resulted in excitation of vagal sensory ganglia that receive hypoxic inputs from the gills and innervate the hindbrain. We argue that gill chemoreceptors are homologues of both pulmonary and carotid body chemoreceptors in mammals.

neuroscience↗

bioRxiv2024

Oxygen chemoreceptor inhibition by dopamine D2 receptors in isolated zebrafish gills

Dopamine is an essential modulator of oxygen sensing and control of ventilation and was the first neurotransmitter described in the carotid body. Little is known of the evolutionary significance of dopamine in oxygen sensing, or whether it plays a similar role in anamniotes. In the model vertebrate, zebrafish (Danio rerio), presynaptic dopamine D2 receptor expression was demonstrated in gill neuroepithelial cells (NECs)--analogues of mammalian oxygen chemoreceptors; however, a mechanism for dopamine and D2 in oxygen sensing in the gills had not been defined. The present study tested the hypothesis that presynaptic D2 receptors provide a feedback mechanism that attenuates the chemoreceptor response to hypoxia. Using an isolated gill preparation from Tg(elavl3:GCaMP6s) zebrafish, we measured hypoxia-induced changes in intracellular Ca2+ concentration ([Ca2+]i) in NECs and postsynaptic neurons. Activation of D2 with dopamine or specific D2 agonist, quinpirole, decreased hypoxic responses in NECs; whereas D2 antagonist, domperidone, had the opposite effect. Addition of SQ22536, an adenylyl cyclase (AC) inhibitor, decreased the effect of hypoxia on [Ca2+]i, similar to dopamine. Activation of AC by forskolin partially recovered the suppressive effect of dopamine on the Ca2+ response to hypoxia. Further, we demonstrate that the response to hypoxia in postsynaptic sensory neurons was dependent upon innervation with NECs, and was subject to modulation by activation of presynaptic D2. Our results provide the first evidence of neurotransmission of the hypoxic signal at the NEC-nerve synapse in the gill and suggest that a presynaptic, modulatory role for dopamine in oxygen sensing arose early in vertebrate evolution. Key pointsO_LIFor the first time, we present an experimental model that permits imaging of intracellular Ca2+ in identified vertebrate oxygen chemoreceptors using GCaMP in a whole/intact sensing organ. C_LIO_LIThe hypoxic response of zebrafish chemoreceptors is attenuated by dopamine through a mechanism involving D2 receptors and adenylyl cyclase. C_LIO_LIZebrafish oxygen chemoreceptors send a hypoxic signal to postsynaptic (sensory) neurons. C_LIO_LIPostsynaptic neuronal responses to hypoxia are modulated by presynaptic D2 receptors, suggesting a link between chemoreceptor inhibition by dopamine and modulation of the hypoxic ventilatory response. C_LIO_LIOur results suggests that a modulatory role for dopamine in oxygen sensing arose early in vertebrate evolution. C_LI

physiology↗

bioRxiv2023

Identification of signalling pathways involved in gill regeneration in zebrafish

The occurrence of regeneration of the organs involved in respiratory gas exchange amongst vertebrates is heterogeneous. In aquatic animals, such as amphibians and fishes, the gills regenerate completely following resection or amputation; whereas in mammals, only partial, facultative regeneration of lung tissue occurs following injury. Given the homology between gills and lungs, the capacity of gill regeneration in aquatic species is of major interest in determining the underlying molecular or signalling pathways involved in respiratory organ regeneration. In the present study, we used adult zebrafish (Danio rerio) to characterize signalling pathways involved in the early stages of gill regeneration. Regeneration of the gills, including the gas exchange surfaces, was induced by resection of the gill filaments and observed over a period of up to 10 days. We screened for the effects of the drugs, SU5402, dorsomorphin, and LY411575, which inhibit FGF, BMP or Notch signalling, respectively, on development of the blastema. Exposure to each drug for 5 days significantly reduced blastema formation in regenerates, compared to unresected controls. In separate experiments, and under normal conditions of regeneration, we used quantitative real-time PCR and observed an increased expression of genes encoding for the bone morphogenetic factor, Bmp2b, fibroblast growth factor, Fgf8a, a transcriptional regulator (Her6) involved in Notch signalling, and Sonic Hedgehog (Shha), in regenerating gills at 10 day post-resection, compared to unresected controls. In situ hybridization confirmed that all four genes were expressed in regenerating gill tissue. This study implicates BMP, FGF, Notch and Shh signalling in gill regeneration in zebrafish. SUMMARY STATEMENTGill regeneration in zebrafish is mediated by multiple signalling pathways, including BMP, FGF, Notch and Sonic Hedgehog.

developmental biology↗

bioRxiv2022

Seasonal changes in membrane structure and excitability in central neurons of goldfish (Carassius auratus) under constant environmental conditions

Seasonal modifications in the structure of cellular membranes occur as an adaptive measure to withstand exposure to prolonged environmental change. Little is known about whether such changes may occur independently of external cues, such as photoperiod or temperature, or how they may impact the central nervous system (CNS). We compared membrane properties of central neurons isolated from the retina of goldfish (Carassius auratus), an organism well-adapted to extreme environmental change, during the summer and winter months. Goldfish were maintained in a facility under constant environmental conditions throughout the year. Analysis of whole-retina phospholipid composition using mass spectrometry-based lipidomics revealed a two-fold increase in phosphatidylethanolamine species during the winter, suggesting an increase in cell membrane fluidity. Atomic force microscopy was used to produce localized, nanoscale-force deformation of neuronal membranes. Measurement of Youngs modulus indicated increased membrane stiffness (or decreased elasticity) in neurons isolated during the winter. Voltage-clamp electrophysiology was used to assess physiological changes in neurons between seasons. Winter neurons displayed a hyperpolarized reversal potential (Vrev) and a significantly lower input resistance (Rin) compared to summer neurons. This was indicative of a decrease in membrane excitability during the winter. Subsequent measurement of intracellular Ca2+ activity using Fura-2 microspectrofluorometry confirmed a reduction in action potential activity, including duration and action potential profile, in neurons isolated during the winter. These studies demonstrate chemical and biophysical changes that occur in central neurons of goldfish throughout the year without exposure to seasonal cues, and suggest a novel mechanism of seasonal regulation of CNS activity. SUMMARY STATEMENTCentral neurons isolated from the retina of goldfish held under constant environmental conditions undergo seasonal changes in membrane structure and excitability.

neuroscience↗