Five Trk receptors in the zebrafish

Neural crest cells are sensitive to radiation-induced DNA damage

Radiation-induced DNA damage introduces mutations that have various deleterious effects, which may lead to apoptosis and carcinogenesis. Different tissues and cell types exhibit varying degrees of sensitivity to radiation-induced DNA damage, which is often attributed to the frequency of cell division. In this study, we showed that irradiation affects early zebrafish embryos in a manner that is not explained by direct DNA damage and repair nor by the frequency of cell division. Zebrafish embryos irradiated at 2 h post fertilization showed drastic apoptosis, mainly in the head region, during organogenesis. Herein, we show that these apoptotic cells did not show aneuploidy or micronuclei, and that not all descendants of the same cells with the same DNA damage were necessarily apoptotic. Finally, we demonstrate that apoptotic cells have various origins and that neural crest cells have a sensitive cell fate. Our results suggest the existence of a radiation damage response mechanism other than those previously described, the elucidation of which may inform strategies for greater protection against radiation injury.

rbfox1 loss in zebrafish drives HPI axis hyperactivation via bdnf/trkb2 pathway and allostatic overload during later life

Mutations in the RBFOX1 gene are associated with psychiatric disorders but how RBFOX1 influences psychiatric disorder vulnerability remains unclear. Recent studies showed that RBFOX proteins mediate the alternative splicing of PAC1, a critical HPA axis activator. Further, RBFOX1 dysfunction is linked to dysregulation of BDNF/TRKB, a pathway promoting neuroplasticity, neuronal survival, and stress resilience. Hence, RBFOX1 dysfunction may increase psychiatric disorder vulnerability via HPA axis dysregulation, leading to disrupted development and allostatic overload. To test this hypothesis, we generated a zebrafish rbfox1 loss-of-function (LoF) line and examined behavioural and molecular effects during development. We found that rbfox1 LoF mutants exhibited hyperactivity, impulsivity and heightened arousal, alongside alterations in proliferation - traits associated with neurodevelopmental and stress-related disorders. In adults, loss of rbfox1 function led to decreased fertility and survival, consistent with allostatic overload. At the molecular level, at larval stages rbfox1 LoF disrupted the expression of bdnf, trkb2, pac1a-hop and HPI axis genes. Pharmacological intervention targeting TRKB restored HPI axis and bdnf/trkb2 gene expression. In adults, dysregulation of bdnf/trkb2 and HPI axis genes was only seen following acute stress exposure. Our findings revealed a fundamental role for RBFOX1 in integrating stress responses through its regulation of BDNF/TRKB and neuroendocrine signalling. By uncovering how RBFOX1 modulates stress adaptation from development to adulthood, our study provides novel insights into the molecular basis of stress resilience and psychiatric disease vulnerability, suggesting RBFOX1 loss predisposes to psychiatric diseases through HPA axis hyperactivation during development, impairing adaptation and heightening vulnerability to allostatic overload in later life.

Neurotrophins and their receptors in the peripheral nervous system and non-nervous tissue of fish

Trophic factors, such as neurotrophins, are fundamental for cellular processes including differentiation, growth, survival, and regeneration. These molecules exhibit significant morphological and phylogenetic conservation throughout the animal kingdom, indicating conserved functions. In fish, the oldest and most diverse group of vertebrates, neurotrophins, and their receptors play pivotal roles not only within the central nervous system but also in various peripheral tissues. They are distributed in mechanosensory, muscle, skin, respiratory, circulatory, digestive, endocrine, urinary, reproductive, and immune systems, suggesting their involvement in the development and maintenance of all tissues/organs/systems. Despite this broad distribution, studies focusing on these molecules outside of the central nervous system have been limited to just 12 fish species. These investigations have revealed diverse expression patterns across different ages and tissues/organs/systems, expanding our comprehension of their functions beyond the central and peripheral nervous systems. Notably, BDNF and NT-3 are prominently expressed outside the central nervous system, particularly in mechanosensory and digestive tissues, whereas NGF is predominantly observed in mechanosensory and urinary systems. The expression and localization of neurotrophins and their receptors vary among organs, underscoring tissue-specific roles. Further research is imperative to decipher the precise functions and mechanisms of action of neurotrophins and their receptors in diverse fish tissues. Enhanced efforts are needed to include a broader range of fish species in these studies to advance our understanding of these agents in complex vertebrates, thereby shedding light on tissue development, regeneration, and maintenance, with potential implications for addressing organ-related issues.

Localization of Piezo 1 and Piezo 2 in Lateral Line System and Inner Ear of Zebrafish (Danio rerio)

Piezo proteins have been identified as mechanosensitive ion channels involved in mechanotransduction. Several ion channel dysfunctions may be associated with diseases (including deafness and pain); thus, studying them is critical to understand their role in mechanosensitive disorders and to establish new therapeutic strategies. The current study investigated for the first time the expression patterns of Piezo proteins in zebrafish octavolateralis mechanosensory organs. Piezo 1 and 2 were immunoreactive in the sensory epithelia of the lateral line system and the inner ear. Piezo 1 (28.7 ± 1.55 cells) and Piezo 2 (28.8 ± 3.31 cells) immunopositive neuromast cells were identified based on their ultrastructural features, and their overlapping immunoreactivity to the s100p specific marker (28.6 ± 1.62 cells), as sensory cells. These findings are in favor of Piezo proteins' potential role in sensory cell activation, while their expression on mantle cells reflects their implication in the maintenance and regeneration of the neuromast during cell turnover. In the inner ear, Piezo proteins' colocalization with BDNF introduces their potential implication in neuronal plasticity and regenerative events, typical of zebrafish mechanosensory epithelia. Assessing these proteins in zebrafish could open up new scenarios for the roles of these important ionic membrane channels, for example in treating impairments of sensory systems.

Jnk1 and downstream signalling hubs regulate anxiety-like behaviours in a zebrafish larvae phenotypic screen

Current treatments for anxiety and depression show limited efficacy in many patients, indicating the need for further research into the underlying mechanisms. JNK1 has been shown to regulate anxiety- and depressive-like behaviours in mice, however the effectors downstream of JNK1 are not known. Here we compare the phosphoproteomes from wild-type and Jnk1-/- mouse brains and identify JNK1-regulated signalling hubs. We next employ a zebrafish (Danio rerio) larvae behavioural assay to identify an antidepressant- and anxiolytic-like (AA) phenotype based on 2759 measured stereotypic responses to clinically proven antidepressant and anxiolytic (AA) drugs. Employing machine learning, we classify an AA phenotype from extracted features measured during and after a startle battery in fish exposed to AA drugs. Using this classifier, we demonstrate that structurally independent JNK inhibitors replicate the AA phenotype with high accuracy, consistent with findings in mice. Furthermore, pharmacological targeting of JNK1-regulated signalling hubs identifies AKT, GSK-3, 14-3-3 ζ/ε and PKCε as downstream hubs that phenocopy clinically proven AA drugs. This study identifies AKT and related signalling molecules as mediators of JNK1-regulated antidepressant- and anxiolytic-like behaviours. Moreover, the assay shows promise for early phase screening of compounds with anti-stress-axis properties and for mode of action analysis.

Neurotrophins and Trk Neurotrophin Receptors in the Retina of Adult Killifish (Nothobranchius guentheri)

Specific subpopulations of neurons in nerve and sensory systems must be developed and maintained, and this is accomplished in significant part by neurotrophins (NTs) and the signaling receptors on which they act, called tyrosine protein kinase receptors (Trks). The neurotrophins-tyrosine protein kinase receptors (NTs/Trks) system is involved in sensory organ regulation, including the visual system. An NTs/Trks system alteration is associated with neurodegeneration related to aging and diseases, including retinal pathologies. An emergent model in the field of translational medicine, for instance, in aging study, is the annual killifish belonging to the Nothobranchius genus, thanks to its short lifespan. Members of this genus, such as Nothobranchius guentheri, and humans share a similar retinal stratigraphy. Nevertheless, according to the authors' knowledge, the occurrence and distribution of the NTs/Trks system in the retina of N. guentheri has never been investigated before. Therefore, the present study aimed to localize neurotrophin BDNF, NGF, and NT-3 and TrkA, TrkB, and TrkC receptors in the N. guentheri retina using the immunofluorescence method. The present investigation demonstrates, for the first time, the occurrence of the NTs/Trks system in N. guentheri retina and, consequently, the potential key role of these proteins in the biology and survival of the retinal cells.

Transcription Pattern of Neurotrophic Factors and Their Receptors in Adult Zebrafish Spinal Cord

In vertebrates, neurotrophins and their receptors play a fundamental role in the central and peripheral nervous systems. Several studies reported that each neurotrophin/receptor signalling pathway can perform various functions during axon development, neuronal growth, and plasticity. Previous investigations in some fish species have identified neurotrophins and their receptors in the spinal cord under physiological conditions and after injuries, highlighting their potential role during regeneration. In our study, for the first time, we used an excellent animal model, the zebrafish (Danio rerio), to compare the mRNA localization patterns of neurotrophins and receptors in the spinal cord. We quantified the levels of mRNA using qPCR, and identified the transcription pattern of each neurotrophin/receptor pathway via in situ hybridization. Our data show that ngf/trka are the most transcribed members in the adult zebrafish spinal cord.

Intravitreal Administration of rhNGF Enhances Regenerative Processes in a Zebrafish Model of Retinal Degeneration

Nerve growth factor (NGF) is the best characterized neurotrophin, and it is known to play an important role in ocular homeostasis. Here, we demonstrated the expression of NGF receptors in adult zebrafish retina and optimized a light-induced retina degeneration (LID) zebrafish model that mimics human cone-rod disorders, demonstrating that intravitreal (IV) administration of rhNGF can boost zebrafish retinal regeneration in this model. Adult zebrafish retinae exposed to 60 h of light irradiation (60 h LID) displayed evident reduction of outer nuclear layer (ONL) thickness and cell number with presence of apoptotic cells. Retinal histologic evaluation at different time points showed that IV therapeutic injection of rhNGF resulted in an increase of ONL thickness and cell number at late time points after damage (14 and 21 days post injury), ultimately accelerating retinal tissue recovery by driving retinal cell proliferation. At a molecular level, rhNGF activated the ERK1/2 pathway and enhanced the regenerative potential of Müller glia gfap- and vim-expressing cells by stimulating at early time points the expression of the photoreceptor regeneration factor Drgal1-L2. Our results demonstrate the highly conserved nature of NGF canonical pathway in zebrafish and thus support the use of zebrafish models for testing new compounds with potential retinal regenerative properties. Moreover, the pro-regenerative effects of IV-injected NGF that we observed pave the way to further studies aimed at evaluating its effects also in mammals, in order to expedite the development of novel rhNGF-based therapeutic approaches for ophthalmological disorders.

The BDNF/TrkB Neurotrophin System in the Sensory Organs of Zebrafish

The brain-derived neurotrophic factor (BDNF) was discovered in the last century, and identified as a member of the neurotrophin family. BDNF shares approximately 50% of its amino acid with other neurotrophins such as NGF, NT-3 and NT-4/5, and its linear amino acid sequences in zebrafish (Danio rerio) and human are 91% identical. BDNF functions can be mediated by two categories of receptors: p75NTR and Trk. Intriguingly, BDNF receptors were highly conserved in the process of evolution, as were the other NTs’ receptors. In this review, we update current knowledge about the distribution and functions of the BDNF-TrkB system in the sensory organs of zebrafish. In fish, particularly in zebrafish, the distribution and functions of BDNF and TrkB in the brain have been widely studied. Both components of the system, associated or segregated, are also present outside the central nervous system, especially in sensory organs including the inner ear, lateral line system, retina, taste buds and olfactory epithelium.

Localization of Neurotrophin Specific Trk Receptors in Mechanosensory Systems of Killifish (Nothobranchius guentheri)

Neurotrophins (NTs) and their signal-transducing Trk receptors play a crucial role in the development and maintenance of specific neuronal subpopulations in nervous and sensory systems. NTs are supposed to regulate two sensory systems in fish, the inner ear and the lateral line system (LLS). The latter is one of the major mechanosensory systems in fish. Considering that annual fishes of the genus Nothobranchius, with their short life expectancy, have become a suitable model for aging studies and that the occurrence and distribution of neurotrophin Trk receptors have never been investigated in the inner ear and LLS of killifish (Nothobranchius guentheri), our study aimed to investigate the localization of neurotrophin-specific Trk receptors in mechanosensory systems of N. guentheri. For histological and immunohistochemical analysis, adult specimens of N. guentheri were processed using antibodies against Trk receptors and S100 protein. An intense immunoreaction for TrkA and TrkC was found in the sensory cells of the inner ear as well as in the hair cells of LLS. Moreover, also the neurons localized in the acoustic ganglia displayed a specific immunoreaction for all Trk receptors (TrkA, B, and C) analyzed. Taken together, our results demonstrate, for the first time, that neurotrophins and their specific receptors could play a pivotal role in the biology of the sensory cells of the inner ear and LLS of N. guentheri and might also be involved in the hair cells regeneration process in normal and aged conditions.

Expression of the neurotrophic tyrosine kinase receptors, ntrk1 and ntrk2a, precedes expression of other ntrk genes in embryonic zebrafish

Background

The neurotrophic tyrosine kinase receptor (Ntrk) gene family plays a critical role in the survival of somatosensory neurons. Most vertebrates have three Ntrk genes each of which encode a Trk receptor: TrkA, TrkB, or TrkC. The function of the Trk receptors is modulated by the p75 neurotrophin receptors (NTRs). Five ntrk genes and one p75 NTR gene (ngfrb) have been discovered in zebrafish. To date, the expression of these genes in the initial stages of neuron specification have not been investigated.

Purpose

The present work used whole mount in situ hybridization to analyze expression of the five ntrk genes and ngfrb in zebrafish at a timepoint when the first sensory neurons of the zebrafish body are being established (16.5 hpf). Because expression of multiple genes were not found at this time point, we also checked expression at 24 hpf to ensure the functionality of our six probes.

Results

At 16.5 hpf, we found tissue specific expression of ntrk1 in cranial ganglia, and tissue specific expression of ntrk2a in cranial ganglia and in the spinal cord. Other genes analyzed at 16.5 hpf were either diffuse or not detected. At 24 hpf, we found expression of both ntrk1 and ntrk2a in the spinal cord as well as in multiple cranial ganglia, and we identified ngfrb expression in cranial ganglia at 24 hpf. ntrk2b, ntrk3a and ntrk3b were detected in the developing brain at 24 hpf.

Conclusion

These data are the first to demonstrate that ntrk1 and ntrk2a are the initial neurotrophic tyrosine kinase receptors expressed in sensory neurons during the development of the zebrafish body, and the first to establish expression patterns of ngfrb during early zebrafish development. Our data indicate co-expression of ntrk1, ntrk2a and ngfrb, and we speculate that these overlapping patterns indicate relatedness of function.

Feeding and food availability modulate brain-derived neurotrophic factor, an orexigen with metabolic roles in zebrafish

Emerging findings point to a role for brain-derived neurotrophic factor (BDNF) on feeding in mammals. However, its role on energy balance is unclear. Moreover, whether BDNF regulates energy homeostasis in non-mammals remain unknown. This research aimed to determine whether BDNF is a metabolic peptide in zebrafish. Our results demonstrate that BDNF mRNAs and protein, as well as mRNAs encoding its receptors trkb2, p75ntra and p75ntrb, are detectable in the zebrafish brain, foregut and liver. Intraperitoneal injection of BDNF increased food intake at 1, 2 and 6 h post-administration, and caused an upregulation of brain npy, agrp and orexin, foregut ghrelin, and hepatic leptin mRNAs, and a reduction in brain nucb2. Fasting for 7 days increased bdnf and p75ntrb mRNAs in the foregut, while decreased bdnf, trkb2, p75ntra and p75ntrb mRNAs in the brain and liver. Additionally, the expression of bdnf and its receptors increased preprandially, and decreased after a meal in the foregut and liver. Finally, we observed BDNF-induced changes in the expression and/or activity of enzymes involved in glucose and lipid metabolism in the liver. Overall, present results indicate that BDNF is a novel regulator of appetite and metabolism in fish, which is modulated by energy intake and food availability.

Cholinergic System and NGF Receptors: Insights from the Brain of the Short-Lived Fish Nothobranchius furzeri

Nerve growth factor (NGF) receptors are evolutionary conserved molecules, and in mammals are considered necessary for ensuring the survival of cholinergic neurons. The age-dependent regulation of NTRK1/NTRKA and p75/NGFR in mammalian brain results in a reduced response of the cholinergic neurons to neurotrophic factors and is thought to play a role in the pathogenesis of neurodegenerative diseases. Here, we study the age-dependent expression of NGF receptors (NTRK1/NTRKA and p75/NGFR) in the brain of the short-lived teleost fish Nothobranchius furzeri. We observed that NTRK1/NTRKA is more expressed than p75/NGFR in young and old animals, although both receptors do not show a significant age-dependent change. We then study the neuroanatomical organization of the cholinergic system, observing that cholinergic fibers project over the entire neuroaxis while cholinergic neurons appear restricted to few nuclei situated in the equivalent of mammalian subpallium, preoptic area and rostral reticular formation. Finally, our experiments do not confirm that NTRK1/NTRKA and p75/NGFR are expressed in cholinergic neuronal populations in the adult brain of N. furzeri. To our knowledge, this is the first study where NGF receptors have been analyzed in relation to the cholinergic system in a fish species along with their age-dependent modulation. We observed differences between mammals and fish, which make the African turquoise killifish an attractive model to further investigate the fish specific NGF receptors regulation.

Neuroanatomical distribution and functions of brain-derived neurotrophic factor in zebrafish (Danio rerio) brain

Brain-derived neurotrophic factor (BDNF) is an extensively studied protein that is evolutionarily conserved and widely distributed in the brain of vertebrates. It acts via its cognate receptors TrkB and p75^NTR and plays a central role in the developmental neurogenesis, neuronal survival, proliferation, differentiation, synaptic plasticity, learning and memory, adult hippocampal neurogenesis, and brain regeneration. BDNF has also been implicated in a plethora of neurological disorders. Hence, understanding the processes that are controlled by BDNF and their regulating mechanisms is important. Although, BDNF has been thoroughly studied in the mammalian models, contradictory effects of its functions have been reported on several occasions. These contradictory effects may be attributed to the sheer complexity of the mammalian brain. The study of BDNF and its associated functions in a simpler vertebrate model may provide some clarity about the effects of BDNF on the neurophysiology of the brain. Keeping that in mind, this review aims at summarizing the current knowledge about the distribution of BDNF and its associated functions in the zebrafish brain. The main focus of the review is to give a comparative overview of BDNF distribution and function in zebrafish and mammals with respect to distinct life stages. We have also reviewed the regulation of bdnf gene in zebrafish and discussed its role in developmental and adult neurogenesis.

Neurotrophin receptor Ntrk2b function in the maintenance of dopamine and serotonin neurons in zebrafish

Neurotrophins and their receptors have highly conserved evolutionary lineage in vertebrates including zebrafish. The NTRK2 receptor has two isoforms in zebrafish, Ntrk2a and Ntrk2b. The spatio-temporal expression pattern of bdnf and ntrk2b in the zebrafish brain was studied using in situ hybridization. The robust and corresponding expression pattern of ntrk2b to bdnf suggests that ntrk2b is the key receptor for bdnf in the zebrafish brain, unlike its duplicate isoform ntrk2a. To study ntrk2b function, two different genetic strategies, the TILLING mutant and morpholino oligonucleotides (MO), were used. Specific subsets of the dopaminergic and serotonergic neuronal populations were affected in the mutants and morphants. The mutant showed anxiety- like behavior both in larval and adult stages. Our results consistently indicate that BDNF/NTRK2 signaling has a significant role in the development and maintenance of aminergic neuronal populations. Therefore, the ntrk2b-deficient zebrafish is well suited to study mechanisms relevant for psychiatric disorders attributed to a dysfunctional monoaminergic system.

BDNF, Brain, and Regeneration: Insights from Zebrafish

Zebrafish (Danio rerio) is a teleost fish widely accepted as a model organism for neuroscientific studies. The adults show common basic vertebrate brain structures, together with similar key neuroanatomical and neurochemical pathways of relevance to human diseases. However, the brain of adult zebrafish possesses, differently from mammals, intense neurogenic activity, which can be correlated with high regenerative properties. Brain derived neurotrophic factor (BDNF), a member of the neurotrophin family, has multiple roles in the brain, due also to the existence of several biologically active isoforms, that interact with different types of receptors. BDNF is well conserved in the vertebrate evolution, with the primary amino acid sequences of zebrafish and human BDNF being 91% identical. Here, we review the available literature regarding BDNF in the vertebrate brain and the potential involvement of BDNF in telencephalic regeneration after injury, with particular emphasis to the zebrafish. Finally, we highlight the potential of the zebrafish brain as a valuable model to add new insights on future BDNF studies.

TrkB receptor antagonism inhibits stab injury induced proliferative response in adult zebrafish (Danio rerio) brain

The Tropomyosin related kinase B (TrkB) receptor, is known to promote neuronal maturation, differentiation, maintenance and survival through its cognate ligands Brain derived neurotrophic factor (BDNF) and neurotrophin 4 (NT4). BDNF, NT4 and TrkB are highly conserved proteins among vertebrates. Although the role of TrkB during brain development is well established, its role in adult neurogenesis and brain regeneration awaits thorough investigation. In this study, we used the zebrafish stab wound injury model to determine whether the injury induced regeneration response in the telencephalon region is governed by TrkB or not. We induced stab wound injury in the mid-dorsal region of telencephalon of ANA-12 (selective TrkB antagonist) treated and non-treated zebrafish brain and examined the proliferation activity in selected brain regions using immunohistochemistry. We found that proliferation activity was significantly low in ANA-12 injected injured fishes as compared to vehicle control injured fishes. Other major findings of the study include the temporal pattern of proliferation activity after an injury and activation of adult neural stem cells (aNSCs) situated distantly apart from the injury site in the adult zebrafish brain.

Role of brain-derived neurotrophic factor during the regenerative response after traumatic brain injury in adult zebrafish

Several mammalian animal models of traumatic brain injury have been used, mostly rodents. However, reparative mechanisms in mammalian brain are very limited, and newly formed neurons do not survive for long time. The brain of adult zebrafish, a teleost fish widely used as vertebrate model, possesses high regenerative properties after injury due to the presence of numerous stem cells niches. The ventricular lining of the zebrafish dorsal telencephalon is the most studied neuronal stem cell niche because its dorso-lateral zone is considered the equivalent to the hippocampus of mammals which contains one of the two constitutive neurogenic niches of mammals. To mimic TBI, stab wound in the dorso-lateral telencephalon of zebrafish was used in studies devoted to fish regenerative properties. Brain-derived neurotrophic factor, which is known to play key roles in the repair process after traumatic brain lesions, persists around the lesioned area of injured telencephalon of adult zebrafish. These results are extensively compared to reparative processes in rodent brain. Considering the complete repair of the damaged area in fish, it could be tempting to consider brain-derived neurotrophic factor as a factor contributing to create a permissive environment that enables the establishment of new neuronal population in damaged brain.

Morpho-Functional Features of the Gonads of Danio rerio: the Role of Brain-Derived Neurotrophic Factor

Zebrafish, a suitable and widely used teleost fish model in basic biomedical research, displays morphophysiological features of adult gonads that share some commonalities with those of mammalian species. In mammals, gametogenesis is regulated, among several factors, by brain-derived neurotrophic factor (BDNF). This neurotrophin has a well-established role in the developing and adult nervous system, as well as gonads development and functions in vertebrate species. We hypothesize that BDNF has a role also in the gonadal functions of zebrafish. At this purpose, we investigated BDNF and its receptors p75 and TrkB in the ovary and testis of adult zebrafish, kept under laboratory conditions. Our results display (1) the expression of BDNF mRNA and pro-BDNF protein outside of the nervous system, specifically in the ovary and testis; (2) the presence of pro-BDNF in primary oocytes and follicular layer, and p75 in follicular cells; (3) the localization of pro-BDNF in type B spermatogonia, and Sertoli cells in testis. Altogether, these data lead us to consider that BDNF is involved in the gonadal function of adult zebrafish, and mainly in the adult ovary. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 301:140-147, 2018. © 2017 Wiley Periodicals, Inc.

An ancient neurotrophin receptor code; a single Runx/Cbfβ complex determines somatosensory neuron fate specification in zebrafish

In terrestrial vertebrates such as birds and mammals, neurotrophin receptor expression is considered fundamental for the specification of distinct somatosensory neuron types where TrkA, TrkB and TrkC specify nociceptors, mechanoceptors and proprioceptors/mechanoceptors, respectively. In turn, Runx transcription factors promote neuronal fate specification by regulating neurotrophin receptor and sensory receptor expression where Runx1 mediates TrkA+ nociceptor diversification while Runx3 promotes a TrkC+ proprioceptive/mechanoceptive fate. Here, we report in zebrafish larvae that orthologs of the neurotrophin receptors in contrast to terrestrial vertebrates mark overlapping and distinct subsets of nociceptors suggesting that TrkA, TrkB and TrkC do not intrinsically promote nociceptor, mechanoceptor and proprioceptor/mechanoceptor neuronal fates, respectively. While we find that zebrafish Runx3 regulates nociceptors in contrast to terrestrial vertebrates, it shares a conserved regulatory mechanism found in terrestrial vertebrate proprioceptors/mechanoceptors in which it promotes TrkC expression and suppresses TrkB expression. We find that Cbfβ, which enhances Runx protein stability and affinity for DNA, serves as an obligate cofactor for Runx in neuronal fate determination. High levels of Runx can compensate for the loss of Cbfβ, indicating that in this context Cbfβ serves solely as a signal amplifier of Runx activity. Our data suggests an alteration/expansion of the neurotrophin receptor code of sensory neurons between larval teleost fish and terrestrial vertebrates, while the essential roles of Runx/Cbfβ in sensory neuron cell fate determination while also expanded are conserved.

Our perception of the external world comes from our senses. Often overlooked the skin is our largest sensory organ. Specialized neurons located in the dorsal root ganglion (DRG), which innervate the body, and trigeminal ganglion (TG), which innervate the face, sense the somatosensory perceptions: light touch, temperature, pain (nociceptors) and muscle/limb position (proprioception) via nerve endings that project to the skin. These neurons receive and relay information from these diverse stimuli through distinct subclasses of neurons. Since these neurons arise from common lineages, they provide an excellent system to study how neurons develop and diversify into different subtypes. Runx transcription factors have been shown in terrestrial vertebrates (birds and mammals) to be instrumental in specifying nociceptor and proprioceptor populations by regulating the expression of a class of genes that code for the neurotrophin receptors, which are thought to be essential for specifying these neuronal fates. In our study we show that mechanisms by which Runx transcription factors regulate neurotrophin receptor expression are conserved between zebrafish and terrestrial vertebrates, yet the type of neuron specified by these genes are different such that in zebrafish the neurotrophin receptor TrkC is expressed in a nociceptor lineage instead of the proprioceptor/mechanoreceptor lineage as in terrestrial vertebrates. These data demonstrate that the specification of neuronal lineages is not fundamental to a given neurotrophin receptor but has adapted and evolved from the time fish and terrestrial vertebrates diverged 350 million years ago. Furthermore we show in fish that zebrafish Runx3 has properties that are divided between Runx1 and Runx3 in terrestrial vertebrates. Finally we show that the Runx co-factor Cbfβ is essential for its function, but the high level of Runx3 expression can overcome the loss of Cbfβ, demonstrating that Cbfβ in this context serves solely as a signal amplifier of Runx3 activity.

Neurotrophins and their Trk-receptors in the cerebellum of zebrafish

Neurotrophins (NTs) and their specific Trk-receptors are key molecules involved in the regulation of survival, proliferation, and differentiation of central nervous system during development and adulthood in vertebrates. In the present survey, we studied the expression and localization of neurotrophins and their Trk-receptors in the cerebellum of teleost fish Danio rerio (zebrafish). Teleostean cerebellum is composed of a valvula, body and vestibulolateral lobe. Valvula and body show the same three-layer structure as cerebellar cortex in mammals. The expression of NTs and Trk-receptors in the whole brain of zebrafish has been studied by Western blotting analysis. By immunohistochemistry, the localization of NTs has been observed mainly in Purkinje cells; TrkA and TrkB-receptors in cells and fibers of granular and molecular layers. TrkC was faintly detected. The occurrence of NTs and Trk-receptors suggests that they could have a synergistic action in the cerebellum of zebrafish. J. Morphol. 277:725-736, 2016. © 2016 Wiley Periodicals, Inc.

BDNF-TrkB axis regulates migration of the lateral line primordium and modulates the maintenance of mechanoreceptor progenitors

BDNF and its specialized receptor TrkB are expressed in the developing lateral line system of zebrafish, but their role in this organ is unknown. To tackle this problem in vivo, we used transgenic animals expressing fluorescent markers in different cell types of the lateral line and combined a BDNF gain-of-function approach by BDNF mRNA overexpression and by soaking embryos in a solution of BDNF, with a loss-of-function approach by injecting the antisence ntrk2b-morpholino and treating embryos with the specific Trk inhibitor K252a. Subsequent analysis demonstrated that the BDNF-TrkB axis regulates migration of the lateral line primordium. In particular, BDNF-TrkB influences the expression level of components of chemokine signaling including Cxcr4b, and the generation of progenitors of mechanoreceptors, at the level of expression of Atoh1a-Atp2b1a.

Brain-derived neurotrophic factor: mRNA expression and protein distribution in the brain of the teleost Nothobranchius furzeri

BDNF (brain-derived neurotrophic factor) is a member of the neurotrophin family and it is implicated in regulating brain development and function. The BDNF gene organization and coding sequence are conserved in all vertebrates. The present survey was conducted in a teleost fish, Nothobranchius furzeri, because it is an emerging model of aging studies due to its short lifespan and shows the high rate of adult neurogenesis typical of anamniotes. The present survey reports: 1) the identification and characterization of the cDNA fragment encoding BDNF protein, and 2) the localization of BDNF in the whole brain. BDNF mRNA expression was assessed by in situ hybridization, by employing an antisense RNA probe; BDNF protein was detected by employing a sensitive immunohistochemical technique, along with highly specific affinity-purified antibodies to BDNF. Both BDNF mRNA and protein were detected in neurons and glial cells of all regions of the brain of N. furzeri. Interestingly, BDNF was localized also in brain areas involved in adult neurogenic activities, suggesting a specific role for this neurotrophic factor in controlling cell proliferation. These results provide baseline information for future studies concerning BDNF involvement in the aging processes of the teleost brain.

Zebrafish crypt neurons project to a single, identified mediodorsal glomerulus

Crypt neurons are a third type of olfactory receptor neurons with a highly unusual "one cell type--one receptor" mode of expression, the same receptor being expressed by the entire population of crypt neurons. Attempts to identify the target region(s) of crypt neurons have been inconclusive so far. We report that TrkA-like immunoreactivity specifically labeled somata, axons, and terminals of zebrafish crypt neurons and reveal a single glomerulus, mdg2 of the dorsomedial group, as target glomerulus of crypt neurons. Injection of a fluorescent tracing dye into the mdg2 glomerulus retrogradely labeled mostly crypt neurons, as assessed by quantitative morphometry, whereas no crypt neurons were found after injections in neighboring glomeruli. Our data provide strong evidence that crypt neurons converge onto a single glomerulus, and thus form a labeled line consisting of a single sensory cell type, a single olfactory receptor and a single target glomerulus.

New transgenic reporters identify somatosensory neuron subtypes in larval zebrafish

To analyze somatosensory neuron diversity in larval zebrafish, we identified several enhancers from the zebrafish and pufferfish genomes and used them to create five new reporter transgenes. Sequential deletions of three of these enhancers identified small sequence elements sufficient to drive expression in zebrafish trigeminal and Rohon-Beard (RB) neurons. One of these reporters, using the Fru.p2x3-2 enhancer, highlighted a somatosensory neuron subtype that expressed both the p2rx3a and pkcα genes. Comparison with a previously described trpA1b reporter revealed that it highlighted the same neurons as the Fru.p2x3-2 reporter. To determine whether neurons of this subtype possess characteristic peripheral branching morphologies or central axon projection patterns, we analyzed the morphology of single neurons. Surprisingly, although these analyses revealed diversity in peripheral axon branching and central axon projection, PKCα/p2rx3a/trpA1b-expressing RB cells did not possess obvious characteristic morphological features, suggesting that even within this molecularly defined subtype, individual neurons may possess distinct properties. The new transgenes created in this study will be powerful tools for further characterizing the molecular, morphological, and developmental diversity of larval somatosensory neurons.

Coordinate development of skin cells and cutaneous sensory axons in zebrafish

Peripheral sensory axons innervate the epidermis early in embryogenesis to detect touch stimuli. To characterize the time course of cutaneous innervation and the nature of interactions between sensory axons and skin cells at early developmental stages, we conducted a detailed analysis of cutaneous innervation in the head, trunk, and tail of zebrafish embryos and larvae from 18 to 78 hours postfertilization. This analysis combined live imaging of fish expressing transgenes that highlight sensory neurons and skin cells, transmission electron microscopy (TEM), and serial scanning electron microscopy (sSEM). In zebrafish, the skin initially consists of two epithelial layers, and all of the axons in the first wave of innervation are free endings. Maturation of the epithelium coincides with, but does not depend on, its innervation by peripheral sensory axons. We found that peripheral axons initially arborize between the two epithelial skin layers, but not within the basal lamina, as occurs in other organisms. Strikingly, as development proceeds, axons become tightly enveloped within basal keratinocytes, an arrangement suggesting that keratinocytes may serve structural or functional roles, akin to Schwann cells, in somatosensation mediated by these sensory neurons.

Robo2 determines subtype-specific axonal projections of trigeminal sensory neurons

How neurons connect to form functional circuits is central to the understanding of the development and function of the nervous system. In the somatosensory system, perception of sensory stimuli to the head requires specific connections between trigeminal sensory neurons and their many target areas in the central nervous system. Different trigeminal subtypes have specialized functions and downstream circuits, but it has remained unclear how subtype-specific axonal projection patterns are formed. Using zebrafish as a model system, we followed the development of two trigeminal sensory neuron subtypes: one that expresses trpa1b, a nociceptive channel important for sensing environmental chemicals; and a distinct subtype labeled by an islet1 reporter (Isl1SS). We found that Trpa1b and Isl1SS neurons have overall similar axon trajectories but different branching morphologies and distributions of presynaptic sites. Compared with Trpa1b neurons, Isl1SS neurons display reduced branch growth and synaptogenesis at the hindbrain-spinal cord junction. The subtype-specific morphogenesis of Isl1SS neurons depends on the guidance receptor Robo2. robo2 is preferentially expressed in the Isl1SS subset and inhibits branch growth and synaptogenesis. In the absence of Robo2, Isl1SS afferents acquire many of the characteristics of Trpa1b afferents. These results reveal that subtype-specific activity of Robo2 regulates subcircuit morphogenesis in the trigeminal sensory system.

Brain-derived neurotrophic factor mediates non-cell-autonomous regulation of sensory neuron position and identity

During development, neurons migrate considerable distances to reside in locations that enable their individual functional roles. Whereas migration mechanisms have been extensively studied, much less is known about how neurons remain in their ideal locations. We sought to identify factors that maintain the position of postmigratory dorsal root ganglion neurons, neural crest derivatives for which migration and final position play an important developmental role. We found that an early developing population of sensory neurons maintains the position of later born dorsal root ganglia neurons in an activity-dependent manner. Further, inhibiting or increasing the function of brain-derived neurotrophic factor induces or prevents, respectively, migration of dorsal root ganglia neurons out of the ganglion to locations where they acquire a new identity. Overall, the results demonstrate that neurotrophins mediate non-cell-autonomous maintenance of position and thereby the identity of differentiated neurons.

Expression of brain-derived neurotrophic factor and TrkB in the lateral line system of zebrafish during development

The neuromasts of the lateral line system are regarded as a model to study the mechanisms of hearing, deafness, and ototoxicity. The neurotrophins (NTs), especially brain-derived neurotrophic factor (BDNF), and its signaling receptor TrkB are involved in the development and maintenance of neuromasts. To know the period in which the BDNF/TrkB complex has more effects in the neuromast biology, the age-related changes were studied. Normal zebrafish from 10 to 180 days post-fertilization (dpf), as well as transgenic ET4 zebrafish 10 and 20 dpf, was analyzed using qRT-PCR, western blot, and immunohistochemistry. BDNF and TrkB mRNAs followed a parallel course, peaking at 20 dpf, and thereafter progressively decreased. Specific immunoreactivity for BDNF and TrkB was found co-localized in all hairy cells of neuromasts in 20 and 30 dpf; then, the number of immunoreactive cells decreased, and by 180 dpf BDNF remains restricted to a subpopulation of hairy cells, and TrkB to a few number of sensory and non-sensory cells. At all ages examined, TrkB immunoreactivity was detected in sensory ganglia innervating the neuromasts. The present results demonstrate that there is a parallel time-related decline in the expression of BDNF and TrkB in zebrafish. Also, the patterns of cell expression suggest that autocrine/paracrine mechanisms for this NT system might occur within the neuromasts. Because TrkB in lateral line ganglia did not vary with age, their neurons are potentially capable to respond to BDNF during the entire lifespan of zebrafish.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

The genome sequence of the protostome Daphnia pulex encodes respective orthologues of a neurotrophin, a Trk and a p75NTR: evolution of neurotrophin signaling components and related proteins in the bilateria

Background

Neurotrophins and their Trk and p75NTR receptors play an important role in the nervous system. To date, neurotrophins, Trk and p75NTR have only been found concomitantly in deuterostomes. In protostomes, homologues to either neurotrophin, Trk or p75NTR are reported but their phylogenetic relationship to deuterostome neurotrophin signaling components is unclear. Drosophila has neurotrophin homologues called Spätzles (Spz), some of which were recently renamed neurotrophins, but direct proof that these are deuterostome neurotrophin orthologues is lacking. Trks belong to the receptor tyrosine kinase (RTK) family and among RTKs, Trks and RORs are closest related. Flies lack Trks but have ROR and ROR-related proteins called NRKs playing a neurotrophic role. Mollusks have so far the most similar proteins to Trks (Lymnaea Trk and Aplysia Trkl) but the exact phylogenetic relationship of mollusk Trks to each other and to vertebrate Trks is unknown. p75NTR belongs to the tumor necrosis factor receptor (TNFR) superfamily. The divergence of the TNFR families in vertebrates has been suggested to parallel the emergence of the adaptive immune system. Only one TNFR representative, the Drosophila Wengen, has been found in protostomes. To clarify the evolution of neurotrophin signaling components in bilateria, this work analyzes the genome of the crustacean Daphnia pulex as well as new genetic data from protostomes.

Results

The Daphnia genome encodes a neurotrophin, p75NTR and Trk orthologue together with Trkl, ROR, and NRK-RTKs. Drosophila Spz1, 2, 3, 5, 6 orthologues as well as two new groups of Spz proteins (Spz7 and 8) are also found in the Daphnia genome. Searching genbank and the genomes of Capitella, Helobdella and Lottia reveals neurotrophin signaling components in other protostomes.

Conclusion

It appears that a neurotrophin, Trk and p75NTR existed at the protostome/deuterostome split. In protostomes, a "neurotrophin superfamily" includes Spzs and neurotrophins which respectively form two paralogous families. Trks and Trkl proteins also form closely related paralogous families within the protostomian RTKs, whereby Trkls are absent in deuterostomes. The finding of p75NTR in several protostomes suggests that death domain TNFR superfamily proteins appeared early in evolution.

Neurotrophin and Trk neurotrophin receptors in the inner ear of Salmo salar and Salmo trutta

Neurotrophins (NTs) and their signal transducing Trk receptors play a critical role in the development and maintenance of specific neuronal populations in the nervous system of higher vertebrates. They are responsible for the innervation of the inner ear cochlear and vestibular sensory epithelia. Neurotrophins and Trks are also present in teleosts but their distribution in the inner ear is unknown. Thus, in the present study, we used Western-blot analysis and immunohistochemistry to investigate the expression and cell localization of both NTs and Trk receptors in the inner ear of alevins of Salmo salar and Salmo trutta. Western-blot analysis revealed the occurrence of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), but not nerve growth factor (NGF), as well as all three Trk receptors, i.e. TrkA, TrkB and TrkC, the estimated molecular weights of which were similar to those expected for mammals. Specific immunoreactivity for neurotrophins was detected mainly in the sensory epithelia. In particular, BDNF immunoreactivity was found in the maculae of the utricle and saccule, whereas NT-3 immunoreactivity was present in the sensory epithelium of the cristae ampullaris. As a rule the sensory epithelia of the inner ear lacked immunoreactivity for Trks, thus excluding possible mechanisms of autocrinia and/or paracrinia. By contrast, overlapping subpopulations of neurons in the statoacoustic ganglion expressed TrkA (about 15%), TrkB (about 65%) and TrkC (about 45%). The present results demonstrate that, as in mammals and birds, the inner ear of teleosts expresses the components of the neurotrophin-Trk system, but their roles remain to be elucidated.

Cell-autonomous TrkB signaling in presynaptic retinal ganglion cells mediates axon arbor growth and synapse maturation during the establishment of retinotectal synaptic connectivity

BDNF contributes to the activity-dependent establishment and refinement of visual connectivity. In Xenopus, BDNF applications in the optic tectum influence retinal ganglion cell (RGC) axon branching and promote synapse formation and stabilization. The expression patterns of BDNF and TrkB suggest that BDNF specifically regulates the maturation of RGC axons at the target. It is possible, however, that BDNF modulates retinotectal synaptic connectivity by differentially influencing presynaptic RGC axons and postsynaptic tectal cells. Here, we combined single-cell expression of a dominant-negative TrkB-enhanced green fluorescent protein (GFP) fusion protein with confocal microscopy imaging in live Xenopus tadpoles to differentiate between presynaptic and postsynaptic actions of BDNF. Disruption of TrkB signaling in individual RGCs influenced the branching and synaptic maturation of presynaptic axon arbors. Specifically, GFP-TrkB.T1 overexpression increased the proportion of axons with immature, growth cone-like morphology, decreased axon branch stability, and increased axon arbor degeneration. In addition, GFP-TrkB.T1 overexpression reduced the number of red fluorescent protein-synaptobrevin-labeled presynaptic specializations per axon terminal. In contrast, overexpression of GFP-TrkB.T1 in tectal neurons did not alter synaptic number or the morphology or dynamic behavior of their dendritic arbors. Electron microscopy analysis revealed a significant decrease in the number of mature synaptic profiles and in the number of docked synaptic vesicles at retinotectal synapses made by RGC axons expressing GFP-TrkB.T1. Together, our results demonstrate that presynaptic TrkB signaling in RGCs is a key determinant in the establishment of visual connectivity and indicate that changes in tectal neuron synaptic connectivity are secondary to the BDNF-elicited enhanced stability and growth of presynaptic RGCs.

Formation and evolution of the chordate neurotrophin and Trk receptor genes

Neurotrophins are structurally related neurotrophic polypeptide factors that regulate neuronal differentiation and are essential for neuronal survival, neurite growth and plasticity. It has until very recently been thought that the neurotrophin system appeared with the vertebrate species, but identification of a cephalochordate neurotrophin receptor (Trk), and more recently neurotrophin sequences in several genomes of deuterostome invertebrates, show that the system already existed at the stem of the deuterostome group. Comparative genomics supports the hypothesis that two whole genome duplications produced many of the vertebrate gene families, among those the neurotrophin and Trk families. It remains to be proven to what extent the whole genome duplications have driven macroevolutionary change, but it appears certain that the formation of the multi-gene copy neurotrophin and Trk receptor families at the stem of vertebrates has provided a foundation from which the various functions and pleiotropic effects produced by each of the four extant neurotrophins have evolved.

Mutations that affect the survival of selected amacrine cell subpopulations define a new class of genetic defects in the vertebrate retina

Amacrine neurons are among the most diverse cell classes in the vertebrate retina. To gain insight into mechanisms vital to the production and survival of amacrine cell types, we investigated a group of mutations in three zebrafish loci: kleks (kle), chiorny (chy), and bergmann (bgm). Mutants of all three genes display a severe loss of selected amacrine cell subpopulations. The numbers of GABA-expressing amacrine interneurons are sharply reduced in all three mutants, while cell loss in other amacrine cell subpopulations varies and some cells are not affected at all. To investigate how amacrine cell loss affects retinal function, we performed electroretinograms on mutant animals. While the kle mutation mostly influences the function of the inner nuclear layer, unexpectedly the chy mutant phenotype also involves a loss of photoreceptor cell activity. The precise ration and arrangement of amacrine cell subpopulations suggest that cell-cell interactions are involved in the differentiation of this cell class. To test whether defects of such interactions may be, at least in part, responsible for mutant phenotypes, we performed mosaic analysis and demonstrated that the loss of parvalbumin-positive amacrine cells in chy mutants is due to extrinsic (cell-nonautonomous) causes. The phenotype of another amacrine cell subpopulation, the GABA-positive cells, does not display a clear cell-nonautonomy in chy animals. These results indicate that environmental factors, possibly interactions among different subpopulations of amacrine neurons, are involved in the development of the amacrine cell class.

Origin and evolution of the Trk family of neurotrophic receptors

Among the numerous tyrosine kinase receptors, those belonging to the Trk family are distinctively involved in the development of complex traits within the vertebrate nervous system. Until recently, the lack of a proper Nt/Trk system in invertebrates has lead to the belief that they were a vertebrate innovation. Recent data, however, have challenged the field, and proved that bona fide Trk receptors do exist in invertebrates. Here, we review and discuss the evolutionary history of the Trk receptor family, and draw a comprehensive scenario that situates the origin of the Nt/Trk signalling prior to the origin of vertebrates. Probably, a ProtoTrk receptor was invented by means of domain and exon shuffling from pieces of ancient genes, generating the unique combination of domains found in extant Trk receptors. It is suggestive to propose that subtle protein mutations, gene duplications, and co-options in particular territories of a primitive Nt/Trk system were instrumental to the development of a complex vertebrate nervous system.

Both 5' and 3' flanks regulate Zebrafish brain-derived neurotrophic factor gene expression

BACKGROUND

Precise control of developmental and cell-specific expression of the brain-derived neurotrophic factor (BDNF) gene is essential for normal neuronal development and the diverse functions of BDNF in the adult organism. We previously showed that the zebrafish BDNF gene has multiple promoters. The complexity of the promoter structure and the mechanisms that mediate developmental and cell-specific expression are still incompletely understood.

RESULTS

Comparison of pufferfish and zebrafish BDNF gene sequences as well as 5' RACE revealed three additional 5' exons and associated promoters. RT-PCR with exon-specific primers showed differential developmental and organ-specific expression. Two exons were detected in the embryo before transcription starts. Of the adult organs examined, the heart expressed a single 5' exon whereas the brain, liver and eyes expressed four of the seven 5' exons. Three of the seven 5' exons were not detectable by RT-PCR. Injection of promoter/GFP constructs into embryos revealed distinct expression patterns. The 3' flank profoundly affected expression in a position-dependent manner and a highly conserved sequence (HCS1) present in 5' exon 1c in a dehancer-like manner.

CONCLUSIONS

The zebrafish BDNF gene is as complex in its promoter structure and patterns of differential promoter expression as is its murine counterpart. The expression of two of the promoters appears to be regulated in a temporally and/or spatially highly circumscribed fashion. The 3' flank has a position-dependent effect on expression, either by affecting transcription termination or post-transcriptional steps. HCS1, a highly conserved sequence in 5' exon 1c, restricts expression to primary sensory neurons. The tools are now available for detailed genetic and molecular analyses of zebrafish BDNF gene expression.

Differential expression of ?-aminobutyric acid type B receptor subunit mRNAs in the developing nervous system and receptor coupling to adenylyl cyclase in embryonic neurons

gamma-Aminobutyric acid type B receptors (GABA(B)Rs) mediate both slow inhibitory synaptic activity in the adult nervous system and motility signals for migrating embryonic cortical cells. Previous papers have described the expression of GABA(B)Rs in the adult brain, but the expression and functional significance of these gene products in the embryo are largely unknown. Here we examine GABA(B)R expression from rat embryonic day 10 (E10) to E18 compared with adult and ask whether embryonic cortical neurons contain functional GABA(B)R. GABA(B)R1 transcript levels greatly exceed GABA(B)R2 levels in the developing neural tube at E11, and olfactory bulb and striatum at E17 but equalize in most regions of adult nervous tissue, except for the glomerular and granule cell layers of the main olfactory bulb and the striatum. Consistent with expression differences, the binding affinity of GABA for GABA(B)Rs is significantly lower in adult striatum compared with cerebellum. Multiple lines of evidence from in situ hybridization, RNase protection, and real-time PCR demonstrate that GABA(B)R1a, GABA(B)R1b, GABA(B)R1h (a subunit subtype, lacking a sushi domain, that we have identified in embryonic rat brain), GABA(B)R2, and GABA(B)L transcript levels are not coordinately regulated. Despite the functional requirement for a heterodimer of GABA(B)R subunits, the expression of each subunit mRNA is under independent control during embryonic development, and, by E18, GABA(B)Rs are negatively coupled to adenylyl cyclase in neocortical neurons. The presence of embryonic GABA(B)R transcripts and protein and functional receptor coupling indicates potentially important roles for GABA(B)Rs in modulation of synaptic transmission in the developing embryonic nervous system.

Immunohistochemical localization of BDNF-, TrkB- and TrkA-like proteins in the teleost lateral line system

The lateral line system, formed of both superficial (pit organs) and canal neuromasts, is one of the major mechanosensory systems in fish. It has always been assumed that this system depends on neurotrophins and their cognate Trk receptors for development and maintenance, as has been shown in other mechanosensitive systems of vertebrates. However, until nowthis issue has not been specifically addressed. In this study we used immunohistochemistry to investigate the occurrence and localization both of neurotrophins (NGF-, BDNF- and NT-3-like) and of Trk-like proteins (TrkA-, TrkB-, TrkC-like) in alevins of Salmo salar and S. trutta. All cells in the pit organs of S. salar displayed strong immunoreactivity for TrkB-like and BDNF-like, whereas they were restricted to the hair cells in S. trutta. The hair, supporting and mantle cells of S. salar, and the mantle cells of S. trutta, also expressed TrkA-like immunoreactivity. In the canal neuromasts BDNF-, TrkA- and TrkB-like proteins were present in all cells, without differences between species. NGF-, NT-3- and TrkC-like immunoreactivity were never detected. The present results suggest that mechanoreceptive hair cells, as well as supporting cells, in the lateral line system are under the control of the BDNF-TrkB-like complex, and probably of ligands of TrkA-like receptors.

Brain-derived neurotrophic factor and TrkB tyrosine kinase receptor gene expression in zebrafish embryo and larva

The genes that encode the neurotrophin family of secreted polypeptides and the Trk family of high affinity neurotrophin transmembrane protein tyrosine kinase receptors are induced at the time of neurogenesis in mammals and are known to play critical roles in nervous system development. We show here that in contrast to mammals, the genes encoding the neurotrophin brain-derived neurotrophic factor (BDNF) and the neurotrophin receptor TrkB are expressed throughout embryonic development in the zebrafish. At the embryonic stages preceding transcription of endogenous genes all cells contain BDNF transcripts and immunoreactive BDNF and the trkB transcripts lack the region that encodes a kinase domain. As development proceeds, progressively fewer cells contain BDNF transcripts and by the time of neurogenesis the trkB transcripts encode a kinase-domain. In the 4-day-old larva, a small subset of specialized sensory cells on the surface and cells in deeper structures including the gill arches, fin, and cloaca express the BDNF gene at high levels in a promoter-specific fashion. This progressive restriction of BDNF gene expression must involve an extinction of BDNF gene transcription in some and induction of high levels of transcription in a promoter-specific fashion in other cells.

TrkA and TrkB neurotrophin receptor immunoreactivity in the teleost (Scorpaena porcus) endocrine pancreas

Mammalian-like and specific neurotrophins, as well as their cognate tyrosine kinase Trk-like receptors have been identified in teleosts. They are mainly distributed in neuronal tissues, but evidence suggests that some non-neuronal tissues also express Trks. In this study we used immunohistochemistry to investigate the occurrence and cell distribution of Trks in the pancreas of teleosts (Scorpaena porcus). Immunoreactivity for TrkA and TrkB, but not for TrkC receptors, was found in the scorpionfish pancreas. TrkA-like positive cells were exclusively observed within pancreatic islets, among insulin- and glucagon-containing cells, but apparently did not co-localize with these hormones. TrkB immunostaining was found in islet cells, presumably colocalized with glucagon, as well as in some cells of the exocrine portion. These data provide a morphological basis for a role of the TrkA and TrkB ligands in the endocrine pancreas of teleosts.

Programmed cell death in zebrafish rohon beard neurons is influenced by TrkC1/NT-3 signaling

Rohon Beard (RB) cells are embryonic primary sensory neurons that are removed by programmed cell death during larval development in zebrafish. RB somatosensory functions are taken over by neurons of the dorsal root ganglia (DRG), suggesting that RB cell death may be triggered by the differentiation of these ganglia, as has been proposed to be the case in Xenopus. However, here we show that the timing of RB cell death correlates with reduced expression of trkC1, the receptor for neurotrophin NT-3, but not with the appearance of DRG, which differentiate only after most RB cells die. trkC1 is expressed in subpopulations of RB neurons during development, and cell death is initiated only in trkC1-negative neurons, suggesting a role for TrkC1 and its ligand, NT-3, in RB cell survival. In support of this, antibodies that deplete NT-3 induce RB cell death while exogenous application of NT-3 reduces death. In addition, we show that RB cell death can be prevented using a caspase inhibitor, zVADfmk, showing that during normal development, RB cells die by a caspase-dependent programmed cell death pathway possibly triggered by reduced signaling via TrkC1.

Inherited retinal degeneration and apoptosis in mutant zebrafish

The mechanism of retinal cell death was studied in mutant zebrafish (Danio rerio) which undergo inherited degeneration of the retina and the brain. The shrunken head (shr(m33)) mutation was isolated as part of a large scale mutagenesis screen. The yellow head (yhd) mutation arose spontaneously among inbred wild type zebrafish. Although the mutants share many morphological features, including small eyes, a small brain and an enlarged pericardial sac, crossing shr(m33) and yhd heterozygotes results in phenotypically normal fish. The retinae of both mutant lines of fish begin to develop normally and then undergo massive degenerative changes. Pyknotic cells first appear in the retina of the shr(m33) fish by 3 days post-fertilization and in the yhd fish by 1.5 days post-fertilization. By 5 days post-fertilization the outer nuclear layer containing the photoreceptor cells has largely disappeared in both mutants. The inner nuclear layer and ganglion cell layer are also severely affected. By 6-7 days post-fertilization, the retina has been largely cleared of pyknotic cells by retinal pigment epithelial cells and by rare macrophage-like cells. Both mutations are lethal by 7-8 days post-fertilization. Two independent measures, TdT-mediated dUTP-biotin nick end label (TUNEL) and transmission electron microscopy, indicate that the pyknotic cells in the mutant retinae are apoptotic. Apoptosis is very rarely observed during normal development of the teleost retina and was not observed in age-matched wild type zebrafish retinae examined for comparison. Our results indicate that a genetic defect can induce massive apoptosis in cell populations that do not normally undergo apoptosis during development.

Co-localization of Trk neurotrophin receptors and regulatory peptides in the endocrine cells of the teleostean stomach

Recently it has been observed that a subpopulation of gut endocrine cells in vertebrates express Trk-like proteins, suggesting that neurotrophins could regulate the synthesis and storage of amines and peptides of these cells. Nevertheless, the peptides and amines present in the endocrine cells that express Trks have not been characterized. In this study we used immunohistochemistry to investigate the occurrence of Trk-like proteins (TrkA-like, TrkB-like and TrkC-like) and the possible co-localization of these with peptides and/or biogenic amines in the endocrine cells of the stomach of three teleost (bass, gilt-head and scorpionfish). No TrkA-like immunoreactivity (IR) was detected in the stomach of these species, whereas TrkB-like IR and TrkC-like IR were observed in numerous cells of the gastric epithelium. TrkB-like immunoreactive cells were present in all three species examined, and were particularly abundant in the blind sac. Conversely, TrkC-like immunoreactive cells were found only in the bass stomach, apparently co-localized with TrkB-like IR. TrkB-like IR was found co-localized with somatostatin IR in scorpionfish, and with somatostatin and CGRP IR in gilt-head and bass. Gastric endocrine cells expressing 5-HT, glucagon, insulin, met-, leu-enkephalin, substance P, PYY, VIP, CCK, NPY, bombesin and motilin were unreactive for Trk-like proteins. The present results provide direct evidence for the occurrence of Trk-like neurotrophin receptor proteins in a subpopulation of the teleostean gastric endocrine cells and suggest that neurotrophins could regulate, as in neurons, the expression of some neuropeptides such as somatostatin and CGRP.