Molecular and Cellular Neuroscience

Voltage-gated sodium channels (VGSCs) are conventionally described as heterotrimers composed of one alpha and two beta subunits. However, the patterns of co-expression of alpha- and beta-subunits in neurons remain unclear. In the present study, we report that alpha- (Nav1.1, Nav1.2, and Nav1.6) and beta- (beta-1 and beta-2) subunits are densely expressed in axon initial segments (AISs) of neurons in the neocortex, hippocampus and cerebellum at postnatal days 14-15 (P14-15) and 8-9 weeks (8-9W). These distributions are largely unique and partially overlapping among brain regions. Notably, in the neocortex and hippocampus, AISs of presumptive parvalbumin-positive inhibitory neurons are positive for Nav1.1 and beta-1, whereas those of excitatory ones are positive for Nav1.2 and beta-2. Similarly, AISs of cerebellar basket cells, which are inhibitory neurons, are positive for Nav1.1 and beta-1, whereas those of granule cells, which are excitatory neurons, are positive for Nav1.2 and beta-2. Nav1.6 is expressed in many of these neurons. Some subunits exhibited distinct distribution patterns at the two postnatal stages analyzed, possibly because of their developmental changes of subcellular localizations. Taken together, these results indicate that combinations of VGSC subunits are largely unique among different neuronal subpopulations. These findings provide a useful reference for understanding the distribution and interactions of VGSC subunits in the brain.

Electrical transmission is mediated by intercellular channels that cluster into structures known as gap junctions (GJ). In vertebrates, GJ channels are encoded by the gene family of connexin (Cx) proteins that assemble as hexamers, termed hemichannels, in the pre- and postsynaptic membranes, and that subsequently dock to form GJ channels. Auditory contacts on the fish Mauthner cells serve as model to study the properties and organization of vertebrate electrical synapses. Electrical transmission at these synapses is mediated by multiple co-existing GJs at which the presence of intercellular channels is regulated by a molecular scaffold. Zebrafish contain four homologs of the neuronal Cx36: Cx35.5 and Cx35.1 (gjd2a and b, respectively), and Cx34.1 and Cx34.7 (gjd1a and b). Cx mutations suggested that GJs are formed by heterotypic channels made of presynaptic Cx35.5 and postsynaptic Cx34.1. Using transgenic fish in which Cxs were tagged, we found that a second Cx, Cx34.7, is present together with Cx34.1 on the postsynaptic side at some but not all GJs at these terminals. When exogenously expressed, both Cx34.1 and Cx34.7 formed heterotypic functional channels with Cx35.5, each with substantially different voltage-dependent properties, indicating they can serve differential functions. However, we previously demonstrated that electrical transmission is lost in Cx34.1 but not Cx34.7 null mutants, suggesting that Cx34.7 cannot compensate for the loss of Cx34, despite the intrinsic ability of Cx34.1 and Cx34.7 to create functional channels. The findings reveal an unanticipated functional organization in the electrical synapse, where Cx34.1 is obligatory and Cx34.7 accessory, roles that appear to be defined by the postsynaptic molecular scaffold, with two postsynaptic Cxs possibly assembling under specific functional contexts. Thus, our results indicate that electrical synapses share an organizational motif with chemical synapses, akin to how they combine postsynaptic receptor types to modify synaptic function.

Insulin-like growth factor-1 (IGF-1) plays a critical role in neuronal signaling. Disrupted insulin/IGF-1 signaling is implicated in Alzheimers disease, among other conditions, yet its specific influence on glutamate receptor-mediated calcium responses remains unclear. We examined the impacts of IGF-1 on glutamate receptor function in primary rat neurons monitored for intraneuronal calcium following stimulation with glutamate, AMPA, or NMDA/glycine. Pharmacological blockers (CNQX for AMPA receptors, APV for NMDA receptors, and nimodipine for L-type calcium channels) were applied to define receptor-specific contributions. In hippocampal neurons, IGF-1 and insulin altered responses to glutamate in different directions, with IGF-1 tending to evoke and enhanced response. In neocortical neurons, by contrast, IGF-1 consistently reduced glutamate- and AMPA-evoked calcium peaks, suggesting an inhibitory effect on AMPA receptors. To rule out effects on voltage-gated calcium channels downstream of AMPA receptors, we tested effects of IGF-1 on depolarization with potassium chloride; calcium elevation in this case was unaffected by IGF-1. Likewise, IGF-1 did not inhibit responses to NMDA/glycine; and IGF-1 did not affect glutamate responses in the presence of CNQX, a selective AMPA receptor blocker. These findings, combined with the observation that IGF-1 effects persisted in the presence of APV (an NMDA receptor antagonist), indicate that the inhibition of glutamate responses by IGF-1 is mediated by suppression of AMPA receptor activity. IGF-1 may thus contribute to normal neurophysiology, and given the role that glutamate receptors play in excitotoxicity, IGF-1 may confer neuroprotection in the neocortex. Disruption of IGF-1 signaling, as seen in states resembling insulin resistance, may therefore worsen glutamate-driven excitotoxicity and contribute to adverse outcomes.

Tau protein, the primary component in neurofibrillary tangles characteristic of Alzheimers Disease and related dementia disorders, normally regulates microtubule growth and stability. While tau dysfunction contributes to the progression of tauopathies, the role of microtubules in disease has remained unclear. Through forward genetic screening in Caenorhabditis elegans tauopathy models, we found multiple tubulin gene mutations that rescue tau-mediated neurodegeneration. Whole animal behavioral and in vitro biochemical assays were employed to characterize mutation-driven effects on neuron function, neurodegeneration, and effects on tubulin and tau proteins as well as microtubule function. Mutant tubulin genes were found to confer different levels of suppression correlating with the level of mutant gene expression. Mutant tubulins did not drastically alter total tau protein levels, tau phosphorylation or aggregation, however tau-induced neurodegeneration was rescued. The suppression of tau toxicity by tubulin gene mutations cannot be explained by changes in tau or tubulin expression, tau phosphorylation, or tau aggregation state. Rather the tubulin mutations appear to act by influencing global microtubule properties. In vitro experiments using C. elegans tubulin in semi-isolated and isolated contexts have indicated changes to microtubule properties without observable changes to tau-tubulin affinity. This work suggests that manipulation of microtubules can rescue tauopathy even when pathological tau species persist, supporting the importance of understanding microtubule contributions to disease progression and investigation into microtubule targeted gene therapy or small molecule approaches for tauopathy intervention.

Nociception is an essential response for organisms to avoid potential harm and promote survival. Its molecular determinants are largely conserved across Eumetazoa. TRPV receptors are polymodal ion channels exhibiting selective peripheral expression and functional coupling that underpins nociception and pain modulation in complex organisms. However, the execution of protective behaviours triggered by TRPVs is also found in species with a simpler nervous organisation, thus encouraging their investigation in invertebrate model organisms to increase understanding of animal nociception. Cephalopods represent an interesting invertebrate phylum with respect to the evolution of the nervous system, whose complexity suggests it might support pain-like states that exist in vertebrates. This possibility is reflected by the inclusion of cephalopods in the UK and EU animal welfare legislations. Despite this, there is poor characterisation of cephalopod molecular nociceptors. For this reason, we used in silico analysis to identify two TRPV channels in Octopus vulgaris genome (Ovtrpv1 and Ovtrpv2). We validated the putative transcript sequences and highlighted prevalent expression in sensory tissues. We investigated the functional competence of these TRPVs by heterologously expressing Ovtrpv1 and Ovtrpv2 cDNA into Caenorhabditis elegans null mutants of the orthologous genes, ocr-2 and osm-9 respectively. Ovtrpvs successfully rescued the aversive response to chemical and mechanical noxious stimuli in the C. elegans mutants, suggesting these receptors are polymodal nociceptors. Additionally, complementary investigation using Xenopus laevis oocytes showed Ovtrpv1 and Ovtrpv2 form an active heteromeric channel gated by nicotinamide. This study highlights Ovtrpvs as an important route to better understand nociceptive detection in cephalopods.

Dopamine signaling through dopamine 1 receptors (D1R) and dopamine 2 receptors (D2R) regulates hippocampal synaptic plasticity underlying learning and memory, yet their subcellular localization within the hippocampus is unknown. Here we performed electron microscopic immunocytochemistry to elucidate the distribution of D1R and D2R in subregions of the mouse hippocampus. In CA1 and CA3 stratum radiatum (SR), D1R- and D2R-immunoreactivity was found primarily on pyramidal cell dendritic spines and unmyelinated axons, and to a lesser extent in axon terminals and glia. In both regions, D1R-labeled terminals formed predominantly asymmetric (excitatory-type) synapses on dendritic spines, whereas D2R-labeled terminals formed mainly symmetric (inhibitory-type) synapses on pyramidal cell dendritic shafts. In the dentate gyrus (DG) hilus, D1R-labeling was almost exclusively found in unmyelinated axons and glia. D2R immunoreactivity in the hilus similarly was present in unmyelinated axons and glia but was also detected in dendritic spines originating from mossy cells and in terminals forming symmetric synapses. These findings indicate that dopamine receptors are positioned to influence excitatory and inhibitory signaling in the murine hippocampus. As D1R and D2R exert opposing effects on neuronal signaling, their localization on pyramidal neuron compartments provides a structural substrate for bidirectional modulation of synaptic plasticity and pyramidal cell activity. In addition, the presence of D2Rs on inhibitory terminals contacting pyramidal neurons and hilar interneurons suggests a role in regulating inhibitory circuitry within the hippocampus.

Homeostatic plasticity of intrinsic excitability (IE) in the visual system has been essentially shown at the cortical level but whether thalamic nuclei also express homeostatic plasticity of IE is unknown. We show here that 4 days of monocular deprivation (MD) at eye opening induces a homeostatic change in IE in dorsal lateral geniculate nucleus (dLGN) neurons. Neurons recorded in the dLGN region activated by the deprived eye are more excitable than neurons recorded in the dLGN region activated by the open eye. No significant changes were observed following 7 days of MD, however. Enhanced excitability in neurons from the deprived side after 4 days of MD was associated with a reduced Kv1-dependent LTP-IE, a smaller voltage ramp, and a reduced inter-spike interval, suggesting that Kv1 channels are down-regulated in deprived dLGN neurons. Furthermore, the ankyrin G signal of the axon initial segment was larger in deprived dLGN neurons compared with open ones, indicating that Nav1 channel number also undergoes homeostatic regulation, and Kv1.1 channel signals were lower in deprived neurons compared to open ones. In addition, electrical coupling was found to be strengthened in neurons displaying enhanced IE following either brief (4 days) or long (10 days) MD. These results suggest that homeostatic and Hebbian plasticity in the dLGN share common expression mechanisms involving the regulation of Kv1 channels, Nav1 channels and electrical coupling between relay neurons.

Mutations in human LIS1 cause lissencephaly, a severe developmental brain malformation. Although most stud-ies focus on development, LIS1 is also expressed in adult mouse tissues. We previously induced LIS1 knockout (iKO) in adult mice using a Cre-Lox approach with an actin promoter driving CreERT2 expression. This proved to be rapidly lethal, with evidence pointing toward nervous system dysfunction. CreERT2 activity was observed in astrocytes, brainstem and spinal motor neurons, and axons and Schwann cells in the sciatic and phrenic nerves, suggesting dysfunctional cardiorespiratory and motor circuits. However, it is unclear how LIS1 knockout in these different cell types contributes to the lethal phenotype. We now report that LIS1 depletion from astro-cytes is not lethal to mice (male or female), although glial fibrillary protein (GFAP) expression is increased in all LIS1-depleted astrocytes. In contrast, LIS1 depletion from projection neurons causes motor deficits and rapid lethality in both males and females. This is accompanied by progressive, widespread axonal degeneration along the entire length of both motor and sensory axons. Interestingly, sensory neurons harvested from iKO mice ini-tially extend axons in culture but soon develop axonal swellings and fragmentation, indicating axonal degenera-tion. LIS1 is a prominent regulator of cytoplasmic dynein 1 (dynein, hereafter), a microtubule motor whose dis-ruption can cause both cortical malformations and later-onset neurodegenerative diseases, such as Charcot-Marie-Tooth disease. Our results raise the possibility that LIS1 depletion, through disruption of dynein function in mature axons, may lead to Wallerian-like axon degeneration without traumatic nerve injury.

Mechanosensing involves proteins detecting mechanical changes in the cytoskeleton or at cell adhesion sites. These interactions initiate signaling cascades that produce biochemical effects such as post-translational modifications or cytoskeletal rearrangements. Filamin is a ubiquitous mechanosensing protein that binds actin filaments and senses pulling forces within the cytoskeleton. Drosophila Filamin (Cheerio) is structurally similar to mammalian Filamin, with roles in egg chamber development, embryo cellularization, and integrity of muscle attachment sites and Z discs in Drosophila indirect flight muscles (IFMs). Here we report a potential novel binding partner of Drosophila Filamins: the death-associated protein kinase Drak that functions as a myosin light chain kinase. We found that Drak biochemically bound to an open mutant of Filamin that resembles the mechanically activated form partially bound to wild type Filamin and did not bind to closed mutant of Filamin. The interaction site was mapped to the intrinsically unfolded C-terminal region of Drak. To study the functional role of Drak-Filamin interaction, we studied two developmental events where Drak has been earlier shown to be expressed and where Filamin also functions: early embryonic cellularization and indirect flight muscle development at pupal stages. We found partial colocalization between Drak-GFP and Filamin-mCherry during the initiation of cellularization furrow, and at the time of myotube attachment site maturation in tendon cells. However, functionally we could not show direct correlation between Filamin and Drak. Our studies reveal interesting new expression patterns of Drak during Drosophila development and provide detailed information about Filamin localization during IFM development.

Mitochondrial morphology and function are critical determinants of neuronal function and survival, with disruptions in mitochondrial dynamics often preceding the overt neuronal dysfunction seen in neurodegenerative diseases such as Alzheimers disease, Huntingtons disease and Parkinsons disease. The kynurenine pathway accounts for 95% of dietary tryptophan catabolism and many of the metabolites are neuroactive, including redox-active 3-hydroxykynurenine (3-HK). 3-HK is present under normal physiological conditions in the central nervous system (CNS) and is elevated during inflammation. While supraphysiological levels of 3-HK have been associated with neurotoxicity, the effects of physiological concentrations on neuronal cells, and specifically their mitochondria, remain poorly understood. Here we assessed viability, ATP levels and redox status to determine cellular health and function in neuronal cells exposed to physiological levels of 3-HK, alongside confocal imaging and transcriptomic profiling, finding significant alterations in mitochondrial function and morphology. Interestingly, a biphasic influence of 3-HK on mitochondrial morphology was observed, with an elongated network as well as decreased surface area and volume being observed only at the lowest concentration of 3-HK, reflecting normal physiological levels. At the highest 3-HK concentration tested, reflecting an inflammatory situation, an increased number of mitochondria were present, accompanied by increased activation of caspase-3/7 and enhanced production of mitochondrial superoxide. These results highlight a previously unknown role for 3-HK in regulating mitochondrial function and structure, possibly through altered fission and fusion events, suggesting that subtle changes in kynurenine pathway metabolism may contribute to early mitochondrial dysfunction in neurological disease.

Vestibular neurons are core elements of the pathways involved in vestibulo-motor functions, such as vestibulo-spinal and vestibulo-ocular reflexes. To meet behavioral needs, electrophysiological neuronal properties are adequately adapted to the sensory-motor computation sustaining these distinct vestibular reflexes. During frog metamorphosis, there is a complete reorganization of the posturo-locomotor system while the oculomotor system remains minimally changed, probably associated to so far unknown changes in vestibular neuronal properties. We used this unique model to investigate the central developmental mechanisms underlying such a reconfiguration of vestibular-associated behaviors. Central vestibular neurons exhibit two types of electrophysiological phenotypes: tonic neurons with a continuous discharge and phasic neurons with a transitory discharge mainly due to the activation of Kv1.1 channel. Electrophysiological recordings and Kv1.1 immunolabeling of vestibulospinal (VS) and vestibulo-ocular (VO) neurons at both larval and juvenile stages revealed that the majority of VS neurons exhibited a tonic discharge in larvae but a phasic discharge in juvenile, while VO neurons remained mainly tonic throughout development. Changes in phasic and tonic neurons proportions in VS population are partly explained by neurogenesis. But we provide evidences that an electrophysiological phenotype switch is a concomitant developmental mechanism participating in the maturation of these central vestibular neurons. All together our results showed that the maturation process in central vestibular neuronal groups is highly related to the metamorphosis-induced remodeling of vestibulo-motor functions they are involved in, with the ultimate purpose of ensuring an adequate adaptation of neuronal elements properties to the developmental changes of behavioral constrains.

The rat vestibular system plays a critical role in anti-gravity responses such as the tail-lift reflex and the air-righting reflex. In a previous study in male rats, we obtained evidence that these two reflexes depend on the function of non-identical populations of vestibular sensory hair cells (HC). Here, we caused graded lesions in the vestibular system of female rats by exposing the animals to several different doses of an ototoxic chemical, 3,3-iminodipropionitrile (IDPN). After exposure, we assessed the anti-gravity responses of the rats and then assessed the loss of type I HC (HCI) and type II HC (HCII) in the central and peripheral regions of the crista, utricle and saccule. As expected, we recorded a dose-dependent loss of vestibular function and loss of HCs. The relationship between hair cell loss and functional loss was examined using non-linear models fitted by orthogonal distance regression. The results indicated that both the tail-lift reflex and the air-righting reflexes mostly depend on HCI function. However, a different dependency was found on the epithelium triggering the reflex: while the tail-lift response is sensitive to loss of crista and/or utricle HCIs, the air-righting response rather depends on utricular and/or saccular integrity.

The nasal epithelium is a complex tissue composed of both respiratory and olfactory tissue, and is constantly exposed to environmental insults, including toxins and pathogens. The main olfactory epithelium (MOE) serves as the critical site for olfaction, or sense of smell. Dysfunction at this critical barrier tissue can result in partial or total loss of olfactory function, resulting in significant impact to quality of life. The MOE is heterogeneous, comprised of many cell types including olfactory sensory neurons, support cells, and immune cells. It is not well understood how these diverse cell types in the MOE interact to regulate this tissue during homeostasis, and during times of injury and inflammation. We investigated how environmental olfactory exposures impact cell type specific transcriptional responses in the mouse MOE. We performed single-cell RNA sequencing (scRNA-seq) of the MOE following controlled environmental exposure to both well-known odorants and allergens. We identified major cell types and subtypes within the MOE, and identified transcriptional changes in response to the olfactory exposures. We identified transcriptional changes in OSNs, sustentacular cells, and resident immune cells to each condition. This indicated that environmental olfactory exposures drive changes to multiple cell types in the MOE. To our knowledge, this is the first study to identify effects of environmental olfactory exposures on cell-type specific transcription at homeostasis. These findings highlight the potential importance of multi-cellular interactions and communication in regulation of the olfactory epithelium.

Lack of social interaction results in various behavioral abnormalities in rodents, including increased anxiety levels, altered sociability, and impaired cognitive ability. Epigenetic factors regulate gene expression, however, how they contribute to juvenile social isolation (jSI)-induced behavioral alterations remains largely unknown. Here, we focused on the nucleus accumbens (NAc), a critical brain region of the reward system that regulates motivation-related behaviors. We first performed RNA-seq on neuronal nuclei and found alterations in genes related to neuronal function, as well as in transcriptional and epigenetic regulation. Protein-protein interaction (PPI) analysis of differentially expressed genes (DEGs) showed that top key nodes among down-regulated genes include membrane receptors (Ntrk2, Grin3a, and Grik1) and an apoptosis regulator (Bcl2). To further investigate whether jSI-induced gene expression alterations are mediated by histone modifications, we next performed CUT&Tag for four histone modifications (H3K4me1, H3K4me3, H3K27ac, and H3K27me3), and the results implied that epigenetic alterations may also play a role in neuronal function as well as transcriptional regulation. Reanalysis of previously published RNA-seq data on the manipulation of histone modification-associated factors (including Kdm6b, Brd4, and Setd1a) suggested that these enzymes were probably involved in jSI-induced gene expression alterations. Taken together, our comprehensive analysis implies the involvement of histone modification regulation in jSI-related alterations of gene expression in NAc.

Plasma Membrane Calcium ATPase (PMCA) is essential for maintaining intracellular calcium homeostasis. Previously, we used constitutive PMCA downregulation in Drosophila melanogaster dopaminergic neurons as a model to increase intracellular calcium and mimic early neuronal alterations associated with Parkinsons disease. Here, we examined the mechanisms underlying the effects mediated by the conditional, adult-specific downregulation of PMCA in dopaminergic neurons in Drosophila melanogaster, both in vivo and in primary neuronal cultures. Adult-specific conditional silencing of PMCA in dopaminergic neurons reduced lifespan but to a lesser extent than the constitutive model and impaired locomotor performance. At the cellular level, PMCA-downregulated dopaminergic neurons exhibited elevated basal calcium, indicating disrupted calcium regulation. This was associated with a progressive increase in presynaptic vesicles and extracellular dopamine levels, suggesting enhanced neurotransmitter release. Notably, the synaptic active zone structure was preserved, indicating primarily functional rather than structural alterations. In primary neuronal cultures, PMCA downregulation reduced dopaminergic neuron survival and induced transient increases in neurite branching. Together, these findings show that PMCA downregulation leads to calcium dysregulation and presynaptic dysfunction without overt neurodegeneration in vivo, while promoting premature neuronal death in culture, indicating increased vulnerability and supporting a pre-degenerative state in which synaptic alterations precede neuronal loss.

In adult superficial dorsal horn, 90% of Reelin (Reln+) and 70% of Disabled-1 (Dab1+) neurons co-express the transcription factor LIM-homeobox 1-beta (Lmx1b+) and therefore are glutamatergic neurons. Here we asked if embryonic Reln+Lmx1b+ and Dab1+Lmx1b+ dorsal horn neurons are derived from Lmx1b-expressing early-born dI5 or late-born dILB dorsal neurons. On Embryonic day (E)11.5, Reln+ and Dab1+ neurons appear to be part of the migration of early-born dI5 Lmx1b-expressing neurons. Between E12.5-E15.5, the lateral Reln+Lmx1b+ and Dab1+Lmx1b+ neurons migrate circumferentially along the rim of what will become the superficial dorsal horn, whereas medial Reln+Lmx1b+ and Dab1+Lmx1b+ neurons move into the dorsal midline and then migrate into lamina V. The small, late-born dILB Reln+Lmx1b+ and Dab1-Lmx1b+ neurons fill the superficial dorsal horn. In Reln mutants, large Dab1+Lmx1b+ neurons were mispositioned in lamina I and at the border between the superficial and deep dorsal horn. To confirm the identity of the circumferential and midline Reln+Lmx1b+ and Dab1+Lmx1b+ neurons, we asked if they expressed the transcription factor Zfhx3, a marker of dI5 projection neurons. We detected examples of Reln+Lmx1b+Zfhx3+ and Dab1+Lmx1b+Zfhx3+ projection neurons that migrated along the outer rim of the superficial dorsal horn and others that migrated from the midline into lamina V. Taken together, our study demonstrates that the larger Reln+Lmx1b+Zfhx3+ and Dab+Lmx1b+Zfhx3+ neurons represent two subsets of dI5 projections neurons, whereas smaller Reln+Lmx1b+ and Dab1+Lmx1b+ neurons concentrated in lamina II are likely dILB interneurons.

Alzheimers disease (AD) is a devastating neurodegenerative disorder characterized by memory loss and a decline in cognitive function. Hallmarks of AD include an age-dependent accumulation of toxic amyloid beta (A{beta}) 42 in the brain, energy dyshomeostasis caused by mitochondrial dysfunction, and iron overload. However, the role of iron overload and mitochondrial dysfunction in AD pathology is unknown and their precise relationship with A{beta} 42 toxicity in AD pathology is unclear. C. elegans provide a powerful model system to untangle and clarify these relationships. In this study, we quantify the temperature-dependence of iron toxicity (16, 20 and 25C) in neurons and muscle of C. elegans that overexpress A{beta} 42. We found that A{beta} 42, regardless of the cell-type expression, caused accelerated paralysis compared to age-matched WT worms with the greatest degree of paralysis observed at an elevated temperature (25C). Moreover, the combination of iron toxicity and A{beta} 42 results in an enhanced paralytic phenotype at 16C. Thus, iron exposure potentiates A{beta} toxicity observed at low temperatures. Iron toxicity stimulated both maximum (State 3) and leak (State 4) respiration in WT and A{beta} 42 worms. A{beta} 42 worms also exhibited increased leak respiration at baseline that was further exacerbated by iron toxicity. Iron burden and sensitivity increased A{beta} 42 peptide toxicity. A{beta} 42 worms exhibited reduced levels of Ca, Zn, Mn, and K. Overall, our results suggest that iron potentiates A{beta} toxicity at low temperature and enhances A{beta} peptide mediated mitochondrial bioenergetic dysfunction in C. elegans. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=140 SRC="FIGDIR/small/714217v1_ufig1.gif" ALT="Figure 1"> View larger version (29K): org.highwire.dtl.DTLVardef@9eaf46org.highwire.dtl.DTLVardef@542eforg.highwire.dtl.DTLVardef@16d9678org.highwire.dtl.DTLVardef@1b1b16d_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LITemperature stress modulates the synergetic interactions of iron toxicity and A{beta} 42 pathology C_LIO_LIIron sensitivity drives increased cell-type specific A{beta} 42 pathology C_LIO_LIEnergy dyshomeostasis via impaired mitochondrial function and increased proton leak contributes to iron- and A{beta}-induced pathology C_LI

Hypoxic-ischemic injury is a major cause of olfactory dysfunction, yet the cellular and morphological mechanisms underlying this sensory loss remain poorly understood. Here, we investigated the structural, cellular, and functional effects of acute hypoxic exposure on the olfactory system of adult zebrafish (Danio rerio) of both sexes, a model organism with remarkable neuroregenerative capacity. Fish were subjected to 15 minutes of acute severe hypoxia (0.8 mg/L dissolved oxygen) and assessed at 1 and 5 days post-hypoxia (dph). We evaluated olfactory function by means of cadaverine-evoked aversive behavioral assays. Structural and morphological integrity and inflammation of the olfactory epithelium (OE) and olfactory bulb (OB) were characterized using immunohistochemistry, histological stainings, and a 2,3,5-triphenyltetrazolium chloride (TTC) colorimetric assay. Acute hypoxic exposure impaired olfactory-mediated behaviors without affecting locomotion or exploratory behavior. In the peripheral OE, hypoxia caused neurodegeneration, disruption of the nasal mucus layer, and robust leukocytic infiltration. We observed reduced mitochondrial dehydrogenase activity in the olfactory bulb (OB) along with reactive astrogliosis. Olfactory function recovered by 5 days, coinciding with full restoration of OE morphology, and supported by a strong proliferative response. These findings reveal a coordinated degenerative and regenerative response to hypoxia across the olfactory axis, with implications for understanding hypoxia-induced sensory loss and neural repair. SIGNIFICANCEThis work addresses an important gap in knowledge regarding the mechanisms linking hypoxic insult and olfactory dysfunction. By using adult zebrafish, an extraordinarily regenerative vertebrate, it also provides insight into neuronal repair and regenerative processes supporting olfactory recovery. The novelty of our study resides in that, to our knowledge, there are no studies that provide a comprehensive characterization of the effects of hypoxia in the olfactory system across molecular, histological, and functional levels. These findings advance our understanding of hypoxia-induced sensory neurodegeneration and regeneration, and highlight the zebrafish olfactory system as a powerful model for investigating neural repair mechanisms relevant to hypoxic-ischemic brain injury.

Neural circuit assembly relies on different neuronal subtypes coming together to form a functional circuit. The question of how the appropriate number of each subtype is integrated into an emerging circuit remains relatively unknown. To answer this question, we used the mouse retina to uncover the molecular mechanisms responsible for neuron subtype integration in a developing circuit. In the mammalian retina, bipolar neurons are a class of interneurons that relay visual information from photoreceptors to ganglion cells. Extensive studies have shown there are 15 distinct bipolar subtypes: 6 types of OFF cone bipolars, 8 types of ON cone bipolars, and 1 type of rod bipolar. During retinal development, bipolar neurons are born in excess and through programmed cell death, a precise number of each subtype remains to give rise to the retinal circuit. Although this process has been well-described, little is known about the key molecules responsible for bipolar subtype integration in the developing retina. Our work uncovered a new role for the autism-associated risk gene, Protocadherin 9 (Pcdh9) in bipolar subtype integration. Deletion of Pcdh9 using a floxed allele leads to loss of OFF and ON cone bipolars; however, disruption in the extracellular binding of Pcdh9 leads to selective loss of ON cone bipolars but not rod bipolars. Moreover, we found this later function of Pcdh9 is mediated by homophilic interactions between ON cone bipolars and their known synaptic partners. Taken together, our work revealed a new role for Pcdh9 in bipolar subtype integration during retinal development. SUMMARY STATEMENTNeural circuits are comprised of multiple neuronal subtypes where a specific number need to come together to give rise to a functional circuit. Although this is a critical process during neurodevelopment, little is known about the molecular mechanisms that determines the precise number of each subtype during circuit development. In the present study, we identified the autism risk gene, Protocadherin 9 as a critical molecule in subtype integration of bipolar neurons within the developing mouse retina. Using newly generated mouse lines, we found distinct requirements of Pcdh9 to promote survival in different bipolar subtypes during retinal circuit assembly. The significance of this work is that it shed lights into how different neuronal subtypes are integrated in nascent neural circuits.

The brain is one of the most complex animal organs but the development of the many different neuron types remains enigmatic. A set of brain-specific transcription factors is known to be involved in brain patterning but their specific contributions remain to be elucidated in most cases, including foxQ2II. This transcription factor is known to be conserved in anterior neuroectodermal patterning of most animals while it has been lost from vertebrates. However, the contribution of foxQ2II-positive neurons to the adult brain has remained enigmatic. Here, we use an enhancer trap, immunostainings and our newly established beetle brainbow system to categorize Tc-foxQ2II-positive neurons into nine clusters with different projection patterns. All clusters contain neurons with the fast activating neurotransmitters acetylcholine and glutamate while no Tc-foxQ2II positive neuron is GABA-ergic or serotonin-positive. Interestingly, we found that many dopaminergic neurons were Tc-foxQ2II positive and we homologize them with dopaminergic neurons of the PPL2c, PPM1 and PPL1 cluster described in the Drosophila brain. Our results show that Tc-foxQ2II marks subsets of fast-acting interneurons contributing to the higher order brain centers mushroom bodies and central complex. Taken together, our work expands the known functional range of foxQ2 genes from sensory and neurosecretory cell specification to interneurons involved in the function of higher order brain centers.