Neuronal synchrony is often achieved by electrical synapses, which are mediated by intercellular channels, known as gap junctions. Electrical transmission can be modulated by intracellular pathways, including direct effects on connexins/innexins, the pore forming subunits of gap junctions, or alterations to the electrical properties of the postsynaptic neuron. Reproduction in the sea snail, Aplysia, is controlled by electrically coupled bag cell neurons. Upon brief synaptic input, these neuroendocrine cells undergo a synchronous burst of action potentials, known as the afterdischarge, culminating in the secretion of egg-laying hormone into the bloodstream. Near the start of the afterdischarge, cAMP levels are elevated. While it is well-established that cAMP inhibits K+ channels, thereby increasing excitability, nothing is known of the effects of this second messenger on electrical transmission in bag cell neurons. Here, we used pairs of cultured bag cell neurons and whole-cell recording to investigate the impact of cAMP on electrical coupling. Presynaptic action potentials consistently evoked postsynaptic electrotonic potentials, while presynaptic hyperpolarization was transferred accordingly to the postsynaptic cell. Delivery of a membrane-permeable cAMP analogue increased the electrotonic potential by ~25%, which was accompanied by a similar change in coupling coefficient. In addition, input resistance was also augmented by ~100%. These outcomes suggest cAMP may enhance electrical transmission, and as a consequence, influence synchronous firing during the afterdischarge.

Cyclic AMP influences electrotonic transmission between coupled neuroendocrine cells

The adult subventricular zone (SVZ) niche contains neural stems cells (NSCs) that maintain and regenerate the brain by producing neurons, astrocytes and oligodendrocytes throughout life. Directly adjacent to the SVZ is the striatum; a region that is important in movement regulation and is rich in GABAergic neurons. While striatal neurons are known to project into NSC-rich SVZ niche, the significance of striatal neuron - SVZ NSC cell-cell communication is not currently known. I hypothesize postnatal SVZ NSCs fates are regulated by striatal neurons. To address this question, I cultured postnatal primary murine SVZ NSCs in control media or media conditioned by primary striatal neurons. My data show striatal neurons secrete soluble molecules that instruct SVZ NSCs to differentiate into oligodendrocytes and neurons without affecting astrocyte formation or NPC proliferation. To further investigate the pro-oligodendrogenic influence of the striatal neuron secretome, we cultured SVZ oligodendrocyte precursor cells (OPCs) in media conditioned by striatal neurons. Our data demonstrate striatal neuron conditioned media causes an increase in OPC differentiation into oligodendrocytes. Using RNA-seq datasets of purified striatal neurons and SVZ NSCs, I then computationally predicted over 80 striatal neuron ligands that have the potential to regulate SVZ NSC biology. In summary, my results suggest striatal GABAergic neurons may play an important role in regulating SVZ NSCs. Future work will determine which striatal neuron ligands are responsible for the observed pro-neurogenic and -oligodendrogenic effects and identify the in vivo significance of these molecules in the SVZ niche.

Striatal neurons regulate neural stem cell fates through secreted molecules

Dendritic spines are postsynaptic structures that arise from dynamic protrusions. Spine formation is a critical facet of synapse formation and aberrant spine properties are a hallmark of many neurodevelopmental disorders. Our lab previously found that expression levels of channel-forming protein pannexin 1 (PANX1) decrease during the period of spine formation. Panx1 knockout (KO) increased spine density and PANX1 overexpression reduced their stability and density. Unpublished results from our lab reveal that probenecid, a PANX1 blocker and Health Canada- and FDA-approved drug, recapitulated the effects of Panx1 KO. Early postnatal acute and chronic administration of probenecid led to increased somatosensory layer 5 cortical neuron spine density in situ. The current study aims to investigate the mechanism underlying probenecid regulation of dendritic spines, and capitalizes on our previous finding that probenecid interferes with a physical interaction between PANX1 and microtubule-stabilizing protein. To gain insight into spine cytoskeleton dynamics during development, we treated neurons with probenecid and imaged microtubule and actin dynamics within spines using microtubule plus-end binding probe EB3-GFP and actin-binding protein Lifeact. In parallel, we monitored spine dynamics using membrane-bound mCherry to understand the relationship between microtubule and actin dynamics and overall spine movement. This work will advance our understanding of dendritic spine development with implications for understanding the cellular mechanisms underlying neurodevelopmental diseases.

Probing dendritic spine cytoskeleton dynamics with the pannexin 1 blocker probenecid

Spinal cord injury (SCI) results in neurodegeneration and damage to the spinal neural circuitry. To date, effective neuronal replacement and functional restoration of spinal circuit remain challenging. Transplantation of neural precursor cells (NPCs) offers a promising approach for neuronal replacement in SCI. However, our studies indicate limited neurogenesis by transplanted NPCs in the hostile milieu of SCI. We identified that injury-induced upregulation of chondroitin sulfate proteoglycans (CSPGs) hinders survival and integration of engrafted NPCs after SCI by signaling through two main receptors, LAR and PTP-σ. Here, we have evaluated the therapeutic potential of co-blocking LAR and PTP-σ by ILP and ISP peptides, respectively, in conjunction with transplantation of human directly reprogrammed NPCs (drNPCs) with the capacity to generate spinal neurons. Our in vitro studies confirmed ILP/ISP co-treatment restores the inhibitory effects of CSPGs on drNPCs and promotes their neuronal differentiation, maturation, and synaptogenesis. In rat SCI, systemic delivery of ILP/ISP significantly promoted long-term survival and biodistribution of human drNPCs, enhanced their neuronal differentiation and complexity, and functional recovery after SCI. Our transcriptomic analysis identified that CSPGs impede neuronal differentiation by inactivating the Wnt/β-catenin pathway, which was verified in our in vitro studies. Altogether, we have developed a new targeted, translationally feasible strategy with the potential to optimize neuronal replacement and functional recovery after SCI.

Suppressing CSPGs receptors enhances spinal specific neuronal replacement by human directly reprogrammed neural precursor cells and improves functional recovery after spinal cord injury

Huntington disease (HD) is monogenic and caused by a CAG repeat expansion in the IT15 gene, which results in the production of a mutant huntingtin protein (mHTT). In addition to the production of this pathogenic variant, HD patients generate half the amount of wild type huntingtin (wtHTT). Although wtHTT has a well-established role in neurodevelopment, as its knockout is embryonic lethal, its exact role in the adult brain is not well understood. Recent literature suggests a major role for wtHTT in synaptic function in adult neurons, specifically loss of wtHTT impairs presynaptic vesicle release as well as postsynaptic receptor localization. We hypothesized that these synaptic impairments may lead to defects in NMDAR-dependent long term potentiation (LTP)-induced changes in hippocampal wtHTT-lowered neurons. Chemical LTP (cLTP) was induced using glycine in primary hippocampal cultured neurons and wtHTT levels were lowered by siRNA application. Both presynaptic synaptophysin (SYP) and postsynaptic PSD-95 were labelled by immunofluorescence. Using super-resolution microscopy, we quantified the density of SYP and PSD-95 puncta from individual dendrites at the nanoscale level. cLTP induction revealed an activity-dependant decrease in SYP, but not PSD-95, density in wtHTT-lowered cells. These findings indicate a potential switch from synaptic potentiation to depression and subsequent loss of synapses during hippocampal NMDAR activation due to wtHTT lowering. Understanding these outcomes is essential as many HD genetic therapies in development further lower patient wtHTT levels.

Huntingtin reduction unmasks an active form of NMDAR-dependent synapse loss

Disruptions in the Cntnap2 gene are known to cause language impairments and symptoms associated with autism spectrum disorder (ASD) in humans. Importantly, knocking out this gene in rodents results in ASD-like symptoms that involve auditory processing deficits. This study used in vitro electrophysiology to examine alterations at a cellular level in the auditory cortex of a Cntnap2-/- rat model, hypothesizing that Cntnap2 is essential for maintaining intrinsic neuronal properties and excitability in the auditory cortex. Whole-cell patch clamp recordings were performed in brain slices from juvenile (P8-12 and P18-21) and adult (P70-90) wildtype (Cntnap2+/+) and knockout (Cntnap2-/-) rats. Intrinsic membrane properties, spontaneous EPSCs and firing patterns of cortical pyramidal cells were assessed. Action potentials were significantly larger in Cntnap2-/- rats at P8-12 and P18-21, and Cntnap2-/- rats at both P8-12 and P18-21 exhibited smaller half-widths compared to wildtypes. Compared to wildtypes, Cntnap2-/- rats exhibited larger sEPSCs at P8-12, but they were smaller at P18-21 and P70-90. The Cntnap2-/- rats also had higher sEPSC frequencies at P70-90, but there were no differences at the younger ages. These results indicate that intrinsic cell properties and activity are altered in Cntnap2-/- rats, but these differences are not consistent across the ages. These experiments will provide novel insights into how Cntnap2 impacts auditory processing at a cellular level, and shed light on the neural mechanisms underlying altered auditory processing seen in Cntnap2-/- rats.

Electrophysiological properties of neurons in the primary auditory cortex of the Cntnap2 KO rat model for autism

In the classical view, the postsynaptic NMDAR (postNMDAR) acts as a coincidence detector and causes synaptic strengthening when connected neurons are simultaneously active, a concept known as Hebbian plasticity. However, NMDARs may also be expressed presynaptically (preNMDARs), where they control both short- and long-term plasticity. We recently showed that preNMDARs regulate evoked and spontaneous release by two independent signalling cascades. Here, we explore the roles of pre- and postNMDARs in synaptic plasticity. To achieve sparse genetic deletion of NMDARs, we created the triple transgenic mouse model Emx1Cre/+;Ai9tdTom/+;NR1flox/flox. Using MNI-NMDA uncaging in acute slices (postnatal days 11 to 18), we verified that postNMDARs were deleted in pyramidal cells (PCs) (0.020 ± 0.5 pA, n=31 vs. control -48 ± 6 pA, n=19; p<0.001). We previously showed that AP5 or Ro25-6981 reduce spontaneous release rates in PCs by blocking preNMDARs. However, these drugs did not affect mEPSC frequency in NMDAR deletion mice (AP5 85% ± 10%, n=10, vs. control 60% ± 20%, n=4, p=0.19; Ro 89% ± 15%, n=7, vs. control 66% ± 10%, n=8; p=0.53), consistent with successful preNMDAR deletion. We also observed that NMDAR deletion caused distinct changes in PC morphology, which we are quantifying by 3D reconstruction and spine counts. With this NMDAR deletion mouse model established, we will next use paired recordings to show how pre- and postNMDARs differentially influence short and long-term synaptic plasticity.

Exploring the location dependence of NMDA receptors in synaptic plasticity

Ferroptosis is a newly identified-form of programmed cell death, which is triggered by excess intracellular iron and deficient antioxidant defense, leading to iron-dependent lipid peroxidation. One molecular pathway of ferroprosis occurs through NCOA4, a shuttle protein that transports cytosolic ferritin to autophagosomes for degradation, resulting in the release of iron from ferritin, which subsequently stimulates lipid peroxidation. In spinal cord injury (SCI), one of the immediate consequences of trauma is rupture of blood vessels. Elevated level of iron due to infiltration of red blood cells to the site of injury increases the possibility that ferroptosis might be involved in secondary damage associated with SCI. However, the role of ferroptosis in SCI remains unclear. We have previously shown that iron accumulation in CD11b+ macrophages is seen rapidly after SCI. We now show increased expression of NCOA4 in the first two weeks after contusion injury in mice. NCOA4 was expressed in microglia/macrophages at the site of SCI lesions and was associated with reduced ferritin. We found that some NCOA4+ cells showed signs of cell death. In addition, protein levels of the antioxidant enzyme glutathione peroxidase 4 (GPX4) remains unchanged after SCI, indicating it may be insufficient to handle increased lipid peroxidation that causes ferroptosis. Treatment with a ferroptosis inhibitor (UAMC-3203), showed a small but significant improvement in locomotor recovery indicated by the BMS score and subscore. These findings indicate that NCOA4 may contribute to ferroptosis mediated iron toxicity in SCI.

The role of ferroptosis in iron toxicity following spinal cord injury

By sensing neuronal signalling, astrocytes release gliotransmitters triggered by intracellular calcium (Ca2 ) transients. This is involved in the regulation of synaptic plasticity. To improve our understanding of layer-5 (L5) neocortical astrocytes in mouse visual cortex, we investigate their developmental profile between postnatal days (P) 9 - 22. Using 2-photon microscopy in acute slices, astrocytes were targeted for patching with Sulforhodamine 101. L5 astrocyte Vm was -82 ± 0.4 mV and Rinput was 34 ± 2 MΩ (n = 127). Over the ages P9-22, Vm increased (Pearson's r = 0.271, p < 0.01) and Rinput decreased (r = -0.189, p < 0.05). Within L5, astrocytes exhibited diverse electrophysiological responses due to voltage steps. Hierarchical clustering indicated two or possibly more response classes, with one class showing a steady, time-invariant conductance and the others a slowly activating component. This slowly activating conductance vanished with age (Spearman's rho = -0.44, p < 0.05, n = 22). We next investigated the development of spontaneous Ca2 transients, visualized with Fluo-5F (200 µM) or AAV-GCaMP6f. We found that Ca2 transients in different compartments of an astrocyte decorrelated with age (r = -0.57, p < 0.01, n = 20), which might be explained by development and maturation of processes. Gap-junction coupling and morphological reconstructions over development were also investigated using Alexa 488 loading. This study provides a foundation for our future work to understand how astrocytes in visual cortex L5 regulate neocortical plasticity at excitatory synapses.

Developmental profile of visual cortex astrocytes

The ability to store new information and to adapt to new environments is a crucial neurobiological process underlying our everyday life. This capability has been linked to modifications of the efficacy of synaptic transmission that are partly due to activity-dependent structural alterations of dendritic spines. We propose a computational pipeline to reconstruct and analyse dendritic spines from confocal microscopy images into a 3D mesh model. First, we acquired confocal, high resolution images of distal dendritic segments from adult born neurons in the olfactory bulb (OB) after odour learning using a simple and complex go/no-go odour discrimination paradigm and after sensory deprivation. The reconstruction of dendritic spines images was performed in 4 steps i) correction of image stacks via deconvolution and diffraction correction, ii) 3D segmentation, iii) reconstruction into a 3D mesh and iv) automatic extraction of spines and measure of features. Recent papers suggest that clusterization instead of classification is more appropriate to describe spines because spine morphologies are continuous. Thus, instead of manually categorizing spines, we assessed several spine features such as the length, surface, open angle, etc., and analysed these features with machine learning. We performed dimension reduction and we then identified some clusters associated with spines' morphological features. We report a significant increase in surface and in open angle as well as a decrease in Hull ratio in the mice of odour learning group as compared to control animals.

Clustering dendritic spines morphological properties after odour stimulation and learning

Cholinergic modulation of the prefrontal cortex is essential for higher brain function. Changes in cholinergic markers have been reported in brains of healthy aged adults and those with dementia. Disturbances in fibers, enzymes, and receptors are hypothesized to underlie age-related cognitive dysfunctions; however, their integrated impact on cortical cholinergic signalling is not well understood. Here, we use optogenetics and whole cell electrophysiology to investigate functional properties of cholinergic synapses across a broad age range in adulthood. We use prefrontal brain slices from transgenic mice expressing excitatory opsin in cholinergic basal forebrain neurons whose axons project to the cortex and release acetylcholine when stimulated with light. We demonstrate that pyramidal neurons in the aged brain show changes in the properties of their responses to optogenetically-released acetylcholine. Direct excitation of cholinergic receptors on pyramidal neurons declines with age. Ongoing experiments are probing the impact of aging on specific contributions of presynaptic and postsynaptic components of the synapse. In addition, we are investigating age-dependence in the sensitivity of cholinergic signalling to allosteric modulation and other treatment interventions. Understanding the impact of aging on synaptic mechanisms of cholinergic transmission is important to understand cognitive resilience and gain a better appreciation for effective targets to treat cognitive impairment.

Functional impact of aging on mouse prefrontal cholinergic synapses: an optophysiological investigation

Schizophrenia (SZ) is a psychiatric disease affecting about 1% of the population. Increasing evidence shows that cortical alterations of the maturation and function of inhibitory systems inducing perturbations in the excitatory/inhibitory (E/I) balance are likely to be involved in the pathogenesis of SZ. We propose that a delay of the GABA switch from excitatory to inhibitory during neurodevelopment could explain the perturbations of the E/I balance and represent a cellular phenotype of SZ. To assess this assumption, we aim to use a multimodal imaging approach including Digital Holographic Microscopy (DHM) to explore the neuronal functional maturation in in vitro cellular models. During early neurodevelopmental stages, GABA-mediated depolarization induces calcium (Ca2+) influx through voltage-gated Ca2+-channels. As a first step, Ca2+ responses to GABA stimulation have then been monitored at different ages in primary cultures of rat cortical neurons. The percentage of neurons responding to GABA decreased overtime suggesting a loss of GABA depolarizing action during neuronal maturation. We are now evaluating the relationship between Ca2+ responses and the label-free DHM measurements of transmembrane water fluxes related to GABAA receptors and (N)KCC cotransporters activities. We aim to use this methodology to explore the maturation of iPSC-derived cortical neurons obtained from patients with SZ, high-risk offspring and controls. Such non-invasive detections of functional alterations during the neurodifferentiation process could help to identify risk biomarkers of SZ.

Exploration of inhibitory system maturation in neuronal cultures with multimodal imaging techniques: toward a methodology to identify cellular biomarkers of schizophrenia

Underline digital video library