Cellular And Synaptic Physiology Of Hippocampal Interneurons

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

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Abstract

Although cortical and hippocampal GABAergic inhibitory interneurons represent only 20% of the total cortical cell population their anatomical diversity is unparalleled in the mammalian central nervous system; for example there are currently upwards of 20 acknowledged distinct members within the CA1 hippocampal formation alone. Their anatomical diversity is rich, with the morphologies of many cell types remaining local to a particular subfield, while other cell types extend wide arbor dendrites and axons that cross numerous cortical and hippocampal layers and subfields. Inhibitory interneurons often demonstrate exquisite targeting of their axons to differential postsynaptic structures. For example, axons can target selective subcellular domains (e.g. the perisomatic, axon initial segment or specific dendritic domains) to compartmentalize or time electrical activity in either a positive or negative manner. Alternatively, axons can make projections several millimeters in length, to innervate thousands of postsynaptic targets to co-ordinate the activity of both homogeneous and distributed neuronal ensembles. Moreover, each cortical interneuron subtype is unique in its proliferative history, migration during corticogenesis as well as postnatal integration into cortical circuitry. Indeed several developmentally regulated neural circuit disorders such as epilepsy, schizophrenia and autism are likely associated with deficits in the numbers and function of distinct interneuron cohorts. For all of these reasons inhibitory interneurons have recently become the intense focus of investigators drawn from a wide variety of backgrounds. Work in my Section over the last year has largely focused on two main aspects of inhibitory interneuron function: (1) We have continued our study of glutamatergic and GABAergic synaptic transmission made onto inhibitory interneurons and their downstream targets within the hippocampal formation. (2) We are also using genetic approaches to examine the role of inhibitory interneurons in the onset and progression of circuit degradation in a mouse models of Alzheimer's disease, and schizophrenia. (3). In a multi investigator collaboration we are developing virally mediated research tools that facilitate the exploration of the role of inhibitory interneurons across species from mouse to humans. We use a multi-parametric approach of cortical and hippocampal development and circuit function, which has been an extremely fruitful research avenue and is a perfect example of a research strategy well suited to the intramural environment. Resilient Hippocampal Gamma Rhythmogenesis and Parvalbumin-Expressing Interneuron Function Before and After Plaque Burden in 5xFAD Alzheimers Disease Model. Recent studies have implicated impaired Parvalbumin Fast-Spiking Interneuron (PVIN) function as a precipitating factor underlying abnormalities in network synchrony, oscillatory rhythms, and cognition associated with Alzheimers disease (AD). However, a complete developmental investigation of potential gamma deficits, induced by commonly used carbachol or kainate in ex vivo slice preparations, within AD model mice is lacking. We examined gamma oscillations using field recordings in acute hippocampal slices from 5xFAD and control mice, through the period of developing pathology, starting at 3 months of age, when there is minimal plaque presence in the hippocampus, through to 12+ months of age, when plaque burden is high. In addition, we examined PVIN participation in gamma rhythms using targeted cell-attached recordings of genetically-reported PVINs, in both wild type and mutant mice. In parallel, a developmental immunohistochemical characterisation probing the PVIN-associated expression of PV and perineuronal nets (PNNs) was compared between control and 5xFAD mice. Remarkably, this comprehensive longitudinal evaluation failed to reveal any obvious correlations between PVIN deficits (electrical and molecular), circuit rhythmogenesis (gamma frequency and power), and A deposits/plaque formation. By 612 months, 5xFAD animals have extensive plaque formation throughout the hippocampus. However, a deficit in gamma oscillatory power was only evident in the oldest 5xFAD animals (12+ months), and only when using kainate, and not carbachol, to induce the oscillations. We found no difference in PV firing or phase preference during kainate-induced oscillations in younger or older 5xFAD mice compared to control, and a reduction of PV and PNNs only in the oldest 5xFAD mice. The lack of a clear relationship between PVIN function, network rhythmicity, and plaque formation in our study highlights an unexpected resilience in PVIN function in the face of extensive plaque pathology associated with this model, calling into question the presumptive link between PVIN pathology and Alzheimers progression. In a collaboration with Paul Worley's research team at Johns Hopkins we have recently discovered a novel biomarker-authenticated model of schizophrenia which implicates the loss of function of the neuronal pentraxin NPTX2. Schizophrenia is a polygenetic disorder whose clinical onset is often associated with behavioral stress. Here, we present a model of disease pathogenesis that builds on our observation that the synaptic immediate early gene , the neuronal pentraxin NPTX2 is reduced in cerebrospinal fluid of individuals with recent onset schizophrenia. NPTX2 plays an essential role in maintaining excitatory homeostasis by adaptively enhancing circuit inhibition. NPTX2 function requires activity-dependent exocytosis and dynamic shedding at synapses and is coupled to circadian behavior. Behavior-linked NPTX2 trafficking is abolished by mutations that disrupt select activity-dependent plasticity mechanisms of excitatory neurons. Modeling NPTX2 loss of function results in failure of parvalbumin interneurons in their adaptive contribution to behavioral stress, and animals exhibit multiple neuropsychiatric domains. Because the genetics of schizophrenia encompasses diverse proteins that contribute to excitatory synapse plasticity, the identified vulnerability of NPTX2 function can provide a framework for assessing the impact of genetics and the intersection with stress. In a multi investigator collaboration with the lab of Gord Fishell and Jordan Dimidschstein we have developed a versatile viral toolkit for functional discovery in the nervous system. The ability to precisely control transgene expression is essential for basic research and clinical applications. Adeno-associated viruses (AAVs) are non-pathogenic and can be used to drive stable expression in virtually any tissue, cell type, or species, but their limited genomic payload results in a trade-off between the transgenes that can be incorporated and the complexity of the regulatory elements controlling their expression. Resolving these competing imperatives in complex experiments inevitably results in compromises. Here, we assemble an optimized viral toolkit (VTK) that addresses these limitations and allows for efficient combinatorial targeting of cell types. Moreover, their modular design explicitly enables further refinements. We achieve this in compact vectors by integrating structural improvements of AAV vectors with innovative molecular tools. Using these tools we have illustrated the potential of this approach through a systematic demonstration of their utility for targeting cell types and querying their biology using a wide array of genetically encoded tools.