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Hippocampal CA1 Pyramidal Neurons Display Sublayer and Circuitry Dependent Degenerative Expression Profiles in Aged Female Down Syndrome Mice

ABSTRACT: People with Down syndrome (DS) have intellectual disability (ID) and develop hallmark Alzheimer’s disease (AD) pathology during midlife. There are several circuits underlying memory and executive function in the DS and AD brain that are particularly vulnerable and degenerate early in disease, most notably the septohippocampal circuit and the trisynaptic loop in the hippocampus. A fundamental lack of knowledge exists as to the etiology and mechanisms of disease progression within these critical circuits vulnerable to degeneration in DS, AD, and relevant models. This is compounded by new evidence that suggests spatial localization of neurons has profound effects on activity and innervation within the hippocampal CA1 region. We postulated gene expression changes in a DS mouse model, at a time that these circuits of input to the CA1 are degraded, would have a significant effect on gene expression in CA1 pyramidal neurons. Further, this dysfunction may be specific to spatial localization and innervation. Laser capture microscopy on pyramidal neurons from CA1 was performed, isolating the entire CA1 pyramidal neuron layer, which was compared to select populations of deep and superficial pyramidal neurons from CA1a (distal CA1, adjacent to the subiculum). RNA-seq and bioinformatic analysis was performed where profound differences in dysregulation in the DS mouse model based on their spatial location and postulated circuitry were examined.

ORGANISM(S): Mus musculus

PROVIDER: GSE264258 | GEO | 2024/08/26

REPOSITORIES: GEO

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Project description:Down syndrome (DS) is the most prevalent genetic abnormality, occurring in ~1/700 live births caused by triplication of human chromosome 21 (HSA21). DS individuals have a multitude of phenotypic changes in peripheral systems and most notably, are cognitively impaired, with deficits in learning, language and memory acquisition and consolidation. Further, individuals with DS develop Alzheimer’s disease (AD) pathology during early middle-age (mid-30’s), although the underlying mechanism(s) driving this pathology onset are not well understood. Contributing to cognitive impairment in DS, morphological deficits are present in cortical lamination and distribution, with Layer III (L3) and Layer V (L5) pyramidal neurons showing reduced proliferation and a disorganized laminar structure. As such, examining cortical L3 and L5 pyramidal neurons in DS individuals with AD pathology may elucidate alternate driver mechanisms of disease onset. We examined DS and age-matched control (CTR) brains from individuals without intellectual disabilities or dementia using postmortem frontal cortex (BA9) via single population low input RNA-sequencing. We specifically targeted L3 and L5 pyramidal neurons to understand circuitry-based alterations in DS/AD individuals to elucidate mechanistic drivers of disease pathology. We show convergent and divergent gene expression changes in L3 and L5 pyramidal neurons which may underlie cognitive deficits and degenerative pathology associated with aging in the DS brain. We pinpoint alterations in HSA21 gene expression as well as a multitude of convergent differentially expressed genes (DEGs) and relevant pathways via bioinformatic inquiry in the DS brains that underlie mechanisms of degeneration. Distinctly, convergent gene expression reveals a neurotoxic phenotype, with multiple canonical pathways of dysregulation and disease functions linked to the convergent DEGs, including downregulation of the CLEAR signaling pathway and significantly increased activation of the neuroinflammation signaling pathway. We identify several target genes including mitogen activated protein kinase 1 and 3 (MAPK1/3), calcium voltage-gated channel subunit alpha1 A (CACNA1A) and superoxide dismutase 1 (SOD1), which exhibit overlap in multiple pathways and disease functions and/or display dysfunctional protein-protein network interactions for further examination. Novel targets identified by our single population approach in L3 and L5 pyramidal neurons may be drivers of the disease mechanism and therefore may be therapeutic candidates for amelioration of degenerative pathology that targets memory and executive function circuits in DS/AD.

Project description:Neurodegenerative brain disorders become more common in the aged. Most of these disorders are associated with or caused by selective death of certain neuronal subpopulations. The mechanisms underlying the differential vulnerability of certain neuronal populations are still largely unexplored and few neuroprotective treatments are available to date. Elucidation of these mechanisms may lead to a greater understanding of the pathogenesis and treatment of neurodegenerative diseases. Moreover, preconditioning by a short seizure confers neuroprotection following a subsequent prolonged seizure. Our goal is to identify pathways that confer vulnerability and resistance to neurotoxic conditions by comparing the basal and preconditioned gene expression profiles of three differentially vulnerable hippocampal neuron populations. Hippocampal CA1 and CA3 pyramidal neurons are highly susceptible to seizures and ischemia, whereas dentate gyrus granule cells are relatively resistant. A brief preconditioning seizure confers protection to the pyramidal cells. We will first determine gene expression profiles of untreated rat CA1 and CA3 pyramidal cells, and dentate granule cells, using laser capture microscopy to obtain region-specific neuronal mRNA. We will then determine the effect of a brief preconditioning seizure, which is neuroprotective in CA1 and CA3, on these expression profiles. We hypothesize that common molecular mechanisms exist in neurons that determine their susceptibility to seizure-induced injury. Intrinsic differences in gene expression exist between hippocampal glutamatergic CA1 and CA3 pyramidal neurons, on the one hand, and dentate granule cells on the other, which contribute to the greater susceptibility of pyramidal neurons to degeneration in experimental stroke and epilepsy. We specifically hypothesize that differences in basal energy metabolism genes may confer differential susceptibility to neurodegeneration produced by seizures and ischemia. Anesthetized animals will be sacrificed by decapitation, and frozen 10 micron sections will be lightly stained with cresyl violet to identify cell layers in the hippocampus. Approximately 1000 neurons from each of the three cell layers will be isolated by LCM. Poly-A RNA will be amplified using a modified Eberwine protocol. The quality of our aRNA will be evaluated by quantitative RT-PCR of GluR6 and KA2 mRNA levels before we send the samples to the Center for labeling and hybridization to Affymetrix rat 230A arrays. We will provide a one-round amplification cDNA product to the center for labeling and hybridization. This protocol is identical to a previously approved study by Jim Greene in our laboratory.

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Hippocampal CA1 Pyramidal Neurons Display Sublayer and Circuitry Dependent Degenerative Expression Profiles in Aged Female Down Syndrome Mice

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