Biology subjects

Gouras, G. K.

Publications and source records attributed to Gouras, G. K..

6 recordsLinked to original sources

bioRxiv2023

Intracellular Amyloid-β in the Normal Rat Brain and Human Subjects

Amyloid-beta (A{beta}) is a normal product of neuronal activity, and the two most common variants are 40 or 42 residues long. Of these, the 42 residue-version (A{beta}42) is normally less abundant but more prone to self-aggregate, and is thought to cause Alzheimers disease (AD). Much knowledge about AD-pathogenesis comes from the study of rodents made to model aspects of the disease by expressing AD-relevant human transgenes, like human amyloid precursor protein (APP) containing mutations that drive up A{beta} production or increase the A{beta}42/40 ratio and thereby causes AD. Yet, when it comes to the normal expression of A{beta}42 in rodent brains, surprisingly little is known. Here we characterize the expression of A{beta}42 throughout the brain of normal, outbred Wistar rats, including animals from 3-18 months of age. We find that intracellular A{beta}42 (iA{beta}42) is present in neurons located throughout the brain at all ages of normal Wistar rats, but that the levels vary greatly between brain regions. In cortex, we observe the highest levels of iA{beta}42 in neurons that are part of layer II of the entorhinal cortex (EC), along with neurons in the hippocampus, closely followed by neurons in the somatosensory cortex. Among subcortical structures, we observe the highest levels iA{beta}42 in the locus coeruleus. In order to explore whether the striking presence of iA{beta}42 in rat EC also holds true in human subject free of neurological disease, we examined EC of six such cases ranging from ages 20-88 years. In all six cases, we find that iA{beta}42 is present in EC layer II-neurons. Our findings support two conclusions about iA{beta}42. First, iA{beta}42 is present in neurons of wild-type Wistar rats and is restricted to the same structures where iA{beta} accumulates, and A{beta}-plaques form, in a much used AD model based on Wistar rats (the McGill-R-Thy1-APP rat model). The difference between wild-type Wistar rats and these AD model rats, with respect to A{beta}42, is therefore a quantitative one rather that a qualitative one. This indicates that the McGill rat model in fact models the underlying wild-type neuronal population-specific vulnerability to A{beta}42-accumulation. Second, because the McGill rat model closely mimics the human AD-associated spatiotemporal sequence of amyloid plaque deposition, this model may offer a useful representation of the pre-plaque neuronal accumulation of iA{beta}42. Our findings in human cases are in line with prior findings, and substantiate the notion that neurons in layer II of EC are vulnerable to accumulation of iA{beta}42.

pathology↗

bioRxiv2023

Proteomic analysis across patient iPSC-based models and human post-mortem hippocampal tissue reveals early cellular dysfunction, progression, and prion-like spread of Alzheimer s disease pathogenesis

The hippocampus is a primary region affected in Alzheimers disease (AD). Because AD postmortem brain tissue is not available prior to symptomatic stage, we lack understanding of early cellular pathogenic mechanisms. To address this issue, we examined the cellular origin and progression of AD pathogenesis in patient-based model systems including iPSC-derived brain cells transplanted into the mouse brain hippocampus. Notably, proteomic analysis of the graft enabled the identification of proteomic alterations in AD patient brain cells, associated with increased levels of {beta}-sheet structures and A{beta}42 peptides. Interestingly, the host cells surrounding the AD graft also presented alterations in cellular biological pathways. Furthermore, proteomic analysis across human iPSC-based models and human post-mortem hippocampal tissue projected coherent longitudinal cellular changes indicative of disease progression from early to end stage AD. Our data showcase patient-based models to study the cellular origin, progression, and prion-like spread of AD pathogenesis. Highlights- AD patient iPSC-derived brain cells survive in the hippocampus of immunodeficient mice 6 months post-transplantation. - Proteomic analysis of the grafts reveals profound alterations in cellular biological pathways in iPSC-derived hippocampal cells despite absence of senile plaques. - Proteomic alterations within transplanted AD iPSC-derived hippocampal cells are reminiscent of early/prodromal AD. - AD-grafted cells induce proteomic changes in host mouse cells.

neuroscience↗

bioRxiv2022

Apolipoprotein E intersects with amyloid-β within neurons

Apolipoprotein E4 (ApoE4) is the most important genetic risk factor for Alzheimers disease (AD). Among the earliest changes in AD is endosomal enlargement in neurons, which was reported as enhanced in ApoE4 carriers. ApoE is thought to be internalized into endosomes of neurons, while {beta}-amyloid (A{beta}) accumulates within neuronal endosomes early in AD. However, it remains unknown whether ApoE and A{beta} intersect intracellularly. We show that internalized astrocytic ApoE localizes mostly to lysosomes in neuroblastoma cells and astrocytes, while in neurons it preferentially localizes to endosomes-autophagosomes of neurites. In AD transgenic neurons, astrocyte-derived ApoE intersects intracellularly with amyloid precursor protein (APP)/A{beta}. Moreover, ApoE4 increases the levels of endogenous and internalized A{beta}42 in neurons. Taken together, we demonstrate differential localization of ApoE in neurons, astrocytes and neuron-like cells, and show that internalized ApoE intersects with APP/A{beta} in neurons, which may be of considerable relevance to AD.

neuroscience↗

bioRxiv2022

Lowering levels of reelin in entorhinal cortex layer II-neurons results in lowered levels of intracellular amyloid-β.

Projection neurons in the anterolateral part of entorhinal cortex layer II (alEC LII) are the predominant cortical site for hyperphosphorylation of tau (p-tau) and formation of neurofibrillary tangles (NFTs) in brains of subjects with early-stage Alzheimers Disease (AD). A majority of alEC LII-neurons are unique among cortical excitatory neurons by expressing the protein reelin (Re+). In AD patients, and a rat model for AD overexpression mutated human APP, these Re+ excitatory projection-neurons are prone to accumulate intracellular amyloid-{beta} (iA{beta}). Biochemical pathways that involve reelin-signaling regulate levels of p-tau, and iA{beta} has been shown to impair such reelin-signaling. We therefore used the rat model and set out to assess whether accumulation of iA{beta} in Re+ alEC LII projection neurons relates to the fact that these neurons express reelin. Here we show that in Re+ alEC LII-neurons, reelin and iA{beta}42 engage in a direct protein-protein interaction, and that microRNA-mediated lowering of reelin-levels in these neurons leads to a concomitant reduction of non-fibrillar iA{beta} ranging across three levels of aggregation. Our experiments are carried out several months before plaque pathology emerges in the rat model, and the reduction of iA{beta} occurs without any substantial associated changes in human APP-levels. We propose a model positioning reelin in a sequence of changes in functional pathways in Re+ alEC LII-neurons, explaining the region and neuron-specific initiation of AD pathology. SignificanceAnterolateral entorhinal cortex layer II (EC LII) neurons are the predominant cortical site for hyperphosphorylation of tau (p-tau) and formation of neurofibrillary tangles (NFTs) in brains of subjects with early-stage Alzheimers disease (AD). The same neurons are prone to very early accumulation of non-fibrillary forms of amyloid-{beta} in the context of AD, and are unique among cortical excitatory neurons by expressing the protein reelin. We show that in such alEC LII-neurons, reelin and iA{beta}42 engage in a direct protein-protein interaction, and that selectively lowering levels of reelin leads to a concomitant reduction of non-fibrillar A{beta}. We propose a model positioning reelin in a sequence of changes in functional pathways in reelin-expressing EC LII neurons, explaining the region and neuron specific initiation of AD.

pathology↗

bioRxiv2022

Aβ/APP-induced hyperexcitability and dysregulation of homeostatic synaptic plasticity in models of Alzheimer's disease

The proper function of the nervous system is dependent on the appropriate timing of neuronal firing. Synapses continually undergo rapid activity-dependent modifications that require feedback mechanisms to maintain network activity within a window in which communication is energy efficient and meaningful. Homeostatic synaptic plasticity (HSP) and homeostatic intrinsic plasticity (HIP) are such negative feedback mechanisms. Accumulating evidence implicates that Alzheimers disease (AD)-related amyloid precursor protein (APP) and its cleavage product amyloid-beta (A{beta}) play a role in the regulation of neuronal network activity, and in particular HSP. AD features impaired neuronal activity with regional early hyper-activity and A{beta}-dependent hyperexcitability has also been demonstrated in AD transgenic mice. We demonstrate similar hyper-activity in AD transgenic neurons in culture that have elevated levels of both human APP and A{beta}. To examine the individual roles of APP and A{beta} in promoting hyperexcitability we used an APP construct that does not generate A{beta}, or elevated A{beta} levels independently of APP. Increasing either APP or A{beta} in wild type (WT) neurons leads to increased frequency and amplitude of calcium transients. Since HSP/HIP mechanisms normally maintain a setpoint of activity, we examined whether homeostatic synaptic/intrinsic plasticity was altered in AD transgenic neurons. Using methods known to induce HSP/HIP, we demonstrate that APP protein levels are regulated by chronic modulation of activity and show that AD transgenic neurons have an impaired response to global changes in activity. Further, AD transgenic compared to WT neurons failed to adjust the length of their axon initial segments (AIS), an adaptation known to alter excitability. Thus, we present evidence that both APP and A{beta} influence neuronal activity and that mechanisms of HSP/HIP are disrupted in neuronal models of AD.

neuroscience↗

bioRxiv2020

APOE4 affects basal and NMDAR mediated protein synthesis in neurons by perturbing calcium homeostasis

Apolipoprotein E (APOE), one of the primary lipoproteins in the brain has three isoforms in humans - APOE2, APOE3, and APOE4. APOE4 is the most well-established risk factor increasing the pre-disposition for Alzheimers disease. The presence of the APOE4 allele alone is shown to cause synaptic defects in neurons and recent studies have identified multiple pathways directly influenced by APOE4. However, the mechanisms underlying APOE4 induced synaptic dysfunction remain elusive. Here, we report that the acute exposure of primary cortical neurons to APOE4 leads to a significant decrease in global protein synthesis. APOE4 treatment also abrogates the NMDA mediated translation response indicating an impairment of synaptic signaling. Importantly, we demonstrate that both APOE3 and APOE4 generate a distinct translation response which is closely linked to their respective calcium signature. Acute exposure to APOE3 causes a short burst of calcium through NMDARs in neurons leading to an initial decrease in protein synthesis which quickly recovers. Contrarily, APOE4 leads to a sustained increase in calcium levels by activating both NMDARs and L-VGCCs, thereby causing sustained translation inhibition through eEF2 phosphorylation, which in turn disrupts NMDAR response. Thus, we show that APOE4 affects basal and activity mediated protein synthesis response in neurons by affecting calcium homeostasis. We propose this as a possible mechanism to explain the synaptic dysfunction caused by APOE4. Highlights / SummaryO_LIAPOE3 treatment causes a short burst of calcium through NMDARs, leading to an acute increase in eEF2 phosphorylation which eventually recovers to basal levels. C_LIO_LIGlobal translation follows a similar temporal profile of initial inhibition followed by recovery in APOE3 treated neurons, thus unaffecting the NMDA mediated translation response. C_LIO_LIAPOE4 treatment activates both NMDARs and L-VGCCs leading to a marked elevation in calcium levels, thus causing sustained increase in eEF2 phosphorylation as well as global translation inhibition. C_LIO_LIHence, the NMDA mediated response is perturbed, potentially causing a stress-related phenotype in APOE4 treated neurons. C_LIO_LIThus, different calcium signatures and sources lead to distinct temporal profiles of translation. C_LI

neuroscience↗