Biology subjects

Parsons, M. P.

Publications and source records attributed to Parsons, M. P..

4 recordsLinked to original sources

bioRxiv2022

Absence of the bile acid enzyme CYP8B1 increases brain chenodeoxycholic acid and reduces neuronal excitotoxicity in mice

BackgroundBile acids (BAs), which act in the liver-brain axis, are liver-derived signaling molecules found in the brain. However, how they modulate neurological function remains largely unknown. MethodsTo assess the role of BAs in the brain, we generated mice with absent 12-hydroxylase (Cyp8b1), a BA synthesis enzyme, and determined if brain BA levels were altered in these mice, and if and how this may modulate neuronal function. ResultsThe absence of CYP8B1 increased brain levels of the primary BA chenodeoxycholic acid (CDCA), and decreased ischemic stroke infarct area. Furthermore, CDCA administration reduced ischemic stroke lesion area in wild-type mice. Excitotoxicity due to elevated extra-cellular glutamate contributes to neuronal death in ischemic stroke. Neurons from Cyp8b1-/- mice showed reduced susceptibility to glutamate-induced toxicity, and exogenous CDCA reduced glutamate-induced toxicity in neurons from wild-type mice. These data suggest that CDCA-mediated decreases in excitotoxic neuronal death contributes to the reduced stroke lesion area in Cyp8b1-/- mice. Aberrant N-methyl-D-aspartate receptor (NMDAR) over-activation contributes to excitotoxicity. CDCA decreased NMDAR-mediated excitatory post-synaptic currents (EPSCs) in wild-type brain slices, by reducing over-activation of the NMDAR subunit GluN2B. In line with this, synaptic NMDAR activity was also decreased in Cyp8b1-/- brain slices. Expression level and synaptic distribution of GluN2B were unaltered in Cyp8b1-/- mice, suggesting that CDCA may directly antagonize GluN2B-containing NMDARs. ConclusionsOur data suggests that CDCA acts in the liver-brain axis and decreases the aberrant over-activation of neuronal GluN2B-containing NMDARs, contributing to neuroprotection.

physiology↗

bioRxiv2022

Atypical NMDA Receptors Limit Synaptic Plasticity in the Adult Ventral Hippocampus

N-methyl-D-aspartate receptors (NMDARs) assemble as functionally diverse heterotetramers. Incorporation of the GluN3A subunit into NMDARs alters conventional NMDAR properties by reducing both magnesium sensitivity and calcium permeability. GluN1 together with GluN3A can also form functional receptors that lack a glutamate binding site and instead serve as excitatory glycine receptors (eGlyRs). GluN3A expression is high in early development but naturally declines to low levels in most brain regions by adulthood. Interestingly, GluN3A expression remains elevated in the CA1 of the adult ventral hippocampus (VH), but not in the dorsal hippocampus (DH). The DH and VH are now well-understood to play very different functional roles, with the DH being primarily involved in cognitive functions and the VH in emotional processing. Why GluN3A persists in the adult VH, and the impact its presence has on glutamatergic neurotransmission in the VH is currently unknown. Here, we show that GluN3A remains elevated both at synaptic and extrasynaptic locations in the adult VH, assembling as GluN1/GluN2/GluN3A NMDARs with reduced magnesium sensitivity, as well as GluN1/GluN3A eGlyRs. By comparing various synaptic properties in the DH and VH of wild-type (WT) and GluN3A knockout (KO) mice, we demonstrate that GluN3A persistence in the VH attenuates glutamate release, limits postsynaptic calcium influx through NMDARs, and reduces the magnitude of NMDAR-dependent long-term potentiation. In comparison, GluN3A KO had relatively little effect on these same properties in the DH. In all, our data demonstrate that GluN3A persistence in the VH represents a key modulator of VH excitability and therefore may play a central role in emotional processing.

neuroscience↗

bioRxiv2022

Huntingtin is essential for synaptic plasticity in the adult hippocampus

Huntingtin (HTT), an exceptionally large protein with hundreds of interacting partners within the central nervous system, has been extensively studied due to its role in Huntingtons disease (HD) pathology. HD is a monogenic disorder caused by a polyglutamine repeat expansion in the HTT gene, which results in the production of a pathogenic mutant huntingtin (mHTT) protein, and toxic effects of this mutant protein in the context of HD have been well-established. Less-established, however, is the role of wild type HTT (wtHTT) in the adult brain, particularly in areas outside the corticostriatal pathway. wtHTT has previously been suggested to play a vital role in cellular functions that promote synapse homeostasis, such as fast axonal transport of synaptic cargo, vesicle replenishment and receptor localization and stability. Synaptic dysfunction precedes and predicts cell death in many neurodegenerative diseases including HD (termed synaptopathies) and whether proper synaptic transmission can be maintained without wtHTT in extrastriatal brain areas such as the hippocampus remains unknown. Consequences of wtHTT reduction in the adult brain are of particular importance as clinical trials for many non-selective HTT-lowering therapies for HD are underway, which are unable to distinguish between mHTT and wtHTT, and therefore reduce levels of both proteins. We investigated the consequences of wtHTT loss of function in the CA3-CA1 pathway of the adult hippocampus using a conditional knockout mouse model and found that 1-2 month deletion of wtHTT in excitatory hippocampal neurons inhibits post-tetanic potentiation and completely abolishes NMDA receptor-dependent long-term potentiation in these animals. These data reveal a novel role of wtHTT as an essential regulator of short- and long-term plasticity in the adult hippocampus.

neuroscience↗

bioRxiv2022

Asymmetric dysregulation of glutamate dynamics across the synaptic cleft in a mouse model of Alzheimer disease

Most research on glutamate spillover focuses on the deleterious consequences of postsynaptic glutamate receptor overactivation. However, two decades ago, it was noted that the glial coverage of hippocampal synapses is asymmetric: astrocytic coverage of postsynaptic sites exceeds coverage of presynaptic sites by a factor of four. The fundamental relevance of this glial asymmetry remains poorly understood. Here, we used the glutamate biosensor iGluSnFR, and restricted its expression to either CA3 or CA1 neurons to visualize glutamate dynamics at pre- and postsynaptic microenvironments, respectively. We demonstrate that inhibition of the primarily astrocytic glutamate transporter-1 (GLT-1) slows glutamate clearance to a greater extent at presynaptic compared to postsynaptic membranes. GLT-1 expression was reduced early in a mouse model of AD, resulting in slower glutamate clearance rates at presynaptic but not postsynaptic membranes that opposed presynaptic short-term plasticity. Overall, our data demonstrate that the presynapse is particularly vulnerable to GLT-1 dysfunction and may have implications for presynaptic impairments in a variety of brain diseases.

neuroscience↗