TRPM2 in the Central Nervous System: Physiological Role and Critical Regulatory Pathways

TRPM2 is a non-selective cation channel which is permeable to calcium. Although expression is highest in the brain, the physiological role for TRPM2 in neurons was unknown. Furthermore, our understanding of the pathways regulating TRPM2 channel function required further investigation. In this thesis, we identified that TRPM2 is required for NMDAR-dependent long-term depression (LTD). No change in NMDAR expression or function was observed following genetic deletion of TRPM2. Instead, the loss of NMDAR-LTD in TRPM2 knockout mice results from diminished GSK-3β activation. We next examined whether age in vitro could facilitate TRPM2 currents. We demonstrate that diminished glutathione with age results in the loss of basal TRPM2 channel inhibition. We subsequently demonstrate that TRPM2 currents are enhanced by oligomeric Aβ, a peptide proposed to initiate the majority of toxic effects observed in Alzheimer’s disease. Potentiation of TRPM2 may involve Fyn, a tyrosine kinase implicated in oxidative stress and neurotoxicity. We demonstrate that Fyn is capable of interacting with and phosphorylating TRPM2. Acute application of Fyn through the patch pipette potentiates TRPM2 currents, and a Fyn(39-57) mimetic peptide significantly attenuates currents in cultured hippocampal neurons. These results are the first to establish a role for TRPM2 in neurons, and also implicate TRPM2 in neurodegenerative disorders such as Alzheimer’s disease

Loss of glutathione homeostasis associated with neuronal senescence facilitates TRPM2 channel activation in cultured hippocampal pyramidal neurons

<p>Abstract</p> <p>Background</p> <p>Glutathione (GSH) plays an important role in neuronal oxidant defence. Depletion of cellular GSH is observed in neurodegenerative diseases and thereby contributes to the associated oxidative stress and Ca²⁺dysregulation. Whether depletion of cellular GSH, associated with neuronal senescence, directly influences Ca²⁺permeation pathways is not known. Transient receptor potential melastatin type 2 (TRPM2) is a Ca²⁺permeable non-selective cation channel expressed in several cell types including hippocampal pyramidal neurons. Moreover, activation of TRPM2 during oxidative stress has been linked to cell death. Importantly, GSH has been reported to inhibit TRPM2 channels, suggesting they may directly contribute to Ca²⁺dysregulation associated with neuronal senescence. Herein, we explore the relation between cellular GSH and TRPM2 channel activity in long-term cultures of hippocampal neurons.</p> <p>Results</p> <p>In whole-cell voltage-clamp recordings, we observe that TRPM2 current density increases in cultured pyramidal neurons over time in vitro. The observed increase in current density was prevented by treatment with NAC, a precursor to GSH synthesis. Conversely, treatment of cultures maintained for 2 weeks in vitro with L-BSO, which depletes GSH by inhibiting its synthesis, augments TRPM2 currents. Additionally, we demonstrate that GSH inhibits TRPM2 currents through a thiol-independent mechanism, and produces a 3.5-fold shift in the dose-response curve generated by ADPR, the intracellular agonist for TRPM2.</p> <p>Conclusion</p> <p>These results indicate that GSH plays a physiologically relevant role in the regulation of TRPM2 currents in hippocampal pyramidal neurons. This interaction may play an important role in aging and neurological diseases associated with depletion of GSH.</p

Dependence of NMDA/GSK-3β Mediated Metaplasticity on TRPM2 Channels at Hippocampal CA3-CA1 Synapses

Transient receptor potential melastatin 2 (TRPM2) is a calcium permeable non-selective cation channel that functions as a sensor of cellular redox status. Highly expressed within the CNS, we have previously demonstrated the functional expression of these channels in CA1 pyramidal neurons of the hippocampus. Although implicated in oxidative stress-induced neuronal cell death, and potentially in neurodegenerative disease, the physiological role of TRPM2 in the central nervous system is unknown. Interestingly, we have shown that the activation of these channels may be sensitized by co-incident NMDA receptor activation, suggesting a potential contribution of TRPM2 to synaptic transmission. Using hippocampal cultures and slices from TRPM2 null mice we demonstrate that the loss of these channels selectively impairs NMDAR-dependent long-term depression (LTD) while sparing long-term potentiation. Impaired LTD resulted from an inhibition of GSK-3β, through increased phosphorylation, and a reduction in the expression of PSD95 and AMPARs. Notably, LTD could be rescued in TRPM2 null mice by recruitment of GSK-3β signaling following dopamine D2 receptor stimulation. We propose that TRPM2 channels play a key role in hippocampal synaptic plasticity

The Transient Receptor Potential Melastatin 2 (TRPM2) Channel Contributes to beta-Amyloid Oligomer-Related Neurotoxicity and Memory Impairment

In Alzheimer\u27s disease, accumulation of soluble oligomers of beta-amyloid peptide is known to be highly toxic, causing disturbances in synaptic activity and neuronal death. Multiple studies relate these effects to increased oxidative stress and aberrant activity of calcium-permeable cation channels leading to calcium imbalance. The transient receptor potential melastatin 2 (TRPM2) channel, a Ca2+-permeable nonselective cation channel activated by oxidative stress, has been implicated in neurodegenerative diseases, and more recently in amyloid-induced toxicity. Here we show that the function of TRPM2 is augmented by treatment of cultured neurons with beta-amyloid oligomers. Aged APP/PS1 Alzheimer\u27s mouse model showed increased levels of endoplasmic reticulum stress markers, protein disulfide isomerase and phosphorylated eukaryotic initiation factor 2 alpha, as well as decreased levels of the presynaptic marker synaptophysin. Elimination of TRPM2 in APP/PS1 mice corrected these abnormal responses without affecting plaque burden. These effects of TRPM2 seem to be selective for beta-amyloid toxicity, as ER stress responses to thapsigargin or tunicamycin in TRPM2(-/-) neurons was identical to that of wild-type neurons. Moreover, reduced microglial activation was observed in TRPM2(-/-)/APP/PS1 hippocampus compared with APP/PS1 mice. In addition, age-dependent spatial memory deficits in APP/PS1 mice were reversed in TRPM2(-/-)/APP/PS1 mice. These results reveal the importance of TRPM2 for beta-amyloid neuronal toxicity, suggesting that TRPM2 activity could be potentially targeted to improve outcomes in Alzheimer\u27s disease

The Prion Protein Ligand, Stress-Inducible Phosphoprotein 1, Regulates Amyloid-beta Oligomer Toxicity

In Alzheimer\u27s disease (AD), soluble amyloid-beta oligomers (A beta Os) trigger neurotoxic signaling, at least partially, via the cellular prion protein (PrPC). However, it is unknown whether other ligands of PrPC can regulate this potentially toxic interaction. Stress-inducible phosphoprotein 1 (STI1), an Hsp90 cochaperone secreted by astrocytes, binds to PrPC in the vicinity of the A beta O binding site to protect neurons against toxic stimuli. Here, we investigated a potential role of STI1 in A beta O toxicity. We confirmed the specific binding of A beta Os and STI1 to the PrP and showed that STI1 efficiently inhibited A beta O binding to PrP in vitro (IC50 of similar to 70 nM) and also decreased A beta O binding to cultured mouse primary hippocampal neurons. Treatment with STI1 prevented A beta O-induced synaptic loss and neuronal death in mouse cultured neurons and long-term potentiation inhibition in mouse hippocampal slices. Interestingly, STI1-haploinsufficient neurons were more sensitive to A beta O-induced cell death and could be rescued by treatment with recombinant STI1. Noteworthy, both A beta O binding to PrPC and PrPC-dependent A beta O toxicity were inhibited by TPR2A, the PrPC-interacting domain of STI1. Additionally, PrPC-STI1 engagement activated alpha 7 nicotinic acetylcholine receptors, which participated in neuroprotection against A beta O-induced toxicity. We found an age-dependent upregulation of cortical STI1 in the APPswe/PS1dE9 mouse model of AD and in the brains of AD-affected individuals, suggesting a compensatory response. Our findings reveal a previously unrecognized role of the PrPC ligand STI1 in protecting neurons in AD and suggest a novel pathway that may help to offset A beta O-induced toxicity

Scientific overview: CSCI-CITAC annual general meeting and young investigator's forum 2011

In 2011, members of the Clinician Investigator Trainee Association of Canada - Association des cliniciens-chercheurs en formation du Canada (CITAC-ACCFC) and the Canadian Society for Clinician Investigators (CSCI) held a joint Annual General Meeting (AGM) and Young Investigator Forum (YIF) September 12-14 in Ottawa, ON, Canada. The theme of the meeting was “The Role of Government and Regulatory Organizations in Shaping the Environment of the Clinician Scientist”. The meeting was well attended by established clinician scientists and clinician investigator trainees from across Canada. The aim of this scientific overview is to highlight the research presented by trainees at both the oral plenary session as well as the poster presentation sessions of this meeting. This work covers a wide variety of medical disciplines, focusing on translational medicine, from the basic sciences to clinical application

Canadian Clinician Investigator Training in the 21st Century

Purpose: Enhancing clinician-investigator (CI) training at Canadian medical schools is urgently needed to bolster the dwindling work force of medical professionals carrying out patient-oriented research in a wide array of medical fields. The purpose of this study is to obtain, from the 15 Canadian medical schools that offer one or more CI training programs, data on the number of trainees, funding levels, attrition rates or other important metrics to evaluate the outcomes of such training efforts. Methods: All Canadian CI programs were surveyed to collect demographic information for the academic year 2010-2011 and compared this to historical data collected by the Association of Faculties of Medicine of Canada (AFMC) and MD/PhD program funding data from the Canadian Institutes of Health Research (CIHR). Results: Over the past decade, enrolment in Canadian CI training programs has increased approximately four-fold. Program-specific funding (CIHR) has also increased, but nearly 50% of MD/PhD trainees are still not supported through dedicated CIHR funding. Conclusion: It is too early to know to what extent this increase in both CI and funding will sustain the workforce of Canadian researchers carrying out patient-oriented research. Monitoring of CI training demographics across Canada, beyond this baseline study, will be essential to measure outcomes from CI training programs and to guide response from funding bodies and policy-makers