Role of TRPV4 channels in astroglial swelling and ion dysregulation in the ischemic brain

• Applicant: Dr. Iris Alvarez Merz
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 529623901

Astrocytes are the principal cell type controlling brain homeostasis, including extracellular ion regulation and neurotransmitter clearance. Dysregulations of the intracellular sodium concentration disturbs the inwardly-directed sodium gradient established across the cell membrane and prevents astrocytes from performing their physiological functions. The sodium gradient is maintained by the sodium/potassium ATPase, the major ATP consumer of the brain. Thus, energy deprivation upon ischemic conditions leads to the breakdown of the ionic homeostasis and buildup of intracellular sodium concentration in both astrocytes and neurons. Astrocytic sodium increments contribute to cell swelling and formation of brain edema. Moreover, the increase in astroglial sodium is a main pathomechanism leading to acute injury in the ischemic brain, although the underlying pathways involved have been only partially elucidated. A new candidate contributing to astrocytic swelling and sodium fluctuations is the non-selective cation channel transient receptor potential vanilloid 4 (TRPV4). Here, I propose to elucidate its role in the generation of astrocyte ion loading and volume changes in acute mouse brain tissue slices using two different protocols to mimic ischemic conditions in the brain. Special focus will be set on astroglial perisynaptic processes and on endfeet that envelop blood vessels, as both are dynamic astrocytic compartments that regulate synaptic transmission and brain blood flow, respectively. Complementary mechanisms that could contribute to astrocytic accumulation of sodium besides a direct TRPV4-channel-mediated sodium influx will be addressed. For that, widefield, confocal and multiphoton microscopy will be used, together with the sodium indicator ION-NaTRIUM-Green-2-AM (ING2). Moreover, rapid fluorescence lifetime imaging microscopy (rapidFLIM) will allow quantitative, dynamic imaging of sodium signals in astrocytes under swelling-inducing conditions. Finally, patch-clamp recordings from astrocytes will be used to characterise membrane potential and membrane current shifts due to TRPV4 activation during metabolic stress. Thus, this project will shed light on the involvement of TRPV4 channels in pathogenic astroglial swelling and ion dysregulation under brain ischemia.

• DFG Programme: WBP Position