Andrew Jo, University of Utah
Ophthalmology and Visual Sciences
All cells, including retinal neurons and glia, must sense and adapt to physical changes in their local environment (e.g. changes in osmotic pressure). Osmotic water flux can cause aberrant cell volume changes, which can contribute to tissue damage, edema, and neuronal hyperexcitability and excitotoxicity. We hypothesized that force-sensitive proteins enable retinal cells to monitor their physical form (e.g. volume) and help maintain homeostasis by regulating cell volume. To test this, we first investigated the properties of cell morphology when cells were bathed in solutions with different tonicities. Under these conditions, we measured changes in cell volume due to osmosis in retinal ganglion cells (RGCs) and Müller glia. We found RGCs were unable to actively adjust their volume, whereas Müller glia reduced their swollen volume in the continued presence of hypotonicity. The regulation of cellular volume often involves calcium signals. We tested whether calcium plays a role in the regulation of retinal cell volume. Free calcium within the cells was sequestered using cytosolic BAPTA, which decreased the extent of hypotonic swelling. This demonstrates that calcium elevations increase the extent of cell swelling. Because cell volume was dependent on calcium, which was elevated by membrane stretch, we hypothesized that the osmosensitive cation channel TRPV4 would transduce osmotic pressure and contribute to cell volume regulation. In an experiment using a selective TRPV4 inhibitor, the extent of hypotonic-induced swelling was reduced. Thus, the opening of TRPV4 leads to a calcium influx that exacerbated cellular swelling. In addition, we tested the idea that TRPV1 cation channel is involved in responses to osmotic stimuli. In an experiment using a selective TRPV1 antagonist, preliminary results show that the extent of hypotonic-induced swelling decreased. This signals that TRPV1 may have a role in volume regulation in retinal neurons and glia. Thus, inhibition of these force-sensitive protein channels might alleviate the deleterious effects of volume changes in pathological contexts. Our findings therefore have implications for our understanding of retinal mechanotransduction and osmoregulation as well as provide a mechanistic framework for developing new therapeutic strategies aimed at blinding conditions that involve mechanical stress and cellular morphology.