OSMOTICALLY INDUCED TRANSLOCATION OF ∆N-TRPV1 CHANNELS IN SUPRAOPTIC NEURONS

Abstract

The physiological mechanisms involved in regulating extracellular osmolality are critical to understand how mammals cope with dehydration and water deprivation. Osmoregulation is a crucial homeostatic process in mammals that functions to maintain a physiological setpoint of extracellular osmolality. Magnocellular neurosecretory cells (MNCs) of the hypothalamus sense changes in external osmolality and transduce changes in cell volume into depolarizing currents and action potential (AP) firing, leading to the release of the hormone vasopressin (VP), which prevents water loss from the kidneys. MNCs lack the normal volume regulatory mechanisms present in other cells, and possess mechanosensitive channels called ∆N-TRPV1 that enable cation influx and MNC plasma membrane depolarization upon activation by cell shrinkage that involves a Ca2+-dependent isoform of the enzyme phospholipase C (PLC) called PLCδ1. Sustained exposure (i.e., longer than 1 hour) to high osmolality causes structural and functional adaptations in MNCs, like somatic hypertrophy, channel translocation, and changes in gene expression. We propose a potential mechanism for MNC hypertrophy and for osmotically induced ∆N-TRPV1 translocation in MNCs that may provide insight into the underlying mechanisms of long-term MNC osmoregulation. Isolated MNCs treated with hyperosmotic saline for 1 hour elicited somatic hypertrophy as well as a significant increase in the number of ∆N-TRPV1 channels present on the plasma membrane. Both the increase in plasma membrane ∆N-TRPV1 and MNC hypertrophy were shown to be reversible once isolated MNCs were returned to isosmotic solution. The increase in plasma membrane ∆N-TRPV1 and hypertrophy were observed to be PLC- and protein kinase C (PKC)-dependent, and both required SNARE-mediated exocytosis and movement of vesicles from the Golgi, as inhibitors of PLC, PKC, the SNARE complex, and the Golgi all prevented the osmotically induced increase in plasma membrane ∆N-TRPV1 as well as hypertrophy. It was also observed that the retrieval of ∆N-TRPV1 from the plasma membrane as well as the recovery from hypertrophy require dynamin-mediated endocytosis, as using an inhibitor of dynamin prevented the retrieval of ∆N-TRPV1 from the plasma membrane and the recovery from hypertrophy. Finally, we report that mice that lack PLCδ1 fail to elicit an increase in plasma membrane ∆N-TRPV1 or hypertrophy in response to high external osmolality, suggesting that this enzyme is necessary for these processes. This project will help to elucidate the potential mechanisms underlying osmosensory transduction and osmoregulation in MNCs.