TRPV4 and cAMP mediated ion transport in the porcine choroid plexus

Shehab Ahmed, Purdue University

Abstract

Hydrocephalus is a medical condition characterized by a buildup of cerebrospinal fluid which causes hydrostatic pressure to increase resulting neuronal destruction and can ultimately cause death. Hydrocephalus is seen in both the pediatric population and adults. Treatment of hydrocephalus usually involves surgical placement of a relocation system to drain the fluid into the abdominal cavity. Hydrocephalus may be caused by mechanical obstruction of the outflow of CSF from the ventricles or by faulty reabsorption. It can be also caused by CSF overproduction by the choroid plexus found in the lateral, third, and fourth ventricles of the brain. The choroid plexus is composed of a high resistance monolayer epithelium which surrounds a network of capillaries. Its primary function is to regulate transport of ions and water that control the production and movement of CSF. Therefore it is important to understand the mechanism of CSF production by the choroid plexus. Recently, a stable porcine choroid plexus (PCP-R) epithelial cell line with a high transepithelial resistance (TER) was developed that provides an important model to study regulation of CSF production. Ussing style electrophysiology was used to measure short circuit current (SCC) to characterize stimulated transepithelial ion transport in confluent PCP-R cells. GSK1016790, a TRPV4 agonist, was used to understand the role of TRPV4 in CSF production by the choroid plexus using PCP-R cell model. TRPV4 activation produces a sustained ion transport response that is consistent with an increase in cation secretion and/or anion absorption which is accompanied by a reversible decrease in TER. The effect of the agonist on both SCC and TER was blocked by HC067047, a TRPV4 antagonist, showing that the sustained ion transport and TER change is TRPV4 specific. TRPV4 mediated ion flux was inhibited by CFTR inhibitor II GlyH-101, a cell permeable inhibitor of the cAMP activated chloride channel CFTR, when added on either side of the membrane and was not accompanied by a TER reversal which showed that CFTR is activated by TRPV4 mediated ion flux. TMEM16A, a calcium activated chloride channel, was speculated to be located in that basal membrane as T16Ainh-AO1, a membrane permeable TMEM16A inhibitor, reversed the TRPV4 mediated ion flux when added on either side of the membrane. Slight reversal in TER was observed when T16Ainh-AO1 was added on the apical side. Apamin, a differential inhibitor of calcium activated small conductance potassium channel 1, 2 and 3 (SK1, SK2 and SK3) had no effect on the TRPV4 mediated ion flux. Whereas, fluoxetine, a membrane permeable inhibitor of SK1, SK2 and SK3 channel, inhibited the TRPV4 mediated ion flux and TER change. Bumetanide, an inhibitor of the sodium-potassium-chloride cotransporter reversed TRPV4 mediated ion flux when added on the apical membrane but not on the basal membrane indicating a possible K+ secretion via SK1 and/or SK4/IK channels and Cl- absorption through CFTR and TMEM16A channels. Acetazolamide, a carbonic anhydrase inhibitor and a compound used to treat hydrocephalus had no effect on the TRPV4 mediated ion flux. cAMP is an intracellular mediator involved in neuromodulator effects, inflammatory responses and other regulatory mechanisms and is constitutively activated by forskolin. In PCP-R cells, forskolin stimulated an increase in transepithelial ion flux that is consistent with an increase in cation absorption and/or anion secretion. Forskolin mediated ion transport was inhibited by CFTR inhibitor II GlyH-101 when added on either side of the membrane. No change in TER was observed. No effect on forskolin mediated ion flux was observed when T16Ainh-A01, apamin or fluoxetine were added. Forskolin stimulated transport is partially inhibited by 1 mM BaCl2. Barium chloride is a general inhibitor of K+ channels. No change in TER was observed.

Degree

M.S.

Advisors

Blazer-Yost, Purdue University.

Subject Area

Biology|Neurosciences

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