Role of TRP Channels in Mediating the Calcium Signaling Response of Brain Endothelial Cells to Mechanical Stretch

Traumatic brain injury (TBI) often results in disruption of the blood brain barrier (BBB), which is an integral component to maintaining the central nervous system homeostasis. Recently cytosolic calcium levels ([Ca2+]i), observed to elevate following TBI, have been shown to influence endothelial barrier integrity. However, the mechanism by which TBI-induced calcium signaling alters the endothelial barrier remains unknown. In the present study, an in vitro BBB model was utilized to address this issue. Exposure of cells to biaxial mechanical stretch, in the range expected for TBI, resulted in a rapid cytosolic calcium increase. Modulation of intracellular and extracellular Ca2+ reservoirs indicated that Ca2+ influx is the major contributor for the [Ca2+]i elevation. Application of pharmacological inhibitors was used to identify the calcium-permeable channels involved in the stretch-induced Ca2+ influx. Antagonist of transient receptor potential (TRP) channel subfamilies, TRPC and TRPP, demonstrated a reduction of the stretch-induced Ca2+ influx. RNA silencing directed at individual TRP channel subtypes revealed that TRPC1 and TRPP2 largely mediate the stretch-induced Ca2+ response. In addition, we found that nitric oxide (NO) levels increased as a result of mechanical stretch, and that inhibition of TRPC1 and TRPP2 abolished the elevated NO synthesis. Further, as myosin light chain (MLC) phosphorylation and actin cytoskeleton rearrangement are correlated with endothelial barrier disruption, we investigated the effect mechanical stretch had on the myosin-actin cytoskeleton. We found that phosphorylated MLC was increased significantly by 10 minutes post-stretch, and that inhibition of TRP channel activity or NO synthesis both abolished this effect. In addition, actin stress fibers formation significantly increased 2 minutes post-stretch, and was abolished by treatment with TRP channel inhibitors. These results suggest that, in brain endothelial cells, TRPC1 and TRPP2 are activated by TBI-mechanical stress and initiate actin-myosin contraction, which may lead to disruption of the BBB

Emerging role of the calcium-activated, small conductance, SK3 K ⁺ channel in distal tubule function: Regulation by TRPV4

The Ca2+-activated, maxi-K (BK) K+ channel, with low Ca2+-binding affinity, is expressed in the distal tubule of the nephron and contributes to flow-dependent K+ secretion. In the present study we demonstrate that the Ca2+-activated, SK3 (KCa2.3) K + channel, with high Ca2+-binding affinity, is also expressed in the mouse kidney (RT-PCR, immunoblots). Immunohistochemical evaluations using tubule specific markers demonstrate significant expression of SK3 in the distal tubule and the entire collecting duct system, including the connecting tubule (CNT) and cortical collecting duct (CCD). In CNT and CCD, main sites for K+ secretion, the highest levels of expression were along the apical (luminal) cell membranes, including for both principal cells (PCs) and intercalated cells (ICs), posturing the channel for Ca2+- dependent K+ secretion. Fluorescent assessment of cell membrane potential in native, split-opened CCD, demonstrated that selective activation of the Ca2+-permeable TRPV4 channel, thereby inducing Ca2+ influx and elevating intracellular Ca2+ levels, activated both the SK3 channel and the BK channel leading to hyperpolarization of the cell membrane. The hyperpolarization response was decreased to a similar extent by either inhibition of SK3 channel with the selective SK antagonist, apamin, or by inhibition of the BK channel with the selective antagonist, iberiotoxin (IbTX). Addition of both inhibitors produced a further depolarization, indicating cooperative effects of the two channels on Vm. It is concluded that SK3 is functionally expressed in the distal nephron and collecting ducts where induction of TRPV4-mediated Ca2+ influx, leading to elevated intracellular Ca2+ levels, activates this high Ca2+- affinity K+ channel. Further, with sites of expression localized to the apical cell membrane, especially in the CNT and CCD, SK3 is poised to be a key pathway for Ca2+-dependent regulation of membrane potential and K+ secretion. © 2014 Berrout et al

Immunohistochemical staining for SK3 and aquaporin-2 (AQP2) in WT mouse kidney sections.

<p>Top Panel (A–C): A low-magnification transverse section (5 µm) of the mouse kidney is shown. Discrete labeling is shown for staining for aquaporin-2 (<b>A.</b> AQP2, red), a marker of the collecting ducts, SK3 (<b>B.</b> SK3, green), and a merger of both channels (<b>C.</b> Merge, yellow-organge for co-localization of AQP2 and SK3). Labeling is apparent for SK3 in both the cortex (label C) and medullary (label M) (dashed line shows cortical-medullary demarcation). <b>Middle Pannel (D–F):</b> Magnified view of the yellow inset box from A. SK3 co-localizes with all AQP2-postive tubules as show by the yellow-orange images (F., asterisk). SK3 staining is also apparent in AQP2-negative structures including other tubular structures (F., arrows) and smaller secondary structures (possibly vascular structures, F., arrow heads). <b>Bottom Panel (G–H):</b> Magnified view of staining in the presence of SK3 blocking peptide. All SK3 staining is abolished demonstrating specificity of our anti-SK3 antibody. Scale bar is 50 µm.</p

Antibodies and markers used for immunohistochemistry.

<p>Antibodies and markers used for immunohistochemistry.</p

Immunohistochemical staining of SK3 in the collecting duct.

<p>Section (5 µm) from WT mouse kidney showing staining for AQP2 (red), a marker of PCs in collecting duct, and SK3 (green). <b>Panels A, C, and E</b> are low magnification views of a cross-section through a CCD identified by AQP2 staining. <b>Panels B, D, and F</b> represent a magnified view of the inset area from <b>A</b> (yellow inset box). <b>Panel B</b> shows strong AQP2 staining along the luminal border of PCs (5–6 cells), but not of the ICs (2 cells without staining). As shown in <b>D</b> and <b>F</b>, strong staining of SK3 is evident along the luminal border of all cells, both PCs and ICs. Variable, but weak staining, is also apparent along the abluminal border of some cells. However, the staining is most pronounced along the luminal border for both PCs and ICs, although typically stronger in PCs, as indicated by the SK3 fluorescence line intensity profiles across (luminal to abluminal direction) two cells identified as PC and IC (<b>Panel G</b>). <b>H</b>. Relative mean intensity profiles (± SEM) across the cells from all sections showing the maximal values across the luminal border (Apical) and abluminal border (Basal) and the minimal values within the cytoplasm (Cytosol). The mean values are given for both PCs (n = 37) and ICs (n = 12) from all sections analyzed. The maximal luminal intensity is much greater than the abluminal intensity (*P<0.02) indicating dominant expression at the luminal border. Scale bar is 10 µm.</p

Effect of TRPV4-mediated activation of SK3 channels on membrane potential, Vm.

<p><b>A.</b> Fluorescence image of a split-open CCD loaded with the voltage-sensitive fluorescence dye, DiSBAC₂(3), showing loading of all cells. The fluorescence intensity is an index of Vm and is presented as relative fluorescence units (RFU). <b>B.</b> Effect of 50 mM K⁺ (High K⁺) application on Vm of CCD cells showing the expected membrane depolarization (increased RFU). <b>C.</b> Effect of 300 nM apamin or 50 nM IbTX application on Vm in basal conditions showing little or no effect of either apamin (Apa) or IbTX in the basal state (TRPV4 not activated). <b>D.</b> Effect of TRPV4 activation with GSK101 (50 nM) leading to membrane hyperpolarization of Vm (decreased RFU), as expected for SK3 and BK activation. Subsequent application of either 300 nM apamin or 50 nM IbTX now induce a marked depolarization of Vm (increased RFU) demonstrating inhibition of SK3 and BK, respectively. <b>E.</b> Summary graph showing mean changes in Vm in basal conditions upon addition of High K⁺ (High K⁺, n = 44 cells), 300 nM apamin, or 50 nM IbTX (Left panel, Basal). Right panel (GSK101: TRPV4 Activation) shows the results after activation of TRPV4 (Ca²⁺ influx). Both apamin and IbTX now bring about a significant depolarization of Vm (*P<0.01 compared to Basal). The combine addition of both apamin and IbTX (Apa + IbTX) displays an enhanced depolarization compared to addition of apamin or IbTX alone (**P<0.01). The number in parentheses is the number of cells for each group (n).</p

Immunohistochemical staining of SK3 in the distal convoluted tubule (DCT).

<p>Mouse (WT) kidney section (5 µm) showing staining for the sodium-calcium exchanger (NCX, red), a marker of DCT, especially the later portion (DCT2), and SK3 (green). The heavy NCX staining of the upper portion of the tubule in <b>Panel A</b> (within yellow inset box) is consistent with the DCT2 segment with the weaker, more basolateral staining in the lower half of the tubule indicating this is the connecting tubule (CNT) (see text for details). Higher resolution image of the DCT2 (<b>D</b>) shows strong staining of SK3 along the luminal border with more variable, weaker staining along the abluminal border. The merged image clearly identifies SK3 staining of the DCT (<b>F</b>). In the CNT segment (<b>A.</b>, labeled CNT), SK3 staining was also apparent along the luminal border with abluminal staining appearing weaker. Scale bar is 10 µm.</p

SK3 expression in WT mouse kidney.

<p><b>A.</b> RT-PCR analysis using whole kidney mRNA extracts revealed prominent bands of the appropriate size on agarose gels for both SK3 (473 bp) and BKα (318 bp), demonstrating expression of both of these channels in the kidney. SK3 primers were selected to cross the exon 2 and exon 3 borders to rule out amplification of intron sequences from genomic DNA. The electropherogram for SK3 is shown with both nucleotide sequences (NT) and amino acid sequences (AA) indicated for the segment across the exon border region, demonstrating that the PCR product does not originate from genomic DNA. 100-bp marker standards are shown (Lane M). <b>B.</b> Western blot of WT mouse kidney-SK3. SK3 protein is expressed as a single band near 90 kD in mouse kidney. SK3 blocking peptide (SK3-BP) was used as a control to verify antibody specificity which, as shown, abolished binding of the anti-SK3 antibody (right lane). Alpha-tubulin expression was used as a loading control (lower panel).</p