Intracellular Ca2+ signaling from pulsed focused ultrasound is initiated by mechanical activation of TRPC1 Na+ currents

Authors

• RM Lorsung
• JA Frank
• SR Burks

Abstract

Targeted pulsed focused ultrasound (pFUS) increases stem cell homing to tissues by establishing a trophic microenvironment through calcium-dependent cyclooxygenase-2 (COX2) signaling. In kidneys and skeletal muscle, both voltage-gated L-type Ca2+ channels (LTCC) and Ca2+/Na+-conducting transient receptor potential (TRP) C1 are essential for pFUS-induced COX2 signaling. pFUS is suggested to directly activate both mechanically- and voltage-gated channels, but the mechanisms of intracellular Ca2+ flux initiation remains unknown. Here, immunostaining and co-immunoprecipitation from mouse kidney and muscle revealed complexes containing TRPC1 and LTCC, but not ORAI1 (confers TRPC1 Ca2+ sensitivity). This suggests TRPC1 is not sensitive to LTCC Ca2+ fluxes. Mouse TCMK1 kidney and C2C12 muscle cells were treated in vitro with pFUS at 1 MHz (4 MPa; 10 ms pulses; 5%-duty-cycle). Fluorescent ionophore imaging revealed both intracellular Ca2+ and Na+ transients during pFUS. Verapamil (LTCC inhibitor) blocked Ca2+, but not Na+ transients. Furthermore, partial Na+ replacement with Cs+ reduced Ca2+ transient magnitude. Na+ transients were unaffected by tetrodotoxin (voltage-gated Na+ channel blocker). Suppressing TRPC1 with shRNA decreased both Ca2+ and Na+ transient magnitudes. These data demonstrate without ORAI1, TRPC1 is opened mechanically and conducts primarily Na+ to locally depolarize the membrane, opening nearby LTCC. LTCC is the origin of intracellular Ca2+ necessary for COX2 upregulation, but pFUS at parameters used for stem cell therapies does not directly activate LTCC by altering the electrical state of the cell; LTCC opens secondarily to mechanically-activated TRPC1 currents. Future studies will investigate the source of mechanical forces which activate mechanically-sensitive moieties.

Scientific Focus Area: Biomedical Engineering and Biophysics

This page was last updated on Friday, March 26, 2021