*6.2. Stretch-Activated Channel Inhibition*

In addition to micro-tears in the sarcolemma, increased expression and activity of stretch-activated non-selective ion channels (SACnsc) have been implicated in calcium influx and overload in muscular dystrophy. In skeletal muscle, the transient receptor potential channel (TRPC) family was identified as a therapeutic target for decreasing extracellular calcium entry in DMD. Inhibition of TRPC1 and TRPC4 expression with antisense oligonucleotides decreased calcium entry measured by patch-clamping in myofibers from *mdx* mice [168]. Further, transgenic expression of a dominant negative form of TRPC3 in *mdx* and *Scgd*-/- mice decreased the dystrophic phenotype in skeletal muscle, including decreased fibrosis, serum CK, and occurrence of central nuclei [16]. In the heart, increased expression of TRPC1 has been implicated in the increased slow force response in *mdx* mice, caused by slow calcium influx leading to increased force production over several minutes of myocyte stretch [160]. Blockage of this channel with stretch-activated channel blocker GsMTx-4 decreased resting calcium concentration [160]. Further research is needed to explore whether decreased SAC activity or expression in the heart can delay or inhibit the dilated phenotype of DMD cardiomyopathy.

#### *6.3. RyR2 Stabilization*

RyR2 hypersensitivity has been implicated in calcium overload within the dystrophic cardiac myocyte [71]. Calstabin2 is a subunit of RyR2, which stabilizes the closed state of this channel [169]. Phosphorylation [169] or oxidative stress leading to nitrosylation [161] inhibits the association of calstabin2 with RyR2, increasing SR calcium leak. Treatment with N-acetyl cysteine to prevent nitrosylation or RyR2 stabilizer Rycal S107 prevented depletion of calstabin2 and decreased production of calcium sparks and depolarization in *mdx* cardiac myocytes [161]. Treatment of *mdx* mice with Rycal S107 also significantly reduced arrhythmias [161]. Similar studies in skeletal muscle found RyR1 stabilization to improve muscle strength, exercise tolerance, and muscle histopathology [58,170]. Whether long-term treatment to stabilize RyR2 in the heart can lead to improved cardiac outcomes in DMD requires further investigation.
