P.136DNAJB6 integrates sarcomere protein homeostasis and myogenic signaling in coordinated skeletal muscle stress response

Abstract

Mutations in DNAJB6, encoding a HSP40 co-chaperone, cause LGMD1D, an adult onset progressive myopathy with vacuolar and aggregate myopathology. DNAJB6 has two isoforms both of which contain LGMD1D mutation residues, DNAJB6a which localizes to the nucleus and DNAJB6b which diffusely localizes throughout the cell. The function of each isoform in normal muscle, and how disease associated mutations cause myopathy, is unknown. DNAJB6a is a tumor suppressor and chaperones a multiprotein complex that maintains GSK3β in a dephosphorylated, active, state. DNAJB6b localizes to the Z disc in muscle and may mediate LGMD1D pathogenesis. Prior studies indicate DNAJB6 is important for sarcomeric protein homeostasis. We recently found DNAJB6 not only facilitates protein quality control, but also negatively regulates myogenic signaling through its client, GSK3β. In addition, DNAJB6b mutations inhibit myogenic signaling through augmented GSK3β activity, and GSK3β inhibition improves pathology and strength in LGMD1D mice. The mechanism connecting DNAJB6′s role at the sarcomere with myogenic signaling is unclear. Using an in-vivo imaging strategy in skeletal muscle of live mice, we demonstrate DNAJB6b localizes to the Z disk at baseline, but after myofibrillar injury, it accumulates on the M band and within nuclei. GSK3β, DNAJB6′s myogenic signaling client, also localizes diffusely at baseline, but is excluded from the nucleus following myofibrillar injury, where it is known to suppress hypertrophic signaling through NFATc3. These findings suggest DNAJB6 participates in an integrated stress response in skeletal muscle, coordinating sarcomere repair with modulation of myogenic signaling pathways. In contrast, mutant DNAJB6b mislocalizes to myonuclei and the M band at baseline, and in LGMD1D mice, GSK3β persists within myonuclei after injury. These changes in GSK3β and LGMD1D mutant DNAJB6 localization suggest an uncoupling of sarcomere repair and myogenic signaling in LGMD1D.