Mechanisms underlying distinct subcellular localization and regulation of epithelial long myosin light-chain kinase splice variants

Intestinal epithelia express two long myosin light chain kinase (MLCK) splice variants, MLCK1 and MLCK2 that differ by the absence of a complete immunoglobulin-like (Ig) domain 3 within MLCK2. Only MLCK1 is associated with the perijunctional actomyosin ring at steady state, and this localization is enhanced by inflammatory stimuli including tumor necrosis factor (TNF). Here we sought to identify MLCK1 domains that direct perijunctional MLCK1 localization and their relevance to disease. Ileal biopsies from Crohn's disease patients demonstrated preferential increases in MLCK1 expression and perijunctional localization relative to healthy controls. In contrast to MLCK1, MLCK2 expressed in intestinal epithelia is predominantly associated with basal stress fibers, and the two isoforms have distinct effects on epithelial migration and barrier regulation. MLCK1(Ig1-4) and MLCK(Ig1-3), but not MLCK2(Ig1-4) or MLCK1(Ig3), directly bind to F-actin in vitro and direct perijunctional recruitment in intestinal epithelial cells. Further study showed that Ig1 is unnecessary, but that, like Ig3, the unstructured linker between Ig1 and Ig2 (Ig1/2us) is essential for recruitment. Despite being unable to bind F-actin or direct recruitment independently, Ig3 does have dominant negative functions that allow it to displace perijunctional MLCK1, increase steady-state barrier function, prevent TNF-induced MLCK1 recruitment, and attenuate TNF-induced barrier loss. These data define the minimal domain required for MLCK1 localization and provide mechanistic insight into the MLCK1 recruitment process. Overall, the results create a foundation for development of molecularly targeted therapies that target key MLCK1 domains to prevent recruitment, restore barrier function, and limit inflammatory bowel disease progression.