Non-muscle myosin II (NM II), an actin-binding protein that has actin cross-linking and contractile properties, was found to be one of the major cytoskeletal proteins in TM, CM, and HUVECs. NM II activity is regulated by the phosphorylation of its light and heavy chains.
37 NM II and actin together represent the major contractile proteins of smooth muscle cells that generate force and play a fundamental role in cell adhesion, migration, cell architecture, and mechanobiology.
37,38 Myosin II–regulated actin stress fibers provide the force for a wide array of motile and morphogenetic processes in eukaryote cells.
38,39 In TM cells and in the outflow pathway, direct inhibitors of myosin II activity, including blebbistatin and 2,3-butanedione 2-monoxime, have been shown to induce cell shape changes, decreased actin stress fibers, focal adhesions, and morphologic changes, concomitant with increased aqueous humor outflow facility.
15,40 NM II activity is regulated predominantly by myosin light chain kinase and Rho kinase-mediated phosphorylation,
37 and both Rho kinase and myosin light chain kinase inhibitor-mediated suppression of phosphorylation of the regulatory subunit of myosin II also increase aqueous humor outflow facility in association with changes in cell shape, actin stress fibers, and morphologic changes.
19,41 These observations suggest that NM II activity plays a significant role in the regulation of aqueous humor outflow facility and homeostasis of IOP. Additionally, as we have suggested earlier,
40 NM II may serve as an important molecular target for the novel treatment of glaucoma. Moreover, because NM II activity is controlled by various extracellular stimuli and intracellular mechanisms,
31 the regulation of NM II activity in the outflow pathway may play a vital role in both homeostasis of aqueous outflow and pathophysiology of glaucoma. Caldesmon, a known negative regulator of myosin II ATPase activity and smooth muscle contraction, was identified as one of the constituents of TM cell cytoskeletal fraction. Overexpression of caldesmon in aqueous humor outflow pathway has been demonstrated to increase outflow facility,
42 further strengthening the importance of myosin II and its contractile activity in the regulation of outflow facility.