Role of the S100 Family of Proteins in Lens Physiology and Cataract

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PROJECT SUMMARY The ocular lens plays a crucial role in focusing incident light on to the retina, with aberrations in lens transparency and mechanical properties leading to impairment of vision. Despite continuing effort, our understanding of the various molecular pathways governing lens cytoarchitecture, homeostasis, deformability, clarity, and the etiological mechanisms of cataract remains incomplete. We recently identified that several members of the S100 family of calcium binding proteins are abundantly expressed and exhibit distinct distribution profiles in the lens. While expression of S100A6 and S100A10 distributed to both the lens epithelium and fibers, S100A4 distributes discretely to lens fiber cells. Absence of S100A4 but not S100A6 or S100A10 leads to opalescent late-onset cataract formation in a mouse model, together with the robust upregulation of S100A5, the expression of which was otherwise undetectable in the wild type lens. Other than these new data on S100A4, we currently have no knowledge of the role played by the S100 family of proteins in lens function. S100A4 is expressed in a cell and tissue specific manner and is involved in the pathophysiology of various diseases. The physiological function of S100A4 however, is poorly understood. A well-understood molecular function of S100A4 is its interaction with non-muscle myosin II (NM II), especially NM IIA, and regulation of NM II assembly, actin polymerization, contractile characteristics, cell migration, and plasticity. NM II plays a crucial role in lens cytoarchitecture, mechanics, and function. Though NM IIA mutations are associated with cataract development in humans, little is known about regulation of NM II activity in the context of lens function. While lenses derived from mature S100A4 null mouse lenses (2 to 6 month old) remain clear and maintain normal growth and size, those from six month-old mice exhibit impairments in assembly and phosphorylation of NM IIA, polymerization of actin, and alterations in the levels of aquaporin-5, CLIC5, connexin-50, cell adhesive proteins (integrin-β1, NrCAM, ZO-1) and α-crystallin, culminating in opalescent cataract formation in eight month-old mice. Based on these preliminary and novel findings, we hypothesize that S100A4 plays a crucial role in regulating NM II assembly in the lens, and that the absence of S100A4 disrupts NM II assembly and phosphorylation, actin polymerization, mechanical properties, and osmotic homeostasis in the lens, leading to late onset cataract formation. To test this hypothesis, in three interrelated specific aims, we will investigate: 1) the mechanistic role of S100A4 in regulation of lens NM II assembly and phosphorylation, actin polymerization, cell adhesion, and lens tensile properties, 2) the disruption of osmoregulation and solute carrier transport mechanisms underlying cataractogenesis in S100A4 null lenses, and 3) the effects of rescuing expression of S100A4 in S100A4 null mouse lens fibers (using a transgenic approach) on cytoskeletal integrity and lens phenotype. These novel and interrelated studies are expected to uncover the significance of S100A4 and other members of the S100 family proteins in lens function, which are known to participate in the pathophysiology of several diseases.