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Zhen Meng, Maria Edman-Woolcott, Chuanqing Ding, Sarah F Hamm-Alvarez; Dysfunction of Rab3D Is Associated with Increased Secretion of Tear Cathepsin S, a Tear Biomarker of Sjögren's Syndrome. Invest. Ophthalmol. Vis. Sci. 2014;55(13):6303.
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Rab3 and Rab27 are two subfamilies of Rab proteins identified in many cell types, including lacrimal gland acinar cells (LGACs), which facilitate secretory vesicle (SV) exocytosis. LGACs produce and release tear proteins to the ocular surface. We explored the role that changes in Rab3D and/or Rab27 may play in the altered tear cathepsin S (CtsS) levels observed in non-obese diabetic (NOD) mouse, a Sjögren’s syndrome (SS) disease model. Increased CtsS abundance and activity in NOD LGACs and tears were reported in our earlier study. Our clinical studies support CtsS as a tear biomarker of SS.
Mice deficient in Rab27a (ashen), Rab27b (27bKO), Rab27a and b (DKO), Rab3D (3DKO) as well as parent C57BL/6 (C57) were used, in addition to NOD, and BALB/c mice. qRT-PCR was performed with mRNA extracted from whole mouse lacrimal gland (LG) or LGACs obtained with laser capture microdissection. Tear fluid was collected from mice by stimulating LGs with carbachol in situ, and the activities of several tear proteins were analyzed including CtsS and β-hexosaminidase (β-hex).
Compared with C57, Rab27-deficient mice (ashen, 27bKO and DKO) had significantly lower CtsS and higher β-hex activities in tears. However, 3DKO mice had significantly higher CtsS but equivalent β-hex activities in tears. In NOD mice LGACs, Rab3D but not Rab27 gene levels were significantly lower than that in BALB/c. In NOD mouse, CtsS had both a higher gene expression level in LGACs and increased activity in mouse tears, whereas β-hex remained unchanged.
Some secretion of CtsS and β-hex occurs though both Rab27- and Rab3D-enriched SVs. However, our data suggest that CtsS is secreted mainly through Rab27-enriched SVs, while β-hex is secreted mainly through Rab3D-enriched SVs. When exocytosis of one set of SV is impaired, proteins may be secreted from the other subset through compensatory mechanisms. In our mouse model, decreased expression of Rab3D but not Rab27 in LGACs causes the expression of CtsS, a SS tear biomarker, to increase. Studies from other groups have shown that Rab3D protein levels and protein distribution are altered in acinar cells from lacrimal and salivary glands from patients with SS. These findings collectively suggest that Rab3D dysregulation is implicated in increased secretion of the CtsS biomarker in disease and that both represent potential therapeutic targets.
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