Role of lacrimal gland myoepithelial cells in dry eye disease

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Chronic inflammation of the lacrimal gland (LG), as occurs in Sjögren’s syndrome, is the leading cause of aqueous-­ deficient dry eye disease (DED). The mechanisms leading to insufficient LG secretion are still not completely understood. The LG is composed of acinar, myoepithelial, and ductal cells, with acini and myoepithelial cells (MEC) forming the secretory units. MECs express several muscle proteins, such as alpha smooth muscle actin (SMA) and calponin, and are therefore able to contract. MECs are best studied in the mammary gland where their contraction is shown to be crucial for milk production and contraction is mainly controlled by the neuropeptide oxytocin. Despite their potential critical role in LG secretion, very little is known about MEC contraction in this tissue, nor is the impact of chronic inflammation of the LG on these cells. One of goals of the current proposal is to fill this gap in knowledge. Our preliminary studies show that murine and human LG MECs express the oxytocin receptor (OXTR) and contract in response to oxytocin stimulation. Furthermore, we show that MECs in chronically inflamed LG are atrophied, with down-­regulation of contractile proteins SMA and calponin and the OXTR and MECs from these glands do not contract in response to oxytocin stimulation. We previously reported that interleukin-1 (IL-1) inhibits neurotransmitter release from LG efferent nerves leading to DED. We also reported activation of the stress activated c-Jun N-terminal kinase (JNK) and metalloproteinases 2 (MMP2) in chronically inflamed LGs and that inhibition of either pathway restored LG secretion and tears output in animal models of DED. Numerous studies showed that denervation of muscle tissues leads to tissue atrophy and degradation of muscle contractile proteins via the ubiquitin/proteasome pathways. Based on these findings, we hypothesize that in chronic DED, proinflammatory cytokines inhibit neurotransmitter release from LG efferent nerves creating a denervated-like tissue and that they trigger degradation of the OXTR and MEC myofilament proteins. This degradation translates into a loss of contractibility of MECs thus further exacerbating the effect of the loss of neural input on the secretory units of the LG. We further hypothesize that the JNK, MMP2, and ubiquitin/proteasome pathways mediate the effects of proinflammatory cytokines on MEC functions. We will use both in vitro (sorted MEC cells) as well as animal models of DED to investigate how proinflammatory cytokines interfere with oxytocin-induced contraction of LG MECs. We will use unbiased RNA-seq and quantitative global proteomics techniques to identify novel pathways that are altered in MECs in chronically inflamed LGs. At the completion of these studies, we will have established a role for the MEC, an important and yet understudied cell in the LG, and the oxytocin signaling system in the pathogenesis of aqueous-deficient DED and identified potential novel therapeutic targets.