Investigating the role for Utrophin in age-related decline of the Merkel lineage

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Our sense of touch enables numerous behaviors fundamental to human existence, allowing us to eat, communicate and survive. Deficits in tactile responsiveness are thought to contribute to the decline of postural stability and hand grip, and the resulting increase in falling frequency, which is a major factor determining quality of life and the ability to live independently for the elderly. In mammals, different tactile qualities (curvature, texture and vibration) are encoded by touch receptors residing in the skin with distinct physiological properties and morphological end-organs; however, the cellular and molecular mechanisms underlying this diversity are largely unknown. Furthermore, the regulation of skin stem cells that are responsible for maintaining the turnover of cellular mechanoreceptors that perceive gentle touch, such as Merkel cells, is unknown. Our laboratory previously characterized a population of epithelial stem cells that reside in epidermal touch domes in the skin, termed touch dome stem cells (TDSCs), that are responsible for maintaining the Merkel lineage during homeostasis. The long-term goal of this proposal is to define the cellular and molecular basis for TDSC maintenance of the Merkel lineage. Our preliminary data chronicles a dramatic age-related decline in Merkel cell numbers in human (40 – 90 years of age) and murine (2 – 32 months of age) skin. However, TDSC numbers remain unchanged. We confirmed that age-related loss of Merkel cells is not due to precocious exit of mature Merkel cells or defects in sensory afferent innervation but is due to diminished TDSC progenitor capacity to replenish the Merkel lineage. Leveraging this age-related deficit in TDSC progenitor capacity, we performed proteomics and mRNA profiling of TDSCs from young versus aged mouse skin and identified the focal adhesion-associated protein Utrophin (Utrn) to be dramatically downregulated in aged TDSCs. Utrn null (Utrn-/-) mice at postnatal day 20 displayed equal numbers of Merkel cells compared to Wt mice, excluding a role for Utrn in Merkel cell development. However, Merkel cell numbers were reduced by 50% in 2-month-old Utrn-/- mice providing genetic evidence for a functional role for Utrn in age-related maintenance of the Merkel lineage by TDSCs. We identified the Nrg1-Erbb2 signaling axis as a regulator of Utrn expression in mouse and human epithelial keratinocytes in vitro and TDSCs in vivo. These preliminary findings support our central hypothesis: Nrg1-Erbb2 regulation of Utrn expression is required for TDSC maintenance of Merkel cell homeostasis. To test our hypothesis, we will employ mouse models to genetically disrupt or rescue Utrn expression and assess its impact on Merkel cell homeostasis. Using in vivo lineage tracing tools and a Nrg1 conditional allele we will investigate the cellular mechanisms underlying TDSC maintenance of the Merkel lineage and upstream regulation of Nrg1 expression in the TD niche. Finally, we will interrogate the molecular mechanisms for Utrn regulation and downstream effectors of Utrn function in TDSCs. Our goal is to uncover first of its kind genetic mechanisms for stem cell regulation of the Merkel lineage.