Project Summary / Abstract Our skin provides a barrier that protects us against the environment. Normal skin function is thought to depend on sebaceous glands (SGs), which are paired appendages typically associated with hair follicles. The main purpose of SGs is to secrete sebum, a lipid-rich substance that acts on the skin to regulate barrier function and hydration, while possibly possessing anti-oxidant and anti-microbial properties. Aberrant SGs have been associated with skin pathologies such as acne and scarring alopecia, but their functional roles in disease remain unclear. Although numerous genes and signaling pathways have been implicated in maintaining SGs, few studies have characterized the specific mechanisms by which these factors modulate SG stem cell differentiation, and whether these effects are direct or indirect. Our previous studies have found that Notch signaling simultaneously exerts opposing forces on SGs: Whereas Notch directly promotes SG stem cell differentiation, this pathway also indirectly suppresses SGs from the surface interfollicular epidermis. We also observed that a PPARγ/K5 double-positive population likely identifies the immediate progenitors that give rise to differentiated sebocytes in the gland. This proposal seeks to identify some of the complex factors that regulate SG stem cells, to test the role of SGs in normal skin function, and to determine whether the PPARγ/K5 double-positive population exhibits stem cell properties. In Aim 1, we will examine how Notch directly promotes SG stem cell differentiation, and evaluate novel genes that may modulate SG function. In Aim 2, we will determine how Notch also indirectly suppresses SG differentiation, and test whether upregulation of epigen functionally links Notch disruption in the epidermis with SG expansion. In Aim 3, we will assess whether PPARγ is required for SG maintenance, and whether cells expressing this transcription factor can act as SG stem cells. Finally, we will characterize the localization of PPARγ/K5 double-positive cells in both normal and pathological human skin. Altogether, these studies will elucidate how SG stem cells are regulated, both locally and from a distance, while potentially uncovering novel roles for SG function in the skin.