ProjectSummary Thelongtermobjectiveofthisproposalistounderstandhowtheactivityofstemcellsisproperlyregulatedto maintainhomeostasisandtissueintegrity.Thehairfollicle,oneoftheimportantskinappendages,isanideal paradigmtoaddressthisproblem.Hairfolliclesundergocyclesofgrowth(anagen),destruction(catagen)and rest(telogen)phases.Hairfolliclestemcells(HFSCs)arelocatedinapermanentprotrusionofthehairfollicle, a structure known as the bulge. HFSCs in the bulge cycle infrequently. During normal homeostasis, HFSCs only proliferate in a very transient window of anagen, while remaining quiescent during all the other phases. HFSCs can also become activated upon wounding. Disregulation of HFSC activity results in severe consequences. For example, alopecia (hair loss) and delayed wound healing may arise from inefficient activationofHFSCs.Onthecontrary,skintumors,suchasbasalcellcarcinomaandsquamouscellcarcinoma, can derive from HFSC hyperproliferation. Stem cell activity is heavily influenced by their microenvironment, knownastheniche.Traditionally,studiesaboutnichefocusonlyonthesurroundingheterologouscelltypes, i.e.,cellsoriginatedfromadifferentlineage.Recentstudiesincludingmyowndiscovertheimportanceofstem cellprogenyasnichecomponentsinseveralvertebrateandinvertebratestemcellsystems,whichispreviously unrecognized.Inthehairfollicle,mypreliminarystudieshaveidentifiedtwoimportantprogenypopulationsas criticalregulatorsforHFSCproliferation.Thecentralhypothesistobetestedbythisproposalisthatfeedback regulationfromHFSCprogenyiscrucialfortheproperbehaviorandactivityofHFSCs.Thishypothesiswillbe testedinthisgrantapplicationbyexperimentsthat:1)examinecandidatesignalsexpressedbytheprogeny2) determinethecontributionsoftheprogenytoHFSCactivationunderphysiologicalandpathologicalconditions and3)identifynovelfunctionalfactorsexpressedbytheprogenytoregulateHFSCs.Candidatesignalswillbe investigated during the mentored phase. The contributions of progeny under dynamic conditions as well as identifiednovelfactorsexpressedbytheprogenywillbefollowupduringtheindependentphase.Successful completion of the proposed experiments will significantly advance our understanding of the cell types and signals that regulate HFSC proliferation and quiescence. In addition, these proposed studies will potentially leadtothedevelopmentoftherapeutictreatmentsforskindisordersassociatewithaberrantstemcellactivity. Mylongtermcareergoalistoleadasuccessful,independent,andwellfundedlaboratorystudyingskinand stem cell biology. My graduate and postdoctoral training up to date has prepared me technically and intellectuallytodeveloprigorousresearchprojects.Thiscareerdevelopmentawardandmyproposedresearch plan will further provide me with opportunities to expand my knowledge in skin stem cell biology and mouse genetics,gainnewskillsinbioinformaticsanalysis,mouseembryomanipulation,imagebasedFACSanalysis, and further accumulate experience to improve mentoring, presentation, and writing skills, all of which are critical to my future success as an independent researcher. The reagents generated during the mentored phase will also help to build up my research program in the independent phase. The Rockefeller University together with its two neighboring institutions, Memorial SloanKettering Cancer Center and Weill Cornell Medical College, offer a prime research environment and many workshops and courses to support my proposed research and my career development. I will have constant interactions with my mentor Dr. Elaine Fuchs,mycollaboratorDr.OlivierElemento,andtheskinandmousedevelopmentalbiologycommunitiesin the New York area. Together they will assess my progress and provide critique or advice. In summary, the proposed studies and career development plan will better prepare me for my independent scientific career, ensurethatIachievemylongtermcareergoals,andallowmetomakecontinuouscontributionstowardsour understandingofhowstemcellactivityisregulatedinhomeostasisanddisease.