Project Grant R00AR080757

EPITENON-DERIVED PROGENITOR CELLS IN TENDON HEALING AND ADAPTATION - PROJECT SUMMARY THOUGH THOUGHT TO SERVE MANY IMPORTANT FUNCTIONS IN OVERALL TENDON FUNCTION, INCLUDING FACILITATING BOTH HEALING AND ADAPTATION TO MECHANICAL LOAD, THE TRUE IDENTITY AND FUNCTION OF THE EPITENON HAS REMAINED ELUSIVE DUE TO A LACK OF GENETIC MARKERS THAT SPECIFICALLY TARGET EPITENON CELLS. IN PRELIMINARY STUDIES, WE IDENTIFIED A NOVEL POPULATION OF GLAST-LINEAGE (GLASTU') CELLS IN THE EPITENON THAT CONTRIBUTE TO BOTH TENDON HEALING FOLLOWING ACUTE INJURY AND TENDON ADAPTATION IN RESPONSE TO MECHANICAL OVERLOAD BY DIFFERENTIATING INTO SCLERAXIS (SCX)EXPRESSING TENOCYTES. IDENTIFICATION OF A GENETIC MARKER FOR EPITENON CELLS AS WELL AS DEMONSTRATION OF THEIR CAPACITY FOR TENOGENIC DIFFERENTIATION HAS OPENED AN EXCITING NEW AVENUE OF TENDON RESEARCH WITH THE LONG-TERM GOAL OF UNDERSTANDING THE ROLE THAT EPITENON CELLS PLAY IN REGULATING OVERALL TENDON HOMEOSTASIS AND TO IDENTIFY WAYS TO LEVERAGE EPITENON CELL BEHAVIOR TO IMPROVE TENDON HEALTH. DURING THE K99 PHASE OF THIS AWARD, WE ESTABLISHED VOLUNTARY WHEEL RUNNING AS A TOOL FOR MODELING ADAPTIVE TENDON GROWTH IN MICE AND GENERATED NUMEROUS SOPHISTICATED GENETIC MOUSE MODELS TO FACILITATE THE STUDY OF HOW GLASTU" EPITENON PROGENITORS AND SEX-EXPRESSING TENOCYTES COORDINATE TO AFFECT TISSUE-LEVEL ADAPTATION. TO FURTHER EXPLORE THIS RELATIONSHIP DURING R00 PHASE OF THIS AWARD, THE PROPOSED STUDIES BUILD ON OUR PRIOR WORK TO TEST THE CENTRAL HYPOTHESIS THAT GLAS TU" EPITENON CELLS ARE AN INDISPENSABLE SOURCE OF TENOGENIC PROGENITOR CELLS FOR TENDON ADAPTATION AND THAT SIGNALING BETWEEN EPITENON CELLS AND TENOCYTES IS CRITICAL FOR THE TENOGENIC RESPONSE OF GLASTLIC PROGENITORS. COMBINING GENETIC LINEAGE TRACING WITH INTEGRATED SPATIAL/SINGLE-CELL RNA-SEQUENCING, WE WILL CREATE A COMPREHENSIVE SPATIAL AND TEMPORAL ATLAS THAT DEFINES THE PATHWAYS REGULATING THE COORDINATED MECHANORESPONSE OF EPITENON CELLS, TENOCYTES, AND GLASTU" PROGENITOR CELLS (AIM 1A), TO MECHANISTICALLY TEST THE HYPOTHESIS THAT COORDINATION BETWEEN EPITENON CELLS AND TENOCYTES IS REQUIRED FOR THE PROPER TENDON ADAPTIVE RESPONSE TO LOAD, WE WILL INDUCIBLY DEPLETE EITHER EPITENON CELLS OR TENOCYTES PRIOR TO TENDON LOAD IN COMPLEMENTARY EXPERIMENTS (AIM 1 B) AND ASSESS HOW DISRUPTION OF GLASTU, EPITENON CELL/TENOCYTE COMMUNICATION AFFECTS THE OVERALL TENDON ADAPTIVE RESPONSE. COLLECTIVELY, THESE DATA WILL PROVIDE THE FIRST COMPREHENSIVE CHARACTERIZATION OF EPITENON CELLS AND THEIR FUNCTION IN TENDON BIOLOGY.