The initial segment of the epididymis functions to maintain a specialized luminal fluid environment that is crucial for sperm maturation and therefore male fertility. Key regulators of initial segment function are testicular luminal fluid factors and without these factors the initial segment undergoes apoptosis and fails to function. Therefore, this proposal will address the mechanisms by which testicular luminal fluid factors regulate initial segment function. Findings from our published and preliminary studies allowed us to formulate the "lumicrine" hypothesis: Growth factors, of Sertoli cell and/or germ cell origin, enter the lumen of the seminiferous tubule, pass out of the testis via the rete testis and efferent ducts, enter the epididymal duct and interact with their cognate receptors located on the apical surface of initial segment cells. Here, second messenger pathways are activated, which result in activation of transcription factors and transactivation of genes. These genes are important for (1) protection of the initial segment from apoptosis and (2) protection of maturing spermatozoa from oxidative stress and providing a specialized luminal fluid microenvironment for their maturation. The proposal will focus specifically on examining the role of testicular luminal fluid growth factors, such as members of the fibroblast growth factor (FGF) family and neurotrophin family, in regulating genes and signal transduction pathways that are involved in initial segment function, and therefore male fertility. Specifically, it is proposed: (1) to test the hypothesis that luminal FGFs and neurotrophins maintain the expression of pro-survival pathways and protective genes in the initial segment. (2) to test the hypothesis that active FGF receptor FGFR1 Illc and the neurotrophin receptor complex TrkC/p75, which are specific to the principal cells of the initial segment, maintain the expression of pro-survival pathways and protective genes. (3) to test the hypothesis that activation of FGFR1 Illc and TrkC/p75 receptors is modulated by the adaptor protein FGF receptor substrate 2 (FRS2/SNT1), and is necessary to maintain the expression of pro-survival pathways and protective genes. (4) to test the hypothesis that normal FGFR1 Illc and TrkC/p75 receptor function in the initial segment are important for male fertility. This proposal is part of a long term goal to understand the mechanisms by which the epididymis maintains an optimal luminal microenvironment for sperm maturation and survival, and therefore male fertility.