Pathogenesis of heat-induced infertility in mammals

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In male mammals, high ambient temperatures reduce normal and motile sperm and fertility. Bos taurus (European/British) bulls are more susceptible than Bos indicus (zebu). The paradigm is that mammalian testes operate near hypoxia, increased testicular temperature increases metabolism, but not blood flow and testes become hypoxic. However, our recent data refute that.***My overall hypothesis is that deleterious effects of increased testicular temperature are primarily due to hyperthermia and not hypoxia.****There are 3 objectives:***Objective 1: Determine effects of increased testicular temperature in B. taurus and B. indicus bulls on testicular blood flow and oxygenation. Testicular temperatures will be serially increased (34 to 40ºC). We expect no anerobic metabolism and B. indicus bulls to have a significantly smaller increase in testicular blood flow.****Objective 2: Determine effects of increased testicular temperature on mRNA and protein expression of specific gene targets in Sertoli and Leydig cells. Bulls will be castrated before and varying times after scrotal insulation. Cell-sorting (FACS) will be done to isolate Sertoli and Leydig and cells, using GATA4 and 3HSD antigens, respectively. We will isolate mRNA, create cDNA and do western blotting and immunohistochemistry to detect target genes and their proteins, as well as RNA sequencing (discovery approach). We expect Sertoli and Leydig cells to be differentially affected by heat stress (up- or down-regulation of various genes and their proteins).****Objective 3: Determine: 1) associations between environmental and testicular temperatures; 2) effects of increased testicular temperature on semen quality, fertilization, development and epigenetic effects on sperm and embryos; and 3) differences among breeding bulls in ability to maintain semen quality and fertility under field conditions with warm ambient temperatures. We will monitor temperature with intratesticular sensors in testes of beef bulls; some will be used for natural breeding (~6 wk), with location (e.g. shade versus non-shade) measured with GPS and sire performance determined by calf birth dates and parentage testing. We will increase testicular temperature with scrotal insulation. Semen will be collected, assessed and used for in vitro fertilization. We expect testicular temperatures will be directly affected by ambient temperatures. Increases in testicular temperature beyond a threshold (which will vary among bulls) will reduce motility, morphology and fertilizing ability and cause epigenetic changes in sperm and embryos.****New knowledge generated in my research program will be used to develop evidence-based biomarkers of susceptibility/resistance to testicular hyperthermia and ways to prevent or mitigate deleterious effects on spermatogenesis, sperm and fertility. This program will generate well-trained HQP specialized in reproductive biology, with a broad range of skills, preparing them for careers in academia, industry and elsewhere.******