NAD Metabolism in Male Reproductive Aging

Project Summary Male age is positively correlated with an overall decline of testicular function, reduced sperm numbers and declining sperm quality, which can negatively affect pregnancy outcome and child development. This is relevant, because in the U.S. the average age of first-time fathers has been steadily increasing since 1980. There is a normal aging-related decline of NAD levels that may provide a plausible explanation for reduced testicular function in aging men, but this hypothesis has been difficult to test due to the absence of suitable laboratory animal models. We developed a transgenic mouse with inducible overexpression of the enzyme hACMSD, which leads to dependency of the animals on dietary intake of vitamin B3 (niacin) for their NAD synthesis (ANDY mouse, Cell Reports, 2018), similar to humans. Dietary niacin restriction of ANDY mice leads to low NAD levels in young males, reminiscent of levels seen in chronologically old animals and similar to the situation in aging men. Our preliminary data show that suboptimal NAD levels in chronologically young ANDY mice resulted in behavioral, metabolic, and physical signs of chronological ageing in mice, along with oligozoospermia and poor sperm quality. All of these signs were reversible by oral niacin supplementation, which restored normal NAD levels. The objective of the proposed investigations is to determine to what extent NAD deficiency in ANDY mice recapitulates pathophysiological effects of the aging process in the testis. To test the central hypothesis that the decline in NAD levels functionally links the aging process with testicular function, three specific aims are proposed. Specific Aim 1 will test the hypothesis that low body NAD levels result in phenotypes characteristic of advanced age by comparing NAD-deficient young mice with chronologically old mice and young controls using an accepted and standardized panel of analyses to assess the healthspan of mice. Specific Aim 2 will test the hypothesis that in aging and NAD-deficient mice inhibition of NAD-dependent metabolic pathways will block spermatogenesis, and that this block is reversible. Specific Aim 3 will define testicular gene expression patterns caused by NAD deficiency and in chronological aging. Utilizing an innovative deep sequencing approach, we expect to identify deviations from normal gene expression patterns seen in young males that are common to both aged and NAD-deficient males. In summary, the proposed investigations should reveal functions of NAD metabolism in organismic and reproductive aging and shed light on mechanisms that lead to NAD deficiency in chronologically old males. Outcomes should provide a rational basis for pharmacological targeting of aging-related testicular decline by vitamin B3 supplementation.