Paternal age is positively correlated with reduced sperm chromatin quality and higher numbers of DNA strand breaks which can negatively affect pregnancy outcome and child development. There is a normal aging-related decline of NAD levels in humans that may provide a plausible explanation for reduced sperm quality in aging men, but this hypothesis has been difficult to test due to the absence of suitable laboratory animal models. To address this important problem, we developed a transgenic mouse with inducible overexpression of the enzyme hACMSD, which leads to an acquired 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 adjustably reduced NAD levels, reminiscent of niacin deficiency possible in humans. Our preliminary data show indeed that suboptimal NAD levels in ANDY mice resulted in elevated numbers of DNA strand breaks and poor chromatin maturation in sperm, which were reversible by oral niacin supplementation, indicating that niacin may be a micronutrient of previously unrecognized importance for male reproduction. The main objective of this project is to test the overarching hypothesis, that low metabolic NAD+ levels lead to reduced chromatin quality and DNA integrity in sperm, using the ANDY mouse model. To this end, we propose to 1) measure the extent to which reduced glutathione (GSH) levels resulting from low NADPH stores in niacin-deficient ANDY spermatids cause elevated oxidative stress. Characterization of open DNA ends in sperm and spermatids using a sequencing approach will determine the contribution of oxidative DNA damage to sperm; 2) determine the impact of niacin deficiency on the activity of NAD-dependent enzymes involved in sperm chromatin development, such as ADP-ribose transferases and sirtuins, and how that results in poor chromatin quality that makes sperm vulnerable to exogenous DNA damage. We will also evaluate reversibility of NAD deficiency by nutritional niacin supplementation to increase testicular NAD and NADP levels and to correct poor sperm quality in ANDY mice. Furthermore, funding of this project would enhance the infrastructure at Utah State University needed to provide undergraduate students, including from underrepresented minorities, with biochemical and biomedical research opportunities. These will enable students, including underrepresented minority and beginning students, to experience a wide range of modern research techniques and to acquire skills of critical thinking, data evaluation and science presentation, which is a training they would otherwise not have access to.
(Relevance to Public Health) The age of first-time fathers is steadily increasing in the U.S., but paternal age is positively correlated with reduced sperm chromatin quality and higher numbers of DNA strand breaks, which can negatively affect pregnancy outcome and child development. There is a normal aging-related decline of NAD levels in humans that may provide a plausible explanation for reduced sperm quality in aging men, which will be investigated in this project using a novel transgenic mouse in which low NAD levels can be lowered to model the situation in aging men. Based on the preliminary results, the expected results will inform the development of male nutritional supplementation strategies with dietary NAD precursor micronutrients of the vitamin B3 group with the goal to increase the chance that children are born healthy.
