Germ Cell-Specific Fatty Acid Beta-Oxidation is Required for Quantitatively Normal Spermatogenesis in the Mouse

Grace H. Neiswanger¹, Emma A, Gilbert¹, Bryan A. Niedenberger¹, Cameron A. Schmidt⁴, Jessica M. Ellis^2;3 , and Christopher B. Geyer^1;3

1.     Department of Anatomy & Cell Biology, Brody School of Medicine at East Carolina University, Greenville, NC, USA

2.     Department of Physiology, Brody School of Medicine at East Carolina University, Greenville, NC, USA

3.     East Carolina Diabetes and Obesity Institute, East Carolina University, Greenville, NC, USA

4.     Department of Biology, East Carolina University, Greenville, NC, USA


Abstract Text:

Over the past several decades, there is growing concern that both sperm quality and quantity in men living in developed countries have declined. Although the extent and causes of this decline remain unclear, there are indisputable links between reduced male fertility and consumption of the ‘Western diet’, with low nutritional value but high in processed sugars and fats. Although high-fat diets (HFDs) are correlated with male sub- and infertility, the mechanistic roles of dietary fatty acids (FAs) on spermatogenesis and male fertility remain largely undefined. FAs are oxidized in mitochondria as both a rich fuel source for ATP production and to generate critical cellular intermediates, including the pool of acetyl groups for critical chromatin remodeling, which in male germ cells can exert considerable impact on both gene expression and transgenerational epigenetic inheritance. Surprisingly, we found that oxidation levels of long chain FAs in testes are as high as in other organs with a known preference for FAs as a fuel source including the liver. The oxidation of long chain FAs ( >12 carbons) requires three enzymes that catalyze their transport into mitochondria. We discovered that protein levels for one of these essential enzymes, ‘carnitine palmitoyltransferase 2’ (CPT2), were upregulated in male germ cells entering meiosis. To examine the requirement for FA oxidation in spermatogenesis and male fertility, we generated Cpt2 male germ cell knockout (Cpt2 gcKO) mice. Compared to controls, Cpt2 gcKO males had ~25% smaller testes, significantly increased germ cell apoptosis and disruption of the timing of male germ cell development, and a dramatic ~50% reduction in cauda epididymal sperm. Despite these considerable defects, Cpt2 gcKO mice are fertile, revealing that FA oxidation is an important, but not critical contributor to normal male germline development. Ongoing work to define the mechanistic roles of male germ cell FA oxidation will be informative for identifying FA dietary recommendations for men who wish to maximize their sperm quality and fertility, supplementing media formulations with FAs to improve the efficiency of in vitro spermatogenesis, and analyzing effects of FAO on acetylation of retained histones in the sperm epigenome that may transmit diet-induced changes during transgenerational inheritance.