103 In Utero Exposure to Maternal Heat Stress Impairs Leydig Cell Development and Function in the Boar

Abstract

Abstract Due to the nearly exclusive use of artificial insemination, a typical U.S. boar sires ~8,398 progeny per year which is a 99.7% greater gene transmission rate compared with a sow (~23.1 progeny/year). Therefore, the boar has the greatest cumulative impact on swine reproductive performance and drives genetic progress in the herd. However, boar fertility is threatened by climate change. While boar studs are cooled to prevent heat stress, it is cost prohibitive to efficiently cool the six million pregnant sows that are exposed annually, leading to in utero heat stress (IUHS) of developing progeny. The IUHS of the boar reduces the number of pre-spermatogonia in neonatal testes and decimates subsequent semen quality by reducing both total sperm count (24% decrease) and increasing the proportion of abnormal sperm (42% increase). These data suggest that IUHS programs a lifetime of testis dysfunction reducing the breeding potential of boars. However, the biological mechanisms mediating this effect are undefined. The objective of this study was to determine how IUHS affects Leydig cell development and the testicular proteome. Pregnant sows were exposed to either heat stress (28 to 38°C; n = 18) or thermal neutral (17 to 22 °C; n = 18) conditions from day 27 to 57 of gestation. Neonatal boars exposed to IUHS (n = 23) or in utero thermal neutral (IUTN; n = 29) conditions were castrated at 3 days of age. Testes were preserved for histological analysis and proteomics. Leydig cells within IUHS testes were hypertrophic compared with IUTN controls (147.3 ± 1.6 µm2 versus 132.8 ± 1.4 µm2; P < 0.0001). Moreover, IUHS decreased the number of Leydig cells (Per microscopy field) compared with IUTN males (72.7 ± 2.0 Leydig cells/field versus 93.5 ± 1.8 Leydig cells/field). The testicular proteome was interrogated in a subset of animals (n = 8/treatment). A total of 519 proteins (32% of those detected) were differentially abundant between IUHS and IUTN testes (278 proteins down-regulated, 241 proteins up-regulated) including proteins important for Sertoli (e.g., insulin-like growth factor binding protein), Leydig (e.g., CYP19), and germ (e.g., glyceraldehyde 3-phosphate dehydrogenase) cell function. Notably, gene ontology analysis revealed that both steroid & cholesterol biosynthesis pathways were enriched (P < 0.0001) indicating differential expression of proteins needed for testicular steroidogenesis. Notably, CYP19 (enzyme expressed in porcine Leydig cells that converts testosterone to 17β-estradiol) was down-regulated in IUHS testes. Therefore, these data suggest that IUHS leads to Leydig cell dysfunction in boars. The importance of Leydig cells to male reproduction is unequivocal; thus, IUHS represents a mounting threat to male fertility, especially in the face of climate change.