Type 1 diabetes is associated with human male reproductive dysfunction, including reports of reduced fertility and poor sperm quality (1
). Mechanisms for this association are widely debated. Studies in rodent models suggest mechanisms including oxidative stress, DNA damage to sperm, altered hormonal profiles, and abnormal progression through spermatogenesis (5
). More recently, investigations into human sperm samples from diabetic males show an increase in nuclear and mitochondrial damage (1
), suggesting that hyperglycemia may cause oxidative stress and free radical damage to sperm DNA. Clinical data from in vitro fertilization clinics show that sperm from diabetic patients are able to fertilize oocytes at similar rates compared with sperm from nondiabetic patients. Pregnancy rates, however, are significantly lower when these embryos are transplanted, suggesting that a diabetic environment damages sperm cells, causing poor sperm quality (2
). It is still unclear whether diabetes affects male fertility at the early stages of spermatogenesis or at the level of mature sperm cells. In light of new data that show insulin expression by cells in the testis (7
), it is now unclear whether the effects of diabetes on fertility are mediated through testicular insulin insufficiency or through systemic effects of diabetes.
In this study, we use the Akita mouse model to study the effects of insulin deficiency on gonadal function. The Akita mouse is a model of type 1 diabetes resulting from a mutation in the ins2
gene. Unlike most other organisms, mice and rats have two functional insulin genes located on separate chromosomes. Ins1
arose from a duplication of the ins2
ancestral gene ~20 million years ago and has since been retained in the mouse and rat genome (8
). Murine ins2
is thus orthologous to the human insulin gene. Deletion of both genes results in pup lethality shortly after birth (9
). The mutation of the ins2
gene results in a misfolded protein product, which accumulates in the endoplasmic reticulum (ER), causing ER stress and, ultimately, death of the insulin-producing β-cells of the pancreas (10
). The diabetic Akita mouse is thus similar to humans with type 1 diabetes, which is caused by an autoimmune destruction of the β-cells of the pancreas (11
). The Akita model also displays a severe onset of diabetes, similar to untreated diabetes in human adults, and lacks the drug-induced toxicity of a streptozocin-induced model.
The Akita mouse model provides a unique way to study insulin function in the reproductive tract, given the production of mutant nonfunctional insulin by both the pancreas and the testes. It is the goal of this study to determine the mechanism of infertility in the insulin-deficient Akita mouse and distinguish the requirement of testicular insulin from that of pancreatic insulin in the maintenance of spermatogenesis.
Although both heterozygous and homozygous mice develop hyperglycemia, Akita homozygotes develop severe hyperglycemia by age 3 weeks and, thus, are infertile at age 8–9 weeks, as opposed to Akita heterozygotes, which become infertile at approximately age 6 months. Previous studies on the Akita mice show that sperm from Akita mice fertilize fewer embryos, and those embryos that do fertilize are developmentally impaired at the blastocyst stage, indicating a paternal effect of diabetes on sperm quality (12
). Although an association with type 1 diabetes and infertility exists, the significance of insulin and normoglycemia to the reproductive system has not been fully characterized.
More recent studies have begun to characterize the function of insulin and insulin receptor in testes and sperm. Murine testes and sperm have both been shown to produce transcripts of insulin (7
), suggesting a significant function for insulin in the reproductive tract and an increased potential for diabetes to affect male fertility. While insulin expression in human testes has not yet been examined, human sperm have been shown to release insulin in a dose-dependent manner in response to glucose (13
). Insulin receptor has also been detected in the midpiece of sperm cells, implicating an autocrine function for the secretion of insulin by sperm cells and demonstrating the importance of proper insulin signaling in the reproductive tract (14
). In this study, we show that Akita mice homozygous for a mutation in the ins2
gene are completely infertile and have a reduced testis size and abnormal testis and sperm morphology. By administering subcutaneous insulin pellets, we are able to rescue Akita homozygous infertility with exogenous insulin pellets. We also see that plasma insulin cannot cross the blood-testis barrier, demonstrating that the exogenous insulin does not directly rescue fertility.