The epididymis represents a pivotal organ for sperm maturation and male fertility maintenance. During the epididymal journey, sperm cells undergo morphological and molecular changes that need to acquire the morpho-functional skills necessary for successful oocyte fertilization. Not last, a great enrichment of the spermatozoa RNA payload occurs via an epithelium-derived epididymosome transfer. Currently, circular RNAs (circRNAs), a class of non-coding RNAs (ncRNAs), are acquiring a prominent role in the setting of sperm quality parameters. In this regard, they are considered potential targets in several male infertility conditions. Despite their consolidated role, few notions are known regarding the alleged epididymal backsplicing activity. In the current review, we discuss the main aspects of spermatozoa maturation along the epididymis and the circRNA role in the field of male reproduction. We also report the most recent findings on the circRNA biogenesis that occurs in the epididymal duct, providing new fascinating evidence on epididymal-derived circRNAs. Finally, we show preliminary compelling data on epididymal backsplicing by exploiting the experimental mouse model of aging. Collectively, these data evidence a remarkable role of the epididymis in remodeling the circRNA payload and in shaping its profile in maturating spermatozoa.

Leydig cells (LCs) represent a somatic testicular population responsible for testosterone synthesis, a hormone essential for spermatogenesis and male fertility. The obesity condition impairs LC steroidogenic activity, contributing to testicular oxidative stress and male reproductive dysfunctions. Using a high-fat-diet (HFD) murine model, we investigated the regulatory role of the nuclear factor of activated T cells 5 (NFAT5s) in the obesity-induced LC damage and the resulting alterations in intergenerationally inherited sperm circRNA cargo. Our findings reveal a significant upregulation of both circNFAT5 and NFAT5 protein levels in HFD testis. This molecular signature correlated with decreased antioxidant defense system, increased LC apoptosis, and impaired steroidogenesis. In vitro experiments, performed in TM3 cells, confirmed that NFAT5 nuclear shuttling drives proapoptotic gene activation, while NFAT5 silencing promotes LC survival. The analysis of HFD progeny (F1H) revealed a full recovery of testis oxidative status and LC apoptosis, linked with the recovery of NFAT5 expression. However, a steroidogenic deficiency persisted in F1H offspring. Notably, HFD and F1H epididymides exhibited NFAT5 overexpression concomitantly with impaired sperm morphology, motility, viability, and altered sperm circRNA profiles alongside a deregulated 4-hydroxy-2-nonenal (4HNE) profile, a marker of sperm oxidative stress. Lastly, an enhanced FUS-related amplification of circRNA perturbations was highlighted in F1H spermatozoa. Collectively, our findings reveal a dual functional role of NFAT5 as a testicular regulator of LC fate and an epididymal sentinel of metabolic stress, in turn linking paternal obesity to the persistent transmission of sperm epigenetic anomalies across the offspring.