In mammalian testes, premeiotic spermatogonia respond to retinoic acid (RA) by completing an essential lengthy differentiation program before initiating meiosis. The molecular and cellular changes directing these developmental processes remain largely undefined. This wide gap in knowledge is due to two critical unresolved technical challenges: 1) lack of robust and reliable in vitro models to study differentiation and meiotic initiation; 2) lack of methods to isolate large and pure populations of male germ cells at each stage of differentiation and at meiotic initiation. Here, we report a facile in vitro differentiation and meiotic initiation system that can be readily manipulated, including the use of chemical agents that cannot be safely administered to live animals. In addition, we present a transgenic mouse model enabling fluorescence-activated cell sorting (FACS)-based isolation of millions of spermatogonia at specific developmental stages as well as meiotic spermatocytes.

The foundation of mammalian spermatogenesis is provided by undifferentiated spermatogonia, comprised of spermatogonial stem cells (SSCs) and transit-amplifying progenitors that differentiate in response to retinoic acid (RA) and are committed to enter meiosis. Our laboratory recently reported that the foundational populations of SSCs, undifferentiated progenitors, and differentiating spermatogonia are formed in the neonatal testis in part based on their differential responsiveness to RA. Here, we expand on those findings to define the extent to which RA responsiveness during steady-state spermatogenesis in the adult testis regulates spermatogonial fate. Our results reveal both progenitor and differentiating spermatogonia throughout the testis are capable of responding to exogenous RA, but their resulting fates were quite distinct – undifferentiated progenitors precociously differentiated and proceeded into meiosis on a normal timeline, while differentiating spermatogonia were unable to hasten their entry into meiosis. This reveals spermatogonia responding to RA must still complete the 8.6 day differentiation program prior to their entry into meiosis. Addition of exogenous RA enriched testes with preleptotene and pachytene spermatocytes one and two seminiferous cycles later, respectively, supporting recent clinical studies reporting increased sperm production and enhanced fertility in subfertile men on long-term RA analog treatment. Collectively, our results reveal a well-buffered system exists within mammalian testes to regulate spermatogonial RA exposure, that exposed undifferentiated progenitors can precociously differentiate, but must complete a normal-length differentiation program prior to entering meiosis, and that daily RA treatments increased numbers of advanced germ cells by directing undifferentiated progenitors to continuously differentiate.

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What is known The foundation of mammalian spermatogenesis is provided by undifferentiated spermatogonia that proliferate before responding to retinoic acid (RA). RA converts undifferentiated into differentiating spermatogonia committed to entering meiosis. This RA‐regulated fate transition is essential for maintaining normal ratios of spermatogonia during steady‐state spermatogenesis in adult testes. Imbalances in spermatogonial populations often result in infertility – enhanced differentiation leads to eventual germline depletion, while impaired differentiation increases the likelihood of developing testicular cancer. Study Objective Define the extent cell autonomous and non‐autonomous mechanisms in adult mammalian spermatogonia regulate their response to RA. Hypothesis Only spermatogonia primed to differentiate will respond to RA and precociously progress through spermatogenesis. Methodology Adult mice were treated in vivo with vehicle (DMSO), RA, or talarozole (RA catabolism inhibitor) and euthanized 12‐48 hours later. Testes were removed and various assays used to assess populations for responsiveness to RA and capacity to precociously differentiate and enter meiosis. Concurrently, a model of synchronized spermatogenesis was used to define the potential of sequential subsets of differentiating spermatogonia to respond to RA and precociously enter meiosis. In complementary ex vivo experiments, mouse testes and patient‐derived testicular biopsies were cultured with DMSO‐alone, RA, and/or talarozole for 6‐24 hours to evaluate RA responsiveness and changes in spermatogonia fate using immunostaining and qRT‐PCR. Main results Exogenous RA elicited a rapid (12 hour) and significant increase in expression of the RA‐responsive gene ‘stimulated by retinoic acid 8’ (STRA8) in undifferentiated (ZBTB16+) spermatogonia. STRA8 expression decreased over the next 36 hours, such that numbers of STRA8+/ZBTB16+ spermatogonia were comparable in testes from vehicle‐ and RA‐treated mice. This transient increase in STRA8 affected spermatogonia at all stages of spermatogenesis. Exogenous RA also elicited STRA8 expression in differentiating (KIT+) spermatogonia in adults and synchronized mice. Despite this capacity for responding to exogenous RA, obvious precocious progression was not observed within the testis. Talarozole treatment elicited STRA8 expression in spermatogonia akin to RA treatments. Finally, qRT‐PCR showed adult mouse and human testes regulate responsiveness to RA similarly, based on mRNA expression profiles. Conclusion In response to exogenous RA, undifferentiated spermatogonia transiently upregulate expression of RA‐responsive genes, but this did not result in precocious progression into meiosis. Implications The adult mammalian testis is a well‐buffered system in which exogenous RA has little effect on cell fate, thus preserving the normal timing and progression of spermatogenesis.

Chemotherapy is a mainstay of cancer therapy. Unfortunately, while chemotherapy can profoundly impact disease free survival, it’s often accompanied with devastating side effects, including peripheral neuropathy. Indeed, 30-40% of patients treated with neurotoxic chemotherapy develop long-term and often debilitating chemotherapy-induced peripheral neuropathy (CIPN). Unfortunately, there are currently no preventative measures for CIPN and while it is transitory in some patients, for others the side effects can persist for months or even years after the cessation of chemotherapy. Recent work suggests that cellular senescence, which is robustly induced by chemotherapy, contributes to CIPN. Senescent cells are typically characterized by increased CDKn2a (i.e., p16) expression, increased SA-β-gal hydrolyzation, and expression of the senescence-associated secretory phenotype (SASP) that can influence multiple cell types in the microenvironment. Through utilization of a mouse model that employs paclitaxel (PTX), we find that PTX robustly induces senescence in the hindpaws and dorsal root ganglia (DRG) of mice that display loss of peripheral axons and decreased response to mechanical stimuli. To address the role of senescence in CIPN, we utilized the INKATTAC mouse that allows for inducible elimination senescent cells. Using this model, we find that the elimination of senescent cells rescues CIPN. Further, the use of senolytics, drugs that kill senescent cells, also rescues CIPN, raising the possibility that we can treat patients with CIPN. To address the mechanism behind CIPN we have carried out single cell RNA-Seq to identify the population of senescent cells senescing in response to chemotherapy. These analyses will allow us to understand the mechanisms that drive CIPN and may lead to new treatments for patients suffering from CIPN. Citation Format: Taylor Malachowski, Ganesh Raut, Satarupa Mullick Bagchi, Shelia Stewart. Chemotherapy induced senescence drives peripheral neuropathy. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4796.