Differential responsiveness of spermatogonia to retinoic acid dictates precocious differentiation but not meiotic entry during steady-state spermatogenesis†

The foundation of mammalian spermatogenesis is provided by undifferentiated spermatogonia, which comprise 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 the spermatogonial fate. Our results reveal that 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 that the 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 that 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 the numbers of advanced germ cells by directing undifferentiated progenitors to continuously differentiate.

Abstract 4796: Chemotherapy induced senescence drives peripheral neuropathy

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.

A high-content flow cytometry and dual CRISPR-Cas9 based platform to quantify genetic interactions

Probing epistasis between two genes can be a critical first step in identifying the molecular players in a cellular pathway. The advent of CRISPR-Cas mediated genetic screen has enabled studying of these genetic interactions at a genomic scale. However, when combining depletion of two genes using CRISPR Cas9, reduced targeting efficiencies due to competition for Cas loading and recombination in the cloning step have emerged as key challenges. Moreover, given conventional CRISPR screens typically involve comparison between the initial and final time point, it is difficult to parse the time kinetics with which a perturbed genetic interaction impacts viability, and it also becomes challenging to assess epistasis with essential genes. Here, we discuss a high-throughput flow-based approach to study genetic interactions. By utilizing two different Cas9 orthologs and monitoring viability at multiple time points, this approach helps to effectively mitigate the limitations of Cas9 competition and enables assessment of genetic interactions with both essential and non-essential genes at a high temporal resolution.

Modeling mammalian spermatogonial differentiation and meiotic initiation in vitro

In mammalian testes, premeiotic spermatogonia respond to retinoic acid 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 unresolved technical challenges: (1) lack of robust and reliable in vitro models to study differentiation and meiotic initiation; and (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-based isolation of millions of spermatogonia at specific developmental stages as well as meiotic spermatocytes.