Heterogeneity of Spermatogonial Stem Cells

Comparison of POU5F1 gene expression and protein localization in two differentiated and undifferentiated spermatogonial stem cells

The POU domain, class 5, transcription factor 1 (POU5F1), plays a vital role in creating pluripotency and maintaining self-renewal of the spermatogonial stem cells (SSCs). In this experimental research, the gene and protein expression of POU5F1 in two populations of differentiated and undifferentiated spermatogonia were examined, by immunohistochemistry (IMH), immunocytochemistry (ICC) and Fluidigm real-time RT-PCR. Our study was extended with online databases and the creation of PPI networks. The results indicated that the POU5F1 protein was localized in the basal compartment of seminiferous tubules. Under in vitro conditions, isolated SSC colonies were ICC-positive for the POU5F1, but the protein expression level of POU5F1 in the undifferentiated populations was higher than that in differentiated. A significant POU5F1 mRNA expression was seen in passage 4 compared to passage 0 for both populations. POU5F1 has a significantly higher mRNA expression in undifferentiated SSCs than that in differentiated SSCs, also in mESCs than in SSC-like cells. Bioinformatic analysis on POU5F1 shows its impressive connection with other genes involved in spermatogonia differentiation. These results support the advanced investigations of spermatogonia differentiation, both in vitro and in vivo. A better understanding of the POU5F1 gene and its function during differentiation will give the scientific community an open perspective for the development of direct differentiation of SSC to other male germline cells which is very important in infertility treatment.

Non-neutral clonal selection and its potential role in mammalian germline stem cell dysfunction with advancing age

The concept of natural selection, or "survival of the fittest", refers to an evolutionary process in nature whereby traits emerge in individuals of a population through random gene alterations that enable those individuals to better adapt to changing environmental conditions. This genetic variance allows certain members of the population to gain an advantage over others in the same population to survive and reproduce in greater numbers under new environmental pressures, with the perpetuation of those advantageous traits in future progeny. Here we present that the behavior of adult stem cells in a tissue over time can, in many respects, be viewed in the same manner as evolution, with each stem cell clone being representative of an individual within a population. As stem cells divide or are subjected to cumulative oxidative damage over the lifespan of the organism, random genetic alterations are introduced into each clone that create variance in the population. These changes may occur in parallel to, or in response to, aging-associated changes in microenvironmental cues perceived by the stem cell population. While many of these alterations will be neutral or silent in terms of affecting cell function, a small fraction of these changes will enable certain clones to respond differently to shifts in microenvironmental conditions that arise with advancing age. In some cases, the same advantageous genetic changes that support survival and expansion of certain clones over others in the population (viz. non-neutral competition) could be detrimental to the downstream function of the differentiated stem cell descendants. In the context of the germline, such a situation would be devastating to successful propagation of the species across generations. However, even within a single generation, the “evolution” of stem cell lineages in the body over time can manifest into aging-related organ dysfunction and failure, as well as lead to chronic inflammation, hyperplasia, and cancer. Increased research efforts to evaluate stem cells within a population as individual entities will improve our understanding of how organisms age and how certain diseases develop, which in turn may open new opportunities for clinical detection and management of diverse pathologies.

Integration and gene co-expression network analysis of scRNA-seq transcriptomes reveal heterogeneity and key functional genes in human spermatogenesis

Spermatogenesis is a complex process of cellular division and differentiation that begins with spermatogonia stem cells and leads to functional spermatozoa production. However, many of the molecular mechanisms underlying this process remain unclear. Single-cell RNA sequencing (scRNA-seq) is used to sequence the entire transcriptome at the single-cell level to assess cell-to-cell variability. In this study, more than 33,000 testicular cells from different scRNA-seq datasets with normal spermatogenesis were integrated to identify single-cell heterogeneity on a more comprehensive scale. Clustering, cell type assignments, differential expressed genes and pseudotime analysis characterized 5 spermatogonia, 4 spermatocyte, and 4 spermatid cell types during the spermatogenesis process. The UTF1 and ID4 genes were introduced as the most specific markers that can differentiate two undifferentiated spermatogonia stem cell sub-cellules. The C7orf61 and TNP can differentiate two round spermatid sub-cellules. The topological analysis of the weighted gene co-expression network along with the integrated scRNA-seq data revealed some bridge genes between spermatogenesis's main stages such as DNAJC5B, C1orf194, HSP90AB1, BST2, EEF1A1, CRISP2, PTMS, NFKBIA, CDKN3, and HLA-DRA. The importance of these key genes is confirmed by their role in male infertility in previous studies. It can be stated that, this integrated scRNA-seq of spermatogenic cells offers novel insights into cell-to-cell heterogeneity and suggests a list of key players with a pivotal role in male infertility from the fertile spermatogenesis datasets. These key functional genes can be introduced as candidates for filtering and prioritizing genotype-to-phenotype association in male infertility.

Spermatogonial Stem Cell Numbers Are Reduced by Transient Inhibition of GDNF Signaling but Restored by Self-Renewing Replication when Signaling Resumes

One cause of human male infertility is a scarcity of spermatogonial stem cells (SSCs) in testes with Sertoli cells that neither produce adequate amounts of GDNF nor form the Sertoli-Sertoli junctions that form the blood-testis barrier (BTB). These patients raise the issue of whether a pool of SSCs, depleted due to inadequate GDNF stimulation, will expand if normal signaling is restored. Here, we reduce adult mouse SSC numbers by 90% using a chemical-genetic approach that reversibly inhibits GDNF signaling. Signal resumption causes all remaining SSCs to replicate immediately, but they primarily form differentiating progenitor spermatogonia. Subsequently, self-renewing replication restores SSC numbers. Testicular GDNF levels are not increased during restoration. However, SSC replication decreases as numbers of SSCs and progenitors increase, suggesting important regulatory interactions among these cells. Finally, sequential loss of SSCs and then pachytene spermatocytes causes dissolution of the BTB, thereby recapitulating another important characteristic of some infertile men.

Short time exposure to low concentration of zinc oxide nanoparticles up-regulates self-renewal and spermatogenesis-related gene expression

Extensive application of zinc oxide (ZnO) nanoparticles (NPs) in everyday life results in increased exposure to these NPs. Spermatogonial stem cells (SSCs) guarantee sperm production throughout the male reproductive life by providing a balance between self-renewal and differentiation. We used an in vitro platform to investigate the ZnO NPs effects on SSCs. We successfully synthesized ZnO NPs. In order to investigate these NPs, we isolated SSCs from mouse testes and cultured them in vitro. Our results confirmed the uptake of ZnO NPs by the cultured SSCs. We observed a dose- and time-dependent decrease in SSC viability. Both spherical and nanosheet ZnO NPs had the same cytotoxic effects on the SSCs, irrespective of their shapes. Moreover, we have shown that short time (one day) exposure of SSCs to a low concentration of ZnO NPs (10 μg/mL) promoted expressions of specific genes (Plzf, Gfr α1 and Bcl6b) for SSC self-renewal and differentiation genes (Vasa, Dazl, C-kit and Sycp3) expressed by spermatogonia during spermatogenesis. Our study provides the first insight into ZnO NPs function in SSCs and suggests a new function for ZnO NPs in the male reproductive system. We demonstrated that ZnO NPs might promote spermatogenesis via upregulation of gene expression related to SSC self-renewal and differentiation at low concentrations. Additional research should clarify the possible effect of ZnO NPs on the SSC genome and its effects on human SSCs.