Differential RA responsiveness directs formation of functionally distinct spermatogonial populations at the initiation of spermatogenesis in the mouse

Abstract

In the mammalian testis, sustained spermatogenesis relies on spermatogonial stem cells (SSCs); their progeny either remain as stem cells (self-renewal) or proliferate and differentiate to enter meiosis in response to retinoic acid (RA). Here, we sought to uncover elusive mechanisms regulating a key switch fundamental to spermatogonial fate: the capacity of spermatogonia to respond to RA. Using the developing mouse testis as a model, we found that spermatogonia and precursor prospermatogonia exhibit a heterogeneous capacity to respond to RA with at least two underlying causes. First, progenitor spermatogonia are prevented from responding to RA by catabolic activity of cytochrome P450 family 26 enzymes. Second, a smaller subset of undifferentiated spermatogonia enriched for SSCs exhibit catabolism-independent RA insensitivity. Moreover, for the first time, we observed that precursor prospermatogonia are heterogeneous and comprise subpopulations that exhibit the same differential RA responsiveness found in neonatal spermatogonia. We propose a novel model by which mammalian prospermatogonial and spermatogonial fates are regulated by their intrinsic capacity to respond (or not) to the differentiation signal provided by RA before, and concurrent with, the initiation of spermatogenesis.

Nutrient restriction, inducer of yeast meiosis, induces mammalian meiotic initiation

“…1E; fig. S8) 23 , this data suggests that nutrient restriction is a critical signal to work with RA in activating meiotic genes.…”

“…Together, these data suggest crucial roles of nutrient restriction by itself or by synergizing with RA in inducing meiotic gene expression; in the absence of nutrient restriction, RA alone is not sufficient (see the effect of in vivo RA treatment on meiotic gene expression despite Stra8 activation in fig. S8) 23 .…”

Unique Epigenetic Programming Distinguishes Regenerative Spermatogonial Stem Cells in the Developing Mouse Testis

“…Finally, we previously suggested that the initial, foundational pool of SSCs that forms in the postnatal mouse testis may derive from a distinct subpopulation of prospermatogonia that become uniquely programmed during late fetal and early postnatal stages such that they are predetermined to form the foundational SSCs 8,9,21,26,28,72 . Our characterization of the epigenetic landscape within foundational SSCs now provides the first insight into the type of epigenetic programming that may underpin such a predetermination mechanism.…”

“…Studies based on detection of specific marker proteins 18,19 , lineage tracing 17 , or bulk 20 or single-cell 21,22,23 RNA-sequencing (RNA-seq), have confirmed that undifferentiated spermatogonia display heterogeneous patterns of gene expression, indicating distinct spermatogonial subpopulations including SSCs, progenitors, transitory cells and cells undergoing cell death 9,10,11,24,25,26 . Expression of the Id4-eGfp transgene marks a majority of undifferentiated spermatogonia, including transplantable/regenerative SSCs 27 .…”

Single cell analyses of development in the modern era

“…Two papers apply live imaging to demonstrate the role of a signal transduction pathway in differentiation (Deathridge et al, 2019) or to demonstrate stochasticity in fate choice (Antolovićet al, 2019); several papers build transcriptomic maps of specific developing tissues that will prove useful resources and reveal new hypotheses for developmental regulation (Combes et al, 2019;Martin et al, 2019;Hulin et al, 2019;Li et al, 2019a,b); several studies computationally reconstruct developmental dynamics (Bastidas-Ponce et al, 2019;Delile et al, 2019;Guo and Li, 2019;Prior et al, 2019;van Gurp et al, 2019); and one study benchmarks in vitro differentiation to in vivo development and develops new computational tools to do so (Edri et al, 2019). Taking a slightly different approach, other papers use single cell analyses to characterize cell behaviors during morphogenesis (Amini et al, 2019;Yang et al, 2019), to investigate how cells respond to DNA or tissue damage (Miermont et al, 2019;Dell'Orso et al, 2019), and to understand how heterogeneities in gene and protein concentration influence development (Papadopoulos et al, 2019;Reznik et al, 2019;Velte et al, 2019).…”