Nutrient restriction synergizes with retinoic acid to induce mammalian meiotic initiation in vitro

Abstract

The molecular machinery and chromosome structures carrying out meiosis are frequently conserved from yeast to mammals. However, signals initiating meiosis appear divergent: while nutrient restriction induces meiosis in the yeast system, retinoic acid (RA) and its target Stra8 have been shown to be necessary but not sufficient to induce meiotic initiation in mammalian germ cells. Here, we use primary culture of mouse undifferentiated spermatogonia without the support of gonadal somatic cells to show that nutrient restriction in combination with RA is sufficient to induce Stra8- and Spo11-dependent meiotic gene and chromosome programs that recapitulate the transcriptomic and cytologic features of in vivo meiosis. We demonstrate that neither nutrient restriction nor RA alone exerts these effects. Moreover, we identify a distinctive network of 11 nutrient restriction-upregulated transcription factor genes, which are associated with early meiosis in vivo and whose expression does not require RA. Our study proposes a conserved model, in which nutrient restriction induces meiotic initiation by upregulating key transcription factor genes for the meiotic gene program and provides an in vitro platform for meiotic induction that could facilitate research and haploid gamete production.

Meiosis initiation: a story of two sexes in all creatures great and small

“…mTOR complexes are known to play a role as amino acid sensors, allowing cancer cells to adapt to the harsh environment by epigenetic shift and metabolic reprogramming [130,131]. This recapitulates the amino acid restriction within the gonad that surrounds germ cells which could explain thereactivation of meiotic genes [87,132]. Notably, chromatin reorganisation can be affected by nutrient availability.…”

“…Interestingly, although meiosis initiation is regulated by multiple signalling pathways within multicellular organisms, it has been reported that nutrient restriction and RA stimulation can activate the meiotic program through activation of a set of key transcription factors in vitro [ 87 ]. This observation is reminiscent of meiosis initiation in yeast, which is primarily nutrient and metabolism-driven.…”

“…For example, Shimamoto et al [ 100 ] reported that primary oocytes bypassed meiotic arrest at diplotene stage in prophase I without inducing hypoxia, indicating that hypoxia serves as one of the major factors maintaining oocyte dormancy which is mediated by nuclear localisation of FOXO3. Utilisation of these tools has led to recent appreciation of the evolutionarily conserved role of nutrient-deprivation in meiosis initiation which, combined with careful studies of meiosis-activating transcriptional networks, has led to landmark discoveries allowing experimental modelling of meiotic initiation in murine cells [ 78 , 87 , 97 , 98 ]. Now that meiosis initiation can be modelled and manipulated, the direct contribution of specific cellular process during meiosis initiation must be established.…”

“…It is likely that metabolic stress, in concert with RA signalling, act to activate meiosis in mammals [ 108 ]. As mentioned in previous sections, Wang et al [ 87 ] reported based on the scRNA-seq data that nutrient starvation is likely to be the metabolic stress inducing switching from glycolysis to mitochondrial oxidative phosphorylation. In vivo , this metabolic stress, which in the male is further contributed to by the blood-testis barrier (BTB), is required for SSC differentiation [ 109 , 110 ].…”

“…Recently uncovered links between nutrient deprivation and mammalian meiosis [ 87 ] raise a series of important questions. For many years, dogma stated that the Weismann barrier (a distinction between ‘ immortal' germ cell lineages that generate gametes and ‘disposable' somatic cells) explained the very tight control of meiotic gene expression, which was thought to limit transcription exclusively to germ cells.…”