Maternal H3K36 and H3K27 HMTs protect germline development via regulation of the transcription factor LIN-15B

Abstract

Maternally synthesized products play critical roles in the development of offspring. A premier example is the Caenorhabditis elegans H3K36 methyltransferase MES-4, which is essential for germline survival and development in offspring. How maternal MES-4 protects the germline is not well understood, but its role in H3K36 methylation hinted that it may regulate gene expression in primordial germ cells (PGCs). We tested this hypothesis by profiling transcripts from nascent germlines (PGCs and their descendants) dissected from wild-type and mes-4 mutant (lacking maternal and zygotic MES-4) larvae. mes-4 nascent germlines displayed downregulation of some germline genes, upregulation of some somatic genes, and dramatic upregulation of hundreds of genes on the X chromosome. We demonstrated that upregulation of one or more genes on the X is the cause of germline death by generating and analyzing mes-4 mutants that inherited different endowments of X chromosome(s). Intriguingly, removal of the THAP transcription factor LIN-15B from mes-4 mutants reduced X misexpression and prevented germline death. lin-15B is X-linked and misexpressed in mes-4 PGCs, identifying it as a critical target for MES-4 repression. The above findings extend to the H3K27 methyltransferase MES-2/3/6, the C. elegans version of polycomb repressive complex 2. We propose that maternal MES-4 and PRC2 cooperate to protect germline survival by preventing synthesis of germline-toxic products encoded by genes on the X chromosome, including the key transcription factor LIN-15B.

SIN3 acts in distinct complexes to regulate the germline transcriptional program inC. elegans

“…MRG-1, a component of several chromatin complexes in addition to SIN3S (Chen et al, 2010; Huang et al, 2017; Smith et al, 2013; Yochum and Ayer, 2002; Baytek et al, 2022), also contributes to silencing of the X in the C. elegans germline (Takasaki et al, 2007). Comparison of our list of sin-3(syb2172 ) upregulated genes on the X with a list of MES-3 and MRG-1 targets on the X (Cockrum and Strome, 2022) revealed commonly upregulated genes in the three mutants (37), as well as a larger set genes uniquely in common between mes-3 and mrg-1, as previously described ( Figure 4B) (Cockrum and Strome, 2022). In addition, 48 genes were upregulated in sin-3 mutants only: these may be unique targets of SIN3 on the X (Figure 4B).…”

“…One of these commonly upregulated genes is lin-15B , encoding a THAP domain transcription factor also upregulated in mes-4 mutants. Loss of lin-15B in mes-4 mutants was shown to reduces X misexpression and prevent germline death (Cockrum and Strome, 2022). We found that lin-15B (RNAi) had no effect on the fertility of sin-3 mutants (Figure S5B), while its upregulation in hda-3 mutants is not associated with any obvious germline defects, suggesting that additional mechanisms contribute to sterility in the absence of sin-3 .…”

“…(B) Venn Diagram showing overlap between X-linked genes upregulated in sin-3(syb2172), mrg-1(qa6200) and mes-3(bn199) mutants. mrg-1 and mes-3 gene lists are from (Cockrum and Strome, 2022). (C) Decreased dosage of wildtype sin-3 enhances mes-2 sterility.…”

“…This may be partly accounted for by the significantly smaller number of SIN-3 target genes on the X. A critical target for repression by MES proteins on the X is the lin-15B THAP domain transcription factor, and its inactivation prevents germline death in mes mutants (Cockrum and Strome, 2022). While lin-15B is also upregulated in both sin-3 and hda-3 mutant germlines, its inactivation in sin-3 mutants had no effect on fertility, suggesting that in this context its upregulation alone does not contribute to sterility.…”

“…A critical target for repression by MES proteins on the X is the lin-15B THAP domain transcription factor, and its inactivation prevents germline death in mes mutants (Cockrum and Strome, 2022).…”