LET-418/Mi2 and SPR-5/LSD1 cooperatively prevent somatic reprogramming of C. elegans germline stem cells

Abstract

Throughout their journey to forming new individuals, germline stem cells must remain totipotent, particularly by maintaining a specific chromatin structure. However, the place epigenetic factors occupy in this process remains elusive. So far, "sensitization" of chromatin by modulation of histone arrangement and/or content was believed to facilitate transcription-factor-induced germ cell reprogramming. Here, we demonstrate that the combined reduction of two epigenetic factors suffices to reprogram C. elegans germ cells. The histone H3K4 demethylase SPR-5/LSD1 and the chromatin remodeler LET-418/Mi2 function together in an early process to maintain germ cell status and act as a barrier to block precocious differentiation. This epigenetic barrier is capable of limiting COMPASS-mediated H3K4 methylation, because elevated H3K4me3 levels correlate with germ cell reprogramming in spr-5; let-418 mutants. Interestingly, germ cells deficient for spr-5 and let-418 mainly reprogram as neurons, suggesting that neuronal fate might be the first to be derepressed in early embryogenesis.

Graphical abstract

Figure 1

SPR-5 Interacts with LET-418 and MEP-1 In Vivo (A) Coimmunoprecipitation of SPR-5 and LET-418 in embryonic extracts using anti-LET-418 or anti-SPR-5 antibodies. wt, wild-type; spr-5, spr-5(by134) null allele. (B) Coimmunoprecipitation of LET-418 using anti-SPR-5 (SPR-5 IP) antibodies with (+) or without (−) DNaseI/ethidium bromide (DNase+EtBr) pretreatment. (C) Coimmunoprecipitation of LET-418 and FLAG-tagged MEP-1, using anti-SPR-5 and anti-FLAG antibodies, in embryonic extracts of a control wild-type strain (−) or a strain stably expressing MEP-1::3xFLAG::GFP (+). See also Figure S1.

Figure 2

Simultaneous Downregulation of SPR-5 and LET-418 Causes Sterility and Germline Tumor Formation (A) Double spr-5(by134); let-418(n3536) mutants are maternal-effect sterile. Percentage of fertile adults, at 20°C, over the indicated generations; ≥100 adults were counted per strain per generation. (B) Nomarski pictures of the indicated genotypes 4 days postbirth. (C) DAPI staining of the indicated mutant germlines 6 days postbirth. Double spr-5; let-418 mutants develop a germline tumor. Plain arrows, oocytes; empty arrows, sperm; gray arrows, embryos; striped arrows, endomitotic oocytes. Scale bar = 20 μm. (D) Enlarged view of the squared zones from (C), showing DAPI-stained nuclei for the indicated genotypes. Fine dashed circles, larger nuclei with decondensed chromatin; long dashed circles, smaller nuclei different from wild-type germ cells. Scale bar = 20 μm. All strains in (B)–(D) displayed the unc-46(e177) allele in their background. See also Figure S2.

Figure 3

spr-5 let-418 Germline Tumors Replicate Rapidly while Losing Pluripotency BrdU incorporation assay in single or double spr-5(by134) let-418(s1617) mutants 4 or 7 days postbirth at 20°C. Gonad immunostaining against BrdU (green) and the P granule component PGL-1 (red) plus DAPI staining of the DNA content (blue). Empty arrows, PGL-1-positive, BrdU-positive cells; plain arrows, PGL-1-negative, BrdU-positive cells; right panel squares, enlargements of the boxed areas on their left. All these strains contained the unc-46(e177) mutation in their genetic background. Scale bar = 20 μm. See also Figure S3 and Table S2.

Figure 4

spr-5 let-418 Germ Cells Reprogram toward a Neuronal Fate (A) Ectopic expression of the pan-neuronal reporter unc-119p::gfp within the spr-5(by134) let-418(RNAi) germinal gonad. spr-5(by134) or control worms (wild-type) expressing the unc-119p::gfp reporter were grown on let-418(RNAi) at 20°C and monitored for ectopic GFP expression in their gonads at 5, 7, 11 and 13 days postbirth. The percentage of worms ectopically expressing unc-119p::GFP in their gonads is indicated within each representative picture. Dashed lines, gonad limits; n.d., not determined; ^∗, distal tip (when observable). Scale bar = 20 μm. (B) Nomarski pictures of unc-119p-driven GFP-positive cells in the spr-5(by134) let-418(RNAi) germlines 7 days postbirth. Scale bar = 10 μm. See also Figure S4.

Figure 5

The Cooperative Control of COMPASS-Dependent H3K4 Methylation Levels by SPR-5 and LET-418 Is Linked to Germ Cell Status (A) Germline tumorigenesis of spr-5 let-418 mutants is partially rescued by set-2 and wdr-5.1(RNAi). DAPI staining of wild-type and double spr-5(by134) let-418(s1617) null mutants grown on control, set-2, or wdr-5.1(RNAi) 7 days postbirth at 20°C. Right panels show an enlarged view of the right arm of spr-5; let-418 mutant gonads. Dashed lines show the position of the somatic gonad. ^∗, distal tip (when visible). Scale bar = 20 μm. (B) P-granule-negative cells of the spr-5(by134); let-418(s1617) germlines show elevated H3K4me3 levels, which are partially rescued by set-2(RNAi) and wdr-5.1(RNAi). Immunostaining of control or spr-5(by134); let-418(s1617) germlines after exposure to control, set-2, or wdr-5.1(RNAi) 7 days postbirth at 20°C. Full arrows show P-granule (GLH-2)-positive germ cells, whereas empty arrows point to P-granule-negative cells. Scale bar = 20 μm. (C) Quantification of (A), showing the percentage of cells with elevated H3K4me3 signal in P-granule-positive (light gray bars) or P-granule-negative (dark gray bars) cells. Results are represented as the mean percentage of cells with high H3K4me3 signal, counted manually using the ImageJ cell counting tool on immunostaining pictures, between two biological replicates. Error bars = SD; p values: t test (1), p = 0.00025; t test (2), p = 0.00012. All the strains present in this figure contained the unc-46(e177) mutation in their genetic background.

Figure 6

Loss of Fertility in spr-5(by134) Single Mutants Is Also Associated with Germ Cell Reprogramming and Elevated H3K4me3 Levels (A) Percentage of worms ectopically expressing the pan-neuronal reporter unc-119p::gfp in the germline of spr-5(by134) fertile and sterile individuals of a same generation 7 days postbirth at 20°C. Two representative pictures are presented. n, total number counted. t test p value = 0.0025 (^∗∗). White dashed lines show gonad borders. ^∗, distal tip; &, background fluorescence of the intestine; white arrow, position of the vulva. Scale bar = 20 μm. (B) Side-by-side immunostaining showing H3K4me3 (green) an GLH-2(red) levels in dissected gonads from fertile and sterile spr-5(by134) individuals from (A). Left gonad (white full line), fertile worm; right gonad (white dashed line), sterile worm. Scale bar = 20 μm. See also Figure S6 and Table S5.