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. 2015 May 15;142(10):1745-55.
doi: 10.1242/dev.121434.

CSR-1 and P Granules Suppress Sperm-Specific Transcription in the C. Elegans Germline

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Free PMC article

CSR-1 and P Granules Suppress Sperm-Specific Transcription in the C. Elegans Germline

Anne C Campbell et al. Development. .
Free PMC article

Abstract

Germ granules (P granules) in C. elegans are required for fertility and function to maintain germ cell identity and pluripotency. Sterility in the absence of P granules is often accompanied by the misexpression of soma-specific proteins and the initiation of somatic differentiation in germ cells. To investigate whether this is caused by the accumulation of somatic transcripts, we performed mRNA-seq on dissected germlines with and without P granules. Strikingly, we found that somatic transcripts do not increase in the young adult germline when P granules are impaired. Instead, we found that impairing P granules causes sperm-specific mRNAs to become highly overexpressed. This includes the accumulation of major sperm protein (MSP) transcripts in germ cells, a phenotype that is suppressed by feminization of the germline. A core component of P granules, the endo-siRNA-binding Argonaute protein CSR-1, has recently been ascribed with the ability to license transcripts for germline expression. However, impairing CSR-1 has very little effect on the accumulation of its mRNA targets. Instead, we found that CSR-1 functions with P granules to prevent MSP and sperm-specific mRNAs from being transcribed in the hermaphrodite germline. These findings suggest that P granules protect germline integrity through two different mechanisms, by (1) preventing the inappropriate expression of somatic proteins at the level of translational regulation, and by (2) functioning with CSR-1 to limit the domain of sperm-specific expression at the level of transcription.

Keywords: C. elegans; CSR-1; Germ granules; Germline; Germline sex determination; P granules; Sperm; Spermatogenesis.

Figures

Fig. 1.
Fig. 1.
Comparison of wild-type and P-granule RNAi expression profiles. (A) Average germline expression of genes following control (black line) and P-granule (pink-red line, running average) RNAi (log scale). Genes are ranked from low to high expression based on the control RNAi dataset. Top box shows position of genes significantly up- or downregulated by P-granule RNAi. *RNAi targets (glh-4, pgl-3 and pgl-1, respectively). (B) Comparison of expression changes following P-granule RNAi in published enriched gene datasets (see supplementary material Table S1). Box, upper and lower quartiles; line, median; dots, mean; bars, s.d.
Fig. 2.
Fig. 2.
Comparison of wild-type and csr-1 RNAi expression profiles. (A) Average germline expression of genes following control (black line) and csr-1 (pink-red line, running average) RNAi (log scale). Genes are ranked from low to high expression, based on the control RNAi dataset. Top box shows position of genes significantly up- or downregulated by csr-1 RNAi. *RNAi target RNAi (csr-1). (B) Comparison of expression changes following csr-1 RNAi in published enriched gene datasets (see supplementary material Table S1). Box, upper and lower quartiles; line, median; dots, mean; bars, s.d.
Fig. 3.
Fig. 3.
Correlation of germline expression changes between P-granule and csr-1-depleted worms. (A) Proportional Venn diagram comparing both set 1 (less stringent) and set 2 (highly stringent) datasets from csr-1 and P-granule RNAi experiments. (B) Correlation plots comparing csr-1 and P-granule RNAi log2 fold changes.
Fig. 4.
Fig. 4.
Distal expansion of msp-3 expression following P-granule and csr-1 RNAi. (A) Representative images show the left gonad arm in fixed whole worms at L4, young adult and day 3 adult stages. Expansion of msp-3 expression is observed following either P-granule or csr-1 RNAi. (B) msp-3 expansion (red) following P-granule or csr-1 RNAi pushes back or suppresses the expression of LIN-41::GFP, an early marker of oocyte development. Red, msp-3 FISH probe; blue, DAPI/DNA; green, PGL-1::GFP in A and LIN-41::GFP in B. Dashed lines indicate domain of msp-3 expression.
Fig. 5.
Fig. 5.
Model for P-granule and CSR-1 function in the hermaphrodite germline. (A) In germ cells, CSR-1 and PGL-1/3 repress alg-3/4 and sperm-specific transcription. As PGL-1/3 begins to disassemble in primary spermatocytes, factors like ALG-3/4 and IFE-1 promote the transcription and translation of sperm-specific mRNAs. When spermatogenesis completes in C. elegans hermaphrodites, P-granule and CSR-1 expression promotes oocyte development and maturation. Dotted lines as previously proposed in Amiri et al. (2001); Conine et al. (2013). (B) P granules and CSR-1 repress her-1, fog-3 and sperm-specific transcription. When P-granule or CSR-1 function is impaired, her-1 transcription is increased, promoting fog-3 expression and sperm transcription.

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