Epigenetic regulation of stem cell maintenance in the Drosophila testis via the nucleosome-remodeling factor NURF

Abstract

Regulation of stem cells depends on both tissue-specific transcriptional regulators and changes in chromatin organization, yet the coordination of these events in endogenous niches is poorly understood. In the Drosophila testis, local JAK-STAT signaling maintains germline and somatic stem cells (GSCs and cyst progenitor cells, or CPCs) in a single niche. Here we show that epigenetic regulation via the nucleosome-remodeling factor (NURF) complex ensures GSC and CPC maintenance by positively regulating JAK-STAT signaling, thereby preventing premature differentiation. Conversely, NURF is not required in early differentiating daughter cells of either lineage. Because three additional ATP-dependent chromatin remodelers (ACF, CHRAC, and dMi-2/NuRD) are dispensable for stem cell maintenance in the testis, epigenetic regulation of stem cells within this niche may rely primarily on NURF. Thus, local signals cooperate with specific chromatin-remodeling complexes in intact niches to coordinately regulate a common set of target genes to prevent premature stem cell differentiation.

Figure 1. Nurf301 is a unique member of the NURF chromatin remodeling complex and is required for GSC maintenance in the Drosophila testis stem cell niche

(A) The Drosophila testis apex. Approximately 10 GSCs make broad contact with the hub. GSCs divide asymmetrically to produce gonialblast (GB) daughters that are displaced away from the hub. GBs undergo four rounds of mitosis giving rise to 16-interconnected spermatogonia, which further differentiate into spermatocytes. Approximately two CPCs flank each GSC and contact the hub through thin cytoplasmic extensions. CPCs divide asymmetrically, giving rise to non-mitotic somatic cyst cells that encyst differentiating germ cells. (B) NURF is one of three ISWI-containing ATP-dependent chromatin remodeling complexes in Drosophila. The Nurf301 subunit is unique to the NURF complex and is essential for its function. ACF and CHRAC are distinct from NURF and share a common subunit: ACF1. (C–D) Confocal section of a wild-type testis apex immunostained with germline specific marker anti-Vasa (red), the DNA counterstain DAPI (blue), and (C) anti-ISWI (green, inset) or (D) anti-Nurf301 (green, inset). (C) ISWI is expressed in GSCs (Vasa-positive and contacting the hub, one indicated, arrowhead), CPCs (Vasa-negative and within two cell diameters of the hub, one indicated, arrow) and the hub (*). (D) Nurf301 is expressed in all cell types in the testis apex including GSCs (arrowhead), CPCs (arrow), and the hub (*). (E–F) Testes immunostained with anti-Vasa (red), the hub marker anti-Armadillo (green), the fusome marker anti-1B1 (green) and DAPI (blue). (E) A wild-type testis with a rosette of GSCs surrounding the hub (*). GSCs contain spherical fusomes (green, arrowhead) whereas differentiating spermatogonia contain elongated and branched fusomes (open arrowhead). (F) Projection of three confocal sections of a testis with reduced Nurf301 function (nurf301⁴/nurf301¹²) showing differentiating spermatogonia (black outline) containing a branched fusome (open arrowhead) and contacting the hub (white outline). Bars, 10 microns.

Figure 2. Nurf301 is required cell-autonomously for GSC maintenance. (A–B)

Larval testes immunostained with anti-Vasa (red), anti-Armadillo (green), anti-1B1 (green), and DAPI (blue). (A) Larval testes heterozygous for nurf301 (nurf301² / +) have a wild-type number of GSCs (one indicated, arrowhead) surrounding the hub (*). Numerous differentiating daughters (outlined) are displaced away from the hub. (B) Larval testes homozygous mutant for nurf301 (nurf301² / nurf301³) have significantly fewer GSCs (one indicated, arrowhead) surrounding the hub (*) and a smaller zone of differentiating spermatogonia (outlined). (C–F) Confocal sections of testes immunostained with anti-GFP (green, inset) and anti-Vasa (red); hubs denoted with an asterisk. (C–D) Wild-type GSC clones (arrowheads, outlined) and their differentiating daughters (outlined), identified by the absence of GFP and the presence of Vasa, are detected (C) 4 days and (D) 8 days ACI. (E) nurf301 mutant GSC clones are found at the hub 4 days ACI, but are no longer detected 8 days ACI (F). (G) Testis immunostained with anti-β-galactosidase (red) and a differentiation marker bam-GFP (green). bam-GFP is not normally expressed in GSCs (yellow outline) but is detected in differentiating spermatogonia (white outline). Two nurf301 mutant GSCs (β-galactosidase-negative, white outline, arrowheads) surround the hub (*). One nurf301 GSC expresses bam-GFP (outlined, open arrowhead) while the other is bam-GFP-negative (outlined, arrowhead), indicating that nurf301 null GSCs differentiate prematurely in the niche. (G') bam-GFP (green) channel alone. Bars, 10 microns.

Figure 3. Nurf301 is required cell-autonomously for CPC maintenance. (A-D)

Confocal sections of testes immunostained with anti-GFP (green), anti-Vasa (red), and DAPI (blue) containing positively-marked wild-type (A, B) or nurf301 mutant (C, D) CPC clones (arrow) and their differentiating daughters (open arrowhead) identified by the presence of GFP, absence of Vasa, and their position within the tissue. (A–B) Wild-type CPC clones and their daughters are detected at (A) 4 and (B) 10 days ACI. nurf301 mutant CPC clones are detected 4 days ACI (C), but are not maintained at 10 days ACI (D). Differentiating mutant CPC daughters are occasionally still found 10 days ACI (open arrowhead). Inset; higher magnification and adjacent confocal section of the boxed area in D showing a mutant CPC daughter cell encysting a differentiating germline cell. (E) Restoring STAT92E to nurf301 null CPCs partially rescues their stem cell loss phenotype. Bars, 10 microns.

Figure 4. NURF is the sole ISWI-containing chromatin remodeling complex required for stem cell maintenance in the Drosophila testis

(A–B) Testes immunostained with anti-ISWI (green), anti-Vasa (red), and DAPI (blue). An asterisk denotes the hub. (A) In wild-type testes, ISWI is expressed at comparable levels in CPCs (arrows) and GSCs (arrowhead). (B) ISWI-RNAi knockdown in CPCs and their daughters reduces ISWI expression in CPCs (arrows) compared to GSCs (arrowhead). (C–D) Testes immunostained with anti-Vasa (red), CPC marker anti-Zfh-1 (green), and DAPI (blue) expressing (C) GFP-RNAi or (D) ISWI-RNAi in CPCs (arrows) and their daughters. When compared to controls (C), a reduction in ISWI leads to fewer CPCs around the hub (*) (D). (E) acf1 homozygous mutant testis immunostained with anti-Vasa (red), anti-Armadillo (green), anti-1B1 (green), and DAPI (blue) containing a wild-type number of GSCs (one indicated, arrowhead) surrounding the hub (*). (F) acf1 homozygous mutant testis immunostained with anti-Vasa (red), anti-Zfh-1 (green), and DAPI (blue) containing a wild-type number of CPCs (one indicated, arrow) surrounding the hub (*). (G) ISWI-RNAi knockdown in CPCs and their daughters leads to a significant decrease in CPCs [n.s. (not significant), p-value > 0.05; *p-value < 0.001]. (H) acf1 homozygous mutant testes have the same number of GSCs and CPCs as wild-type (n.s., p-value > 0.05). Data are represented as mean +/− SEM. Bars, 10 microns.

Figure 5. NURF regulates JAK-STAT signaling to maintain GSCs and CPCs in the Drosophila testis stem cell niche

(A) nurf301 heterozygous testis immunostained with anti-Vasa (red), anti-dSTAT92E (green), and DAPI (blue). STAT92E is expressed primarily in GSCs (arrowhead) surrounding the hub (*), whereas gonialblasts have reduced STAT92E levels (open arrowhead). (A') STAT92E channel alone. (B) Testis immunostained with anti-Vasa (magenta), anti-GFP (green), anti-dSTAT92E (red), and DAPI (blue) containing a single negatively marked nurf301² homozygous mutant GSC (arrow) indicated by the presence of Vasa and absence of GFP, 4 days ACI. The homozygous nurf301 GSC has reduced levels of STAT92E protein compared to the wild-type neighboring GSCs (one indicated, arrowhead) but similar levels to a neighboring wild-type gonialblast (open arrowhead). An asterisk denotes the hub. (B') STAT92E channel alone. (C–D) Testes immunostained with anti-Vasa (red), anti-Armadillo (green), anti-1B1 (green), and DAPI (blue). (C) socs36E^PZ1647 homozygous mutant testes have a reduced number of GSCs (arrowhead) surrounding the hub (*). (D) Testes homozygous for socs36E^PZ1647 and heterozygous for nurf301³ have a near wild-type number of GSCs (one indicated, arrowhead) surrounding the hub (*). (E) nurf301 null GSCs have significantly less dSTAT92E staining compared to neighboring wild-type GSCs (*, p-value < 0.05). (F) Removing one copy of stat92E or nurf301 partially rescues the socs36E^PZ1647 GSC loss phenotype (*, p-value < 0.001). Data are represented as mean +/− SEM. Bars, 10 microns.