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. 2017 Jul 11;9(1):342-354.
doi: 10.1016/j.stemcr.2017.05.034. Epub 2017 Jun 29.

Redox Homeostasis Plays Important Roles in the Maintenance of the Drosophila Testis Germline Stem Cells

Sharon Wui Sing Tan  1 , Qian Ying Lee  1 , Belinda Shu Ee Wong  1 , Yu Cai  2 , Gyeong Hun Baeg  3
Affiliations

Redox Homeostasis Plays Important Roles in the Maintenance of the Drosophila Testis Germline Stem Cells

Sharon Wui Sing Tan et al. Stem Cell Reports. .

Abstract

Oxidative stress influences stem cell behavior by promoting the differentiation, proliferation, or apoptosis of stem cells. Thus, characterizing the effects of reactive oxygen species (ROS) on stem cell behavior provides insights into the significance of redox homeostasis in stem cell-associated diseases and efficient stem cell expansion for cellular therapies. We utilized the Drosophila testis as an in vivo model to examine the effects of ROS on germline stem cell (GSC) maintenance. High levels of ROS induced by alteration in Keap1/Nrf2 activity decreased GSC number by promoting precocious GSC differentiation. Notably, high ROS enhanced the transcription of the EGFR ligand spitz and the expression of phospho-Erk1/2, suggesting that high ROS-mediated GSC differentiation is through EGFR signaling. By contrast, testes with low ROS caused by Keap1 inhibition or antioxidant treatment showed an overgrowth of GSC-like cells. These findings suggest that redox homeostasis regulated by Keap1/Nrf2 signaling plays important roles in GSC maintenance.

Keywords: Drosophila; EGFR signaling; Keap1/Nrf2 signaling; antioxidant; differentiation; germline stem cell; redox homeostasis.

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Figures

Figure 1
Figure 1
Paraquat Treatment Induces ROS in the Drosophila Testis (A) Schematic of the Drosophila testis. GSC, germline stem cell; CySC, cyst stem cell. (B–D″) ROS detection assay (B) shows an increase in ROS upon paraquat treatment. Error bar denotes SEM from three independent experiments; Student's t test (∗∗p < 0.01). ROS levels were monitored by using DHE probe in (C–C″) control testis and (D–D″) treated testis. (E–F′) ROS levels were monitored by using the in vivo ROS reporter GstD1-GFP in (E and E′) control testis and (F and F′) treated testis. (G and H) TUNEL assay shows that paraquat treatment does not induce cell death. Asterisks in images indicate hub cells. Scale bars, 10 μm.
Figure 2
Figure 2
High Levels of ROS Decrease GSC Number (A–B′) Control (A and A′) and paraquat-treated (B and B′) testes stained with FasIII and Vasa. (C) Paraquat-treated testes show a decrease in GSC number. (D and E) Control (D) and treated (E) testes stained with Bam. Solid lines indicate the distance between hub cells and differentiating germ cells. (F) Quantification of the distance between hub cells and Bam-positive germ cells. (G–H′) Control (G and G′) and treated (H and H′) testes stained with Spectrin. Arrows indicate branching fusomes. Branching fusomes are found near hub cells in treated testis. (I) Quantification of spectrosomes observed in control and treated testes. (J and K) Control (J) and treated (K) testes stained with pH3, a mitotic marker. Arrows indicate pH3-positive GSCs. (L) Treated testes show a decrease in pH3-positive GSCs. Error bars in bar graphs denote SEM from three independent experiments; Student's t test (∗∗p < 0.01, ∗∗∗p < 0.001). Asterisks in images indicate hub cells. Scale bars, 10 μm.
Figure 3
Figure 3
Keap1/Nrf2 Activity Affects GSC Homeostasis by Modulating ROS Levels (A–C″) Control testes (A–A″), cncCRNAi-expressing testes (B–B″), and Keap1-expressing testes (C–C″). (A, B, C) DHE staining. Dotted area marks the apical tip of testis in which early-stage germ and cyst cells reside. (A′, B′, C′) Bam staining. Dashed lines indicate the distance between hub cells and differentiating germ cells. (A″, B″, C″) Spectrin staining. Arrows indicate branching fusomes. (D) Testes overexpressing Keap1 or cncCRNAi show a decrease in GSC number compared with control. (E) Distance between hub cells and Bam-positive germ cells. (F) Control FRT clones. Arrow indicates a GFP-negative germ cell. (G) cncC mutant clones. Arrow indicates cncC mutant clone showing higher DHE staining than neighboring cells. (H) Control FRT clones. Arrow indicates a GFP-negative GSC. (I) cncC mutant clones. cncC mutant GSC is not observed, but cncC mutant germ cell clone is detected (dotted area). (J) Clonal analysis suggests that CncC activity is required for GSCs to maintain their stemness. Error bars in (D) and (E) denote SEM from three independent experiments; Student's t test (∗∗∗p < 0.001). Asterisks in images indicate hub cells. Scale bars, 10 μm.
Figure 4
Figure 4
Redox Status Influences GSC Homeostasis (A–H) Bam-positive germ cells in testes expressing ND42RNAi or ND75RNAi (A–C) are detected closer to hub cells compared with those in control testes. Removing one copy of (D) cncC or (E) sod2 further enhances ND75RNAi phenotype. GSC differentiation induced by ND75 knockdown is suppressed by co-expression of (F) CncC, (G) Sod2, or (H) Catalase. Dashed lines indicate the distance between hub cells and differentiating germ cells. Asterisks indicate hub cells. Scale bars, 10 μm. (I) Distance between hub cells and Bam-positive germ cells in testes with different genotypes. Error bar is SEM from three independent experiments; Student's t test (∗∗p < 0.01 and ∗∗∗p < 0.001).
Figure 5
Figure 5
EGFR Signaling Is Involved in High ROS-Mediated GSC Differentiation (A–C′) Control testis (A and A′), cncCRNAi-expressing testes (B and B′), and Keap1-expressing testes (C and C′). Testes with excessive ROS show an increased phospho-Erk1/2 expression compared with control. Arrows indicate p-Erk1/2-positive cyst cells. (D and E) High levels of ROS increase phospho-Erk1/2 levels. (F and G) Control (F) and ND75RNAi (G) testes. ND75 inhibition promotes GSC differentiation. (H and I) ND75RNAi phenotype is suppressed by removing one copy of Egfr or stet. (J) Distance between hub cells and Bam-positive germ cells in testes with different genotypes. (K) High ROS enhance the transcription of spitz. (L–M′) Control (L and L′) and ND75RNAi (M and M′) testes. spitz promoter-driven lacZ expression is highly detected in early-stage germ cells (arrows) in ND75RNAi testis. Error bars in graphs denote SEM from three independent experiments; Student's t test (∗p < 0.05, ∗∗p < 0.01, and ∗∗∗p < 0.001). Asterisks in images indicate hub cells, and dashed lines indicate the distance between hub cells and differentiating germ cells. Scale bars, 10 μm.
Figure 6
Figure 6
Low Levels of ROS Facilitate GSC Growth Control testes heterozygous for keap1 (A, A′, A″, C, C′, E, E′, H, H′, J, J′); testes heterozygous for keap1 that further express keap1RNAi under the control of nos-Gal4 (B, B′, B″, D, D′, F, F′, I, I′, K, K′). (B–B″) Reduced ROS cause enlarged testes containing excessive DAPI-positive cells, indicative of active cell proliferation. Keap1 inhibition results in a drastic increase in the number of (D and D′) Tj-positive cyst cells, (F, F′, G) Zfh1-positive early-stage cyst cells, (I and I′) pH3-postive germ cells, and (K, K′, L) esg-lacZ-positive early-stage germ cells. (M and N) Treatment of the antioxidants glutathione or vitamin C causes enlarged testes. Error bars in (G) and (L) denote SEM from three independent experiments; Student's t test (∗∗∗p < 0.001). Asterisks in images indicate hub cells. Scale bars, 100 μm (A″, B″) and 10 μm (C′, D′, E′, F′, H′, I′, J′, K′, M, N).
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