MPK-1 ERK controls membrane organization in C. elegans oogenesis via a sex-determination module

Abstract

Tissues that generate specialized cell types in a production line must coordinate developmental mechanisms with physiological demand, although how this occurs is largely unknown. In the Caenorhabditis elegans hermaphrodite, the developmental sex-determination cascade specifies gamete sex in the distal germline, while physiological sperm signaling activates MPK-1/ERK in the proximal germline to control plasma membrane biogenesis and organization during oogenesis. We discovered repeated utilization of a self-contained negative regulatory module, consisting of NOS-3 translational repressor, FEM-CUL-2 (E3 ubiquitin ligase), and TRA-1 (Gli transcriptional repressor), which acts both in sex determination and in physiological demand control of oogenesis, coordinating these processes. In the distal germline, where MPK-1 is not activated, TRA-1 represses the male fate as NOS-3 functions in translational repression leading to inactivation of the FEM-CUL-2 ubiquitin ligase. In the proximal germline, sperm-dependent physiological MPK-1 activation results in phosphorylation-based inactivation of NOS-3, FEM-CUL-2-mediated degradation of TRA-1 and the promotion of membrane organization during oogenesis.

Figure 1. Summary of adult hermaphrodite germline development and the functions of the NOS-3/FEM-CUL-2/TRA-1 regulatory module

(a) Dissected adult hermaphrodite gonad arm stained for activated MPK-1 (dpMPK-1, red) and germ cell nuclear morphology (DAPI, blue). For all Figures, distal (marked with *) is to the left, proximal to the right. In the distal region, germ cell sexual fate is specified (male/sperm or female/oocyte) while in the medial and proximal germline, activated MPK-1 is necessary for physiological control of the oogenesis production-line, including membrane organization. (b) Schematic representation of the adult hermaphrodite germline, showing a surface view of hexagonally organized cells and their membranes from the distal tip through end pachytene (medial-to-loop), and an internal view of growing diplotene to diakinesis oocytes in the proximal region. Cross-section show cells on the surface of the gonadal tube, each with an interior opening to the Rachis that provides cytoplasm to growing oocytes; the interior surface can be visualized by anilin staining with non-staining windows representing the interior openings. (c) Part of the pathway for germline sex determination is shown, with the NOS-3/FEM-CUL-2/TRA-1 module highlighted in blue. (d) Model for physiological control of membrane organization during oogenesis. The NOS-3/FEM-CUL-2/TRA-1 module is regulated by MPK-1 ERK, active (ON) during ongoing oogenesis in the presence of sperm/MSP signaling and inactive (OFF) in the absence of sperm/MSP signaling when oogenesis is arrested. (See also Figure S6, S8, and S9; Table S1)

Figure 2. NOS-3 partially restores membrane organization of oogeneic pachytene cells in mpk-1 null adult hermaphrodites

Dissected gonads from strains expressing GFP::PH(PLCd), stained with anti-GFP, to visualize cell membranes (green) and DAPI to show nuclear morphology (blue). (a) Wild-type adult hermaphrodite germline, with hexagonally arranged germ cells on the surface from the distal end to the loop and growing oocytes proximally. (b) mpk-1(ga117) null mutant germline with massive disruption of membrane organization in the region of pachytene arrested nuclei. (c) nos-3(oz231); mpk-1(ga117) germline displaying substantial restoration of cell membranes around pachytene arrested nuclei. Insets (b’) pachytene arrested nuclei lacking organized membranes; (c’) organized membranes around individual or multiple pachytene nuclei. Scale bar, 20 microns, for all figures. (See also Figure S1)

Figure 3. NOS-3 is phosphorylated in vitro by active murine ERK2 at Serine 644 and Threonine 648

(a-c) A schematic protein domain structure of NOS-3. Green box marks the ERK docking site and the red box marks the Nanos domain. The arrows indicate the phospho-acceptors. (b) and (c) indicate the two truncated versions of the protein that were tested in in vitro kinase assays. (d) Autoradiograph of kinase assays with full length and the two truncated versions of NOS-3 reveals that the C-terminal domain of NOS-3 is phosphorylated by active murine ERK2 in vitro. (e) An anti-HIS western blot of the proteins tested in (d). (See also Figure S2)

Figure 4. NOS-3 is phosphorylated in vivo at S644 and T648 in an mpk-1 dependent manner

(a) Western blot analysis of pNOS-3 from lysates of wild-type, mpk-1(ga117) and nos-3(oz231) null mutant worms reveals that NOS-3 is phosphorylated on S644 and/or T648 as detected by the phospho-epitope specific antibody; in wild-type a band at about 90kDa is detected, but not in nos-3(0), showing specificity for NOS-3, and not in mpk-1(0), indicating mpk-1 dependence. (b) Western blot analysis of non-pNOS-3 on lysates from wild-type, mpk-1(0) and nos-3(0) mutant worms shows that unphosphorylated NOS-3 accumulates in mpk-1(0) and wild-type worms at about 90kDa. The absence of signal in nos-3(0) indicates specificity of the non-pNOS-3 antibody. (c), (d) and (e) Dissected gonads from wild-type adult hermaphrodites stained for pNOS-3 (c, green), non-p-NOS-3 (d, green) and total NOS-3 (e, green), dpMPK-1 (red) and nuclear morphology (blue). pNOS-3, non-pNOS-3 and total NOS-3 are largely cytoplasmically localized. (c) pNOS-3 accumulation mimics dpMPK-1, detected from mid-pachytene through the end of oogenesis. (d) non-pNOS-3 (green) accumulates in a manner reciprocal to dpMPK-1 (red) and pNOS-3, detected from the distal tip through mid-pachytene. Non-pNOS-3 was not observed in proximal pachytene and in diplotene/ diakinesis oocytes, where residual staining was equivalent to the background observed in nos-3(0) (Figure S3c). (e) Total NOS-3 (green) shows homogeneous accumulation throughout the oogenic germline. (See also Figure S3)

Figure 5. Phospho-mimetic NOS-3 mutant Ser644Glu and Thr648Glu reduces FBF binding in vitro and FEM-3 accumulates during oogenesis and is repressed in the distal germline by NOS-3

(a) Autoradiograph (top) showing binding analysis of recombinant Flag-tagged wild-type, S644/T648 sites mutated to alanine (phospho-null) or S644/T648 sites mutated to glutamic acid (phospho-mimetic) NOS-3 with S³⁵ labeled FBF-1 (Supplemental Methods). Wild-type and phospho-null NOS-3 bind FBF-1 very robustly (lanes 2 and 3), while phospho-mimetic NOS-3 results in a reduction of FBF-1 binding (lane 4) by 14 fold compared to wild-type. In replicate experiments, FBF-1 binding was reduced 17 fold or great than 20 fold in two additional trials (data not shown). Photo (bottom) shows Coommassie Blue staining of the input recombinant NOS-3 proteins used. (b and c) Adult hermaphrodite gonads stained for FEM-3 (green) and dpMPK-1 (red). (b) In wild-type, FEM-3, which is largely cytoplasmic, accumulates from mid-pachytene through the end of oogenesis mimicking that of dpMPK-1. FEM-3 is not detected in the distal germline. (c) In nos-3(oz231), FEM-3 accumulates throughout the germline, consistent with NOS-3 functioning in translational repression of fem-3 in the distal germline of wild-type. Dashed line marks the region of fem-3 translation repression by NOS-3. (See also Figure S4)

Figure 6. TRA-1 nuclear accumulation during MPK-1 activation

Dissected adult hermaphrodite gonads stained for TRA-1 (green) and dpMPK-1 (red). (a) In wild-type, TRA-1 is detected in nuclei (inset a’, arrow) from the distal tip until mid-pachytene, a region that lacks dpMPK-1. From mid-pachytene through proximal pachytene, a region containing high dpMPK-1, nuclear TRA-1 is not detected (inset a”, arrowhead). (b) In mpk-1(ga117), nuclear TRA-1 (arrows) is detected throughout the germline including all proximal pachytene arrested nuclei. (c) In nos-3(oz231) nuclear TRA-1 is not detected from the distal tip through the end of pachytene, although the pattern of MPK-1 activation is normal. (d) In fem-3(e1996) null mated with wild-type males, resulting in the normal pattern of MPK-1 activation and ongoing oogenesis, nuclear TRA-1 (arrows) is observed throughout, including proximal pachytene containing dpMPK-1. (See also Figure S5)

Figure 7. Phospho-mimetic NOS-3 mutant Ser644Glu and Thr648Glu is unable to mediate translational repression of fem-3 mRNA in vivo

Dissected gonads from nos-3(oz231) strains containing wild-type or mutant NOS-3 low-copy integrated transgenes stained for FEM-3 (green), FLAG-tagged NOS-3 (red) and nuclear morphology (blue)(also see Figure S5). (a) The wild-type NOS-3 transgene (ozIs27) rescues the nos-3(0) background restoring distal repression of fem-3 mRNA translation. (b) The NOS-3 transgene containing the phospho-mimetic site changes (S644/T648>E; ozIs29) results in FEM-3 accumulation throughout the germline, indicating that phospho-mimetic NOS-3 is unable to translationally repress fem-3 mRNA. (c) The NOS-3 transgene containing the non-phosphorylatable site changes (S644/T438>A; ozIs28) fails to accumulate FEM-3 throughout the germline, consistent with continuous translational repression of fem-3 mRNA by this mutant form of NOS-3. For all three transgenes, driven by the nos-3 promoter, FLAG::NOS-3 accumulates throughout the germline similar to endogenous NOS-3 and the level of protein accumulation is similar (Figure 4e, data not shown).