BLMP-1/Blimp-1 Regulates the Spatiotemporal Cell Migration Pattern in C. elegans

Spatiotemporal regulation of cell migration is crucial for animal development and organogenesis. Compared to spatial signals, little is known about temporal signals and the mechanisms integrating the two. In the Caenorhabditis elegans hermaphrodite, the stereotyped migration pattern of two somatic distal tip cells (DTCs) is responsible for shaping the gonad. Guidance receptor UNC-5 is necessary for the dorsalward migration of DTCs. We found that BLMP-1, similar to the mammalian zinc finger transcription repressor Blimp-1/PRDI-BF1, prevents precocious dorsalward turning by inhibiting precocious unc-5 transcription and is only expressed in DTCs before they make the dorsalward turn. Constitutive expression of blmp-1 when BLMP-1 would normally disappear delays unc-5 transcription and causes turn retardation, demonstrating the functional significance of blmp-1 down-regulation. Correct timing of BLMP-1 down-regulation is redundantly regulated by heterochronic genes daf-12, lin-29, and dre-1, which regulate the temporal fates of various tissues. DAF-12, a steroid hormone receptor, and LIN-29, a zinc finger transcription factor, repress blmp-1 transcription, while DRE-1, the F-Box protein of an SCF ubiquitin ligase complex, binds to BLMP-1 and promotes its degradation. We have therefore identified a gene circuit that integrates the temporal and spatial signals and coordinates with overall development of the organism to direct cell migration during organogenesis. The tumor suppressor gene product FBXO11 (human DRE-1 ortholog) also binds to PRDI-BF1 in human cell cultures. Our data suggest evolutionary conservation of these interactions and underscore the importance of DRE-1/FBXO11-mediated BLMP-1/PRDI-BF1 degradation in cellular state transitions during metazoan development.


Introduction
Cell migration is important for organogenesis and development of animals. Numerous extracellular guidance cues and receptors for the spatial control of cell migration have been identified and characterized [1]. However, little is known about the temporal regulation of cell migration and how the spatial and temporal signals are coordinated to generate a specific and reproducible pattern of cell migration during development.
The bilobed gonad of C. elegans hermaphrodites develops from a four-cell primordium positioned in the ventral midbody [2]. The shape of the two symmetrical U-shaped gonadal arms is determined by the migratory paths of the two distal tip cells (DTC), leader cells found at the tip of each arm [3]. The DTCs undergo three sequential phases of migration and re-orient twice during the three larval developmental stages, thus providing a paradigm for the study of the spatio-temporal regulation of cell migration in vivo [2,3]. In phase I during the L2 and early L3 stages, the anterior and posterior DTCs move centrifugally along the ventral body wall muscles towards the head or tail, respectively ( Figure 1A). In phase II, they turn 90 degrees and move from the ventral to the dorsal muscles, then, during phase III, they again turn 90 degrees and move centripetally along the dorsal body wall muscles and halt in the mid-body. Both orthogonal turns occur during the late L3 stage. The timing of these turns is regulated by a redundant function of the heterochronic genes daf-12, dre-1, and lin-29 [4]. A single mutation in any of these three genes has no effect on DTC migration, but mutation of 2 or 3 of the genes delays the L3-specific DTC migration pattern, which fails to take place even in L4 or the adult. lin-29, daf-12, and dre-1 encode, respectively, a zinc-finger transcription factor, a steroid hormone receptor similar to the vertebrate vitamin D and liver-X receptor, and an F-Box protein of an SCF ubiquitin ligase complex [4][5][6], indicating that a complex mechanism, involving steroid hormone signaling, gene transcription, and protein degradation, is responsible for the temporal control of the dorsal turn. However, how these three genes function to do so is unclear.
The dorsal migration of DTCs is regulated by the guidance receptors UNC-5 and UNC-40 (a homolog of Deleted in Colorectal Cancer) [7][8][9], which drive DTCs to move away from the ventrally localized UNC-6 to the dorsal side [10,11]. Dorsally localized UNC-129/TGF-b also promotes DTC dorsal migration through UNC-5 and UNC-40 receptors [12]. Mutations in these genes disrupt the ventral-to-dorsal migration of DTCs. unc-40 appears to be transcribed in the DTCs throughout their migration [7], whereas unc-5 is transcriptionally up-regulated at the time when the dorsal turn is initiated [13]. Precocious expression of unc-5 cDNA at the time when DTCs have not yet turned dorsalward induces unc-6-dependent precocious dorsal migration [13]. These data show that unc-5 is both necessary and sufficient for DTC dorsal migration and that the increase in unc-5 transcription is responsible, at least in part, for the initiation of DTC dorsal migration [8,9,13]. However, how unc-5 is temporally regulated to direct dorsal migration precisely at the late L3 stage is unclear.
In this study, we performed genetic screening for mutants defective in DTC migration and isolated blmp-1 mutants. Our results showed that blmp-1 is a heterochronic gene that acts with daf-12, dre-1, and lin-29 in a regulatory circuit to control the correct timing of DTC migration. We also showed that this regulatory circuit is, at least in part, conserved in C. elegans and humans.

blmp-1 mutants show defective DTC migration
To identify genes that are important for the spatiotemporal regulation of DTC migration, we performed genetic screening for mutants defective in DTC migration, as described in the Materials and Methods, and isolated alleles tp5 and tk41. A genetic complementation test showed that tp5 and tk41 were allelic, and that either allele failed to complement the previously identified mutation dpy-24(s71) (dpy stands for dumpy, shorter than wildtype) [14].
The blmp-1 gene encodes a protein with 27% identity to mouse B lymphocyte-induced maturation protein 1 (Blimp-1) and 26% identity to human positive regulatory domain I-binding factor (PRDI-BF1) ( Figure S2). Both Blimp-1 and PRDI-BF1 are thought to act predominantly as transcription repressors and are essential for the terminal differentiation of B and T cells [17,18]. As shown in Figure 2, BLMP-1, like Blimp-1 and PRDI-BF1, is predicted to contain a positive regulatory (PR) domain, a nuclear localization signal (NLS), and five Kruppel-type [(Cys)2-(His)2] zinc fingers. The zinc fingers of both Blimp-1 and PRDI-BF1 have been shown to bind to target DNA and are essential for their transcriptional repression activities [19][20][21].
Alleles s71, tk41, and tp5 have, respectively, a non-sense mutation in codon 281, 381, or 434, and are predicted to encode truncated proteins without zinc fingers (Figure 2 and S2). The deletion allele tm548, which was isolated by a reverse genetic approach (National Bioresource Project), has an 810 bp deletion, removing part of exon 3 and part of intron 3 (Figure 2 and S2) and may result in a truncated BLMP-1 protein containing the first 254 amino acids of BLMP-1 and 17 amino acids encoded by the third intron.

DTCs in blmp-1 mutants undergo a precocious dorsal turn
The four blmp-1 mutants, s71, tk41, tp5, and tm548, were found to have a similar set of defects, including a DTC migration abnormality (shown for blmp-1(s71) in Figure 1), a weak dumpy phenotype (shown for blmp-1(s71) in Figure S3A), and a partially penetrant embryonic lethality (shown for all four in Figure S3B). Wild-type blmp-1 genomic DNA rescued the dumpy phenotype and embryonic lethality of the blmp-1(s71) mutant ( Figure S3B), showing that these defects were caused by the loss of blmp-1.
The DTC migration patterns of these mutants were varied, but shared the common feature that the mutant DTCs had a shorter centrifugal phase I migration path and executed the dorsal turn at a point closer to the mid-body than wild-type DTCs (Figure 1B-G,  Table S1), suggesting either slower movement and/or precocious initiation of the dorsal turn. However, we timed the movement of the DTCs during phase I migration and found that the blmp-1 mutant DTCs did not migrate significantly slower than the wildtype DTCs (Table S2). In addition, some DTCs migrated obliquely with respect to the dorsal-ventral axis until they reached the dorsal muscle ( Figure 1D); such a migratory route is probably due to the simultaneous execution of centrifugal phase I and dorsal phase II migrations, support for a precocious execution of the dorsal turn at the time when phase I migration normally occurs.
To examine whether the abnormal DTC migration pattern of the blmp-1 mutants was indeed caused by a precocious dorsal turn, we performed a time-course analysis of DTC migration in the wild-type and blmp-1(s71) mutant, using the division stages of the vulval precursor cell P6.p as temporal developmental markers, as described previously [22]. P6.p is generated in mid L1, undergoes three rounds of cell division during L3, and gives rise to eight descendants that constitute the vulva [23]. Figure 3A shows

Author Summary
The migratory path of DTCs determines the shape of the C. elegans gonad. How the spatiotemporal migration pattern is regulated is not clear. We identified a conserved transcription factor BLMP-1 as a central component of a gene regulatory circuit required for the spatiotemporal control of DTC migration. BLMP-1 levels regulate the timing of the DTC dorsal turn, as high levels delay the turn and low levels result in an early turn. We identify and characterize upstream regulators that control BLMP-1 levels. These regulators function in two ways, i.e. by destabilization of BLMP-1 through ubiquitin-mediated proteolysis and by transcriptional repression of the blmp-1 gene to down-regulate BLMP-1. Interestingly, blmp-1 also negatively controls these regulators. Our data suggest that a dietary signal input acts together with a double-negative feedback loop to switch DTCs from the ''blmp-1-on'' to the ''blmp-1-off'' state, promoting their dorsal turn. Furthermore, we show that some protein interactions in the circuit are conserved in C. elegans and humans. Our work defines a novel function of the conserved blmp-1 gene in the temporal control of cell migration, and establishes a gene regulatory circuit that integrates the temporal and spatial inputs to direct cell migration during organogenesis.
representative DIC images of wild-type (a, b, e, f) and blmp-1(s71) (c, d g, h) posterior gonadal arms in early L3 (top panels) and late L3 (bottom panels). Figure 3B shows that, in the wild-type, the DTCs in more than 90% of worms underwent ventral-to-dorsal migration at the four-P6.p cell stage and the DTCs in none made a dorsal turn before P6.p cell division, whereas, in the s71 mutant, the anterior DTCs in 36% of worms and the posterior DTCs in 66% of worms had turned dorsalward before P6.p divided. These results demonstrate that loss of blmp-1 causes a precocious initiation of DTC dorsal migration (early L3, rather than late L3) and that blmp-1 functions to prevent a precocious DTC dorsal turn. The developmental stage in which each migration phase occurs is indicated on the right. The solid line and arrow show, respectively, the path and direction of each migration phase. (B-F) DIC images of adult wild-type (B) and blmp-1(s71) (C-F) gonads. The black line shows the migratory path, starting from the asterisk, the DTC is indicated by the black arrowhead, and the white arrowhead indicates the position at which the DTC initiated dorsal phase II migration. In (C-F), the DTC had a shorter phase I migratory path (distance between the asterisk and white arrowhead) than the wildtype DTC (B); in addition, the DTC in (C) had a longer phase III migration path, that in (D) had a slanted phase II migration path (see text for details), and that in (E) moved in the opposite direction during phase III migration. (F) The DTC had a similar migratory trajectory to that shown in (E), except that it failed to stay on the dorsal side during phase III migration. The gonadal arm shown in B-E is posterior, while that in F is anterior, as the defect was only observed in this arm, as shown in Table S1. The picture in F was flipped 180 degrees so that its migration trajectory could be easily compared to those in B-E. Scale bar 40 mm. (G) Percentage of the indicated blmp-1 mutants and transgenic worms with a DTC migration defect (shown in Figure 1C BLMP-1 is present in the DTC before the dorsal turn To determine the localization of BLMP-1 and explore its function, we raised polyclonal antibodies against bacterially expressed recombinant BLMP-1 (Materials and Methods) and used the affinity-purified antibodies to stain whole-mount animals. The results showed that BLMP-1 was localized in the nucleus and was detected in hypodermal, vulval, and intestinal cells ( Figure 4A), as well as DTCs ( Figure 4B). We next stained the mutant embryos with the blmp-1 mutations (s71, tm548, tp5 and tk71), which are loss-of-function recessive alleles by genetic tests (Materials and Methods). Little or no signals were detected in these mutant embryos as shown in representative images in Figure 4C, demonstrating the specificity of the antibodies. Notably, BLMP-1 was seen in DTCs prior to the mid L3 larval stage (two P6.pdescent cells), but not during, or after, mid L3 stage, after the DTCs had undergone the dorsal turn ( Figure 4B). This result and the precocious dorsal turn phenotype of the blmp-1 mutant support a model in which BLMP-1 functions in DTCs before mid L3 stage to prevent these cells from undergoing a precocious dorsal turn.
The localization of BLMP-1 in DTCs supports the model that BLMP-1 controls the timing of DTC dorsal migration in a cellautonomous fashion. We then tested whether expression of blmp-1 cDNA in DTCs under the control of the lag-2 promoter P lag-2 is sufficient to rescue the DTC migration defect of the blmp-1 mutant. P lag-2 drives gene expression in DTCs, but not in body wall muscle or hypodermis, the structures on which DTCs migrate [24]. In blmp-1 mutants carrying the P lag-2 blmp-1 transgene, the percentage of worms with abnormal anterior or posterior DTC migration was reduced, respectively, from 82% to 20% and from 93% to 23% ( Figure 1G), similar levels to those seen when blmp-1 cDNA was expressed under the control of the blmp-1 endogenous promoter P blmp-1 (40% or 19% for the anterior and posterior DTC, respectively; Figure 1G). This result further supports a cellautonomous role of blmp-1 in DTC migration. Table 1. Genetic interactions between blmp-1 and the heterochronic genes lin-29, dre-1, and daf-12 in the timing of the DTC dorsal turn.

Genotype
Dorsal turn a (%) Wild-type Premature Retarded blmp-1(s71)   blmp-1 represses unc-5 transcription to prevent the DTCs from dorsalward turning Next, we tested whether blmp-1 may prevent dorsalward turning of DTCs by regulating a dorsal-ventral guidance system at the early larval stage. The dorsal migration of DTCs is regulated by the guidance receptors UNC-5 and UNC-40 (a homolog of Deleted in Colorectal Cancer) [7][8][9], which drive DTCs to move away from the ventrally localized UNC-6 [10][11]. The observations that blmp-1, unc-5 and unc-40 function cell-autonomously in the control of DTC migration [7,9] ( Figure 1G) suggest that blmp-1 may prevent DTC precocious dorsalward turning by regulating unc-5 and/or unc-40. unc-40 appears to be transcribed in the DTCs throughout their migration [7], whereas unc-5 is transcriptionally up-regulated at the time when the dorsal turn is initiated [13]. Using the transgene P unc-5(1 kb) gfp, in which an approximately 1 kb sequence upstream of the unc-5 coding sequence was used to drive the GFP reporter, we confirmed this unc-5 expression pattern, i.e. unc-5 was expressed during and after, but not before, the DTC dorsal turn ( Figure S4). Interestingly, this temporal pattern of unc-5 transcription is complementary to that of BLMP-1 expression in DTCs, as BLMP-1 was present in DTCs only before the dorsal turn ( Figure 4B). This raised the possibility that BLMP-1 inhibits unc-5 transcription and thus prevents DTCs from turning dorsalward. If this were the case, the correct timing of the disappearance of BLMP-1 from DTCs would alleviate this transcriptional inhibition and allow unc-5 transcription and the DTC dorsal turn. We tested this hypothesis by altering the temporal expression pattern of blmp-1 and examining their effects on unc-5 transcription.
At the early L3 stage when the P6.p cell has not yet divided, no wildtype worms contained DTCs that had turned dorsalward or showed any expression of the P unc-5(1 kb) ::gfp transgene (Figure 5Ba), whereas, in some blmp-1(s71) mutants, DTCs at the same stage had performed the dorsal turn and displayed precocious unc-5 transcription (Figure 5Bb). Thus, loss of blmp-1 causes precocious unc-5 expression, which coincides with the precocious dorsalward turning of DTCs.
Next, we tested whether the blmp-1 precocious dorsal turn phenotype required unc-5 by examining and comparing the DTC dorsal migration patterns of the blmp-1 and unc-5 single mutants and the blmp-1; unc-5 double mutant. Approximately 48% anterior DTCs and 83% posterior DTCs failed to turn dorsalward in the unc-5(e53) mutant (Table S3). No precocious dorsal turn was observed in the blmp-1 (s71); unc-5 double mutant, showing that the unc-5(e53) mutation blocked the precocious dorsal turn phenotype of the blmp-1 (s71) mutant and that the blmp-1 precocious dorsal turn phenotype required unc-5. Constitutive expression of blmp-1 delays dorsal turn and inhibits unc-5 expression Because BLMP-1 is present in the DTC before the dorsal turn, we examined the significance of blmp-1 down-regulation and its effect on the initiation of DTC dorsalward turning. To this end, we overexpressed blmp-1 using the transgene P blmp-1 ::blmp-1::gfp, in which the fusion protein BLMP-1::GFP was expressed under the control of the promoter P blmp-1 . In the resulting transgenic line, 10% of transgenic worms had DTCs that displayed normal BLMP-1::GFP down-regulation, underwent a normal dorsal turn ( Figure 5A, a, b), whereas 90% of transgenic worms showed persistent BLMP-1::GFP expression, even at the L4 stage, showing that BLMP-1::GFP was not appropriately down-regulated in these worms, and these worms showed no sign of dorsal turn ( Figure 5A, d, e). This retarded turn phenotype was in contrast to the precocious dorsal migration phenotype caused by loss of blmp-1 ( Figures 1C-G). Collectively, these results show that the BLMP-1 level is important regulation for DTC dorsal migration and that the timely disappearance of BLMP-1 allows the DTCs to turn dorsalward, hence switching their migration phase from centrifugal phase I migration to ventral-to-dorsal dorsal phase II migration.
To test whether blmp-1 overexpression might repress unc-5 expression in DTCs and thus resulted in the retarded turning phenotype, we expressed the transgene P blmp-1 ::blmp-1::gfp in worms carrying the P unc-5(1 kb) ::mCherry reporter. In the resulting transgenic line, 10% of transgenic worms had DTCs that displayed normal BLMP-1::GFP down-regulation, underwent a normal dorsal turn, and showed P unc-5(1 kb) ::mCherry expression ( Figure 5A, a-c), whereas 90% of transgenic worms showed persistent BLMP-1::GFP expression, and these worms showed no sign of dorsal turn or unc-5 expression ( Figure 5A, d-f). Thus, constitutive blmp-1 expression in the late larval stage represses unc-5 transcription and blocks the DTC dorsal turn. These results confirmed the causal relationship of the reciprocal patterns of BLMP-1 expression and unc-5 transcription in DTCs and support the model in which BLMP-1 inhibits unc-5 transcription and thus prevents DTCs from turning dorsalward during early larval development.
Using chromatin immunoprecipitation coupled with highthroughput DNA sequencing (ChIP-seq), the modENCODE Consortium has shown that BLMP-1 binds to the upstream sequence of lin-29, but not that of daf-12 or dre-1, at the early larval stage [26]. This suggested that BLMP-1 might negatively regulate lin-29 expression. To test this possibility, we analyzed and compared lin-29 transcription in the wild-type and blmp-1 mutants using the transcriptional reporter P lin-29 ::gfp, in which gfp was expressed under the control of the lin-29 promoter P lin-29 ( Figure 5Ca). We confirm that the transcription of lin-29 starts at approximately mid L3 stage and continues during L4 [4,27]. We found that knockdown of blmp-1 using RNAi resulted in precocious expression of P lin-29 ::gfp at the L2 stage (Figure 5Cb,c), showing that blmp-1 represses lin-29 transcription at the L2 stage.
To investigate the significance of this blmp-1-mediated lin-29 down-regulation during DTC phase I migration in L2, we precociously expressed lin-29 under the control of the P lag-2 promoter using the P lag-2 ::lin-29 transgene. In the wild-type animals carrying this transgene, 27% and 9.5% of the anterior or posterior DTCs, respectively, underwent a precocious dorsal turn ( Figure 1G). This result shows the functional importance of blmp-1-mediated repression of lin-29 transcription in preventing precocious DTC dorsal migration during early larval development.
The heterochronic genes daf-12, dre-1, and lin-29 act redundantly to down-regulate BLMP-1 levels in the mid and late L3 stages The observation that the constitutive expression of blmp-1 in DTCs throughout larval development prevented them from performing the dorsal turn ( Figure 5A) highlights the importance of BLMP-1 down-regulation in promoting the dorsal turn. Two observations suggested that daf-12, lin-29, and dre-1 might be responsible for BLMP-1 down-regulation. First, like worms constitutively expressing blmp-1 (Figure 5Ad-f), the double and triple mutants of daf-12, lin-29, and dre-1 had a retarded DTC migration phenotype (Table 1). Second, loss of blmp-1 partially suppressed the retarded phenotype of the double and triple mutants ( Table 1), showing that the defect in the double and triple mutants required blmp-1 activity. We therefore examined BLMP-1 levels in mutants defective in daf-12, lin-29 and/or dre-1 using immunostaining with anti-BLMP-1 antibody.

LIN-29 and DAF-12 repress blmp-1 transcription to promote the DTC dorsal turn
To determine how BLMP-1 was down-regulated by lin-29, daf-29, and dre-1, we used two gfp reporters in an in vivo expression assay to examine whether blmp-1 expression was repressed at the transcriptional level by transcription factors LIN-29 and DAF-12 and at the post-transcriptional level by the F-Box protein DRE-1. We first generated a transcriptional reporter P blmp-1 ::dgfp ( Figure 6A), in which dGFP (destabilized GFP) was controlled by the P blmp-1 promoter. dGFP contains a PEST sequence and has a shorter half-life than normal GFP [28] and therefore allows sensitive detection of promoter activity. In animals carrying the integrated P blmp-1 ::dgfp transgene, the DTCs of 75.6% of worms expressed GFP in the early and mid L3 stages, while only 4.3% expressed GFP in the late L3 and L4 stages ( Figure 6B and 6C), suggesting that blmp-1 transcription is strongly repressed during, and after, late L3 stage.
Next, we tested whether daf-12 or lin-29 was responsible for blmp-1 transcriptional repression by examining P blmp-1 ::dgfp transgene expression in the absence of daf-12 and/or lin-29. As shown in Figure 6C, in early and mid L3, loss of either daf-12 or lin-29 did not affect the percentage of worms with DTCs expressing dgfp. However, during the late L3 and L4 stages, loss of lin-29, but not daf-12, increased the percentage of worms with DTCs expressing dGFP in late L3 and L4. For example, only 4.3% or 2.4% of wildtype or daf-12 mutant worms, respectively, had DTCs expressing dGFP, whereas 39.6% of lin-29(RNAi) worms had dGFPexpressing DTCs. This result shows a differential requirement for lin-29 (stronger) and daf-12 (weaker) for blmp-1 transcriptional repression during, and after, the late L3 stage ( Figure 6C). In addition, 78.4% of the daf-12(rh61rh411); lin-29(RNAi) double mutants had DTCs expressing dGFP ( Figure 6C). These results suggest that daf-12 plays a non-essential, but auxiliary, role in the repression of blmp-1 transcription.
We also tested the involvement of dre-1 in blmp-1 transcriptional repression. No effect on the P blmp-1 ::dgfp transcription level was observed when the dre-1(dh99) mutation was introduced into the wild-type or the daf-12mutant ( Figure 6C). Thus, dre-1 probably plays no role in blmp-1 transcriptional repression. Interestingly, blmp-1 seems to be required for its own expression. During early and mid L3 stages, approximately 75% of the wild-type worms had DTCs expressing dGFP from the P blmp-1 ::dgfp transgene, but only 9% of the blmp-1(s71) worms had DTCs expressing dGFP. DRE-1 functions in an SCF ubiquitin ligase complex to regulate BLMP-1 levels Previous genetic and biochemical data have shown that the F-Box protein DRE-1 functions in an SCF ubiquitin ligase complex, which contains CUL-1 (a cullin scaffold protein), SKP-1 (a SKP1like adaptor that binds to the F-Box protein), and RBX-1 (an RBX ring finger that bridges to the E2 ubiquitin conjugating enzyme), and is important for the temporal control of somatic and gonad development [4] and the timing of tail spike cell death [29]. This raised the possibility that DRE-1 may regulate BLMP-1 stability through ubiquitin-mediated proteolysis. Like dre-1 RNAi, RNAi for skp-1, rbx-1, or cul-1 caused a retarded DTC migration phenotype in the daf-12(rh61rh411) single mutant [4] (Table S4), consistent with a model in which DRE-1 targets protein(s) for proteolysis in an SCF ubiquitin ligase complex during the temporal regulation of the DTC dorsal turn.
Using the transgene P lag-2 ::gfp::blmp-1, in which BLMP-1 was tagged with GFP and expressed under the control of the P lag-2 promoter ( Figure 7A), we next examined whether DRE-1 destabilized BLMP-1. Two independent transgenic lines carrying an extrachromosomal transgene array were generated. In line 1, approximately 27% of worms expressed GFP in DTCs during early L3 and the percentage decreased to 16.9% during mid L3 to L4, showing approximately 38% down-regulation, and a similar level of down-regulation (35.5%) was observed in line 2 (Figure 7Aa,b and C). In contrast, no down-regulation was observed in the control line carrying the integrated transgene qIs56[P lag-2 ::gfp], in which gfp was expressed under the control of the P lag-2 promoter (Figure 7Ba,b and C). These results show that BLMP-1 levels drop significantly when entering the mid L3 stage. Although the difference may be due to differences in sensitivity of the gfp reporters, given the previous result that blmp-1 transcriptional repression was detected in late, but not mid, L3 stage, this suggests that BLMP-1 degradation occurs approximately 3 hours earlier than transcriptional repression.
Next, we examined whether dre-1 was required for this decrease in GFP::BLMP-1 fusion protein levels. In two independent transgenic lines of the dre-1 mutant, the percentage of worms with DTCs expressing GFP::BLMP-1 during the early L3 stage was similar to that seen during the mid L3-to-L4 stage (Figure 7Ac,d, and C), showing that dre-1 is required for GFP::BLMP-1 degradation. Since loss of dre-1 completely blocked the decrease in GFP::BLMP-1 levels, it is unlikely that any other gene acts in a redundant fashion with dre-1 to regulate BLMP-1 stability.

DRE-1 and BLMP-1 physically interact, as do their mammalian orthologs FBXO11 and PRDI-BF1
The F-box protein of an SCF ligase complex binds to substrates and targets them for ubiquitin-mediated proteolysis [30]. To test the idea that BLMP-1 might be the target of DRE-1 in the SCF DRE-1 ligase complex, we examined whether DRE-1 and BLMP-1 physically interacted by co-immunoprecipitation in mammalian cell cultures. When Myc-tagged dre-1 and HA-tagged blmp-1 were transfected into HEK293T cells, then lysates were subjected to immunoprecipitation with anti-Myc antibody and Western blotting with anti-HA antibody, BLMP-1 was co-immunoprecipitated with DRE-1, while no signal was seen in the singly transfected controls ( Figure 8A). The reciprocal experiment using immunoprecipitation with anti-HA antibody and Western blotting with anti-Myc antibody showed that DRE-1 was co-immunoprecipitated with BLMP-1 ( Figure 8B). These results demonstrate that DRE-1 binds to BLMP-1 in HEK293T cells.
We next examined whether FBXO11 and PRDI-BF1, the respective human orthologs of DRE-1 and BLMP-1, also associate in HEK293T cells and found that Myc-tagged FBXO11 was coimmunoprecipitated with HA-tagged PRDI-BF1 in lysate of cells co-transfected with both, but not in lysate of cells transfected with only Myc-tagged FBXO11 ( Figure 8C). This result shows association of FBXO11 and PRDI-BF1 in human cell cultures. In addition, HA-tagged PRDI-BF1 also pulled down endogenous CUL1 in co-immunoprecipitation experiments in HEK293T cells ( Figure 8C), demonstrating the association of FBXO11, BLMP-1, and CUL1 in an SCF complex and suggesting that FBXO11 may regulate PRDI-BF1 stability through ubiquitin-mediated proteolysis. These results demonstrate a conserved interaction between DRE-1/FBXO11 and BLMP-1/PRDI-BF1 in both humans and C. elegans. The regulation of BLMP-1/PRDI-BF1 stability by DRE-1/FBXO11 in an SCF ubiquitin ligase complex may also be conserved in evolution.

blmp-1 acts in a heterochronic pathway to regulate the timing of the DTC dorsal turn
In this study, we identified a conserved transcription factor, BLMP-1, as an essential component of the heterochronic hierarchy during gonad development, and provided evidence that BLMP-1 levels are critical for the timing of DTC dorsal migration during gonadogenesis. BLMP-1 was present in DTCs only before the dorsal turn and disappeared when the DTC was about to make the dorsal turn. Loss of blmp-1 resulted in precocious unc-5 transcription and DTC dorsal migration, whereas constitutive expression of blmp-1 delayed both events. These data show that blmp-1 controls the temporal identity of DTCs by preventing them from undergoing a precocious dorsal turn. In addition, our results provide a molecular mechanism by which daf-12, dre-1, and lin-29 promote the correct timing of DTC dorsal turn by timely downregulation of blmp-1. In this model, lin-29 and daf-12 repress blmp-1 transcription, which abolishes the synthesis of blmp-1 mRNA, while dre-1 mediates BLMP-1 degradation. Together, these two negative regulatory systems result in the efficient elimination of blmp-1 activity, and thus alleviate the repression of the DTC dorsal turn.
daf-12, dre-1, and lin-29 function in parallel to reduce BLMP-1 levels prior to the DTC dorsal turn Interestingly, daf-12, dre-1, and lin-29 appeared to contribute to BLMP-1 down-regulation to different extents. For example, in the dre-1; lin-29, dre-1; daf-12, and lin-29; daf-12 double mutants, 100%, 20%, or 5% of worms, respectively, showed persistent BLMP-1 expression beyond late L3 in the immunostaining experiment. This suggests that, of these three genes, loss of dre-1 has the strongest effect on BLMP-1 down-regulation and loss of daf-12 has the weakest effect. Using transcriptional and translational gfp reporters, it appeared that the DRE-1-dependent protein degradation process occurred at the mid L3 stage, slightly earlier than the transcriptional repression mediated by lin-29 and daf-12 (late L3 stage), although this difference could be due to different sensitivities of the gfp reporters. The notion that lin-29 is more significant than daf-12 in the repression of blmp-1 was further supported by the observation that, using the P blmp-1 ::dgfp reporter, loss of lin-29 increased blmp-1 transcription in the late L3 to L4 stages, but loss of daf-12 had no detectable effect on blmp-1 expression ( Figure 6C). Further experiments are necessary to test whether LIN-29 may inhibit blmp-1 transcription by directly binding to the blmp-1 genomic sequence.
It is intriguing that only 20% of the dre-1; daf-12 double mutants had DTCs with detectable persistent BLMP-1, whereas 98% of these double mutants showed the retarded phenotype (Table 1). It is possible that, in the mutant, BLMP-1 persists at a low level beyond the detection limit of our system, but sufficient to block lin-29 transcription and therefore abolish the DTC dorsal turn in the absence of daf-12. On the other hand, the retarded DTC migration phenotype of the lin-29; daf-12 double mutant may not be caused solely by persistent BLMP-1 at the undetectable low level, but also by the loss of both transcription activators lin-29 and  daf-12, which function in a redundant fashion in the transcriptional activation of unc-5 (see below and Figure S5).
As shown in Figure 5 A, BLMP-1::GFP in worms carrying P blmp-1 ::blmp-1::gfp was present and persisted beyond late L3 stage and the transgenic worms frequently showed a retarded DTC migration defect. A similar result was observed in worms carrying P lag-2 ::blmp-1::gfp (Huang and Wu, unpublished results). Interestingly, wild-type worms carrying P lag-2 ::blmp-1 had normal DTC migration ( Figure 1G), suggesting that BLMP-1 in the transgenic worms was properly degraded beyond L3 stage. These data and the observation that GFP::BLMP-1 expressed from the transgene P lag-2 ::gfp::blmp-1 was degraded suggest that GFP tag at N-terminus or C-terminus make a difference in GFP fusion BLMP-1 degradation.

Regulation of unc-5 transcription
Using ChIP-seq, the modENCODE Consortium showed that BLMP-1 binds to the upstream sequence of unc-5 in vivo [26], suggesting that it might inhibit unc-5 expression by binding to the regulatory region of the gene. In addition, using the P unc-5(4.6 kb) ::gfp transgene, we found that inactivation of either daf-12 or lin-29 slightly reduced unc-5 transcription, while loss of both genes completely abolished it ( Figure S5). These results are consistent with the notion that the transcription factors DAF-12 and LIN-29 act together in a redundant fashion to activate unc-5 transcription and promote the DTC dorsal turn. The consensus DAF-12 binding sequence can also be identified in the unc-5 promoter (T.F. Huang and Y.C. Wu, unpublished results), raising the possibility that DAF-12 may directly bind to unc-5 for its transcriptional activation.

A double-negative feedback loop involving blmp-1 and lin-29 contributes to the DTC dorsal turn
The DTC dorsal turn can be characterized by a specific transition in which the DTC moves irreversibly from phase I centrifugal migration into phase II dorsal migration. Phase I centrifugal migration occurs in the ''blmp-1-on'' state (high BLMP-1 and low UNC-5 levels) and phase II dorsal migration occurs in the''blmp-1-off'' state (low BLMP-1 and high UNC-5 levels). A double-negative feedback loop, in which two genes mutually repress each other directly or indirectly, is commonly utilized to generate switch-like bistable responses during the progression of cellular and developmental processes [31]. Our data suggest that lin-29 and blmp-1 act in a double negative feedback loop that helps maintain DTCs in one of the bistable ''blmp-1-on'' and ''blmp-1off'' states. On the basis of our results, we propose a molecular model for the switch-like process of the DTC dorsal turn. In L2 and early L3, BLMP-1 is expressed and represses lin-29 transcription (Figure 9). The double-negative feedback loop keeps DTCs in the ''blmp-1-on'' state, thus repressing DTC dorsal migration. In late L2, the hormones known as dafachronic acids (DA) are generated from dietary cholesterol and bind to the ligand binding domain of DAF-12 [32]. The DAF-12-DA complex promotes the L2-to-L3 transition [32], but is insufficient to repress blmp-1 transcription. During early to mid L3, dre-1 expression is initiated [4], which reduces BLMP-1 levels, probably through ubiquitin-mediated proteolysis. The decrease in BLMP-1 levels shifts the steady state toward low BLMP-1 levels and high LIN-29 levels (blmp-1 off). In late L3, accumulation of LIN-29 locks the DTCs in the ''blmp-1-off'' state through the negative feedback loop, thus allowing DTCs to switch to dorsal migration. This gene regulatory circuit integrates the temporal and spatial signals and coordinates with overall development of the organism to direct DTC dorsal migration during organogenesis.
Expression of lin-29 is negatively regulated by lin-42 [33]. LIN-42 is similar to the Period (Per) family of circadian rhythm proteins and functions as a member of the heterochronic pathway, regulating temporal cell identities [33,34]. However, how lin-42 regulates the expression of lin-29 is not clear. lin-42 also genetically interacts with daf-12. During development, when conditions are unfavorable, due to starvation, crowding or high temperature, daf-12 promotes diapauses and formation of duaer larvae [5,35,36]. Previous studies show that daf-12 represses expression of lin-42 during dauer formation [37] and that lin-42 antagonizes the dauer-inducing signal from ligand-free DAF-12 [38]. However, whether daf-12 and lin-42 also regulate each other in the timing of DTC dorsalward turn during gonadaogenesis needs to be examined in the future.

blmp-1 is important for DTC migration along the A-P axis
In addition to ventral-to-dorsal migration, blmp-1 affects DTC migration along the AP axis. As shown in Figure 1D, some DTCs migrated obliquely with respect to the dorsal-ventral axis in blmp-1 mutants, suggesting simultaneous execution of centrifugal phase I and dorsal phase II movement. This result implies that the mutual exclusion of dorsalward and centrifugal migration in wild type animals was impaired in the blmp-1 mutants. Moreover, as shown in Figure 1E and 1F and Table S1, the reversal of migration direction was frequently observed in the centripetal phase III migration of blmp-1 mutants. Expression of unc-5 by the lag-2 promoter from transgene P lag-2 ::unc-5 resulted in DTC migration defects, including the reversal of the migration direction during the centripetal phase III migration (Huang, T.F. and Wu, Y.C., unpublished results), similar to those seen in blmp-1 mutants (Table  S1). Further experiments are necessary to test whether the phase III DTC migration defect of blmp-1 mutants may be caused by an accumulative high level of unc-5 in the DTCs.

blmp-1 is essential for multiple cellular and developmental processes
Previous studies have shown that blmp-1 mutants have a dumpy phenotype, a weak cuticle sensitive to oxidative stress, and defective adult alae, showing that blmp-1 is important for normal body length, oxidative stress resistance, and alae formation [39]. In addition, blmp-1 is important for pharynx and [40] male tail morphogenesis [41]. Under Nomarski optics, we confirmed the blmp-1 alae defect, as 40% of blmp-1(s71) mutants had incomplete alae and the rest had no alae in the adult stage ( Figure S3C). The alae are continuous ridged cuticular structures synthesized by the lateral epidermal seam cells at the larval to adult (L/A) transition and serve as a specific marker of adult fate. The seam cells undergo asymmetric cell divisions at each larval stage and exit the cell cycle at the L/A transition. The blmp-1(s71) mutant was found to have a normal number of seam cells at the late L4 and early adult stage ( Figure S3D), indicating that seam cell division was normal. These results and the abnormal alae phenotype suggest that blmp-1 is essential for the adult, but not the larval, fate of the epidermal seam cells.
The dre-1 mutant shows precocious formation of adult alae [4]. We therefore tested the genetic interaction between dre-1 and blmp-1 in the seam cell heterochronic hierarchy. About 8% of dre-1(dh99) mutants had adult alae at the early L4 stage (Figure S3D), confirming a role of dre-1 in preventing the adult fate of the seam cell [4]. At the adult stage, 35% of dre-1(dh99) mutants had incomplete alae (Figure S3Cc and D). Interestingly, the blmp-1; dre-1 double mutant had a weaker defect in alae formation than the blmp-1 single mutant, as 23% of double mutants and 60% of blmp-1 single mutants had no alae ( Figure S3C and D). This result shows that dre-1 partially suppresses the blmp-1 alae abnormality and, therefore, blmp-1 may genetically act upstream of, or in parallel to, dre-1 in the specification of the adult seam cell fate, at least in terms of alae formation.
blmp-1(tm548 or s71) mutants have a weak uncoordinated movement phenotype [39]. Although blmp-1 expression has been previously observed in neurons using reporter transgene assays [26], we failed to detect BLMP-1 in neurons. It is possible that our anti-BLMP-1antibody system was not sensitive enough to detect a low amount of protein in neurons. Alternatively, the neuronal expression of blmp-1 revealed by the transgene may be ectopic and not reflect the endogenous expression pattern.
Like the blmp-1 mutations, lin-42 RNAi causes both a precocious DTC dorsal turn [33] and precocious lin-29 expression in L2, one stage earlier than in the wild-type [33]. However, the lin-42(RNAi) and blmp-1 mutations induce precocious dorsal migration at different larval stages, i.e. in L2 for lin-42(RNAi) [33] and early L3 in the case of blmp-1 (Figure 3). Similar to loss of blmp-1, precocious expression of lin-29 under the control of the lag-2 promoter caused the DTCs to undergo a precocious dorsal turn in early L3, but not L2. These results suggest that lin-42 regulates most, if not all, of the genes necessary for the DTC dorsal turn, while blmp-1 regulates the temporal expression of only a subset, including lin-29.
The conserved interactions between BLMP-1/PRDI-BF1 and DRE-1/FBXO11 DRE-1 has two human orthologs FBXO10 and FBXO11, which are localized in the cytoplasm and nucleus, respectively [29]. FBXO10 and DRE-1 mediate, respectively, the degradation of anti-apoptotic protein Bcl2 or CED-9 to promote the death of a cell [29]. Recently, FBXO 11 has been shown to target the prooncogene product BCL6 for degradation and to be inactivated in diffuse large B cell lymphomas [42]. BCL6, a zinc finger transcription factor, regulates the transcription of a variety of genes involved in B cell development, differentiation, and activation [43,44] and is overexpressed in the majority of patients with aggressive diffuse large B cell lymphoma [45]. Using the Clustal Omega multiple sequence alignment website http://www. ebi.ac.uk/Tools/msa/clustalo/, we found that Bcl6 shares sequence similarity with BLMP-1 (20%) and PRDI-BF1 (24%). These observations and our result showing that DRE-1/FBXO11 and BLMP-1/PRDI-BF1 are associated in human cell cultures indicate that DRE-1, FBXO10 and FBXO11 target different proteins for degradation in different cellular or developmental contexts, i.e. DRE-1 targets BLMP-1 and CED-9, FBXO10 targets Bcl2 and FBXO11 targets PRDF-BF1 and Bcl6. It will be interesting to determine how target specification is regulated.

Genetics
To test whether the tk41, tm548, s71 or tp5 mutation is recessive or dominant, homozygous mutant hermaphrodites were crossed to wild-type males carrying the integrated sur-5::gfp transgene. The cross-progeny hermaphrodites carrying the sur-5::gfp transgene were scored for the dumpy (Dpy) and DTC migration phenotypes using Nomarski microscopy. In these crosses, all heterozygous cross-progeny hermaphrodites were wild-type, showing that these mutations are recessive.
In complementation tests, the tk41, tm548 or tp5 homozygous hermaphrodites were crossed to s71 males carrying the sur-5::gfp transgene, the cross-progeny hermaphrodites with the transgene were scored for the Dpy and DTC migration phenotypes using Nomarski microscopy. In these crosses, all of the mutations tested failed to complement s71.

Genetic screen
We performed genetic screening for mutants defective in DTC migration under a dissecting microscope, as described in Nishiwaki, 1999, and isolated the mutation tk41. In an independent screen for mutants with a DTC migration defect using Nomarski microscopy, we isolated the mutation tp5.

Analysis of the DTC migration phenotype
The DTC migratory patterns of the wild-type and mutants were determined by observing the shape of the gonadal arms in the adult stage. Worms were mounted on a 4% agar pad containing 20 mM NaN 3 and observed on a microscope with Nomarski optics. For the time course analysis of DTC migration, wild-type and blmp-1(s71) mutants at the indicated time points were collected and scored under the Nomarski microscope.

DNA construct
To obtain the 59 end of blmp-1 cDNA, the first three exons were amplified by RT-PCR using a forward primer corresponding to the SL1 sequence and the reverse primer blmp-1_exon_3/r (see Table S5 for detailed information). The PCR product was fused with the blmp-1 cDNA fragment from the yk487b7 clone by fusion PCR [49], and the resulting product was cloned into the vector pGEM-T Easy (Promega) to generate pYW687, containing the full-length blmp-1 cDNA.
Two unc-5 transcriptional gfp constructs were generated. The P unc-5(4.6 kb) DNA fragment was PCR-amplified from C. elegans genomic DNA using primers P unc-5 _4.6 kb/f and P unc-5 _4.6 kb/r and inserted into the vector pGEM-T Easy. The P unc-5(4.6 kb) fragment of the resulting construct was then inserted into the vector pPD95.77 (A. Fire) via SphI/SalI sites to generate P unc-5(4.6 kb) ::gfp. A similar approach was used to generate P unc-5(1 kb) ::gfp. The P unc-5(1 kb) fragment was PCR-amplified from genomic DNA using primers P unc-5 _1 kb/f and P unc-5 _1 kb/r and cloned into the vector pGEM-T Easy, then the P unc-5(1 kb) fragment was excised from the resulting plasmid and inserted into the vector pPD95.75 (A. Fire) via the XmaI site. To generate P unc-5(1 kb) ::mCherry, the gfp fragment of the plasmid P unc-5(1 kb) ::gfp was replaced by mCherry cDNA.
The P blmp-1 ::dgfp fragment was generated by fusion PCR by fusing two PCR products corresponding to P blmp-1 and the region containing dgfp and the unc-54 39 UTR. P blmp-1 was PCR-amplified from genomic DNA using primers D24-5end5kb and d24Pgfp/r, and the region containing dgfp and unc-54 39UTR was PCRamplified from pPD95.75PEST (pYW807), which contains dgfp, using primers GFP/f and D. The two PCR products were then mixed and fused by fusion PCR using primers d24-5kbf-nest and D9. The P blmp-1 ::blmp-1::gfp fragment was generated by fusion PCR by fusing two PCR products corresponding to blmp-1 genomic DNA and the region containing gfp and unc-54 39 UTR from the pPD95.75 plasmid [50]. blmp-1 genomic DNA was PCR-amplified from wild-type genomic DNA using primers D24-5end5kb and d24gfp/r, and the region containing gfp and the unc-54 39 UTR was PCR-amplified from the pPD95.75 plasmid using primers GFP/f and D, then the two PCR products were fused by fusion PCR using primers d24-5kbf-nest and D9.
The primers used in this work are listed in Table S6.
The transgenes used in this work were listed in Table S5.
HEK293T cell culture and drug treatments HEK293T cells were grown in Dulbecco's modified Eagle's medium (Thermo Inc) containing 10% fetal bovine serum, 100 U/ml of penicillin, and 100 mg/ml of streptomycin (all from Life technologies) and were transfected with the indicated plasmids using Lipofectamine 2000 (Invitrogen). MG132 (20 mM) was added to the medium 2 h before the cells were harvested to block protein degradation by the proteasome.

Co-immunoprecipitation and immunoblotting
Cells were harvested using pre-chilled PBS and lysed using 1% NP-40 lysis buffer with protease inhibitor (Roche). HA-tagged or Myc-tagged fusion proteins were immunoprecipitated by incubation overnight at 4uC with anti-HA or anti-Myc agarose beads (Sigma) and bound protein identified by Western blotting using antibodies against HA (Covance), Myc (BD Biosciences), or CUL1 (Zymed). Antibodies against tubulin (Sigma) or GAPDH (Abnova) were used on the loading controls. Figure S1 Molecular Cloning of blmp-1. (A) The genetic map near the dpy-24/blmp-1 locus on chromosome I is shown above. The cosmid clones shown in the middle and the open reading frames of the cosmid F25D7 shown below were tested for their abilities to rescue the blmp-1(s71) mutant phenotypes. The gene structure and transcription direction (indicated by an arrow) for each open reading frame is shown. (B) Rescue of the phenotype responsible for the DTC migration defect, dumpy phenotype or embryonic lethality of blmp-1(s71) mutants by germline transformation using genomic DNA clones. Plus sign, rescue; minus sign, no rescue. n.d., not determined. (TIFF) Figure S2 Sequence alignment of BLMP-1 and related proteins. Sequence alignment of BLMP-1, mouse Blimp-1, and human PRDI-BF1. The light gray and black shading indicates, respectively, residues that are identical in two or three proteins, while the dark gray shading with white letters indicate similar residues in two proteins. The PR domain is indicated by double underlining and the zinc fingers by circles, the predicted NLS is boxed by a rectangle, and the positions of the blmp-1 mutant alleles are indicated by arrowheads. (TIFF) Figure S3 Loss of blmp-1 results in partial penetrant embryonic lethality, a dumpy phenotype, and a defect in alae formation. (A) blmp-1 mutants are dumpy. DIC images of wild-type and blmp-1(s71) worms 24 h after reaching adulthood. Scale bar, 100 mm. (B) blmp-1 embryos have a partial penetrant embryonic lethality. The table shows the percentages of embryos that did not hatch after 24 h. The progeny derived from three worms was counted for each genotype. (C) DIC images of wild-type (a), blmp-1(s71) (b), dre-1(dh99) (c) and blmp-1(s71); dre-1(dh99) (d) adults. Alae are indicated by arrows. Scale bar, 10 mm. (D) The seam cell phenotype of the indicated mutants. The seam cell numbers and alae were scored at the indicated developmental stages. The seam cell-specific marker SCM::GFP allows visualization of seam cell nuclei and was used to assay seam cell number. The apical junction marker AJM-1::GFP was also present in the transgene. (TIFF) Figure S4