C. elegans PAT-9 is a nuclear zinc finger protein critical for the assembly of muscle attachments
© Liu et al.; licensee BioMed Central Ltd. 2012
Received: 17 April 2012
Accepted: 22 May 2012
Published: 22 May 2012
Caenorhabditis elegans sarcomeres have been studied extensively utilizing both forward and reverse genetic techniques to provide insight into muscle development and the mechanisms behind muscle contraction. A previous genetic screen investigating early muscle development produced 13 independent mutant genes exhibiting a Pat (p aralyzed and a rrested elongation at the t wo-fold length of embryonic development) muscle phenotype. This study reports the identification and characterization of one of those genes, pat-9.
Positional cloning, reverse genetics, and plasmid rescue experiments were used to identify the predicted C. elegans gene T27B1.2 (recently named ztf-19) as the pat-9 gene. Analysis of pat-9 showed it is expressed early in development and within body wall muscle lineages, consistent with a role in muscle development and producing a Pat phenotype. However, unlike most of the other known Pat gene family members, which encode structural components of muscle attachment sites, PAT-9 is an exclusively nuclear protein. Analysis of the predicted PAT-9 amino acid sequence identified one putative nuclear localization domain and three C2H2 zinc finger domains. Both immunocytochemistry and PAT-9::GFP fusion expression confirm that PAT-9 is primarily a nuclear protein and chromatin immunoprecipitation (ChIP) experiments showed that PAT-9 is present on certain gene promoters.
We have shown that the T27B1.2 gene is pat-9. Considering the Pat-9 mutant phenotype shows severely disrupted muscle attachment sites despite PAT-9 being a nuclear zinc finger protein and not a structural component of muscle attachment sites, we propose that PAT-9 likely functions in the regulation of gene expression for some necessary structural or regulatory component(s) of the muscle attachment sites.
The nematode C. elegans provides an established, developmentally well-documented, and evolutionarily conserved system to study muscle structure, development, and function [1, 2]. The C. elegans sarcomere, the basic muscle contraction unit, has been studied for decades revealing a highly organized structure consisting of several hundred proteins, yet new components are still being identified [2–6]. In C. elegans sarcomeres, myosin thick filaments are organized around M-lines and actin thin filaments are anchored to the dense bodies, structures analogous to the vertebrate Z-disk. The dense bodies and M-lines are sites of attachment for body wall muscle cells to the basement membrane, thus transmitting the force of muscle contraction and allowing movement . The overall mechanism of C. elegans muscle function is highly evolutionarily conserved and many of the known proteins have vertebrate orthologs within vertebrate muscle costameres or non-muscle focal adhesions [1, 2, 6, 8].
Many of the components necessary for C. elegans muscle attachments were identified by immunological approaches or through genetic screening for mutants exhibiting disorganized myofilaments, paralysis, and/or embryonic arrest [4, 9, 10]. Genes required for muscle development and function are grouped into two main phenotypic classes of mutants, Pat (p aralyzed and a rrested elongation at the t wo-fold length of embryonic development) and Unc (Uncoordinated), with some genes capable of producing both phenotypes depending on the nature of the mutation [10, 11]. Both of these phenotypic classes comprise proteins that localize specifically to the M-lines, dense bodies or both, and much of their organized assembly into functional sarcomeres has been characterized (reviewed in [1, 7]). A fourth type of localization for some sarcomeric proteins is exemplified by ZYX-1, UNC-95, UNC-97, and UNC-98, which are found both at the sarcomere and in the nucleus, supporting an additional role for the sarcomere as a platform for mediating signal transduction to the nucleus to influence gene expression [12–15].
In a continuing effort to identify new components required for muscle attachment site assembly, we have focused on characterizing the Pat group of C. elegans mutants. Mutations in the genes that encode the membrane-associated components of the muscle attachment sites, such as unc-52, pat-2, pat-3, unc-112, unc-97, pat-4 and pat-6, display the most severe Pat phenotype, in which neither thin filaments nor thick filaments are assembled into the myofilament lattice . Previous studies indicate that PAT-4, UNC-97 and PAT-6 serve as intermediary members of the linkage formed between the integrins and myofilaments [16–18]. Here, we identified and characterized a novel gene in the Pat family, pat-9[10, 19]. In pat-9 mutants, the recruitment of both actin thin filaments and myosin thick filaments to the muscle cell membrane appears to be disrupted. Similar to other Pat genes, pat-9 is expressed in C. elegans body wall muscle during embryogenesis. Unlike other genes in the Pat family that are structural and functional components of muscle attachments, pat-9 encodes a nuclear localized C2H2 zinc finger transcription factor, suggesting a potential regulatory role, as opposed to a sarcomeric structural function for PAT-9.
The pat-9 gene is encoded by T27B1.2 (ztf-19)
PAT-9 is localized to nuclei through all developmental stages
PAT-9 has a nuclear localization signal (NLS)
PAT-9 associates with gene promoters
PAT-9 is required for the assembly of functional muscle attachments
We have shown that the pat-9 gene is the predicted gene T27B1.2, renamed ztf-19, which encodes a nuclear C2H2 type zinc finger transcription factor. Thus, PAT-9 is the only exclusively nuclear protein among all pat genes that have been identified and characterized. In mammals, many DNA-binding transcriptional regulators contain C2H2 zinc fingers . Similarly, in C. elegans C2H2 zinc finger transcription regulators play critical roles in many biological aspects, including body wall muscle development, synaptic transmission, and egg-laying behavior and fertility [15, 24, 25]. The characterization of PAT-9 suggests it functions as a transcription factor for one or more factors required in muscle attachment site assembly. Lending support to this hypothesis, a yeast one-hybrid binding assay showed that PAT-9/ZTF-19 bound to five promoters, daf-3 tbx-2 cog-1 let-7, and mir-76. Furthermore, two of the genes, daf-3 and tbx-2, were shown to be bound by PAT-9 in vivo. Interestingly, both genes are transcriptional regulators themselves involved in C. elegans pharynx development, while daf-3 is additionally involved in dauer formation, indicating that PAT-9 has multiple roles during development including roles outside of body wall muscle development [26–28].
TBX-2 is a transcription factor specifically required for development of ABa-derived pharyngeal muscles . TBX-2 is expressed from early embryo to adult stages and tbx-2::gfp expression was also observed in body wall muscles at the comma stage and later, in 2- to 3-fold embryos similar to the expression of pat-9. As with pat-9, there is a muscle defect in tbx-2 mutants at the embryo stage. Interestingly, RNAi analysis reveals that tbx-2 is critical for early larval development and normal locomotion. These animals have a less severe phenotype than the pat-9 RNAi animals that have embryo lethality and larval arrest, but are still consistent with TBX-2 being a downstream target of PAT-9 [29, 30].
DAF-3 is a transcriptional regulator negatively regulated by TGF-β signaling . Loss of DAF-3 by genetic null does not result in locomotion or embryonic lethality phenotypes, but rather, dauer defective animals . The other predicted targets of PAT-9/ZTF-19 are unlikely candidates for producing the Pat phenotype; cog-1 is expressed in neurons , let-7 is a heterochronic switch gene only expressed in L3, L4 and adults , and the mir-76 microRNA is expressed in early embryos and adult neurons but not body wall muscle lineages . This expression information along with the ChIP data has enabled us to hypothesize that PAT-9 is involved in the regulation of both tbx-2 and daf-3 expression, however, neither gene is responsible for the Pat phenotype of pat-9 mutants, leaving the critical downstream target(s) of PAT-9 yet to be identified.
PAT-9 is required for proper organization of myofilaments and for recruitment to the M line. In pat-9 mutants, UNC-52/perlecan, the most upstream molecule of the muscle attachment assembly pathway, is present in the basal lamina; however, PAT-3/integrin is not well organized at the basal membrane. Subsequently, PAT-4/ILK and DEB-1/vinculin are not recruited properly to the nascent attachment sites. The correct assembly of dense bodies and M lines are dependant upon the recruitment of each protein component in a distinct order, such that failure of one protein to assemble hinders the recruitment of all other proteins downstream in the pathway [16, 17]. Therefore it is reasonable to make the prediction that the molecular components at muscle attachment sites downstream of integrin, including UNC112, UNC-97/PINCH, and PAT-6/actopaxin, would not be assembled properly at the attachment sites of pat-9 mutants. In both pat-4 and pat-6 mutants, the PAT-3/integrin is not affected, since both proteins are downstream of PAT-3 in the assembly pathway. However, in pat-9 mutants integrin is not properly organized at the basal membrane, which directly causes the disruption of the integrity of the attachment complex and expression of UNC-52 is not enough to initiate the assembly of muscle at attachment sites. Since integrins have two-way signaling, the lack of PAT-9 causes the loss of a critical inside out signaling mechanism, compounding the Pat-9 phenotype.
The assembly of proteins forming muscle attachments is a highly ordered process, but is also regulated at the transcriptional level by transcription factors. Our study shows that PAT-9/ZTF-19 is indispensable for establishing and maintaining the integrity of muscle attachment sites and most likely functions at the level of transcription regulation.
Strains and genetics
Standard methods were used for culturing C. elegans. The following strains were used: wild-type worms were N2 strain of the Bristol variety; RW1385 (mnDp1(X; V)/+V; unc-3(e151) pat-9(st558) X). SNP mapping was performed as described . RW1385 worms were mated with Hawaiian CB4856 males and Unc non-Pat recombinants were isolated and used for PCR and RFLP analysis around known SNPs.
Because homozygote pat-9 is lethal, a segment of duplication of the X chromosome (mnDp1) containing wild-type pat-9 and unc-3 genes was fused to one of its V chromosomes. This strain segregates 25% pats, 25% early arrested embryos containing two duplications and 50% normal progeny with the same genotype as RW1385. For the pat-9 rescue line (+/+V; unc-3(e151) pat-9(st558) X; pat-9::gfp), the rescue animal lost the duplication mnDp1 and is homozygous for unc-3 and pat-9. The Pat phenotype was rescued by a wild-type pat-9 gene in an extrachromosomal array and was homozygous for unc-3, yielding an Unc phenotype and coiled tail. The extrachromosomal arrays contain the dominant transformation marker pRF4(rol-6); therefore the rescued strain segregates either rollers with coiled tail or Pats.
All PCR fragments used to generate expression plasmids were first subcloned into pGEM-T easy (Promega) and sequenced. All oligonucleotide primers are listed in Additional file 1: Table S3. Cosmid T27b1 was verified to contain the whole T27b1.2 gene by sequencing with primers BW-580, BW-581, BW-582 and BW-583. For the initial transformation rescue experiment, the pat-9 3.3 kb promoter and coding region were amplified separately with primer pairs (BW-608, BW-610), and (BW-611, BW-612) respectively, using cosmid T27b1 as the template. Two PCR fragments have a 500 bp overlap. The initial pat-9 full-length cDNA was generated by RT-PCR using Superscript III/Platinum Taq one-step RT-PCR system (Invitrogen) and primer pair (BW-631, BW-632). Total RNA was isolated from N2 worms using a TRIZOL (Invitrogen) based method with slight modification: 800ul Trizol was added to 200ul packed worms, vortexed, and incubated at room temperature for 20 min. The reaction was centrifuged at full speed at 4°C for 10 min, the supernatant containing the RNA was removed to a new tube, purified as per manufacturers instructions, and RNA was suspended in 10 μl DEPC-H2O.
To study the expression and localization of PAT-9, a pat-9::gfp fusion was constructed as follows. First, the 3.3 kb pat-9 promoter was PCR-amplified using primers BW-613 and BW-630. This fragment was digested with PstI and KpnI, and then replaced the myo-3 promoter of pPD118.20 (Addgene, Fire Lab C. elegans Vector Kit), to make P pat-9 ::gfp. The pat-9 genomic fragment including 980 bp downstream of the 6th exon was PCR-amplified using primers BW631 and BW596, digested with KpnI and DraIII, and then replaced the KpnI – DraIII region of P pat-9 ::gfp, to make P pat-9 ::pat-9. Finally, gfp was amplified and digested with KpnI, and inserted into P pat-9 ::pat-9, to make P pat-9 ::pat-9::gfp. The pat-9 promoter and genomic coding region were amplified using cosmid T27b1 as a template; gfp was amplified using vector pPD118.20 as a template.
The following transgene plasmid constructs for NLS identification were made in the pPD118.20 vector with gfp fused in-frame to the carboxyl terminus of the pat-9 coding sequence. Deletions of pat-9 were amplified from the pat-9 cDNA and cloned in-frame with gfp by NotI and KpnI digestion. The following primer pairs were used for PCR amplification: pat-9 C (primer 2 and primer 3), pat-9CNLS (primer), pat-9zf1(PJ-101, PJ-105), pat-9zf123NLS (primer 1 and primer 4). The above PCR fragments were digested with NotI and KpnI and inserted into the vector pPD118.20 NotI and KpnI site to make P myo-3 ::pat-9 C::gfp, P myo-3 ::zf1::gfp and P myo-3 ::zf123::gfp respectively. All plasmids were verified by sequencing. Transgenic lines were made using the standard microinjection approach with pRF4(rol-6) as an injection marker.
A partial pat-9 cDNA fragment (carboxyl terminal, amino acids 176–470) was PCR amplified (using primers PJ-117 and PJ-118) from the pat-9 cDNA, digested with NdeI and NotI, sub-cloned into a NdeI and NotI digested pET23a plasmid (Novagen), and transformed into E. coli BL21(DE3) bacteria for expression. Recombinant protein was purified under denaturing conditions using Talon Resin (Clontech) as per manufacturer’s instructions, dialyzed with PBS, and used as an immunogen. Rabbit polyclonal antibodies were generated at the Immune Resource Center at the University of Illinois at Urbana-Champaign. Antiserum was affinity purified against recombinant PAT-9 C-terminal protein covalently linked to cyanogen bromide-activated sepharose (GE Healthcare) and analyzed by immunoblotting for specificity (Additional file 1: Figure S2).
RNAi for all of the nine candidate pat-9 genes were performed as previously described by dsRNA injection [17, 37] and additionally for T27b1.2 by feeding bacteria induced to express double-stranded RNA to the worms . The cDNA clones yk64f5 yk414e5 yk443f12 yk483d11 yk668g10 yk782b11, and yk839g10 were kindly provided by Dr. Yuji Kohara, National Institute of Genetics, Mishima, Shizuoka, Japan. RT-PCR from N2 RNA was performed to obtain the cDNAs for T25D1.2 (BW-570, BW-571) and F59C12.3 (BW-572, BW-573). For injection RNAi, sense and antisense RNA were generated by in vitro transcription using T3 and T7 RNA polymerase (Promega). The RNAs were mixed (1 μg/μl), injected into N2 hermaphrodites, and eggs laid between 12 – 48 h after injection were scored for the Pat phenotype. For feeding RNAi, the T27b1.2 cDNA was cloned between the two T7 promoters of vector pL4440 (Open Biosystems). The plasmid was transformed into competent HT115(DE3) bacteria, grown up from single colonies in 2x YT media containing tetracycline and kanamycin, and expression of dsRNA was induced with 1 mM IPTG when the culture reached OD600 = 0.3 ~ 0.4. NGM plates, made fresh with 50 μg/ml kanamycin, 12.5 μg/μl tetracycline and 0.4 mM IPTG, were seeded with the induced cultures and set at room temperature overnight. L4-stage hermaphrodite worms were placed on the plates and incubated for 48 h at 18°C after which 5 worms were transferred separately onto plates seeded with the same bacteria and allowed to lay eggs for 24 h at 18°C before being removed. Progeny from both the first plates and second plates were scored for a Pat phenotype and any other developmental abnormalities after another 24 h.
The ChIP experiments were performed essentially as described . N2 worms were grown in 500 ml S medium at 20°C with shaking—producing mixed stage cultures, collected, washed, and fixed with formaldehyde. Approximately 0.4 ml of packed N2 worms were used for each ChIP experiment and each ChIP replicate experiment started with a new liquid culture. After fixation, soluble chromatin was generated by sonication, resulting in an average of 750–500 bp DNA fragments as determined by agarose gel. For the immunoprecipitation step, 10% of the soluble chromatin was removed and used as input control and the remaining chromatin was divided into two equal pools. The PAT-9 was added (1:100) to one tube and NRS (1:100) was added to the second sample as a control for non-specific binding. IPs proceeded overnight at 4°C with end-over-end rotation. The immune/chromatin complexes were collected using pre-locked protein A/G agarose (Santa Cruz Biotech). After washing extensively, the bound DNA was purified and subjected to qPCR using SYBR Green and a BioRad I-Cycler. PCR primers were designed to the 5′ regulatory regions of frg-1 (PJ-111, PJ-112), daf-3 (PJ-113, PJ-114), and tbx-2 (PJ-115, PJ-116), and tested for conditions resulting in a single PCR product of the expected size.
Populations of embryos were fixed and stained as previously described . The following monoclonal antibodies were obtained from the Developmental Studies Hybridoma Bank and diluted as indicated: MH2 (1:100), MH25 (1:250), MH24 (1:200), and MH27 (1:1500) (developed by R. H. Waterston), and 5–6 (1:200) (developed by H. F. Epstein). The mouse polyclonal PAT-4 antibody  and rabbit polyclonal PAT-9 antibody were generated in the lab as described. Antibodies were diluted for staining in PBS supplemented with 0.5% tween-20 and 30% normal goat serum. Affinity-purified goat anti-mouse IgG conjugated to rhodamine, diluted 1:100 (Chemicon International) or AlexaFluor594 goat anti-rabbit IgG, diluted 1:800 (Invitrogen Corp) was used as a secondary antibody.
Adult worm immunostaining
Immunostaining on adult animals was essentially as described . Mixed stage N2 worms were harvested and washed thoroughly in PBS to remove bacteria. Worms were suspended in 4% paraformaldehyde in 100 mM sodium phosphate buffer and quick frozen on dry ice. After thawing on ice, animals were incubated on ice for 1 h, washed three times in 1% triton X-100, 100 mM tris (pH 7.5), then incubated in 1% triton X-100, 100 mM tris (pH 7.5) and 1% β-mercaptoethanol at 37°C for 2 h. Worms were washed three times in 10 mM NaBO3 (pH 9.2), incubated for 1 h in 10 mM NaBO3 + 0.3% H2O2, washed 3 times with 10 mM NaBO3 (pH 9.2) and stored for further processing in AbA buffer (1 × PBS, 0.1% triton X-100, 1% BSA, 0.05% NaN3). The PAT-9 antibody was diluted 1:500 in AbA buffer, incubated overnight at 4°C, washed three times with AbB buffer (1 × PBS, 0.1% triton X-100, 0.1% BSA, 0.05% NaN3) and incubated with secondary antibody (Alexa 488 goat anti-rabbit) at 4°C overnight. After washing with AbB three times, the pellets were mounted on slides for fluorescence microscopy.
The authors thank Dr. Yuji Kohara (the National Institute of Genetics, Mishima, Shizuoka, Japan) for providing cDNA clones, and Dr. Andy Fire (Stanford University) and Dr. Phil Newmark (University of Illinois at Urbana-Champaign) for plasmid vectors. The monoclonal antibodies were developed by R. H. Waterston and obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by The University of Iowa, Department of Biology, Iowa City, IA 52242. This work was funded by a National Institutes of Health Grant [RO1HD38464] to BDW and institutional support from the University of Illinois at Urbana-Champaign for BDW and PLJ.
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