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RESEARCH |
Institute for Molecular Bioscience, The University of Queensland, Brisbane, Queensland 4072, Australia, 1 Human Molecular Genetics Laboratory, Prince Henrys Institute of Medical Research, Monash Medical Centre, Clayton, Victoria 3168, Australia 2 Murdoch Childrens Research Institute and Department of Paediatrics, University of Melbourne, Royal Childrens Hospital, Parkville, Victoria 3052, Australia
Correspondence should be addressed to P Koopman; Email: p.koopman{at}imb.uq.edu.au
| Abstract |
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| Introduction |
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Of the various cell lineages making up the testis, only the Sertoli cells express Sry (Albrecht & Eicher 2001, Sekido et al. 2004). Sertoli cells are known to influence differentiation of other testicular cell types and thus play a critical inductive role in testis development (Burgoyne et al. 1988; reviewed by Wilhelm & Koopman 2006). One of the first critical steps in the testis-determining pathway is activation of Sox9 (Vidal et al. 2001, Chaboissier et al. 2004, Barrionuevo et al. 2006). However, the genetic pathways downstream of Sry that regulate sex determination and Sertoli cell differentiation remain largely unclear.
To further understand the regulatory pathways governing these processes, it is necessary to identify novel genes involved in sex-specific differentiation of the gonad. Many researchers have used expression-screening strategies, including microarray studies, in an effort to identify a plethora of genes showing sexually dimorphic expression in gonads. We decided to further explore the spatiotemporal expression pattern of a novel gene with accession no. BC019731, which was shown in two such studies to be up-regulated at 11.5 dpc XY gonads when compared with XX gonads (Nef et al. 2005, Beverdam & Koopman 2006).
Tmem184a spans a 9.3 kb region on chromosome 5 and was initially identified in a mouse cDNA library screen (Strausberg et al. 2002), with subsequent updated annotations (Nef et al. 2005). TMEM184a has a predicted kinase activity; however, its amino acid sequence does not align to any known domains and hence the function of this protein is unknown. Mouse Tmem184a has putative orthologs in a number of mammalian species, including human (Homo sapiens; Q6ZMB5), chimpanzee (Pan troglodytes; LOC735493), dog (Canis familiaris; LOC489896), and rat (Rattus norvegicus; RGD1306702). Homology between these putative orthologs is more than 80%, and hence sufficiently high to be confident that the genes are orthologs and potentially have conserved function across species. In mouse, only one paralogous gene (4732495E13Rik/Tmem184b) has been identified (sharing putative domains of around 80% homology), with the two gene products making up a predicted MAP kinase-activating protein family of two members.
The involvement of kinase regulatory pathways during testis differentiation is poorly understood and has the potential to shed light on several unexplained aspects of these morphological events. As a first step toward elucidating the role of Tmem184a during mouse developments, we studied in detail its expression pattern and found it to be specifically up-regulated within the testis shortly after gonadal sex determination, with continued testis-specific expression into adulthood. The restricted expression of this gene in Sertoli cells suggests a role for the encoded protein in the development and/or continued function of that cell type.
| Materials and Methods |
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RNA isolation, cDNA synthesis, and SYBR green real-time RT-PCR
Fetal gonads were collected by dissection, pooled according to sex and developmental stage (one litter per pool), and total RNA was isolated using the SV Total RNA Isolation System (Promega) as per manufacturers instructions. cDNA was synthesized from 1 µg total RNA by RT (Superscript III, Invitrogen) using random primers (Promega) according to manufacturers instructions. Relative cDNA levels were analyzed by quantitative real-time RT-PCR (qRT-PCR) using an ABIPrism-7000 Sequence Detector System.
For statistical significance, all qRT-PCR experiments were performed in triplicate and repeated thrice on separate biological samples, each representing one pooled litter (710 littermates/sex), and finally represented as mean ± S.E.M. of the three individual experiments. Samples were analyzed in 25 µl reactions containing SYBR Green PCR Master Mix (Applied Biosystems, Foster City, CA, USA), 3.75 µM each forward and reverse primers, and 1 µl cDNA, prepared as described using an ABIPrism-7000 Sequence Detector System. PCR was conducted over 40 cycles of 95 ° C for 15 s and 60 ° C for 1 min in a two-step thermal cycle, preceded by an initial 10-min step at 95 ° C to activate the Amplitaq Gold DNA polymerase. The primers were designed using the Universal Probe library tool (http://www.roche-applied-science.com/sis/rtpcr/upl/adc.jsp) as previously described (Beverdam & Koopman 2006) and for this study, the primers used were rtTmem184a.F: 5'-CTGCATTGTGAAACCCGTTA and rtTmem184a.R: 5'-GGTAGCCGCTGTGGATGT; rtSox9.F: 5'-AGTACCCGCATCTGCACAAC and rtSox9.R: 5'-TACT-TGTAATCGGGGTGGTCT; and rtOct4.F: 5'-TGC-GGAGGGATGGCATACTG and rtOct4.R: 5'-GCACA-GGGCTCAGAGGAGGTTC. To adjust for variation in input cDNA, samples were normalized against 18S RNA using the ΔCt method. 18S primers used were rt18S.F: 5'-GATCCATTGGAGGGCAAGTCT and rt18S.R: 5'-CCAA-GATCCAACTACGAGCTTTTT. Dissociation curve analyses were performed.
Whole-mount and section in situ hybridization (WISH/SISH)
Embryos and dissected gonads/mesonephroi were fixed in 4% paraformaldehyde (PFA) in PBS for several hours at 4 ° C. WISH with digoxygenin (DIG)-labeled RNA probes was carried out essentially as described by Hargrave et al.(2006). For SISH samples, whole embryos were processed and mounted in paraffin wax and stored at 4 ° C. SISH was performed on 7 µm sagittal sections that were dewaxed, rehydrated, and incubated in 5 µM/ml proteinase K for 20 min at room temperature (RT), after which samples were washed in PBS. Sections were re-fixed with 4% PFA for 10 min at RT, acetylated, and pre-hybridized with hybridization solution (50% formamide, 5 x SSC, 5 x Denhardts solution, 250 µg/ml yeast RNA, and 500 µg/ml herring sperm DNA) for 2 h at RT. Hybridization (0.5 µg/ml probe in hybridization solution) was performed overnight at 60 ° C. Slides were washed in 5 x SSC for 5 min, 0.2 x SSC for 1 h at 60 ° C, 0.2 x SSC for 5 min at RT, and NT buffer (150 mM NaCl, 50 mM TrisHCl (pH 7.5)) for 5 min at RT, before blocking for 2 h with blocking solution (10% heat-inactivated sheep serum in NT buffer) in a humidified chamber. Anti-DIG antibody (Roche) in blocking solution (1:2000) was added to the slides and incubated overnight at 4 ° C. Unbound antibodies were removed by washing thrice in NT buffer. Section were equilibrated in NTM buffer (100 mM NaCl, 100 mM TrisHCl (pH 9.5), 50 mM MgCl2) and incubated in color solution (3.5 µl BCIP (Roche), 3.5 µl NBT (Roche) per ml NTM buffer) until staining was detected.
A 273 bp Tmem184a fragment was cloned from position 419692 (accession no. BC019731) into pGEM-T Easy vector (Promega). Primers used were Tmem184a.F: 5'-TTCCCCAGCCAGGTGTATTCAG and Tmem184a.R: 5'-TGCGTTTCTCTATGTTGCGACTC. The probes for Amh, Sox9, and Oct4 were made as previously described (Münsterberg & Lovell-Badge 1991, Wright et al. 1995, Schepers et al. 2003).
| Results |
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| Discussion |
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The 449 amino acid TMEM184A protein contains seven putative transmembrane helices, strongly suggesting a role in transmembrane signaling pathways acting in or on developing Sertoli cells. It is possible that Tmem184a may mediate signaling between germ cells and Sertoli cells, or between other somatic cells and Sertoli cells, or alternatively within regulatory pathways governing architectural compartmentalization of the testis. In addition, the expression of Tmem184a in the adult testis suggests a continuing role for TMEM184A function. Although detectable in other tissues in both fetal and adult mice, the expression of Tmem184a was significantly higher in testes, strengthening the concept that TMEM184A may elicit specific functions limited to the testis, potentially mediating a male-specific cascade of phosphorylation during Sertoli cell differentiation and/or function. In addition, the restricted expression pattern of Tmem184a makes this gene an excellent target for functional analysis by gene targeting in mice.
Due to the observed tissue-specific expression of Tmem184a and its up-regulation from 11.5 dpc, we hypothesize a close transcriptional regulation with the testis-determining factor SRY, either directly or indirectly. After the onset of Sry expression in the pre-Sertoli cell lineage around 10.5 dpc, the testis enters an active phase of proliferation and differentiation, which dictates both inter- and intracellular signaling pathways, almost certainly involving kinase pathways and phosphorylation cascades. However, as Tmem184a expression persists into adulthood, we hypothesize that TMEM184A function is distinct from sex determination, but is integral to testis differentiation. Further studies will focus on determining protein structure and sub-cellular localization in order to better understand the likely cellular functions of TMEM184A protein. It will be especially beneficial to elucidate whether TMEM184A is a cellular transduction molecule or kinase receptor, ultimately allowing for extensive in vitro and ex vivo studies after the characterization of effector molecules.
| Acknowledgements |
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| Footnotes |
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| References |
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