Functional analysis of Mmd2 and related PAQR genes during sex determination in mice

Functional analysis of Mmd2 and related PAQR genes during sex determination in mice | bioRxiv /* */ /* */ <!-- <!-- /*! * yepnope1.5.4 * (c) WTFPL, GPLv2 */ (function(a,b,c){function d(a){return"[object Function]"==o.call(a)}function e(a){return"string"==typeof a}function f(){}function g(a){return!a||"loaded"==a||"complete"==a||"uninitialized"==a}function h(){var a=p.shift();q=1,a?a.t?m(function(){("c"==a.t?B.injectCss:B.injectJs)(a.s,0,a.a,a.x,a.e,1)},0):(a(),h()):q=0}function i(a,c,d,e,f,i,j){function k(b){if(!o&&g(l.readyState)&&(u.r=o=1,!q&&h(),l.onload=l.onreadystatechange=null,b)){"img"!=a&&m(function(){t.removeChild(l)},50);for(var d in y[c])y[c].hasOwnProperty(d)&&y[c][d].onload()}}var j=j||B.errorTimeout,l=b.createElement(a),o=0,r=0,u={t:d,s:c,e:f,a:i,x:j};1===y[c]&&(r=1,y[c]=[]),"object"==a?l.data=c:(l.src=c,l.type=a),l.width=l.height="0",l.onerror=l.onload=l.onreadystatechange=function(){k.call(this,r)},p.splice(e,0,u),"img"!=a&&(r||2===y[c]?(t.insertBefore(l,s?null:n),m(k,j)):y[c].push(l))}function j(a,b,c,d,f){return q=0,b=b||"j",e(a)?i("c"==b?v:u,a,b,this.i++,c,d,f):(p.splice(this.i++,0,a),1==p.length&&h()),this}function k(){var a=B;return a.loader={load:j,i:0},a}var l=b.documentElement,m=a.setTimeout,n=b.getElementsByTagName("script")[0],o={}.toString,p=[],q=0,r="MozAppearance"in l.style,s=r&&!!b.createRange().compareNode,t=s?l:n.parentNode,l=a.opera&&"[object Opera]"==o.call(a.opera),l=!!b.attachEvent&&!l,u=r?"object":l?"script":"img",v=l?"script":u,w=Array.isArray||function(a){return"[object Array]"==o.call(a)},x=[],y={},z={timeout:function(a,b){return b.length&&(a.timeout=b[0]),a}},A,B;B=function(a){function b(a){var a=a.split("!"),b=x.length,c=a.pop(),d=a.length,c={url:c,origUrl:c,prefixes:a},e,f,g;for(f=0;f<d;f++)g=a[f].split("="),(e=z[g.shift()])&&(c=e(c,g));for(f=0;f<b;f++)c=x[f](c);return c}function g(a,e,f,g,h){var i=b(a),j=i.autoCallback;i.url.split(".").pop().split("?").shift(),i.bypass||(e&&(e=d(e)?e:e[a]||e[g]||e[a.split("/").pop().split("?")[0]]),i.instead?i.instead(a,e,f,g,h):(y[i.url]?i.noexec=!0:y[i.url]=1,f.load(i.url,i.forceCSS||!i.forceJS&&"css"==i.url.split(".").pop().split("?").shift()?"c":c,i.noexec,i.attrs,i.timeout),(d(e)||d(j))&&f.load(function(){k(),e&&e(i.origUrl,h,g),j&&j(i.origUrl,h,g),y[i.url]=2})))}function h(a,b){function c(a,c){if(a){if(e(a))c||(j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}),g(a,j,b,0,h);else if(Object(a)===a)for(n in m=function(){var b=0,c;for(c in a)a.hasOwnProperty(c)&&b++;return b}(),a)a.hasOwnProperty(n)&&(!c&&!--m&&(d(j)?j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}:j[n]=function(a){return function(){var b=[].slice.call(arguments);a&&a.apply(this,b),l()}}(k[n])),g(a[n],j,b,n,h))}else!c&&l()}var h=!!a.test,i=a.load||a.both,j=a.callback||f,k=j,l=a.complete||f,m,n;c(h?a.yep:a.nope,!!i),i&&c(i)}var i,j,l=this.yepnope.loader;if(e(a))g(a,0,l,0);else if(w(a))for(i=0;i (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0];var j=d.createElement(s);var dl=l!='dataLayer'?'&l='+l:'';j.src='//www.googletagmanager.com/gtm.js?id='+i+dl;j.type='text/javascript';j.async=true;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-M677548'); Skip to main content Home About Submit ALERTS / RSS Search for this keyword Advanced Search New Results Functional analysis of Mmd2 and related PAQR genes during sex determination in mice Liang Zhao , Ella Thomson , Ee Ting Ng , Enya Longmuss , Terje Svingen , Stefan Bagheri-Fam , Alexander Quinn , Vincent R. Harley , Leonard C. Harrison , Emanuele Pelosi , Peter Koopman doi: https://doi.org/10.1101/2021.07.10.451886 Liang Zhao 1 Institute for Molecular Bioscience, The University of Queensland , Brisbane, Queensland 4072, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Ella Thomson 1 Institute for Molecular Bioscience, The University of Queensland , Brisbane, Queensland 4072, Australia 2 Centre for Clinical Research, The University of Queensland, Royal Brisbane & Women’s Hospital , Herston, Queensland 4029, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Ee Ting Ng 1 Institute for Molecular Bioscience, The University of Queensland , Brisbane, Queensland 4072, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Enya Longmuss 1 Institute for Molecular Bioscience, The University of Queensland , Brisbane, Queensland 4072, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Terje Svingen 3 National Food Institute, Technical University of Denmark , 2800 Kongens Lyngby, Denmark Find this author on Google Scholar Find this author on PubMed Search for this author on this site Stefan Bagheri-Fam 4 Centre for Endocrinology and Metabolism, Hudson Institute of Medical Research, Monash Medical Centre , Melbourne, Victoria 3168, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Alexander Quinn 1 Institute for Molecular Bioscience, The University of Queensland , Brisbane, Queensland 4072, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Vincent R. Harley 4 Centre for Endocrinology and Metabolism, Hudson Institute of Medical Research, Monash Medical Centre , Melbourne, Victoria 3168, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Leonard C. Harrison 5 The Walter and Eliza Hall Institute of Medical Research , Parkville, Victoria 3052, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Emanuele Pelosi 1 Institute for Molecular Bioscience, The University of Queensland , Brisbane, Queensland 4072, Australia 2 Centre for Clinical Research, The University of Queensland, Royal Brisbane & Women’s Hospital , Herston, Queensland 4029, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site For correspondence: p.pelosi{at}uq.edu.au Peter Koopman 1 Institute for Molecular Bioscience, The University of Queensland , Brisbane, Queensland 4072, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Abstract Full Text Info/History Metrics Supplementary material Preview PDF Abstract Sex determination in eutherian mammals is controlled by the Y-linked gene Sry , which drives the formation of testes in male embryos. Despite extensive study, the genetic steps linking Sry action and male sex determination remain largely unknown. Here, we focused on Mmd2 , a gene that encodes a member of the progestin and adipoQ receptor (PAQR) family. We show that Mmd2 is expressed during the sex-determining period in XY but not XX gonads, specifically in the Sertoli cell lineage which orchestrates early testis development. Analysis of knockout mice deficient in Sox9 and Sf1 revealed that Mmd2 operates downstream of these known sex-determining genes. However, when we used CRISPR to ablate Mmd2 in the mouse, fetal testis development appeared to progress normally. To determine if other genes might have compensated for the loss of Mmd2 , we identified the closely related PAQR family members Paqr8 and Mmd as also being expressed during testis development. We used CRISPR to generate mouse strains deficient in Paqr8 and Mmd , but both knockout lines appeared phenotypically normal and fertile. Finally, we generated Mmd2 ; Mmd and Mmd2 ; Paqr8 double-null embryos and again observed normal testis development. These results may reflect functional redundancy among these factors. Our findings highlight the difficulties involved in identifying genes with a functional role in sex determination and gonadal development through expression screening and loss-of-function analyses of individual candidate genes, and may help to explain the paucity of genes in which variations have been found to cause human disorders/differences of sex development. Introduction In most eutherian mammals, male sex determination is regulated by the sex-determining region Y gene Sry ( Gubbay et al., 1990 ; Koopman et al., 1991 ; Sinclair et al., 1990 ). When expressed in fetal genital ridges, the transcription factor SRY, together with co-factor SF1 (also known as NR5A1), triggers the expression of Sox9 ( Sekido and Lovell-Badge, 2008 ), which drives the differentiation of Sertoli cells and hence the differentiation of testes ( Vidal et al., 2001 ). In the absence of Sry expression or subsequent upregulation of Sox9 , the bipotential gonads develop instead into ovaries. Despite three decades of research and significant progress in the field, many aspects of the molecular mechanisms regulating fetal testis development remain unclear. As a result, the cause of most 46,XY disorders/differences of sex development (DSD) remains unknown ( Eggers et al, 2016 ). The progestin and adipoQ receptor (PAQR) family of proteins consists of 11 membrane receptors that are found throughout eukaryotes and in some eubacteria, and show high conservation between human and mouse ( Tang et al., 2005 ). One member of this family, Mmd2 (also known as Paqr10 ), has previously been identified in a number of screening studies as being expressed in pre-Sertoli cells of the developing testis, but not in the ovary ( Menke and Page, 2002 ; Beverdam and Koopman, 2006 ; Bouma et al., 2007 ; Cory et al., 2007 ; Jameson et al., 2012 ; Nef et al., 2005 ; Small et al., 2005 ; Zhao et al., 2018 ). In view of these consistent observations, we decided to investigate a possible role for Mmd2 in testis development. Previous studies have described roles for Mmd2 in the developing pancreas and brain. The encoded protein was shown to act in mitochondria to regulate pancreatic endocrine cell development/survival ( Gonez et al., 2008 ). It also functions during gliogenesis, where it was found to localize to the Golgi apparatus, regulating ERK/MAPK signaling by tethering and activating Ras proteins, and maintaining their localization within the Golgi ( Jin et al., 2012a , 2012b ). The possible link with ERK/MAPK signalling may be relevant in the context of male sex determination, given that a number of regulators of this pathway are known to play a role in testis development ( Pearlman et al., 2010 ; Bogani et al., 2009 ; Warr et al., 2016 ). Further, Mmd2 is regulated by SOX9 to control the proliferation of glial precursors within the spinal cord ( Kang et al., 2012 ), raising the possibility that it may also be regulated by SOX9 during testis development. Accordingly, Mmd2 is among 1903 genes identified as potential SOX9 targets by chromatin immunoprecipitation of mouse fetal testis extracts ( Li et al., 2014 ), although a direct regulatory relationship was not confirmed experimentally. In the present study, we studied in detail the expression dynamics of Mmd2 in the developing mouse fetal gonads and established Mmd2 as one of the earliest Sertoli cell markers. We confirmed that MMD2 is localized to the Golgi apparatus in Sertoli-like cells and acts downstream of Sox9 . However, fetal testes appeared normal in mice lacking Mmd2 alone and in combination with Mmd and Paqr8 ; two PAQR family members expressed in the developing gonads. These results may reflect functional redundancy within the PAQR family during gonadal development. Materials and methods Mice Mouse embryos were collected from timed matings of the relevant strains, with noon of the day when the mating plug was observed designated 0.5 dpc. Embryos were sexed by PCR ( McFarlane et al., 2013 ) and by morphological assessment of the gonads (12.5–15.5 dpc embryos). To ensure correct staging at 10.5 and 11.5 dpc, tail somites (ts) were counted as described ( Hacker et al., 1995 ): ts 7–9 at 10.5 dpc and ts 17–19 at 11.5 dpc. W e strain ( Buehr et al., 1993 ), Wt1-RG red-green reporter strain ( Zhao et al., 2014b ), AMH-Cre; Sox9 flox/flox mice (Barrionueo et al., 2009), and Cited2 -knockout strain ( Barbera et al., 2002 ) have been described previously. To generate the Mmd2 flox allele, two lox P sites were inserted into the Mmd2 locus flanking exons 3 and 4 by gene targeting (Ozgene). The Mmd2 flox allele was recombined using a CMV-Cre allele ( Schwenk et al., 1995 ) to generate the Mmd2 -null allele. The CMV-Cre allele was subsequently excluded from the line by selective breeding. The Mmd -null and Paqr8 -null alleles were generated using the Alt-R CRISPR/Cas9 system (IDT). Briefly, for each deletion, a pair of crRNAs were separately annealed with tracrRNA. Each duplex RNA was incubated with Cas9 protein to form Cas9 ribonucleoprotein (RNP). Two Cas9 RNP preparations were then mixed 1:1 and subsequently microinjected into B6 one-cell embryos. In the microinjection cocktail, the final concentration of crRNA and tracrRNA was 15 ng/μl with Cas9 protein at 60 ng/μl. Injected embryos were incubated overnight to the two-cell stage and surgically transferred to pseudopregnant CD1 females as described ( Zhao et al., 2014a ). Sequences of crRNAs and genotyping primers are described in Supplementary Table 1. All mutant strains were maintained on a C57BL/6 background. Animal experimentation was approved and carried out according to the guidelines established by the University of Queensland Animal Ethics Committee. Quantitative RT-PCR (RT-qPCR) For expression analysis in wild type CD1, AMH- Cre ; Sox9 flox/flox , or Cited2 -knockout mice, fetal gonads were collected with mesonephros removed. For expression analysis in Mmd2 , Mmd , or Paqr8 single or double knock-out gonads, fetal gonads with mesonephroi were pooled for analysis. RT-qPCR analysis was as described previously ( Zhao et al., 2015 ), and primer sequences are provided in Supplementary Table 1. In situ hybridization (ISH) Embryos and embryonic gonads were dissected and fixed in 4% paraformaldehyde (PFA) in phosphate-buffered saline (PBS) solution at 4°C overnight and subsequently washed with PBS. Whole-mount ISH was carried out as described ( Hargrave et al., 2006 ). For section ISH, embryos were mounted in paraffin and sectioned at 7 μm using a microtome. Section ISH was carried out as described ( Zhao et al., 2017 ). In situ probe was generated using PCR (primer sequences described in Supplementary Table 1). Cell Sorting Multiple pairs (n >4) of fetal gonads (removed from the mesonephroi) were dissected from Wt1-RG embryos ( Zhao et al., 2014b ) at 13.5 dpc. Gonadal cells were dissociated and sorted for mCherry and EGFP fluorescence as described ( Zhao et al., 2017 ). Immunofluorescence staining of transfected cells The coding region of mouse Mmd2 was cloned into pcDNA3 to generate pcDNA-Mmd2. This was transfected into 15P-1 ( Paquis-Flucklinger et al., 1993 ) or TM4 ( Mather, 1980 ) cells, two mouse Sertoli-like cell lines. Transfected cells were immuno-stained as described ( Neumann et al., 2008 ). Primary antibodies were used at the following dilutions: mouse anti-MMD2 (Abcam), 1:100; and rabbit anti-Giantin (Abcam) 1:200. Immunofluorescence staining of mouse tissue sections Immunofluorescence was performed on 7-μm paraffin sections as described ( Zhao et al., 2015 ). Images were captured on a LSM710 confocal microscope (Zeiss). Primary antibodies were used at the following dilutions: goat-anti-AMH (Santa Cruz), 1:500; rabbit anti-HSD3b (Transgenic Inc.), 1:500; goat anti-FOXL2 (NovusBio), 1:100; and mouse anti-SOX9 (Abnova), 1:200. Results Mmd2 is specifically up-regulated in Sertoli cells in the developing fetal testis We first determined the expression of Mmd2 in male and female mouse gonads from 10.5 to 15.5 dpc by RT-qPCR. Mmd2 mRNA was undetectable in XX gonads at all developmental stages, whereas XY gonads showed an increasing level of mRNA from 10.5 dpc to 15.5 dpc ( Fig 1A ). These results were confirmed by whole-mount in situ hybridization (WISH) where no expression was evident in the ovaries ( Fig 1B ). Conversely, Mmd2 started to be expressed in the undifferentiated testis from around 11.5 dpc, and staining increased in intensity until 15.5 dpc ( Fig 1B ). Download figure Open in new tab Figure 1. Mmd2 is specifically up-regulated in Sertoli cells in the developing mouse testis. (A) RT-qPCR showing Mmd2 expression specifically up-regulated in mouse fetal testis from 11.5 to 15.5 dpc. Mean ± s.e.m., n = 3. (B) Whole-mount in situ hybridisation suggesting Mmd2 expression restricted in testis cords from 12.5 dpc onwards in fetal testes. n = 3-5 per sex per stage; representative images are shown. (C) Section in situ hybridisation at 13.5 dpc confirming that Mmd2 expression was predominantly detected within testis cords (n = 3; a representative image is shown). (D) Whole-mount in situ hybridisation showing the presence of Mmd2 transcripts in the W e fetal testis (n = 4; representative images are shown). (E) RT-qPCR analysis on Wt1-RG sorted cell populations confirming that Mmd2 was predominantly expressed in XY RG Hi cell population enriched for Sertoli cells. Cells from multiple pairs of fetal gonads were pooled and sorted. Mean ± s.e.m. of triplicate RT-qPCR reactions. ** P < 0.01 (Sidak’s multiple comparisons test). Mmd2 expression localized to the testis cords ( Fig 1B ), with in situ hybridization signal suggesting that Mmd2 marks Sertoli cells that are situated between the central germ cells and the peripheral layer of myoid cells ( Fig 1C ). To address whether Mmd2 is expressed in germline or somatic cells, we also analysed expression in the We mouse strain. This line harbours a mutation in the germ cell marker c-kit , resulting in gonads lacking germ cells, but retaining somatic cells. WISH showed no difference in staining between wild type and W e XY gonads, confirming Mmd2 expression within the somatic cells of the testis cords ( Fig 1D ). To further define the specific cell type expressing Mmd2 , we used the Wt1-RG red green reporter mouse strain ( Zhao et al., 2014b ). We previously showed that Sertoli, fetal Leydig and germ cell populations can be identified by sorting via mCherry and EGFP fluorescence, and analysis of specific lineage markers ( Zhao et al., 2017 ). In XY gonads, cells showing strong expression of the RG reporter proteins are Sertoli cells as confirmed by expression of markers including Sox9 and Amh . Cells with low levels of RG are Leydig cells, characterized by the specific expression of Hsd3b . RG-negative cells are germ cells, expressing the marker Mvh ( Zhao et al., 2017 ). According to these criteria, Mmd2 transcript was strongly associated with Sertoli cells, with very low levels present in the fetal Leydig population, and none in XY germ cells or any XX gonadal cell populations ( Fig 1E ). Hence, the timing and cell type-specificity of Mmd2 expression are consistent with a role in regulating sex determination. Mmd2 expression is dependent on SOX9 and SF1 Sox9 is one of the major downstream effectors of SRY and plays a critical role in sex determination. To determine whether Mmd2 acts up- or downstream of SOX9, we used the AMH- Cre ; Sox9 flox/flox mouse line, in which Sox9 is conditionally deleted in the somatic Sertoli cells. At 13.5 dpc, ablation of Sox9 led to a 43% reduction of Mmd2 expression compared to wild type ( Fig. 2A ), suggesting expression is downstream of, and partially dependent on, SOX9 within these cells. This finding is consistent with previous studies showing that Mmd2 is a SOX9 target in the spinal cord and testes ( Kang et al., 2012 ; Li et al., 2014 ). Download figure Open in new tab Figure 2. Mmd2 expression is dependent on Sox9 and Sf1 . RT-qPCR analysis showing significant down-regulation of Mmd2 in either AMH-Cre; Sox9 flox/flox (13.5 dpc; A) or Cited2 −/− fetal testes (12.5 dpc; B), which have reduced Sf1 expression. (A) n = 3 (XY KO), 4 (XY WT), or 5 (XX WT). KO, knockout; WT, wild type. (B) n = 3. Mean ± s.e.m., ** P < 0.01 (Sidak’s multiple comparisons test). SF1 is an important transcription factor involved in sex determination and development. Together with SRY, SF1 upregulates Sox9 expression, and acts with SOX9 to activate transcription of a number of genes involved in testis development ( De Santa et al., 1998 ; Wilson et al., 2005 ; Wilhelm et al., 2007 ; Sekido et al., 2008 ; Kashimada et al., 2011 ). Mice deleted for Sf1 lack gonads, making them unsuitable for this study ( Luo et al., 1994 ; Sadovsky et al., 1995 ). Therefore, to determine if Mmd2 expression was also dependent on SF1, we used the Cited2 −/− mouse model. Cited2 interacts with the transcription factor WT1 within the embryonic gonads, to regulate the expression of Sf1 ( Buaas and Swain, 2009 ). A Cited2−/− line shows hypomorphic Sf1 expression that can be used to study the development of the testis ( Combes et al., 2010 ). We found that Cited2 −/− gonads showed a significant reduction in Mmd2 expression compared to wild type at 12.5 dpc, suggesting that Mmd2 is to some extent dependent on, and downstream of, SF1. MMD2 localizes to the Golgi complex in Sertoli-like cells The MMD2 protein has variously been shown to localize to the Golgi complex or to mitochondria ( Jin et al., 2012a ; Jin et al., 2012b ; Gonez et al., 2008 ). We examined the cellular localization of MMD2 in vitro using the Sertoli-like cell lines 15p-1 and TM4, transfected with an Mmd2 expression construct. MMD2 co-localized with the Golgi apparatus marker Giantin in both cell lines ( Figure 3 ). However, no co-localization was evident using the mitochondrion-specific dye Mitotracker (data not shown). These results indicate that MMD2 localizes to the Golgi complex of Sertoli cells. Download figure Open in new tab Figure 3. MMD2 localises to Golgi apparatus in mouse Sertoli-like cell lines. MMD2 co-localised with Giantin, a Golgi marker, in transfected 15P-1 or TM4 cells. Scale bar, 10 μm. Ablation of Mmd2 does not affect testis determination and development To investigate the function of Mmd2 in testis determination or development, we generated a mouse line in which exons 3 and 4 of Mmd2 were flanked with loxP recombination sites, and bred these mice with a line ubiquitously expressing Cre recombinase (CMV-Cre; Schwenk et al., 1995 ) to functionally ablate one ( Mmd2 +/− ) or both ( Mmd2 −/− ) alleles ( Fig 4A ). Gene expression analysis confirmed ablation of Mmd2 , showing a 50% reduction in intact transcript levels in Mmd2 +/− XY gonads compared to wild-type, and no detectable expression of intact transcripts in Mmd2 −/− XY gonads ( Fig 4B ). Download figure Open in new tab Figure 4. Fetal gonads develop normally in mice lacking Mmd2 . (A) Mmd2 flox allele was generated by inserting two loxP sites flanking exons 3 and 4. Mmd2 − (null) allele was generated by in vivo recombination using the CMV-Cre line. (B) RT-qPCR analysis confirming the absence of Mmd2 expression in Mmd2 −/− fetal gonads. Mean ± s.e.m., n = 3 (XX +/+ ; XY +/− ), 4 (XY +/+ ; XY −/− ; and XX −/− ), or 5 (XX +/− ). (C) Immunofluorescence analysis showing unperturbed expression of sex markers, including SOX9, FOXL2 (upper panel), HSD3β and AMH (bottom panel), in Mmd2 −/− fetal gonads. Mmd2 +/− fetal gonads were included as controls. Scale bar, 50 μm. Mmd2 +/− and Mmd2 −/− mice were viable, with no gross phenotypic abnormalities. Embryonic gonads were also grossly normal, with no evidence of gonadal sex reversal or morphological abnormalities. Mutant gonads were collected at 13.5dpc and analyzed by RT-qPCR for expression of a range of lineage marker genes, but no significant differences were seen between wild-type, Mmd2 +/− , and Mmd2 −/− gonads (Supplementary Figure S1). Sertoli cell markers SOX9 and AMH were analyzed at the protein level by immunofluorescence. In XY testes, both SOX9 and AMH appeared similar in intensity and localization between Mmd2 −/− samples and Mmd2 +/− controls, whereas they were not detected in XX ovaries, as expected ( Fig 4C ). The Leydig cell marker HSD3B was also assessed and showed no difference in expression between XY Mmd2 +/− and XY Mmd2 −/− testes, whereas HSD3B was absent in XX Mmd2+/− ovaries, as expected, and in XX Mmd2−/− ovaries ( Fig 4C ). Finally, we analyzed the expression of ovarian marker FOXL2 to confirm there was no ectopic development of the granulosa cell lineage within the embryonic Mmd2 −/− testes. Like in XY Mmd2 +/− testes, no FOXL2 expression was detected in XY Mmd2 −/− testes, while FOXL2 was expressed throughout the XX Mmd2 +/− and Mmd2 −/− ovaries ( Fig 4C ). Hence, loss of Mmd2 caused no detectable impairment of testis development. Expression of other PAQR family members in fetal testes The lack of an obvious phenotype in XY Mmd2 −/− gonads raised the possibility that other members of the PAQR family might compensate for the loss of MMD2. To test this hypothesis, we focused on two other Paqr genes, Mmd ( Paqr11 ) and Paqr8 , based on their known function and expression profile. We surveyed our published RNA-Seq dataset derived from analysis of fetal gonads from 10.5-13.5dpc ( Zhao et al., 2018 ), to identify other Paqr genes expressed in mouse fetal gonads (Supplementary Figure 2). We found that Mmd was robustly expressed in both male and female gonads, peaking between 10.5 and 11.5 dpc, a critical window of sex determination (Supplementary Figure S2). In addition, functional redundancy between MMD2 and MMD has previously been reported in zebrafish and in vitro experiments ( Huang et al., 2012 ; Jin et al., 2012a ). A second gene, Paqr8 , showed consistently higher expression in the male gonad from 11.5 to 13.5dpc, when testis differentiation takes place (Supplementary Figure S2). We first validated the expression profiles of Mmd and Paqr8 using RT-qPCR on 10.5 to 15.5 dpc gonads. We confirmed that Mmd was expressed at similar levels in both XX and XY gonads and was especially strong around the time of sex determination ( Fig 5A ). Also, in support of RNA-seq data, Paqr8 was expressed more strongly in XY gonads compared to XX in the period 11.5 – 13.5 dpc ( Fig 5B ). Download figure Open in new tab Figure 5. Mmd and Paqr8 are expressed in somatic cells in mouse fetal gonads. (A, B) RT-qPCR showing Mmd (A) and Paqr8 (B) expression up-regulating in mouse fetal testis from 11.5 to 15.5 dpc. Mean ± s.e.m., n = 3. (C, D) RT-qPCR analysis on Wt1-RG sorted cell populations confirming that both Mmd and Paqr8 were expressed in somatic cells in mouse fetal gonads. Within fetal testes, both genes were expressed predominantly in XY RG Hi cell population enriched for Sertoli cells. Cells from multiple pairs of fetal gonads were pooled and sorted. (E, F) RT-qPCR showing expression of Mmd (E) and Paqr8 (F) in gonads from XY and XX mice that were wild type (+/+), heterozygous (+/−) or null (−/−) for Mmd2 . Mean ± s.e.m. of triplicate RT-qPCR reactions. ** = p <0.01 (Sidak’s multiple comparisons test); ns = not significant. We then identified the cell types expressing Mmd and Paqr8 using the Wt1-RG mouse line. Like Mmd2 , both Mmd and Paqr8 were highly expressed in the Sertoli cells of the testis ( Fig 5C, D ). However, expression was more widespread than Mmd2 , as both Mmd and Paqr8 were also expressed in fetal Leydig cells and somatic ovarian cells, though at lower levels ( Fig 5C, D ). The expression of Mmd and Paqr8 was measured in the Mmd2 mutant gonads by RT-qPCR to determine if loss of Mmd2 prompted a compensatory response resulting in their up-regulation. We observed a small but significant increase of Mmd expression in Mmd2 −/− males, but not females ( Fig 5E ). A trend of increased expression was also observed for Paqr8 in Mmd2 −/− testes ( Fig 5F ), although it was not statistically significant. To investigate if Mmd or Paqr8 are expressed in a SOX9-dependent manner, as is Mmd2 , we analyzed their expression in the AMH-Cre; Sox9 flox/flox line. No significant reduction was observed (data not shown), suggesting that Mmd and Paqr8 expression is independent of SOX9 in fetal testes. Ablation of either Mmd or Paqr8 function in mice does not affect fetal testis development To investigate whether Mmd or Paqr8 are required for sex determination or gonadal development, we used CRISPR-Cas9 genome editing to create mouse lines lacking the function of each gene ( Mmd −/− : Fig 6A and Paqr8 −/− : Fig 7A ). Gene expression profile analyses using primers specific for the deleted exons confirmed successful disruption of Mmd and Paqr8 respectively ( Figs 6B, 7B ). Interestingly, we observed an upregulation of Mmd2 and Paqr8 in Mmd −/− testes, although significant only for Mmd2 ( Fig 6B ). Similarly, Mmd2 and Mmd were modestly upregulated in the Paqr8 testes, but this was not significant (p = 0.054) ( Fig 7B ). Download figure Open in new tab Figure 6. Development of Mmd -null fetal gonads appears normal. (A) Generation of Mmd − (null) allele via CRISPR in mouse zygotes. cr-5’/3’, crRNA-5’/3’. (B) RT-qPCR analysis confirming lack of Mmd expression in the knockout gonads. Note that Mmd2 and Paqr8 appeared to be up-regulated in Mmd -null gonads. Mean ± s.e.m., n = 3 (XY +/− ), 4 (XY +/+ ; XX +/+ ; XX +/− ; and XX −/− ), or 5 (XY −/− ). * = p < 0.05 (Sidak’s multiple comparisons test). (C) Immunofluorescence analysis showing similar expression of sex markers, including SOX9, FOXL2 (upper panel), HSD3β and AMH (bottom panel), in Mmd +/− and Mmd −/− fetal gonads. Scale bar, 50 μm. Download figure Open in new tab Figure 7. Fetal gonads in Paqr8 -null mice develop normally. (A) Generation of Paqr8 − (null) allele via CRISPR in mouse zygotes. cr-5’/3’, crRNA-5’/3’. (B) RT-qPCR analysis confirming lack of Paqr8 expression in the knockout gonads. Mean ± s.e.m., n = 4. Mmd and Mmd2 were not significantly up-regulated in Paqr8 -null fetal testes. (C) Immunofluorescence analysis showing similar expression of sex markers, including SOX9, FOXL2 (upper panel), AMH, and HSD3β (bottom panel), in Paqr8 +/− and Paqr8 −/− fetal gonads. Scale bar, 50 μm. Loss of Mmd or Paqr8 function did not cause any gross defects in gonad development in either sex. Expression of a wide range of marker genes was unaffected, as assessed by RT-qPCR (Supplementary Figures S3, S4). Immunofluorescence analysis of key gonadal differentiation markers did not reveal any significant difference between Mmd −/− or Paqr8 −/− gonads and controls ( Figs 6C, 7C ). We conclude that each of these factors individually is, like Mmd2 , dispensable for sex determination and gonadal development. Combined deletion of Mmd2 and Mmd or Paqr8 function does not disrupt fetal testis development Although mice lacking the function of either Mmd2 , Mmd , or Paqr8 showed no gonadal defects, it remained possible that combined loss-of-function of these genes might affect sex development. To address this possibility, we generated Mmd2 ; Mmd and Mmd2 ; Paqr8 double knockout embryos by breeding the relevant mouse lines, and analyzed their gonads at 13.5dpc. Gene expression analysis confirmed the expected genetic disruption in each case ( Fig 8A ). Download figure Open in new tab Figure 8. Double knockout of Mmd2 / Mmd and Mmd2 / Paqr8 does not disrupt sex determination. (A) RT-qPCR analysis confirming ablation of Mmd2 in combination with Mmd or Paqr8 in double knockout gonads. Mean ± s.e.m., n = 4. (B, C) Immunofluorescence analysis showing similar expression of sex markers, including SOX9, FOXL2 (upper panel), HSD3β and AMH (bottom panel), in Mmd2 / Mmd and Mmd2 / Paqr8 double knockout XY (B) and XX (C) gonads compared to wild type controls. Scale bar, 50 μm. However, similar to the three singly-targeted lines, no gross fetal gonadal phenotype was observed. Immunofluorescence analysis was performed to assess the expression of the Sertoli cell marker SOX9 and the granulosa cell marker FOXL2 in 13.5 dpc gonads ( Fig 8B ). No difference was observed between double homozygous deleted mice and double heterozygous controls, and this was further confirmed by analysis of the expression profile of a range of male and female markers including Sox9 , Hsd3b , Amh , Cyp11a1 , Foxl2 , and Fst (Supplementary Figures S5, S6). These results indicate that the combined ablation of Mmd2 with either Mmd or Paqr8 does not affect sex determination or early gonadal development in the mouse. Discussion Despite extensive study since the discovery of Sry , our understanding of the regulatory networks of sex determination in mammals remains patchy, with relatively few functionally important genes having been identified. As a result, many cases of human disorders/differences of sex development (DSD) remain undiagnosed. Efforts to fill in the blanks and assemble genes into pathways have mostly revolved around molecular strategies aimed at identifying targets of key transcription factors known to play a role in gonadal development ( Li et al., 2014 , Garcia-Ortiz et al., 2009 ), genomic analysis of DNA from individuals with DSD to identify causative variants ( Croft et al., 2018 , Sreenivasan et al., 2018 ), and expression screening strategies in model organisms such as mice ( Jameson et al., 2012 , Zhao et al., 2018 ). The logic underpinning expression screening appears robust and is summarized as follows. If a gene is expressed in the developing testes but not ovaries, or vice versa, and/or if its expression is confined to a time interval in which critical sex determination or differentiation decisions are occurring, then such a gene is likely to play a functional role in those events. This likelihood is further increased if the gene can be shown to associate with a cell lineage known to orchestrate key events in sex differentiation, in this case the Sertoli cell lineage. In this study, we focused our attention on Mmd2 , a gene that we found to match all of these criteria. It was therefore surprising to find that loss of Mmd2 function had no discernible effect on sex determination or the development of testes in mice. Although we cannot exclude the possibility that Mmd2 expression is functionally unrelated to early testis development, the logic described above led us to seek other explanations for the lack of phenotype. We reasoned that lack of MMD2 may be compensated within the testes by the expression of other PAQR family members. To test this hypothesis, we generated null mouse lines for the related genes Mmd and Paqr8 using CRISPR. Mmd is functionally redundant with Mmd2 in the developing heart in zebrafish ( Huang et al., 2012 ), and Paqr8 was chosen due to its expression profile showing a greater upregulation in XY gonads at the time of sex determination – a profile unique among the Paqr genes. Both genes were co-expressed with Mmd2 during testis development. Neither Mmd −/− nor Paqr8 −/− mice showed evidence of sex reversal or disruption of gonadal development, and double ablation of Mmd2 / Mmd and Mmd2 / Paqr8 did not result in impaired sex development. Therefore, deletion of the Paqr candidates most likely to play a role in sex determination and gonadal differentiation, either singly or pairwise with Mmd2 , had no discernible effect in this system. It is possible that some impairment or phenotypic changes occurred that we were unable to detect in the mutant mice. For each strain generated in this study, we applied a comprehensive battery of qualitative and quantitative markers of gonadal development. However, gonadal development is strongly canalized, and so it remains formally possible that the program of gonadal development was weakened, but perhaps only transiently and not so much as to cause a failure to meet a threshold required for sex reversal or gonadal dysgenesis. The loss-of-function mutants were created on a C57BL/6 genetic background, which is known to be susceptible to perturbations of the sex determination program ( Correa et al., 2012 ), suggesting that this possibility is unlikely. Nonetheless, it remains possible that an overt phenotype might have been exposed by crossing the mice with other partially compromised strains such as Sox9 heterozygous mutant mice ( Barrionuevo et al., 2006 ; Bagheri-Fam et al., 2008 ). We consider it most likely that functional redundancy exists between the three factors. While Mmd2 and Mmd have been shown to act redundantly in the zebrafish heart ( Huang et al., 2012 ), loss of both together did not compromise mouse gonadal development. This issue may be resolved by the creation of triple mutant mice deficient in Mmd2 , Mmd and Paqr8 together. Even so, it remains possible that other PAQR factors might be capable of masking a phenotype in the triple mutants. Our analysis showed that several Paqr genes are expressed during gonadal development in mice, notably Paqr7, which shows some degree of preferential expression in testes, and Paqr1 and − 2 , which are expressed at a moderate level in gonads of both sexes (Supplementary Figure S2; Zhao et al., 2018 ). In addition, it is possible that one or more Paqr genes would show compensatory upregulation in the absence of MMD2, MMD, and PAQR8 or combinations thereof, consistent with the compensatory upregulation among Mmd , Mmd2 and Paqr8 detected in the present study. Certainly, our findings highlight the difficulties involved in identifying members of multi-gene families with a functional role in sex determination and gonadal development through loss-of-function analyses of individual genes. The localization of the MMD2 protein to the Golgi apparatus within the Sertoli cells may reflect a role in regulating the MAPK signaling pathway within the gonad, consistent with the known role of the Golgi as a regulatory node for various signaling cascades ( Wei and Seeman, 2009 ). Further, MMD2, MMD and PAQR8 have previously been found to be involved in MAPK activation by regulating MAPK1/3 phosphorylation ( Liu et al., 2012 ; Kasubuchi et al., 2017 ). MAPK signaling is critically important for the proper expression of the male-determining gene, Sry, which is activated at 10.5 dpc within the pre-Sertoli cells. Targeted loss of components within this pathway, including MAP3K4, MAP2K, and GADD45γ, have resulted in various levels of dysregulation of Sry , producing impaired sex development ranging from ovotestis formation to complete XY sex reversal ( Bogani D 2009 ; Warr et al., 2016 ; Warr et al., 2012 ). If MMD2, MMD, and PAQR8 do act redundantly in this system, this may reflect regulation of a critical pathway that needs to be protected from the effects of single gene mutation. Consistent with this concept, we note that a recent study investigated the role of 30 genes that were highly enriched in the testis and believed to be important for testis development and fertility ( Lu et al., 2019 ). The authors found no obvious phenotype nor fertility defects when single knock-out mice were generated, suggesting a high degree of functional redundancy in this system. The results of our study may also help to explain the paucity of DSD genes that have been positively identified by variant analysis. Currently, around 60% of 46,XY DSD cases have an unknown genetic cause ( Eggers et al., 2016 ), and the identity and role of relevant genes and pathways are still poorly understood. As our understanding of complex disorders increases, it is becoming clear that many congenital conditions have a multigenic component. Regulatory networks with a high degree of redundancy are likely to mask potentially deleterious mutations. Further, generation of mouse models can be complicated by the known differences in dosage sensitivity to loss-of-function thresholds between mice and humans ( Barrionuevo et al., 2006 , Uda et al., 2004 ). More sensitive “omics” methods, including improved computational strategies, the ability to generate more genetically complex mouse models using CRISPR technologies, and/or the development of appropriate high-throughput cell-based assays may hold the key to illuminating the spectrum of molecular causes of human DSD. Acknowledgements We thank Dr Johnny Huang (Queensland Facility for Advanced Genome Editing) for zygote microinjections. Confocal microscopy was performed at the Australian Cancer Research Foundation/Institute for Molecular Bioscience Cancer Biology Imaging Facility. This work was supported by grants from the Australian Research Council and the National Health and Medical Research Council (NHMRC) of Australia. PK received salary support as a Senior Principal Research Fellow of the NHMRC. Footnotes ↵ * Joint first authors ↵ ǂ Joint last authors References ↵ Barbera JP , Rodriguez TA , Greene ND , Weninger WJ , Simeone A , Copp AJ , Beddington RS , Dunwoodie S . Folic acid prevents exencephaly in Cited2 deficient mice . Hum Mol Genet. 2002 ; 11 : 283 – 293 . OpenUrl CrossRef PubMed Web of Science ↵ Bagheri-Fam S , Sim H , Bernard P , Jayakody I , Taketo MM , Scherer G , Harley VR . Loss of Fgfr2 leads to partial XY sex reversal . Dev Biol. 2008 ; 314 : 71 – 83 . 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