Sexy splicing: regulatory interplays governing sex determination from Drosophila to mammals

Role of Alternative Splicing in Sex Determination in Vertebrates

During the process of sex determination, a germ-cell-containing undifferentiated gonad is converted into either a male or a female reproductive organ. Both the composition of sex chromosomes and the environment determine sex in vertebrates. It is assumed that transcription level regulation drives this cascade of mechanisms; however, transcription factors can alter gene expression beyond transcription initiation by controlling pre-mRNA splicing and thereby mRNA isoform production. Using the key time window in sex determination and gonad development in mice, it has been reported that new non-transcriptional events, such as alternative splicing, could play a key role in sex determination in mammals. We know the role of key regulatory factors, like WT1(+/-KTS) or FGFR2(b/c) in pre-mRNA splicing and sex determination, indicating that important steps in the vertebrate sex determination process probably operate at a post-transcriptional level. Here, we discuss the role of pre-mRNA splicing regulators in sex determination in vertebrates, focusing on the new RNA-seq data reported from mice fetal gonadal transcriptome.

Sex determination without sex chromosomes

With or without sex chromosomes, sex determination is a synthesis of many molecular events that drives a community of cells towards a coordinated tissue fate. In this review, we will consider how a sex determination pathway can be engaged and stabilized without an inherited genetic determinant. In many reptilian species, no sex chromosomes have been identified, yet a conserved network of gene expression is initiated. Recent studies propose that epigenetic regulation mediates the effects of temperature on these genes through dynamic post-transcriptional, post-translational and metabolic pathways. It is likely that there is no singular regulator of sex determination, but rather an accumulation of molecular events that shift the scales towards one fate over another until a threshold is reached sufficient to maintain and stabilize one pathway and repress the alternative pathway. Investigations into the mechanism underlying sex determination without sex chromosomes should focus on cellular processes that are frequently activated by multiple stimuli or can synthesize multiple inputs and drive a coordinated response. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Sex differences in spiders: from phenotype to genomics

Sexual reproduction is pervasive in animals and has led to the evolution of sexual dimorphism. In most animals, males and females show marked differences in primary and secondary sexual traits. The formation of sex-specific organs and eventually sex-specific behaviors is defined during the development of an organism. Sex determination processes have been extensively studied in a few well-established model organisms. While some key molecular regulators are conserved across animals, the initiation of sex determination is highly diverse. To reveal the mechanisms underlying the development of sexual dimorphism and to identify the evolutionary forces driving the evolution of different sexes, sex determination mechanisms must thus be studied in detail in many different animal species beyond the typical model systems. In this perspective article, we argue that spiders represent an excellent group of animals in which to study sex determination mechanisms. We show that spiders are sexually dimorphic in various morphological, behavioral, and life history traits. The availability of an increasing number of genomic and transcriptomic resources and functional tools provides a great starting point to scrutinize the extensive sexual dimorphism present in spiders on a mechanistic level. We provide an overview of the current knowledge of sex determination in spiders and propose approaches to reveal the molecular and genetic underpinnings of sexual dimorphism in these exciting animals.

Beyond Transcription: Roles of Transcription Factors in Pre-mRNA Splicing

Whereas individual steps of protein-coding gene expression in eukaryotes can be studied in isolation in vitro, it has become clear that these steps are intimately connected within cells. Connections not only ensure quality control but also fine-tune the gene expression process, which must adapt to environmental changes while remaining robust. In this review, we systematically present proven and potential mechanisms by which sequence-specific DNA-binding transcription factors can alter gene expression beyond transcription initiation and regulate pre-mRNA splicing, and thereby mRNA isoform production, by (i) influencing transcription elongation rates, (ii) binding to pre-mRNA to recruit splicing factors, and/or (iii) blocking the association of splicing factors with pre-mRNA. We propose various mechanistic models throughout the review, in some cases without explicit supportive evidence, in hopes of providing fertile ground for future studies.

Alternative splicing mechanisms orchestrating post-transcriptional gene expression: intron retention and the intron-rich genome of apicomplexan parasites

Apicomplexan parasites including Toxoplasma gondii and Plasmodium species have complex life cycles that include multiple hosts and differentiation through several morphologically distinct stages requiring marked changes in gene expression. This review highlights emerging evidence implicating regulation of mRNA splicing as a mechanism to prime these parasites for rapid gene expression upon differentiation. We summarize the most important insights in alternative splicing including its role in regulating gene expression by decreasing mRNA abundance via 'Regulated Unproductive Splicing and Translation'. As a related but less well-understood mechanism, we discuss also our recent work suggesting a role for intron retention for precluding translation of stage specific isoforms of T. gondii glycolytic enzymes. We additionally provide new evidence that intron retention might be a widespread mechanism during parasite differentiation. Supporting this notion, recent genome-wide analysis of Toxoplasma and Plasmodium suggests intron retention is more pervasive than heretofore thought. These findings parallel recent emergence of intron retention being more prevalent in mammals than previously believed, thereby adding to the established roles in plants, fungi and unicellular eukaryotes. Deeper mechanistic studies of intron retention will provide important insight into its role in regulating gene expression in apicomplexan parasites and more general in eukaryotic organisms.

Representing sex in the brain, one module at a time

Sexually dimorphic behaviors, qualitative or quantitative differences in behaviors between the sexes, result from the activity of a sexually differentiated nervous system. Sensory cues and sex hormones control the entire repertoire of sexually dimorphic behaviors, including those commonly thought to be charged with emotion such as courtship and aggression. Such overarching control mechanisms regulate distinct genes and neurons that in turn specify the display of these behaviors in a modular manner. How such modular control is transformed into cohesive internal states that correspond to sexually dimorphic behavior is poorly understood. We summarize current understanding of the neural circuit control of sexually dimorphic behaviors from several perspectives, including how neural circuits in general, and sexually dimorphic neurons in particular, can generate sexually dimorphic behaviors, and how molecular mechanisms and evolutionary constraints shape these behaviors. We propose that emergent themes such as the modular genetic and neural control of dimorphic behavior are broadly applicable to the neural control of other behaviors.

Identification of male gametogenesis expressed genes from the scallop Nodipecten subnodosus by suppressive subtraction hybridization and pyrosequencing

Despite the great advances in sequencing technologies, genomic and transcriptomic information for marine non-model species with ecological, evolutionary, and economical interest is still scarce. In this work we aimed to identify genes expressed during spermatogenesis in the functional hermaphrodite scallop Nodipecten subnodosus (Mollusca: Bivalvia: Pectinidae), with the purpose of obtaining a panel of genes that would allow for the study of differentially transcribed genes between diploid and triploid scallops in the context of meiotic arrest and reproductive sterility. Because our aim was to isolate genes involved in meiosis and other testis maturation-related processes, we generated suppressive subtractive hybridization libraries of testis vs. inactive gonad. We obtained 352 and 177 ESTs by clone sequencing, and using pyrosequencing (454-Roche) we maximized the identified ESTs to 34,276 reads. A total of 1,153 genes from the testis library had a blastx hit and GO annotation, including genes specific for meiosis, spermatogenesis, sex-differentiation, and transposable elements. Some of the identified meiosis genes function in chromosome pairing (scp2, scp3), recombination and DNA repair (dmc1, rad51, ccnb1ip1/hei10), and meiotic checkpoints (rad1, hormad1, dtl/cdt2). Gene expression analyses in different gametogenic stages in both sexual regions of the gonad of meiosis genes confirmed that the expression was specific or increased towards the maturing testis. Spermatogenesis genes included known testis-specific ones (kelch-10, shippo1, adad1), with some of these known to be associated to sterility. Sex differentiation genes included one of the most conserved genes at the bottom of the sex-determination cascade (dmrt1). Transcript from transposable elements, reverse transcriptase, and transposases in this library evidenced that transposition is an active process during spermatogenesis in N. subnodosus. In relation to the inactive library, we identified 833 transcripts with functional annotation related to activation of the transcription and translation machinery, as well as to germline control and maintenance.

Global regulation of alternative splicing by adenosine deaminase acting on RNA (ADAR)

Alternative mRNA splicing is a major mechanism for gene regulation and transcriptome diversity. Despite the extent of the phenomenon, the regulation and specificity of the splicing machinery are only partially understood. Adenosine-to-inosine (A-to-I) RNA editing of pre-mRNA by ADAR enzymes has been linked to splicing regulation in several cases. Here we used bioinformatics approaches, RNA-seq and exon-specific microarray of ADAR knockdown cells to globally examine how ADAR and its A-to-I RNA editing activity influence alternative mRNA splicing. Although A-to-I RNA editing only rarely targets canonical splicing acceptor, donor, and branch sites, it was found to affect splicing regulatory elements (SREs) within exons. Cassette exons were found to be significantly enriched with A-to-I RNA editing sites compared with constitutive exons. RNA-seq and exon-specific microarray revealed that ADAR knockdown in hepatocarcinoma and myelogenous leukemia cell lines leads to global changes in gene expression, with hundreds of genes changing their splicing patterns in both cell lines. This global change in splicing pattern cannot be explained by putative editing sites alone. Genes showing significant changes in their splicing pattern are frequently involved in RNA processing and splicing activity. Analysis of recently published RNA-seq data from glioblastoma cell lines showed similar results. Our global analysis reveals that ADAR plays a major role in splicing regulation. Although direct editing of the splicing motifs does occur, we suggest it is not likely to be the primary mechanism for ADAR-mediated regulation of alternative splicing. Rather, this regulation is achieved by modulating trans-acting factors involved in the splicing machinery.

Regulation and expression of sexual differentiation factors in embryonic and extragonadal tissues of Atlantic salmon

Background

The products of cyp19, dax, foxl2, mis, sf1 and sox9 have each been associated with sex-determining processes among vertebrates. We provide evidence for expression of these regulators very early in salmonid development and in tissues outside of the hypothalamic-pituitary-adrenal/gonadal (HPAG) axis. Although the function of these factors in sexual differentiation have been defined, their roles in early development before sexual fate decisions and in tissues beyond the brain or gonad are essentially unknown.

Results

Bacterial artificial chromosomes containing salmon dax1 and dax2, foxl2b and mis were isolated and the regulatory regions that control their expression were characterized. Transposon integrations are implicated in the shaping of the dax and foxl2 loci. Splice variants for cyp19b1 and mis in both embryonic and adult tissues were detected and characterized. We found that cyp19b1 transcripts are generated that contain 5'-untranslated regions of different lengths due to cryptic splicing of the 3'-end of intron 1. We also demonstrate that salmon mis transcripts can encode prodomain products that present different C-termini and terminate before translation of the MIS hormone. Regulatory differences in the expression of two distinct aromatases cyp19a and cyp19b1 are exerted, despite transcription of their transactivators (ie; dax1, foxl2, sf1) occurring much earlier during embryonic development.

Conclusions

We report the embryonic and extragonadal expression of dax, foxl2, mis and other differentiation factors that indicate that they have functions that are more general and not restricted to steroidogenesis and gonadogenesis. Spliced cyp19b1 and mis transcripts are generated that may provide regulatory controls for tissue- or development-specific activities. Selection of cyp19b1 transcripts may be regulated by DAX-1, FOXL2 and SF-1 complexes that bind motifs in intron 1, or by signals within exon 2 that recruit splicing factors, or both. The potential translation of proteins bearing only the N-terminal MIS prodomain may modulate the functions of other TGF β family members in different tissues. The expression patterns of dax1 early in salmon embryogenesis implicate its role as a lineage determination factor. Other roles for these factors during embryogenesis and outside the HPAG axis are discussed.

Early life stress increases anxiety-like behavior in Balb c mice despite a compensatory increase in levels of postnatal maternal care

A better understanding of the molecular and cellular mechanisms by which early life stress (ELS) modifies brain development and adult behavior is necessary for diagnosing and treating psychopathology associated with exposure to ELS. For historical reasons, most of the work in rodents has been done in rats and attempts to establish robust and reproducible paradigms in the mouse have proven to be challenging. Here we show that under normal rearing conditions, increased levels of postnatal maternal care are associated with a decrease in anxiety-like behavior in BALB/cByj offspring. Brief daily pup-dam separation (BDS) during the postnatal period was associated with increased postnatal maternal care but was surprisingly associated with increased anxiety-like behavior in adult offspring, providing the first example in which offspring receiving higher levels of postnatal maternal care are more anxious in adulthood. Plasma corticosterone levels were elevated in BDS pups even 3 h after the pups were reunited with the dam, suggesting that this paradigm represents a form of early life stress. We also show that levels of total RNA and DNA in the hippocampus reach a peak at postnatal day 14 and that exposure to BDS seems to inhibit this developmental growth spurt. We propose that exposure to stress during the postnatal period overrides the ability of high levels of postnatal maternal care to program anxiety-like behavior by inhibiting the normal growth spurt that characterizes this period.

Regulation, expression and characterization of aromatase (cyp19b1) transcripts in ovary and testis of rainbow trout (Oncorhynchus mykiss)

Cytochrome P450 aromatase is the key enzyme in the pathway that converts androgens to estrogens. The enzyme functions in the smooth endoplasmic reticulum in a complex with NADPH-cytochrome P450 reductase. In teleost fish, at least two separate loci, cyp19a and cyp19b, encode distinct aromatase isoforms. The activity of cyp19a and cyp19b are predominantly associated with the ovary and brain, respectively, although their expression is not confined solely to these tissues. We found that at least five cyp19b1 transcripts with different 5'-UTRs are generated in the ovary and testis of rainbow trout. Regulation for selection of these variants may be through signals present in exon 2 that recruit alternative splicing factors. Also, binding elements for FOXL2 and SF-1 located within the cyp19b1 intron 1 may influence formation of transcripts that contain the 3'-end of the intron. Another transcript devoid of the exon 2 methionine initiator codon may utilize other downstream in-frame start codons. Less developed stages of ovarian and testicular tissues express only the intron-containing transcripts whereas precocious and more mature gonads express all five cyp19b1 messages. The function of these different 5'-UTRs may be for regulation of cyp19b1 at particular developmental stages or to specify control in distinct gonadal cell-types.

Male-specific expression of Sox9 during gonad development of crocodile and mouse is mediated by alternative splicing of its proline-glutamine-alanine rich domain

The initial trigger for sexual differentiation is regulated by multiple ways during embryonic development. In vertebrates, chromosome-based mechanisms generally known as genetic sex determination are prevalent; however, some species, such as many reptilians, display temperature-dependent sex determination. The Sry-related transcription factor, Sox9, which is expressed by an evolutionary conserved gene, has been shown to be a key player in the process of sex determination. In the present study, we report the identification and expression of crocodile homolog of Sox9 (cpSox9) from the Indian Mugger, Crocodylus palustris. We show that cpSox9 undergoes extensive alternative splicing around the proline-glutamine-alanine rich transactivation domain that results in cpSox9 variants with presumably impaired or reduced transactivation potential. The multiple isoforms were also detected in various embryonic tissues, with some of them displaying a differential expression profile. With respect to sex differentiation, a putative unspliced full-length cpSox9 could be detected only in the genital ridge-adrenal-mesonephros complex of male, but not female embryos during the temperature-sensitive period. Importantly, we further show that this phenomenon was not restricted to the temperature-dependent sex determination species C. palustris, but was also observed in the mouse, a species exhibiting genetic sex determination. Thus, the present study describes, for the first time, a complete coding locus of Sox9 homolog from a temperature-dependent sex determination species. More importantly, we demonstrate an evolutionarily conserved role of alternative splicing resulting in transcriptional diversity and male-sex specific expression of Sox9 during testis development in vertebrates (i.e. irrespective of their underlying sex-determination mechanisms).

Can Drosophila melanogaster represent a model system for the detection of reproductive adverse drug reactions?

Once a molecule is identified as a potential drug, the detection of adverse drug reactions is one of the key components of its development and the FDA approval process. We propose using Drosophila melanogaster to screen for reproductive adverse drug reactions in the early stages of drug development. Compared with other non-mammalian models, D. melanogaster has many similarities to the mammalian reproductive system, including putative sex hormones and conserved proteins involved in genitourinary development. Furthermore, the D. melanogaster model would present significant advantages in time efficiency and cost-effectiveness compared with mammalian models. We present data on methotrexate (MTX) reproductive adverse events in multiple animal models, including fruit flies, as proof-of-concept for the use of the D. melanogaster model.

Heterogeneous nuclear ribonucleoprotein G regulates splice site selection by binding to CC(A/C)-rich regions in pre-mRNA

Almost every protein-coding gene undergoes pre-mRNA splicing, and the majority of these pre-mRNAs are alternatively spliced. Alternative exon usage is regulated by the transient formation of protein complexes on the pre-mRNA that typically contain heterogeneous nuclear ribonucleoproteins (hnRNPs). Here we characterize hnRNP G, a member of the hnRNP class of proteins. We show that hnRNP G is a nuclear protein that is expressed in different concentrations in various tissues and that interacts with other splicing regulatory proteins. hnRNP G is part of the supraspliceosome, where it regulates alternative splice site selection in a concentration-dependent manner. Its action on alternative exons can occur without a functional RNA-recognition motif by binding to other splicing regulatory proteins. The RNA-recognition motif of hnRNP G binds to a loose consensus sequence containing a CC(A/C) motif, and hnRNP G preferentially regulates alternative exons where this motif is clustered in close proximity. The X-chromosomally encoded hnRNP G regulates different RNAs than its Y-chromosomal paralogue RNA-binding motif protein, Y-linked (RBMY), suggesting that differences in alternative splicing, evoked by the sex-specific expression of hnRNP G and RBMY, could contribute to molecular sex differences in mammals.

Presumptive pre-Sertoli cells express genes involved in cell proliferation and cell signalling during a critical window in early testis differentiation

In mammals, the pre-Sertoli cell of the male genital ridge is the first cell type to display sex specific differentiation and differential gene expression. The genetic cascade driving the differentiation of pre-Sertoli cells and ultimately testis formation is beginning to be unravelled, but many questions remain. A better understanding of the transcriptome of pre-Sertoli cells immediately after sex determination is essential in order to further understand this differentiation process. A mouse model expressing Red Fluorescent Protein (RFP) under the control of a hybrid mouse/pig SRY promoter (HybSRYp-RFP) was used to purify cells from embryonic day 12.0 (e12.0) male genital ridges. To compare the transcriptomes of HybSRYp-RFP cell populations versus age matched whole female genital ridges, RNA was extracted and used to generate molecular probes that were hybridized onto Affymetrix Mouse Genome 430 2.0 micro-arrays. The expression of genes considered markers for pre-Sertoli cells, including Sox9, Mis, Dhh and Fgf9 were identified within the HybSRYp-RFP expressing cell population, while markers for germ cells (Oct4, SSEA-1) and endothelial cells (Ntrk3) were not identified. In contrast, markers for ovarian somatic cell expression, including Fst and Bmp2, were identified as overexpressed within the ovarian cell population. In a general fashion, genes identified as 2.5-fold over expressed in HybSRYp-RFP expressing cells coded notably for cell signalling and extra cellular proteins. The expression of Sox10, Stc2, Fgf18, Fgf13 and Wnt6 were further characterized via whole mount in situ hybridization (WISH) on male and female genital ridges between e11.5 and e14.5. Sox10, Fgf18, Fgf13 and Stc2 gene expression was detected within the male genital ridges while Wnt6 was found diffusely within both the male and female genital ridges. These data represent the earliest comprehensive microarray expression analysis of purified presumptive pre-Sertoli cells available to date.

Sry and the hesitant beginnings of male development

In mammals, Sry (sex-determining region Y gene) is the master regulator of male sex determination. The discovery of Sry in 1990 was expected to provide the key to unravelling the network of gene regulation underlying testis development. Intriguingly, no target gene of SRY protein has yet been discovered, and the mechanisms by which it mediates its developmental functions are still elusive. What is clear is that instead of the robust gene one might expect as the pillar of male sexual development, Sry function hangs by a thin thread, a situation that has profound biological, medical and evolutionary implications.

Molecular patterns of sex determination in the animal kingdom: a comparative study of the biology of reproduction

Determining sexual fate is an integral part of reproduction, used as a means to enrich the genome. A variety of such regulatory mechanisms have been described so far and some of the more extensively studied ones are being discussed. For the insect order of Hymenoptera, the choice lies between uniparental haploid males and biparental diploid females, originating from unfertilized and fertilized eggs accordingly. This mechanism is also known as single-locus complementary sex determination (slCSD). On the other hand, for Dipterans and Drosophila melanogaster, sex is determined by the ratio of X chromosomes to autosomes and the sex switching gene, sxl. Another model organism whose sex depends on the X:A ratio, Caenorhabditis elegans, has furthermore to provide for the brief period of spermatogenesis in hermaphrodites (XX) without the benefit of the "male" genes of the sex determination pathway. Many reptiles have no discernible sex determining genes. Their sexual fate is determined by the temperature of the environment during the thermosensitive period (TSP) of incubation, which regulates aromatase activity. Variable patterns of sex determination apply in fish and amphibians. In birds, while sex chromosomes do exist, females are the heterogametic (ZW) and males the homogametic sex (ZZ). However, we have yet to decipher which of the two (Z or W) is responsible for the choice between males and females. In mammals, sex determination is based on the presence of two identical (XX) or distinct (XY) gonosomes. This is believed to be the result of a lengthy evolutionary process, emerging from a common ancestral autosomal pair. Indeed, X and Y present different levels of homology in various mammals, supporting the argument of a gradual structural differentiation starting around the SRY region. The latter initiates a gene cascade that results in the formation of a male. Regulation of sex steroid production is also a major result of these genetic interactions. Similar observations have been described not only in mammals, but also in other vertebrates, emphasizing the need for further study of both normal hormonal regulators of sexual phenotype and patterns of epigenetic/environmental disruption.

Systematic identification of factors involved in post-transcriptional processes in wheat grain

Post-transcriptional processing of primary transcripts can significantly affect both the quantity and the structure of mature mRNAs and the corresponding protein products. It is an important mechanism of gene regulation in animals, yeast and plants. Here we have investigated the interactive networks of pre-mRNA processing factors in the developing grain of wheat (Triticum aestivum), one of the world's major food staples. As a first step we isolated a homologue of the plant specific AtRSZ33 splicing factor, which has been shown to be involved in the early stages of embryo development in Arabidopsis. Real-time PCR showed that the wheat gene, designated TaRSZ38, is expressed mainly in young, developing organs (flowers, root, stem), and expression peaks in immature grain. In situ hybridization and immunodetection revealed preferential abundance of TaRSZ38 in mitotically active tissues of the major storage organ of the grain, the endosperm. The protein encoded by TaRSZ38 was subsequently used as a starting bait in a two-hybrid screen to identify additional factors in grain that are involved in pre-mRNA processing. Most of the identified proteins showed high homology to known splicing factors and splicing related proteins, supporting a role for TaRSZ38 in spliceosome formation and 5' site selection. Several clones were selected as baits in further yeast two-hybrid screens. In total, cDNAs for 16 proteins were isolated. Among these proteins, TaRSZ22, TaSRp30, TaU1-70K, and the large and small subunits of TaU2AF, are wheat homologues of known plant splicing factors. Several, additional proteins are novel for plants and show homology to known pre-mRNA splicing, splicing related and mRNA export factors from yeast and mammals.

WT1 interacts with the splicing protein RBM4 and regulates its ability to modulate alternative splicing in vivo

Wilm's tumor protein 1 (WT1), a protein implicated in various cancers and developmental disorders, consists of two major isoforms: WT1(-KTS), a transcription factor, and WT1(+KTS), a post-transcriptional regulator that binds to RNA and can interact with splicing components. Here we show that WT1 interacts with the novel splicing regulator RBM4. Each protein was found to colocalize in nuclear speckles and to cosediment with supraspliceosomes in glycerol gradients. RBM4 conferred dose-dependent and cell-specific regulation of alternative splicing of pre-mRNAs transcribed from several reporter genes. We found that overexpressed WT1(+KTS) abrogated this effect of RBM4 on splice-site selection, whereas WT1(-KTS) did not. We conclude that the (+KTS) form of WT1 is able to inhibit the effect of RBM4 on alternative splicing.

Multiple Alternative Splicing and Differential Expression of dmrt1 During Gonad Transformation of the Rice Field Eel1

Morphologically distinct males and females are observed throughout the animal kingdom. Why and how sex evolved and is maintained in most living organisms remains a key question in cellular and evolutionary biology. Here we report that four isoforms of dmrt1 (dsx- and mab3-related transcription factor 1) are generated in testis, ovotestis, and ovary by alternative splicing in the rice field eel, a fresh water fish that undergoes natural sex reversal from female to male during its life cycle. These transcripts encode four different size proteins with 301, 196, 300, and 205 amino acids. Like fly doublesex splicing, the dmrt1 of the rice field eel is also alternatively spliced at the 3' region, which generates diverse isoforms in gonads by alternative use of 3' sequences. Not only is dmrt1 expressed specifically in gonads, but its multiple isoforms are differentially coexpressed in gonadal epithelium during gonad transformation. Expression levels of a and b isoforms of dmrt1 ranged from low to high (ovary < ovotestis I < ovotestis II < ovotestis III < testis), based on comparisons of mean values from real-time fluorescent quantitative reverse transcription-polymerase chain reaction analysis. The overall expression level of dmrt1 b was much lower than that of dmrt1 a. Expression of dmrt1 d was not only low, but it also did not change significantly during sex transformation. The differential expression of dmrt1 isoforms may also be regulated by their 3' untranslated regions (UTRs), although these 3' UTRs do not contribute to intracellular localization of the Dmrt1 protein. These results provide new insight into roles of regulation at the level of splicing of dmrt1 in governing the sex differentiation cascade.

New insights into the regulation of mammalian sex determination and male sex differentiation

In mammals, sex development is a genetically and hormonally controlled process that begins with the establishment of chromosomal or genetic sex (XY or XX) at conception. At approximately 6 to 7 weeks of human gestation or embryonic day e11.5 in the mouse, expression of the Y chromosome-linked sex determining gene called SRY (described in detail in this chapter) then initiates gonadal differentiation, which is the formation of either a testis (male) or an ovary (female). Male sex differentiation (development of internal and external reproductive organs and acquisition of male secondary sex characteristics) is then controlled by three principal hormones produced by the testis: Mullerian inhibiting substance (MIS) or anti-Mullerian hormone (AMH), testosterone, and insulin-like factor 3 (INSL3). In the absence of these critical testicular hormones, female sex differentiation ensues. This sequential, three-step process of mammalian sex development is also known as the Jost paradigm. With the advent of modern biotechnologies over the past decade, such as transgenics, array-based gene profiling, and proteomics, the field of mammalian sex determination has witnessed a remarkable boost in the understanding of the genetics and complex molecular mechanisms that regulate this fundamental biological event. Consequently, a number of excellent reviews have been devoted to this topic. The purpose of the present chapter is to provide an overview of selected aspects of mammalian sex determination and differentiation with an emphasis on studies that have marked this field of study.

DAX-1, an unusual orphan receptor at the crossroads of steroidogenic function and sexual differentiation

The unusual orphan member of the nuclear hormone receptor superfamily DAX-1 (NR0B1) owes its name to its double role in human pathology. On one side, duplications in Xp21, containing the DAX-1 gene, cause phenotypic sex reversal in XY individuals. On the other side, DAX-1 gene mutations are responsible for adrenal hypoplasia congenita, invariably associated with hypogonadotropic hypogonadism. DAX-1 functions as a global negative regulator of steroid hormone production by repressing the expression of multiple genes involved in the steroidogenic pathway. Here we review the mechanism of DAX-1 function in adrenal and gonadal differentiation, with special emphasis on recent results showing the critical role of DAX-1 protein misfolding in the pathogenesis of adrenal hypoplasia congenita.