The steroidogenic factor 1 (SF-1) protein is a transcription factor involved in sex determination by controlling the activity of genes related to the reproductive glands or gonads and adrenal glands. [5] This protein is encoded by the NR5A1 gene, a member of the nuclear receptor subfamily, located on the long arm of chromosome 9 at position 33.3. It was originally identified as a regulator of genes encoding cytochrome P450 steroid hydroxylases, however, further roles in endocrine function have since been discovered. [6]
The NR5A1 gene encodes a 461-amino acid protein that shares several conserved domains consistent with members of the nuclear receptor subfamily. [6] The N-terminal domain includes two zinc fingers and is responsible for DNA binding via specific recognition of target sequences. Variations of AGGTCA DNA motifs allows SF-1 to interact with the major groove of the DNA helix and monomerically bind. [7] Following binding, trans-activation of target genes depends on recruitment of co-activators such as SRC-1, GRIP1, PNRC, or GCN5. Other critical domains of SF-1 include a proline-rich hinge region, ligand-binding domain, and a C-terminal activation domain for transcriptional interactions. A 30-amino acid extension of the DNA-binding domain known as the A-box stabilizes monomeric binding by acting as a DNA anchor. The hinge region can undergo post-transcriptional and translational modifications such as phosphorylation by cAMP-dependent kinase, that further enhance stability and transcriptional activity. [8]
SF-1 is considered an orphan receptor as high-affinity naturally occurring ligands have yet to be identified.
Analysis of mouse SF-1 cDNA revealed sequence similarities with Drosophila fushi tarazu factor I (FTZ-F1) which regulates the fushi tarazu homeobox gene. [9] Several other FTZ-F1 homologs have been identified that implicate high level of sequence conservation among vertebrates and invertebrates. For example, SF-1 cDNA shares an identical 1017 base-pair sequence with embryonal long terminal repeat-binding protein (ELP) cDNA isolated from embryonal carcinoma cells, differing only in their terminal ends. [9]
SF-1 expression is localized to adult steroidogenic tissues correlating with known expression profiles of steroid hydroxylases. Using in situ hybridization with SF-1 cRNA specific probe detected gene transcripts in adrenocortical cells, Leydig cells, and ovarian theca and granulosa cells. [9] SF-1 specific antibody studies confirmed expression profile of SF-1 in rats [10] and humans [11] corresponding to sites of transcript detection.
Genetic sex in mammals is determined by the presence or absence of the Y chromosome at fertilization. Sexually dimorphic development of embryonic gonads into testes or ovaries is activated by the SRY gene product. [12] Sexual differentiation is then directed by hormones produced by embryonic testes, the presence of ovaries, or complete absence of gonads. SF-1 transcripts initially localize to the urogenital ridge before SF-1 expressing cells resolve into distinct adrenocortical and gonadal precursors that ultimately give rise to adrenal cortex and gonads.
SF-1 transcripts precede the onset of SRY expression in the fetal testes hinting at gonadal developmental role. SRY influences the differentiation of the fetal testes into distinct compartments: testicular cords and interstitial region containing Leydig cells. [12] Increase in SF-1 protein and detection in the steroidogenic Leydig cells and testicular cords coincides with development.
However, in the ovaries, gonadal sexual differentiation is facilitated by reductions in SF-1 transcript and protein. SF-1 levels is strongly expressed at the onset of follicular development in theca and granulosa cells which precedes expression of the aromatase enzyme responsible for estrogen biosynthesis.
Embryonic mouse SF-1 transcripts have been discovered to localize within regions of the developing diencephalon and subsequently in the ventromedial hypothalamic nucleus (VMH) suggesting roles beyond steroidogenic maintenance. [9]
RT-PCR approaches have detected transcripts of mice FTZ-F1 gene in the placenta and spleen; and SF-1 transcripts in the human placenta. [13]
Transcription capacity of SF-1 can be influenced by post-translational modification. Specifically, phosphorylation of serine 203 is mediated by cyclin-dependent kinase 7. Mutations to CDK7 prevent interaction with the basal transcription factor, TFIIH, and formation of CDK-activating kinase complex. This inactivity has shown to repress phosphorylation of SF-1 and SF-1-dependent transcription. [14]
SF-1 is a critical regulator of reproduction, regulating the transcription of key genes involved in sexual development and reproduction, most notably StAR and P450SCC. It can form a transcriptional complex with TDF to up-regulate transcription of the Sox9 gene. Its targets include genes at every level of the hypothalamic-pituitary-gonadal axis, as well as many genes involved in gonadal and adrenal steroidogenesis. [15]
SF-1 has been identified as a transcriptional regulator for an array of different genes related to sex determination and differentiation, reproduction, and metabolism via binding to their promoters. For example, SF-1 controls expression of Amh gene in Sertoli cells, whereby the presence or absence of the gene product affects development of Müllerian structures. Increased AMH protein levels leads to regression of such structures. [6] Leydig cells express SF-1 to regulate transcription of steroidogenesis and testosterone biosynthesis genes causing virilization in males.
First identified as a regulator of steroid hydroxylases within adrenocortical cells, studies aimed to define localization and expression of SF-1 have since revealed enzyme activity within other steroidogenic cells. [6]
species | Gene | Cell/Tissue |
---|---|---|
rat | P450scc | granulosa cells |
mouse | P450scc | Y1 adrenocortical cells |
bovine | Oxytocin | ovary |
mouse | StAR | MA-10 Leydig cells |
The Müllerian inhibiting substance (MIS or AMH) gene within Sertoli cells contains a conserved motif identical to the optimal binding sequence for SF-1. Gel mobility shift experiments and use of SF-1-specific polyclonal antibodies established binding complexes of SF-1 to MIS, [16] however, other studies suggest the MIS promoter is repressed and not activated by SF-1 binding.
Gonadotrope-specific element, or GSE, in the promoter of the gene encoding α-subunit of glycoproteins (α-GSU) resembles the SF-1 binding sires. Studies have implicated SF-1 as an upstream regulator of a collection of genes required for gonadotrope function via GSE. [17]
SF-1 knockout mice displayed profound defects in the VMH suggesting potential target genes at the site. Target genes have yet to be identified due to difficulties in studying gene expression in neurons.
Several approaches used targeted gene disruption in mouse embryonic stem cells with the aim of identifying potential target genes of SF-1. The different targeting strategies include disruption to exons encoding for the zinc finger motif, disruption of a 3’-exon and targeted mutation of the initiator methionine. The corresponding observed phenotypic effects on endocrine development and function were found to be quite similar. [6]
Sf-1 knockout mice displayed diminished corticosterone levels while maintaining elevated ACTH levels. Observed morphological changes and DNA fragmentation was consistent with apoptosis and structural regression resulting in the death of all mice within 8 days after birth. [18]
Sf-1 function was determined to be necessary for development of primary steroidogenic tissue as evidenced by complete lack of adrenal and gonadal glands in the knockout. Male to female sex reversal of genitalia was also observed. [19]
Mutations in NR5A1 can produce intersex genitals, absence of puberty, and infertility. It is one cause of arrest of ovarian function in women <40 years of age, which occurs in 1% of all women.
Two SF-1 variants associated with primary adrenal failure and complete gonadal dysgenesis have been reported as caused by NR5A1 mutations. One reported case was found to have de novo heterozygous p.G35E change to the P-box domain. [20] The affected region allows for DNA binding specificity through interactions with regulatory response elements of target genes. This p.G35E change may have a mild competitive or dominant negative effect on transactivation resulting in severe gonadal defects and adrenal dysfunction. Similarly, homozygous p.R92Q change within the A-box interfered with monomeric binding stability and reduced functional activity. [20] This change requires mutations to both allele to display phenotypic effects as heterozygous carriers showed normal adrenal function.
Missense, in-frame and frameshift mutations of NR5A1 have been found in families with 46,XY disorders of sex development, 46,XX gonadal dysgenesis and 46,XX primary ovarian insufficiency. 46,XY individuals may have ambiguous or female genitals. Individuals of either karyotype may not enter puberty, although expression of the phenotype, penetrance, fertility, and modes of inheritance can vary. Some mutations are dominant, some are recessive. [21]
Heterozygous NR5A1 changes are emerging as a frequent contributor in 46, XY complete gonadal dysgenesis. [20] In affected individuals, sexual development does not match their chromosomal makeup. Males, despite having 46, XY karyotype, develop female external genitalia, as well as uterus and fallopian tubes, along with gonadal defects rendering them nonfunctional. [22] NR5A1 mutations have also been linked to partial gonadal dysgenesis, whereby affected individuals have ambiguous genitalia, urogenital sinus, absent or rudimentary Müllerian structures, and other abnormalities. [20]
Typically, these genetic changes are frameshift, nonsense, or missense mutations that alter DNA-binding and gene transcription. While many are de novo, one-third of cases have been maternally inherited in a similar manner as X-linked inheritance. Furthermore, one report of homozygous missense mutation p.D293N within the ligand-binding domain of SF-1 revealed autosomal recessive inheritance was also possible. [21]
Analysis of NR5A1 in men with non-obstructive male factor infertility found those with gene changes had more severe forms of infertility and lower testosterone levels. [23] These changes affected the hinge region of SF-1. It is important to note further studies are required to establish the relationship between SF-1 changes and infertility.
SF-1 has also been shown to interact with:
Androgen insensitivity syndrome (AIS) is a difference in sex development involving hormonal resistance due to androgen receptor dysfunction.
Lipoid congenital adrenal hyperplasia is an endocrine disorder that is an uncommon and potentially lethal form of congenital adrenal hyperplasia (CAH). It arises from defects in the earliest stages of steroid hormone synthesis: the transport of cholesterol into the mitochondria and the conversion of cholesterol to pregnenolone—the first step in the synthesis of all steroid hormones. Lipoid CAH causes mineralocorticoid deficiency in affected infants and children. Male infants are severely undervirilized causing their external genitalia to look feminine. The adrenals are large and filled with lipid globules derived from cholesterol.
Sex-determining region Y protein (SRY), or testis-determining factor (TDF), is a DNA-binding protein encoded by the SRY gene that is responsible for the initiation of male sex determination in therian mammals. SRY is an intronless sex-determining gene on the Y chromosome. Mutations in this gene lead to a range of disorders of sex development with varying effects on an individual's phenotype and genotype.
The steroidogenic acute regulatory protein, commonly referred to as StAR (STARD1), is a transport protein that regulates cholesterol transfer within the mitochondria, which is the rate-limiting step in the production of steroid hormones. It is primarily present in steroid-producing cells, including theca cells and luteal cells in the ovary, Leydig cells in the testis and cell types in the adrenal cortex.
Gonadal dysgenesis is classified as any congenital developmental disorder of the reproductive system in humans. It is atypical development of gonads in an embryo. One type of gonadal dysgenesis is the development of functionless, fibrous tissue, termed streak gonads, instead of reproductive tissue. Streak gonads are a form of aplasia, resulting in hormonal failure that manifests as sexual infantism and infertility, with no initiation of puberty and secondary sex characteristics.
The follicle-stimulating hormone receptor or FSH receptor (FSHR) is a transmembrane receptor that interacts with the follicle-stimulating hormone (FSH) and represents a G protein-coupled receptor (GPCR). Its activation is necessary for the hormonal functioning of FSH. FSHRs are found in the ovary, testis, and uterus.
The luteinizing hormone/choriogonadotropin receptor (LHCGR), also lutropin/choriogonadotropin receptor (LCGR) or luteinizing hormone receptor (LHR), is a transmembrane receptor found predominantly in the ovary and testis, but also many extragonadal organs such as the uterus and breasts. The receptor interacts with both luteinizing hormone (LH) and chorionic gonadotropins and represents a G protein-coupled receptor (GPCR). Its activation is necessary for the hormonal functioning during reproduction.
Cholesterol side-chain cleavage enzyme is commonly referred to as P450scc, where "scc" is an acronym for side-chain cleavage. P450scc is a mitochondrial enzyme that catalyzes conversion of cholesterol to pregnenolone. This is the first reaction in the process of steroidogenesis in all mammalian tissues that specialize in the production of various steroid hormones.
The adrenocorticotropic hormone receptor or ACTH receptor also known as the melanocortin receptor 2 or MC2 receptor is a type of melanocortin receptor (type 2) which is specific for ACTH. A G protein–coupled receptor located on the external cell plasma membrane, it is coupled to Gαs and upregulates levels of cAMP by activating adenylyl cyclase. The ACTH receptor plays a role in immune function and glucose metabolism.
The liver receptor homolog-1 (LRH-1) also known as totipotency pioneer factor NR5A2 is a protein that in humans is encoded by the NR5A2 gene. LRH-1 is a member of the nuclear receptor family of intracellular transcription factors.
DAX1 is a nuclear receptor protein that in humans is encoded by the NR0B1 gene. The NR0B1 gene is located on the short (p) arm of the X chromosome between bands Xp21.3 and Xp21.2, from base pair 30,082,120 to base pair 30,087,136.
WNT4 is a secreted protein that, in humans, is encoded by the WNT4 gene, found on chromosome 1. It promotes female sex development and represses male sex development. Loss of function may have consequences, such as female to male sex reversal.
Transcription factor GATA-4 is a protein that in humans is encoded by the GATA4 gene.
Paired box gene 8, also known as PAX8, is a protein which in humans is encoded by the PAX8 gene.
Transcription factor GATA-6, also known as GATA-binding factor 6 (GATA6), is protein that in humans is encoded by the GATA6 gene. The gene product preferentially binds (A/T/C)GAT(A/T)(A) of the consensus binding sequence.
Transcriptional-regulating factor 1 is a protein that in humans is encoded by the TRERF1 gene.
The fetal endocrine system is one of the first systems to develop during prenatal development of a human individual. The endocrine system arises from all three embryonic germ layers. The endocrine glands that produce the steroid hormones, such as the gonads and adrenal cortex, arise from the mesoderm. In contrast, endocrine glands that arise from the endoderm and ectoderm produce the amine, peptide, and protein hormones.
About 10–15% of human couples are infertile, unable to conceive. In approximately in half of these cases, the underlying cause is related to the male. The underlying causative factors in the male infertility can be attributed to environmental toxins, systemic disorders such as, hypothalamic–pituitary disease, testicular cancers and germ-cell aplasia. Genetic factors including aneuploidies and single-gene mutations are also contributed to the male infertility. Patients with nonobstructive azoospermia or oligozoospermia show microdeletions in the long arm of the Y chromosome and/or chromosomal abnormalities, each with the respective frequency of 9.7% and 13%. A large percentage of human male infertility is estimated to be caused by mutations in genes involved in primary or secondary spermatogenesis and sperm quality and function. Single-gene defects are the focus of most research carried out in this field.
Steroidogenic acute regulatory protein is a protein that in humans is encoded by the STAR gene.
Melanocortin 2 receptor accessory protein is a transmembrane accessory protein that in humans is encoded by the MRAP gene located in chromosome 21q22.11. Alternate splicing of the MRAP mRNA generates two functionally isoforms MRAP-α and MRAP-β.