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K L Parker

Publications and source records attributed to K L Parker.

At least 19 recordsLinked to original sources
PubMed2001

Pituitary-specific knockout of steroidogenic factor 1.

Knockout mice lacking the orphan nuclear receptor steroidogenic factor 1 (SF-1) revealed its essential roles at multiple levels of endocrine development and function. These SF-1 knockout mice lacked adrenal glands and gonads, thereby manifesting adrenal insufficiency and sex reversal of their internal and external genitalia. Their pituitary gonadotropes failed to express several markers of normal differentiated function, and they lacked a specific hypothalamic nucleus, the ventromedial hypothalamic nucleus (VMH). Using the Cre-loxP system, we generated mice whose gene encoding SF-1 was inactivated specifically in the anterior pituitary. These pituitary-specific SF-1 knockout mice were sterile and never matured sexually. Their gonads weighed only approximately 5% of the weight of wild-type gonads. SF-1 immunoreactivity was absent in the anterior pituitary but was unaffected in the adrenal cortex, validating the selectivity of the gene targeting strategy. Consistent with an important role of SF-1 in gonadotropes, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were markedly decreased in the pituitary-specific SF-1 knockout mice. The pituitary-specific SF-1 knockout mice are a novel genetic model of hypogonadotropic hypogonadism and establish essential roles of SF-1 in gonadotropin expression.

Animals↗
PubMed2001

Approaches to define the role of SF-1 at different levels of the hypothalamic-pituitary-steroidogenic organ axis.

Targeted gene disruption has produced knockout mice lacking the orphan nuclear receptor steroidogenic factor 1 (SF-1). These SF-1 knockout mice lacked adrenal glands and gonads, resulting in adrenocortical insufficiency and sex reversal of their internal and external genitalia. They also had impaired expression of pituitary gonadotropins and agenesis of the ventromedial hypothalamic nucleus (VMH), confirming roles of SF-1 at multiple levels of the hypothalamic-pituitary-steroidogenic tissue axis. Using the Cre-loxP system, we now have generated mice in which SF-1 is inactivated selectively in the anterior pituitary. These pituitary-specific SF-1 knockout mice were sterile and failed to exhibit sexual maturation. Histologically, their gonads were markedly hypoplastic, weighing only approximately 5% of the gonads of wild-type mice. Consistent with an important role of SF-1 in gonadotropes, there were no cells in the pituitary gland that expressed either follicle-stimulating hormone (FSH) or luteinizing hormone (LH). These pituitary-specific SF-1 knockout mice are a novel genetic model of hypogonadotropic hypogonadism and establish essential roles of SF-1 in gonadotropin expression.

Adrenal Glands↗
PubMed2001

SF-1: a critical mediator of steroidogenesis.

Studies in knockout mice have established that the orphan nuclear receptor steroidogenic factor 1 (SF-1) plays essential roles in the development and function of the primary steroidogenic organs. These SF-1 knockout mice lacked adrenal glands and gonads, causing adrenocortical insufficiency and sex reversal of their internal and external genitalia. They also had impaired expression of pituitary gonadotropins and agenesis of the ventromedial hypothalamic nucleus (VMH), confirming roles of SF-1 at all three levels of the hypothalamic-pituitary-steroidogenic organ axis. Ongoing experiments are directed at developing methods to inactivate SF-1 in a tissue-specific manner.

Adrenal Glands↗
PubMed2001

Hypomorphic phenotype in mice with pituitary-specific knockout of steroidogenic factor 1.

The bacteriophage Cre recombinase provides a powerful approach for tissue-specific gene inactivation. Using a Cre transgene driven by the common alpha subunit of glycoprotein hormones (alphaGSU-Cre), we have previously inactivated steroidogenic factor 1 (SF-1) in the anterior pituitary, causing hypogonadotropic hypogonadism with sexual infantilism, sterility, and severe gonadal hypoplasia. We now explore the molecular mechanisms underlying a hypomorphic gonadal phenotype in mice carrying two floxed SF-1 alleles (F/F) relative to mice carrying one recombined and one floxed allele (F/R). Because their Cre-mediated disruption of the locus encoding SF-1 was less efficient, alphaGSU-Cre, F/F mice retained some gonadotropin-expressing cells in the anterior pituitary, thereby stimulating some gonadal function. This novel in vivo model for exploring the effects of differing levels of gonadotropins on gonadal development highlights the need for careful genotype-phenotype comparisons in studies using Cre recombinase to produce tissue-specific knockouts.

Animals↗
PubMed2001

Genes essential for early events in gonadal development.

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. The basic underlying principle of sexual development is that genetic sex--determined at fertilization by the presence or absence of the Y chromosome--directs the embryonic gonads to differentiate into either testes or ovaries. Thereafter, hormones produced by the testes direct the developmental program that leads to male sexual differentiation. In the absence of testicular hormones, the female pathway of sexual differentiation occurs. Recent studies have defined key roles in gonadal development for two transcription factors: Wilms' tumor suppressor 1 (WT1) and steroidogenic factor 1 (SF-1). After presenting a brief overview of gonadal development and sexual differentiation, this chapter reviews the studies that led to the isolation and characterization of WT1 and SF-1, and then discusses how interactions between these two genes may mediate their key roles in a common developmental pathway.

Animals↗
PubMed2001

TFIIAalpha/beta-like factor is encoded by a germ cell-specific gene whose expression is up-regulated with other general transcription factors during spermatogenesis in the mouse.

TFIIAalpha/beta-like factor (ALF) is a testis-specific counterpart of the large subunit of human general transcription factor TFIIA. Northern analysis shows that ALF mRNA first appears in mouse testis at Postnatal Day 14. Similarly, expression of the general transcription factors TBP, TRF2, TFIIAalpha/beta, TFIIAgamma, and TFIIIB(90) is also increased beginning at Postnatal Day 14, suggesting that there is a coordinated induction of many general transcription factors during male germ cell differentiation. Analysis of male germ cells separated by Staput sedimentation shows that ALF is present in pachytene spermatocytes and haploid spermatids. In addition, in situ hybridization experiments with adult mouse testis shows that ALF is present in haploid spermatids. Searches of the human genome sequence database using the basic local alignment search tool reveal that the ALF and TFIIAalpha/beta(GTF2A1) genes are both composed of nine exons, whereas the TFIIAgamma (GTF2A2) gene is composed of five exons. Furthermore, nucleotide and amino acid comparisons among human and mouse ALF, TFIIAalpha/beta, and TFIIAgamma cDNA sequences show that ALF has diverged more rapidly than either TFIIAalpha/beta or TFIIAgamma. Finally, the ALF and SBLF (Stoned B-Like Factor) sequences present in the chimeric SALF cDNA are both present on human chromosome 2, and an analysis of the corresponding genes suggests a model for the formation of SALF.

Amino Acid Sequence↗
PubMed2001

Expression profiles of SF-1, DAX1, and CYP17 in the human fetal adrenal gland: potential interactions in gene regulation.

Cytochrome P450 17alpha-hydroxylase/17-20 lyase (P450(C17)) is a critical branchpoint enzyme for steroid hormone biosynthesis. During human gestation, P450(C17) is required for the production of dehydroepiandrostenedione sulfate by the fetal adrenal cortex and for testicular production of androgens that mediate male sexual differentiation. In this study, we investigate the regulation of the human CYP17 gene by two orphan nuclear receptors, steroidogenic factor 1 (SF-1) and DAX1. In human embryos, SF-1 and DAX1 are expressed throughout the developing adrenal cortex from its inception at 33 days post conception (dpc). In contrast, P450(C17) expression, which commences between 41 and 44 dpc, is limited to the fetal zone. The 5'-flanking region of the human CYP17 gene contains three functional SF-1 elements that collectively mediate a > or =25-fold induction of promoter activity by SF-1. In constructs containing all three functional SF-1 elements, DAX1 inhibited this activation by > or =55%. In the presence of only one or two SF-1 elements, DAX1 inhibition was lost even though SF-1 transactivation persisted. These data suggest that efficient repression of SF-1-mediated activation of the human CYP17 gene by DAX1 requires multiple SF-1 elements. Opposing effects of SF-1 and DAX1 may fine tune the differential responses of various SF-1 target genes in different endocrine tissues.

Adrenal Glands↗
PubMed2001

Steroidogenic factor 1 (SF1) is essential for pituitary gonadotrope function.

Knockout mice lacking the orphan nuclear receptor steroidogenic factor 1 (SF1) exhibit a complex endocrine phenotype that includes adrenal and gonadal agenesis, impaired expression of pituitary gonadotropins, and absence of the ventromedial hypothalamic nucleus (VMH). These multiple defects complicate efforts to delineate primary versus secondary effects of SF1 deficiency in different tissues, such that its direct role in gonadotropes remains uncertain. To define this role, we have expressed Cre recombinase driven by the promoter region of the common alpha subunit of glycoprotein hormones (alpha GSU), thereby inactivating a loxP-modified SF1 locus in the anterior pituitary gland. Although pituitary-specific SF1 knockout mice were fully viable, they were sterile and failed to develop normal secondary sexual characteristics. Their adrenal glands and VMH appeared normal histologically, but their testes and ovaries were severely hypoplastic. alpha GSU-Cre, loxP mice had normal levels of most pituitary hormones, but had markedly decreased expression of LH and FSH. Treatment with exogenous gonadotropins stimulated gonadal steroidogenesis, inducing germ cell maturation in males and follicular and uterine maturation in females--establishing that the gonads can respond to gonadotropins. The pituitary-specific SF1 knockout mice are a novel genetic model of hypogonadotropic hypogonadism that establishes essential role(s) of SF1 in pituitary gonadotropes.

Animals↗
PubMed2000

Steroidogenic factor 1 (SF-1) is essential for ovarian development and function.

The orphan nuclear receptor steroidogenic factor 1 (SF-1) was identified originally as a key regulator of the tissue-specific expression of the cytochrome P450 steroid hydroxylases. Hints at considerably broader roles for SF-1 came from analyses of its expression pattern in mouse embryos. As anticipated, SF-1 was expressed in the adrenal glands and gonads from their early stages of development. Surprisingly, SF-1 also was expressed outside of the primary steroidogenic tissues in the anterior pituitary and hypothalamus. SF-1 knockout mice dramatically confirmed its multiple essential roles in vivo. These mice lacked adrenal glands and gonads, leading to adrenocortical insufficiency and male-to-female sex reversal of their internal and external genitalia. SF-1 knockout mice also had impaired pituitary expression of gonadotropins and agenesis of the ventromedial hypothalamic nucleus (VMH), confirming roles of SF-1 at all three levels of the hypothalamic-pituitary-gonadal axis. With some focus on the ovary, this review summarizes experiments that have defined essential roles of SF-1 in endocrine development, and highlights important areas for future studies.

Animals↗
PubMed2000

The LIM homeobox gene Lhx9 is essential for mouse gonad formation.

During mammalian embryonic development, the ovaries and testes develop from somatic cells of the urogenital ridges as indifferent gonads, harbouring primordial germ cells that have migrated there. After sex determination of the gonads, the testes produce testosterone and anti-Mullerian hormone which mediate male sexual differentiation, and the female developmental pathway ensues in their absence. Here we show that transcripts of the LIM homeobox gene Lhx9 are present in urogenital ridges of mice at embryonic day 9.5; later they localize to the interstitial region as morphological differentiation occurs. In mice lacking Lhx9 function, germ cells migrate normally, but somatic cells of the genital ridge fail to proliferate and a discrete gonad fails to form. In the absence of testosterone and anti-Mullerian hormone, genetically male mice are phenotypically female. The expression of steroidogenic factor 1 (Sf1), a nuclear receptor essential for gonadogenesis, is reduced to minimal levels in the Lhx9-deficient genital ridge, indicating that Lhx9 may lie upstream of Sf1 in a developmental cascade. Unlike mice lacking other genes that mediate early stages of gonadogenesis, Lhx9 mutants do not exhibit additional major developmental defects. Thus, LHX9 mutations may underlie certain forms of isolated gonadal agenesis in humans.

Animals↗
PubMed2000

Developmental roles of the steroidogenic acute regulatory protein (StAR) as revealed by StAR knockout mice.

Steroidogenic acute regulatory protein (StAR) is essential for adrenal and gonadal steroidogenesis, stimulating the translocation of cholesterol to the inner mitochondrial membrane where steroidogenesis commences. StAR mutations in humans cause congenital lipoid adrenal hyperplasia (lipoid CAH), an autosomal recessive condition with severe deficiencies of all classes of steroid hormones. We previously described StAR knockout mice that mimic many features of lipoid CAH patients. By keeping StAR knockout mice alive with corticosteroid replacement, we now examine the temporal effects of StAR deficiency on the structure and function of steroidogenic tissues. The adrenal glands, affected most severely at birth, exhibited progressive increases in lipid deposits with aging. The testes of newborn StAR knockout mice contained scattered lipid deposits in the interstitial region, presumably in remnants of fetal Leydig cells. By 8 weeks of age, the interstitial lipid deposits worsened considerably and were associated with Leydig cell hyperplasia. Despite these changes, germ cells in the seminiferous tubules appeared intact histologically, suggesting that the StAR knockout mice retained some capacity for androgen biosynthesis. Sperm maturation was delayed, and the germ cells exhibited histological features of apoptosis, consistent with suboptimal androgen production. Immediately after birth, the ovaries of StAR knockout mice appeared normal. After the time of normal puberty, however, prominent lipid deposits accumulated in the interstitial region, accompanied by marked luteinization of stromal cells and incomplete follicular maturation that ultimately culminated in premature ovarian failure. These studies provide the first systematic evaluation of the developmental consequences of StAR deficiency in the various steroidogenic organs.

Adrenal Cortex Hormones↗
PubMed2000

Approaches to define the role of SF-1 at different levels of the hypothalamic-pituitary-steroidogenic organ axis.

Targeted gene disruption has produced knockout mice globally deficient in the orphan nuclear receptor steroidogenic factor 1 (SF-1). These SF-1 knockout mice lacked adrenal glands and gonads and consequently exhibited adrenocortical insufficiency and sex reversal of their internal and external genitalia. They also had impaired expression of gonadtropins in the anterior pituitary gonadotropes and agenesis of the ventromedial hypothalamic nucleus (VMH), confirming roles of SF-1 at all three levels of the hypothalamic-pituitary-steroidogenic organ axis. Ongoing experiments are directed at using evolving techniques for tissue-specific gene inactivation to define the roles of SF-1 within discrete sites of the hypthalamic-pituitary-steroidogenic organ axis.

Animals↗
PubMed1999

Activation of the promoter of the orphan receptor SHP by orphan receptors that bind DNA as monomers.

Small heterodimer partner (SHP) is an orphan nuclear receptor that lacks a conventional DNA binding domain. It interacts with several other members of the nuclear receptor superfamily and inhibits receptor transactivation. In order to characterize the regulation of SHP expression, a number of receptors and other transcription factors were tested for effects on the SHP promoter. Among these, the orphan receptor steroidogenic factor-1 (SF-1) was found to potently transactivate the SHP promoter. Detailed footprinting studies show that the SHP promoter contains at least five SF-1 binding sites, and mutagenesis studies demonstrate each of the three strongest binding sites is required for SF-1 transactivation. SHP is coexpressed with SF-1 in adrenal glands, but is also expressed in tissues that lack SF-1, including liver. However, liver expresses a close relative of SF-1, the orphan fetoprotein transcription factor (FTF), and FTF can also transactivate the SHP promoter. These results suggest that alterations in the levels or activities of SF-1 or FTF could modulate SHP expression in appropriate tissues and thereby affect a variety of receptor dependent signaling pathways.

Binding Sites↗
PubMed1999

Genes essential for early events in gonadal development.

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. The basic underlying principle of sexual development is that genetic sex-determined at fertilization by the presence or absence of the Y chromosome--directs the embryonic gonads to differentiate into either testes or ovaries. Thereafter, hormones produced by the testes direct the developmental program that leads to male sexual differentiation. In the absence of testicular hormones, the female pathway of sexual differentiation occurs. Recent studies have defined key roles in gonadal development for two transcription factors: Wilms' tumor suppressor 1 (WT1) and steroidogenic factor 1 (SF-1). After presenting a brief overview of gonadal development and sexual differentiation, this paper reviews the studies that led to the isolation and characterization of WT1 and SF-1, and then discusses how interactions between these two genes may mediate their key roles in a common developmental pathway.

Animals↗
PubMed1999

Expression of steroidogenic factor 1 and Wilms' tumour 1 during early human gonadal development and sex determination.

The transcription factors SF-1 and WT1 play pivotal roles in mammalian gonadal development and sexual differentiation. In human embryos, both SF-1 and WT1 are expressed when the indifferent gonadal ridge first forms at 32 days post-ovulation. As the sex cords develop - providing morphological evidence of testis differentiation - SF-1 localises predominantly to developing Sertoli cells in the sex cords, whereas WT1 retains a broader pattern of expression. Later, SF-1 localises predominantly to steroidogenic Leydig cells, and WT1 localises to the sex cords. In the ovary, SF-1 and WT1 transcripts persist in the gonadal ridge from the earliest developmental stages throughout the critical period of sex determination. These studies, which delineate for the first time the sequential expression profiles of SF-1 and WT1 during human gonadal development, provide a framework for understanding human sex reversal phenotypes associated with their mutations.

DNA-Binding Proteins↗
PubMed1999

Gene interactions in gonadal development.

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. Although the maturation of sexual function and reproduction occurs after birth, essentially all of the critical developmental steps take place during embryogenesis. Temporally, these steps can be divided into two different phases: sex determination, the initial event that determines whether the gonads will develop as testes or ovaries; and sexual differentiation, the subsequent events that ultimately produce either the male or the female sexual phenotype. A basic tenet of sexual development in mammals is that genetic sex--determined by the presence or absence of the Y chromosome--directs the embryonic gonads to differentiate into either testes or ovaries. Thereafter, hormones produced by the testes direct the developmental program leading to male sexual differentiation. In the absence of testicular hormones, the pathway of sexual differentiation is female. This chapter reviews the anatomic and cellular changes that constitute sexual differentiation and discusses SRY and other genes, including SF-1, WT1, DAX-1, and SOX9, that play key developmental roles in this process. Dose-dependent interactions among these genes are critical for sex determination and differentiation.

Animals↗
PubMed1999

Developmental and hormonal regulation of murine scavenger receptor, class B, type 1.

The scavenger receptor, class B, type I (SR-BI), is the predominant receptor that supplies plasma cholesterol to steroidogenic tissues in rodents. We showed previously that steroidogenic factor-1 (SF-1) binds a sequence in the human SR-BI promoter whose integrity is required for high-level SR-BI expression in cultured adrenocortical tumor cells. We now provide in vivo evidence that SF-1 regulates SR-BI. During mouse embryogenesis, SR-BI mRNA was initially expressed in the genital ridge of both sexes and persisted in the developing testes but not ovary. This sexually dimorphic expression profile of SR-BI expression in the gonads mirrors that of SF-1. No SR-BI mRNA was detected in the gonadal ridge of day 11.5 SF-1 knockout embryos. Both SR-BI and SF-1 mRNA were expressed in the cortical cells of the nascent adrenal glands. These studies directly support SF-1 participating in the regulation of SR-BI in vivo. We examined the effect of cAMP on SR-BI mRNA and protein in mouse adrenocortical (Y1-BS1) and testicular carcinoma Leydig (MA-10) cells. The time courses of induction were strikingly similar to those described for other cAMP- and SF-1-regulated genes. Addition of lipoproteins reduced SR-BI expression in Y1-BS1 cells, an effect that was reversed by administration of cAMP analogs. SR-BI mRNA and protein were expressed at high levels in the adrenal glands of knockout mice lacking the steroidogenic acute regulatory protein; these mice have extensive lipid deposits in the adrenocortical cells and high circulating levels of ACTH. Taken together, these studies suggest that trophic hormones can override the suppressive effect of cholesterol on SR-BI expression, thus ensuring that steroidogenesis is maintained during stress.

Adrenal Glands↗