Neonatal mice possessing an Sry transgene show a masculinized pattern of progesterone receptor expression in the brain independent of sex chromosome status

To assess the relative roles of sex chromosome genes and gonadal steroid hormones in producing sex differences in progesterone receptor (PR) expression in the forebrain of neonatal mice, we used mice in which the Sry gene had been deleted from the Y-chromosome and inserted as a transgene on an autosome in both XX and XY genotypes. Levels of PR immunoreactivity (PRir) in the anteroventral periventricular nucleus, the medial preoptic nucleus, and the ventromedial nucleus were significantly higher in mice that possessed an Sry transgene compared with mice that lacked an Sry transgene, regardless of their complement of sex chromosomes (XX vs. XY). This result suggests that sexual differentiation of PR expression in these regions is likely controlled mainly by gonadal hormones, not by the genetic sex of the brain cells. No differences in PRir were detected between wild-type XY mice with the Sry gene on the Y-chromosome and XY mice with the Sry transgene, suggesting that testicular hormones produced in these two genotypes have comparable effects on neural tissue.     

A yin-yang effect between sex chromosome complement and sex hormones on the immune response

Sex chromosome complement, by determining whether an ovary or testis develops, exerts indirect hormone-mediated effects on the development of sex-specific traits. However, this does not preclude more direct effects that are independent of gonadal hormones. To look for gonadal hormone-independent effects in sexually dimorphic immune responses, we used mice in which the testis determinant Sry has been moved from the Y chromosome to an autosome, thus allowing the production of mice that differ in sex chromosome complement while having the same gonadal type. This model permits comparison of XX and XY mice with ovaries or testes. These mice were immunized with an autoantigen, and draining lymph node cells were assessed for autoantigen-specific proliferative responses and cytokine production. Surprisingly, we found that the male complement of sex chromosomes (XY) was relatively stimulatory, whereas male sex hormones were inhibitory, for this immune response. This is the first experimental evidence of a compensatory yin-yang effect of sex chromosome complement and sex hormones on a biologic process.     

Minireview: Sex chromosomes and brain sexual differentiation

The brains of males and females differ, not only in regions specialized for reproduction, but also in other regions (controlling cognition, for example) where sex differences are not necessarily expected. Moreover, males and females are differentially susceptible to neurological and psychiatric disease. What are the origins of these sex differences? Two major sources of sexually dimorphic information could lead to sex differences in brain function. Male and female brain cells carry a different complement of sex chromosome genes and are influenced throughout life by a different mix of gonadal hormones. Until recently all sex differences in the brain have been attributed to the differential action of gonadal hormones. Recent findings, however, suggest that brain cells that differ in their genetic sex are not equivalent, and that difference may contribute to sex differences in brain function. Here we discuss evidence for sex chromosome effects on both neural and nonneural systems, which together provide support for the idea that XX and XY cells differentiate even before they are influenced by gonadal hormones, and even if they are exposed to similar levels of gonadal steroids. Fortunately, new model systems for studying sex chromosome effects have recently been developed, and they should help in testing further the role of sex chromosome genes.     

Male Japanese quails with female brains do not show male sexual behaviors

During embryonic development, gonadal steroid hormones (androgens and estrogens) are thought to organize the sexual differentiation of the brain in the heterogametic sexes of higher vertebrates (males in mammals, females in birds). Brain differentiation of the homogametic sexes is thought to proceed by default, not requiring sex hormones for sex-specific organization. In gallinaceous birds such as the Japanese quail, female brain organization is thought to develop via estrogen-dependent demasculinization of a default male brain phenotype. We performed male donor-to-female host (MF), female-to-male (FM), male-to-male (MM), and female-to-female (FF) isotopic, isochronic transplantation of the forebrain primordium in Japanese quail embryos before gonadal differentiation had occurred; brain chimeras had a forebrain (including the hypothalamus) originating exclusively from donor cells. MM, FF, and MF chimeras all showed sexual behavior governed by the genetic sex of the host. In contrast, FM chimeras (genetically female forebrain, all other tissues genetically male) showed no mounting and only rudimentary crowing behavior. Although MM, FF, MF, and FM chimeras all showed host-typical production of steroid hormones during embryonic life, only FM chimeras were hypogonadal, had atypical low levels of circulating testosterone in adulthood, and showed reduction (crowing) or absence (mounting) of reproductive behaviors. Morphological features of the medial preoptic nucleus (a sexually dimorphic brain area) also were not male-like in FM males. These data demonstrate a brain-intrinsic, genetically determined component that organizes the sex-typical production of gonadal hormones in adulthood and call for a reevaluation of the mechanisms underlying brain sexual differentiation in other higher-vertebrate species.     

HIV protease inhibitor ritonavir induces lipoatrophy in male mice

We investigated the effects of the HIV protease inhibitor ritonavir on body composition, serum lipids, and gene expression in C57BL/6 mice. Dual-energy X-ray absorptiometry measurements in ritonavir-treated male mice revealed whole-body lipoatrophy. In female mice fat reduction was restricted to the gonadal depot. A histopathological analysis showed no visible abnormalities in liver or adipose tissue from ritonavir-treated mice, although adipocytes were significantly smaller in diameter. Serum triglyceride levels were increased in ritonavir-treated male mice. Ritonavir was coadministered with the peroxisome proliferator-activated receptor alpha (PPARalpha) agonist gemfibrozil and the PPARgamma agonist rosiglitazone for 8 weeks. Neither drug alleviated the hypertriglyceridemia or lipoatrophy in ritonavir-treated male mice. Rather, gemfibrozil exacerbated the lipoatrophy. Ritonavir reduced basal expression of two PPARalpha target genes in liver, as well as the PPARgamma target gene phosphoenolpyruvate carboxykinase (PEPCK) in adipose tissues. Ritonavir partially inhibited induction of PPAR target genes by gemfibrozil and rosiglitazone. Gemfibrozil induced expression of fatty acid oxidation genes in liver, and this induction was less substantial when ritonavir was coadministered. Similarly, rosiglitazone induced expression of uncoupling protein-1, uncoupling protein-2, and PEPCK in adipose tissues, and this effect was partially inhibited by ritonavir. Thus, the effects of ritonavir on serum triglycerides and body composition may be due, at least in part, to an inhibition of PPAR function.     

Sex differences in health and aging: a dialog between the brain and gonad?

Women live longer than men in virtually all circumstances. However, a more common pattern among animals is that one sex lives longer under some conditions, the other lives longer under other conditions. In laboratory mice, interventions that extend longevity are surprisingly often sex-specific in their effects. Understanding these conditional sex differences could provide mechanistic insight into how longevity could be modulated in humans. One way that longevity can be consistently enhanced is by inhibiting reproduction or eliminating the capacity to reproduce. Thus, there appears to be a mechanistic link between gonadal activity and longevity. There also appears to be a mechanistic link between some types of neuroendocrine signaling and longevity. Combining these two observations suggest that communication between the brain and gonad is a ripe avenue for further exploring longevity-assurance mechanisms. Also, because the timing and activity of specific brain–gonad endocrine differs between the sexes, neuroendocrine linkages between the brain and gonad, particularly among the less obvious hormones such as activin and inhibin, could provide additional insight into mechanisms of sex differences in aging.

     

Sexual dimorphism of gonadal development

Sexual dimorphism is a term describing morphological differences between the sexes, but is often extended to include all differences observed between females and males. Sex differentiation in vertebrates is by definition sexually dimorphic and starts at the level of the sex chromosomes. In this review the sexual dimorphism of gonadal differentiation is discussed, with a focus on human development. In the embryo, the indifferent gonadal anlagen harbours four different cell lineages with bipotential fates dependent on the sex of the individual. The different paths taken by these cell lineages in male and female development are reviewed, along with other sexually dimorphic features of gonadal development. These include sex-determining genes, timing of events, dependence on germ cells, spatial organization of stromal cells, steroidogenic cells types, and other aspects.     

Sex-specific genetic effects influence variation in body composition

Aims/hypothesis  Despite well-known sex differences in body composition it is not known whether sex-specific genetic or environmental effects contribute to these differences. Methods  We assessed body composition in 2,506 individuals, from a young Dutch genetic isolate participating in the Erasmus Rucphen Family study, by dual-energy X-ray absorptiometry and anthropometry. We used variance decomposition procedures to partition variation of body composition into genetic and environmental components common to both sexes and to men and women separately and calculated the correlation between genetic components in men and women. Results  After accounting for age, sex and inbreeding, heritability ranged from 0.39 for fat mass index to 0.84 for height. We found sex-specific genetic effects for fat percentage (fat%), lean mass, lean mass index (LMI) and fat distribution, but not for BMI and height. Genetic correlations between sexes were significantly different from 1 for fat%, lean mass, LMI, android fat, android:gynoid fat ratio and WHR, indicating that there are sex-specific genes contributing to variation of these traits. Genetic variance was significantly higher in women for the waist, hip and thigh circumference and WHR, implying that genes account for more variance of fat distribution in women than in men. Environmental variance was significantly higher in men for the android:gynoid fat ratio. Conclusions/interpretation  Sex-specific genetic effects underlie sexual dimorphism in several body composition traits. The findings are relevant for studies on the relationship of body composition with common diseases like cardiovascular disease and type 2 diabetes and for genetic association studies. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorised users.     

Pituitary growth hormone network responses are sexually dimorphic and regulated by gonadal steroids in adulthood

There are well-recognized sex differences in many pituitary endocrine axes, usually thought to be generated by gonadal steroid imprinting of the neuroendocrine hypothalamus. However, the recognition that growth hormone (GH) cells are arranged in functionally organized networks raises the possibility that the responses of the network are different in males and females. We studied this by directly monitoring the calcium responses to an identical GH-releasing hormone (GHRH) stimulus in populations of individual GH cells in slices taken from male and female murine GH-eGFP pituitary glands. We found that the GH cell network responses are sexually dimorphic, with a higher proportion of responding cells in males than in females, correlated with greater GH release from male slices. Repetitive waves of calcium spiking activity were triggered by GHRH in some males, but were never observed in females. This was not due to a permanent difference in the network architecture between male and female mice; rather, the sex difference in the proportions of GH cells responding to GHRH were switched by postpubertal gonadectomy and reversed with hormone replacements, suggesting that the network responses are dynamically regulated in adulthood by gonadal steroids. Thus, the pituitary gland contributes to the sexually dimorphic patterns of GH secretion that play an important role in differences in growth and metabolism between the sexes.     

Gene expression profiling identifies liver X receptor alpha as an estrogen-regulated gene in mouse adipose tissue

Estrogens reduce adipose tissue mass in both humans and animals. The molecular mechanisms for this effect are, however, not well characterized. We took a gene expression profiling approach to study the direct effects of estrogen on mouse white adipose tissue (WAT). Female ovariectomized mice were treated for 10, 24 and 48 h with 17beta-estradiol or vehicle. RNA was extracted from gonadal fat and hybridized to Affymetrix MG-U74Av2 arrays. 17beta-Estradiol was shown to decrease mRNA expression of liver X receptor (LXR) alpha after 10 h of treatment compared with the vehicle control. The expression of several LXRalpha target genes, such as sterol regulatory element-binding protein 1c, apolipoprotein E, phospholipid transfer protein, ATP-binding cassette A1 and ATP-binding cassette G1, was similarly decreased. We furthermore identified a 1.5 kb LXRalpha promoter fragment that is negatively regulated by estrogen. Several genes involved in lipogenesis and lipolysis were identified as novel targets that could mediate estrogenic effects on adipose tissue. Finally, we show that ERalpha is the main estrogen receptor expressed in mouse white adipose tissue (WAT) with mRNA levels several hundred times higher than those of ERbeta mRNA.     

Tissue-specific expression of receptor-interacting protein in aging mouse

Receptor-interacting protein (RIP) is a well-characterized coregulator for nuclear receptors. Here, we report the expression of RIP as two isoforms with molecular weights of 140 kDa and 137 kDa in liver and kidney, but only as one isoform of 140 kDa in lung, adipose tissue, prostate and testis of mice. The levels of both the isoforms decreased in liver and kidney of old mice compared with adult mice. The expression of RIP140 in kidney was relatively lower in old males than females. In contrast, adipose tissue showed remarkably higher levels of RIP140 in old than adult mice of both sexes. Thus, the expression of RIP varied with the type of tissue, sex and age of mice, suggesting differences in its function as a coregulator.     

Neural,not gonadal,origin of brain sex differences in a gynandromorphic finch

In mammals and birds, sex differences in brain function and disease are thought to derive exclusively from sex differences in gonadal hormone secretions. For example, testosterone in male mammals acts during fetal and neonatal life to cause masculine neural development. However, male and female brain cells also differ in genetic sex; thus, sex chromosome genes acting within cells could contribute to sex differences in cell function. We analyzed the sexual phenotype of the brain of a rare gynandromorphic finch in which the right half of the brain was genetically male and the left half genetically female. The neural song circuit on the right had a more masculine phenotype than that on the left. Because both halves of the brain were exposed to a common gonadal hormone environment, the lateral differences indicate that the genetic sex of brain cells contributes to the process of sexual differentiation. Because both sides of the song circuit were more masculine than that of females, diffusible factors such as hormones of gonadal or neural origin also likely played a role in sexual differentiation.     

Sexually dimorphic expression of the growth hormone-releasing hormone gene is not mediated by circulating gonadal hormones in the adult rat

The sexual dimorphism characterizing GH secretion in the rat is thought to be related to differences in the hypothalamic synthesis and release of the GH-regulating peptides, GH-releasing hormone (GHRH), and somatostatin. Therefore, the influence of gender and sex steroid hormones on hypothalamic expression of the GHRH gene in adult rats were examined. GHRH messenger RNA (mRNA) levels were measured in individual rat hypothalami by Northern hybridization analysis using a 32P-labeled complementary DNA encoding rat GHRH. Destruction of hypothalamic GHRH neurons by neonatal treatment with monosodium glutamate caused similar 3-fold reductions in the levels of GHRH mRNA in adult male and female animals. In three separate experiments, hypothalamic GHRH mRNA concentrations in male rats were 2- to 3-fold greater than in randomly cycling females (four or five rats per group; P less than 0.01). In spite of the greater abundance of GHRH mRNA abundance in the male rat hypothalamus, circulating gonadal steroids lacked the ability to modulate GHRH gene expression in adult animals, since neither gonadectomy nor pharmacological sex steroid replacement changed GHRH mRNA levels in the hypothalamus of male and female adult rats. Furthermore, GHRH mRNA concentrations in female rats were similar during the proestrus, estrus, and diestrus phase of the estrous cycle. Also, GH inhibited hypothalamic GHRH gene expression in a sex-specific manner. Exposure to high levels of GH secreted by the MtTW15 tumor for 4 weeks reduced GHRH mRNA concentrations 7-fold in male rats (P less than 0.001) but only 2-fold in females (P less than 0.05). These studies demonstrate that GHRH gene expression in the rat hypothalamus is sexually dimorphic. Basal mRNA levels are greater in male rats, and expression in male hypothalami is more sensitive to feedback inhibition by GH than expression in females. There is no evidence for regulation of GHRH mRNA levels by either testosterone or estrogen in adult rats. These gender differences in GHRH gene expression likely contribute to the generation of a sex-specific pattern of GH secretion.     

Genes involved in sex determination and differentiation

Chromosomal sex is established at fertilization by the presence of an X or Y chromosome. The first step of male and female development is gonadal specialization in testes or ovaries; all other processes that follow result from secondary effects produced by testis and ovary hormones. Gonadal determination and differentiation and the development of external genitalia involve time- and tissue-specific expression of genes forming a gene cascade. Those genes, their expression profile and their role in the pathological manifestations related to gonadal and external genitalia development will be discussed in this review.