Cooperation of sex chromosomal genes and endocrine influences for hypothalamic sexual differentiation

Age and sex mediated effects of estrogen and Β3-adrenergic receptor on cardiovascular pathophysiology

Sex differences are consistently identified in determining the prevalence, manifestation, and response to therapies in several systemic disorders, including those affecting the cardiovascular (CV), skeletal muscle, and nervous system. Interestingly, such differences are often more noticeable as we age. For example, premenopausal women experience a lower risk of CV disease than men of the same age. While at an advanced age, with menopause, the risk of cardiovascular diseases and adverse outcomes increases exponentially in women, exceeding that of men. However, this effect appears to be reversed in diseases such as pulmonary hypertension, where women are up to seven times more likely than men to develop an idiopathic form of the disease with symptoms developing ten years earlier than their male counterparts. Explaining this is a complex question. However, several factors and mechanisms have been identified in recent decades, including a role for sex hormones, particularly estrogens and their related receptors. Furthermore, an emerging role in these sex differences has also been suggested for β-adrenergic receptors (βARs), which are essential regulators of mammalian physiology. It has in fact been shown that βARs interact with estrogen receptors (ER), providing further demonstration of their involvement in determining sexual differences. Based on these premises, this review article focused on the β3AR subtype, which shows important activities in adipose tissue but with new and interesting roles in regulating the function of cardiomyocytes and vascular cells. In detail, we examined how β3AR and ER signaling are intertwined and whether there would be sex- and age-dependent specific effects of these receptor systems.

Evaluating different models of maternal stress on stress-responsive systems in prepubertal mice

Introduction

Maternal adversity during pregnancy influences neurodevelopment in human and model animal offspring. Adversity can result from stressors coming from many different directions ranging from environmental to nutritional and physiological to immune (e.g., infection). Most stressors result in fetal overexposure to glucocorticoids that have been directly linked to long- and short-term negative impacts on neurological health of offspring. Neuropsychiatric diseases postulated to have fetal origins are diverse and include such things cardiovascular disease, obesity, affective disorders, and metabolic and immune disorders.

Methods

The experiments in the current study compare 3 stressors: prenatal exposure to dexamethasone (DEX), maternal high fat diet (HFD), and maternal caloric restriction (CR). Offspring of mothers with these treatments were examined prepubertally to evaluate stress responsiveness and stress-related behaviors in in male and female mice.

Results

Prenatal exposure to synthetic glucocorticoid, DEX, resulted in decreased neonatal body weights, reduced social interaction behavior, and hypoactive stress response offspring exposed to maternal DEX. Maternal CR resulted in decreased body weights and social interaction behavior in males and females and increased anxiety-like behavior and acute stress response only in males. HFD resulted in altered body weight gain in both sex offspring with decreased anxiety-like behavior in a female-biased manner.

Discussion

The idea that glucocorticoid responses to different stressors might serve as a common stimulus across stress paradigms is insufficient, given that different modes of prenatal stress produced differential effects. Opposite nutritional stressors produced similar outcomes for anxiety-like behavior in both sexes, social-like behavior in females, and a hyperactive adrenal stress response in males. One common theme among the three models of maternal stress (DEX, CR, and HFD) was consistent data showing their role in activating the maternal and fetal immune response. By tuning in on the more immediate immunological aspect on the developing fetus (e.g., hormones, cytokines), additional studies may tease out more direct outcomes of maternal stress in rodents and increase their translational value to human studies.

Female aging: when translational models don't translate

For many pathologies associated with aging, female patients present with higher morbidity and more frequent adverse events from treatments compared to male patients. While preclinical models are the foundation of our mechanistic understanding of age-related diseases, the most common models fail to recapitulate archetypical female aging trajectories. For example, while over 70% of the top age-related diseases are influenced by the systemic effects of reproductive senescence, we found that preclinical studies that include menopausal phenotypes modeling those seen in humans make up <1% of published aging biology research. The long-term impacts of pregnancy, birthing and breastfeeding are also typically omitted from preclinical work. In this Perspective, we summarize limitations in the most commonly used aging models, and we provide recommendations for better incorporating menopause, pregnancy and other considerations of sex in vivo and in vitro. Lastly, we outline action items for aging biology researchers, journals, funding agencies and animal providers to address this gap.

Evaluation of Gender Differences in Response to Photobiomodulation Therapy, Including Laser Acupuncture: A Narrative Review and Implication to Precision Medicine

Background: The influence of gender is significant in the manifestation and response to many diseases and in the treatment strategy. Photobiomodulation (PBM) therapy, including laser acupuncture, is an evidence-based treatment and disease prevention modality that has shown promising efficacy for a myriad of chronic and acute diseases. Anecdotal experience and limited clinical trials suggest gender differences exist in treatment outcomes to PBM therapy. There is preliminary evidence that gender may be as important as skin color in the individual response to PBM therapy. Purpose: To conduct a literature search of publications addressing the effects of gender differences in PBM therapy, including laser acupuncture, to provide a narrative review of the findings, and to explore potential mechanisms for the influence of gender. Methods: A narrative review of the literature on gender differences in PBM applications was conducted using key words relating to PBM therapy and gender. Results: A total of 13 articles were identified. Of these articles, 11 have direct experimental investigations into the response difference in gender for PBM, including laser acupuncture. A variety of cadaver, human, and experimental studies demonstrated results that gender effects were significant in PBM outcome responses, including differences in tendon structural and mechanical outcomes, and mitochondrial gene expression. One cadaver experiment showed that gender was more important than skin tone. The physiologic mechanisms directing gender differences are explored and postulated. Conclusions: The review suggests that to address the requirements of a proficient precision medicine-based strategy, it is important for PBM therapy to consider gender in its treatment plan and dosing prescription. Further research is warranted to determine the correct dose for optimal gender treatment, including gender-specific treatment plans to improve outcomes, taking into account wavelength, energy exposure, intensity, and parameters related to the deliverance of treatment to each anatomical location.

Estradiol-dependent axogenesis and Ngn3 expression are determined by XY sex chromosome complement in hypothalamic neurons

Hypothalamic neurons show sex differences in neuritogenesis, female neurons have longer axons and higher levels of the neuritogenic factor neurogenin 3 (Ngn3) than male neurons in vitro. Moreover, the effect of 17-β-estradiol (E2) on axonal growth and Ngn3 expression is only found in male-derived neurons. To investigate whether sex chromosomes regulate these early sex differences in neuritogenesis by regulating the E2 effect on Ngn3, we evaluated the growth and differentiation of hypothalamic neurons derived from the “four core genotypes” mouse model, in which the factors of “gonadal sex” and “sex chromosome complement” are dissociated. We showed that sex differences in neurite outgrowth are determined by sex chromosome complement (XX > XY). Moreover, E2 increased the mRNA expression of Ngn3 and axonal length only in XY neurons. ERα/β expressions are regulated by sex chromosome complement; however, E2-effect on Ngn3 expression in XY neurons was only fully reproduced by PPT, a specific ligand of ERα, and prevented by MPP, a specific antagonist of ERα. Together our data indicate that sex chromosomes regulate early development of hypothalamic neurons by orchestrating not only sex differences in neuritogenesis, but also regulating the effect of E2 on Ngn3 expression through activation of ERα in hypothalamic neurons.

Gonadal hormone-independent sex differences in GABAA receptor activation in rat embryonic hypothalamic neurons

BACKGROUND AND PURPOSE

GABA_A receptor functions are dependent on subunit composition, and, through their activation, GABA can exert trophic actions in immature neurons. Although several sex differences in GABA-mediated responses are known to be dependent on gonadal hormones, few studies have dealt with sex differences detected before the critical period of brain masculinisation. In this study, we assessed GABA_A receptor functionality in sexually segregated neurons before brain hormonal masculinisation.

EXPERIMENTAL APPROACH

Ventromedial hypothalamic neurons were obtained from embryonic day 16 rat brains and grown in vitro for 2 days. Calcium imaging and electrophysiology recordings were carried out to assess GABA_A receptor functional parameters.

KEY RESULTS

GABA_A receptor activation elicited calcium entry in immature hypothalamic neurons mainly through L-type voltage-dependent calcium channels. Nifedipine blocked calcium entry more efficiently in male than in female neurons. There were more male than female neurons responding to GABA, and they needed more time to return to resting levels. Pharmacological characterisation revealed that propofol enhanced GABA_A -mediated currents and blunted GABA-mediated calcium entry more efficiently in female neurons than in males. Testosterone treatment did not erase such sex differences. These data suggest sex differences in the expression of GABA_A receptor subtypes.

CONCLUSION AND IMPLICATIONS

GABA-mediated responses are sexually dimorphic even in the absence of gonadal hormone influence, suggesting genetically biased differences. These results highlight the importance of GABA_A receptors in hypothalamic neurons even before hormonal masculinisation of the brain.

Impact of X/Y genes and sex hormones on mouse neuroanatomy

Biological sex influences brain anatomy across many species. Sex differences in brain anatomy have classically been attributed to differences in sex chromosome complement (XX versus XY) and/or in levels of gonadal sex steroids released from ovaries and testes. Using the four core genotype (4CG) mouse model in which gonadal sex and sex chromosome complement are decoupled, we previously found that sex hormones and chromosomes influence the volume of distinct brain regions. However, recent studies suggest there may be more complex interactions between hormones and chromosomes, and that circulating steroids can compensate for and/or mask underlying chromosomal effects. Moreover, the impact of pre vs post-pubertal sex hormone exposure on this sex hormone/sex chromosome interplay is not well understood. Thus, we used whole brain high-resolution ex-vivo MRI of intact and pre-pubertally gonadectomized 4CG mice to investigate two questions: 1) Do circulating steroids mask sex differences in brain anatomy driven by sex chromosome complement? And 2) What is the contribution of pre- versus post-pubertal hormones to sex-hormone-dependent differences in brain anatomy? We found evidence of both cooperative and compensatory interactions between sex chromosomes and sex hormones in several brain regions, but the interaction effects were of low magnitude. Additionally, most brain regions affected by sex hormones were sensitive to both pre- and post-pubertal hormones. This data provides further insight into the biological origins of sex differences in brain anatomy.

Sex in basic research: concepts in the cardiovascular field

Women and men, female and male animals and cells are biologically different, and acknowledgement of this fact is critical to advancing medicine. However, incorporating concepts of sex-specific analysis in basic research is largely neglected, introducing bias into translational findings, clinical concepts and drug development. Research funding agencies recently approached these issues but implementation of policy changes in the scientific community is still limited, probably due to deficits in concepts, knowledge and proper methodology. This expert review is based on the EUGenMed project (www.eugenmed.eu) developing a roadmap for implementing sex and gender in biomedical and health research. For sake of clarity and conciseness, examples are mainly taken from the cardiovascular field that may serve as a paradigm for others, since a significant amount of knowledge how sex and oestrogen determine the manifestation of many cardiovascular diseases (CVD) has been accumulated. As main concepts for implementation of sex in basic research, the study of primary cell and animals of both sexes, the study of the influence of genetic vs. hormonal factors and the analysis of sex chromosomes and sex specific statistics in genome wide association studies (GWAS) are discussed. The review also discusses methodological issues, and analyses strength, weaknesses, opportunities and threats in implementing sex-sensitive aspects into basic research.

Neuroendocrine disruption of organizational and activational hormone programming in poikilothermic vertebrates

In vertebrates, sexual differentiation of the reproductive system and brain is tightly orchestrated by organizational and activational effects of endogenous hormones. In mammals and birds, the organizational period is typified by a surge of sex hormones during differentiation of specific neural circuits; whereas activational effects are dependent upon later increases in these same hormones at sexual maturation. Depending on the reproductive organ or brain region, initial programming events may be modulated by androgens or require conversion of androgens to estrogens. The prevailing notion based upon findings in mammalian models is that male brain is sculpted to undergo masculinization and defeminization. In absence of these responses, the female brain develops. While timing of organizational and activational events vary across taxa, there are shared features. Further, exposure of different animal models to environmental chemicals such as xenoestrogens such as bisphenol A-BPA and ethinylestradiol-EE2, gestagens, and thyroid hormone disruptors, broadly classified as neuroendocrine disrupting chemicals (NED), during these critical periods may result in similar alterations in brain structure, function, and consequently, behaviors. Organizational effects of neuroendocrine systems in mammals and birds appear to be permanent, whereas teleost fish neuroendocrine systems exhibit plasticity. While there are fewer NED studies in amphibians and reptiles, data suggest that NED disrupt normal organizational-activational effects of endogenous hormones, although it remains to be determined if these disturbances are reversible. The aim of this review is to examine how various environmental chemicals may interrupt normal organizational and activational events in poikilothermic vertebrates. By altering such processes, these chemicals may affect reproductive health of an animal and result in compromised populations and ecosystem-level effects.

Effects of Sex Steroids in the Human Brain

Sex steroids are thought to play a critical developmental role in shaping both cortical and subcortical structures in the human brain. Periods of profound changes in sex steroids invariably coincide with the onset of sex differences in mental health vulnerability, highlighting the importance of sex steroids in determining sexual differentiation of the brain. Yet, most of the evidence for the central effects of sex steroids relies on non-human studies, as several challenges have limited our understanding of these effects in humans: the lack of systematic assessment of the human sex steroid metabolome, the different developmental trajectories of specific sex steroids, the impact of genetic variation and epigenetic changes, and the plethora of interactions between sex steroids, sex chromosomes, neurotransmitters, and other hormonal systems. Here we review how multimodal strategies may be employed to bridge the gap between the basic and clinical understanding of sex steroid-related changes in the human brain.

Sexual dimorphism of cardiopulmonary regulation in the arcuate nucleus of the hypothalamus

The arcuate nucleus of the hypothalamus (ANH) interacts with other hypothalamic nuclei, forebrain regions, and downstream brain sites to affect autonomic nervous system outflow, energy balance, temperature regulation, sleep, arousal, neuroendocrine function, reproduction, and cardiopulmonary regulation. Compared to studies of other ANH functions, how the ANH regulates cardiopulmonary function is less understood. Importantly, the ANH exhibits structural and functional sexually dimorphic characteristics and contains numerous neuroactive substances and receptors including leptin, neuropeptide Y, glutamate, acetylcholine, endorphins, orexin, kisspeptin, insulin, Agouti-related protein, cocaine and amphetamine-regulated transcript, dopamine, somatostatin, components of renin-angiotensin system and gamma amino butyric acid that modulate physiological functions. Moreover, several clinically relevant disorders are associated with ANH ventilatory control dysfunction. This review highlights how ANH neurotransmitter systems and receptors modulate breathing differently in male and female rodents. Results highlight the significance of the ANH in cardiopulmonary regulation. The paucity of studies in this area that will hopefully spark investigations of sexually dimorphic ANH-modulation of breathing.

Sex differences in the brain–an interplay of sex steroid hormones and sex chromosomes

Although considerable progress has been made in our understanding of brain function, many questions remain unanswered. The ultimate goal of studying the brain is to understand the connection between brain structure and function and behavioural outcomes. Since sex differences in brain morphology were first observed, subsequent studies suggest different functional organization of the male and female brains in humans. Sex and gender have been identified as being a significant factor in understanding human physiology, health and disease, and the biological differences between the sexes is not limited to the gonads and secondary sexual characteristics, but also affects the structure and, more crucially, the function of the brain and other organs. Significant variability in brain structures between individuals, in addition to between the sexes, is factor that complicates the study of sex differences in the brain. In this review, we explore the current understanding of sex differences in the brain, mostly focusing on preclinical animal studies.

The importance of having two X chromosomes

Historically, it was thought that the number of X chromosomes plays little role in causing sex differences in traits. Recently, selected mouse models have been used increasingly to compare mice with the same type of gonad but with one versus two copies of the X chromosome. Study of these models demonstrates that mice with one X chromosome can be strikingly different from those with two X chromosomes, when the differences are not attributable to confounding group differences in gonadal hormones. The number of X chromosomes affects adiposity and metabolic disease, cardiovascular ischaemia/reperfusion injury and behaviour. The effects of X chromosome number are likely the result of inherent differences in expression of X genes that escape inactivation, and are therefore expressed from both X chromosomes in XX mice, resulting in a higher level of expression when two X chromosomes are present. The effects of X chromosome number contribute to sex differences in disease phenotypes, and may explain some features of X chromosome aneuploidies such as in Turner and Klinefelter syndromes.

The Influence of Gonadal Steroid Hormones on Immunoreactive Kisspeptin in the Preoptic Area and Arcuate Nucleus of Developing Agonadal Mice with a Genetic Disruption of Steroidogenic Factor 1

Kisspeptin, a regulator of reproductive function and puberty in mammals, is expressed in the rostral (anteroventral) periventricular nucleus (AVPV) and arcuate nucleus (Arc), and its expression is at least partially regulated by estradiol in rodents. The aim of the present study was to determine contributions of genetic factors and gonadal steroid hormones to the sexual differentiation of kisspeptin-immunoreactive (kisspeptin-ir) cell populations in the AVPV and Arc during postnatal development using agonadal steroidogenic factor 1 (SF-1) knockout (KO) mice. To examine the effects of gonadal hormones on pubertal development of kisspeptin neurons, SF-1 KO mice were treated with estradiol benzoate (EB) from postnatal day (P)25 to P36, and their brains were examined at P36. No sex differences were observed in the SF-1 KO mice during postnatal development and after treatment with EB - which failed to increase the number of kisspeptin-ir cells at P36 to the levels found in wild-type (WT) control females. This suggests that specific time periods of estradiol actions or other factors are needed for sexual differentiation of the pattern of immunoreactive kisspeptin in the AVPV. Kisspeptin immunoreactivity in the Arc was significantly higher in gonadally intact WT and SF-1 KO females than in male mice at P36 during puberty. Further, in WT and SF-1 KO females, but not in males, adult levels were reached at P36. This suggests that maturation of the kisspeptin system in the Arc differs between sexes and is regulated by gonad-independent mechanisms.

Cellular and molecular mechanisms of sexual differentiation in the mammalian nervous system

Neuroscientists are likely to discover new sex differences in the coming years, spurred by the National Institutes of Health initiative to include both sexes in preclinical studies. This review summarizes the current state of knowledge of the cellular and molecular mechanisms underlying sex differences in the mammalian nervous system, based primarily on work in rodents. Cellular mechanisms examined include neurogenesis, migration, the differentiation of neurochemical and morphological cell phenotype, and cell death. At the molecular level we discuss evolving roles for epigenetics, sex chromosome complement, the immune system, and newly identified cell signaling pathways. We review recent findings on the role of the environment, as well as genome-wide studies with some surprising results, causing us to re-think often-used models of sexual differentiation. We end by pointing to future directions, including an increased awareness of the important contributions of tissues outside of the nervous system to sexual differentiation of the brain.

A path forward in the debate over health impacts of endocrine disrupting chemicals

Several recent publications reflect debate on the issue of "endocrine disrupting chemicals" (EDCs), indicating that two seemingly mutually exclusive perspectives are being articulated separately and independently. Considering this, a group of scientists with expertise in basic science, medicine and risk assessment reviewed the various aspects of the debate to identify the most significant areas of dispute and to propose a path forward. We identified four areas of debate. The first is about the definitions for terms such as "endocrine disrupting chemical", "adverse effects", and "endocrine system". The second is focused on elements of hormone action including "potency", "endpoints", "timing", "dose" and "thresholds". The third addresses the information needed to establish sufficient evidence of harm. Finally, the fourth focuses on the need to develop and the characteristics of transparent, systematic methods to review the EDC literature. Herein we identify areas of general consensus and propose resolutions for these four areas that would allow the field to move beyond the current and, in our opinion, ineffective debate.

Gonadectomy prior to puberty decreases normal parental behavior in adult mice

Sex steroid hormones secreted by gonads influence development and expression of many behaviors including parental behaviors. The capacity to display many behaviors develops under the influence of sex steroid hormones; it begins with gonadal differentiation and lasts through puberty. The timing of gonadectomy may have important and long lasting effects on the organization and activation of neural circuits regulating the expression of different behaviors. The present study investigated the importance of exposure to endogenous gonadal steroid hormones during pubertal period/adolescence on parental behavior in adult mice. Male and female WT mice were gonadectomized either before puberty (25 days of age) or after puberty (60 days of age) and tested for parental behavior with and without estradiol benzoate (EB) replacement in adulthood. Additional groups of mice were gonadectomized at P25 and supplemented with estradiol (females) or testosterone (males) during puberty. Female mice gonadectomized after puberty or gonadectomized before puberty and supplemented with estradiol during puberty, displayed better pup directed parental behaviors in comparison to mice gonadectomized at 25 days of age regardless of treatment with estradiol in adulthood. However, mice treated with EB in adulthood displayed better non-pup directed nest building behavior than when they were tested without EB treatment regardless of sex and time of gonadectomy. To examine whether the sensitivity to sex steroid hormones was altered due to differences in time without gonads prior to the testing, mice were also tested for female sex behavior and there were no differences between mice gonadectomized at P25 or P60, although this could not completely rule out the possibility that parental behavior is more sensitive to prolonged absence of steroid hormones than female sex behavior. These results suggest that the absence of gonads and thereby the absence of appropriate gonadal steroid hormones during puberty/adolescence may have a profound effect on pup directed parental behaviors in adult mice.

Conceptual frameworks and mouse models for studying sex differences in physiology and disease: why compensation changes the game

A sophisticated mechanistic understanding of physiology and disease requires knowledge of how sex-biasing factors cause sex differences in phenotype. In therian mammals, all sex differences are downstream of the unequal effects of XX vs. XY sex chromosomes. Three major categories of sex-biasing factors are activational and organizational effects of gonadal hormones, and sex chromosome effects operating outside of the gonads. These three types of effects can be discriminated from each other with established experimental designs and animal models. Two important mouse models, which allow conclusions regarding the sex-biasing effects of sex chromosome complement, interacting with gonadal hormone effects, are the Four Core Genotypes model and the XY* model. Chromosome Y consomic strains give information about the role of the Y chromosome. An important recent change in sexual differentiation theory is the increasing realization that sex-biasing factors can counteract the effects of each other, reducing rather than producing sex differences in phenotype. This change in viewpoint rationalizes a change in experimental strategies for dissecting sex chromosome effects. The overall goal is to understand the sexome, defined as the sum of effects of sex-biasing factors on gene systems and networks.

Gender-dependent resiliency to stressful and metabolic challenges following prenatal exposure to high-fat diet in the p66(Shc-/-) mouse

Metabolic stressful challenges during susceptible time windows, such as fetal life, can have important implications for health throughout life. Deletion of the p66^Shc gene in mice leads to reduced oxidative stress (OS), resulting in a healthy and lean phenotype characterized by increased metabolic rate, resistance to high-fat diet (HFD)-induced obesity and reduced emotionality at adulthood. Here we hypothesize that p66^Shc−/− (KO) adult offspring might be protected from the detrimental effects induced by maternal HFD administered before and during pregnancy. To test such hypothesis, we fed p66^Shc+/+ (WT) and KO females with HFD for 13 weeks starting on 5 weeks of age until delivery and tested adult male and female offspring for their metabolic, neuroendocrine, and emotional profile. Prenatal diet affected stress responses and metabolic features in a gender-dependent fashion. In particular, prenatal HFD increased plasma leptin levels and decreased anxiety-like behavior in females, while increasing body weight, particularly in KO subjects. KO mice were overall characterized by metabolic resiliency, showing a blunted change in glycemia levels in response to glucose or insulin challenges. However, in p66^Shc−/− mice, prenatal HFD affected glucose tolerance response in an opposite manner in the two genders, overriding the resilience in males and exacerbating it in females. Finally, KO females were protected from the disrupting effect of prenatal HFD on neuroendocrine response. These findings indicate that prenatal HFD alters the emotional profile and metabolic functionality of the adult individual in a gender-dependent fashion and suggest that exposure to high-caloric food during fetal life is a stressful condition interfering with the developmental programming of the adult phenotype. Deletion of the p66^Shc gene attenuates such effects, acting as a protective factor.

Disruption of fetal hormonal programming (prenatal stress) implicates shared risk for sex differences in depression and cardiovascular disease

Comorbidity of major depressive disorder (MDD) and cardiovascular disease (CVD) represents the fourth leading cause of morbidity and mortality worldwide, and women have a two times greater risk than men. Thus understanding the pathophysiology has widespread implications for attenuation and prevention of disease burden. We suggest that sex-dependent MDD-CVD comorbidity may result from alterations in fetal programming consequent to the prenatal maternal environments that produce excess glucocorticoids, which then drive sex-dependent developmental alterations of the fetal hypothalamic-pituitary-adrenal (HPA) axis circuitry impacting mood, stress regulation, autonomic nervous system (ANS), and the vasculature in adulthood. Evidence is consistent with the hypothesis that disruptions of pathways associated with gamma aminobutyric acid (GABA) in neuronal and vascular development and growth factors have critical roles in key developmental periods and adult responses to injury in heart and brain. Understanding the potential fetal origins of these sex differences will contribute to development of novel sex-dependent therapeutics.

Nutrient-dependent/pheromone-controlled adaptive evolution: a model

BACKGROUND

The prenatal migration of gonadotropin-releasing hormone (GnRH) neurosecretory neurons allows nutrients and human pheromones to alter GnRH pulsatility, which modulates the concurrent maturation of the neuroendocrine, reproductive, and central nervous systems, thus influencing the development of ingestive behavior, reproductive sexual behavior, and other behaviors.

METHODS

THIS MODEL DETAILS HOW CHEMICAL ECOLOGY DRIVES ADAPTIVE EVOLUTION VIA: (1) ecological niche construction, (2) social niche construction, (3) neurogenic niche construction, and (4) socio-cognitive niche construction. This model exemplifies the epigenetic effects of olfactory/pheromonal conditioning, which alters genetically predisposed, nutrient-dependent, hormone-driven mammalian behavior and choices for pheromones that control reproduction via their effects on luteinizing hormone (LH) and systems biology.

RESULTS

Nutrients are metabolized to pheromones that condition behavior in the same way that food odors condition behavior associated with food preferences. The epigenetic effects of olfactory/pheromonal input calibrate and standardize molecular mechanisms for genetically predisposed receptor-mediated changes in intracellular signaling and stochastic gene expression in GnRH neurosecretory neurons of brain tissue. For example, glucose and pheromones alter the hypothalamic secretion of GnRH and LH. A form of GnRH associated with sexual orientation in yeasts links control of the feedback loops and developmental processes required for nutrient acquisition, movement, reproduction, and the diversification of species from microbes to man.

CONCLUSION

An environmental drive evolved from that of nutrient ingestion in unicellular organisms to that of pheromone-controlled socialization in insects. In mammals, food odors and pheromones cause changes in hormones such as LH, which has developmental affects on pheromone-controlled sexual behavior in nutrient-dependent reproductively fit individuals across species of vertebrates.

Cell-autonomous sex determination outside of the gonad

BACKGROUND

The classic model of sex determination in mammals states that the sex of the individual is determined by the type of gonad that develops, which in turn determines the gonadal hormonal milieu that creates sex differences outside of the gonads. However, XX and XY cells are intrinsically different because of the cell-autonomous sex-biasing action of X and Y genes.

RESULTS

Recent studies of mice, in which sex chromosome complement is independent of gonadal sex, reveal that sex chromosome complement has strong effects contributing to sex differences in phenotypes such as metabolism. Adult mice with two X chromosomes (relative to mice with one X chromosome) show dramatically greater increases in body weight and adiposity after gonadectomy, irrespective of their gonadal sex. When fed a high-fat diet, XX mice develop striking hyperinsulinemia and fatty liver, relative to XY mice. The sex chromosome effects are modulated by the presence of gonadal hormones, indicating an interaction of the sex-biasing effects of gonadal hormones and sex chromosome genes.

CONCLUSIONS

Other cell-autonomous sex chromosome effects are detected in mice in many phenotypes. Birds (relative to eutherian mammals) are expected to show more widespread cell-autonomous sex determination in non-gonadal tissues, because of ineffective sex chromosome dosage compensation mechanisms.

Sexual differentiation in the developing mouse brain: contributions of sex chromosome genes

Neural sexual differentiation begins during embryogenesis and continues after birth for a variable amount of time depending on the species and brain region. Because gonadal hormones were the first factors identified in neural sexual differentiation, their role in this process has eclipsed investigation of other factors. Here, we use a mouse with a spontaneous translocation that produces four different unique sets of sex chromosomes. Each genotype has one normal X-chromosome and a unique second sex chromosome creating the following genotypes: XY(*x) , XX, XY(*) , XX(Y) (*) . This Y(*) mouse line is used by several laboratories to study two human aneuploid conditions: Turner and Klinefelter syndromes. As sex chromosome number affects behavior and brain morphology, we surveyed brain gene expression at embryonic days 11.5 and 18.5 to isolate X-chromosome dose effects in the developing brain as possible mechanistic changes underlying the phenotypes. We compared gene expression differences between gonadal males and females as well as individuals with one vs. two X-chromosomes. We present data showing, in addition to genes reported to escape X-inactivation, a number of autosomal genes are differentially expressed between the sexes and in mice with different numbers of X-chromosomes. Based on our results, we can now identify the genes present in the region around the chromosomal break point that produces the Y(*) model. Our results also indicate an interaction between gonadal development and sex chromosome number that could further elucidate the role of sex chromosome genes and hormones in the sexual differentiation of behavior.

Developmental profile and sexually dimorphic expression of kiss1 and kiss1r in the fetal mouse brain

The hypothalamic-pituitary-gonadal axis (HPG) is a complex neuroendocrine circuit involving multiple levels of regulation. Kisspeptin neurons play essential roles in controlling the HPG axis from the perspectives of puberty onset, oscillations of gonadotropin releasing hormone (GnRH) neuron activity, and the pre-ovulatory LH surge. The current studies focus on the expression of kisspeptin during murine fetal development using in situ hybridization (ISH), quantitative reverse transcription real-time PCR (QPCR), and immunocytochemistry. Expression of mRNA coding for kisspeptin (KISS1) and its receptor KISS1R was observed at embryonic (E) day 13 by ISH. At E13 and other later ages examined, Kiss1 signal in individual cells within the arcuate nucleus (ARC) appeared stronger in females than males. ISH examination of agonadal steroidogenic factor-1 (Sf1) knockout mice revealed that E17 XY knockouts (KO) resembled wild-type (WT) XX females. These findings raise the possibility that gonadal hormones modulate the expression of Kiss1 in the ARC prior to birth. The sex and genotype differences were tested quantitatively by QPCR experiments in dissected hypothalami from mice at E17 and adulthood. Females had significantly more Kiss1 than males at both ages, even though the number of cells detected by ISH was similar. In addition, QPCR revealed a significant difference in the amount of Kiss1 mRNA in Sf1 mice with WT XY mice expressing less than XY KO and XX mice of both genotypes. The detection of immunoreactive KISS1 in perikarya of the ARC at E17 indicates that early mRNA is translated to peptide. The functional significance of this early expression of Kiss1 awaits elucidation.

Volumetric parcellation methodology of the human hypothalamus in neuroimaging: normative data and sex differences

There is increasing evidence regarding the importance of the hypothalamus for understanding sex differences in relation to neurological, psychiatric, endocrine and sleep disorders. Although different in histology, physiology, connections and function, multiple hypothalamic nuclei subserve non-voluntary functions and are nodal points for the purpose of maintaining homeostasis of the organism. Thus, given the critical importance of hypothalamic nuclei and their key multiple roles in regulating basic functions, it is important to develop the ability to conduct in vivo human studies of anatomic structure, volume, connectivity, and function of hypothalamic regions represented at the level of its nuclei. The goals of the present study were to develop a novel method of semi-automated volumetric parcellation for the human hypothalamus that could be used to investigate clinical conditions using MRI and to demonstrate its applicability. The proposed new method subdivides the hypothalamus into five parcels based on visible anatomic landmarks associated with specific nuclear groupings and was confirmed using two ex vivo hypothalami that were imaged in a 7 T (7 T) scanner and processed histologically. Imaging results were compared with histology from the same brain. Further, the method was applied to 44 healthy adults (26 men; 18 women, comparable on age, handedness, ethnicity, SES) to derive normative volumes and assess sex differences in hypothalamic regions using 1.5 T MRI. Men compared to women had a significantly larger total hypothalamus, relative to cerebrum size, similar for both hemispheres, a difference that was primarily driven by the tuberal region, with the sex effect size being largest in the superior tuberal region and, to a lesser extent, inferior tuberal region. Given the critical role of hypothalamic nuclei in multiple chronic diseases and the importance of sex differences, we argue that the use of the novel methodology presented here will allow for critical investigations of these disorders and further delineation of potential treatments, particularly sex-specific approaches to gene and drug discoveries that involve hypothalamic nuclei.

Hormonal programming across the lifespan

Hormones influence countless biological processes across an animal's lifespan. Many hormone-mediated events occur within developmental sensitive periods, during which hormones have the potential to cause permanent tissue-specific alterations in anatomy and physiology. There are numerous selective critical periods in development with different targets being affected during different periods. This review outlines the proceedings of the Hormonal Programming in Development session at the US-South American Workshop in Neuroendocrinology in August 2011. Here we discuss how gonadal steroid hormones impact various biological processes within the brain and gonads during early development and describe the changes that take place in the aging female ovary. At the cellular level, hormonal targets in the brain include neurons, glia, or vasculature. On a genomic/epigenomic level, transcription factor signaling and epigenetic changes alter the expression of critical hormone receptor genes across development and following ischemic brain insult. In addition, organizational hormone exposure alters epigenetic processes in specific brain nuclei and may be an important mediator of sexual differentiation of the neonatal brain. Brain targets of hormonal programming, such as the paraventricular nucleus of the hypothalamus, may be critical in influencing the development of peripheral targets, such as the ovary. Exposure to excess hormones can cause abnormalities in the ovary during development leading to polycystic ovarian syndrome (PCOS). Exposure to excess androgens during fetal development also has a profound effect on the development of the male reproductive system. In addition, increased activity of the sympathetic nerve and stress during early life have been linked to PCOS symptomology in adulthood. Finally, we describe how age-related decreases in fertility are linked to high levels of nerve growth factor (NGF), which enhances sympathetic nerve activity and alters ovarian function.

Sex-specific modulation of gene expression networks in murine hypothalamus

The hypothalamus contains nuclei and cell populations that are critical in reproduction and that differ significantly between the sexes in structure and function. To examine the molecular and genetic basis for these differences, we quantified gene expression in the hypothalamus of 39 pairs of adult male and female mice belonging to the BXD strains. This experimental design enabled us to define hypothalamic gene coexpression networks and provided robust estimates of absolute expression differences. As expected, sex has the strongest effect on the expression of genes on the X and Y chromosomes (e.g., Uty, Xist, Kdm6a). Transcripts associated with the endocrine system and neuropeptide signaling also differ significantly. Sex-differentiated transcripts often have well delimited expression within specific hypothalamic nuclei that have roles in reproduction. For instance, the estrogen receptor (Esr1) and neurokinin B (Tac2) genes have intense expression in the medial preoptic and arcuate nuclei and comparatively high expression in females. Despite the strong effect of sex on single transcripts, the global pattern of covariance among transcripts is well preserved, and consequently, males and females have well matched coexpression modules. However, there are sex-specific hub genes in functionally equivalent modules. For example, only in males is the Y-linked gene, Uty, a highly connected transcript in a network that regulates chromatin modification and gene transcription. In females, the X chromosome paralog, Kdm6a, takes the place of Uty in the same network. We also find significant effect of sex on genetic regulation and the same network in males and females can be associated with markedly different regulatory loci. With the exception of a few sex-specific modules, our analysis reveals a system in which sets of functionally related transcripts are organized into stable sex-independent networks that are controlled at a higher level by sex-specific modulators.

Evidence that sex chromosome genes affect sexual differentiation of female sexual behavior

Female receptivity including the immobile hormone-dependent lordosis posture is essential for successful reproduction in rodents. It is well documented that lordosis is organized during the perinatal period when the actions of androgens decrease the males' ability to display this behavior in adulthood. Conversely the absence of androgens, and the presence of low levels of prepubertal estrogens, preserve circuitry that regulates this behavior in females. The current study set out to determine whether sex chromosomal genes are involved in the differentiation of this behavior. An agonadal mouse model was used to test this hypothesis. The SF-1 gene (Nr5a1) is required for development of gonads and adrenal glands, and knockout mice are consequently not exposed to endogenous gonadal steroids. Thus contributions of sex chromosome genes can be disassociated from the actions of estrogens. Use of this model reveals a direct genetic contribution from sex chromosomes in the display of lordosis and other female-typical sexual behavior patterns. It is likely that the concentrations of gonadal steroids present during normal male development modify the actions of sex chromosome genes on the potential to display female sexual behavior.

Sexual differentiation of the rodent brain: dogma and beyond

Steroid hormones of gonadal origin act on the neonatal brain to produce sex differences that underlie adult reproductive physiology and behavior. Neuronal sex differences occur on a variety of levels, including differences in regional volume and/or cell number, morphology, physiology, molecular signaling, and gene expression. In the rodent, many of these sex differences are determined by steroid hormones, particularly estradiol, and are established by diverse downstream effects. One brain region that is potently organized by estradiol is the preoptic area (POA), a region critically involved in many behaviors that show sex differences, including copulatory and maternal behaviors. This review focuses on the POA as a case study exemplifying the depth and breadth of our knowledge as well as the gaps in understanding the mechanisms through which gonadal hormones produce lasting neural and behavioral sex differences. In the POA, multiple cell types, including neurons, astrocytes, and microglia are masculinized by estradiol. Multiple downstream molecular mediators are involved, including prostaglandins, various glutamate receptors, protein kinase A, and several immune signaling molecules. Moreover, emerging evidence indicates epigenetic mechanisms maintain sex differences in the POA that are organized perinatally and thereby produce permanent behavioral changes. We also review emerging strategies to better elucidate the mechanisms through which genetics and epigenetics contribute to brain and behavioral sex differences.

The end of gonad-centric sex determination in mammals

The 20th-century theory of mammalian sex determination states that the embryo is sexually indifferent until the differentiation of gonads, after which sex differences in phenotype are caused by the differential effects of gonadal hormones. However, this theory is inadequate because some sex differences precede differentiation of the gonads and/or are determined by non-gonadal effects of the sexual inequality in the number and type of sex chromosomes. In this article, I propose a general theory of sex determination, which recognizes multiple parallel primary sex-determining pathways initiated by genes or factors encoded by the sex chromosomes. The separate sex-specific pathways interact to synergize with or antagonize each other, enhancing or reducing sex differences in phenotype.

What a difference an X or Y makes: sex chromosomes, gene dose, and epigenetics in sexual differentiation

A modern general theory of sex determination and sexual differentiation identifies the factors that cause sexual bias in gene networks, leading to sex differences in physiology and disease. The primary sex-biasing factors are those encoded on the sex chromosomes that are inherently different in the male and female zygotes. These factors, and downstream factors such as gonadal hormones, act directly on tissues to produce sex differences and antagonize each other to reduce sex differences. Recent studies of mouse models such as the four core genotypes have begun to distinguish between the direct effects of sex chromosome complement (XX vs. XY) and hormonal effects. Several lines of evidence implicate epigenetic processes in the control of sex differences, although a great deal of information is needed about sex differences in the epigenome.

Steroidogenic factor 1 and the central nervous system

Steroidogenic factor 1 (SF-1; officially designated NR5a1) is a member of a nuclear receptor superfamily with important roles in the development of endocrine systems. Studies with global and tissue-specific (i.e. central nervous system) knockout mice have revealed several roles of SF-1 in brain. These include morphological effects on the development of the ventromedial nucleus of the hypothalamus and functional effects on body weight regulation through modulation of physical activity, anxiety-like behaviours and female sexual behaviours. Although such defects are almost certainly a result of the absence of SF-1 acting as a transcription factor in the hypothalamus, global SF-1 knockout mice also represent a model for studying the sex differences in the brain that develop in the absence of exposure to foetal sex steroid hormones as a result of the absence of gonads. In the present review, current knowledge of the roles of SF-1 protein in the central nervous system is discussed.

Reframing sexual differentiation of the brain

In the twentieth century, the dominant model of sexual differentiation stated that genetic sex (XX versus XY) causes differentiation of the gonads, which then secrete gonadal hormones that act directly on tissues to induce sex differences in function. This serial model of sexual differentiation was simple, unifying and seductive. Recent evidence, however, indicates that the linear model is incorrect and that sex differences arise in response to diverse sex-specific signals originating from inherent differences in the genome and involve cellular mechanisms that are specific to individual tissues or brain regions. Moreover, sex-specific effects of the environment reciprocally affect biology, sometimes profoundly, and must therefore be integrated into a realistic model of sexual differentiation. A more appropriate model is a parallel-interactive model that encompasses the roles of multiple molecular signals and pathways that differentiate males and females, including synergistic and compensatory interactions among pathways and an important role for the environment.