Slov Vet Res 2010; 47 (4): 167-70 UDC 577.25:612.4:599.323.4
Review Article
GONADAL HORMONE INDEPENDENT SEX DIFFERENCES IN STEROIDOGENIC FACTOR 1 KNOCKOUT MICE BRAIN
Tomaž Budefeld1*, Stuart A. Tobet2, Gregor Majdič1,3
1Center for Animal Genomics, Veterinary faculty, University of Ljubljana, Gerbičeva 60, 1000 Ljubljana, Slovenia; 2College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO, 80523-1680, USA; 3Institute of Physiology, Medical School, University of Maribor, Slomškov trg 15, 2000 Maribor, Slovenia
Corresponding author, E-mail: tomaz.budefeld@vf.uni-lj.si
Summary: Sex differences in brain morphology have been described in a number of species including humans. Gonadal hormones were shown to provide a major influence on brain sexual differentiation more than 50 years ago. A growing number of studies is providing evidence for roles of genetic factors, in particular sex chromosome complement, on brain sexual differentiation in mammals. In this review, hormone-independent brain sexual differentiation, with the emphasis on mice with a disruption of the SF-1 gene (SF-1 knockout, SF-1 KO) are discussed.
Keywords: brain sexual differentiation; sex chromosomes; SF-1 KO mice; preoptic area and hypothalamus; neuronal NO synthase; calbindin D-28k; neuropeptide Y
Introduction
Permanent effect of sex hormones on sex specific brain development was clearly described in 1959 when Phoenix and coworkers reported the organizational effect of prenatal exposure to exogenous testosterone on brain function in adult guinea pig females (1). Today it is widely accepted that gonadal steroid hormones are the major factors that shape brain development and function in a sex specific manner.
Male and female cells differ in their genetic blueprint consisting of autosomal and sex chromosomes. However, the direct effects of genes located on specific chromosomes on brain sexual differentiation are difficult to study due to the multiple factors contributing to sex specific brain structure and function. In the last decade, development of specific mouse genetic models has energized studies of direct effects of genetic factors on brain sexual development.
Received: 13 September 2010
Accepted for publication: 15 November 2010
Genetic factors in brain sexual development and function
Genes encoded on sex chromosomes are expressed in the brain where they could regulate expression of neural proteins, presumably in a sex specific manner. For example, Sry gene expression was found in tyrosine hydroxylase expressing neurons in the substantia nigra of adult mice and rats (2). However, the effects of genetic factors during brain sexual development are often masked by the actions of sex steroids.
One way to study genetic effects independently of steroid hormones is to study them in experimental systems before exposure to steroid hormones. In mice, gonadal primordia differentiate into testes or ovaries between embryonic day 10.5 and 12.5. Testes become steroidogenically active on embryonic day 13.0, while ovaries do not secrete steroid hormones until the first week after birth. Consequently, sex differences developed in the brain before gonad-al differentiation must be caused by genetic factors. Genomic studies of brains from embryonic mice before or shortly after gonadal differentiation indeed found sex differences in gene expression of X and Y linked genes (3, 4) suggesting that sex chromo-
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somes could have a role in brain sexual differentiation during early embryonic development. A study by Carruth and co-workers (5) showed sex differences in the expression of tyrosine hydroxylase in mesencephalic tissue slices dissected from the fetal mouse brain before gonadal development. A different approach to study genetic influences on brain sexual development is using animal models that are exposed to the same gonadal steroid hormones but differ in sex chromosome complements. Such are four core genotype (FCG) mice, developed by translocation of the Sry gene onto the autosome together with the deletion of the Sry from the Y chromosome
(6).	Comparisons of XX and XY gonadal males and XX and XY gonadal females showed the effect of sex chromosome complement on density of fibers expressing arginine vasopressin in the lateral septum
(7)	and on some behavioral traits (8).
SF-1 KO mice; an animal model for studying gonadal hormone independent brain sexual differentiation
Steroidogenic factor 1 (SF-1) is a transcription factor that regulates expression of a plethora of genes involved in development and function of endocrine organs. In mice with disruption of the SF-1 gene (SF-1 knockout, SF-1 KO) gonadal and adrenal primordia regress early during development (9). The absence of adrenal glands makes the SF-1 KO genotype lethal after birth. With adrenal transplantation, SF-1 KO mice can be kept alive into adulthood (10) and since these mice are not exposed to the endogenous go-nadal steroids, they represent a useful animal model for studying genetic and hormonal contributions to brain sexual development independently. Studies of SF-1 KO mice in adulthood have identified some sex differences in brain morphology and behavior traits in which genetic factors, acting independently or in concert with gonadal hormones, are likely contributors (11, 12).
The anteroventral periventricular nucleus (AVPV) in mice is sexually dimorphic for many traits such as volume, number of neurons (13) and in the size of chemically defined neuronal populations (e.g. number of cells expressing tyrosine hydroxylase or kisspeptin) (14, 15). All of the sex differences in the AVPV can be manipulated by changing gonadal hormonal milieu, showing the effect of gonadal hormones. In our study (11) we found that the number of neurons expressing neuronal nitric oxide syn-thase (nNOS) was higher in wild type (WT) males in
comparison to WT females. A similar sex difference was found in SF-1 KO mice and since SF-1 KO mice are not exposed to gonadal hormones this suggests gonadal hormone independent sexual differentiation. We also studied expression of nNOS in the medial preoptic area (POA) where sex differences in brain morphology have been described previously in various species (rev. in (16)). As in the AVPV, SF-1 KO mice males had higher nNOS immunoreactive area than females, similar to the sex difference in WT mice (11). The stands in stark contrast to the grouping of cells containing immunoreactive calbi-ndin in the same POA, for which the sex difference found was completely hormone-dependent (found in WT, but not KO) (11).
The ventromedial hypothalamus (VMH) is involved in regulation of various behaviors and endocrine processes (17). Structure of the VMH is sexually dimorphic and most sex differences have been found to be gonadal hormone dependent. Studies of VMH sex differences in SF-1 KO mice are complicated by alterations in its cyto-architecture (18). Nevertheless, similar sex differences in the number of cal-bindin D-28k immunopositive cells were found in WT and SF-1 KO mice suggesting gonadal hormone independent sexual differentiation (11).
Neuropeptide Y (NPY) is an orexigenic metabolic peptide, highly expressed in the arcuate nucleus, from where NPY neurons project to the paraventricular nucleus (rev. in (19)). NPY is also expressed in many other brain areas including the lateral septum where its function is not yet known. There are suggestions that the lateral septum may have important role(s) in social/ affiliative behaviors or in the regulation of food intake, at least in female rats (20). Interestingly, we found gonadal hormone independent sex difference in NPY expression in the lateral septum, similarly to previously described sexual dimorphism in arginine vasopressin expression in this area (7).
Conclusions
It is widely accepted that gonadal steroid hormones are the major factor influencing sex-dependent brain development. However recent studies have indicated that some genes may also have effects on shaping the brain acting independently or together with gonadal hormones in synergistic or antagonistic manners during development. Initial studies with two mouse models, SF-1 KO and FCG mice have already revealed several sex differences that are likely
Gonadal hormone independent sex differences in steroidogenic factor 1 knockout mice brain
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dependent on sex chromosome gene complement, and future studies with these and other models will undoubtedly further reveal an interplay between sex hormones and genetic factors in shaping male or female brain structure and function.
Acknowledgements
This work was supported by NIH grant MH61376 (SA.T. and G.M.), ICGEB grant CRP SLO 06/02, ARRS (Slovenian research agency) grants P4-0053 and J7-2093 (G.M.).
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OD SPOLNIH ŽLEZ NEODVISNE SPOLNE RAZLIKE V MOŽGANIH PRI MIŠIH BREZ GENA SF-1
T. Budefeld, S. A. Tobet, G. Majdič
Povzetek: Spolne razlike v možganih so prisotne pri številnih živalskih vrstah vključno z ljudmi. Vplive spolnih hormonov na razvoj možganov so potrdili že pred 50 leti, medtem ko vplive genoma, natančneje spolnih kromosomov, ugotavljamo šele v zadnjem desetletju. V tem preglednem članku so opisani nekateri primeri vplivov genov na spolno diferenciacijo možganov, s poudarkom na miših brez gena SF-1 (SF-1 knockout, SF-1 KO), ki predstavljajo poskusni model miši za proučevanje genetskih vplivov pri spolnem razvoju možganov.
Ključne besede: spolna diferenciacija možganov; spolni kromosomi; miši brez gena SF-1; predoptično področje in hipo-talamus; živčna sintaza dušikovih oksidov; kalbindin D-28k, nevropeptid Y