Sex is not the only aspect of life governed by the hypothalamus. The structure, located at the center of the brain, accounts for just 5 percent of brain mass, yet controls life-and-death matters such as appetite, wakefulness, body temperature, and fear responses, in addition to the sex drive.

But it is sex that has thus far been the most fruitful focus for developmental biologist Holly Ingraham's studies of genes and proteins that guide hypothalamic development. Ingraham has defined the role of a protein, steroidogenic factor 1 (SF-1), which when deficient, prevents development of a normal endocrine system. Mice lacking both copies of the SF-1 gene are missing gonads and adrenals as a result, and they also have impaired pituitary function and alterations in the hypothalamus, Ingraham and others have found.

Sugar and spice and puppy dog tails aside, it might seem clear enough what makes little boys and little girls, but even so, one in every several thousand children born is anatomically ambiguous and not easily assigned to one sex or the other. The causes of inter-sex phenomena are varied and can affect individuals with normal XX or XY sex chromosome allotments. In some instances inter-sex characteristics are not recognized until long after birth. For either XY or XX children born with a mutant SF-1 gene, it is a life-threatening failure to control fluid volume and stress responses -- due to the absence of adrenal steroids -- that first brings the infants in for medical attention. But humans endowed with XY chromosomes and only half a normal dose of SF-1 also have a female "phenotype" -- they appear female despite their chromosomal maleness.

Before she began to focus on the role of SF-1 in the hypothalamus, Ingraham analyzed mice in which the SF-1 genes were "knocked out" and traced the biochemical and developmental chain of events that explains the XY female phenotype.

Ovaries and testes derive from the same "indifferent gonadal stage" in the developing embryo, and only begin to become distinct organs after about seven weeks into gestation. Basically, a lack of sex hormones leads to female development, while the synthesis of testosterone in the male fetus leads to male sexual development.

In the absence of SF-1 and sex hormones, indifferent gonads never develop into testes in the XY fetus. In addition to lacking testosterone, the SF-1-deficient XY fetus lacks two other important male developmental hormones, Mullerian inhibiting substance (MIS), which causes the embryonic female reproductive tract to atrophy, and the insulin-like hormone 3 (INSL3), which causes the testes to descend. Such an infant, born with a vagina and labia and no signs of maleness, appears female. (Strangely, in female mice equipped with extra SF-1 genes, Ingraham has found that the ovaries descend.)

Ingraham now is focusing on other SF-1 functions. "I think the pathway of sex determination and sexual differentiation has been pretty well worked out," she says. "To me, a more interesting question now is, how does gender identity get set up?"

This is where the hypothalamus comes back into the picture. Preliminary studies by other researchers have suggested that in certain ways the hypothalamus may be "sexually dimorphic," exhibiting anatomical differences in men and women. The expectation among many scientists is that such differences will be reflected in behavioral tendencies that differ between sexes. A comparatively noncontroversial difference is gender identity: Most men feel like men; most women feel like women.

Still, many individuals so strongly identify with the opposite gender that they change their anatomic sex -- undergoing difficult surgery, taking hormones for a lifetime, and facing myriad other adjustment and acceptance issues.

Ingraham, with her genetic manipulations of SF-1 in mice, and armed with new molecular labeling techniques that make it possible to more easily highlight both proteins of interest and the tendrils of interconnected nerve cells in slices of brain tissue, hopes to trace genes to behavior, although she admits that it might be difficult to detect behaviors in mice that reflect deviations in normal gender identity.

Even so, she already has connected some of the changes in hypothalamic development that result from SF-1 mutation to changes in characteristics unrelated to sexual dimorphism -- obesity and inactivity. The obesity she has observed may be due to the fact that the hypothalamus of a mouse deficient in SF-1 is also deficient in a secreted growth factor called BDNF, needed by certain hypothalamic nerves in order to survive. Indeed, Ingraham has observed that SF-1-deficient mice do not form all the usual connections between the hypothalamus and other brain structures. Additional study may reveal a more precise molecular explanation for the couch-potato behavior, Ingraham hopes.

In addition to MIS, INSL3, testosterone and BDNF, levels of many other proteins may turn out be influenced by SF-1. This is because SF-1 is a nuclear receptor, a type of protein that governs the activity of many different genes. Nuclear receptors act within the cell nucleus. They typically become activated when hormones enter the cell, cross into the nucleus, and attach to the receptor, although their activation also depends on the attachment of other "cofactor" molecules.

SF-1 is unusual among known nuclear receptors in that it is key for normal development, and, according to Ingraham, because it is one of the most important "orphan" nuclear receptors, one for which no natural hormone activator is known. Ingraham is also investigating a very closely related orphan nuclear receptor, liver-related homologue-1 (LRH-1), which plays a role in converting bile acids to cholesterol in the liver. This alone makes LRH-1 a potentially interesting target for drugs aimed at preventing clogged arteries, but just as tantalizing to Ingraham is the fact that LRH is also involved in controlling the production of aromatase -- the enzyme needed to convert testosterone to estrogen.

Ingraham has described a role for non-hormone molecules that affect LRH-1 activity, and perhaps the activity of SF-1 and other orphan receptors. In addition, even though SF-1 appears to have no natural hormone that attaches to it, UCSF X-ray crystallographer Robert Fletterick's laboratory team showed that it does have a pocket that resembles the site where hormones dock onto other nuclear receptors. The structural analysis helped to explain why the receptor is constitutively active, even with no hormone to bind. Further study by School of Pharmacy chemists and collaborators Irwin "Tack" Kuntz and Kip Guy showed that certain pharmaceutical compounds fit into SF-1's pocket and affect its activity.

Given the progress to date, from Ingraham's perspective, the study of SF-1, LRH-1 and other orphans, both in terms of basic biological research and potential clinical applications, appears primed to yield rewarding discoveries for years to come. --Jeffrey Norris