Sex-determining genes on mouse autosomes identified by linkage analysis of C57BL/6J-YPOS sex reversal.

A powerful approach for identifying mammalian primary (gonadal) sex determination genes is the molecular genetic analyses of sex reversal conditions (that is, XX individuals with testicular tissue and XY individuals with ovarian tissue). Here we determined the number and chromosomal location of autosomal and X-linked genes that cause sex reversal in C57BL/6J (B6) mice carrying a Y chromosome of Mus domesticus poschiavinus origin (YPOS). B6 XYPOS mice develop either as females with exclusively ovarian tissue or as true hermaphrodites with ovarian and testicular tissue. In contrast, the YPOS chromosome is fully masculinizing on most other inbred strain backgrounds. B6-YPOS sex reversal appears to result from the incompatibility of the Sry (sex determining region, Y chromosome) allele carried on the YPOS chromosome with B6-derived autosomal or X-linked loci. We found strong evidence for the location of one gene, designated tda1 (testis-determining, autosomal 1), at the distal end of Chromosome (Chr) 4 and a second gene, tda2, in the central region of Chr 2. A third gene, tda3, on Chr 5 is implicated, but the evidence here is not as strong. We suggest that B6 alleles at these loci predispose XYPOS fetuses to ovarian tissue development, but no single locus or combination of loci is necessary and sufficient to cause sex reversal. The TDA proteins may regulate Sry expression or form complexes with the SRY protein to regulate other genes, or the tda genes may be activated or repressed by the SRY protein.

Assay of oxysterol-binding protein in a mouse fibroblast, cell-free system. Dissociation constant and other properties of the system.

Procedures for determining a 7.5 S oxysterol-binding protein in the cytosol fraction of cultured mouse fibroblasts were developed. The methods involved precipitation of cytosolic proteins between 0.3 and 0.4 saturation with (NH4)2SO4, incubation of the proteins with 25-hydroxy[3H]cholesterol at 0 degrees C and analysis by velocity sedimentation of 7.5 S radioactivity or of specific binding using dextran-charcoal to adsorb free sterol. By these means it was shown that binding of the ligand to the protein in a citric acid-phosphate buffer was optimal at pH 5.5 and that the sedimentation rate of the complex was greater at pH 7.4 (7.5 S) than at pH 5.5 (6.9 S). The binding protein was essentially saturated at a diol concentration of about 20 X 10(-9) M. The apparent Kd of the sterol-protein complex was approximately 3.9 X 10(-9) M. Cholesterol did not bind to 25-hydroxycholesterol-binding sites on the 7.5 S protein, whereas several oxysterols that are potent suppressors of 3-hydroxy-3-methylglutaryl CoA reductase also inhibited the binding of 25-hydroxycholesterol. One of these sterols, 5 alpha-cholest-8(14)-en-3 beta-ol-15-one was shown to compete for sites occupied by 25-hydroxycholesterol.

A proteolytic fragment of the oxysterol receptor which retains oxysterol binding activity.

The structural organization of the oxysterol receptor, postulated to be involved in the regulation of 3-hydroxy-3-methylglutaryl CoA reductase and cholesterol biosynthesis in mammalian cells, has been explored by limited proteolysis with trypsin, alpha-chymotrypsin, and endoproteinase GluC. Treatment with each of these proteases converts the receptor from a homodimer of approximately 95 kDa subunits to a 44-kDa form, based on hydrodynamic measurements by sucrose density gradient centrifugation and gel filtration chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of photoaffinity-labeled preparations indicates that the oxysterol binding site is on a 28-kDa fragment within the 44-kDa limit form of the receptor. The limit proteolytic form exhibits the high affinity and structural specificity for oxysterols of the native dimeric receptor with an increase in the rate constant of association for 25-hydroxycholesterol. The proteolytic form also shows an increased binding affinity for nonspecific DNA, but no sequence specificity for the oxysterol regulatory element from the reductase gene was detected.

Sex reversal in C57BL/6J-YPOS mice corrected by a Sry transgene.

C57BL/6J mice carrying a Mus domesticus poschiavinus Y chromosome (YPOS) develop as females with ovarian tissue or as hermaphrodites with ovarian and testicular tissue. We tested the hypothesis that the Y-linked component of this inherited sex reversal is caused by the M. d. poschiavinus Y-linked testis determining gene (symbolized Tdy or Sry) by examining gonadal development in C57BL/6J XYPOS mice carrying a M. musculus allele of Sry as a transgene. We found that in the presence of the transgene, XYPOS mice developed exclusively testicular tissue. This result indicates that the Sry allele carried on the YPOS chromosome is responsible for development of ovarian tissue in the C57BL/6J inbred strain background. We discuss this finding in light of DNA polymorphisms present in Sry alleles carried by various M. domesticus and M. musculus Y chromosomes. In addition, we present a hypothesis concerning the timing of expression of the testicular and ovarian determining genes in the developing fetal gonad based on the organization of ovarian and testicular tissue in ovotestes.

Different forms of the oxysterol-binding protein. Binding kinetics and stability.

Based upon measurements of the sedimentation coefficient and the Stokes radii, three forms of the oxysterol-binding protein were identified. The unliganded binding protein was the largest (7.7 S, Stokes radius = 71.6 A, Mr = 236,000) was relatively asymmetric (f/f0 = 1.7), and was composed of at least three subunits. Binding of 25-hydroxycholesterol was associated with a reduction in the size of the protein (7.5 S, Stokes radius = 50 A, Mr approximately 169,000) and an increase in symmetry (f/f0 = 1.4), due to the loss of a subunit of Mr approximately 67,000. At pH 6 or lower, the Mr = 169,000 sterol-protein complex was altered so that reversible dissociation to give a smaller (4.2 S, Stokes radius = 53 A, Mr = 97,000) more asymmetric (f/f0 = 1.8) sterol-protein complex occurred when it was sedimented in a sucrose gradient buffered at pH 7.4 containing 0.3 M KCl and 2.5 M urea. Irreversible dissociation of the 7.5 S, Mr = 169,000 form to a 4.2 S form occurred spontaneously when the complex in whole cytosol buffered at pH 7.8 was allowed to stand overnight at 0 degree C, or when the partially purified complex was incubated at pH 5.5 at 0 degree C for several days. The partially purified, unliganded binding protein was unstable at 0 degree C (approximately 75% loss of binding activity in 24 h) whereas the liganded protein was stable for 7 days at 0 degree C although irreversible conversion to a 4.2 S form occurred under some conditions. Rates of sterol binding and dissociation were increased in the presence of 2.5 M urea at pH 7.4 or when the pH was lowered to 5.5 Kd values were not greatly altered under the various incubation conditions.

Binding of 25-hydroxycholesterol and cholesterol to different cytoplasmic proteins.

Studies were carried out to determine whether or not oxygenated derivatives of cholesterol (e.g., 25-hydroxycholesterol) that specifically suppress the activity of 3-hydroxy-3-methylglutaryl-CoA reductase [mevalonate:NADP(+) oxidoreductase (CoA-acylating), EC 1.1.1.34], bind to a soluble component of the cytoplasm different from that which binds the nonsuppressor, cholesterol. Density gradient fractionation of the cytosolic fraction isolated from L cell cultures that had been incubated with low concentrations of 25-hydroxy[26,27-(3)H]cholesterol or [1,2-(3)H]cholesterol provided evidence for the existence of at least two different sterol-binding proteins. Bound cholesterol sedimented in a sucrose density gradient as two or more broad bands with coefficients of approximately 9 S and 21 S. Two relatively narrow bands of bound 25-hydroxycholesterol had sedimentation coefficients of 5 S and 8 S. Preincubation of the cells with a relatively high concentration of unlabeled 25-hydroxycholesterol altered the banding pattern of the 25-hydroxy[(3)H]cholesterol taken up during a subsequent incubation period by decreasing the size of the major (8S) band. Under these conditions, cholesterol did not affect the banding pattern of 25-hydroxy[(3)H]cholesterol. The density gradient banding pattern of bound [(3)H]cholesterol was only slightly affected by preincubating the cells with unlabeled cholesterol or 25-hydroxycholesterol. Both sterols appeared to be bound to proteins because the bound sterols were eliminated from cytosol that had been heated at 100 degrees , and their sedimentation coefficients were altered by proteolysis.

Purification, subunit structure, and DNA binding properties of the mouse oxysterol receptor.

A cytosolic receptor protein for oxygenated sterols, postulated to be involved in the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and cholesterol biosynthesis, has been purified from mouse L cell cytosol greater than 3,600-fold in its undenatured form and to apparent homogeneity upon further electrophoresis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified 7.5 S receptor appears to be a dimer with similar or identical subunits of Mr 95,000. Proteolytic cleavage by an endogenous factor(s) gives rise to a 4.2 S form of the receptor which is resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis into a heterogeneous mixture of ligand binding fragments of Mr 30,000-60,000. This 4.2 S form of the receptor retains high affinity for the oxysterol ligand and exhibits a more rapid oxysterol binding rate than the 7.5 S form. The 7.5 S form of the receptor binds to DNA-cellulose at low salt concentrations at neutral pH, and its affinity increases at low pH or in the presence of Zn2+. Receptor preparations from mouse liver were purified approximately 900-fold by the same purification procedure, but this was accompanied by conversion of the 7.5 S liver receptor to a approximately 4 S form, Mr approximately 55,000.

Correlation between oxysterol binding to a cytosolic binding protein and potency in the repression of hydroxymethylglutaryl coenzyme A reductase.

Support for the role of a cytosolic oxysterol-binding protein in the regulation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was obtained by correlating the relative binding affinities of a wide range of oxysterols to their potency in suppressing HMG-CoA reductase activity in mouse fibroblast cell cultures. Forty-seven oxysterols encompassing a 100-fold range of activity in both assays were tested and the two parameters were closely correlated for 35 of the sterols. Twelve sterols showed poor binding when compared to their ability to suppress HMG-CoA reductase activity in cell cultures. Among these were seven sterols with a ketone function at C-3. For this group, the discrepancy could be explained by their rapid conversion within cells to the 3 beta-hydroxy derivatives which have a much higher affinity for the binding protein. One sterol with 3-keto-4-ene grouping was not reduced to its 3 beta-hydroxy derivative in cells and thereby showed no discrepancy in the two assays. The remaining five sterols exhibiting discordant activities in the two tests contained 4,4-dimethyl moieties and were relatively weak suppressors of HMG-CoA reductase activity. Cellular metabolism of these sterols was not detected. Possible reasons for their apparent inactivity in the binding assay are discussed.