Photoaffinity labelled rat androgen-binding protein and human sex hormone steroid-binding protein bind specifically to rat germ cells

Microarray analysis of androgen-regulated gene expression in testis: the use of the androgen-binding protein (ABP)-transgenic mouse as a model

BACKGROUND

Spermatogenesis is an androgen-dependent process, yet the molecular mechanisms of androgens' actions in testis are poorly understood. Transgenic mice overexpressing rat androgen-binding protein (ABP) in their testes have reduced levels of intratesticular androgens and, as a result, show a progressive impairment of spermatogenesis. We used this model to characterize changes in global gene expression in testis in response to reduced bioavailability of androgens.

METHODS

Total RNA was extracted from testes of 30-day old transgenic and wild-type control mice, converted to cRNA, labeled with biotin, and hybridized to oligonucleotide microarrays. Microarray results were confirmed by real-time reverse transcription polymerase chain reaction.

RESULTS

Three-hundred-eighty-one genes (3.05% of all transcripts represented on the chips) were up-regulated and 198 genes (1.59%) were down-regulated by at least a factor of 2 in the androgen-deficient animals compared to controls. Genes encoding membrane proteins, intracellular signaling molecules, enzymes, proteins participating in the immune response, and those involved in cytoskeleton organization were significantly overrepresented in the up-regulated group. Among the down-regulated transcripts, those coding for extracellular proteins were overrepresented most dramatically, followed by those related to proteolysis, cell adhesion, immune response, and growth factor, cytokine, and ion channel activities. Transcripts with the greatest potential impact on cellular activities included several transcription factors, intracellular signal transducers, secreted signaling molecules and enzymes, and various cell surface molecules. Major nodes in the up-regulated network were IL-6, AGT, MYC, and A2M, those in the down-regulated network were IL-2, -4, and -10, MAPK8, SOCS1, and CREB1.

CONCLUSION

Microarray analysis followed by gene ontology profiling and connectivity analysis identified several functional groups of genes and individual genes responding to sustained reduction of androgen levels in the mouse testis. These include genes whose products function as transcription factors, cell surface molecules including ion channels, extra- and intracellular signaling molecules, and secreted enzymes with the potential of regulating cell-to-cell attachment. The transcription factors CREB1 (down-regulated) and MYC (up-regulated) may mediate the most important initial phases of the testicular response to reduced levels of androgens. These results suggest specific avenues for further research that will lead to a better understanding of how androgens regulate spermatogenesis.

Testosterone-immunopositive primordial germ cells in the testis of the bullfrog, Rana catesbeiana

In amphibia, steroidogenesis remains quiescent in distinct seasonal periods, but the mechanism by which spermatogenesis is maintained under low steroidogenic conditions is not clear. In the present study, testosterone location in the testes of Rana catesbeiana was investigated immunohistochemically during breeding (summer) and nonbreeding (winter) periods. In winter, the scarce interstitial tissue exhibited occasional testosterone immunopositivity in the interstitial cells but the cytoplasm of primordial germ cells (PG cells) was clearly immunopositive. By contrast, in summer, PG cells contained little or no immunoreactivity whereas strong immunolabelling was present in the well-developed interstitial tissue. These results suggest that PG cells could retain testosterone during winter. This androgen reservoir could be involved in the control of early spermatogenesis in winter and/or to guarantee spermiogenesis and spermiation in the next spring/summer. The weak or negative immunoreaction in the summer PG cells might reflect consumption of androgen reservoir by the intense spermatogenic activity from spring to summer. Thus, besides acting as stem cells, PG cells of R. catesbeiana could exert an androgen regulatory role during seasonal spermatogenesis.

Dynamics of testicular germ cell apoptosis in normal mice and transgenic mice overexpressing rat androgen-binding protein

The number and type of testicular germ cells undergoing apoptosis in different age groups of mice (from 7 to 360 days of age) was determined and compared in age-matched wild type (WT) control and in a transgenic (TG) mice homozygous to rat androgen binding protein (ABP) using flow cytometry. Flow cytometric quantification revealed that the total number of germ cells undergoing apoptosis did not differ significantly in WT and TG mice up to Day 14. From Day 21 to Day 60, the number of germ cells undergoing apoptosis was consistently higher in TG than in WT mice. Starting from Day 90, the number of germ cells undergoing apoptosis in TG mice was lower than controls until Day 360. In 21-60 days old TG mice, spermatogonia, S-Phase cells, and primary spermatocytes are the cell types undergoing apoptosis at significantly greater numbers than those in WT mice. However, starting from day 60, the total number of spermatids undergoing apoptosis was significantly lower in TG mice than in age-matched WT controls. TdT-mediated dUTP-biotin nick end labeling (TUNEL) in testicular sections from TG mice of 21 and 30 days of age confirmed the presence of increased numbers of apoptotic germ cells compared to their age matched controls. These data indicate that the continuous presence of greater than physiological concentrations of ABP in the mouse testis has a biphasic effect on the frequency of apoptosis in germ cells. The initial pre-pubertal increase in testicular germ cell apoptosis may result from direct or indirect actions of ABP and is likely to determine the subsequent life-death balance of germ cell populations in TG mice, whereas the subsequent reduction may result from maturation depletion. A wave of apoptosis during the pre-pubertal period is required for normal spermatogenesis to develop, and our data indicate that this apoptotic wave may be regulated by ABP and/or androgens.

Proposed mechanism for sperm chromatin condensation/decondensation in the male rat

Condensation of sperm chromatin occurs after spermatozoa have left the caput epididymis and are in transit to the cauda epididymis, during which time large numbers of disulfide bonds are formed. The formation of these disulfide bonds requires the repeated oxidation of the cofactor, NAD(P)H. To date, the means by which this oxidation is achieved has yet to be elucidated. Spermatozoa lose the bulk of their cytoplasm prior to leaving the testis; and, as a result, any shuttle systems for removing and transferring reducing equivalents into the mitochondria are unlikely to be operational. In an apparent preparation for the loss of cytoplasm, however, the following events occur during spermatogenesis. First, androgen-binding protein (ABP) is produced by the Sertoli cells of the testis; second, high affinity binding sites for ABP are inserted into the membrane surrounding the nucleus; and third, a nuclear location is acquired for the enzyme, 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD). We propose that after the loss of cytoplasm, the nuclear region of spermatozoa is directly accessible to constituents contained in the lumen of the caput epididymis. As a consequence, luminal ABP attaches itself to the nuclear membrane via its binding sites, and is internalized. After internalization, ABP exerts its principle function, which is to bind to luminal 5alpha-dihydrotestosterone (5alpha-DHT), thereby ensuring its availability to the enzyme, 3alpha-HSD. In the conversion of 5alpha-DHT to 3alpha-androstanediol (3alpha-Diol), NAD(P)H is oxidized. Spermatozoa that reach the cauda epididymis have fully condensed chromatin. In addition, the nuclear region retains appreciable amounts of 5alpha-DHT and 3alpha-Diol, both bound to ABP. During fertilization, the bound 3alpha-Diol is converted back to 5alpha-DHT, reducing equivalents are transferred to NAD(P)+, and disulfide bonds are broken.IVF clinics report that spermatozoa with incompletely condensed chromatin have a low percentage of fertilization. If our proposed mechanism for chromatin condensation/decondensation is borne out by further research, IVF clinics might consider preincubating spermatozoa with 5alpha-DHT in order to increase the efficiency of fertilization.

Effects of androgen-binding protein (ABP) on spermatid Tnp1 gene expression in vitro

In vitro studies were designed to determine whether Sertoli cell-delivered ABP could act on spermatogenetic events, whether such an action could occur via a paracrine or a juxtacrine pathway and whether sex steroids could be involved in this action. ABP delivery to germ cells was achieved using an in vitro model based on recombinant rat ABP-producing mouse Sertoli cells cocultivated with rat spermatids. Using semi-quantitative RT-PCR, the expression of the Tnp 1 gene encoding the Transition Protein 1, involved in the histone to protamine replacement during spermatid nuclear transformation, was analyzed. Our results provide clear evidence that Sertoli cell-derived ABP acts on spermatids by modifying the TP1 mRNA level. This outcome, strictly requiring juxtacrine conditions, is obtained in the absence of sex steroid hormones. To our knowledge this is the first evidence of an effect of ABP itself on male germ cells.

Dynamics of testicular germ cell proliferation in normal mice and transgenic mice overexpressing rat androgen-binding protein: a flow cytometric evaluation

Transgenic mice carrying rat androgen-binding protein (ABP) genomic DNA express high amounts of testicular ABP and develop a progressive impairment of spermatogenesis. To understand the mechanism of these changes, we have studied the pattern of testicular germ cell proliferation from 7 to 360 days of age in wild-type (WT) control and transgenic homozygous (ABP-TG) mice by flow cytometry after labeling DNA in isolated germ cells with propidium iodide. At all ages studied, the body weight of the ABP-TG mice was lower than that of age-matched WT controls. Significantly reduced testicular weight and total germ cell number in the ABP-TG mice were evident from Day 30 and Day 60, respectively. Flow cytometric analysis of isolated germ cells revealed that the number of germ cells undergoing proliferation (S-phase cells) was identical in WT control and ABP-TG mice up to Day 14. Subsequently, the number of germ cells in S-phase was consistently higher in ABP-TG than in WT mice. The number of primary spermatocytes was significantly increased starting from Day 60, and the numbers of round and elongated spermatids were significantly reduced in the ABP-TG animals from Day 21 and Day 60 onwards, respectively. Immunocytometry for intracellular ABP at 90 days of age revealed that the percentage of ABP-containing germ cells was greater in ABP-TG than in WT mice. The continuous presence of ABP in mouse seminiferous tubules at greater than physiological concentrations facilitates the formation of primary spermatocytes but impairs subsequent transformation to round and elongated spermatids. Based on our observations and the analysis of the available literature, the most likely mechanism for production of these effects is sustained reduction in the bioavailability of androgens.

Establishment of a mouse Sertoli cell line producing rat androgen-binding protein (ABP)

The ultimate goal of this study was to compare the fate of rat testicular germ cells cocultured with mouse Sertoli cells that either do or do not produce rat androgen-binding protein (ABP). As a first step, we stably transfected a rat ABP expression construct into an immortalized mouse Sertoli cell line (TM4), which does not produce ABP when growing on plastic without hormones. The transfection of the pRc/CMV- rat ABP cDNA expression vector containing a neomycin resistance gene was made by either the liposome method (Dotap) or by polyethyleneimine transfection (PEI) into TM4 cell cultures. Neomycin-resistant clones were selected by adding Geneticin to the culture medium for 3 weeks. Analysis of over 25 clones revealed the presence of recombinant rat ABP when cell extracts and culture media were probed with a rabbit polyclonal antibody raised against rat testicular ABP, indicating the translation and secretion of a protein similar to rat testicular ABP. Transfected TM4 cells maintain the secretion of rat ABP for more than 40 days, with immunopositive rat ABP localized within cytoplasmic granules in the Golgi region and along cytoplasmic processes in TM4 transfected with either vector. Electron microscopic study revealed a higher development of cytoplasmic organelles involved in protein secretion.

Plasma membrane receptor for beta-lactoglobulin and retinol-binding protein in murine hybridomas

The aim of the present work was to study the binding of [125I]-BLGA (beta-lactoglobulin variant A) to the plasma membrane fraction of hybrid cells. This binding increased as a function of time with on-rate and off-rate constant at 4.47 +/- 0.18 x 10(6) M-1 min-1 and 0.17 +/- 0.07 min-1, respectively (n = 3). The saturation study showed a single binding site type corresponding to a Kd at 8.26 +/- 2.98 nM and 14.02 +/- 2.61 x 10(12) sites per mg of the plasma membrane protein (n = 3). Competitive of binding BLGA was observed with BLGA, complexed with retinol and also with RBP (retinol-binding protein). Gel filtration of [125I]-BLGA incubated with Triton X-100 solubilized membrane showed the formation of a ligand-receptor complex. Cross-linking of the tracer to plasma membrane showed a complex with a M(r) at 69 kDa, suggesting a receptor M(r) of 51 kDa, as seen by autoradiography of SDS-PAGE.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.

Retinol free and retinol complexed beta-lactoglobulin binding sites in bovine germ cells

A high affinity specific binding site for bovine beta-lactoglobulin (BLG) was identified in bovine germ cell plasma membrane enriched fractions. Binding was found to be reversible and pH-dependent with maximum binding occurring at pH 5. The on-rate and off-rate constants were 2.26 +/- 0.8 x 10(5) M(-1) min(-1) (n = 3) and 0.016 +/- 0.004 min(-1) (n = 3), respectively. Scatchard analysis showed a single class of binding sites, with 12.38 +/- 4.62 x 10(12) sites per mg of membrane protein (n = 3) and a dissociation constant (K(D)) estimated at 26.43 +/- 2.68 nM. There was inhibition of iodinated-BLG (variant A) (125I-BLGA) binding to germ cell plasma membrane enriched fractions in the presence of unlabelled BLG variant A, BLG variant B, retinol complexed BLGA and human retinol-binding protein. Inhibition was observed neither with BSA nor with lactoferrin. 125I-BLGA incubated with a Triton X-100 solubilized plasma membrane fraction formed a high molecular mass complex in Superose 12B gel filtration. This receptor complex disappeared in the presence of unlabelled BLGA and in the presence of 10 mM EDTA. The results suggest that germ cell plasma membrane may contain a receptor which is capable of binding either retinol free or retinol complexed BLGA.

Endocytosis of androgen-binding protein (ABP) by spermatogenic cells

To test whether Sertoli cell-secreted ABP could serve as steroid carrier to the germ cell (GC) lineage, radiolabeled ABP and SHBG and gold SHBG were used for binding studies and for internalization studies based on transmission electron microscope analyses and autoradiography of the radiolabeled samples. The data clearly showed that: (1) rat and human germ cells possess a single class of binding sites for rat ABP and human SHBG respectively (Kd of 0.78 and 0.56 nM); (2) 1.7 x 10(10) and 2.7 x 10(10) sites/mg protein was found in the corresponding plasma membrane preparations; (3) the receptor peak was eluted in the same position as dextran blue: 2000 kDa (M(r) = 2 x 106) for labeled rat ABP; (4) in the whole GC lineage, the labeled ligand was internalized through an endocytic pathway involving clathrin coated structures and the distribution was similar throughout the maturation step, however striking differences in the internalization rate were revealed with regard to the maturation step; and (5) this internalization occurred even in ligated seminiferous tubules, via the Sertoli cells cytoplasm. When isolated rat GC were incubated in the presence of ABP, a dose dependent increase in labeled secreted protein was observed for spermatocytes (50-250%) whereas ABP had no effect on spermatids. Addition of steroids and ABP caused a 200 and 50% increase in labeled secreted proteins for spermatocytes and spermatids respectively. 2-D SDS-PAGE analysis revealed that ABP alone increased the secretion of specific spermatocyte proteins whereas steroids in the presence of ABP resulted in the synthesis of new spermatocyte secreted proteins. Taken together these results strongly suggest that ABP may be required for spermatogenesis either as a steroid transmembrane carrier or on its own.

Binding of sex steroid binding protein to plasma membranes of human testis

The existence of a specific binding site for sex steroid binding protein (SBP or SHBG) was detected on plasma membranes prepared from the testis of a patient affected by a variant form of testicular feminization. A binding technique using [125I]SBP as a tracer allowed us to identify a single set of binding sites, characterized by a Kd of 1.917 x 10(-11) M. The maximum number of binding sites was 5.2 fmol/mg membrane protein. Membranes were also prepared from a sample of genital skin from the same patient, but no binding for [125I]SBP was detectable. The evidence of the SBP membrane receptor in the testis of a patient affected by Morris syndrome extends our knowledge about the tissue distribution of the SBP receptor and suggests the possible implication of SBP and its recognition system in a disorder related to peripheral androgen insensitivity.

Interaction of sex hormone-binding globulin with plasma membranes from the rat epididymis and other tissues

The binding of human sex hormone-binding globulin (hSHBG) to plasma membranes prepared from the adult rat epididymis and other potential target and non-target tissues was examined. Specific binding sites were detected in the epididymis, testis, prostate, skeletal muscle and liver. The first three organs exhibited a higher (KD approx. 0.1 nM; Bmax approx. 0.05-0.10 pmol/mg membrane protein, Site I) and a lower (KD approx. 5 nM; Bmax approx. 1.0-2.5 pmol/mg membrane protein, Site II) affinity binding site. Only Site I was detected in muscle membranes and only Site II was detected in membranes isolated from liver. Specific binding was not detectable in either spleen or brain. Regional distribution of hSHBG binding sites occurred in the epididymis. Both Site I and Site II were present in the proximal caput and distal cauda. The distal caput and proximal cauda contained only Site II; no specific binding was detected in the corpus. Binding of hSHBG to epididymal membranes was time- and temperature-dependent. The presence of Ca2+ did not affect binding. Non-liganded [125I]-labeled hSHBG can bind to both sites in epididymal membranes. The affinity of hSHBG for Site I increased 2-fold when it was complexed with 5 alpha-dihydrotestosterone, testosterone or estradiol. The hSHBG-androgen complex had little effect on Site II versus steroid-free SHBG. However, the affinity of the hSHBG-estradiol complex for these sites was increased 10-fold. Cortisol, which has a low affinity for hSHBG, did not influence its binding to either the higher or lower affinity membrane sites.

Structure, function, and regulation of androgen-binding protein/sex hormone-binding globulin

Despite over 20 years of research, the functions of ABP and SHBG remain elusive. The major reason for this lack of knowledge has been the unavailability of natural mutants with clinical defects for study. There is strong evidence that these binding proteins do act to modulate the gene regulatory actions of nuclear sex steroid receptors by controlling the availability of androgens and estrogens. In plasma, SHBG controls the metabolic clearance rate of sex steroids. In addition there is strong evidence that they have a much broader function. The identification of plasma membrane receptors in target tissues and the finding of homologous domains in several developmental proteins support other functions. Moreover, other experiments suggest the proteins may actually be hormones or growth factors. These findings are not compatible with a model that has the proteins only regulating free steroid hormone levels. Obviously, much more experimentation will be necessary to reveal the functions of ABP and SHBG. The recent discoveries have offered several clues to their functions and open new routes for study. These experiments, coupled with newly developed techniques, such as gene knockout by homologous recombination, make one optimistic that the functions of these unique proteins will be deciphered in the near future.

Transcobalamin II--cobalamin binding sites are present on rabbit germ cells

Specific binding sites for rabbit transcobalamin II have been found on isolated adult rabbit germ cells. Scatchard analysis revealed a single class of binding sites for [57Co]cyanocobalamin-transcobalamin II with an association constant (Ka) of 1.3 x 10(10) M-1 and 700 sites per cell. Binding was reversible, saturable and calcium dependent. Electron microscope radioautography following incubation with iodinated transcobalamin II at 4 degrees C led to a detectable labeling mainly restricted to the plasma membrane.