Histochemistry and cytochemistry of nuclear receptors

How technical progress reshaped behavioral neuroendocrinology during the last 50 years… and some methodological remarks

The first issue of Hormones and Behavior was published 50 years ago in 1969, a time when most of the techniques we currently use in Behavioral Endocrinology were not available. Researchers have during the last 5 decades developed techniques that allow measuring hormones in small volumes of biological samples, identify the sites where steroids act in the brain to activate sexual behavior, characterize and quantify gene expression correlated with behavior expression, modify this expression in a specific manner, and manipulate the activity of selected neuronal populations by chemogenetic and optogenetic techniques. This technical progress has considerably transformed the field and has been very beneficial for our understanding of the endocrine controls of behavior in general, but it did also come with some caveats. The facilitation of scientific investigations came with some relaxation of methodological exigency. Some critical controls are no longer performed on a regular basis and complex techniques supplied as ready to use kits are implemented without precise knowledge of their limitations. We present here a selective review of the most important of these new techniques, their potential problems and how they changed our view of the hormonal control of behavior. Fortunately, the scientific endeavor is a self-correcting process. The problems have been identified and corrections have been proposed. The next decades will obviously be filled with exciting discoveries in behavioral neuroendocrinology.

Impact of heterodimerization on intracellular localization of the ecdysteroid receptor (EcR)

Initially, nuclear import of the ecdysteroid receptor (EcR) in vertebrate cells (CHO-K1 and COS-7) does not afford a heterodimerization partner. Later on, EcR is retained in the nucleus only in the presence of a heterodimerization partner. Ultraspiracle (Usp) is more efficient compared to its vertebrate orthologue RXR and leads to an exclusively nuclear localization of EcR even in the absence of ligand. The DNA binding domain of the heterodimerization partner is important for retainment of EcR in the nucleus as shown by Usp4 (Usp(R130C)), which has lost its DNA binding capability. The C-terminal end of Usp (Usp(Delta205-508)) encompassing the C-terminal part of the D-domain and the E- and F-domains are essential for retainment of EcR in the nucleus. Nuclear localization is further influenced by cell-specific factors, since hormone and heterodimerization stabilizes the EcR protein in a cell-specific way.

Heat-induced antigen retrieval: mechanisms and application to histochemistry

Since the introduction of the fluorescence-labeled antibody method by Coons et al. [Immunological properties of antibody containing a fluorescent group. Proc Soc Exp Biol Med 47, 200-2002], many immunohistochemical methods have been refined to obtain high sensitivity with low background staining at both light and electron microscopic levels. Heat-induced antigen retrieval (HIAR) reported by Shi et al. in the early 1990s has greatly contributed to immunohistochemical analysis for formalin-fixed and paraffin-embedded (FFPE) materials, particularly in the field of pathology. Although antigen retrieval techniques including enzyme digestion, treatment with protein denaturants and heating have been considered tricky and mysterious techniques, the mechanisms of HIAR have been rapidly elucidated. Heating cleaves crosslinks (methylene bridges) and add methylol groups in formaldehyde-fixed proteins and nucleic acids and extends polypeptides to unmask epitopes hidden in the inner portion of antigens or covered by adjacent macromolecules. In buffers having an appropriate pH and ion concentration, epitopes are exposed without entangling the extended polypeptides during cooling process, since polypeptides may strike a balance between hydrophobic attraction force and electrostatic repulsion force. Recent studies have demonstrated that HIAR is applicable for immunohistochemistry with various kinds of specimens, i.e., FFPE materials, frozen sections, plastic-embedded specimens, and physically fixed tissues at both the light- and electron-microscopic levels, and have suggested that the mechanism of HIAR is common to aldehyde-fixed and aldehyde-unfixed materials. Furthermore, heating has been shown to be effective for flow cytometry, nucleic acid histochemistry (fluorescein in situ hybridization (FISH), in situ hybridization (ISH), and terminal deoxynucleotidyl transferase-mediated nick labeling (TUNEL)), and extraction and analysis of macromolecules in both FFPE archive materials and specimens processed by other procedures. In this article, we review mechanism of HIAR and application of heating in both immunohistochemistry and other histochemical reactions.

Application of heat-induced antigen retrieval to aldehyde-fixed fresh frozen sections

We applied the heat-induced antigen retrieval (HIAR) to aldehyde-fixed fresh frozen sections based on a new approach (i.e., a rapid and complete immobilization of antigen followed by heating). Frozen sections were fixed with 10% formalin in 0.1 M cacodylate buffer (pH 7.4) containing 25 mM CaCl(2) for 30 min, or with 0.5% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4) for 1 min at room temperature, and then autoclaved in 20 mM Tris-HCl buffer (pH 9.0) for 10 min at 120 C. Both fixatives yielded good tissue structure after autoclaving. In the sections fixed with formalin containing CaCl(2), 20 of 22 antigens located in the nucleus, cytoplasm, membranes, and extracellular matrix greatly recovered their antigenicity after autoclaving; only two antigens exhibited stronger immunoreaction in acetone-fixed fresh frozen sections than these sections. Heating also retrieved the immunoreactivity of at least 14 antigens in the sections fixed with glutaraldehyde. We used the similar procedures to localize ligand-free estrogen receptor alpha (ERalpha) and glucocorticoid receptors (GR). Mouse uterine cells exhibited almost the same nuclear ERalpha immunostaining regardless of the hormonal status in glutaraldehyde-fixed fresh frozen sections and unliganded GR was localized mainly in the nucleus of mouse hepatocytes in fresh frozen sections fixed with 20% formalin containing 50 or 75 mM CaCl(2) at 40 C, after autoclaving. These results demonstrate that HIAR is useful for the immunohistochemistry of many antigens in aldehyde-fixed fresh frozen sections.

Ultraspiracle promotes the nuclear localization of ecdysteroid receptor in mammalian cells

The heterodimer consisting of ecdysteroid receptor (EcR) and ultraspiracle (USP), both of which are members of the nuclear receptor superfamily, is considered to be the functional ecdysteroid receptor. Here we analyzed the subcellular distribution of EcR and USP fused to fluorescent proteins. The experiments were carried out in mammalian COS-7, CHO-K1 and HeLa cells to facilitate investigation of the subcellular trafficking of EcR and USP in the absence of endogenous expression of these two receptors. The distribution of USP tagged with a yellow fluorescent protein (YFP-USP) was almost exclusively nuclear in all cell types analyzed. The nuclear localization remained constant for at least 1 day after the first visible signs of expression. In contrast, the intracellular distribution of EcR tagged with a yellow fluorescent protein (YFP-EcR) varied and was dependent on time and cell type, although YFP-EcR alone was also able to partially translocate into the nuclear compartment. Coexpression of YFP-EcR with USP tagged with a cyan fluorescent protein (CFP-USP) resulted in exclusively nuclear localization of both proteins in all cell types analyzed. The USP-induced nuclear localization of YFP-EcR was stable for at least 20 hours. These experiments suggest that USP has a profound effect on the subcellular distribution of EcR.

Green fluorescent proteins in receptor research: An emerging tool for drug discovery

In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.

Pituitary expression of type I and type II glucocorticoid receptors during chicken embryonic development and their involvement in growth hormone cell differentiation

Glucocorticoids can induce somatotroph differentiation in vitro and in vivo during chick embryonic and rat fetal development. In the present study, we identified the nuclear receptors involved in somatotroph differentiation and examined their ontogeny and cellular distribution during pituitary development in the chicken embryo. Several steroids were tested for their ability to induce GH cell differentiation. Only glucocorticoids and aldosterone were effective at low nanomolar concentrations, suggesting involvement of both type I (mineralocorticoid) and type II (glucocorticoid) receptors (MR and GR, respectively). ZK98299 and spironolactone (GR and MR antagonists, respectively) when used alone were unable to block corticosterone or aldosterone (2 nm)-induced somatotroph differentiation. However, ZK98299 and spironolactone in combination abolished corticosterone or aldosterone (2 nm)-induced somatotroph differentiation. When used separately, both antagonists attenuated induction of GH mRNA by corticosterone. Spironolactone alone blocked somatotroph differentiation induced by 0.2 nm corticosterone or aldosterone, indicating that corticosteroids at subnanomolar concentrations act only through the MR. GR protein was detected in pituitary extracts as early as embryonic d 8, whereas MR protein was readily detectable only around d 12. GR were expressed in greater than 95% of all pituitary cells, whereas MR were expressed in about 40% of all pituitary cells. Dual-label immunofluorescence revealed that the majority of somatotrophs on d 12 expressed MR. Given the high affinity of corticosteroids for MR and that corticosteroid concentrations during embryonic development are in the subnanomolar range, expression of MR may constitute a significant developmental event during somatotroph differentiation.

Leptin receptors are developmentally regulated in rat pituitary and hypothalamus

We have previously reported that leptin is expressed in adult rat brain and pituitary gland, though the role of leptin in these sites has not been determined. Leptin mRNA is developmentally regulated in the brain and pituitary of male and female rats during early postnatal development, suggesting a role in the maturation of the brain-pituitary system. Here, we sought to extend our previous studies by evaluating (1) the ontogeny of leptin receptor mRNA levels in rat brain and pituitary and (2) pituitary leptin protein levels in neonatal and pre-pubertal rats. Pituitary leptin concentration was highest shortly after birth (postnatal day (PD) 4, 25 ng/mg protein) and fell significantly throughout postnatal development and into adulthood (PD 60, 3.5 ng/mg protein; P<0.005) coincident with a decline in pituitary leptin mRNA levels. Significant age-related effects on leptin receptor mRNA levels were also observed in the pituitary and the hypothalamus of male and female rats using semi-quantitative RT-PCR analysis. In the pituitary, the short form (OBRa) mRNA levels were highest in neonatal rats (PD 4) but declined throughout postnatal development (PD 4-22) paralleling the fall in pituitary leptin mRNA and protein levels. The long form (OBRb) mRNA levels were unaffected by age between PD 4 and 22. In contrast, hypothalamic, levels of OBRb mRNA were very low to undetectable shortly after birth (PD 4) and rose significantly between PD 4 and 14/22 while levels of OBRa mRNA were not significantly different between PD 4 and 22. Immunohistochemical detection of leptin receptor immunoreactivity (all forms) revealed the presence of OBR-like protein in pituitary and hypothalamus as early as PD 4. Cortical leptin receptor mRNA levels were similar throughout early postnatal development. No gender-related differences in leptin receptor mRNA levels were noted in brain or pituitary. In conclusion, these data, together with our previous work, indicate that the neonatal pituitary gland expresses leptin and leptin receptors at levels far in excess of those observed in mature rats. The pituitary is thus quite different from adipose tissue, hypothalamus and cerebral cortex, in which neonatal leptin expression is lowest at birth. Since neonatal pituitary leptin receptor expression is also elevated, it is possible that pituitary-derived leptin plays some role in the development of the hypothalamic-pituitary system.

In vivo tissue-specific regulation of the human papillomavirus type 18 early promoter by estrogen, progesterone, and their antagonists

Human papillomavirus type 18 is a causative agent of epithelial cancers in the uterine cervix. We show here that estrogen and progesterone activate beta-galactosidase expression from the early promoter of this virus in the genital epithelia of transgenic mice. Ovariectomy caused suppression of transgene expression exclusively in vagina and cervix epithelia. Beta-galactosidase expression could be restored in ovariectomized females by administration of estrogen, alone or in combination with progesterone. Further, rescue of transgene expression was inhibited by the estrogen antagonist tamoxifen and the anti-progesterone RU486, suggesting that this was a specific effect.