Chapter 11 - Steroidogenic Enzymes in the Brain: Morphological Aspects

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Abstract

It is now well documented that brain tissue is capable of synthesizing de novo bioactive steroids, named neurosteroids, which are involved in the regulation of various functions in the brain, including behavioral, neuroendocrine and metabolic processes. In this chapter, we have summarized the current knowledge about the expression of enzymes involved in the biosynthesis and metabolism of steroids in the brain with special emphasis on the morphological localization of those enzymes. The results obtained following use of immunocytochemistry and/or in situ hybridization indicate that the enzymes are expressed in both neurons and glial cells distributed throughout the brain with some species-related variations. As observed at the electron microscopic level, the enzymes localized in the neurons or glial cells are not associated with any specific organelles, being distributed throughout the cytoplasm. Since usually one nerve cell expresses only one enzyme, it might be suggested that the neuroactive steroid synthesis that requires the action of several enzymes involves the intervention of several cells (neurons and/or glial cells). More work is required to fully establish the migration of precursors and active steroids in the brain.

Introduction

The steroid hormones exert a determinant role in brain development while regulating various physiological functions such as locomotion, feeding, sexual behaviors, learning and memory (for a review, see McEwen, 1994). It has been long believed that the steroids acting on the brain were originating from steroidogenic glands, including gonads, adrenal cortex and plasma. In the early eighties, it was shown that the brain is capable of de novo biosynthesis of steroids. The first clue of neurosteroidogenesis came from the observations that pregnenolone, dehydroepiandrosterone (DHEA), and their sulphate derivatives accumulate in the brain of castrated and adrenalectomized rats (Corpéchot et al., 1981). Moreover, Robel et al., (1986) showed that circadian variations of steroid concentrations in brain tissue are not synchronized with those of steroids measured in peripheral blood. As a result, the term neurosteroids was proposed to designate the steroids synthesized in the brain, either de novo from cholesterol or by in situ metabolism of blood-borne precursors (Baulieu, 1997, Robel and Baulieu, 1985). The concept of neurosteroidogenesis has been subsequently supported by morphological data showing the occurrence of steroidogenic enzymes or their mRNAs by immunohistochemistry and in situ hybridization, respectively (Do Rego et al., 2007, Dupont et al., 1994, Le Goascogne et al., 1987, Mellon and Vaudry, 2001, Mensah-Nyagan et al., 1994, Pelletier et al., 2003, Mensah-Nyagan et al., 1999, Pelletier et al., 2007, Robel and Baulieu, 1985). The morphological approach has largely contributed to accurately determine the brain areas involved in the biosynthesis and metabolism of active neurosteroids as well as to identify the cell type (neurons/glial cells) expressing the different enzymes.

The aim of this chapter is to summarize the current research about the expression of steroidogenic enzymes in the brain with special emphasis on their cellular localization.

Section snippets

Cytochrome P450 side-chain cleavage

The first step in the synthesis of all steroid hormones is the conversion of cholesterol to pregnenolone by a mitochondrial enzyme, cytochrome P450 side-chain cleavage (scc) (Lieberman et al., 1984). This enzyme was first observed by Le Goascogne et al. (1987) who detected by immunocytochemistry the presence of immunoactive glial cells in the white matter throughout the rat brain. P450 scc protein has also been detected by Western blot analysis in the cerebellum and by immunochemistry in

3β-Hydroxysteroid dehydrogenase

3β-Hydroxysteroid dehydrogenase (3β-HSD) is a membrane-bound mitochondrial enzyme that catalyzes the conversion of Δ5-3β-hydroxysteroid into Δ4-3β-ketosteroids, leading to the formation of progesterone from pregnenolone and androstenedione from DHEA. In human, two isoforms of 3β-HSD have been characterized: type 1 is mainly expressed in the placenta, but also found in the skin and mammary gland (Luu The et al., 1989, Rheaume et al., 1991) while type 2 is predominantly expressed in the adrenal

Cytochrome P450C17

Cytochrome P450C17, also termed 17α-hydroxylase/C17.20 lyase, catalyzes the hydroxylation of C21 steroids (pregnenolone, progesterone) into C19 steroids (DHEA and androstenedione, respectively).

Although the presence of DHEA and androstenedione in the brain is well documented (Akwa et al., 1991, Corpechot et al., 1981, Jo et al., 1989, Lanthier and Patwardhan, 1986, Matsunaga et al., 2001, Mensah-Nyagan et al., 1996, Robel et al., 1986, Soma et al., 2004), the biosynthetic pathways leading to

17β-Hydroxysteroid dehydrogenase

The enzyme 17β-hydroxysteroid dehydrogenase (17β-HSD) catalyzes the interconversion between the active and inactive forms of specific steroidal hormones in the final steps of their biosynthesis (Labrie et al., 2000). They are named according to their ability to catalyze oxidation or reduction of the 17-hydroxy or 17-keto functions of specific physiologically relevant steroids. Up to now, 15 types of 17β-HSDs are reported in vertebrates (Moeller and Adamski, 2009). With the exception of

5α-Reductase

The enzyme 5α-reductase catalyzes the transformation of progesterone, T and 11-deoxycorticosterone into 5α-dihydroprogesterone, 5α-dihydrotestosterone and 5α-dihydrodeoxycorticosterone, respectively. Two 5α-reductase isozymes encoded by two distinct genes, designated type 1 and type 2, have been identified in rodents (Berman and Russell, 1993, Mahendroo et al., 1996, Normington and Russell, 1992), monkey (Levy et al., 1995) and human (Andersson et al., 1991, Labrie et al., 1992, Russell and

3α-Hydroxysteroid dehydrogenase

3α-Hydroxysteroid dehydrogenase (3α-HSD) is a bifunctional enzyme that interconverts, in a reversible manner, the 5α-reduced steroids (5α-DHT and 5α-dihydroprogesterone (5α-DHP) into 3α-androstanediol and 3α,5α-tetrahydroprogesterone, respectively). 3α-HSD also catalyzes the reversible conversion of dihydrodeoxycorticosterone into tetrahydrodeoxycorticosterone. In humans, multiple cDNAs encoding various proteins structurally related to 3α-HSD have been reported (Qin et al., 1993). However, to

20α-Hydroxysteroid dehydrogenase

The enzyme 20α-hydroxysteroid dehydrogenase (20α-HSD) catalyzes the conversion of progesterone into its inactive form, 20α-hydroxyprogesterone (Zhang et al., 2000). In the brain of several species, conversion of progesterone into 20α-reduced metabolites has been reported, indicating that some brain areas have 20α-HSD activity (Karavolas and Hodges, 1990). Recently, with a cRNA probe that has been successfully used to localize 20α-HSD by in situ hybridization in a variety of mouse tissues (

Cytochrome P450 aromatase

Cytochrome P450 aromatase (P450arom) catalyzes the conversion of C19 androgens (androstenedione and testosterone) into C18 estrogens (estrone and estradiol, respectively). The presence of P450arom mRNA in the brain of mammals has been demonstrated by RT-PCR and in situ hybridization histochemistry (Harada and Yamada, 1992, Hojo et al., 2004, Lephart et al., 1992). In rat and mouse, intense expression of the P450arom gene occurs in the cerebral cortex, medial preoptic nucleus, bed nucleus of the

11β-Hydroxysteroid dehydrogenase

The interconversion of glucocorticoids from their inactive (11-dehydrocorticosterone, cortisone) to their active forms (corticosterone, cortisol) is catalyzed by the enzyme 11β-hydroxysteroid dehydrogenase (11β-HSD) in specific tissues. Molecular cloning of the cDNAs encoding 11β-HSD revealed the existence of two isoforms of the enzyme, 11β-HSD type 1 and type 2, in humans and rodents (Tannin et al., 1991, Zhou et al., 1995). 11β-HSD type 1 is a bidirectional enzyme, predominantly displaying

Conclusion

Since the first observation by Beaulieu and his collaborators (Corpéchot et al., 1981) that some hormonal steroids were in higher concentrations in the brain than in blood, there have been a large number of reports on the expression of steroidogenic enzymes in the brain. So far, most of the enzymes involved in the biosynthesis and metabolism of all the categories of steroid hormones (sex steroids and glucocorticoids) have been found to be expressed in the brain of many representative species of

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