Synthesis of progesterone in Schwann cells: regulation by sensory neurons

Genomic and Non-genomic Action of Neurosteroids in the Peripheral Nervous System

Since the former evidence of biologic actions of neurosteroids in the central nervous system, also the peripheral nervous system (PNS) was reported as a structure affected by these substances. Indeed, neurosteroids are synthesized and active in the PNS, exerting many important actions on the different cell types of this system. PNS is a target for neurosteroids, in their native form or as metabolites. In particular, old and recent evidence indicates that the progesterone metabolite allopregnanolone possesses important functions in the PNS, thus contributing to its physiologic processes. In this review, we will survey the more recent findings on the genomic and non-genomic actions of neurosteroids in nerves, ganglia, and cells forming the PNS, focusing on the mechanisms regulating the peripheral neuron-glial crosstalk. Then, we will refer to the physiopathological significance of the neurosteroid signaling disturbances in the PNS, in to identify new molecular targets for promising pharmacotherapeutic approaches.

Changes in neurosteroidogenesis during demyelination and remyelination in cuprizone-treated mice

Changes of neurosteroids may be involved in the pathophysiology of multiple sclerosis (MS). The present study investigated whether changes of neurosteroidogenesis also occurred in the grey and white matter regions of the brain in mice subjected to cuprizone-induced demyelination. Accordingly, we compared the expression of neurosteroidogenic proteins, including steroidogenic acute regulatory protein (StAR), voltage-dependent anion channel (VDAC) and 18 kDa translocator protein (TSPO), as well as neurosteroidogenic enzymes, including the side chain cleavage enzyme (P450scc), 3β-hydroxysteroid dehydrogenase/isomerase and 5α-reductase (5α-R), during the demyelination and remyelination periods. Using immunohistochemistry and a quantitative polymerase chain reaction, we demonstrated a decreased expression of StAR, P450scc and 5α-R with respect to an increase astrocytic and microglial reaction and elevated levels of tumor necrosis factor (TNF)α during the cuprizone demyelination period in the hippocampus, cortex and corpus callosum. These parameters, as well as the glial reaction, were normalised after 2 weeks of spontaneous remyelination in regions containing grey matter. Conversely, persistent elevated levels of TNFα and low levels of StAR and P450scc were observed during remyelination in corpus callosum white matter. We conclude that neurosteroidogenesis/myelination status and glial reactivity are inversely related in the hippocampus and neocortex. Establishing a cause and effect relationship for the measured variables remains a future challenge for understanding the pathophysiology of MS.

Steroids and Brain, a Rising Bio-Medical Domain: a Perspective

Some newly described steroid-related compounds, also found in the rest of the body, are formed and active in the central nervous system, particularly in the brain. Some are of pharmacological and physiopathological interest. We specifically report on two compounds, "MAP4343," a new neurosteroid very efficient antidepressant, and "FKBP52," a protein component of hetero-oligomeric steroid receptors that we found involved in cerebral function, including in Alzheimer's disease.

Distribution and localization of 3β-hydroxysteroid dehydrogenase (3β-HSD) in the brain and its regions of the catfish Heteropneustes fossilis

In vertebrates, steroids are synthesized de novo in the central and peripheral nervous system, independent of peripheral steroidogenic glands, such as the adrenal, gonads and placenta. 3β-Hydroxysteroid dehydrogenase/Δ5-Δ4-isomerase (3β-HSD) is a key steroidogenic enzyme in vertebrate gonads, placenta and adrenal. It mediates the oxidation and isomerization reactions of progesterone from pregnenolone, 17-hydroxyprogesterone from 17-hydroxypregnenolone and androstenedione from dehydroepiandrosterone. In the present study, we examined the expression of 3β-HSD cDNA by real time-PCR and localization of the mRNA by in situ hybridization in the brain and its regions during the different phases of the reproductive cycle of the catfish Heteropneustes fossilis. Further, 3β-HSD activity was assayed biochemically to show seasonal variations. We showed significant seasonal and sexual dimorphic changes in the levels of transcript abundance in the whole brain and its regions. In whole brain, level was the highest in post-spawning phase and lowest in spawning phase in males. In females, there was a progressive increase through resting phase to pre-spawning phase, a decline in the spawning phase and increase in the post-spawning phase. In the preparatory phase, the highest transcript level was seen in medulla oblongata and the lowest in pituitary in males. In females, the level was the highest in the hypothalamus and lowest in olfactory bulb and pituitary. However, in the pre-spawning phase, in males it was the highest in telencephalon and hypothalamus and lowest in pituitary. In females, the highest transcript level was in olfactory bulb and lowest in pituitary. 3β-HSD enzyme activity showed significant seasonal variation in the brain, the highest in the resting phase and lowest in the preparatory and spawning phases. In situ hybridization showed the presence of 3β-HSD transcript was especially high in the cerebellum region. The presence of 3β-HSD in the brain may indicate steroidogenesis in the catfish brain.

Comparative aspects of neurosteroidogenesis: From fish to mammals

It is now clearly established that the central and peripheral nervous systems have the ability to synthesize de novo steroids referred to as neurosteroids. The major evidence for biosynthesis of neuroactive steroids by nervous tissues is based on the expression of enzymes implicated in the formation of steroids in neural cells. The aim of the present review is to summarize the current knowledge regarding the presence of steroidogenic enzymes in the brain of vertebrates and to highlight the very considerable contribution of Professor Kazuyoshi Tsutsui in this domain. The data indicate that expression of steroid-producing enzymes in the brain appeared early during vertebrate evolution and has been preserved from fish to mammals.

Progesterone synthesis in the nervous system: implications for myelination and myelin repair

Progesterone is well known as a female reproductive hormone and in particular for its role in uterine receptivity, implantation, and the maintenance of pregnancy. However, neuroendocrine research over the past decades has established that progesterone has multiple functions beyond reproduction. Within the nervous system, its neuromodulatory and neuroprotective effects are much studied. Although progesterone has been shown to also promote myelin repair, its influence and that of other steroids on myelination and remyelination is relatively neglected. Reasons for this are that hormonal influences are still not considered as a central problem by most myelin biologists, and that neuroendocrinologists are not sufficiently concerned with the importance of myelin in neuron functions and viability. The effects of progesterone in the nervous system involve a variety of signaling mechanisms. The identification of the classical intracellular progesterone receptors as therapeutic targets for myelin repair suggests new health benefits for synthetic progestins, specifically designed for contraceptive use and hormone replacement therapies. There are also major advantages to use natural progesterone in neuroprotective and myelin repair strategies, because progesterone is converted to biologically active metabolites in nervous tissues and interacts with multiple target proteins. The delivery of progesterone however represents a challenge because of its first-pass metabolism in digestive tract and liver. Recently, the intranasal route of progesterone administration has received attention for easy and efficient targeting of the brain. Progesterone in the brain is derived from the steroidogenic endocrine glands or from local synthesis by neural cells. Stimulating the formation of endogenous progesterone is currently explored as an alternative strategy for neuroprotection, axonal regeneration, and myelin repair.

Differential gene expression in motor and sensory Schwann cells in the rat femoral nerve

Phenotypic differences in Schwann cells (SCs) may help to guide axonal regeneration down motor or sensory specific pathways following peripheral nerve injury. The goal of this study was to identify phenotypic markers for SCs harvested from the cutaneous (sensory) and quadriceps (motor) branches of the rat femoral nerve and to study the effects of expansion culture on the expression patterns of these motor or sensory phenotypic markers. RNA was extracted from SCs harvested from the motor and sensory branches of the rat femoral nerve and analyzed using Affymetrix Gene Chips (Rat Genome 230 v2.0 Array A). Genes that were upregulated in motor SCs compared with the sensory SCs or vice versa were identified, and the results were verified for a subset of genes using quantitative real-time polymerase chain reaction (qRT-PCR). The expression levels of the "phenotype-specific" genes were then evaluated in SC expansion cultures at various time points over 30 days by qRT-PCR to determine the effect of expansion on SC phenotype. Expression levels of the phenotype-specific genes were significantly altered after expansion culture for both the motor and the sensory markers compared with fresh nerve tissue. These results indicate that both motor and sensory SC gene expression patterns are disrupted during expansion in vitro and may affect the ability of SCs to express phenotype-specific genes after transplantation.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Schwann cells from human neurofibromas show increased proliferation rates under the influence of progesterone

Neurofibromatosis type 1 (NF1) is a hereditary disease caused by mutations of the NF1 gene at 17q11.2. Loss of the NF1 gene product in Schwann cells leads to the development of benign nerve sheath tumors. These neurofibromas may occur at any time but tend to arise during periods of hormonal imbalance, suggesting that hormones influence neurofibroma growth. As steroid hormone levels rise during these times, we hypothesized that progesterone has proliferative effects on neurofibroma-derived Schwann cells. We chose specific medium conditions for selective proliferation of NF (+/-) and NF (-/-) cells from human neurofibromas. Genetic characterization was not performed, but former works have shown that under the conditions used (+/-) and (-/-) cells can be selected. Different progesterone concentrations were added at different days with BrdU-staining was performed to investigate proliferation rates and DAB-staining to identify a progesterone receptor. We could demonstrate that Schwann cells from human neurofibromas express progesterone receptors. These cells show elevated proliferation rates (highest in NF(-/-) cells) under progesterone, whereas normal human Schwann cells were not affected. These data suggest that progesterone plays an important role in the development of neurofibromas in NF1.

Regulatory role of cytochrome P450scc and pregnenolone in myelination by rat Schwann cells

To investigate the production of steroid hormones by Schwann cells and to examine the regulation of steroid hormone production during myelination, cultures of rat Schwann cells were differentiated into their myelinating phenotype in the absence of neurons with dibutyryl cAMP (db-cAMP). During this process, the expression of P450scc (involved in steroid biosynthesis) was elevated at both the mRNA and protein levels as evident in RT-PCR, Western blots, and immunostaining. Labeling of the cells with [14C] acetate revealed enhanced production of pregnenolone during differentiation into the myelinating phenotype. Disruption of P450scc's activity with an inhibitor diminished the extent of differentiation into the myelinating phenotype as levels of mRNA and protein expression of myelin protein zero (P0) declined. However, the effect was reversed with the addition of pregnenolone. Furthermore, when the differentiating cultures were treated with pregnenolone, mRNA expression of P0 was upregulated, suggesting the stimulation of the differentiation process. Together, these results provide evidence for Schwann cells as a major producer of steroid hormones and pregnenolone production by P450scc as an important regulatory step during myelination.

Novel perspectives for progesterone in hormone replacement therapy, with special reference to the nervous system

The utility and safety of postmenopausal hormone replacement therapy has recently been put into question by large clinical trials. Their outcome has been extensively commented upon, but discussions have mainly been limited to the effects of estrogens. In fact, progestagens are generally only considered with respect to their usefulness in preventing estrogen stimulation of uterine hyperplasia and malignancy. In addition, various risks have been attributed to progestagens and their omission from hormone replacement therapy has been considered, but this may underestimate their potential benefits and therapeutic promises. A major reason for the controversial reputation of progestagens is that they are generally considered as a single class. Moreover, the term progesterone is often used as a generic one for the different types of both natural and synthetic progestagens. This is not appropriate because natural progesterone has properties very distinct from the synthetic progestins. Within the nervous system, the neuroprotective and promyelinating effects of progesterone are promising, not only for preventing but also for reversing age-dependent changes and dysfunctions. There is indeed strong evidence that the aging nervous system remains at least to some extent sensitive to these beneficial effects of progesterone. The actions of progesterone in peripheral target tissues including breast, blood vessels, and bones are less well understood, but there is evidence for the beneficial effects of progesterone. The variety of signaling mechanisms of progesterone offers exciting possibilities for the development of more selective, efficient, and safe progestagens. The recognition that progesterone is synthesized by neurons and glial cells requires a reevaluation of hormonal aging.

3beta-Hydroxysteroid dehydrogenase/5-ene-4-ene isomerase mRNA expression in rat brain: effect of pseudopregnancy and traumatic brain injury

Evidence that endogenous progesterone (PROG) is neuroprotective after traumatic brain injury (TBI) is supported by the findings that pseudopregnant female rats present less edema and achieve better functional recovery than do male rats. PROG in the nervous system may originate from steroidogenic glands or can be locally synthesized. 3beta-Hydroxysteroid dehydrogenase/5-ene-4-ene isomerase (3beta-HSD) is the key enzyme in the biosynthesis of PROG. In the present study, we investigated the effects of pseudopregnancy and TBI on brain 3beta-HSD mRNA expression and on PROG levels. Twenty-four hours after bilateral contusion of the medial prefrontal cortex of rats, 3beta-HSD mRNA expression was analyzed by in situ hybridization while PROG levels were measured by gas chromatography/mass spectrometry. Similar levels of 3beta-HSD mRNA expression were observed in males and pseudopregnant females in the non-injured groups. At this time point, there was a significant decrease in the 3beta-HSD mRNA expression in the contusion site within the frontal cortex in both males and pseudopregnant females. In all other regions analyzed, 3beta-HSD mRNA expression was not affected by TBI and there was no difference between males and pseudopregnant females. The high decrease in the expression of the 3beta-HSD mRNA in the lesion site 24 h after TBI suggests a possible decrease in locally synthesized PROG in lesion site without change in the other brain regions. This decrease has less impact in pseudopregnant females since they have high plasmatic and brain levels of PROG compared to males.

Steroids and glial cell function

Hormonal and locally produced steroids act in the nervous system as neuroendocrine regulators, as trophic factors and as neuromodulators and have a major impact on neural development and function. Glial cells play a prominent role in the local production of steroids and in the mediation of steroid effects on neurons and other glial cells. In this review, we examine the role of glia in the synthesis and metabolism of steroids and the functional implications of glial steroidogenesis. We analyze the mechanisms of steroid signaling on glia, including the role of nuclear receptors and the mechanisms of membrane and cytoplasmic signaling mediated by changes in intracellular calcium levels and activation of signaling kinases. Effects of steroids on functional parameters of glia, such as proliferation, myelin formation, metabolism, cytoskeletal reorganization, and gliosis are also reviewed, as well as the implications of steroid actions on glia for the regulation of synaptic function and connectivity, the regulation of neuroendocrine events, and the response of neural tissue to injury.

Rapid inhibition of Ca2+ influx by neurosteroids in murine embryonic sensory neurones

The non-genomic role of neuroactive steroids on [Ca2+]i transients induced by GABA receptor activation was investigated in cultured dorsal root ganglia (DRG) neurones at embryonic stage E13. [Ca2+]i measurements were performed with Fura-2 fast fluorescence microfluorimetry. Application of the GABAA receptor agonist muscimol (Musci) evoked an increase in [Ca2+]i, confirming the excitatory effect of GABA at this embryonic stage. The muscimol-induced [Ca2+]i response was inhibited by progesterone (Proges) and its primary metabolite allopregnanolone (Allo) in a rapid, reversible and dose-dependent manner. These calcium transients were suppressed in the absence of external Ca2+ or in the presence of Ni2+ + Cd2+ suggesting an involvement of voltage-activated Ca2+ channels. In contrast, none of these steroids affected the resting [Ca2+]i nor exhibited any inhibitory effect on 50 mM KCl-induced [Ca2+]i increases. In view of the well-established potentiation of GABAA receptor by direct binding of neurosteroids, the inhibitory effects described in this study seem to involve distinct mechanisms. This new inhibitory effect of progesterone is observed at low and physiological concentrations, is rapid and independent of RU38486, an antagonist of the classic progesterone receptor, probably involving a membrane receptor. Using RT-PCR, we demonstrated the expression of progesterone receptor membrane component 1 (Pgrmc1), encoding 25-Dx, a membrane-associated progesterone binding protein in DRG neurones at different stages of development. In conclusion, we describe for the first time a rapid effect of progestins on embryonic DRG neurones involving an antagonistic effect of progesterone and allopregnanolone on GABAA receptors.

Myelin disorders: Causes and perspectives of Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth (CMT) disease is a common hereditary neuropathy that causes progressive distally pronounced muscle weakness and can lead to life-long disability in patients. In most cases, the disorder has been associated with a partial duplication of human chromosome 17 (CMT1A), causing 1.5-fold overexpression of the peripheral myelin protein 22 kDa (PMP22). Increased PMP22 gene dosage results in demyelination, secondary axonal loss, and neurogenic muscle atrophy. Experimental therapeutic approaches based on the role of progesterone and ascorbic acid in myelin formation recently have reached preclinical proof-of-principle trials in rodents. It was shown that progesterone receptor antagonists can reduce PMP22 overexpression and clinical severity in a CMT1A rat model. Furthermore, ascorbic acid treatment reduced premature death and demyelination in a CMT1A mouse model. Thus, basic research has opened up new vistas for the understanding and treatment of hereditary neuropathies.

Sex hormones and brain aging

Sex steroids exert pleiotropic effects in the nervous system, preserving neural function and promoting neuronal survival. Therefore, the age-related decrease in sex steroids may have a negative impact on neural function. Progesterone, testosterone and estradiol prevent neuronal loss in the central nervous system in different experimental animal models of neurodegeneration. Furthermore, progesterone and its reduced derivatives dihydroprogesterone and tetrahydroprogesterone reduce aging-associated morphological abnormalities of myelin and aging-associated myelin fiber loss in rat peripheral nerves. However, the results from hormone replacement studies in humans are thus far inconclusive. A possible alternative to hormonal replacement therapy is to increase local steroidogenesis by neural tissues, which express enzymes for steroid synthesis and metabolism. Proteins involved in the intramitochondrial trafficking of cholesterol, the first step in steroidogenesis, such as the peripheral-type benzodiazepine receptor and the steroidogenic acute regulatory protein, are up-regulated in the nervous system after injury. Furthermore, steroidogenic acute regulatory protein expression is increased in the brain of 24-month-old rats compared with young adult rats. This suggests that brain steroidogenesis may be modified in adaptation to neurodegenerative conditions and to the brain aging process. Furthermore, recent studies have shown that local formation of estradiol in the brain, by the enzyme aromatase, is neuroprotective. Therefore, steroidogenic acute regulatory protein, peripheral-type benzodiazepine receptor and aromatase are attractive pharmacological targets to promote neuroprotection in the aged brain.

Glia-neuron crosstalk in the neuroprotective mechanisms of sex steroid hormones

Proteins involved in the intramitochondrial trafficking of cholesterol, the first step in steroidogenesis, such as the steroidogenic acute regulatory protein (StAR) and the peripheral-type benzodiazepine receptor (PBR), are upregulated in the nervous system after injury. Accordingly, a local increase in the levels of steroids, such as pregnenolone and progesterone, is observed following traumatic injury in the brain and spinal cord. The expression and activity of aromatase, the enzyme that synthesizes estradiol, is also increased in injured brain areas and its inhibition results in an increased neurodegeneration. These findings suggest that an increase in steroidogenesis is part of an overall mechanism used by the nervous tissue to cope with neurodegenerative conditions. Neural steroidogenesis is the result of a coordinated interaction of neurons and glia. For example, after neural injury, there is an upregulation of StAR in neurons and of PBR in microglia and astroglia. Aromatase is expressed in neurons under basal conditions and is upregulated in reactive astrocytes after injury. Some of the steroids produced by glia are neuroprotective. Progesterone and progesterone derivatives produced by Schwann cells, promote myelin formation and the remyelination and regeneration of injured nerves. In the central nervous system, the steroids produced by glia regulate synaptic function, affect anxiety, cognition, sleep and behavior, and exert neuroprotective and reparative roles. In addition, glial cells are targets for steroids and mediate some of the effects of these molecules on neurons, including the regulation of survival and regeneration.

Sex-related differences in the regeneration of sensory axons and recovery of nociception after peripheral nerve crush in the rat

Sex-related differences regarding the regeneration of nociceptive axons and the recovery of nociception after sural nerve crush injury were examined in rats. The elongation rate of the fastest regenerating sensory axons in females started to increase after the first 6 days. This resulted in about 15% greater axon elongation distance at 8 days after crush in female than in male rats as determined by the nerve pinch test. The number of regenerating sensory axons in female and male rats, however, was not different. The recovery of nociception in the instep started earlier and was more extensive in females than in males during the entire 24-week recovery period, so that the pain sensitive area was finally about 20% larger in females than in males. Although ovariectomy significantly reduced plasma estradiol concentration in female rats, it did not change the elongation distance of regenerating nociceptive axons, which remained significantly greater than in male rats. Elimination of the cells in the distal nerve segment by freezing revealed that a more effective cell support in the distal nerve segment is probably responsible for faster regeneration of nociceptive axons in females than in males, rather than the circulating female sex hormones.

The synthesis of glycoprotein Po and peripheral myelin protein 22 in sciatic nerve of male rats is modulated by testosterone metabolites

Glycoprotein Po (Po) and peripheral myelin protein 22 (PMP22) are two proteins playing a crucial physiological role in the maintenance of the multilamellar structure of peripheral myelin. We here demonstrate that the removal of circulating androgens by orchidectomy induces a significant decrease of the synthesis of Po and PMP22 in the rat sciatic nerve. In case of Po, this effect may be counteracted by the subsequent treatment with testosterone metabolites, dihydrotestosterone or 5alpha-androstan-3alpha,17beta-diol (3alpha-diol). Experiments have been consequently performed in order to evaluate the role of androgen receptor (AR) in the control of Po synthesis. In vivo treatment with flutamide (i.e., an antagonist of AR) induces a decrease of the synthesis of this myelin protein in the sciatic nerve of intact male rats confirming a role for this steroid receptor. On the contrary, PMP22 seems not to be under the control of AR, but a role for GABAA receptor may be proposed. This concept is based on the findings that: (a) only 3alpha-diol, which is able to interact with GABAA receptor, is effective in stimulating the synthesis of PMP22 in the sciatic nerve of castrated male rats, and (b) flutamide treatment is ineffective in decreasing the protein levels in intact male rats. The observations here reported clearly show similarities and dissimilarities with the effects exerted by other members of neuroactive steroid family, like for instance progesterone derivatives, which will be discussed in text.

Local synthesis and dual actions of progesterone in the nervous system: neuroprotection and myelination

Progesterone (PROG) is synthesized in the brain, spinal cord and peripheral nerves. Its direct precursor pregnenolone is either derived from the circulation or from local de novo synthesis as cytochrome P450scc, which converts cholesterol to pregnenolone, is expressed in the nervous system. Pregnenolone is converted to PROG by 3beta-hydroxysteroid dehydrogenase (3beta-HSD). In situ hybridization studies have shown that this enzyme is expressed throughout the rat brain, spinal cord and dorsal root ganglia (DRG) mainly by neurons. Macroglial cells, including astrocytes, oligodendroglial cells and Schwann cells, also have the capacity to synthesize PROG, but expression and activity of 3beta-HSD in these cells are regulated by cellular interactions. Thus, Schwann cells convert pregnenolone to PROG in response to a neuronal signal. There is now strong evidence that P450scc and 3beta-HSD are expressed in the human nervous system, where PROG synthesis also takes place. Although there are only a few studies addressing the biological significance of PROG synthesis in the brain, the autocrine/paracrine actions of locally synthesized PROG are likely to play an important role in the viability of neurons and in the formation of myelin sheaths. The neuroprotective effects of PROG have recently been documented in a murine model of spinal cord motoneuron degeneration, the Wobbler mouse. The treatment of symptomatic Wobbler mice with PROG for 15 days attenuated the neuropathological changes in spinal motoneurons and had beneficial effects on muscle strength and the survival rate of the animals. PROG may exert its neuroprotective effects by regulating expression of specific genes in neurons and glial cells, which may become hormone-sensitive after injury. The promyelinating effects of PROG were first documented in the mouse sciatic nerve and in co-cultures of sensory neurons and Schwann cells. PROG also promotes myelination in the brain, as shown in vitro in explant cultures of cerebellar slices and in vivo in the cerebellar peduncle of aged rats after toxin-induced demyelination. Local synthesis of PROG in the brain and the neuroprotective and promyelinating effects of this neurosteroid offer interesting therapeutic possibilities for the prevention and treatment of neurodegenerative diseases, for accelerating regenerative processes and for preserving cognitive functions during aging.

Downregulation of steroidogenic acute regulatory protein (StAR) gene expression by cyclic AMP in cultured Schwann cells

Steroidogenic acute regulatory protein (StAR) plays a key role in the availability of cholesterol to the inner mitochondrial membrane, where the first step of steroidogenesis, its conversion to pregnenolone, takes place. Here, we demonstrate for the first time that the StAR gene is also expressed in the rat sciatic nerve and in cultured Schwann cells. The addition to the culture medium of the cAMP-elevating agent forskolin or of the cAMP analogue 8Br-cAMP produced a time-course extinction of StAR gene expression. An inverse relationship was demonstrated between StAR gene expression and the intracellular cAMP content. Accordingly, pharmacological inhibition of the activities of Schwann cell adenylyl cyclase or of phosphodiesterase IV resulted in modifications of StAR gene expression. Since StAR gene expression is stimulated by cAMP in classical steroidogenic cells, our work is the first demonstration of a negative regulation of StAR gene by cAMP.

Actions of progesterone and its 5alpha-reduced metabolites on the major proteins of the myelin of the peripheral nervous system

The sciatic nerve, and the Schwann cells in particular, are able to synthesize progesterone and possess the enzymes forming the 5alpha-reduced and the 3alpha-5alpha-reduced derivatives of progesterone: dihydroprogesterone and tetrahydroprogesterone. Moreover, the progesterone receptor (PR) is present in the sciatic nerve and in Schwann cell cultures. These facts suggest that progesterone and its derivatives might play a role in the control of the synthesis of the two major proteins of the peripheral nervous system (PNS): the glycoprotein Po (Po) and peripheral myelin protein 22 (PMP22). We have shown that: (a) dihydroprogesterone enhances the low mRNA levels of Po in the sciatic nerve of aged male rats; (b) progesterone and its derivatives stimulate the gene expression of Po in the sciatic nerve of adult rats and in Schwann cell cultures; (c) tetrahydroprogesterone increases PMP22 gene expression in the sciatic nerve of adult rats and in Schwann cell cultures. In additional experiments, utilizing agonists and antagonists of PR and GABAA receptor, we have observed that progesterone and its derivatives control Po gene expression via the PR, while tetrahydroprogesterone modulates the expression of PMP22 through the GABAA receptor.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

Steroid hormones and neurosteroids in normal and pathological aging of the nervous system

Without medical progress, dementing diseases such as Alzheimer's disease will become one of the main causes of disability. Preventing or delaying them has thus become a real challenge for biomedical research. Steroids offer interesting therapeutical opportunities for promoting successful aging because of their pleiotropic effects in the nervous system: they regulate main neurotransmitter systems, promote the viability of neurons, play an important role in myelination and influence cognitive processes, in particular learning and memory. Preclinical research has provided evidence that the normally aging nervous system maintains some capacity for regeneration and that age-dependent changes in the nervous system and cognitive dysfunctions can be reversed to some extent by the administration of steroids. The aging nervous system also remains sensitive to the neuroprotective effects of steroids. In contrast to the large number of studies documenting beneficial effects of steroids on the nervous system in young and aged animals, the results from hormone replacement studies in the elderly are so far not conclusive. There is also little information concerning changes of steroid levels in the aging human brain. As steroids present in nervous tissues originate from the endocrine glands (steroid hormones) and from local synthesis (neurosteroids), changes in blood levels of steroids with age do not necessarily reflect changes in their brain levels. There is indeed strong evidence that neurosteroids are also synthesized in human brain and peripheral nerves. The development of a very sensitive and precise method for the analysis of steroids by gas chromatography/mass spectrometry (GC/MS) offers new possibilities for the study of neurosteroids. The concentrations of a range of neurosteroids have recently been measured in various brain regions of aged Alzheimer's disease patients and aged non-demented controls by GC/MS, providing reference values. In Alzheimer's patients, there was a general trend toward lower levels of neurosteroids in different brain regions, and neurosteroid levels were negatively correlated with two biochemical markers of Alzheimer's disease, the phosphorylated tau protein and the beta-amyloid peptides. The metabolism of dehydroepiandrosterone has also been analyzed for the first time in the aging brain from Alzheimer patients and non-demented controls. The conversion of dehydroepiandrosterone to Delta5-androstene-3beta,17beta-diol and to 7alpha-OH-dehydroepiandrosterone occurred in frontal cortex, hippocampus, amygdala, cerebellum and striatum of both Alzheimer's patients and controls. The formation of these metabolites within distinct brain regions negatively correlated with the density of beta-amyloid deposits.

Progesterone and its metabolites increase myelin basic protein expression in organotypic slice cultures of rat cerebellum

We have previously shown that progesterone (PROG) is synthesized by Schwann cells and promotes myelin formation in the peripheral nervous system (PNS). We now report that this neurosteroid also stimulates myelination in organotypic slice cultures of 7-day-old (P7) rat and mouse cerebellum. Myelination was evaluated by immunofluorescence analysis of the myelin basic protein (MBP). After 7 days in culture (7DIV), we found that adding PROG (2(-5) x 10(-5) M) to the culture medium caused a fourfold increase in MBP expression when compared to control slices. The effect of PROG on MBP expression involves the classical intracellular PROG receptor (PR): the selective PR agonist R5020 significantly increased MBP expression and the PR antagonist mifepristone (RU486) completely abolished the effect of PROG on this MBP expression. Moreover, treatment of P7-cerebellar slice cultures from PR knockout (PRKO) mice with PROG had no significant effect on MBP expression. PROG was metabolized in the cerebellar slices to 5alpha-dihydroprogesterone (5alpha-DHP) and to the GABAA receptor-active metabolite 3alpha,5alpha-tetrahydroprogesterone (3alpha,5alpha-THP, allopregnanolone). The 5alpha-reductase inhibitor L685-273 partially inhibited the effect of PROG, and 3alpha,5alpha-THP (2(-5) x 10(-5) M) significantly stimulated the MBP expression, although to a lesser extent than PROG. The increase in MBP expression by 3alpha,5alpha-THP involved GABAA receptors, as it could be inhibited by the selective GABAA receptor antagonist bicuculline. These findings suggest that progestins stimulate MBP expression and consequently suggest an increase in CNS myelination via two signalling systems, the intracellular PR and membrane GABAA receptors, and they confirm a new role of GABAA receptors in myelination.

Developmental expression of genes involved in neurosteroidogenesis: 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase in the rat brain

In the central nervous system, neurosteroids, in particular progesterone, have neurotrophic and neuroprotective effects. We thus decided to study the developmental expression of 3beta-hydroxysteroid-dehydrogenase/Delta(5)-Delta(4) isomerase (3betaHSD), an enzyme that converts pregnenolone to progesterone, in the male rat brain at 0, 7, 14, and 70 d after birth. 3betaHSD mRNA was widely distributed throughout the brain, as shown by in situ hybridization. At all ages, the same cerebral structures were labeled, but the intensity of the hybridization signal constantly decreased during postnatal development. As the hippocampus is of particular interest because of its neuronal plasticity, we chose to quantify the changes in 3betaHSD mRNA levels as well as progesterone and pregnenolone concentrations in this structure. Quantitative in situ hybridization confirmed a decrease in the expression of 3betaHSD mRNA with progressing age, as revealed by a significant reduction in the density of silver grains per cell in the CA1 layer. This decrease was confirmed by semiquantitative RT-PCR on hippocampal samples. Concentrations of hippocampal pregnenolone and progesterone measured by gas chromatography/mass spectrometry were highest on the day of birth and lower at the other ages. Plasma concentrations of these steroids were lower than those in the hippocampus, suggesting that they may have been mostly synthesized in situ since the day of birth. These results demonstrate variations in the expression of a gene coding for an enzyme critically involved in progesterone synthesis in the hippocampus throughout postnatal development.

3 beta-hydroxysteroid dehydrogenase isomerase (3beta-HSD) activity in the rat sciatic nerve: kinetic analysis and regulation by steroids

We have shown that progesterone (PROG) has a stimulatory effect on myelin formation after sciatic nerve injury. PROG is synthesized from pregnenolone (PREG) by the enzyme 3 beta-hydroxysteroid dehydrogenase isomerase (3beta-HSD). At the occasion of the 15th International Symposium of the Journal of the Steroid Biochemistry and Molecular Biology, we presented some of our recent results demonstrating, expression and activity of the enzyme 3beta-HSD in the rat sciatic nerve. We determined the kinetic properties of 3beta-HSD and its regulation by PROG and estradiol. The expression of 3beta-HSD protein was assessed by Western-blot analysis, and the 3beta-HSD activity was evaluated by incubating homogenates with [3H]-PREG as substrate and NAD(+) as cofactor. Levels of steroids formed were calculated either by extrapolation of the relationship between the tritiated peaks obtained by thin layer chromatography (TLC) and the initial amount of PREG, or by gas chromatography-mass spectrometry (GC-MS) determination. A rapid increase in PROG formation was found between 0 and 50min of incubation and no significant change was observed between 1 and 4h. The calculated K(m) value was close to the values obtained for the 3beta-HSD types I and IV isoforms. Trilostane caused a potent inhibition of the rate of conversion of PREG to PROG. When we tested the effects of progesterone and estradiol on 3beta-HSD activity, a significant inhibition was obtained.

Increased expression of 3 beta-hydroxysteroid dehydrogenase mRNA in dorsal root ganglion neurons of adult rats following peripheral nerve injury

3beta-hydroxysteroid dehydrogenase (3beta-HSD) is an enzyme that converts pregnenolone to progesterone. It has been believed that 3beta-HSD is simply a converting enzyme of female steroid hormone. Recently, 3beta-HSD expressing cells were identified in the spinal cord. Steroid synthesis in the nervous system may indicate that steroid plays a role in the nervous system. We report here the increased expression of 3beta-HSD mRNA in the dorsal root ganglion (DRG) after peripheral nerve injury using reverse transcription-polymerase chain reaction and in situ hybridization histochemistry techniques. We detected only a few 3beta-HSD signals in the naïve DRG, and found that 3beta-HSD mRNA expression increased 3 days after injury and this increase was still observed at 14 days. Our results suggest that progesterone may have a role in the process against neuronal injury or in regeneration in the peripheral nervous system.

Characterization and regulation of the 3beta-hydroxysteroid dehydrogenase isomerase enzyme in the rat sciatic nerve

In the peripheral nervous system, progesterone (PROG) has a stimulatory effect on myelination. It could be derived from local synthesis, as Schwann cells in culture express the 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and convert pregnenolone (PREG) to PROG. Although 3beta-HSD mRNA can be detected by RT-PCR in peripheral nerves, the activity of the enzyme has so far not been demonstrated and characterized in nerve tissue. In this study, we show that homogenates prepared from rat sciatic nerves contain a functional 3beta-HSD enzyme and we have analysed its kinetic properties and its regulation by steroids. The activity of 3beta-HSD in homogenates was evaluated using 3H-labelled PREG as a substrate and NAD+ as a cofactor, the levels of steroids formed were calculated either by extrapolating the relationship between tritiated peaks obtained by TLC to the initial amount of PREG, or by gas chromatography/mass spectrometry determination. A rapid increase in PROG formation was found between 0 and 50 min of incubation and no further significant changes were observed between 1 and 4 h. The calculated Km value (1.06 +/- 0.19 microm) was close to the values described for the 3beta-HSD type-I and type-IV isoforms. Trilostane, a competitive inhibitor of the 3beta-HSD caused a potent inhibition of the rate of conversion of PREG to PROG (IC50 = 4.06 +/- 2.58 microm). When the effects of different steroids were tested, both oestradiol and PROG significantly inhibited the conversion of PREG to PROG.

3 Beta-hydroxysteroid dehydrogenase expression in rat spinal cord

In adult male rats, 3beta-hydroxysteroid dehydrogenase/delta5-delta4-isomerase (3beta-HSD) expressing cells were identified in the spinal cord from the cervical to the sacral segments. An in situ hybridization study, using an oligonucleotide common to the four known isoforms of rat 3beta-HSD, revealed its mRNA in gray matter. Measurements of optical densities in autoradiograms showed the following regional distribution: dorsal horn (layers I-III) > central canal (layer X) > or = ventral horn (layers VIII-IX) > ventral funiculus = lateral funiculus. At the cellular level, the number of grains was higher on the large motoneurons than on small neurons of the dorsal horn, but the grain density per cell was similar. Further evidence for the expression of 3beta-HSD in the spinal cord was obtained by western blot analysis, which revealed an immunoreactive protein of approximately 45 kDa in the dorsal and ventral parts of the spinal cord. Castration and adrenalectomy did not influence the expression of 3beta-HSD mRNA and protein. Gas chromatography/mass spectrometry measurements showed higher levels of pregnenolone and progesterone in the spinal cord than in the plasma. After castration and adrenalectomy, their levels remained elevated in the spinal cord, suggesting that these neurosteroids may be synthesized locally. The wide distribution of 3beta-HSD, and the high levels of pregnenolone and progesterone in the spinal cord even after castration and adrenalectomy, strongly suggest a potential endogenous production of progesterone and an important signalling function of this steroid in the spinal cord.

Progesterone protects against necrotic damage and behavioral abnormalities caused by traumatic brain injury

A single injection of progesterone can attenuate cerebral edema when administered during the first 24 h after traumatic brain injury (TBI) in rats but this regimen may not always produce functional benefits. In this experiment, we sought to find the duration of progesterone administration needed to facilitate both behavioral and morphological recovery. Male rats received bilateral contusions of the medial prefrontal cortex and were given progesterone (4 mg/kg) or vehicle for 3 or 5 days postoperatively. Both the 3- and the 5-day progesterone regimens reduced the size of injury- induced necrosis and cell loss, with the 5-day schedule being most effective. With regard to behavioral outcome, only 5 days of progesterone injections resulted in improved spatial learning performance and reduced sensory neglect. These results show that 5 days of postinjury progesterone treatment are needed to reduce significantly the neuropathological and behavioral abnormalities found in a rodent model of TBI. These benefits of progesterone, in the absence of any known side effects, provide further support for clinical testing of this neurosteroid.

Myelin P0: new knowledge and new roles

Protein zero (P0) is an integral transmembrane glycoprotein that serves as the major protein component of peripheral nerve myelin and is a member of the immunoglobulin (IgG) gene superfamily. As a cell adhesion molecule, P0 mediates homophilic adhesive interactions between Schwann cell plasma membranes and is a key structural constituent of both the major dense line and intraperiod line of compact myelin. Both the extracellular and cytoplasmic domains contribute to these interactions and evidence indicates that the post-translational modifications of the molecule, including glycosylation, acylation and phosphorylation, play an important modulatory role in adhesion and likely in the proper trafficking of P0 from the endoplasmic reticulum to the plasma membrane as well. Structural and genetic studies indicate that mutations in P0 producing human demyelinating diseases probably do so by perturbing or preventing homophilic interactions during myelination, or by producing cellular toxicity or an unstable myelin sheath. A variety of transcription factors, growth factors and neurosteroids both directly and indirectly influence P0 gene expression during maturation of the myelinating Schwann cell. Besides its structural function in myelin, P0 may have roles in the delivery of other Schwann cell proteins to their proper location, especially at or near nodes of Ranvier, and in neuronal-glial interactions.

Type 2 deiodinase in the peripheral nervous system: induction in the sciatic nerve after injury

Thyroid hormones are essential for the development and function of the brain and also for the maturation and repair of the peripheral nervous system. In the brain, most of the 3,5,3'-triiodothyronine is locally produced by 5'-deiodination of thyroxine catalyzed by the type 2 deiodinase. The absence of any information about thyroid hormone metabolism in the peripheral nervous system prompted us to study the expression of type 2 deiodinase (mRNA and activity) in the peripheral nervous system. Expression of type 2 deiodinase mRNA was very low in the sciatic nerve of rats until day 5 after birth, then increased from day 10 to 35-45 and gradually decreased afterwards, down to the low basal levels observed in the adult. A lesion of the sciatic nerve in the adult induced an increase in type 2 deiodinase mRNA and activity. After a cryolesion, the stimulation was observed as early as 4 h and mRNA levels increased until 24-48 h, then gradually declined down to basal levels around 28 days, when regeneration and functional recovery were completed. After a permanent transection, up-regulation of type 2 deiodinase persisted in both proximal and distal segments until the end of the experiment (28 days). Transection and cryolesion were also followed by increased type 2 deiodinase mRNA expression in the ipsilateral L4/L6 dorsal root ganglia within 24 h. Both mRNA and activity were found in the peripheral nerve sheaths but not in the internal compartment of the intact or injured nerve. Cultured fibroblasts from the sciatic nerve expressed type 2 deiodinase 4 h after stimulation by 10 microM forskolin, whereas purified Schwann cells did not. The present study provides evidence that the peripheral nervous system has its own system responsible for the local production of 3,5,3'-triiodothyronine, which may play a key role during the regeneration process.

Induction of type 3 iodothyronine deiodinase by nerve injury in the rat peripheral nervous system

Thyroid hormones are essential for the development and repair of the peripheral nervous system. The type 2 deiodinase, which is responsible for the activation of T(4) into T(3), is induced in injured sciatic nerve. To obtain information on the type 3 deiodinase (D3) responsible for the degradation of thyroid hormones, we looked for its expression (mRNA and activity) in the sciatic nerve after injury. D3 was undetectable in the intact sciatic nerve of adult rats, but was rapidly and highly increased in the distal and proximal segments after nerve lesion. After cryolesion, D3 up-regulation disappeared after 3 d in the proximal segment, whereas it was sustained for 10 d in the distal segment, then declined to reach basal levels after 28 d, when functional recovery was completed. After a transsection preventing the nerve regeneration, up-regulation of D3 persisted up to 28 d at high levels in the distal segment. D3 was expressed in peripheral connective sheaths and in the internal endoneural compartment. D3 mRNA was inducible by 12-O-tetradecanoylphorbol-13-acetate in cultured fibroblasts or Schwann cells. In conclusion, induction of D3 in the peripheral nervous system after injury may play an important role during the regeneration process by adjusting intracellular T(3) levels.

Progesterone synthesis and myelin formation in peripheral nerves

Progesterone is synthesized in the nervous system by neurons and glial cells. Because of their simple structure, plasticity and capacity of regeneration, peripheral nerves are particularly well suited for studying the biosynthesis, mechanisms of action and effects of the hormone. Schwann cells, the myelinating glial cells in the peripheral nervous system, synthesize progesterone in response to a diffusible neuronal signal. In peripheral nerves, the local synthesis of progesterone plays an important role in the formation of myelin sheaths. This has been shown in vivo, after cryolesion of the mouse sciatic nerve, and in vitro, in cocultures of Schwann cells and sensory neurons. Schwann cells also express an intracellular receptor for progesterone, which thus functions as an autocrine signalling molecule. Progesterone may promote myelination by activating the expression of genes coding for transcription factors (Krox-20) and/or for myelin proteins (P0, PMP22). Recently, it has been proposed that progesterone may indirectly regulate myelin formation by influencing gene expression in neurons. Steroid hormones also influence the proliferation of Schwann cells: estradiol becomes a potent mitogen for Schwann cells when levels of cAMP are elevated and glucocorticosteroids have been shown to increase the mitogenic effects of peptide growth factors.

Neuroactive steroids and peripheral myelin proteins

The present review summarizes observations obtained in our laboratories which underline the importance of neuroactive steroids (i.e., progesterone (PROG), dihydroprogesterone (5alpha-DH PROG), tetrahydroprogesterone (3alpha, 5alpha-TH PROG), testosterone (T), dihydrotestosterone (DHT) and 5alpha-androstan-3alpha,17beta-diol (3alpha-diol)) in the control of the gene expression of myelin proteins (i.e. glycoprotein Po (Po) and the peripheral myelin protein 22 (PMP22)) in the peripheral nervous system. Utilizing different in vivo (aged and adult male rats) and in vitro (Schwann cell cultures) experimental models, we have observed that neuroactive steroids are able to stimulate the mRNA levels of Po and PMP22. The effects of these neuroactive steroids, which are able to interact with classical (progesterone receptor, PR, and androgen receptor, AR) and non-classical (GABA(A) receptor) steroid receptors is further supported by our demonstration in sciatic nerve and/or Schwann cells of the presence of these receptors. On the basis of the observations obtained in the Schwann cells cultures, we suggest that the stimulatory effect of neuroactive steroids on Po is acting through PR, while that on PMP22 needs the GABA(A) receptor. The present findings might be of importance for the utilization of specific receptor ligands as new therapeutical approaches for the rebuilding of the peripheral myelin, particularly in those situations in which the synthesis of Po and PMP22 is altered (i.e. demyelinating diseases like Charcot-Marie-Tooth type 1A and type 1B, hereditary neuropathy with liability to pressure palsies and the Déjérine-Sottas syndrome, aging, and after peripheral injury).

Rapid effects of triiodothyronine on immediate-early gene expression in Schwann cells

In the peripheral nervous system, triiodothyronine (T3) plays an important role in the development and regeneration of nerve fibers and in myelin formation. However, the target genes of T3 in peripheral nerves remain to be identified. We investigated whether T3 activated genes of transcription factors in Schwann cells. Expression of egr-1 (krox-24), egr-2 (krox-20), egr-3, c-jun, junB, c-fos, fosB, fra-1, fra-2, and CREB genes was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) in Schwann cells isolated from neonatal rat sciatic nerves and in the cell lines MSC-80 (mouse Schwann cells), NIH-3T3 (mouse fibroblasts), and CHO (Chinese hamster ovary cells). Some of these transcription factors have been shown to be involved in Schwann cell differentiation. T3 triggered a rapid (15-30 min), transient (1-2-h) and strong (6- to 15-fold) stimulation of Egr-1, Egr-2, Egr-3, Jun B, c-Fos, and Fos B mRNA expression in Schwann cells. In contrast, expression of c-Jun, Fra-1, Fra-2, and CREB mRNA was not affected by T3. The stimulatory effects of T3 could be abolished by adding actinomycin D. T3 triggered the same pattern of gene stimulation in the mouse Schwann cell line MSC80, but not in the NIH-3T3 and CHO cell lines. Serum activated all the genes that responded to T3 and in addition fra-1 and fra-2, but not c-jun and CREB. Immunoblotting showed that the increase in Egr-1 and c-Fos mRNA levels was accompanied by an increase in the corresponding proteins. In addition, shifts of the protein bands indicated a posttranslational modification of the two proteins. These effects of T3 are likely to be mediated by the intracellular T3 receptor, as the D-isomer RT3 and T0, which do not bind to T3 receptors, proved ineffective. The present data suggested that T3 may regulate Schwann cell functions and differentiation by transiently activating the expression of specific transcription factors.

Progesterone stimulates Krox-20 gene expression in Schwann cells

The gene of the zinc finger transcription factor Krox-20 (Egr-2) is expressed in Schwann cells and plays an important role in myelination of peripheral nerves. We have shown that progesterone promotes myelination in the regenerating sciatic nerve and in cocultures of Schwann cells and sensory neurones. To determine whether progesterone regulates Krox-20 expression, we measured its effects on Krox-20 mRNA levels in the MSC80 mouse Schwann cell line by semi-quantitative RT-PCR. Although low levels of Krox-20 mRNA are detectable in MSC80 cells cultured in defined medium, treatment with 10(-6) M progesterone induces a rapid (15 min) and transient increase in the levels of Krox-20 mRNA. Lower doses of progesterone (10(-9), 10(-8) and 10(-7) M) are also effective in increasing Krox-20 mRNA. Other steroids including testosterone, dexamethasone, and estradiol are ineffective when added to the culture medium at 10(-6) M for 1 h. The induction of Krox-20 mRNA was also observed with the selective progesterone agonist Organon 2058 and was abolished by treating the MSC80 Schwann cells with the progesterone antagonist RU486, indicating that progesterone induces Krox-20 mRNA expression by binding to its intracellular receptor. The induction of Krox-20 by progesterone was also demonstrated in primary cultures of Schwann cells isolated from neonatal rat sciatic nerves, at the mRNA level by RT-PCR and at the protein level by immunohistochemistry. As Krox-20 is a necessary step for the initiation of myelin formation in peripheral nerves, its stimulation by progesterone suggests an important signalling function for this steroid in myelination.