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

Testosterone Reduces Myelin Abnormalities in the Wobbler Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal motoneuron degenerative disease that is associated with demyelination. The Wobbler (WR) mouse exhibits motoneuron degeneration, gliosis and myelin deterioration in the cervical spinal cord. Since male WRs display low testosterone (T) levels in the nervous system, we investigated if T modified myelin-relative parameters in WRs in the absence or presence of the aromatase inhibitor, anastrozole (A). We studied myelin by using luxol-fast-blue (LFB) staining, semithin sections, electron microscopy and myelin protein expression, density of IBA1+ microglia and mRNA expression of inflammatory factors, and the glutamatergic parameters glutamine synthetase (GS) and the transporter GLT1. Controls and WR + T showed higher LFB, MBP and PLP staining, lower g-ratios and compact myelin than WRs and WR + T + A, and groups showing the rupture of myelin lamellae. WRs showed increased IBA1+ cells and mRNA for CD11b and inflammatory factors (IL-18, TLR4, TNFαR1 and P2Y12R) vs. controls or WR + T. IBA1+ cells, and CD11b were not reduced in WR + T + A, but inflammatory factors' mRNA remained low. A reduction of GS+ cells and GLT-1 immunoreactivity was observed in WRs and WR + T + A vs. controls and WR + T. Clinically, WR + T but not WR + T + A showed enhanced muscle mass, grip strength and reduced paw abnormalities. Therefore, T effects involve myelin protection, a finding of potential clinical translation.

Effects of endocrine disrupting chemicals on myelin development and diseases

In the central and peripheral nervous systems, myelin is essential for efficient conduction of action potentials. During development, oligodendrocytes and Schwann cells differentiate and ensure axon myelination, and disruption of these processes can contribute to neurodevelopmental disorders. In adults, demyelination can lead to important disabilities, and recovery capacities by remyelination often decrease with disease progression. Among environmental chemical pollutants, endocrine disrupting chemicals (EDCs) are of major concern for human health and are notably suspected to participate in neurodevelopmental and neurodegenerative diseases. In this review, we have combined the current knowledge on EDCs impacts on myelin including several persistent organic pollutants, bisphenol A, triclosan, heavy metals, pesticides, and nicotine. Besides, we presented several other endocrine modulators, including pharmaceuticals and the phytoestrogen genistein, some of which are candidates for treating demyelinating conditions but could also be deleterious as contaminants. The direct impacts of EDCs on myelinating cells were considered as well as their indirect consequences on myelin, particularly on immune mechanisms associated with demyelinating conditions. More studies are needed to describe the effects of these compounds and to further understand the underlying mechanisms in relation to the potential for endocrine disruption.

Physiopathological Role of Neuroactive Steroids in the Peripheral Nervous System

Peripheral neuropathy (PN) refers to many conditions involving damage to the peripheral nervous system (PNS). Usually, PN causes weakness, numbness and pain and is the result of traumatic injuries, infections, metabolic problems, inherited causes, or exposure to chemicals. Despite the high prevalence of PN, available treatments are still unsatisfactory. Neuroactive steroids (i.e., steroid hormones synthesized by peripheral glands as well as steroids directly synthesized in the nervous system) represent important physiological regulators of PNS functionality. Data obtained so far and here discussed, indeed show that in several experimental models of PN the levels of neuroactive steroids are affected by the pathology and that treatment with these molecules is able to exert protective effects on several PN features, including neuropathic pain. Of note, the observations that neuroactive steroid levels are sexually dimorphic not only in physiological status but also in PN, associated with the finding that PN show sex dimorphic manifestations, may suggest the possibility of a sex specific therapy based on neuroactive steroids.

Physiopathological role of the enzymatic complex 5α-reductase and 3α/β-hydroxysteroid oxidoreductase in the generation of progesterone and testosterone neuroactive metabolites

The enzymatic complex 5α-reductase (5α-R) and 3α/3β-hydroxysteroid oxidoreductase (HSOR) is expressed in the nervous system, where it transforms progesterone (PROG) and testosterone (T) into neuroactive metabolites. These metabolites regulate myelination, brain maturation, neurotransmission, reproductive behavior and the stress response. The expression of 5α-R and 3α-HSOR and the levels of PROG and T reduced metabolites show regional and sex differences in the nervous system and are affected by changing physiological conditions as well as by neurodegenerative and psychiatric disorders. A decrease in their nervous tissue levels may negatively impact the course and outcome of some pathological events. However, in other pathological conditions their increased levels may have a negative impact. Thus, the use of synthetic analogues of these steroids or 5α-R modulation have been proposed as therapeutic approaches for several nervous system pathologies. However, further research is needed to fully understand the consequences of these manipulations, in particular with 5α-R inhibitors.

Sex differences in steroid levels and steroidogenesis in the nervous system: Physiopathological role

The nervous system, in addition to be a target for steroid hormones, is the source of a variety of neuroactive steroids, which are synthesized and metabolized by neurons and glial cells. Recent evidence indicates that the expression of neurosteroidogenic proteins and enzymes and the levels of neuroactive steroids are different in the nervous system of males and females. We here summarized the state of the art of neuroactive steroids, particularly taking in consideration sex differences occurring in the synthesis and levels of these molecules. In addition, we discuss the consequences of sex differences in neurosteroidogenesis for the function of the nervous system under healthy and pathological conditions and the implications of neuroactive steroids and neurosteroidogenesis for the development of sex-specific therapeutic interventions.

Neurosteroid Metabolites of Gonadal Steroid Hormones in Neuroprotection: Implications for Sex Differences in Neurodegenerative Disease

Gonadal steroid hormones are neurotrophic and neuroprotective. These effects are modulated by local metabolism of the hormones within the brain. Such control is necessary to maintain normal function, as several signaling pathways that are activated by gonadal steroid hormones in the brain can also become dysregulated in disease. Metabolites of the gonadal steroid hormones—particularly 3α-hydroxy, 5α-reduced neurosteroids—are synthesized in the brain and can act through different mechanisms from their parent steroids. These metabolites may provide a mechanism for modulating the responses to their precursor hormones, thereby providing a regulatory influence on cellular responses. In addition, there is evidence that the 3α-hydroxy, 5α-reduced neurosteroids are neuroprotective in their own right, and therefore may contribute to the overall protection conferred by their precursors. In this review article, the rapidly growing body of evidence supporting a neuroprotective role for this class of neurosteroids will be considered, including a discussion of potential mechanisms that may be involved. In addition, we explore the hypothesis that differences between males and females in local neurosteroid production may contribute to sex differences in the development of neurodegenerative disease.

Androgen and estrogen actions on male physical activity: a story beyond muscle

Physical inactivity is a pandemic that contributes to several chronic diseases and poses a significant burden on health care systems worldwide. The search for effective strategies to combat sedentary behavior has led to an intensification of the research efforts to unravel the biological substrate controlling activity. A wide body of preclinical evidence makes a strong case for sex steroids regulating physical activity in both genders, albeit the mechanisms implicated remain unclear. The beneficial effects of androgens on muscle as well as on other peripheral functions might play a role in favoring adaptation to exercise. Alternatively or in addition, sex steroids could act on specific brain circuitries to boost physical activity. This review critically discusses the evidence supporting a role for androgens and estrogens stimulating male physical activity, with special emphasis on the possible role of peripheral and/or central mechanisms. Finally, the potential translation of these findings to humans is briefly discussed.

Effects of Strength Training and Anabolic Steroid in the Peripheral Nerve and Skeletal Muscle Morphology of Aged Rats

Thirty male 20-month-old Wistar rats were divided into groups: IC—initial control (n = 6), FC—final control (n = 6), AC—anabolic hormone control (n = 6), ST—strength trained (n = 6) and STA—strength trained with anabolic hormone (n = 6). All groups were submitted to adaptation, familiarization and maximum load carrying test (MLCT). Strength training (6–8×/session with loads of 50%–100% MLCT, 3×/week and pause of 120 s) was performed in ladder climbing (LC) for 15 weeks. The administration of testosterone propionate (TP) was performed 2×/week (10 mg/kg) in animals in the AC and STA groups. After the experimental period, animals were euthanized and the tibial nerve and plantaris muscle removed and prepared for electron transmission and histochemistry. To compare the groups we used one-way ANOVA (post hoc Bonferroni), student’s t-tests for pre vs. post (dependent and independent variables) comparisons and significance level set at p ≤ 0.05. The following significant results were found: (a) aging decreased the number of myelinated axon fibers; (b) use of isolated TP increased the diameter of myelinated fibers, along with increased thickness of myelin sheath; (c) ST increased area of myelinated and unmyelinated fibers, together with the myelin sheath. These changes made it possible to increase the area occupied by myelinated fibers keeping their quantity and also reduce the interstitial space; and (d) association of anabolic steroid and ST increased the area of unmyelinated axons and thickness of the myelin sheath. Compared to ST, both strategies have similar results. However, Schwann cells increased significantly only in this strategy.

Androgen Mediated Regulation of Endoplasmic Reticulum-Associated Degradation and its Effects on Prostate Cancer

The endoplasmic reticulum (ER) comprises thirty percent of the newly translated proteins in eukaryotic cells. The quality control mechanism within the ER distinguishes between properly and improperly folded proteins and ensures that unwanted proteins are retained in the ER and subsequently degraded through ER-associated degradation (ERAD). Besides cleaning of misfolded proteins ERAD is also important for physiological processes by regulating the abundance of normal proteins of the ER. Thus it is important to unreveal the regulation patterns of ERAD. Here, we describe that ERAD pathway is regulated by androgen, where its inhibitor SVIP was downregulated, all other ERAD genes were upregulated. Consistently, androgen treatment increased the degradation rate of ERAD substrates. Using several independent techniques, we showed that this regulation is through androgen receptor transactivation. ERAD genes found to be upregulated in prostate cancer tissues and silencing expression of Hrd1, SVIP, and gp78 reduced the in vitro migration and malignant transformation of LNCaP cells. Our data suggests that expression levels of ERAD components are regulated by androgens, that promotes ERAD proteolytic activity, which is positively related with prostate tumorigenesis.

Differential motor and sensory functional recovery in male but not female adult rats is associated with remyelination rather than axon regeneration after sciatic nerve crush

Peripheral nerve functional recovery after injuries relies on both axon regeneration and remyelination. Both axon regeneration and remyelination require intimate interactions between regenerating neurons and their accompanying Schwann cells. Previous studies have shown that motor and sensory neurons are intrinsically different in their regeneration potentials. Moreover, denervated Schwann cells accompanying myelinated motor and sensory axons have distinct gene expression profiles for regeneration-associated growth factors. However, it is unknown whether differential motor and sensory functional recovery exists. If so, the particular one among axon regeneration and remyelination responsible for this difference remains unclear. Here, we aimed to establish an adult rat sciatic nerve crush model with the nonserrated microneedle holders and measured rat motor and sensory functions during regeneration. Furthermore, axon regeneration and remyelination was evaluated by morphometric analysis of electron microscopic images on the basis of nerve fiber classification. Our results showed that Aα fiber-mediated motor function was successfully recovered in both male and female rats. Aδ fiber-mediated sensory function was partially restored in male rats, but completely recovered in female littermates. For both male and female rats, the numbers of regenerated motor and sensory axons were quite comparable. However, remyelination was diverse among myelinated motor and sensory nerve fibers. In detail, Aβ and Aδ fibers incompletely remyelinated in male, but not female rats, whereas Aα fibers fully remyelinated in both sexes. Our result indicated that differential motor and sensory functional recovery in male but not female adult rats is associated with remyelination rather than axon regeneration after sciatic nerve crush.

Neuroactive steroids and the peripheral nervous system: An update

In the present review we summarize observations to date supporting the concept that neuroactive steroids are synthesized in the peripheral nervous system, regulate the physiology of peripheral nerves and exert notable neuroprotective actions. Indeed, neuroactive steroids have been recently proposed as therapies for different types of peripheral neuropathy, like for instance those occurring during aging, chemotherapy, physical injury and diabetes. Moreover, pharmacological tools able to increase the synthesis of neuroactive steroids might represent new interesting therapeutic strategy to be applied in case of peripheral neuropathy.

Neuroregenerative Effect of Oxandrolone: A Case Report

Patient: Male, 25

Final Diagnosis: Charcot-Marie-Tooth 1

Symptoms: Muscular • spasticity

Medication: Oxandrolone

Clinical Procedure: Neural and muscle biopsies

Specialty: Neurology

Role of neuroactive steroids in the peripheral nervous system

Several reviews have so far pointed out on the relevant physiological and pharmacological role exerted by neuroactive steroids in the central nervous system. In the present review we summarize observations indicating that synthesis and metabolism of neuroactive steroids also occur in the peripheral nerves. Interestingly, peripheral nervous system is also a target of their action. Indeed, as here reported neuroactive steroids are physiological regulators of peripheral nerve functions and they may also represent interesting therapeutic tools for different types of peripheral neuropathy.

GABA and neuroactive steroid interactions in glia: new roles for old players?

In recent years it has becoming clear that glial cells of the central and peripheral nervous system play a crucial role from the earliest stages of development throughout adult life. Glial cells are important for neuronal plasticity, axonal conduction and synaptic transmission. In this respect, glial cells are able to produce, uptake and metabolize many factors that are essential for neuronal physiology, including classic neurotransmitters and neuroactive steroids. In particular, neuroactive steroids, which are mainly synthesized by glial cells, are able to modulate some neurotransmitter receptors affecting both glia and neurons. Among the signaling systems that are specialized for neuron-glial communication, we can include neurotransmitter GABA.The main focus of this review is to illustrate the cross-talk between neurons and glial cells in terms of GABA neurotransmission and actions of neuroactive steroids. To this purpose, we will review the presence of the different GABA receptors in the glial cells of the central and peripheral nervous system. Then, we will discuss their modulation by some neuroactive steroids.

GABA synthesis in Schwann cells is induced by the neuroactive steroid allopregnanolone

Recent evidence showed that neurotransmitters are synthesised in glial cells, such as the Schwann cells, which form myelin sheaths in the PNS. While the presence of GABA type A (GABA-A) receptors has been previously demonstrated in these cells, the evidence of GABA synthesis remained still elusive. In an attempt to demonstrate the presence of GABA in rat Schwann cells, we adopted a strategy, using several integrated neurochemical, molecular as well as immunocytochemical approaches. We first demonstrated the presence of glutamic acid decarboxylase of 67 kDa (GAD67) in Schwann cells, a crucial enzyme of the GABA synthesis mechanism. Second, we demonstrated that GABA is synthesized and localized in Schwann cells. As the third step we showed that allopregnanolone (10 nM), a potent allosteric modulator of GABA-A receptors, stimulates GABA synthesis through increased levels of GAD67 in Schwann cells. Analysis of intracellular signalling mechanisms revealed that the protein kinase A pathway, through enhanced cAMP levels and cAMP response element binding protein phosphorylation, modulates the allosteric action of allopregnanolone at the GABA-A receptor in Schwann cells. Our findings are the first to demonstrate that this GABA mechanism is active in Schwann cells thus establishing new potential therapeutic targets to control Schwann cell biology, which may prove useful in the treatment of several neurodegenerative disorders.

Seasonal rewiring of the songbird brain: an in vivo MRI study

The song control system (SCS) of songbirds displays a remarkable plasticity in species where song output changes seasonally. The mechanisms underlying this plasticity are barely understood and research has primarily been focused on the song nuclei themselves, largely neglecting their interconnections and connections with other brain regions. We investigated seasonal changes in the entire brain, including the song nuclei and their connections, of nine male starlings (Sturnus vulgaris). At two times of the year, during the breeding (April) and nonbreeding (July) seasons, we measured in the same subjects cellular attributes of brain regions using in vivo high-resolution diffusion tensor imaging (DTI) at 7 T. An increased fractional anisotropy in the HVC-RA pathway that correlates with an increase in axonal density (and myelination) was found during the breeding season, confirming multiple previous histological reports. Other parts of the SCS, namely the occipitomesencephalic axonal pathway, which contains fiber tracts important for song production, showed increased fractional anisotropy due to myelination during the breeding season and the connection between HVC and Area X showed an increase in axonal connectivity. Beyond the SCS we discerned fractional anisotropy changes that correlate with myelination changes in the optic chiasm and axonal organization changes in an interhemispheric connection, the posterior commissure. These results demonstrate an unexpectedly broad plasticity in the connectivity of the avian brain that might be involved in preparing subjects for the competitive and demanding behavioral tasks that are associated with successful reproduction.

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.

Neuroactive steroids and peripheral neuropathy

Peripheral neuropathy, either inherited or acquired, represents a very common disorder for which effective clinical treatments are not available yet. Observations here summarized indicate that neuroactive steroids, such as progesterone, testosterone and their reduced metabolites, might represent a promising therapeutic option. Peripheral nerves are able to synthesize and metabolize neuroactive steroids and are a target for these molecules, since they express classical and non-classical steroid receptors. Neuroactive steroids modulate the expression of key transcription factors for Schwann cell function, regulate Schwann cell proliferation and promote the expression of myelin proteins involved in the maintenance of myelin multilamellar structure, such as myelin protein zero and peripheral myelin protein 22. These actions may result in the protection and regeneration of peripheral nerves affected by different forms of pathological alterations. Indeed, neuroactive steroids are able to counteract biochemical, morphological and functional alterations of peripheral nerves in different experimental models of neuropathy, including the alterations caused by aging, diabetic neuropathy and physical injury. Therefore, neuroactive steroids, pharmacological agents able to increase their local synthesis and synthetic ligands for their receptors have a promising potential for the treatment of different forms of peripheral neuropathy.

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.

GABA receptor-mediated effects in the peripheral nervous system: A cross-interaction with neuroactive steroids

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the adult mammalian central nervous system (CNS), exerts its action via an interaction with specific receptors (e.g., GABAA and GABAB). These receptors are expressed not only in neurons but also on glial cells of the CNS, which might represent a target for the allosteric action of neuroactive steroids. Herein, we have demonstrated first that in the peripheral nervous system (PNS), the sciatic nerve and myelin-producing Schwann cells express both GABAA and GABAB receptors. Specific ligands, muscimol and baclofen, respectively, control Schwann-cell proliferation and expression of some specific myelin proteins (i.e., glycoprotein P0 and peripheral myelin protein 22 [PMP22]). Moreover, the progesterone (P) metabolite allopregnanolone, acting via the GABAA receptor, can influence PMP22 synthesis. In addition, we demonstrate that P, dihydroprogesterone, and allopregnanolone influence the expression of GABAB subunits in Schwann cells. The results suggest, at least in the myelinating cells of the PNS, a cross-interaction within the GABAergic receptor system, via GABAA and GABAB receptors and neuroactive steroids.

Peripheral nerves: a target for the action of neuroactive steroids

Peripheral nervous system possesses both classical and non-classical steroid receptors and consequently may represent a target for the action of neuroactive steroids. The present review summarizes the state of art of this intriguing field of research reporting data which indicate that neuroactive steroids, like for instance progesterone, dihydroprogesterone, tetrahydroprogesterone, dihydrotestosterone and 3alpha-diol, stimulate the expression of two important proteins of the myelin of peripheral nerves, the glycoprotein P0 (P0) and the peripheral myelin protein 22 (PMP22). Interestingly, the mechanisms by which neuroactive steroids exert their effects involve classical steroid receptors, like for instance progesterone and androgen receptors, in case of P0 and non-classical steroid receptors, like GABA(A) receptor, in case of PMP22. Moreover, neuroactive steroids not only control the expression of these specific myelin proteins, but also influence the morphology of myelin sheaths and axons suggesting that these molecules may represent an interesting new therapeutic approach to maintain peripheral nerve integrity during neurodegenerative events.