Neurons regulate Schwann cell genes by diffusible molecules

Autophagy-targeting modulation to promote peripheral nerve regeneration

Nerve regeneration following traumatic peripheral nerve injuries and neuropathies is a complex process modulated by diverse factors and intricate molecular mechanisms. Past studies have focused on factors that stimulate axonal outgrowth and myelin regeneration. However, recent studies have highlighted the pivotal role of autophagy in peripheral nerve regeneration, particularly in the context of traumatic injuries. Consequently, autophagy-targeting modulation has emerged as a promising therapeutic approach to enhancing peripheral nerve regeneration. Our current understanding suggests that activating autophagy facilitates the rapid clearance of damaged axons and myelin sheaths, thereby enhancing neuronal survival and mitigating injury-induced oxidative stress and inflammation. These actions collectively contribute to creating a favorable microenvironment for structural and functional nerve regeneration. A range of autophagy-inducing drugs and interventions have demonstrated beneficial effects in alleviating peripheral neuropathy and promoting nerve regeneration in preclinical models of traumatic peripheral nerve injuries. This review delves into the regulation of autophagy in cell types involved in peripheral nerve regeneration, summarizing the potential drugs and interventions that can be harnessed to promote this process. We hope that our review will offer novel insights and perspectives on the exploitation of autophagy pathways in the treatment of peripheral nerve injuries and neuropathies.

New Frontiers in Peripheral Nerve Regeneration: Concerns and Remedies

Topical advances in studying molecular and cellular mechanisms responsible for regeneration in the peripheral nervous system have highlighted the ability of the nervous system to repair itself. Still, serious injuries represent a challenge for the morphological and functional regeneration of peripheral nerves, calling for new treatment strategies that maximize nerve regeneration and recovery. This review presents the canonical view of the basic mechanisms of nerve regeneration and novel data on the role of exosomes and their transferred microRNAs in intracellular communication, regulation of axonal growth, Schwann cell migration and proliferation, and stromal cell functioning. An integrated comprehensive understanding of the current mechanistic underpinnings will open the venue for developing new clinical strategies to ensure full regeneration in the peripheral nervous system.

Review : Axons, Schwann Cells, and Neurotrophic Factors

As most elegantly confirmed by the recent success in deriving mice with null mutations in the genes for specific neurotrophic factors or their respective receptors, it is clear that neurotrophic factors alone or in combination are essential for the development of many classes of neurons. Specific neurotrophic factors have now been characterized that have actions on primary sensory afferents, sympathetic and parasym pathetic neurons, and motor neurons—the major contributors to the axon bundles that comprise the periph eral nervous system. The peripheral tissues or "end organs" that these neurons innervate have traditionally been thought of as the key source of neurotrophic factor support, but it is now evident that this "target- derived neurotrophic factor hypothesis" has restricted validity. Rather, the totality of neurotrophic support required to promote the survival, maturation, and maintenance of a neuron appears to be derived not only from targets, but also from support cells and possibly even neurons themselves. In this article, we review the role played by multiple sources of neurotrophic factors, especially factors derived from non-neuronal cells, not only in development, but also in the maintenance and regenerative responses of the adult PNS. In par ticular, we focus on neurotrophic factors of the neurotrophin family and ciliary neurotrophic factor. The Neuro scientist 1:192-199, 1995

■ REVIEW : Gene Expression in Nerve Regeneration

Injury of peripheral nerve in mammals leads to a complex but stereotypical pattern of histological events that comprise a highly reproducible sequence of degenerative reactions (Wallerian degeneration) succeeded by regenerative responses. These reactions are based on a corresponding sequence of cellular and mo lecular interactions that, in turn, reflect the differential expression of specific genes with functions in nerve degeneration and repair. We report on more than 60 genes and their products that show a specific pattern of regulation following peripheral nerve lesion. The group of regulated genes encoding, e.g., transcription factors, growth factors and their receptors, cytokines, neuropeptides, myelin proteins and lipid carriers, and cytoskeletal proteins as well as extracellular matrix and cell adhesion molecules. We describe and compare the distinct time-courses and cellular origin of expression and further discuss established or putative mo lecular interrelationships and functions with respect to the contribution of these genes/gene products to the molecular regeneration program of the PNS. NEUROSCIENTIST 3:112-122, 1997

Gene expression profiling studies in regenerating nerves in a mouse model for CMT1X: uninjured Cx32-knockout peripheral nerves display expression profile of injured wild type nerves

X-linked Charcot-Marie-Tooth disease (CMT1X) is an inherited peripheral neuropathy caused by mutations in GJB1, the human gene for Connexin32 (Cx32). This present study uses Ilumina Ref8-v2 BeadArray to examine the expression profiles of injured and uninjured sciatic nerves at 5, 7, and 14 days post-crush injury (dpi) from Wild Type (WT) and Cx32-knockout (Cx32KO) mice to identify the genes and signaling pathways that are dysregulated in the absence of Schwann cell Cx32. Given the assumption that loss of Schwann cell Cx32 disrupts the regeneration and maintenance of myelinated nerve leading to a demyelinating neuropathy in CMT1X, we initially hypothesized that nerve crush injury would result in significant increases in differential gene expression in Cx32KO mice relative to WT nerves. However, microarray analysis revealed a striking collapse in the number of differentially expressed genes at 5 and 7 dpi in Cx32KO nerves relative to WT, while uninjured and 14 dpi time points showed large numbers of differentially regulated genes. Further comparisons within each genotype showed limited changes in Cx32KO gene expression following crush injury when compared to uninjured Cx32KO nerves. By contrast, WT nerves exhibited robust changes in gene expression at 5 and 7 dpi with no significant differences in gene expression by 14dpi relative to uninjured WT nerve samples. Taken together, these data suggest that the gene expression profile in uninjured Cx32KO sciatic nerve strongly resembles that of a WT nerve following injury and that loss of Schwann cell Cx32 leads to a basal state of gene expression similar to that of an injured WT nerve. These findings support a role for Cx32 in non-myelinating and regenerating populations of Schwann cells in normal axonal maintenance in re-myelination, and regeneration of peripheral nerve following injury. Disruption of Schwann cell-axonal communication in CMT1X may cause dysregulation of signaling pathways that are essential for the maintenance of intact myelinated peripheral nerves and to establish the necessary conditions for successful regeneration and remyelination following nerve injury.

Bone marrow stem cells in facial nerve regeneration from isolated stumps

INTRODUCTION

Severe lesions in the facial nerve may have extensive axonal loss and leave isolated stumps that impose technical difficulties for nerve grafting.

METHODS

We evaluated bone marrow stem cells (BMSC) in a silicone conduit for rat facial nerve regeneration from isolated stumps. Group A utilized empty silicone tubes; in groups B-D, the tube was filled with acellular gel; and, in groups C and D, undifferentiated BMSC (uBMSC) or Schwann-like cells differentiated from BMSC (dBMSC) were added, respectively. Compound muscle action potentials (CMAPs) were measured, and histology was evaluated.

RESULTS

Groups C and D had the highest CMAP amplitudes. Group C had shorter CMAP durations than groups A, B, and D. Distal axonal number and density were increased in group C compared with groups A and B.

CONCLUSIONS

Regeneration of the facial nerve was improved by both uBMSC and dBMSC in rats, yet uBMSC was associated with superior functional results.

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.

Transient lysosomal activation is essential for p75 nerve growth factor receptor expression in myelinated Schwann cells during Wallerian degeneration

Myelinated Schwann cells in the peripheral nervous system express the p75 nerve growth factor receptor (p75NGFR) as a consequence of Schwann cell dedifferentiation during Wallerian degeneration. p75NGFR has been implicated in the remyelination of regenerating nerves. Although many studies have shown various mechanisms underlying Schwann cell dedifferentiation, the molecular mechanism contributing to the re-expression of p75NGFR in differentiated Schwann cells is largely unknown. In the present study, we found that lysosomes were transiently activated in Schwann cells after nerve injury and that the inhibition of lysosomal activation by chloroquine or lysosomal acidification inhibitors prevented p75NGFR expression at the mRNA transcriptional level in an ex vivo Wallerian degeneration model. Lysosomal acidification inhibitors suppressed demyelination, but not axonal degeneration, thereby suggesting that demyelination mediated by lysosomes may be an important signal for inducing p75NGFR expression. Tumor necrosis factor-α (TNF-α) has been suggested to be involved in regulating p75NGFR expression in Schwann cells. In this study, we found that removing TNF-α in vivo did not significantly suppress the induction of both lysosomes and p75NGFR. Thus, these findings suggest that lysosomal activation is tightly correlated with the induction of p75NGFR in demyelinating Schwann cells during Wallerian degeneration.

Hyaluronan tetrasaccharide promotes regeneration of peripheral nerve: in vivo analysis by film model method

Hyaluronan (HA) is known to inhibit neurons from regenerating in the central nervous system. However, hyaluronan tetrasaccharide (HA4) was found in in vitro experiments to promote outgrowth of neurons. To investigate the promotion by HA4 of nerve regeneration in vivo, we analyzed outgrowth of regenerating axons treated with HA4, using a film model method. After the common peroneal nerve in mice was transected, the proximal end of cut nerve was placed on a sheet of thin plastic film, immersed in several drops of HA4 solution, covered with another sheet of film, and then kept in vivo. Six hours after the procedure, terminal sprouts had grown out from ending bulbs formed at the cut end of parent nerve administered with HA4 solution 100 or 1000 μg/mL, while no sprouts were observed in groups treated with 10 μg/mL of HA4 or in controls. On the 2nd day after axotomy (day 2), many regenerating axons in the group treated with 100 μg/mL of HA4 extended onto the flat film for a longer distance than those treated with 1000 μg/mL of HA4 and controls. With the optimal dose of HA4 (100 μg/mL), axonal outgrowth was significantly (p<0.01) greater than that in controls at each time point. Schwann cells appeared migrating from parent nerve onto the film from day 3 as well as in controls. Thus, enhanced outgrowth of regenerating axons and normal behavior of migratory Schwann cells suggested that HA4 promoted regeneration of neurons without the mediation of Schwann cells.

Neurosteroids: synthesis and functions in the central and peripheral nervous systems

Some steroids are synthesized within the central and peripheral nervous systems, mostly by glial cells. These are known as neurosteroids. In the brain, neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3 alpha,5 alpha-reduced metabolite 3 alpha,5 alpha-tetrahydroprogesterone (allopregnanolone) activates the type A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rat brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phophodiesterase (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct precursor pregnenolone are synthesized by Schwann cells. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

‘Cross-talk’ between Schwannian stroma and neuroblasts promotes neuroblastoma tumor differentiation and inhibits angiogenesis

Neuroblastoma (NB) tumors with abundant Schwannian stroma have a differentiated phenotype, low vascularity, and are associated with a favorable prognosis. These observations have led to the hypothesis that 'cross-talk' between Schwann cells and neuroblasts influences the biology and clinical behavior of NB tumors. In support of this hypothesis, laboratory studies have shown that factors secreted by Schwann cells are capable of promoting NB differentiation, inhibiting angiogenesis, and impairing NB growth. Recently, using a novel NB xenograft model that was designed to directly investigate the affects of infiltrating Schwann cells, we demonstrated that infiltrating mouse Schwann cells can directly impact the phenotype of human NB xenografts in vivo. Taken together, these studies indicate that tumor-stroma interactions are critical in determining the biology of NB tumors. Further research investigating the molecules involved in the 'cross-talk' between Schwann cells and neuroblasts may lead to new treatment strategies that will modify tumor biology and alter the clinically aggressive nature of Schwannian stroma-poor NB tumors.

PRP-1 Protective Effect against Central and Peripheral Neurodegeneration following n. ischiadicus Transection

We investigated the action of the new hypothalamic proline-rich peptide (PRP-1), normally produced by neurosecretory cells of hypothalamic nuclei (NPV and NSO), 3 and 4 weeks following rat sciatic nerve transection. The impulse activity flow of interneurons (IN) and motoneurons (MN) on stimulation of mixed (n. ischiadicus), flexor (n. gastrocnemius--G) and extensor (n. peroneus communis--P) nerves of both injured and symmetric intact sides of spinal cord (SC) was recorded in rats with daily administration of PRP-1 (for a period of 3 weeks) and without it (control). On the injured side of SC in control, there were no responses of IN and MN on ipsilateral G and P stimulation, while responses were elicited on contralateral nerve stimulation. The neuron responses on the intact side of SC were revealed in a reverse ratio. Thus, there were no effects upon stimulation of the injured nerve distal stump in the control because of the absence of fusion between transected nerve stumps. This was also testified by the atrophy of the distal stump and the absence of motor activity of the affected limb. In PRP-1-treated animals, the responses of SC IN and MN in postaxotomy 3 weeks on the injured side of SC at ipsilateral nerve stimulation and on the intact side at contralateral nerve stimulation were recorded because of the obvious fusion of the severed nerve stumps. The histochemical data confirmed the electrophysiological findings. Complete coalescence of transected fibers together with restoration of the motor activity of the affected limb provided evidence for reinnervation on the injured side. Thus, it may be concluded that PRP-1 promotes nerve regeneration and may be used clinically to improve the outcome of peripheral nerve primary repair.

Schwann cells and endoneurial extracellular matrix molecules as potential cues for sorting of regenerated axons: a review

Besides very well elaborated microsurgical management of severed peripheral nerves, the clinical results of functional recovery following surgical repair of mixed nerves are disappointing. An improvement of functional recovery after peripheral nerve lesion requires the accurate regeneration of axons to their original target tissues and structures. Therefore, better clinical results could be obtained by a greater understanding of the cellular and molecular biology of selective nerve regeneration. The studies concerning Schwann cells and their endoneurial extracellular matrix as potent cues for selective promotion and influence of regenerating motor and sensory axons are reviewed. Knowledge of the sorting mechanisms of regenerated motor and sensory axons is needed not only for improvement of functional recovery, but also for the development of biocompatible nerve prostheses.

Rapid axoglial signaling mediated by neuregulin and neurotrophic factors

During peripheral nervous system development, Schwann cells are precisely matched to the axons that they support. This is mediated by axonal neuregulins that are essential for Schwann cell survival and differentiation. Here, we show that sensory and motor axons rapidly release heparin-binding forms of neuregulin in response to Schwann cell-derived neurotrophic factors in a dose-dependent manner. Neuregulin release occurs within minutes, is saturable, and occurs from axons that were isolated using a newly designed chamber slide apparatus. Although NGF and glial cell line-derived neurotrophic factor (GDNF) were the most potent neurotrophic factors to release neuregulin from sensory neurons, GDNF and BDNF were most potent for motor neurons and were the predominant neuregulin-releasing neurotrophic factors produced by cultured Schwann cells. Comparable levels of neuregulin could be released at a similar rate from neurons after protein kinase C activation with the phorbol ester, phorbol 12-myristate 13-acetate, which has also been shown to promote the cleavage and release of neuregulin from its transmembrane precursor. The rapid release of neuregulin from axons in response to Schwann cell-derived neurotrophic factors may be part of a spatially restricted system of communication at the axoglial interface important for proper peripheral nerve development, function, and repair.

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.

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.

Expression of protein zero is increased in lesioned axon pathways in the central nervous system of adult zebrafish

The immunoglobulin superfamily molecule protein zero (P0) is important for myelin formation and may also play a role in adult axon regeneration, since it promotes neurite outgrowth in vitro. Moreover, it is expressed in the regenerating central nervous system (CNS) of fish, but not in the nonregenerating CNS of mammals. We identified a P0 homolog in zebrafish. Cell type-specific expression of P0 begins in the ventromedial hindbrain and the optic chiasm at 3-5 days of development. Later (at 4 weeks) expression has spread throughout the optic system and spinal cord. This is consistent with a role for P0 in CNS myelination during development. In the adult CNS, glial cells constitutively express P0 mRNA. After an optic nerve crush, expression is increased within 2 days in the entire optic pathway. Expression peaks at 1 to 2 months and remains elevated for at least 6 months postlesion. After enucleation, P0 mRNA expression is also upregulated but fails to reach the high levels observed in crush-lesioned animals at 4 weeks postlesion. Spinal cord transection leads to increased expression of P0 mRNA in the spinal cord caudal to the lesion site. The glial upregulation of P0 mRNA expression after a lesion of the adult zebrafish CNS suggests roles for P0 in promoting axon regeneration and remyelination after injury.

Central and peripheral nerve regeneration by transplantation of Schwann cells and transdifferentiated bone marrow stromal cells

In contrast to the peripheral nervous system (PNS), little structural and functional regeneration of the central nervous system (CNS) occurs spontaneously following injury in adult mammals. The inability of the CNS to regenerate is mainly attributed to its own inhibitorial environment such as glial scar formation and the myelin sheath of oligodendrocytes. Therefore, one of the strategies to promote axonal regeneration of the CNS is to experimentally modify the environment to be similar to that of the PNS. Schwann cells are the myelinating glial cells in the PNS, and are known to play a key role in Wallerian degeneration and subsequent regeneration. Central nervous system regeneration can be elicited by Schwann cell transplantation, which provides a suitable environment for regeneration. The underlying cellular mechanism of regeneration is based upon the cooperative interactions between axons and Schwann cells involving the production of neurotrophic factors and other related molecules. Furthermore, tight and gap junctional contact between the axon and Schwann cell also mediates the molecular interaction and linking. In this review, the role of the Schwann cell during the regeneration of the sciatic (representing the PNS) and optic (representing the CNS) nerves is explained. In addition, the possibility of optic nerve reconstruction by an artificial graft of Schwann cells is also described. Finally, the application of cells not of neuronal lineage, such as bone marrow stromal cells (MSCs), in nerve regeneration is proposed. Marrow stromal cells are known as multipotential stem cells that, under specific conditions, differentiate into several kinds of cells. The strategy to transdifferentiate MSCs into the cells with a Schwann cell phenotype and the induction of sciatic and optic nerve regeneration are described.

Early activation of transcription factor expression in Schwann cells by progesterone

Progesterone (PROG) promotes the myelination of sciatic nerves during regeneration after cryolesion. But, little is known about the molecular mechanisms by which the hormone exerts its effects. This could be initiated by the regulation of transcription factor expression in Schwann cells, which produce the myelin sheaths in the peripheral nervous system. We investigated by RT-PCR whether PROG activated expression of transcription factors: Egr-1 (Krox-24) Egr-2 (Krox-20), Egr-3, c-jun, jun B, jun D, c-Fos, Fos B, Fra-1, Fra-2, CREB, ATF 4, SCIP and Sox-10 in cultured Schwann cells. PROG triggered a quick (visible as soon as 15 min), strong (6 to 18-fold) and transient (1-2 h) stimulation of Egr-1, Egr-2, Egr-3 and Fos B genes expression. Expression of other genes remained unaffected by PROG treatment. The same expression pattern was obtained in the MSC 80 line (mouse Schwann cells), but not in the NIH-3T3 and CHO lines. Estradiol and testosterone induced different patterns of transcription factor gene activation in Schwann cells. Serum stimulated all genes activated by PROG in addition c-fos, fra-1 and fra-2. The PROG effects were blocked by Actinomycin D and by RU 486. This suggests that the activation of these genes occurs at the transcriptional level via the interaction of the hormone with its cognate receptor. Thus, PROG can regulate Schwann cell functions and differentiation by transiently activating specific transcription factors.

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.

cAMP is involved in the differentiation of human teratocarcinoma cells

The generation of fully differentiated post-mitotic human neuronal cells from stem cells (human teratocarcinoma (hNT2) cells) might enable the development of a co-culture model of human neurons with human Schwann cells (SCs). This co-culture model is an important tool to study formation of myelin sheaths. However, the thin process of the post-mitotic human neuronal cells formed under known culture conditions do not provide a good substrate for human SCs to start myelination. We optimized the culture conditions of these cells to obtain axons with a larger diameter. Western blotting and immunofluorescence studies were performed to confirm the neuronal status of the cells and diameter of the processes. In this study, we show that addition of cAMP-inducing factors to hNT2 cells resulted in rapid morphological changes including the development of processes with a larger diameter.

Synthesis of progesterone in Schwann cells: regulation by sensory neurons

In peripheral nerves, progesterone synthesized by Schwann cells has been implicated in myelination. In spite of such an important function, little is known of the regulation of progesterone biosynthesis in the nervous system. We show here that in rat Schwann cells, expression of the 3 beta-hydroxysteroid dehydrogenase and formation of progesterone are dependent on neuronal signal. Levels of 3 beta-hydroxysteroid dehydrogenase mRNA and synthesis of [3H]progesterone from [3H]pregnenolone were low in purified Schwann cells prepared from neonatal rat sciatic nerves. However, when Schwann cells were cultured in contact with sensory neurons, both expression and activity of the 3 beta-hydroxysteroid dehydrogenase were induced. Regulation of 3 beta-hydroxysteroid dehydrogenase expression by neurons was also demonstrated in vivo in the rat sciatic nerve. 3 beta-hydroxysteroid dehydrogenase mRNA was present in the intact nerve, but could no longer be detected 3 or 6 days after cryolesion, when axons had degenerated. After 15 days, when Schwann cells made new contact with the regenerating axons, the enzyme was re-expressed. After nerve transection, which does not allow axonal regeneration, 3 beta-hydroxysteroid dehydrogenase mRNA remained undetectable. The regulation of 3 beta-hydroxysteroid dehydrogenase mRNA after lesion was similar to the regulation of myelin protein zero (P0) and peripheral myelin protein 22 (PMP22) mRNAs, supporting an important role of locally formed progesterone in myelination.

Reciprocal interactions between perisynaptic Schwann cells and regenerating nerve terminals at the frog neuromuscular junction

The perisynaptic Schwann cell (PSC) has gained recent attention with respect to its roles in synaptic function, remodeling, and regeneration at the vertebrate neuromuscular junction (NMJ). Here we test the hypothesis that, following nerve injury, processes extended by PSCs guide regenerating nerve terminals (NTs) in vivo, and that the extension of sprouts by PSCs is triggered by the arrival of regenerating NTs. Frog NMJs were double-stained with a fluorescent dye, FM4-64, for NTs, and fluorescein isothiocyanate (FITC)-tagged peanut agglutinin (PNA) for PSCs. Identified NMJs were imaged in vivo repeatedly for several months after nerve injury. PSCs sprouted profusely beginning 3-4 weeks after nerve transection and, as reinnervation progressed, regenerating NTs closely followed the preceding PSC sprouts, which could extend tens to hundreds of microns beyond the original synaptic site. The pattern of reinnervation was dictated by PSC sprouts, which could form novel routes joining neighboring junctions or develop into new myelinated axonal pathways. In contrast to mammals, profuse PSC sprouting in frog muscles was not seen in response to axotomy alone, and did not occur at chronically denervated NMJs. Instead, sprouting coincided with the arrival of regenerating NTs. Immunofluorescent staining revealed that in muscle undergoing reinnervation 4 weeks after axotomy, 91% of NMJs bore PSC sprouts, compared to only 6% of NMJs in muscle that was chronically denervated for 4 weeks. These results suggest that reciprocal interactions between regenerating NTs and PSCs govern the process of reinnervation at frog NMJs: regenerating NTs induce PSCs to sprout, and PSC sprouts, in turn, lead and guide the elaboration of NTs.

Progesterone synthesized by Schwann cells during myelin formation regulates neuronal gene expression

Previously, progesterone was found to regulate the initiation and biosynthetic rate of myelin synthesis in Schwann cell/neuronal cocultures. The mRNA for cytochrome P450scc (converts cholesterol to pregnenolone), 3beta-hydroxysteroid dehydrogenase (3beta-HSD, converts pregnenolone to progesterone), and the progesterone receptor were found to be markedly induced during active myelin synthesis. However, the cells in the cocultures responsible for these changes were not identified. In this study, in situ hybridization was used to determine the localization of the enzymes responsible for steroid biosynthesis. The mRNA for cytochrome P450scc and 3beta-HSD were detected only in actively myelinating cocultures and were localized exclusively in the Schwann cells. Using immunocytochemistry, with minimal staining of the Schwann cells, we found the progesterone receptor in the dorsal root ganglia (DRG) neurons. The progesterone receptor in the neurons translocated into the nuclei of these cells when progesterone was added to neuronal cultures or during myelin synthesis in the cocultures. Additionally, a marked induction of the progesterone receptor was found in neuronal cultures after the addition of progesterone. The induction of various genes in the neurons was also investigated using mRNA differential display PCR in an attempt to elucidate the mechanism of steroid action on myelin synthesis. Two novel genes were induced in neuronal cultures by progesterone. These genes, along with the progesterone receptor, were also induced in cocultures during myelin synthesis, and their induction was blocked by RU-486 (a progesterone receptor antagonist). These genes were not induced in Schwann cells cultured alone after the addition of progesterone. These results suggest that progesterone is synthesized in Schwann cells and that it can indirectly regulate myelin formation by activating transcription via the classical steroid receptor in the DRG neurons.

Signals for proinflammatory cytokine secretion by human Schwann cells

Wallerian degeneration is a post-traumatic process of the peripheral nervous system whereby damaged axons and their surrounding myelin sheaths are phagocytosed by infiltrating leukocytes. Our studies indicate that Schwann cells could initiate the process of Wallerian degeneration by releasing proinflammatory cytokines involved in leukocyte recruitment and differentiation including IL-1beta, MCP-1, IL-8 and IL-6. A comparison of the secretory pattern between nerve explants and cultured Schwann cells showed that each cytokine was differentially regulated by growth factor deprivation or axonal membrane fragments. Since Wallerian-like degeneration occurs in a wide variety of peripheral neuropathies, Schwann cell-mediated cytokine production may play an important role in many disease processes.

A protein with the characters of a zinc-finger is implicated in the differentiation of Schwann cells

During the development of the PNS, Schwann cells (SC) differentiate into myelinating and nonmyelinating cells, implying regulation by different transcription factors such as ZF proteins. Employing an original strategy using monoclonal antibodies specifically directed against the conserved ZF motif, we have identified a new ZF protein of 55 kDa present in rat sciatic nerve extract (SCp55). We used polyclonal antibodies and cloned cDNA to characterize the expression of SCp55 by immunohistochemistry and in situ hybridization. This protein is specific for SC and shows differential expression both during development and between the two SC phenotypes. When they differentiate the protein is first induced in myelinating SC and then in nonmyelinating SC. The nature of this protein together with its differential expression suggests that it is a transcription factor that may have a role in the development of SC.

Functional Ca2+ and Na+ channels on mouse Schwann cells cultured in serum-free medium: regulation by a diffusible factor from neurons and by cAMP

Regulation of expression of functional voltage-gated ion channels for inward currents was studied in Schwann cells in organotypic cultures of dorsal root ganglia from E19 mouse embryos maintained in serum-free medium. Of the Schwann cells that did not contact axons, 46.5% expressed T-type Ca2+ conductances (ICaT). Two days or more after excision of the ganglia, and consequent disappearance of neurites, ICaT were detectable in only 10.9% of the cells, and the marker 04 disappeared. On Schwann cells deprived of neurons, T- (but not L-) type Ca2+ conductances were re-induced by weakly hydrolysable analogues of cAMP, and by forskolin (an activator of adenylyl cyclase) after long-term treatment (4 days). With CPT cAMP (0.1-2 mM), 8Br cAMP, db cAMP or forskolin (0.01 or 0.1 mM), the proportion of cells with ICaT was not significantly different from the proportion in the cultures with neurons. These agents also induced expression in some cells of tetrodotoxin-resistant Na+ currents, which were rarely induced by neurons, but 04 was not re-induced by cAMP analogue treatments that re-induced ICaT. Inward currents (Ba2+ or Na+) were partly restored (P < 0.05) on Schwann cells cultured for 6-7 days beneath a filter bearing cultured neurons. In contrast, addition of neuron-conditioned medium was ineffective. The results suggest that neurons activate, via diffusible and degradable factors, a subset of Schwann cell cAMP pathways leading to expression of IcaT, and activate additional non-cAMP pathways that lead to expression of 04.

Selective inhibition of early axonal regeneration by myelin-associated glycoprotein

When the distal stump of a transected peripheral nerve is brought into the vicinity of the proximal nerve stump, the regenerating axons advance toward it across the gap. Similar results are obtained when a predegenerated nerve segment is used. However, when a nerve segment subjected to proximal axotomy 7 days earlier (7-day nerve segment) was placed close to the proximal end of a freshly cut nerve at a distance of less than 1.5 mm, there were neither regenerating axons nor sprouts. The same inhibition of axonal regeneration was also exhibited when a nerve segment subjected to axotomy 9 to 14 days earlier was used. To examine the inhibitory effect of the nerve segments on established regenerating axons, we positioned a 7-day nerve segment in close apposition to a proximal nerve end at 2 or 3 days after transection. The growth of the 3-day-old regenerating axons, already ensheathed by Schwann cells, was not disturbed, but the 2-day-old regenerating axons, consisting of naked axons, were eliminated by the 7-day nerve segment. It is assumed that the findings reflect a mechanism serving to eliminate abundant sprouts and immature axons, probably conferring optimum regeneration and maturation of outgrowing pioneer axons. The inhibitory effect on abundant sprouts and immature axons was completely blocked by local application of antibodies to myelin-associated glycoprotein (MAG). The MAG-containing cells appeared at 6 to 12 days after axotomy.

Neurosteroids: expression of functional 3beta-hydroxysteroid dehydrogenase by rat sensory neurons and Schwann cells

Steroids which are synthesized within the nervous system, such as progesterone, have been termed 'neurosteroids'. Levels of progesterone are much larger in peripheral nerves of rats and mice than in plasma, and persist after removal of the steroidogenic endocrine glands. Schwann cells are a source of progesterone: when isolated from embryonic dorsal root ganglia, they can synthesize progesterone from pregnenolone, the obligate precursor of all steroids. Locally produced progesterone has been shown to play an important role in myelination of peripheral nerve. We show here that sensory neurons from embryonic dorsal root ganglia also express 3beta-hydroxysteroid dehydrogenase and can convert [3H]pregnenolone to [3H]progesterone. Moreover, when cultured under different conditions and incubated for 24 h in the presence of 100 nM [3H]pregnenolone, they produce 5-10 times more [3H]progesterone than Schwann cells. The conversion of pregnenolone to progesterone by neurons is further increased by a diffusible factor produced by Schwann cells. Sensory neurons can also metabolize progesterone to 5alpha-dihydroprogesterone, but unlike Schwann cells, they do not produce 3alpha,5alpha-tetrahydroprogesterone, a potent positive allosteric modulator of gamma-aminobutyric acid type A receptors. We also show that cells isolated from the adult nervous system still have the capacity to convert [3H]pregnenolone to progesterone and its 5alpha-reduced metabolites: neurons and Schwann cells purified from dorsal root ganglia of 6 week old male rats show a similar pattern of pregnenolone metabolism to cells isolated from 18 day old embryos. These findings further support the important role of progesterone in the development and regeneration of the peripheral nervous system.

Effects of delayed re-innervation on the expression of c-erbB receptors by chronically denervated rat Schwann cells in vivo

We propose that chronically denervated Schwann cells may be less able to respond to axonal signals than their acutely denervated counterparts, and that this lack of sensitivity may be one reason why axons fail to regenerate into chronically denervated nerve stumps. To test this proposal we have used in situ hybridization, and quantitative and qualitative immunohistochemistry to compare the expression of c-erbB2 and c-erbB4 receptors in Schwann cells denervated for up to 6 months in vivo, with that seen in Schwann cells denervated for similar periods of time but then exposed to regenerating axons. The results were correlated with the extent of axonal regeneration in each experimental group as assessed from transverse sections which had been double-immunolabelled using anti S-100 and anti-beta tubulin III antibodies. Since c-erbBs are receptors for neuronally derived neuregulins we probed the appropriate axotomised DRG neurons for expression of GGF2 mRNA. When the denervated distal stumps were anastomosed to acutely transected proximal stumps, GGF expression in DRGs increased transiently during the first week: we assume that secreted GGF2 derived from regrowing axon sprouts would have been available to Schwann cells in all distal stumps. Endoneurial cell proliferation (predominantly Schwann cell proliferation); levels of expression of c-erbB receptors by Schwann cells, and the degree to which axons regenerated into the distal stumps all decreased as the period of prior denervation increased: the longer the time of denervation, the lower the expression of c-erbBs in Schwann cells, and the smaller the percentage of bands of Bungner which were re-innervated.

HNMP-1: a novel hematopoietic and neural membrane protein differentially regulated in neural development and injury

The hnmp-1 (hematopoietic neural membrane protein) gene encodes a protein with striking similarity to the tetra-transmembrane-spanning protein encoded by pmp22. hnmp-1 was cloned from an elutriated human monocyte library and is expressed in various human hematopoietic and lymphoid lineages as well as adult mouse spleen and thymus. In the mouse nervous system, HNMP-1 mRNA is temporally expressed by Schwann cells during sciatic nerve myelination. Dorsal root ganglia sensory and spinal cord alpha-motoneurons acquire HNMP-1 protein selectively throughout development. In the fiber tracts of the spinal cord and in sciatic nerve, HNMP-1 protein is axon-associated. Additionally a rapid and sustained level of HNMP-1 expression is observed in response to acute PNS injury. HNMP-1 is constituitively induced in sciatic nerve of Trembler J mice, which are mutant for pmp22 and have a demyelinating/hypomyelinating phenotype. The expression pattern of HNMP-1 suggests a possible role for this molecule during active myelination.

HNMP-1: a novel hematopoietic and neural membrane protein differentially regulated in neural development and injury

The hnmp-1 (hematopoietic neural membrane protein) gene encodes a protein with striking similarity to the tetra-transmembrane-spanning protein encoded by pmp22. hnmp-1 was cloned from an elutriated human monocyte library and is expressed in various human hematopoietic and lymphoid lineages as well as adult mouse spleen and thymus. In the mouse nervous system, HNMP-1 mRNA is temporally expressed by Schwann cells during sciatic nerve myelination. Dorsal root ganglia sensory and spinal cord alpha-motoneurons acquire HNMP-1 protein selectively throughout development. In the fiber tracts of the spinal cord and in sciatic nerve, HNMP-1 protein is axon-associated. Additionally a rapid and sustained level of HNMP-1 expression is observed in response to acute PNS injury. HNMP-1 is constituitively induced in sciatic nerve of Trembler J mice, which are mutant for pmp22 and have a demyelinating/hypomyelinating phenotype. The expression pattern of HNMP-1 suggests a possible role for this molecule during active myelination.

Inducible receptors

While regulation of receptor function is known to occur at many levels (e.g. transcriptional, post-translational), it is generally perceived that a tissue either expresses or does not express a particular receptor in an all-or-none fashion. Many pathological (e.g. tissue injury) and physiological (e.g. angiogenesis) processes have, however, been shown to be associated with the transcriptional induction of specific receptors. Induced receptors are not confined to any particular class, but range from G protein-coupled receptors to receptor tyrosine kinases. The potential implications of de novo receptor expression are profound with respect to potential novel therapeutic targets in specific disease states. Further, this observation may explain unexpected side-effects in the pharmacotherapy of existing disease states. In this article Lucy Donaldson, Michael Hanley and Amparo Villablanca discuss circumstances under which de novo receptor induction has been described, potential mechanisms of induction and the implications for pharmacology.

Actin plays a role in both changes in cell shape and gene-expression associated with Schwann cell myelination

Schwann cell (SC) differentiation into a myelinating cell requires concurrent interactions with basal lamina and an axon destined for myelination. As SCs differentiate, they undergo progressive morphological changes and initiate myelin-specific gene expression. We find that disrupting actin polymerization with cytochalasin D (CD) inhibits myelination of SC/neuron co-cultures. Basal lamina is present, neurons are healthy, and the inhibition is reversible. Electron microscopic analysis reveals that actin plays a role at two stages of SC differentiation. At 0.75-1.0 microg/ml CD, SCs do not differentiate and appear as "rounded" cells in contact with axons. This morphology is consistent with disruption of actin filaments and cell shape changes. However, at 0.25 microg/ml CD, SCs partially differentiate; they elongate and segregate axons but generally fail to form one-to-one relationships and spiral around the axon. In situ hybridizations reveal that SCs in CD-treated cultures do not express mRNAs encoding the myelin-specific proteins 2',3'-cyclic nucleotide phosphodiesterase (CNP), myelin-associated glycoprotein (MAG), and P0. Our results suggest that at the lower CD dose, SCs commence differentiation as evidenced by changes in cell shape but are unable to elaborate myelin lamellae because of a lack of myelin-specific mRNAs. We propose that F-actin influences myelin-specific gene expression in SCs.

The cellular and molecular basis of peripheral nerve regeneration

Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.

Actin plays a role in both changes in cell shape and gene-expression associated with Schwann cell myelination

Schwann cell (SC) differentiation into a myelinating cell requires concurrent interactions with basal lamina and an axon destined for myelination. As SCs differentiate, they undergo progressive morphological changes and initiate myelin-specific gene expression. We find that disrupting actin polymerization with cytochalasin D (CD) inhibits myelination of SC/neuron co-cultures. Basal lamina is present, neurons are healthy, and the inhibition is reversible. Electron microscopic analysis reveals that actin plays a role at two stages of SC differentiation. At 0.75-1.0 microg/ml CD, SCs do not differentiate and appear as "rounded" cells in contact with axons. This morphology is consistent with disruption of actin filaments and cell shape changes. However, at 0.25 microg/ml CD, SCs partially differentiate; they elongate and segregate axons but generally fail to form one-to-one relationships and spiral around the axon. In situ hybridizations reveal that SCs in CD-treated cultures do not express mRNAs encoding the myelin-specific proteins 2',3'-cyclic nucleotide phosphodiesterase (CNP), myelin-associated glycoprotein (MAG), and P0. Our results suggest that at the lower CD dose, SCs commence differentiation as evidenced by changes in cell shape but are unable to elaborate myelin lamellae because of a lack of myelin-specific mRNAs. We propose that F-actin influences myelin-specific gene expression in SCs.

Axonal regeneration through acellular muscle grafts

The management of peripheral nerve injury remains a major clinical problem. Progress in this field will almost certainly depend upon manipulating the pathophysiological processes which are triggered by traumatic injuries. One of the most important determinants of functional outcome after the reconstruction of a transected peripheral nerve is the length of the gap between proximal and distal nerve stumps. Long defects (> 2 cm) must be bridged by a suitable conduit in order to support axonal regrowth. This review examines the cellular and acellular elements which facilitate axonal regrowth and the use of acellular muscle grafts in the repair of injuries in the peripheral nervous system.

The regulation of Krox-20 expression reveals important steps in the control of peripheral glial cell development

The zinc finger transcription factor gene Krox-20 is expressed in Schwann cells and is required for the myelination of peripheral nerves. We show that the regulation of Krox-20 expression in peripheral glial cells reveals three important steps in the development and differentiation of these cells. (i) Expression of Krox-20 in Schwann cells requires continuous neuronal signalling via direct axonal contact. Therefore Krox-20 appears to be a key component of the transduction cascade linking axonal signalling to myelination. (ii) Krox-20 inducibility is acquired by Schwann cells at the time that they are formed from their precursors. Diffusible factor(s) synthesised by the neural tube can mediate this transition and can be mimicked by NDFbeta or a combination of CNTF and bFGF. Furthermore, the neural tube activity is blocked by a hybrid protein containing the NDF-binding domain of the ErbB4 receptor, strongly implicating NDF in the physiological transition. (iii) In sensory ganglia, the microenvironment is capable of negatively regulating Krox-20, presumably by preventing the conversion of satellite glial cells toward a Schwann cell-like phenotype.

Neurofilament phosphorylation is modulated by myelination

Axons undergo substantial changes in radial growth during the course of development. Recent evidence suggests that axonal diameter may be controlled by the state of neurofilament (NF) phosphorylation. Using dorsal root ganglion (DRG)-Schwann cell co-cultures, we provide direct evidence that phosphorylation of NF is regulated by myelination. NF phosphorylation increased upon myelination of DRG neurons by Schwann cells. The increase in NF phosphorylation was reflected both as an increase in immunoreactivity with the antibody SMI31, specific for phosphorylation-dependent NF epitopes, and a concomitant decrease in immunoreactivity with SMI32, specific for nonphosphorylated NF epitopes. The increase in NF phosphorylation induced by myelination in the neuron-glia co-cultures was similar to NF phosphorylation seen in sciatic nerve extracts of mice with normal myelination compared to Trembler J mouse littermates in which myelination of peripheral nerves is compromised. Using an in situ gel kinase assay, we have detected changes in individual NF kinase activities during myelination. In particular, a 35-kDa kinase activity was induced by myelination, whereas a 42-kDa kinase decreased in activity. We discuss the possibility that these and other kinases may be involved in signaling processes between neurons and glia during myelination.

Progesterone as a neurosteroid: actions within the nervous system

1. Some progesterone is synthesized within both the central and the peripheral nervous systems, where it regulates neurotransmission and important glial functions, such as the formation of myelin. Progesterone can thus be designated a "neurosteroid." 2. Steroids act not only on the brain, but also on peripheral nerves, which offer many advantages to study the biological significance of locally produced neurosteroids: their remarkable plasticity and regenerative capacity and their relatively simple structure. 3. By using the regenerating mouse sciatic nerve as a model, we have shown that progesterone synthesized by rat Schwann cells promotes the formation of new myelin sheaths. Progesterone also increases the number of myelinated axons when added at a low concentration to cocultures of Schwann cells and sensory neurons. 4. These findings show a function on myelination for locally produced progesterone and suggest a new pharmacological approach of myelin repair.

Cultured Schwann cells constitutively express the myelin protein P0

It is widely thought that mammalian Schwann cells do not express Po, the major glycoprotein in peripheral myelin, unless they are induced to do so by axonal signals that can be mimicked by agents that trigger cAMP signaling pathways. In contrast, we find that cultured Schwann cells make large amounts of Po without the addition of any axonal-like signal, provided they have not been exposed to serum during the culture process. We also report that glial growth factor/neuregulin inhibits this constitutive Po expression. Myelin basic protein is regulated in a similar way. We suggest that expression of Po by Schwann cells before the onset of myelination may be prevented by inhibitory signals within the nerve, rather than by the absence of a positive signal from axons.

Progesterone synthesis and myelin formation by Schwann cells

Progesterone is shown here to be produced from pregnenolone by Schwann cells in peripheral nerves. After cryolesion of the sciatic nerve in male mice, axons regenerate and become myelinated. Blocking either the local synthesis or the receptor-mediated action of progesterone impaired remyelination. Administration of progesterone or its precursor, pregnenolone, to the lesion site increased the extent of myelin sheath formation. Myelination of axons was also increased when progesterone was added to cultures of rat dorsal root ganglia. These observations indicate a role for locally produced progesterone in myelination, demonstrate that progesterone is not simply a sex steroid, and suggest a new therapeutic approach to promote myelin repair.