Role of progesterone in peripheral nerve repair.

Progesterone is synthesized in the peripheral nervous system in glial cells. The functions of progesterone are indicated by the findings that it stimulates neurite outgrowth from dorsal root ganglia sensory neurones in explant cultures, accelerates the maturation of the regenerating axons in cryolesioned sciatic nerve, and enhances the remyelination of regenerated nerve fibres. The formation of myelin sheaths around axons is a sexually dimorphic process, as the sheaths are thicker in female than in male regenerating nerves. The progesterone-induced myelination is probably mediated by progesterone receptors, as it is impaired by mifepristone (RU486), a progesterone antagonist. The stimulation of neurite growth in the peripheral nervous system may be mediated by a progesterone metabolite, 5alpha-tetrahydroprogesterone, through GABA(A) receptors.

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.

In vivo proliferative pattern of trembler hypomyelinating Schwann cells is modified in culture: an experimental analysis.

Trembler mouse, a Schwann cell mutation, is characterized by severe hypomyelination of peripheral nerves, high Schwann cell proliferation and the presence of a multilayered basal lamina which surrounds them. In contrast with their continuous in vivo division, mutant Schwann cells prepared from 15-day sciatic nerves display a lower proliferation rate in cell culture than normal Schwann cells. However, quiescent Trembler Schwann cells are still able to respond, as normal Schwann cells, to exogenous mitogens, such as nerve extracts and myelin-enriched fractions. In addition, both normal and Trembler Schwann cells proliferate in response to Trembler serum. Fibroblast growth factor is not the mitogenic factor which stimulates mutant Schwann cell proliferation in vivo, since it is absent in Trembler serum and poorly concentrated in Trembler adult sciatic nerves. Our results suggest that, in vivo, the serum of Trembler mouse probably contains mitogenic factors, not yet characterized, which may trigger the permanent division of mutant Schwann cells, in contrast to the quiescent state of these cells in the nerves of normal mice.

Alteration of ethanolamine glycerophospholipid turnover in trembler dysmyelinating mutant: An analysis of the sciatic nerve by biochemistry and radioautography.

The turnover of phospholipids was compared in peripheral nerves of Trembler dysmelinating mutant and control mice, after intraperitoneal and local injection of labeled ethanolamine. In the mutant sciatic nerve, neurochemical analysis showed that [(14)C]ethanolamine is incorporated into EGP (ethanolamine glycerophospholipids) of the sciatic nerve at a much higher rate in Trembler mutant than in control mice. Furthermore the decay rate of (14)C-labeled EGP is faster in Trembler than in normal animals. The accelerated turnover of EGP in Trembler sciatic nerve affects the diacyl-EGP while the renewal of the alkenylacyl-EGP (plasmalogens) is slower than in controls. Quantitative radioautographic study at the ultrastructural level corroborate that the initial increase of the label in Trembler nerve fibers was different in axons, Schwann cells and myelin sheaths. EM radioautographs reveal indeed that the high label content observed in Trembler axons takes place preferentially in the myelinated portions of axons and drops within 1 week. In both myelinated and unmyelinated segments of the axons, the majority of the radioactivity was contained in axolemma and smooth axoplasmic reticulum. The 10-fold increase of label found in the myelin sheath of Trembler nerve fibers at 1 day raises the question of the origin of the labeled EGP, either by a stimulated synthesis in Schwann cells or by transfer from axonally transported phospholipids. In contrast, the label of axons, Schwann cells and myelin sheaths of control nerve remains stable during the same period.

In vitro evidence for a neurite growth-promoting activity in Trembler mouse serum.

Basal lamina components, such as heparan sulfate proteoglycan (HSPG) and laminin play an important role in neuritic outgrowth for CNS and PNS neurons in culture. The mutant mouse 'Trembler' is characterized by hypomyelinization and production of an excess of basal lamina layers around Schwann cells in peripheral nerves. In order to analyse whether or not the serum of the mutant animals contains neurite outgrowth-promoting factors, we cultured rat spinal cord neurons in the presence of Trembler serum. Under these conditions, the outgrowth of neurites was increased approx. 2 times as compared to control serum. Trembler serum induces neuritic outgrowth characterized both by an increase in number of primary neurites emerging from the nerve cell body as well as by an increase in peripheral branching of neurites. To characterize the factors implicated in this increase we added antibodies directed against HSPG or laminin to the mutant serum. As a result, the increase in neuritic outgrowth was reduced or abolished in both cases. Trembler effect on neurite growth disappeared when the number of the non-neuronal cells was reduced, suggesting that the mutant serum did not act directly on neurons but by the intermediary action of non-neuronal cells.

Acetylcholine receptors and acetylcholinesterase activity in soleus muscle of trembler dysmyelinating mutant: a cytochemical and biochemical analysis.

We performed comparative biochemical and morphological studies of trembler and control soleus muscles. In the mutant, small multiple endplates were observed on some muscle fibers. The acetylcholine receptor (AChR) concentration and the acetylcholinesterase (AChE) activity of the muscle were not modified in the mutant. Our results suggest that both AChR and AChE levels are similar in trembler and control soleus but that these molecules are localized differently in the sarcolemma of the mutant muscles.

[Motor innervation of the Trembler mouse]. / Innervation motrice de la souris Trembler.

The effects on muscle fibres and particularly on their motor end-plates, by axons whose Schwann cells fail to form and maintain a normal myelin sheath, were examined by morphological and biochemical techniques. In the soleus (slow type), the innervation area was more extensive along the muscle fibre; the most frequent modifications were: abnormal axonal branching pattern, swellings of the terminal branches and terminal and preterminal sprouting. Additional clusters of ACh receptors were frequent and AChE activity was unaffected. In the fast part of the gastrocnemius, neuromuscular junctions were slightly affected: sprouting was slight and all the muscle fibres had a single cluster of AChE receptors; AChE activity was modified. In trembler mouse, the alteration of axon-Schwann cell relationships interacted with the regulatory processes ensuring the maintenance and stability of muscle fibre innervation.

Contribution of axonal transport to the renewal of myelin phospholipids in peripheral nerves. I. Quantitative radioautographic study.

Kinetics of phospholipid constituents transferred from the axon to the myelin sheath were studied in the oculomotor nerve (OMN) and the ciliary ganglion (CG) of chicken. Axons of the OMN were loaded with transported phospholipids after an intracerebral injection of [2-3H]glycerol or [3H]labeled choline. Quantitative electron microscope radioautography revealed that labeled lipids were transported in the axons mainly associated with the smooth endoplasmic reticulum. Simultaneously, the labeling of the myelin sheath was found in the Schmidt-Lanterman clefts and the inner myelin layers. The outer Schwann cell cytoplasm and the outer myelin layers contained some label with [methyl-3H]choline, but virtually none with [2-3H]glycerol. With time the radioactive lipids were redistributed throughout and along the whole myelin sheath. Since [2-3H]glycerol incorporated into phospholipids is practically not re-utilized, the occurrence of label in myelin results from a translocation of entire phospholipid molecules and from their preferential insertion into Schmidt-Lanterman clefts. In this way, the axon-myelin transfer of phospholipid contributes rapidly to the renewal of a limited pool of phospholipids in the inner myelin layers. When [methyl-3H]choline was used as precursor of phospholipids, the rapid appearance of the label in the inner myelin layers was interpreted also as an axon-myelin transfer of labeled phospholipids. However, the additional labeling of the outer Schwann cell cytoplasm adjacent to Schmidt-Lanterman clefts and of the outer myelin layers reflects a local re-incorporation of the base released from the axon. By these two processes, the axon contributes to purvey the inner myelin layers with new phospholipids and the Schwann cells with new choline molecules.

The smooth endoplasmic reticulum: structure and role in the renewal of axonal membrane and synaptic vesicles by fast axonal transport.

The spatial arrangement of the smooth endoplasmic reticulum (SER) was studied in 0.5-2 mum thick sections of rat spinal and chick ciliary ganglia previously impregnated with heavy metal salts. Electron microscopy at low (10-5 V) or high (10-6 V) voltage showed the impregnated SER as a continuous system extending probably from the perikaryon to the axon terminal. Tubules of the SER, which were running in a parallel direction with the axon, were occasionally seen in close apposition with the axonal membrane. Moreover in the preterminal region, anastomosed tubules of the SER formed a subsurface 'primary network' and gave rise to a deeper 'secondary network' made of thinner tubules; synaptic vesicles bulging at the tip of thin tubules of the SER were frequently observed. To specify the role played by the SER in the fast axonal transport, chicken ciliary ganglia were slighty compressed and radioautographed 3 h after the intracerebral injection of [3-H]lysine. Quantitative analysis of the silver grain distribution indicated that labeled proteins, rapidly conveyed down the axon, piled up in regions containing an accumulation of SER profiles. On the basis of these results, it is concluded that: (1) the SER appears as a continuous intraaxonal pathway bridging the perikaryon and the axon terminal; (2) the SER conveys macromolecular components with the fast axonal transport; (3) the conveyed macromolecules, which are delivered to the axonal membrane and to the synaptic vesicles, are probably transferred by means of connections with the SER.