Atypical axon-Schwann cell relationships in the common peroneal nerve of the dystrophic mouse: an ultrastructural study

LAMA2 Neuropathies: Human Findings and Pathomechanisms From Mouse Models

Merosin deficient Congenital Muscular Dystrophy (MDC1A), or LAMA2-related muscular dystrophy (LAMA2-RD), is a recessive disorder resulting from mutations in the LAMA2 gene, encoding for the alpha-2 chain of laminin-211. The disease is predominantly characterized by progressive muscular dystrophy affecting patient motor function and reducing life expectancy. However, LAMA2-RD also comprises a developmentally-associated dysmyelinating neuropathy that contributes to the disease progression, in addition to brain abnormalities; the latter often underappreciated. In this brief review, we present data supporting the impact of peripheral neuropathy in the LAMA2-RD phenotype, including both mouse models and human studies. We discuss the molecular mechanisms underlying nerve abnormalities and involved in the laminin-211 pathway, which affects axon sorting, ensheathing and myelination. We conclude with some final considerations of consequences on nerve regeneration and potential therapeutic strategies.

Distinct roles for laminin globular domains in laminin alpha1 chain mediated rescue of murine laminin alpha2 chain deficiency

Background

Laminin α2 chain mutations cause congenital muscular dystrophy with dysmyelination neuropathy (MDC1A). Previously, we demonstrated that laminin α1 chain ameliorates the disease in mice. Dystroglycan and integrins are major laminin receptors. Unlike laminin α2 chain, α1 chain binds the receptors by separate domains; laminin globular (LG) domains 4 and LG1-3, respectively. Thus, the laminin α1 chain is an excellent tool to distinguish between the roles of dystroglycan and integrins in the neuromuscular system.

Methodology/Principal Findings

Here, we provide insights into the functions of laminin α1LG domains and the division of their roles in MDC1A pathogenesis and rescue. Overexpression of laminin α1 chain that lacks the dystroglycan binding LG4-5 domains in α2 chain deficient mice resulted in prolonged lifespan and improved health. Importantly, diaphragm and heart muscles were corrected, whereas limb muscles were dystrophic, indicating that different muscles have different requirements for LG4-5 domains. Furthermore, the regenerative capacity of the skeletal muscle did not depend on laminin α1LG4-5. However, this domain was crucial for preventing apoptosis in limb muscles, essential for myelination in peripheral nerve and important for basement membrane assembly.

Conclusions/Significance

These results show that laminin α1LG domains and consequently their receptors have disparate functions in the neuromuscular system. Understanding these interactions could contribute to design and optimization of future medical treatment for MDC1A patients.

Biological role of dystroglycan in Schwann cell function and its implications in peripheral nervous system diseases

Dystroglycan is a central component of the dystrophin-glycoprotein complex (DGC) that links extracellular matrix with cytoskeleton, expressed in a variety of fetal and adult tissues. Dystroglycan plays diverse roles in development and homeostasis including basement membrane formation, epithelial morphogenesis, membrane stability, cell polarization, and cell migration. In this paper, we will focus on biological role of dystroglycan in Schwann cell function, especially myelination. First, we review the molecular architecture of DGC in Schwann cell abaxonal membrane. Then, we will review the loss-of-function studies using targeted mutagenesis, which have revealed biological functions of each component of DGC in Schwann cells. Based on these findings, roles of dystroglycan in Schwann cell function, in myelination in particular, and its implications in diseases will be discussed in detail. Finally, in view of the fact that understanding the role of dystroglycan in Schwann cells is just beginning, future perspectives will be discussed.

Role of integrins in peripheral nerves and hereditary neuropathies

Interactions between Schwann cells and extracellular matrix on one surface, and axons on the other, are required for correct myelination in the developing peripheral nervous system. Integrins are transmembrane proteins that mediate the former in association with other surface receptors. This review focuses on the role that integrins play in the development of the peripheral nervous system, and in inherited human peripheral neuropathies. Here we describe recent findings on integrin signaling to different intracellular pathways, focusing on cell adhesion, migration, and polarization. Then we use information derived from recent experiments of targeted mutagenesis in mice to show that, consistent with temporally regulated expression, different integrins serve multiple roles in developing nerve.

Laminin alpha1 chain improves laminin alpha2 chain deficient peripheral neuropathy

Absence of laminin alpha2 chain leads to a severe form of congenital muscular dystrophy (MDC1A) associated with peripheral neuropathy. Hence, future therapies should be aimed at alleviating both muscle and neurological dysfunctions. Pre-clinical studies in animal models have mainly focused on ameliorating the muscle phenotype. Here we show that transgenic expression of laminin alpha1 chain in muscles and the peripheral nervous system of laminin alpha2 chain deficient mice reduced muscular dystrophy and largely corrected the peripheral nerve defects. The presence of laminin alpha1 chain in the peripheral nervous system resulted in near-normal myelination, restored Schwann cell basement membranes and improved rotarod performance. In summary, we postulate that laminin alpha1 chain is an excellent substitute for laminin alpha2 chain in multiple tissues and suggest that treatment with laminin alpha1 chain may be beneficial for MDC1A in humans.

Both laminin and Schwann cell dystroglycan are necessary for proper clustering of sodium channels at nodes of Ranvier

Nodes of Ranvier are specialized axonal domains, at which voltage-gated sodium channels cluster. How axons cluster molecules in discrete domains is mostly unknown. Both axons and glia probably provide constraining mechanisms that contribute to domain formation. Proper sodium channel clustering in peripheral nerves depends on contact from Schwann cell microvilli, where at least one molecule, gliomedin, binds the sodium channel complex and induces its clustering. Furthermore, mice lacking Schwann cell dystroglycan have aberrant microvilli and poorly clustered sodium channels. Dystroglycan could interact at the basal lamina or at the axonglial surface. Because dystroglycan is a laminin receptor, and laminin 2 mutations [merosin-deficient congenital muscular dystrophy (MDC1A)] cause reduced nerve conduction velocity, we asked whether laminins are involved. Here, we show that the composition of both laminins and the dystroglycan complex at nodes differs from that of internodes. Mice defective in laminin 2 have poorly formed microvilli and abnormal sodium clusters. These abnormalities are similar, albeit less severe, than those of mice lacking dystroglycan. However, mice lacking all Schwann cell laminins show severe nodal abnormalities, suggesting that other laminins compensate for the lack of laminin 2. Thus, although laminins are located at a distance from the axoglial junction, they are required for proper clustering of sodium channels. Laminins, through their specific nodal receptors and cytoskeletal linkages, may participate in the formation of mechanisms that constrain clusters at nodes. Finally, abnormal sodium channel clusters are present in a patient with MDC1A, providing a molecular basis for the reduced nerve conduction velocity in this disorder.

Human diseases reveal novel roles for neural laminins

Extracellular matrix molecules such as laminins have a central role in regulating cell behaviour. However, our understanding of their functions in the mammalian nervous system is incomplete. It is important to establish these functions, both for an understanding of normal development and to devise strategies to enhance repair. Here, we review how insights gained from human diseases caused by genetic mutations in laminins or their receptors have revealed significant and sometimes unexpected roles for laminins in neural stem cells, migrating neurons and myelinating glia, in both the PNS and CNS.

Laminins and their receptors in Schwann cells and hereditary neuropathies

This review focuses on the influence of laminins, mediated through laminin receptors present on Schwann cells, on peripheral nerve development and pathology. Laminins influence multiple aspects of cell differentiation and tissue morphogenesis, including cell survival, proliferation, cytoskeletal rearrangements, and polarity. Peripheral nerves are no exception, as shown by the discovery that defective laminin signals contribute to the pathogenesis of diverse neuropathies such as merosin-deficient congenital muscular dystrophy and Charcot-Marie-Tooth 4F, neurofibromatosis, and leprosy. In the last 5 years, advanced molecular and cell biological techniques and conditional mutagenesis in mice began revealing the role of different laminins and receptors in developing nerves. In this way, we are starting to explain morphological and pathological observations beginning at the start of the last century. Here, we review these recent advances and show how the roles of laminins and their receptors are surprisingly varied in both time and place.

Rho kinase regulates schwann cell myelination and formation of associated axonal domains

The myelin sheath forms by the spiral wrapping of a glial membrane around an axon. The mechanisms involved are poorly understood but are likely to involve coordinated changes in the glial cell cytoskeleton. Because of its key role as a regulator of the cytoskeleton, we investigated the role of Rho kinase (ROCK), a major downstream effector of Rho, in Schwann cell morphology, differentiation, and myelination. Pharmacologic inhibition of ROCK activity results in loss of microvilli and stress fibers in Schwann cell cultures and strikingly aberrant myelination in Schwann cell-neuron cocultures; there was no effect on Schwann cell proliferation or differentiation. Treated Schwann cells branch aberrantly and form multiple, small, independent myelin segments along the length of axons, each with associated nodes and paranodes. This organization partially resembles myelin formed by oligodendrocytes rather than the single long myelin sheath characteristic of Schwann cells. ROCK regulates myosin light chain phosphorylation, which is robustly, but transiently, activated at the onset of myelination. These results support a key role of Rho through its effector ROCK in coordinating the movement of the glial membrane around the axon at the onset of myelination via regulation of myosin phosphorylation and actomyosin assembly. They also indicate that the molecular machinery that promotes the wrapping of the glial membrane sheath around the axon is distributed along the entire length of the internode.

Expression of laminin receptors in schwann cell differentiation: evidence for distinct roles

Schwann cells require laminin-2 throughout nerve development, because mutations in the alpha2 chain in dystrophic mice interfere with sorting of axons before birth and formation of myelin internodes after birth. Mature Schwann cells express several laminin receptors, but their expression and roles in development are poorly understood. Therefore, we correlated the onset of myelination in nerve and synchronized myelinating cultures to the appearance of integrins and dystroglycan in Schwann cells. Only alpha6beta1 integrin is expressed before birth, whereas dystroglycan and alpha6beta4 integrin appear perinatally, just before myelination. Although dystroglycan is immediately polarized to the outer surface of Schwann cells, alpha6beta4 appears polarized only after myelination. We showed previously that Schwann cells lacking beta1 integrin do not relate properly to axons before birth. Here we show that the absence of beta1 before birth is not compensated by other laminin receptors, whereas coexpression of both dystroglycan and beta4 integrin is likely required for beta1-null Schwann cells to myelinate after birth. Finally, both beta1-null and dystrophic nerves contain bundles of unsorted axons, but they are predominant in different regions: in spinal roots in dystrophic mice and in nerves in beta1-null mice. We show that differential compensation by laminin-1, but not laminin receptors may partially explain this. These data suggest that the action of laminin is mediated by beta1 integrins during axonal sorting and by dystroglycan, alpha6beta1, and alpha6beta4 integrins during myelination.

Unique role of dystroglycan in peripheral nerve myelination, nodal structure, and sodium channel stabilization

Dystroglycan is a central component of the dystrophin-glycoprotein complex implicated in the pathogenesis of several neuromuscular diseases. Although dystroglycan is expressed by Schwann cells, its normal peripheral nerve functions are unknown. Here we show that selective deletion of Schwann cell dystroglycan results in slowed nerve conduction and nodal changes including reduced sodium channel density and disorganized microvilli. Additional features of mutant mice include deficits in rotorod performance, aberrant pain responses, and abnormal myelin sheath folding. These data indicate that dystroglycan is crucial for both myelination and nodal architecture. Dystroglycan may be required for the normal maintenance of voltage-gated sodium channels at nodes of Ranvier, possibly by mediating trans interactions between Schwann cell microvilli and the nodal axolemma.

Dysmyelinating sensory-motor neuropathy in merosin-deficient congenital muscular dystrophy

A 20-year-old man with mild myopathy, external ophthalmoparesis, epilepsy, and diffuse white matter hyperintensity in the brain on magnetic resonance imaging had partial merosin deficiency in muscle and absent merosin in the endoneurium. Motor and sensory nerve conduction velocities were slow. Nerve biopsy showed reduction of large myelinated fibers, short internodes, enlarged nodes, excessive variability of myelin thickness, tomacula, and uncompacted myelin, but no evidence of segmental demyelination, naked axons, or onion bulbs. Thus, in congenital muscular dystrophy, merosin expression may be dissociated in different tissues, and the neuropathy is sensory-motor and due to abnormal myelinogenesis.

Glial cell line-derived neurotrophic factor alters axon schwann cell units and promotes myelination in unmyelinated nerve fibers

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in the development and maintenance of a subset of dorsal root ganglion sensory neurons. We administered high-dose exogenous recombinant human GDNF (rhGDNF) daily to adult rats to examine its effect on unmyelinated axon-Schwann cell units in intact peripheral nerves. In rhGDNF-treated animals, there was a dramatic proliferation in the Schwann cells of unmyelinated fibers, which resulted in the segregation of many unmyelinated axons into a 1:1 relationship with Schwann cells and myelination of normally unmyelinated small axons. This study demonstrates that the administration of high doses of a growth factor to adult rats can change the phenotype of nerve fibers from unmyelinated to myelinated.

Effects of nerve segment supernatants on cultured Schwann cell proliferation and laminin production

Mouse sciatic nerves were transected and 3 hr to 16 days later proximal segments were removed and homogenized. Supernatants of these segments or of normal sciatic nerves were added to Schwann cells maintained in Dulbecco's modified Eagle's medium (DMEM) + 15% fetal calf serum (FCS). After 6 days, Schwann cells were solubilized and the protein content was measured using a Bio-Rad (Melville, NY) protein assay. Samples containing the same amounts of protein were then applied to microtiter plates and the laminin content was determined by enzyme-linked immunosorbent assay (ELISA). Lysates of cultures treated with 24 hr proximal segment supernatants contained significantly higher levels of laminin than those prepared from other intervals, from distal segments, or from control nerves. Increased surface and cytoplasmic anti-laminin immunoreactivity also was found in Schwann cells treated with 24 hr supernatants. To identify the source(s) of this effect, proximal segments removed 24 hr after transection were bisected; supernatants were prepared from each half and tested. Significant increases in laminin production were produced by supernatants from both halves. When supernatants from proximal and distal halves were compared, the latter produced significantly higher laminin levels. Electron microscopic examination of both halves showed that distal halves contained sprouting neurites and growth cones ensheathed by Schwann cells which had a basal lamina and resembled those seen during development and regeneration. Proximal halves appeared normal. Schwann cell proliferation also was compared in supernatant-treated cultures by using a bromodeoxy-uridine (BrdU) ELISA. The 24 hr and 2 day supernatants increased Schwann cell proliferation significantly; 12 hr, 4 day, and 8 day supernatants produced smaller increases. Our observations suggest that axons undergoing early regenerative changes are one of several possible sources of substance(s) in our proximal segment supernatants which increased Schwann cell proliferation and laminin production.

Comparison of demyelination and neural degeneration in spiral and Scarpa's ganglia of C57BL/6 mice

Spiral and Scarpa's (vestibular) ganglia of female C57BL/6 mice, ranging in age from 1.5 to 32 months, were examined by light and electron microscopy. The spiral ganglia of C57BL/6 mice undergo sweeping neuronal losses. Based on cytological characteristics that repeat themselves in both young and old mice, we divided perikaryal demyelination and degeneration of spiral ganglia into four arbitary stages: 1) incipient demyelination, myelin sheaths begin to loosen and unravel; 2) contact, the partially demyelinated perikarya abut as the extracellular matrix disappear; 3) clumping, clusters of naked perikarya clump together, cytoplasmic processes surround the exterior of the clump; and 4) resorption, the clumps gradually disintegrate, leaving at completion large, fluid-filled spaces and few cellular remnants. In Scarpa's ganglia, a small number of neurons demyelinate and clump, but none degenerate or undergo resorption.

Microheterogeneity of myelin basic proteins in the partially myelinated spinal roots of the Bar Harbor 129 ReJ muscular dystrophic mouse

The protein composition of normal spinal roots and the partially myelinated spinal roots of Bar Harbor 129 ReJ dystrophic mice was analyzed by 2D-gel electrophoresis which resolved basic proteins. The normal roots contained proteins with mobilities identical to two of the three 18.5 kDa and two of the three 14 kDa myelin basic protein spots resolved in purified spinal cord myelin suggesting that normal root myelin may have some of the characteristics of CNS myelin. In contrast dystrophic roots contained spots with mobilities identical to only one of the spots resolved for each myelin basic protein. The possibility that the difference in microheterogeneity may be responsible for the decreased myelination is discussed.

Progressive axonopathy: an inherited neuropathy of boxer dogs. 2. The nature and distribution of the pathological changes

This report describes the neuropathology of progressive axonopathy (PA), an autosomal recessive inherited neuropathy of Boxer dogs, which affects CNS and PNS. The nerve roots contain numerous myelin bubbles and proximal paranodal axonal swellings containing vesicles, vesiculo-tubular profiles and disorganized neurofilaments. The myelin sheath overlying such swellings is often attenuated. As the disease develops there are progressive changes in the myelin sheath with thinning at paranodal and internodal locations, loss of myelin from lengths of axon and the formation of short internodes with disproportionately thin sheaths. The abnormalities show a very definite selectivity for nerve roots and proximal nerves. Conversely, the frequency of degeneration and regeneration is greater distally except in the cervical ventral roots which contain numerous regenerating clusters. In the CNS numerous axonal spheroids are found in the lateral and ventral columns of the spinal cord and in various brain stem nuclei, particularly the superior olives, accessory cuneate nuclei and lateral lemniscus and its nucleus. Axonal degeneration which occurs mainly in the cord shows no obvious tract or proximal/distal selectivity. The optic pathways are also involved, predominantly adjacent to the chiasma. The autonomic nervous system is affected and distal limb muscles show varying, but usually minor, degrees of neurogenic atrophy. The condition, which has no obvious direct parallel in human or veterinary medicine, shows gross disturbances of axon-glial inter-relationships in both CNS and PNS.

Differences in density and distribution of surface glycoconjugates between normal and dystrophic mouse Schwann cells detected by statistical analyses of lectin-ferritin binding

Cultures of Schwann cells and neurons from dorsal root ganglia of normal (C57bl/6J +/+) and dystrophic (C57bl/6J dy2j/dy2j) mice were labeled with wheat germ agglutinin (WGA) and Ricinus communis agglutinin (RCA-I) conjugated to ferritin. Statistical methods were used to compare the regional densities and distribution characteristics of lectin binding in these two types of Schwann cells, which differ in their capacities to ensheath and myelinate axons in vivo and in cultures. Regional variations in lectin binding densities and distributions were observed in both types of Schwann cells. WGA-ferritin was bound at lower densities in dystrophic mouse Schwann cells than in corresponding regions of normal cells. In both normal and dystrophic cells, WGA-ferritin was distributed at greater densities on the free surfaces of Schwann cells than on the substrate-associated surfaces. WGA-ferritin was clustered in all regions of both normal and dystrophic mouse cells. RCA-ferritin densities did not differ significantly between corresponding regions of normal and dystrophic mouse Schwann cells. However, in normal mouse Schwann cells, the density of RCA-ferritin was significantly greater in the thinner, peripheral processes of Schwann cells than in thicker perinuclear regions of the cells. Differences in the degree of RCA-ferritin clustering were also detected between normal and dystrophic Schwann cells. These results indicate that regional differences in the density and distributions of cell surface glycoconjugates occur in Schwann cells of normal and dystrophic mice.

Structure of motor nerve terminals in chickens with hereditary muscular dystrophy

The structure and size of 1-week to 1-year-old normal (line 412) and dystrophic (line 413) chicken motor nerve terminals were studied using combined pre- and postsynaptic histologic endplate staining. The main result is that adult dystrophic terminals have abnormal structure and are significantly smaller than normal. These differences occurred progressively during development. At 1 week ex ovo, dystrophic motor nerve terminals were similar to normals in size and appearance. By 8 weeks, differences between normal and dystrophic terminal size and structural organization began to emerge. Qualitatively, beginning at 8 weeks and becoming more frequent by 1 year of age (the endpoint of this study), dystrophic motor endplates differed from normal in having: generally smaller synaptic boutons, often separated by extremely thin branching interconnectives; increasing incidence of multiple innervation; and frequent occurrences of apparent partial or total denervation, terminal sprouting, and reinnervation.

Schwann cells fail to differentiate when co-cultured in contact with PC12 neurites

Schwann cells derived from mouse or rat dorsal root ganglia (DRG) were co-cultured with either DRG neurons or nerve growth factor (NGF)-responsive PC12 pheochromocytoma cells for up to 7 weeks. When Schwann cells were grown in the presence of DRG neurites, they displayed normal ensheathing behavior and produced basal laminae and small diameter collagen fibrils within 5-19 days in vitro. However, when Schwann cells were co-cultured in direct contact with PC12 cells and without DRG neurons, they largely failed to ensheath PC12 neurites, and failed to assemble either basal lamina or small diameter collagen fibrils at any point during 7 weeks. Schwann cell proliferation continued in the presence of PC12 neurites, indicating that PC12 cells produced a mitogenic activity for Schwann cells functionally similar to previously described neurite-associated activities. These results demonstrate that Schwann cell contact with PC12 cells does not elicit the final morphogenetic events in Schwann cells (ensheathment, basal lamina formation and collagen fibril assembly) that normally occur when Schwann cells are co-cultured in contact with DRG neurons.

Normal basal laminas are realized on dystrophic Schwann cells in dystrophic in equilibrium shiverer chimera nerves

Multiple discontinuities are observed in the basal laminas of Schwann cells in mature dystrophic mice. To explore the pathogenesis of this abnormality we have exploited a dystrophic in equilibrium shiverer mouse chimera preparation in which both the basal lamina phenotype and the genotype of myelin-forming Schwann cells can be determined. If the basal lamina abnormality were to arise from an intrinsic deficiency of the dystrophic Schwann cell itself, only those Schwann cells of dystrophic genotype could express the mutant phenotype, whereas the coexisting population of shiverer Schwann cells should express typically normal basal laminas. No such distinction was observed; rather both dystrophic and shiverer Schwann cells were found to express relatively normal basal laminas and two pathogenetic mechanisms remain theoretical possibilities. The dystrophic Schwann cell population may be intrinsically defective but also may be rescued by obtaining the normal product of the dy locus synthesized by the coexisting shiverer cells. Alternatively, an extra Schwann cell deficiency existing within dystrophic mice may be normalized by shiverer cells and the normal intrinsic potential of both dystrophic and shiverer Schwann cells can then be realized. Regardless of the exact mechanism underlying these findings, some extracellularly mediated influence, emanating in vivo from shiverer cells, is capable of ameliorating the basal lamina deficiency typically expressed by dystrophic Schwann cells.

Hypomyelination in the peripheral nervous system of shiverer mice and in shiverer in equilibrium normal chimaera

In shiverer mice, the P1 component of myelin basic protein (MBP) is deficient in both the central nervous system (CNS) and peripheral nervous system (PNS) but compact myelin is more grossly defective in the CNS. In the PNS, myelin exhibits a normal periodic structure, and although examples of subtle abnormalities of shiverer Schwann cell ultrastructure have been described previously, myelin thickness has been reported as unremarkable when observed by light microscopy. We report a quantitative investigation of the myelin sheath thickness of shiverer Schwann cells in which a mild but apparently consistent hypomyelination of axons ensheathed by shiverer Schwann cells was observed. This abnormality was expressed both in the peripheral nerves of a homozygous shiverer mouse and in the shiverer Schwann cells populating the mosaic nerves of a mature shiverer in equilibrium normal mouse chimaera. In addition, multiple interlamellar gaps was found to be a highly consistent feature of shiverer myelin. These observations extend the description of the peripheral nerve defects expressed in shiverer mice and further define these abnormalities as direct consequences of the shiverer Schwann cells' intrinsic genotype. In light of these results, a significant role for P1 in the formation and/or maintenance of normal myelin in the PNS is suggested.

Myelin proteins and collagen in the spinal roots and sciatic nerves of muscular dystrophic mice

Proteins of lumbosacral spinal roots and sciatic nerves of adult dystrophic mice (Bar Harbor 129REJ dydy) were analyzed by SDS gel electrophoresis to determine if identifiable proteins were affected. All peripheral nerve myelin proteins (P0 glycoprotein, P1 and Pr basic proteins, X protein and a high-molecular-weight protein) were decreased in the roots but not in the sciatic nerves. Central nervous system myelin proteins were not increased in either roots or sciatic nerves. Although dystrophic spinal roots and sciatic nerves contained less collagen, Type I, III and V collagens were present.

Effects of crush injury on the abnormalities in the spinal roots and peripheral nerves of dystrophic mice

Lumbosacral spinal roots and peroneal nerves in dystrophic and control mice were crushed and allowed to regenerate. Six weeks after crush injury, the dystrophic roots no longer showed the typical groups of unensheathed axons that characterize the uncrushed roots. Thus, the location of this ensheathment defect in the spinal roots cannot be the exclusive mechanism responsible for its development. Crush injury and regeneration also tended to correct a second abnormality in the peripheral nervous system of dystrophic mice: the discontinuities in the Schwann cell basal laminas. Because the regenerated nerves contained increased amounts of collagen, the results of this study support the evidence from tissue culture experiments that the extracellular matrix may be involved in the pathogenesis of these disorders. However, the outcome of the present in vivo experiments indicates that genetically normal fibroblasts are not required for this change to occur.

Quantitative studies of the abnormal axon-Schwann cell relationship in the peripheral motor and sensory nerves of the dystrophic mouse

The nature and extent of abnormal axon-Schwann cell relationships in peripheral portions of dystrophic motor and sensory nerves were quantitatively evaluated between 1 and 9 months of age using teased fibres and electron micrographs. The results show that in the dystrophic (dy/dy) common peroneal (CPN) and tibial nerves (TN), and less in the dy/dy sural nerve (SN): (1) the number of Schwann cell nuclei associated with myelinated axons is increased with respect to normal; (2) the average internodal length is correspondingly reduced; (3) the average dystrophic internode elongates roughly in parallel with the average normal internode, and with the dystrophic limb; the longitudinal growth of the dystrophic limb is normal; (4) the variation of internodal length is greater than normal; it does not increase with age; (5) the incidence of the nodes of Ranvier which are wider than the normal 3 micrometers limit does not increase with age; and (6) the number of myelinated axons is reduced in the dy/dy CPN and TN but not in the dy/dy SN; it shows no change with age. These data indicate that: (1) in the dy/dy peripheral nerves (PNS) the abnormal axon-Schwann cell relationships and the reduced number of myelinated axons have been established prior to 1 month of age, thereafter progressive degenerative processes do not appear to take place, and (2) the dy/dy sensory nerves are less affected than the motor ones.

Regeneration and reinnervation of the dystrophic mouse soleus muscle. A light- and electron-microscopic study

The regeneration and reinnervation of the dystrophic mouse soleus muscle was investigated in response to a double crush-lesion, which causes degeneration of muscle fibres leaving the innervation intact. In normal and dystrophic muscles, injury produced degeneration of muscle fibres, proliferation and fusion of muscle satellite cells, and growth and reinnervation of regenerating fibres. Four, 6 and 21 days after injury, regenerating dystrophic fibres were 50% smaller in cross-sectional area than regenerating normal fibres and showed several pathological changes. Nerve terminal morphology was initially unaffected by the crush, and nerve terminals were associated with degenerating muscle fibres 2 days after injury and with regenerating muscle fibres 6-28 days after crushing. In intact muscles dystrophic endplates were longer and showed increased ultraterminal sprouting compared to normal endplates. At 28 days after crushing normal nerve terminal sprouting was significantly increased compared to the contralateral control. The extent of nerve terminal sprouting and endplate length in dystrophic muscles was not affected by the degeneration and subsequent regeneration of the muscle fibres. We conclude that a proportion of dystrophic mouse soleus muscle fibres can regenerate after a crush when the innervation is left intact.

Factors affecting schwann cell basal lamina formation in cultures of dorsal root ganglia from mice with muscular dystrophy

Pure populations of sensory neurons (N), Schwann cells (S) and fibroblasts (Fb) were established in culture from normal and dystrophic (dy) mice in order to investigate the cellular origin(s) of the peripheral nervous system abnormalities present in murine muscular dystrophy. These cell types were placed together in various combinations and their subsequent interactions were monitored with the light and electron microscope. The formation of the basal lamina (BL) which in normal tissue, completely surrounds the external aspect of the Schwann cell (when in contact with axons) was documented by morphometric analysis of electron micrographs. Defects in Schwann cell BL formation, observed throughout the PNS of the dy mouse in vivo, were used as a marker for the expression of the dystrophic abnormality in culture. Initially mature cultures of dy tissues containing only S and N (SN) without Fb were examined and found to contain an incomplete BL that surrounded only 82.8 +/- 12.2% of the externally directed plasmalemma of axon-related Schwann cells. The following recombination cultures were established: (1) normal S were placed on dystrophic N; (2) dystrophic S were placed on dystrophic N; (3) dystrophic S were placed on normal N; and (4) normal Fb were added to a dystrophic SN culture. After a 5-week period, the BL formed by normal S in direct contact with dystrophic N was thick and continuous (97.7 +/- 2.2 coverage). On the other hand, in culture situations (without Fb) containing dystrophic S in contact with either dystrophic or normal neurites, the BL coverage was considerably less (58.5 +/- 14.8% and 55.4 +/- 13.2%, respectively). The addition of normal Fb obtained from sciatic nerve explants to dystrophic SN cultures in time resulted in the formation of a morphologically complete BL (98.9 +/- 1.4% coverage). We conclude that neuronal signal(s) are adequate to induce complete BL formation by Schwann cells in the dystrophic tissue but that dystrophic Schwann cells are incapable of forming a complete BL. Furthermore, this deficiency of dy Schwann cells is apparently corrected by the presence of normal Fb by an unknown mechanism.

X-ray diffraction from striated muscles and nerves in normal and dystrophic mice

The structure of striated muscle (thick and thin filaments, filament lattice, and collagen), peripheral nerve myelin, and tendon collagen were studied in tissues from dystrophic and normal mice using small-angle x-ray diffraction. There were increases in the amount of disorganized tissue in the dystrophic mice, and the time course of the changes was monitored over the first 42 weeks of life. As the dystrophic mice became older, the contractile apparatus of the muscles appeared to atrophy, while the amount of collagen increased. In general, the molecular structure and packing appeared to remain unchanged as the disease progressed, although changes in the relative amounts and the organization of proteins were noted. In both normal and dystrophic mice, the collagen periodicity (65.7 nm) was 2% smaller when detected in muscle tissue compared with that detected in tendon tissue.

Factors influencing the release of proteins by cultured Schwann cells

Cultured rat schwann cells grown in association with sensory neurons when labeled with [(3)H]leucinem, [(3)H]glucosamine, or [(35)S]methionine release labeled polypeptides into the culture medium. Analysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) of the culture medium reveals a reproducible pattern of more than 20 polypeptides with molecular weights ranging from 15,000 to more than 250,000. Five major polypeptides (apparent molecular weights 225,000, 210,000, 90,000, 66,000, 50,000, and 40,000) account for approximately 40 percent of the leucine or methionine radioactivity in medium polypeptide. Schwann cells grown in a serum-free defined medium, in which schwann cells do not relate normally to axons, release approximately four times less labeled medium polypeptides tha cultures grown in medium supplemented with serum and chick embryo extract. In addition, there is a qualitative difference in the pattern of medium polypeptides resolved by SDS-PAGE, so that a single polypeptide (mol wt 40,000) accounts for nearly all of the label in medium polypeptides. Switching of cultures grown in defined medium to supplemented medium for 2 d results in a fourfold increase in the amount of labeled polypeptides appearing in the culture medium, and a return to the normal pattern of medium polypeptides appearing in the culture medium, and a return to the normal pattern of medium polypeptides as resolved by SDS-PAGE. This change in the pattern of polypeptides release by schwann cells is accompanied by changes in the association between schwann cells and axons. An early step in the establishment of normal axon-schwann cell relations appears to be an inward migration of schwann cells into axonal bundles and spreading of schwann cells along neurites. These changes are evident within 48 h after medium shift. Our results thus suggest that the release of proteins by schwann cells may be important for the development of normal axonal ensheathment.

Myelination-dependent axonal membrane specializations demonstrated in insufficiently myelinated nerves of the dystrophic mouse

"Dystrophic' mice of the 129/ReJ-Dy strain have a genetic defect affecting Schwann cell proliferation. Spinal nerve roots of these animals contain myelinated and unmyelinated axons in addition to groups of large "amyelinated' axons. In affected regions of the spinal roots, myelinated axons are missing their myelin sheaths. Where the myelination terminates or begins, half-nodes are created. Freeze-fracture analysis of these half-nodes shows that only the myelinated side contains rows of dimeric particles in the axonal P-face of the paranode. The P-face on the amyelinated side of a half-node, and the remainder of the amyelinated axon. contains a dense even distribution of particles, many of which are the size of dimeric-particle subunits, but only a few of which are arranged into short rows. As the long circumferential rows are not found on the unmyelinated side of the myelinated side of the half-node we conclude that the paranodal rows of dimeric particles are dependent upon myelination for their organization.

A quantitative ultrastructural study of dorsal root regeneration

Regeneration in rat lumbo-sacral dorsal roots was studied 5-71 days following crush lesions. Wallerian degeneration occurred up to 20 days. At 11 days degenerating myelin was found in both Schwann cells and macrophages. Myelination was first observed 4 mm central to the crush at 7 days, and myelin became compact when the mesaxon exhibited 3.5 turns about the axon (about 11 days post-operatively). At 71 days, 69% of all fibres were myelinated, compared with 36% in normal roots. An example of 2 axons myelinating within the same Schwann cell occurred at 20 days. In normal roots curvilinear relationships were found between axon diameter and fibre diameter, myelin thickness and axon diameter, and between g and fibre diameter. In contrast, linear relationships between these parameters occurred in post-operative roots up to 71 days. Curvilinearity returned at 71 days. Alterations in the relationship between axon diameter and myelin thickness during regeneration indicated that myelin growth lagged behind axon growth throughout, but was more noticeable in larger calibre fibres. By 71 days, larger fibres exhibited disproportionately thin myelin, whilst small fibres possessed abnormally thick myelin compared to normal fibres of similar calibre. Regeneration was limited by axons failing to make successful central synaptic connections and by the poor metabolic response of dorsal root ganglion cells to sectioning of their central processes.

Schwann cells proliferate but fail to differentiate in defined medium

Primary cultures of dorsal root ganglia cells from 18- to 21-day rodent embryos were studied for their ability to express Schwann cell function in a defined medium lacking serum and embryo extract. It was confirmed that Schwann cells, but not fibroblasts, are able to proliferate in response to contact with axons when cultured in this defined medium. We here report that in this medium, however, differentiation of Schwann cells was arrested before completion of ensheathment and before initiation of myelin formation. Electron microscopic analysis confirmed this ensheathment failure and showed that the extracellular matrix components (basal lamina and thin collagenous fibrils) normally produced by axon-related Schwann cells had not been formed. This absence of extracellular matrix, as well as the presence in the Schwann cell of an increased cytoplasmic granularity (observed in the light microscope) and numerous distended cisterns of rough endoplasmic reticulum, suggest a failure in Schwann cell secretion. However, within one week after addition of serum and embryo extract to the culture medium, the ensheathment failure was corrected and myelination occurred; electron microscopic observations showed the presence of basal lamina and collagen fibrils in association with Schwann cells. These results suggest the presence in serum or embryo extract (or both) of factors necessary for the full expression of Schwann cell function (although a similar requirement is not present for the expression of oligodendrocyte function in culture). We propose that these observations indicate a linkage between Schwann cell secretion and axonal ensheathment, including myelin formation.

Abnormalities expressed in long-term cultures of dorsal root ganglia from the dystrophic mouse

We have compared the development in long-term tissue culture of dorsal root ganglia taken from normal and dystrophic mice. Cultures were prepared from late fetal (15--20 days) or neonatal mice of either the C57BL/6 dy2j/dy2j dystrophic (dy) or C57BL/6J +/+ (control) strain and maintained until fully myelinated (5 weeks or more). Analysis by light and electron microscopy indicated that the substantial ensheathment failure present in certain dy nerve roots in vivo is not expressed in cultures; myelination and Schwann cell numbers are comparable to control cultures. On the other hand, many of the subtle abnormalities more recently described in distal parts of peripheral nerves of dystrophic mice are expressed in the dy cultures. These include: (a) discontinuity in the basal lamina surrounding both myelin-forming and non-myelinating Schwann cells: (b) elongated nodes of Ranvier occurring along otherwise well myelinated nerve fibers; (c) relatively short myelin internodes that are increased in thickness as well as irregularities of internode length along a nerve fiber; (d) Schwann cell nuclei substantially displaced from the central point of myelin internodes; and (e) occasional regions of incomplete ensheathment of unmyelinated nerve fibers. In discussing these observations, we present arguments that the dy nerve lesion may be explained by the presence of an abnormality in the extracellular matrix of the peripheral nerve tissues of the dy mouse.