Investigation of cranial and other nerves in the mouse with muscular dystrophy

Mouse Extraocular Muscles and the Musculotopic Organization of Their Innervation

The organization of extraocular muscles (EOMs) and their motor nuclei was investigated in the mouse due to the increased importance of this model for oculomotor research. Mice showed a standard EOM organization pattern, although their eyes are set at the side of the head. They do have more prominent oblique muscles, whose insertion points differ from those of frontal-eyed species. Retrograde tracers revealed that the motoneuron layout aligns with the general vertebrate plan with respect to nuclei and laterality. The mouse departed in some significant respects from previously studied species. First, more overlap between the distributions of muscle-specific motoneuronal pools was present in the oculomotor nucleus (III). Furthermore, motoneuron dendrites for each pool filled the entire III and extended beyond the edge of the abducens nucleus (VI). This suggests mouse extraocular motoneuron afferents must target specific pools based on features other than dendritic distribution and nuclear borders. Second, abducens internuclear neurons are located outside the VI. We concluded this because no unlabeled abducens internuclear neurons were observed following lateral rectus muscle injections and because retrograde tracer injections into the III labeled cells immediately ventral and ventrolateral to the VI, not within it. This may provide an anatomical substrate for differential input to motoneurons and internuclear neurons that allows rodents to move their eyes more independently. Finally, while soma size measurements suggested motoneuron subpopulations supplying multiply and singly innervated muscle fibers are present, markers for neurofilaments and perineuronal nets indicated overlap in the size distributions of the two populations. Anat Rec, 302:1865-1885, 2019. © 2019 American Association for Anatomy.

A single point mutation in the LN domain of LAMA2 causes muscular dystrophy and peripheral amyelination

Mutations in the gene encoding the basal lamina (BL) component laminin α2 (LAMA2) cause merosin-deficient congenital muscular dystrophy 1A (MDC1A), a complex disorder that includes hypomyelination and myodegeneration. In dystrophia muscularis (dy) mice bearing Lama2 mutations, myofibers and Schwann cells fail to assemble stable BLs, which are thought to be crucial for myofiber survival and Schwann cell differentiation. Here, we describe defects in a new allele of Lama2 in mice, nmf417, in which a point mutation substitutes Arg for Cys79 at a universally conserved CxxC motif in the laminin N-terminal (LN) domain; this domain mediates laminin-laminin interactions. nmf417 homozygosity caused progressive myodegeneration and severe peripheral amyelination in nerve roots, similar to previous Lama2 mutations, but without the pervasive BL thinning previously associated with the disorder. In direct contrast to the previously characterized dy and dy2J alleles, nmf417 homozygous myofibers frequently had thickened BLs. Severe amyelination in nmf417-mutant nerve roots suggested complete laminin 2 inactivation for Schwann cells, although myelinated fibers had normal BLs. The results reveal crucial roles for the LN domain CxxC motif in both nerve and muscle, but challenge expected relationships between LN-domain function, Ln2 activity and BL stability. The nmf417 mutation provides a defined animal model in which to investigate mechanisms and treatments for moderate forms of MDC1A.

Vimentin and desmin expression in degenerating and regenerating dystrophic murine muscles

The distribution of the intermediate filament proteins (IFP) desmin and vimentin was studied in gastrocnemius, plantaris and soleus muscles of the dystrophic mouse strain ReJ 129 during postnatal development. Special attention was paid to the overall morphological changes in the distribution of these cytoskeletal constituents in degenerating and regenerating muscle fibres. In contrast to their normal counterparts, the dystrophic mice (ReJ 129 dy/dy) appeared to develop four types of distinct muscle fibres with immunohistochemically detectable aberrant IFP patterns. The distribution of desmin IFP differed in the dystrophic muscle fibres as compared to the normal fibres in that juxtanuclear aggregates of IFP were frequently seen. In contrast to the recent literature we conclude that these cells are regenerated myofibres exhibiting defective nuclear migration.

Effects of early immobilization on the functional capacity of dystrophic (ReJ 129 dy/dy) mouse leg muscles

Hindleg muscles of dystrophic (ReJ 129 dy/dy) mice were immobilized during the second post-natal week. Two months after remobilization histopathological features and isometric force characteristics of the m. extensor digitorum longus (EDL) and the m. tibialis anterior (TA) were studied. As a result of early transient immobilization significant differences were observed in muscle morphology and isometric force compared with untreated dystrophic muscles. Restriction of dynamic use of the muscles during this second post-natal week largely prevents the muscles of dystrophic mice from becoming affected by the disease process. Even after two months of remobilization pathology appears to be reduced.

Inherited neuromuscular diseases in the mouse. A review of the literature

There are several neuromuscular disorders affecting the human being. Most of these are poorly understood and lack and effective treatment. Due to the limitation of experimental manipulation in "anima nobili", inherited neuromuscular diseases in laboratory animals constitute a valuable source of scientific information. Amongst several animal species affected by neuromuscular disorders the house mouse is of particular interest because of its small size, short pregnancy and low costs of maintanence. In the present review 20 murine mutants with diseases affecting peripheral nerves, skeletal muscles and motor end-plates are tabulated. Genetic, clinical and pathological aspects are discussed aiming to provide information about these mutants which might be of great interest as animal models for human neuromuscular diseases.

Evidence on hypomyelination of central nervous system in murine muscular dystrophy

To elucidate the disturbances of myelin metabolism in the nerve tissue of murine muscular dystrophy, the lipid composition of and the developmental changes in 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) and cholesterol ester hydrolase (CEHase) activities in the purified CNS myelin of dystrophic mice were determined. Several kinds of lipids, total galactolipid and cerebroside sulfatide levels were significantly reduced as compared with controls. Total cholesterol levels in the spinal cord of dystrophic mice were moderately higher. CEHase and, to a lesser degree, CNPase activities were reduced in the purified myelin of the CNS of the dystrophic mice. The reduced myelin CEHase activity in dystrophic mice suggests that impairment of hydrolysis of steryl esters may be important in the development of hypomyelination of the CNS.

Activity of 2'3'-cyclic-nucleotide 3'-phosphodiesterase and content of PO protein in the peripheral nervous system of the dystrophic mouse and chicken

To investigate whether various myelin markers could detect pathological changes in myelination, the activity of 2'-3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and the level of PO protein were compared in the peripheral nervous system (PNS) of the dystrophic mouse and chicken and their phenotypically normal siblings. The same comparison was made for subcellular fractions of these nerves. The level of PO protein and the activity of CNP were normal in the spinal roots of sciatic nerves of the 129B6F1/Jdy/dy strain of dystrophic mouse. These parameters were slightly but significantly lower in the spinal roots of the 129REJ/dy/dy strain of dystrophic mouse. These results suggest that the 129B6F1/Jdy/dy mutant and the 129REJ/dy/dy mutant mouse might not have the same type of biochemical abnormality in their PNS. The specific activity of CNP and the proportion of PO protein increased significantly in the various regions of the PNS of 1-day-old chicks when compared to that of the 18-day-old embryos. Subcellular fractionation of peripheral nerves indicated that these components were enriched in the crude myelin fraction. These results support the conclusion that PO protein and CNP are suitable markers of myelinogenesis in the PNS of the chick. These parameters were normal in the line 413 dystrophic chicken. These results do not support the conclusion of a systemic membrane defect in the PNS of the dystrophic mouse and chicken.

Postnatal growth and differentiation of muscle fibres in the mouse. II. A histochemical and morphometrical investigation of dystrophic muscle

Postnatal development of three hind legs muscles, the soleus, plantaris, and gastrocnemius, of dystrophic mice (ReJ 129) was investigated with histochemical and morphometric methods. The results were compared with normal postnatal development. Especially during the second week postnatally, there was severe fibre necrosis with no apparent preference for any particular fibre type. This period of necrosis was shortly followed by a wave or regeneration during the third week that could not, however, compensate for the loss of fibres. In dystrophic animals of 4-5 months of age, the number of fibres was reduced by 40-70%. Cross sectional areas of dystrophic muscles rarely, if ever, exceeded values for normal animals 14 days of age, while body weights were also drastically reduced. Growth and differentiation of the nonaffected fibres proceeded almost normally during the first month. During the second month, the "slow' fibres in the soleus muscle, and the "fast-oxidative-glycolytic' fibres in the plantaris muscle were hypertrophied, while, incidentally, some "fast-glycolytic' fibres showed hypertrophy; but in this case the average size of the fibre type was not increased. After two months, a general fibre atrophy was observed. The fate of the regenerated fibres was difficult to trace, especially in muscles older than one month. It is assumed that a number of them were capable of developing into "adult' fibre types histochemically. During the course of the disease the percentage of "intermediate' fibres increased markedly, whereas nearly all "fast-glycolytic' fibres disappeared. Because of these shifts in fibre profiles, the plantaris and the gastrocnemius muscles obtained a rather "juvenile' and "oxidative' aspect. Changes in the histochemical character of the soleus muscle were less spectacular. In dystrophic muscles, no new fibre types were found, compared with normal muscles. Rather, fibre types were present at the wrong moment, or occurred in quantities unusual for the age concerned.

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.

A freeze-fracture study of normal and dystrophic C57BL mouse muscle

Freeze-fracture replicas of normal and dystrophic C57BL mouse muscle and kidney were examined to see whether here was a deficit in plasmalemmal particles which others suggest is a feature of dystrophies. When compared with normal membranes there was an increase in the particle density in dystrophic extensor digitorum longus muscle, a decrease in dystrophic soleus muscle, and no change in dystrophic kidney. Therefore there was not a general deficit in intramembrane particles in this dystrophic tissue. Indirect evidence supported the hypothesis that abnormalities in dystrophic mouse muscles are caused by abnormal motor input. The density of indentations, parallel to the T-tubule, on the flat surface of the terminal cisternae can be modulated by the motor nerve. Changes were found in indentation density in dystrophic muscle which were similar to changes seen after transection of the spinal cord in the mid-thoracic region. There were parallel changes in contractile properties and indentation density in dystrophic fibers.

Glycolytic, pentose-phosphate shunt and transaminase enzymes in gastrocnemius muscle, liver, heart, and brain of two mouse mutants, 129 J-dy and A2g-adr, with abnormal muscle function

Aldolase and phosphoglycerate kinase activity were markedly reduced in muscle from two mouse mutants, 129 J-dy and A2G-adr, with abnormal muscle development. The pentose-phosphate shunt enzymes, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were both greatly increased in the gastrocnemius of 129 J-dy mice, but only the former was slightly increased in A2G-adr muscle. Alanine and aspartate aminotransferase activities were normal or low in 129 J-dy muscle but increased to approximately 200% in A2G-adr muscle. Liver from 129 J-dy mice showed increased activity of glucose-6-phosphate dehydrogenase. These findings are compatible with the well-recognised lipid involvement in the 129 J-dy mutant but indicate that an abnormality of amino acid metabolism in relation to energy supply is probably more important in the A2G-adr mutant.

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.

Motor innervation of the gastrocnemius muscle of wobbler and dystrophic mice

Innervation of the gastrocnemius muscle of mice from the wobbler, dystrophic and C57/BL colonies has been studied. It was found that phenotypically normal mice from each of the colonies did not differ in their innervation properties, hence suggesting no heterozygote penetrance. However, the end-plate complexity increased with age of normal mice. Functional terminal innervation ratio for both the wobbler and dystrophic gastrocnemius muscle was raised above that of the normal and many dystrophic end-plates also appeared abnormal. A study of mice from the wobbler colony manifesting a late onset hind-limb muscle degeneration (Wr/HLD) has been included and the results suggest a relatively benign form of spinal muscular atrophy.

The relationship of Schwann cell migration in vitro to injury, using normal, Wobbler, and dystrophic mice

A quantitative in vitro study of the cellular outgrowth from mouse sciatic nerve explants has been carried out using both normal untreated nerves and nerves taken at various times after the in vivo application of trauma. The results obtained have been compared with the results for outgrowth from sciatic nerve of two neurological mutants: 'the Wobbler mouse' and the 'ReJ 129 Dystrophic mouse'. It has been shown that the in vitro response to peripheral injury does reflect the activity known to occur in vivo. Outgrowth from explants of mutant mouse nerves, although differing from normal has been found to be less profuse than that occurring after mechanical nerve injury.

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

Several atypical features of myelination of the peripheral nervous system are reported in common peroneal nerve of dystrophic mice (129 Re J dy/dy): (i) central nervous system-like contact between myelin sheaths of adjacent nerve fibres; (ii) nodes and internodes of myelinated fibres enwrapped with cytoplasmic processes of Schwann cells from adjacent nerve fibres; (iii) Schwann cells of adjacent nerve fibres co-operating in formation of a single myelin sheath; and (iv) a single Schwann cell myelinating two separate axons. In view of the presence of similar features of myelination in the central nervous system, where the myelin producing cells lack basement membrane, we suggest that in the dystrophic peripheral nerves the development of these features can be attributed to the partial deficiency of the Schwann cell basement membrane. Two types of widened nodes of Ranvier are also identified: (i) nodes with paranodal damage; and (ii) nodes without paranodal damage. In addition, abnormal features of myelination are described which are likely to represent altered Schwann cell/axon relationships during demyelination and remyelination and/or decreased myelinating ability of Schwann cells. We interpret these findings as indicating a metabolic disorder of Schwann cells. They provide an experimental model for the investigation of factors involved in the origin and maintenance of the structural organization of peripheral nerve.

Muscular dystrophy in the mouse: neuromuscular transmission and the concept of functional denervation

The results of recent investigations by ourselves and others indicate that no form of denervation exists to any remarkable degree in dystrophic mouse skeletal muscles. This conclusion is based on the following information: Dystrophic nerve terminals liberate normal amounts of transmitter both spontaneously and during impulse-mediated activity. The characteristics of the release process, the size of the available store of transmitter, and the probability of release of transmitter in response to the invasion of an action potential appear to be normal. The sensitivity of the postsynaptic membrane to the transmitter is normal. Action potential generation in response to both direct and indirect excitation is normal. There is no unequivocal pharmacologic evidence of denervation in dystrophic skeletal muscle, even though dystrophic muscle fibers respond to surgical denervation in a normal fashion. Nerve terminal sprouting is extensive, but there is no evidence of collateral reinnervation.

Abnormalities of the sciatic nerves of dystrophic mice with reference to the large U-axons

A statistical study using regression analysis was used to evaluate the density of axonal organelles in dystrophic peripheral nerves. The slope of the density of neurotubules (NT) in myelinated (M-) axons was different from that in small unmyelinated (U-) axons. The slope of the density of NT in large U-axons (larger than 1.5 micron in diameter) was similar to that of the M-axons in both the dystrophic and control mice. There was a higher density of NT in the dystrophic M-axons than in the controls in the anterior, posterior and mixed nerves of the sciatic nerve. There was also a higher density of NT in the dystrophic small U-axons than in the controls. There was a higher density of neurofilaments (NF) of M-axons in the dystrophic mice than in the controls. On the contrary, the NF of small U-axons were lower in density in the dystrophic mice. These results were different from our previous reports, which were observed in the distal part, depending on when the groups of U-axons were divided (Okada, Mizuhira and Nakamura 1976a).

Electrophysiological observations on the spinal cord of the normal and dystrophic mouse

A method of carrying out electrophysiological experiments on the mouse spinal cord is described. The conduction velocity in the spinal dorsal roots (DR) of the normal mouse was in the range 10-100 m sec-1 and in the ventral roots (VR) 50-70 m sec-1. In the dystrophic mutant (129 ReJ dy/dy) the conduction velocity for both roots was usually in the range 0.1-2.0 m sec-1. Reflexes from DR to VR were recorded in both mutant and wild type animals and it was concluded that the delays in the mutant reflex were probably due to the slower conduction velocity in the roots. Postsynaptic inhibition and presynaptic inhibition were demonstrated and records were made from Renshaw cells and intracellularly from motoneurones. Delayed activity in spinal reflex paths, probably of supraspinal origin, was more pronounced in the dystrophic mutant. It is concluded that if the dystrophic mouse mutant were to be regarded as a model for human disease then similar reflex delays should be demonstrated in human subjects with muscular dystrophy.

Dysmyelination in the sciatic nerves of dystrophic mice

Ultrastructural alterations were observed in the sciatic nerve of dystrophic mice. Myelin sheaths were abnormal in shape, abruptly ceased beyond a node of Ranvier, leaving the axon naked. These changes were seen in both afferent and efferent nerve fibres. Apparent embryonal Schwann cells and Schwann cells which were associated with increased lysosomes in the cytoplasm were observed in the proximal portion. There is a relative decrease in Schwann cells in the cross-section of the radicular parts, and a relative increase in the distal parts. The mean number of neurotubules per unit area was smaller while the mean number of the neurofilaments was larger in U-axons, in the dystrophic mice than in the controls. In M-fibres, neurotubules and neurofilaments showed no significant difference between systrophic and control mice.

Structural alterations of the junctional region in extraocular muscle of dystrophic mice. I. Modifications of sole-plate nuclei

Sole-plate nuclei of the C57Bl/6Jdy2j dystrophic mouse showed apparent selective susceptibility to various forms of structural alteration. Pyknosis and chromatin fragmentation were seen in addition to vacuolar and membranous nuclear inclusions. These were often associated with neuromuscular junctions with markedly reduced or virtually absent junctional folding. Membranous proliferations also occurred nearby sole-plate nuclei of such flattened junctions.