Tomacula in MAG-deficient mice

Hereditary spastic paraplegia and prominent sensorial involvement: think MAG mutations!

Homozygous mutations in MAG, encoding the myelin-associated glycoprotein, a transmembrane component of the myelin sheath, have been associated with SPG 75 recessive spastic paraplegia. Here, we report the first patient with two compound heterozygous novel MAG mutations (p.A151V and p.S373R) and early developmental delay with a progressive complex phenotype characterized by spastic paraplegia, peripheral sensorimotor neuropathy, intellectual disability, and sensorial dysfunctions with severe optic atrophy and hearing involvement. Brain imaging showed progressive global cerebellar atrophy. We propose that complex hereditary spastic paraplegia, with axonal and demyelinating polyneuropathy, sensorial impairment and intellectual disability might suggest MAG mutations.

Avian Riboflavin Deficiency: An Acquired Tomaculous Neuropathy

The finding of tomacula, focal areas of sausage-shaped hypermyelination in peripheral nerves, is reported for the first time in avian riboflavin deficiency. Day-old, meat-type chickens were fed a riboflavin-deficient diet (1.8 mg/kg) and were killed on postnatal days 6, 11, 16, and 21, while control chickens were fed a conventional diet containing 5.0 mg/kg riboflavin. Tomacula were found in sciatic and brachial nerves from day 11 onward, became more frequent and prominent with increasing time, and preceded the onset of segmental demyelination.

Exome sequencing links corticospinal motor neuron disease to common neurodegenerative disorders

Hereditary spastic paraplegias (HSPs) are neurodegenerative motor neuron diseases characterized by progressive age-dependent loss of corticospinal motor tract function. Although the genetic basis is partly understood, only a fraction of cases can receive a genetic diagnosis, and a global view of HSP is lacking. By using whole-exome sequencing in combination with network analysis, we identified 18 previously unknown putative HSP genes and validated nearly all of these genes functionally or genetically. The pathways highlighted by these mutations link HSP to cellular transport, nucleotide metabolism, and synapse and axon development. Network analysis revealed a host of further candidate genes, of which three were mutated in our cohort. Our analysis links HSP to other neurodegenerative disorders and can facilitate gene discovery and mechanistic understanding of disease.

Motor and sensory neuropathy due to myelin infolding and paranodal damage in a transgenic mouse model of Charcot-Marie-Tooth disease type 1C

Charcot-Marie-Tooth disease type 1C (CMT1C) is a dominantly inherited motor and sensory neuropathy. Despite human genetic evidence linking missense mutations in SIMPLE to CMT1C, the in vivo role of CMT1C-linked SIMPLE mutations remains undetermined. To investigate the molecular mechanism underlying CMT1C pathogenesis, we generated transgenic mice expressing either wild-type or CMT1C-linked W116G human SIMPLE. Mice expressing mutant, but not wild type, SIMPLE develop a late-onset motor and sensory neuropathy that recapitulates key clinical features of CMT1C disease. SIMPLE mutant mice exhibit motor and sensory behavioral impairments accompanied by decreased motor and sensory nerve conduction velocity and reduced compound muscle action potential amplitude. This neuropathy phenotype is associated with focally infolded myelin loops that protrude into the axons at paranodal regions and near Schmidt-Lanterman incisures of peripheral nerves. We find that myelin infolding is often linked to constricted axons with signs of impaired axonal transport and to paranodal defects and abnormal organization of the node of Ranvier. Our findings support that SIMPLE mutation disrupts myelin homeostasis and causes peripheral neuropathy via a combination of toxic gain-of-function and dominant-negative mechanisms. The results from this study suggest that myelin infolding and paranodal damage may represent pathogenic precursors preceding demyelination and axonal degeneration in CMT1C patients.

The PMP22 gene and its related diseases

Peripheral myelin protein-22 (PMP22) is primarily expressed in the compact myelin of the peripheral nervous system. Levels of PMP22 have to be tightly regulated since alterations of PMP22 levels by mutations of the PMP22 gene are responsible for >50 % of all patients with inherited peripheral neuropathies, including Charcot-Marie-Tooth type-1A (CMT1A) with trisomy of PMP22, hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of PMP22, and CMT1E with point mutations of PMP22. While overexpression and point-mutations of the PMP22 gene may produce gain-of-function phenotypes, deletion of PMP22 results in a loss-of-function phenotype that reveals the normal physiological functions of the PMP22 protein. In this article, we will review the basic genetics, biochemistry and molecular structure of PMP22, followed by discussion of the current understanding of pathogenic mechanisms involving in the inherited neuropathies with mutations in PMP22 gene.

Conduction block in PMP22 deficiency

Patients with PMP22 deficiency present with focal sensory and motor deficits when peripheral nerves are stressed by mechanical force. It has been hypothesized that these focal deficits are due to mechanically induced conduction block (CB). To test this hypothesis, we induced 60-70% CB (defined by electrophysiological criteria) by nerve compression in an authentic mouse model of hereditary neuropathy with liability to pressure palsies (HNPP) with an inactivation of one of the two pmp22 alleles (pmp22(+/-)). Induction time for the CB was significantly shorter in pmp22(+/-) mice than that in pmp22(+/+) mice. This shortened induction was also found in myelin-associated glycoprotein knock-out mice, but not in the mice with deficiency of myelin protein zero, a major structural protein of compact myelin. Pmp22(+/-) nerves showed intact tomacula with no segmental demyelination in both noncompressed and compressed conditions, normal molecular architecture, and normal concentration of voltage-gated sodium channels by [(3)H]-saxitoxin binding assay. However, focal constrictions were observed in the axonal segments enclosed by tomacula, a pathological hallmark of HNPP. The constricted axons increase axial resistance to action potential propagation, which may hasten the induction of CB in Pmp22 deficiency. Together, these results demonstrate that a function of Pmp22 is to protect the nerve from mechanical injury.

CXCR4 chemokine receptor signaling mediates pain hypersensitivity in association with antiretroviral toxic neuropathy

Nucleoside reverse transcriptase inhibitors (NRTIs) are known to produce painful neuropathies and to enhance states of pain hypersensitivity produced by HIV-1 infection. It has also been observed that in some neuropathic pain models, chemokines and their receptors are upregulated, perhaps contributing to the pain state. In order to understand if chemokines are involved in NRTI-mediated sensory neuropathies, we treated rats with the anti-retroviral drug, 2',3'-dideoxycytidine (ddC), which is known to produce an extended period of hyperalgesia and allodynia. Using in situ hybridization, we observed that under normal conditions, CXCR4 chemokine receptors were widely expressed by satellite glia in the dorsal root ganglia (DRG) and Schwann cells in the sciatic nerve. A limited number of DRG neurons also expressed CXCR4 receptors. The chemokine SDF-1/CXCL12 was similarly expressed in glial cells in the DRG and peripheral nerve. Following a single administration of ddC, expression levels of CXCR4 mRNA in glia and neurons and SDF-1 mRNA in glia increased considerably. The functional nature of increased CXCR4 mRNA expression was confirmed by measuring SDF-1 induced [Ca2+]i increases in acutely isolated DRG neurons and glia. In contrast, the expression of the chemokine receptors CCR2 and CCR5 did not change following ddC treatment. Pain hypersensitivity produced by ddC could be inhibited by treatment with the CXCR4 antagonist, AMD3100. Hence, we postulate that NRTIs produce pain hypersensitivity through the upregulation of CXCR4 signaling in the DRG. Increased numbers of CXCR4 receptors would also explain the synergism observed between NRTI treatment and the proalgesic effects of HIV-1 infection.

Neuroactive steroids prevent peripheral myelin alterations induced by diabetes

The effect of the neuroactive steroids progesterone, dihydroprogesterone and tetrahydroprogesterone on myelin abnormalities induced by diabetes was studied in the sciatic nerve of adult male rats treated with streptozotocin. Streptozotocin increased blood glucose levels and decreased body weight gain, parameters not affected by steroids. Streptozotocin increased the number of fibers with myelin infoldings in the axoplasm, 8 months after the treatment. Chronic treatment for 1 month with progesterone and dihydroprogesterone resulted in a significant reduction in the number of fibers with myelin infoldings to control levels. Treatment with tetrahydroprogesterone did not significantly affect this myelin alteration. These results suggest that neuroactive steroids such as progesterone and dihydroprogesterone may represent therapeutic alternatives to counteract peripheral myelin alterations induced by diabetes.

Early paranodal myelin swellings (tomacula) in an avian riboflavin deficiency model of demyelinating neuropathy

INTRODUCTION

Disruption of the complex architectural and molecular organization of the paranodal region of myelinated peripheral nerve fiber may initiate the evolving time dependent process of segmental demyelination. In support of this notion was the finding of focal paranodal myelin swellings (tomacula) due to redundant folding of myelin sheaths, early in the time course of an avian riboflavin deficiency model of demyelinating neuropathy.

METHODS

Newborn broiler meat chickens were maintained either on a routine diet containing 5.0 mg/kg riboflavin (control group) or a riboflavin-deficient diet containing 1.8 mg/kg riboflavin. Riboflavin concentrations in the liver were measured at postnatal day 11. Peripheral nerves were morphologically examined at days 6, 11, 16 and 21 using light and electron microscopy and teased nerve fiber techniques.

RESULTS

Riboflavin-deficient chickens showed signs of a neuropathy from days 8 and pathological examination of peripheral nerves revealed a demyelinating neuropathy with paranodal tomacula formation starting on day 11. Paranodal tomacula consisted of redundant myelin infoldings or outfoldings, increased in size and frequency after day 11. After day 16, the paranodal swellings showed prominent degenerative changes accompanied by an increased frequency of myelinated fibers showing demyelination.

CONCLUSION

Tomacula due to redundant myelin folds are generally considered a remyelination phenomenon, yet in this avian riboflavin deficiency model of demyelination, the paranodal tomacula occurred early in the course of demyelination.

Peripheral nervous system and central nervous system pathology in rapidly progressive lower motor neuron syndrome with immunoglobulin M anti-GM1 ganglioside antibody

Pathological studies, including novel teased peripheral nerve fiber studies, were performed in a patient who presented with a rapidly progressive, lower motor neuron syndrome and high titer of immunoglobulin M anti-GM1 ganglioside antibody. In the central nervous system, there was a severe loss of motor neurons and central chromatolysis with ubiquitin immunopositive cytoplasmic inclusions in residual motor neurons. In the peripheral nervous system, axonal degeneration of myelinated fibers in the anterior nerve roots was evident. Pathologic evidence of sensory nerve involvement was also found despite the absence of clinical or electrophysiological sensory abnormalities. Sectional studies of single myelinated nerve fibers from an antemortem sural nerve biopsy showed remyelination and globular paranodal swellings due to focal complex myelin folding and degeneration in 13% of fibers. Postmortem studies of the sural nerves 4 weeks later showed paranodal demyelination (90% of fibers), but no paranodal swellings and similar findings were present in samples of the ulnar, radial, median, tibial, and common peroneal nerves. Paranodal abnormalities of enlargement of the adaxonal space, myelin degeneration, and axonal compaction were found on cross-sectional studies of individual teased fibers, which on conventional light microscopic assessment appeared normal. These changes suggest a disturbance of paranodal axonal-myelin adhesion due to binding of the anti-GM1 ganglioside antibody to the common epitope known to be present on the myelin sheath and nodal axolemma in the paranodal region of both motor and sensory nerves.

Pathology of a mouse mutation in peripheral myelin protein P0 is characteristic of a severe and early onset form of human Charcot-Marie-Tooth type 1B disorder

Mutations in the gene of the peripheral myelin protein zero (P0) give rise to the peripheral neuropathies Charcot-Marie-Tooth type 1B disease (CMT1B), Déjérine-Sottas syndrome, and congenital hypomyelinating neuropathy. To investigate the pathomechanisms of a specific point mutation in the P0 gene, we generated two independent transgenic mouse lines expressing the pathogenic CMT1B missense mutation Ile106Leu (P0sub) under the control of the P0 promoter on a wild-type background. Both P0sub-transgenic mouse lines showed shivering and ultrastructural abnormalities including retarded myelination, onion bulb formation, and dysmyelination seen as aberrantly folded myelin sheaths and tomacula in all nerve fibers. Functionally, the mutation leads to dispersed compound muscle action potentials and severely reduced conduction velocities. Our observations support the view that the Ile106Leu mutation acts by a dominant-negative gain of function and that the P0sub-transgenic mouse represents an animal model for a severe, tomaculous form of CMT1B.