Myelin disorders: Causes and perspectives of Charcot-Marie-Tooth neuropathy

Combined molecular, structural and memory data unravel the destructive effect of tramadol on hippocampus

Tramadol is a synthetic analogue of codeine and stimulates neurodegeneration in several parts of the brain that leads to various behavioral impairments. Despite the leading role of hippocampus in learning and memory as well as decreased function of them under influence of tramadol, there are few studies analyzing the effect of tramadol administration on gene expression profiling and structural consequences in hippocampus region. Thus, we sought to determine the effect of tramadol on both PC12 cell line and hippocampal tissue, from gene expression changes to structural alterations. In this respect, we investigated genome-wide mRNA expression using high throughput RNA-seq technology and confirmatory quantitative real-time PCR, accompanied by stereological analysis of hippocampus and behavioral assessment following tramadol exposure. At the cellular level, PC12 cells were exposed to 600μM tramadol for 48 hrs, followed by the assessments of ROS amount and gene expression levels of neurotoxicity associated with neurodegenerative pathways such as apoptosis and autophagy. Moreover, the structural and functional alteration of the hippocampus under chronic exposure to tramadol was also evaluated. In this regard, rats were treated with tramadol at doses of 50 mg/kg for three consecutive weeks. In vitro data revealed that tramadol provoked ROS production and caused the increase in the expression of autophagic and apoptotic genes in PC12 cells. Furthermore, in-vivo results demonstrated that tramadol not only did induce hippocampal atrophy, but it also triggered microgliosis and microglial activation, causing upregulation of apoptotic and inflammatory markers as well as over-activation of neurodegeneration. Tramadol also interrupted spatial learning and memory function along with long-term potentiation (LTP). Taken all together, our data disclosed the neurotoxic effects of tramadol on both in vitro and in-vivo. Moreover, we proposed a potential correlation between disrupted biochemical cascades and memory deficit under tramadol administration.

EXOSC8 mutations alter mRNA metabolism and cause hypomyelination with spinal muscular atrophy and cerebellar hypoplasia

The exosome is a multi-protein complex, required for the degradation of AU-rich element (ARE) containing messenger RNAs (mRNAs). EXOSC8 is an essential protein of the exosome core, as its depletion causes a severe growth defect in yeast. Here we show that homozygous missense mutations in EXOSC8 cause progressive and lethal neurological disease in 22 infants from three independent pedigrees. Affected individuals have cerebellar and corpus callosum hypoplasia, abnormal myelination of the central nervous system or spinal motor neuron disease. Experimental downregulation of EXOSC8 in human oligodendroglia cells and in zebrafish induce a specific increase in ARE mRNAs encoding myelin proteins, showing that the imbalanced supply of myelin proteins causes the disruption of myelin, and explaining the clinical presentation. These findings show the central role of the exosomal pathway in neurodegenerative disease.

The exosome is responsible for mRNA degradation, which is an important step in the regulation of gene expression. Here the authors report that homozygous missense mutations in the exosome subunit, EXOSC8, may cause neurodegenerative disease in infants through the dysregulation of myelin expression.

Peripheral nerve injury modulates neurotrophin signaling in the peripheral and central nervous system

Peripheral nerve injury disrupts the normal functions of sensory and motor neurons by damaging the integrity of axons and Schwann cells. In contrast to the central nervous system, the peripheral nervous system possesses a considerable capacity for regrowth, but regeneration is far from complete and functional recovery rarely returns to pre-injury levels. During development, the peripheral nervous system strongly depends upon trophic stimulation for neuronal differentiation, growth and maturation. The perhaps most important group of trophic substances in this context is the neurotrophins (NGF, BDNF, NT-3 and NT-4/5), which signal in a complex spatial and timely manner via the two structurally unrelated p75(NTR) and tropomyosin receptor kinase (TrkA, Trk-B and Trk-C) receptors. Damage to the adult peripheral nerves induces cellular mechanisms resembling those active during development, resulting in a rapid and robust increase in the synthesis of neurotrophins in neurons and Schwann cells, guiding and supporting regeneration. Furthermore, the injury induces neurotrophin-mediated changes in the dorsal root ganglia and in the spinal cord, which affect the modulation of afferent sensory signaling and eventually may contribute to the development of neuropathic pain. The focus of this review is on the expression patterns of neurotrophins and their receptors in neurons and glial cells of the peripheral nervous system and the spinal cord. Furthermore, injury-induced changes of expression patterns and the functional consequences in relation to axonal growth and remyelination as well as to neuropathic pain development will be reviewed.

An integrated approach to characterize transcription factor and microRNA regulatory networks involved in Schwann cell response to peripheral nerve injury

Background

The regenerative response of Schwann cells after peripheral nerve injury is a critical process directly related to the pathophysiology of a number of neurodegenerative diseases. This SC injury response is dependent on an intricate gene regulatory program coordinated by a number of transcription factors and microRNAs, but the interactions among them remain largely unknown. Uncovering the transcriptional and post-transcriptional regulatory networks governing the Schwann cell injury response is a key step towards a better understanding of Schwann cell biology and may help develop novel therapies for related diseases. Performing such comprehensive network analysis requires systematic bioinformatics methods to integrate multiple genomic datasets.

Results

In this study we present a computational pipeline to infer transcription factor and microRNA regulatory networks. Our approach combined mRNA and microRNA expression profiling data, ChIP-Seq data of transcription factors, and computational transcription factor and microRNA target prediction. Using mRNA and microRNA expression data collected in a Schwann cell injury model, we constructed a regulatory network and studied regulatory pathways involved in Schwann cell response to injury. Furthermore, we analyzed network motifs and obtained insights on cooperative regulation of transcription factors and microRNAs in Schwann cell injury recovery.

Conclusions

This work demonstrates a systematic method for gene regulatory network inference that may be used to gain new information on gene regulation by transcription factors and microRNAs.

MicroRNAs modulate Schwann cell response to nerve injury by reinforcing transcriptional silencing of dedifferentiation-related genes

In the peripheral nervous system, Schwann cells (SCs) surrounding damaged axons undergo an injury response that is driven by an intricate transcriptional program and is critical for nerve regeneration. To examine whether these injury-induced changes in SCs are also regulated posttranscriptionally by miRNAs, we performed miRNA expression profiling of mouse sciatic nerve distal segment after crush injury. We also characterized the SC injury response in mice containing SCs with disrupted miRNA processing due to loss of Dicer. We identified 87 miRNAs that were expressed in mouse adult peripheral nerve, 48 of which were dynamically regulated after nerve injury. Most of these injury-regulated SC miRNAs were computationally predicted to inhibit drivers of SC dedifferentiation/proliferation and thereby re-enforce the transcriptional program driving SC remyelination. SCs deficient in miRNAs manifested a delay in the transition between the distinct differentiation states required to support peripheral nerve regeneration. Among the miRNAs expressed in adult mouse SCs, miR-34a and miR-140 were identified as functional regulators of SC dedifferentiation/proliferation and remyelination, respectively. We found that miR-34a interacted with positive regulators of dedifferentiation and proliferation such as Notch1 and Ccnd1 to control cell cycle dynamics in SCs. miR-140 targeted the transcription factor Egr2, a master regulator of myelination, and modulated myelination in DRG/SC cocultures. Together, these results demonstrate that SC miRNAs are important modulators of the SC regenerative response after nerve damage.

A new long term in vitro model of myelination

Besides in vivo models, co-cultures systems making use of Rat dorsal root ganglion explants/Schwann cells (SC) are widely used to essentially study myelination in vitro. In the case of animal models of demyelinating diseases, it is expected to reproduce a pathological process; conversely the co-cultures are primarily developed to study the myelination process and in the aim to use them to replace animals in experiences of myelin destruction or functional disturbances. We describe (in terms of protein expression kinetic) a new in vitro model of sensory neurons/SC co-cultures presenting the following advantages: both sensory neurons and SC originate from the same individual; sensory neurons and SC being dissociated, they can be co-cultured in monolayer, allowing an easier microscope observation; the co-culture can be maintained in a serum-free medium for at less three months, allowing kinetic studies of myelin formation both at a molecular and cellular level. Optimizing culture conditions permits to use 96-well culture plates; image analyses conducted with an automatic image analyzer allows rapid, accurate and quantitative expression of results. Finally, this system was proved by measuring the apparition of myelin protein to mimic in vitro the physiological process of in vivo myelination.

Chapter 24: the coming of molecular biology and its impact on clinical neurology

Although the chemical study of the nervous system dates back well into the 19th century, molecular biology and especially molecular neurobiology only began to be established in the second half of the 20th century. This chapter reviews their impact on clinical neuroscience during the 50 years since Watson and Crick published their seminal paper. After a short review of the part played by F.O. Schmitt in establishing molecular neuroscience the chapter outlines work that led to a detailed understanding of the biochemical structure and function of nerve cell membranes and their embedded channel proteins, receptors, and other molecules. The chapter then turns to the numerous pathologies that result from disorders of these elements: the various channel and gap-junction pathologies. The chapter continues with a discussion of some of the diseases caused by defective DNA, especially the trinucleotide repeat expansion diseases (TREDs) and ends with a short account of the development of molecular approaches to prion diseases, myasthenia gravis, and the neurodegenerative diseases of old age. Francis Bacon said long ago that "knowledge is power." The hope is that increasing molecular knowledge will help cure some of the human suffering seen in the neurological ward and clinic.

Role of mitofusin 2 mutations in the physiopathology of Charcot-Marie-Tooth disease type 2A

Charcot-Marie-Tooth disease (CMT) is the most common form of hereditary peripheral neuropathy. The main axonal form of CMT, CMT2A, preferentially affects peripheral neurons with the longest neurites. CMT2A has been recently linked to mutations in the mitofusin 2 (Mfn2) gene. Mfn2 participates in mitochondrial fusion a process that together with mitochondrial fission, contributes to mitochondrial morphology. Many hypotheses have been postulated to understand how mutations in Mfn2 lead to CMT2A. In this review, we will describe the physiological role of Mfn2, the pathophysiology of CMT2A and current hypotheses about the deleterious role of mutant Mfn2 in neuronal function.

Level and localization of polysialic acid is critical for early peripheral nerve regeneration

PolySia, the most striking post-translational modification of the neural cell adhesion molecule, is down-regulated during postnatal development. After peripheral nerve lesion, polySia is located on neuronal and glial cells normally not synthesizing polySia. However, structural consequences of reduced polySia content for peripheral nerve regeneration have not yet been clear. Furthermore, the contribution of sialyltransferases ST8SiaII and ST8SiaIV for the up-regulation of polySia has not been studied so far. In order to investigate the impact of polySia on regeneration processes of myelinated axons, we examined mouse mutants retaining only one functional sialyltransferase allele. In the absence of ST8SiaII, quantification of myelinated axons revealed a significant decrease in number and size of regenerated fibers without impairment of remyelination. In contrast, St8SiaIV deficiency resulted in increased fiber outgrowth and axonal maturation. Western blot analysis demonstrated that both ST8SiaII and St8SiaIV direct up-regulation of polySia. Cell-specific induction of polySia in myelinating Schwann cells and on regenerated axons in the presence of ST8SiaIV, but not ST8SiaII, indicates that not only the amount of polySia but also its cellular localization has a high impact on the regeneration progress of peripheral nerves.

Peripheral regeneration

Whereas the central nervous system (CNS) usually cannot regenerate, peripheral nerves regenerate spontaneously after injury because of a permissive environment and activation of the intrinsic growth capacity of neurons. Functional regeneration requires axon regrowth and remyelination of the regenerated axons by Schwann cells. Multiple factors including neurotrophic factors, extracellular matrix (ECM) proteins, and hormones participate in Schwann cell dedifferentiation, proliferation, and remyelination. We describe the current understanding of peripheral axon regeneration and focus on the molecules and potential mechanisms involved in remyelination.