Animal models of immune-mediated neuropathies

PURPOSE OF REVIEW: This article gives an overview on animal models for immune-mediated demyelinating disorders of the peripheral nervous system. As insight into human disease is mainly based on biopsy material and ex-vivo analysis, an understanding of the pathogenetic mechanism of these complex and heterogeneous disorders is mainly based on animal models. RECENT FINDINGS: Besides experimental autoimmune neuritis in rats, recent efforts to establish this model in mice are discussed. In addition, models for spontaneous autoimmune neuropathies and secondary immune reactions in degenerative disorders of the peripheral nervous system are reviewed. SUMMARY: Recently described animal models offer the possibility to analyse the complex interaction of genetic and immunological factors. The entire panel of animal models for immune-mediated disorders of the peripheral nervous system provides a rational basis for studying the mechanisms of pathogenesis and new immunotherapeutic strategies for human autoimmune demyelinating neuropathies.     

Charcot–Marie–Tooth neuropathies: Current gene therapy advances and the route toward translation

Charcot–Marie–Tooth (CMT) neuropathies are a group of genetically and phenotypically heterogeneous disorders that predominantly affect the peripheral nervous system. Unraveling the genetic and molecular mechanisms, as well as the cellular effects of CMT mutations, has facilitated the development of promising gene therapy approaches. Proposed gene therapy treatments for CMTs include virally or non-virally mediated gene replacement, addition, silencing, modification, and editing of genetic material. For most CMT neuropathies, gene- and disease- and even mutation-specific therapy approaches targeting the neuronal axon or myelinating Schwann cells may be needed, due to the diversity of underlying cellular and molecular-genetic mechanisms. The efficiency of gene therapies to improve the disease phenotype has been tested mostly in vitro and in vivo rodent models that reproduce different molecular and pathological aspects of CMT neuropathies. In the next stage, bigger animal models, in particular non-human primates, provide important insights into the translatability of the proposed administration and dosing, demonstrating scale-up potential and safety. The path toward clinical trials is faced with further challenges but is becoming increasingly feasible owing to the progress and knowledge gained from clinical applications of gene therapies for other neurological disorders, as well as the emergence of sensitive outcome measures and biomarkers in patients with CMT neuropathies.     

Clinical features and molecular genetics of hereditary peripheral neuropathies

Hereditary peripheral neuropathies are the most common monogenetically inherited diseases of the nervous system. The prevalence of the Hereditary Motor and Sensory Neuropathy Type 1A (HMSN 1A or Charcot-Marie-Tooth Neuropathy 1A, CMT1A) alone is estimated to be as high as 1/5000. In 1991, a duplication on chromosome 17p11.2 was identified as the causative genetic defect of CMT1A. Since then causative mutations in 17 genes have been identified. This review summarises the clinical and molecular genetic features of primary inherited neuropathies. It is aimed primarily at clinicians and geneticists. Therefore less emphasis is placed on the pathology and the (often unknown) underlying biological disease mechanisms.     

Disease mechanisms and potential therapeutic strategies in Charcot–Marie–Tooth disease

Until 10 years ago, the genetic basis of Charcot–Marie–Tooth (CMT) disease was largely unknown. With the finding of an intrachromosomal duplication on chromosome 17 in 1991, associated with the most commonly found subtype CMT1A, and the discovery of a point mutation in the peripheral myelin protein-22 (pmp22) gene in the Trembler mouse in 1992, the groundwork was laid down for a novel chapter in the elucidation of the molecular basis of this large group of peripheral neuropathies. In the meantime, several different genes have been found to be associated with different forms of demyelinating and axonal forms of CMT. In this review, we will summarize what is known today about the genetics of this group of disease which constitute the most common known monogenetic disorder affecting the nervous system in man, the animal models that have been generated, and what we have learned about the underlying disease mechanisms. Furthermore, we will review how this gain of knowledge about CMT may open new avenues to the development of novel treatment strategies.     

Peripheral nervous system neuroimmunology seen by a neuro-pathologist

In most dysimmune neuropathies, historically the microscopical lesions were described prior to immunological studies. The latter along with neuropathological studies have found some immune, albeit incomplete, explanations of the mechanisms of these lesions which we will describe in two main syndromes: the primitive auto-immune inflammatory peripheral polyneuropathies (GBS and CIDP) and polyneuropathies induced by a monoclonal dysglobulinemia. In some patients who have to be discussed case by case pathology (nerve biopsy) will confirm the diagnosis, may help to delineate the molecular anomalies and identify lesional mechanisms. We will review the high variability of nerve lesions which is characteristic of dysimmune neuropathies. Pathological studies confirm that both humoral and cellular immune reactions against Schwann cell and/or axonal antigens are implicated in primitive dysimmune neuropathies due to a dysfunction or failure of immune tolerance mechanisms. In case of a polyneuropathy associated to a monoclonal dysglobulinemia, pathological and immunological studies have shown that in many patients, the dysglobulinemia did harm the peripheral nerve; knowledge of the pathological lesions and their mechanisms is of major interest for orienting specific treatments.     

Molecular basis of inherited neuropathies

Considerable advances in our knowledge of the most frequently encountered group of inherited neuropathies, Charcot-Marie-Tooth neurpathy (CMT) and related disorders, have recently been made by genetic studies demonstrating that these disorders are caused by duplication, deletion or point mutations of specific genes of the peripheral myelin. The present classification of CMT and related disorders is based on a combination of clinical, neurophysiological, and genetic findings, and new genes and distinct mutations responsible for different clinical phenotypes are continuously being added. The genes that encode peripheral myelin protein of 22 kDa, protein zero, connexin-32 and early growth response-2 are the genes known to be involved in the pathogenesis of inherited neuropathies. Overexpression or underexpression of peripheral myelin protein of 22 kDa are causative for the most frequent forms of CMT-CMT1A and hereditary neuropathy with liability to pressure palsies--but the mechanisms that lead to incorrect myelin formation and maintenance are still unknown. Point mutations in the myelin genes can determine a loss of function, but in some cases an aberrant protein can act through a dominant negative or a toxic gain of function mechanism, disrupting the regular and precise relationship between the different myelin genes. Animal and in-vitro models of inherited neuropathies have been developed and will probably give the information that is necessary to clarify the pathogenetic mechanisms of demyelination.     

Foot pad skin biopsy in mouse models of hereditary neuropathy

Numerous transgenic and knockout mouse models of human hereditary neuropathies have become available over the past decade. We describe a simple, reproducible, and safe biopsy of mouse skin for histopathological evaluation of the peripheral nervous system (PNS) in models of hereditary neuropathies. We compared the diagnostic outcome between sciatic nerve and dermal nerves found in skin biopsy (SB) from the hind foot. A total of five animal models of different Charcot-Marie-Tooth neuropathies, and one model of congenital muscular dystrophy associated neuropathy were examined. In wild type mice, dermal nerve fibers were readily identified by immunohistochemistry, light, and electron microscopy and they appeared similar to myelinated fibers in sciatic nerve. In mutant mice, SB manifested myelin abnormalities similar to those observed in sciatic nerves, including hypomyelination, onion bulbs, myelin outfolding, redundant loops, and tomacula. In many strains, however, SB showed additional abnormalities--fiber loss, dense neurofilament packing with lower phosphorylation status, and axonal degeneration-undetected in sciatic nerve, possibly because SB samples distal nerves. SB, a reliable technique to investigate peripheral neuropathies in human beings, is also useful to investigate animal models of hereditary neuropathies. Our data indicate that SB may reveal distal axonal pathology in mouse models and permits sequential follow-up of the neuropathy in an individual mouse, thereby reducing the number of mice necessary to document pathology of the PNS.     

The contribution of magnetic resonance imaging in the differential diagnosis of optic nerve damage

In this paper we review the findings of magnetic resonance imaging (MRI) in optic neuritis and visual dysfunction due to other optic neuropathies. With advances in MRI technology, it has become possible to visualise optic nerve pathology. STIR and RARE sequences, contrast-enhanced sequences, and phased array surface coils are technical developments that provide fine anatomical detail and that are sensitive to pathological changes. MRI can offer information in the differential diagnosis of optic neuropathies, the monitoring of their treatment, and in some instances should provide new insights into the underlying pathophysiological mechanisms.     

Epilepsy and autism spectrum disorders: Are there common developmental mechanisms?

Autistic spectrum disorders (ASD) and epilepsies are heterogeneous disorders that have diverse etiologies and pathophysiologies. The high rate of co-occurrence of these disorders suggest potentially shared underlying mechanisms. A number of well-known genetic disorders share epilepsy and autism as prominent phenotypic features, including tuberous sclerosis, Rett syndrome, and fragile X. In addition, mutations of several genes involved in neurodevelopment, including ARX, DCX, neuroligins and neuropilin2 have been identified in children with epilepsy, ASD or often both. Finally, in animal models, early-life seizures can result in cellular and molecular changes that could contribute to learning and behavioral disabilities as seen in ASD. Increased understanding of the common genetic, molecular and cellular mechanisms of ASD and epilepsy may provide insight into their underlying pathophysiology and elucidate new therapeutic approaches of both conditions.     

Treatment of peripheral neuropathies with neutrotrophic factors: animal models and clinical trials

In vitro experiments and works with knock-out mice have demonstrated the physiological importance of neurotrophic factors (NF) in the development and the survival of peripheral nervous system neurons. Therefore, NF may be useful in the treatment of peripheral neuropathies. These pathologies may be more amenable than central nervous diseases to the systemic delivery of NF. Indeed, NF can readily access peripheral nerves from blood whereas penetration into the central nervous system is limited by the blood-brain barrier. The objectives of NF treatment are: 1) to compensate a putative deficiency of NF associated with the pathogenesis of some neuropathies, such as diabetic neuropathy; 2) to stop or slow disease progression by acting on the biochemical pathways involved in the neurodegenerative cascade; and 3) to enhance the physiological compensatory mechanism of axonal sprouting. The efficacy of treatment with NF has been demonstrated in animal models mimicking various neuropathies, such as neuropathies related to diabetes or treatment with chemotherapeutic agents. However, a phase 3 trial in diabetic neuropathy and a phase 2 trial in HIV-related neuropathy have failed to demonstrate any substantial effect of treatment with NGF. In this review, we discuss the factors that may explain these negative results. A major limitation of systemic administration is the poor bioavailability of NF due to their short half-life. Alternative modes of delivery may be more appropriate than systemic administration of the recombinant protein. In particular, muscular-based gene therapy allows the delivery of sustained levels of neurotrophic factor into the circulation. This strategy has shown to be effective in animal models of motor and sensory neuropathies. Another promising treatment is the use of small molecules that induce the endogenous synthesis of NF, such as xaliprodene or 4-methylcathecol.     

Modèles animaux dans la maladie de Charcot-Marie-Tooth et applications de la compréhension de la maladie chez l’homme

Charcot-Marie-Tooth neuropathies (CMT) are inherited neuromuscular disorders caused by length-dependent neurodegeneration of peripheral nerves. More than 900 mutations in 60 different genes are responsible for Charcot-Marie-Tooth neuropathy. Despite significant progress in therapeutic strategies, the disease remains incurable. The increasing number of genes linked to the disease, and their considerable clinical and genetic heterogeneity renders the development of these strategies particularly challenging. In this context, cellular and animals models provide powerful tools. Efficient motor and sensory tests have been developed to assess the behavioral phenotype in transgenic animal models (rodents and fly). When these models reproduce a phenotype comparable to CMT, they allow therapeutic approaches and the discovery of modifiers and biomarkers. The majority of these models concern the demyelinating form (type 1) of the disease. The axonal form (type 2) is less common. Both forms can further be divided into multiple subtypes reflecting the heterogeneity of the disease. In this review, we describe the most convincing transgenic rodent and fly models of CMT and how some of them led to clinical trials.     

Molecular cell biology of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT), also named hereditary motor and sensory neuropathies, includes a clinically and genetically heterogeneous group of disorders affecting the peripheral nervous system. Traditionally, the different classes of CMT have been divided into demyelinating forms (CMT1, CMT3, and CMT4) and axonal forms (CMT2), a clinically very useful distinction. However, investigations of the underlying molecular and cellular disease mechanisms, mainly accomplished using cell culture and animal models, as well as specific re-examination of appropriate patient cohorts, have revealed that the pathological signs of myelinopathies and axonopathies are often intermingled. These findings, although only recently fully appreciated, are not surprising given the dependence and intimate cellular interactions of Schwann cells and neurons, mainly during nerve development and, as indicated by the pathology of CMT, also in the adult organism. This review is intended to summarize our current knowledge about the molecular and cellular basis of CMT, with a particular emphasis on the role of Schwann cell/axon interactions. Such a view is particularly timely since approximately ten genes have now been identified as culprits in different forms of CMT. This collection revealed novel crucial players in the interplay between Schwann cells and neurons. The analysis of these genes and their encoded proteins will provide additional insights into the molecular and cellular basis of neuropathies and valuable information about the biology and interactions of Schwann cells, their associated neurons, endoneurial fibroblasts, and the nerve-surrounding and protecting perineurial sheath. Electronic Publication     

Amyloid-induced neurofibrillary tangle formation in Alzheimer''s disease: insight from transgenic mouse and tissue-culture models

Of all forms of dementia, Alzheimer's disease is the most prevalent. It is histopathologically characterized by beta-amyloid-containing plaques, tau-containing neurofibrillary tangles, reduced synaptic density and neuronal loss in selected brain areas. For the rare familial forms of Alzheimer's disease, pathogenic mutations have been identified in both the gene encoding the precursor of the Abeta peptide, APP, itself and in the presenilin genes which encode part of the APP-protease complex. For the more frequent sporadic forms of Alzheimer's disease, the pathogenic trigger has not been unambiguously identified. Whether Abeta is again the main cause remains to be heavily discussed. In a related disorder termed frontotemporal dementia, which is characterized by tangles in the absence of beta-amyloid deposition, mutations have been identified in tau which also lead to neurodegeneration and dementia. For Alzheimer's disease the existence of familial forms lead to the proposition of the amyloid cascade hypothesis, which claims that beta-amyloid causes or enhances the tangle pathology. In this review, we describe tau transgenic mouse models in which aspects of the tau-associated pathology, including tangle formation, has been achieved. Moreover, tau transgenic mouse and tissue-culture models were used to test the amyloid cascade hypothesis. In addition, we discuss alternative hypotheses to explain the sporadic forms. The animal and tissue-culture models will provide insight into the underlying biochemical mechanisms of tau aggregation and nerve cell degeneration. These mechanisms may be partially shared between sporadic Alzheimer's disease, the familial forms and frontotemporal dementia. Eventually, Alzheimer's disease may be redefined based on biochemical events rather than phenotype.     

Impaired intracellular trafficking is a common disease mechanism of PMP22 point mutations in peripheral neuropathies

The most common forms of hereditary motor and sensory neuropathies (HMSN) or Charcot-Marie-Tooth disease (CMT) are associated with mutations affecting myelin genes in the peripheral nervous system. A minor subgroup of CMT type 1A (CMT1A) is caused by point mutations in the gene encoding the peripheral myelin protein 22 (PMP22). To study the mechanisms by which these mutations cause the CMT pathology, we transiently transfected COS7 and Schwann cells with wild-type and PMP22 expression constructs carrying six representative dominant or de novo point mutations and one putative recessive point mutation. All but one of the first group of mutant PMP22 proteins failed to be incorporated into the plasma membrane and were retained in intracellular compartments of transfected cells. Surprisingly, the recessive PMP22 mutation produced a protein that was also mildly impaired in trafficking. Thus, our results suggest a common disease mechanism underlying the pathology of CMT1A due to PMP22 point mutations.     

Molecular mechanisms of cognitive and behavioral comorbidities of epilepsy in children

Intellectual and developmental disabilities (IDDs) such as autistic spectrum disorders (ASDs) and epilepsies are heterogeneous disorders that have diverse etiologies and pathophysiologies. The high rate of co-occurrence of these disorders, however, suggests potentially shared underlying mechanisms. A number of well-known genetic disorders share epilepsy, intellectual disability, and autism as prominent phenotypic features, including tuberous sclerosis complex, Rett syndrome, and fragile X syndrome. In addition, mutations of several genes involved in neurodevelopment, including ARX, DCX, neuroligins, and neuropilin 2 have been identified in children with epilepsy, IDDs, ASDs, or a combination of thereof. Finally, in animal models, early life seizures can result in cellular and molecular changes that could contribute to learning and behavioral disabilities. Increased understanding of the common genetic, molecular, and cellular mechanisms of IDDs, ASDs, and epilepsy may provide insight into their underlying pathophysiology and elucidate new therapeutic approaches for these conditions.     

Charcot-marie-tooth disease and related neuropathies: molecular basis for distinction and diagnosis.

Great advances have been made in understanding the molecular basis of Charcot-Marie-Tooth disease (CMT) and related neuropathies, namely Dejerine-Sottas disease (DSD), hereditary neuropathy with liability to pressure palsies (HNPP) and congenital hypomyelination (CH). The number of newly uncovered mutations and identified genetic loci is rapidly increasing, and, as a consequence, the classification of these disorders is becoming more complicated. Molecular genetics, animal models, and transfected cell studies are shedding light on function and dysfunction of proteins involved in hereditary myelinopathies-peripheral myelin protein 22 (PMP22), myelin protein zero (PO), connexin 32 (Cx32), and early growth response 2 (EGR2). Gene dosage effect, loss of function, gain of toxic function, and dominant negative effect are possible mechanisms whereby different gene mutations may exert their detrimental action on peripheral nerves. A tentative rational approach to clinical and molecular diagnosis based on genotype-phenotype correlation analysis is described.     

Conditioning lesions before or after spinal cord injury recruit broad genetic mechanisms that sustain axonal regeneration: superiority to camp-mediated effects

Previous studies indicate that peripheral nerve conditioning lesions significantly enhance central axonal regeneration via modulation of cAMP-mediated mechanisms. To gain insight into the nature and temporal dependence of neural mechanisms underlying conditioning lesion effects on central axonal regeneration, we compared the efficacy of peripheral sciatic nerve crush lesions to cAMP elevations (in lumbar dorsal root ganglia) on central sensory axonal regeneration when administered either before or after cervical spinal cord lesions. We found significantly greater effects of conditioning lesions compared to cAMP elevations on central axonal regeneration when combined with cellular grafts at the lesion site and viral neurotrophin delivery; further, these effects persisted whether conditioning lesions were applied prior to or shortly after spinal cord injury. Indeed, conditioning lesions recruited extensively greater sets of genetic mechanisms of possible relevance to axonal regeneration compared to cAMP administration, and sustained these changes for significantly greater time periods through the post-lesion period. We conclude that cAMP-mediated mechanisms account for only a portion of the potency of conditioning lesions on central axonal regeneration, and that recruitment of broader genetic mechanisms can extend the effect and duration of cellular events that support axonal growth.