Transgenic Mouse Models of CMT1A and HNPP

Anesthetic Considerations for Patients with Hereditary Neuropathy with Liability to Pressure Palsies: A Narrative Review

Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant demyelinating neuropathy characterized by an increased susceptibility to peripheral nerve injury from trauma, compression, or shear forces. Patients with this condition are unique, necessitating distinct considerations for anesthesia and surgical teams. This review describes the etiology, prevalence, clinical presentation, and management of HNPP and presents contemporary evidence and recommendations for optimal care for HNPP patients in the perioperative period. While the incidence of HNPP is reported at 7-16:100,000, this figure may be an underestimation due to underdiagnosis, further complicating medicolegal issues. With the subtle nature of symptoms associated with HNPP, patients with this condition may remain unrecognized during the perioperative period, posing significant risks. Several aspects of caring for this population, including anesthetic choices, intraoperative positioning, and monitoring strategy, may deviate from standard practices. As such, a tailored approach to caring for this unique population, coupled with meticulous preoperative planning, is crucial and requires a multidisciplinary approach.

The LITAF/SIMPLE I92V sequence variant results in an earlier age of onset of CMT1A/HNPP diseases

Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) represent the most common heritable neuromuscular disorders. Molecular diagnostics of CMT1A/HNPP diseases confirm clinical diagnosis, but their value is limited to the clinical course and prognosis. However, no biomarkers of CMT1A/HNPP have been identified. We decided to explore if the LITAF/SIMPLE gene shared a functional link to the PMP22 gene, whose duplication or deletion results in CMT1A and HNPP, respectively. By studying a large cohort of CMT1A/HNPP-affected patients, we found that the LITAF I92V sequence variant predisposes patients to an earlier age of onset of both the CMT1A and HNPP diseases. Using cell transfection experiments, we showed that the LITAF I92V sequence variant partially mislocalizes to the mitochondria in contrast to wild-type LITAF which localizes to the late endosome/lysosomes and is associated with a tendency for PMP22 to accumulate in the cells. Overall, this study shows that the I92V LITAF sequence variant would be a good candidate for a biomarker in the case of the CMT1A/HNPP disorders.

Regulation of PMP22 mRNA by G3BP1 affects cell proliferation in breast cancer cells

Background

Regulation of mRNAs is one way to control protein levels and thereby important cellular processes such as growth, invasion and apoptosis. G3BPs constitute a family of mRNA-binding proteins, shown to be overexpressed in several cancer types, including breast, colon and pancreas cancer. G3BP has been reported to both stabilize and induce degradation of specific mRNAs.

Results

Here, we show that G3BP1, but not G3BP2, supports proliferation of several breast cancer cell lines. Global gene expression analyses of G3BP1- and G3BP2-depleted cells indicate that primarily G3BP1, and much less G3BP2, influences mRNA expression levels. Peripheral myelin protein 22 (PMP22) was one gene that was significantly influenced by G3BP1 depletion which led to a 2–3 fold increased expression. Depletion of PMP22 resulted in increased proliferation and the G3BP1-mediated effect on proliferation was not seen upon PMP22-depletion.

Conclusions

This indicates a novel role for G3BP1 in the regulation of cell proliferation in breast cancer cells, perhaps via a regulatory effect on PMP22 expression.

Myelin is dependent on the Charcot-Marie-Tooth Type 4H disease culprit protein FRABIN/FGD4 in Schwann cells

Studying the function and malfunction of genes and proteins associated with inherited forms of peripheral neuropathies has provided multiple clues to our understanding of myelinated nerves in health and disease. Here, we have generated a mouse model for the peripheral neuropathy Charcot-Marie-Tooth disease type 4H by constitutively disrupting the mouse orthologue of the suspected culprit gene FGD4 that encodes the small RhoGTPase Cdc42-guanine nucleotide exchange factor Frabin. Lack of Frabin/Fgd4 causes dysmyelination in mice in early peripheral nerve development, followed by profound myelin abnormalities and demyelination at later stages. At the age of 60 weeks, this was accompanied by electrophysiological deficits. By crossing mice carrying alleles of Frabin/Fgd4 flanked by loxP sequences with animals expressing Cre recombinase in a cell type-specific manner, we show that Schwann cell-autonomous Frabin/Fgd4 function is essential for proper myelination without detectable primary contributions from neurons. Deletion of Frabin/Fgd4 in Schwann cells of fully myelinated nerve fibres revealed that this protein is not only required for correct nerve development but also for accurate myelin maintenance. Moreover, we established that correct activation of Cdc42 is dependent on Frabin/Fgd4 function in healthy peripheral nerves. Genetic disruption of Cdc42 in Schwann cells of adult myelinated nerves resulted in myelin alterations similar to those observed in Frabin/Fgd4-deficient mice, indicating that Cdc42 and the Frabin/Fgd4-Cdc42 axis are critical for myelin homeostasis. In line with known regulatory roles of Cdc42, we found that Frabin/Fgd4 regulates Schwann cell endocytosis, a process that is increasingly recognized as a relevant mechanism in peripheral nerve pathophysiology. Taken together, our results indicate that regulation of Cdc42 by Frabin/Fgd4 in Schwann cells is critical for the structure and function of the peripheral nervous system. In particular, this regulatory link is continuously required in adult fully myelinated nerve fibres. Thus, mechanisms regulated by Frabin/Fgd4-Cdc42 are promising targets that can help to identify additional regulators of myelin development and homeostasis, which may crucially contribute also to malfunctions in different types of peripheral neuropathies.

Sensorimotor and cognitive function of a NEFL(P22S) mutant model of Charcot-Marie-Tooth disease type 2E

Charcot-Marie-Tooth (CMT) disease is the most frequently encountered hereditary disease causing sensorimotor neuropathies and slowly progressive muscle weakness and atrophy. The P22S mutation of the NEFL gene encoding the light polypeptide neurofilament (NFL) is associated with CMT. To understand more clearly the pathogenesis of sensorimotor dysfunction in CMT, we generated transgenic mice with the NEFL(P22S) mutation under the tet-off tetracycline regulated system with involvement of the Thy1 neuron-specific promoter. NEFL(P22S) transgenic mice exhibited extended duration of the hindlimb clasping response and gait anomalies, as well as sensorimotor deficits in stationary beam and suspended bar tests. In addition, the NEFL(P22S) mice were deficient in the reversal phase of left-right discrimination learning in a water maze. This model mimics some aspects of human CMT pathology and provides an opportunity of ameliorating CMT symptoms with experimental therapies.

Biology of peripheral inherited neuropathies: Schwann cell axonal interactions

Development and maintenance of PNS myelin depends on continual signaling from axons ensheathed by myelin. Recent advances have demonstrated the roles of neuregulin 1 type III, Erb2/3 and intracellular signal transduction pathways in inducing Schwann cell myelination. Alternatively, maintenance of myelinated axons depends on healthy myelinating Schwann cells. Axonal degeneration is a feature of virtually all inherited demyelinating neuropathies and in many cases is more responsible for clinical impairment than the primary demyelination. Signaling mechanisms through which demyelinating Schwann cells damage axons are not well understood. In this review several examples of potential mechanisms by which demyelinating neuropathies damage axons will be presented. Understanding the molecular basis of Schwann cell-axonal interactions will not only increase the understanding of PNS biology but also identify therapeutic targets for inherited neuropathies.

AFM combines functional and morphological analysis of peripheral myelinated and demyelinated nerve fibers

Demyelination of the myelinated peripheral or central axon is a common pathophysiological step in the clinical manifestation of several human diseases of the peripheral and the central nervous system such as the majority of Charcot-Marie-Tooth syndromes and multiple sclerosis, respectively. The structural degradation of the axon insulating myelin sheath has profound consequences for ionic conduction and nerve function in general, but also affects the micromechanical properties of the nerve fiber. We have for the first time investigated mechanical properties of rehydrated, isolated peripheral nerve fibers from mouse using atomic force microscopy (AFM). We have generated quantitative maps of elastic modulus along myelinated and demyelinated axons, together with quantitative maps of axon topography. This study shows that AFM can combine functional and morphological analysis of neurological tissue at the level of single nerve fibers.

Comparison of Schwann cell and sciatic nerve transcriptomes indicates that mouse is a valid model for the human peripheral nervous system

High-throughput gene expression analyses of murine models of the peripheral nervous system (PNS), and its cellular components, have yielded enormous amounts of expression data of the PNS in various conditions. These data provided clues for future research directions to further decipher this complex organ in relation to acquired and inherited PNS diseases. Various studies addressing the validity of mouse models for human conditions in other tissues and cell types have indicated that in many cases the mouse model only poorly represents the human situation. To determine how well the mouse can serve as model to study the biological processes occurring in the PNS, we compared the gene expression profiles that we generated for mouse and human sciatic nerve and cultured Schwann cells derived thereof. A two-way analysis based on the differentially expressed genes between the sciatic nerve and the cultured Schwann cell, and which takes into account the differential expression between mouse and man, indicates that the human PNS is well represented by that of the mouse in terms of the "biological processes" ontology.

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.

Steroid hormones and neurosteroids in normal and pathological aging of the nervous system

Without medical progress, dementing diseases such as Alzheimer's disease will become one of the main causes of disability. Preventing or delaying them has thus become a real challenge for biomedical research. Steroids offer interesting therapeutical opportunities for promoting successful aging because of their pleiotropic effects in the nervous system: they regulate main neurotransmitter systems, promote the viability of neurons, play an important role in myelination and influence cognitive processes, in particular learning and memory. Preclinical research has provided evidence that the normally aging nervous system maintains some capacity for regeneration and that age-dependent changes in the nervous system and cognitive dysfunctions can be reversed to some extent by the administration of steroids. The aging nervous system also remains sensitive to the neuroprotective effects of steroids. In contrast to the large number of studies documenting beneficial effects of steroids on the nervous system in young and aged animals, the results from hormone replacement studies in the elderly are so far not conclusive. There is also little information concerning changes of steroid levels in the aging human brain. As steroids present in nervous tissues originate from the endocrine glands (steroid hormones) and from local synthesis (neurosteroids), changes in blood levels of steroids with age do not necessarily reflect changes in their brain levels. There is indeed strong evidence that neurosteroids are also synthesized in human brain and peripheral nerves. The development of a very sensitive and precise method for the analysis of steroids by gas chromatography/mass spectrometry (GC/MS) offers new possibilities for the study of neurosteroids. The concentrations of a range of neurosteroids have recently been measured in various brain regions of aged Alzheimer's disease patients and aged non-demented controls by GC/MS, providing reference values. In Alzheimer's patients, there was a general trend toward lower levels of neurosteroids in different brain regions, and neurosteroid levels were negatively correlated with two biochemical markers of Alzheimer's disease, the phosphorylated tau protein and the beta-amyloid peptides. The metabolism of dehydroepiandrosterone has also been analyzed for the first time in the aging brain from Alzheimer patients and non-demented controls. The conversion of dehydroepiandrosterone to Delta5-androstene-3beta,17beta-diol and to 7alpha-OH-dehydroepiandrosterone occurred in frontal cortex, hippocampus, amygdala, cerebellum and striatum of both Alzheimer's patients and controls. The formation of these metabolites within distinct brain regions negatively correlated with the density of beta-amyloid deposits.

Myelination and motor coordination are increased in transferrin transgenic mice

Myelin deficiency in the central nervous system (CNS) can cause severe disabling conditions. Most of the transgenic mice models overexpressing myelin components have limitations for investigators of myelin deficiency and myelin therapy as they severely alter CNS architecture. It has been postulated that transferrin (Tf) is involved in oligodendrocyte (OL) maturation and myelinogenesis. Because Tf is not an intrinsic myelin constituent, we decided to investigate if its overexpression could have an impact on the myelination process without affecting myelin integrity. We generated transgenic mice containing the complete human Tf gene specifically overexpressed in OLs. This overexpression leads to more than a 30% increase in myelin components, such as galactolipids, phospholipids, and proteins. Electron microscopy showed that myelin is structurally normal in terms of thickness and compaction. Behavior analysis showed that mice do not display significant modifications in their locomotion and cognitive and emotional abilities. Furthermore, in one of the genetic background, animals presented a significant increase in motor coordination. We did not find any modification in OL number during early postnatal development, suggesting that Tf does not act on OL proliferation. In addition, the levels of iron and ferritin remained unchanged in the brain of transgenic mice compared to control mice. Our findings indicate that, besides its known iron transport function, Tf is able to influence myelination process and induce behavioral improvements in mice.

Close relationship between motor impairments and loss of functional motoneurons in a Charcot-Marie-Tooth type 1A model

Charcot-Marie-Tooth disease type 1A is the most frequent hereditary neuropathy affecting the peripheral nervous system. A partial duplication of chromosome 17 (17p11.2) involving the PMP22 gene is responsible for dysmyelination-demyelination processes leading to motor and sensory impairments. Murine models of this disease are now widely used to investigate the mechanisms occurring at the behavioural and physiological levels. In this study, adult transgenic mice (6 months old) having integrated 7 copies of the human PMP22 gene were used to compare the motor performance, evaluated by using a complex locomotor test (the rotarod test), with both the number of functional motoneurons innervating the soleus muscle and the level of myelination in the sciatic nerve. Two levels of motor deficits were detected and led us to divide the population into two subgroups. In both impaired groups, the level of motor deficit was strongly correlated with the number of functional motoneurons evaluated by retrograde labeling from the muscle, but not with the number of myelinated fibers or the thickness of the myelin sheath (g-ratio). It therefore appears that the number of motor units may be a key element in motor impairments observed in Charcot-Marie-Tooth disease type 1A disease. These findings may have implications for therapeutic procedures, which should focus on the survival of the motoneuronal pool and/or the maintenance of functional neuro-muscular connexions to reduce motor impairments in humans.

Tight junction proteins

A fundamental function of epithelia and endothelia is to separate different compartments within the organism and to regulate the exchange of substances between them. The tight junction (TJ) constitutes the barrier both to the passage of ions and molecules through the paracellular pathway and to the movement of proteins and lipids between the apical and the basolateral domains of the plasma membrane. In recent years more than 40 different proteins have been discovered to be located at the TJs of epithelia, endothelia and myelinated cells. This unprecedented expansion of information has changed our view of TJs from merely a paracellular barrier to a complex structure involved in signaling cascades that control cell growth and differentiation. Both cortical and transmembrane proteins integrate TJs. Among the former are scaffolding proteins containing PDZ domains, tumor suppressors, transcription factors and proteins involved in vesicle transport. To date two components of the TJ filaments have been identified: occludin and claudin. The latter is a protein family with more than 20 members. Both occludin and claudins are integral proteins capable of interacting adhesively with complementary molecules on adjacent cells and of co-polymerizing laterally. These advancements in the knowledge of the molecular structure of TJ support previous physiological models that exhibited TJ as dynamic structures that present distinct permeability and morphological characteristics in different tissues and in response to changing natural, pathological or experimental conditions.

Inducible site-specific recombination in myelinating cells

To explore the function of genes expressed by myelinating cells we have developed a model system that allows for the inducible ablation of predetermined genes in oligodendrocytes and Schwann cells. The Cre/loxP recombination system provides the opportunity to generate tissue-specific somatic mutations in mice. We have used a fusion protein between the Cre recombinase and a mutated ligand-binding domain of the human estrogen receptor (CreER(T)) to obtain inducible, site-specific recombination. CreER(T) expression was placed under the transcriptional control of the regulatory sequences of the myelin proteolipid protein (PLP) gene, which is abundantly expressed in oligodendrocytes and to a lesser extent in Schwann cells. The CreER(T) fusion protein translocated to the nucleus and mediated the recombination of a LacZ reporter transgene in myelinating cells of PLP/CreER(T) mice injected with the synthetic steroid tamoxifen. In untreated animals CreER(T) remained cytoplasmic, and there was no evidence of recombination. The PLP/ CreER(T) animals should be very useful in elucidating and distinguishing a particular gene's function in the formation and maintenance of the myelin sheath and in analyzing mature oligodendrocyte function in pathological conditions.

Proliferation of Schwann cells and regulation of cyclin D1 expression in an animal model of Charcot-Marie-Tooth disease type 1A

Overexpression of PMP22 is responsible for the most common form of inherited neuropathy, Charcot-Marie-Tooth disease (CMT) type 1A. The PMP22-transgenic rat (CMT rat) is an animal model of CMT1A, and its peripheral nerves show the characteristic features of ongoing demyelination and remyelination that is also seen in CMT1A patients. Since Schwann cell proliferation is a prominent feature of peripheral nerves in inherited peripheral neuropathies, we examined proliferation and the expression of cyclin D1 in CMT rats. D-type cyclins are required for the initial steps in cell division and nuclear import is crucial for the function of cyclin D1 in promoting cell proliferation. Like normal myelinating Schwann cells in wild-type rats, remyelinating Schwann cells in CMT rats show perinuclear cyclin D1 expression. Schwann cells with nuclear cyclin D1 expression, as well as proliferating Schwann cells, were both associated with demyelinated axonal segments. Supernumerary onion bulb Schwann cells, however, do not express cyclin D1 and were not proliferating. Thus, cyclin D1 expression and its subcellular localization correlate directly with distinct physiological states of Schwann cells in this animal model of CMT1A.

Progesterone synthesis and myelin formation in peripheral nerves

Progesterone is synthesized in the nervous system by neurons and glial cells. Because of their simple structure, plasticity and capacity of regeneration, peripheral nerves are particularly well suited for studying the biosynthesis, mechanisms of action and effects of the hormone. Schwann cells, the myelinating glial cells in the peripheral nervous system, synthesize progesterone in response to a diffusible neuronal signal. In peripheral nerves, the local synthesis of progesterone plays an important role in the formation of myelin sheaths. This has been shown in vivo, after cryolesion of the mouse sciatic nerve, and in vitro, in cocultures of Schwann cells and sensory neurons. Schwann cells also express an intracellular receptor for progesterone, which thus functions as an autocrine signalling molecule. Progesterone may promote myelination by activating the expression of genes coding for transcription factors (Krox-20) and/or for myelin proteins (P0, PMP22). Recently, it has been proposed that progesterone may indirectly regulate myelin formation by influencing gene expression in neurons. Steroid hormones also influence the proliferation of Schwann cells: estradiol becomes a potent mitogen for Schwann cells when levels of cAMP are elevated and glucocorticosteroids have been shown to increase the mitogenic effects of peptide growth factors.

Identification of a positive regulatory element in the myelin-specific promoter of the PMP22 gene

Over- and under expression of the 22 kDa peripheral myelin protein (PMP22) results in dysmyelinating peripheral neuropathies, such as Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy, with the liability to pressure palsies (HNPP). Expression of the PMP22 gene is driven by two alternative promoters, P1 and P2, with transcripts originating from P1 associated with peripheral nerve myelination by Schwann cells. Transient transfections of constructs containing P1 (3.5 kb) or P2 (2.5 kb) resulted in high levels of reporter gene expression in the RT4-D6P2T schwannoma cell line. Serial deletions of P1 revealed that region P1-A (-105 to -43), situated upstream of the minimal promoter, contained a positive regulatory element. The 62 bp P1-A region conferred in cis a sevenfold increase in expression of luciferase driven by a heterologous promoter in an orientation-dependent manner. Interspecies comparison of the P1-A region revealed a 98% degree of identity between the human, mouse, and rat sequences. A prominent sequence-dependent DNA-protein complex (C-I) was detected in electrophoretic mobility shift assays with P1-A using RT4-D6P2T nuclear extract and was localized to a minimal 21 bp region within P1-A. Site-directed mutagenesis of this region revealed nucleotides at positions -46 to -43 as being necessary for formation of C-I. Functional analysis of the mutated P1-A element indicated that positions -46 and -45 were essential for transactivation mediated by this element. Characterization of the transacting factor(s) interacting with this key regulatory element will shed light on its role in regulating peripheral nerve myelination.

Behavioural profiling of a murine Charcot-Marie-Tooth disease type 1A model

Different features of motor behaviour were studied on a transgenic mouse model of Charcot-Marie-Tooth's disease (CMT). Mutants with 4 or 7 copies of the human PMP22 gene leading to a phenotype significantly close to CMT's disease type 1A were compared with control animals. The aim of the study was to validate this transgenic model and to characterise the impairments occurring in the various lines. Three main types of analysis were performed in 2-month-old mice without any peculiar visible deficit: (i) a study of standardised clinical tests (SHIRPA protocol) demonstrated that only a few motor deficits were expressed; (ii) a measurement of general spontaneous activity by means of a commercial video-tracking system was performed and revealed that the main spontaneous activities were identical in the three lines with, however, some slight localised modifications; and, (iii) by contrast, the three lines respond very differently to the footprints, grip strength, splay test and rotarod test. Even in lines with a significantly limited copy number of the transgene, we observed and quantified impairments. In conclusion, mutants of CMT1A seem to be a very pertinent model of this human pathology and will certainly be useful for therapeutic procedures and for theoretical studies on this disease.

Membrane topology of peripheral myelin protein 22

Peripheral myelin protein 22 (PMP22) is a structural component of compact peripheral nerve myelin and is likely to play a role in the modulation of cell proliferation and cell spreading. Molecular genetics revealed that mutations affecting the PMP22 gene are responsible for the most common forms of hereditary motor and sensory neuropathies in humans. Computer analysis predicts a tetraspan-membrane structure for the PMP22 protein. We have assessed the topology of PMP22 experimentally using chimeric proteins consisting of different PMP22 domains fused to reporter genes and internally tagged molecules. Based on in vitro transcription/translation assays and immunohistochemical analysis of transfected cells, we propose that PMP22 can adopt a non-tetraspan topology that has functional implications in normal and disease processes.