Peripheral neuropathy in the twitcher mouse: accumulation of extracellular matrix in the endoneurium and aberrant expression of ion channels

Graded Elevation of c-Jun in Schwann Cells In Vivo: Gene Dosage Determines Effects on Development, Remyelination, Tumorigenesis, and Hypomyelination

Schwann cell c-Jun is implicated in adaptive and maladaptive functions in peripheral nerves. In injured nerves, this transcription factor promotes the repair Schwann cell phenotype and regeneration and promotes Schwann-cell-mediated neurotrophic support in models of peripheral neuropathies. However, c-Jun is associated with tumor formation in some systems, potentially suppresses myelin genes, and has been implicated in demyelinating neuropathies. To clarify these issues and to determine how c-Jun levels determine its function, we have generated c-Jun OE/+ and c-Jun OE/OE mice with graded expression of c-Jun in Schwann cells and examined these lines during development, in adulthood, and after injury using RNA sequencing analysis, quantitative electron microscopic morphometry, Western blotting, and functional tests. Schwann cells are remarkably tolerant of elevated c-Jun because the nerves of c-Jun OE/+ mice, in which c-Jun is elevated ∼6-fold, are normal with the exception of modestly reduced myelin thickness. The stronger elevation of c-Jun in c-Jun OE/OE mice is, however, sufficient to induce significant hypomyelination pathology, implicating c-Jun as a potential player in demyelinating neuropathies. The tumor suppressor P19ARF is strongly activated in the nerves of these mice and, even in aged c-Jun OE/OE mice, there is no evidence of tumors. This is consistent with the fact that tumors do not form in injured nerves, although they contain proliferating Schwann cells with strikingly elevated c-Jun. Furthermore, in crushed nerves of c-Jun OE/+ mice, where c-Jun levels are overexpressed sufficiently to accelerate axonal regeneration, myelination and function are restored after injury.SIGNIFICANCE STATEMENT In injured and diseased nerves, the transcription factor c-Jun in Schwann cells is elevated and variously implicated in controlling beneficial or adverse functions, including trophic Schwann cell support for neurons, promotion of regeneration, tumorigenesis, and suppression of myelination. To analyze the functions of c-Jun, we have used transgenic mice with graded elevation of Schwann cell c-Jun. We show that high c-Jun elevation is a potential pathogenic mechanism because it inhibits myelination. Conversely, we did not find a link between c-Jun elevation and tumorigenesis. Modest c-Jun elevation, which is beneficial for regeneration, is well tolerated during Schwann cell development and in the adult and is compatible with restoration of myelination and nerve function after injury.

Substrate reduction therapy for Krabbe's disease

Krabbe's disease (KD) is a lysosomal storage disorder in which galactosylceramide, a major glycosphingolipid of myelin, and psychosine (galactose-sphingosine) cannot be adequately metabolized because of a deficiency in galactosylceramidase. Substrate reduction therapy (SRT) has been tested in preclinical studies. The premise of SRT is to reduce the synthesis of substrates that are not adequately digested so that the substrate burden is lowered, resulting in less accumulation of unmetabolized material. SRT is used for Gaucher's disease, in which inhibitors of the terminal biosynthetic step are used. Unfortunately, an inhibitor for the final step of galactosylceramide biosynthesis, i.e., UDP glycosyltransferase 8 (a.k.a. UDP-galactose ceramide galactosyltransferase), has not been found. Approaches that inhibit an earlier biosynthetic step or that lessen the substrate burden by other means, such as genetic manipulations, have been tested in the twitcher mouse model of KD. Either as a stand-alone therapy or in combination with other approaches, SRT slowed the disease course, indicating that this approach has potential therapeutic value. For instance, in individuals with adult-onset disease, SRT theoretically could lessen the production of substrates so that residual enzymatic activity could adequately manage the lower substrate burden. In more severe forms of disease, SRT theoretically could be part of a combination therapy. However, SRT has the potential to impair normal function by reducing the synthesis of galactosylceramide to levels that impede myelin function, or SRT could have other deleterious effects. Thus, multiple issues need to be resolved before this approach is ready for testing in humans. © 2016 Wiley Periodicals, Inc.

Two factor-based reprogramming of rodent and human fibroblasts into Schwann cells

Schwann cells (SCs) generate the myelin wrapping of peripheral nerve axons and are promising candidates for cell therapy. However, to date a renewable source of SCs is lacking. In this study, we show the conversion of skin fibroblasts into induced Schwann cells (iSCs) by driving the expression of two transcription factors, Sox10 and Egr2. iSCs resembled primary SCs in global gene expression profiling and PNS identity. In vitro, iSCs wrapped axons generating compact myelin sheaths with regular nodal structures. Conversely, iSCs from Twitcher mice showed a severe loss in their myelinogenic potential, demonstrating that iSCs can be an attractive system for in vitro modelling of PNS diseases. The same two factors were sufficient to convert human fibroblasts into iSCs as defined by distinctive molecular and functional traits. Generating iSCs through direct conversion of somatic cells offers opportunities for in vitro disease modelling and regenerative therapies.

Schwann cells (SCs) myelinate peripheral nerve axons and offer opportunities for the treatment of injuries and demyelinating diseases but reliable and renewable sources of these cells are hard to come by. Here the authors reprogram rat, mouse and human fibroblasts into Schwann cells using two transcription factors.

Mechanism-based combination treatment dramatically increases therapeutic efficacy in murine globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD, Krabbe disease) is a lysosomal storage disease (LSD) caused by a deficiency in galactocerebrosidase (GALC) activity. In the absence of GALC activity, the cytotoxic lipid, galactosylsphingosine (psychosine), accumulates in the CNS and peripheral nervous system. Oligodendrocytes and Schwann cells are particularly sensitive to psychosine, thus leading to a demyelinating phenotype. Although hematopoietic stem-cell transplantation provides modest benefit in both presymptomatic children and the murine model (Twitcher), there is no cure for GLD. In addition, GLD has been relatively refractory to virtually every experimental therapy attempted. Here, Twitcher mice were simultaneously treated with CNS-directed gene therapy, substrate reduction therapy, and bone marrow transplantation to target the primary pathogenic mechanism (GALC deficiency) and two secondary consequences of GALC deficiency (psychosine accumulation and neuroinflammation). Simultaneously treating multiple pathogenic targets resulted in an unprecedented increase in life span with improved motor function, persistent GALC expression, nearly normal psychosine levels, and decreased neuroinflammation. Treating the primary pathogenic mechanism and secondary targets will likely improve therapeutic efficacy for other LSDs with complex pathological and clinical presentations.

Multipotent stromal cells alleviate inflammation, neuropathology, and symptoms associated with globoid cell leukodystrophy in the twitcher mouse

Globoid cell leukodystrophy (GLD) is a common neurodegenerative lysosomal storage disorder caused by a deficiency in galactocerebrosidase (GALC), an enzyme that cleaves galactocerebroside during myelination. Bone marrow transplantation has shown promise when administered to late-onset GLD patients. However, the side effects (e.g., graft vs. host disease), harsh conditioning regimens (e.g., myelosuppression), and variable therapeutic effects make this an unsuitable option for infantile GLD patients. We previously reported modest improvements in the twitcher mouse model of GLD after intracerebroventricular (ICV) injections of a low-dose of multipotent stromal cells (MSCs). Goals of this study were to improve bone marrow-derived MSC (BMSC) therapy for GLD by increasing the cell dosage and comparing cell type (e.g., transduced vs. native), treatment timing (e.g., single vs. weekly), and administration route (e.g., ICV vs. intraperitoneal [IP]). Neonatal twitcher mice received (a) 2 × 10(5) BMSCs by ICV injection, (b) 1 × 10(6) BMSCs by IP injection, (c) weekly IP injections of 1 × 10(6) BMSCs, or (d) 1 × 10(6) lentiviral-transduced BMSCs overexpressing GALC (GALC-BMSC) by IP injection. All treated mice lived longer than untreated mice. However, the mice receiving peripheral MSC therapy had improved motor function (e.g., hind limb strength and rearing ability), twitching symptoms, and weight compared to both the untreated and ICV-treated mice. Inflammatory cell, globoid cell, and apoptotic cell levels in the sciatic nerves were significantly decreased as a result of the GALC-BMSC or weekly IP injections. The results of this study indicate a promising future for peripheral MSC therapy as a noninvasive, adjunct therapy for patients affected with GLD.

Increased activation of the epidermal growth factor receptor in transgenic mice overexpressing epigen causes peripheral neuropathy

In the mammalian nervous system, axons are commonly surrounded by myelin, a lipid-rich sheath that is essential for precise and rapid conduction of nerve impulses. In the peripheral nervous system (PNS), myelin sheaths are formed by Schwann cells which wrap around individual axons. While the tyrosine kinase receptors ERBB2 and ERBB3 are established mediators of peripheral myelination, less is known about the functions of the related epidermal growth factor receptor (EGFR) in the regulation of PNS myelination. Here, we report a peripheral neurodegenerative disease caused by increased EGFR activation. Specifically, we characterize a symmetric and distally pronounced, late-onset muscular atrophy in transgenic mice overexpressing the EGFR ligand epigen. Histological examination revealed a demyelinating neuropathy and axon degeneration, and molecular analysis of signaling pathways showed reduced protein kinase B (PKB, AKT) activation in the nerves of Epigen-tg mice, indicating that the muscular phenotype is secondary to PNS demyelination and axon degeneration. Crossing of Epigen-tg mice into an EGFR-deficient background revealed the pathology to be completely EGFR-dependent. This mouse line provides a new model for studying molecular events associated with early stages of peripheral neuropathies, an essential prerequisite for the development of successful therapeutic interventions.

Axonopathy is a compounding factor in the pathogenesis of Krabbe disease

Loss-of-function of the lysosomal enzyme galactosyl-ceramidase causes the accumulation of the lipid raft-associated sphingolipid psychosine, the disruption of postnatal myelination, neurodegeneration and early death in most cases of infantile Krabbe disease. This work presents a first study towards understanding the progression of axonal defects in this disease using the Twitcher mutant mouse. Axonal swellings were detected in axons within the mutant spinal cord as early as 1 week after birth. As the disease progressed, more axonopathic profiles were found in other regions of the nervous system, including peripheral nerves and various brain areas. Isolated mutant neurons recapitulated axonal and neuronal defects in the absence of mutant myelinating glia, suggesting an autonomous neuronal defect. Psychosine was sufficient to induce axonal defects and cell death in cultures of acutely isolated neurons. Interestingly, axonopathy in young Twitcher mice occurred in the absence of demyelination and of neuronal apoptosis. Neuronal damage occurred at later stages, when mutant mice were moribund and demyelinated. Altogether, these findings suggest a progressive dying-back neuronal dysfunction in Twitcher mutants.

Transplant outcomes in leukodystrophies

Hematopoietic stem cell transplantation (HSCT) has been used for three decades as therapy for lysosomal storage diseases. Stable engraftment following transplantation has the potential to provide a source of an enzyme for the life of a patient. Recombinant enzyme is available for disorders that do not have a primary neurologic component. However, for diseases affecting the central nervous system (CNS), intravenous enzyme is ineffective due to its inability to cross the blood-brain barrier. For selected lysosomal disorders, including metachromatic leukodystrophy and globoid cell leukodystrophy, disease phenotype and the extent of disease at the time of transplantation are of fundamental importance in determining outcomes. Adrenoleukodystrophy is an X-linked, peroxisomal disorder, and in approximately 40% of cases a progressive, inflammatory condition develops in the CNS. Early in the course of the disease, allogeneic transplantation can arrest the disease process in cerebral adrenoleukodystrophy, while more advanced patients do poorly. In many of these cases, the utilization of cord blood grafts allows expedient transplantation, which can be critical in achieving optimal outcomes.