A nonsense mutation in myelin protein zero causes congenital hypomyelination neuropathy through altered P0 membrane targeting and gain of abnormal function.

Protein zero (P0) is the major structural protein in peripheral myelin, and mutations in the Myelin Protein Zero (Mpz) gene produce wide-ranging hereditary neuropathy phenotypes. To gain insight in the mechanisms underlying a particularly severe form, congenital hypomyelination (CH), we targeted mouse Mpz to encode P0Q215X, a nonsense mutation associated with the disease, that we show escapes nonsense mediated decay and is expressed in CH patient nerves. The knock-in mice express low levels of the resulting truncated protein, producing a milder phenotype when compared to patients, allowing to dissect the subtle pathogenic mechanisms occurring in otherwise very compromised peripheral myelin. We find that P0Q215X does not elicit an unfolded protein response, which is a key mechanism for other pathogenic MPZ mutations, but is instead in part aberrantly trafficked to non-myelin plasma membranes and induces defects in radial sorting of axons by Schwann cells. We show that the loss of the C-terminal Tyr-Ala-Met-Leu motif is responsible for P0 mislocalization, as its addition is able to restore correct P0Q215X trafficking in vitro. Lastly, we show that P0Q215X acts through dose-dependent gain of abnormal function, as wild-type P0 is unable to rescue the hypomyelination phenotype. Collectively, these data indicate that alterations at the premyelinating stage, linked to altered targeting of P0, may be responsible for CH, and that different types of gain of abnormal function produce the diverse neuropathy phenotypes associated with MPZ, supporting future allele-specific therapeutic silencing strategies.

Neuregulin 1 type III improves peripheral nerve myelination in a mouse model of congenital hypomyelinating neuropathy.

Myelin sheath thickness is precisely regulated and essential for rapid propagation of action potentials along myelinated axons. In the peripheral nervous system, extrinsic signals from the axonal protein neuregulin 1 (NRG1) type III regulate Schwann cell fate and myelination. Here we ask if modulating NRG1 type III levels in neurons would restore myelination in a model of congenital hypomyelinating neuropathy (CHN). Using a mouse model of CHN, we improved the myelination defects by early overexpression of NRG1 type III. Surprisingly, the improvement was independent from the upregulation of Egr2 or essential myelin genes. Rather, we observed the activation of MAPK/ERK and other myelin genes such as peripheral myelin protein 2 and oligodendrocyte myelin glycoprotein. We also confirmed that the permanent activation of MAPK/ERK in Schwann cells has detrimental effects on myelination. Our findings demonstrate that the modulation of axon-to-glial NRG1 type III signaling has beneficial effects and improves myelination defects during development in a model of CHN.

Novel bicistronic lentiviral vectors correct ß-Hexosaminidase deficiency in neural and hematopoietic stem cells and progeny: implications for in vivo and ex vivo gene therapy of GM2 gangliosidosis.

The favorable outcome of in vivo and ex vivo gene therapy approaches in several Lysosomal Storage Diseases suggests that these treatment strategies might equally benefit GM2 gangliosidosis. Tay-Sachs and Sandhoff disease (the main forms of GM2 gangliosidosis) result from mutations in either the HEXA or HEXB genes encoding, respectively, the α- or ß-subunits of the lysosomal ß-Hexosaminidase enzyme. In physiological conditions, α- and ß-subunits combine to generate ß-Hexosaminidase A (HexA, αß) and ß-Hexosaminidase B (HexB, ßß). A major impairment to establishing in vivo or ex vivo gene therapy for GM2 gangliosidosis is the need to synthesize the α- and ß-subunits at high levels and with the correct stoichiometric ratio, and to safely deliver the therapeutic products to all affected tissues/organs. Here, we report the generation and in vitro validation of novel bicistronic lentiviral vectors (LVs) encoding for both the murine and human codon optimized Hexa and Hexb genes. We show that these LVs drive the safe and coordinate expression of the α- and ß-subunits, leading to supranormal levels of ß-Hexosaminidase activity with prevalent formation of a functional HexA in SD murine neurons and glia, murine bone marrow-derived hematopoietic stem/progenitor cells (HSPCs), and human SD fibroblasts. The restoration/overexpression of ß-Hexosaminidase leads to the reduction of intracellular GM2 ganglioside storage in transduced and in cross-corrected SD murine neural progeny, indicating that the transgenic enzyme is secreted and functional. Importantly, bicistronic LVs safely and efficiently transduce human neurons/glia and CD34+ HSPCs, which are target and effector cells, respectively, in prospective in vivo and ex vivo GT approaches. We anticipate that these bicistronic LVs may overcome the current requirement of two vectors co-delivering the α- or ß-subunits genes. Careful assessment of the safety and therapeutic potential of these bicistronic LVs in the SD murine model will pave the way to the clinical development of LV-based gene therapy for GM2 gangliosidosis.

PERK inhibition delays neurodegeneration and improves motor function in a mouse model of Marinesco-Sjögren syndrome.

Marinesco-Sjögren syndrome (MSS) is a rare, early onset, autosomal recessive multisystem disorder characterized by cerebellar ataxia, cataracts and myopathy. Most MSS cases are caused by loss-of-function mutations in the gene encoding SIL1, a nucleotide exchange factor for the molecular chaperone BiP which is essential for correct protein folding in the endoplasmic reticulum. Woozy mice carrying a spontaneous Sil1 mutation recapitulate key pathological features of MSS, including cerebellar atrophy with degeneration of Purkinje cells and progressive myopathy. Because the PERK branch of the unfolded protein response is activated in degenerating neurons of woozy mice, and inhibiting PERK-mediated translational attenuation has shown protective effects in protein-misfolding neurodegenerative disease models, we tested the therapeutic efficacy of GSK2606414, a potent inhibitor of PERK. Mice were chronically treated with GSK2606414 starting from a presymptomatic stage, and the effects were evaluated on biochemical, histopathological and clinical readouts. GSK2606414 delayed Purkinje cell degeneration and the onset of motor deficits, prolonging the asymptomatic phase of the disease; it also reduced the skeletal muscle abnormalities and improved motor performance during the symptomatic phase. The protein but not the mRNA level of ORP150, a nucleotide exchange factor which can substitute for SIL1, was increased in the cerebellum of GSK2606414-treated woozy mice, suggesting that translational recovery promoted the synthesis of this alternative BiP co-factor. Targeting PERK signaling may have beneficial disease-modifying effects in carriers of SIL1 mutations.

Generation of a LacZ reporter transgenic mouse line for the stereological analysis of oligodendrocyte loss in galactosylceramidase deficiency.

Krabbe's disease is a leukodystrophy resulting from deficiency of galactosylceramidase and the accumulation of galactosylsphingosine (psychosine) in the nervous system. Psychosine is believed to cause central demyelination by killing oligodendrocytes. Quantitative analysis of this process is lacking. To address this, we generated a new transgenic reporter twitcher line in which myelinating oligodendrocytes are genetically marked by the expression of LacZ under control of the myelin basic protein (MBP) promoter. MBP-LacZ-twitcher transgenic mice were used for unbiased stereological quantification of ß-galactosidase+ oligodendrocytes in the spinal cord. As expected, we found decreased numbers of these cells in mutant cords, paralleling the severity of clinical disease. The decrease of oligodendrocytes does not correlate well with the increase of psychosine. The new MBP-LacZ-twitcher line will be a useful genetic tool for measuring changes in oligodendrocyte numbers in different regions of the mutant CNS and in preclinical trials of therapies to prevent demyelination. © 2016 Wiley Periodicals, Inc.

Therapeutic advantages of combined gene/cell therapy strategies in a murine model of GM2 gangliosidosis.

Genetic deficiency of ß-N-acetylhexosaminidase (Hex) functionality leads to accumulation of GM2 ganglioside in Tay-Sachs disease and Sandhoff disease (SD), which presently lack approved therapies. Current experimental gene therapy (GT) approaches with adeno-associated viral vectors (AAVs) still pose safety and efficacy issues, supporting the search for alternative therapeutic strategies. Here we leveraged the lentiviral vector (LV)-mediated intracerebral (IC) GT platform to deliver Hex genes to the CNS and combined this strategy with bone marrow transplantation (BMT) to provide a timely, pervasive, and long-lasting source of the Hex enzyme in the CNS and periphery of SD mice. Combined therapy outperformed individual treatments in terms of lifespan extension and normalization of the neuroinflammatory/neurodegenerative phenotypes of SD mice. These benefits correlated with a time-dependent increase in Hex activity and a remarkable reduction in GM2 storage in brain tissues that single treatments failed to achieve. Our results highlight the synergic mode of action of LV-mediated IC GT and BMT, clarify the contribution of treatments to the therapeutic outcome, and inform on the realistic threshold of corrective enzymatic activity. These results have important implications for interpretation of ongoing experimental therapies and for design of more effective treatment strategies for GM2 gangliosidosis.

PERK inhibition attenuates the abnormalities of the secretory pathway and the increased apoptotic rate induced by SIL1 knockdown in HeLa cells.

Loss-of-function mutations in the SIL1 gene are linked to Marinesco-Sjögren syndrome (MSS), a rare multisystem disease of infancy characterized by cerebellar and skeletal muscle degeneration. SIL1 is a ubiquitous adenine nucleotide exchange factor for the endoplasmic reticulum (ER) chaperone BiP. The complexity of mechanisms by which loss of SIL1 causes MSS is not yet fully understood. We used HeLa cells to test the hypothesis that impaired protein folding in the ER due to loss of SIL1 could affect secretory trafficking, impairing the transport of cargoes essential for the function of MSS vulnerable cells. Immunofluorescence and ultrastructural analysis of SIL1-knocked-down cells detected ER chaperone aggregation, enlargement of the Golgi complex, increased autophagic vacuoles, and mitochondrial swelling. SIL1-interefered cells also had delayed ER-to-plasma membrane transport with retention of Na+/K+-ATPase and procollagen-I in the ER and Golgi, and increased apoptosis. The PERK pathway of the unfolded protein response was activated in SIL1-interfered cells, and the PERK inhibitor GSK2606414 attenuated the morphological and functional alterations of the secretory pathway, and significantly reduced cell death. These results indicate that loss of SIL1 is associated with alterations of secretory transport, and suggest that inhibiting PERK signalling may alleviate the cellular pathology of SIL1-related MSS.