Changes in the axo-glial junctions of the optic nerves of cuprizone-treated mice

Adult-onset depletion of sulfatide leads to axonal degeneration with relative myelin sparing

3-O-sulfogalactosylceramide (sulfatide) constitutes a class of sphingolipids that comprise about 4% of myelin lipids in the central nervous system. Previously, our group characterized a mouse with sulfatide's synthesizing enzyme, cerebroside sulfotransferase (CST), constitutively disrupted. Using these mice, we demonstrated that sulfatide is required for establishment and maintenance of myelin, axoglial junctions, and axonal domains and that sulfatide depletion results in structural pathologies commonly observed in Multiple Sclerosis (MS). Interestingly, sulfatide is reduced in regions of normal appearing white matter (NAWM) of MS patients. Sulfatide reduction in NAWM suggests depletion occurs early in disease development and consistent with functioning as a driving force of disease progression. To closely model MS, an adult-onset disease, our lab generated a "floxed" CST mouse and mated it against the PLP-creERT mouse, resulting in a double transgenic mouse that provides temporal and cell-type specific ablation of the Cst gene (Gal3st1). Using this mouse, we demonstrate adult-onset sulfatide depletion has limited effects on myelin structure but results in the loss of axonal integrity including deterioration of domain organization accompanied by axonal degeneration. Moreover, structurally preserved myelinated axons progressively lose the ability to function as myelinated axons, indicated by the loss of the N1 peak. Together, our findings indicate that sulfatide depletion, which occurs in the early stages of MS progression, is sufficient to drive the loss of axonal function independent of demyelination and that axonal pathology, which is responsible for the irreversible loss of neuronal function that is prevalent in MS, may occur earlier than previously recognized.

Validating visual evoked potentials as a preclinical, quantitative biomarker for remyelination efficacy

Many biomarkers in clinical neuroscience lack pathological certification. This issue is potentially a significant contributor to the limited success of neuroprotective and neurorestorative therapies for human neurological disease - and is evident even in areas with therapeutic promise such as myelin repair. Despite the identification of promising remyelinating candidates, biologically validated methods to demonstrate therapeutic efficacy or provide robust preclinical evidence of remyelination in the central nervous system are lacking. Therapies with potential to remyelinate the central nervous system constitute one of the most promising and highly anticipated therapeutic developments in the pipeline to treat multiple sclerosis and other demyelinating diseases. The optic nerve has been proposed as an informative pathway to monitor remyelination in animals and human subjects. Recent clinical trials using visual evoked potential (VEP) have had promising results, but without unequivocal evidence about the cellular and molecular basis for signal changes on VEP, the interpretation of these trials is constrained. The VEP was originally developed and utilized in the clinic as a diagnostic tool but its use as a quantitative method for assessing therapeutic response requires certification of its biological specificity. Here, using the tools of experimental pathology we demonstrate that quantitative measurements of myelination using both histopathological measures of nodal structure and ultrastructural assessments correspond to VEP latency in both inflammatory and chemical models of demyelination. VEP latency improves after treatment with a tool remyelinating compound (clemastine), mirroring both quantitative and qualitative myelin assessment. Furthermore, clemastine does not improve VEP latency following demyelinating injury when administered to a transgenic animal incapable of forming new myelin. Therefore, using the capacity for therapeutic enhancement and biological loss of function we demonstrate conclusively that VEP measures myelin status and is thereby a validated tool for preclinical verification of remyelination.

Structural adaption of axons during de- and remyelination in the Cuprizone mouse model

Multiple Sclerosis is an autoimmune disorder causing neurodegeneration mostly in young adults. Thereby, myelin is lost in the inflammatory lesions leaving unmyelinated axons at a high risk to degenerate. Oligodendrocyte precursor cells maintain their regenerative capacity into adulthood and are able to remyelinate axons if they are properly activated and differentiate. Neuronal activity influences the success of myelination indicating a close interplay between neurons and oligodendroglia. The myelination profile determines the distribution of voltage-gated ion channels along the axon. Here, we analyze the distribution of the sodium channel subunit Nav1.6 and the ultrastructure of axons after cuprizone-induced demyelination in transgenic mice expressing GFP in oligodendroglial cells. Using this mouse model, we found an increased number of GFP-expressing oligodendroglial cells compared to untreated mice. Analyzing the axons, we found an increase in the number of nodes of Ranvier in mice that had received cuprizone. Furthermore, we found an enhanced portion of unmyelinated axons showing vesicles in the cytoplasm. These vesicles were labeled with VGlut1, indicating that they are involved in axonal signaling. Our results highlight the flexibility of axons towards changes in the glial compartment and depict the structural changes they undergo upon myelin removal. These findings might be considered if searching for new neuroprotective therapies that aim at blocking neuronal activity in order to avoid interfering with the process of remyelination.