Multiple Sclerosis - Disease Pathology
Multiple sclerosis also called as T cell mediated autoimmune disease, is a condition where the body’s immune system identifies the myelin fatty sheath around the nerves as an antigen and destroys them. The destruction of the myelin sheath is through inflammatory response. The inflammatory process initiates with T cell multiplication induced by cytokine such as IL-12, IL-23 and IL-4. Differentiation of T cells into phenotypes further exacerbates the inflammatory response. This inflammation driven loss of myelin sheath alters the electrical flow of the neurons which affects brain and spinal cord functions. As a result, this condition affects gait, balance, cognitive functions, causes muscle weakness and spasms and bowel and bladder incontinence. Loss of myelin sheath also leads to further neuropathy. The histology of brain in multiple sclerosis has been observed with regions of concentrated T cells and macrophages present as focal areas called plaques. Both oligodendrocyte death and astrogliosis are observed across the brain with neuroaxonal loss in cortico-spinal tracts.
Zebrafish as a validated model for Multiple Sclerosis
Multiple sclerosis model is well established in zebrafish. Brain and spinal cord histology in zebrafish model reveals cells involved in pathology such as T cells, macrophages, oligodendrocytes, astrocytes and axons. Some of these cells can be quantified with differential stain such as haematoxylin and eosin and luxol fast blue, while the rest of the pathology can be studied with targeted fluorescent stain. Histological scoring of neuropathy and rate of oligodendrocyte remyelination can be quantified to study the progress of the condition. Similarly, cytokines and other inflammatory biomarkers can be quantified as an mRNA expression profile or protein quantified using ELISA. Additional parameters that can be quantified are phenotype locomotion tests, muscle weakness by swim motion test, predator avoidance memory for cognition capability, numbness and pain response measures can also be quantified. Hence the zebrafish model of multiple sclerosis would be a suitable model for screening for compounds for a wide range of discovery strategies.
MS Zebrafish Model Design for Therapeutic Strategies
In zebrafish, the condition of multiple sclerosis is induced by injecting antigens that are present on the oligodendrocytes along with adjuvants. As a result, the similar inflammatory process is created in the animal. T cell differentiating cytokines are produced initially and subsets of T cells are made mounting a strong immune response against the oligodendrocytes in the animal. This is followed by demyelination of the nerves that leads to similar pathology presentation in the animal as observed in human conditions of multiple sclerosis.
Multiple Sclerosis Discovery Strategies using Zebrafish
Immunomodulation: Immunomodulation can be measured by biomarkers analysis over a time course study. The therapeutic effect is observed within the first four hours of dosing, however long-term effect on disease progress and pathology can be measured as well.
Antioxidants and Anti-inflammatory agents: In some cases, both antioxidant ability and anti-inflammatory capacity is contributed by a single molecule and in other cases as multi drug therapies. Antioxidant ability can be observed at cellular level and rescue from oxidation can be quantified in the brain and spinal cord.
T Cell migration inhibitors and count reduction agents: Both T cells and their subsets can be counted in a whole animal imaging method. Also, localization of immune cells can be correlated with drug rescue activity. In zebrafish, the spleen, thymus and bone marrow can be specifically evaluated for the turnover of B and T cell
Cytokine Inhibitors: In zebrafish, heightened inflammation is observed as a phenotype such as darker skin, red spots and hyperactivity. Reduction in cytokine can be measured through specific assays however a continuous 24-hour video tracking of the zebrafish can be done to track long term improvements in disease condition.
Oligodendrocyte progenitor driven remyelination: Remyelination and glial cell regeneration can be measured through histopathology using cellular and molecular markers. Further the remyelination can also be correlated for phenotype rescue for disease modification. Typically, the model shows a small amount of regeneration, but the rate is too slow. Therapies that can actively regenerate the myelin sheath can be identified by quantifying the rate of remyelination.