Axonal damage and neuronal loss are key factors leading to irreversible neurological impairment in patients with MS. Early axonal damage in MS is associated with the infiltration of immune cells, which release neurotoxic cytokines to injure axons. Prolonged injury can induce Wallerian degeneration, a process of neuronal apoptosis, resulting in damage that propagates along axons and leads to the cell body loss. It is critical to visualize and differentiate these two types of axonal damage, so that therapeutic approaches can be used effectively to targeting each pathological condition. Diffusion Tensor Imaging (DTI) is a clinically available imaging modality. Our previous studies have demonstrated that DTI can detect Wallerian degeneration as decreased axial diffusivity ( ?||, sensitive to axonal damage), which is followed by increased radial diffusivity (?-, sensitive to myelin loss) in some days or weeks later. Thus, it is critical to know if DTI can be used to differentiate immune-induced damage from Wallerian degeneration. We hypothesized that the immune-induced axonal damage is closely associated with simultaneously decreased || and increased;while the Wallerian degeneration, results in decreased ?-, which is then followed by increased. We will examine our hypothesis using an animal model of MS, the experimental autoimmune encephalomyelitis (EAE) on the slow Wallerian Degeneration (WldS) mutant and wild-type mice. Our preliminary longitudinal DTI on mouse EAE visual system supported our hypothesis. It has been speculated that immune-induced damage and Wallerian degeneration may have different inflammatory processes. Gd-enhanced T1-weighted imaging (Gd-T1WI) is commonly used for MS to reveal the immune-induced Blood-Brain Barrier (BBB) breakdown. However, Wallerian degeneration activates the residual microglia cells but might not necessarily induce BBB breakdown. We examined degenerative optic nerves after retinal ischemia and found that the injured nerves, though observable with DTI, appeared normal in Gd-T1WI. This finding suggests a fundamental limitation of Gd-T1WI to characterize axonal damage in MS and EAE. We will investigate the time course of DTI in EAE combined with Gd-T1WI (Aim 1). Since optic nerves are axons originated from retinal ganglion cells (RGC), we will evaluate whether DTI in optic nerves predicts the RGC loss in EAE mice (Aim 3). Beyond the pathological examinations, we will also evaluate the relation of DTI and neurofunctional deficits (Aim 2). Visual evoked potential (VEP) will be measured to quantify the visual pathway conductivity. Mn2+enhanced MRI (MEMRI) will also be used to monitor the axonal transport of RGC. In our preliminary studies, both VEP and MEMRI-derived axonal transport were significantly delayed in EAE mice. The findings of this study will provide significant clinical relevance as to move therapeutic strategies toward a better efficient and direct treatment to prevent permanent disability.
The findings of this project will have significant clinical relevance. We will explore the DTI as imaging biomarkers for immune-induced axonal damage and Wallerian degeneration. We will also explore the changes of DTI in association with neural functional outcomes. The findings of this study will provide significant clinical relevance as to move therapeutic strategies toward a better efficient and direct treatment to prevent permanent disability for Multiple Sclerosis.
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