Stepchild or Prodigy? Neuroprotection in Multiple Sclerosis (MS) Research

Abstract

Multiple sclerosis (MS) is an autoimmune disorder of the central nervous system (CNS) and characterized by the infiltration of immune cells, demyelination and axonal loss. Loss of axons and nerve fiber pathology are widely accepted as correlates of neurological disability. Hence, it is surprising that the development of neuroprotective therapies has been neglected for a long time. A reason for this could be the diversity of the underlying mechanisms, complex changes in nerve fiber pathology and the absence of biomarkers and tools to quantify neuroregenerative processes. Present therapeutic strategies are aimed at modulating or suppressing the immune response, but do not primarily attenuate axonal pathology. Yet, target-oriented neuroprotective strategies are essential for the treatment of MS, especially as severe damage of nerve fibers mostly occurs in the course of disease progression and cannot be impeded by immune modulatory drugs. This review shall depict the need for neuroprotective strategies and elucidate difficulties and opportunities.

Keywords: axonal damage; degeneration; multiple sclerosis; neuroprotection; regeneration.

Figure 1

Incorporation of damaged nerve fibers by macrophages/microglia. Activated macrophages (Ma)/microglia (Mi) (A,B) phagocytose axolytic axons (A, axA) and myelin debris (A, MyD) within murine EAE spinal cord lesions. EAE = experimental autoimmune encephalomyelitis.

Figure 2

Myelination patterns of nerve fibers: (A) demyelinating nerve fibers (deNF); (B) olidodendrocytes (Odc); and (B,C) remyelinating nerve fibers (reNF).

Figure 3

Comparison of healthy (A) versus EAE lesion (B) tissue taken from the murine spinal cord. EAE = experimental autoimmune encephalomyelitis.

Figure 4

Types of nerve fiber pathology: (A) axolytic axons (axA); (B) axonal transection as observed after staining for hypophosphorylated neurofilaments (arrows) with typical “ovoid” formation (inset); and (C–E) fine and early nerve fiber pathology: a decrease in the nearest neighbor neurofilament distance (C, deNNND), enlargement of the inner tongue (D, arrow) and a nude nerve fiber (nudeNF) with enlarged mitochondria (E, arrow).