Axonal Damage in Multiple Sclerosis - POTENTIAL MECHANISMS OF DAMAGE

Axonal Damage Consequent to Direct Cytotoxic Attack

The exact mechanisms that underlie the progression of axonal damage are largely unknown. In vitro experimental evidence has shown CD4⁺ T cells can contribute to neurodegeneration in a mouse model of Parkinson’s Disease (PD).¹²⁰ In addition, polyclonally activated CD4⁺/CD8⁺ effector cells align along axons and the soma of human neurons, causing neuronal cell death in vitro, but not death of oligodendrocytes or astrocytes.¹²¹ It has been proposed that loss of spinal motor neurons is the consequence of CD4⁺ and CD8⁺ T-cell infiltration in animal models of demyelination.¹²² However, the number of CD8⁺ T cells correlates with the extent of axonal damage in MS and the detection of CD8⁺ T cells in the brain,¹²³ blood,¹²⁴ and CSF¹²⁵ of patients, in excess of CD4⁺ T cells,¹²³ have suggested CD8⁺ T cells as important effectors of axonal damage. Indeed, accumulation of APP in damaged axons correlates with the number of macrophages and CD8⁺ T cells within MS lesions,¹⁵ CD8⁺ T cells closely interact with demyelinated axons in MS lesions¹²⁶ and are capable of inducing direct damage to cultured murine neurons.¹²⁷ Dying spinal motor neurons are surrounded by CD4⁺ and CD8⁺ T cells in EAE and postmortem MS tissue.¹²² Consistent with the proposed cytotoxic role of CD8⁺ T cells, depletion of CD4⁺ T cells in MS patients shows no improvement on relapse rate or inflammatory activity when visualized by MRI.^128,129 Depletion of both CD8⁺ and CD4⁺ T cells, however, decreases relapse rate and new lesion formation but provides only little improvement of neurological symptoms.^56,130 This suggests that CD8⁺ T cells are potential mediators of axonal damage. One possibility is that CD8⁺ T cells may cause damage indirectly, by targeting oligodendrocytes and myelin. For example, CD8⁺ T cells have been found to mediate the lysis of cultured murine oligodendrocytes¹³¹ and myelin in cerebellar slice cultures with axonal damage occurring as a bystander effect.¹³² The alternative possibility is that CD8⁺ T cells and possibly natural killer cells directly damage the axon due to the release of the cytolytic molecule perforin, a membrane pore-forming protein found in intracellular granules within these cells.¹³³ The evidence to support the role for perforin in MS disease progression is substantiated from both the analysis of MS blood samples and the phenotype of mice with genetic ablation of the gene encoding for perforin. For example, high numbers of myelin basic protein (MBP)-reactive perforin mRNA expressing blood mononuclear cells (MNCs) are detected in MS patients¹³⁴ and increased perforin expression is found in CD4⁺ T cells during MS disease exacerbation¹³⁵ and correlates with the extent of brain lesions detected by MRI.¹³⁶ Importantly, perforin-deficient mice are protected from axonal damage in the spinal cord and suggest that this might be a prominent mechanism of axonal degeneration.¹³⁷

Axonal injury is therefore, at least in part, independent of demyelinating activity, and this concept may bear important implications for future therapeutic strategies aimed at preventing axonal loss.

Antibody/Complement-Mediated Lesion

We have previously discussed experimental evidence linking GM pathology, characterized by axonal loss and empty myelin sheaths to the injection of NFL into mice, thereby suggesting an antibody-mediated mechanism of damage.⁴³ Furthermore, adoptive transfer of transiently expressed axonal glycoprotein 1 (TAG-1)-specific T cells into rats elicited a disease course with a preferential GM pathology, with WM demyelination only occurring after MOG antibody injection,¹³⁸ thereby reflecting GM disease detected by early MRI measurements in MS patients. In further support of the hypothesis that neurodegeneration may underlie the pathogenesis of MS are also the findings of autoantibodies to neuronal and axonal antigens in the serum and CSF in a subset of MS patients, as well as the presence of meningeal B-cell infiltrates.⁴⁴ The autoantibodies and antigens include neurofilament,¹³⁹ axolemma enriched fractions,¹⁴⁰ neurofascin,¹⁴¹ and contactin 2/TAG-1.¹³⁸ However, some of these axonal or neuronal targets, namely intracytoplasmic proteins, may represent markers of injury rather than direct mediators of attack. Thus, neurodegeneration may underlie disease progression and WM demyelination, at least in a subset of patients.

Damage Caused by Dysfunctional Glial-Neuronal Crosstalk

The axonal degeneration detected in mice with deletion of genes encoding for myelin proteins suggested the existence of a bidirectional trophic support between myelin and neurons, affecting the long-term survival of neurons.⁵¹ For example, neurons and oligodendrocytes can exchange small metabolites necessary for myelin membrane lipid synthesis.¹⁴² The axonal degeneration detected in Plp-null mice, for instance, has been proposed to result from impaired transport of the nicotinamide adenine dinucleotide⁺-dependent deacetylase sirtuin 2, a cytoplasmic deacetylase implicated in axo-glial support. Similarly, ablation of a crucial peroxisomal enzyme in oligodendrocytes has been associated with dramatic axonal degeneration¹⁴³ and the subsequent presence of inflammatory infiltrates.¹⁴⁴ Thus, the loss of trophic support and deregulation of energy metabolism may destabilize metabolically isolated axons, resulting in their damage.⁵¹