Canine degenerative myelopathy : perspectives from aging, microglia and neurofilaments
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Canine degenerative myelopathy (DM) is a late-onset neurodegenerative disease similar to some forms of amyotrophic lateral sclerosis (ALS). Mutations in superoxide dismutase 1 (SOD1) are a risk factor for disease; however, disease penetrance is also age-related. Microglia, the immune effector cells of the CNS, undergo phenotypic changes with age that may predispose to neurodegeneration. Furthermore, they have been directly implicated in disease pathogenesis in ALS. We tested the hypotheses that 1) microglia in aged, neurologically normal dogs have increased activation toward a neurotoxic phenotype compared to adult dogs and 2) microglia in DM-affected dogs will transition from a neuroprotective (M2) to neurotoxic (M1) phenotype as disease progresses and 3) increased fractalkine, a chemotactic molecule for microglia, will correlate with increased M2 microglia in DM-affected dogs. We found that aged, neurologically normal dogs had increased numbers of M1 and M2-activated microglia; however, total M2 microglia per motor neuron was reduced from adult dogs. Furthermore, compared to adult dogs, aged canine spinal cord had increased expression of genes associated with phagocytosis and proteolysis, which also are elevated in ALS and Alzheimer's disease models. Within the context of DM, microglia in close proximity to motor neurons were increased at all disease stages. M2 microglia per motor neuron were increased in early stages of DM, whereas the number of M1 microglia per motor neuron were indistinguishable from aged controls at all stages of disease. Fractalkine levels did not change throughout disease progression. These findings collectively suggested that aged canine microglia are highly activated with reduced neuroprotective microglia near motor neurons. In the context of DM, concomitant loss of motor function and increased M2 microglia, without a change in M1 microglia, suggested that SOD1E40K M2 microglia could have contributed to neurodegeneration through a toxic gain of function. Currently, there is no definitive ante-mortem diagnostic test for DM. We tested the hypothesis that phosphorylated neurofilament heavy (pNF-H), an abundant structural protein of myelinated motor axons, is readily detectable in CSF and serum of DM-affected dogs, and can be used as a diagnostic biomarker for DM. We identified pNF-H as a sensitive and specific diagnostic test for SOD1E40K-associated canine DM. This work may provide the first ante-mortem diagnostic test for canine DM.
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