Mild Microglial Responses in the Cortex and Perivascular Macrophage Infiltration in Subcortical White Matter in Dogs with Age-Related Dementia Modelling Prodromal Alzheimer's Disease

Background: Microglia contribute to Alzheimer's disease (AD) pathogenesis by clearing amyloid-beta (A beta) and driving neuroinflammation. Domestic dogs with age-related dementia (canine cognitive dysfunction (CCD)) develop cerebral amyloidosis like humans developing AD, and studying such dogs can provide novel information about microglial response in prodromal AD. Objective: The aim was to investigate the microglial response in the cortical grey and the subcortical white matter in dogs with CCD versus age-matched cognitively normal dogs. Methods: Brains from aged dogs with CCD and age-matched controls without dementia were studied. Cases were defined by dementia rating score. Brain sections were stained for A beta, thioflavin S, hyperphosphorylated tau, and the microglial-macrophage ionized calcium binding adaptor molecule 1 (Iba1). Results were correlated to dementia rating score and tissue levels of A beta. Results: Microglial numbers were higher in the A beta plaque-loaded deep cortical layers in CCD versus control dogs, while the coverage by microglial processes were comparable. A beta plaques were of the diffuse type and without microglial aggregation. However, a correlation was found between the % Iba1 area and insoluble A beta(42) and N-terminal pyroglutamate modified A beta(N3pE)-42. The % Iba1 area was higher in white matter, showing phosphorylation of S396 tau, versus grey matter. Perivascular macrophage infiltrates were abundant in the white matter particularly in CDD dogs. Conclusion: The results from this study of the microglial-macrophage response in dogs with CCD are suggestive of relatively mild microglial responses in the A beta plaque-loaded deep cortical layers and perivascular macrophage infiltrates in the subcortical white matter, in prodromal AD.

Automated summary

The study found higher microglial numbers in the A beta plaque-loaded deep cortical layers in dogs with CCD compared to controls, with comparable microglial process coverage. The percentage of Iba1 area was higher in white matter, showing phosphorylation of S396 tau. Overall, dogs with CCD showed relatively mild microglial-macrophage responses.