Microglial Expression of the Wnt Signaling Modulator DKK2 Differs between Human Alzheimer’s Disease Brains and Mouse Neurodegeneration Models

Abstract

Wnt signaling is crucial for synapse and cognitive function. Indeed, deficient Wnt signaling is causally related to increased expression of DKK1, an endogenous negative Wnt regulator, and synapse loss, both of which likely contribute to cognitive decline in Alzheimer’s disease (AD). Increasingly, AD research efforts have probed the neuroinflammatory role of microglia, the resident immune cells of the CNS, which have furthermore been shown to be modulated by Wnt signaling. The DKK1 homolog DKK2 has been previously identified as an activated response and/or disease-associated microglia (DAM/ARM) gene in a mouse model of AD. Here, we performed a detailed analysis of DKK2 in mouse models of neurodegeneration, and in human AD brain. In APP/PS1 and APP^NL-G-F AD mouse model brains as well as in SOD1^G93A ALS mouse model spinal cords, but not in control littermates, we demonstrated significant microgliosis and microglial Dkk2 mRNA upregulation in a disease-stage-dependent manner. In the AD models, these DAM/ARM Dkk2⁺ microglia preferentially accumulated close to βAmyloid plaques. Furthermore, recombinant DKK2 treatment of rat hippocampal primary neurons blocked WNT7a-induced dendritic spine and synapse formation, indicative of an anti-synaptic effect similar to that of DKK1. In stark contrast, no such microglial DKK2 upregulation was detected in the postmortem human frontal cortex from individuals diagnosed with AD or pathologic aging. In summary, the difference in microglial expression of the DAM/ARM gene DKK2 between mouse models and human AD brain highlights the increasingly recognized limitations of using mouse models to recapitulate facets of human neurodegenerative disease.

Significance Statement

The endogenous negative Wnt regulator Dkk2 is significantly upregulated at the mRNA level in microglia of Alzheimer’s disease (AD) mouse models, implying that microglia derived Dkk2 protein may detrimentally contribute to a reduced Wnt signaling tone in the AD brain, a known pathophysiological manifestation. Indeed, recombinant DKK2 prevented Wnt-dependent synapse formation in cultured neurons. However, DKK2 upregulation was not recapitulated in postmortem human AD brains. The success of neurodegeneration animal models has relied on pathophysiology that for the most part correctly modelled human disease. Increasingly, however, limitations to the validity of mouse models to recapitulate human neurodegenerative disease have become apparent, as evidenced by the present study by the difference in microglial DKK2 expression between AD mouse models and human AD brain.