Age-dependent effects on microglia-mediated control of neuronal activity

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Project Summary/Abstract This proposal addresses a novel microglia-controlled neuronal activity feedback mechanism and its potential contribution to aging-dependent neuropathology. Aging in mice and humans is associated with alterations to neuronal circuit excitability and function, increased seizure susceptibility, and neurodegeneration. Our recent studies have identified microglia as novel regulators of neuronal activity and function, maintaining homeostatic levels of neuronal activation, thereby serving as brake pads for hyperexcitation. Preliminary evidence suggests this neuroprotective function may be altered in aging, potentially as a consequence of inflammatory microglia activation associated with aging and neurodegenerative disease. I hypothesize that microglia play a critical role in aberrant neuronal responses and dysfunction in the aging brain. In support of this idea, we found that microglia are able to regulate neuronal activity in an activity dependent manner by responding to ATP released during neuronal activation and metabolizing it into adenosine, thereby suppressing neuronal activity. We further found that age-associated increase in pro-inflammatory gene expression in microglia is associated with changes in this mechanism. To investigate my hypothesis, I will first elucidate the exact mechanisms of microglia-mediated neurosuppression, using the healthy adult striatum as a model. Microglia express the rate-limiting enzyme, CD39, which controls ATP to AMP degradation, while AMP to adenosine degradation is accomplished by CD73, which is expressed in microglia but also by striatal D2 medium spiny neurons. This suggests that microglia may either be independently sufficient to produce adenosine by expressing both CD39 and CD73, or this mechanism requires microglia-neuron interactions. To investigate this, we propose to generate transgenic mouse models to identify the relative contributions of CD73 found on microglia versus D2 medium spiny neurons. Microglial sufficiency of adenosine production may implicate this as a brain-wide mechanism of neuroprotection against aberrant neuronal activation. Additionally, to investigate the presynaptic and postsynaptic specificity of this microglial mechanism, we will use transgenic mouse models lacking adenosine receptor either on projection neurons or D1 medium spiny neurons. Prevention of adenosine-based neuronal activity inhibition on either the presynapse or postsynapse will recapitulate the effects seen with loss of microglial adenosine generation. Using these models, we will assess neuronal activation by immediate early gene expression, open field locomotor activity, and seizure susceptibility. We further demonstrated that expression of key genes involved in microglia driven neuronal activity modulation are downregulated in aged animals. However, this has yet to be specifically observed in microglia. Therefore, we proposed to perform gene expression analysis specifically in microglia of different brain regions and animals of different ages. This information is critical to our understanding of the cellular mechanisms by which microglia regulate neuronal function and may help to identify key regulators of this pathway, leading to the development of novel approaches for the treatment of age-associated disorders.