Summary Astrocytes are the most abundant type of glial cell in the CNS and play vital roles in all facets of brain physiology. Recent studies from our lab identified five molecularly and functionally distinct astrocyte subpopulations in the brain. One of these subpopulations is specifically labeled by the cell surface maker CD51 and is endowed with enhanced synaptogenic function, leading us to hypothesize that CD51+ astrocytes play an important role in functioning brain circuits. To determine how CD51+ astrocytes contribute to circuit function, we created a CD51-FLP mouse line that allows us selectively label and manipulate this population of astrocytes in the brain. To decipher the mechanisms that regulate CD51+ astrocytes, we found that the transcription factor Sox9 is enriched in this subpopulation. Preliminary studies in a newly generated mouse line that specifically eliminates Sox9 in astrocytes revealed that this transcription factor selectively regulates astrocyte morphology and circuit function in the hippocampus and olfactory bulb in an aging-dependent manner. Together, these new mouse lines will enable us to uncover how CD51+ astrocytes regulate brain circuits and reveal novel roles for Sox9 in controlling astrocyte diversity and function during aging. Based on the strength of these preliminary observations, we propose the following specific aims. In specific aim 1, we will define the anatomical, morphological and physiological properties of CD51+ and CD51- astrocytes in the aging brain and decipher how they contribute to circuit function in the hippocampus and olfactory bulb. In specific aim 2, we will use our astrocyte-specific Sox9 knockout mouse to decipher how it regulates astrocyte function in the aging brain and how these changes in astrocytes impact circuit function in the hippocampus and olfactory bulb. In specific aim 3 we will determine how Sox9 regulates astrocyte diversity during aging and integrate our findings with Sox9 ChIP-Seq to decipher Sox9 target gene networks.