Mitophagy Dependent Regulation of Mammary Gland Differentiation - SUMMARY Mitochondria operate as a central hub for many metabolic processes by sensing and responding to the cellular environment to maintain homeostasis. Consequently, their disruption is a key factor in the onset and progression of many human conditions, including metabolic disorders, neurodegenerative diseases, and cancer. Mitochondrial homeostasis is primarily maintained through the recycling of damaged mitochondria by targeted autophagy, termed mitophagy. Mitophagy is tissue-specific and occurs in response to both cellular stress and differentiation cues. Differentiation-cued mitophagy is often termed programmed mitophagy and has recently gained attention for its contribution to epigenetic status, cell fate decisions, metabolic adaptation and differentiation. Although these and other effects have been attributed to mitophagy, little is known about the upstream signaling pathways that induce mitophagy to meet specific cellular needs. Distinct morphological differences in mitochondria exist during the post-natal stages of mammary gland development. This suggests that mitophagy plays an important to the development of this tissue. Identifying the mechanism by which mitochondrial homeostasis is maintained during mammary gland development will provide much needed insight into the broader role of mitochondrial adaptation in normal development and disease. We have shown that Singleminded-2s (SIM2s; expressed from Sim2), is differentially expressed during mammary gland development and is a key regulator of functional mammary gland differentiation. Our recent results utilizing mammary gland- specific over- and under-expressing Sim2s transgenic mice show that SIM2s is required for functional lactation, and does so, in part, through direct interaction with the PRKN mitophagy complex. Based on these new results, we hypothesize that mitophagy-dependent mitochondrial adaptation is essential for mammary gland functional differentiation and that SIM2s is required to maintain mitochondrial homeostasis. To address this hypothesis we propose two Specific Aims. In Aim 1, we will determine the mitophagy-driven metabolic transition required for mammary epithelial cell differentiation by crossing the mito-QC mouse model with MMTV-Sim2s and Sim2fl/fl mice to assess mitophagy and mitochondrial architecture and metabolic adaptation. In Aim 2, we will define the physical basis for, and functional outcomes of, interactions between SIM2s, ATM, PINK1/PRKN, and LC3 in mitophagy and mammary gland differentiation. Successful completion of this proposal will provide insight into heretofore unknown mechanisms of mitochondrial adaptation under physiological conditions. We expect results from these studies will help define the mechanism of mitochondrial adaptation in mammary gland development, lactation, and cancer.
