Session 1: Mitochondrial architecture and behavior

We study the biology of mitochondria in cellular homeostasis, stress and disease. Derived from bacteria, mitochondria have retained a reduced genome and evolved to become one of the primary metabolic hubs of eukaryotic cells. Our current research is focused on how mitochondrial DNA copy number and transmission are controlled in cells, how mitochondrial behavior, metabolism and cell behavior are intertwined and how the mitochondrial inner membrane is differentiated into distinct domains. By broadly addressing the fundamental mechanisms governing mitochondrial behavior and organization, we continue to illuminate how they contribute to pathogenesis.

Programming of mitochondrial metabolism is a critical hallmark of activating macrophages and controls immune cell function. Mitochondria do not function as autonomous organelles. Instead, they can communicate with many - if not all - other metabolic organelles. If and how organelles combine their response to ensure metabolic programming remains unclear. We established a multispectral-organelle imaging approach (OrgaPlexing) that allows us, for the first time, to visualize up to 6 key metabolic organelles in primary macrophages. This approach revealed dynamic organelle changes upon bacterial stimulation: organelles underwent a zonation response, arranging themselves in rings emanating from the nucleus and formed defined two-, three-, four-way organelle interactions. Amongst the latter, we identified organelle hubs containing mitochondria, the ER, peroxisomes and lipid droplets. The functional units were tuned by the major mitochondrial fission factor Drp1 and controlled inflammatory lipid metabolism and production of the immunomodulatory lipid messenger PGE₂. Together, this project builds the first organelle-interaction map in primary immune cells and reveals a novel function of Drp1 in controlling inflammatory macrophage signaling.

Mitochondrial function and their ability to communicate with the rest of the cell depends on outer mitochondrial membrane proteins. These proteins are translated in the cytosol and must be targeted and inserted into the outer membrane, a process poorly understood in mammals. With a series of genome-wide screens probing the biogenesis of outer membrane proteins with diverse topologies, we discover numerous factors involved in outer membrane targeting and insertion. We define a set of rules linking substrates to their biogenesis pathways and preventing mislocalization, largely determined by their topologies. Specifically, we identify both a novel cytosolic chaperone required for the biogenesis of signal-anchored and multi-pass transmembrane proteins and a new function for a classic chaperone in alpha-helical outer membrane protein targeting. Cumulatively, our work sheds light on the complexity of pathways involved in outer mitochondrial membrane protein biogenesis and lays the foundation for future mechanistic studies.

Molecular control of RNA folding and modification is crucial for cellular function across the tree of life, with RNA playing active roles in fundamental processes such as translation. Little is known about the mechanisms by which mitochondrial ribosomal RNA (rRNA) is folded and modified to generate subunits exhibiting high translational fidelity in evolutionarily divergent organisms. Here, we have combined cryo-EM, human genome editing tools, and yeast genetics to determine structural principles underlying the formation of functional rRNA in mitochondria. These structures illuminate the mechanistic basis for how GTPases are employed to control rRNA folding events and how mitoribosomal proteins play active roles during these transitions. Together, our results uncover both conserved principles and species-specific adaptations that govern the maturation of rRNA in different organisms. Thus, our analysis provides a vignette for how molecular complexity and diversity can evolve in large ribonucleoprotein assemblies.