Myo19 tethers mitochondria to endoplasmic reticulum-associated actin to promote mitochondrial fission.

Mitochondrial homeostasis requires a dynamic balance of fission and fusion. The actin cytoskeleton promotes fission, and we found that the mitochondrially localized myosin, myosin 19 (Myo19), is integral to this process. Myo19 knockdown induced mitochondrial elongation, whereas Myo19 overexpression induced fragmentation. This mitochondrial fragmentation was blocked by a Myo19 mutation predicted to inhibit ATPase activity and strong actin binding but not by mutations predicted to affect the working stroke of the motor that preserve ATPase activity. Super-resolution imaging indicated a dispersed localization of Myo19 on mitochondria, which we found to be dependent on metaxins. These observations suggest that Myo19 acts as a dynamic actin-binding tether that facilitates mitochondrial fragmentation. Myo19-driven fragmentation was blocked by depletion of either the CAAX splice variant of the endoplasmic reticulum (ER)-anchored formin INF2 or the mitochondrially localized F-actin nucleator Spire1C (a splice variant of Spire1), which together polymerize actin at sites of mitochondria-ER contact for fission. These observations imply that Myo19 promotes fission by stabilizing mitochondria-ER contacts; we used a split-luciferase system to demonstrate a reduction in these contacts following Myo19 depletion. Our data support a model in which Myo19 tethers mitochondria to ER-associated actin to promote mitochondrial fission.

Building the next generation of virtual cells to understand cellular biology.

Cell science has made significant progress by focusing on understanding individual cellular processes through reductionist approaches. However, the sheer volume of knowledge collected presents challenges in integrating this information across different scales of space and time to comprehend cellular behaviors, as well as making the data and methods more accessible for the community to tackle complex biological questions. This perspective proposes the creation of next-generation virtual cells, which are dynamic 3D models that integrate information from diverse sources, including simulations, biophysical models, image-based models, and evidence-based knowledge graphs. These virtual cells would provide statistically accurate and holistic views of real cells, bridging the gap between theoretical concepts and experimental data, and facilitating productive new collaborations among researchers across related fields.

INF2-mediated actin polymerization at ER-organelle contacts regulates organelle size and movement.

Proper regulation of organelle dynamics is critical for cellular function, but the mechanisms coordinating multiple organelles remain poorly understood. Here we show that actin polymerization mediated by the endoplasmic reticulum (ER)-anchored formin INF2 acts as a master regulator of organelle morphology and movement. Using high-resolution imaging, we demonstrate that INF2-polymerized actin filaments assemble at ER contact sites on mitochondria, endosomes, and lysosomes just prior to their fission. Genetic manipulation of INF2 activity alters the size, shape and motility of all three organelles. Our findings reveal a conserved mechanism by which the ER uses actin polymerization to control diverse organelles, with implications for understanding organelle dysfunction in neurodegenerative and other diseases. This work establishes INF2-mediated actin assembly as a central coordinator of organelle dynamics and inter-organelle communication.

Imaging of the Cytoskeleton Using Live and Fixed Drosophila Tissue Culture Cells.

In recent years, the convergence of multiple technologies and experimental approaches has led to the expanded use of cultured Drosophila cells as a model system. Their ease of culture and maintenance, susceptibility to RNA interference, and imaging characteristics have led to extensive use in both traditional experimental approaches and high-throughput RNAi screens. Here we describe Drosophila S2 cell culture and preparation for live-cell and fixed-cell fluorescence microscopy and scanning electron microscopy.

Inclusion: an individual pursuit requiring organization-level investment to sustain it.

If this was not happening in the midst of the COVID-19 pandemic, I imagine that I would be speaking these words instead of writing them on my laptop. Even so, I am so jazzed for this opportunity! No word or phrase describes what I am feeling in this moment in receiving the 2021 American Society for Cell Biology Prize for Excellence in Inclusivity. It is certainly an honor to be recognized in this way. I am grateful to the Howard Hughes Medical Institute for awarding me additional resources to keep on keeping on. My approach to finding the connection between people and their science certainly could use the monetary support. Resources open doors. At the same time that I am grateful for the attention, I am not exactly sure what to do with the spotlight. Importantly, there are a host of other folks out there also doing amazing things who have never been recognized. Let's work to ensure that their contributions are supported, appreciated, and recognized. Instead of focusing the spotlight on me, I would rather redirect it to recognize my foundational influences. I also hope to encourage the need for institutional approaches beyond celebrating individual accomplishment.