Mitochondrial morphology controls fatty acid utilization by changing CPT1 sensitivity to malonyl-CoA.

Changes in mitochondrial morphology are associated with nutrient utilization, but the precise causalities and the underlying mechanisms remain unknown. Here, using cellular models representing a wide variety of mitochondrial shapes, we show a strong linear correlation between mitochondrial fragmentation and increased fatty acid oxidation (FAO) rates. Forced mitochondrial elongation following MFN2 over-expression or DRP1 depletion diminishes FAO, while forced fragmentation upon knockdown or knockout of MFN2 augments FAO as evident from respirometry and metabolic tracing. Remarkably, the genetic induction of fragmentation phenocopies distinct cell type-specific biological functions of enhanced FAO. These include stimulation of gluconeogenesis in hepatocytes, induction of insulin secretion in islet ß-cells exposed to fatty acids, and survival of FAO-dependent lymphoma subtypes. We find that fragmentation increases long-chain but not short-chain FAO, identifying carnitine O-palmitoyltransferase 1 (CPT1) as the downstream effector of mitochondrial morphology in regulation of FAO. Mechanistically, we determined that fragmentation reduces malonyl-CoA inhibition of CPT1, while elongation increases CPT1 sensitivity to malonyl-CoA inhibition. Overall, these findings underscore a physiologic role for fragmentation as a mechanism whereby cellular fuel preference and FAO capacity are determined.

A non-canonical Raf function is required for dorsal-ventral patterning during Drosophila embryogenesis.

Proper embryonic development requires directional axes to pattern cells into embryonic structures. In Drosophila, spatially discrete expression of transcription factors determines the anterior to posterior organization of the early embryo, while the Toll and TGFß signalling pathways determine the early dorsal to ventral pattern. Embryonic MAPK/ERK signaling contributes to both anterior to posterior patterning in the terminal regions and to dorsal to ventral patterning during oogenesis and embryonic stages. Here we describe a novel loss of function mutation in the Raf kinase gene, which leads to loss of ventral cell fates as seen through the loss of the ventral furrow, the absence of Dorsal/NFκB nuclear localization, the absence of mesoderm determinants Twist and Snail, and the expansion of TGFß. Gene expression analysis showed cells adopting ectodermal fates much like loss of Toll signaling. Our results combine novel mutants, live imaging, optogenetics and transcriptomics to establish a novel role for Raf, that appears to be independent of the MAPK cascade, in embryonic patterning.

An Optogenetic Method to Study Signal Transduction in Intestinal Stem Cell Homeostasis.

Homeostasis in adult organs involves replacement of cells from a stem cell pool maintained in specialized niches regulated by extracellular signals. This cell-to-cell communication employs signal transduction pathways allowing cells to respond with a variety of behaviors. To study these cellular behaviors, signaling must be perturbed within tissues in precise patterns, a technique recently made possible by the development of optogenetic tools. We developed tools to study signal transduction in vivo in an adult fly midgut stem cell model where signaling was regulated by the application of light. Activation was achieved by clustering of membrane receptors EGFR and Toll, while inactivation was achieved by clustering the downstream activators ERK/Rolled and NFκB/Dorsal in the cytoplasm, preventing nuclear translocation and transcriptional activation. We show that both pathways contribute to stem and transit amplifying cell numbers and affect the lifespan of adult flies. We further present new approaches to overcome overexpression phenotypes and novel methods for the integration of optogenetics into the already-established genetic toolkit of Drosophila.

WNT Signaling in Disease.

Developmental signaling pathways control a vast array of biological processes during embryogenesis and in adult life. The WNT pathway was discovered simultaneously in cancer and development. Recent advances have expanded the role of WNT to a wide range of pathologies in humans. Here, we discuss the WNT pathway and its role in human disease and some of the advances in WNT-related treatments.