PLOS Biology: New Articles

1. Caspase 3 and caspase 7 promote cytoprotective autophagy and the DNA damage response during non-lethal stress conditions in human breast cancer cells

   by Gayathri Samarasekera, Nancy E. Go, Courtney Choutka, Jing Xu, Yuka Takemon, Jennifer Chan, Michelle Chan, Shivani Perera, Samuel Aparicio, Gregg B. Morin, Marco A. Marra, Suganthi Chittaranjan, Sharon M. Gorski

   Cell stress adaptation plays a key role in normal development and in various diseases including cancer. Caspases are activated in response to cell stress, and growing evidence supports their function in non-apoptotic cellular processes. A role for effector caspases in promoting stress-induced cytoprotective autophagy was demonstrated in Drosophila, but has not been explored in the context of human cells. We found a functionally conserved role for effector caspase 3 (CASP3) and caspase 7 (CASP7) in promoting starvation or proteasome inhibition-induced cytoprotective autophagy in human breast cancer cells. The loss of CASP3 and CASP7 resulted in an increase in PARP1 cleavage, reduction in LC3B and ATG7 transcript levels, and a reduction in H2AX phosphorylation, consistent with a block in autophagy and DNA damage-induced stress response pathways. Surprisingly, in non-lethal cell stress conditions, CASP7 underwent non-canonical processing at two calpain cleavage sites flanking a PARP1 exosite, resulting in stable CASP7-p29/p30 fragments. Expression of CASP7-p29/p30 fragment(s) could rescue H2AX phosphorylation in the CASP3 and CASP7 double knockout background. Strikingly, yet consistent with these phenotypes, the loss of CASP3 and CASP7 exhibited synthetic lethality with BRCA1 loss. These findings support a role for human caspases in stress adaptation through PARP1 modulation and reveal new therapeutic avenues for investigation.
2. RIPK1 is required for ZBP1-driven necroptosis in human cells

   by Oluwamuyiwa T. Amusan, Shuqi Wang, Chaoran Yin, Heather S. Koehler, Yixun Li, Tencho Tenev, Rebecca Wilson, Benjamin Bellenie, Ting Zhang, Jian Wang, Chang Liu, Kim Seong, Seyedeh L. Poorbaghi, Joseph Yates, Yuchen Shen, Jason W. Upton, Pascal Meier, Siddharth Balachandran, Hongyan Guo

   Necroptosis initiated by the host sensor Z-NA binding protein 1 (ZBP1) is essential for host defense against a growing number of viruses, including herpes simplex virus 1 (HSV-1). Studies with HSV-1 and other necroptogenic stimuli in murine settings have suggested that ZBP1 triggers necroptosis by directly complexing with the kinase RIPK3. Whether this is also the case in human cells, or whether additional co-factors are needed for ZBP1-mediated necroptosis, is unclear. Here, we show that ZBP1-induced necroptosis in human cells requires RIPK1. We have found that RIPK1 is essential for forming a stable and functional ZBP1-RIPK3 complex in human cells, but is dispensable for the formation of the equivalent murine complex. The receptor-interacting protein (RIP) homology interaction motif (RHIM) in RIPK3 is responsible for this difference between the 2 species, because replacing the RHIM in human RIPK3 with the RHIM from murine RIPK3 is sufficient to overcome the requirement for RIPK1 in human cells. These observations describe a critical mechanistic difference between mice and humans in how ZBP1 engages in necroptosis, with important implications for treating human diseases.
3. Macrophage invasion into the <i>Drosophila</i> brain requires JAK/STAT-dependent MMP activation in the blood–brain barrier

   by Bente Winkler, Dominik Funke, Christian Klämbt

   The central nervous system is well-separated from external influences by the blood–brain barrier. Upon surveillance, infection or neuroinflammation; however, peripheral immune cells can enter the brain where they often cause detrimental effects. To invade the brain, immune cells not only have to breach cellular barriers, but they also need to traverse associated extracellular matrix barriers. Neither in vertebrates nor in invertebrates is it fully understood how these processes are molecularly controlled. We recently established Drosophila melanogaster as a model to elucidate peripheral immune cell invasion into the brain. Here, we show that neuroinflammation leads to the expression of Unpaired cytokines that activate the JAK/STAT signaling pathway in glial cells of the blood–brain barrier. This in turn triggers the expression of matrix metalloproteinases enabling remodeling of the extracellular matrix enclosing the fly brain and a subsequent invasion of immune cells into the brain. Our study demonstrates conserved mechanisms underlying immune cell invasion of the nervous system in invertebrates and vertebrates and could, thus, further contribute to understanding of JAK/STAT signaling during neuroinflammation.
4. Neural development goes retro: Gags as essential modulators of synapse formation

   by Yung-Heng Chang, Josh Dubnau

   Neurodevelopment requires dynamic control of synapse number. A new study in PLOS Biology reveals that the gag protein of Copia, an active retrotransposon, forms virus-like capsids that transfer its own RNA across the Drosophila neuromuscular junction. Here, Copia acts antagonistically with Arc, another retrotransposon gag protein, to regulate synapse formation. Neurodevelopment requires dynamic control of synapse number. This Primer highlights a new study in PLOS Biology which reveals that the gag protein of Copia, an active retrotransposon, forms virus-like capsids that transfer its own RNA across the Drosophila neuromuscular junction, to regulate synapse formation.
5. Gap junctions allow transfer of metabolites between germ cells and somatic cells to promote germ cell growth in the <i>Drosophila</i> ovary

   by Caroline Vachias, Camille Tourlonias, Louis Grelée, Nathalie Gueguen, Yoan Renaud, Parvathy Venugopal, Graziella Richard, Pierre Pouchin, Emilie Brasset, Vincent Mirouse

   Gap junctions allow the exchange of small molecules between cells. How this function could be used to promote cell growth is not yet fully understood. During Drosophila ovarian follicle development, germ cells, which are surrounded by epithelial somatic cells, undergo massive growth. We found that this growth depends on gap junctions between these cell populations, with a requirement for Innexin4 and Innexin2, in the germ cells and the somatic cells, respectively. Translatomic analyses revealed that somatic cells express enzymes and transporters involved in amino acid metabolism that are absent in germ cells. Among them, we identified a putative amino acid transporter required for germline growth. Its ectopic expression in the germline can partially compensate for its absence or the one of Innexin2 in somatic cells. Moreover, affecting either gap junctions or the import of some amino acids in somatic cells induces P-bodies in the germ cells, a feature usually associated with an arrest of translation. Finally, in somatic cells, innexin2 expression and gap junction assembly are regulated by the insulin receptor/PI3K kinase pathway, linking the growth of the two tissues. Overall, these results support the view that metabolic transfer through gap junction promotes cell growth and illustrate how such a mechanism can be integrated into a developmental program, coupling growth control by extrinsic systemic signals with the intrinsic coordination between cell populations.