Zarfin, A. | bioRxiv Science

bioRxiv2024

Cathepsin X is a conserved cell death protein involved in algal response to environmental stress

Phytoplankton play a crucial role in global primary production and can form vast blooms in aquatic ecosystems. Bloom demise and the rapid turnover of phytoplankton are suggested to involve programmed cell death (PCD) induced by diverse environmental stressors. However, fundamental knowledge of the PCD molecular components in algae and protists in general remains elusive. Previously, we revealed that early oxidation in the chloroplast predicted subsequent cell death or survival in isogenic subpopulations that emerged following H2O2 treatment in the diatom Phaeodactylum tricornutum. Here, we performed transcriptome analysis of sorted sensitive oxidized cells and resilient reduced cells, to discover genes linked to their contrasting fates. By cross-comparison with a large-scale mutant screen in the green alga Chlamydomonas reinhardtii, we identified functionally relevant conserved PCD gene candidates, including the cysteine protease cathepsin X/Z (CPX). CPX mutants in P. tricornutum CPX1 and C. reinhardtii CEP12 both exhibited profound resilience to oxidative stress, supporting a conserved function in algal PCD. P. tricornutum cpx1 mutants, generated using CRISPR-Cas9, also exhibited resilience to the toxic diatom-derived infochemical cyanogen bromide. Phylogenetic and predictive structural analyses show that CPX is highly conserved in eukaryotes, and algae of the green and red lineages exhibit strong structural similarity to human cathepsin CTSZ. CPX is expressed by diverse algae across the oceans and during toxic Pseudo-nitzschia blooms, supporting its ecological importance. Elucidating PCD components in algae sheds light on the evolutionary origin of PCD in unicellular organisms, and on the cellular strategies employed by the population to cope with stressful conditions.

microbiology↗

bioRxiv2023

Expression of modified FcγRI enables myeloid cells to elicit robust tumor-specific cytotoxicity

Despite the central role of T cells in tumor immunity, attempts to harness their cytotoxic capacity as a therapy have met limited efficacy, partially as a result of the suppressive microenvironment which limits their migration and activation. In contrast, myeloid cells massively infiltrate tumors and are well adapted to survive these harsh conditions. While they are equipped with cell-killing abilities, they often adopt an immunosuppressive phenotype upon migration to tumors. Therefore, the questions of how to modify their activation programming against cancer, and what signaling cascades should be activated in myeloid cells to elicit their cytotoxicity have remained unclear. Here, we found that activation of IgM-induced signaling in myeloid cells results in secretion of lytic granules and massive tumor cell death. These findings open venues for designing novel immunotherapy by equipping monocytes with chimeric receptors that target tumor antigens and consequently, signal through IgM receptor. Nonetheless, we found that myeloid cells do not express the antibody-derived portion used to recognize the tumor antigen due to the induction of an ER stress response. To overcome this limitation, we designed chimeric receptors that are based on the high-affinity Fc{gamma}RI for IgG. Incubation of macrophages expressing these receptors along with tumor-binding IgG induced massive tumor cell killing and secretion of reactive oxygen species and Granzyme B. Overall, this work highlights the challenges involved in genetically reprogramming the signaling in myeloid cells and provides a framework for endowing myeloid cells with antigen-specific cytotoxicity.

immunology↗