Analysis of autophagic and necrotic cell death in Dictyostelium

A High-Throughput, Multi-Cell Phenotype Assay for the Identification of Novel Inhibitors of Chemotaxis/Migration

Chemotaxis and cell migration are fundamental, universal eukaryotic processes essential for biological functions such as embryogenesis, immunity, cell renewal, and wound healing, as well as for pathogenesis of many diseases including cancer metastasis and chronic inflammation. To identify novel chemotaxis inhibitors as probes for mechanistic studies and leads for development of new therapeutics, we developed a unique, unbiased phenotypic chemotaxis-dependent Dictyostelium aggregation assay for high-throughput screening using rapid, laser-scanning cytometry. Under defined conditions, individual Dictyostelium secrete chemoattractants, migrate, and aggregate. Chemotaxis is quantified by laser-scanning cytometry with a GFP marker expressed only in cells after chemotaxis/multi-cell aggregation. We applied the assay to screen 1,280 known compounds in a 1536-well plate format and identified two chemotaxis inhibitors. The chemotaxis inhibitory activities of both compounds were confirmed in both Dictyostelium and in human neutrophils in a directed EZ-TAXIscan chemotaxis assay. The compounds were also shown to inhibit migration of two human cancer cell lines in monolayer scratch assays. This test screen demonstrated that the miniaturized assay is extremely suited for high-throughput screening of very large libraries of small molecules to identify novel classes of chemotaxis/migratory inhibitors for drug development and research tools for targeting chemotactic pathways universal to humans and other systems.

c-di-GMP induction of Dictyostelium cell death requires the polyketide DIF-1

Two inducers, DIF-1 and c-di-GMP, each separately shown to play a major role in Dictyostelium cell death induction in vitro, in fact cooperate. A similar cooperation with polyketides might occur for c-di-GMP effects in other situations and organisms, in particular in innate immunity and cell death in animal cells.

Cell death in the model organism Dictyostelium, as studied in monolayers in vitro, can be induced by the polyketide DIF-1 or by the cyclical dinucleotide c-di-GMP. c-di-GMP, a universal bacterial second messenger, can trigger innate immunity in bacterially infected animal cells and is involved in developmental cell death in Dictyostelium. We show here that c-di-GMP was not sufficient to induce cell death in Dictyostelium cell monolayers. Unexpectedly, it also required the DIF-1 polyketide. The latter could be exogenous, as revealed by a telling synergy between c-di-GMP and DIF-1. The required DIF-1 polyketide could also be endogenous, as shown by the inability of c-di-GMP to induce cell death in Dictyostelium HMX44A cells and DH1 cells upon pharmacological or genetic inhibition of DIF-1 biosynthesis. In these cases, c-di-GMP–induced cell death was rescued by complementation with exogenous DIF-1. Taken together, these results demonstrated that c-di-GMP could trigger cell death in Dictyostelium only in the presence of the DIF-1 polyketide or its metabolites. This identified another element of control to this cell death and perhaps also to c-di-GMP effects in other situations and organisms.

Mycobacterium avium uses apoptotic macrophages as tools for spreading

BACKGROUND

Mycobacterium avium (MAC) lives and replicates in macrophages and causes disseminated disease in immunocompromised individuals. As a host response to control disease, many macrophages become apoptotic a few days after MAC infection. In this study, we hypothesized that MAC can survive autophagic and apoptotic macrophages and spread.

METHODS

Electron, time-lapse video, fluorescence microscopy. Apoptosis was determined by ELISA and TUNEL assays. Autophagy was seen by migration of LC3-1.

RESULTS

Apoptotic macrophages harbor chiefly viable MAC. MAC escapes both the vacuole and the macrophage once apoptosis is triggered, leaving the bacteria free to infect nearby macrophages in the process of spreading. In addition, some MAC species will have apoptotic bodies and are released in healthy macrophages following apoptotic body ingestion. Because autophagy precedes apoptosis, it was established that heat-killed MAC, and viable MAC induces autophagy in macrophages at similar rates, but MAC still survives.

CONCLUSION

MAC spreading from cell-to-cell is triggered by the macrophage's attempt to kill the bacterium, undergoing apoptosis.