Developmental Cell Death in Dictyostelium Does Not Require Paracaspase

Abstract

Apoptotic cell death often requires caspases. Caspases are part of a family of related molecules including also paracaspases and metacaspases. Are molecules of this family generally involved in cell death? More specifically, do non-apoptotic caspase-independent types of cell death require paracaspases or metacaspases? Dictyostelium discoideum lends itself well to answering these questions because 1) it undergoes non-apoptotic developmental cell death of a vacuolar autophagic type and 2) it bears neither caspase nor metacaspase genes and apparently only one paracaspase gene. This only paracaspase gene can be inactivated by homologous recombination. Paracaspase-null clones were thus obtained in each of four distinct Dictyostelium strains. These clones were tested in two systems, developmental stalk cell death in vivo and vacuolar autophagic cell death in a monolayer system mimicking developmental cell death. Compared with parent cells, all of the paracaspase-null cells showed unaltered cell death in both test systems. In addition, paracaspase inactivation led to no alteration in development or interaction with a range of bacteria. Thus, in Dictyostelium, vacuolar programmed cell death in development and in a monolayer model in vitro would seem not to require paracaspase. To our knowledge, this is the first instance of developmental programmed cell death shown to be independent of any caspase, paracaspase or metacaspase. These results have implications as to the relationship in evolution between cell death and the caspase family.Caspases, a subset of cysteine proteases, play an essential role in apoptotic death in animals (1, 2). The caspase family includes not only caspases but also other members called paracaspases and metacaspases. Paracaspases were identified through their remote homology with caspases, and metacaspases were identified in turn through their remote homology with paracaspases (3). Within eukaryotes, paracaspases are found in animals and Dictyostelium discoideum and metacaspases are found in plants, fungi, and some protists (3, 4).

A necrotic cell death model in a protist

“…Necrotic death of atg1-1 Dictyostelium cells is one of the very few instances where a developmental trigger leads in a cell line to death in the absence of genes encoding bcl-2 family members, or caspase or metacaspase genes (its only paracaspase gene is not involved in cell death 37 and Lam et al, in preparation). This cell line can undergo neither autophagy nor apoptosis, thus ensuring minimal interference from other types of cell death with its necrotic cell death.…”

Autophagy and cell death in model organisms

“…In addition, caspase activity is not required for Dictyostelium cell death 36 and the single Dictyostelium paracaspase gene that has been identified is not required, either for autophagic cell death or for necrotic cell death. 37,38 Therefore, apoptosis does not interfere with autophagic cell death in this organism. Dictyostelium displays developmental cell death, 39 which can be mimicked under in vitro monolayer conditions, where Dictyostelium cells are subjected to starvation and the differentiation-inducing factor (DIF-1).…”

Does Autophagy Contribute To Cell Death?

“…2,89 In the social amoeba Dictyostelium discoidium, programmed cell death is caspase independent and notable for an autophagic vacuolar morphology, reminiscent of a type 2 process. [90][91][92] This cell death can be induced in Dictyostelium through the combined effects of starvation and the production of a morphogen called Differentiation Inducing Factor (DIF). However, studies in a mutant strain unable to produce DIF demonstrate that starvation alone can continue to induce autophagy without triggering cell death.…”