An Autocrine Negative Feedback Loop Inhibits Dictyostelium discoideum Proliferation through Pathways Including IP3/Ca2

Polyphosphate uses mTOR, pyrophosphate, and Rho GTPase components to potentiate bacterial survival in Dictyostelium

IMPORTANCE

Although most bacteria are quickly killed after phagocytosis by a eukaryotic cell, some pathogenic bacteria escape death after phagocytosis. Pathogenic Mycobacterium species secrete polyP, and the polyP is necessary for the bacteria to prevent their killing after phagocytosis. Conversely, exogenous polyP prevents the killing of ingested bacteria that are normally killed after phagocytosis by human macrophages and the eukaryotic microbe Dictyostelium discoideum. This suggests the possibility that in these cells, a signal transduction pathway is used to sense polyP and prevent killing of ingested bacteria. In this report, we identify key components of the polyP signal transduction pathway in D. discoideum. In cells lacking these components, polyP is unable to inhibit killing of ingested bacteria. The pathway components have orthologs in human cells, and an exciting possibility is that pharmacologically blocking this pathway in human macrophages would cause them to kill ingested pathogens such as Mycobacterium tuberculosis.

A circuit for secretion-coupled cellular autonomy in multicellular eukaryotic cells

Cancers represent complex autonomous systems, displaying self-sufficiency in growth signaling. Autonomous growth is fueled by a cancer cell's ability to "secrete-and-sense" growth factors (GFs): a poorly understood phenomenon. Using an integrated computational and experimental approach, here we dissect the impact of a feedback-coupled GTPase circuit within the secretory pathway that imparts secretion-coupled autonomy. The circuit is assembled when the Ras-superfamily monomeric GTPase Arf1, and the heterotrimeric GTPase Giαβγ and their corresponding GAPs and GEFs are coupled by GIV/Girdin, a protein that is known to fuel aggressive traits in diverse cancers. One forward and two key negative feedback loops within the circuit create closed-loop control, allow the two GTPases to coregulate each other, and convert the expected switch-like behavior of Arf1-dependent secretion into an unexpected dose-response alignment behavior of sensing and secretion. Such behavior translates into cell survival that is self-sustained by stimulus-proportionate secretion. Proteomic studies and protein-protein interaction network analyses pinpoint GFs (e.g., the epidermal GF) as key stimuli for such self-sustenance. Findings highlight how the enhanced coupling of two biological switches in cancer cells is critical for multiscale feedback control to achieve secretion-coupled autonomy of growth factors.

A PKC that controls polyphosphate levels, pinocytosis and exocytosis, regulates stationary phase onset in Dictyostelium

Many cells can pause their growth cycle, a topic much enriched by studies of the stationary phase (SP) of model microorganisms. While several kinases are implicated in SP onset, a possible role for protein kinase C remains unknown. We show that Dictyostelium discoideum cells lacking pkcA entered SP at a reduced cell density, but only in shaking conditions. Precocious SP entry occurs because extracellular polyphosphate (polyP) levels reach a threshold at the lower cell density; adding exopolyphosphatase to pkcA- cells reverses the effect and mimics wild type growth. PkcA's regulation of polyP depended on inositol hexakisphosphate kinase and phospholipase D. PkcA- mutants also had higher actin levels, higher rates of exocytosis and lower pinocytosis rates. Postlysosomes were smaller and present in fewer pkcA- cells, compared to the wildtype. Overall, the results suggest that a reduced PkcA level triggers SP primarily because cells do not acquire or retain nutrients as efficiently, thus mimicking, or amplifying, the conditions of actual starvation.