The Dictyostelium MAPK ERK1 is phosphorylated in a secondary response to early developmental signaling

Abstract

Previous reports have suggested that the two mitogen-activated protein kinases (MAPKs) in Dictyostelium discoideum, ERK1 and ERK2, can be directly activated in response to external cAMP even though these MAPKs play different roles in the developmental life cycle. To better characterize MAPK regulation, the levels of phosphorylated MAPKs were analyzed in response to external signals. Only ERK2 was rapidly phosphorylated in response to the chemoattractants, cAMP and folate. In contrast, the phosphorylation of ERK1 occurred as a secondary or indirect response to these stimuli and this phosphorylation was enhanced by cell-cell interactions, suggesting that other external signals can activate ERK1. The phosphorylation of ERK1 or ERK2 did not require the function of the other MAPK in these responses. Folate stimulation of a chimeric population of erk1− and gα4− cells revealed that the phosphorylation of ERK1 could be mediated through an intercellular signal other than folate. Loss of ERK1 function suppressed the developmental delay and the deficiency in anterior cell localization associated with gα5− mutants suggesting that ERK1 function can be down regulated through Gα5 subunit-mediated signaling. However, no major changes in the phosphorylation of ERK1 were observed in gα5− cells suggesting that the Gα5 subunit signaling pathway does not regulate the phosphorylation of ERK1. These findings suggest that the activation of ERK1 occurs as a secondary response to chemoattractants and that other cell-cell signaling mechanisms contribute to this activation. Gα5 subunit signaling can down regulate ERK1 function to promote prestalk cell development but not through major changes to the level of phosphorylated ERK1.

Extracellular polyphosphate signals through Ras and Akt to prime Dictyostelium discoideum cells for development

“…When nutrient levels are sufficiently low, polyphosphate also induces aggregation. Hadwiger, 2015). Perhaps as cells become aggregation competent, in part due to polyphosphate-induced CsA expression, Erk1 activation becomes stronger because of the increase in cell-cell interactions, as Erk1 activation aligns with peak CsA levels.…”

“…Erk1 is activated in a delayed response to extracellular cAMP and folic acid (Schwebs and Hadwiger, 2015). Polyphosphate induced Erk1 phosphorylation after 24 h in 100% HL5, with no significant induction after 2 and 12 h, suggesting that Erk1 phosphorylation is a secondary or indirect response, perhaps after aggregation competence is obtained and development is initiated (Fig.…”

“…We next investigated the role of Erk1 activation in polyphosphatemediated induction of the GDT by using a monoclonal antibody that recognizes phosphorylation of the conserved TxY motif (Schwebs and Hadwiger, 2015). Erk1 is activated in a delayed response to extracellular cAMP and folic acid (Schwebs and Hadwiger, 2015).…”

“…For blots with total protein loading controls, identical aliquots of the protein samples were electrophoresed on gels, and proteins were visualized via Coomassie Blue stain. Mccc1 loading control staining for phospho-Erk1 blots was performed as previously described (Davidson et al, 2013;Schwebs and Hadwiger, 2015). Anti-PkbA and anti-PkgB were kind gifts from Pascale Charest, and were each incubated in TBS containing 5% BSA for 1 h at 1:2000.…”

Acanthamoeba and Dictyostelium Use Different Foraging Strategies

“…Foraging cells exhibit substantial meandering during chemotaxis and do not display the elongated morphology typical of aggregating cells. However, both cAMP and folate chemotaxis responses require G proteins that couple to cell surface receptors and many of the downstream cellular responses are quite similar including the transient accumulations of cAMP and cGMP, cytoplasmic influx of calcium, and the activation of regulatory proteins such as mitogen activated protein kinases (MAPKs) (ERK1 and ERK2) (Hadwiger et al 1994; Kumagai et al 1991; Maeda et al 1996; Schwebs and Hadwiger 2015). …”