Erythrocyte scaffolding protein p55/MPP1 functions as an essential regulator of neutrophil polarity

Abstract

As mediators of innate immunity, neutrophils respond to chemoattractants by adopting a highly polarized morphology. Efficient chemotaxis requires the formation of one prominent pseudopod at the cell front characterized by actin polymerization, while local inhibition suppresses the formation of rear and lateral protrusions. This asymmetric control of signaling pathways is required for directional migration along a chemotactic gradient. Here, we identify the MAGUK protein p55/MPP1 as a mediator of the frontness signal required for neutrophil polarization. We developed a p55 knockout (p55 ؊/؊ ) mouse model, and demonstrate that p55 ؊/؊ neutrophils form multiple transient pseudopods upon chemotactic stimulation, and do not migrate efficiently in vitro. Upon agonist stimulation, p55 is rapidly recruited to the leading edge of neutrophils in mice and humans. Total F-actin polymerization, along with Rac1 and RhoA activation, appear to be normal in p55 ؊/؊ neutrophils. Importantly, phosphorylation of Akt is significantly decreased in p55 ؊/؊ neutrophils upon chemotactic stimulation. The activity of immunoprecipitated phosphatidylinositol 3-kinase ␥ (PI3K␥), responsible for chemoattractantinduced synthesis of PIP3 and Akt phosphorylation, is unperturbed in p55 ؊/؊ neutrophils. Although the total amount of PIP3 is normal in p55 ؊/؊ neutrophils, PIP3 is diffusely localized and forms punctate aggregates in activated p55 ؊/؊ neutrophils, as compared to its accumulation at the leading edge membrane in the wild type neutrophils. Together, these results show that p55 is required for neutrophil polarization by regulating Akt phosphorylation through a mechanism that is independent of PI3K␥ activity.Akt ͉ erythrocyte p55 ͉ MPP1 ͉ MAGUK

Essential Function of Protein 4.1G in Targeting of Membrane Protein Palmitoylated 6 into Schmidt-Lanterman Incisures in Myelinated Nerves

“…Although MPP6 was previously shown not to interact with 4.1R (44), we demonstrate that it interacts with 4.1G in myelinating Schwann cells. The existence of an MPP6-4.1G complex in mouse Schwann cells is analogous to the MPP1 (p55)-4.1R complex found in erythrocytes (22,29,33), CASK-4.1N interaction in neurons (3), and MPP6-4.1B interaction in epithelial cells (34). In the 4.1G Ϫ/Ϫ mouse sciatic nerves, the total amount of MPP6 was significantly reduced and the protein was detected in the cytoplasm near the Schwann cell nuclei, which is abnormal.…”

Membrane Rafts in the Erythrocyte Membrane: A Novel Role of MPP1p55

“…Using a MPP1 knockout (p55 -/-) mouse model, Quinn et al ( 2009 ) showed that, upon agonist-stimulation of neutrophils, MPP1 is rapidly recruited to the leading edge. Neutrophils of knockout mouse do not migrate efficiently in vitro and form multiple pseudopods upon chemotactic stimulation, in contrast to normal mouse neutrophils, which form a single pseudopod at the cell front required for effi cient chemotaxis.…”

MPP1/p55 gene deletion in a hemophilia A patient with ectrodactyly and severe developmental defects

“…Previously, we generated MPP1 knockout mice showing phenotypes unrelated to erythrocyte stability including defects in neutrophil polarity and Akt signaling. 9 With further breeding of the MPP1 KO mouse colony on a C57BL/6 genetic background, we observed developmental abnormalities with increasing frequency. To date, no human case of genetic abnormality in the MPP1 family has been reported.…”

“…We compared the developmental defects seen in the patient ( Figure 1A-D) with our established MPP1 null mouse model. 9 Sequential backcrosses onto wild type (WT) C57BL/6 mice were performed for at least 10 generations. No discernible limb development abnormality analogous to the patient's condition was observed.…”