INPP4's role in the regulation of BCR signaling and B cell metabolic activity

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Date
2020-01-13
Authors
Ste-Croix, Marie Ashley
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Abstract
Inositol polyphosphate 4-phosphatase (INPP4) regulates the PI3K signaling pathway by hydrolysis of the D-4 phosphate of phosphatidylinositol (3,4) bisphosphate (PI(3,4)P2) to produce PI(3)P1. The PH-domain containing proteins Akt and Btk differentially bind to PI(3,4)P2 and PI(3,4,5)P3 produced from PI3K activation, and each contribute to different and equally important downstream signaling pathways activated during B cell antigen receptor (BCR) signaling. However, there is limited understanding of how PI(3,4)P2 hydrolysis affects these specific downstream signaling pathways, and no studies have examined how INPP4 aids in regulation of lymphocyte activation. It is hypothesized that INPP4 regulates BCR signaling and cell metabolic activity via the hydrolysis of PI(3,4)P2. Various biochemical and analytical techniques (confocal microscopy and ECAR assays) were utilized to determine how PI(3,4)P2 depletion by INPP4A overexpression affects the regulation of Akt phosphorylation, Btk/PLCγ phosphorylation and calcium flux, and B cell metabolism during BCR stimulation. INPP4A WT overexpression in transduced human B cells negatively affected Akt phosphorylation as well as some downstream targets, such as GSK-3β and PRAS40. This decrease in Akt phosphorylation was observed to be a result of Akt being physically hindered from recruiting to the plasma membrane due to significant depletion in PI(3,4)P2 levels; visualized via under confocal microscopy analysis of Akt-EGFP transfected INPP4A WT cells. Glycolysis assays determined the decrease in Akt activation may have a significantly impaired ability to increase extracellular acidification rate upon BCR stimulation in these INPP4A WT cells. In contrast, the Btk/PLCγ branch, and the subsequent calcium signaling response during BCR activation, was not affected in INPP4A WT transduced B cells. Current results show that depletion of one of the specific products of PI3K, PI(3,4)P2, can selectively affect the activation of one of the downstream phosphoinositide-binding proteins. Understanding the mechanisms of these pathways and how they are affected could lead to better therapeutic research against consequence of INPP4 dysregulation in B cells. Future experiments will further define the mechanisms by which PI(3,4)P2 depletion selectively affects these downstream signaling events and determine their impact on metabolic programming and B cells function in vivo.
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Keywords
B cells, INPP4, PI3K, BCRs, Lymphocytes
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