PI(4,5)P2 determines the threshold of mechanical force–induced B cell activation

Abstract

B lymphocytes use B cell receptors (BCRs) to sense the chemical and physical features of antigens. The activation of isotype-switched IgG-BCR by mechanical force exhibits a distinct sensitivity and threshold in comparison with IgM-BCR. However, molecular mechanisms governing these differences remain to be identified. In this study, we report that the low threshold of IgG-BCR activation by mechanical force is highly dependent on tethering of the cytoplasmic tail of the IgG-BCR heavy chain (IgG-tail) to the plasma membrane. Mechanistically, we show that the positively charged residues in the IgG-tail play a crucial role by highly enriching phosphatidylinositol (4,5)-biphosphate (PI(4,5)P2) into the membrane microdomains of IgG-BCRs. Indeed, manipulating the amounts of PI(4,5)P2 within IgG-BCR membrane microdomains significantly altered the threshold and sensitivity of IgG-BCR activation. Our results reveal a lipid-dependent mechanism for determining the threshold of IgG-BCR activation by mechanical force.

Electrostatic interactions: From immune receptor assembly to signaling

“…The basic residues in the IgG tail interact with PI(4,5)P2 at the PM, and abrogation of this interaction through mutation of the basic residues lead to dissociation of from the PM, albeit without evident spontaneous signaling . Basic residue mutations also reduced the enrichment between PI(4,5)P2 and the IgG‐BCR microdomains and reduction of PI(4,5)P2 at the PM through enzymatic manipulation confirmed that the interaction between the IgG‐BCR CT and PI(4,5)P2 is required for the low mechanical force threshold observed for the activation of the IgG‐BCR . The lower sensitivity of IgG isotype‐switched memory B cells to force due to electrostatic interactions between IgG basic residues and PI(4,5)P2 reveals how memory B cells show fast antigen recall responses.…”

“…While the IgM cytoplasmic tail of the BCR is very short and is not implicated in BCR signaling, the mouse IgG‐BCR cytoplasmic tail contains membrane‐proximal basic residues and is highly hydrophobic, and recruits Grb2 when ligand bound (Figure ) . FRET studies have shown that the IgG cytoplasmic tail is associated to the PM in resting B cells through electrostatic interactions provided by the membrane‐proximal basic as well as hydrophobic residues . In quiescent B cells, IgG‐BCR membrane microdomains are specifically enriched in PI(4,5)P2, while PI(3,4,5)P3, PS and PA are neither enriched nor depleted but may also provide charge for PM binding .…”

“…FRET studies have shown that the IgG cytoplasmic tail is associated to the PM in resting B cells through electrostatic interactions provided by the membrane‐proximal basic as well as hydrophobic residues . In quiescent B cells, IgG‐BCR membrane microdomains are specifically enriched in PI(4,5)P2, while PI(3,4,5)P3, PS and PA are neither enriched nor depleted but may also provide charge for PM binding . Similar to the CD3ε cytoplasmic tail, NMR studies have showed that the IgG tail adopts an alpha helical structure in presence of acidic phospholipids (POPG bicelles) leading to deep insertion of the immunoglobulin tail tyrosine (ITT) within the hydrophobic core pointing to a general feature of membrane association and mode of activation for ITAM and ITAM‐like bearing receptors .…”

“…The lower sensitivity of IgG isotype‐switched memory B cells to force due to electrostatic interactions between IgG basic residues and PI(4,5)P2 reveals how memory B cells show fast antigen recall responses. However, these studies did not investigate the role of the electrostatic interactions that regulate kinase recruitment to and/or phosphorylation of both the IgG‐BCR and CD97a/b accessory signaling heterodimer …”

Mechanical activation of TWIK-related potassium channel by nanoscopic movement and rapid second messenger signaling

“…4) adds in vivo support to a growing list of anionic lipids that set force sensing thresholds. For example, PIP2 sets the threshold for mechanical B-cell activation 59 . Likewise, sphingosine-1-phosphate (S1P), an anionic lipid similar to PA, regulates pain in mice 60 .…”