Disturbances in PP2A methylation and one-carbon metabolism compromise Fyn distribution, neuritogenesis, and APP regulation

Goce Taleski,Diana Schuhmacher,Henry Su,Jean-Marie Sontag,Estelle Sontag

doi:10.1074/jbc.ra120.016069

Abstract

The nonreceptor protein tyrosine kinase Fyn and protein Ser/Thr phosphatase 2A (PP2A) are major multifunctional signaling molecules. Deregulation of Fyn and altered PP2A methylation are implicated in cancer and Alzheimer's disease (AD). Here, we tested the hypothesis that the methylation state of PP2A catalytic subunit, which influences PP2A subunit composition and substrate specificity, can affect Fyn regulation and function. Using Neuro-2a (N2a) neuroblastoma cell models, we first show that methylated PP2A holoenzymes containing the Bα subunit coimmunoprecipitate and copurify with Fyn in membrane rafts. PP2A methylation status regulates Fyn distribution and Fyn-dependent neuritogenesis, likely in part by affecting actin dynamics. A methylation-incompetent PP2A mutant fails to interact with Fyn. It perturbs the normal partitioning of Fyn and amyloid precursor protein (APP) in membrane microdomains, which governs Fyn function and APP processing. This correlates with enhanced amyloidogenic cleavage of APP, a hallmark of AD pathogenesis. Conversely, enhanced PP2A methylation promotes the nonamyloidogenic cleavage of APP in a Fyn-dependent manner. Disturbances in one-carbon metabolic pathways that control cellular methylation are associated with AD and cancer. Notably, they induce a parallel loss of membrane-associated methylated PP2A and Fyn enzymes in N2a cells and acute mouse brain slices. One-carbon metabolism also modulates Fyn-dependent process outgrowth in N2a cells. Thus, our findings identify a novel methylation-dependent PP2A/Fyn signaling module. They highlight the underestimated importance of cross talks between essential metabolic pathways and signaling scaffolds that are involved in normal cell homeostasis and currently being targeted for cancer and AD treatment.

Highlights

Fyn is a member of the Src family kinases (SFKs) of nonreceptor protein tyrosine kinases that modulate a plethora of key cellular functions, including growth, survival, adhesion, migration, and differentiation (1)
Based on the reported in vitro binding of catalytic “C” subunit of PP2A (PP2Ac) to Src (29), an SFK structurally closely related to Fyn, we first assessed the existence of phosphatase 2A (PP2A)–Fyn protein complexes using a series of coimmunoprecipitation assays
Western blot analyses revealed that endogenous Fyn was present in HA–PP2Ac immunoprecipitates prepared from WT but not L309Δ-expressing or control N2a cell lysates (Fig. 1B), suggesting that Fyn preferentially associates with B-containing methylated PP2A holoenzymes

Summary

Introduction

Fyn is a member of the Src family kinases (SFKs) of nonreceptor protein tyrosine kinases that modulate a plethora of key cellular functions, including growth, survival, adhesion, migration, and differentiation (1). Western blot analyses revealed that endogenous Fyn was present in HA–PP2Ac immunoprecipitates prepared from WT but not L309Δ-expressing or control N2a cell lysates (Fig. 1B), suggesting that Fyn preferentially associates with B-containing methylated PP2A holoenzymes.

Results

Conclusion