14-3-3 protein sigma isoform co-localizes with phosphorylated α-synuclein in Lewy bodies and Lewy neurites in patients with Lewy body disease

14-3-3 phosphorylation inhibits 14-3-3θ's ability to regulate LRRK2 kinase activity

LRRK2 mutations are among the most common genetic causes for Parkinson's disease (PD), and toxicity is associated with increased kinase activity. 14-3-3 proteins are key interactors that regulate LRRK2 kinase activity. Phosphorylation of the 14-3-3θ isoform at S232 is dramatically increased in human PD brains. Here we investigate the impact of 14-3-3θ phosphorylation on its ability to regulate LRRK2 kinase activity. Both wildtype and the non-phosphorylatable S232A 14-3-3θ mutant reduced the kinase activity of wildtype and G2019S LRRK2, whereas the phosphomimetic S232D 14-3-3θ mutant had minimal effects on LRRK2 kinase activity, as determined by measuring autophosphorylation at S1292 and T1503 and Rab10 phosphorylation. However, wildtype and both 14-3-3θ mutants similarly reduced the kinase activity of the R1441G LRRK2 mutant. 14-3-3θ phosphorylation did not promote global dissociation with LRRK2, as determined by co-immunoprecipitation and proximal ligation assays. 14-3-3s interact with LRRK2 at several phosphorylated serine/threonine sites, including T2524 in the C-terminal helix, which can fold back to regulate the kinase domain. Interaction between 14-3-3θ and phosphorylated T2524 LRRK2 was important for 14-3-3θ's ability to regulate kinase activity, as wildtype and S232A 14-3-3θ failed to reduce the kinase activity of G2019S/T2524A LRRK2. Molecular modeling showed that 14-3-3θ phosphorylation causes a partial rearrangement of its canonical binding pocket, thus affecting the interaction between 14-3-3θ and the C-terminus of LRRK2. We conclude that 14-3-3θ phosphorylation destabilizes the interaction of 14-3-3θ with LRRK2 at T2524, which consequently promotes LRRK2 kinase activity.

Stabilization of 14-3-3 protein-protein interactions with Fusicoccin-A decreases alpha-synuclein dependent cell-autonomous death in neuronal and mouse models

Synucleinopathies are a group of neurodegenerative diseases without effective treatment characterized by the abnormal aggregation of alpha-synuclein (aSyn) protein. Changes in levels or in the amino acid sequence of aSyn (by duplication/triplication of the aSyn gene or point mutations in the encoding region) cause familial cases of synucleinopathies. However, the specific molecular mechanisms of aSyn-dependent toxicity remain unclear. Increased aSyn protein levels or pathological mutations may favor abnormal protein-protein interactions (PPIs) that could either promote neuronal death or belong to a coping response program against neurotoxicity. Therefore, the identification and modulation of aSyn-dependent PPIs can provide new therapeutic targets for these diseases. To identify aSyn-dependent PPIs we performed a proximity biotinylation assay based on the promiscuous biotinylase BioID2. When expressed as a fusion protein, BioID2 biotinylates by proximity stable and transient interacting partners, allowing their identification by streptavidin affinity purification and mass spectrometry. The aSyn interactome was analyzed using BioID2-tagged wild-type (WT) and pathological mutant E46K aSyn versions in HEK293 cells. We found the 14-3-3 epsilon isoform as a common protein interactor for WT and E46K aSyn. 14-3-3 epsilon correlates with aSyn protein levels in brain regions of a transgenic mouse model overexpressing WT human aSyn. Using a neuronal model in which aSyn cell-autonomous toxicity is quantitatively scored by longitudinal survival analysis, we found that stabilization of 14-3-3 protein-proteins interactions with Fusicoccin-A (FC-A) decreases aSyn-dependent toxicity. Furthermore, FC-A treatment protects dopaminergic neuronal somas in the substantia nigra of a Parkinson's disease mouse model. Based on these results, we propose that the stabilization of 14-3-3 epsilon interaction with aSyn might reduce aSyn toxicity, and highlight FC-A as a potential therapeutic compound for synucleinopathies.

Fibril treatment changes protein interactions of tau and α-synuclein in human neurons

In several neurodegenerative disorders, the neuronal proteins tau and α-synuclein adopt aggregation-prone conformations capable of replicating within and between cells. To better understand how these conformational changes drive neuropathology, we compared the interactomes of tau and α-synuclein in the presence or the absence of recombinant fibril seeds. Human embryonic stem cells with an inducible neurogenin-2 transgene were differentiated into glutamatergic neurons expressing (1) WT 0N4R tau, (2) mutant (P301L) 0N4R tau, (3) WT α-synuclein, or (4) mutant (A53T) α-synuclein, each genetically fused to a promiscuous biotin ligase (BioID2). Neurons expressing unfused BioID2 served as controls. After treatment with fibrils or PBS, interacting proteins were labeled with biotin in situ and quantified using mass spectrometry via tandem mass tag labeling. By comparing interactions in mutant versus WT neurons and in fibril- versus PBS-treated neurons, we observed changes in protein interactions that are likely relevant to disease progression. We identified 45 shared interactors, suggesting that tau and α-synuclein function within some of the same pathways. Potential loci of shared interactions include microtubules, Wnt signaling complexes, and RNA granules. Following fibril treatment, physiological interactions decreased, whereas other interactions, including those between tau and 14-3-3 η, increased. We confirmed that 14-3-3 proteins, which are known to colocalize with protein aggregates during neurodegeneration, can promote or inhibit tau aggregation in vitro depending on the specific combination of 14-3-3 isoform and tau sequence.

In Silico Studies on GCP-Lys-OMe as a Potential 14-3-3σ Homodimer Stabilizer

14-3-3 sigma is a vital negative cell cycle regulator. Its expression is consistently downregulated in many types of cancer through gene promoter hypermethylation or proteasomal degradation. 14-3-3 sigma needs to form a homodimer to be functional, while dimers are less prone to degradation than monomers. This suggests that a homodimer stabilizer may increase the tumor suppressive activities of 14-3-3 sigma. However, no known homodimer stabilizer of 14-3-3 sigma has been reported to date. Therefore, this study attempts to test the potential capability of GCP-Lys-OMe (previously reported to bind at the dimer interface of 14-3-3 zeta isoform), to bind and stabilize the 14-3-3 sigma homodimer. In silico docking of GCP-Lys-OMe on 14-3-3 sigma showed more favorable interaction energy (−9.63 kcal/mole) to the dimer interface than 14-3-3 zeta (−7.73 kcal/mole). Subsequent 100 ns molecular dynamics simulation of the GCP-Lys-OMe/14-3-3 sigma complex revealed a highly stable interaction with an average root-mean-square deviation of 0.39 nm (protein backbone) and 0.77 nm (ligand atoms). More contacts between residues at the homodimer interface and a smaller coverage of conformational space of protein atoms were detected for the bound form than for the apo form. These results suggest that GCP-Lys-OMe is a potential homodimer stabilizer of 14-3-3 sigma.

Neuroprotective effect of paeoniflorin in the mouse model of Parkinson's disease through α-synuclein/protein kinase C δ subtype signaling pathway

OBJECTIVES

Paeoniflorin, an active component of Radix Paeoniae Alba, has a neuroprotective effect in Parkinson's animal models. However, its mechanism of action remains to be determined.

METHODS

In this study, we hypothesized that the neuroprotective effect of paeoniflorin occurs through the α-synuclein/protein kinase C δ subtype (PKC-δ) signaling pathway. We tested our hypothesis in the 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-induced mouse model of Parkinson's disease. We evaluated the effects of paeoniflorin on the expression levels of signal components of the α-synuclein/PKC-δ pathway, cellular apoptosis and motor performance.

RESULTS

Our results demonstrated that paeoniflorin restored the motor performance impairment caused by MPTP, inhibited apoptosis, and protected the ultrastructure of neurons. Paeoniflorin treatment also resulted in the dose-dependent upregulation of an antiapoptotic protein, B-cell lymphoma-2, at the mRNA and protein levels, similar to the effects of the positive control, selegiline. In contrast, paeoniflorin treatment downregulated the expression of pro-apoptotic proteins BCL2-Associated X2, α-synuclein, and PKC-δ at the mRNA and protein levels, as well as the level of the activated form of nuclear factor kappa B (p-NF-κB p65).

CONCLUSIONS

Thus, our results showed that paeoniflorin exerts its neuroprotective effect by regulating the α-synuclein/PKC-δ signaling pathway to reduce neuronal apoptosis.

Similarity of the non-amyloid-β component and C-terminal tail of monomeric and tetrameric alpha-synuclein with 14-3-3 sigma

Alpha-synuclein (αSyn) is often described as a predominantly disordered protein that has a propensity to self-assemble into toxic oligomers that are found in patients with Parkinson's and Alzheimer's diseases. αSyn's chaperone behavior and tetrameric structure are proposed to be protective against toxic oligomerization. In this paper, we extended the previously proposed similarity between αSyn and 14-3-3 proteins to the α-helical tetrameric species of αSyn in detail. 14-3-3 proteins are a family of well-folded proteins with seven human isoforms, and function in signal transduction and as molecular chaperones. We investigated protein homology, using sequence alignment, amyloid, and disorder prediction, as well as three-dimensional visualization and protein-interaction networks. Our results show sequence homology and structural similarity between the aggregation-prone non-amyloid-β component (NAC) residues Val-52 to Gly-111 in αSyn and 14-3-3 sigma residues Leu-12 to Gly-78. We identified an additional region of sequence homology in the C-terminal region of αSyn (residues Ser-129 to Asp-135) and a C-terminal loop of 14-3-3 between helix αH and αI (residues Ser-209 to Asp-215). This data indicates αSyn shares conserved domain architecture with small heat shock proteins. We show predicted regions of high amyloidogenic propensity and intrinsic structural disorder in αSyn coincide with amyloidogenic and disordered predictions for 14-3-3 proteins. The homology in the NAC region aligns with residues involved in dimer- and tetramerization of the non-amyloidogenic 14-3-3 proteins. Because 14-3-3 proteins are generally not prone to misfolding, our results lend further support to the hypothesis that the NAC region is critical to the assembly of αSyn into the non-toxic tetrameric state.

Cholesterol-containing lipid nanodiscs promote an α-synuclein binding mode that accelerates oligomerization

Dysregulation of the biosynthesis of cholesterol and other lipids has been implicated in many neurological diseases, including Parkinson's disease. Misfolding of α-synuclein (α-Syn), the main actor in Parkinson's disease, is associated with changes in a lipid environment. However, the exact molecular mechanisms underlying cholesterol effect on α-Syn binding to lipids as well as α-Syn oligomerization and fibrillation remain elusive, as does the relative importance of cholesterol compared to other factors. We probed the interactions and fibrillation behaviour of α-Syn using styrene-maleic acid nanodiscs, containing zwitterionic and anionic lipid model systems with and without cholesterol. Surface plasmon resonance and thioflavin T fluorescence assays were employed to monitor α-Syn binding, as well as fibrillation in the absence and presence of membrane models. 1 H-15 N-correlated NMR was used to monitor the fold of α-Syn in response to nanodisc binding, determining individual residue apparent affinities for the nanodisc-contained bilayers. The addition of cholesterol inhibited α-Syn interaction with lipid bilayers and, however, significantly promoted α-Syn fibrillation, with a more than a 20-fold reduction of lag times before fibrillation onset. When α-Syn bilayer interactions were analysed at an individual residue level by solution-state NMR, we observed two different effects of cholesterol. In nanodiscs made of DOPC, the addition of cholesterol modulated the NAC part of α-Syn, leading to stronger interaction of this region with the lipid bilayer. In contrast, in the nanodiscs comprising DOPC, DOPE and DOPG, the NAC part was mostly unaffected by the presence of cholesterol, while the binding of the N and the C termini was both inhibited.

14-3-3σ and Its Modulators in Cancer

14-3-3σ is an acidic homodimer protein with more than one hundred different protein partners associated with oncogenic signaling and cell cycle regulation. This review aims to highlight the crucial role of 14-3-3σ in controlling tumor growth and apoptosis and provide a detailed discussion on the structure-activity relationship and binding interactions of the most recent 14-3-3σ protein-protein interaction (PPI) modulators reported to date, which has not been reviewed previously. This includes the new fusicoccanes stabilizers (FC-NAc, DP-005), fragment stabilizers (TCF521-123, TCF521-129, AZ-003, AZ-008), phosphate-based inhibitors (IMP, PLP), peptide inhibitors (2a-d), as well as inhibitors from natural sources (85531185, 95911592). Additionally, this review will also include the discussions of the recent efforts by a different group of researchers for understanding the binding mechanisms of existing 14-3-3σ PPI modulators. The strategies and state-of-the-art techniques applied by various group of researchers in the discovery of a different chemical class of 14-3-3σ modulators for cancer are also briefly discussed in this review, which can be used as a guide in the development of new 14-3-3σ modulators in the near future.

Intrinsic disorder associated with 14-3-3 proteins and their partners

Protein-protein interactions (PPIs) mediate a variety of cellular processes and form complex networks, where connectivity is achieved owing to the "hub" proteins whose interaction with multiple protein partners is facilitated by the intrinsically disordered protein regions (IDPRs) and posttranslational modifications (PTMs). Universal regulatory proteins of the eukaryotic 14-3-3 family nicely exemplify these concepts and are the focus of this chapter. The extremely wide interactome of 14-3-3 proteins is characterized by high levels of intrinsic disorder (ID) enabling protein phosphorylation and consequent specific binding to the well-structured 14-3-3 dimers, one of the first phosphoserine/phosphothreonine binding modules discovered. However, high ID enrichment also challenges structural studies, thereby limiting the progress in the development of small molecule modulators of the key 14-3-3 PPIs of increased medical importance. Besides the well-known structural flexibility of their variable C-terminal tails, recent studies revealed the strong and conserved ID propensity hidden in the N-terminal segment of 14-3-3 proteins (~40 residues), normally forming the α-helical dimerization region, that may have a potential role for the dimer/monomer dynamics and recently reported moonlighting chaperone-like activity of these proteins. We review the role of ID in the 14-3-3 structure, their interactome, and also in selected 14-3-3 complexes. In addition, we discuss approaches that, in the future, may help minimize the disproportion between the large amount of known 14-3-3 partners and the small number of 14-3-3 complexes characterized with atomic precision, to unleash the whole potential of 14-3-3 PPIs as drug targets.