Protein Kinase C Isoforms Differentially Phosphorylate Human Choline Acetyltransferase Regulating Its Catalytic Activity

Alzheimer's Disease and Green Tea: Epigallocatechin-3-Gallate as a Modulator of Inflammation and Oxidative Stress

Alzheimer's disease (AD) is the most common cause of dementia, characterised by a marked decline of both memory and cognition, along with pathophysiological hallmarks including amyloid beta peptide (Aβ) accumulation, tau protein hyperphosphorylation, neuronal loss and inflammation in the brain. Additionally, oxidative stress caused by an imbalance between free radicals and antioxidants is considered one of the main risk factors for AD, since it can result in protein, lipid and nucleic acid damage and exacerbate Aβ and tau pathology. To date, there is a lack of successful pharmacological approaches to cure or even ameliorate the terrible impact of this disease. Due to this, dietary compounds with antioxidative and anti-inflammatory properties acquire special relevance as potential therapeutic agents. In this context, green tea, and its main bioactive compound, epigallocatechin-3-gallate (EGCG), have been targeted as a plausible option for the modulation of AD. Specifically, EGCG acts as an antioxidant by regulating inflammatory processes involved in neurodegeneration such as ferroptosis and microglia-induced cytotoxicity and by inducing signalling pathways related to neuronal survival. Furthermore, it reduces tau hyperphosphorylation and aggregation and promotes the non-amyloidogenic route of APP processing, thus preventing the formation of Aβ and its subsequent accumulation. Taken together, these results suggest that EGCG may be a suitable candidate in the search for potential therapeutic compounds for neurodegenerative disorders involving inflammation and oxidative stress, including Alzheimer's disease.

A multidimensional computational exploration of congenital myasthenic syndrome causing mutations in human choline acetyltransferase

Missense mutations of human choline acetyltransferase (CHAT) are mainly associated with congenital myasthenic syndrome (CMS). To date, several pathogenic mutations have been reported, but due to the rarity and genetic complexity of CMS and difficult genotype-phenotype correlations, the CHAT mutations, and their consequences are underexplored. In this study, we systematically sift through the available genetic data in search of previously unreported pathogenic mutations and use a dynamic in silico model to provide structural explanations for the pathogenicity of the reported deleterious and undetermined variants. Through rigorous multiparameter analyses, we conclude that mutations can affect CHAT through a variety of different mechanisms: by disrupting the secondary structure, by perturbing the P-loop through long-range allosteric interactions, by disrupting the domain connecting loop, and by affecting the phosphorylation process. This study provides the first dynamic look at how mutations affect the structure and catalytic activity in CHAT and highlights the need for further genomic research to better understand the pathology of CHAT.

Ubiquitin-Proteasome Modulating Dolabellanes and Secosteroids from Soft Coral Clavularia flava

We performed a high-content screening (HCS) assay aiming to discover bioactive molecules with proteasome inhibitory activity. By structural elucidation, we identified six compounds purified from soft coral Clavularia flava, which potentiates proteasome inhibition. Chemical structure elucidation revealed they are dolabellane- and secosteroid-based compounds including a new dolabellane, clavinflol C (1), three known dolabellanes, stolonidiol (2), stolonidiol-17-acetate (3), and clavinflol B (4) as well as two new secosteroids, 3β,11-dihydroxy-24-methyl-9,11-secocholest-5-en-9,23-dione (5) and 3β,11-dihydroxy-24-methylene-9,11-secocholest-5-en-9,23-dione (6). All six compounds show less cytotoxicity than those of known proteasome inhibitors, bortezomib and MG132. In summary, the high-content measurements of control inhibitors, bortezomib and MG132, manifest the highest ratio >2 in high-content measurement. Of the isolated compounds, 2 and 5 showed higher activity, followed by 3 and 6, and then 1 and 4 exhibited moderate inhibition.

Acetylcholine signaling system in progression of lung cancers

The neurotransmitter acetylcholine (ACh) acts as an autocrine growth factor for human lung cancer. Several lines of evidence show that lung cancer cells express all of the proteins required for the uptake of choline (choline transporter 1, choline transporter-like proteins) synthesis of ACh (choline acetyltransferase, carnitine acetyltransferase), transport of ACh (vesicular acetylcholine transport, OCTs, OCTNs) and degradation of ACh (acetylcholinesterase, butyrylcholinesterase). The released ACh binds back to nicotinic (nAChRs) and muscarinic receptors on lung cancer cells to accelerate their proliferation, migration and invasion. Out of all components of the cholinergic pathway, the nAChR-signaling has been studied the most intensely. The reason for this trend is due to genome-wide data studies showing that nicotinic receptor subtypes are involved in lung cancer risk, the relationship between cigarette smoke and lung cancer risk as well as the rising popularity of electronic cigarettes considered by many as a "safe" alternative to smoking. There are a small number of articles which review the contribution of the other cholinergic proteins in the pathophysiology of lung cancer. The primary objective of this review article is to discuss the function of the acetylcholine-signaling proteins in the progression of lung cancer. The investigation of the role of cholinergic network in lung cancer will pave the way to novel molecular targets and drugs in this lethal malignancy.

Stolonidiol: Synthesis, Target Identification, and Mechanism for Choline Acetyltransferase Activation

Stolonidiol, a marine natural product, has been reported to potentiate the activity of choline acetyltransferase (ChAT), the enzyme that produces the neurotransmitter acetylcholine. Here we report the total synthesis of stolonidiol starting from (R)-(+)-limonene. To identify the mechanism by which ChAT activity is increased, we sought to identify the biological target of stolonidiol. We show that stolonidiol binds to the phorbol ester binding site of protein kinase C (PKC), induces translocation of PKC to the cell membrane, and activates kinase activity. Furthermore, we confirmed the increase in ChAT activity observed upon treatment of cells with stolonidiol and show that this effect is mediated by PKC. Collectively, our data strongly suggest that PKC activation by stolonidiol is responsible for the resulting potentiation of ChAT activity.

Enhanced ubiquitination and proteasomal degradation of catalytically deficient human choline acetyltransferase mutants

Choline acetyltransferase (ChAT) is essential for cholinergic neuron function as it mediates synthesis of the neurotransmitter acetylcholine. ChAT mutations have been linked to the neuromuscular disorder congenital myasthenic syndrome (CMS). One CMS-related ChAT mutation, V18M, reduces enzyme activity and cellular protein levels, and is positioned within a highly conserved proline-rich motif with the sequence 14 PKLPVPP20 . We demonstrate that N-terminal truncation that includes this proline-rich motif, as well as mutation of prolines-17/19 together to alanine (P17A/P19A), dramatically reduces ChAT steady-state protein levels and cellular activity when expressed in cholinergic SN56 neural cells. The in vitro activity of bacterially expressed recombinant P17A/P19A-ChAT is also reduced, although this is not caused by changes in protein secondary structure or thermal stability. Treatment of SN56 cells with the proteasome inhibitor MG132 increases cellular P17A/P19A-ChAT steady-state protein levels, and by immunoprecipitation we found that ChAT is ubiquitinated and that polyubiquitination of P17A/P19A-ChAT is increased compared to wild-type (WT) ChAT. Using a novel fluorescent-biorthogonal pulse-chase protocol in SN56 cells, we determined that the protein half-life of P17A/P19A-ChAT (2.2 h) is substantially reduced compared to WT-ChAT (19.7 h). Lastly, we show that two CMS-related ChAT mutants (V18M and A513T) have enhanced ubiquitination, and that treatment with MG132 can partially restore both the steady-state protein levels as well as cellular activity of some CMS-mutant ChAT. These results identify a novel mechanism for regulation of ChAT through the ubiquitin-proteasome system that is influenced by the conserved N-terminal proline-rich motif of ChAT and may be implicated in CMS pathology. Choline acetyltransferase (ChAT) synthesizes acetylcholine in cholinergic neurons. In this study we find that steady-state protein levels of human 69-kDa ChAT are regulated by the ubiquitin-proteasome system. Mutation of a highly conserved N-terminal proline-rich motif in human 69-kDa ChAT reduces both cellular ChAT protein levels, through enhanced ubiquitination and proteasomal degradation, and enzyme activity. Ubiquitination of catalytically deficient congenital myasthenic syndrome (CMS)-mutant ChAT is increased in cells, and importantly proteasome inhibition partially restores steady-state protein levels as well as cellular activity of some CMS-mutant ChAT proteins.

Neuronal AChE splice variants and their non-hydrolytic functions: redefining a target of AChE inhibitors?

AChE enzymatic inhibition is a core focus of pharmacological intervention in Alzheimer's disease (AD). Yet, AChE has also been ascribed non-hydrolytic functions, which seem related to its appearance in various isoforms. Neuronal AChE presents as a tailed form (AChE-T) predominantly found on the neuronal synapse, and a facultatively expressed readthough form (AChE-R), which exerts short to medium-term protective effects. Notably, this latter form is also found in the periphery. While these non-hydrolytic functions of AChE are most controversially discussed, there is evidence for them being additional targets of AChE inhibitors. This review aims to provide clarification as to the role of these AChE splice variants and their interplay with other cholinergic parameters and their being targets of AChE inhibition: AChE-R is particularly involved in the mediation of (anti-)apoptotic events in cholinergic cells, involving adaptation of various cholinergic parameters and a time-dependent link to the expression of neuroprotective factors. The AChE-T C-terminus is central to AChE activity regulation, while isolated AChE-T C-terminal fragments mediate toxic effects via the α7 nicotinic acetylcholine receptor. There is direct evidence for roles of AChE-T and AChE-R in neurodegeneration and neuroprotection, with these roles involving AChE as a key modulator of the cholinergic system: in vivo data further encourages the use of AChE inhibitors in the treatment of neurodegenerative conditions such as AD since effects on both enzymatic activity and the enzyme's non-hydrolytic functions can be postulated. It also suggests that novel AChE inhibitors should enhance protective AChE-R, while avoiding the concomitant up-regulation of AChE-T.

Hippocampal protein kinase C family members in spatial memory retrieval in the mouse

Although a few individual members of the protein kinase C (PKC) family were studied in spatial memory no systematic approach was carried out to concomitantly determine all described PKC family members in spatial memory of the mouse. It was therefore the aim of the current study to link hippocampal PKCs to memory retrieval in the Morris water maze (MWM). CD1 mice were trained (n=9) or untrained (n=9) in the MWM, hippocampi were taken 6h following the test for memory retrieval and PKCs were determined in mouse hippocampi by immunoblotting. The trained animals learned the spatial memory task and kept memory at the probe trial. PKCs alpha and epsilon were comparable between groups while PKCs beta, delta, gamma (two forms, i.e. two bands on Western blotting), zeta (2 forms) were higher in trained mice and theta (2 forms) were lower in trained mice. PKC gamma (1 form) was significantly correlating with the time spent in the target quadrant (r=0.7933; P=0.0188). Changes of hippocampal levels of PKCs beta, delta, gamma, zeta and theta were paralleling memory retrieval of the MWM task but correlations revealed that spatial memory retrieval was only linked to one form of PKC gamma. Results are also in agreement with a recent publication showing that PKM zeta is not required for memory formation. These findings may be relevant for the interpretation of previous work and the design of future work on the protein kinase C family in spatial memory of the mouse.

Protein kinase C activators as synaptogenic and memory therapeutics

The last decade has witnessed a rapid progress in understanding of the molecular cascades that may underlie memory and memory disorders. Among the critical players, activity of protein kinase C (PKC) isoforms is essential for many types of learning and memory and their dysfunction, and is critical in memory disorders. PKC inhibition and functional deficits lead to an impairment of various types of learning and memory, consistent with the observations that neurotoxic amyloid inhibits PKC activity and that transgenic animal models with PKCbeta deficit exhibit impaired capacity in cognition. In addition, PKC isozymes play a regulatory role in amyloid production and accumulation. Restoration of the impaired PKC signal pathway pharmacologically results in an enhanced memory capacity and synaptic remodeling / repair and synaptogenesis, and, therefore, represents a potentially important strategy for the treatment of memory disorders, including Alzheimer's dementia. The PKC activators, especially those that are isozyme-specific, are a new class of drug candidates that may be developed as future memory therapeutics.

Protein kinase C isoforms differentially phosphorylate Ca(v)1.2 alpha(1c)

The regulation of Ca(2+) influx through the phosphorylation of the L-type Ca(2+) channel, Ca(v)1.2, is important for the modulation of excitation-contraction (E-C) coupling in the heart. Ca(v)1.2 is thought to be the target of multiple kinases that mediate the signals of both the renin-angiotensin and sympathetic nervous systems. Detailed biochemical information regarding the protein phosphorylation reactions involved in the regulation of Ca(v)1.2 is limited. The protein kinase C (PKC) family of kinases can modulate cardiac contractility in a complex manner, such that contractility is either enhanced or depressed and relaxation is either accelerated or slowed. We have previously reported that Ser(1928) in the C-terminus of alpha(1c) was a target for PKCalpha, -zeta, and -epsilon phosphorylation. Here, we report the identification of seven PKC phosphorylation sites within the alpha(1c) subunit. Using phospho-epitope specific antibodies to Ser(1674) and Ser(1928), we demonstrate that both sites within the C-terminus are phosphorylated in HEK cells in response to PMA. Phosphorylation was inhibited with a PKC inhibitor, bisindolylmaleimide. In Langendorff-perfused rat hearts, both Ser(1674) and Ser(1928) were phosphorylated in response to PMA. Phosphorylation of Ser(1674), but not Ser(1928), is PKC isoform specific, as only PKCalpha, -betaI, -betaII, -gamma, -delta, and -theta, but not PKCepsilon, -zeta, and -eta, were able to phosphorylate this site. Our results identify a molecular mechanism by which PKC isoforms can have different effects on channel activity by phosphorylating different residues.

Phosphorylation of the Aspergillus oryzae Woronin body protein, AoHex1, by protein kinase C: evidence for its role in the multimerization and proper localization of the Woronin body protein

Woronin body, a specialized peroxisome, is a unique organelle involved in septal pore sealing and protecting filamentous fungus from excessive cytoplasmic bleeding. We recently characterized the Aohex1 gene encoding the major protein of the Woronin body in the fungus Aspergillus oryzae. Although three-dimensional microscopy revealed plugging of the septal pore by Woronin body, the mechanism of its formation remains unknown. We report here a reduction in the oligomeric forms (dimeric and tetrameric) of AoHex1 upon l-phosphatase treatment, which indicated that AoHex1 phosphorylation in vivo facilitates its oligomerization. Concomitant with the presence of a highly conserved predicted PKC (protein kinase C)-phosphorylatable site (Ser151), the recombinant AoHex1 was phosphorylated by PKC in vitro and the administration of the PKC inhibitors, bisindolylmaleimide I and chelerythrine, resulted in the reduction of the oligomeric forms of AoHex1 in vivo. While spherical dot-like Woronin bodies were visualized by expressing the dsred2-Aohex1 and egfp (enhanced green fluorescent protein)-Aohex1 constructs in A. oryzae, treatment with the PKC inhibitors caused an abnormal localization to ring-like structures. In addition to the reduced phosphorylation of the mutagenized recombinant AoHex1[S151A] (Ser151 to alanine substitution) by PKC in vitro, the overexpression of Aohex1[S151A] as dsred2 fusion against the wild-type background also showed reduction of the oligomeric forms of the endogenous AoHex1 and its perturbed localization to ring-like structures in vivo. In conclusion, the present study implicates the relevance of PKC-dependent phosphorylation of the Woronin body protein, AoHex1, for its multimerization and proper localization.

Substrate binding and catalytic mechanism of human choline acetyltransferase

Choline acetyltransferase (ChAT) catalyzes the synthesis of the neurotransmitter acetylcholine from choline and acetyl-CoA, and its presence is a defining feature of cholinergic neurons. We report the structure of human ChAT to a resolution of 2.2 A along with structures for binary complexes of ChAT with choline, CoA, and a nonhydrolyzable acetyl-CoA analogue, S-(2-oxopropyl)-CoA. The ChAT-choline complex shows which features of choline are important for binding and explains how modifications of the choline trimethylammonium group can be tolerated by the enzyme. A detailed model of the ternary Michaelis complex fully supports the direct transfer of the acetyl group from acetyl-CoA to choline through a mechanism similar to that seen in the serine hydrolases for the formation of an acyl-enzyme intermediate. Domain movements accompany CoA binding, and a surface loop, which is disordered in the unliganded enzyme, becomes localized and binds directly to the phosphates of CoA, stabilizing the complex. Interactions between this surface loop and CoA may function to lower the KM for CoA and could be important for phosphorylation-dependent regulation of ChAT activity.

A model for dynamic regulation of choline acetyltransferase by phosphorylation

Choline acetyltransferase (ChAT) synthesizes the neurotransmitter acetylcholine (ACh) and is a phenotypic marker for cholinergic neurons. Cholinergic neurons in brain are involved in cognitive function, attentional processing and motor control, and decreased ChAT activity is found in several neurological disorders including Alzheimer's disease. Dysregulation of ChAT and cholinergic communication is also associated with some spontaneous point-mutations in ChAT that alter its substrate binding kinetics, or by disruption of signaling pathways that could regulate protein kinases for which ChAT is a substrate. It has been identified recently that the catalytic activity and subcellular distribution of ChAT, and its interaction with other cellular proteins, can be modified by phosphorylation of the enzyme by protein kinase-C and Ca2+/calmodulin-dependent protein kinase II; these kinases appear also to mediate some of the effects of beta-amyloid peptides on cholinergic neuron functions, including the effects on ChAT. This review outlines a new model for the regulation of cholinergic transmission at the level of the presynaptic terminal that is mediated by hierarchically-regulated, multi-site phosphorylation of ChAT.

Two methods for large-scale purification of recombinant human choline acetyltransferase

Choline acetyltransferase (ChAT) catalyzes the transfer of an acetyl group from acetyl-CoA to choline to produce the neurotransmitter acetylcholine (ACh). We have produced large quantities of pure human ChAT using two different bacterial expression systems. In the first, ChAT is fused to a chitin-binding domain via a self-cleavable linker allowing the release of ChAT without the use of proteases. In the second, ChAT is fused to a hexahistidine (His6) tag at the N-terminus with a linker incorporating a TEV protease cleavage site. In both cases, pure ChAT was produced that has a final specific activity of approximately 50 micromol ACh/min/mg and is suitable for structural characterization. Analysis of purified ChAT by Western blots and mass spectrometry revealed that the C-terminal 15 amino acids were slowly removed by endogenous proteolytic activity, to produce a stable 615 residue protein. Furthermore, we show that purified recombinant human ChAT is highly prone to oxidation, leading to the formation of covalent dimers and/or a loss of catalytic activity. Kinetic parameters of our purified proteins were obtained and, when compared to previously published constants for human placental ChAT, we found that recombinant human ChAT displays lower values for Michaelis and inhibition constants for ACh, which may be due to the complete absence of post-translational modifications.