Presynaptic Calmodulin targets: lessons from structural proteomics

CaV2.1 mediates presynaptic dysfunction induced by amyloid β oligomers

Synaptic dysfunction is an early pathological phenotype of Alzheimer's disease (AD) that is initiated by oligomers of amyloid β peptide (Aβos). Treatments aimed at correcting synaptic dysfunction could be beneficial in preventing disease progression, but mechanisms underlying Aβo-induced synaptic defects remain incompletely understood. Here, we uncover an epithelial sodium channel (ENaC) - CaV2.3 - protein kinase C (PKC) - glycogen synthase kinase-3β (GSK-3β) signal transduction pathway that is engaged by Aβos to enhance presynaptic CaV2.1 voltage-gated Ca2+ channel activity, resulting in pathological potentiation of action-potential-evoked synaptic vesicle exocytosis. We present evidence that the pathway is active in human APP transgenic mice in vivo and in human AD brains, and we show that either pharmacological CaV2.1 inhibition or genetic CaV2.1 haploinsufficiency is sufficient to restore normal neurotransmitter release. These findings reveal a previously unrecognized mechanism driving synaptic dysfunction in AD and identify multiple potentially tractable therapeutic targets.

Delineating the Molecular Basis of the Calmodulin‒bMunc13-2 Interaction by Cross-Linking/Mass Spectrometry-Evidence for a Novel CaM Binding Motif in bMunc13-2

Exploring the interactions between the Ca²⁺ binding protein calmodulin (CaM) and its target proteins remains a challenging task. Members of the Munc13 protein family play an essential role in short-term synaptic plasticity, modulated via the interaction with CaM at the presynaptic compartment. In this study, we focus on the bMunc13-2 isoform expressed in the brain, as strong changes in synaptic transmission were observed upon its mutagenesis or deletion. The CaM–bMunc13-2 interaction was previously characterized at the molecular level using short bMunc13-2-derived peptides only, revealing a classical 1–5–10 CaM binding motif. Using larger protein constructs, we have now identified for the first time a novel and unique CaM binding site in bMunc13-2 that contains an N-terminal extension of a classical 1–5–10 CaM binding motif. We characterize this motif using a range of biochemical and biophysical methods and highlight its importance for the CaM–bMunc13-2 interaction.

The Calmodulin Binding Region of the Synaptic Vesicle Protein Mover Is Required for Homomeric Interaction and Presynaptic Targeting

Neurotransmitter release is mediated by an evolutionarily conserved machinery. The synaptic vesicle (SV) associated protein Mover/TPRGL/SVAP30 does not occur in all species and all synapses. Little is known about its molecular properties and how it may interact with the conserved components of the presynaptic machinery. Here, we show by deletion analysis that regions required for homomeric interaction of Mover are distributed across the entire molecule, including N-terminal, central and C-terminal regions. The same regions are also required for the accumulation of Mover in presynaptic terminals of cultured neurons. Mutating two phosphorylation sites in N-terminal regions did not affect these properties. In contrast, a point mutation in the predicted Calmodulin (CaM) binding sequence of Mover abolished both homomeric interaction and presynaptic targeting. We show that this sequence indeed binds Calmodulin, and that recombinant Mover increases Calmodulin signaling upon heterologous expression. Our data suggest that presynaptic accumulation of Mover requires homomeric interaction mediated by regions distributed across large areas of the protein, and corroborate the hypothesis that Mover functionally interacts with Calmodulin signaling.

iTRAQ-based quantitative proteomics reveals the neuroprotection of rhubarb in experimental intracerebral hemorrhage

ETHNOPHARMACOLOGICAL RELEVANCE

Rhubarb is a traditional Chinese medicine(TCM), that possesses neuroprotective, anti-inflammatory, antibacterial, antioxidative, purgative and anticancer properties, and has been used to treat intracerebral hemorrhage (ICH) and many other diseases.

AIMS OF THE STUDY

This study aimed to investigate the changes of brain protein in ICH rats treated with rhubarb and to explore the multi-target mechanism of rhubarb in the treatment of ICH via bioinformatics analysis of differentially expressed proteins (DEPs).

MATERIALS AND METHODS

Rats were subjected to collagenase-induced ICH and then treated orally with 3 or 12 g/kg rhubarb daily for 2 days following ICH. After sacrifice, total protein of brain tissue was extracted, and isobaric tag for relative and absolute quantification (iTRAQ)-based liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis was employed to quantitatively identify of the DEPs in two treatment groups compared with the vehicle group. The DEPs were analyzed by Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and STRING databases. Bioinformatics Analysis Tool for Molecular mechanism of TCM (BATMAN-TCM) was used to predict the target of rhubarb and western blotting was used for verification.

RESULTS

In total, 1356 proteins were identified with a 1% false discovery rate (FDR). Among them, 55 DEPs were significantly altered in the sham, vehicle, low dose rhubarb group (LDR, 3 g/kg), and high dose rhubarb group (HDR, 12 g/kg). Enrichment analysis of GO annotations indicated that rhubarb mainly regulated expression of some neuron projection proteins involved in the response to drug and nervous system development. The dopaminergic synapse pathway was found to be the most significant DEP in the combined analysis of the KEGG and BATMAN-TCM databases. Based on the results of the STRING analysis, oxidative stress (OS), calcium binding protein regulation, vascularization, and energy metabolism were important in the rhubarb therapeutic process.

CONCLUSION

Rhubarb achieves its effects mainly through the dopaminergic synapse pathway in ICH treatment. The ICH-treating mechanisms of rhubarb may also involve anti-OS, calcium binding protein regulation, angiogenic regulation, and energy metabolism improvement. This study adds new evidence to clinical applications of rhubarb for ICH.

Current advances of carbene-mediated photoaffinity labeling in medicinal chemistry

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites. In particular, carbene-mediated photoaffinity labeling (cmPAL) has been widely used in drug target identification owing to its excellent photolabeling efficiency, minimal steric interference and longer excitation wavelength. Specifically, diazirines, which are among the precursors of carbenes and have higher carbene yields and greater chemical stability than diazo compounds, have proved to be valuable photolabile reagents in a diverse range of biological systems. This review highlights current advances of cmPAL in medicinal chemistry, with a focus on structures and applications for identifying small molecule-protein and macromolecule-protein interactions and ligand-gated ion channels, coupled with advances in the discovery of targets and inhibitors using carbene precursor-based biological probes developed in recent decades.

Conformational Ensembles of Calmodulin Revealed by Nonperturbing Site-Specific Vibrational Probe Groups

Seven native residues on the regulatory protein calmodulin, including three key methionine residues, were replaced (one by one) by the vibrational probe amino acid cyanylated cysteine, which has a unique CN stretching vibration that reports on its local environment. Almost no perturbation was caused by this probe at any of the seven sites, as reported by CD spectra of calcium-bound and apo calmodulin and binding thermodynamics for the formation of a complex between calmodulin and a canonical target peptide from skeletal muscle myosin light chain kinase measured by isothermal titration. The surprising lack of perturbation suggests that this probe group could be applied directly in many protein–protein binding interfaces. The infrared absorption bands for the probe groups reported many dramatic changes in the probes’ local environments as CaM went from apo- to calcium-saturated to target peptide-bound conditions, including large frequency shifts and a variety of line shapes from narrow (interpreted as a rigid and invariant local environment) to symmetric to broad and asymmetric (likely from multiple coexisting and dynamically exchanging structures). The fast intrinsic time scale of infrared spectroscopy means that the line shapes report directly on site-specific details of calmodulin’s variable structural distribution. Though quantitative interpretation of the probe line shapes depends on a direct connection between simulated ensembles and experimental data that does not yet exist, formation of such a connection to data such as that reported here would provide a new way to evaluate conformational ensembles from data that directly contains the structural distribution. The calmodulin probe sites developed here will also be useful in evaluating the binding mode of calmodulin with many uncharacterized regulatory targets.