Targeting the PDZ domains of molecular scaffolds of transmembrane ion channels

Fecal microbiota transplantation alleviated Alzheimer's disease-like pathogenesis in APP/PS1 transgenic mice

Alzheimer's disease (AD) is the most common dementia in the elderly. Treatment for AD is still a difficult task in clinic. AD is associated with abnormal gut microbiota. However, little is known about the role of fecal microbiota transplantation (FMT) in AD. Here, we evaluated the efficacy of FMT for the treatment of AD. We used an APPswe/PS1dE9 transgenic (Tg) mouse model. Cognitive deficits, brain deposits of amyloid-β (Aβ) and phosphorylation of tau, synaptic plasticity as well as neuroinflammation were assessed. Gut microbiota and its metabolites short-chain fatty acids (SCFAs) were analyzed by 16S rRNA sequencing and ¹H nuclear magnetic resonance (NMR). Our results showed that FMT treatment could improve cognitive deficits and reduce the brain deposition of amyloid-β (Aβ) in APPswe/PS1dE9 transgenic (Tg) mice. These improvements were accompanied by decreased phosphorylation of tau protein and the levels of Aβ40 and Aβ42. We observed an increases in synaptic plasticity in the Tg mice, showing that postsynaptic density protein 95 (PSD-95) and synapsin I expression were increased after FMT. We also observed the decrease of COX-2 and CD11b levels in Tg mice after FMT. We also found that FMT treatment reversed the changes of gut microbiota and SCFAs. Thus, FMT may be a potential therapeutic strategy for AD.

Fructooligosaccharides Ameliorating Cognitive Deficits and Neurodegeneration in APP/PS1 Transgenic Mice through Modulating Gut Microbiota

Alzheimer's disease (AD) is closely related to gut microbial alteration. Prebiotic fructooligosaccharides (FOS) play major roles by regulating gut microbiota. The present study aimed to explore the effect and mechanism of FOS protection against AD via regulating gut microbiota. Male Apse/PSEN 1dE9 (APP/PS1) transgenic (Tg) mice were administrated with FOS for 6 weeks. Cognitive deficits and amyloid deposition were evaluated. The levels of synaptic plasticity markers including postsynaptic density protein 95 (PSD-95) and synapsin I, as well as phosphorylation of c-Jun N-terminal kinase (JNK), were determined. The intestinal microbial constituent was detected by 16S rRNA sequencing. Moreover, the levels of glucagon-like peptide-1 (GLP-1) in the gut and GLP-1 receptor (GLP-1R) in the brain were measured. The results indicated that FOS treatment ameliorated cognitive deficits and pathological changes in the Tg mice. FOS significantly upregulated the expression levels of synapsin I and PSD-95, as well as decreased phosphorylated level of JNK. The sequencing results showed that FOS reversed the altered microbial composition. Furthermore, FOS increased the level of GLP-1 and decreased the level of GLP-1R in the Tg mice. These findings indicated that FOS exerted beneficial effects against AD via regulating the gut microbiota-GLP-1/GLP-1R pathway.

Molecular determinants of beta-adrenergic signaling to voltage-gated K+ channels in the cerebral circulation

Voltage-gated K⁺ (K_v ) channels are major determinants of membrane potential in vascular smooth muscle cells (VSMCs) and regulate the diameter of small cerebral arteries and arterioles. However, the intracellular structures that govern the expression and function of vascular K_v channels are poorly understood. Scaffolding proteins including postsynaptic density 95 (PSD95) recently were identified in rat cerebral VSMCs. Primarily characterized in neurons, the PSD95 scaffold has more than 50 known binding partners, and it can mediate macromolecular signaling between cell-surface receptors and ion channels. In cerebral arteries, Shaker-type K_v 1 channels appear to associate with the PSD95 molecular scaffold, and PSD95 is required for the normal expression and vasodilator influence of members of this K⁺ channel gene family. Furthermore, recent findings suggest that the β1-subtype adrenergic receptor is expressed in cerebral VSMCs and forms a functional vasodilator complex with K_v 1 channels on the PSD95 scaffold. Activation of β1-subtype adrenergic receptors in VSMCs enables protein kinase A-dependent phosphorylation and opening of K_v 1 channels in the PSD95 complex; the subsequent K⁺ efflux mediates membrane hyperpolarization and vasodilation of small cerebral arteries. Early evidence from other studies suggests that other families of K_v channels and scaffolding proteins are expressed in VSMCs. Future investigations into these macromolecular complexes that modulate the expression and function of K_v channels may reveal unknown signaling cascades that regulate VSMC excitability and provide novel targets for ion channel-based medications to optimize vascular tone.

Role of Severe Acute Respiratory Syndrome Coronavirus Viroporins E, 3a, and 8a in Replication and Pathogenesis

Viroporins are viral proteins with ion channel (IC) activity that play an important role in several processes, including virus replication and pathogenesis. While many coronaviruses (CoVs) encode two viroporins, severe acute respiratory syndrome CoV (SARS-CoV) encodes three: proteins 3a, E, and 8a. Additionally, proteins 3a and E have a PDZ-binding motif (PBM), which can potentially bind over 400 cellular proteins which contain a PDZ domain, making them potentially important for the control of cell function. In the present work, a comparative study of the functional motifs included within the SARS-CoV viroporins was performed, mostly focusing on the roles of the IC and PBM of E and 3a proteins. Our results showed that the full-length E and 3a proteins were required for maximal SARS-CoV replication and virulence, whereas viroporin 8a had only a minor impact on these activities. A virus missing both the E and 3a proteins was not viable, whereas the presence of either protein with a functional PBM restored virus viability. E protein IC activity and the presence of its PBM were necessary for virulence in mice. In contrast, the presence or absence of the homologous motifs in protein 3a did not influence virus pathogenicity. Therefore, dominance of the IC and PBM of protein E over those of protein 3a was demonstrated in the induction of pathogenesis in mice.IMPORTANCE Collectively, these results demonstrate key roles for the ion channel and PBM domains in optimal virus replication and pathogenesis and suggest that the viral viroporins and PBMs are suitable targets for antiviral therapy and for mutation in attenuated SARS-CoV vaccines.

Synaptic vesicle tethering and the CaV2.2 distal C-terminal

Evidence that synaptic vesicles (SVs) can be gated by a single voltage sensitive calcium channel (CaV2.2) predict a molecular linking mechanism or "tether" (Stanley, 1993). Recent studies have proposed that the SV binds to the distal C-terminal on the CaV2.2 calcium channel (Kaeser et al., 2011; Wong et al., 2013) while genetic analysis proposed a double tether mechanism via RIM: directly to the C terminus PDZ ligand domain or indirectly via a more proximal proline rich site (Kaeser et al., 2011). Using a novel in vitro SV pull down binding assay, we reported that SVs bind to a fusion protein comprising the C-terminal distal third (C3, aa 2137-2357; Wong et al., 2013). Here we limit the binding site further to the last 58 aa, beyond the proline rich site, by the absence of SV capture by a truncated C3 fusion protein (aa 2137-2299). To test PDZ-dependent binding we generated two C terminus-mutant C3 fusion proteins and a mimetic blocking peptide (H-WC, aa 2349-2357) and validated these by elimination of MINT-1 or RIM binding. Persistence of SV capture with all three fusion proteins or with the full length C3 protein but in the presence of blocking peptide, demonstrated that SVs can bind to the distal C-terminal via a PDZ-independent mechanism. These results were supported in situ by normal SV turnover in H-WC-loaded synaptosomes, as assayed by a novel peptide cryoloading method. Thus, SVs tether to the CaV2.2 C-terminal within a 49 aa region immediately prior to the terminus PDZ ligand domain. Long tethers that could reflect extended C termini were imaged by electron microscopy of synaptosome ghosts. To fully account for SV tethering we propose a model where SVs are initially captured, or "grabbed," from the cytoplasm by a binding site on the distal region of the channel C-terminal and are then retracted to be "locked" close to the channel by a second attachment mechanism in preparation for single channel domain gating.

Mechanism underlying selective regulation of G protein-gated inwardly rectifying potassium channels by the psychostimulant-sensitive sorting nexin 27

G protein-gated inwardly rectifying potassium (GIRK) channels are important gatekeepers of neuronal excitability. The surface expression of neuronal GIRK channels is regulated by the psychostimulant-sensitive sorting nexin 27 (SNX27) protein through a class I (-X-Ser/Thr-X-Φ, where X is any residue and Φ is a hydrophobic amino acid) PDZ-binding interaction. The G protein-insensitive inward rectifier channel (IRK1) contains the same class I PDZ-binding motif but associates with a different synaptic PDZ protein, postsynaptic density protein 95 (PSD95). The mechanism by which SNX27 and PSD95 discriminate these channels was previously unclear. Using high-resolution structures coupled with biochemical and functional analyses, we identified key amino acids upstream of the channel's canonical PDZ-binding motif that associate electrostatically with a unique structural pocket in the SNX27-PDZ domain. Changing specific charged residues in the channel's carboxyl terminus or in the PDZ domain converts the selective association and functional regulation by SNX27. Elucidation of this unique interaction site between ion channels and PDZ-containing proteins could provide a therapeutic target for treating brain diseases.

PDZ domains and their binding partners: structure, specificity, and modification

PDZ domains are abundant protein interaction modules that often recognize short amino acid motifs at the C-termini of target proteins. They regulate multiple biological processes such as transport, ion channel signaling, and other signal transduction systems. This review discusses the structural characterization of PDZ domains and the use of recently emerging technologies such as proteomic arrays and peptide libraries to study the binding properties of PDZ-mediated interactions. Regulatory mechanisms responsible for PDZ-mediated interactions, such as phosphorylation in the PDZ ligands or PDZ domains, are also discussed. A better understanding of PDZ protein-protein interaction networks and regulatory mechanisms will improve our knowledge of many cellular and biological processes.

A C-terminal class I PDZ binding motif of EspI/NleA modulates the virulence of attaching and effacing Escherichia coli and Citrobacter rodentium

Enteropathogenic Escherichia coli induces characteristic attaching-effacing (A/E) lesions on the intestinal mucosa during infection. The locus of enterocyte effacement is essential for A/E lesion formation and encodes a type III secretion system that translocates multiple effector proteins into the host cell. Following translocation, EspI/NleA localizes to the Golgi. Using the yeast two-hybrid system (Y2HS) and PSD-95/Disk-large/ZO-1 (PDZ)-domain protein array overlays, we identified 15 putative host-interacting partners of EspI. All but two of the target proteins contained PDZ domains. Examination of the EspI amino acid sequence revealed a C-terminal consensus class I PDZ binding motif. Deletion of the last 7 amino acids of EspI to generate EspI(DeltaC7) abrogated the Y2HS interaction between EspI and 5 of the 6 putative host cell target proteins tested. Deletion of the EspI PDZ binding motif also resulted in delayed trafficking of EspI to the Golgi. Using a mouse model of infection, we showed that Citrobacter rodentium expressing truncated EspI(DeltaC7) was attenuated when in competition with C. rodentium expressing full-length EspI. Overall, these results suggested that EspI may modulate the virulence of A/E pathogens by binding host PDZ-domain proteins.