Specific interaction of the PDZ domain protein PICK1 with the COOH terminus of protein kinase C-alpha

Exploring the role of AMPA receptor auxiliary proteins in synaptic functions and diseases

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) ionotropic glutamate receptors (AMPARs) mediate rapid excitatory synaptic transmission in the mammalian brain, primarily driven by the neurotransmitter glutamate. The modulation of AMPAR activity, particularly calcium-permeable AMPARs (CP-AMPARs), is crucially influenced by various auxiliary subunits. These subunits are integral membrane proteins that bind to the receptor's core and modify its functional properties, including ion channel kinetics and receptor trafficking. This review comprehensively catalogs all known AMPAR auxiliary proteins, providing vital insights into the biochemical mechanisms governing synaptic modulation and the specific impact of CP-AMPARs compared to their calcium-impermeable AMPA receptor (CI-AMPARs). Understanding the complex interplay between AMPARs and their auxiliary subunits in different brain regions is essential for elucidating their roles in cognitive functions such as learning and memory. Importantly, alterations in these auxiliary proteins' expression, function or interactions have been implicated in various neurological disorders. Aberrant signaling through CP-AMPARs, in particular, is associated with severe synaptic dysfunctions across neurodevelopmental, neurodegenerative and psychiatric conditions. Targeting the distinct properties of AMPAR-auxiliary subunit complexes, especially those involving CP-AMPARs, could disclose new therapeutic strategies, potentially allowing for more precise interventions in treating complex neuronal disorders.

PICK1 links KIBRA and AMPA receptor subunit GluA2 in coiled-coil-driven supramolecular complexes

The human memory-associated protein KIBRA regulates synaptic plasticity and trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors, and is implicated in multiple neuropsychiatric and cognitive disorders. How KIBRA forms complexes with and regulates AMPA receptors remains unclear. Here, we show that KIBRA does not interact directly with the AMPA receptor subunit GluA2, but that protein interacting with C kinase 1 (PICK1), a key regulator of AMPA receptor trafficking, can serve as a bridge between KIBRA and GluA2. In contrast, KIBRA can form a complex with GluA1 independent of PICK1. We identified structural determinants of KIBRA-PICK1-AMPAR complexes by investigating interactions and cellular expression patterns of different combinations of KIBRA and PICK1 domain mutants. We find that the PICK1 BAR domain, a coiled-coil structure, is sufficient for interaction with KIBRA, whereas mutation of the PICK1 BAR domain disrupts KIBRA-PICK1-GluA2 complex formation. In addition, KIBRA recruits PICK1 into large supramolecular complexes, a process which requires KIBRA coiled-coil domains. These findings reveal molecular mechanisms by which KIBRA can organize key synaptic signaling complexes.

Protein interacting with C-kinase 1 (PICK1) regulates synaptic function and reversal learning in a mouse model for schizophrenia

Protein interacting with C-kinase 1 (PICK1) is important for synaptic plasticity through directing transport of glutamate receptors and other proteins. PICK1 gene variants have been associated with schizophrenia. To examine the role of PICK1 in schizophrenia-related behaviors, mice with a mutation in the PICK1 lipid-interacting BAR domain were characterized. Male Pick1-S262T mice had disrupted AMPA receptor (AMPAR) subunit GluA1 and GluA2 protein expression in the hippocampus and prefrontal cortex (PFC). Young adult, but not juvenile (P21), Pick1-S262T mice showed decreased hippocampal synaptic transmission and deficits in long-term depression (LTD). Mutant males also had deficits in reversal learning in the Morris water maze (MWM). These observations suggest that the Pick1-S262T mutation affects AMPAR trafficking, disrupting synaptic transmission and plasticity, as well as cognitive flexibility, a core neuropsychological deficit in schizophrenia. This work suggests possible mechanisms by which a known schizophrenia susceptibility gene could contribute to clinical features of the disorder.

Postsynaptic Density Proteins and Their Role in the Trafficking of Group I Metabotropic Glutamate Receptors

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system that regulates multiple different forms of synaptic plasticity, including learning and memory. Glutamate transduces its signal by activating ionotropic glutamate receptors and metabotropic glutamate receptors (mGluRs). Group I mGluRs belong to the G protein-coupled receptor (GPCR) family. Regulation of cell surface expression and trafficking of the glutamate receptors represents an important mechanism that assures proper transmission of information at the synapses. There is growing evidence implicating dysregulated glutamate receptor trafficking in the pathophysiology of several neuropsychiatric disorders. The postsynaptic density (PSD) region consists of many specialized proteins which are assembled beneath the postsynaptic membrane of dendritic spines. Many of these proteins interact with group I mGluRs and have essential roles in group I mGluR-mediated synaptic function and plasticity. This review provides up-to-date information on the molecular determinants regulating cell surface expression and trafficking of group I mGluRs and discusses the role of few of these PSD proteins in these processes. As substantial evidences link mGluR dysfunction and maladaptive functioning of many PSD proteins to the pathophysiology of various neuropsychiatric disorders, understanding the role of the PSD proteins in group I mGluR trafficking may provide opportunities for the development of novel therapeutics in multiple neuropsychiatric disorders.

PICK1 links KIBRA and AMPA receptors in coiled-coil-driven supramolecular complexes

The human memory-associated protein KIBRA regulates synaptic plasticity and trafficking of AMPA-type glutamate receptors, and is implicated in multiple neuropsychiatric and cognitive disorders. How KIBRA forms complexes with and regulates AMPA receptors remains unclear. Here, we show that KIBRA does not interact directly with the AMPA receptor subunit GluA2, but that PICK1, a key regulator of AMPA receptor trafficking, can serve as a bridge between KIBRA and GluA2. We identified structural determinants of KIBRA-PICK1-AMPAR complexes by investigating interactions and cellular expression patterns of different combinations of KIBRA and PICK1 domain mutants. We find that the PICK1 BAR domain, a coiled-coil structure, is sufficient for interaction with KIBRA, whereas mutation of the BAR domain disrupts KIBRA-PICK1-GluA2 complex formation. In addition, KIBRA recruits PICK1 into large supramolecular complexes, a process which requires KIBRA coiled-coil domains. These findings reveal molecular mechanisms by which KIBRA can organize key synaptic signaling complexes.

A vital role for PICK1 in the differential regulation of metabotropic glutamate receptor internalization and synaptic AMPA receptor endocytosis

Group I metabotropic glutamate receptors (mGluRs) play important roles in many neuronal processes and are believed to be involved in synaptic plasticity underlying the encoding of experience, including classic paradigms of learning and memory. These receptors have also been implicated in various neurodevelopmental disorders, such as Fragile X syndrome and autism. Internalization and recycling of these receptors in the neuron are important mechanisms to regulate the activity of the receptor and control the precise spatio-temporal localization of these receptors. Applying a "molecular replacement" approach in hippocampal neurons derived from mice, we demonstrate a critical role for protein interacting with C kinase 1 (PICK1) in regulating the agonist-induced internalization of mGluR1. We show that PICK1 specifically regulates the internalization of mGluR1 but it does not play any role in the internalization of the other member of group I mGluR family, mGluR5. Various regions of PICK1 viz., the N-terminal acidic motif, PDZ domain and BAR domain play important roles in the agonist-mediated internalization of mGluR1. Finally, we demonstrate that PICK1-mediated internalization of mGluR1 is critical for the resensitization of the receptor. Upon knockdown of endogenous PICK1, mGluR1s stayed on the cell membrane as inactive receptors, incapable of triggering the MAP-kinase signaling. They also could not induce AMPAR endocytosis, a cellular correlate for mGluR-dependent synaptic plasticity. Thus, this study unravels a novel role for PICK1 in the agonist-mediated internalization of mGluR1 and mGluR1-mediated AMPAR endocytosis that might contribute to the function of mGluR1 in neuropsychiatric disorders.

Hypoxia-induced degradation of PICK1 by RBCK1 promotes the proliferation of nasopharyngeal carcinoma cells

AIMS

Hypoxia is an important feature of nasopharyngeal carcinoma (NPC). "Protein interacting with PRKCA 1" (PICK1) is commonly downregulated in human malignancies and is functionally related to poor prognosis. However, there is a limited understanding of the upstream mechanisms regulating PICK1 currently.

MAIN METHODS

PICK1 and HIF-1α expression levels were analyzed by Immunohistochemistry (IHC), western blotting, and quantitative real-time PCR assay. Protein stability and ubiquitin assays were used to investigate PICK1 protein degradation. Immunofluorescence and co-immunoprecipitation assays were used to demonstrate the interaction between RBCK1 and PICK1. Gene knockdown by siRNA transfection was used to investigate the role of HIF-1α and RBCK1 in hypoxia-induced PICK1 degradation. Cell Counting Kit-8 (CCK-8), 5-Ethynyl-2'-deoxyuridine (EdU) assays and subcutaneous xenograft nude models were used to explore the roles of RBCK1 and PICK1 in NPC cell proliferation.

KEY FINDINGS

PICK1 expression in NPC tissue was negatively relative to that of HIF-1α. HIF-1α downregulated PICK1 expression by facilitating its ubiquitination by the E3 ligases RANBP2-type and C3HC4-type zinc finger containing 1 (RBCK1), thereby enhancing proteasome-mediated PICK1 degradation. RBCK1 knockdown inhibited NPC cell proliferation, which was ameliorated by double knockdown of RBCK1/PICK1.

SIGNIFICANCE

These data provide evidence for an NPC cell adaptation mechanism to hypoxia, where HIF-1α regulates RBCK1, which targets PICK1 for degradation to promote cell proliferation.

Enhanced activity of Alzheimer disease-associated variant of protein kinase Cα drives cognitive decline in a mouse model

Exquisitely tuned activity of protein kinase C (PKC) isozymes is essential to maintaining cellular homeostasis. Whereas loss-of-function mutations are generally associated with cancer, gain-of-function variants in one isozyme, PKCα, are associated with Alzheimer's disease (AD). Here we show that the enhanced activity of one variant, PKCα M489V, is sufficient to rewire the brain phosphoproteome, drive synaptic degeneration, and impair cognition in a mouse model. This variant causes a modest 30% increase in catalytic activity without altering on/off activation dynamics or stability, underscoring that enhanced catalytic activity is sufficient to drive the biochemical, cellular, and ultimately cognitive effects observed. Analysis of hippocampal neurons from PKCα M489V mice reveals enhanced amyloid-β-induced synaptic depression and reduced spine density compared to wild-type mice. Behavioral studies reveal that this mutation alone is sufficient to impair cognition, and, when coupled to a mouse model of AD, further accelerates cognitive decline. The druggability of protein kinases positions PKCα as a promising therapeutic target in AD.

The Scaffold Protein PICK1 as a Target in Chronic Pain

Well-tolerated and effective drugs for treating chronic pain conditions are urgently needed. Most chronic pain patients are not effectively relieved from their pain and suffer from debilitating drug side effects. This has not only drastic negative consequences for the patients' quality of life, but also constitute an enormous burden on society. It is therefore of great interest to explore new potent targets for effective pain treatment with fewer side effects and without addiction liability. A critical component of chronic pain conditions is central sensitization, which involves the reorganization and strengthening of synaptic transmission within nociceptive pathways. Such changes are considered as maladaptive and depend on changes in the surface expression and signaling of AMPA-type glutamate receptors (AMPARs). The PDZ-domain scaffold protein PICK1 binds the AMPARs and has been suggested to play a key role in these maladaptive changes. In the present paper, we review the regulation of AMPARs by PICK1 and its relation to pain pathology. Moreover, we highlight other pain-relevant PICK1 interactions, and we evaluate various compounds that target PICK1 and have been successfully tested in pain models. Finally, we evaluate the potential on-target side effects of interfering with the action of PICK1 action in CNS and beyond. We conclude that PICK1 constitutes a valid drug target for the treatment of inflammatory and neuropathic pain conditions without the side effects and abuse liability associated with current pain medication.

Protein kinase Cγ in cerebellar Purkinje cells regulates Ca2+-activated large-conductance K+ channels and motor coordination

The cerebellum, the site where protein kinase C (PKC) was discovered, contains the highest amount of PKCγ in the central nervous system. PKCγ in the cerebellum is exclusively confined to Purkinje cells (PCs), sole outputs from the cerebellar cortex. Systemic PKCγ-knockout mice show impaired motor coordination; however, the cause of motor defects remains unknown. Here we show that activation of PKCγ suppresses the Ca²⁺-activated large-conductance K⁺ (BK) channels located along the PC dendrites. A consequential increase in the membrane resistance attenuates electrical signal decay during propagation, resulting in an altered complex spike waveform. Our results suggest that synaptically activated PKCγ in PCs plays a critical role in motor coordination by negative modulation of BK currents.

The cerebellum, the site where protein kinase C (PKC) was first discovered, contains the highest amount of PKC in the central nervous system, with PKCγ being the major isoform. Systemic PKCγ-knockout (KO) mice showed impaired motor coordination and deficient pruning of surplus climbing fibers (CFs) from developing cerebellar Purkinje cells (PCs). However, the physiological significance of PKCγ in the mature cerebellum and the cause of motor incoordination remain unknown. Using adeno-associated virus vectors targeting PCs, we showed that impaired motor coordination was restored by re-expression of PKCγ in mature PKCγ-KO mouse PCs in a kinase activity–dependent manner, while normal motor coordination in mature Prkcg^fl/fl mice was impaired by the Cre-dependent removal of PKCγ from PCs. Notably, the rescue or removal of PKCγ from mature PKCγ-KO or Prkcg^fl/fl mice, respectively, did not affect the CF innervation profile of PCs, suggesting the presence of a mechanism distinct from multiple CF innervation of PCs for the motor defects in PKCγ-deficient mice. We found marked potentiation of Ca²⁺-activated large-conductance K⁺ (BK) channel currents in PKCγ-deficient mice, as compared to wild-type mice, which decreased the membrane resistance, resulting in attenuation of the electrical signal during the propagation and significant alterations of the complex spike waveform. These changes in PKCγ-deficient mice were restored by the rescue of PKCγ or pharmacological suppression of BK channels. Our results suggest that PKCγ is a critical regulator that negatively modulates BK currents in PCs, which significantly influences PC output from the cerebellar cortex and, eventually, motor coordination.

A Novel Peripheral Action of PICK1 Inhibition in Inflammatory Pain

Chronic pain is a major healthcare problem that impacts one in five adults across the globe. Current treatment is compromised by dose-limiting side effects including drowsiness, apathy, fatigue, loss of ability to function socially and professionally as well as a high abuse liability. Most of these side effects result from broad suppression of excitatory neurotransmission. Chronic pain states are associated with specific changes in the efficacy of synaptic transmission in the pain pathways leading to amplification of non-noxious stimuli and spontaneous pain. Consequently, a reversal of these specific changes may pave the way for the development of efficacious pain treatment with fewer side effects. We have recently described a high-affinity, bivalent peptide TAT-P4-(C5)2, enabling efficient targeting of the neuronal scaffold protein, PICK1, a key protein in mediating chronic pain sensitization. In the present study, we demonstrate that in an inflammatory pain model, the peptide does not only relieve mechanical allodynia by targeting PICK1 involved in central sensitization, but also by peripheral actions in the inflamed paw. Further, we assess the effects of the peptide on novelty-induced locomotor activity, abuse liability, and memory performance without identifying significant side effects.

Conventional protein kinase C in the brain: repurposing cancer drugs for neurodegenerative treatment?

Protein Kinase C (PKC) isozymes are tightly regulated kinases that transduce a myriad of signals from receptor-mediated hydrolysis of membrane phospholipids. They play an important role in brain physiology, and dysregulation of PKC activity is associated with neurodegeneration. Gain-of-function mutations in PKCα are associated with Alzheimer's disease (AD) and mutations in PKCγ cause spinocerebellar ataxia (SCA) type 14 (SCA14). This article presents an overview of the role of the conventional PKCα and PKCγ in neurodegeneration and proposes repurposing PKC inhibitors, which failed in clinical trials for cancer, for the treatment of neurodegenerative diseases.

Regulation of acid-sensing ion channels by protein binding partners

Acid-sensing ion channels (ASICs) are a family of proton-gated cation channels that contribute to a diverse array of functions including pain sensation, cell death during ischemia, and more broadly to neurotransmission in the central nervous system. There is an increasing interest in understanding the physiological regulatory mechanisms of this family of channels. ASICs have relatively short N- and C-termini, yet a number of proteins have been shown to interact with these domains both in vitro and in vivo. These proteins can impact ASIC gating, localization, cell-surface expression, and regulation. Like all ion channels, it is important to understand the cellular context under which ASICs function in neurons and other cells. Here we will review what is known about a number of these potentially important regulatory molecules.

PICK1 Deficiency Exacerbates Sepsis-Associated Acute Kidney Injury

We performed in vitro and in vivo experiments to explore the role of protein kinase C-binding protein 1 (PICK1), an intracellular transporter involved in oxidative stress-related neuronal diseases, in sepsis-related acute kidney injury (AKI). Firstly, PCR, western blotting, and immunohistochemistry were used to observe the expression of PICK1 after lipopolysaccharide- (LPS-) induced AKI. Secondly, by inhibiting PICK1 in vivo and silencing PICK1 in vitro, we further explored the effect of PICK1 on AKI. Finally, the relationship between PICK1 and oxidative stress and the related mechanisms were explored. We found that the expression of PICK1 was increased in LPS-induced AKI models both in vitro and in vivo. PICK1 silencing significantly aggravated LPS-induced apoptosis, accompanied by ROS production in renal tubular epithelial cells. FSC231, a PICK1-specific inhibitor, aggravated LPS-induced kidney injury. Besides, NAC (N-acetylcysteine), a potent ROS scavenger, significantly inhibited the PICK1-silencing-induced apoptosis. In conclusion, PICK1 might protect renal tubular epithelial cells from LPS-induced apoptosis by reducing excessive ROS, making PICK1 a promising preventive target in LPS-induced AKI.

mGluR1 signaling in cerebellar Purkinje cells: Subcellular organization and involvement in cerebellar function and disease

The cerebellum is essential for the control, coordination, and learning of movements, and for certain aspects of cognitive function. Purkinje cells are the sole output neurons in the cerebellar cortex and therefore play crucial roles in the diverse functions of the cerebellum. The type 1 metabotropic glutamate receptor (mGluR1) is prominently enriched in Purkinje cells and triggers downstream signaling pathways that are required for functional and structural plasticity, and for synaptic responses. To understand how mGluR1 contributes to cerebellar functions, it is important to consider not only the operational properties of this receptor, but also its spatial organization and the molecular interactions that enable its proper functioning. In this review, we highlight how mGluR1 and its related signaling molecules are organized into tightly coupled microdomains to fulfill physiological functions. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunction in ataxias of human patients and mouse models.

Multisite NHERF1 phosphorylation controls GRK6A regulation of hormone-sensitive phosphate transport

The type II sodium-dependent phosphate cotransporter (NPT2A) mediates renal phosphate uptake. The NPT2A is regulated by parathyroid hormone (PTH) and fibroblast growth factor 23, which requires Na⁺/H⁺ exchange regulatory factor-1 (NHERF1), a multidomain PDZ-containing phosphoprotein. Phosphocycling controls the association between NHERF1 and the NPT2A. Here, we characterize the critical involvement of G protein–coupled receptor kinase 6A (GRK6A) in mediating PTH-sensitive phosphate transport by targeted phosphorylation coupled with NHERF1 conformational rearrangement, which in turn allows phosphorylation at a secondary site. GRK6A, through its carboxy-terminal PDZ recognition motif, binds NHERF1 PDZ1 with greater affinity than PDZ2. However, the association between NHERF1 PDZ2 and GRK6A is necessary for PTH action. Ser¹⁶², a PKCα phosphorylation site in PDZ2, regulates the binding affinity between PDZ2 and GRK6A. Substitution of Ser¹⁶² with alanine (S¹⁶²A) blocks the PTH action but does not disrupt the interaction between NHERF1 and the NPT2A. Replacement of Ser¹⁶² with aspartic acid (S¹⁶²D) abrogates the interaction between NHERF1 and the NPT2A and concurrently PTH action. We used amber codon suppression to generate a phosphorylated Ser¹⁶²(pSer¹⁶²)-PDZ2 variant. K_D values determined by fluorescence anisotropy indicate that incorporation of pSer¹⁶² increased the binding affinity to the carboxy terminus of GRK6A 2-fold compared with WT PDZ2. Molecular dynamics simulations predict formation of an electrostatic network between pSer¹⁶² and Asp¹⁸³ of PDZ2 and Arg at position −1 of the GRK6A PDZ-binding motif. Our results suggest that PDZ2 plays a regulatory role in PTH-sensitive NPT2A-mediated phosphate transport and phosphorylation of Ser¹⁶² in PDZ2 modulates the interaction with GRK6A.

Residue-Level Contact Reveals Modular Domain Interactions of PICK1 Are Driven by Both Electrostatic and Hydrophobic Forces

PICK1 is a multi-domain scaffolding protein that is uniquely comprised of both a PDZ domain and a BAR domain. While previous experiments have shown that the PDZ domain and the linker positively regulate the BAR domain and the C-terminus negatively regulates the BAR domain, the details of internal regulation mechanisms are unknown. Molecular dynamics (MD) simulations have been proven to be a useful tool in revealing the intramolecular interactions at atomic-level resolution. PICK1 performs its biological functions in a dimeric form which is extremely computationally demanding to simulate with an all-atom force field. Here, we use coarse-grained MD simulations to expose the key residues and driving forces in the internal regulations of PICK1. While the PDZ and BAR domains do not form a stable complex, our simulations show the PDZ domain preferentially interacting with the concave surface of the BAR domain over other BAR domain regions. Furthermore, our simulations show that the short helix in the linker region can form interactions with the PDZ domain. Our results reveal that the surface of the βB-βC loop, βC strand, and αA-βD loop of the PDZ domain can form a group of hydrophobic interactions surrounding the linker helix. These interactions are driven by hydrophobic forces. In contrast, our simulations reveal a very dynamic C-terminus that most often resides on the convex surface of the BAR domain rather than the previously suspected concave surface. These interactions are driven by a combination of electrostatic and hydrophobic interactions.

Noncanonical Sequences Involving NHERF1 Interaction with NPT2A Govern Hormone-Regulated Phosphate Transport: Binding Outside the Box

Na+/H+ exchange factor-1 (NHERF1), a multidomain PDZ scaffolding phosphoprotein, is required for the type II sodium-dependent phosphate cotransporter (NPT2A)-mediated renal phosphate absorption. Both PDZ1 and PDZ2 domains are involved in NPT2A-dependent phosphate uptake. Though harboring identical core-binding motifs, PDZ1 and PDZ2 play entirely different roles in hormone-regulated phosphate transport. PDZ1 is required for the interaction with the C-terminal PDZ-binding sequence of NPT2A (-TRL). Remarkably, phosphocycling at Ser290 distant from PDZ1, the penultimate step for both parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF23) regulation, controls the association between NHERF1 and NPT2A. PDZ2 interacts with the C-terminal PDZ-recognition motif (-TRL) of G Protein-coupled Receptor Kinase 6A (GRK6A), and that promotes phosphorylation of Ser290. The compelling biological puzzle is how PDZ1 and PDZ2 with identical GYGF core-binding motifs specifically recognize distinct binding partners. Binding determinants distinct from the canonical PDZ-ligand interactions and located "outside the box" explain PDZ domain specificity. Phosphorylation of NHERF1 by diverse kinases and associated conformational changes in NHERF1 add more complexity to PDZ-binding diversity.

A high-affinity, bivalent PDZ domain inhibitor complexes PICK1 to alleviate neuropathic pain

Maladaptive plasticity involving increased expression of AMPA-type glutamate receptors is involved in several pathologies, including neuropathic pain, but direct inhibition of AMPARs is associated with side effects. As an alternative, we developed a cell-permeable, high-affinity (~2 nM) peptide inhibitor, Tat-P₄ -(C5)₂ , of the PDZ domain protein PICK1 to interfere with increased AMPAR expression. The affinity is obtained partly from the Tat peptide and partly from the bivalency of the PDZ motif, engaging PDZ domains from two separate PICK1 dimers to form a tetrameric complex. Bivalent Tat-P₄ -(C5)₂ disrupts PICK1 interaction with membrane proteins on supported cell membrane sheets and reduce the interaction of AMPARs with PICK1 and AMPA-receptor surface expression in vivo. Moreover, Tat-P₄ -(C5)₂ administration reduces spinal cord transmission and alleviates mechanical hyperalgesia in the spared nerve injury model of neuropathic pain. Taken together, our data reveal Tat-P₄ -(C5)₂ as a novel promising lead for neuropathic pain treatment and expand the therapeutic potential of bivalent inhibitors to non-tandem protein-protein interaction domains.

Rac1 is a downstream effector of PKCα in structural synaptic plasticity

Structural and functional plasticity of dendritic spines is the basis of animal learning. The rapid remodeling of actin cytoskeleton is associated with spine enlargement and shrinkage, which are essential for structural plasticity. The calcium-dependent protein kinase C isoform, PKCα, has been suggested to be critical for this actin-dependent plasticity. However, mechanisms linking PKCα and structural plasticity of spines are unknown. Here, we examine the spatiotemporal activation of actin regulators, including small GTPases Rac1, Cdc42 and Ras, in the presence or absence of PKCα during single-spine structural plasticity. Removal of PKCα expression in the postsynapse attenuated Rac1 activation during structural plasticity without affecting Ras or Cdc42 activity. Moreover, disruption of a PDZ binding domain within PKCα led to impaired Rac1 activation and deficits in structural spine remodeling. These results demonstrate that PKCα positively regulates the activation of Rac1 during structural plasticity.

The Host GTPase Arf1 and Its Effectors AP1 and PICK1 Stimulate Actin Polymerization and Exocytosis To Promote Entry of Listeria monocytogenes

Listeria monocytogenes is a foodborne bacterium that causes gastroenteritis, meningitis, or abortion. Listeria induces its internalization (entry) into some human cells through interaction of the bacterial surface protein InlB with its host receptor, the Met tyrosine kinase. InlB and Met promote entry through stimulation of localized actin polymerization and exocytosis. How actin cytoskeletal changes and exocytosis are controlled during entry is not well understood. Here, we demonstrate important roles for the host GTPase Arf1 and its effectors AP1 and PICK1 in actin polymerization and exocytosis during InlB-dependent uptake. Depletion of Arf1 by RNA interference (RNAi) or inhibition of Arf1 activity using a dominant-negative allele impaired InlB-dependent internalization, indicating an important role for Arf1 in this process. InlB stimulated an increase in the GTP-bound form of Arf1, demonstrating that this bacterial protein activates Arf1. RNAi and immunolocalization studies indicated that Arf1 controls exocytosis and actin polymerization during entry by recruiting the effectors AP1 and PICK1 to the plasma membrane. In turn, AP1 and PICK1 promoted plasma membrane translocation of both Filamin A (FlnA) and Exo70, two host proteins previously found to mediate exocytosis during InlB-dependent internalization (M. Bhalla, H. Van Ngo, G. C. Gyanwali, and K. Ireton, Infect Immun 87:e00689-18, 2018, https://doi.org/10.1128/IAI.00689-18). PICK1 mediated recruitment of Exo70 but not FlnA. Collectively, these results indicate that Arf1, AP1, and PICK1 stimulate exocytosis by redistributing FlnA and Exo70 to the plasma membrane. We propose that Arf1, AP1, and PICK1 are key coordinators of actin polymerization and exocytosis during infection of host cells by Listeria.

Cellular and pathophysiological consequences of Arp2/3 complex inhibition: role of inhibitory proteins and pharmacological compounds

The actin-related protein complex 2/3 (Arp2/3) generates branched actin networks important for many cellular processes such as motility, vesicular trafficking, cytokinesis, and intercellular junction formation and stabilization. Activation of Arp2/3 requires interaction with actin nucleation-promoting factors (NPFs). Regulation of Arp2/3 activity is achieved by endogenous inhibitory proteins through direct binding to Arp2/3 and competition with NPFs or by binding to Arp2/3-induced actin filaments and disassembly of branched actin networks. Arp2/3 inhibition has recently garnered more attention as it has been associated with attenuation of cancer progression, neurotoxic effects during drug abuse, and pathogen invasion of host cells. In this review, we summarize current knowledge on expression, inhibitory mechanisms and function of endogenous proteins able to inhibit Arp2/3 such as coronins, GMFs, PICK1, gadkin, and arpin. Moreover, we discuss cellular consequences of pharmacological Arp2/3 inhibition.

Structural Basis of Protein Kinase Cα Regulation by the C-Terminal Tail

Protein kinase C (PKC) isoenzymes are multi-modular proteins activated at the membrane surface to regulate signal transduction processes. When activated by second messengers, PKC undergoes a drastic conformational and spatial transition from the inactive cytosolic state to the activated membrane-bound state. The complete structure of either state of PKC remains elusive. We demonstrate, using NMR spectroscopy, that the isolated Ca²⁺-sensing membrane-binding C2 domain of the conventional PKCα interacts with a conserved hydrophobic motif of the kinase C-terminal region, and we report a structural model of the complex. Our data suggest that the C-terminal region plays a dual role in regulating the PKC activity: activating, through sensitization of PKC to intracellular Ca²⁺ oscillations; and auto-inhibitory, through its interaction with a conserved positively charged region of the C2 domain.

Regulating transcriptional activity by phosphorylation: A new mechanism for the ARX homeodomain transcription factor

Aristaless-related homeobox (ARX) gene encodes a paired-type homeodomain transcription factor with critical roles in development. Here we identify that ARX protein is phosphorylated. Using mass spectrometry and in vitro kinase assays we identify phosphorylation at serines 37, 67 and 174. Through yeast-2-hybrid and CoIP we identified PICK1 (Protein interacting with C kinase 1) binding with the C-terminal region of ARX. PICK1 is a scaffold protein known to facilitate phosphorylation of protein partners by protein kinase C alpha (PRKCA). We confirm that ARX is phosphorylated by PRKCA and demonstrate phosphorylation at serine 174. We demonstrate that phosphorylation is required for correct transcriptional activity of the ARX protein using transcriptome-wide analysis of gene expression of phospho-null mutants (alanines replacing serines) compared to ARX wild-type (ARX-WT) overexpressed in pancreatic alpha TC cells. Compared to untransfected cells, ARX-WT overexpression significantly altered expression of 70 genes (Log2FC >+/-1.0, P-value <0.05). There were fewer genes with significantly altered expression compared to untransfected cells with the double phospho-null mutant Ser37Ala+Ser67Ala (26%) and Ser174Ala (39%), respectively. We demonstrate that the c-terminal region of ARX required to bind PICK1 causes a shift in PICK1 subcellular localisation to the nucleus to co-locate with the ARX protein, and truncation of this C-terminal region leads to the same loss of transcriptional activation as S174A mutant. In conclusion, we show that ARX is phosphorylated at several sites and that this modification affects its transcriptional activity.

An Aplysia-like synaptic switch for rapid protection against ethanol-induced synaptic inhibition in a mammalian habit circuit

Decades of work in Aplysia californica established the general rule that principles of synaptic plasticity and their molecular mechanisms are evolutionarily conserved from mollusks to mammals. However, an exquisitely sensitive, activity-dependent homosynaptic mechanism that protects against the depression of neurotransmitter release in Aplysia sensory neuron terminals has, to date, not been uncovered in other animals, including mammals. Here, we discover that depression at a mammalian synapse that is implicated in habit formation and habit learning acceleration by ethanol, the fast-spiking interneuron (FSI) to medium spiny principal projection neuron (MSN) synapse of the dorsolateral striatum, is subject to this type of synaptic protection. We show that this protection against synaptic depression is calcium- and PDZ domain interaction-dependent. These findings support activity dependent protection against synaptic depression as an Aplysia-like synaptic switch in mammals that may represent a leveraging point for treating alcohol use disorders.

PKCα integrates spatiotemporally distinct Ca2+ and autocrine BDNF signaling to facilitate synaptic plasticity

The Protein Kinase C (PKC) enzymes have long been established as critical for synaptic plasticity. However, it is unknown whether Ca²⁺-dependent PKC isozymes are activated in dendritic spines during plasticity, and if so, how this synaptic activity is encoded by PKC. Here, using newly-developed, isozyme-specific sensors, we demonstrate that classic isozymes are activated to varying degrees and with unique kinetics. PKCα is activated robustly and rapidly in stimulated spines and is the only isozyme required for structural plasticity. This specificity, depends on a PDZ-binding domain present only in PKCα. The activation of PKCα during plasticity requires both NMDAR Ca²⁺-flux and autocrine BDNF-TrkB signaling, two pathways that differ vastly in their spatiotemporal scales of signaling. Our results suggest that by integrating these signals, PKCα combines a measure of recent, nearby synaptic plasticity with local synaptic input, enabling complex cellular computations such as heterosynaptic facilitation of plasticity necessary for efficient hippocampal-dependent learning.

PICK1 inhibits the E3 ubiquitin ligase activity of Parkin and reduces its neuronal protective effect

Parkin functions as a multipurpose E3 ubiquitin ligase, and Parkin loss of function is associated with both sporadic and familial Parkinson's disease (PD). We report that the Bin/Amphiphysin/Rvs (BAR) domain of protein interacting with PRKCA1 (PICK1) bound to the really interesting new gene 1 (RING1) domain of Parkin and potently inhibited the E3 ligase activity of Parkin by disrupting its interaction with UbcH7. Parkin translocated to damaged mitochondria and led to their degradation in neurons, whereas PICK1 robustly inhibited this process. PICK1 also impaired the protective function of Parkin against stresses in SH-SY5Y cells and neurons. The protein levels of several Parkin substrates were reduced in young PICK1-knockout mice, and these mice were resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-mediated toxicity. Taken together, the results indicate that PICK1 is a potent inhibitor of Parkin, and the reduction of PICK1 enhances the protective effect of Parkin.

Protein kinase Cα gain-of-function variant in Alzheimer's disease displays enhanced catalysis by a mechanism that evades down-regulation

Conventional protein kinase C (PKC) family members are reversibly activated by binding to the second messengers Ca²⁺ and diacylglycerol, events that break autoinhibitory constraints to allow the enzyme to adopt an active, but degradation-sensitive, conformation. Perturbing these autoinhibitory constraints, resulting in protein destabilization, is one of many mechanisms by which PKC function is lost in cancer. Here, we address how a gain-of-function germline mutation in PKCα in Alzheimer's disease (AD) enhances signaling without increasing vulnerability to down-regulation. Biochemical analyses of purified protein demonstrate that this mutation results in an ∼30% increase in the catalytic rate of the activated enzyme, with no changes in the concentrations of Ca²⁺ or lipid required for half-maximal activation. Molecular dynamics simulations reveal that this mutation has both localized and allosteric effects, most notably decreasing the dynamics of the C-helix, a key determinant in the catalytic turnover of kinases. Consistent with this mutation not altering autoinhibitory constraints, live-cell imaging studies reveal that the basal signaling output of PKCα-M489V is unchanged. However, the mutant enzyme in cells displays increased sensitivity to an inhibitor that is ineffective toward scaffolded PKC, suggesting the altered dynamics of the kinase domain may influence protein interactions. Finally, we show that phosphorylation of a key PKC substrate, myristoylated alanine-rich C-kinase substrate, is increased in brains of CRISPR-Cas9 genome-edited mice containing the PKCα-M489V mutation. Our results unveil how an AD-associated mutation in PKCα permits enhanced agonist-dependent signaling via a mechanism that evades the cell's homeostatic down-regulation of constitutively active PKCα.

PICK1 deficiency exacerbates sepsis-associated acute lung injury and impairs glutathione synthesis via reduction of xCT

The role of oxidative stress has been well documented in the development of sepsis-induced acute lung injury (ALI). Protein interaction with C-kinase 1 (PICK1) participates in oxidative stress-related neuronal diseases. However, its function in lung infections and inflammatory diseases is not known. We therefore sought to investigate whether PICK1 is involved in sepsis-induced ALI. Cecal ligation and puncture (CLP) was performed in anesthetized wild type (WT) and PICK1 knock out (KO, PICK1^-/-) mice with C57BL/6 background. At the time of CLP, mice were given fluid resuscitation. Mouse lungs were harvested at 24 and 72 h for Western blot analysis, qRT-PCR, BALF analysis, Hematoxylin and Eosin staining, TUNEL staining, maleimide staining, flow cytometry analysis, GCL, GSH, GSSG and cysteine levels measurement. A marked elevation of PICK1 mRNA and protein level were demonstrated in lung tissue, which was accompanied by increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) and consumption of glutathione (GSH). N-acetylcysteine (NAC), buthionine sulfoximine (BSO) and GSH-monoethyl ester (GSH-MEE) were injected into mice via caudal vein to regulate glutathione (GSH) level in lung. Alterations of lung GSH content induced PICK1 level change after CLP challenge. In PICK1^-/- underwent with CLP, lung injury and survival were significantly aggravated compared with wild-type mice underwent with CLP. Concomitantly, CLP-induced lung cell apoptosis was exacerbated in PICK1^-/- mice. The level of xCT, other than PKCα, in lung tissue was significantly lowered in PICK1^-/- but not in wild type that underwent CLP surgery. Meanwhile, Nrf2 activation, which dominating xCT expression, was inhibited in PICK1-/- but not in wild type mice that underwent CLP surgery, as well. Moreover, higher level of PICK1 was detected in PBMCs of septic patients than healthy controls. Taken together, PICK1 plays a pivotal role in sepsis-induced ALI by regulating GSH synthesis via affecting the substrate-specific subunit of lung cystine/glutamate transporter, xCT.

Protein kinase C regulates AMPA receptor auxiliary protein Shisa9/CKAMP44 through interactions with neuronal scaffold PICK1

Synaptic α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole‐propionate (AMPA) receptors are essential mediators of neurotransmission in the central nervous system. Shisa9/cysteine‐knot AMPAR modulating protein 44 (CKAMP44) is a transmembrane protein recently found to be present in AMPA receptor‐associated protein complexes. Here, we show that the cytosolic tail of Shisa9/CKAMP44 interacts with multiple scaffold proteins that are important for regulating synaptic plasticity in central neurons. We focussed on the interaction with the scaffold protein PICK1, which facilitates the formation of a tripartite complex with the protein kinase C (PKC) and thereby regulates phosphorylation of Shisa9/CKAMP44 C‐terminal residues. This work has implications for our understanding of how PICK1 modulates AMPAR‐mediated transmission and plasticity and also highlights a novel function of PKC.

Conventional protein kinase C in the brain: 40 years later

Protein kinase C (PKC) is a family of enzymes whose members transduce a large variety of cellular signals instigated by the receptor-mediated hydrolysis of membrane phospholipids. While PKC has been widely implicated in the pathology of diseases affecting all areas of physiology including cancer, diabetes, and heart disease-it was discovered, and initially characterized, in the brain. PKC plays a key role in controlling the balance between cell survival and cell death. Its loss of function is generally associated with cancer, whereas its enhanced activity is associated with neurodegeneration. This review presents an overview of signaling by diacylglycerol (DG)-dependent PKC isozymes in the brain, and focuses on the role of the Ca2+-sensitive conventional PKC isozymes in neurodegeneration.

Gain-of-function mutations in protein kinase Cα (PKCα) may promote synaptic defects in Alzheimer's disease

Alzheimer's disease (AD) is a progressive dementia disorder characterized by synaptic degeneration and amyloid-β (Aβ) accumulation in the brain. Through whole-genome sequencing of 1345 individuals from 410 families with late-onset AD (LOAD), we identified three highly penetrant variants in PRKCA, the gene that encodes protein kinase Cα (PKCα), in five of the families. All three variants linked with LOAD displayed increased catalytic activity relative to wild-type PKCα as assessed in live-cell imaging experiments using a genetically encoded PKC activity reporter. Deleting PRKCA in mice or adding PKC antagonists to mouse hippocampal slices infected with a virus expressing the Aβ precursor CT100 revealed that PKCα was required for the reduced synaptic activity caused by Aβ. In PRKCA(-/-) neurons expressing CT100, introduction of PKCα, but not PKCα lacking a PDZ interaction moiety, rescued synaptic depression, suggesting that a scaffolding interaction bringing PKCα to the synapse is required for its mediation of the effects of Aβ. Thus, enhanced PKCα activity may contribute to AD, possibly by mediating the actions of Aβ on synapses. In contrast, reduced PKCα activity is implicated in cancer. Hence, these findings reinforce the importance of maintaining a careful balance in the activity of this enzyme.

Functional and Physical Interaction of Diacylglycerol Kinase ζ with Protein Kinase Cα Is Required for Cerebellar Long-Term Depression

The balance between positive and negative regulators required for synaptic plasticity must be well organized at synapses. Protein kinase Cα (PKCα) is a major mediator that triggers long-term depression (LTD) at synapses between parallel fibers and Purkinje cells in the cerebellum. However, the precise mechanisms involved in PKCα regulation are not clearly understood. Here, we analyzed the role of diacylglycerol kinase ζ (DGKζ), a kinase that physically interacts with PKCα as well as postsynaptic density protein 95 (PSD-95) family proteins and functionally suppresses PKCα by metabolizing diacylglycerol (DAG), in the regulation of cerebellar LTD. In Purkinje cells of DGKζ-deficient mice, LTD was impaired and PKCα was less localized in dendrites and synapses. This impaired LTD was rescued by virus-driven expression of wild-type DGKζ, but not by a kinase-dead mutant DGKζ or a mutant lacking the ability to localize at synapses, indicating that both the kinase activity and synaptic anchoring functions of DGKζ are necessary for LTD. In addition, experiments using another DGKζ mutant and immunoprecipitation analysis revealed an inverse regulatory mechanism, in which PKCα phosphorylates, inactivates, and then is released from DGKζ, is required for LTD. These results indicate that DGKζ is localized to synapses, through its interaction with PSD-95 family proteins, to promote synaptic localization of PKCα, but maintains PKCα in a minimally activated state by suppressing local DAG until its activation and release from DGKζ during LTD. Such local and reciprocal regulation of positive and negative regulators may contribute to the fine-tuning of synaptic signaling.

SIGNIFICANCE STATEMENT

Many studies have identified signaling molecules that mediate long-term synaptic plasticity. In the basal state, the activities and concentrations of these signaling molecules must be maintained at low levels, yet be ready to be boosted, so that synapses can undergo synaptic plasticity only when they are stimulated. However, the mechanisms involved in creating such conditions are not well understood. Here, we show that diacylglycerol kinase ζ (DGKζ) creates optimal conditions for the induction of cerebellar long-term depression (LTD). DGKζ works by regulating localization and activity of protein kinase Cα (PKCα), an important mediator of LTD, so that PKCα effectively responds to the stimulation that triggers LTD.

Actin Out: Regulation of the Synaptic Cytoskeleton

The small size of dendritic spines belies the elaborate role they play in excitatory synaptic transmission and ultimately complex behaviors. The cytoskeletal architecture of the spine is predominately composed of actin filaments. These filaments, which at first glance might appear simple, are also surprisingly complex. They dynamically assemble into different structures and serve as a platform for orchestrating the elaborate responses of the spine during spinogenesis and experience-dependent plasticity. Multiple mutations associated with human neurodevelopmental and psychiatric disorders involve genes that encode regulators of the synaptic cytoskeleton. A major, unresolved question is how the disruption of specific actin filament structures leads to the onset and progression of complex synaptic and behavioral phenotypes. This review will cover established and emerging mechanisms of actin cytoskeletal remodeling and how this influences specific aspects of spine biology that are implicated in disease.

PDZ Protein Regulation of G Protein-Coupled Receptor Trafficking and Signaling Pathways

G protein-coupled receptors (GPCRs) contribute to the regulation of every aspect of human physiology and are therapeutic targets for the treatment of numerous diseases. As a consequence, understanding the myriad of mechanisms controlling GPCR signaling and trafficking is essential for the development of new pharmacological strategies for the treatment of human pathologies. Of the many GPCR-interacting proteins, postsynaptic density protein of 95 kilodaltons, disc large, zona occludens-1 (PDZ) domain-containing proteins appear most abundant and have similarly been implicated in disease mechanisms. PDZ proteins play an important role in regulating receptor and channel protein localization within synapses and tight junctions and function to scaffold intracellular signaling protein complexes. In the current study, we review the known functional interactions between PDZ domain-containing proteins and GPCRs and provide insight into the potential mechanisms of action. These PDZ domain-containing proteins include the membrane-associated guanylate-like kinases [postsynaptic density protein of 95 kilodaltons; synapse-associated protein of 97 kilodaltons; postsynaptic density protein of 93 kilodaltons; synapse-associated protein of 102 kilodaltons; discs, large homolog 5; caspase activation and recruitment domain and membrane-associated guanylate-like kinase domain-containing protein 3; membrane protein, palmitoylated 3; calcium/calmodulin-dependent serine protein kinase; membrane-associated guanylate kinase protein (MAGI)-1, MAGI-2, and MAGI-3], Na(+)/H(+) exchanger regulatory factor proteins (NHERFs) (NHERF1, NHERF2, PDZ domain-containing kidney protein 1, and PDZ domain-containing kidney protein 2), Golgi-associated PDZ proteins (Gα-binding protein interacting protein, C-terminus and CFTR-associated ligand), PDZ domain-containing guanine nucleotide exchange factors (GEFs) 1 and 2, regulator of G protein signaling (RGS)-homology-RhoGEFs (PDZ domain-containing RhoGEF and leukemia-associated RhoGEF), RGS3 and RGS12, spinophilin and neurabin-1, SRC homology 3 domain and multiple ankyrin repeat domain (Shank) proteins (Shank1, Shank2, and Shank3), partitioning defective proteins 3 and 6, multiple PDZ protein 1, Tamalin, neuronal nitric oxide synthase, syntrophins, protein interacting with protein kinase C α 1, syntenin-1, and sorting nexin 27.

Dynamics and Membrane Interactions of Protein Kinase C

Protein kinase C (PKC) is a family of Ser/Thr kinases that regulate a multitude of cellular processes through participation in the phosphoinositide signaling pathway. Significant research efforts have been directed at understanding the structure, function, and regulatory modes of the enzyme since its discovery and identification as the first receptor for tumor-promoting phorbol esters. The activation of PKC involves a transition from the cytosolic autoinhibited latent form to the membrane-associated active form. The membrane recruitment step is accompanied by the conformational rearrangement of the enzyme, which relieves autoinhibitory interactions and thereby allows PKC to phosphorylate its targets. The multidomain structure and intrinsic flexibility of PKC present remarkable challenges and opportunities for the biophysical and structural biology studies of this class of enzymes and their interactions with membranes, the major focus of this Current Topic. I will highlight the recent advances in the field, outline the current challenges, and identify areas where biophysics and structural biology approaches can provide insight into the isoenzyme-specific regulation of PKC activity.

Structure of Dimeric and Tetrameric Complexes of the BAR Domain Protein PICK1 Determined by Small-Angle X-Ray Scattering

PICK1 is a neuronal scaffolding protein containing a PDZ domain and an auto-inhibited BAR domain. BAR domains are membrane-sculpting protein modules generating membrane curvature and promoting membrane fission. Previous data suggest that BAR domains are organized in lattice-like arrangements when stabilizing membranes but little is known about structural organization of BAR domains in solution. Through a small-angle X-ray scattering (SAXS) analysis, we determine the structure of dimeric and tetrameric complexes of PICK1 in solution. SAXS and biochemical data reveal a strong propensity of PICK1 to form higher-order structures, and SAXS analysis suggests an offset, parallel mode of BAR-BAR oligomerization. Furthermore, unlike accessory domains in other BAR domain proteins, the positioning of the PDZ domains is flexible, enabling PICK1 to perform long-range, dynamic scaffolding of membrane-associated proteins. Together with functional data, these structural findings are compatible with a model in which oligomerization governs auto-inhibition of BAR domain function.

A Place at the Table: LTD as a Mediator of Memory Genesis

Resolving how our brains encode information requires an understanding of the cellular processes taking place during memory formation. Since the 1970s, considerable effort has focused on determining the properties and mechanisms underlying long-term potentiation (LTP) at glutamatergic synapses and how these processes influence initiation of new memories. However, accumulating evidence suggests that long-term depression (LTD) of synaptic strength, particularly at glutamatergic synapses, is a bona fide learning and memory mechanism in the mammalian brain. The known range of mechanisms capable of inducing LTD has been extended to those including NMDAR-independent forms, neuromodulator-dependent LTD, synaptic depression following stress, and non-synaptically induced forms. The examples of LTD observed at the hippocampal CA1 synapse to date demonstrate features consistent with LTP, including homo- and heterosynaptic expression, extended duration beyond induction (several hours to weeks), and association with encoding of distinct types of memories. Canonical mechanisms through which synapses undergo LTD include activation of phosphatases, initiation of protein synthesis, and dynamic regulation of presynaptic glutamate release and/or postsynaptic glutamate receptor endocytosis. Here, we will discuss the pre- and postsynaptic changes underlying LTD, recent advances in the identification and characterization of novel mechanisms underlying LTD, and how engagement of these processes constitutes a cellular analog for the genesis of specific types of memories.

The intellectual disability protein RAB39B selectively regulates GluA2 trafficking to determine synaptic AMPAR composition

RAB39B is a member of the RAB family of small GTPases that controls intracellular vesicular trafficking in a compartment-specific manner. Mutations in the RAB39B gene cause intellectual disability comorbid with autism spectrum disorder and epilepsy, but the impact of RAB39B loss of function on synaptic activity is largely unexplained. Here we show that protein interacting with C-kinase 1 (PICK1) is a downstream effector of GTP-bound RAB39B and that RAB39B-PICK1 controls trafficking from the endoplasmic reticulum to the Golgi and, hence, surface expression of GluA2, a subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). The role of AMPARs in synaptic transmission varies depending on the combination of subunits (GluA1, GluA2 and GluA3) they incorporate. RAB39B downregulation in mouse hippocampal neurons skews AMPAR composition towards non GluA2-containing Ca²⁺-permeable forms and thereby alters synaptic activity, specifically in hippocampal neurons. We posit that the resulting alteration in synaptic function underlies cognitive dysfunction in RAB39B-related disorders.

Mutations in the RAB39B gene, which encodes a protein involved in vesicular trafficking, are associated with intellectual disability, but the impact of RAB39B loss of function on synaptic activity is not known. Here the authors show that RAB39B interacts with PICK1, and that this interaction is critical for the translocation of AMPA receptor subunits into the Golgi.

Trafficking of kainate receptors

Ionotropic glutamate receptors (iGluRs) mediate the vast majority of excitatory neurotransmission in the central nervous system of vertebrates. In the protein family of iGluRs, kainate receptors (KARs) comprise the probably least well understood receptor class. Although KARs act as key players in the regulation of synaptic network activity, many properties and functions of these proteins remain elusive until now. Especially the precise pre-, extra-, and postsynaptic localization of KARs plays a critical role for neuronal function, as an unbalanced localization of KARs would ultimately lead to dysregulated neuronal excitability. Recently, important advances in the understanding of the regulation of surface expression, function, and agonist-dependent endocytosis of KARs have been achieved. Post-translational modifications like PKC-mediated phosphorylation and SUMOylation have been reported to critically influence surface expression and endocytosis, while newly discovered auxiliary proteins were shown to shape the functional properties of KARs.

Protein interacting with C-kinase 1 (PICK1) binding promiscuity relies on unconventional PSD-95/discs-large/ZO-1 homology (PDZ) binding modes for nonclass II PDZ ligands

PDZ domain proteins control multiple cellular functions by governing assembly of protein complexes. It remains unknown why individual PDZ domains can bind the extreme C terminus of very diverse binding partners and maintain selectivity. By employing NMR spectroscopy, together with molecular modeling, mutational analysis, and fluorescent polarization binding experiments, we identify here three structural mechanisms explaining why the PDZ domain of PICK1 selectively binds >30 receptors, transporters, and kinases. Class II ligands, including the dopamine transporter, adopt a canonical binding mode with promiscuity obtained via differential packing in the binding groove. Class I ligands, such as protein kinase Cα, depend on residues upstream from the canonical binding sequence that are likely to interact with flexible loop residues of the PDZ domain. Finally, we obtain evidence that the unconventional ligand ASIC1a has a dual binding mode involving a canonical insertion and a noncanonical internal insertion with the two C-terminal residues forming interactions outside the groove. Together with an evolutionary analysis, the data show how unconventional binding modes might evolve for a protein recognition domain to expand the repertoire of functionally important interactions.

Pick1 modulates ephrinB1-induced junctional disassembly through an association with ephrinB1

Members of the Eph family have been implicated in the formation of cell-cell boundaries, cell movement, and positioning during development in the context of cancer progression. De-regulation of this signaling system is linked to the promotion of more aggressive and metastatic tumor phenotypes in a large variety of human cancers, including breast, lung, and prostate cancer, melanoma, and leukemia. Thus, it is interesting to consider the case of cancer progression where de-regulation of the Eph/ephrin signaling system results in invasion and metastasis. Here, we present evidence that Pick1, one of the essential components of the adherens junction, recovers ephrinB1-induced cell-cell de-adhesion. Loss of Pick1 leads to dissociation of epithelial cells via disruption of the adherens junction, a phenotype similar to ephrinB1 overexpression. In addition, overexpressed ephrinB1-induced disruption of the adherens junction is rescued via binding to Pick1. These data indicate that Pick1 is involved in regulating the cell-cell junction in epithelial cells, and this may influence therapeutic strategy decisions with regards to cell adhesion molecules in metastatic disease.

The protein interacting with C-kinase (PICK1) interacts with and attenuates parkin-associated endothelial-like (PAEL) receptor-mediated cell death

The parkin-associated endothelial-like receptor (PAELR, GPR37) is an orphan G protein-coupled receptor that interacts with and is degraded by parkin-mediated ubiquitination. Mutations in parkin are thought to result in PAELR accumulation and increase neuronal cell death in Parkinson's disease. In this study, we find that the protein interacting with C-kinase (PICK1) interacts with PAELR. Specifically, the Postsynaptic density protein-95/Discs large/ZO-1 (PDZ) domain of PICK1 interacted with the last three residues of the c-terminal (ct) located PDZ motif of PAELR. Pull-down assays indicated that recombinant and native PICK1, obtained from heterologous cells and rat brain tissue, respectively, were retained by a glutathione S-transferase fusion of ct-PAELR. Furthermore, coimmunoprecipitation studies isolated a PAELR-PICK1 complex from transiently transfected cells. PICK1 interacts with parkin and our data showed that PICK1 reduces PAELR expression levels in transiently transfected heterologous cells compared to a PICK1 mutant that does not interact with PAELR. Finally, PICK1 over-expression in HEK293 cells reduced cell death induced by PAEALR over-expression during rotenone treatment and these effects of PICK1 were attenuated during inhibition of the proteasome. These results suggest a role for PICK1 in preventing PAELR-induced cell toxicity.

Changes in the GRIP 1&2 scaffolding proteins in the cerebellum of the ataxic stargazer mouse

Glutamate receptor-interacting proteins (GRIP1&2) and protein-interacting with C kinase-1 (PICK1) are synaptic scaffold proteins associated with the stabilization and recycling of synaptic GluA2-, 3- and 4c-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). PICK1-mediated phosphorylation of GluA serine880 uncouples GRIP1&2 leading to AMPAR endocytosis, important in mediating forms of synaptic plasticity underlying learning and memory. Ataxic and epileptic stargazer mice possess a mutation in the CACNG2 gene encoding the transmembrane AMPAR-regulatory protein (TARP)-γ2 (stargazin). TARPs are AMPAR-auxiliary subunits required for efficient AMPAR trafficking to synapses. Stargazin is abundantly expressed in the cerebellum and its loss results in severe deficits in AMPAR trafficking to cerebellar synapses, particularly at granule cell (GC) synapses, leading to the ataxic phenotype of stargazers. However, how the stargazin mutation impacts on the expression of other AMPAR-interacting scaffold proteins is unknown. This study shows a significant increase in GRIP1&2, but not PICK1, levels in whole tissue and synapse-enriched extracts from stargazer cerebella. Post-embedding immunogold-cytochemistry electron microscopy showed GRIP1&2 levels were unchanged at mossy fiber-GC synapses in stargazers, which are silent due to virtual total absence of synaptic and extrasynaptic GluA2/3-AMPARs. These results indicate that loss of synaptic AMPARs at this excitatory synapse does not affect GRIP1&2 expression within the postsynaptic region of mossy fiber-GC synapses. Interestingly, increased GRIP and reduced GluA2-AMPARexpression also occur in cerebella of autistic patients. Further research establishing the role of elevated cerebellar GRIP1&2 in stargazers may help identify common cellular mechanisms in the comorbid disorders ataxia, epilepsy and autism leading to more effective treatment strategies.

AMPA receptor pHluorin-GluA2 reports NMDA receptor-induced intracellular acidification in hippocampal neurons

NMDA receptor activation promotes endocytosis of AMPA receptors, which is an important mechanism underlying long-term synaptic depression. The pH-sensitive GFP variant pHluorin fused to the N terminus of GluA2 (pH-GluA2) has been used to assay NMDA-mediated AMPA receptor endocytosis and recycling. Here, we demonstrate that in somatic and dendritic regions of hippocampal neurons a large fraction of the fluorescent signal originates from intracellular pH-GluA2, and that the decline in fluorescence in response to NMDA and AMPA primarily describes an intracellular acidification, which quenches the pHluorin signal from intracellular receptor pools. Neurons expressing an endoplasmic reticulum-retained mutant of GluA2 (pH-GluA2 ΔC49) displayed a larger response to NMDA than neurons expressing wild-type pH-GluA2. A similar NMDA-elicited decline in pHluorin signal was observed by expressing cytosolic pHluorin alone without fusion to GluA2 (cyto-pHluorin). Intracellular acidification in response to NMDA was further confirmed by using the ratiometric pH indicator carboxy-SNARF-1. The NMDA-induced decline was followed by rapid recovery of the fluorescent signal from both cyto-pHluorin and pH-GluA2. The recovery was sodium-dependent and sensitive to Na(+)/H(+)-exchanger (NHE) inhibitors. Moreover, recovery was more rapid after shRNA-mediated knockdown of the GluA2 binding PDZ domain-containing protein interacting with C kinase 1 (PICK1). Interestingly, the accelerating effect of PICK1 knockdown on the fluorescence recovery was eliminated in the presence of the NHE1 inhibitor zoniporide. Our results indicate that the pH-GluA2 recycling assay is an unreliable assay for studying AMPA receptor trafficking and also suggest a role for PICK1 in regulating intracellular pH via modulation of NHE activity.

Interaction of PICK1 with C-terminus of growth hormone-releasing hormone receptor (GHRHR) modulates trafficking and signal transduction of human GHRHR

Release of growth hormone (GH) from the somatotroph is regulated by binding GH-releasing hormone (GHRH) to its cognate receptor (GHRHR), one of the members of the G protein-coupled receptor (GPCR) superfamily. Proteins bound to the carboxy (C)-terminus of GPCR have been reported to regulate intracellular trafficking and function of the receptor; however, no functionally significant protein associated with GHRHR has been reported. We have identified a protein interacting with C-kinase 1 (PICK1) as a binding partner of GHRHR. In vitro binding assay revealed the PDZ-domain of PICK1 and the last four amino acid residues of GHRHR were prerequisite for the interaction. Further, in vivo association of these proteins was confirmed. Immunostaining data of a stable cell line expressing GHRHR with or without PICK1 suggested the C-terminus of GHRHR promoted cell surface expression of GHRHR and PICK1 affected the kinetics of the cell surface expression of GHRHR. Furthermore, cAMP production assay showed the C-terminus of GHRHR is involved in the regulation of receptor activation, and the interaction of GHRHR with PICK1 may influence intensities of the signal response after ligand stimulation. Thus, the interaction of the C-terminus of GHRHR with PICK1 has a profound role in regulating the trafficking and the signaling of GHRHR. [Supplementary Figure: available only at http://dx.doi.org/10.1254/jphs.12287FP].