α4β2 nicotinic acetylcholine receptor downregulates D3 dopamine receptor expression through protein kinase C activation

Protein kinase C: A potential therapeutic target for endothelial dysfunction in diabetes

Protein kinase C (PKC) is a family of serine/threonine protein kinases that play an important role in many organs and systems and whose activation contributes significantly to endothelial dysfunction in diabetes. The increase in diacylglycerol (DAG) under high glucose conditions mediates PKC activation and synthesis, which stimulates oxidative stress and inflammation, resulting in impaired endothelial cell function. This article reviews the contribution of PKC to the development of diabetes-related endothelial dysfunction and summarizes the drugs that inhibit PKC activation, with the aim of exploring therapeutic modalities that may alleviate endothelial dysfunction in diabetic patients.

Neural regulation of ILC2s in allergic airway inflammation

The sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) regulate the effector functions of group 2 innate lymphoid cells (ILC2s) through β2 adrenergic receptor (ADRB2) and nicotinic/muscarinic cholinergic receptor signaling, respectively. To further maintain the critical balance between host-protective and pathogenic type 2 inflammation in the lungs, neuropeptides neuromedin B (NMB) and neuromedin U (NMU) function to suppress or promote ILC2 responses in synergy with IL-33/IL-25, respectively. Additionally, the release of ATP into the extracellular environment in response to cell death caused by challenge to the airway epithelial barrier quickly becomes converted into adenosine, which helps keep the inflammatory response in check by suppressing ILC2 responses. Besides neurotransmitter and neuropeptides derived from other cells, ILC2s further regulate allergic airway inflammation through the production of acetylcholine (ACh) and calcitonin gene-related peptide (CGRP). In this article we review the neuromodulation of ILC2s through cholinergic and adrenergic signaling, neuropeptides, and adenosine and its role in allergic airway inflammation. Furthermore, we discuss the potential clinical utility of targeting these pathways for therapeutic goals and address directions for future research.

The Role of Dopamine D3 Receptors in Tobacco Use Disorder: A Synthesis of the Preclinical and Clinical Literature

Tobacco smoking is a significant cause of preventable morbidity and mortality globally. Current pharmacological approaches to treat tobacco use disorder (TUD) are only partly effective and novel approaches are needed. Dopamine has a well-established role in substance use disorders, including TUD, and there has been a long-standing interest in developing agents that target the dopaminergic system to treat substance use disorders. Dopamine has 5 receptor subtypes (DRD1 to DRD5). Given the localization and safety profile of the dopamine receptor D3 (DRD3), it is of therapeutic potential for TUD. In this chapter, the preclinical and clinical literature investigating the role of DRD3 in processes relevant to TUD will be reviewed, including in nicotine reinforcement, drug reinstatement, conditioned stimuli and cue-reactivity, executive function, and withdrawal. Similarities and differences in findings from the animal and human work will be synthesized and findings will be discussed in relation to the therapeutic potential of targeting DRD3 in TUD.

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.

β-Arrestin1 and GPCR kinase2 play permissive roles in Src-mediated endocytosis of α4β2 nicotinic ACh receptors

BACKGROUND AND PURPOSE

The α4β2 nicotinic ACh receptor (nAChR), a subtype of the ligand-gated ion channel, is abundantly expressed in the brain and is implicated in several neurological disorders. The endocytosis of nAChRs plays important roles in the pathogenesis of neurological diseases, but the underlying molecular mechanisms remain poorly understood.

EXPERIMENTAL APPROACH

Loss-of-function approaches and mutants of α4β2 nAChRs that display different endocytic properties were used to identify the cellular components and processes responsible for endocytosis. The signalling cascade that leads to endocytosis was deduced via protein interactions in predicted cellular components. The endocytosis of α4β2 nAChRs was determined and crosschecked using an ELISA and radioligand assay.

KEY RESULTS

Endocytosis of α4β2 nAChRs occurred through clathrin-mediated endocytosis in a dynamin-dependent manner. 14-3-3η-dependent Src-mediated phosphorylation of the nAChR α4 subunit at Y575 was required for nAChR endocytosis, and this occurred with the assistance of β-arrestin1 and GPCR kinase 2 (GRK2) without the need for kinase activity. Endocytosis triggered the mouse double minute 2 homologue-mediated ubiquitination and subsequent down-regulation of α4β2 nAChRs.

CONCLUSIONS AND IMPLICATIONS

α4β2 nAChR, an ionophore receptor, employs the metabotropic signalling pathway required for endocytosis, which leads to ubiquitination and down-regulation. Further, GRK2 and β-arrestin1, usually associated with GPCR signalling, are involved in the endocytosis of α4β2 nAChRs via different mechanisms. Considering the functional and pathological implications of nAChR endocytosis, results obtained in this study are crucial for the progression of basic research and clinical investigations.

Roles of the Functional Interaction between Brain Cholinergic and Dopaminergic Systems in the Pathogenesis and Treatment of Schizophrenia and Parkinson's Disease

Most physiologic processes in the brain and related diseases involve more than one neurotransmitter system. Thus, elucidation of the interaction between different neurotransmitter systems could allow for better therapeutic approaches to the treatments of related diseases. Dopaminergic (DAergic) and cholinergic neurotransmitter system regulate various brain functions that include cognition, movement, emotion, etc. This review focuses on the interaction between the brain DAergic and cholinergic systems with respect to the pathogenesis and treatment of schizophrenia and Parkinson’s disease (PD). We first discussed the selection of motor plans at the level of basal ganglia, the major DAergic and cholinergic pathways in the brain, and the receptor subtypes involved in the interaction between the two signaling systems. Next, the roles of each signaling system were discussed in the context of the negative symptoms of schizophrenia, with a focus on the α7 nicotinic cholinergic receptor and the dopamine D₁ receptor in the prefrontal cortex. In addition, the roles of the nicotinic and dopamine receptors were discussed in the context of regulation of striatal cholinergic interneurons, which play crucial roles in the degeneration of nigrostriatal DAergic neurons and the development of L-DOPA-induced dyskinesia in PD patients. Finally, we discussed the general mechanisms of nicotine-induced protection of DAergic neurons.

A Selective α7 Nicotinic Acetylcholine Receptor Agonist, PNU-282987, Attenuates ILC2s Activation and Alternaria-Induced Airway Inflammation

Background

The anti-inflammatory effect of an α7nAChR agonist, PNU-282987, has previously been explored in the context of inflammatory disease. However, the effects of PNU-282987 on type 2 innate lymphoid cells (ILC2s)-mediated allergic airway inflammation has not yet been established.

Aims

To determine the effects of PNU-282987 on the function of ILC2s in the context of IL-33– or Alternaria Alternata (AA)– induced airway inflammation.

Methods

PNU-282987 was administered to mice that received recombinant IL-33 or AA intranasal challenges. Lung histological analysis and flow cytometry were performed to determine airway inflammation and the infiltration and activation of ILC2s. The previously published α7nAChR agonist GTS-21 was employed as a comparable reagent. ILC2s were isolated from murine lung tissue and cultured in vitro in the presence of IL-33, IL-2, and IL-7 with/without either PNU-282987 or GTS-21. The expression of the transcription factors GATA3, IKK, and NF-κB were also determined.

Results

PNU-282987 and GTS-21 significantly reduced goblet cell hyperplasia in the airway, eosinophil infiltration, and ILC2s numbers in BALF, following IL-33 or AA challenge. In vitro IL-33 stimulation of isolated lung ILC2s showed a reduction of GATA3 and Ki67 in response to PNU-282987 or GTS-21 treatments. There was a significant reduction in IKK and NF-κB phosphorylation in the PNU-282987–treated group when compared to the GTS-21–treated ILC2s.

Conclusion

PNU-282987 inhibits ILC2-associated airway inflammation, where its effects were comparable to that of GTS-21.

Metabotropic signaling cascade involved in α4β2 nicotinic acetylcholine receptor-mediated PKCβII activation

Nicotinic acetylcholine receptors (nAChRs) belong to the ionophore receptor family, which regulates plasma membrane conductance to Na⁺, K⁺, and Ca²⁺ ions. Some studies, however, have shown that nAChRs also employ second messengers for intracellular signaling. We previously showed that α4β2 nAChR mediates the translocation of protein kinase CβII (PKCβII) from the cytoplasm to the plasma membrane, which is a typical activation marker for PKCβII. In this study, we investigated the molecular mechanisms underlying PKCβII activation through α4β2 nAChR. α4β2 nAChR is the most abundant nAChR subtype and is implicated in various brain functions and diseases. Putative α4β2 nAChR signaling components were identified by knockdown or chemical inhibition of candidate proteins, and the signaling cascade was deduced by protein interactions in predicted cellular components. α4β2 nAChR-mediated PKCβII translocation was found to occur in an ionophore activity-independent manner. Nicotinic stimulation of α4β2 nAChR activated Src in a β-arrestin1 and 14-3-3η-dependent manner. Activated Src phosphorylated the tyrosine residue(s) on Syk molecules, which in turn interacted with phospholipase C γ1 to trigger the translocation of PKCβII to the cell membrane by elevating cellular diacylglycerol levels. The activated PKCβII in turn exerted a positive feedback effect on Src activation, suggesting that α4β2 nAChR signaling is amplified by a positive feedback loop. These findings provide novel information for unveiling the previously unclear metabotropic second messenger-based signal transduction pathway of nAChRs.