Direct SUMOylation of M1 muscarinic acetylcholine receptor increases its ligand-binding affinity and signal transduction

The Activation of Muscarinic Acetylcholine Receptors Protects against Neuroinflammation in a Mouse Model through Attenuating Microglial Inflammation

Neuroinflammation is a critical factor that contributes to neurological impairment and is closely associated with the onset and progression of neurodegenerative diseases. In the central nervous system (CNS), microglia play a pivotal role in the regulation of inflammation through various signaling pathways. Therefore, mitigating microglial inflammation is considered a promising strategy for restraining neuroinflammation. Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and exhibit clear neuroprotective effects in various disease models. However, whether the activation of mAChRs can harness benefits in neuroinflammation remains largely unexplored. In this study, the anti-inflammatory effects of mAChRs were found in a neuroinflammation mouse model. The expression of various cytokines and chemokines was regulated in the brains and spinal cords after the administration of mAChR agonists. Microglia were the primary target cells through which mAChRs exerted their anti-inflammatory effects. The results showed that the activation of mAChRs decreased the pro-inflammatory phenotypes of microglia, including the expression of inflammatory cytokines, morphological characteristics, and distribution density. Such anti-inflammatory modulation further exerted neuroprotection, which was found to be even more significant by the direct activation of neuronal mAChRs. This study elucidates the dual mechanisms through which mAChRs exert neuroprotective effects in central inflammatory responses, providing evidence for their application in inflammation-related neurological disorders.

Overview of the regulation of the class IA PI3K/AKT pathway by SUMO

The phosphatidylinositol-3-kinase (PI3K)/AKT pathway is a major regulator of metabolism, migration, survival, proliferation, and antiviral immunity. Both an overactivation and an inhibition of the PI3K/AKT pathway are related to different pathologies. Activation of this signaling pathway is tightly controlled through a multistep process and its deregulation can be associated with aberrant post-translational modifications including SUMOylation. Here, we review the complex modulation of the PI3K/AKT pathway by SUMOylation and we discuss its putative incvolvement in human disease.

Loss of Cholinergic Receptor Muscarinic 1 (CHRM1) Protein in the Hippocampus and Temporal Cortex of a Subset of Individuals with Alzheimer’s Disease, Parkinson’s Disease, or Frontotemporal Dementia: Implications for Patient Survival

Background: Dysfunction of cholinergic neurotransmission is a hallmark of Alzheimer’s disease (AD); forming the basis for using acetylcholine (ACh) esterase (AChE) inhibitors to mitigate symptoms of ACh deficiency in AD. The Cholinergic Receptor Muscarinic 1 (CHRM1) is highly expressed in brain regions impaired by AD. Previous analyses of postmortem AD brains revealed unaltered CHRM1 mRNA expression compared to normal brains. However, the CHRM1 protein level in AD and other forms of dementia has not been extensively studied. Reduced expression of CHRM1 in AD patients may explain the limited clinical efficacy of AChE inhibitors. Objective: To quantify CHRM1 protein in the postmortem hippocampus and temporal cortex of AD, Parkinson’s disease (PD), and frontotemporal dementia (FTD) patients. Methods: Western blotting was performed on postmortem hippocampus (N = 19/73/7/9: unaffected/AD/FTD/PD) and temporal cortex (N = 9/74/27: unaffected/AD/PD) using a validated anti-CHRM1 antibody. Results: Quantification based on immunoblotting using a validated anti-CHRM1 antibody revealed a significant loss of CHRM1 protein level (<50%) in the hippocampi (78% AD, 66% PD, and 85% FTD) and temporal cortices (56% AD and 42% PD) of dementia patients. Loss of CHRM1 in the temporal cortex was significantly associated with early death (<65–75 years) for both AD and PD patients. Conclusion: Severe reduction of CHRM1 in a subset of AD and PD patients can explain the reported low efficacy of AChE inhibitors as a mitigating treatment for dementia patients. Based on this study, it can be suggested that future research should prioritize therapeutic restoration of CHRM1 protein levels in cholinergic neurons.

SUMOylation and Major Depressive Disorder

Since the discovery of the small ubiquitin-like modifier (SUMO) protein in 1995, SUMOylation has been considered a crucial post-translational modification in diverse cellular functions. In neurons, SUMOylation has various roles ranging from managing synaptic transmitter release to maintaining mitochondrial integrity and determining neuronal health. It has been discovered that neuronal dysfunction is a key factor in the development of major depressive disorder (MDD). PubMed and Google Scholar databases were searched with keywords such as 'SUMO', 'neuronal plasticity', and 'depression' to obtain relevant scientific literature. Here, we provide an overview of recent studies demonstrating the role of SUMOylation in maintaining neuronal function in participants suffering from MDD.

Post-Translational Modifications in GPCR Internalization

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors that serve as the most important drug targets. Classically, GPCR internalization has been considered to lead to receptor desensitization. However, many studies over the past decade have reported that internalized membrane receptors can trigger distinct signal activation. The "internalized activation" provides a completely new understanding for the receptor internalization, the mechanism of physiology/pathology and novel drug targets for precision medicine. GPCR internalization undergoes a series of strict regulations, especially by post-translational modifications (PTMs). Here, this review summarizes different PTMs in GPCR internalization and analyzes their significance in GPCR internalization dynamics, internalization routes, post-internalization fates and related diseases, which will offer new insights into the regulatory mechanism of GPCR signaling and novel drug targets for precision medicine.

Roles of receptor phosphorylation and Rab proteins in G protein-coupled receptor function and trafficking

The G Protein-Coupled Receptors form the most abundant family of membrane proteins and are crucial physiological players in the homeostatic equilibrium, which we define as health. They also participate in the pathogenesis of many diseases and are frequent targets of therapeutic intervention. Considering their importance, it is not surprising that different mechanisms regulate their function, including desensitization, resensitization, internalization, recycling to the plasma membrane, and degradation. These processes are modulated in a highly coordinated and specific way by protein kinases and phosphatases, ubiquitin ligases, protein adaptors, interaction with multifunctional complexes, molecular motors, phospholipid metabolism, and membrane distribution. This review describes significant advances in the study of the regulation of these receptors by phosphorylation and endosomal traffic (where signaling can take place); we revisited the bar code hypothesis and include two additional observations: a) that different phosphorylation patterns seem to be associated with internalization and endosome sorting for recycling or degradation, and b) that, surprisingly, phosphorylation of some G protein-coupled receptors appears to be required for proper receptor insertion into the plasma membrane. Significance Statement G protein-coupled receptor phosphorylation is an early event in desensitization/ signaling switching, endosomal traffic, and internalization. These events seem crucial for receptor responsiveness, cellular localization, and fate (recycling/ degradation) with important pharmacological/ therapeutic implications. Phosphorylation sites vary depending on the cells in which they are expressed and on the stimulus that leads to such covalent modification. Surprisingly, evidence suggests that phosphorylation also seems to be required for proper insertion into the plasma membrane for some receptors.

Regulation of Serotonin 1A Receptor SUMOylation by SENP2 and PIASxα

Serotonin 1A receptors (5-HT1ARs) are implicated in the control of mood, cognition, and memory and in various neuropsychiatric disorders such as depression and anxiety. As such, understanding the regulation of 5-HT1ARs will inform the development of better treatment approaches. We previously demonstrated 5-HT1ARs are SUMOylated by SUMO1 in the rat brain. Agonist stimulation increased SUMOylation and was further enhanced when combined with 17β-estradiol-3-benzoate (EB), which are treatments that cause the transient and prolonged desensitization of 5-HT1AR signaling, respectively. In the current study, we identified the protein inhibitor of activated STAT (PIAS)xα as the enzyme that facilitates SUMOylation, and SENP2 as the protein that catalyzes the deSUMOylation of 5-HT1ARs. We demonstrated that PIASxα significantly increased in the membrane fraction of rats co-treated with EB and an agonist, compared to either the EB-treated or vehicle-treated groups. The acute treatment with an agonist alone shifted the location of SENP2 from the membrane to the cytoplasmic fraction, but it has little effect on PIASxα. Hence, two separate mechanisms regulate SUMOylation and the activity of 5-HT1ARs by an agonist and EB. The effects of EB on 5-HT1AR SUMOylation and signaling may be related to the higher incidence of mood disorders in women during times with large fluctuations in estrogens. Targeting the SUMOylation of 5-HT1ARs could have important clinical relevance for the therapy for several neuropsychiatric disorders in which 5-HT1ARs are implicated.

The new fate of internalized membrane receptors: Internalized activation

Classically, the fate of internalized membrane receptors includes receptor degradation and receptor recycling. However, recent findings have begun to challenge these views. Much research demonstrated that many internalized membrane receptors can trigger distinct signal activation rather than being desensitized inside the cell. Here, we introduce the concept of "internalized activation" which not only represents a new mode of receptor activation, but also endows the new fate for receptor internalization (from death to life). The new activation mode and fate of membrane receptor are ubiquitous and have unique theoretical significance. We systematically put forward the features, process, and regulation of "internalized activation" and its significance in signal transduction and diseases. "Internalized activation" will provide a completely new understanding for the theory of receptor activation, internalization and novel drug targets for precision medicine.

SUMOylation of synaptic and synapse-associated proteins: An update

SUMOylation is a post-translational modification that regulates protein signalling and complex formation by adjusting the conformation or protein-protein interactions of the substrate protein. There is a compelling and rapidly expanding body of evidence that, in addition to SUMOylation of nuclear proteins, SUMOylation of extranuclear proteins contributes to the control of neuronal development, neuronal stress responses and synaptic transmission and plasticity. In this brief review we provide an update of recent developments in the identification of synaptic and synapse-associated SUMO target proteins and discuss the cell biological and functional implications of these discoveries.

Post-Translational Modifications of G Protein-Coupled Receptors Control Cellular Signaling Dynamics in Space and Time

G protein-coupled receptors (GPCRs) are a large family comprising >800 signaling receptors that regulate numerous cellular and physiologic responses. GPCRs have been implicated in numerous diseases and represent the largest class of drug targets. Although advances in GPCR structure and pharmacology have improved drug discovery, the regulation of GPCR function by diverse post-translational modifications (PTMs) has received minimal attention. Over 200 PTMs are known to exist in mammalian cells, yet only a few have been reported for GPCRs. Early studies revealed phosphorylation as a major regulator of GPCR signaling, whereas later reports implicated a function for ubiquitination, glycosylation, and palmitoylation in GPCR biology. Although our knowledge of GPCR phosphorylation is extensive, our knowledge of the modifying enzymes, regulation, and function of other GPCR PTMs is limited. In this review we provide a comprehensive overview of GPCR post-translational modifications with a greater focus on new discoveries. We discuss the subcellular location and regulatory mechanisms that control post-translational modifications of GPCRs. The functional implications of newly discovered GPCR PTMs on receptor folding, biosynthesis, endocytic trafficking, dimerization, compartmentalized signaling, and biased signaling are also provided. Methods to detect and study GPCR PTMs as well as PTM crosstalk are further highlighted. Finally, we conclude with a discussion of the implications of GPCR PTMs in human disease and their importance for drug discovery. SIGNIFICANCE STATEMENT: Post-translational modification of G protein-coupled receptors (GPCRs) controls all aspects of receptor function; however, the detection and study of diverse types of GPCR modifications are limited. A thorough understanding of the role and mechanisms by which diverse post-translational modifications regulate GPCR signaling and trafficking is essential for understanding dysregulated mechanisms in disease and for improving and refining drug development for GPCRs.

UBC-9 Acts in GABA Neurons to Control Neuromuscular Signaling in C. elegans

Regulation of excitatory to inhibitory signaling balance is essential to nervous system health and is maintained by numerous enzyme systems that modulate the activity, localization, and abundance of synaptic proteins. SUMOylation is a key post-translational regulator of protein function in diverse cells, including neurons. There, its role in regulating synaptic transmission through pre- and postsynaptic effects has been shown primarily at glutamatergic central nervous system synapses, where the sole SUMO-conjugating enzyme Ubc9 is a critical player. However, whether Ubc9 functions globally at other synapses, including inhibitory synapses, has not been explored. Here, we investigated the role of UBC-9 and the SUMOylation pathway in controlling the balance of excitatory cholinergic and inhibitory GABAergic signaling required for muscle contraction in Caenorhabditis elegans. We found inhibition or overexpression of UBC-9 in neurons modestly increased muscle excitation. Similar and even stronger phenotypes were seen with UBC-9 overexpression specifically in GABAergic neurons, but not in cholinergic neurons. These effects correlated with accumulation of synaptic vesicle-associated proteins at GABAergic presynapses, where UBC-9 and the C. elegans SUMO ortholog SMO-1 localized, and with defects in GABA-dependent behaviors. Experiments involving expression of catalytically inactive UBC-9 [UBC-9(C93S)], as well as co-expression of UBC-9 and SMO-1, suggested wild type UBC-9 overexpressed alone may act via substrate sequestration in the absence of sufficient free SUMO, underscoring the importance of tightly regulated SUMO enzyme function. Similar effects on muscle excitation, GABAergic signaling, and synaptic vesicle localization occurred with overexpression of the SUMO activating enzyme subunit AOS-1. Together, these data support a model in which UBC-9 and the SUMOylation system act at presynaptic sites in inhibitory motor neurons to control synaptic signaling balance in C. elegans. Future studies will be important to define UBC-9 targets at this synapse, as well as mechanisms by which UBC-9 and the SUMO pathway are regulated.