Cholesterol-dependent endocytosis of GPCRs: implications in pathophysiology and therapeutics

Sharma²

2023

Front. Mol. Biosci.

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The biosynthesis of cholesterol, an essential component of higher eukaryotic membranes, was worked out by Konrad Bloch (and Feodor Lynen) in the 1960s and they received the Nobel Prize around that time in recognition of their pioneering contributions. An elegant consequence of this was a hypothesis proposed by Konrad Bloch (the Bloch hypothesis) which suggests that each subsequent intermediate in the cholesterol biosynthesis pathway is superior in supporting membrane function in higher eukaryotes relative to its precursor. In this review, we discuss an autosomal recessive metabolic disorder, known as Smith-Lemli-Opitz syndrome (SLOS), associated with a defect in the Kandutsch-Russell pathway of cholesterol biosynthesis that results in accumulation of the immediate precursor of cholesterol in its biosynthetic pathway (7-dehydrocholesterol) and an altered cholesterol to total sterol ratio. Patients suffering from SLOS have several developmental, behavioral and cognitive abnormalities for which no drug is available yet. We characterize SLOS as a manifestation of the Bloch hypothesis and review its molecular etiology and current treatment. We further discuss defective Hedgehog signaling in SLOS and focus on the role of the serotonin1A receptor, a representative neurotransmitter receptor belonging to the GPCR family, in SLOS. Notably, ligand binding activity and cellular signaling of serotonin1A receptors are impaired in SLOS-like condition. Importantly, cellular localization and intracellular trafficking of the serotonin1A receptor (which constitute an important determinant of a GPCR cellular function) are compromised in SLOS. We highlight some of the recent developments and emerging concepts in SLOS pathobiology and suggest that novel therapies based on trafficking defects of target receptors could provide new insight into treatment of SLOS.

Section: Cholesterol: An Essential and Abundant Lipid In Higher Eukar...mentioning

confidence: 99%

Section: Defective Hedgehog Signaling In Slosmentioning

confidence: 99%

Section: Endocytosis and Cellular Trafficking: New Paradigm In Slos R...mentioning

confidence: 99%

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Smith-Lemli-Opitz syndrome: A pathophysiological manifestation of the Bloch hypothesis

Sharma²

2023

Front. Mol. Biosci.

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“…G-protein-coupled receptors (GPCRs) are the largest and the most diverse class of proteins in higher eukaryotic plasma membranes that facilitate the transfer of a variety of signals from outside the cell to the cellular interior. − They are characterized by seven transmembrane helices interconnected with variable extracellular and intracellular domains. , GPCRs function as signaling hubs controlling multiple cellular responses that include cellular metabolism, neurotransmission, growth, immune response, and cellular differentiation. As a result, GPCRs act as major therapeutic targets, , and account for ∼40% of current drug targets across all clinical areas. , The serotonin 1A receptor is a representative neurotransmitter GPCR, expressed mainly in neuronal cells, and is implicated in anxiety, depression, stress, learning deficiency, cognition, and mood disorders. − For this reason, the serotonin 1A receptor represents a major drug target in developing drugs against neuropsychiatric disorders such as depression, anxiety, and even cancer. , Notably, the serotonin 1A receptor is one of the best-characterized GPCRs in terms of lipid dependence of its ligand binding, G-protein coupling, organization, dynamics, oligomerization, signaling, and endocytosis. ,− …”

Section: Introductionmentioning

confidence: 99%

Synergistic and Competitive Lipid Interactions in the Serotonin_1A Receptor Microenvironment

Mohole

Sengupta

2022

ACS Chem. Neurosci.

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The interaction of lipids with G-protein-coupled receptors (GPCRs) has been shown to modulate and dictate several aspects of GPCR organization and function. Diverse lipid interaction sites have been identified from structural biology, bioinformatics, and molecular dynamics studies. For example, multiple cholesterol interaction sites have been identified in the serotonin 1A receptor, along with distinct and overlapping sphingolipid interaction sites. How these lipids interact with each other and what is the resultant effect on the receptor is still not clear. In this work, we have analyzed lipid−lipid crosstalk at the receptor of the serotonin 1A receptor embedded in a membrane bilayer that mimics the neuronal membrane composition by long coarse-grain simulations. Using a set of similarity coefficients, we classified lipids that bind at the receptor together as synergistic cobinding, and those that bind individually as competitive. Our results show that certain lipids interact with the serotonin 1A receptor in synergy with each other. Not surprisingly, the ganglioside GM1 and cholesterol show a synergistic cobinding, along with the relatively uncommon GM1-phosphatidylethanolamine (PE) and cholesterol-PE synergy. In contrast, certain lipid pairs such as cholesterol and sphingomyelin appear to be in competition at several sites, despite their coexistence in lipid nanodomains. In addition, we observed intralipid competition between two lipid tails, with the receptor exhibiting increased interactions with the unsaturated lipid tails. We believe our work represents an important step in understanding the diversity of GPCR−lipid interactions and exploring synergistic cobinding and competition in natural membranes.

“…We reported that the serotonin 1A receptor internalizes primarily via clathrin‐mediated endocytosis and recycles back to the plasma membrane. Interestingly, we recently showed that the endocytosis and trafficking of the serotonin 1A receptor are highly dependent on membrane cholesterol content 44–47 . However, the role of sphingolipids in serotonin 1A receptor endocytosis remains unexplored.…”

Section: Introductionmentioning

confidence: 99%

Metabolic depletion of sphingolipids inhibits agonist‐induced endocytosis of the serotonin_1A receptor

Kumar

Sarkar

2022

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G protein-coupled receptors (GPCRs) are vital cellular signaling machinery and currently represent $40% drug targets. Endocytosis of GPCRs is an important process that allows stringent spatiotemporal control over receptor population on the cell surface. Although the role of proteins in GPCR endocytosis is well addressed, the contribution of membrane lipids in this process is rather unexplored. Sphingolipids are essential functional lipids in higher eukaryotes and are implicated in several neurological functions. To understand the role of sphingolipids in GPCR endocytosis, we subjected cells expressing human serotonin 1A receptors (an important neurotransmitter GPCR involved in cognitive and behavioral functions) to metabolic sphingolipid depletion using fumonisin B 1 , an inhibitor of sphingolipid biosynthetic pathway. Our results, using flow cytometric analysis and confocal microscopic imaging, show that sphingolipid depletion inhibits agonist-induced endocytosis of the serotonin 1A receptor in a concentration-dependent manner, which was restored when sphingolipid levels were replenished. We further show that there was no change in the internalization of transferrin, a marker for clathrin-mediated endocytosis, under sphingolipiddepleted condition, highlighting the specific requirement of sphingolipids for endocytosis of serotonin 1A receptors. Our results reveal the regulatory role of sphingolipids in GPCR endocytosis and highlight the importance of neurotransmitter receptor trafficking in health and disease.endocytosis, fumonisin B 1 , G protein-coupled receptor, serotonin 1A receptor, sphingolipids, sphingomyelin | INTRODUCTIONG protein-coupled receptors (GPCRs) represent the largest class of integral membrane proteins and have a characteristic seven transmembrane domain architecture involved in signal transduction across the plasma membrane. [1][2][3][4] Because of their versatile role in multiple physiological functions, GPCRs are major drug targets across all clinical areas. [4][5][6][7] The intracellular signaling responses mediated by GPCRs are initiated at the plasma membrane and the downstream signaling cascades are spatiotemporally regulated in a stringent manner.In this context, endocytosis offers a key desensitization mechanism, by internalization of the receptor from the plasma membrane into the cellular interior, to restrict GPCR signaling responses in a strict spatiotemporal regime. [8][9][10] Emerging evidences have suggested that endocytosis of GPCRs could simultaneously facilitate non-canonical receptor-mediated signaling even from the intracellular receptor pool. [11][12][13][14] Endocytosis is concerted by a number of membraneassociated and cytoplasmic proteins that help in cargo assembly, Abbreviations: BCA, bicinchoninic acid; FB 1 , fumonisin B 1 ; FCS, fetal calf serum; GPCR, G protein-coupled receptor; HEK-5-HT 1A R, HEK-293 cells stably expressing N-terminal myctagged human serotonin 1A receptors; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; PBS, phosphate buffered...