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Deep Mutagenesis of a Transporter for Uptake of a Non-Native Substrate Identifies Conformationally Dynamic Regions

ABSTRACT: The serotonin transporter, SERT, catalyzes serotonin reuptake at the synapse to terminate neurotransmission via an alternating access mechanism, and SERT inhibitors are the most widely prescribed antidepressants. Here, deep mutagenesis is used to determine the effects of nearly all amino acid substitutions on human SERT surface expression and transport of the fluorescent substrate analogue APP+, identifying many mutations that enhance APP+ import. Comprehensive simulations of the entire ion-coupled import process reveal that while binding of the native substrate, serotonin, reduces free energy barriers between conformational states to promote SERT dynamics, the conformational free energy landscape in the presence of APP+ instead resembles Na+ bound-SERT, with a higher free energy barrier for isomerization to an inward-facing state. The deep mutational scan for SERT-catalyzed import of APP+ finds mutations that promote the necessary conformational changes that would otherwise be facilitated by the native substrate. Indeed, hundreds of gain-of-function mutations for APP+ import are found along the permeation pathway, most notably mutations that favor opening of a solvent-exposed intracellular vestibule. The mutagenesis data support the simulated mechanism in which the neurotransmitter and a symported sodium share a common cytosolic exit pathway to achieve coupling. Furthermore, the mutational landscape for SERT surface trafficking, which likely filters out misfolded sequences, reveals that residues along the permeation pathway are mutationally tolerant, providing plausible evolutionary pathways for changes in transporter properties while maintaining folded structure.

ORGANISM(S): Homo sapiens

PROVIDER: GSE109499 | GEO | 2019/07/31

REPOSITORIES: GEO

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Project description:The serotonin transporter (SERT, SLC6A4), belonging to Na+ and Cl- dependent transporters, functions to regulate the availability of serotonin (5-HT) in various tissues including brain, platelets, and intestine. Alterations in 5-HT levels and/or SERT expression have been implicated in pathophysiological conditions including anxiety-like behavior, obesity, cardiovascular disorders and GI disorders such as IBD, IBS, and diarrhea. Accordingly, SERT deficiency in mice is pleiotropic with many different phenotypic traits. Transgenic mice with global deletion of SERT exhibit increased anxiety, abnormal GI motility, and obesity associated with hyperglycemia. Additionally, these mice are more susceptible to intestinal inflammation. The mechanism by which SERT deficiency underlies GI related disturbances and exacerbates the severity of gut inflammation is not known. Thus, we evaluated the ileal mRNA expression to identify potential intestinal targets that contribute to disease pathogenesis when the SERT gene is non-functional. For these studies, RNA was extracted from ileum mucosa from age matched (9-10 wk) control and SERT KO mice by Trizol and was subjected to Mouse mRNA and OneArray Profiling. Our microarray data revealed 492 significantly upregulated and 293 downregulated genes in SERT KO ileum. Pathway analysis revealed significant enrichment in cell cycle, signaling, chemotaxis, and epithelial transporters. Genes of interest were validated by RT-PCR, which included CYP1A1 (~20 fold decrease, p < 0.05), CLDN8 (~30% decrease, p < 0.05), and HMGCS2 (~7 fold increase, p < 0.05). Similar changes were found in colon. A remarkable decrease in expression of Cyp1a1, which encodes a cytochrome P450 enzyme, provides a novel role of serotonergic pathways in drug and xenobiotic detoxification. CLDN8 encodes for claudin 8, which is downregulated in patients with Crohn’s disease. Deficiency of HMGCS2, the mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase, is linked to hypoglycemia. Our data provides novel clues for differential expression of genes affected by 5-HT and/or SERT deficiency in the intestine.

Project description:Adipocytes are dynamic cells that have critical functions to maintain body energy homeostasis. Function of adipocyte is affected by adipogenic differentiation, and exogenous stimulation of biochemical factors such as serotonin and TNF-. Serotonin (5-hydroxytriptamine, 5-HT) is a biogenic monoamine, which known to be involved in lipid metabolism in adipocytes. The TNF- is a proinflammatory cytokines involved in adipocyte biology. Here we investigated the global transcriptome alterations of porcine intramuscular adipocytes at undifferentiated preadipocyte, differentiated adipocytes state, and adipocytes after stimulation with serotonin and TNF- for 12 h by using Agilent’s Porcine (V2) one-color Gene Expression Microarray 4 × 44. Transcriptome analysis revealed that the expression of 443, 261, and 249 genes were altered after differentiation, and after serotonin and TNF- stimulation, respectively. Differentiation process of adipocytes are found to be involved with expression changes of APP, HNF4A, ESR1, EGR1, SRC, HNF1A, FN1, ALB, STAT3, CBL, CEBPB, AR, FOS, CFTR, PAN2, PTPN6, VDR, PPARG, STAT5A and NCOA3 genes which are enriched in the ‘PPAR signaling pathway’ and ‘insulin resistance pathway’. Dose-dependent serotonin stimulation resulted in a decreased fat accumulation in the adipocytes as well as decreased adipocyte proliferation. Serotonin-induced differentially expressed genes in adipocytes were participated in the significant enrichment of GPCR ligand-binding, Cell chemotaxis, blood coagulation and complement pathways, metabolism of lipid and lipoproteins, regulation of lipid metabolism by PPARA; and lipid digestion, mobilization and transport pathways. TNF-α-induced transcriptional modification follows the similar trend, and alteration of many genes shared between both serotonin- and TNF-α stimulations. Our results provide new insights of transcriptome modifications associated with adipogenesis, and exogenous stimulation of serotonin- and TNF- to the porcine intramuscular adipocytes.

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Deep Mutagenesis of a Transporter for Uptake of a Non-Native Substrate Identifies Conformationally Dynamic Regions

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