S-palmitoylation regulates biogenesis of core glycosylated wild-type and F508del CFTR in a post-ER compartment

Palmitoylation regulates norepinephrine transporter trafficking and expression and is potentially involved in the pathogenesis of postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is an adrenergic signaling disorder characterized by excessive plasma norepinephrine, postural tachycardia, and syncope. The norepinephrine transporter (NET) modulates adrenergic homeostasis via reuptake of extracellular catecholamines and is implicated in the pathogenesis of adrenergic and neurological disorders. Previous research has outlined that NET activity and trafficking is modulated via reversible post-translational modifications like phosphorylation and ubiquitylation. S-palmitoylation, or the addition of a 16-carbon saturated fatty acid, is another post-translational modification responsible for numerous biological mechanisms. In this study, we reveal that NET is dynamically palmitoylated and inhibition of this modification with the palmitoyl acyltransferase (DHHC) inhibitor, 2-bromopalmitate (2BP), results in decreased NET palmitoylation within 90 min of treatment. This result was followed closely with a reduction in transport capacity, cell surface, and total cellular NET expression after 120 min of treatment. Increasing 2BP concentrations and treatment time revealed a nearly complete loss of total NET protein. Co-expression with individual DHHCs revealed a single DHHC enzyme, DHHC1, promoted WT hNET palmitoylation and elevated NET protein levels. The POTS associated NET mutant, A457P, exhibits dramatically decreased transport capacity and cell surface levels which we have confirmed in the current study. In an attempt to recover A457P NET expression we co-expressed the A457P variant with DHHC1 to drive expression as seen with the WT protein but instead saw an increase in NET N-terminal immuno-detectable fragments. Further investigation of A457P NET palmitoylation and surface expression is necessary, but our preliminary novel findings reveal palmitoylation as a mechanism of NET regulation and suggest that dysregulation of this process may contribute to the pathogenesis of POTS.

Palmitoylation Regulates Human Serotonin Transporter Activity, Trafficking, and Expression and Is Modulated by Escitalopram

In the central nervous system, serotonergic signaling modulates sleep, mood, and cognitive control. During serotonergic transmission, the synaptic concentration of serotonin is tightly controlled in a spatial and temporal manner by the serotonin transporter (SERT). Dysregulation of this process is implicated in the pathogenesis of major-depressive, obsessive-compulsive, and autism-spectrum disorders, which makes SERT a primary target for prescription therapeutics, most notably selective serotonin reuptake inhibitors (SSRIs). S-Palmitoylation, the reversible addition of a 16-carbon fatty acid to proteins, is an increasingly recognized dynamic post-translational modification responsible for modulating protein kinetics, trafficking, and localization patterns in response to physiologic/cellular stimuli. In this study, we reveal that human SERTs are a target for palmitoylation, and using the irreversible palmitoyl acyltransferase inhibitor 2-bromopalmitate (2BP), we have identified several associated functions. Using a lower dose of 2BP in shorter time frames, inhibition of palmitoylation was associated with reductions in SERT Vmax, without changes in Km or surface expression. With higher doses of 2BP for longer time intervals, inhibition of palmitoylation was consistent with the loss of cell surface and total SERT protein, suggesting palmitoylation is an important mechanism in regulating SERT trafficking and maintenance of SERT protein through biogenic or anti-degradative processes. Additionally, we have identified that treatment with the SSRI escitalopram decreases SERT palmitoylation analogous to 2BP, reducing SERT surface expression and transport capacity. Ultimately, these results reveal that palmitoylation is a major regulatory mechanism for SERT kinetics and trafficking and may be the mechanism responsible for escitalopram-induced internalization and ultimately decreased cellular SERT protein levels.

Palmitoylation regulates human serotonin transporter activity, trafficking, and expression and is modulated by escitalopram

In the central nervous system, serotonergic signaling modulates sleep, mood, and cognitive control. During neuronal transmission, the synaptic concentration of serotonin is tightly controlled in a spatial and temporal manner by the serotonin transporter (SERT). Dysregulation of serotonergic signaling is implicated in the pathogenesis of major-depressive, obsessive-compulsive, and autism-spectrum disorders, which makes SERT a primary target for prescription therapeutics, most notably selective-serotonin reuptake inhibitors (SSRIs). S-palmitoylation is an increasingly recognized dynamic post-translational modification, regulating protein kinetics, trafficking, and localization patterns upon physiologic/cellular stimuli. In this study, we reveal that human SERTs are a target for palmitoylation, and using the irreversible palmitoyl acyl-transferase inhibitor, 2-bromopalmitate (2BP) we have identified several associated functions. Using a lower dose of 2BP in shorter time frames, inhibition of palmitoylation was associated with reductions in SERT V max , without changes in K m or surface expression. With higher doses of 2BP for longer time intervals, inhibition of palmitoylation was consistent with the loss of cell surface and total SERT protein, suggesting palmitoylation is an important mechanism in regulating SERT trafficking and maintenance of SERT protein through biogenic or anti-degradative processes. Additionally, we have identified that treatment with the SSRI escitalopram decreases SERT palmitoylation analogous to 2BP, reducing SERT surface expression and transport capacity. Ultimately, these results reveal palmitoylation is a major regulatory mechanism for SERT kinetics and trafficking and may be the mechanism responsible for escitalopram-induced internalization and loss of total SERT protein.

The Impact of Endoplasmic Reticulum-Associated Protein Modifications, Folding and Degradation on Lung Structure and Function

The accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER) causes ER stress and induces the unfolded protein response (UPR) and other mechanisms to restore ER homeostasis, including translational shutdown, increased targeting of mRNAs for degradation by the IRE1-dependent decay pathway, selective translation of proteins that contribute to the protein folding capacity of the ER, and activation of the ER-associated degradation machinery. When ER stress is excessive or prolonged and these mechanisms fail to restore proteostasis, the UPR triggers the cell to undergo apoptosis. This review also examines the overlooked role of post-translational modifications and their roles in protein processing and effects on ER stress and the UPR. Finally, these effects are examined in the context of lung structure, function, and disease.

S-acylation modulates the function of the apical sodium-dependent bile acid transporter in human cells

The ileal apical sodium-dependent bile acid transporter (ASBT) is crucial for the enterohepatic circulation of bile acids. ASBT function is rapidly regulated by several posttranslational modifications. One reversible posttranslational modification is S-acylation, involving the covalent attachment of fatty acids to cysteine residues in proteins. However, whether S-acylation affects ASBT function and membrane expression has not been determined. Using the acyl resin-assisted capture method, we found that the majority of ASBT (∼80%) was S-acylated in ileal brush border membrane vesicles from human organ donors, as well as in HEK293 cells stably transfected with ASBT (2BT cells). Metabolic labeling with alkyne-palmitic acid (100 μm for 15 h) also showed that ASBT is S-acylated in 2BT cells. Incubation with the acyltransferase inhibitor 2-bromopalmitate (25 μm for 15 h) significantly reduced ASBT S-acylation, function, and levels on the plasma membrane. Treatment of 2BT cells with saturated palmitic acid (100 μm for 15 h) increased ASBT function, whereas treatment with unsaturated oleic acid significantly reduced ASBT function. Metabolic labeling with alkyne-oleic acid (100 μm for 15 h) revealed that oleic acid attaches to ASBT, suggesting that unsaturated fatty acids may decrease ASBT's function via a direct covalent interaction with ASBT. We also identified Cys-314 as a potential S-acylation site. In conclusion, these results provide evidence that S-acylation is involved in the modulation of ASBT function. These findings underscore the potential for unsaturated fatty acids to reduce ASBT function, which may be useful in disorders in which bile acid toxicity is implicated.

Trafficking and function of the cystic fibrosis transmembrane conductance regulator: a complex network of posttranslational modifications

Posttranslational modifications add diversity to protein function. Throughout its life cycle, the cystic fibrosis transmembrane conductance regulator (CFTR) undergoes numerous covalent posttranslational modifications (PTMs), including glycosylation, ubiquitination, sumoylation, phosphorylation, and palmitoylation. These modifications regulate key steps during protein biogenesis, such as protein folding, trafficking, stability, function, and association with protein partners and therefore may serve as targets for therapeutic manipulation. More generally, an improved understanding of molecular mechanisms that underlie CFTR PTMs may suggest novel treatment strategies for CF and perhaps other protein conformational diseases. This review provides a comprehensive summary of co- and posttranslational CFTR modifications and their significance with regard to protein biogenesis.

Co- and Post-Translational Protein Folding in the ER

The biophysical rules that govern folding of small, single-domain proteins in dilute solutions are now quite well understood. The mechanisms underlying co-translational folding of multidomain and membrane-spanning proteins in complex cellular environments are often less clear. The endoplasmic reticulum (ER) produces a plethora of membrane and secretory proteins, which must fold and assemble correctly before ER exit - if these processes fail, misfolded species accumulate in the ER or are degraded. The ER differs from other cellular organelles in terms of the physicochemical environment and the variety of ER-specific protein modifications. Here, we review chaperone-assisted co- and post-translational folding and assembly in the ER and underline the influence of protein modifications on these processes. We emphasize how method development has helped advance the field by allowing researchers to monitor the progression of folding as it occurs inside living cells, while at the same time probing the intricate relationship between protein modifications during folding.

The physiology of protein S-acylation

Protein S-acylation, the only fully reversible posttranslational lipid modification of proteins, is emerging as a ubiquitous mechanism to control the properties and function of a diverse array of proteins and consequently physiological processes. S-acylation results from the enzymatic addition of long-chain lipids, most typically palmitate, onto intracellular cysteine residues of soluble and transmembrane proteins via a labile thioester linkage. Addition of lipid results in increases in protein hydrophobicity that can impact on protein structure, assembly, maturation, trafficking, and function. The recent explosion in global S-acylation (palmitoyl) proteomic profiling as a result of improved biochemical tools to assay S-acylation, in conjunction with the recent identification of enzymes that control protein S-acylation and de-acylation, has opened a new vista into the physiological function of S-acylation. This review introduces key features of S-acylation and tools to interrogate this process, and highlights the eclectic array of proteins regulated including membrane receptors, ion channels and transporters, enzymes and kinases, signaling adapters and chaperones, cell adhesion, and structural proteins. We highlight recent findings correlating disruption of S-acylation to pathophysiology and disease and discuss some of the major challenges and opportunities in this rapidly expanding field.