Acetylation stimulates the epithelial sodium channel by reducing its ubiquitination and degradation

The cardioprotective role of sirtuins is mediated in part by regulating KATP channel surface expression

Sirtuins are NAD+-dependent deacetylases with beneficial roles in conditions relevant to human health, including metabolic disease, type II diabetes, obesity, cancer, aging, neurodegenerative diseases, and cardiac ischemia. Since ATP-sensitive K+ (KATP) channels have cardioprotective roles, we investigated whether they are regulated by sirtuins. Nicotinamide mononucleotide (NMN) was used to increase cytosolic NAD+ levels and to activate sirtuins in cell lines, isolated rat and mouse cardiomyocytes or insulin-secreting INS-1 cells. KATP channels were studied with patch clamping, biochemistry techniques, and antibody uptake experiments. NMN led to an increase in intracellular NAD+ levels and an increase in the KATP channel current, without significant changes in the unitary current amplitude or open probability. An increased surface expression was confirmed using surface biotinylation approaches. The rate of KATP channel internalization was diminished by NMN, which may be a partial explanation for the increased surface expression. We show that NMN acts via sirtuins since the increased KATP channel surface expression was prevented by blockers of SIRT1 and SIRT2 (Ex527 and AGK2) and mimicked by SIRT1 activation (SRT1720). The pathophysiological relevance of this finding was studied using a cardioprotection assay with isolated ventricular myocytes, in which NMN protected against simulated ischemia or hypoxia in a KATP channel-dependent manner. Overall, our data draw a link between intracellular NAD+, sirtuin activation, KATP channel surface expression, and cardiac protection against ischemic damage.

The Epithelial Sodium Channel-An Underestimated Drug Target

Epithelial sodium channels (ENaC) are part of a complex network of interacting biochemical pathways and as such are involved in several disease states. Dependent on site and type of mutation, gain- or loss-of-function generated symptoms occur which span from asymptomatic to life-threatening disorders such as Liddle syndrome, cystic fibrosis or generalized pseudohypoaldosteronism type 1. Variants of ENaC which are implicated in disease assist further understanding of their molecular mechanisms in order to create models for specific pharmacological targeting. Identification and characterization of ENaC modifiers not only furthers our basic understanding of how these regulatory processes interact, but also enables discovery of new therapeutic targets for the disease conditions caused by ENaC dysfunction. Numerous test compounds have revealed encouraging results in vitro and in animal models but less in clinical settings. The EMA- and FDA-designated orphan drug solnatide is currently being tested in phase 2 clinical trials in the setting of acute respiratory distress syndrome, and the NOX1/ NOX4 inhibitor setanaxib is undergoing clinical phase 2 and 3 trials for therapy of primary biliary cholangitis, liver stiffness, and carcinoma. The established ENaC blocker amiloride is mainly used as an add-on drug in the therapy of resistant hypertension and is being studied in ongoing clinical phase 3 and 4 trials for special applications. This review focuses on discussing some recent developments in the search for novel therapeutic agents.

Histone deacetylase inhibitors (HDACi) increase expression of KCa2.3 (SK3) in primary microvascular endothelial cells

The small conductance calcium-activated potassium channel (KCa2.3) has long been recognized for its role in mediating vasorelaxation through the endothelium-derived hyperpolarization (EDH) response. Histone deacetylases (HDACs) have been implicated as potential modulators of blood pressure and histone deacetylase inhibitors (HDACi) are being explored as therapeutics for hypertension. Herein, we show that HDACi increase KCa2.3 expression when heterologously expressed in HEK cells and endogenously expressed in primary cultures of human umbilical vein endothelial cells (HUVECs) and human intestinal microvascular endothelial cells (HIMECs). When primary endothelial cells were exposed to HDACi, KCa2.3 transcripts, subunits, and functional current are increased. Quantitative RT-PCR (qPCR) demonstrated increased KCa2.3 mRNA following HDACi, confirming transcriptional regulation of KCa2.3 by HDACs. By using pharmacological agents selective for different classes of HDACs, we discriminated between cytoplasmic and epigenetic modulation of KCa2.3. Biochemical analysis revealed an association between the cytoplasmic HDAC6 and KCa2.3 in immunoprecipitation studies. Specifically inhibiting HDAC6 increases expression of KCa2.3. In addition to increasing the expression of KCa2.3, we show that nonspecific inhibition of HDACs causes an increase in the expression of the molecular chaperone Hsp70 in endothelial cells. When Hsp70 is inhibited in the presence of HDACi, the magnitude of the increase in KCa2.3 expression is diminished. Finally, we show a slower rate of endocytosis of KCa2.3 as a result of exposure of primary endothelial cells to HDACi. These data provide the first demonstrated approach to increase KCa2.3 channel number in endothelial cells and may partially account for the mechanism by which HDACi induce vasorelaxation.

Paraoxonase 2 is an ER chaperone that regulates the epithelial Na+ channel

The mammalian paraoxonases (PONs) have been linked to protection against oxidative stress. However, the physiological roles of members in this family (PON1, PON2, and PON3) are still being characterized. PON2 and PON3 are expressed in the aldosterone-sensitive distal nephron of the kidney and have been shown to negatively regulate expression of the epithelial sodium channel (ENaC), a trimeric ion channel that orchestrates salt and water homeostasis. To date, the nature of this phenomenon has not been explored. Therefore, to investigate the mechanism by which PON2 regulates ENaC, we expressed PON2 along with the ENaC subunits in fisher rat thyroid (FRT) cells, a system that is amenable to biochemical analyses of ENaC assembly and trafficking. We found that PON2 primarily resides in the endoplasmic reticulum (ER) in FRT cells, and its expression reduces the abundance of each ENaC subunit, reflecting enhanced subunit turnover. In contrast, no effect on the levels of mRNAs encoding the ENaC subunits was evident. Inhibition of lysosome function with chloroquine or NH4Cl did not alter the inhibitory effect of PON2 on ENaC expression. In contrast, PON2 accelerates ENaC degradation in a proteasome-dependent manner and acts before ENaC subunit ubiquitination. As a result of enhanced ENaC subunit ubiquitination and degradation, both channel surface expression and ENaC-mediated Na+ transport in FRT cells were reduced by PON2. Together, our data suggest that PON2 functions as an ER chaperone to monitor ENaC biogenesis and redirects the channel for ER-associated degradation.

Deacetylation as a receptor-regulated direct activation switch for pannexin channels

Activation of Pannexin 1 (PANX1) ion channels causes release of intercellular signaling molecules in a variety of (patho)physiological contexts. PANX1 can be activated by G protein-coupled receptors (GPCRs), including α1-adrenergic receptors (α1-ARs), but how receptor engagement leads to channel opening remains unclear. Here, we show that GPCR-mediated PANX1 activation can occur via channel deacetylation. We find that α1-AR-mediated activation of PANX1 channels requires Gαq but is independent of phospholipase C or intracellular calcium. Instead, α1-AR-mediated PANX1 activation involves RhoA, mammalian diaphanous (mDia)-related formin, and a cytosolic lysine deacetylase activated by mDia - histone deacetylase 6. HDAC6 associates with PANX1 and activates PANX1 channels, even in excised membrane patches, suggesting direct deacetylation of PANX1. Substitution of basally-acetylated intracellular lysine residues identified on PANX1 by mass spectrometry either prevents HDAC6-mediated activation (K140/409Q) or renders the channels constitutively active (K140R). These data define a non-canonical RhoA-mDia-HDAC6 signaling pathway for GαqPCR activation of PANX1 channels and uncover lysine acetylation-deacetylation as an ion channel silencing-activation mechanism.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Histone Deacetylases in Kidney Physiology and Acute Kidney Injury

Histone deacetylases (HDACs) are part of the epigenetic machinery that regulates transcriptional processes. The current paradigm is that HDACs silence gene expression via regulation of histone protein lysine deacetylation, or by forming corepressor complexes with transcription factors. However, HDACs are more than just nuclear proteins, and they can interact and deacetylate a growing number of nonhistone proteins to regulate cellular function. Cancer-field studies have shown that deranged HDAC activity results in uncontrolled proliferation, inflammation, and fibrosis; all pathologies that also may occur in kidney disease. Over the past decade, studies have emerged suggesting that HDAC inhibitors may prevent and potentially treat various models of acute kidney injury. This review focuses on the physiology of kidney HDACs and highlights the recent advances using HDAC inhibitors to potentially treat kidney disease patients.

High salt intake induces collecting duct HDAC1-dependent NO signaling

We reported that high salt (HS) intake stimulates renal collecting duct (CD) endothelin (ET) type B receptor (ETBR)/nitric oxide (NO) synthase 1β (NOS1β)-dependent NO production inhibiting the epithelial sodium channel (ENaC) promoting natriuresis. However, the mechanism underlying the HS-induced increase of NO production is unclear. Histone deacetylase 1 (HDAC1) responds to increased fluid flow, as can occur in the CD during HS intake. The renal inner medulla (IM), in particular the IMCD, has the highest NOS1 activity within the kidney. Hence, we hypothesized that HS intake provokes HDAC1 activation of NO production in the IM. HS intake for 1 wk significantly increased HDAC1 abundance in the IM. Ex vivo treatment of dissociated IM from HS-fed mice with a selective HDAC1 inhibitor (MS-275) decreased NO production with no change in ET-1 peptide or mRNA levels. We further investigated the role of the ET-1/ETBR/NOS1β signaling pathway with chronic ETBR blockade (A-192621). Although NO was decreased and ET-1 levels were elevated in the dissociated IM from HS-fed mice treated with A-192621, ex vivo MS-275 did not further change NO or ET-1 levels suggesting that HDAC1-mediated NO production is regulated at the level or downstream of ETBR activation. In split-open CDs from HS-fed mice, patch clamp analysis revealed significantly higher ENaC activity after MS-275 pretreatment, which was abrogated by an exogenous NO donor. Moreover, flow-induced increases in mIMCD-3 cell NO production were blunted by HDAC1 or calcium inhibition. Taken together, these findings indicate that HS intake induces HDAC1-dependent activation of the ETBR/NO pathway contributing to the natriuretic response.

Post-Translational Modification and Natural Mutation of TRPC Channels

Transient Receptor Potential Canonical (TRPC) channels are homologues of Drosophila TRP channel first cloned in mammalian cells. TRPC family consists of seven members which are nonselective cation channels with a high Ca²⁺ permeability and are activated by a wide spectrum of stimuli. These channels are ubiquitously expressed in different tissues and organs in mammals and exert a variety of physiological functions. Post-translational modifications (PTMs) including phosphorylation, N-glycosylation, disulfide bond formation, ubiquitination, S-nitrosylation, S-glutathionylation, and acetylation play important roles in the modulation of channel gating, subcellular trafficking, protein-protein interaction, recycling, and protein architecture. PTMs also contribute to the polymodal activation of TRPCs and their subtle regulation in diverse physiological contexts and in pathological situations. Owing to their roles in the motor coordination and regulation of kidney podocyte structure, mutations of TRPCs have been implicated in diseases like cerebellar ataxia (moonwalker mice) and focal and segmental glomerulosclerosis (FSGS). The aim of this review is to comprehensively integrate all reported PTMs of TRPCs, to discuss their physiological/pathophysiological roles if available, and to summarize diseases linked to the natural mutations of TRPCs.

CBP mediated DOT1L acetylation confers DOT1L stability and promotes cancer metastasis

Background and Aim: DOT1L regulates various genes involved in cancer onset and progression by catalyzing H3K79 methylation, but how DOT1L activity itself is regulated is unclear. Here, we aimed to identify specific DOT1L post-translational modifications that might regulate DOT1L activity and thus impact on colorectal cancer (CRC) progression. Methods: We conducted affinity purification and mass spectrometry to explore DOT1L post-translational modifications. We then established transwell migration and invasion assays to specifically investigate the role of DOT1L(K358) acetylation on CRC cellular behavior in vitro and a bioluminescence imaging approach to determine the role of DOT1L(K358) acetylation in CRC metastasis in vivo. We performed chromatin immunoprecipitation to identify DOT1L acetylation-controlled target genes. Finally, we used immunohistochemical staining of human tissue arrays to examine the relevance of DOT1L(K358) acetylation in CRC progression and metastasis and the correlation between DOT1L acetylation and CBP. Results: We found that CBP mediates DOT1L K358 acetylation in human colon cancer cells and positively correlates with CRC stages. Mechanistically, DOT1L acetylation confers DOT1L stability by preventing the binding of RNF8 to DOT1L and subsequent proteasomal degradation, but does not affect its enzyme activity. Once stabilized, DOT1L can catalyze the H3K79 methylation of genes involved in epithelial-mesenchymal transition, including SNAIL and ZEB1. An acetylation mimic DOT1L mutant (Q358) could induce a cancer-like phenotype in vitro, characterized by metastasis and invasion. Finally, DOT1L(K358) acetylation correlated with CRC progression and a poor survival rate as well as with high CBP expression. Conclusions: DOT1L acetylation by CBP drives CRC progression and metastasis. Targeting DOT1L deacetylation signaling is a potential therapeutic strategy for DOT1L-driven cancers.

Membrane trafficking pathways regulating the epithelial Na+ channel

Renal Na⁺ reabsorption, facilitated by the epithelial Na⁺ channel (ENaC), is subject to multiple forms of control to ensure optimal body blood volume and pressure through altering both the ENaC population and activity at the cell surface. Here, the focus is on regulating the number of ENaCs present in the apical membrane domain through pathways of ENaC synthesis and targeting to the apical membrane as well as ENaC removal, recycling, and degradation. Finally, the mechanisms by which ENaC trafficking pathways are regulated are summarized.

Trichostatin A blocks aldosterone-induced Na+ transport and control of serum- and glucocorticoid-inducible kinase 1 in cortical collecting duct cells

BACKGROUND AND PURPOSE

Aldosterone stimulates epithelial Na⁺ channel (ENaC)-dependent Na⁺ retention in the cortical collecting duct (CCD) of the kidney by activating mineralocorticoid receptors that promote expression of serum and glucocorticoid-inducible kinase 1 (SGK1). This response is critical to BP homeostasis. It has previously been suggested that inhibiting lysine deacetylases (KDACs) can post-transcriptionally disrupt this response by promoting acetylation of the mineralocorticoid receptor. The present study critically evaluates this hypothesis.

EXPERIMENTAL APPROACH

Electrometric and molecular methods were used to define the effects of a pan-KDAC inhibitor, trichostatin A, on the responses to a physiologically relevant concentration of aldosterone (3 nM) in murine mCCD_cl1 cells.

KEY RESULTS

Aldosterone augmented ENaC-induced Na⁺ absorption and increased SGK1 activity and abundance, as expected. In the presence of trichostatin A, these responses were suppressed. Trichostatin A-induced inhibition of KDAC was confirmed by increased acetylation of histone H3, H4, and α-tubulin. Trichostatin A did not block the electrometric response to insulin, a hormone that activates SGK1 independently of increased transcription, indicating that trichostatin A has no direct effect upon the SGK1/ENaC pathway.

CONCLUSIONS AND IMPLICATIONS

Inhibition of lysine de-acetylation suppresses aldosterone-dependent control over the SGK1-ENaC pathway but does not perturb post-transcriptional signalling, providing a physiological basis for the anti-hypertensive action of KDAC inhibition seen in vivo.

Dynamic changes in histone deacetylases following kidney ischemia-reperfusion injury are critical for promoting proximal tubule proliferation

Deranged histone deacetylase (HDAC) activity causes uncontrolled proliferation, inflammation, fibrosis, and organ damage. It is unclear whether deranged HDAC activity results in acute kidney injury in the renal hypoperfusion model of bilateral ischemia-reperfusion injury (IRI) and whether in vivo inhibition is an appropriate therapeutic approach to limit injury. Male mice were implanted with intraperitoneal osmotic minipumps containing vehicle, the class I HDAC inhibitor, MS275, or the pan-HDAC inhibitor, trichostatin A (TSA), 3 days before sham/bilateral IRI surgery. Kidney cortical samples were analyzed using histological, immunohistochemical, and Western blotting techniques. HDAC-dependent proliferation rate was measured in immortalized rat epithelial cells and primary mouse or human proximal tubule (PT) cells. There were dynamic changes in cortical HDAC localization and abundance following IRI including a fourfold increase in HDAC4 in the PT. HDAC inhibition resulted in a significantly higher plasma creatinine, increased kidney damage, but reduced interstitial fibrosis compared with vehicle-treated IRI mice. HDAC-inhibited mice had reduced interstitial α-smooth muscle actin, fibronectin expression, and Sirius red-positive area, suggesting that IRI activates HDAC-mediated fibrotic pathways. In vivo proliferation of the kidney epithelium was significantly reduced in TSA-treated, but not MS275-treated, IRI mice, suggesting class II HDACs mediate proliferation. Furthermore, HDAC4 activation increased proliferation of human and mouse PTs. Kidney HDACs are activated during IRI with isoform-specific expression patterns. Our data point to mechanisms whereby IRI activates HDACs resulting in fibrotic pathways but also activation of PT proliferation and repair pathways. This study demonstrates the need to develop isoform-selective HDAC inhibitors for the treatment of renal hypoperfusion-induced injury.

Histone deacetylase inhibitor LMK235 attenuates vascular constriction and aortic remodelling in hypertension

Here, we report that LMK235, a class I and histone deacetylase (HDAC6)‐preferential HDAC inhibitor, reduces hypertension via inhibition of vascular contraction and vessel hypertrophy. Angiotensin II‐infusion mice and spontaneously hypertensive rats (SHRs) were used to test the anti‐hypertensive effect of LMK235. Daily injection of LMK235 lowered angiotensin II‐induced systolic blood pressure (BP). A reduction in systolic BP in SHRs was observed on the second day when SHRs were treated with 3 mg/kg LMK235 every 3 days. However, LMK235 treatment did not affect angiotensin‐converting enzyme 1 and angiotensin II receptor mRNA expression in either hypertensive model. LMK235, acting via the nitric oxide pathway, facilitated the relaxing of vascular contractions induced by a thromboxane A2 agonist in the rat aortic and mesenteric artery ring test. In addition, LMK235 increased nitric oxide production in HUVECs and inhibited the increasing of aortic wall thickness in both animal hypertensive models. LMK235 decreased the enhanced cell cycle‐related genes cyclin D1 and E2F3 in angiotensin II‐infusion mice and restored the decreased p21 expression. In addition, LMK235 suppressed calcium calmodulin‐dependent protein kinase II (CaMKII) α, which is related to vascular smooth muscle cell proliferation. Inhibition or knockdown of HDAC5 blocked the CaMKIIα‐induced cell cycle gene expression. Immunoprecipitation demonstrated that class I HDACs were involved in the inhibition of CaMKII α‐induced HDAC4/5 by LMK235. We suggest that LMK235 should be further investigated for its use in the development of new therapeutic options to treat hypertension via reducing vascular hyperplasia or vasoconstriction.

BK channel deacetylation by SIRT1 in dentate gyrus regulates anxiety and response to stress

Previous genomic studies in humans indicate that SIRT1, a nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylase, is involved in anxiety and depression, but the mechanisms are unclear. We previously showed that SIRT1 is highly activated in the nuclear fraction of the dentate gyrus of the chronically stressed animals and inhibits memory formation and increases anhedonic behavior during chronic stress, but specific functional targets of cytoplasmic SIRT1 are unknown. Here, we demonstrate that SIRT1 activity rapidly modulates intrinsic and synaptic properties of the dentate gyrus granule cells and anxiety behaviors through deacetylation of BK channel α subunits in control animals. Chronic stress decreases BKα channel membrane expression, and SIRT1 activity has no rapid effects on synaptic transmission or intrinsic properties in the chronically stressed animal. These results suggest SIRT1 activity rapidly modulates the physiological function of the dentate gyrus, and this modulation participates in the maladaptive stress response.

Diankun Yu et al. show that deacetylase SIRT1 rapidly modulates synaptic properties of the dentate gyrus granule cells and anxiety behaviors through deacetylation of BK channel α subunits. This study provides mechanistic insight into how SIRT1 regulates fight-or-flight stress response.

Gene expression and promoter characterization of heat-shock protein 90B gene (HSP90B) in the model unicellular green alga Chlamydomonas reinhardtii

Molecular chaperones or heat shock proteins are a large protein family with important functions in every cellular organism. Among all types of the heat shock proteins, information on the ER-localized HSP90 protein (HSP90B) and its encoding gene is relatively scarce in the literature, especially in photosynthetic organisms. In this study, expression profiles as well as promoter sequence of the HSP90B gene were investigated in the model green alga Chlamydomonas reinhardtii. We have found that HSP90B is strongly induced by heat and ER stresses, while other short-term exposure to abiotic stresses, such as salinity, dark-to-light transition or light stress does not appear to affect the expression. Promoter truncation analysis as well as chromatin immunoprecipitation using the antibodies recognizing histone H3 and acetylated histone H3, revealed a putative core constitutive promoter sequence between -1 to -253 bp from the transcription start site. Our results also suggested that the nucleotides upstream of the core promoter may contain repressive elements such as putative repressor binding site(s).

Utilizing Optimized Tools to Investigate PTM Crosstalk: Identifying Potential PTM Crosstalk of Acetylated Mitochondrial Proteins

Post-translational modification (PTM) crosstalk is recognized as a major cell-regulatory mechanism, and studies of several proteins have validated the premise that PTMs work in concert. Previous work by our group investigated the potential PTM crosstalk on proteins in the EGFR-Ras-c-Fos axis by utilizing a comprehensive set of PTM reagents termed Signal-Seeker toolkits. In this study, these tools were used to investigate the potential PTM crosstalk that occurs in acetylated mitochondrial proteins in response to a mitochondrial stress-inducing agent hydrogen peroxide (H₂O₂). Mitochondrial protein acetylation has been shown to participate in PTM crosstalk as exemplified by the regulation of the pyruvate dehydrogenase complex via kinase, phosphatase, acetyltransferase, and deacetylase activities. Changes in the acetylated state of mitochondrial proteins were investigated, in response to H₂O₂, using a novel anti acetyl lysine (Ac-K) antibody. Signal-Seeker PTM detection tools were used to validate the acetylation state of ten mitochondrial targets, as well as their endogenous acetylation state in response to H₂O₂. Importantly, the endogenous acetylation, ubiquitination, SUMOylation 2/3, and tyrosine phosphorylation state of four target mitochondrial proteins were also investigated with the toolkit. Each of the four proteins had unique PTM profiles, but diverging acetylation and ubiquitin or SUMO 2/3 signals appeared to be a common theme. This proof-of-concept study identifies the Signal-Seeker toolkits as a useful tool to investigate potential PTM crosstalk.

MDM2 Degrades Deacetylated Nucleolin Through Ubiquitination to Promote Glioma Stem-Like Cell Enrichment for Chemotherapeutic Resistance

Glioblastoma multiforme (GBM) is the most fatal of all brain cancers, and the standard care protocol for GBM patients is surgical tumor resection followed by radiotherapy and temozolomide (TMZ)-mediated chemotherapy. However, tumor recurrence frequently occurs, and recurrent GBM exhibits more malignancy and less sensitivity in response to chemotherapy. The malignancy and drug resistance primarily reflect the small population of glioma stem-like cells (GSC). Therefore, understanding the mechanism that controls GSC enrichment is important to benefit the prognosis of GBM patients. Nucleolin (NCL), which is responsible for ribosome biogenesis and RNA maturation, is overexpressed in gliomas. However, the role of NCL in GSC development and drug resistance is still unclear. In this study, we demonstrate that NCL attenuated GSC enrichment to enhance the sensitivity of GBM cells in response to TMZ. In GSC enrichment, NCL was significantly reduced at the protein level as a result of decreased protein stability. In particular, the inhibition of HDAC activity by suberoylanilide hydroxamic acid rescued NCL acetylation accompanied by the loss of mouse double minute 2 homolog (MDM2)-mediated ubiquitination. In addition, we found that NCL ubiquitination resulted from the activation of STAT3- and JNK-mediated signaling in GSC. Moreover, NCL inhibited the formation of stem-like spheres by attenuating the expression of Sox2, Oct4, and Bmi1. Furthermore, NCL sensitized the response of GBM cells to TMZ. Based on these findings, NCL expression is a potential indicator to predict chemotherapeutic efficiency in GBM patients.

Restoration of mutant hERG stability by inhibition of HDAC6

The human ether-a-go-go-related gene (hERG) encodes the α subunit of a rapidly activating delayed-rectifier potassium (I_Kr) channel. Mutations of the hERG cause long QT syndrome type 2 (LQT2). Acetylation of lysine residues occurs in a subset of non-histone proteins and this modification is controlled by both histone acetyltransferases and deacetylases (HDACs). The aim of this study was to clarify effects of HDAC(s) on wild-type (WT) and mutant hERG proteins. WThERG and two trafficking-defective mutants (G601S and R752W) were transiently expressed in HEK293 cells, which were treated with a pan-HDAC inhibitor Trichostatin A (TSA) or an isoform-selective HDAC6 inhibitor Tubastatin A (TBA). Both TSA and TBA increased protein levels of WThERG and induced expression of mature forms of the two mutants. Immunoprecipitation showed an interaction between HDAC6 and immature forms of hERG. Coexpression of HDAC6 decreased acetylation and, reciprocally, increased ubiquitination of hERG, resulting in its decreased expression. siRNA against HDAC6, as well as TBA, exerted opposite effects. Immunochemistry revealed that HDAC6 knockdown increased expression of the WThERG and two mutants both in the endoplasmic reticulum and on the cell surface. Electrophysiology showed that HDAC6 knockdown or TBA treatment increased the hERG channel current corresponding to the rapidly activating delayed-rectifier potassium current (I_Kr) in HEK293 cells stably expressing the WT or mutants. Three lysine residues (K116, K495 and K757) of hERG were predicted to be acetylated. Substitution of these lysine residues with arginine eliminated HDAC6 effects. In HL-1 mouse cardiomyocytes, TBA enhanced endogenous ERG expression, increased I_Kr, and shortened action potential duration. These results indicate that hERG is a substrate of HDAC6. HDAC6 inhibition induced acetylation of hERG which counteracted ubiquitination leading its stabilization. HDAC6 inhibition may be a novel therapeutic option for LQT2.

Utilizing a Comprehensive Immunoprecipitation Enrichment System to Identify an Endogenous Post-translational Modification Profile for Target Proteins

It is now well-appreciated that post-translational modifications (PTMs) play an integral role in regulating a protein's structure and function, which may be essential for a given protein's role both physiologically and pathologically. Enrichment of PTMs is often necessary when investigating the PTM status of a target protein, because PTMs are often transient and relatively low in abundance. Many pitfalls are encountered when enriching for a PTM of a target protein, such as buffer incompatibility, the target protein antibody is not IP-compatible, loss of PTM signal, and others. The degree of difficulty is magnified when investigating multiple PTMs like acetylation, ubiquitination, SUMOylation 2/3, and tyrosine phosphorylation for a given target protein. Studying a combination of these PTMs may be necessary, as crosstalk between PTMs is prevalent and critical for protein regulation. Often, these PTMs are studied in different lysis buffers and with unique inhibitor compositions. To simplify the process, a unique denaturing lysis system was developed that effectively isolates and preserves these four PTMs; thus, enabling investigation of potential crosstalk in a single lysis system. A unique filter system was engineered to remove contaminating genomic DNA from the lysate, which is a problematic by-product of denaturing buffers. Robust affinity matrices targeting each of the four PTMs were developed in concert with the buffer system to maximize the enrichment and detection of the endogenous states of these four PTMs. This comprehensive PTM detection toolset streamlines the process of obtaining critical information about whether a protein is modified by one or more of these PTMs.

Proteome-wide lysine acetylation identification in developing rice (Oryza sativa) seeds and protein co-modification by acetylation, succinylation, ubiquitination, and phosphorylation

Protein lysine acetylation is a highly conserved post-translational modification with various biological functions. However, only a limited number of acetylation sites have been reported in plants, especially in cereals, and the function of non-histone protein acetylation is still largely unknown. In this report, we identified 1003 lysine acetylation sites in 692 proteins of developing rice seeds, which greatly extended the number of known acetylated sites in plants. Seven distinguished motifs were detected flanking acetylated lysines. Functional annotation analyses indicated diverse biological processes and pathways engaged in lysine acetylation. Remarkably, we found that several key enzymes in storage starch synthesis pathway and the main storage proteins were heavily acetylated. A comprehensive comparison of the rice acetylome, succinylome, ubiquitome and phosphorylome with available published data was conducted. A large number of proteins carrying multiple kinds of modifications were identified and many of these proteins are known to be key enzymes of vital metabolic pathways. Our study provides extending knowledge of protein acetylation. It will have critical reference value for understanding the mechanisms underlying PTM mediated multiple signal integration in the regulation of metabolism and development in plants.

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.