SNX17 (Sorting Nexin 17) Mediates Atrial Fibrillation Onset Through Endocytic Trafficking of the Kv1.5 (Potassium Voltage-Gated Channel Subfamily A Member 5) Channel

A multi-trait GWAS identifies novel genes influencing albuminuria

BACKGROUND

Albuminuria is common and is associated with increased risks of end-stage kidney disease and cardiovascular diseases, yet its underlying mechanism remains obscure. Previous genome-wide association studies (GWAS) for albuminuria did not consider gene pleiotropy and primarily focused on European ancestry populations. This study adopted a multi-trait analysis of GWAS (MTAG) approach to jointly analyze two vital kidney traits, estimated glomerular filtration rate (eGFR) and urine albumin-to-creatinine ratio (UACR) to identify and prioritize the genes associated with UACR.

METHODS

Data from the Taiwan Biobank from 2012 to 2023 were analyzed. GWAS of UACR and eGFR were performed separately and the summary statistics from these GWAS were jointly analyzed using MTAG. The polygenic risk scores (PRS) of UACR were constructed for validation. The UACR-associated loci were further fine-mapped and prioritized based on their deleteriousness, eQTL associations and relatedness to Mendelian kidney diseases.

RESULTS

MTAG analysis of the UACR revealed 15 genetic loci, including 12 novel loci. The PRS for UACR was significantly associated with urinary albumin level (P < .001) and microalbuminuria (P = .001-.045). A list of priority genes was generated. Twelve genes with high priority included the albumin endocytic receptor gene LRP2 and ciliary gene IFT172.

CONCLUSIONS

The findings of this multi-trait GWAS suggest that primary cilia play a role in sensing mechanical stimuli, leading to albumin endocytosis. The priority list of genes warrants further translational investigation to reduce albuminuria.

ER stress modulates Kv1.5 channels via PERK branch in HL-1 atrial myocytes: Relevance to atrial arrhythmogenesis and the effect of tetramethylpyrazine

Endoplasmic reticulum (ER) stress is implicated in cardiac arrhythmia whereas the associated mechanisms remain inadequately understood. Kv1.5 channels are essential for atrial repolarization. Whether ER stress affects Kv1.5 channels is unknown. This study aimed to elucidate the response of Kv1.5 channels to ER stress by clarifying the unfolded protein response (UPR) branch responsible for the channel modulation. In addition, the effect of tetramethylpyrazine (TMP) on Kv1.5 channels was studied. Patch clamp and western-blot results revealed that exposure of HL-1 atrial myocytes to ER stress inducer tunicamycin upregulates Kv1.5 expression, increases Kv1.5 channel current (I Kur ) (14.91 ± 1.11 vs. 6.11 ± 1.31 pA/pF, P < 0.001), and shortened action potential duration (APD) (APD90: 82.79 ± 5.25 vs.121.11 ± 6.72 ms, P < 0.01), which could be reverted by ER stress inhibitors. Blockade of the PERK branch while not IRE1 and ATF6 branches of UPR downregulated Kv1.5 expression, accompanied by a decreased I Kur (9.03 ± 0.99 pA/pF) and a prolonged APD90 (113.69 ± 4.41 ms) (P < 0.01). PERK-mediated increases of Kv1.5 expression and I Kur were also observed in HL-1 cells incubated with thapsigargin. TMP suppressed the enhancement of I Kur (10.52 ± 0.97 vs. 17.52 ± 2.25 pA/pF, P < 0.05), prevented the shortening of APD (APD90: 110.16 ± 5.36 vs. 84.84 ± 4.58 ms, P < 0.05), and inhibited the upregulation of Kv1.5 triggered by ER stress. Our study suggests that ER stress induces upregulation and activation of Kv1.5 channels in atrial myocytes through the PERK branch of UPR. TMP prevents Kv1.5 upregulation/activation and the resultant APD shortening by inhibiting ER stress. These results may shed light on the mechanisms of atrial arrhythmogenesis and the antiarrhythmic effect of the traditional Chinese herb TMP.

SNX17 Mediates Dendritic Spine Maturation via p140Cap

Sorting nexin17 (SNX17) is a member of the sorting nexin family, which plays a crucial role in endosomal trafficking. Previous research has shown that SNX17 is involved in the recycling or degradation of various proteins associated with neurodevelopmental and neurological diseases in cell models. However, the significance of SNX17 in neurological function in the mouse brain has not been thoroughly investigated. In this study, we generated Snx17 knockout mice and observed that the homozygous deletion of Snx17 (Snx17-/-) resulted in lethality. On the other hand, heterozygous mutant mice (Snx17+/-) exhibited anxiety-like behavior with a reduced preference for social novelty. Furthermore, Snx17 haploinsufficiency led to impaired synaptic transmission and reduced maturation of dendritic spines. Through GST pulldown and interactome analysis, we identified the SRC kinase inhibitor, p140Cap, as a potential downstream target of SNX17. We also demonstrated that the interaction between p140Cap and SNX17 is crucial for dendritic spine maturation. Together, this study provides the first in vivo evidence highlighting the important role of SNX17 in maintaining neuronal function, as well as regulating social novelty and anxiety-like behaviors.

Emerging Role of Sorting Nexin 17 in Human Health and Disease

The distortion of the cellular membrane transport pathway has a profound impact on cell dynamics and can drive serious physiological consequences during the process of cell sorting. SNX17 is a member of the Sorting Nexin (SNX) family and plays a crucial role in protein sorting and transport in the endocytic pathway. SNX17, SNX27, and SNX31 belong to the SNX-FERM subfamily and possess the FERM domain, which can assist in endocytic transport and lysosomal degradation. The binding partners of SNX27 have been discovered to number over 100, and SNX27 has been linked to the development of Alzheimer's disease progression, tumorigenesis, cancer progression, and metastasis. However, the role and potential mechanisms of SNX17 in human health and disease remain poorly understood, and the function of SNX17 has not been fully elucidated. In this review, we summarize the structure and basic functions of SNX protein, focusing on providing current evidence of the role and possible mechanism of SNX17 in human neurodegenerative diseases and cardiovascular diseases.

Biochemical Structure and Function of TRAPP Complexes in the Cardiac System

Trafficking protein particle (TRAPP) is well reported to play a role in the trafficking of protein products within the Golgi and endoplasmic reticulum. Dysfunction in TRAPP has been associated with disorders in the nervous and cardiovascular systems, but the majority of literature focuses on TRAPP function in the nervous system solely. Here, we highlight the known pathways of TRAPP and hypothesize potential impacts of TRAPP dysfunction on the cardiovascular system, particularly the role of TRAPP as a guanine-nucleotide exchange factor for Rab1 and Rab11. We also review the various cardiovascular phenotypes associated with changes in TRAPP complexes and their subunits.

The cellular pathways that maintain the quality control and transport of diverse potassium channels

Potassium channels are multi-subunit transmembrane proteins that permit the selective passage of potassium and play fundamental roles in physiological processes, such as action potentials in the nervous system and organismal salt and water homeostasis, which is mediated by the kidney. Like all ion channels, newly translated potassium channels enter the endoplasmic reticulum (ER) and undergo the error-prone process of acquiring post-translational modifications, folding into their native conformations, assembling with other subunits, and trafficking through the secretory pathway to reach their final destinations, most commonly the plasma membrane. Disruptions in these processes can result in detrimental consequences, including various human diseases. Thus, multiple quality control checkpoints evolved to guide potassium channels through the secretory pathway and clear potentially toxic, aggregation-prone misfolded species. We will summarize current knowledge on the mechanisms underlying potassium channel quality control in the secretory pathway, highlight diseases associated with channel misfolding, and suggest potential therapeutic routes.

The genome of a hadal sea cucumber reveals novel adaptive strategies to deep-sea environments

How organisms cope with coldness and high pressure in the hadal zone remains poorly understood. Here, we sequenced and assembled the genome of hadal sea cucumber Paelopatides sp. Yap with high quality and explored its potential mechanisms for deep-sea adaptation. First, the expansion of ACOX1 for rate-limiting enzyme in the DHA synthesis pathway, increased DHA content in the phospholipid bilayer, and positive selection of EPT1 may maintain cell membrane fluidity. Second, three genes for translation initiation factors and two for ribosomal proteins underwent expansion, and three ribosomal protein genes were positively selected, which may ameliorate the protein synthesis inhibition or ribosome dissociation in the hadal zone. Third, expansion and positive selection of genes associated with stalled replication fork recovery and DNA repair suggest improvements in DNA protection. This is the first genome sequence of a hadal invertebrate. Our results provide insights into the genetic adaptations used by invertebrate in deep oceans.

Sorting nexin 17 increases low-density lipoprotein receptor-related protein 4 membrane expression: A novel mechanism of acetylcholine receptor aggregation in myasthenia gravis

Myasthenia gravis (MG) is characterized by autoimmune damage to the postsynaptic membrane of the neuromuscular junction (NMJ) with impaired postsynaptic acetylcholine receptor (AChR) aggregation. Low-density lipoprotein receptor-related protein 4 (LRP4) plays an important role in AChR aggregation at endplate membranes via the Agrin–LRP4–muscle-specific receptor tyrosine kinase (MuSK) cascade. Sorting nexin 17 (SNX17) regulates the degradation and recycling of various internalized membrane proteins. However, whether SNX17 regulates LRP4 remains unclear. Therefore, we examined the regulatory effects of SNX17 on LRP4 and its influence on AChR aggregation in MG. We selected C2C12 myotubes and induced LRP4 internalization via stimulation with anti-LRP4 antibody and confirmed intracellular interaction between SNX17 and LRP4. SNX17 knockdown and overexpression confirmed that SNX17 promoted MuSK phosphorylation and AChR aggregation by increasing cell surface LRP4 expression. By establishing experimental autoimmune MG (EAMG) mouse models, we identified that SNX17 upregulation improved fragmentation of the AChR structure at the NMJ and alleviated leg weakness in EAMG mice. Thus, these results reveal that SNX17 may be a novel target for future MG therapy.

Sorting nexins as a promising therapeutic target for cardiovascular disorders: An updated overview

Sorting nexins (SNXs) are involved in sorting the protein cargo within the endolysosomal system. Recently, several studies have shown the role of SNXs in cardiovascular pathology. SNXs exert both physiologic and pathologic functions in the cardiovascular system by regulating protein sorting and trafficking, maintaining protein homeostasis, and participating in multiple signaling pathways. SNX deficiency results in blood pressure response to dopamine 5 receptor [D5R] stimulation. SNX knockout protected against atherosclerosis lesions by suppressing foam cell formation. Moreover, SNXs can act as endogenous anti-arrhythmic agents via maintenance of calcium homeostasis. Overexpression SNXs also can reduce cardiac fibrosis in atrial fibrillation. The SNX-STAT3 interaction in cardiac cells promoted heart failure. SNXs may have the potential to act as a pharmacological target against specific cardiovascular diseases.

The Role of Sorting Nexin 17 in Cardiac Development

Sorting nexin 17 (SNX17), a member of sorting nexin (SNX) family, acts as a modulator for endocytic recycling of membrane proteins. Results from our previous study demonstrated the embryonic lethality of homozygous defect of SNX17. In this study, we investigated the role of SNX17 in rat fetal development. Specifically, we analyzed patterns of SNX17 messenger RNA (mRNA) expression in multiple rat tissues and found high expression in the cardiac outflow tract (OFT). This expression was gradually elevated during the cardiac OFT morphogenesis. Homozygous deletion of the SNX17 gene in rats resulted in mid-gestational embryonic lethality, which was accompanied by congenital heart defects, including the double-outlet right ventricle and atrioventricular and ventricular septal defects, whereas heterozygotes exhibited normal fetal development. Moreover, we found normal migration distance and the number of cardiac neural crest cells during the OFT morphogenesis. Although cellular proliferation in the cardiac OFT endocardial cushion was not affected, cellular apoptosis was significantly suppressed. Transcriptomic profiles and quantitative real-time PCR data in the cardiac OFT showed that SNX17 deletion resulted in abnormal expression of genes associated with cardiac development. Overall, these findings suggest that SNX17 plays a crucial role in cardiac development.

SNX17 protects the heart from doxorubicin-induced cardiotoxicity by modulating LMOD2 degradation

Anthracyclines including doxorubicin (DOX) are still the most widely used and efficacious antitumor drugs, although their cardiotoxicity is a significant cause of heart failure. Despite considerable efforts being made to minimize anthracycline-induced cardiac adverse effects, little progress has been achieved. In this study, we aimed to explore the role and underlying mechanism of SNX17 in DOX-induced cardiotoxicity. We found that SNX17 was downregulated in cardiomyocytes treated with DOX both in vitro and in vivo. DOX treatment combined with SNX17 interference worsened the damage to neonatal rat ventricular myocytes (NRVMs). Furthermore, the rats with SNX17 deficiency manifested increased susceptibility to DOX-induced cardiotoxicity (myocardial damage and fibrosis, impaired contractility and cardiac death). Mechanistic investigation revealed that SNX17 interacted with leiomodin-2 (LMOD2), a key regulator of the thin filament length in muscles, via its C-TERM domain and SNX17 deficiency exacerbated DOX-induced cardiac systolic dysfunction by promoting aberrant LMOD2 degradation through lysosomal pathway. In conclusion, these findings highlight that SNX17 plays a protective role in DOX-induced cardiotoxicity, which provides an attractive target for the prevention and treatment of anthracycline induced cardiotoxicity.

The function of SUMOylation and its crucial roles in the development of neurological diseases

Neurological diseases are relatively complex diseases of a large system; however, the detailed mechanism of their pathogenesis has not been completely elucidated, and effective treatment methods are still lacking for some of the diseases. The SUMO (small ubiquitin-like modifier) modification is a dynamic and reversible process that is catalyzed by SUMO-specific E1, E2, and E3 ligases and reversed by a family of SENPs (SUMO/Sentrin-specific proteases). SUMOylation covalently conjugates numerous cellular proteins, and affects their cellular localization and biological activity in numerous cellular processes. A wide range of neuronal proteins have been identified as SUMO substrates, and the disruption of SUMOylation results in defects in synaptic plasticity, neuronal excitability, and neuronal stress responses. SUMOylation disorders cause many neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, and Huntington's disease. By modulating the ion channel subunit, SUMOylation imbalance is responsible for the development of various channelopathies. The regulation of protein SUMOylation in neurons may provide a new strategy for the development of targeted therapeutic drugs for neurodegenerative diseases and channelopathies.

Year in Review in Cardiac Electrophysiology

In the past year, there have been numerous advances in our understanding of arrhythmia mechanisms, diagnosis, and new therapies. We have seen advances in basic cardiac electrophysiology with data suggesting that secretoneurin may be a biomarker for patients at risk of ventricular arrhythmias, and we have learned of the potential role of an NPR-C (natriuretic peptide receptor-C) in atrial fibrosis and the role of an atrial specific 2-pore potassium channel TASK-1 as a therapeutic target for atrial fibrillation. We have seen studies demonstrating the role of sensory neurons in sleep apnea-related atrial fibrillation and the association between bariatric surgery and atrial fibrillation ablation outcomes. Artificial intelligence applied to electrocardiography has yielded estimates of age, sex, and overall health. We have seen new tools for collection of patient-centered outcomes following catheter ablation. There have been significant advances in the ability to identify ventricular tachycardia termination sites through high-density mapping of deceleration zones. We have learned that right ventricular dysfunction may be a predictor of survival benefit after implantable cardioverter-defibrillator implantation in patients with nonischemic cardiomyopathy. We have seen further insights into the role of His bundle pacing on improving outcomes. As our understanding of cardiac laminopathies advances, we may have new tools to predict arrhythmic event rates in gene carriers. Finally, we have seen numerous advances in the treatment of arrhythmias in patients with congenital heart disease.

Microtubule polymerization state and clathrin-dependent internalization regulate dynamics of cardiac potassium channel: Microtubule and clathrin control of KV1.5 channel

Ion channel trafficking powerfully influences cardiac electrical activity as it regulates the number of available channels at the plasma membrane. Studies have largely focused on identifying the molecular determinants of the trafficking of the atria-specific K_V1.5 channel, the molecular basis of the ultra-rapid delayed rectifier current I_Kur. Besides, regulated K_V1.5 channel recycling upon changes in homeostatic state and mechanical constraints in native cardiomyocytes has been well documented. Here, using cutting-edge imaging in live myocytes, we investigated the dynamics of this channel in the plasma membrane. We demonstrate that the clathrin pathway is a major regulator of the functional expression of K_V1.5 channels in atrial myocytes, with the microtubule network as the prominent organizer of K_V1.5 transport within the membrane. Both clathrin blockade and microtubule disruption result in channel clusterization with reduced membrane mobility and internalization, whereas disassembly of the actin cytoskeleton does not. Mobile K_V1.5 channels are associated with the microtubule plus-end tracking protein EB1 whereas static K_V1.5 clusters are associated with stable acetylated microtubules. In human biopsies from patients in atrial fibrillation associated with atrial remodeling, drastic modifications in the trafficking balance occurs together with alteration in microtubule polymerization state resulting in modest reduced endocytosis and increased recycling. Consequently, hallmark of atrial K_V1.5 dynamics within the membrane is clathrin- and microtubule- dependent. During atrial remodeling, predominance of anterograde trafficking activity over retrograde trafficking could result in accumulation ok K_V1.5 channels in the plasma membrane.

An implantable system for long-term assessment of atrial fibrillation substrate in unanesthetized rats exposed to underlying pathological conditions

Atrial fibrillation (AF) is a progressive arrhythmia with underlying mechanisms that are not fully elucidated, partially due to lack of reliable and affordable animal models. Here, we introduce a system for long-term assessment of AF susceptibility (substrate) in ambulatory rats implanted with miniature electrodes on the atrium. Rats were subjected to excessive aldosterone (Aldo) or solvent only (Sham). An additional group was exposed to myocardial infarction (MI). AF substrate was tested two- and four-weeks post implantation and was also compared with implanted rats early post-implantation (Base). Aldo and MI increased the AF substrate and atrial fibrosis. In the MI group only, AF duration was correlated with the level of atrial fibrosis and was inversely correlated with systolic function. Unexpectedly, Shams also developed progressive AF substrate relative to Base individuals. Further studies indicated that serum inflammatory markers (IL-6, TNF-alpha) were not elevated in the shams. In addition, we excluded anxiety\depression due to social-isolation as an AF promoting factor. Finally, enhanced biocompatibility of the atrial electrode did not inhibit the gradual development of AF substrate over a testing period of up to 8 weeks. Overall, we successfully validated the first system for long-term AF substrate testing in ambulatory rats.

Cold-inducible RNA-binding protein modulates atrial fibrillation onset by targeting multiple ion channels

BACKGROUND

Atrial fibrillation (AF), the most common sustained arrhythmia, significantly increases cardiovascular and cerebrovascular morbidity and mortality. The pathogenesis and treatment of AF remain a major challenge in the field of cardiology. We previously found that cold-inducible RNA-binding protein (CIRP) regulated ventricular repolarization by posttranscriptionally regulating Kv4.2/4.3 ion channels in rats, but the role of CIRP in AF is not clear.

OBJECTIVE

The purpose of this study was to confirm that CIRP participates in atrial electrophysiological remodeling and AF occurrence by regulating atrial channels posttranscriptionally.

METHODS

Programmed intra-atrial stimulation was used to induce AF in wild-type or transcription activator-like effector nucleases-based CIRP knockout (KO) rats. Atrial optical mapping, patch clamp, Western blotting, RNA immunoprecipitation, and luciferase reporter assays were performed to evaluate the underlying mechanism of atrial electrical remodeling.

RESULTS

First, we observed a shortened atrial effective refractory period and increased susceptibility to AF in CIRP KO rats. Second, atria-specific CIRP delivery through an adeno-associated viral vector serotype 9 prolonged the atrial effective refractory period and attenuated AF development in CIRP KO rats. Third, we observed the shortened action potential duration and enhanced expression of Kv1.5 and Kv4.2/4.3 in KO rats. The transient outward current blocker 4-Aminopyridine and ultrarapid component of the delayed rectifier current blocker Diphenyl phosphine oxide-1 restored the shortened action potential duration in KO atria. Finally, we demonstrated that CIRP suppressed Kv1.5 and Kv4.2/4.3 expression by directly targeting their 3'-untranslated regions.

CONCLUSION

CIRP plays a protective role in preventing AF onset through the posttranscriptional regulation of Kv1.5 and Kv4.2/4.3.