Hepatic connexin 32 associates with nonalcoholic fatty liver disease severity

Accumulation of α-synuclein in hepatocytes in nonalcoholic steatohepatitis and its usefulness in pathological diagnosis

BACKGROUNDS

Nonalcoholic steatohepatitis (NASH) is characterized by fat deposition, inflammation, and hepatocellular damage. The diagnosis of NASH is confirmed pathologically, and hepatocyte ballooning is an important finding for definite diagnosis. Recently, α-synuclein deposition in multiple organs was reported in Parkinson's disease. Since it was reported that α-synuclein is taken up by hepatocytes via connexin 32, the expression of α-synuclein in the liver in NASH is of interest. The accumulation of α-synuclein in the liver in NASH was investigated. Immunostaining for p62, ubiquitin, and α-synuclein was performed, and the usefulness of immunostaining in pathological diagnosis was examined.

METHODS

Liver biopsy tissue specimens from 20 patients were evaluated. Several antibodies against α-synuclein, as well as antibodies against connexin 32, p62, and ubiquitin were used for immunohistochemical analyses. Staining results were evaluated by several pathologists with varying experience, and the diagnostic accuracy of ballooning was compared.

RESULTS

Polyclonal α-synuclein antibody, not the monoclonal antibody, reacted with eosinophilic aggregates in ballooning cells. Expression of connexin 32 in degenerating cells was also demonstrated. Antibodies against p62 and ubiquitin also reacted with some of the ballooning cells. In the pathologists' evaluations, the highest interobserver agreement was obtained with hematoxylin and eosin (H&E)-stained slides, followed by slides immunostained for p62 and α-synuclein, and there were cases with different results between H&E staining and immunostaining CONCLUSION: These results indicate the incorporation of degenerated α-synuclein into ballooning cells, suggesting the involvement of α-synuclein in the pathogenesis of NASH. The combination of immunostaining including polyclonal α-synuclein may contribute to improving the diagnosis of NASH.

Effects of acute and chronic disease on cell junctions in mouse liver

Cell junctions, including anchoring, occluding and communicating junctions, play an indispensable role in tissue architecture and homeostasis. Consequently, malfunctioning of cell junctions is linked with a wide range of disorders, including in liver. The present study was set up to investigate the effects of acute and chronic disease induced by chemical compounds on hepatic cell junctions in mice. Mice were either overdosed with paracetamol or repeatedly administered carbon tetrachloride followed by sampling at 24 hours or 8 weeks, respectively. mRNA and protein expression levels of adherens, gap and tight junction components were measured in liver using reverse transcription quantitative real-time polymerase chain reaction analysis and immunoblot techniques, respectively. It was found that protein levels of the adherens junction building blocks β-catenin and γ-catenin, the gap junction components Cx26 and Cx32, and the tight junction constituent zonula occludens 2 were decreased, while mRNA levels of the adherens junction building block E-cadherin, and the tight junction constituent zonula occludens 2 and claudin 1 were upregulated following paracetamol overdosing. Repeated administration of carbon tetrachloride increased protein levels of E-cadherin, β-catenin, Cx26, Cx32, Cx43 and claudin 1. The latter was reflected at the mRNA level. In conclusion, acute and chronic liver disease have different effects on cell junctions in liver.

Connexin-Based Channels in the Liver

Connexin proteins oligomerize in hexameric structures called connexin hemichannels, which then dock to form gap junctions. Gap junctions direct cell-cell communication by allowing the exchange of small molecules and ions between neighboring cells. In this way, hepatic gap junctions support liver homeostasis. Besides serving as building blocks for gap junctions, connexin hemichannels provide a pathway between the intracellular and the extracellular environment. The activation of connexin hemichannels is associated with acute and chronic liver pathologies. This article discusses the role of gap junctions and connexin hemichannels in the liver. © 2022 American Physiological Society. Compr Physiol 12:1-17, 2022.

Acute hazard assessment of silver nanoparticles following intratracheal instillation, oral and intravenous injection exposures

With ever-increasing production and use of nanoparticles (NPs), there is a necessity to evaluate the probability of consequential adverse effects in individuals exposed to these particles. It is now understood that a proportion of NPs can translocate from primary sites of exposure to a range of secondary organs, with the liver, kidneys and spleen being some of the most important. In this study, we carried out a comprehensive toxicological profiling (inflammation, changes in serum biochemistry, oxidative stress, acute phase response and histopathology) of Ag NP induced adverse effects in the three organs of interest following acute exposure of the materials at identical doses via intravenous (IV), intratracheal (IT) instillation and oral administration. The data clearly demonstrated that bioaccumulation and toxicity of the particles were most significant following the IV route of exposure, followed by IT. However, oral exposure to the NPs did not result in any changes that could be interpreted as toxicity in any of the organs of interest within the confines of this investigation. The finding of this study clearly indicates the importance of the route of exposure in secondary organ hazard assessment for NPs. Finally, we identify Connexin 32 (Cx32) as a novel biomarker of NP-mediated hepatic damage which is quantifiable both (in vitro) and in vivo following exposure of physiologically relevant doses.

cGAS-STING Signaling Pathway and Liver Disease: From Basic Research to Clinical Practice

The cGAS-STING signaling pathway is an autoimmune inflammatory pathway that can trigger the expression of a series of inflammatory factors represented by type 1 interferon. Recent studies have found that the cGAS-STING signaling pathway played a significant role in liver physiology and was closely related to the progress of liver diseases. For example, activating the cGAS-STING signaling pathway could significantly inhibit hepatitis B virus (HBV) replication in vivo. Moreover, the cGAS-STING signaling pathway was also closely associated with tumor immunity in hepatocellular carcinoma (HCC). This review summarized the role of the cGAS-STING signaling pathway in several common liver diseases, especially the current application of the cGAS-STING signaling pathway in liver disease treatment, and prospected its future research, which provided a new idea for understanding and treating liver diseases.

The Inside-Out of End-Stage Liver Disease: Hepatocytes are the Keystone

Chronic liver injury results in cirrhosis and end-stage liver disease (ESLD) which represents a leading cause of death worldwide, affecting people in their most productive years of life. Medical therapy can extend life, but the only definitive treatment is liver transplantation (LT). However, LT remains limited by access to quality donor organs and suboptimal long-term outcomes. The degeneration from healthy-functioning livers to cirrhosis and ESLD involves a dynamic process of hepatocyte damage, diminished hepatic function, and adaptation. However, the mechanisms responsible for deterioration of hepatocyte function and ultimately hepatic failure in man are poorly understood. We review the current understanding of cirrhosis and ESLD as a dynamic process and outline the current mechanisms associated with the development of hepatic failure from the clinical manifestations to energy adaptations, regeneration, and regulation of nuclear transcription factors. A new generation of therapeutics could target stabilization of hepatocyte differentiation and function to avoid the need for transplantation in patients with cirrhosis and ESLD.

Collagen I Modifies Connexin-43 Hemichannel Activity via Integrin α2β1 Binding in TGFβ1-Evoked Renal Tubular Epithelial Cells

Chronic Kidney Disease (CKD) is associated with sustained inflammation and progressive fibrosis, changes that have been linked to altered connexin hemichannel-mediated release of adenosine triphosphate (ATP). Kidney fibrosis develops in response to increased deposition of extracellular matrix (ECM), and up-regulation of collagen I is an early marker of renal disease. With ECM remodeling known to promote a loss of epithelial stability, in the current study we used a clonal human kidney (HK2) model of proximal tubular epithelial cells to determine if collagen I modulates changes in cell function, via connexin-43 (Cx43) hemichannel ATP release. HK2 cells were cultured on collagen I and treated with the beta 1 isoform of the pro-fibrotic cytokine transforming growth factor (TGFβ1) ± the Cx43 mimetic Peptide 5 and/or an anti-integrin α2β1 neutralizing antibody. Phase microscopy and immunocytochemistry observed changes in cell morphology and cytoskeletal reorganization, whilst immunoblotting and ELISA identified changes in protein expression and secretion. Carboxyfluorescein dye uptake and biosensing measured hemichannel activity and ATP release. A Cytoselect extracellular matrix adhesion assay assessed changes in cell-substrate interactions. Collagen I and TGFβ1 synergistically evoked increased hemichannel activity and ATP release. This was paralleled by changes to markers of tubular injury, partly mediated by integrin α2β1/integrin-like kinase signaling. The co-incubation of the hemichannel blocker Peptide 5, reduced collagen I/TGFβ1 induced alterations and inhibited a positive feedforward loop between Cx43/ATP release/collagen I. This study highlights a role for collagen I in regulating connexin-mediated hemichannel activity through integrin α2β1 signaling, ahead of establishing Peptide 5 as a potential intervention.

Proteomic screening of plasma identifies potential noninvasive biomarkers associated with significant/advanced fibrosis in patients with nonalcoholic fatty liver disease

Noninvasive biomarkers are clinically useful for evaluating liver fibrosis stage in patients with nonalcoholic fatty liver disease (NAFLD). The aim of the present study was to compare plasma proteins in patients with early nonalcoholic steatohepatitis (NASH) (F0-F1) versus NASH with significant/advanced fibrosis (F2–F4) to determine whether candidate proteins could be used as potential noninvasive biomarkers. Nineteen biopsy-proven NAFLD patients including ten early NASH patients and nine NASH patients with significant/advanced fibrosis were enrolled in the present study. High-resolution proteomics screening of plasma was performed with the SCIEX TripleTOF 5600 System. Proteins were quantified using two different software platforms, Progenesis Qi and Scaffold Q+, respectively. Progenesis Qi analysis resulted in the discovery of 277 proteins compared with 235 proteins in Scaffold Q+. Five consensus proteins (i.e. Complement component C7; α-2-macroglobulin; Complement component C8 γ chain; Fibulin-1; α-1-antichymotrypsin) were identified. Complement component C7 was three-fold higher in the NASH group with significant/advanced fibrosis (F2–F4) compared with the early NASH (F0-F1) group (q-value = 3.6E-6). Complement component C7 and Fibulin-1 are positively correlated with liver stiffness (P=0.000, P=0.002, respectively); whereas, Complement component C8 γ chain is negatively correlated (P=0.009). High levels of Complement C7 are associated with NASH with significant/advanced fibrosis and Complement C7 is a perfect classifier of patients included in this pilot study. Further studies will be needed in a larger validation cohort to confirm the utility of complement proteins as biomarkers or mechanistic determinants of NASH with significant/advanced fibrosis.

Intercellular Transmission of Hepatic ER Stress in Obesity Disrupts Systemic Metabolism

Endoplasmic reticulum stress (ERS) has a pathophysiological role in obesity-associated insulin resistance. Yet, the coordinated tissue response to ERS remains unclear. Increased connexin 43 (Cx43)-mediated intercellular communication has been implicated in tissue-adaptive and -maladaptive response to various chronic stresses. Here, we demonstrate that in hepatocytes, ERS results in increased Cx43 expression and cell-cell coupling. Co-culture of ER-stressed "donor" cells resulted in intercellular transmission of ERS and dysfunction to ERS-naive "recipient" cells ("bystander response"), which could be prevented by genetic or pharmacologic suppression of Cx43. Hepatocytes from obese mice were able to transmit ERS to hepatocytes from lean mice, and mice lacking liver Cx43 were protected from diet-induced ERS, insulin resistance, and hepatosteatosis. Taken together, our results indicate that in obesity, the increased Cx43-mediated cell-cell coupling allows intercellular propagation of ERS. This novel maladaptive response to over-nutrition exacerbates the tissue ERS burden, promoting hepatosteatosis and impairing whole-body glucose metabolism.

Targeting gap junctional intercellular communication by hepatocarcinogenic compounds

Gap junctions in liver, as in other organs, play a critical role in tissue homeostasis. Inherently, these cellular constituents are major targets for systemic toxicity and diseases, including cancer. This review provides an overview of chemicals that compromise liver gap junctions, in particular biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. The focus in this review is placed upon the mechanistic scenarios that underlie these adverse effects. Further, the potential use of gap junctional activity as an in vitro biomarker to identify non-genotoxic hepatocarcinogenic chemicals is discussed.

Hepatic gap junctions amplify alcohol liver injury by propagating cGAS-mediated IRF3 activation

Alcohol-related liver disease (ALD) accounts for the majority of cirrhosis and liver-related deaths worldwide. Activation of IFN-regulatory factor (IRF3) initiates alcohol-induced hepatocyte apoptosis, which fuels a robust secondary inflammatory response that drives ALD. The dominant molecular mechanism by which alcohol activates IRF3 and the pathways that amplify inflammatory signals in ALD remains unknown. Here we show that cytoplasmic sensor cyclic guanosine monophosphate-adenosine monophosphate (AMP) synthase (cGAS) drives IRF3 activation in both alcohol-injured hepatocytes and the neighboring parenchyma via a gap junction intercellular communication pathway. Hepatic RNA-seq analysis of patients with a wide spectrum of ALD revealed that expression of the cGAS-IRF3 pathway correlated positively with disease severity. Alcohol-fed mice demonstrated increased hepatic expression of the cGAS-IRF3 pathway. Mice genetically deficient in cGAS and IRF3 were protected against ALD. Ablation of cGAS in hepatocytes only phenocopied this hepatoprotection, highlighting the critical role of hepatocytes in fueling the cGAS-IRF3 response to alcohol. We identified connexin 32 (Cx32), the predominant hepatic gap junction, as a critical regulator of spreading cGAS-driven IRF3 activation through the liver parenchyma. Disruption of Cx32 in ALD impaired IRF3-stimulated gene expression, resulting in decreased hepatic injury despite an increase in hepatic steatosis. Taken together, these results identify cGAS and Cx32 as key factors in ALD pathogenesis and as potential therapeutic targets for hepatoprotection.