Medical Research Section 1
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Human iPSC-hepatocyte modeling of alpha-1 antitrypsin heterozygosity reveals metabolic dysregulation and cellular heterogeneity. Andrew A. Wilson
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The Recruitment-Secretory Block (“R-SB”) Phenomenon and Endoplasmic Reticulum Storage Diseases. Francesco Callea
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Diploid hepatocytes drive physiological liver renewal in adult humans. Olaf Bergmann
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Aging and liver disease. David A. Brenner
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Why do some Alpha's develop liver disease and others not.
Human iPSC-hepatocyte modeling of alpha-1 antitrypsin heterozygosity reveals metabolic dysregulation and cellular heterogeneity
Andrew A. Wilson
This study demonstrate that MZ iHeps exhibit a cellular phenotype intermediate to genetically matched ZZ and MM comparators. Through multi-omics profiling, we have shown that AAT processing is deranged in both ZZ and MZ iHeps and associated with downstream metabolic dysregulation, impaired mitochondrial function, and cellular heterogeneity characterized by branch-specific UPR gene expression. These findings provide important insight into the mechanistic underpinnings of ZAAT-driven hepatocyte injury that could contribute to the increased risk of clinical liver disease observed among both individuals heterozygous and homozygous for the Z mutation
The Recruitment-Secretory Block (“R-SB”) Phenomenon and
Endoplasmic Reticulum Storage Diseases
Francesco Callea
In this article, we review the biological and clinical implication of the Recruitment-Secretory Block (“R-SB”) phenomenon. The phenomenon refers to the reaction of the liver with regard to protein secretion in conditions of clinical stimulation. Our basic knowledge of the process is due to
the experimental work in animal models. Under basal conditions, the protein synthesis is mainly carried out by periportal (zone 1) hepatocytes that are considered the “professional” synthesizing protein cells. Under stimulation, mid-lobular and centro-lobular (zones 2 and 3) hepatocytes, are progressively recruited according to lobular gradients and contribute to the increase of synthesis and secretion. The block of secretion, operated by exogenous agents, causes intracellular retention of all secretory proteins. The Pi MZ phenotype of Alpha-1-antitrypsin deficiency (AATD) has turned out to be the key for in vivo studies of the reaction of the liver, as synthesis and block of secretion are concomitant. Indeed, the M fraction of AAT is stimulated for synthesis and regularly exported while the Z fraction is mostly retained within the cell. For that reason, the phenomenon has been designated “Recruitment-Secretory Block” (“R-SB”). The “R-SB” phenomenon explains why: (a)
the MZ individuals can correct the serum deficiency; (b) the resulting immonohistochemical and electron microscopic (EM) patterns are very peculiar and specific for the diagnosis of the Z mutation in tissue sections in the absence of genotyping; (c) the term carrier is no longer applicable for the
heterozygous condition as all Pi MZ individuals undergo storage and the storage predisposes to liver damage. The storage represents the true elementary lesion and consequently reflects the phenotypegenotype
correlation; (d) the site and function of the extrahepatic AAT and the relationship between intra and extracellular AAT; (e) last but not least, the concept of Endoplasmic Reticulum Storage Disease (ERSD) and of a new disease, hereditary hypofibrinogenemia with hepatic storage (HHHS).
In the light of the emerging phenomenon, described in vitro, namely that M and Z AAT can form heteropolymers within hepatocytes as well as in circulation, we have reviewed the whole clinical and experimental material collected during forty years, in order to evaluate to what extent the
polymerization phenomenon occurs in vivo. The paper summarizes similarities and differences between AAT and Fibrinogen as well as between the related diseases, AATD and HHHS. Indeed, fibrinogen gamma chain mutations undergo an aggregation process within the RER of hepatocytes
similar to AATD. In addition, this work has clarified the intriguing phenomenon underlying a new syndrome, hereditary hypofibrinogenemia and hypo-APO-B-lipoproteinemia with hepatic storage of fibrinogen and APO-B lipoproteins. It is hoped that these studies could contribute to future research and select strategies aimed to simultaneously correct the hepatocytic storage, thus preventing the liver damage and the plasma deficiency of the two proteins.
Diploid hepatocytes drive physiological liver renewal
in adult humans
Olaf Bergmann
Graphical abstract
Based on radiocarbon birth dating, a
comprehensive model of hepatocyte
renewal in humans shows that the liver
remains a young organ. Hepatocytes
display a continuous and lifelong
turnover, which is highly dependent on
their ploidy level.
Retrospective 14C birth dating establishes the age of the human liver (<3 years)
Renewal rates of adult hepatocytes are independent of subject age
Diploid hepatocytes show more than 7-fold higher birth rates compared with polyploid
Contribution of higher ploidy levels to the diploid hepatocyte pool is limited
Aging and liver disease
David A. Brenner
The volume and blood flow of the liver gradually decrease with aging. According to studies using ultrasound, the liver volume decreases by 20–40% as one gets older. Such changes are related to a decline in the blood flow in the liver. Individuals aged 65 years or higher showed an approximately 35% decrease in the blood volume of the liver compared with those aged less than 40 years. Meanwhile, the studies that scanned the liver with radioisotopes observed a decrease not in the total liver volume but in the mass of the functional liver cells. Studies have reported mixed results about aging-induced changes in the liver. Humans show a slight decrease in the serum albumin concentration or maintain the normal level in the natural aging process. The neural fat and cholesterol volumes in the liver gradually expand as one gets older, and the blood cholesterol, high-density lipoprotein cholesterol, and neutral fat levels also increase over time. Meanwhile, the metabolism of the low-density lipoprotein cholesterol decreases by 35%. The serum γ-glutamyltransferase and alkaline phosphatase levels are elevated with aging. Although the serum aminotransferase maintains the normal level, the serum bilirubin is gradually reduced, as humans get older.
Aging-related changes in liver cells include volume changes, polyploidy (polyploidy nuclei), accumulation of dense bodies (lipofuscin) inside liver cells, a decreased area of smooth endoplasmic reticulum, and a declining number and dysfunction of mitochondria. The volume of the liver cells gradually increases as they approach maturity, but starts to decrease due to aging. Lipofuscins are highly cross-linked undegradable protein aggregates that are formed when proteins damaged and denatured by oxidative stress are not degraded inside the liver cells. Such lipofuscins cause increased generation of reactive oxygen species (ROS) in cells and reduced cell survivability. As a result of aging, hepatocyte polyploidy tends to occur more frequently over time, which is accompanied by a decreased number and dysfunction of mitochondria, and results in a decline in the ATP level. Also, the area of smooth endoplasmic reticulum is reduced, causing decreased generation of smooth endoplasmic reticulum and reducing the synthesis of microsomal proteins in the liver.
Why do some Alpha's develop liver disease and others not
Variants in autophagy genes MTMR12 and FAM134A are putative modifiers of the hepatic phenotype in α1-antitrypsin deficiency
Tafaleng, Edgar N.1; Li, Jie2; Wang, Yan1; Hidvegi, Tunda2; Soto-Gutierrez, Alex1; Locke, Adam E.2; Nicholas, Thomas J.2; Wang, Yung-Chun2; Pak, Stephen2; Cho, Michael H.3; Silverman, Edwin K3; Silverman, Gary A.2; Jin, Sheng Chih2; Fox, Ira J.1; Perlmutter, David H.2
Abstract
Background and Aims:
In the classical form of α1-antitrypsin deficiency, a misfolded variant α1-antitrypsin Z accumulates in the endoplasmic reticulum of liver cells and causes liver cell injury by gain-of-function proteotoxicity in a sub-group of affected homozygotes but relatively little is known about putative modifiers. Here, we carried out genomic sequencing in a uniquely affected family with an index case of liver failure and 2 homozygous siblings with minimal or no liver disease. Their sequences were compared to sequences in well-characterized cohorts of homozygotes with or without liver disease, and then candidate sequence variants were tested for changes in the kinetics of α1-antitrypsin variant Z degradation in iPS-derived hepatocyte-like cells derived from the affected siblings themselves.
Approach and Results:
Specific variants in autophagy genes MTMR12 and FAM134A could each accelerate the degradation of α1-antitrypsin variant Z in cells from the index patient, but both MTMR12 and FAM134A variants were needed to slow the degradation of α1-antitrypsin variant Z in cells from a protected sib, indicating that inheritance of both variants is needed to mediate the pathogenic effects of hepatic proteotoxicity at the cellular level. Analysis of homozygote cohorts showed that multiple patient-specific variants in proteostasis genes are likely to explain liver disease susceptibility at the population level.
Conclusions:
These results validate the concept that genetic variation in autophagy function can determine susceptibility to liver disease in α1-antitrypsin deficiency and provide evidence that polygenic mechanisms and multiple patient-specific variants are likely needed for proteotoxic pathology.
Note: This is not an open publication