Smooth muscle actin isoforms: A tug of war between contraction and compliance

Dynamics of immune functional mechanisms and their roles in promoting host resistance to Haemonchus contortus infection in suckling lambs

An RNA-seq dataset of 68-day-old lambs from two sheep breeds was analysed to identify differentially expressed genes involved in biological processes, molecular functions, cellular components, and pathways associated with resistance against Haemonchus contortus. Naïve Santa Ines (resistant breed) and Ile de France (susceptible breed) suckling lambs were serially infected with increasing numbers of H. contortus infective larvae (5400 in total) and had their abomasal mucosa sampled for sequencing. Gene Ontology annotation and enrichment analysis generated a list of 312 biological processes, 56 cellular components, and 106 molecular functions significantly enriched terms. Santa Ines lambs had significantly higher expression of genes involved in inflammatory responses, B-cell activation and receptor signalling pathways, complement activation, and angiogenesis. Most of the cellular components were associated with B-cell receptor complex, Fc-epsilon receptor I complex, and MHC class II protein (MHCII) complex terms. Additionally, most of the molecular functions were associated with IgE binding and receptor activity, IgA and immunoglobulin receptor binding. Santa Ines lambs showed a robust tissue repair mechanism, mediated mainly by fibroblast proliferation and collagen biosynthesis, epithelial cell maturation and structure maintenance, and strong smooth muscle contraction. Here, we present a detailed picture of the biological processes, and a comprehensive understanding of the gene architecture involved in the resistance trait of young lambs against H. contortus. The cluster of genes reported here might be useful for identifying target-based markers that could provide early selection of resistant sheep to H. contortus infection.

Development of multilayered artificial urethra graft for urethroplasty

To enhance the treatment of patients' urethral defects, such as strictures and hypospadias, we investigated the potential of using artificial urethral tissue. Our study aimed to generate this tissue and assess its effectiveness in a rabbit model. Two types of bioprinted grafts, based on methacrylated gelatin-silk fibroin (GelMA-SF) hydrogels, were produced: acellular, as well as loaded with autologous rabbit stem cells. Rabbit adipose stem cells (RASC) were differentiated toward smooth muscle in the GelMA-SF hydrogel, while rabbit buccal mucosa stem cells (RBMC), differentiated toward the epithelium, were seeded on its surface, forming two layers of the cell-laden tissue. The constructs were then reinforced with polycaprolactone-polylactic acid meshes to create implantable multilayered artificial urethral grafts. In vivo experiments showed that the cell-laden tissue integrated into the urethra with less fibrosis and inflammation compared to its acellular counterpart. Staining to trace the implanted cells confirmed integration into the host organism 3 months postsurgery.

Pancreatic Tumor Organoid-Derived Factors from Cachectic Patients Disrupt Contractile Smooth Muscle Cells

Patients with pancreatic cancer often suffer from cachexia and experience gastrointestinal symptoms that may be related to intestinal smooth muscle cell (SMC) dysfunction. We hypothesized that pancreatic tumor organoids from cachectic patients release factors that perturb the SMC's contractile characteristics. Human visceral SMCs were exposed to conditioned medium (CM) from the pancreatic tumor organoid cultures of cachectic (n = 2) and non-cachectic (n = 2) patients. Contractile proteins and markers of inflammation, muscle atrophy, and proliferation were evaluated by qPCR and Western blot. SMC proliferation and migration were monitored by live cell imaging. The Ki-67-positive cell fraction was determined in the intestinal smooth musculature of pancreatic cancer patients. CM from the pancreatic tumor organoids of cachectic patients did not affect IL-1β, IL-6, IL-8, MCP-1, or Atrogin-1 expression. However, CM reduced the α-SMA, γ-SMA, and SM22-α levels, which was accompanied by a reduced SMC doubling time and increased expression of S100A4, a Ca2+-binding protein associated with the synthetic SMC phenotype. In line with this, Ki-67-positive nuclei were increased in the intestinal smooth musculature of patients with a low versus high L3-SMI. In conclusion, patient-derived pancreatic tumor organoids release factors that compromise the contractile SMC phenotype and increase SMC proliferation. This may contribute to the frequently observed gastrointestinal motility problems in these patients.

PIEZO1 loss-of-function compound heterozygous mutations in the rare congenital human disorder Prune Belly Syndrome

Prune belly syndrome (PBS), also known as Eagle-Barret syndrome, is a rare, multi-system congenital myopathy primarily affecting males. Phenotypically, PBS cases manifest three cardinal pathological features: urinary tract dilation with poorly contractile smooth muscle, wrinkled flaccid ventral abdominal wall with skeletal muscle deficiency, and intra-abdominal undescended testes. Genetically, PBS is poorly understood. After performing whole exome sequencing in PBS patients, we identify one compound heterozygous variant in the PIEZO1 gene. PIEZO1 is a cation-selective channel activated by various mechanical forces and widely expressed throughout the lower urinary tract. Here we conduct an extensive functional analysis of the PIEZO1 PBS variants that reveal loss-of-function characteristics in the pressure-induced normalized open probability (NPo) of the channel, while no change is observed in single-channel currents. Furthermore, Yoda1, a PIEZO1 activator, can rescue the NPo defect of the PBS mutant channels. Thus, PIEZO1 mutations may be causal for PBS and the in vitro cellular pathophysiological phenotype could be rescued by the small molecule, Yoda1. Activation of PIEZO1 might provide a promising means of treating PBS and other related bladder dysfunctional states.

Fibroblast Activation Protein Alpha (FAPα) in Fibrosis: Beyond a Perspective Marker for Activated Stromal Cells?

The development of tissue fibrosis is a complex process involving the interaction of multiple cell types, which makes the search for antifibrotic agents rather challenging. So far, myofibroblasts have been considered the key cell type that mediated the development of fibrosis and thus was the main target for therapy. However, current strategies aimed at inhibiting myofibroblast function or eliminating them fail to demonstrate sufficient effectiveness in clinical practice. Therefore, today, there is an unmet need to search for more reliable cellular targets to contribute to fibrosis resolution or the inhibition of its progression. Activated stromal cells, capable of active proliferation and invasive growth into healthy tissue, appear to be such a target population due to their more accessible localization in the tissue and their high susceptibility to various regulatory signals. This subpopulation is marked by fibroblast activation protein alpha (FAPα). For a long time, FAPα was considered exclusively a marker of cancer-associated fibroblasts. However, accumulating data are emerging on the diverse functions of FAPα, which suggests that this protein is not only a marker but also plays an important role in fibrosis development and progression. This review aims to summarize the current data on the expression, regulation, and function of FAPα regarding fibrosis development and identify promising advances in the area.

Abnormally elevated expression of ACTA2 of circular smooth muscle leads to hyperactive contraction in aganglionic segments of HSCR

BACKGROUND

Actin Alpha 2 (ACTA2) is expressed in intestinal smooth muscle cells (iSMCs) and is associated with contractility. Hirschsprung disease (HSCR), one of the most common digested tract malformations, shows peristaltic dysfunction and spasm smooth muscles. The arrangement of the circular and longitudinal smooth muscle (SM) of the aganglionic segments is disorganized. Does ACTA2, as a marker of iSMCs, exhibit abnormal expression in aganglionic segments? Does the ACTA2 expression level affect the contraction function of iSMCs? What are the spatiotemporal expression trends of ACTA2 during different developmental stages of the colon?

METHODS

Immunohistochemical staining was used to detect the expression of ACTA2 in iSMCs of children with HSCR and Ednrb-/- mice, and the small interfering RNAs (siRNAs) knockdown technique was employed to investigate how Acta2 affected the systolic function of iSMCs. Additionally, Ednrb-/- mice were used to explore the changes in the expression level of iSMCs ACTA2 at different developmental stages.

RESULTS

The expression of ACTA2 is higher in circular SM in the aganglionic segments of HSCR patients and Ednrb-/- mice than in normal control children and mice. Down regulation of Acta2 weakens the contraction ability of intestinal smooth muscle cells. Abnormally elevated expression of ACTA2 of circular smooth muscle occurs since embryonic day 15.5 (E15.5d) in aganglionic segments of Ednrb-/- mice.

CONCLUSIONS

Abnormally elevated expression of ACTA2 in the circular SM leads to hyperactive contraction, which may cause the spasm of aganglionic segments in HSCR.

Smooth Muscle-Alpha Actin R149C Pathogenic Variant Downregulates Integrin Recruitment at Cell-Matrix Adhesions and Decreases Cellular Contractility

Thoracic aortic aneurysm is found in patients with ACTA2 pathogenic variants. ACTA2 missense variants are associated with impaired aortic smooth muscle cell (SMC) contraction. This study tested the hypothesis that the Acta2R149C/+ variant alters actin isoform expression and decreases integrin recruitment, thus, reducing aortic contractility. Stress relaxation measurements in thoracic aortic rings showed two functional regimes with a reduction of stress relaxation in the aorta from Acta2R149C/+ mice at low tension, but not at high tension values. Contractile responses to phenylephrine and potassium chloride were 50% lower in Acta2R149C/+ mice than in wild-type (WT) mice. Additionally, SMC were immunofluorescently labeled for specific proteins and imaged by confocal or total internal reflection fluorescence microscopy. The quantification of protein fluorescence of Acta2R149C/+ SMC showed a downregulation in smooth muscle α-actin (SMα-actin) and a compensatory upregulation of smooth muscle γ-actin (SMγ-actin) compared to WT cells. These results suggest that downregulation of SMα-actin leads to reduced SMC contractility, while upregulation of SMγ-actin may lead to increased SMC stiffness. Decreased α5β1 and α2β1 integrin recruitment at cell-matrix adhesions further reduce the ability of mutant cells to participate in cell-matrix crosstalk. Collectively, the results suggest that mutant Acta2R149C/+ aortic SMC have reduced contractility and interaction with the matrix, which are potential long-term contributing factors to thoracic aortic aneurysms.

Heterozygous Actg2R257C mice mimic the phenotype of megacystis microcolon intestinal hypoperistalsis syndrome

BACKGROUND

Megacystis microcolon intestinal hypoperistalsis syndrome (MMIHS) is a rare and serious congenital disorder with poor outcomes, where a heterozygous missense mutation is present in the ACTG2 gene. Here, we aimed to investigate the pathogenesis of ACTG2 in MMIHS.

METHODS

A cohort with 20 patients with MMIHS was screened. Actg2^R257C heterozygous mutant mice were generated using the CRISPR/Cas9 system. Gastrointestinal (GI) motility, voluntary urination, collagen gel contraction, and G-actin/F-actin analysis were performed.

KEY RESULTS

The R257C variant of ACTG2 most frequently occurred in patients with MMIHS and demonstrated the typical symptoms of MMIHS. Actg2^R257C heterozygous mutant mice had dilated intestines and bladders. The functional assay showed a prolonged total time of GI transit and decreased urine spot area. Collagen gel contraction assay and G-actin/F-actin analysis indicated that mutant mice showed reduced area of contraction of smooth muscle cells (SMCs) and impaired actin polymerization.

CONCLUSIONS & INFERENCES

A mouse model demonstrating MMIHS-like symptoms was generated. The Actg2^R257C heterozygous variant impairs SMCs contraction by interfering with actin polymerization, leading to GI motility disorders.

Intestinal Pathology in Patients With Pathogenic ACTG2-Variant Visceral Myopathy: 16 Patients From 12 Families and Review of the Literature

BACKGROUND

Dominant gamma-smooth muscle actin gene (ACTG2) variants cause clinically diverse forms of visceral myopathy. Many patients undergo intestinal resection or biopsy before identification of their genetic defect. The pathology of ACTG2-variant visceral myopathy has not been evaluated systematically.

METHODS

Glass slides, ultrastructural images, molecular genetic reports, and clinical records from 16 patients with pathogenic (15) or likely pathogenic (1) ACTG2 variants were reviewed and compared with surgical specimens from controls (no evidence of a primary myopathy or pseudo-obstruction due to Hirschsprung disease) and published descriptions.

RESULTS

The variable clinical manifestations in our cohort matched those in the literature. Only non-specific light and electron microscopic findings observed in non-myopathic controls were encountered in 13 of 16 patients. The remaining 3 patients harbored hyalinized cytoplasmic inclusions in smooth muscle cells and 1 of them had polyglucosan bodies in the muscularis propria.

CONCLUSIONS

Apart from hyalinized inclusions, which were only observed in 3/16 patients, intestinal pathology in the majority of patients with ACTG2 variants is not indicative of an underlying visceral myopathy. Molecular testing should be considered even when no diagnostic intestinal pathology is identified.

MYL9 deficiency is neonatal lethal in mice due to abnormalities in the lung and the muscularis propria of the bladder and intestine

Class II myosin complexes are responsible for muscle contraction as well as other non-sarcomeric contractile functions in cells. Myosin heavy chain molecules form the core of these structures, while light chain molecules regulate their stability and function. MYL9 is a light chain isoform that is thought to regulate non-sarcomeric myosin. However, whether this in only in specific cell types or in all cells remains unclear. To address this, we generated MYL9 deficient mice. These mice die soon after birth with abnormalities in multiple organs. All mice exhibited a distended bladder, shortening of the small intestine and alveolar overdistension in the lung. The Myl9 allele in these mice included a LacZ reporter knockin that allowed for mapping of Myl9 gene expression. Using this reporter, we show that MYL9 expression is restricted to the muscularis propria of the small intestine and bladder, as well as in the smooth muscle layer of the bronchi in the lung and major bladder vessels in all organs. This suggests that MYL9 is important for the function of smooth muscle cells in these organs. Smooth muscle dysfunction is therefore likely to be the cause of the abnormalities observed in the intestine, bladder and lung of MYL9 deficient mice and the resulting neonatal lethality.

DLEU2 modulates proliferation, migration and invasion of platelet-derived growth factor-BB (PDGF-BB)-induced vascular smooth muscle cells (VSMCs) via miR-212-5p/YWHAZ axis

DLEU2 has been proved to act as an oncogene in a variety of cancers, but its role in cardiovascular diseases is dearth of research. Thus, this study mainly discussed the effect and possible mechanism of DLEU2 on platelet-derived growth factor-BB (PDGF-BB)-triggered vascular smooth muscle cell (VSMC) injury. To obtain authentic results, the expressions of target genes in atherosclerosis serum were determined by reverse transcription quantitative PCR (RT-qPCR) and the protein levels were evaluated by Western blot. PDGF-BB was used to simply simulate the biological characteristics of VSMCs in vitro. The effect of DLEU2 on the biological behavior of PDGF-BB-induced VSMCs was analyzed by gain- and loss-of-function assays. Bioinformatics analysis, dual luciferase reporter assay, and Pearson correlation method were conducted to determine the relationship between target genes. The role of DLEU2/miR-212-5p/ YWHAZ (tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein zeta) axis in PDGF-BB-induced VSMCs was verified by rescue experiments. As a result, DLEU2 and YWHAZ were up-regulated, and miR-212-5p was down-regulated in atherosclerosis serum. Overexpressed DLEU2 facilitated the biological behavior of PDGF-BB-induced VSMCs, whilst siDLEU2 did the opposite. Moreover, overexpressed DLEU2 promoted proliferating cell nuclear antigen (PCNA) expression but repressed α-smooth muscle actin (α-SMA) and Calponin expressions, while it also enhanced YWHAZ expression via suppressing miR-212-5p. MiR-212-5p mimic and siYWHAZ reversed the effects of overexpressed DLEU2 on above biological characteristics and protein expressions in PDGF-BB-induced VSMCs, while the regulatory effect of miR-212-5p mimic was partially offset by overexpressed YWHAZ. Collectively, DLEU2 modulates PDGF-BB-induced VSMC injury via miR-212-5p/YWHAZ axis in atherosclerosis.

Deoxynivalenol interferes with intestinal motility via injuring the contractility of enteric smooth muscle cells: A novel hazard to the gastrointestinal tract by environmental toxins

Deoxynivalenol (DON) is a prevalent Fusarium mycotoxin, occurs predominantly in the global environment, especially in cereals, animal feed and food commodities. The widespread contamination causes a serious risk to human and animal health. DON usually impairs weight gain, which is presumably from its capacity to reduce feed intake by interfering with intestinal motility. To clarify the role of smooth muscle cells (SMCs) contractility in intestinal motility and growth inhibition caused by DON, twelve weaned piglets were firstly divided into two groups to feed control or Fusarium mycotoxin-contaminated (MC) diet. Results showed that the final body weight, average daily gain and average daily feed intake were significantly reduced in piglets fed the MC diet. Exposure to the MC diet also significantly decreased the thickness of smooth muscle layer and SMCs contractile markers expression (myosin heavy chain 11, smooth muscle actin gamma 2, transgelin, calponin 1) in jejunum and ileum of piglets. Furthermore, oral DON supplementation (3 mg/kg body weight) to mice in six consecutive days could significantly inhibit the upper intestinal transit, impede normal defecation and downregulate SMCs contractile markers expression in small intestine. Finally, we generated a porcine enteric smooth muscle cell line (PISMC), and found that DON could depress its contractility by decreasing PISMC proliferation, migration and contractile markers expression. In conclusion, these findings in vivo and in vitro suggest that DON, as a common environmental toxin, can not only reduce proliferative and motile phenotype, but also decrease contractile apparatus components (contractile markers expression) in SMCs, which in turn influences SMCs contractility and then interferes with intestinal motility and growth performance.

Visceral myopathy: clinical syndromes, genetics, pathophysiology, and fall of the cytoskeleton

Visceral smooth muscle is a crucial component of the walls of hollow organs like the gut, bladder, and uterus. This specialized smooth muscle has unique properties that distinguish it from other muscle types and facilitate robust dilation and contraction. Visceral myopathies are diseases where severe visceral smooth muscle dysfunction prevents efficient movement of air and nutrients through the bowel, impairs bladder emptying, and affects normal uterine contraction and relaxation, particularly during pregnancy. Disease severity exists along a spectrum. The most debilitating defects cause highly dysfunctional bowel, reduced intrauterine colon growth (microcolon), and bladder-emptying defects requiring catheterization, a condition called megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS). People with MMIHS often die early in childhood. When the bowel is the main organ affected and microcolon is absent, the condition is known as myopathic chronic intestinal pseudo-obstruction (CIPO). Visceral myopathies like MMIHS and myopathic CIPO are most commonly caused by mutations in contractile apparatus cytoskeletal proteins. Here, we review visceral myopathy-causing mutations and normal functions of these disease-associated proteins. We propose molecular, cellular, and tissue-level models that may explain clinical and histopathological features of visceral myopathy and hope these observations prompt new mechanistic studies.

Pseudo-obstruction-inducing ACTG2R257C alters actin organization and function

Actin γ 2, smooth muscle (ACTG2) R257C mutation is the most common genetic cause of visceral myopathy. Individuals with ACTG2 mutations endure prolonged hospitalizations and surgical interventions, become dependent on intravenous nutrition and bladder catheterization, and often die in childhood. Currently, we understand little about how ACTG2 mutations cause disease, and there are no mechanism-based treatments. Our goal was to characterize the effects of ACTG2R257C on actin organization and function in visceral smooth muscle cells. We overexpressed ACTG2WT or ACTG2R257C in primary human intestinal smooth muscle cells (HISMCs) and performed detailed quantitative analyses to examine effects of ACTG2R257C on (a) actin filament formation and subcellular localization, (b) actin-dependent HISMC functions, and (c) smooth muscle contractile gene expression. ACTG2R257C resulted in 41% fewer, 13% thinner, 33% shorter, and 40% less branched ACTG2 filament bundles compared with ACTG2WT. Curiously, total F-actin probed by phalloidin and a pan-actin antibody was unchanged between ACTG2WT- and ACTG2R257C-expressing HISMCs, as was ultrastructural F-actin organization. ACTG2R257C-expressing HISMCs contracted collagen gels similar to ACTG2WT-expressing HISMCs but spread 21% more and were 11% more migratory. In conclusion, ACTG2R257C profoundly affects ACTG2 filament bundle structure, without altering global actin cytoskeleton in HISMCs.

Generation of an enteric smooth muscle cell line from the pig ileum

Smooth muscle cells (SMCs) play an important role in physiology and production in farm animals such as pigs. Here, we report the generation of a pig SMC line. Our original objective was to establish an enteroendocrine cell line from the pig ileum epithelium through lentiviral transduction of the Simian Virus (SV) 40 large T antigen. However, an initial expression analysis of marker genes in nine cell clones revealed that none of them were enteroendocrine cells or absorptive enterocytes, goblet cells, or Paneth cells, some of the major cell types existing in the ileum epithelium. A more detailed characterization of one clone named PIC7 by RNA-seq showed that these cells expressed many of the known smooth muscle-specific or -enriched genes, including smooth muscle actin alpha 2, calponin 1, calponin 3, myosin heavy chain 11, myosin light chain kinase, smoothelin, tenascin C, transgelin, tropomyosin 1, and tropomyosin 2. Both quantitative PCR and RNA-seq analyses showed that the PIC7 cells had a high expression of mRNA for smooth muscle actin gamma 2, also known as enteric smooth muscle actin. A Western blot analysis confirmed the expression of SV40 T antigen in the PIC7 cells. An immunohistochemical analysis demonstrated the expression of smooth muscle actin alpha 2 filaments in the PIC7 cells. A collagen gel contraction assay showed that the PIC7 cells were capable of both spontaneous contraction and contraction in response to serotonin stimulation. We conclude that the PIC7 cells are derived from an enteric SMC from the pig ileum. These cells may be a useful model for studying the cellular and molecular physiology of pig enteric SMCs. Because pigs are similar to humans in anatomy and physiology, the PIC7 cells may be also used as a model for human intestinal SMCs.

Loss of smooth muscle α-actin effects on mechanosensing and cell-matrix adhesions

Mutations in ACTA2 , encoding smooth muscle α-actin, are a frequent cause of heritable thoracic aortic aneurysm and dissections. These mutations are associated with impaired vascular smooth muscle cell function, which leads to decreased ability of the cell to sense matrix-mediated mechanical stimuli. This study investigates how loss of smooth muscle α-actin affects cytoskeletal tension development and cell adhesion using smooth muscle cells explanted from aorta of mice lacking smooth muscle α-actin. We tested the hypothesis that reduced vascular smooth muscle contractility due to a loss of smooth muscle α-actin decreases cellular mechanosensing by dysregulating cell adhesion to the matrix. Assessment of functional mechanical properties of the aorta by stress relaxation measurements in thoracic aortic rings suggested two functional regimes for Acta2 −/− mice. Lower stress relaxation was recorded in aortic rings from Acta2 −/− mice at tensions below 10 mN compared with wild type, likely driven by cytoskeletal-dependent contractility. However, no differences were recorded between the two groups above the 10 mN threshold, since at higher tension the matrix-dependent contractility may be predominant. In addition, our results showed that at any given level of stretch, transmural pressure is lower in aortic rings from Acta2 −/− mice than wild type mice. In addition, a three-dimensional collagen matrix contractility assay showed that collagen pellets containing Acta2 −/− smooth muscle cells contracted less than the pellets containing the wild type cells. Moreover, second harmonic generation non-linear microscopy revealed that Acta2 −/− cells locally remodeled the collagen matrix fibers to a lesser extent than wild type cells. Quantification of protein fluorescence measurements in cells also showed that in absence of smooth muscle α-actin, there is a compensatory increase in smooth muscle γ-actin. Moreover, specific integrin recruitment at cell–matrix adhesions was reduced in Acta2 −/− cells. Thus, our findings suggest that Acta2 −/− cells are unable to generate external forces to remodel the matrix due to reduced contractility and interaction with the matrix.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Mechanical regulation of myofibroblast phenoconversion and collagen contraction

Activated fibroblasts promote physiological wound repair following tissue injury. However, dysregulation of fibroblast activation contributes to the development of fibrosis by enhanced production and contraction of collagen-rich extracellular matrix. At the peak of their activities, fibroblasts undergo phenotypic conversion into highly contractile myofibroblasts by developing muscle-like features, including formation of contractile actin-myosin bundles. The phenotype and function of fibroblasts and myofibroblasts are mechanically regulated by matrix stiffness using a feedback control system that is integrated with the progress of tissue remodelling. The actomyosin contraction machinery and cell-matrix adhesion receptors are critical elements that are needed for mechanosensing by fibroblasts and the translation of mechanical signals into biological responses. Here, we focus on mechanical and chemical regulation of collagen contraction by fibroblasts and the involvement of these factors in their phenotypic conversion to myofibroblasts.

Sudden coronary death in the young: Evidence of contractile phenotype of smooth muscle cells in the culprit atherosclerotic plaque

BACKGROUND

Culprit coronary atherosclerotic plaques (APs) from young sudden cardiac death (SCD) victims are mostly non-atheromatous, i.e., consisting of proliferative smooth muscle cells (SMCs). Coronary vasospasm has been advocated to explain plaque instability in the absence of thrombosis. Our aim was to characterize the SMC phenotype in the intima and media of coronary arteries from young SCD victims.

METHODS AND RESULTS

A total of 38 coronary artery segments were studied: (a) 18 APs from young (≤40 years old) SCD patients, (b) 9 APs from old (>40 years old) SCD patients, (c) 11 non-atherosclerotic coronary arteries from young patients (≤40 years old). Markers of differentiated SMCs such as α-smooth muscle actin (α-SMA), smooth muscle myosin heavy chains (SMMHCs), and heavy-caldesmon (h-CaD), were assessed in intima and media by immunohistochemistry and quantified morphometrically. In the intima, their expression was higher in non-atherosclerotic arteries (44.37 ± 3.03% for α-SMA, 14.21 ± 2.01% for SMMHCs, 8.90 ± 1.33% for h-CaD) and APs from young SCD victims (38.95 ± 2.29% for α-SMA, 11.92 ± 1.92% for SMMHCs, 8.93 ± 1.12% for h-CaD) compared with old patients (22.01 ± 3.56% for α-SMA, 6.39 ± 0.7% for SMMHCs, 3.00 ± 0.57% for h-CaD; all P statistically significant). The media of non-atherosclerotic arteries and APs from young SCD victims exhibited strong positivity for the differentiation markers unlike that of old patients.

CONCLUSIONS

SMCs of coronary APs as well as from the underlying media from young SCD victims exhibit strong contractile phenotype. In the setting of critical stenosis, both intima and media SMC contractility might contribute to transient coronary spasm leading to myocardial ischemia and SCD.

Smooth muscle: a stiff sculptor of epithelial shapes

Smooth muscle is increasingly recognized as a key mechanical sculptor of epithelia during embryonic development. Smooth muscle is a mesenchymal tissue that surrounds the epithelia of organs including the gut, blood vessels, lungs, bladder, ureter, uterus, oviduct and epididymis. Smooth muscle is stiffer than its adjacent epithelium and often serves its morphogenetic function by physically constraining the growth of a proliferating epithelial layer. This constraint leads to mechanical instabilities and epithelial morphogenesis through buckling. Smooth muscle stiffness alone, without smooth muscle cell shortening, seems to be sufficient to drive epithelial morphogenesis. Fully understanding the development of organs that use smooth muscle stiffness as a driver of morphogenesis requires investigating how smooth muscle develops, a key aspect of which is distinguishing smooth muscle-like tissues from one another in vivo and in culture. This necessitates a comprehensive appreciation of the genetic, anatomical and functional markers that are used to distinguish the different subtypes of smooth muscle (for example, vascular versus visceral) from similar cell types (including myofibroblasts and myoepithelial cells). Here, we review how smooth muscle acts as a mechanical driver of morphogenesis and discuss ways of identifying smooth muscle, which is critical for understanding these morphogenetic events.This article is part of the Theo Murphy meeting issue 'Mechanics of Development'.

Vascular Smooth Muscle Contractile Function Declines With Age in Skeletal Muscle Feed Arteries

Aging induces a progressive decline in vasoconstrictor responses in central and peripheral arteries. This study investigated the hypothesis that vascular smooth muscle (VSM) contractile function declines with age in soleus muscle feed arteries (SFA). Contractile function of cannulated SFA isolated from young (4 months) and old (24 months) Fischer 344 rats was assessed by measuring constrictor responses of denuded (endothelium removed) SFA to norepinephrine (NE), phenylephrine (PE), and angiotensin II (Ang II). In addition, we investigated the role of RhoA signaling in modulation of VSM contractile function. Structural and functional characteristics of VSM cells were evaluated by fluorescence imaging and atomic force microscopy (AFM). Results indicated that constrictor responses to PE and Ang II were significantly impaired in old SFA, whereas constrictor responses to NE were preserved. In the presence of a Rho-kinase inhibitor (Y27632), constrictor responses to NE, Ang II, and PE were significantly reduced in young and old SFA. In addition, the age-group difference in constrictor responses to Ang II was eliminated. ROCK1 and ROCK2 content was similar in young and old VSM cells, whereas pROCK1 and pROCK2 were significantly elevated in old VSM cells. Aging was associated with a reduction in smooth muscle α-actin stress fibers and recruitment of proteins to cell-matrix adhesions. Old VSM cells presented an increase in integrin adhesion to the matrix and smooth muscle γ-actin fibers that was associated with increased cell stiffness. In conclusion, our results indicate that VSM contractile function declined with age in SFA. The decrement in contractile function was mediated in part by RhoA/ROCK signaling. Upregulation of pROCK in old VSM cells was not able to rescue contractility in old SFA. Collectively, these results indicate that changes at the VSM cell level play a central role in the reduced contractile function of aged SFA.

A novel human cell culture model to study visceral smooth muscle phenotypic modulation in health and disease

Adaptation of the smooth muscle cell (SMC) phenotype is essential for homeostasis and is often involved in pathologies of visceral organs (e.g., uterus, bladder, gastrointestinal tract). In vitro studies of the behavior of visceral SMCs under (patho)-physiological conditions are hampered by a spontaneous, uncontrolled phenotypic modulation of visceral SMCs under regular tissue culture conditions. We aimed to develop a new visceral SMC culture model that allows controlled phenotypic modulation. Human uterine SMCs [ULTR and telomerase-immortalized human myometrial cells (hTERT-HM)] were grown to confluency and kept for up to 6 days on regular tissue culture surfaces or basement membrane (BM) matrix-coated surfaces in the presence of 0-10% serum. mRNA and protein expression and localization of SMC-specific phenotype markers and their transcriptional regulators were investigated by quantitative PCR, Western blotting, and immunofluorescence. Maintaining visceral SMCs confluent for 6 days increased α-smooth muscle actin (1.9-fold) and smooth muscle protein 22-α (3.1-fold), whereas smooth muscle myosin heavy chain was only slightly upregulated (1.3-fold). Culturing on a BM matrix-coated surface further increased these proteins and also markedly promoted mRNA expression of γ-smooth muscle actin (15.0-fold), smoothelin (3.5-fold), h-caldesmon (5.2-fold), serum response factor (7.6-fold), and myocardin (8.1-fold). Whereas additional serum deprivation only minimally affected contractile markers, platelet-derived growth factor-BB and transforming growth factor β1 consistently reduced versus increased their expression. In conclusion, we present a simple and reproducible visceral SMC culture system that allows controlled phenotypic modulation toward both the synthetic and the contractile phenotype. This may greatly facilitate the identification of factors that drive visceral SMC phenotypic changes in health and disease.

Reversible differentiation of immortalized human bladder smooth muscle cells accompanied by actin bundle reorganization

Previous studies have shown that phenotypic modulation of smooth muscle cells (SMCs) plays a pivotal role in human diseases. However, the molecular mechanisms underlying the reversible differentiation of SMCs remain elusive particularly because cultured SMCs that reproducibly exhibit bidirectional phenotypic modulation have not been established. Here we established an immortalized human bladder SMC line designated as hBS11. Under differentiation-inducing conditions, hBS11 cells underwent smooth muscle differentiation accompanied by the robust expression of smooth muscle differentiation markers and isoform-dependent reorganization of actin bundles. The cholinergic receptor agonist carbachol increased intracellular calcium in differentiated hBS11 cells in an acetylcholine muscarinic receptor-dependent manner. Differentiated hBS11 cells displayed contractile properties depending on the elevation in the levels of intracellular calcium. Depolarization of membrane potential triggered inward sodium current in differentiated hBS11 cells. However, differentiated hBS11 cells lost the differentiated phenotype and resumed mitosis when re-fed with growth medium. Our study provides direct evidence pertaining to the human bladder SMCs being able to retain the capacity of reversible differentiation and that the reorganization of actin bundles is involved in the reinstatement of contractility. Moreover, we have established a human SMC line retaining high proliferating potential without compromising differentiation potential.

Proteomic analysis and comparison of intra‑ and extracranial cerebral atherosclerosis responses to hyperlipidemia in rabbits

The present study aimed to investigate protein expression levels of intra‑ and extracranial atherosclerosis in rabbits following administration of a high‑fat diet. Rabbits were randomly divided into control (group A; n=9) and high‑fat diet (group B; n=9) groups. At week 12, tissues were sectioned from the common carotid artery (CCA) and middle cerebral artery (MCA). Pathological analysis was performed. Differential protein expression levels were examined by 2‑D gel electrophoresis (2‑DE) and mass spectrometry (MS) analysis and validated by western blotting. Serum lipid levels, the intima‑media thickness (IMT) and degree of atherosclerosis of the CCA and MCA were increased at week 12 in the high‑fat diet group compared with rabbits that received a normal diet. 2‑DE and MS analysis of the protein extracted from CCA and MCA detected >439 different proteins; the expression of 25 proteins was altered, and 8 proteins [albumin A chain, tropomyosin α‑1 chain (TPM1), heat shock protein 70 (HSP70), α‑smooth muscle actin, β‑galactose binding agglutinin, TPM4 isoform 2, cell keratin 9, single octylic acid glyceride β‑2) demonstrated significant alterations in expression levels. Due to limited antibody sources, only three differentially expressed proteins (TPM1, HSP70 and α‑smooth muscle actin) were examined by western blotting. The results of our previous study demonstrated that hyperlipidemia affected the IMT of intracranial and extracranial cerebral arteries. In the present study, protein expression levels of TPM1 and α‑smooth muscle actin from extracranial cerebral arteries were significantly increased compared with intracranial cerebral arteries; however, protein expression levels of HSP70 from intracranial cerebral arteries was increased compared with extracranial cerebral arteries. The differences may be closely associated with cell proliferation and metastasis, and oxidoreduction, in intra‑ and extracranial cerebral atherosclerosis. HSP70 may have protective properties against atherosclerosis via underlying anti‑inflammatory mechanisms, furthermore, differential protein expression levels (TPM1, HSP70 and α‑smooth muscle actin) between intra‑ and extracranial cerebral arteries may facilitate the identification of novel biological markers for the diagnosis and treatment of cerebral arteriosclerosis.

Blockade of HMGB1 preserves vascular homeostasis and improves blood perfusion in rats of acute limb ischemia/reperfusion

Acute limb ischemia is one of the most common peripheral arterial disease, while surgical restoration of blood flow often results in ischemia/reperfusion (I/R) injury. Our previous study revealed the inflammation intensity in arterial tissue, characterized by expression of high mobility group box protein 1 (HMGB1), was contrary to the fluctuation of hemodynamics in reperfusion limbs in a rat model. This study meant to clarify the role of HMGB1 during this process. Laser Doppler perfusion imaging evaluated limb hemodynamics in mean and max perfusion unit (PU). Femoral arterial tissue was collected for molecular biology examination. The results revealed that HMGB1 promoted vascular structure remodeling and vasomotor dysfunction during acute I/R, characterized by degradation of collagenous fibers, disruption of elastic lamellae, intensive inflammation and phenotype transfer of smooth muscle cells. Blockade of HMGB1 preserved vascular homeostasis and improved PUs (PmeanPU<0.001, PmaxPU<0.001). The elevated expression of TNF-α, IL-6, ICAM, VCAM, MMP-2, MMP-9, α-SM actin correlated with HMGB1 positively. In conclusion, HMGB1 promoted vascular remodeling and dysfunction via initiating an inflammation cascade during I/R. Blockade of HMGB1 would preserve vascular homeostasis and facilitate the blood perfusion of ischemic limb.

Matrix metalloproteinases: an emerging role in regulation of actin microfilament system

Matrix metalloproteinases (MMPs) are implicated in many physiological and pathological processes, including contraction, migration, differentiation, and proliferation. These processes all involve cell phenotype changes, known to be accompanied by reorganization of actin cytoskeleton. Growing evidence indicates a correlation between MMP activity and the dynamics of actin system, suggesting their mutual regulation. Here, data on the influence of MMPs on the actin microfilament system, on the one hand, and the dependence of MMP expression and activation on the organization of actin structures, on the other hand, are reviewed. The different mechanisms of putative actin-MMP regulation are discussed.

Expression of epithelial sodium channel (ENaC) and CFTR in the human epidermis and epidermal appendages

A major function of the skin is the regulation of body temperature by sweat secretions. Sweat glands secrete water and salt, especially NaCl. Excreted water evaporates, cooling the skin surface, and Na⁺ ions are reabsorbed by the epithelial sodium channels (ENaC). Mutations in ENaC subunit genes lead to a severe multi-system (systemic) form of pseudohypoaldosteronism (PHA) type I, characterized by salt loss from aldosterone target organs, including sweat glands in the skin. In this study, we mapped the sites of localization of ENaC in the human skin by confocal microscopy using polyclonal antibodies generated against human αENaC. Our results reveal that ENaC is expressed strongly in all epidermal layers except stratum corneum, and also in the sebaceous glands, eccrine glands, arrector pili smooth muscle cells, and intra-dermal adipocytes. In smooth muscle cells and adipocytes, ENaC is co-localized with F-actin. No expression of ENaC was detected in the dermis. CFTR is strongly expressed in sebaceous glands. In epidermal appendages noted, except the eccrine sweat glands, ENaC is mainly located in the cytoplasm. In the eccrine glands and ducts, ENaC and CFTR are located on the apical side of the membrane. This localization of ENaC is compatible with ENaC's role in salt reabsorption. PHA patients may develop folliculitis, miliaria rubra, and atopic dermatitis-like skin lesions, due to sweat gland duct occlusion and inflammation of eccrine glands as a result of salt accumulation.

Localizations of γ-Actins in Skin, Hair, Vibrissa, Arrector Pili Muscle and Other Hair Appendages of Developing Rats

Six isoforms of actins encoded by different genes have been identified in mammals including α-cardiac, α-skeletal, α-smooth muscle (α-SMA), β-cytoplasmic, γ-smooth muscle (γ-SMA), and γ-cytoplasmic actins (γ-CYA). In a previous study we showed the localization of α-SMA and other cytoskeletal proteins in the hairs and their appendages of developing rats (Morioka K., et al. (2011) Acta Histochem. Cytochem. 44, 141–153), and herein we determined the localization of γ type actins in the same tissues and organs by immunohistochemical staining. Our results indicate that the expression of γ-SMA and γ-CYA is suggested to be poor in actively proliferating tissues such as the basal layer of the epidermis and the hair matrix in the hair bulb, and as well as in highly keratinized tissues such as the hair cortex and hair cuticle. In contrast, the expression of γ-actins were high in the spinous layer, granular layer, hair shaft, and inner root sheath, during their active differentiations. In particular, the localization of γ-SMA was very similar to that of α-SMA. It was located not only in the arrector pili muscles and muscles in the dermis, but also in the dermal sheath and in a limited area of the outer root sheath in both the hair and vibrissal follicles. The γ-CYA was suggested to be co-localized with γ-SMA in the dermal sheath, outer root sheath, and arrector pili muscles. Sparsely distributed dermal cells expressed both types of γ-actin. The expression of γ-actins is suggested to undergo dynamic changes according to the proliferation and differentiation of the skin and hair-related cells.

Monoclonal antibodies against muscle actin isoforms: epitope identification and analysis of isoform expression by immunoblot and immunostaining in normal and regenerating skeletal muscle

Higher vertebrates (mammals and birds) express six different highly conserved actin isoforms that can be classified in three subgroups: 1) sarcomeric actins, α-skeletal (α-SKA) and α-cardiac (α-CAA), 2) smooth muscle actins (SMAs), α-SMA and γ-SMA, and 3) cytoplasmic actins (CYAs), β-CYA and γ-CYA. The variations among isoactins, in each subgroup, are due to 3-4 amino acid differences located in their acetylated N-decapeptide sequence. The first monoclonal antibody (mAb) against an actin isoform (α-SMA) was produced and characterized in our laboratory in 1986 (Skalli  et al., 1986) . We have further obtained mAbs against the 5 other isoforms. In this report, we focus on the mAbs anti-α-SKA and anti-α-CAA obtained after immunization of mice with the respective acetylated N-terminal decapeptides using the Repetitive Immunizations at Multiple Sites Strategy (RIMMS). In addition to the identification of their epitope by immunoblotting, we describe the expression of the 2 sarcomeric actins in mature skeletal muscle and during muscle repair after micro-lesions. In particular, we analyze the expression of α-CAA, α-SKA and α-SMA by co-immunostaining in a time course frame during the muscle repair process. Our results indicate that a restricted myocyte population expresses α-CAA and suggest a high capacity of self-regeneration in muscle cells. These antibodies may represent a helpful tool for the follow-up of muscle regeneration and pathological changes.

In vitro construction and in vivo regeneration of esophageal bilamellar muscle tissue

In order to induce esophageal muscle cells’ orientation, the silicon wafer with prototype 1 and prototype 2 was designed. Prototype 1 has micro-channels of 200 µm width and 30 µm depth with 30 µm wide wall as the interval. Prototype 2 has channels of 100 µm width and 30 µm depth with a discontinuous wall which has 30 µm gap for each 100 µm channel. The poly(ester urethane) scaffolds with pattern prototype 1 and prototype 2 were fabricated using solution casting method and abbreviated as PU1 and PU2, respectively. Silk fibroin was grafted individually on PU1 and PU2 surface (PU1-SF, PU2-SF) using our previous protocol, aiming at improving scaffolds’ biocompatibility. The primary esophageal smooth muscle cell was seeded to evaluate the scaffolds’ cytocompatibility in vitro. Characterizations like MTT assay, immunocytochemistry, scanning electron microscope, and Western blotting were applied. After that, poly(ester urethane) scaffolds with double patterns, prototype 1 on the exterior, and prototype 2 in the lumen were implanted into the rabbit esophagous to test the regeneration of the muscle tissue. Results from these preliminary tests showed that the growth and differentiation of primary smooth muscle cells were promoted, but also the muscle tissue with endocircular and exolongitudinal architecture was in regenerating, against non-constitution in the animals without the patterned scaffold or with poly(ester urethane) plane membrane at the defaulted sites. This micro-channel pattern together with silk fibroin grafting and vascular endothelial growth factor coating greatly promoted the regeneration of esophageal muscle with normal histological structure.

Mammalian Actins: Isoform-Specific Functions and Diseases

Actin is the central building block of the actin cytoskeleton, a highly regulated filamentous network enabling dynamic processes of cells and simultaneously providing structure. Mammals have six actin isoforms that are very conserved and thus share common functions. Tissue-specific expression in part underlies their differential roles, but actin isoforms also coexist in various cell types and tissues, suggesting specific functions and preferential interaction partners. Gene deletion models, antibody-based staining patterns, gene silencing effects, and the occurrence of isoform-specific mutations in certain diseases have provided clues for specificity on the subcellular level and its consequences on the organism level. Yet, the differential actin isoform functions are still far from understood in detail. Biochemical studies on the different isoforms in pure form are just emerging, and investigations in cells have to deal with a complex and regulated system, including compensatory actin isoform expression.

ACTG2 variants impair actin polymerization in sporadic Megacystis Microcolon Intestinal Hypoperistalsis Syndrome

Megacystis Microcolon Intestinal Hypoperistalsis Syndrome (MMIHS) is a rare congenital disorder, in which heterozygous missense variants in the Enteric Smooth Muscle actin γ-2 (ACTG2) gene have been recently identified. To investigate the mechanism by which ACTG2 variants lead to MMIHS, we screened a cohort of eleven MMIHS patients, eight sporadic and three familial cases, and performed immunohistochemistry, molecular modeling and molecular dynamics (MD) simulations, and in vitro assays. In all sporadic cases, a heterozygous missense variant in ACTG2 was identified. ACTG2 expression was detected in all intestinal layers where smooth muscle cells are present in different stages of human development. No histopathological abnormalities were found in the patients. Using molecular modeling and MD simulations, we predicted that ACTG2 variants lead to significant changes to the protein function. This was confirmed by in vitro studies, which showed that the identified variants not only impair ACTG2 polymerization, but also contribute to reduced cell contractility. Taken together, our results confirm the involvement of ACTG2 in sporadic MMIHS, and bring new insights to MMIHS pathogenesis.

The defining pathology of the new clinical and histopathologic entity ACTA2-related cerebrovascular disease

INTRODUCTION

Smooth muscle cell contraction is an essential function of arteries and relies on the integrity of the actin-myosin apparatus. The tissue-specific α2-smooth muscle actin, encoded by ACTA2, is predominantly expressed in vascular smooth muscle cells. ACTA2 mutations predispose to development of aortic aneurysms and early onset coronary and cerebrovascular disease. Based on arteriographic findings, a distinct cerebrovascular disease has been proposed for ACTA2 heterozygous patients carrying the R179H mutation.

RESULTS

We present the first integrated analysis of a severely compromised patient with the R179H mutation and define the arterial pathology of ACTA2-related cerebrovascular disease. Histologically, striking morphological abnormalities were present in cerebral arteries of all sizes. Massive intimal smooth muscle cell proliferation, fragmentation of the elastic laminae and medial fibromuscular proliferation characterized large arteries whereas prominent vessel wall thickening, fibrosis and smooth muscle cell proliferation were unique changes in small arteries. The medial fibrosis and smooth muscle cell proliferation explain the characteristic radiologic appearance of "straight arteries" and suggest impaired function of mutant smooth muscle cells. Actin three-dimensional molecular modeling revealed critical positioning of R179 at the interface between the two strands of filamentous actin and destabilization of inter-strand bundling by the R179H mutation, explaining the severe associated phenotype.

CONCLUSIONS

In conclusion, these characteristic clinical and pathologic findings confirm ACTA2-related cerebrovascular disease as a new cerebrovascular disorder for which new therapeutic strategies need to be designed.

Gamma-smooth muscle actin expression is associated with epithelial-mesenchymal transition and stem-like properties in hepatocellular carcinoma

BACKGROUND AND AIMS

The prognosis of hepatocellular carcinoma (HCC) is hampered by frequent tumour recurrence and metastases. Epithelial-Mesenchymal Transition (EMT) is now recognized as a key process in tumour invasion, metastasis and the generation of cancer initiating cells. The morphological identification of EMT in tumour samples from the expression of novel mesenchymal markers could provide relevant prognostic information and aid in understanding the metastatic process.

METHODS

The expression of Smooth Muscle Actins was studied using immunofluorescence and immunohistochemistry assays in cultured liver cells during an induced EMT process and in liver specimens from adult and paediatric HCC series.

RESULTS

We report here that in HCC cell lines treated with TGF-β and in HCC specimens, the expression of αSMA, a known mesenchymal marker of EMT, could never be detected. In addition, our in vitro studies identified the enteric form of SMA, γSMA, as being a marker of EMT. Moreover, this SMA isoform was expressed in 46% of 58 tumours from 42 adult HCC patients and in 90% of 16 tumours from 12 paediatric HCC patients. Interestingly, this expression was significantly correlated with poor tumour differentiation and progenitor cell features characterized by the expression of EpCAM and K19.

CONCLUSION

Taken together, our results support the conclusion that γSMA expression in HCC is strongly correlated with the EMT process, HCC aggressiveness and the identification of cancer stem cells. This correlation suggests that γSMA represents a novel and powerful marker to predict HCC progression.

Idiopathic epiretinal membrane

BACKGROUND

Idiopathic epiretinal membrane (iERM) is a fibrocellular membrane that proliferates on the inner surface of the retina at the macular area. Membrane contraction is an important sight-threatening event and is due to fibrotic remodeling.

METHODS

Analysis of the current literature regarding the epidemiology, clinical features, and pathogenesis of iERM and fibrotic tissue contraction.

RESULTS

Epidemiologic studies report a relationship between iERM prevalence, increasing age, and posterior vitreous detachment. Clinically, iERM progresses through different stages characterized by an increased thickness and wrinkling of the membrane. Pathophysiologically, iERM formation is a fibrotic process in which myofibroblast formation and the deposition of newly formed collagens play key roles. Anomalous posterior vitreous detachment may be a key event initiating the formation of iERM. The age-related accumulation of advanced glycation end products may contribute to anomalous posterior vitreous detachment formation and may also influence the mechanical properties of the iERM.

CONCLUSION

Remodeling of the extracellular matrix at the vitreoretinal interface by aging and fibrotic changes, plays a significant role in the pathogenesis of iERM. A better understanding of molecular mechanisms underlying this process may eventually lead to the development of effective and nonsurgical approaches to treat and prevent vitreoretinal fibrotic diseases.

A role for actin polymerization in persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN) is defined as the failure of normal pulmonary vascular relaxation at birth. Hypoxia is known to impede postnatal disassembly of the actin cytoskeleton in pulmonary arterial myocytes, resulting in elevation of smooth muscle α-actin and γ-actin content in elastic and resistance pulmonary arteries in PPHN compared with age-matched controls. This review examines the original histological characterization of PPHN with attention to cytoskeletal structural remodeling and actin isoform abundance, reviews the existing evidence for understanding the biophysical and biochemical forces at play during neonatal circulatory transition, and specifically addresses the role of the cortical actin architecture, primarily identified as γ-actin, in the transduction of mechanical force in the hypoxic PPHN pulmonary circuit.