Aortic carboxypeptidase-like protein is expressed in collagen-rich tissues during mouse embryonic development

Fibroblast-specific adipocyte enhancer binding protein 1 is a potential pathological trigger and prognostic marker for liver fibrosis independent of etiology

OBJECTIVES

Aortic carboxypeptidase-like protein (ACLP) is an extracellular protein involved in adipogenesis, epithelial-mesenchymal transition, epithelial cell hyperplasia, and collagen fibrogenesis. This study mainly aimed to analyze the potential role of adipocyte enhancer binding protein 1 (AEBP1), the ACLP-encoding gene, as a pathological target or prognostic marker for liver fibrosis regardless of etiology.

METHODS

Dysregulation pattern, clinical relevance, and biological significance of AEBP1 gene in liver fibrosis were analyzed using publicly available transcriptomic profiles, different liver fibrosis mouse models, biological databases, and AEBP1 gene silencing followed by RNA sequencing in human hepatic stellate cells (HSCs).

RESULTS

AEBP1 gene expression was upregulated and positively correlated with liver fibrogenesis independent of etiology, the protein of which was further verified in liver fibrosis mouse models induced by different pathogenic factors. A higher expression of liver AEBP1 gene had the potential to predict poor prognosis in liver fibrosis. Systematic bioinformatic analyses revealed that AEBP1 expression was HSCs-specific and associated with extracellular matrix (ECM) remodeling and its downstream mechanical-chemical signaling transition. AEBP1 knockdown by specific small interfering RNAs (siRNAs) in HSCs inhibited ECM-receptor interaction and immune-related pathways as well as HSC proliferation or activation.

CONCLUSION

A high expression of AEBP1 was specifically associated with liver fibrosis and was related to a poor prognosis and predicted the role of AEBP1 in HSCs, providing a new insight for understanding AEBP1 in liver fibrosis.

Acquisition of new function through gene duplication in the metallocarboxypeptidase family

Gene duplication is a key first step in the process of expanding the functionality of a multigene family. In order to better understand the process of gene duplication and its role in the formation of new enzymes, we investigated recent duplication events in the M14 family of proteolytic enzymes. Within vertebrates, four of 23 M14 genes were frequently found in duplicate form. While AEBP1, CPXM1, and CPZ genes were duplicated once through a large-scale, likely whole-genome duplication event, the CPO gene underwent many duplication events within fish and Xenopus lineages. Bioinformatic analyses of enzyme specificity and conservation suggested a greater amount of neofunctionalization and purifying selection in CPO paralogs compared with other CPA/B enzymes. To examine the functional consequences of evolutionary changes on CPO paralogs, the four CPO paralogs from Xenopus tropicalis were expressed in Sf9 and HEK293T cells. Immunocytochemistry showed subcellular distribution of Xenopus CPO paralogs to be similar to that of human CPO. Upon activation with trypsin, the enzymes demonstrated differential activity against three substrates, suggesting an acquisition of new function following duplication and subsequent mutagenesis. Characteristics such as gene size and enzyme activation mechanisms are possible contributors to the evolutionary capacity of the CPO gene.

Silencing of adipocyte enhancer-binding protein 1 (AEBP1) alleviates renal fibrosis in vivo and in vitro via inhibition of the β-catenin signaling pathway

Renal fibrosis is the common final pathway in many renal diseases regardless of the underlying etiology. Adipocyte enhancer-binding protein 1 (AEBP1) was reported to play a vital role in the development of organ fibrosis, but its role in renal fibrosis has not been reported. Thus, the aim of this study was to investigate the possible function of AEBP1 in renal fibrosis and the mechanism associated with the β-catenin signaling pathway. A total of 83 genes upregulated after unilateral ureteral obstruction (UUO) were screened from two Gene Expression Omnibus (GEO) datasets and subjected to Gene Ontology (GO) function and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Among them, AEBP1 was enriched in collagen binding and the regulation of collagen fibril organization and was confirmed to be upregulated in UUO kidneys and TGF-β1-induced cells. Knockdown of AEBP1 ameliorated renal fibrosis via reducing collagen accumulation, inhibiting epithelial-mesenchymal transition and fibroblast transformation, as evidenced by decreases in the expression of collagen I and III, Col1a1, Col3a1, fibronectin, Snail, α-SMA, as well as collagen-specific staining of kidney tissues, whereas the E-cadherin was increased. Besides, AEBP1 silencing inhibited the expression of β-catenin in nucleus and β-catenin downstream proteins (Axin2, Myc, and Ccnd1). Continuously active β-catenin-S33Y further restored the inhibitory effect of AEBP1 silencing on renal fibrosis. These findings indicate that knockdown of AEBP1 could potentially slow down renal fibrosis by blocking the β-catenin signaling pathway, highlighting the potential of AEBP1 as a therapeutic target for renal fibrosis.

Epitenon-derived cells comprise a distinct progenitor population that contributes to both tendon fibrosis and regeneration following acute injury

Flexor tendon injuries are common and heal poorly owing to both the deposition of function- limiting peritendinous scar tissue and insufficient healing of the tendon itself. Therapeutic options are limited due to a lack of understanding of the cell populations that contribute to these processes. Here, we identified a bi-fated progenitor cell population that originates from the epitenon and goes on to contribute to both peritendinous fibrosis and regenerative tendon healing following acute tendon injury. Using a combination of genetic lineage tracing and single cell RNA-sequencing (scRNA-seq), we profiled the behavior and contributions of each cell fate to the healing process in a spatio-temporal manner. Branched pseudotime trajectory analysis identified distinct transcription factors responsible for regulation of each fate. Finally, integrated scRNA-seq analysis of mouse healing with human peritendinous scar tissue revealed remarkable transcriptional similarity between mouse epitenon- derived cells and fibroblasts present in human peritendinous scar tissue, which was further validated by immunofluorescent staining for conserved markers. Combined, these results clearly identify the epitenon as the cellular origin of an important progenitor cell population that could be leveraged to improve tendon healing.

ACLP promotes activation of cancer-associated fibroblasts and tumor metastasis via ACLP-PPARγ-ACLP feedback loop in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy with poor prognosis. Its fibrotic tumor microenvironment (TME) plays a crucial role in promoting tumor invasion and metastasis, which eventually leads to a dismal 5-year survival rate in PDAC patients. Aortic carboxypeptidase-like protein (ACLP) promotes tissue fibrosis in benign diseases. However, its role in cancer-associated fibrosis remains unelucidated. Here, we show that ACLP was mainly expressed in cancer-associated fibroblasts (CAFs) but not in cancer cells and highly expressed in PDAC tissues. High ACLP expression was correlated with poor overall survival. Moreover, ACLP expression in PDAC patients with liver metastases was higher than that in PDAC patients without liver metastases. By detecting activation marker expression and CAF contractility and motility, we found that ACLP promoted CAF activation in PDAC, leading to TME fibrosis. Furthermore, ACLP-activated CAFs could promote cancer cell invasion in vitro and tumor metastasis in vivo. Mechanistically, ACLP promotes the expressions of MMP1 and MMP3 in CAFs, thus promoting PDAC invasion and metastasis. Intriguingly, we identified an ACLP-PPARγ-ACLP feedback loop in PDAC CAFs. Abatement of this feedback loop might be a promising approach in CAF-targeting PDAC treatment.

The Ehlers–Danlos Syndromes against the Backdrop of Inborn Errors of Metabolism

The Ehlers–Danlos syndromes are a group of multisystemic heritable connective tissue disorders with clinical presentations that range from multiple congenital malformations, over adolescent-onset debilitating or even life-threatening complications of connective tissue fragility, to mild conditions that remain undiagnosed in adulthood. To date, thirteen different EDS types have been recognized, stemming from genetic defects in 20 different genes. While initial biochemical and molecular analyses mainly discovered defects in genes coding for the fibrillar collagens type I, III and V or their modifying enzymes, recent discoveries have linked EDS to defects in non-collagenous matrix glycoproteins, in proteoglycan biosynthesis and in the complement pathway. This genetic heterogeneity explains the important clinical heterogeneity among and within the different EDS types. Generalized joint hypermobility and skin hyperextensibility with cutaneous fragility, atrophic scarring and easy bruising are defining manifestations of EDS; however, other signs and symptoms of connective tissue fragility, such as complications of vascular and internal organ fragility, orocraniofacial abnormalities, neuromuscular involvement and ophthalmological complications are variably present in the different types of EDS. These features may help to differentiate between the different EDS types but also evoke a wide differential diagnosis, including different inborn errors of metabolism. In this narrative review, we will discuss the clinical presentation of EDS within the context of inborn errors of metabolism, give a brief overview of their underlying genetic defects and pathophysiological mechanisms and provide a guide for the diagnostic approach.

Aortic carboxypeptidase-like protein regulates vascular adventitial progenitor and fibroblast differentiation through myocardin related transcription factor A

The vascular adventitia contains numerous cell types including fibroblasts, adipocytes, inflammatory cells, and progenitors embedded within a complex extracellular matrix (ECM) network. In response to vascular injury, adventitial progenitors and fibroblasts become activated and exhibit increased proliferative capacity and differentiate into contractile cells that remodel the ECM. These processes can lead to vascular fibrosis and disease progression. Our previous work established that the ECM protein aortic carboxypeptidase-like protein (ACLP) promotes fibrotic remodeling in the lung and is activated by vascular injury. It is currently unknown what controls vascular adventitial cell differentiation and if ACLP has a role in this process. Using purified mouse aortic adventitia Sca1+ progenitors, ACLP repressed stem cell markers (CD34, KLF4) and upregulated smooth muscle actin (SMA) and collagen I expression. ACLP enhanced myocardin-related transcription factor A (MRTFA) activity in adventitial cells by promoting MRTFA nuclear translocation. Sca1 cells from MRTFA-null mice exhibited reduced SMA and collagen expression induced by ACLP, indicating Sca1 cell differentiation is regulated in part by the ACLP-MRTFA axis. We determined that ACLP induced vessel contraction and increased adventitial collagen in an explant model. Collectively these studies identified ACLP as a mediator of adventitial cellular differentiation, which may result in pathological vessel remodeling.

The Ehlers-Danlos syndromes

The Ehlers-Danlos syndromes (EDS) are a heterogeneous group of hereditary disorders of connective tissue, with common features including joint hypermobility, soft and hyperextensible skin, abnormal wound healing and easy bruising. Fourteen different types of EDS are recognized, of which the molecular cause is known for 13 types. These types are caused by variants in 20 different genes, the majority of which encode the fibrillar collagen types I, III and V, modifying or processing enzymes for those proteins, and enzymes that can modify glycosaminoglycan chains of proteoglycans. For the hypermobile type of EDS, the molecular underpinnings remain unknown. As connective tissue is ubiquitously distributed throughout the body, manifestations of the different types of EDS are present, to varying degrees, in virtually every organ system. This can make these disorders particularly challenging to diagnose and manage. Management consists of a care team responsible for surveillance of major and organ-specific complications (for example, arterial aneurysm and dissection), integrated physical medicine and rehabilitation. No specific medical or genetic therapies are available for any type of EDS.

Mechanisms of aortic carboxypeptidase-like protein secretion and identification of an intracellularly retained variant associated with Ehlers-Danlos syndrome

Aortic carboxypeptidase-like protein (ACLP) is a collagen-binding extracellular matrix protein that has important roles in wound healing and fibrosis. ACLP contains thrombospondin repeats, a collagen-binding discoidin domain, and a catalytically inactive metallocarboxypeptidase domain. Recently, mutations in the ACLP-encoding gene, AE-binding protein 1 (AEBP1), have been discovered, leading to the identification of a new variant of Ehlers-Danlos syndrome causing connective tissue disruptions in multiple organs. Currently, little is known about the mechanisms of ACLP secretion or the role of post-translational modifications in these processes. We show here that the secreted form of ACLP contains N-linked glycosylation and that inhibition of glycosylation results in its intracellular retention. Using site-directed mutagenesis, we determined that glycosylation of Asn-471 and Asn-1030 is necessary for ACLP secretion and identified a specific N-terminal proteolytic ACLP fragment. To determine the contribution of secreted ACLP to extracellular matrix mechanical properties, we generated and mechanically tested wet-spun collagen ACLP composite fibers, finding that ACLP enhances the modulus (or stiffness), toughness, and tensile strength of the fibers. Some AEBP1 mutations were null alleles, whereas others resulted in expressed proteins. We tested the hypothesis that a recently discovered 40-amino acid mutation and insertion in the ACLP discoidin domain regulates collagen binding and assembly. Interestingly, we found that this protein variant is retained intracellularly and induces endoplasmic reticulum stress identified with an XBP1-based endoplasmic reticulum stress reporter. Our findings highlight the importance of N-linked glycosylation of ACLP for its secretion and contribute to our understanding of ACLP-dependent disease pathologies.

Bi-allelic AEBP1 mutations in two patients with Ehlers-Danlos syndrome

The Ehlers-Danlos syndromes (EDSs) are a clinically and molecularly diverse group of heritable connective tissue disorders caused by defects in a wide range of genes. Recently, bi-allelic loss-of-function mutations in the adipocyte enhancer-binding protein 1 (AEBP1) gene were reported in three families with an autosomal recessive EDS-like condition characterized by thin and hyperextensible skin, poor wound healing with prominent atrophic scarring, joint hypermobility and osteoporosis. Using whole exome sequencing, we identified novel bi-allelic AEBP1 variants in two unrelated adult patients, previously diagnosed with an undefined EDS type, which shows important clinical resemblance to several other EDS subtypes. Our patients present with similar cutaneous and musculoskeletal features as the previously reported patients. They also show unreported clinical features, including pectus deformity, premature aged appearance, sparse and frizzled hair, fatigue and pain. AEBP1 is ubiquitously expressed and encodes the secreted aortic carboxypeptidase-like protein (ACLP) that can bind fibrillar collagens and assist in collagen polymerization. Transmission electron microscopy studies on the patients' skin biopsies show ultrastructural alterations in collagen fibril diameter and appearance, underscoring an important role for ACLP in collagen fibril organization. This report further expands the clinical, molecular and ultrastructural spectrum associated with AEBP1 defects and highlights the complex and variable phenotype associated with this new EDS variant.

Expanding the Clinical and Mutational Spectrum of Recessive AEBP1-Related Classical-Like Ehlers-Danlos Syndrome

Ehlers-Danlos syndrome (EDS) comprises clinically heterogeneous connective tissue disorders with diverse molecular etiologies. The 2017 International Classification for EDS recognized 13 distinct subtypes caused by pathogenic variants in 19 genes mainly encoding fibrillar collagens and collagen-modifying or processing proteins. Recently, a new EDS subtype, i.e., classical-like EDS type 2, was defined after the identification, in six patients with clinical findings reminiscent of EDS, of recessive alterations in AEBP1, which encodes the aortic carboxypeptidase⁻like protein associating with collagens in the extracellular matrix. Herein, we report on a 53-year-old patient, born from healthy second-cousins, who fitted the diagnostic criteria for classical EDS (cEDS) for the presence of hyperextensible skin with multiple atrophic scars, generalized joint hypermobility, and other minor criteria. Molecular analyses of cEDS genes did not identify any causal variant. Therefore, AEBP1 sequencing was performed that revealed homozygosity for the rare c.1925T>C p.(Leu642Pro) variant classified as likely pathogenetic (class 4) according to the American College of Medical Genetics and Genomics (ACMG) guidelines. The comparison of the patient's features with those of the other patients reported up to now and the identification of the first missense variant likely associated with the condition offer future perspectives for EDS nosology and research in this field.

A biallelic truncating AEBP1 variant causes connective tissue disorder in two siblings

Biallelic variants in the AEBP1 gene cause a novel autosomal-recessive connective tissue disorder (CTD) reminiscent of Ehlers-Danlos Syndrome (EDS). The four previously reported individuals show considerable clinical variability. Unbiased high-throughput sequencing enables the rapid identification of additional cases for such rare entities. We identified the homozygous nonsense variant c.917dup, p.Tyr306* in AEBP1 using clinical exome sequencing in a female individual with previously unsolved CTD. Segregation testing confirmed homozygosity in the clinically affected brother and heterozygous carrier status in the healthy mother. Chromosomal microarray showed that the variant lies in a run of homozygosity, suggesting a common origin of this genomic segment. RT-PCR analysis in the mother revealed a monoallelic expression of the normal transcript supporting a nonsense-mediated mRNA decay and functional nullizygosity as disease mechanism. We describe two individuals from a fourth family with AEBP1-associated CTD. Our results further verify that autosomal-recessive inherited LOF variants in the AEBP1 gene cause clinical features of different EDS subtypes, but also of the marfanoid spectrum. As identification of further individuals is necessary to inform the clinical characterization, we stress the added value of exome sequencing for such rare diseases.

Aortic carboxypeptidase-like protein enhances adipose tissue stromal progenitor differentiation into myofibroblasts and is upregulated in fibrotic white adipose tissue

White adipose tissue expands through both adipocyte hypertrophy and hyperplasia and it is hypothesized that fibrosis or excess accumulation of extracellular matrix within adipose tissue may limit tissue expansion contributing to metabolic dysfunction. The pathways that control adipose tissue remodeling are only partially understood, however it is likely that adipose tissue stromal and perivascular progenitors participate in fibrotic remodeling and also serve as adipocyte progenitors. The goal of this study was to investigate the role of the secreted extracellular matrix protein aortic carboxypeptidase-like protein (ACLP) on adipose progenitor differentiation in the context of adipose tissue fibrosis. Treatment of 10T1/2 mouse cells with recombinant ACLP suppressed adipogenesis and enhanced myofibroblast differentiation, which was dependent on transforming growth factor-β receptor kinase activity. Mice fed a chronic high fat diet exhibited white adipose tissue fibrosis with elevated ACLP expression and cellular fractionation of these depots revealed that ACLP was co-expressed with collagens primarily in the inflammatory cell depleted stromal-vascular fraction (SVF). SVF cells isolated from mice fed a high fat diet secreted increased amounts of ACLP compared to low fat diet control SVF. These cells also exhibited reduced adipogenic differentiation capacity in vitro. Importantly, differentiation studies in primary human adipose stromal cells revealed that mature adipocytes do not express ACLP and exogenous ACLP administration blunted their differentiation potential while upregulating myofibroblastic markers. Collectively, these studies identify ACLP as a stromal derived mediator of adipose progenitor differentiation that may limit adipocyte expansion during white adipose tissue fibrosis.

FACS-Seq analysis of Pax3-derived cells identifies non-myogenic lineages in the embryonic forelimb

Skeletal muscle in the forelimb develops during embryonic and fetal development and perinatally. While much is known regarding the molecules involved in forelimb myogenesis, little is known about the specific mechanisms and interactions. Migrating skeletal muscle precursor cells express Pax3 as they migrate into the forelimb from the dermomyotome. To compare gene expression profiles of the same cell population over time, we isolated lineage-traced Pax3⁺ cells (Pax3^EGFP) from forelimbs at different embryonic days. We performed whole transcriptome profiling via RNA-Seq of Pax3⁺ cells to construct gene networks involved in different stages of embryonic and fetal development. With this, we identified genes involved in the skeletal, muscular, vascular, nervous and immune systems. Expression of genes related to the immune, skeletal and vascular systems showed prominent increases over time, suggesting a non-skeletal myogenic context of Pax3-derived cells. Using co-expression analysis, we observed an immune-related gene subnetwork active during fetal myogenesis, further implying that Pax3-derived cells are not a strictly myogenic lineage, and are involved in patterning and three-dimensional formation of the forelimb through multiple systems.

Bi-allelic Alterations in AEBP1 Lead to Defective Collagen Assembly and Connective Tissue Structure Resulting in a Variant of Ehlers-Danlos Syndrome

AEBP1 encodes the aortic carboxypeptidase-like protein (ACLP) that associates with collagens in the extracellular matrix (ECM) and has several roles in development, tissue repair, and fibrosis. ACLP is expressed in bone, the vasculature, and dermal tissues and is involved in fibroblast proliferation and mesenchymal stem cell differentiation into collagen-producing cells. Aebp1-/- mice have abnormal, delayed wound repair correlating with defects in fibroblast proliferation. In this study, we describe four individuals from three unrelated families that presented with a unique constellation of clinical findings including joint laxity, redundant and hyperextensible skin, poor wound healing with abnormal scarring, osteoporosis, and other features reminiscent of Ehlers-Danlos syndrome (EDS). Analysis of skin biopsies revealed decreased dermal collagen with abnormal collagen fibrils that were ragged in appearance. Exome sequencing revealed compound heterozygous variants in AEBP1 (c.1470delC [p.Asn490_Met495delins(40)] and c.1743C>A [p.Cys581∗]) in the first individual, a homozygous variant (c.1320_1326del [p.Arg440Serfs∗3]) in the second individual, and a homozygous splice site variant (c.1630+1G>A) in two siblings from the third family. We show that ACLP enhances collagen polymerization and binds to several fibrillar collagens via its discoidin domain. These studies support the conclusion that bi-allelic pathogenic variants in AEBP1 are the cause of this autosomal-recessive EDS subtype.

Surface-bound collagen 4 is significantly more stable than collagen 1

Collagen 1 (C1) is commonly used to improve biological responses to implant surfaces. Here, the stability of C1 was compared with collagen 4 (C4) on a mixed macrodiol polyurethane, both adsorbed and covalently bound via acetaldehyde glow discharge polymerization and reductive amination. Substrate specimens were incubated in solutions of C1 and C4. The strength of conjugation was tested by incubation in 8 M urea followed by enzyme linked immunosorbent assays to measure residual C1 and C4. The basal lamina protein, laminin-332 (L332) was superimposed via adsorption on C4-treated specimens. Keratinocytes were grown on untreated, C1-treated, C4-treated, and C4 + L332-treated specimens, followed by measurement of cell area, proliferation, and focal adhesion density. Adsorbed C4 was shown to be significantly more stable than C1 and covalent conjugation conferred even greater stability, with no degradation of C4 over twenty days in 8 M urea. Cell growth was similar for C1 and C4, with no additional benefit conferred by superimposition of L332. The greater resistance of C4 to degradation may be consequent to cysteine residues and disulphide bonds in its non-collagenous domains. The use of C4 on implants, rather than C1, may improve their long-term stability in tissues. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1364-1373, 2017.

Expanding the clinical and genetic heterogeneity of hereditary disorders of connective tissue

Ehlers-Danlos syndrome (EDS) describes a group of clinical entities in which the connective tissue, primarily that of the skin, joint and vessels, is abnormal, although the resulting clinical manifestations can vary widely between the different historical subtypes. Many cases of hereditary disorders of connective tissue that do not seem to fit these historical subtypes exist. The aim of this study is to describe a large series of patients with inherited connective tissue disorders evaluated by our clinical genetics service and for whom a likely causal variant was identified. In addition to clinical phenotyping, patients underwent various genetic tests including molecular karyotyping, candidate gene analysis, autozygome analysis, and whole-exome and whole-genome sequencing as appropriate. We describe a cohort of 69 individuals representing 40 families, all referred because of suspicion of an inherited connective tissue disorder by their primary physician. Molecular lesions included variants in the previously published disease genes B3GALT6, GORAB, ZNF469, B3GAT3, ALDH18A1, FKBP14, PYCR1, CHST14 and SPARC with interesting variations on the published clinical phenotypes. We also describe the first recessive EDS-like condition to be caused by a recessive COL1A1 variant. In addition, exome capture in a familial case identified a homozygous truncating variant in a novel and compelling candidate gene, AEBP1. Finally, we also describe a distinct novel clinical syndrome of cutis laxa and marked facial features and propose ATP6V1E1 and ATP6V0D2 (two subunits of vacuolar ATPase) as likely candidate genes based on whole-genome and whole-exome sequencing of the two families with this new clinical entity. Our study expands the clinical spectrum of hereditary disorders of connective tissue and adds three novel candidate genes including two that are associated with a highly distinct syndrome.

Insider trading: Extracellular matrix proteins and their non-canonical intracellular roles

In metazoans, the extracellular matrix (ECM) provides a dynamic, heterogeneous microenvironment that has important supportive and instructive roles. Although the primary site of action of ECM proteins is extracellular, evidence is emerging for non-canonical intracellular roles. Examples include osteopontin, thrombospondins, IGF-binding protein 3 and biglycan, and relate to roles in transcription, cell-stress responses, autophagy and cancer. These findings pose conceptual problems on how proteins signalled for secretion can be routed to the cytosol or nucleus, or can function in environments with diverse redox, pH and ionic conditions. We review evidence for intracellular locations and functions of ECM proteins, and current knowledge of the mechanisms by which they may enter intracellular compartments. We evaluate the experimental methods that are appropriate to obtain rigorous evidence for intracellular localisation and function. Better insight into this under-researched topic is needed to decipher the complete spectrum of physiological and pathological roles of ECM proteins.

Stable methylation at promoters distinguishes epiblast stem cells from embryonic stem cells and the in vivo epiblasts

Embryonic Stem Cells (ESCs) and Epiblast Stem Cells (EpiSCs) are the in vitro representatives of naïve and primed pluripotency, respectively. It is currently unclear how their epigenomes underpin the phenotypic and molecular characteristics of these distinct pluripotent states. Here, we performed a genome-wide comparison of DNA methylation between ESCs and EpiSCs by MethylCap-Seq. We observe that promoters are preferential targets for methylation in EpiSC compared to ESCs, in particular high CpG island promoters. This is in line with upregulation of the de novo methyltransferases Dnmt3a1 and Dnmt3b in EpiSC, and downregulation of the demethylases Tet1 and Tet2. Remarkably, the observed DNA methylation signature is specific to EpiSCs and differs from that of their in vivo counterpart, the postimplantation epiblast. Using a subset of promoters that are differentially methylated, we show that DNA methylation is established within a few days during in vitro outgrowth of the epiblast, and also occurs when ESCs are converted to EpiSCs in vitro. Once established, this methylation is stable, as ES-like cells obtained by in vitro reversion of EpiSCs display an epigenetic memory that only extensive passaging and sub-cloning are able to almost completely erase.

Aortic carboxypeptidase-like protein (ACLP) enhances lung myofibroblast differentiation through transforming growth factor β receptor-dependent and -independent pathways

Idiopathic pulmonary fibrosis (IPF) is a chronic and fatal lung disease characterized by the overgrowth, hardening, and scarring of lung tissue. The exact mechanisms of how IPF develops and progresses are unknown. IPF is characterized by extracellular matrix remodeling and accumulation of active TGFβ, which promotes collagen expression and the differentiation of smooth muscle α-actin (SMA)-positive myofibroblasts. Aortic carboxypeptidase-like protein (ACLP) is an extracellular matrix protein secreted by fibroblasts and myofibroblasts and is expressed in fibrotic human lung tissue and in mice with bleomycin-induced fibrosis. Importantly, ACLP knockout mice are significantly protected from bleomycin-induced fibrosis. The goal of this study was to identify the mechanisms of ACLP action on fibroblast differentiation. As primary lung fibroblasts differentiated into myofibroblasts, ACLP expression preceded SMA and collagen expression. Recombinant ACLP induced SMA and collagen expression in mouse and human lung fibroblasts. Knockdown of ACLP slowed the fibroblast-to-myofibroblast transition and partially reverted differentiated myofibroblasts by reducing SMA expression. We hypothesized that ACLP stimulates myofibroblast formation partly through activating TGFβ signaling. Treatment of fibroblasts with recombinant ACLP induced phosphorylation and nuclear translocation of Smad3. This phosphorylation and induction of SMA was dependent on TGFβ receptor binding and kinase activity. ACLP-induced collagen expression was independent of interaction with the TGFβ receptor. These findings indicate that ACLP stimulates the fibroblast-to-myofibroblast transition by promoting SMA expression via TGFβ signaling and promoting collagen expression through a TGFβ receptor-independent pathway.

Discoidin domain receptors: unique receptor tyrosine kinases in collagen-mediated signaling

The discoidin domain receptors (DDRs) are receptor tyrosine kinases that recognize collagens as their ligands. DDRs display unique structural features and distinctive activation kinetics, which set them apart from other members of the kinase superfamily. DDRs regulate cell-collagen interactions in normal and pathological conditions and thus are emerging as major sensors of collagen matrices and potential novel therapeutic targets. New structural and biological information has shed light on the molecular mechanisms that regulate DDR signaling, turnover, and function. This minireview provides an overview of these areas of DDR research with the goal of fostering further investigation of these intriguing and unique receptors.

Gastroschisis in mice lacking aortic carboxypeptidase-like protein is associated with a defect in neuromuscular development of the eviscerated intestine

Mice lacking aortic carboxypeptidase-like protein (ACLP) exhibit a gastroschisis (GS) like abdominal wall defect. The objectives of this study were to evaluate the pathophysiological features of GS in ACLP mice and to characterize the neuromuscular development of the eviscerated intestine (EI). ACLP mice were created by heterozygous mating from previously generated mice with targeted disruption of ACLP. Specimens were processed for H&E, and immunohistochemistry for smooth muscle cells [SMC, alpha-smooth muscle actin (alpha-SMA) antibody], interstitial cells of Cajal (ICC, c-kit-antibody), neural crest cells (NCC, Hox-b5-antibody), and enteric neurons (EN, PGP9.5-, alpha-internexin, and synaptophysin antibody). From 47 fetuses genotyped, 13 (27.7%) were wild type, 20 (42.5%) were heterozygous, and 14 (29.8%) were ACLP homozygous. In GS mice, expression of c-kit, Hox-b5, PGP-9.5, alpha-internexin, and synaptophysin were almost completely absent and only faint alpha-SMA expression was seen in the EI. In contrast, c-kit, Hox-b5, PGP9.5, alpha-internexin, synaptophysin, and alpha-SMA expression in intra-abdominal intestine in GS fetuses was the same as control intestine. The defect observed in ACLP mice closely resembles GS. Absence of ICC, NCC, EN, and immature differentiation of SMC supports an associated defect in neuromuscular development that is restricted to the EI.

Aortic carboxypeptidase-like protein is expressed in fibrotic human lung and its absence protects against bleomycin-induced lung fibrosis

The pathological hallmarks of idiopathic pulmonary fibrosis include proliferating fibroblasts and myofibroblasts, as well as excessive collagen matrix deposition. In addition, both myofibroblast contraction and remodeling of the collagen-rich matrix contribute to the abnormal structure and function of the fibrotic lung. Little is known, however, about collagen-associated proteins that promote fibroblast and myofibroblast retention, as well as the proliferation of these cells on the extracellular matrix. In this study, we demonstrate that aortic carboxypeptidase-like protein (ACLP), a collagen-associated protein with a discoidin-like domain, is expressed at high levels in human fibrotic lung tissue and human fibroblasts, and that its expression increases markedly in the lungs of bleomycin-injured mice. Importantly, ACLP-deficient mice accumulated significantly fewer myofibroblasts and less collagen in the lung after bleomycin injury, as compared with wild-type controls, despite equivalent levels of bleomycin-induced inflammation. ACLP that is secreted by lung fibroblasts was retained on fibrillar collagen, and ACLP-deficient lung fibroblasts that were cultured on collagen exhibited changes in cell spreading, proliferation, and contraction of the collagen matrix. Finally, the addition of recombinant discoidin-like domain of ACLP to cultured ACLP-deficient lung fibroblasts restored cell spreading and increased the contraction of collagen gels. Therefore, both ACLP and its discoidin-like domain may be novel targets for anti-myofibroblast-based therapies for the treatment of pulmonary fibrosis.

Enhancement of pituitary adenoma cell invasion and adhesion is mediated by discoidin domain receptor-1

The discoidin domain receptor-1 (DDR1) tyrosine kinases are a family of cell surface receptors that bind to several types of collagen and facilitate cell adhesion that is known association with several cancers. However, no previous study has examined the expression and function of DDR1 in pituitary adenoma. Tissue microarray analysis of DDR1 expression levels in 52 pituitary adenoma tissues revealed that DDR1 expression was significantly related to hormonal background (Kruskal-Wallis test; P < 0.0001). To further elucidate the function of DDR1 in pituitary adenoma, we developed DDR1 over- and under-expressing cell lines using DDR1 clone transfection and short interfering ribonucleic acids (siRNA)-based DDR1 gene silencing, respectively, in a human pituitary adenoma cell line (HP-75). Real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting confirmed that expression of both DDR1 isoforms (DDR1a and DDR1b) was elevated by clone transfection and diminished by siRNA. Matrigel invasion assays revealed that cell invasion was increased in HP-75 cells over-expressing DDR1 and decreased in cells under-expressing DDR1. Consistent with this, zymography revealed that the activation levels of matrix metalloproteinase (MMP)-2 and -9 were increased and decreased in cells over- and under-expressing DDR1, respectively. Examination of in vitro cell adhesion to collagen types I, II, III, and IV with respect to MMP-2 and -9 expression revealed that DDR1 regulated cell adhesion to collagen type I, which was responsible for accelerating secretion of MMP-2 and -9 in DDR1 over-expressing cells. Taken together, these results strongly suggest that DDR1 mediates cell invasion-related signaling between collagen type I and MMP-2 and -9 in pituitary adenoma cells.

Incorporating genetic analyses into birth defects cluster investigations: Strategies for identifying candidate genes

BACKGROUND

Incorporating genetic analyses into birth defect cluster investigations may increase understanding of both genetic and environmental risk factors for the defect. Current constraints of most birth defect cluster investigations make candidate gene selection the most feasible approach. Here, we describe strategies for choosing candidate genes for such investigations, which will also be applicable to more general gene-environment studies.

METHODS

We reviewed publicly available web-based resources for selection of candidate genes and identification of risk factors, as well as publications on different strategies for candidate gene selection.

RESULTS

Candidate gene selection requires consideration of available gene-disease databases, previous epidemiological studies, animal model research, linkage and expression studies, and other resources. We describe general considerations for utilizing available resources, as well as provide an example of a search for candidate genes related to gastroschisis.

CONCLUSIONS

Available web resources could facilitate selection of candidate genes, but selection of optimal candidates will still require a strong understanding of genetics and the pathogenesis of the defect, as well as careful consideration of previous epidemiological studies.

Aortic carboxypeptidase-like protein is regulated by transforming growth factor β in 3T3-L1 preadipocytes

Adipogenesis is characterized by early remodeling of the extracellular matrix, allowing preadipocytes to adopt a more spherical shape and optimize lipid accumulation as they mature. Aortic carboxypeptidase-like protein (ACLP), found in collagen-rich tissues including adipose tissue, is expressed in 3T3-L1 and 3T3-F442A preadipocytes, and is downregulated during adipogenesis. We now report that ACLP is found in medium conditioned by 3T3-L1 preadipocytes. Transforming growth factor (TGF) beta, a known modulator of fibrillar matrix protein production, increased ACLP expression by 2.4+/-0.4-fold (mean+/-SE; n=3) in 3T3-L1 preadipocytes, through a mechanism that requires p42/44 MAPK activity. Addition of TGFbeta to differentiation medium, which inhibits adipogenesis, raised ACLP levels in 3T3-L1 cells. However, sustained expression of ACLP in stable clones of 3T3-L1 or 3T3-F442A preadipocytes did not interfere with adipogenesis.