Midgut Laterality Is Driven by Hyaluronan on the Right

Targeted Analysis of the Size Distribution of Heavy Chain-Modified Hyaluronan with Solid-State Nanopores

The glycosaminoglycan hyaluronan (HA) plays important roles in diverse physiological functions where the distribution of its molecular weight (MW) can influence its behavior and is known to change in response to disease conditions. During inflammation, HA undergoes a covalent modification in which heavy chain subunits of the inter-alpha-inhibitor family of proteins are transferred to its structure, forming heavy chain-HA (HC•HA) complexes. While limited assessments of HC•HA have been performed previously, determining the size distribution of its HA component remains a challenge. Here, we describe a selective method for extracting HC•HA from mixtures that yields material amenable to MW analysis with a solid-state nanopore sensor. After demonstrating the approach in vitro, we validate extraction of HC•HA from osteoarthritic human synovial fluid as a model complex biological matrix. Finally, we apply our technique to pathophysiology by measuring the size distributions of HC•HA and total HA in an equine model of synovitis.

Evo-Devo Mechanobiology: The Missing Link

While the modern framework of evolutionary development (evo-devo) has been decidedly genetic, historic analyses have also considered the importance of mechanics in the evolution of form. With the aid of recent technological advancements in both quantifying and perturbing changes in the molecular and mechanical effectors of organismal shape, how molecular and genetic cues regulate the biophysical aspects of morphogenesis is becoming increasingly well studied. As a result, this is an opportune time to consider how the tissue-scale mechanics that underlie morphogenesis are acted upon through evolution to establish morphological diversity. Such a focus will enable a field of evo-devo mechanobiology that will serve to better elucidate the opaque relations between genes and forms by articulating intermediary physical mechanisms. Here, we review how the evolution of shape is measured and related to genetics, how recent strides have been made in the dissection of developmental tissue mechanics, and how we expect these areas to coalesce in evo-devo studies in the future.

Elastic fibers define embryonic tissue stiffness to enable buckling morphogenesis of the small intestine

During embryonic development, tissues must possess precise material properties to ensure that cell-generated forces give rise to the stereotyped morphologies of developing organs. However, the question of how material properties are established and regulated during development remains understudied. Here, we aim to address these broader questions through the study of intestinal looping, a process by which the initially straight intestinal tube buckles into loops, permitting ordered packing within the body cavity. Looping results from elongation of the tube against the constraint of an attached tissue, the dorsal mesentery, which is elastically stretched by the elongating tube to nearly triple its length. This elastic energy storage allows the mesentery to provide stable compressive forces that ultimately buckle the tube into loops. Beginning with a transcriptomic analysis of the mesentery, we identified widespread upregulation of extracellular matrix related genes during looping, including genes related to elastic fiber deposition. Combining molecular and mechanical analyses, we conclude that elastin confers tensile stiffness to the mesentery, enabling its mechanical role in organizing the developing small intestine. These results shed light on the role of elastin as a driver of morphogenesis that extends beyond its more established role in resisting cyclic deformation in adult tissues.

Age related changes in hyaluronan expression leads to Meibomian gland dysfunction

The prevalence of dry eye disease (DED) ranges from ∼5 to 50 % and its associated symptoms decrease productivity and reduce the quality of life. Approximately 85 % of all DED cases are caused by Meibomian gland dysfunction (MGD). As humans and mice age, their Meibomian glands (MGs) undergo age-related changes resulting in age related-MGD (ARMGD). The precise cause of ARMGD remains elusive, which makes developing therapies extremely challenging. We previously demonstrated that a hyaluronan (HA)-rich matrix exists surrounding the MG, regulating MG morphogenesis and homeostasis. Herein, we investigated whether changes to the HA matrix in the MG throughout life contributes towards ARMGD, and whether altering this HA matrix can prevent ARMGD. For such, HA synthase (Has) knockout mice were aged and compared to age matched wild type (wt) mice. MG morphology, lipid production, PPARγ expression, basal cell proliferation, stem cells, presence of atrophic glands and MG dropout were analyzed at 8 weeks, 6 months, 1 year and 2 years of age and correlated with the composition of the HA matrix. We found that as mice age, there is a loss of HA expression in and surrounding the MGs of wt mice, while, in contrast, Has1-/-Has3-/- mice present a significant increase in HA expression through Has2 upregulation. At 1 year, Has1-/-Has3-/- mice present significantly enlarged MGs, compared to age-matched wt mice and compared to all adult mice. Thus, Has1-/-Has3-/- mice continue to develop new glandular tissue as they age, instead of suffering MG atrophy. At 2 years, Has1-/-Has3-/- mice continue to present significantly larger MGs compared to age-matched wt mice. Has1-/-Has3-/- mice present increased lipid production, increased PPARγ expression and an increase in the number of proliferating cells when compared to wt mice at all-time points analyzed. Taken together, our data shows that a loss of the HA matrix surrounding the MG as mice age contributes towards ARMGD, and increasing Has2 expression, and consequently HA levels, prevents ARMGD in mice.

ROS-Mediated Fragmentation Alters the Effects of Hyaluronan on Corneal Epithelial Wound Healing

A buildup of reactive oxygen species (ROS) occurs in virtually all pathological conditions. Hyaluronan (HA) is a major extracellular matrix component and is susceptible to oxidation by reactive oxygen species (ROS), yet the precise chemical structures of oxidized HA products (oxHA) and their physiological properties remain largely unknown. This study characterized the molecular weight (MW), structures, and physiological properties of oxHA. For this, high-molecular-weight HA (HMWHA) was oxidized using increasing molar ratios of hydrogen peroxide (H2O2) or hypochlorous acid (HOCl). ROS lead to the fragmentation of HA, with the oxHA products produced by HOCl exhibiting an altered chemical structure while those produced by H2O2 do not. HMWHA promotes the viability of human corneal epithelial cells (hTCEpi), while low MWHA (LMWHA), ultra-LMWHA (ULMWHA), and most forms of oxHA do not. HMWHA and LMWHA promote hTCEpi proliferation, while ULMWHA and all forms of oxHA do not. LMWHA and some forms of oxHA promote hTCEpi migration, while HMWHA does not. Finally, all native forms of HA and oxHA produced by HOCl promote in vivo corneal wound healing, while oxHA produced by H2O2 does not. Taken together, our results show that HA fragmentation by ROS can alter the physiological activity of HA by altering its MW and structure.

An atypical basement membrane forms a midline barrier in left-right asymmetric gut development

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1+ midline barrier to separate the two sides of the embryo-without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals are not well understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

TSG6 hyaluronan matrix remodeling dampens the inflammatory response during colitis

In response to tissue injury, changes in the extracellular matrix (ECM) can directly affect the inflammatory response and contribute to disease progression or resolution. During inflammation, the glycosaminoglycan hyaluronan (HA) becomes modified by tumor necrosis factor stimulated gene-6 (TSG6). TSG6 covalently transfers heavy chain (HC) proteins from inter-α-trypsin inhibitor (IαI) to HA in a transesterification reaction and is to date is the only known HC-transferase. By modifying the HA matrix, TSG6 generates HC:HA complexes that are implicated in mediating both protective and pathological responses. Inflammatory bowel disease (IBD) is a lifelong chronic disorder with well-described remodeling of the ECM and increased mononuclear leukocyte influx into the intestinal mucosa. Deposition of HC:HA matrices is an early event in inflamed gut tissue that precedes and promotes leukocyte infiltration. However, the mechanisms by which TSG6 contributes to intestinal inflammation are not well understood. The aim of our study was to understand how the TSG6 and its enzymatic activity contributes to the inflammatory response in colitis. Our findings indicate that inflamed tissues of IBD patients show an elevated level of TSG6 and increased HC deposition and that levels of HA strongly associate with TSG6 levels in patient colon tissue specimens. Additionally, we observed that mice lacking TSG6 are more vulnerable to acute colitis and exhibit an aggravated macrophage-associated mucosal immune response characterized by elevated pro-inflammatory cytokines and chemokines and diminished anti-inflammatory mediators including IL-10. Surprisingly, along with significantly increased levels of inflammation in the absence of TSG6, tissue HA levels in mice were found to be significantly reduced and disorganized, absent of typical "HA-cable" structures. Inhibition of TSG6 HC-transferase activity leads to a loss of cell surface HA and leukocyte adhesion, indicating that the enzymatic functions of TSG6 are a major contributor to stability of the HA ECM during inflammation. Finally, using biochemically generated HC:HA matrices derived by TSG6, we show that HC:HA complexes can attenuate the inflammatory response of activated monocytes. In conclusion, our data suggests that TSG6 exerts a tissue-protective, anti-inflammatory effect via the generation of HC:HA complexes that become dysregulated in IBD.

ELASTIC FIBERS DEFINE EMBRYONIC TISSUE STIFFNESS TO ENABLE BUCKLING MORPHOGENESIS OF THE SMALL INTESTINE

During embryonic development, tissues must possess precise material properties to ensure that cell-generated forces give rise to the stereotyped morphologies of developing organs. However, the question of how material properties are established and regulated during development remains understudied. Here, we aim to address these broader questions through the study of intestinal looping, a process by which the initially straight intestinal tube buckles into loops, permitting ordered packing within the body cavity. Looping results from elongation of the tube against the constraint of an attached tissue, the dorsal mesentery, which is elastically stretched by the elongating tube to nearly triple its length. This elastic energy storage allows the mesentery to provide stable compressive forces that ultimately buckle the tube into loops. Beginning with a transcriptomic analysis of the mesentery, we identified widespread upregulation of extracellular matrix related genes during looping, including genes related to elastic fiber deposition. Combining molecular and mechanical analyses, we conclude that elastin confers tensile stiffness to the mesentery, enabling its mechanical role in organizing the developing small intestine. These results shed light on the role of elastin as a driver of morphogenesis that extends beyond its more established role in resisting cyclic deformation in adult tissues.

Visualizing and manipulating the production and accumulation of hyaluronan for functional assessment in chicken embryos

Hyaluronan (HA) accumulates in the extracellular matrix to regulate organ morphogenesis. The spatiotemporal dynamics of its production and function are poorly understood due to its instability. Here, we present a protocol using the embryonic chicken intestine as a binary in vivo system for HA visualization and manipulation. We describe steps for pharmacological manipulation and in ovo electroporation to target HA production and accumulation. We then detail HA-binding protein assay to detect HA accumulation and quantification of tissue morphology changes. For complete details on the use and execution of this protocol, please refer to Sivakumar et al. (2018)1 and Sanketi et al. (2022).2.

Dysfunctional Extracellular Matrix Remodeling Supports Perianal Fistulizing Crohn's Disease by a Mechanoregulated Activation of the Epithelial-to-Mesenchymal Transition

BACKGROUND AND AIMS

Perianal fistula represents one of the most disabling manifestations of Crohn's disease (CD) due to complete destruction of the affected mucosa, which is replaced by granulation tissue and associated with changes in tissue organization. To date, the molecular mechanisms underlying perianal fistula formation are not well defined. Here, we dissected the tissue changes in the fistula area and addressed whether a dysregulation of extracellular matrix (ECM) homeostasis can support fistula formation.

METHODS

Surgical specimens from perianal fistula tissue and the surrounding region of fistulizing CD were analyzed histologically and by RNA sequencing. Genes significantly modulated were validated by real-time polymerase chain reaction, Western blot, and immunofluorescence assays. The effect of the protein product of TNF-stimulated gene-6 (TSG-6) on cell morphology, phenotype, and ECM organization was investigated with endogenous lentivirus-induced overexpression of TSG-6 in Caco-2 cells and with exogenous addition of recombinant human TSG-6 protein to primary fibroblasts from region surrounding fistula. Proliferative and migratory assays were performed.

RESULTS

A markedly different organization of ECM was found across fistula and surrounding fistula regions with an increased expression of integrins and matrix metalloproteinases and hyaluronan (HA) staining in the fistula, associated with increased newly synthesized collagen fibers and mechanosensitive proteins. Among dysregulated genes associated with ECM, TNFAI6 (gene encoding for TSG-6) was as significantly upregulated in the fistula compared with area surrounding fistula, where it promoted the pathological formation of complexes between heavy chains from inter-alpha-inhibitor and HA responsible for the formation of a crosslinked ECM. There was a positive correlation between TNFAI6 expression and expression of mechanosensitive genes in fistula tissue. The overexpression of TSG-6 in Caco-2 cells promoted migration, epithelial-mesenchymal transition, transcription factor SNAI1, and HA synthase (HAs) levels, while in fibroblasts, isolated from the area surrounding the fistula, it promoted an activated phenotype. Moreover, the enrichment of an HA scaffold with recombinant human TSG-6 protein promoted collagen release and increase of SNAI1, ITGA4, ITGA42B, and PTK2B genes, the latter being involved in the transduction of responses to mechanical stimuli.

CONCLUSIONS

By mediating changes in the ECM organization, TSG-6 triggers the epithelial-mesenchymal transition transcription factor SNAI1 through the activation of mechanosensitive proteins. These data point to regulators of ECM as new potential targets for the treatment of CD perianal fistula.

Pitx2 patterns an accelerator-brake mechanical feedback through latent TGFβ to rotate the gut

The vertebrate intestine forms by asymmetric gut rotation and elongation, and errors cause lethal obstructions in human infants. Rotation begins with tissue deformation of the dorsal mesentery, which is dependent on left-sided expression of the Paired-like transcription factor Pitx2. The conserved morphogen Nodal induces asymmetric Pitx2 to govern embryonic laterality, but organ-level regulation of Pitx2 during gut asymmetry remains unknown. We found Nodal to be dispensable for Pitx2 expression during mesentery deformation. Intestinal rotation instead required a mechanosensitive latent transforming growth factor-β (TGFβ), tuning a second wave of Pitx2 that induced reciprocal tissue stiffness in the left mesentery as mechanical feedback with the right side. This signaling regulator, an accelerator (right) and brake (left), combines biochemical and biomechanical inputs to break gut morphological symmetry and direct intestinal rotation.

The cell surface hyaluronidase TMEM2 plays an essential role in mouse neural crest cell development and survival

Hyaluronan (HA) is a major extracellular matrix component whose tissue levels are dynamically regulated during embryonic development. Although the synthesis of HA has been shown to exert a substantial influence on embryonic morphogenesis, the functional importance of the catabolic aspect of HA turnover is poorly understood. Here, we demonstrate that the transmembrane hyaluronidase TMEM2 plays an essential role in neural crest development and the morphogenesis of neural crest derivatives, as evidenced by the presence of severe craniofacial abnormalities in Wnt1-Cre–mediated Tmem2 knockout (Tmem2^CKO) mice. Neural crest cells (NCCs) are a migratory population of cells that gives rise to diverse cell lineages, including the craniofacial complex, the peripheral nervous system, and part of the heart. Analysis of Tmem2 expression during NCC formation and migration reveals that Tmem2 is expressed at the site of NCC delamination and in emigrating Sox9-positive NCCs. In Tmem2^CKO embryos, the number of NCCs emigrating from the neural tube is greatly reduced. Furthermore, linage tracing reveals that the number of NCCs traversing the ventral migration pathway and the number of post-migratory neural crest derivatives are both significantly reduced in a Tmem2^CKO background. In vitro studies using Tmem2-depleted mouse O9-1 neural crest cells demonstrate that Tmem2 expression is essential for the ability of these cells to form focal adhesions on and to migrate into HA-containing substrates. Additionally, we show that Tmem2-deficient NCCs exhibit increased apoptotic cell death in NCC-derived tissues, an observation that is corroborated by in vitro experiments using O9-1 cells. Collectively, our data demonstrate that TMEM2-mediated HA degradation plays an essential role in normal neural crest development. This study reveals the hitherto unrecognized functional importance of HA degradation in embryonic development and highlights the pivotal role of Tmem2 in the developmental process.

As a major component of the extracellular matrix, hyaluronan is particularly abundant in the extracellular matrix of embryonic tissues, where its expression is dynamically regulated during tissue morphogenetic processes. Tissue levels of hyaluronan are regulated not only by its synthesis but also by its degradation. Curiously, however, mice lacking known hyaluronidase molecules, including HYAL1 and HYAL2, exhibit minimal embryonic phenotypes. As a result, our understanding of the role of the catabolic aspect of hyaluronan metabolism in embryonic development is quite limited. Here, we show that TMEM2, a recently identified hyaluronidase that degrades hyaluronan on the cell surface, plays a critical role in the development of neural crest cells and their derivatives. Our analyses of Tmem2 conditional knockout mice, Tmem2 knock-in reporter mice, and in vitro cell cultures demonstrate that TMEM2 is essential for generating a tissue environment needed for efficient migration of neural crest cells from the neural tube. Our paper reveals for the first time that the degradation of hyaluronan plays a specific regulatory role in embryonic morphogenesis, and that dysregulation of hyaluronan degradation leads to severe developmental defects.

The Development of the Mesenteric Model of Abdominal Anatomy

Recent advances in mesenteric anatomy have clarified the shape of the mesentery in adulthood. A key finding is the recognition of mesenteric continuity, which extends from the oesophagogastric junction to the mesorectal level. All abdominal digestive organs develop within, or on, the mesentery and in adulthood remain directly connected to the mesentery. Identification of mesenteric continuity has enabled division of the abdomen into two separate compartments. These are the mesenteric domain (upon which the abdominal digestive system is centered) and the non-mesenteric domain, which comprises the urogenital system, musculoskeletal frame, and great vessels. Given this anatomical endpoint differs significantly from conventional descriptions, a reappraisal of mesenteric developmental anatomy was recently performed. The following narrative review summarizes recent advances in abdominal embryology and mesenteric morphogenesis. It also examines the developmental basis for compartmentalizing the abdomen into two separate domains along mesenteric lines.

In Ovo Gain- and Loss-of-Function Approaches to Study Gut Morphogenesis

The polarity of cellular components is essential for cellular shape changes, oriented cell migration, and modulating intra- and intercellular mechanical forces. However, many aspects of polarized cell behavior-especially dynamic cell shape changes during the process of morphogenesis-are almost impossible to study in cells cultured in plastic dishes. Avian embryos have always been a treasured model system to study vertebrate morphogenesis for developmental biologists. Avian embryos recapitulate human biology particularly well in the early stages due to their flat disc gastruloids. Since avian embryos can be manipulated in ovo they present paramount opportunities for highly localized targeting of genetic mechanisms during cellular and developmental processes. Here, we review the application of these methods for both gain of function and loss of function of a gene of interest at a specific developmental stage during left-right (LR) asymmetric gut morphogenesis. These tools present a powerful premise to investigate various polarized cellular activities and molecular processes in vivo in a reproducible manner.

Avian Embryos as a Model to Study Vascular Development

The use of live imaging is indispensable for advancing our understanding of vascular morphogenesis. Imaging fixed embryos at a series of distinct developmental time points, although valuable, does not reveal the dynamic behavior of cells, as well as their interactions with the underlying ECM. Due to the easy access of chicken embryos to manipulation and high-resolution imaging, this model has been at the origin of key discoveries. In parallel, known through its extensive use in quail-chick chimera studies, the quail embryo is equally poised to genetic manipulations and paramount to direct imaging of transgenic reporter quails. Here we describe ex ovo time-lapse confocal microscopy of transgenic quail embryo slices to image vascular development during gut morphogenesis. This technique is powerful as it allows direct observation of the dynamic endothelial cell behaviors along the left-right (LR) axis of the dorsal mesentery (DM), the major conduit for blood and lymphatic vessels that serve the gut. In combination with in ovo plasmid electroporation and quail-chick transplantation, these methods have allowed us to study the molecular mechanisms underlying blood vessel assembly during the formation of the intestine. Below we describe our protocols for the generation of embryo slices, ex ovo time-lapse imaging of fluorescently labeled cells, and quail-chick chimeras to study the early stages of gut vascular development.

Extracellular hyaluronate pressure shaped by cellular tethers drives tissue morphogenesis

How tissues acquire complex shapes is a fundamental question in biology and regenerative medicine. Zebrafish semicircular canals form from invaginations in the otic epithelium (buds) that extend and fuse to form the hubs of each canal. We find that conventional actomyosin-driven behaviors are not required. Instead, local secretion of hyaluronan, made by the enzymes uridine 5'-diphosphate dehydrogenase (ugdh) and hyaluronan synthase 3 (has3), drives canal morphogenesis. Charged hyaluronate polymers osmotically swell with water and generate isotropic extracellular pressure to deform the overlying epithelium into buds. The mechanical anisotropy needed to shape buds into tubes is conferred by a polarized distribution of actomyosin and E-cadherin-rich membrane tethers, which we term cytocinches. Most work on tissue morphogenesis ascribes actomyosin contractility as the driving force, while the extracellular matrix shapes tissues through differential stiffness. Our work inverts this expectation. Hyaluronate pressure shaped by anisotropic tissue stiffness may be a widespread mechanism for powering morphological change in organogenesis and tissue engineering.

The development and structure of the mesentery

The position of abdominal organs, and mechanisms by which these are centrally connected, are currently described in peritoneal terms. As part of the peritoneal model of abdominal anatomy, there are multiple mesenteries. Recent findings point to an alternative model in which digestive organs are connected to a single mesentery. Given that direct evidence of this is currently lacking, we investigated the development and shape of the entire mesentery. Here we confirm that, within the abdomen, there is one mesentery in which all abdominal digestive organs develop and remain connected to. We show that all abdominopelvic organs are organised into two, discrete anatomical domains, the mesenteric and non-mesenteric domain. A similar organisation occurs across a range of animal species. The findings clarify the anatomical foundation of the abdomen; at the foundation level, the abdomen comprises a visceral (i.e. mesenteric) and somatic (i.e. musculoskeletal) frame. The organisation at that level is a fundamental order that explains the positional anatomy of all abdominopelvic organs, vasculature and peritoneum. Collectively, the findings provide a novel start point from which to systemically characterise the abdomen and its contents.

Byrnes et al. reconstruct the developing mesentery from digitized embryonic datasets and human and animal cadavers using 3D digital and printed models. They confirm the mesentery remains a continuous organ in and on which all abdominal digestive organs develop and that at the foundation level, the abdomen comprises a mesenteric and non-mesenteric domain.

Reduced hyaluronan cross-linking induces breast cancer malignancy in a CAF-dependent manner

Hyaluronan (HA) cross-linking is a conformational state of HA, a covalent complex between HA and heavy chains (HCs) from inter-α-trypsin inhibitor (I-α-I) mediated by tumor necrosis factor-induced protein 6 (TSG6). Cross-linked HA has been identified as a protective factor in physiological and inflammatory conditions. However, the state of HA cross-linking in tumor microenvironment has not been fully elucidated. As a major constituent of the extracellular matrix (ECM), HA is mainly synthesized by cancer-associated fibroblasts (CAFs). Our study aimed to clarify the role of HA cross-linking in breast cancer malignancy. Compared to normal mammary gland tissues, cross-linked HA levels were significantly decreased in breast cancer and associated with tumor malignancy. When NFbs were activated into CAFs, the levels of cross-linked HA and TSG6 were both suppressed. Through upregulating TSG6, CAFs restored the high level of cross-linked HA and significantly inhibited breast cancer malignancy, whereas NFbs promoted the malignancy when the cross-linked HA level was reduced. Furthermore, the inhibitory role of HA cross-linking in tumor malignancy was directly verified using the synthesized HA-HC complex. Collectively, our study found that the deficiency of cross-linked HA induced breast cancer malignancy in a CAF-dependent manner, suggesting that recovering HA cross-linking may be a potential therapeutic strategy.

The Axenfeld-Rieger Syndrome Gene FOXC1 Contributes to Left-Right Patterning

Precise spatiotemporal expression of the Nodal-Lefty-Pitx2 cascade in the lateral plate mesoderm establishes the left–right axis, which provides vital cues for correct organ formation and function. Mutations of one cascade constituent PITX2 and, separately, the Forkhead transcription factor FOXC1 independently cause a multi-system disorder known as Axenfeld–Rieger syndrome (ARS). Since cardiac involvement is an established ARS phenotype and because disrupted left–right patterning can cause congenital heart defects, we investigated in zebrafish whether foxc1 contributes to organ laterality or situs. We demonstrate that CRISPR/Cas9-generated foxc1a and foxc1b mutants exhibit abnormal cardiac looping and that the prevalence of cardiac situs defects is increased in foxc1a^−/−; foxc1b^−/− homozygotes. Similarly, double homozygotes exhibit isomerism of the liver and pancreas, which are key features of abnormal gut situs. Placement of the asymmetric visceral organs relative to the midline was also perturbed by mRNA overexpression of foxc1a and foxc1b. In addition, an analysis of the left–right patterning components, identified in the lateral plate mesoderm of foxc1 mutants, reduced or abolished the expression of the NODAL antagonist lefty2. Together, these data reveal a novel contribution from foxc1 to left–right patterning, demonstrating that this role is sensitive to foxc1 gene dosage, and provide a plausible mechanism for the incidence of congenital heart defects in Axenfeld–Rieger syndrome patients.

Strong Reduction of the Chain Rigidity of Hyaluronan by Selective Binding of Ca2+ Ions

The biological functions of natural polyelectrolytes are strongly influenced by the presence of ions, which bind to the polymer chains and thereby modify their properties. Although the biological impact of such modifications is well recognized, a detailed molecular picture of the binding process and of the mechanisms that drive the subsequent structural changes in the polymer is lacking. Here, we study the molecular mechanism of the condensation of calcium, a divalent cation, on hyaluronan, a ubiquitous polymer in human tissues. By combining two-dimensional infrared spectroscopy experiments with molecular dynamics simulations, we find that calcium specifically binds to hyaluronan at millimolar concentrations. Because of its large size and charge, the calcium cation can bind simultaneously to the negatively charged carboxylate group and the amide group of adjacent saccharide units. Molecular dynamics simulations and single-chain force spectroscopy measurements provide evidence that the binding of the calcium ions weakens the intramolecular hydrogen-bond network of hyaluronan, increasing the flexibility of the polymer chain. We also observe that the binding of calcium to hyaluronan saturates at a maximum binding fraction of ∼10–15 mol %. This saturation indicates that the binding of Ca²⁺ strongly reduces the probability of subsequent binding of Ca²⁺ at neighboring binding sites, possibly as a result of enhanced conformational fluctuations and/or electrostatic repulsion effects. Our findings provide a detailed molecular picture of ion condensation and reveal the severe effect of a few, selective and localized electrostatic interactions on the rigidity of a polyelectrolyte chain.

Hyaluronan: Metabolism and Function

As a major polysaccharide component of the extracellular matrix, hyaluronan plays essential roles in the organization of tissue architecture and the regulation of cellular functions, such as cell proliferation and migration, through interactions with cell-surface receptors and binding molecules. Metabolic pathways for biosynthesis and degradation tightly control the turnover rate, concentration, and molecular size of hyaluronan in tissues. Despite the relatively simple chemical composition of this polysaccharide, its wide range of molecular weights mediate diverse functions that depend on molecular size and tissue concentration. Genetic engineering and pharmacological approaches have demonstrated close associations between hyaluronan metabolism and functions in many physiological and pathological events, including morphogenesis, wound healing, and inflammation. Moreover, emerging evidence has suggested that the accumulation of hyaluronan extracellular matrix and fragments due to the altered expression of hyaluronan synthases and hyaluronidases potentiates cancer development and progression by remodeling the tumor microenvironment. In addition to the well-known functions exerted by extracellular hyaluronan, recent metabolomic approaches have also revealed that its synthesis can regulate cellular functions via the reprogramming of cellular metabolism. This review highlights the current advances in knowledge on the biosynthesis and catabolism of hyaluronan and describes the diverse functions associated with hyaluronan metabolism.

The twists and turns of left-right asymmetric gut morphogenesis

Many organs develop left-right asymmetric shapes and positions that are crucial for normal function. Indeed, anomalous laterality is associated with multiple severe birth defects. Although the events that initially orient the left-right body axis are beginning to be understood, the mechanisms that shape the asymmetries of individual organs remain less clear. Here, we summarize new evidence challenging century-old ideas about the development of stomach and intestine laterality. We compare classical and contemporary models of asymmetric gut morphogenesis and highlight key unanswered questions for future investigation.

The Inter-α-Trypsin Inhibitor Family: Versatile Molecules in Biology and Pathology

Inter-α-trypsin inhibitor (IαI) family members are ancient and unique molecules that have evolved over several hundred million years of vertebrate evolution. IαI is a complex containing the proteoglycan bikunin to which heavy chain proteins are covalently attached to the chondroitin sulfate chain. Besides its matrix protective activity through protease inhibitory action, IαI family members interact with extracellular matrix molecules and most notably hyaluronan, inhibit complement, and provide cell regulatory functions. Recent evidence for the diverse roles of the IαI family in both biology and pathology is reviewed and gives insight into their pivotal roles in tissue homeostasis. In addition, the clinical uses of these molecules are explored, such as in the treatment of inflammatory conditions including sepsis and Kawasaki disease, which has recently been associated with severe acute respiratory syndrome coronavirus 2 infection in children.

Mechanobiology of vertebrate gut morphogenesis

Approximately a century after D'Arcy Thompson's On Growth and Form, there continues to be widespread interest in the biophysical and mathematical basis of morphogenesis. Particularly over the past 20 years, this interest has led to great advances in our understanding of a broad range of processes in embryonic development through a quantitative, mechanically driven framework. Nowhere in vertebrate development is this more apparent than the development of endodermally derived organs. Here, we discuss recent advances in the study of gut development that have emerged primarily from mechanobiology-motivated approaches that span from gut tube morphogenesis and later organogenesis of the respiratory and gastrointestinal systems.

Molecular and cellular basis of left-right asymmetry in vertebrates

Although the human body appears superficially symmetrical with regard to the left-right (L-R) axis, most visceral organs are asymmetric in terms of their size, shape, or position. Such morphological asymmetries of visceral organs, which are essential for their proper function, are under the control of a genetic pathway that operates in the developing embryo. In many vertebrates including mammals, the breaking of L-R symmetry occurs at a structure known as the L-R organizer (LRO) located at the midline of the developing embryo. This symmetry breaking is followed by transfer of an active form of the signaling molecule Nodal from the LRO to the lateral plate mesoderm (LPM) on the left side, which results in asymmetric expression of Nodal (a left-side determinant) in the left LPM. Finally, L-R asymmetric morphogenesis of visceral organs is induced by Nodal-Pitx2 signaling. This review will describe our current understanding of the mechanisms that underlie the generation of L-R asymmetry in vertebrates, with a focus on mice.

Hyaluronan Accelerates Intestinal Mucosal Healing through Interaction with TSG-6

Hyaluronan (HA) has proven to be beneficial in the treatment of several diseases. Recently, it has been shown that the local application of HA (IBD98E) improves endoscopic and clinical outcomes in subjects with active distal ulcerative colitis (UC). However, the mechanisms by which this polysaccharide exerts its beneficial effects are unclear. Here, we demonstrated that HA treatment in vitro and in vivo improved mucosal healing by accelerating intestinal epithelial regeneration. Indeed, mice treated with HA showed a faster recovery from colitis and reduced endoscopic signs of mucosal inflammation compared to those receiving saline. Furthermore, histological analysis revealed less ulcerated mucosa in mice treated with HA, characterized by re-epithelialized areas. TSG-6, the secreted product of TNF-stimulated gene-6, is an HA-binding protein shown previously to have tissue-protective properties and promote wound healing. Mucosal levels of TSG-6 increased in UC patients compared to the healthy controls and also after wounding in mice. TSG-6 deletion prevented the beneficial properties of HA in mucosal wound repair, suggesting that the interaction of HA with TSG-6 is crucial for intestinal epithelial regeneration. Overall these results are consistent with HA having a therapeutic effect via the promotion of mucosal healing in patients with ulcerative colitis.

The journey of hyaluronan research in the Journal of Biological Chemistry

Hyaluronan has a very simple structure. It is a linear glycosaminoglycan composed of disaccharide units of GlcNAc and d-glucuronic acid with alternating β-1,4 and β-1,3 glycosidic bonds that can be repeated 20,000 or more times, a molecular mass >8 million Da, and a length >20 μm. However, it has a very complex biology. It is a major, ubiquitous component of extracellular matrices involved in everything from fertilization, development, inflammations, to cancer. This JBC Review highlights some of these processes that were initiated through publications in the Journal of Biological Chemistry.

Hyaluronan Works on the Right for Directional Gut Looping

The cellular basis of left-right asymmetric organogenesis remains largely unknown, but signaling events on the left side were thought to be dominant. In this issue of Developmental Cell, however, Sivakumar et al. (2018) suggest that covalent modification of hyaluronan on the right side initiates directional looping of the developing midgut.