Role of CD34 family members in lumen formation in the developing kidney

Differential expression of epithelial and smooth muscle lineage-specific markers of metanephros in one-humped camel foetuses

The development of the metanephros in one-humped camels involves a complex series of interactions between epithelial and mesenchymal cells. As a result, there is a synchronized differentiation process of stromal, vascular and epithelial cell types during glomerulogenesis, angiogenesis and tubulogenesis. In the current work, the metanephros of camel foetuses were divided into four stages where kidneys from each stage were processed and immunoassayed, followed by quantitative analysis to determine target protein intensities throughout metanephrogenesis in the camel. This study demonstrated robust expression of α-smooth muscle actin (α-SMA) in the glomerular mesangium, as well as in interlobular and glomerular arterioles during the earlier stages of development. However, in the late stages, α-SMA expression became more localized around the blood capillaries in both the cortex and medulla. Strong expression of CD34 was observed in the immature glomerular and peritubular endothelial cells within the subcapsular zone, as well as in the glomerular, proximal tubular and distal tubular epithelium of stage one foetuses, although its expression gradually diminished with foetal maturation. The expression pattern of osteopontin was prominently observed in the distal convoluted tubules throughout all stages, however, no expression was detected in the proximal tubules, glomeruli and arterioles. E-cadherin was detected in the developing renal tubular epithelial cells but not in the glomeruli. In conclusion, this study reveals the spatiotemporal distribution of key proteins, including α-SMA, CD34, Osteopontin and E-cadherin, which play a crucial role in metanephrogenesis in camel foetuses.

Rap1 regulates lumen continuity via Afadin in renal epithelia

The continuity of a lumen within an epithelial tubule is critical for its function. We previously found that the F-actin binding protein Afadin is required for timely lumen formation and continuity in renal tubules formed from the nephrogenic mesenchyme in mice. Afadin is a known effector and interactor of the small GTPase Rap1, and in the current study, we examine the role of Rap1 in nephron tubulogenesis. Here, we demonstrate that Rap1 is required for nascent lumen formation and continuity in cultured 3D epithelial spheroids and in vivo in murine renal epithelial tubules derived from the nephrogenic mesenchyme, where its absence ultimately leads to severe morphogenetic defects in the tubules. By contrast, Rap1 is not required for lumen continuity or morphogenesis in renal tubules derived from the ureteric epithelium, which differ in that they form by extension from a pre-existing tubule. We further demonstrate that Rap1 is required for correct localization of Afadin to adherens junctions both in vitro and in vivo. Together, these results suggest a model in which Rap1 localizes Afadin to junctional complexes, which in turn regulates nascent lumen formation and positioning to ensure continuous tubulogenesis.

Polarized transport of membrane and secreted proteins during lumen morphogenesis

A ubiquitous feature of animal development is the formation of fluid-filled cavities or lumina, which transport gases and fluids across tissues and organs. Among different species, lumina vary drastically in size, scale, and complexity. However, all lumen formation processes share key morphogenetic principles that underly their development. Fundamentally, a lumen simply consists of epithelial cells that encapsulate a continuous internal space, and a common way of building a lumen is via opening and enlarging by filling it with fluid and/or macromolecules. Here, we discuss how polarized targeting of membrane and secreted proteins regulates lumen formation, mainly focusing on ion transporters in vertebrate model systems. We also discuss mechanistic differences observed among invertebrates and vertebrates and describe how the unique properties of the Na+/K+-ATPase and junctional proteins can promote polarization of immature epithelia to build lumina de novo in developing organs.

Endoglycan (PODXL2) is proteolytically processed by ADAM10 (a disintegrin and metalloprotease 10) and controls neurite branching in primary neurons

Cell adhesion is tightly controlled in multicellular organisms, for example, through proteolytic ectodomain shedding of the adhesion-mediating cell surface transmembrane proteins. In the brain, shedding of cell adhesion proteins is required for nervous system development and function, but the shedding of only a few adhesion proteins has been studied in detail in the mammalian brain. One such adhesion protein is the transmembrane protein endoglycan (PODXL2), which belongs to the CD34-family of highly glycosylated sialomucins. Here, we demonstrate that endoglycan is broadly expressed in the developing mouse brains and is proteolytically shed in vitro in mouse neurons and in vivo in mouse brains. Endoglycan shedding in primary neurons was mediated by the transmembrane protease a disintegrin and metalloprotease 10 (ADAM10), but not by its homolog ADAM17. Functionally, endoglycan deficiency reduced the branching of neurites extending from primary neurons in vitro, whereas deletion of ADAM10 had the opposite effect and increased neurite branching. Taken together, our study discovers a function for endoglycan in neurite branching, establishes endoglycan as an ADAM10 substrate and suggests that ADAM10 cleavage of endoglycan may contribute to neurite branching.

Deciphering epiblast lumenogenesis reveals proamniotic cavity control of embryo growth and patterning

During the peri-implantation stages, the mouse embryo radically changes its appearance, transforming from a hollow-shaped blastocyst to an egg cylinder. At the same time, the epiblast gets reorganized from a simple ball of cells to a cup-shaped epithelial monolayer enclosing the proamniotic cavity. However, the cavity's function and mechanism of formation have so far been obscure. Through investigating the cavity formation, we found that in the epiblast, the process of lumenogenesis is driven by reorganization of intercellular adhesion, vectoral fluid transport, and mitotic paracellular water influx from the blastocoel into the emerging proamniotic cavity. By experimentally blocking lumenogenesis, we found that the proamniotic cavity functions as a hub for communication between the early lineages, enabling proper growth and patterning of the postimplantation embryo.

Effect of Cell Spreading on Rosette Formation by Human Pluripotent Stem Cell-Derived Neural Progenitor Cells

Neural rosettes (NPC rosettes) are radially arranged groups of cells surrounding a central lumen that arise stochastically in monolayer cultures of human pluripotent stem cell (hPSC)-derived neural progenitor cells (NPC). Since NPC rosette formation is thought to mimic cell behavior in the early neural tube, these rosettes represent important in vitro models for the study of neural tube morphogenesis. However, using current protocols, NPC rosette formation is not synchronized and results are inconsistent among different hPSC lines, hindering quantitative mechanistic analyses and challenging live cell imaging. Here, we report a rapid and robust protocol to induce rosette formation within 6 h after evenly-sized “colonies” of NPC are generated through physical cutting of uniformly polarized NESTIN⁺/PAX6⁺/PAX3⁺/DACH1⁺ NPC monolayers. These NPC rosettes show apically polarized lumens studded with primary cilia. Using this assay, we demonstrate reduced lumenal size in the absence of PODXL, an important apical determinant recently identified as a candidate gene for juvenile Parkinsonism. Interestingly, time lapse imaging reveals that, in addition to radial organization and apical lumen formation, cells within cut NPC colonies initiate rapid basally-driven spreading. Further, using chemical, genetic and biomechanical tools, we show that NPC rosette morphogenesis requires this basal spreading activity and that spreading is tightly regulated by Rho/ROCK signaling. This robust and quantitative NPC rosette platform provides a sensitive system for the further investigation of cellular and molecular mechanisms underlying NPC rosette morphogenesis.

Cells into tubes: Molecular and physical principles underlying lumen formation in tubular organs

Tubular networks, such as the vascular and respiratory systems, transport liquids and gases in multicellular organisms. The basic units of these organs are tubes formed by single or multiple cells enclosing a luminal cavity. The formation and maintenance of correctly sized and shaped lumina are fundamental steps in organogenesis and are essential for organismal homeostasis. Therefore, understanding how cells generate, shape and maintain lumina is crucial for understanding normal organogenesis as well as the basis of pathological conditions. Lumen formation involves polarized membrane trafficking, cytoskeletal dynamics, and the influence of intracellular as well as extracellular mechanical forces, such as cortical tension, luminal pressure or blood flow. Various tissue culture and in vivo model systems, ranging from MDCK cell spheroids to tubular organs in worms, flies, fish, and mice, have provided many insights into the molecular and cellular mechanisms underlying lumenogenesis and revealed key factors that regulate the size and shape of cellular tubes. Moreover, the development of new experimental and imaging approaches enabled quantitative analyses of intracellular dynamics and allowed to assess the roles of cellular and tissue mechanics during tubulogenesis. However, how intracellular processes are coordinated and regulated across scales of biological organization to generate properly sized and shaped tubes is only beginning to be understood. Here, we review recent insights into the molecular, cellular and physical mechanisms underlying lumen formation during organogenesis. We discuss how these mechanisms control lumen formation in various model systems, with a special focus on the morphogenesis of tubular organs in Drosophila.

A sticky wicket: Defining molecular functions for CD34 in hematopoietic cells

The CD34 cell surface antigen is widely expressed in tissues on cells with progenitor-like properties and on mature vascular endothelia. In adult human bone marrow, CD34 marks hematopoietic stem and progenitor cells (HSPCs) starting from the bulk of hematopoietic stem cells with long-term repopulating potential (LT-HSCs) throughout expansion and differentiation of oligopotent and unipotent progenitors. CD34 protein surface expression is typically lost as cells mature into terminal effectors. Because of this expression pattern of HSPCs, CD34 has had a central role in the evaluation or selection of donor graft tissue in HSC transplant (HSCT). Given its clinical importance, it is surprising that the biological functions of CD34 are still poorly understood. This enigma is due, in part, to CD34's context-specific role as both a pro-adhesive and anti-adhesive molecule and its potential functional redundancy with other sialomucins. Moreover, there are also critical differences in the regulation of CD34 expression on HSPCs in humans and experimental mice. In this review, we highlight some of the more well-defined functions of CD34 in HSPCs with a focus on proposed functions most relevant to HSCT biology.

The phospholipid PI(3,4)P2 is an apical identity determinant

Apical-basal polarization is essential for epithelial tissue formation, segregating cortical domains to perform distinct physiological functions. Cortical lipid asymmetry has emerged as a determinant of cell polarization. We report a network of phosphatidylinositol phosphate (PIP)-modifying enzymes, some of which are transcriptionally induced upon embedding epithelial cells in extracellular matrix, and that are essential for apical-basal polarization. Unexpectedly, we find that PI(3,4)P2 localization and function is distinct from the basolateral determinant PI(3,4,5)P3. PI(3,4)P2 localizes to the apical surface, and Rab11a-positive apical recycling endosomes. PI(3,4)P2 is produced by the 5-phosphatase SHIP1 and Class-II PI3-Kinases to recruit the endocytic regulatory protein SNX9 to basolateral domains that are being remodeled into apical surfaces. Perturbing PI(3,4)P2 levels results in defective polarization through subcortical retention of apically destined vesicles at apical membrane initiation sites. We conclude that PI(3,4)P2 is a determinant of apical membrane identity.

Insane in the apical membrane: Trafficking events mediating apicobasal epithelial polarity during tube morphogenesis

The creation of cellular tubes is one of the most vital developmental processes, resulting in the formation of most organ types. Cells have co-opted a number of different mechanisms for tube morphogenesis that vary among tissues and organisms; however, generation and maintenance of cell polarity is fundamental for successful lumenogenesis. Polarized membrane transport has emerged as a key driver not only for establishing individual epithelial cell polarity, but also for coordination of epithelial polarization during apical lumen formation and tissue morphogenesis. In recent years, much work has been dedicated to identifying membrane trafficking regulators required for lumenogenesis. In this review we will summarize the findings from the past couple of decades in defining the molecular machinery governing lumenogenesis both in 3D tissue culture models and during organ development in vivo.

Spatiotemporal heterogeneity and patterning of developing renal blood vessels

The kidney vasculature facilitates the excretion of wastes, the dissemination of hormones, and the regulation of blood chemistry. To carry out these diverse functions, the vasculature is regionalized within the kidney and along the nephron. However, when and how endothelial regionalization occurs remains unknown. Here, we examine the developing kidney vasculature to assess its 3-dimensional structure and transcriptional heterogeneity. First, we observe that endothelial cells (ECs) grow coordinately with the kidney bud as early as E10.5, and begin to show signs of specification by E13.5 when the first arteries can be identified. We then focus on how ECs pattern and remodel with respect to the developing nephron and collecting duct epithelia. ECs circumscribe nephron progenitor populations at the distal tips of the ureteric bud (UB) tree and form stereotyped cruciform structures around each tip. Beginning at the renal vesicle (RV) stage, ECs form a continuous plexus around developing nephrons. The endothelial plexus envelops and elaborates with the maturing nephron, becoming preferentially enriched along the early distal tubule. Lastly, we perform transcriptional and immunofluorescent screens to characterize spatiotemporal heterogeneity in the kidney vasculature and identify novel regionally enriched genes. A better understanding of development of the kidney vasculature will help instruct engineering of properly vascularized ex vivo kidneys and evaluate diseased kidneys.

Nephronectin Regulates Mesangial Cell Adhesion and Behavior in Glomeruli

A critical aspect of kidney function occurs at the glomerulus, the capillary network that filters the blood. The glomerular basement membrane (GBM) is a key component of filtration, yet our understanding of GBM interactions with mesangial cells, specialized pericytes that provide structural stability to glomeruli, is limited. We investigated the role of nephronectin (Npnt), a GBM component and known ligand of α8β1 integrin. Immunolocalization and in situ hybridization studies in kidneys of adult mice revealed that nephronectin is produced by podocytes and deposited into the GBM. Conditional deletion of Npnt from nephron progenitors caused a pronounced increase in mesangial cell number and mesangial sclerosis. Nephronectin colocalized with α8β1 integrin to novel, specialized adhesion structures that occurred at sites of mesangial cell protrusion at the base of the capillary loops. Absence of nephronectin disrupted these adhesion structures, leading to mislocalization of α8β1. Podocyte-specific deletion of Npnt also led to mesangial sclerosis in mice. These results demonstrate a novel role for nephronectin and α8β1 integrin in a newly described adhesion complex and begin to uncover the molecular interactions between the GBM and mesangial cells, which govern mesangial cell behavior and may have a role in pathologic states.

Signaling Networks in Epithelial Tube Formation

Epithelial tubes are crucial to the function of organ systems including the excretory, gastrointestinal, cardiovascular, and pulmonary. Studies in the last two decades using in vitro organotypic systems and a variety of animal models have substantiated a large number of the morphogenetic mechanisms required to form epithelial tubes in development and regeneration. Many of these mechanisms modulate the differentiation and proliferation events necessary for generating the cell movements and changes in cell shape to delineate the wide variety of epithelial tube sizes, lengths, and conformations. For instance, when coupled with oriented cell division, proliferation itself plays a role in changes in tube shape and their directed expansion. Most of these processes are regulated in response to signaling inputs from adjacent cells or soluble factors from the environment. Despite the great deal of recent investigation in this direction, the knowledge we have about the signaling pathways associated with all epithelial tubulogenesis in development and regeneration is still very limited.

Loss of mucin-type O-glycans impairs the integrity of the glomerular filtration barrier in the mouse kidney

The kidney's filtration activity is essential for removing toxins and waste products from the body. The vascular endothelial cells of the glomerulus are fenestrated, flattened, and surrounded by podocytes, specialized cells that support glomerular endothelial cells. Mucin-type core 1-derived O-glycans (O-glycans) are highly expressed on both glomerular capillary endothelial cells and their supporting podocytes, but their biological role is unclear. Biosynthesis of core 1-derived O-glycans is catalyzed by the glycosyltransferase core 1 β1,3-galactosyltransferase (C1galt1). Here we report that neonatal or adult mice with inducible deletion of C1galt1 (iC1galt1^-/-) exhibit spontaneous proteinuria and rapidly progressing glomerulosclerosis. Ultrastructural analysis of the glomerular filtration barrier components revealed that loss of O-glycans results in altered podocyte foot processes. Further analysis indicated that O-glycan is essential for the normal signaling function of podocalyxin, a podocyte foot process-associated glycoprotein. Our results reveal a new function of O-glycosylation in the integrity of the glomerular filtration barrier.