From fate to function: the Drosophila trachea and salivary gland as models for tubulogenesis

Tube formation is a ubiquitous process required to sustain life in multicellular organisms. The tubular organs of adult mammals include the lungs, vasculature, digestive and excretory systems, as well as secretory organs such as the pancreas, salivary, prostate, and mammary glands. Other tissues, including the embryonic heart and neural tube, have requisite stages of tubular organization early in development. To learn the molecular and cellular basis of how epithelial cells are organized into tubular organs of various shapes and sizes, investigators have focused on the Drosophila trachea and salivary gland as model genetic systems for branched and unbranched tubes, respectively. Both organs begin as polarized epithelial placodes, which through coordinated cell shape changes, cell rearrangement, and cell migration form elongated tubes. Here, we discuss what has been discovered regarding the details of cell fate specification and tube formation in the two organs; these discoveries reveal significant conservation in the cellular and molecular events of tubulogenesis.     

Extracellular matrix and cytoskeletal dynamics during branching morphogenesis

Branching morphogenesis is a fundamental developmental process which results in amplification of epithelial surface area for exchanging molecules in organs including the lung, kidney, mammary gland and salivary gland. These complex tree-like structures are built by iterative rounds of simple routines of epithelial morphogenesis, including bud formation, extension, and bifurcation, that require constant remodeling of the extracellular matrix (ECM) and the cytoskeleton. In this review, we highlight the current understanding of the role of the ECM and cytoskeletal dynamics in branching morphogenesis across these different organs. The cellular and molecular mechanisms shared during this morphogenetic process provide insight into the development of other branching organs.     

Extracellular matrix and cytoskeletal dynamics during branching morphogenesis

Branching morphogenesis is a fundamental developmental process which results in amplification of epithelial surface area for exchanging molecules in organs including the lung, kidney, mammary gland and salivary gland. These complex tree-like structures are built by iterative rounds of simple routines of epithelial morphogenesis, including bud formation, extension, and bifurcation, that require constant remodeling of the extracellular matrix (ECM) and the cytoskeleton. In this review, we highlight the current understanding of the role of the ECM and cytoskeletal dynamics in branching morphogenesis across these different organs. The cellular and molecular mechanisms shared during this morphogenetic process provide insight into the development of other branching organs.     

Use of the collagen I-deficient Mov13 mouse mutant to analyse epithelial-mesenchymal tissue interaction

The collagen I-deficient mouse mutant (Mov13 — an embryonic recessive lethal) was used to investigate the function of this major constituent of the extracellular matrix (ECM) in organ development. All epithelial-mesenchymal organs tested as explants (lung, kidney, pancreas, salivary glands, skin) developed normally and, in particular, showed typical branching morphogenesis in the absence of collagen I. It is concluded that the ECM of these organs can organize for proper developmental function in the absence of the major interstitial collagen, but a possible morphogenetic function of other fibrillar collagens (types III and V) cannot be excluded. The only insufficiencies in the mutant were seen in the cornea where deposition and organization of the collagenous stroma was highly inadequate; but even there, development and migration of cells proceeded normally. In summary, the results indicate that ‘cellular’ development in epithelial-mesenchymal organs (including growth, morphogenesis, and differentiation) does not depend on collagen I.     

Multifunctional ECM proteins in bone and teeth

The extracellular matrix (ECM) of all tissues and organs is a highly organized and complex structure unique to the specific organ type. The ECM contains structural and functional proteins that define cellular function, organization, behavior and ultimately organ characteristics and function. The ECM was initially thought to contain only a specific set of secretory proteins. However, our group and several other groups have shown that the ECM contains functional proteins that have been previously defined as solely intracellular. In the present review, we have focused on the ECM of mineralized tissues namely bone and dentin. We provide here, a brief review of some non-classical ECM proteins that have been shown to possess both intra and extracellular roles in the formation of these mineralized matrices.     

Tissue elongation requires oscillating contractions of a basal actomyosin network

Understanding how molecular dynamics leads to cellular behaviours that ultimately sculpt organs and tissues is a major challenge not only in basic developmental biology but also in tissue engineering and regenerative medicine. Here we use live imaging to show that the basal surfaces of Drosophila follicle cells undergo a series of directional, oscillating contractions driven by periodic myosin accumulation on a polarized actin network. Inhibition of the actomyosin contractions or their coupling to extracellular matrix (ECM) blocked elongation of the whole tissue, whereas enhancement of the contractions exaggerated it. Myosin accumulated in a periodic manner before each contraction and was regulated by the small GTPase Rho, its downstream kinase, ROCK, and cytosolic calcium. Disrupting the link between the actin cytoskeleton and the ECM decreased the amplitude and period of the contractions, whereas enhancing cell-ECM adhesion increased them. In contrast, disrupting cell-cell adhesions resulted in loss of the actin network. Our findings reveal a mechanism controlling organ shape and an experimental model for the study of the effects of oscillatory actomyosin activity within a coherent cell sheet.     

Convective tissue movements play a major role in avian endocardial morphogenesis

Endocardial cells play a critical role in cardiac development and function, forming the innermost layer of the early (tubular) heart, separated from the myocardium by extracellular matrix (ECM). However, knowledge is limited regarding the interactions of cardiac progenitors and surrounding ECM during dramatic tissue rearrangements and concomitant cellular repositioning events that underlie endocardial morphogenesis. By analyzing the movements of immunolabeled ECM components (fibronectin, fibrillin-2) and TIE1 positive endocardial progenitors in time-lapse recordings of quail embryonic development, we demonstrate that the transformation of the primary heart field within the anterior lateral plate mesoderm (LPM) into a tubular heart involves the precise co-movement of primordial endocardial cells with the surrounding ECM. Thus, the ECM of the tubular heart contains filaments that were associated with the anterior LPM at earlier developmental stages. Moreover, endocardial cells exhibit surprisingly little directed active motility, that is, sustained directed movements relative to the surrounding ECM microenvironment. These findings point to the importance of large-scale tissue movements that convect cells to the appropriate positions during cardiac organogenesis.     

Morphogenesis of epithelial tubes: Insights into tube formation, elongation, and elaboration

Epithelial tubes are a fundamental tissue across the metazoan phyla and provide an essential functional component of many of the major organs. Recent work in flies and mammals has begun to elucidate the cellular mechanisms driving the formation, elongation, and branching morphogenesis of epithelial tubes during development. Both forward and reverse genetic techniques have begun to identify critical molecular regulators for these processes and have revealed the conserved role of key pathways in regulating the growth and elaboration of tubular networks. In this review, we discuss the developmental programs driving the formation of branched epithelial networks, with specific emphasis on the trachea and salivary gland of Drosophila melanogaster and the mammalian lung, mammary gland, kidney, and salivary gland. We both highlight similarities in the development of these organs and attempt to identify tissue and organism specific strategies. Finally, we briefly consider how our understanding of the regulation of proliferation, apicobasal polarity, and epithelial motility during branching morphogenesis can be applied to understand the pathologic dysregulation of these same processes during metastatic cancer progression.     

Comprehension of ECM-Cell dynamics: A prerequisite for tissue regeneration

Tissue regeneration and cell therapy have an enormous potential in healthcare through the creation of artificial human tissues and organs. The possibility of producing functional replica of tissues and organs can offer a common, solitary solution for various kinds of inflictions. It can also provide an ultimate test model for drug discovery. There exists convincing evidence that if cells are cultured in extra-cellular matrix (ECM) mimicking 3D scaffolds infused with tissue-specific biochemical cues they grow and differentiate to express functionality. However, comprehensive understanding of ECM and its dynamic relation with the growing cells is vital for creating functional tissue models ex vivo. Different medical and non-medical groups all over the world are working towards achieving affordable, user friendly and technically viable solutions for improving our understanding of Cell-ECM dynamics for tissue engineering (TE). Successful TE, an ambitious goal that includes tissue neogenesis in vitro and functional tissue mending (regenerative medicine) in vivo, however involves innumerable challenges. Present review discusses some of the major technical hurdles that hinder the pace of progress in tissue regeneration/engineering (TE).     

Ancestral Vascular Lumen Formation via Basal Cell Surfaces

The cardiovascular system of bilaterians developed from a common ancestor. However, no endothelial cells exist in invertebrates demonstrating that primitive cardiovascular tubes do not require this vertebrate-specific cell type in order to form. This raises the question of how cardiovascular tubes form in invertebrates? Here we discovered that in the invertebrate cephalochordate amphioxus, the basement membranes of endoderm and mesoderm line the lumen of the major vessels, namely aorta and heart. During amphioxus development a laminin-containing extracellular matrix (ECM) was found to fill the space between the basal cell surfaces of endoderm and mesoderm along their anterior-posterior (A-P) axes. Blood cells appear in this ECM-filled tubular space, coincident with the development of a vascular lumen. To get insight into the underlying cellular mechanism, we induced vessels in vitro with a cell polarity similar to the vessels of amphioxus. We show that basal cell surfaces can form a vascular lumen filled with ECM, and that phagocytotic blood cells can clear this luminal ECM to generate a patent vascular lumen. Therefore, our experiments suggest a mechanism of blood vessel formation via basal cell surfaces in amphioxus and possibly in other invertebrates that do not have any endothelial cells. In addition, a comparison between amphioxus and mouse shows that endothelial cells physically separate the basement membranes from the vascular lumen, suggesting that endothelial cells create cardiovascular tubes with a cell polarity of epithelial tubes in vertebrates and mammals.     

A bit of give and take: the relationship between the extracellular matrix and the developing chondrocyte

The extracellular matrix (ECM), once thought to be a static structural component of tissues, is now known to play a complex and dynamic role in a variety of cellular functions in a number of diverse tissues. A significant body of literature attests to the ability of the ECM to communicate both spatial and temporal information to adherent cells, thereby directing cell behavior via interactions between the ECM and cell-surface receptors. Moreover, volumes of experimental data show that a great deal of communication travels in the opposite direction, from the cell to the ECM, allowing for regulation of the cues transmitted by the ECM. As such, the ECM, with respect to its components and their organization, is not a fixed reflection of the state the local microenvironment in which a cell finds itself at a particular time, but rather is able to respond to and effect changes in its local microenvironment. As an example of the developmental consequences of ECM interactions, this review gives an overview of the 'give and take' relationship between the ECM and the cells of the developing skeletal elements, in particular, the chondrocyte.     

Suprastructures of extracellular matrices: paradigms of functions controlled by aggregates rather than molecules

Extracellular matrices (ECM) not only serve as structural scaffolds in organs and tissues, but also determine critical cellular functions through cell-matrix interactions. These are mediated by cell surface receptors that recognise specific structural features of ECMs and, hence, overall physical properties of the acellular environment. ECM structures are subject to hierarchic organisations, which are tightly adapted to the functions of tissues and organs. Only a few specialised tasks are reserved for isolated ECM macromolecules. Instead, molecular ECM components attain their prominent functions only after polymerising into insoluble suprastructural elements, i.e. fibrils, microfibrils, or networks that, in turn, are assembled into regional tissue structures, such as fibres or basement membranes. As an outstanding feature, most, if not all, ECM suprastructures are co-polymers of more than one molecular species that differ in their identity and relative abundance. Thus, ECM suprastructures are composite biological amalgamates. The analogy to metal alloys refers to structural and functional characteristics of ECM composites, which differ from those of each homo-polymeric aggregate. At the tissue level, biological alloys can themselves be assembled into conglomerates that again assume properties distinct from those of each individual alloy. Nevertheless, most studies in matrix biology solely focus on molecular features and mechanisms. Progress has however been made in identifying principles of interactions within suprastructural elements and their functional consequences. We are now only beginning to understand the impact of suprastructural organisation on the assembly and the functions of whole tissues and many fundamental issues in this almost pristine field await discovery.     

Drosophila tracheal morphogenesis: intricate cellular solutions to basic plumbing problems

The cellular architecture of tubular organs suggests striking similarities in the mechanisms of tubulogenesis between species. The formation of the Drosophila respiratory organ (trachea) highlights the basic principles of branch patterning and tube growth that generate a highly elaborate but stereotyped epithelial tubular network. Oriented cell migration, changes in cell shape, selective growth of the apical cell membrane and intracellular lumen formation are essential events in this process. These morphogenetic processes build four structurally distinct classes of tubes that facilitate optimal airflow and gas exchange with target tissues. The molecular players in these plots include attractant and repellent signals, differentiation factors that cause a high diversity of cell fates within the epithelium, and determinants of tube formation and dimensions.     

Functional role of periostin in development and wound repair: implications for connective tissue disease

Integrity of the extracellular matrix (ECM) is essential for maintaining the normal structure and function of connective tissues. ECM is secreted locally by cells and organized into a complex meshwork providing physical support to cells, tissues, and organs. Initially thought to act only as a scaffold, the ECM is now known to provide a myriad of signals to cells regulating all aspects of their phenotype from morphology to differentiation. Matricellular proteins are a class of ECM related molecules defined through their ability to modulate cell-matrix interactions. Matricellular proteins are expressed at high levels during development, but typically only appear in postnatal tissue in wound repair or disease, where their levels increase substantially. Members of the CCN family, tenascin-C, osteopontin, secreted protein acidic rich in cysteine (SPARC), bone sialoprotein, thrombospondins, and galectins have all been classed as matricellular proteins. Periostin, a 90 kDa secreted homophilic cell adhesion protein, was recently added to matricellular class of proteins based on its expression pattern and function during development as well as in wound repair. Periostin is expressed in connective tissues including the periodontal ligament, tendons, skin and bone, and is also prominent in neoplastic tissues, cardiovascular disease, as well as in connective tissue wound repair. This review will focus on the functional role of periostin in tissue physiology. Fundamentally, it appears that periostin influences cell behaviour as well as collagen fibrillogenesis, and therefore exerts control over the structural and functional properties of connective tissues in both health and disease. Periostin is a novel matricellular protein with close homology to Drosophila fasciclin 1. In this review, the functional role of periostin is discussed in the context of connective tissue physiology, in development, disease, and wound repair.     

Basement membranes in skin: unique matrix structures with diverse functions?

The view of extracellular matrix (ECM) has evolved from a merely scaffolding and space filling tissue element to an interface actively controlling cellular activities and tissue functions. A highly specialized form of ECM is the basement membrane (BM), an ubiquitous sheet-like polymeric structure composed of a set of distinct glycoproteins and proteoglycans. In this review we are largely focusing on function and assembly of BM in skin (1) at the dermo-epidermal interface and (2) in the resident micro-vasculature. The role of the non-polymeric components perlecan and particularly nidogen is exemplified by reviewing experiments based on genetic approaches and adequate experimental skin models in vivo and in vitro. While in mice total deficiency of one of these components is eventually developmentally lethal, the severity of the defects varies drastically between tissues and also the skin models recapitulating BM formation in vitro. There is accumulating evidence that this relies on the mechanical properties, the molecular composition of the BM, the adjacent ECM or connective tissue, the dynamics of molecular assembly, and ‘minor’ tissue-specific modifier or adapter components. Though the role of nidogen or perlecan is still remaining a controversial issue, the statements ‘being essential for BM/or not’ should be consequently referred to the developmental, tissue, and functional (e.g., repair) context.     

Bacterial adhesion to animal tissues: protein determinants for recognition of extracellular matrix components

The extracellular matrix (ECM) is present within all animal tissues and organs. Actually, it surrounds the eukaryotic cells composing the four basic tissue types, i.e. epithelial, muscle, nerve and connective. ECM does not solely refer to connective tissue but composes all tissues where its composition, structure and organization vary from one tissue to another. Constituted of the four main fibrous proteins, i.e. collagen, fibronectin, laminin and elastin, ECM components form a highly structured and functional network via specific interactions. From the basement membrane to interstitial matrix, further heterogeneity exists in the organization of the ECM in various tissues and organs also depending on their physiological state. Back to a molecular level, bacterial proteins represent the most significant part of the microbial surface components recognizing adhesive matrix molecules (MSCRAMM). These cell surface proteins are secreted and localized differently in monoderm and diderm–LPS bacteria. While one collagen‐binding domain (CBD) and different fibronectin‐binding domains (FBD1 to 8) have been registered in databases, much remains to be learned on specific binding to other ECM proteins via single or supramolecular protein structures. Besides theinteraction of bacterial proteins with individual ECM components, this review aims at stressing the importance of fully considering the ECM at supramolecular, cellular, tissue and organ levels. This conceptual view should not be overlooked to rigorously comprehend the physiology of bacterial interaction from commensal to pathogenic species.     

A matrix metalloproteinase mediates airway remodeling in Drosophila

Organ size typically increases dramatically during juvenile growth. This growth presents a fundamental tension, as organs need resiliency to resist stresses while still maintaining plasticity to accommodate growth. The extracellular matrix (ECM) is central to providing resiliency, but how ECM is remodeled to accommodate growth is poorly understood. We investigated remodeling of Drosophila respiratory tubes (tracheae) that elongate continually during larval growth, despite being lined with a rigid cuticular ECM. Cuticle is initially deposited with a characteristic pattern of repeating ridges and valleys known as taenidia. We find that for tubes to elongate, the extracellular protease Mmp1 is required for expansion of ECM between the taenidial ridges during each intermolt period. Mmp1 protein localizes in periodically spaced puncta that are in register with the taenidial spacing. Mmp1 also degrades old cuticle at molts, promotes apical membrane expansion in larval tracheae, and promotes tube elongation in embryonic tracheae. Whereas work in other developmental systems has demonstrated that MMPs are required for axial elongation occurring in localized growth zones, this study demonstrates that MMPs can also mediate interstitial matrix remodeling during growth of an organ system.     

Extracellular matrix-mediated remodeling and mechanotransduction in large vessels during development and disease

The vascular extracellular matrix (ECM) is synthesized and secreted during embryogenesis and facilitates the growth and remodeling of large vessels. Proper interactions between the ECM and vascular cells are pivotal for building the vasculature required for postnatal dynamic circulation. The ECM serves as a structural component by maintaining the integrity of the vessel wall while also regulating intercellular signaling, which involves cytokines and growth factors. The major ECM component in large vessels is elastic fibers, which include elastin and microfibrils. Elastin is predominantly synthesized by vascular smooth muscle cells (SMCs) and uses microfibrils as a scaffold to lay down and assemble cross-linked elastin. The absence of elastin causes developmental defects that result in the subendothelial proliferation of SMCs and inward remodeling of the vessel wall. Notably, elastic fiber formation is attenuated in the ductus arteriosus and umbilical arteries. These two vessels function during embryogenesis and close after birth via cellular proliferation, migration, and matrix accumulation. In dynamic postnatal mechano-environments, the elastic fibers in large vessels also serve an essential role in proper signal transduction as a component of elastin-contractile units. Disrupted mechanotransduction in SMCs leads to pathological conditions such as aortic aneurysms that exhibit outward remodeling. This review discusses the importance of the ECM—mainly the elastic fiber matrix—in large vessels during developmental remodeling and under pathological conditions. By dissecting the role of the ECM in large vessels, we aim to provide insights into the role of ECM-mediated signal transduction that can provide a basis for seeking new targets for intervention in vascular diseases.     

ECM Cross-Linking Regulates Invadopodia Dynamics

Invadopodia are membrane protrusions dynamically assembled by invasive cancer cells in contact with the extracellular matrix (ECM). Invadopodia are enriched by the structural proteins actin and cortactin as well as metalloproteases such as MT1-MMP, whose function is to degrade the surrounding ECM. During metastasis, invadopodia are necessary for cancer cell intravasation and extravasation. Although signaling pathways involved in the assembly and function of invadopodia are well studied, few studies address invadopodia dynamics and how the cell-ECM interactions contribute to cell invasion. Using iterative analysis based on time-lapse microscopy and mathematical modeling of invasive cancer cells, we found that cells oscillate between invadopodia presence and cell stasis—termed the “invadopodia state”—and invadopodia absence during cell translocation—termed the “migration state.” Our data suggest that β1-integrin-ECM binding and ECM cross-linking control the duration of each of the two states. By changing the concentration of cross-linkers in two-dimensional and three-dimensional cultures, we generate an ECM in which 0–0.92 of total lysine residues are cross-linked. Using an ECM with a range of cross-linking degrees, we demonstrate that the dynamics of invadopodia-related functions have a biphasic relationship to ECM cross-linking. At intermediate levels of ECM cross-linking (0.39), cells exhibit rapid invadopodia protrusion-retraction cycles and rapid calcium spikes, which lead to more frequent MT1-MMP delivery, causing maximal invadopodia-mediated ECM degradation. In contrast, both extremely high or low levels of cross-linking lead to slower invadopodia-related dynamics and lower ECM degradation. Additionally, β1-integrin inhibition modifies the dynamics of invadopodia-related functions as well as the length of time cells spend in either of the states. Collectively, these data suggest that β1-integrin-ECM binding nonlinearly translates small physical differences in the extracellular environment to differences in the dynamics of cancer cell behaviors. Understanding the conditions under which invadopodia can be reduced by subtle environment-targeting treatments may lead to combination therapies for preventing metastatic spread.