A dynamic and mosaic basement membrane controls cell intercalation in Drosophila ovaries

Linking Basement Membrane and Slit Diaphragm in Drosophila Nephrocytes

Key Points

Background

The glomerular basement membrane and the slit diaphragm are essential parts of the filtration barrier. How these layers collaborate remains unclear. The podocyte-like nephrocytes in Drosophila harbor both a slit diaphragm and a basement membrane, serving as a model to address this critical question.

Methods

Basement membrane components and matrix receptors were silenced using RNA interference in nephrocytes. Slit diaphragms were analyzed using immunofluorescence, followed by automated quantification. Tracer endocytosis was applied for functional readouts.

Results

Immunofluorescence indicated a significant reduction in slit diaphragm density upon loss of laminin and collagen IV components. This was accompanied by reduced expression of fly nephrin and shallower membrane invaginations. Tracer studies revealed that the basement membrane defines properties of the nephrocyte filtration barrier. Acute enzymatic disruption of the basement membrane via collagenase rapidly caused slit diaphragm mislocalization and disintegration, which was independent of cell death. Loss of matrix-interacting receptors, particularly integrins mys and mew, phenocopied basement membrane disruption. Integrins and nephrin colocalized at the slit diaphragm in nephrocytes in a mutually dependent manner, interacting genetically. Human integrin α3 interacted physically with nephrin.

Conclusions

The glomerular basement membrane model in Drosophila nephrocytes reveals that matrix receptor–mediated cues ensure correct positioning of the slit diaphragm and the overall filtration barrier architecture.

An mTurq2-Col4a1 mouse model allows for live visualization of mammalian basement membrane development

Basement membranes (BMs) are specialized sheets of extracellular matrix that underlie epithelial and endothelial tissues. BMs regulate the traffic of cells and molecules between compartments, and participate in signaling, cell migration, and organogenesis. The dynamics of mammalian BMs, however, are poorly understood, largely due to a lack of models in which core BM components are endogenously labeled. Here, we describe the mTurquoise2-Col4a1 mouse in which we fluorescently tag collagen IV, the main component of BMs. Using an innovative planar-sagittal live imaging technique to visualize the BM of developing skin, we directly observe BM deformation during hair follicle budding and basal progenitor cell divisions. The BM's inherent pliability enables dividing cells to remain attached to and deform the BM, rather than lose adhesion as generally thought. Using FRAP, we show BM collagen IV is extremely stable, even during periods of rapid epidermal growth. These findings demonstrate the utility of the mTurq2-Col4a1 mouse to shed new light on mammalian BM developmental dynamics.

Extracellular matrix dynamics: A key regulator of cell migration across length-scales and systems

The interactions between cells and their surrounding extracellular matrix (ECM) are dynamic and play critical roles in cell migration during development, health, and diseases. Recent advances have highlighted the complexity and diversity of ECM compositions, or "matrisomes", of tissues resulting in ECMs of different physical, mechanical, and biochemical properties. Investigating the effects of these properties on cell-ECM interactions in the context of cell migration have led to a better understanding of the principles underlying tissue morphogenesis, wound healing, immune response, or cancer metastasis. These new insights into the interplay between ECM dynamics and cell migration can lead to the identification of unique opportunities for therapeutic interventions.

A Window into Mammalian Basement Membrane Development: Insights from the mTurq2-Col4a1 Mouse Model

Basement membranes (BMs) are specialized sheets of extracellular matrix that underlie epithelial and endothelial tissues. BMs regulate traffic of cells and molecules between compartments, and participate in signaling, cell migration and organogenesis. The dynamics of mammalian BMs, however, are poorly understood, largely due to a lack of models in which core BM components are endogenously labelled. Here, we describe the mTurquoise2-Col4a1 mouse, in which we fluorescently tag collagen IV, the main component of BMs. Using an innovative Planar-Sagittal live imaging technique to visualize the BM of developing skin, we directly observe BM deformation during hair follicle budding and basal progenitor cell divisions. The BM's inherent pliability enables dividing cells to remain attached to and deform the BM, rather than lose adhesion as generally thought. Using FRAP, we show BM collagen IV is extremely stable, even during periods of rapid epidermal growth. These findings demonstrate the utility of the mTurq2-Col4a1 mouse to shed new light on mammalian BM developmental dynamics.

Chondroitin sulfate is required for follicle epithelial integrity and organ shape maintenance in Drosophila

Heparan sulfate (HS) and chondroitin sulfate (CS) are evolutionarily conserved glycosaminoglycans that are found in most animal species, including the genetically tractable model organism Drosophila. In contrast to extensive in vivo studies elucidating co-receptor functions of Drosophila HS proteoglycans (PGs), only a limited number of studies have been conducted for those of CSPGs. To investigate the global function of CS in development, we generated mutants for Chondroitin sulfate synthase (Chsy), which encodes the Drosophila homolog of mammalian chondroitin synthase 1, a crucial CS biosynthetic enzyme. Our characterizations of the Chsy mutants indicated that a fraction survive to adult stage, which allowed us to analyze the morphology of the adult organs. In the ovary, Chsy mutants exhibited altered stiffness of the basement membrane and muscle dysfunction, leading to a gradual degradation of the gross organ structure as mutant animals aged. Our observations show that normal CS function is required for the maintenance of the structural integrity of the ECM and gross organ architecture.

Identification and Expression of Integrins during Testicular Fusion in Spodoptera litura

Integrin members are cell adhesion receptors that bind to extracellular matrix (ECM) proteins to regulate cell-cell adhesion and cell-ECM adhesion. This process is essential for tissue development and organogenesis. The fusion of two testes is a physiological phenomenon that is required for sperm production and effective reproduction in many Lepidoptera. However, the molecular mechanism of testicular fusion is unclear. In Spodoptera litura, two separated testes fuse into a single testis during the larva-to-pupa transformation. We identified five α and five β integrin subunits that were closely associated with testicular fusion. Integrin α1 and α2 belong to the position-specific 1 (PS1) and PS2 groups, respectively. Integrin α3, αPS1/αPS2, and αPS3 were clustered into the PS3 group. Integrin β1 belonged to the insect β group, and β2, β3, and β5 were clustered in the βν group. Among these integrins, α1, α2, α3, αPS1/PS2, αPS3, β1, and β4 subunits were highly expressed when the testes fused. However, their expression levels were much lower before and after the fusion of the testis. The qRT-PCR and immunohistochemistry analyses indicated that integrin β1 mRNA and the protein were highly expressed in the peritoneal sheath of the testis, particularly when the testes fused. These results indicate that integrins might participate in S. litura testicular fusion.

Interommatidial cells build a tensile collagen network during Drosophila retinal morphogenesis

Drosophila compound eye morphogenesis transforms a simple epithelium into an approximate hollow hemisphere comprised of ∼700 ommatidia, packed as tapering hexagonal prisms between a rigid external array of cuticular lenses and a parallel, rigid internal floor, the fenestrated membrane (FM). Critical to vision, photosensory rhabdomeres are sprung between these two surfaces, grading their length and shape accurately across the eye and aligning them to the optical axis. Using fluorescently tagged collagen and laminin, we show that that the FM assembles sequentially, emerging in the larval eye disc in the wake of the morphogenetic furrow as the original collagen-containing basement membrane (BM) separates from the epithelial floor and is replaced by a new, laminin-rich BM, which advances around axon bundles of newly differentiated photoreceptors as they exit the retina, forming fenestrae in this new, laminin-rich BM. In mid-pupal development, the interommatidial cells (IOCs) autonomously deposit collagen at fenestrae, forming rigid, tension-resisting grommets. In turn, stress fibers assemble in the IOC basal endfeet, where they contact grommets at anchorages mediated by integrin linked kinase (ILK). The hexagonal network of IOC endfeet tiling the retinal floor couples nearest-neighbor grommets into a supracellular tri-axial tension network. Late in pupal development, IOC stress fiber contraction folds pliable BM into a hexagonal grid of collagen-stiffened ridges, concomitantly decreasing the area of convex FM and applying essential morphogenetic longitudinal tension to rapidly growing rhabdomeres. Together, our results reveal an orderly program of sequential assembly and activation of a supramolecular tensile network that governs Drosophila retinal morphogenesis.

Visualization of basement membranes by a nidogen-based fluorescent reporter in mice

Basement membranes (BMs) are thin, sheet-like extracellular structures that cover the basal side of epithelial and endothelial tissues and provide structural and functional support to adjacent cell layers. The molecular structure of BMs is a fine meshwork that incorporates specialized extracellular matrix proteins. Recently, live visualization of BMs in invertebrates demonstrated that their structure is flexible and dynamically rearranged during cell differentiation and organogenesis. However, the BM dynamics in mammalian tissues remain to be elucidated. We developed a mammalian BM imaging probe based on nidogen-1, a major BM-specific protein. Recombinant human nidogen-1 fused with an enhanced green fluorescent protein (Nid1-EGFP) retains its ability to bind to other BM proteins, such as laminin, type IV collagen, and perlecan, in a solid-phase binding assay. When added to the culture medium of embryoid bodies derived from mouse ES cells, recombinant Nid1-EGFP accumulated in the BM zone of embryoid bodies, and BMs were visualized in vitro. For in vivo BM imaging, a knock-in reporter mouse line expressing human nidogen-1 fused to the red fluorescent protein mCherry (R26-CAG-Nid1-mCherry) was generated. R26-CAG-Nid1-mCherry showed fluorescently labeled BMs in early embryos and adult tissues, such as the epidermis, intestine, and skeletal muscles, whereas BM fluorescence was unclear in several other tissues, such as the lung and heart. In the retina, Nid1-mCherry fluorescence visualized the BMs of vascular endothelium and pericytes. In the developing retina, Nid1-mCherry fluorescence labeled the BM of the major central vessels; however, the BM fluorescence were hardly observed in the peripheral growing tips of the vascular network, despite the presence of endothelial BM. Time-lapse observation of the retinal vascular BM after photobleaching revealed gradual recovery of Nid1-mCherry fluorescence, suggesting the turnover of BM components in developing retinal blood vessels. To the best of our knowledge, this is the first demonstration of in vivo BM imaging using a genetically engineered mammalian model. Although R26-CAG-Nid1-mCherry has some limitations as an in vivo BM imaging model, it has potential applications in the study of BM dynamics during mammalian embryogenesis, tissue regeneration, and pathogenesis.

Growth anisotropy of the extracellular matrix shapes a developing organ

Final organ size and shape result from volume expansion by growth and shape changes by contractility. Complex morphologies can also arise from differences in growth rate between tissues. We address here how differential growth guides the morphogenesis of the growing Drosophila wing imaginal disc. We report that 3D morphology results from elastic deformation due to differential growth anisotropy between the epithelial cell layer and its enveloping extracellular matrix (ECM). While the tissue layer grows in plane, growth of the bottom ECM occurs in 3D and is reduced in magnitude, thereby causing geometric frustration and tissue bending. The elasticity, growth anisotropy and morphogenesis of the organ are fully captured by a mechanical bilayer model. Moreover, differential expression of the Matrix metalloproteinase MMP2 controls growth anisotropy of the ECM envelope. This study shows that the ECM is a controllable mechanical constraint whose intrinsic growth anisotropy directs tissue morphogenesis in a developing organ.

Epithelial morphogenesis in the Drosophila egg chamber requires Parvin and ILK

Integrins are the major family of transmembrane proteins that mediate cell-matrix adhesion and have a critical role in epithelial morphogenesis. Integrin function largely depends on the indirect connection of the integrin cytoplasmic tail to the actin cytoskeleton through an intracellular protein network, the integrin adhesome. What is currently unknown is the role of individual integrin adhesome components in epithelia dynamic reorganization. Drosophila egg chamber consists of the oocyte encircled by a monolayer of somatic follicle epithelial cells that undergo specific cell shape changes. Egg chamber morphogenesis depends on a developmental array of cell-cell and cell-matrix signalling events. Recent elegant work on the role of integrins in the Drosophila egg chamber has indicated their essential role in the early stages of oogenesis when the pre-follicle cells assemble into the follicle epithelium. Here, we have focused on the functional requirement of two key integrin adhesome components, Parvin and Integrin-Linked Kinase (ILK). Both proteins are expressed in the developing ovary from pupae to the adult stage and display enriched expression in terminal filament and stalk cells, while their genetic removal from early germaria results in severe disruption of the subsequent oogenesis, leading to female sterility. Combining genetic mosaic analysis of available null alleles for both Parvin and Ilk with conditional rescue utilizing the UAS/Gal4 system, we found that Parvin and ILK are required in pre-follicle cells for germline cyst encapsulation and stalk cell morphogenesis. Collectively, we have uncovered novel developmental functions for both Parvin and ILK, which closely synergize with integrins in epithelia.

Adipokine and fat body in flies: Connecting organs

Under conditions of nutritional and environmental stress, organismal homeostasis is preserved through inter-communication between multiple organs. To do so, higher organisms have developed a system of interorgan communication through which one tissue can affect the metabolism, activity or fate of remote organs, tissues or cells. In this review, we discuss the latest findings emphasizing Drosophila melanogaster as a powerful model organism to study these interactions and may constitute one of the best documented examples depicting the long-distance communication between organs. In flies, the adipose tissue appears to be one of the main organizing centers for the regulation of insect development and behavior: it senses nutritional and hormonal signals and in turn, orchestrates the release of appropriate adipokines. We discuss the nature and the role of recently uncovered adipokines, their regulations by external cues, their secretory routes and their modes of action to adjust developmental growth and timing accordingly. These findings have the potential for identification of candidate factors and signaling pathways that mediate conserved interorgan crosstalk.

Basement membrane remodeling guides cell migration and cell morphogenesis during development

Basement membranes (BMs) are thin, dense forms of extracellular matrix that underlie or surround most animal tissues. BMs are enormously complex and harbor numerous proteins that provide essential signaling, mechanical, and barrier support for tissues during their development and normal functioning. As BMs are found throughout animal tissues, cells frequently migrate, change shape, and extend processes along BMs. Although sometimes used only as passive surfaces by cells, studies in developmental contexts are finding that BMs are often actively modified to help guide cell motility and cell morphogenesis. Here, I provide an overview of recent work revealing how BMs are remodeled in remarkably diverse ways to direct cell migration, cell orientation, axon guidance, and dendrite branching events during animal development.