Mechanics of shape and movement of embryonic macrophage clusters - Dr Aparna Ratheesh

Drosophila embryonic macrophages are highly motile phagocytic and secretory cells which are essential for embryonic development. Embryonic migration and dispersal of these cells along pre-determined routes is invariably tied to their functions such that misrouting or delay in macrophage migration has serious consequences for embryogenesis and adult homeostasis. In this talk, I will describe our recent work showing how N-Cadherin mediated cellular rearrangements shape the embryonic macrophage cluster and positions it to allow developmental dispersion.

Two stories on actin form and organisation: a protein that can bend actin in vitro and how considering cell actin cortex mechanics can help us understand the hypermobile Ehlers-Danlos-Syndrome - Dr Darius Koester

The cell plasma membrane and its underlying actin cytoskeleton play a central role in providing mechanical stability, enabling vesicle transport, cell migration and cell division. The general interest of the Köster lab lies in the understanding of the molecular and physical principles that govern these processes at the plasma membrane. We follow two approaches to study these processes systematically, using reconstituted minimal systems of the membrane – cortex interface and cell experiments. This, in combination with high-resolution microscopy and quantitative image analysis, allows us to dissect and understand the interlinked contributions of membrane mechanics, cytoskeletal activity, and membrane organisation. In this seminar, we will highlight two recent studies.

One is about ‘Curly’, an N-terminal part of IQGAP proteins that binds actin and leads to the bending of individual actin filaments into tight rings when anchored to lipid membranes. We identified the Curly module within IQGAP1 (human), rng2 (S. Pombe) and iqg1 (S. Cervisiae), and have developed a theoretical description to understand the underlying mechanism. Membrane anchored Curly alters the stiffness of actin filaments and induces formation of actin rings of radii less than a micrometre. Addition of myosin II motors leads to the contraction of these actin rings, making it a minimal, four component contractile ring system. The discovery of Curly offers a unique way to generate actin filaments of defined curvature and could be used to modify the cell cortex in cells.

The second is on the role of fibroblast interactions with the extracellular matrix (ECM) in the development of hypermobile Ehlers-Danlos syndrome (hEDS). EDS are connective tissue disorders characterised by joint hypermobility, skin hyperextensibility, and tissue fragility. They are primarily considered collagen disorders, as most subtypes result from mutations in collagen-related genes. However, the underlying pathogenic mechanism remain poorly understood, and diagnostic and therapeutic options for EDS are limited, leaving many patients with a diminished quality of life. We developed a native ECM culture system to study the makeup of fibroblasts, their mechanophenotype and the ECM composition. We found distinct alterations in fibroblast adhesion to the ECM and contractility. hEDS fibroblasts exhibited reduced α2β1 integrin expression and a overexpression of αvβ3 integrin, with highly striated structures along actin filaments. These changes coincided with complete fibronectin matrix disintegration and an increased proportion of α-smooth muscle actin-positive cells, indicating a hypercontractile phenotype. In contrast, classical EDS fibroblasts also showed increased αvβ3 and decreased α2β1 expression but exhibited fewer α-smooth muscle actin-positive cells. Based on this we propose a novel pathomechanism for EDS, suggesting that ECM composition changes disrupt cell adhesion and mechanotransduction, ultimately compromising connective tissue integrity.

The cell plasma membrane and its underlying actin cytoskeleton play a central role in providing mechanical stability, enabling vesicle transport, cell migration and cell division. The general interest of the Köster lab lies in the understanding of the molecular and physical principles that govern these processes at the plasma membrane. We follow two approaches to study these processes systematically, using reconstituted minimal systems of the membrane – cortex interface and cell experiments. This, in combination with high-resolution microscopy and quantitative image analysis, allows us to dissect and understand the interlinked contributions of membrane mechanics, cytoskeletal activity, and membrane organisation. In this seminar, we will highlight two recent studies.

One is about ‘Curly’, an N-terminal part of IQGAP proteins that binds actin and leads to the bending of individual actin filaments into tight rings when anchored to lipid membranes. We identified the Curly module within IQGAP1 (human), rng2 (S. Pombe) and iqg1 (S. Cervisiae), and have developed a theoretical description to understand the underlying mechanism. Membrane anchored Curly alters the stiffness of actin filaments and induces formation of actin rings of radii less than a micrometre. Addition of myosin II motors leads to the contraction of these actin rings, making it a minimal, four component contractile ring system. The discovery of Curly offers a unique way to generate actin filaments of defined curvature and could be used to modify the cell cortex in cells.

The second is on the role of fibroblast interactions with the extracellular matrix (ECM) in the development of hypermobile Ehlers-Danlos syndrome (hEDS). EDS are connective tissue disorders characterised by joint hypermobility, skin hyperextensibility, and tissue fragility. They are primarily considered collagen disorders, as most subtypes result from mutations in collagen-related genes. However, the underlying pathogenic mechanism remain poorly understood, and diagnostic and therapeutic options for EDS are limited, leaving many patients with a diminished quality of life. We developed a native ECM culture system to study the makeup of fibroblasts, their mechanophenotype and the ECM composition. We found distinct alterations in fibroblast adhesion to the ECM and contractility. hEDS fibroblasts exhibited reduced α2β1 integrin expression and a overexpression of αvβ3 integrin, with highly striated structures along actin filaments. These changes coincided with complete fibronectin matrix disintegration and an increased proportion of α-smooth muscle actin-positive cells, indicating a hypercontractile phenotype. In contrast, classical EDS fibroblasts also showed increased αvβ3 and decreased α2β1 expression but exhibited fewer α-smooth muscle actin-positive cells. Based on this we propose a novel pathomechanism for EDS, suggesting that ECM composition changes disrupt cell adhesion and mechanotransduction, ultimately compromising connective tissue integrity.