Single-Molecule Force Imaging Reveals That Podosome Formation Requires No Extracellular Integrin-Ligand Tensions or Interactions

Decoding Biomechanical Cues Based on DNA Sensors

Biological systems perceive and respond to mechanical forces, generating mechanical cues to regulate life processes. Analyzing biomechanical forces has profound significance for understanding biological functions. Therefore, a series of molecular mechanical techniques have been developed, mainly including single-molecule force spectroscopy, traction force microscopy, and molecular tension sensor systems, which provide indispensable tools for advancing the field of mechanobiology. DNA molecules with a programmable structure and well-defined mechanical characteristics have attached much attention to molecular tension sensors as sensing elements, and are designed for the study of biomechanical forces to present biomechanical information with high sensitivity and resolution. In this work, a comprehensive overview of molecular mechanical technology is presented, with a particular focus on molecular tension sensor systems, specifically those based on DNA. Finally, the future development and challenges of DNA-based molecular tension sensor systems are looked upon.

Hydrogel-based molecular tension fluorescence microscopy for investigating receptor-mediated rigidity sensing

Extracellular matrix (ECM) rigidity serves as a crucial mechanical cue impacting diverse biological processes. However, understanding the molecular mechanisms of rigidity sensing has been limited by the spatial resolution and force sensitivity of current cellular force measurement techniques. Here we developed a method to functionalize DNA tension probes on soft hydrogel surfaces in a controllable and reliable manner, enabling molecular tension fluorescence microscopy for rigidity sensing studies. Our findings showed that fibroblasts respond to substrate rigidity by recruiting more force-bearing integrins and modulating integrin sampling frequency of the ECM, rather than simply overloading the existing integrin-ligand bonds, to promote focal adhesion maturation. We also demonstrated that ECM rigidity positively regulates the pN force of T cell receptor-ligand bond and T cell receptor mechanical sampling frequency, promoting T cell activation. Thus, hydrogel-based molecular tension fluorescence microscopy implemented on a standard confocal microscope provides a simple and effective means to explore detailed molecular force information for rigidity-dependent biological processes.

Molecular Force Imaging Reveals That Integrin-Dependent Mechanical Checkpoint Regulates Fcγ-Receptor-Mediated Phagocytosis in Macrophages

Macrophages are a type of immune cell that helps eliminate pathogens and diseased cells. Recent research has shown that macrophages can sense mechanical cues from potential targets to perform effective phagocytosis, but the mechanisms behind it remain unclear. In this study, we used DNA-based tension probes to study the role of integrin-mediated forces in FcγR-mediated phagocytosis. The results showed that when the phagocytic receptor FcγR is activated, the force-bearing integrins create a "mechanical barrier" that physically excludes the phosphatase CD45 and facilitates phagocytosis. However, if the integrin-mediated forces are physically restricted at lower levels or if the macrophage is on a soft matrix, CD45 exclusion is significantly reduced. Moreover, CD47-SIRPα "don't eat me" signaling can reduce CD45 segregation by inhibiting the mechanical stability of the integrin barrier. These findings demonstrate how macrophages use molecular forces to identify physical properties and combine them with biochemical signals from phagocytic receptors to guide phagocytosis.

Fine-tuning osteoclastogenesis: An insight into the cellular and molecular regulation of osteoclastogenesis

Osteoclasts, the bone-resorbing cells, are essential for the bone remodeling process and are involved in the pathophysiology of several bone-related diseases. The extensive corpus of in vitro research and crucial mouse model studies in the 1990s demonstrated the key roles of monocyte/macrophage colony-stimulating factor, receptor activator of nuclear factor kappa B ligand (RANKL) and integrin αvβ3 in osteoclast biology. Our knowledge of the molecular mechanisms by which these variables control osteoclast differentiation and function has significantly advanced in the first decade of this century. Recent developments have revealed a number of novel insights into the fundamental mechanisms governing the differentiation and functional activity of osteoclasts; however, these mechanisms have not yet been adequately documented. Thus, in the present review, we discuss various regulatory factors including local and hormonal factors, innate as well as adaptive immune cells, noncoding RNAs (ncRNAs), etc., in the molecular regulation of the intricate and tightly regulated process of osteoclastogenesis. ncRNAs have a critical role as epigenetic controllers of osteoclast physiologic activities, including differentiation and bone resorption. The primary ncRNAs, which include micro-RNAs, circular RNAs, and long noncoding RNAs, form a complex network that affects gene transcription activities associated with osteoclast biological activity. Greater knowledge of the involvement of ncRNAs in osteoclast biological activities will contribute to the treatment and management of several skeletal diseases such as osteoporosis, osteoarthritis, rheumatoid arthritis, etc. Moreover, we further outline potential therapies targeting these regulatory pathways of osteoclastogenesis in distinct bone pathologies.

Novel imaging methods and force probes for molecular mechanobiology of cytoskeleton and adhesion

Detection and conversion of mechanical forces into biochemical signals is known as mechanotransduction. From cells to tissues, mechanotransduction regulates migration, proliferation, and differentiation in processes such as immune responses, development, and cancer progression. Mechanosensitive structures such as integrin adhesions, the actin cortex, ion channels, caveolae, and the nucleus sense and transmit forces. In vitro approaches showed that mechanosensing is based on force-dependent protein deformations and reorganizations. However, the mechanisms in cells remained unclear since cell imaging techniques lacked molecular resolution. Thanks to recent developments in super-resolution microscopy (SRM) and molecular force sensors, it is possible to obtain molecular insight of mechanosensing in live cells. We discuss how understanding of molecular mechanotransduction was revolutionized by these innovative approaches, focusing on integrin adhesions, actin structures, and the plasma membrane.

Recent advances in label-free imaging of cell-matrix adhesions

Cell-matrix adhesions play an essential role in mediating and regulating many biological processes. The adhesion receptors, typically transmembrane integrins, provide dynamic correlations between intracellular environments and extracellular matrixes (ECMs) by bi-directional signaling. In-depth investigations of cell-matrix adhesion and integrin-mediated cell adhesive force are of great significance in biology and medicine. The emergence of advanced imaging techniques and principles has facilitated the understanding of the molecular composition and structure dynamics of cell-matrix adhesions, especially the label-free imaging methods that can be used to study living cell dynamics without immunofluorescence staining. This highlight article aims to give an overview of recent developments in imaging cell-matrix adhesions in a label-free manner. Electrochemiluminescence microscopy (ECLM) and surface plasmon resonance microscopy (SPRM) are briefly introduced and their applications in imaging analysis of cell-matrix adhesions are summarized. Then we highlight the advances in mapping cell-matrix adhesion force based on molecular tension probes and fluorescence microscopy (collectively termed as MTFM). The biomaterials including polyethylene glycol (PEG), peptides and DNA for constructing tension probes in MTFM are summarized. Finally, the outlook and perspectives on the further developments of cell-matrix adhesion imaging are presented.

The circle of life: Phases of podosome formation, turnover and reemergence

Podosomes are highly dynamic actin-rich structures in a variety of cell types, especially monocytic cells. They fulfill multiple functions such as adhesion, mechanosensing, or extracellular matrix degradation, thus allowing cells to detect and respond to a changing environment. These abilities are based on an intricate architecture that enables podosomes to sense mechanical properties of their substratum and to transduce them intracellularly in order to generate an appropriate cellular response. These processes are enabled through the tightly orchestrated interplay of more than 300 different components that are dynamically recruited during podosome formation and turnover. In this review, we discuss the different phases of the podosome life cycle and the current knowledge on regulatory factors that impact on the genesis, activity, dissolution and reemergence of podosomes. We also highlight mechanoregulatory processes that become important during these different stages, on the level of individual podosomes, and also at podosome sub- and superstructures.