Mechanical stimulus alters conformation of type 1 parathyroid hormone receptor in bone cells

Magnetogenetics as a promising tool for controlling cellular signaling pathways

Magnetogenetics emerges as a transformative approach for modulating cellular signaling pathways through the strategic application of magnetic fields and nanoparticles. This technique leverages the unique properties of magnetic nanoparticles (MNPs) to induce mechanical or thermal stimuli within cells, facilitating the activation of mechano- and thermosensitive proteins without the need for traditional ligand-receptor interactions. Unlike traditional modalities that often require invasive interventions and lack precision in targeting specific cellular functions, magnetogenetics offers a non-invasive alternative with the capacity for deep tissue penetration and the potential for targeting a broad spectrum of cellular processes. This review underscores magnetogenetics' broad applicability, from steering stem cell differentiation to manipulating neuronal activity and immune responses, highlighting its potential in regenerative medicine, neuroscience, and cancer therapy. Furthermore, the review explores the challenges and future directions of magnetogenetics, including the development of genetically programmed magnetic nanoparticles and the integration of magnetic field-sensitive cells for in vivo applications. Magnetogenetics stands at the forefront of cellular manipulation technologies, offering novel insights into cellular signaling and opening new avenues for therapeutic interventions.

PTH1R translocation to primary cilia in mechanically-stimulated ostecytes prevents osteoclast formation via regulation of CXCL5 and IL-6 secretion

Osteocytes respond to mechanical forces controlling osteoblast and osteoclast function. Mechanical stimulation decreases osteocyte apoptosis and promotes bone formation. Primary cilia have been described as potential mechanosensors in bone cells. Certain osteogenic responses induced by fluid flow (FF) in vitro are decreased by primary cilia inhibition in MLO-Y4 osteocytes. The parathyroid hormone (PTH) receptor type 1 (PTH1R) modulates osteoblast, osteoclast, and osteocyte effects upon activation by PTH or PTH-related protein (PTHrP) in osteoblastic cells. Moreover, some actions of PTH1R seem to be triggered directly by mechanical stimulation. We hypothesize that PTH1R forms a signaling complex in the primary cilium that is essential for mechanotransduction in osteocytes and affects osteocyte-osteoclast communication. MLO-Y4 osteocytes were stimulated by FF or PTHrP (1-37). PTH1R and primary cilia signaling were abrogated using PTH1R or primary cilia specific siRNAs or inhibitors, respectively. Conditioned media obtained from mechanically- or PTHrP-stimulated MLO-Y4 cells inhibited the migration of preosteoclastic cells and osteoclast differentiation. Redistribution of PTH1R along the entire cilium was observed in mechanically stimulated MLO-Y4 osteocytic cells. Preincubation of MLO-Y4 cells with the Gli-1 antagonist, the adenylate cyclase inhibitor (SQ22536), or with the phospholipase C inhibitor (U73122), affected the migration of osteoclast precursors and osteoclastogenesis. Proteomic analysis and neutralizing experiments showed that FF and PTH1R activation control osteoclast function through the modulation of C-X-C Motif Chemokine Ligand 5 (CXCL5) and interleukin-6 (IL-6) secretion in osteocytes. These novel findings indicate that both primary cilium and PTH1R are necessary in osteocytes for proper communication with osteoclasts and show that mechanical stimulation inhibits osteoclast recruitment and differentiation through CXCL5, while PTH1R activation regulate these processes via IL-6.

Genetically encoded fluorescent biosensors for GPCR research

G protein-coupled receptors (GPCRs) regulate a wide range of physiological and pathophysiological cellular processes, thus it is important to understand how GPCRs are activated and function in various cellular contexts. In particular, the activation process of GPCRs is dynamically regulated upon various extracellular stimuli, and emerging evidence suggests the subcellular functions of GPCRs at endosomes and other organelles. Therefore, precise monitoring of the GPCR activation process with high spatiotemporal resolution is required to investigate the underlying molecular mechanisms of GPCR functions. In this review, we will introduce genetically encoded fluorescent biosensors that can precisely monitor the real-time GPCR activation process in live cells. The process includes the binding of extracellular GPCR ligands, conformational change of GPCR, recruitment of G proteins or β-arrestin, GPCR internalization and trafficking, and the GPCR-related downstream signaling events. We will introduce fluorescent GPCR biosensors based on a variety of strategies such as fluorescent resonance energy transfer (FRET), bioluminescence resonance energy transfer (BRET), circular permuted fluorescent protein (cpFP), and nanobody. We will discuss the pros and cons of these GPCR biosensors as well as their applications in GPCR research.

Neural crest mechanosensors: Seeing old proteins in a new light

Mechanical forces exerted on neural crest cells control their collective migration and differentiation. This perspective discusses our current understanding of neural crest mechanotransduction during cell migration and differentiation. Additionally, we describe proteins that have mechanosensitive functions in other systems, such as mechanosensitive G-protein-coupled receptors, mechanosensitive ion channels, cell-cell adhesion, and cell-matrix-interacting proteins, and highlight that these same proteins have in the past been studied in neural crest development from a purely signaling point of view. We propose that future studies elucidate the mechanosensitive functions these receptors may play in neural crest development and integrate this with their known molecular role.

Primary cilia and PTH1R interplay in the regulation of osteogenic actions

Primary cilia are subcellular structures specialized in sensing different stimuli in a diversity of cell types. In bone, the primary cilium is involved in mechanical sensing and transduction of signals that regulate the behavior of mesenchymal osteoprogenitors, osteoblasts and osteocytes. To perform its functions, the primary cilium modulates a plethora of molecules including those stimulated by the parathyroid hormone (PTH) receptor type I (PTH1R), a master regulator of osteogenesis. Binding of the agonists PTH or PTH-related protein (PTHrP) to the PTH1R or direct agonist-independent stimulation of the receptor activate PTH1R signaling pathways. In turn, activation of PTH1R leads to regulation of bone formation and remodeling. Herein, we describe the structure, function and molecular partners of primary cilia in the context of bone, playing special attention to those signaling pathways that are mediated directly or indirectly by PTH1R in association with primary cilia during the process of osteogenesis.

Translating the Force - mechano-sensing GPCRs

Incorporating mechanical cues into cellular responses allows us to experience our direct environment. Specialized cells can perceive and discriminate between different physical properties such as level of vibration, temperature, or pressure. Mechanical forces are abundant signals that also shape general cellular responses such as cytoskeletal rearrangement, differentiation, or migration and contribute to tissue development and function. The molecular structures that perceive and transduce mechanical forces are specialized cytoskeletal proteins, cell junction molecules, and membrane proteins such as ion channels and metabotropic receptors. G protein-coupled receptors (GPCRs) have attracted attention as metabotropic force receptors as they are among the most important drug targets. This review summarizes the function of mechano-sensitive GPCRs, specifically, the angiotensin II type 1 receptor and adrenergic, apelin, histamine, parathyroid hormone 1, and orphan receptors, focusing particularly on the advanced knowledge gained from adhesion-type GPCRs. We distinguish between shear stress and cell swelling/stretch as the two major types of mechano-activation of these receptors and contemplate the potential contribution of the force-from-lipid and force-from-tether models that have previously been suggested for ion channels.

Mechanical regulation of bone remodeling

Bone remodeling is a lifelong process that gives rise to a mature, dynamic bone structure via a balance between bone formation by osteoblasts and resorption by osteoclasts. These opposite processes allow the accommodation of bones to dynamic mechanical forces, altering bone mass in response to changing conditions. Mechanical forces are indispensable for bone homeostasis; skeletal formation, resorption, and adaptation are dependent on mechanical signals, and loss of mechanical stimulation can therefore significantly weaken the bone structure, causing disuse osteoporosis and increasing the risk of fracture. The exact mechanisms by which the body senses and transduces mechanical forces to regulate bone remodeling have long been an active area of study among researchers and clinicians. Such research will lead to a deeper understanding of bone disorders and identify new strategies for skeletal rejuvenation. Here, we will discuss the mechanical properties, mechanosensitive cell populations, and mechanotransducive signaling pathways of the skeletal system.

Mechanical and chemical activation of GPR68 probed with a genetically encoded fluorescent reporter

G-protein-coupled receptor (GPCR) 68 (GPR68, or OGR1) couples extracellular acidifications and mechanical stimuli to G-protein signaling and plays important roles in vascular physiology, neuroplasticity and cancer progression. Inspired by previous GPCR-based reporters, here, we inserted a cyclic permuted fluorescent protein into the third intracellular loop of GPR68 to create a genetically encoded fluorescent reporter of GPR68 activation we call 'iGlow'. iGlow responds to known physiological GPR68 activators such as fluid shear stress and extracellular acidifications. In addition, iGlow responds to Ogerin, a synthetic GPR68-selective agonist, but not to a non-active Ogerin analog, showing the specificity of iGlow-mediated fluorescence signals. Flow-induced iGlow activation is not eliminated by pharmacological modulation of downstream G-protein signaling, disruption of actin filaments or application of GsMTx4, an inhibitor of certain mechanosensitive ion channels activated by membrane stretch. Deletion of the conserved helix 8, proposed to mediate mechanosensitivity in certain GPCRs, does not eliminate flow-induced iGlow activation. iGlow could be useful to investigate the contribution of GPR68-dependent signaling in health and disease.

Mechanosensitive Regulation of Fibrosis

Cells in the human body experience and integrate a wide variety of environmental cues. A growing interest in tissue mechanics in the past four decades has shown that the mechanical properties of tissue drive key biological processes and facilitate disease development. However, tissue stiffness is not only a potent behavioral cue, but also a product of cellular signaling activity. This review explores both roles of tissue stiffness in the context of inflammation and fibrosis, and the important molecular players driving such processes. During inflammation, proinflammatory cytokines upregulate tissue stiffness by increasing hydrostatic pressure, ECM deposition, and ECM remodeling. As the ECM stiffens, cells involved in the immune response employ intricate molecular sensors to probe and alter their mechanical environment, thereby facilitating immune cell recruitment and potentiating the fibrotic phenotype. This powerful feedforward loop raises numerous possibilities for drug development and warrants further investigation into the mechanisms specific to different fibrotic diseases.

Fluid shear stress-mediated mechanotransduction in circulating leukocytes and its defect in microvascular dysfunction

Leukocytes (neutrophils, monocytes) in the active circulation exhibit multiple phenotypic indicators for a low level of cellular activity, like lack of pseudopods and minimal amounts of activated, cell-adhesive integrins on their surfaces. In contrast, before these cells enter the circulation in the bone marrow or when they recross the endothelium into extravascular tissues of peripheral organs they are fully activated. We review here a multifaceted mechanism mediated by fluid shear stress that can serve to deactivate leukocytes in the circulation. The fluid shear stress controls pseudopod formation via the FPR receptor, the same receptor responsible for pseudopod projection by localized actin polymerization. The bioactivity of macromolecular factors in the blood plasma that interfere with receptor stimulation by fluid flow, such as proteolytic cleavage in the extracellular domain of the receptor or the membrane actions of cholesterol, leads to a defective ability to respond to fluid shear stress by actin depolymerization. The cell reaction to fluid shear involves CD18 integrins, nitric oxide, cGMP and Rho GTPases, is attenuated in the presence of inflammatory mediators and modified by glucocorticoids. The mechanism is abolished in disease models (genetic hypertension and hypercholesterolemia) leading to an increased number of activated leukocytes in the circulation with enhanced microvascular resistance and cell entrapment. In addition to their role in binding to biochemical agonists/antagonists, membrane receptors appear to play a second role: to monitor local fluid shear stress levels. The fluid shear stress control of many circulating cell types such as lymphocytes, stem cells, tumor cells remains to be elucidated.

Osteocytes and Diabetes: Altered Function of Diabetic Osteocytes

PURPOSE OF REVIEW

Diabetes mellitus is a prevalent chronic disease affecting millions of people in the world. Bone fragility is a complication found in diabetic patients. Although osteoblasts and osteoclasts are directly affected by diabetes, herein we focus on how the diabetic state-based on hyperglycemia and accumulation of advanced glycation end products among other features-impairs osteocyte functions exerting deleterious effects on bone.

RECENT FINDINGS

In the last years, several studies described that diabetic conditions cause morphological modifications on lacunar-canalicular system, alterations on osteocyte mechanoreceptors and intracellular pathways and on osteocyte communication with other cells through the secretion of proteins such as sclerostin or RANKL. This article gives an overview of events occurring in diabetic osteocytes. In particular, mechanical responses seem to be seriously affected in these conditions, suggesting that mechanical sensibility could be a target for future research in the field.

Roles of CCN2 as a mechano-sensing regulator of chondrocyte differentiation

Cellular communication network factor 2 (CCN2) is a cysteine-rich secreted matricellular protein that regulates various cellular functions including cell differentiation. CCN2 is highly expressed under several types of mechanical stress, such as stretch, compression, and shear stress, in mesenchymal cells including chondrocytes, osteoblasts, and fibroblasts. In particular, CCN2 not only promotes cell proliferation and differentiation of various cells but also regulates the stability of mRNA of TRPV4, a mechanosensitive ion channel in chondrocytes. Of note, CCN2 behaves like a biomarker to sense suitable mechanical stress, because CCN2 expression is down-regulated when chondrocytes are subjected to excessive mechanical stress. These findings suggest that CCN2 is a mechano-sensing regulator. CCN2 expression is regulated by the activation of various mechano-sensing signaling pathways, e.g., mechanosensitive ion channels, integrin-focal adhesion-actin dynamics, Rho GTPase family members, Hippo-YAP signaling, and G protein-coupled receptors. This review summarizes the characterization of mechanoreceptors involved in CCN2 gene regulation and discusses the role of CCN2 as a mechano-sensing regulator of mesenchymal cell differentiation, with particular focus on chondrocytes.

Regulation of blood flow in small arteries: mechanosensory events underlying myogenic vasoconstriction

As blood flow is proportional to the fourth power of the vascular radius small changes in the diameter of resistance arteries/arterioles following an increase in intraluminal pressure would be expected to substantially increase blood flow. However, arteriolar myocytes display an intrinsic ability to locally regulate blood flow according to metabolic demands by tuning the diameter of small arteries in response to local changes in he-modynamics. Critical to this, observations were made more than 100 years ago that mechanosensitive small arteries exhibit the "myogenic response" or pressure-induced vasoconstriction or vasodilation in re-sponse to increased or decreased intravascular pressure, respectively. Although cellular mechanisms underlying the myogenic response have now been studied extensively, the precise cellular mechanisms under-lying this intriguing phenomenon still remain uncertain. In particular, the biological machinery that senses changes in intravascular pressure in vascular smooth muscle cells have not been unquestionably identified and remain a significant issue in vascular biology to be fully elucidated. As such, this brief review focuses on putative mechanosensors that have been proposed to contribute to myogenic vasoreactivity. Specific attention is paid to the roles of integrins, G protein-coupled receptors, and cadherins.

Mechanical GPCR Activation by Traction Forces Exerted on Receptor N-Glycans

Cells are sensitive to chemical stimulation which is converted into intracellular biochemical signals by the activation of specific receptors. Mechanical stimulations can also induce biochemical responses via the activation of various mechano-sensors. Although principally appreciated for their chemosensory function, G-protein-coupled receptors (GPCRs) may participate in mechano-transduction. They are indirectly activated by the paracrine release of chemical compounds secreted in response to mechanical stimuli, but they might additionally behave as mechano-sensors that are directly stimulated by mechanical forces. Although several studies are consistent with this latter hypothesis, the molecular mechanisms of a potential direct mechanical activation of GPCRs have remained elusive until recently. In particular, investigating the activation of the catecholamine β₂-adrenergic receptor by a pathogen revealed that traction forces directly exerted on the N-terminus of the receptor via N-glycan chains activate specific signaling pathways. These findings open new perspectives in GPCR biology and pharmacology since most GPCRs express N-glycan chains in their N-terminus, which might similarly be involved in the interaction with cell-surface glycan-specific lectins in the context of cell-to-cell mechanical signaling.

Helix 8 is the essential structural motif of mechanosensitive GPCRs

G-protein coupled receptors (GPCRs) are versatile cellular sensors for chemical stimuli, but also serve as mechanosensors involved in various (patho)physiological settings like vascular regulation, cardiac hypertrophy and preeclampsia. However, the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here we show that mechanosensitive histamine H₁ receptors (H₁Rs) are endothelial sensors of fluid shear stress and contribute to flow-induced vasodilation. At the molecular level, we observe that H₁Rs undergo stimulus-specific patterns of conformational changes suggesting that mechanical forces and agonists induce distinct active receptor conformations. GPCRs lacking C-terminal helix 8 (H8) are not mechanosensitive, and transfer of H8 to non-responsive GPCRs confers, while removal of H8 precludes, mechanosensitivity. Moreover, disrupting H8 structural integrity by amino acid exchanges impairs mechanosensitivity. Altogether, H8 is the essential structural motif endowing GPCRs with mechanosensitivity. These findings provide a mechanistic basis for a better understanding of the roles of mechanosensitive GPCRs in (patho)physiology.

GPCRs are versatile cellular sensors for chemical stimuli but the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here authors identify the C-terminal helix 8 (H8) as the essential structural motif endowing H₁R and other GPCRs with mechanosensitivity.

Closer to Nature Through Dynamic Culture Systems

Mechanics in the human body are required for normal cell function at a molecular level. It is now clear that mechanical stimulations play significant roles in cell growth, differentiation, and migration in normal and diseased cells. Recent studies have led to the discovery that normal and cancer cells have different mechanosensing properties. Here, we discuss the application and the physiological and pathological meaning of mechanical stimulations. To reveal the optimal conditions for mimicking an in vivo microenvironment, we must, therefore, discern the mechanotransduction occurring in cells.

Molecular interactions between Neisseria meningitidis and its human host

Neisseria meningitidis is a Gram‐negative bacterium that asymptomatically colonises the nasopharynx of humans. For an unknown reason, N. meningitidis can cross the nasopharyngeal barrier and invade the bloodstream where it becomes one of the most harmful extracellular bacterial pathogen. This infectious cycle involves the colonisation of two different environments. (a) In the nasopharynx, N. meningitidis grow on the top of mucus‐producing epithelial cells surrounded by a complex microbiota. To survive and grow in this challenging environment, the meningococcus expresses specific virulence factors such as polymorphic toxins and MDAΦ. (b) Meningococci have the ability to survive in the extra cellular fluids including blood and cerebrospinal fluid. The interaction of N. meningitidis with human endothelial cells leads to the formation of typical microcolonies that extend overtime and promote vascular injury, disseminated intravascular coagulation, and acute inflammation. In this review, we will focus on the interplay between N. meningitidis and these two different niches at the cellular and molecular level and discuss the use of inhibitors of piliation as a potent therapeutic approach.

GPR68 Senses Flow and Is Essential for Vascular Physiology

Mechanotransduction plays a crucial role in vascular biology. One example of this is the local regulation of vascular resistance via flow-mediated dilation (FMD). Impairment of this process is a hallmark of endothelial dysfunction and a precursor to a wide array of vascular diseases, such as hypertension and atherosclerosis. Yet the molecules responsible for sensing flow (shear stress) within endothelial cells remain largely unknown. We designed a 384-well screening system that applies shear stress on cultured cells. We identified a mechanosensitive cell line that exhibits shear stress-activated calcium transients, screened a focused RNAi library, and identified GPR68 as necessary and sufficient for shear stress responses. GPR68 is expressed in endothelial cells of small-diameter (resistance) arteries. Importantly, Gpr68-deficient mice display markedly impaired acute FMD and chronic flow-mediated outward remodeling in mesenteric arterioles. Therefore, GPR68 is an essential flow sensor in arteriolar endothelium and is a critical signaling component in cardiovascular pathophysiology.

PTH1R signalling regulates the mechanotransduction process of cementoblasts under cyclic tensile stress

Objective

To investigate the regulatory role of type I parathyroid hormone receptor (PTH1R) signalling in the mechanotransduction process of cementoblasts under cyclic tensile stress (CTS).

Materials and methods

Immortalized cementoblast cell line OCCM-30 were employed and subjected to cyclic tensile strain applied by a four-point bending system. The expression of PTHrP and PTH1R, as well as cementoblastic transcription factor Runx-2, Osterix, and extracellular matrix protein COL-1 and OPN were assessed by quantitative real-time polymerase chain reaction and western blot analysis. PTH1R expression was knocked down by siPTH1R transfection, and the alteration of cementoblastic biomarkers expression was examined to evaluate the function of PTH1R. Furthermore, to investigate possible downstream molecules, expression of signal molecule ERK1/2 with or without siPTH1R transfection, and the effect of ERK inhibitor PD98059 on the expression of cementoblastic biomarkers was also examined.

Results

Cyclic tensile strain elevated the expression of PTHrP and PTH1R, as well as cementoblastic biomarkers Runx-2, Osterix, COL-1, and OPN in a time-dependent manner, which was inhibited by siPTH1R transfection. The expression of phosphorylated ERK1/2 was upregulated time-dependently under cyclic stretch, which was also inhibited by siPTH1R transfection, and pretreatment of p-ERK1/2 inhibitor PD98059 undermined the increase of Runx-2, Osterix, COL-1, and OPN prominently.

Conclusion

The findings of the present study indicate that PTH1R signalling plays a regulatory role in the CTS induced cementoblastic differentiation in mature cementoblasts, and ERK1/2 is essentially involved as a downstream intracellular signal molecule in this mechanotransduction process.

In Vivo Osteocyte Mechanotransduction: Recent Developments and Future Directions

PURPOSE OF REVIEW

Mechanical loading is an essential stimulus for skeletal tissues. Osteocytes are primarily responsible for sensing mechanical stimuli in bone and for orchestrating subsequent responses. This is critical for maintaining homeostasis, and responding to injury/disease. The osteocyte mechanotransduction pathway, and the downstream effects it mediates, is highly complex. In vivo models have proved invaluable in understanding this process. This review summarizes the commonly used models, as well as more recently developed ones, and describes how they are used to address emerging questions in the field.

RECENT FINDINGS

Minimally invasive animal models can be used to determine mechanisms of osteocyte mechanotransduction, at the cell and molecular level, while simultaneously reducing potentially confounding responses such as inflammation/wound-healing. The details of osteocyte mechanotransduction in bone are gradually becoming clearer. In vivo model systems are a key tool in pursing this question. Advances in this field are explored and discussed in this review.

Integration of intracellular signaling: Biological analogues of wires, processors and memories organized by a centrosome 3D reference system

BACKGROUND

Myriads of signaling pathways in a single cell function to achieve the highest spatio-temporal integration. Data are accumulating on the role of electromechanical soliton-like waves in signal transduction processes. Theoretical studies strongly suggest feasibility of both classical and quantum computing involving microtubules.

AIM

A theoretical study of the role of the complex composed of the plasma membrane and the microtubule-based cytoskeleton as a system that transmits, stores and processes information.

METHODS

Theoretical analysis presented here refers to (i) the Penrose-Hameroff theory of consciousness (Orchestrated Objective Reduction; Orch OR), (ii) the description of the centrosome as a reference system for construction of the 3D map of the cell proposed by Regolini, (iii) the Heimburg-Jackson model of the nerve pulse propagation along axons' lipid bilayer as soliton-like electro-mechanical waves.

RESULTS AND CONCLUSION

The ideas presented in this paper provide a qualitative model for the decision-making processes in a living cell undergoing a differentiation process.

OUTLOOK

This paper paves the way for the real-time live-cell observation of information processing by microtubule-based cytoskeleton and cell fate decision making.

Adhesion GPCRs as a Putative Class of Metabotropic Mechanosensors

Adhesion GPCRs as mechanosensors. Different aGPCR homologs and their cognate ligands have been described in settings, which suggest that they function in a mechanosensory capacity. For details, see text G protein-coupled receptors (GPCRs) constitute the most versatile superfamily of biosensors. This group of receptors is formed by hundreds of GPCRs, each of which is tuned to the perception of a specific set of stimuli a cell may encounter emanating from the outside world or from internal sources. Most GPCRs are receptive for chemical compounds such as peptides, proteins, lipids, nucleotides, sugars, and other organic compounds, and this capacity is utilized in several sensory organs to initiate visual, olfactory, gustatory, or endocrine signals. In contrast, GPCRs have only anecdotally been implicated in the perception of mechanical stimuli. Recent studies, however, show that the family of adhesion GPCRs (aGPCRs), which represents a large panel of over 30 homologs within the GPCR superfamily, displays molecular design and expression patterns that are compatible with receptivity toward mechanical cues (Fig. 1). Here, we review physiological and molecular principles of established mechanosensors, discuss their relevance for current research of the mechanosensory function of aGPCRs, and survey the current state of knowledge on aGPCRs as mechanosensing molecules.

Shear stress induces Gαq/11 activation independently of G protein-coupled receptor activation in endothelial cells

Mechanochemical signal transduction occurs when mechanical forces, such as fluid shear stress, are converted into biochemical responses within the cell. The molecular mechanisms by which endothelial cells (ECs) sense/transduce shear stress into biological signals, including the nature of the mechanosensor, are still unclear. G proteins and G protein-coupled receptors (GPCRs) have been postulated independently to mediate mechanotransduction. In this study, we used in situ proximity ligation assay (PLA) to investigate the role of a specific GPCR/Gα_q/11 pair in EC shear stress-induced mechanotransduction. We demonstrated that sphingosine 1-phosphate (S1P) stimulation causes a rapid dissociation at 0.5 min of Gα_q/11 from its receptor S1P₃, followed by an increased association within 2 min of GPCR kinase-2 (GRK2) and β-arrestin-1/2 with S1P₃ in human coronary artery ECs, which are consistent with GPCR/Gα_q/11 activation and receptor desensitization/internalization. The G protein activator AlF₄ resulted in increased dissociation of Gα_q/11 from S1P₃, but no increase in association between S1P₃ and either GRK2 or β-arrestin-1/2. The G protein inhibitor guanosine 5'-(β-thio) diphosphate (GDP-β-S) and the S1P₃ antagonist VPC23019 both prevented S1P-induced activation. Shear stress also caused the rapid activation within 7 s of S1P₃/Gα_q/11 There were no increased associations between S1P₃ and GRK2 or S1P₃ and β-arrestin-1/2 until 5 min. GDP-β-S, but not VPC23019, prevented dissociation of Gα_q/11 from S1P₃ in response to shear stress. Shear stress did not induce rapid dephosphorylation of β-arrestin-1 or rapid internalization of S1P₃, indicating no GPCR activation. These findings suggest that Gα_q/11 participates in the sensing/transducing of shear stress independently of GPCR activation in ECs.

Modeling and remodeling effects of intermittent administration of teriparatide (parathyroid hormone 1-34) on bone morphogenetic protein-induced bone in a rat spinal fusion model

Background

Bone morphogenetic protein (BMP)-based tissue engineering has focused on inducing new bone efficiently. However, modeling and remodeling of BMP-induced bone have rarely been discussed. Teriparatide (parathyroid hormone [PTH] 1-34) administration initially increases markers of bone formation, followed by an increase in bone resorption markers. This unique activity would be expected to accelerate the modeling and remodeling of new BMP-induced bone.

Methods

Male Sprague-Dawley rats underwent posterolateral spinal fusion surgery and implantation of collagen sponge containing either 50 μg recombinant human (rh)BMP-2 or saline. PTH 1-34 (60 μg/kg, 3 times/week) or saline injections were continued from preoperative week 2 week to postoperative week 12. The volume and quality of newly formed bone were monitored by in vivo micro-computed tomography and analyses of bone histomorphometry and serum bone metabolism markers were conducted at postoperative week 12.

Results

Microstructural indices of the newly formed bone were significantly improved by PTH 1-34 administration, which significantly decreased the tissue volumes of the fusion mass at postoperative week 12 compared to that at postoperative week 2. Bone histomorphometry and serum analyses showed that PTH administration significantly increased both bone formation and resorption markers. Analysis of the histomorphometry of cortical bone identified predominant periosteal bone resorption and endosteal bone formation.

Conclusions

Long-term intermittent administration of PTH 1-34 significantly accelerated the modeling and remodeling of new BMP-induced bone.

Clinical relevance

Our results suggest that the combined administration of rhBMP-2 and PTH 1-34 facilitates qualitative and quantitative improvements in bone regeneration, by accelerating bone modeling and remodeling.

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The present study found that intermittent administration of PTH 1-34 significantly decreased the TV of new rhBMP-2-induced bone, following the initial formation of a fusion mass equivalent to that of the control group.

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Bone histomorphometry demonstrated predominant bone resorption at the periosteum and bone formation at the endosteum in rats receiving PTH 1-34.

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These results indicated that PTH 1-34 supported modeling of rhBMP-2-induced bone in addition to the remodeling effect which confirmed by bone histomorphometry and serum markers.

Fluid shear stress promotes osteoblast proliferation via the Gαq-ERK5 signaling pathway

Fluid shear stress (FSS) is a ubiquitous mechanical stimulus that potently promotes osteoblast proliferation. Previously, we reported that extracellular signal-regulated kinase 5 (ERK5) is essential for FSS-induced osteoblast proliferation. However, the precise mechanism by which FSS promotes osteoblast proliferation via ERK5 activation is poorly understood. The aim of this study was to determine the critical role of Gαq in FSS-induced ERK5 phosphorylation and osteoblast proliferation, as well as the downstream targets of the Gαq-ERK5 pathway. MC3T3-E1 cells were transfected with 50 nM Gαq siRNA, treated with 5 mM XMD8-92 (a highly selective inhibitor of ERK5 activity), and/or exposed to FSS (12 dyn/cm(2)). Cell proliferation was evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The protein expression levels of Gαq, P-ERK5, ERK5, Cyclin B1, and CDK1 were analyzed by Western blot. Physiological FSS exposure for 60 min remarkably promoted MC3T3-E1 cell proliferation, however, this effect was suppressed by siRNA-mediated Gαq knockdown or inhibition of ERK5 activity by XMD8-92 treatment, suggesting that Gαq and ERK5 might modulate FSS-increased osteoblast proliferation. Furthermore, ERK5 phosphorylation was dramatically inhibited by Gαq siRNA. In addition, our study further revealed that FSS treatment of MC3T3-E1 cells for 60 min markedly upregulated the protein expression levels of Cyclin B1 and CDK1, and this increased expression was predominantly blocked by Gαq siRNA or XMD8-92 treatment. We propose that FSS acts on the Gαq-ERK5 signaling pathway to upregulate Cyclin B1 and CDK1 expression, thereby resulting in MC3T3-E1 cell proliferation. Thus, the Gαq-ERK5 signaling pathway may provide useful information regarding the treatment of bone metabolic disease.

Bone and skeletal muscle: Key players in mechanotransduction and potential overlapping mechanisms

The development and maintenance of skeletal muscle and bone mass is critical for movement, health and issues associated with the quality of life. Skeletal muscle and bone mass are regulated by a variety of factors that include changes in mechanical loading. Moreover, bone mass is, in large part, regulated by muscle-derived mechanical forces and thus by changes in muscle mass/strength. A thorough understanding of the cellular mechanism(s) responsible for mechanotransduction in bone and skeletal muscle is essential for the development of effective exercise and pharmaceutical strategies aimed at increasing, and/or preventing the loss of, mass in these tissues. Thus, in this review we will attempt to summarize the current evidence for the major molecular mechanisms involved in mechanotransduction in skeletal muscle and bone. By examining the differences and similarities in mechanotransduction between these two tissues, it is hoped that this review will stimulate new insights and ideas for future research and promote collaboration between bone and muscle biologists.(1).

CCR5-CCL Axis in PDL during Orthodontic Biophysical Force Application

Tooth movement by application of orthodontic biophysical force primarily reflects the role of soluble molecules released from the periodontal ligament (PDL). Thus far, many factors have been reported to be involved in orthodontic tooth movement (OTM), but key molecules that orchestrate responses of periodontal tissues to biophysical force are still enigmatic. In this in vivo study, in which the upper first molars in rats were moved, differential display–polymerase chain reaction revealed that CC chemokine receptor 5 (CCR5) level was differentially increased during OTM. Strong immunoreactivity for CCR5 was found in the PDL undergoing force application. Moreover, the in vitro compression or tension force application to primary cultured human PDL cells increased the expression of CCR5 and CCR5 ligands. In vitro tension force on human PDL cells did not induce RANKL, an osteoclastogenesis-inducing factor, but did induce the upregulation of IL12, an osteoclast inhibitory factor, and osteoblast differentiation factors, including Runx2, which was attenuated under tension by CCR5 gene silencing whereas augmented with CCR5 ligands. In contrast, in vitro compression force did not induce the expression of osteoprotegerin, a decoy receptor for RANKL and Runx2, but did induce the upregulation of RANKL, which was attenuated under compression by CCR5 gene silencing. These results suggest that the CCR5–CCR5 ligands axis in PDL cells may play a crucial role in the remodeling of periodontal tissues and can be a therapeutic target for achieving efficient OTM.

Role of the Parathyroid Hormone Type 1 Receptor (PTH1R) as a Mechanosensor in Osteocyte Survival

Osteocytes have a major role in the control of bone remodeling. Mechanical stimulation decreases osteocyte apoptosis and promotes bone accrual, whereas skeletal unloading is deleterious in both respects. PTH1R ablation or overexpression in osteocytes in mice produces trabecular bone loss or increases bone mass, respectively. The latter effect was related to a decreased osteocyte apoptosis. Here, the putative role of PTH1R activation in osteocyte protection conferred by mechanical stimulation was assessed. Osteocytic MLO-Y4 cells were subjected to mechanical stimuli represented by hypotonic shock (216 mOsm/kg) or pulsatile fluid flow (8 Hz, 10 dynes/cm(2)) for a short pulse (10 min), with or without PTH1R antagonists or after transfection with specific PTHrP or PTH1R siRNA. These mechanical stimuli prevented cell death induced within 6 hours by etoposide (50 μM), related to PTHrP overexpression; and this effect was abolished by the calcium antagonist verapamil (1 μM), a phospholipase C (PLC) inhibitor (U73122; 10 μM), and a PKA activation inhibitor, Rp-cAMPS (25 μM), in these cells. Each mechanical stimulus also rapidly induced β-catenin stabilization and nuclear ERK translocation, which were inhibited by the PTH1R antagonist PTHrP(7-34) (1 μM), or PTH1R siRNA, and mimicked by PTHrP(1-36) (100 nM). Mechanical stretching by hypotonic shock did not affect cAMP production but rapidly (<1 min) stimulated Ca(i)(2+) transients in PTH1R-overexpressing HEK-293 cells and in MLO-Y4 cells, in which calcium signaling was unaffected by the presence of a PTHrP antiserum or PTHrP siRNA but inhibited by knocking down PTH1R. These novel findings indicate that PTH1R is an important component of mechanical signal transduction in osteocytic MLO-Y4 cells, and that PTH1R activation by PTHrP-independent and dependent mechanisms has a relevant role in the prosurvival action of mechanical stimulus in these cells.

Activity of Bradykinin B2 Receptor Is Regulated by Long-Chain Polyunsaturated Fatty Acids

The molecular and cellular mechanisms by which long-chain polyunsaturated fatty acids (LCPUFA) exert their beneficial effects on cardiovascular health remain obscure. While both LCPUFA and bradykinin (BK) signaling pathway play a role in the cardiovascular system, any direct link between the two is yet to be established. Using picosecond time-resolved fluorescence microscopy and a genetically engineered bradykinin B₂ receptor (B₂R) sensor (B2K-CC), we detected LCPUFA-induced conformational responses in the B₂R similar to those caused by its cognate ligand, BK. The selective B₂R antagonist (HOE-140) blocked the eicosapentaenoic acid (EPA, C20∶5, n-3) induced conformational response of the B2K-CC. Further analysis suggests that LCPUFA are capable of direct, B₂R-dependent activation of extracellular ligand-regulated kinases (ERK). From a wide range of fatty acids studied, varying in chain length, saturation, and position of double bonds, EPA, docosahexaenoic (DHA, C22∶6, n-3), docosadienoic (DDA, C22∶2, n-6), and dihomo-gamma linoleic (DGLA, C20∶3, n-6) fatty acids caused the highest ERK phosphorylation. EPA or DHA dependent ERK phosphorylation was inhibited by the selective B₂R antagonist. We show that LCPUFA stimulates downstream signaling by B₂R such as B₂R-dependent phosphorylation and expression regulation of endothelial nitric-oxide synthase (eNOS). Further analysis indicated that LCPUFA also alters levels of the eNOS transcription factor, kruppel-like factor 2 (KLF2). Moreover we show that EPA increases membrane fluidity on the same time scale as B₂R conformational response, suggesting that partitioning of LCPUFA into bilayer is a primary step required for receptor activation. In summary our data show that LCPUFA activate B₂R receptor at nanomolar concentrations suggesting a novel molecular mechanism by which fatty acids may affect the cardiovascular system.

Structural and biophysical characterisation of G protein-coupled receptor ligand binding using resonance energy transfer and fluorescent labelling techniques

The interaction between ligands and the G protein-coupled receptors (GPCRs) to which they bind has long been the focus of intensive investigation. The signalling cascades triggered by receptor activation, due in most cases to ligand binding, are of great physiological and medical importance; indeed, GPCRs are targeted by in excess of 30% of small molecule therapeutic medicines. Attempts to identify further pharmacologically useful GPCR ligands, for receptors with known and unknown endogenous ligands, continue apace. In earlier days direct assessment of such interactions was restricted largely to the use of ligands incorporating radioactive isotope labels as this allowed detection of the ligand and monitoring its interaction with the GPCR. This use of such markers has continued with the development of ligands labelled with fluorophores and their application to the study of receptor-ligand interactions using both light microscopy and resonance energy transfer techniques, including homogenous time-resolved fluorescence resonance energy transfer. Details of ligand-receptor interactions via X-ray crystallography are advancing rapidly as methods suitable for routine production of substantial amounts and stabilised forms of GPCRs have been developed and there is hope that this may become as routine as the co-crystallisation of serine/threonine kinases with ligands, an approach that has facilitated widespread use of rapid structure-based ligand design. Conformational changes involved in the activation of GPCRs, widely predicted by biochemical and biophysical means, have inspired the development of intramolecular FRET-based sensor forms of GPCRs designed to investigate the events following ligand binding and resulting in a signal propagation across the cell membrane. Finally, a number of techniques are emerging in which ligand-GPCR binding can be studied in ways that, whilst indirect, are able to monitor its results in an unbiased and integrated manner. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

Shear stress activates extracellular signal-regulated kinase 1/2 via the angiotensin II type 1 receptor

Mechanical factors such as strain, pressure, and shear stress are key regulators of cell function, but the molecular mechanisms underlying the detection and responses to such stimuli are poorly understood. Whether the angiotensin II (AngII) AT1 receptor (AT1R) transduces shear stress in endothelial cells (ECs) is unknown. We exposed human umbilical cord endothelial cells (HUVECs) to a shear stress of 0 (control) or 15 dyn/cm(2) for 5 or 10 min. The colocalization of AT1R with caveolin-1 (Cav1), endosomal markers Rab5, EEA1, and Rab7, and lysosomal marker Lamp-1 increased in shear stimulated cells, detected by immunocytochemistry. Shear stress reduced labeling of wild-type mouse ECs (18±3% of unsheared control, P<0.01) but not Cav1(-/-) ECs (90±10%) with fluorescent AngII, confirming that internalization of AT1R requires Cav1. Shear stress activated ERK1/2 2-fold (P<0.01), which was prevented by the AT1R blocker losartan. NADPH oxidase inhibition with apocynin prevented both the colocalization of AT1R with Cav1 and the induction of ERK1/2 by shear stress. Moreover, shear-dependent ERK1/2 activation was minimal in CHO cells expressing an AT1Ra mutant that does not internalize, compared with cells expressing wild-type AT1Ra (P<0.05). Hence, AT1R may be an important transducer of shear stress-dependent activation of ERK1/2.

Real-time detection of G protein activation using monomolecular Gγ FRET sensors

G protein-coupled receptors (GPCRs) are involved in many diseases, and are the target of a large percentage of modern drugs. While only a few fluorescence resonance energy transfer (FRET) sensors have been made for real-time detection of GPCR activation, the human genome encodes roughly 950 GPCRs and a need for a broad real-time detection system is needed. In this study, we developed and characterized a new type of G protein sensor based on the Gγ subunit containing an intra-molecular FRET pair to allow detection of real-time G protein activation in multiple cell lines using time-resolved fluorescence spectroscopy. Our Gγ sensors were able to detect G protein activation to aluminum fluoride, a G protein activator, in human embryonic kidney 293 cells (HEK293). Moreover, our sensors were able to couple to the bradykinin B(2), parathyroid type 1 and adrenergic 2A GPCRs and detect G protein activation in response to the cognate ligands; bradykinin, parathyroid and noradrenaline hormones, respectively. Furthermore, our sensors also detected ligand-independent G protein activation by fluid shear stress. G protein inhibitors, pertussis toxin and guanosine 5' [β-thio] diphosphate, inhibited the FRET response to G protein stimulants indicating that the sensor response is specific to G protein activation. These findings suggest that the described Gγ sensors can be used to monitor real-time G protein activation by a variety of G protein stimulants or inhibitors.

PTH1 receptor is involved in mediating cellular response to long-chain polyunsaturated fatty acids

The molecular pathways by which long chain polyunsaturated fatty acids (LCPUFA) influence skeletal health remain elusive. Both LCPUFA and parathyroid hormone type 1 receptor (PTH1R) are known to be involved in bone metabolism while any direct link between the two is yet to be established. Here we report that LCPUFA are capable of direct, PTH1R dependent activation of extracellular ligand-regulated kinases (ERK). From a wide range of fatty acids studied, varying in chain length, saturation, and position of double bonds, eicosapentaenoic (EPA) and docosahexaenoic fatty acids (DHA) caused the highest ERK phosphorylation. Moreover, EPA potentiated the effect of parathyroid hormone (PTH(1–34)) in a superagonistic manner. EPA or DHA dependent ERK phosphorylation was inhibited by the PTH1R antagonist and by knockdown of PTH1R. Inhibition of PTH1R downstream signaling molecules, protein kinases A (PKA) and C (PKC), reduced EPA and DHA dependent ERK phosphorylation indicating that fatty acids predominantly activate G-protein pathway and not the β-arrestin pathway. Using picosecond time-resolved fluorescence microscopy and a genetically engineered PTH1R sensor (PTH-CC), we detected conformational responses to EPA similar to those caused by PTH(1–34). PTH1R antagonist blocked the EPA induced conformational response of the PTH-CC. Competitive binding studies using fluorescence anisotropy technique showed that EPA and DHA competitively bind to and alter the affinity of PTH1 receptor to PTH(1–34) leading to a superagonistic response. Finally, we showed that EPA stimulates protein kinase B (Akt) phosphorylation in a PTH1R-dependent manner and affects the osteoblast survival pathway, by inhibiting glucocorticoid-induced cell death. Our findings demonstrate for the first time that LCPUFAs, EPA and DHA, can activate PTH1R receptor at nanomolar concentrations and consequently provide a putative molecular mechanism for the action of fatty acids in bone.

Capitalizing on diversity: an integrative approach towards the multiplicity of cellular mechanisms underlying myogenic responsiveness

The intrinsic ability of resistance arteries to respond to transmural pressure is the single most important determinant of their function. Despite an ever-growing catalogue of signalling pathways that underlie the myogenic response, it remains an enigmatic mechanism. The myogenic response's mechanistic diversity has largely been attributed to 'hard-wired' differences across species and vascular beds; however, emerging evidence suggests that the mechanistic basis for the myogenic mechanism is, in fact, 'plastic'. This means that the myogenic response can change quantitatively (i.e. change in magnitude) and qualitatively (i.e. change in mechanistic basis) in response to environmental challenges (e.g. disease conditions). Consequently, understanding the dynamics of how the myogenic response capitalizes on its mechanistic diversity is key to unlocking clinically viable interventions. Using myogenic sphingosine-1-phosphate (S1P) signalling as an example, this review illustrates the remarkable plasticity of the myogenic response. We propose that currently unidentified 'organizational programmes' dictate the contribution of individual signalling pathways to the myogenic response and introduce the concept that certain signalling elements act as 'divergence points' (i.e. as the potential higher level regulatory sites). In the context of pressure-induced S1P signalling, the S1P-generating enzyme sphingosine kinase 1 serves as a divergence point, by orchestrating the calcium-dependent and -independent signalling pathways underlying microvascular myogenic responsiveness. By acting on divergence points, the proposed 'organizational programmes' could form the basis for the flexible recruitment and fine-tuning of separate signalling streams that underlie adaptive changes to the myogenic response and its distinctiveness across species and vascular beds.

Live cell imaging of G protein-coupled receptors

Live cell imaging experiments with G protein-coupled receptors (GPCRs) tagged with fluorescent fusion proteins were originally performed to study trafficking and subcellular location of these important drug targets. In the past decade, however, substantial progress came from improved imaging methods and from the cloning of novel fluorescent fusion proteins. Today, these methods allow to visualize not only GPCR interactions but also, e.g., receptor activation, trafficking between subcellular compartments, and to measure transport kinetics. Here, we summarize recent progress in live cell imaging of GPCRs using a confocal laser scanning microscope.

The interaction of biological factors with mechanical signals in bone adaptation: recent developments

Mechanotransduction in bone is fundamental to proper skeletal development. Deficiencies in signaling mechanisms that transduce physical forces to effector cells can have severe consequences for skeletal integrity. Therefore, a solid understanding of the cellular and molecular components of mechanotransduction is crucial for correcting skeletal modeling and remodeling errors and designing effective therapies. In recent years, progress has been made on many fronts regarding our understanding of bone cell mechanotransduction, including subcellular localization of mechanosensitive components in bone cells, the discovery of mechanosensitive G-protein-coupled receptors, identification of new ion channels and larger pores (eg, hemichannels) involved in physical signal transduction, and cell adhesion proteins, among others. These and other recent mechanisms are reviewed to provide a synthesis of recent experimental findings, in the larger context of whole bone adaptation.

Effect of membrane tension on the physical properties of DOPC lipid bilayer membrane

Molecular dynamics simulations of a dioleoylphosphocholine (DOPC) lipid bilayer were performed to explore its mechanosensitivity. Variations in the bilayer properties, such as area per lipid, volume, thickness, hydration depth (HD), hydration thickness (HT), lateral diffusion coefficient, and changes in lipid structural order were computed in the membrane tension range 0 to 15dyn/cm. We determined that an increase in membrane tension results in a decrease in the bilayer thickness and HD of ~5% and ~5.7% respectively, whereas area per lipid, volume, and HT/HD increased by 6.8%, 2.4%, and 5% respectively. The changes in lipid conformation and orientation were characterized using orientational (S(2)) and deuterium (S(CD)) order parameters. Upon increase of membrane tension both order parameters indicated an increase in lipid disorder by 10-20%, mostly in the tail end region of the hydrophobic chains. The effect of membrane tension on lipid lateral diffusion in the DOPC bilayer was analyzed on three different time scales corresponding to inertial motion, anomalous diffusion and normal diffusion. The results showed that lateral diffusion of lipid molecules is anomalous in nature due to the non-exponential distribution of waiting times. The anomalous and normal diffusion coefficients increased by 20% and 52% when the membrane tension changed from 0 to 15dyn/cm, respectively. In conclusion, our studies showed that membrane tension causes relatively significant changes in the area per lipid, volume, polarity, membrane thickness, and fluidity of the membrane suggesting multiple mechanisms by which mechanical perturbation of the membrane could trigger mechanosensitive response in cells.

Mechanical regulation of signaling pathways in bone

A wide range of cell types depend on mechanically induced signals to enable appropriate physiological responses. The skeleton is particularly dependent on mechanical information to guide the resident cell population towards adaptation, maintenance and repair. Research at the organ, tissue, cell and molecular levels has improved our understanding of how the skeleton can recognize the functional environment, and how these challenges are translated into cellular information that can site-specifically alter phenotype. This review first considers those cells within the skeleton that are responsive to mechanical signals, including osteoblasts, osteoclasts, osteocytes and osteoprogenitors. This is discussed in light of a range of experimental approaches that can vary parameters such as strain, fluid shear stress, and pressure. The identity of mechanoreceptor candidates is approached, with consideration of integrins, pericellular tethers, focal adhesions, ion channels, cadherins, connexins, and the plasma membrane including caveolar and non-caveolar lipid rafts and their influence on integral signaling protein interactions. Several mechanically regulated intracellular signaling cascades are detailed including activation of kinases (Akt, MAPK, FAK), β-catenin, GTPases, and calcium signaling events. While the interaction of bone cells with their mechanical environment is complex, an understanding of mechanical regulation of bone signaling is crucial to understanding bone physiology, the etiology of diseases such as osteoporosis, and to the development of interventions to improve bone strength.

FRET-based detection of M1 muscarinic acetylcholine receptor activation by orthosteric and allosteric agonists

Background and Objective

Muscarinic acetylcholine receptors (mAChRs) are 7-transmembrane, G protein-coupled receptors that regulate a variety of physiological processes and represent potentially important targets for therapeutic intervention. mAChRs can be stimulated by full and partial orthosteric and allosteric agonists, however the relative abilities of such ligands to induce conformational changes in the receptor remain unclear. To gain further insight into the actions of mAChR agonists, we have developed a fluorescently tagged M₁ mAChR that reports ligand-induced conformational changes in real-time by changes in Förster resonance energy transfer (FRET).

Methods

Variants of CFP and YFP were inserted into the third intracellular loop and at the end of the C-terminus of the mouse M₁ mAChR, respectively. The optimized FRET receptor construct (M₁-cam5) was expressed stably in HEK293 cells.

Results

The variant CFP/YFP-receptor chimera expressed predominantly at the plasma membrane of HEK293 cells and displayed ligand-binding affinities comparable with those of the wild-type receptor. It also retained an ability to interact with Gα_q/11 proteins and to stimulate phosphoinositide turnover, ERK1/2 phosphorylation and undergo agonist-dependent internalization. Addition of the full agonist methacholine caused a reversible decrease in M₁ FRET (F_EYFP/F_ECFP) that was prevented by atropine pre-addition and showed concentration-dependent amplitude and kinetics. Partial orthosteric agonists, arecoline and pilocarpine, as well as allosteric agonists, AC-42 and 77-LH-28-1, also caused atropine-sensitive decreases in the FRET signal, which were smaller in amplitude and significantly slower in onset compared to those evoked by methacholine.

Conclusion

The M₁ FRET-based receptor chimera reports that allosteric and orthosteric agonists induce similar conformational changes in the third intracellular loop and/or C-terminus, and should prove to be a valuable molecular reagent for pharmacological and structural investigations of M₁ mAChR activation.

G protein-mediated stretch reception

Mechanosensation and -transduction are important for physiological processes like the senses of touch, hearing, and balance. The mechanisms underlying the translation of mechanical stimuli into biochemical information by activating various signaling pathways play a fundamental role in physiology and pathophysiology but are only poorly understood. Recently, G protein-coupled receptors (GPCRs), which are essential for the conversion of light, olfactory and gustatory stimuli, as well as of primary messengers like hormones and neurotransmitters into cellular signals and which play distinct roles in inflammation, cell growth, and differentiation, have emerged as potential mechanosensors. The first candidate for a mechanosensitive GPCR was the angiotensin-II type-1 (AT(1)) receptor. Agonist-independent mechanical receptor activation of AT(1) receptors induces an active receptor conformation that appears to differ from agonist-induced receptor conformations and entails the activation of G proteins. Mechanically induced AT(1) receptor activation plays an important role for myogenic vasoconstriction and for the initiation of cardiac hypertrophy. A growing body of evidence suggests that other GPCRs are involved in mechanosensation as well. These findings highlight physiologically relevant, ligand-independent functions of GPCRs and add yet another facet to the polymodal activation spectrum of this ubiquitous protein family.

Effect of membrane tension on the electric field and dipole potential of lipid bilayer membrane

The dipole potential of lipid bilayer membrane controls the difference in permeability of the membrane to oppositely charged ions. We have combined molecular dynamics (MD) simulations and experimental studies to determine changes in electric field and electrostatic potential of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid bilayer in response to applied membrane tension. MD simulations based on CHARMM36 force field showed that electrostatic potential of DOPC bilayer decreases by ~45mV in the physiologically relevant range of membrane tension values (0 to 15dyn/cm). The electrostatic field exhibits a peak (~0.8×10(9)V/m) near the water/lipid interface which shifts by 0.9Å towards the bilayer center at 15dyn/cm. Maximum membrane tension of 15dyn/cm caused 6.4% increase in area per lipid, 4.7% decrease in bilayer thickness and 1.4% increase in the volume of the bilayer. Dipole-potential sensitive fluorescent probes were used to detect membrane tension induced changes in DOPC vesicles exposed to osmotic stress. Experiments confirmed that dipole potential of DOPC bilayer decreases at higher membrane tensions. These results are suggestive of a potentially new mechanosensing mechanism by which mechanically induced structural changes in the lipid bilayer membrane could modulate the function of membrane proteins by altering electrostatic interactions and energetics of protein conformational states.

Role of endocrine and paracrine factors in the adaptation of bone to mechanical loading

There appears to be no unique mechanically sensitive pathway by which changes in bone loading regulate bone mass and architecture to ensure adequate structural strength. Rather, strain-derived changes in bone cells activate a number of nonspecific strain-sensitive pathways (including calcium fluxes, prostanoids, nitric oxide, extracellular signal-regulated kinase, and sclerostin), the activities of which are modified by a number of factors (including estrogen receptors) for which this contribution is subsidiary to other purposes. The strain-sensitive pathways modified by these factors interact with a number of other pathways, some of which appear to have specific osteoregulatory potential (eg, the parathyroid hormone pathway), whereas others such as the Wnt pathway appear to be associated primarily with the response mechanisms of proliferation, differentiation, and apoptosis. The outcome of these multiple interactions are stimuli for local bone formation, resorption, or maintenance of the status quo, to maintain existing bone architecture or adapt it to a new mechanical regimen.

Real-time visualization of heterotrimeric G protein Gq activation in living cells

Background

Gq is a heterotrimeric G protein that plays an important role in numerous physiological processes. To delineate the molecular mechanisms and kinetics of signalling through this protein, its activation should be measurable in single living cells. Recently, fluorescence resonance energy transfer (FRET) sensors have been developed for this purpose.

Results

In this paper, we describe the development of an improved FRET-based Gq activity sensor that consists of a yellow fluorescent protein (YFP)-tagged Gγ2 subunit and a Gαq subunit with an inserted monomeric Turquoise (mTurquoise), the best cyan fluorescent protein variant currently available. This sensor enabled us to determine, for the first time, the k_on (2/s) of Gq activation. In addition, we found that the guanine nucleotide exchange factor p63RhoGEF has a profound effect on the number of Gq proteins that become active upon stimulation of endogenous histamine H1 receptors. The sensor was also used to measure ligand-independent activation of the histamine H1 receptor (H1R) upon addition of a hypotonic stimulus.

Conclusions

Our observations reveal that the application of a truncated mTurquoise as donor and a YFP-tagged Gγ2 as acceptor in FRET-based Gq activity sensors substantially improves their dynamic range. This optimization enables the real-time single cell quantification of Gq signalling dynamics, the influence of accessory proteins and allows future drug screening applications by virtue of its sensitivity.

The Load-Bearing Mechanosome Revisited

We introduced the mechanosome hypothesis in 2003 as a heuristic model for investigating mechanotransduction in bone (Pavalko et al., J Cell Biochem, 2003, 88(1):104-112). This model suggested specific approaches for investigating how mechanical information is conveyed from the membrane of the sensor bone cell to the target genes and how this transmitted information from the membrane is converted into changes in transcription. The key concepts underlying the mechanosome hypothesis are that load-induced deformation of bone deforms the sensor cell membrane; embedded in the membrane are the focal adhesion and cadherin-catenin complexes, which in turn are physically connected to the chromatin via a solid-state scaffold. The physical stimulation of the membrane launches multiprotein complexes (mechanosomes) from the adhesion platforms while concomitantly tugging target genes into position for contact with the incoming mechanosomes, the carriers of the mechanical information to the nucleus. The mechanosome is comprised of an adhesion-associated protein and a nucleocytoplasmic shuttling transcription factor. Upon arrival at the target gene, mechanosomes alter DNA conformation and thus influence the interactions between trans-acting proteins along the gene, changing gene activity. Here, we update significant progress related to the mechanosome concept since publication of our original hypothesis. The launching of adhesion- and cytoskeletal-associated proteins into the nucleus toward target genes appears to be a common mechanism for regulating cell response to changes in its mechanical microenvironment.

Live cell imaging of mechanotransduction

Mechanical forces play important roles in the regulation of cellular functions, including polarization, migration and stem cell differentiation. Tremendous advancement in our understanding of mechanotransduction has been achieved with the recent development of imaging technologies and molecular biosensors. In particular, genetically encoded biosensors based on fluorescence resonance energy transfer (FRET) technology have been widely developed and applied in the field of mechanobiology. In this article, we will provide an overview of the recent progress of FRET application in mechanobiology, specifically mechanotransduction. We first introduce fluorescent proteins and FRET technology. We then discuss the mechanotransduction processes in different cells including stem cells, with a special emphasis on the important signalling molecules involved in mechanotransduction. Finally, we discuss methods that can allow the integration of simultaneous FRET imaging and mechanical stimulation to trigger signalling transduction. In summary, FRET technology has provided a powerful tool for the study of mechanotransduction to advance our systematic understanding of the molecular mechanisms by which cells respond to mechanical stimulation.