Metabolically intact nuclei are fluidized by the activity of the chromatin remodeling motor BRG1.

The structure and dynamics of the cell nucleus regulate nearly every facet of the cell. Changes in nuclear shape limit cell motility and gene expression. Although the nucleus is generally seen as the stiffest organelle in the cell, cells can nevertheless deform the nucleus to large strains by small mechanical stresses. Here, we show that the mechanical response of the cell nucleus exhibits active fluidization that is driven by the BRG 1 motor of the SWI/SNF/BAF chromatin-remodeling complex. Atomic force microscopy measurements show that the nucleus alters stiffness in response to the cell substrate stiffness, which is retained after the nucleus is isolated and that the work of nuclear compression is mostly dissipated rather than elastically stored. Inhibiting BRG 1 stiffens the nucleus and eliminates dissipation and nuclear remodeling both in isolated nuclei and in intact cells. These findings demonstrate a novel link between nuclear motor activity and global nuclear mechanics.

Extracellular vimentin is sufficient to promote cell attachment, spreading, and motility by a mechanism involving N-acetyl glucosamine-containing structures.

Vimentin intermediate filaments form part of the cytoskeleton of mesenchymal cells, but under pathological conditions often associated with inflammation, vimentin filaments depolymerize as the result of phosphorylation or citrullination, and vimentin oligomers are secreted or released into the extracellular environment. In the extracellular space, vimentin can bind surfaces of cells and the extracellular matrix, and the interaction between extracellular vimentin and cells can trigger changes in cellular functions, such as activation of fibroblasts to a fibrotic phenotype. The mechanism by which extracellular vimentin binds external cell membranes and whether vimentin alone can act as an adhesive anchor for cells is largely uncharacterized. Here, we show that various cell types (normal and vimentin null fibroblasts, mesenchymal stem cells, and A549 lung carcinoma cells) attach to and spread on polyacrylamide hydrogel substrates covalently linked to vimentin. Using traction force microscopy and spheroid expansion assays, we characterize how different cell types respond to extracellular vimentin. Cell attachment to and spreading on vimentin-coated surfaces is inhibited by hyaluronic acid degrading enzymes, hyaluronic acid synthase inhibitors, soluble heparin or N-acetyl glucosamine, all of which are treatments that have little or no effect on the same cell types binding to collagen-coated hydrogels. These studies highlight the effectiveness of substrate-bound vimentin as a ligand for cells and suggest that carbohydrate structures, including the glycocalyx and glycosylated cell surface proteins that contain N-acetyl glucosamine, form a novel class of adhesion receptors for extracellular vimentin that can either directly support cell adhesion to a substrate or fine-tune the glycocalyx adhesive properties.

Matching mechanical heterogeneity of the native spinal cord augments axon infiltration in 3D-printed scaffolds.

Scaffolds delivered to injured spinal cords to stimulate axon connectivity often match the anisotropy of native tissue using guidance cues along the rostral-caudal axis, but current approaches do not mimic the heterogeneity of host tissue mechanics. Although white and gray matter have different mechanical properties, it remains unclear whether tissue mechanics also vary along the length of the cord. Mechanical testing performed in this study indicates that bulk spinal cord mechanics do differ along anatomical level and that these differences are caused by variations in the ratio of white and gray matter. These results suggest that scaffolds recreating the heterogeneity of spinal cord tissue mechanics must account for the disparity between gray and white matter. Digital light processing (DLP) provides a means to mimic spinal cord topology, but has previously been limited to printing homogeneous mechanical properties. We describe a means to modify DLP to print scaffolds that mimic spinal cord mechanical heterogeneity caused by variation in the ratio of white and gray matter, which improves axon infiltration compared to controls exhibiting homogeneous mechanical properties. These results demonstrate that scaffolds matching the mechanical heterogeneity of white and gray matter improve the effectiveness of biomaterials transplanted within the injured spinal cord.

Microindentation of Fluid-Filled Cellular Domes Reveals the Contribution of RhoA-ROCK Signaling to Multicellular Mechanics.

Cellular mechanics encompass both mechanical properties that resist forces applied by the external environment and internally generated forces applied at the location of cell-cell and cell-matrix junctions. Here, the authors demonstrate that microindentation of cellular domes formed by cell monolayers that locally lift off the substrate provides insight into both aspects of cellular mechanics in multicellular structures. Using a modified Hertz contact equation, the force-displacement curves generated by a micro-tensiometer are used to measure an effective dome stiffness. The results indicate the domes are consistent with the Laplace-Young relationship for elastic membranes, regardless of biochemical modulation of the RhoA-ROCK signaling axis. In contrast, activating RhoA, and inhibiting ROCK both alter the relaxation dynamics of the domes deformed by the micro-tensiometer, revealing an approach to interrogate the role of RhoA-ROCK signaling in multicellular mechanics. A finite element model incorporating a Mooney-Rivlin hyperelastic constitutive equation to describe monolayer mechanics predicts effective stiffness values that are consistent with the micro-tensiometer measurements, verifying previous measurements of the response of cell monolayers to tension. Overall, these studies establish microindentation of fluid-filled domes as an avenue to investigate the contribution of cell-generated forces to the mechanics of multicellular structures.

Extracellular Vimentin as a Target Against SARS-CoV-2 Host Cell Invasion.

Infection of human cells by pathogens, including SARS-CoV-2, typically proceeds by cell surface binding to a crucial receptor. The primary receptor for SARS-CoV-2 is the angiotensin-converting enzyme 2 (ACE2), yet new studies reveal the importance of additional extracellular co-receptors that mediate binding and host cell invasion by SARS-CoV-2. Vimentin is an intermediate filament protein that is increasingly recognized as being present on the extracellular surface of a subset of cell types, where it can bind to and facilitate pathogens' cellular uptake. Biophysical and cell infection studies are done to determine whether vimentin might bind SARS-CoV-2 and facilitate its uptake. Dynamic light scattering shows that vimentin binds to pseudovirus coated with the SARS-CoV-2 spike protein, and antibodies against vimentin block in vitro SARS-CoV-2 pseudovirus infection of ACE2-expressing cells. The results are consistent with a model in which extracellular vimentin acts as a co-receptor for SARS-CoV-2 spike protein with a binding affinity less than that of the spike protein with ACE2. Extracellular vimentin may thus serve as a critical component of the SARS-CoV-2 spike protein-ACE2 complex in mediating SARS-CoV-2 cell entry, and vimentin-targeting agents may yield new therapeutic strategies for preventing and slowing SARS-CoV-2 infection.

Extracellular vimentin as a target against SARS-CoV-2 host cell invasion.

Infection of human cells by pathogens, including SARS-CoV-2, typically proceeds by cell surface binding to a crucial receptor. In the case of SARS-CoV-2, angiotensin-converting enzyme 2 (ACE2) has been identified as a necessary receptor, but not all ACE2-expressing cells are equally infected, suggesting that other extracellular factors are involved in host cell invasion by SARS-CoV-2. Vimentin is an intermediate filament protein that is increasingly recognized as being present on the extracellular surface of a subset of cell types, where it can bind to and facilitate pathogens' cellular uptake. Here, we present evidence that extracellular vimentin might act as a critical component of the SARS-CoV-2 spike protein-ACE2 complex in mediating SARS-CoV-2 cell entry. We demonstrate direct binding between vimentin and SARS-CoV-2 pseudovirus coated with the SARS-CoV-2 spike protein and show that antibodies against vimentin block in vitro SARS-CoV-2 pseudovirus infection of ACE2-expressing cells. Our results suggest new therapeutic strategies for preventing and slowing SARS-CoV-2 infection, focusing on targeting cell host surface vimentin.

Matrix stiffness regulates endosomal escape of uropathogenic E. coli.

Uropathogenic E. coli (UPEC) infection in vivo is characterized by invasion of bladder umbrella epithelial cells followed by endosomal escape and proliferation in the cytoplasm to form intracellular bacterial communities. By contrast, UPEC infection in tissue culture models results in bacteria being trapped within Lamp1-positive endosomes where proliferation is limited. Pharmacological disruption of the actin cytoskeleton has been shown to facilitate UPEC endosomal escape in vitro and extracellular matrix stiffness is a well-characterized physiological regulator of actin dynamics; therefore, we hypothesized that substrate stiffness may play a role in UPEC endosomal escape. Using functionalized polyacrylamide substrates, we found that at physiological stiffness, UPEC escaped the endosome and proliferated rapidly in the cytoplasm of bladder epithelial cells. Dissection of the cytoskeletal signaling pathway demonstrated that inhibition of the Rho GTPase RhoB or its effector PRK1 was sufficient to increase cytoplasmic bacterial growth and that RhoB protein level was significantly reduced at physiological stiffness. Our data suggest that tissue stiffness is a critical regulator of intracellular bacterial growth. Due to the ease of doing genetic and pharmacological manipulations in cell culture, this model system may provide a useful tool for performing mechanistic studies on the intracellular life cycle of uropathogens.

Loss of Vimentin Enhances Cell Motility through Small Confining Spaces.

The migration of cells through constricting spaces or along fibrous tracks in tissues is important for many biological processes and depends on the mechanical properties of a cytoskeleton made up of three different filaments: F-actin, microtubules, and intermediate filaments. The signaling pathways and cytoskeletal structures that control cell motility on 2D are often very different from those that control motility in 3D. Previous studies have shown that intermediate filaments can promote actin-driven protrusions at the cell edge, but have little effect on overall motility of cells on flat surfaces. They are however important for cells to maintain resistance to repeated compressive stresses that are expected to occur in vivo. Using mouse embryonic fibroblasts derived from wild-type and vimentin-null mice, it is found that loss of vimentin increases motility in 3D microchannels even though on flat surfaces it has the opposite effect. Atomic force microscopy and traction force microscopy experiments reveal that vimentin enhances perinuclear cell stiffness while maintaining the same level of acto-myosin contractility in cells. A minimal model in which a perinuclear vimentin cage constricts along with the nucleus during motility through confining spaces, providing mechanical resistance against large strains that could damage the structural integrity of cells, is proposed.

Soft Substrates Containing Hyaluronan Mimic the Effects of Increased Stiffness on Morphology, Motility, and Proliferation of Glioma Cells.

Unlike many other cancer cells that grow in tumors characterized by an abnormally stiff collagen-enriched stroma, glioma cells proliferate and migrate in the much softer environment of the brain, which generally lacks the filamentous protein matrix characteristic of breast, liver, colorectal, and other types of cancer. Glial cell-derived tumors and the cells derived from them are highly heterogeneous and variable in their mechanical properties, their response to treatments, and their properties in vitro. Some glioma samples are stiffer than normal brain when measured ex vivo, but even those that are soft in vitro stiffen after deformation by pressure gradients that arise in the tumor environment in vivo. Such mechanical differences can strongly alter the phenotype of cultured glioma cells. Alternatively, chemical signaling might elicit the same phenotype as increased stiffness by activating intracellular messengers common to both initial stimuli. In this study the responses of three different human glioma cell lines to changes in substrate stiffness are compared with their responses on very soft substrates composed of a combination of hyaluronic acid and a specific integrin ligand, either laminin or collagen I. By quantifying cell morphology, stiffness, motility, proliferation, and secretion of the cytokine IL-8, glioma cell responses to increased stiffness are shown to be nearly identically elicited by substrates containing hyaluronic acid, even in the absence of increased stiffness. PI3-kinase activity was required for the response to hyaluronan but not to stiffness. This outcome suggests that hyaluronic acid can trigger the same cellular response, as can be obtained by mechanical force transduced from a stiff environment, and demonstrates that chemical and mechanical features of the tumor microenvironment can achieve equivalent reactions in cancer cells.

Measuring the Stiffness of Ex Vivo Mouse Aortas Using Atomic Force Microscopy.

Arterial stiffening is a significant risk factor and biomarker for cardiovascular disease and a hallmark of aging. Atomic force microscopy (AFM) is a versatile analytical tool for characterizing viscoelastic mechanical properties for a variety of materials ranging from hard (plastic, glass, metal, etc.) surfaces to cells on any substrate. It has been widely used to measure the stiffness of cells, but less frequently used to measure the stiffness of aortas. In this paper, we will describe the procedures for using AFM in contact mode to measure the ex vivo elastic modulus of unloaded mouse arteries. We describe our procedure for isolation of mouse aortas, and then provide detailed information for the AFM analysis. This includes step-by-step instructions for alignment of the laser beam, calibration of the spring constant and deflection sensitivity of the AFM probe, and acquisition of force curves. We also provide a detailed protocol for data analysis of the force curves.

Bactericidal Activity of Ceragenin CSA-13 in Cell Culture and in an Animal Model of Peritoneal Infection.

Ceragenins constitute a novel family of cationic antibiotics characterized by a broad spectrum of antimicrobial activities, which have mostly been assessed in vitro. Using a polarized human lung epithelial cell culture system, we evaluated the antibacterial activities of the ceragenin CSA-13 against two strains of Pseudomonas aeruginosa (PAO1 and Xen5). Additionally, the biodistribution and bactericidal activity of a CSA-13-IRDye 800CW derivate were assessed using an animal model of peritoneal infection after PAO1 challenge. In cell culture, CSA-13 bactericidal activities against PAO1 and Xen5 were higher than the activities of the human cathelicidin peptide LL-37. Increased CSA-13 activity was observed in polarized human lung epithelial cell cultures subjected to butyric acid treatment, which is known to increase endogenous LL-37 production. Eight hours after intravenous or intraperitoneal injection, the greatest CSA-13-IRDye 800CW accumulation was observed in mouse liver and kidneys. CSA-13-IRDye 800CW administration resulted in decreased bacterial outgrowth from abdominal fluid collected from animals subjected to intraperitoneal PAO1 infection. These observations indicate that CSA-13 may synergistically interact with antibacterial factors that are naturally present at mucosal surfaces and it maintains its antibacterial activity in the infected abdominal cavity. Cationic lipids such as CSA-13 represent excellent candidates for the development of new antibacterial compounds.

Polyelectrolyte-mediated increase of biofilm mass formation.

BACKGROUND: Biofilm formation is associated with various aspects of bacterial and fungal infection. This study was designed to assess the impact of diverse natural polyelectrolytes, such as DNA, F-actin, neurofilaments (NFs), vimentin and purified Pf1 bacteriophage on biofilm formation and swarming motility of select pathogens including Pseudomonas aeruginosa associated with lung infections in CF patients. RESULTS: The bacteriophage Pf1 (1 mg/ml) significantly increased biofilm mass produced by Pseudomonas aeruginosa P14, Escherichia coli RS218 and Bacillus subtilis ATCC6051. DNA, F-actin, NFs and Pf1 also increased biofilm mass of the fungal C. albicans 1409 strain. Addition of F-actin, DNA or Pf1 bacteriophage to 0.5% agar plates increased swarming motility of Pseudomonas aeruginosa Xen5. CONCLUSIONS: The presence of polyelectrolytes at infection sites is likely to promote biofilm growth and bacterial swarming.

Contact-induced apical asymmetry drives the thigmotropic responses of Candida albicans hyphae.

Filamentous hyphae of the human pathogen, Candida albicans, invade mucosal layers and medical silicones. In vitro, hyphal tips reorient thigmotropically on contact with small obstacles. It is not known how surface topography is sensed but hyphae lacking the cortical marker, Rsr1/Bud1, are unresponsive. We show that, on surfaces, the morphology of hyphal tips and the position of internal polarity protein complexes are asymmetrically skewed towards the substratum and biased towards the softer of two surfaces. In nano-fabricated chambers, the Spitzenkörper (Spk) responded to touch by translocating across the apex towards the point of contact, where its stable maintenance correlated with contour-following growth. In the rsr1Δ mutant, the position of the Spk meandered and these responses were attenuated. Perpendicular collision caused lateral Spk oscillation within the tip until after establishment of a new growth axis, suggesting Spk position does not predict the direction of growth in C. albicans. Acute tip reorientation occurred only in cells where forward growth was countered by hyphal friction sufficient to generate a tip force of ∼ 8.7 µN (1.2 MPa), more than that required to penetrate host cell membranes. These findings suggest mechanisms through which the organization of hyphal tip growth in C. albicans facilitates the probing, penetration and invasion of host tissue.

Mechanically stimulated contraction of engineered cardiac constructs using a microcantilever.

The beating heart undergoes cyclic mechanical and electrical activity during systole and diastole. The interaction between mechanical stimulation and propagation of the depolarization wavefront is important for understanding not just normal sinus rhythm, but also mechanically induced cardiac arrhythmia. This study presents a new platform to study mechanoelectrical coupling in a 3-D in vitro model of the myocardium. Cardiomyocytes and cardiac fibroblasts are seeded within extracellular matrix proteins and form constructs constrained by microfabricated tissue gauges that provide in situ measurement of contractile function. The microcantilever of an atomic force microscope is indented into the construct at varying magnitudes and frequencies to cause a coordinated contraction. The results indicate that changes in indentation depth and frequency do not significantly affect the magnitude of contraction, but increasing indentation frequency significantly increases the contractile velocity. Overall, this study demonstrates the validity of this platform as a means to study mechanoelectrical coupling in a 3-D setting, and to investigate the mechanism underlying mechanically stimulated contraction.

Increased levels of sphingosine-1-phosphate in cerebrospinal fluid of patients diagnosed with tick-borne encephalitis.

BACKGROUND: Tick-borne encephalitis (TBE) is a serious acute central nervous system infection that can result in death or long-term neurological dysfunctions. We hypothesize that changes in sphingosine-1-phosphate (S1P) concentration occur during TBE development. METHODS: S1P and interleukin-6 (IL-6) concentrations in blood plasma and cerebrospinal fluid (CSF) were measured using HPLC and ELISA, respectively. The effects of S1P on cytoskeletal structure and IL-6 production were assessed using rat astrocyte primary cultures with and without addition of plasma gelsolin and the S1P receptor antagonist fingolimod phosphate (FTY720P). RESULTS: We report that acute inflammation due to TBE virus infection is associated with elevated levels of S1P and IL-6 in the CSF of infected patients. This elevated concentration is observed even at the earliest neurologic stage of disease, and may be controlled by glucocorticosteroid anti-inflammatory treatment, administered to patients unresponsive to antipyretic drugs and who suffer from a fever above 39°C. In vitro, treatment of confluent rat astrocyte monolayers with a high concentration of S1P (5 µM) results in cytoskeletal actin remodeling that can be prevented by the addition of recombinant plasma gelsolin, FTY720P, or their combination. Additionally, gelsolin and FTY720P significantly decreased S1P-induced release of IL-6. CONCLUSIONS: TBE is associated with increased concentration of S1P and IL-6 in CSF, and this increase might promote development of inflammation. The consequences of increased extracellular S1P can be modulated by gelsolin and FTY720P. Therefore, blocking the inflammatory response at sites of infection by agents modulating S1P pathways might aid in developing new strategies for TBE treatment.

Augmentation of integrin-mediated mechanotransduction by hyaluronic acid.

Changes in tissue and organ stiffness occur during development and are frequently symptoms of disease. Many cell types respond to the stiffness of substrates and neighboring cells in vitro and most cell types increase adherent area on stiffer substrates that are coated with ligands for integrins or cadherins. In vivo cells engage their extracellular matrix (ECM) by multiple mechanosensitive adhesion complexes and other surface receptors that potentially modify the mechanical signals transduced at the cell/ECM interface. Here we show that hyaluronic acid (also called hyaluronan or HA), a soft polymeric glycosaminoglycan matrix component prominent in embryonic tissue and upregulated during multiple pathologic states, augments or overrides mechanical signaling by some classes of integrins to produce a cellular phenotype otherwise observed only on very rigid substrates. The spread morphology of cells on soft HA-fibronectin coated substrates, characterized by formation of large actin bundles resembling stress fibers and large focal adhesions resembles that of cells on rigid substrates, but is activated by different signals and does not require or cause activation of the transcriptional regulator YAP. The fact that HA production is tightly regulated during development and injury and frequently upregulated in cancers characterized by uncontrolled growth and cell movement suggests that the interaction of signaling between HA receptors and specific integrins might be an important element in mechanical control of development and homeostasis.

Compression stiffening of brain and its effect on mechanosensing by glioma cells.

Many cell types, including neurons, astrocytes and other cells of the central nervous system respond to changes in extracellular matrix or substrate viscoelasticity, and increased tissue stiffness is a hallmark of several disease states including fibrosis and some types of cancers. Whether the malignant tissue in brain, an organ that lacks the protein-based filamentous extracellular matrix of other organs, exhibits the same macroscopic stiffening characteristic of breast, colon, pancreatic, and other tumors is not known. In this study we show that glioma cells like normal astrocytes, respond strongly in vitro to substrate stiffness in the range of 100 to 2000 Pa, but that macroscopic (mm to cm) tissue samples isolated from human glioma tumors have elastic moduli on the order of 200 Pa that are indistinguishable from those of normal brain. However, both normal brain and glioma tissues increase their shear elastic moduli under modest uniaxial compression, and glioma tissue stiffens more strongly under compression than does normal brain. These findings suggest that local tissue stiffness has the potential to alter glial cell function, and that stiffness changes in brain tumors might arise not from increased deposition or crosslinking of collagen-rich extracellular matrix but from pressure gradients that form within the tumors in vivo.

Antibacterial activity of the human host defence peptide LL-37 and selected synthetic cationic lipids against bacteria associated with oral and upper respiratory tract infections.

OBJECTIVES: We aim to develop antibacterial peptide mimics resistant to protease degradation, with broad-spectrum activity at sites of infection. METHODS: The bactericidal activities of LL-37, ceragenins CSA-13, CSA-90 and CSA-92 and the spermine-conjugated dexamethasone derivative D2S were evaluated using MIC and MBC measurements. Gingival fibroblast counting, interleukin-8 (IL-8) and lactate dehydrogenase (LDH) release from keratinocytes (HaCat) were used to determine effects on cell growth, pro-inflammatory response and toxicity. RESULTS: All tested cationic lipids showed stronger bactericidal activity than LL-37. Incubation of Staphylococcus aureus with half the MIC of LL-37 led to the appearance of bacteria resistant to its bactericidal effects, but identical incubations with CSA-13 or D2S did not produce resistant bacteria. Cathelicidin LL-37 significantly increased the total number of gingival fibroblasts, but ceragenins and D2S did not alter gingival fibroblast growth. Cationic lipids showed no toxicity to HaCat cells at concentrations resulting in bacterial killing. CONCLUSIONS: These data suggest that cationic lipids such as ceragenins warrant further testing as potential novel antibacterial agents.

Cardiovascular protection by ApoE and ApoE-HDL linked to suppression of ECM gene expression and arterial stiffening.

Arterial stiffening is a risk factor for cardiovascular disease, but how arteries stay supple is unknown. Here, we show that apolipoprotein E (apoE) and apoE-containing high-density lipoprotein (apoE-HDL) maintain arterial elasticity by suppressing the expression of extracellular matrix genes. ApoE interrupts a mechanically driven feed-forward loop that increases the expression of collagen-I, fibronectin, and lysyl oxidase in response to substratum stiffening. These effects are independent of the apoE lipid-binding domain and transduced by Cox2 and miR-145. Arterial stiffness is increased in apoE null mice. This stiffening can be reduced by administration of the lysyl oxidase inhibitor BAPN, and BAPN treatment attenuates atherosclerosis despite highly elevated cholesterol. Macrophage abundance in lesions is reduced by BAPN in vivo, and monocyte/macrophage adhesion is reduced by substratum softening in vitro. We conclude that apoE and apoE-containing HDL promote healthy arterial biomechanics and that this confers protection from cardiovascular disease independent of the established apoE-HDL effect on cholesterol.

Smooth muscle tension induces invasive remodeling of the zebrafish intestine.

The signals that initiate cell invasion are not well understood, but there is increasing evidence that extracellular physical signals play an important role. Here we show that epithelial cell invasion in the intestine of zebrafish meltdown (mlt) mutants arises in response to unregulated contractile tone in the surrounding smooth muscle cell layer. Physical signaling in mlt drives formation of membrane protrusions within the epithelium that resemble invadopodia, matrix-degrading protrusions present in invasive cancer cells. Knockdown of Tks5, a Src substrate that is required for invadopodia formation in mammalian cells blocked formation of the protrusions and rescued invasion in mlt. Activation of Src-signaling induced invadopodia-like protrusions in wild type epithelial cells, however the cells did not migrate into the tissue stroma, thus indicating that the protrusions were required but not sufficient for invasion in this in vivo model. Transcriptional profiling experiments showed that genes responsive to reactive oxygen species (ROS) were upregulated in mlt larvae. ROS generators induced invadopodia-like protrusions and invasion in heterozygous mlt larvae but had no effect in wild type larvae. Co-activation of oncogenic Ras and Wnt signaling enhanced the responsiveness of mlt heterozygotes to the ROS generators. These findings present the first direct evidence that invadopodia play a role in tissue cell invasion in vivo. In addition, they identify an inducible physical signaling pathway sensitive to redox and oncogenic signaling that can drive this process.