Mechanoregulation of proliferation

The molecular determinants of microglial developmental dynamics

Microglia constitute the largest population of parenchymal macrophages in the brain and are considered a unique subset of central nervous system glial cells owing to their extra-embryonic origins in the yolk sac. During development, microglial progenitors readily proliferate and eventually colonize the entire brain. In this Review, we highlight the origins of microglial progenitors and their entry routes into the brain and discuss the various molecular and non-molecular determinants of their fate, which may inform their specific functions. Specifically, we explore recently identified mechanisms that regulate microglial colonization of the brain, including the availability of space, and describe how the expansion of highly proliferative microglial progenitors facilitates the occupation of the microglial niche. Finally, we shed light on the factors involved in establishing microglial identity in the brain.

Optical sensor reveals the hidden influence of cell dissociation on adhesion measurements

Cell adhesion experiments are important in tissue engineering and for testing new biologically active surfaces, prostheses, and medical devices. Additionally, the initial state of adhesion (referred to as nascent adhesion) plays a key role and is currently being intensively researched. A critical step in handling all adherent cell types is their dissociation from their substrates for further processing. Various cell dissociation methods and reagents are used in most tissue culture laboratories (here, cell dissociation from the culture surface, cell harvesting, and cell detachment are used interchangeably). Typically, the dissociated cells are re-adhered for specific measurements or applications. However, the impact of the choice of dissociation method on cell adhesion in subsequent measurements, especially when comparing the adhesivity of various surfaces, is not well clarified. In this study, we demonstrate that the application of a label-free optical sensor can precisely quantify the effect of cell dissociation methods on cell adhesivity, both at the single-cell and population levels. The optical measurements allow for high-resolution monitoring of cellular adhesion without interfering with the physiological state of the cells. We found that the choice of reagent significantly alters cell adhesion on various surfaces. Our results clearly demonstrate that biological conclusions about cellular adhesion when comparing various surfaces are highly dependent on the employed dissociation method. Neglecting the choice of cellular dissociation can lead to misleading conclusions when evaluating cell adhesion data from various sources and comparing the adhesivity of two different surfaces (i.e., determining which surface is more or less adhesive).

The Role of Mechanotransduction in Contact Inhibition of Locomotion and Proliferation

Contact inhibition (CI) represents a crucial tumor-suppressive mechanism responsible for controlling the unbridled growth of cells, thus preventing the formation of cancerous tissues. CI can be further categorized into two distinct yet interrelated components: CI of locomotion (CIL) and CI of proliferation (CIP). These two components of CI have historically been viewed as separate processes, but emerging research suggests that they may be regulated by both distinct and shared pathways. Specifically, recent studies have indicated that both CIP and CIL utilize mechanotransduction pathways, a process that involves cells sensing and responding to mechanical forces. This review article describes the role of mechanotransduction in CI, shedding light on how mechanical forces regulate CIL and CIP. Emphasis is placed on filamin A (FLNA)-mediated mechanotransduction, elucidating how FLNA senses mechanical forces and translates them into crucial biochemical signals that regulate cell locomotion and proliferation. In addition to FLNA, trans-acting factors (TAFs), which are proteins or regulatory RNAs capable of directly or indirectly binding to specific DNA sequences in distant genes to regulate gene expression, emerge as sensitive players in both the mechanotransduction and signaling pathways of CI. This article presents methods for identifying these TAF proteins and profiling the associated changes in chromatin structure, offering valuable insights into CI and other biological functions mediated by mechanotransduction. Finally, it addresses unanswered research questions in these fields and delineates their possible future directions.

Suture causing urethral meatus stricture: A novel animal model of partial bladder outlet obstruction

AIMS

Open surgery is the most commonly used methodological approach for generating a partial bladder outlet obstruction (pBOO) animal model. Surgical suturing closing a part of the urethral meatus induces comparable pathophysiological changes in bladder and renal functions, but the optimum degree of obstruction that closely mimics the clinical pathology of pBOO has not been elucidated. We investigated the optimum obstruction level by performing a comprehensive time-dependent analysis of the stability and reliability of this novel animal model.

METHODS

Six- to eight-week-old female BALB/c mice were divided into three groups according to the degree of urethral meatus stricture (UMS). Non-operated mice served as controls, and a pBOO model generated using the traditional method served as a positive control. A cystometric evaluation and long-term studies were performed to evaluate the validity and reliability of this novel animal model. An additional 35 mice were used to investigate the protein expression levels and histopathological features 24 h and 14 days postoperatively, respectively.

RESULTS

The characteristic cystometry features in the UMS group revealed increased changes in pressure-related parameters compared with the control. The 1/3 UMS model is an optional pBOO animal model because the cystometric evaluation and histopathological studies revealed a striking resemblance between the 1/3 UMS model and the model generated using the traditional open-surgery method.

CONCLUSIONS

The minimally invasive UMS model required less time and produced minimal alterations in pathophysiologically relevant processes compared with the traditional surgery model. Suturing to cause UMS produced effective and repeatable patterns in bladder function investigations in mice.

Mechanoregulation of Wound Healing and Skin Homeostasis

Basic and clinical studies on mechanobiology of cells and tissues point to the importance of mechanical forces in the process of skin regeneration and wound healing. These studies result in the development of new therapies that use mechanical force which supports effective healing. A better understanding of mechanobiology will make it possible to develop biomaterials with appropriate physical and chemical properties used to treat poorly healing wounds. In addition, it will make it possible to design devices precisely controlling wound mechanics and to individualize a therapy depending on the type, size, and anatomical location of the wound in specific patients, which will increase the clinical efficiency of the therapy. Linking mechanobiology with the science of biomaterials and nanotechnology will enable in the near future precise interference in abnormal cell signaling responsible for the proliferation, differentiation, cell death, and restoration of the biological balance. The objective of this study is to point to the importance of mechanobiology in regeneration of skin damage and wound healing. The study describes the influence of rigidity of extracellular matrix and special restrictions on cell physiology. The study also defines how and what mechanical changes influence tissue regeneration and wound healing. The influence of mechanical signals in the process of proliferation, differentiation, and skin regeneration is tagged in the study.

A computational model of the effect of symplastic growth on cell mechanics in a linear leaf blade

The epidermis of a linear leaf, as in Poaceae, is established by parallel files of cells originating from the leaf base. Their feature is symplastic growth where neighboring cell walls adhere and do not slide along each other. We developed a simple mechanical cell-based model for symplastic growth of linear leaf blade. The challenge is to determine what restrictions on cell size symplastic growth creates compared to the free growing cells. We assume an unidirectional growing cell ensemble starting from a meristem-like layer of generative cells and then generating parallel cell rows from every cell of the initial layer. Each cell is characterized by its growth function, and growth of the whole leaf blade is accompanied by mutual adjustment between all the cells. Cells divide once they have reached a threshold area. A mathematical model and its implementation are proposed for computational simulation of 1D symplastic growth of tissues. The question analyzed is how a cell grows in a plant tissue if there is a mechanism for regulating the growth of an isolated growing cell and the behavior of the cell wall matter is elastoplastic. The results of the simulation of linear leaf blade growth are compared to those for a free-growing cell population.

Spatial constraints control cell proliferation in tissues

Control of cell proliferation is a fundamental aspect of tissue formation in development and regeneration. Cells experience various spatial and mechanical constraints depending on their environmental context in the body, but we do not fully understand if and how such constraints influence cell cycle progression and thereby proliferation patterns in tissues. Here, we study the impact of mechanical manipulations on the cell cycle of individual cells within a mammalian model epithelium. By monitoring the response to experimentally applied forces, we find a checkpoint at the G1-S boundary that, in response to spatial constraints, controls cell cycle progression. This checkpoint prevents cells from entering S phase if the available space remains below a characteristic threshold because of crowding. Stretching the tissue results in fast cell cycle reactivation, whereas compression rapidly leads to cell cycle arrest. Our kinetic analysis of this response shows that cells have no memory of past constraints and allows us to formulate a biophysical model that predicts tissue growth in response to changes in spatial constraints in the environment. This characteristic biomechanical cell cycle response likely serves as a fundamental control mechanism to maintain tissue integrity and to ensure control of tissue growth during development and regeneration.

From tissue mechanics to transcription factors

Changes in tissue stiffness are frequently associated with diseases such as cancer, fibrosis, and atherosclerosis. Several recent studies suggest that, in addition to resulting from pathology, mechanical changes may play a role akin to soluble factors in causing the progression of disease, and similar mechanical control might be essential for normal tissue development and homeostasis. Many cell types alter their structure and function in response to exogenous forces or as a function of the mechanical properties of the materials to which they adhere. This review summarizes recent progress in identifying intracellular signaling pathways, and especially transcriptional programs, that are differentially activated when cells adhere to materials with different mechanical properties or when they are subject to tension arising from external forces. Several cytoplasmic or cytoskeletal signaling pathways involving small GTPases, focal adhesion kinase and transforming growth factor beta as well as the transcriptional regulators MRTF-A, NFκB, and Yap/Taz have emerged as important mediators of mechanical signaling.

Skp2-mediated degradation of p27 regulates cell cycle progression in compressed human bladder smooth muscle cells

Bladder outlet obstruction (BOO) results in smooth muscle cell hyperplasia, decreased bladder wall compliance, and lower and upper urinary tract pathology. Mechanical stimulus on detrusor tissue is critical to BOO disease progression. Our previous studies confirm that mechanical stimulus triggers human bladder smooth muscle cell (HBSMC) proliferation. To better understand the signal transduction mechanisms for this process we detected cell cycle machinery of HBSMC (Bose ® Biodynamic, Minnetonka, MN, USA). HBSMCs cultured in scaffolds were subjected to four different pressures (0 cmH2O, 100 cmH2O, 200 cmH2O, and 300 cmH2O) for 24 hours, which were controlled by a BOSE BioDynamic bioreactor. Then we used flow cytometry to examine cell cycle distribution, polymerase chain reaction, and immunoblotting to quantify Skp2, p27, and p21 expression in each group. Additionally, Skp2 was silenced in HBSMCs using small interfering RNA to validate the role of Skp2 in mediating pressure-induced cell cycle progression. Compared with the 0 cmH2O control, HBSMCs in the 200 cmH2O and 300 cmH2O groups exhibited high-level expression of Skp2 gene and low-level expression of p27 protein. However, p21, another downstream signal of Skp2, showed no significant change between groups. In addition, Skp2 silencing abolished increases in cell proliferation induced by pressure. To the best of our knowledge, this is the first report on the functional importance of Skp2 in cyclic hydrodynamic pressure stimulated HBSMC proliferation. The signal transduction mechanism for this process involves p27 as well as p21 signaling pathway.

Human VRK2 (vaccinia-related kinase 2) modulates tumor cell invasion by hyperactivation of NFAT1 and expression of cyclooxygenase-2

Human VRK2 (vaccinia-related kinase 2), a kinase that emerged late in evolution, affects different signaling pathways, and some carcinomas express high levels of VRK2. Invasion by cancer cells has been associated with NFAT1 (nuclear factor of activated T cells) activation and expression of the COX-2 (cyclooxygenase 2) gene. We hypothesized that VRK proteins might play a regulatory role in NFAT1 activation in tumor cells. We demonstrate that VRK2 directly interacts and phosphorylates NFAT1 in Ser-32 within its N-terminal transactivation domain. VRK2 increases NFAT1-dependent transcription by phosphorylation, and this effect is only detected following cell phorbol 12-myristate 13-acetate and ionomycin stimulation and calcineurin activation. This NFAT1 hyperactivation by VRK2 increases COX-2 gene expression through the proximal NFAT1 binding site in the COX-2 gene promoter. Furthermore, VRK2A down-regulation by RNA interference reduces COX-2 expression at transcriptional and protein levels. Therefore, VRK2 down-regulation reduces cell invasion by tumor cells, such as MDA-MB-231 and MDA-MB-435, upon stimulation with phorbol 12-myristate 13-acetate plus ionomycin. These findings identify the first reported target and function of human VRK2 as an active kinase playing a role in regulation of cancer cell invasion through the NFAT pathway and COX-2 expression.

Corneal antifibrotic switch identified in genetic and pharmacological deficiency of vimentin

The type III intermediate filaments (IFs) are essential cytoskeletal elements of mechanosignal transduction and serve critical roles in tissue repair. Mice genetically deficient for the IF protein vimentin (Vim(-/-)) have impaired wound healing from deficits in myofibroblast development. We report a surprising finding made in Vim(-/-) mice that corneas are protected from fibrosis and instead promote regenerative healing after traumatic alkali injury. This reparative phenotype in Vim(-/-) corneas is strikingly recapitulated by the pharmacological agent withaferin A (WFA), a small molecule that binds to vimentin and down-regulates its injury-induced expression. Attenuation of corneal fibrosis by WFA is mediated by down-regulation of ubiquitin-conjugating E3 ligase Skp2 and up-regulation of cyclin-dependent kinase inhibitors p27(Kip1) and p21(Cip1). In cell culture models, WFA exerts G(2)/M cell cycle arrest in a p27(Kip1)- and Skp2-dependent manner. Finally, by developing a highly sensitive imaging method to measure corneal opacity, we identify a novel role for desmin overexpression in corneal haze. We demonstrate that desmin down-regulation by WFA via targeting the conserved WFA-ligand binding site shared among type III IFs promotes further improvement of corneal transparency without affecting cyclin-dependent kinase inhibitor levels in Vim(-/-) mice. This dissociates a direct role for desmin in corneal cell proliferation. Taken together, our findings illuminate a previously unappreciated pathogenic role for type III IF overexpression in corneal fibrotic conditions and also validate WFA as a powerful drug lead toward anti-fibrosis therapeutic development.

The physiological significance of p27(KIP1) expression in detrusor function

PURPOSE

Bladder outlet obstruction results in smooth muscle cell hyperplasia, decreased bladder wall compliance, and lower and upper urinary tract pathology. The cyclin-dependent kinase inhibitor p27(KIP1) regulates bladder smooth muscle cell proliferation in response to bladder outlet obstruction but little is known about its physiological role in the bladder. We investigated the role of p27(KIP1) in the structure and function of the detrusor layer of the bladder wall.

MATERIALS AND METHODS

We used immunoblotting and reverse transcriptase-polymerase chain reaction to examine cell cycle regulation in response to increased mechanical tension in an in vitro model of tension induced smooth muscle cell proliferation and an in vivo model of bladder outlet obstruction. We compared unobstructed bladders of p27(+/+) and p27(-/-) mice (Jackson Laboratory, Bar Harbor, Maine) structurally by histological staining and functionally by in vivo cystometric measurements of bladder capacity, detrusor compliance and detrusor leak point pressure.

RESULTS

Increased tension decreased p27(KIP1) at the protein level in human bladder smooth muscle cells and in intact murine bladder smooth muscle. p27(-/-) mice had bladder smooth muscle cell hyperplasia even in the absence of bladder outlet obstruction. While p27 loss had little effect on detrusor leak point pressure, p27(-/-) mice had significantly decreased bladder capacity and detrusor compliance.

CONCLUSIONS

To our knowledge we provide the first report of the in vivo significance of p27(KIP1) in the regulation of detrusor function using a cystometric approach. We identified a role for p27(KIP1) in protecting against dysregulated smooth muscle cell proliferation, bladder capacity and detrusor compliance under normotensive conditions.

Regional dependency of the vascular smooth muscle cell contribution to the mechanical properties of the pig ascending aortic tissue

BACKGROUND

Dilation and dissection of aneurysmal ascending aortic tissues occur preferentially at the outer curvature of the vessel. In this study we hypothesize that the density and contractile properties of the vascular smooth muscle cells (VSMCs) of the pig ascending aorta (AA) are heterogeneous and could explain the non-uniform remodeling and weakening of the AA during aneurysm formation.

METHODS

Eleven pig AA rings were collected. Two square samples of 15 x 15 mm were taken from each ring from the inner and outer curvature of the AA. Each sample was subjected to equi-biaxial tensile testing in Krebs-Ringer solution maintained at 37 degrees C. Each test consisted of 8 cycles of preconditioning followed by one experimental run from 0% to 30% strain. Phenylephrine (10(-5) M) was added to contract VSMCs. After biaxial testing, samples were paraffin-embedded and stained with hematoxylin-phloxine-saffron (HPS) to quantify VSMC density.

RESULTS

Significant differences in cell density, maximum contractile stress resultant magnitude (MCSRM) and orientation (theta(MCSR)) were found between the inner and outer curvature. The inner curvature had the greatest contraction. The outer curvature had the highest VSMC density with the maximum contraction stress resultant oriented towards the axial direction.

CONCLUSION

VSMC activation with phenylephrine had a significant effect on the stiffness of the pig AA. This effect was independent of location and direction. However, cell orientation, density and contractile properties were dependent on location and suggest variations in the remodeling capabilities, tissue strain and cell phenotype between locations.

Sent to destroy: the ubiquitin proteasome system regulates cell signaling and protein quality control in cardiovascular development and disease

The ubiquitin proteasome system (UPS) plays a crucial role in biological processes integral to the development of the cardiovascular system and cardiovascular diseases. The UPS prototypically recognizes specific protein substrates and places polyubiquitin chains on them for subsequent destruction by the proteasome. This system is in place to degrade not only misfolded and damaged proteins, but is essential also in regulating a host of cell signaling pathways involved in proliferation, adaptation to stress, regulation of cell size, and cell death. During the development of the cardiovascular system, the UPS regulates cell signaling by modifying transcription factors, receptors, and structural proteins. Later, in the event of cardiovascular diseases as diverse as atherosclerosis, cardiac hypertrophy, and ischemia/reperfusion injury, ubiquitin ligases and the proteasome are implicated in protecting and exacerbating clinical outcomes. However, when misfolded and damaged proteins are ubiquitinated by the UPS, their destruction by the proteasome is not always possible because of their aggregated confirmations. Recent studies have discovered how these ubiquitinated misfolded proteins can be destroyed by alternative "specific" mechanisms. The cytosolic receptors p62, NBR, and histone deacetylase 6 recognize aggregated ubiquitinated proteins and target them for autophagy in the process of "selective autophagy." Even the ubiquitination of multiple proteins within whole organelles that drive the more general macro-autophagy may be due, in part, to similar ubiquitin-driven mechanisms. In summary, the crosstalk between the UPS and autophagy highlight the pivotal and diverse roles the UPS plays in maintaining protein quality control and regulating cardiovascular development and disease.