Peak ependymal cell stretch overlaps with the onset locations of periventricular white matter lesions

Deep and periventricular white matter hyperintensities (dWMH/pvWMH) are bright appearing white matter tissue lesions in T2-weighted fluid attenuated inversion recovery magnetic resonance images and are frequent observations in the aging human brain. While early stages of these white matter lesions are only weakly associated with cognitive impairment, their progressive growth is a strong indicator for long-term functional decline. DWMHs are typically associated with vascular degeneration in diffuse white matter locations; for pvWMHs, however, no unifying theory exists to explain their consistent onset around the horns of the lateral ventricles. We use patient imaging data to create anatomically accurate finite element models of the lateral ventricles, white and gray matter, and cerebrospinal fluid, as well as to reconstruct their WMH volumes. We simulated the mechanical loading of the ependymal cells forming the primary brain-fluid interface, the ventricular wall, and its surrounding tissues at peak ventricular pressure during the hemodynamic cycle. We observe that both the maximum principal tissue strain and the largest ependymal cell stretch consistently localize in the anterior and posterior horns. Our simulations show that ependymal cells experience a loading state that causes the ventricular wall to be stretched thin. Moreover, we show that maximum wall loading coincides with the pvWMH locations observed in our patient scans. These results warrant further analysis of white matter pathology in the periventricular zone that includes a mechanics-driven deterioration model for the ventricular wall.

Abstract P413: Stretch-induced Caveolae-mediated Disruption Of Cyclic AMP Microdomains Leads To Arrhythmogenic Ca 2+ Mishandling In Mouse Atrial Myocytes

Background: Atrial fibrillation (AF) often occurs during hypertension and is associated with an increase in cardiomyocyte stretch. Mechanism of ectopic beats, that trigger AF, has been linked to Ca 2+ mishandling and leaky hyperphosphorylated ryanodine receptors (RyRs), while the underlying mechanisms remain elusive. Caveolae membrane structures are involved in cell mechanosensing processes and control the cAMP signaling pathway. We hypothesized that mechanical stretch disrupts caveolae, promoting cAMP production and sarcoplasmic reticulum Ca 2+ leak via augmentation of RyRs phosphorylation. Methods and Results: Cell size analysis and Ca 2+ dynamics measurements were performed by confocal imaging of isolated mouse atrial myocytes. Cell stretch was modeled by hypoosmotic swelling (from 310 mOsM to 220 mOsM to flatten caveolae structures) resulting in a ~30% increase in cell width (p<0.05) with no changes in cell length. Swelling resulted in a biphasic effect on Ca 2+ spark activity: a fast (<10 min of exposure) ~50% increase (p<0.001) followed by a slow decrease to the level observed in isotonic conditions (>30 min of exposure). Similarly, caveolae disruption via cholesterol depletion by 10 mM methyl-β-cyclodextrin (MβCD) led to 2-fold increase in Ca 2+ sparks frequency (p<0.001). Swelling- and MβCD-induced increases in atrial Ca 2+ spark activity were prevented via inhibition of cAMP production by adenylyl cyclases by 0.1mM SQ22536 or cAMP-dependent protein kinase A (PKA) by 1μM H-89. Then, we tested if this mechanism is present in atrial myocytes from pressure-overloaded (4-weeks transaortic constriction, TAC) mice. Atrial myocytes from TAC mice showed a 1.6 times higher Ca 2+ sparks frequency than wild-type myocytes (p<0.01), which was significantly reduced (p<0.01) to wild-type level after incubation with SQ22536. Conclusions: Our findings suggest that cell stretch increases spontaneous Ca 2+ spark activity through the disruption of caveolae and cAMP-mediated augmentation of PKA activity. This mechanism could be involved in the Ca 2+ mishandling and AF in pressure overloaded hearts.

Peak ependymal cell stretch overlaps with the onset location of periventricular white matter lesions

Deep and periventricular white matter hyperintensities (dWMH/pvWMH) are bright appearing white matter tissue lesions in T2-weighted fluid attenuated inversion recovery magnetic resonance images and are frequent observations in the aging human brain. While early stages of these white matter lesions are only weakly associated with cognitive impairment, their progressive growth is a strong indicator for long-term functional decline. DWMHs are typically associated with vascular degeneration in diffuse white matter locations; for pvWMHs, however, no unifying theory exists to explain their consistent onset around the horns of the lateral ventricles. We use patient imaging data to create anatomically accurate finite element models of the lateral ventricles, white and gray matter, and cerebrospinal fluid, as well as to reconstruct their WMH volumes. We simulated the mechanical loading of the ependymal cells forming the primary brain-fluid interface, the ventricular wall, and its surrounding tissues at peak ventricular pressure during the hemodynamic cycle. We observe that both the maximum principal tissue strain and the largest ependymal cell stretch consistently localize in the anterior and posterior horns. Our simulations show that ependymal cells experience a loading state that causes the ventricular wall to be stretched thin. Moreover, we show that maximum wall loading coincides with the pvWMH locations observed in our patient scans. These results warrant further analysis of white matter pathology in the periventricular zone that includes a mechanics-driven deterioration model for the ventricular wall.

A mechanosensing mechanism mediated by IRSp53 controls plasma membrane shape homeostasis at the nanoscale

As cells migrate and experience forces from their surroundings, they constantly undergo mechanical deformations which reshape their plasma membrane (PM). To maintain homeostasis, cells need to detect and restore such changes, not only in terms of overall PM area and tension as previously described, but also in terms of local, nano-scale topography. Here we describe a novel phenomenon, by which cells sense and restore mechanically induced PM nano-scale deformations. We show that cell stretch and subsequent compression reshape the PM in a way that generates local membrane evaginations in the 100 nm scale. These evaginations are recognized by the I-BAR protein IRSp53, which triggers a burst of actin polymerization mediated by Rac1 and Arp2/3. The actin polymerization burst subsequently re-flattens the evagination, completing the mechanochemical feedback loop. Our results demonstrate a new mechanosensing mechanism for PM shape homeostasis, with potential applicability in different physiological scenarios.

Force-induced recruitment of cten along keratin network in epithelial cells

The cytoskeleton provides structural integrity to cells and serves as a key component in mechanotransduction. Tensins are thought to provide a force-bearing linkage between integrins and the actin cytoskeleton; yet, direct evidence of tensin’s role in mechanotransduction is lacking. We here report that local force application to epithelial cells using a micrometer-sized needle leads to rapid accumulation of cten (tensin 4), but not tensin 1, along a fibrous intracellular network. Surprisingly, cten-positive fibers are not actin fibers; instead, these fibers are keratin intermediate filaments. The dissociation of cten from tension-free keratin fibers depends on the duration of cell stretch, demonstrating that the external force favors maturation of cten−keratin network interactions over time and that keratin fibers retain remarkable structural memory of a cell’s force-bearing state. These results establish the keratin network as an integral part of force-sensing elements recruiting distinct proteins like cten and suggest the existence of a mechanotransduction pathway via keratin network.

A Microfluidic Cell Stretch Device to Investigate the Effects of Stretching Stress on Artery Smooth Muscle Cell Proliferation in Pulmonary Arterial Hypertension

A microfluidic cell stretch device was developed to investigate the effects of stretching stress on pulmonary artery smooth muscle cell (PASMC) proliferation in pulmonary arterial hypertension (PAH). The microfluidic device harbors upper cell culture and lower control channels, separated by a stretchable poly(dimethylsiloxane) membrane that acts as a cell culture substrate. The lower channel inlet was connected to a vacuum pump via a digital switch-controlled solenoid valve. For cyclic stretch at heartbeat frequency (80 bpm), the open or close time for each valve was set to 0.38 s. Proliferation of normal PASMCs and those obtained from patients was enhanced by the circumferential stretching stimulation. This is the first report showing patient cells increased in number by stretching stress. These results are consistent with the abnormal proliferation observed in PAH. Circumferential stretch stress was applied to the cells without increasing the pressure inside the microchannel. Our data may suggest that the stretch stress itself promotes cell proliferation in PAH.

A REVIEW OF TWO-DIMENSIONAL IN VITRO CELL STRETCH PLATFORMS

The focus of this paper is to describe the mechanism and behavior of two-dimensional in vitro cell stretch platforms, as well as discussing designs for the evaluation of mechanical properties of cells. It is extremely important to understand the cellular response to extrinsic mechanical forces as living biological system is constantly subjected to mechanical forces in vivo. In addition, this mechanistic understanding of cellular response will provide valuable information towards the design and fabrication of bioengineered tissues and organs, which are expected to replace and/or aid bodily functions. This paper will primarily focus on the development, advantages and limitations of two-dimensional cell stretch platforms.