Cortical Bending Research Articles

Romosozumab rescues impaired bone mass and strength in a murine model of diabetic kidney disease

As international incidence of diabetes and diabetes-driven comorbidities such as chronic kidney disease (CKD) continue to climb, interventions are needed that address the high-risk skeletal fragility of what is a complex disease state. Romosozumab (Romo) is an FDA-approved sclerostin inhibitor that has been shown to increase bone mineral density and decrease fracture rates in osteoporotic patients with mild to severe CKD, but its effect on diabetes-weakened bone is unknown. We aimed to test Romo's performance in a model of combined diabetes and CKD. 6-week old male C57BL/6 mice were randomly divided into control (CON) and disease model (STZ-Ad) groups, using a previously established streptozotocin- and adenine-diet-induced model. After 16 weeks of disease induction, both CON and STZ-Ad groups were subdivided into two treatment groups and given weekly subcutaneous injections of 100 μL vehicle (phosphorus buffered saline, PBS) or 10 mg/kg Romo. Mice were euthanized after 4 weeks of treatment via cardiac exsanguination and cervical dislocation. Hindlimb bones and L4 vertebrae were cleaned of soft tissue, wrapped in PBS-soaked gauze and stored at 20C. Right tibiae, femora, and L4s were scanned via microcomputed tomography; tibiae were then tested to failure in 4-pt bending while L4s were compression tested. Romo treatment significantly increased cortical and trabecular bone mass in both STZ-Ad and CON animals. These morphological improvements created corresponding increases in cortical bending strength and trabecular compression strength, with STZ-Ad treated mice surpassing vehicle CON mice in all trabecular mechanics measures. These results suggest that Romo retains its efficacy at increasing bone mass and strength in diabetic kidney disease.

Polarized branched Actin modulates cortical mechanics to produce unequal-size daughters during asymmetric division

The control of cell shape during cytokinesis requires a precise regulation of mechanical properties of the cell cortex. Only few studies have addressed the mechanisms underlying the robust production of unequal-sized daughters during asymmetric cell division. Here we report that unequal daughter-cell sizes resulting from asymmetric sensory organ precursor divisions in Drosophila are controlled by the relative amount of cortical branched Actin between the two cell poles. We demonstrate this by mistargeting the machinery for branched Actin dynamics using nanobodies and optogenetics. We can thereby engineer the cell shape with temporal precision and thus the daughter-cell size at different stages of cytokinesis. Most strikingly, inverting cortical Actin asymmetry causes an inversion of daughter-cell sizes. Our findings uncover the physical mechanism by which the sensory organ precursor mother cell controls relative daughter-cell size: polarized cortical Actin modulates the cortical bending rigidity to set the cell surface curvature, stabilize the division and ultimately lead to unequal daughter-cell size.

Collision of Expanding Actin Caps with Actomyosin Borders for Cortical Bending and Mitotic Rounding in a Syncytium

The early Drosophila embryo is a large syncytial cell that compartmentalizes mitotic spindles with furrows. Before furrow ingression, an Arp2/3 actin cap forms above each nucleus and is encircled by actomyosin. We investigated how these networks transform a flat cortex into a honeycomb-like compartmental array. The growing caps circularize and ingress upon meeting their actomyosin borders, which become the furrow base. Genetic perturbations indicate that the caps physically displace their borders and, reciprocally, that the borders resist and circularize their caps. These interactions create an actomyosin cortex arrayed with circular caps. The Rac-GEF Sponge, Rac-GTP, Arp3, and actin coat the caps as a growing material that can drive cortical bending for initial furrow ingression. Additionally, laser ablations indicate that actomyosin contraction squeezes the cytoplasm, producing counterforces that swell the caps. Thus, Arp2/3 caps form clearances of the actomyosin cortex and control buckling and swelling of these clearances for metaphase compartmentalization.

Optimal shapes and stresses of adherent cells on patterned substrates

We investigate a continuum mechanical model for an adherent cell on two dimensional adhesive micropatterned substrates. The cell is modeled as an isotropic and homogeneous elastic material subject to uniform internal contractile stresses. The build-up of tension from cortical actin bundles at the cell periphery is incorporated by introducing an energy cost for bending the cell boundary, resulting in a resistance to changes in the local curvature. Integrin-based adhesions are modeled as harmonic springs that pin the cell to adhesive patches of a predefined geometry. Using Monte Carlo simulations and analytical techniques we investigate the competing effects of bulk contractility and cortical bending rigidity in regulating cell shapes on non-adherent regions. We show that the crossover from convex to concave cell edges is controlled by the interplay between contractile stresses and boundary bending rigidity. In particular, the cell boundary becomes concave beyond a critical value of the contractile stress that is proportional to the cortical bending rigidity. Furthermore, the intracellular stresses are found to be largely concentrated at the concave edge of the cell. The model can be used to generate a cell-shape phase diagram for each specific adhesion geometry.

A High-Resolution Shape Fitting and Simulation Demonstrated Equatorial Cell Surface Softening during Cytokinesis and Its Promotive Role in Cytokinesis

Different models for animal cell cytokinesis posit that the stiffness of the equatorial cortex is either increased or decreased relative to the stiffness of the polar cortex. A recent work has suggested that the critical cytokinesis signaling complex centralspindlin may reduce the stiffness of the equatorial cortex by inactivating the small GTPase Rac. To determine if such a reduction occurs and if it depends on centralspindlin, we devised a method to estimate cortical bending stiffness with high spatio-temporal resolution from in vivo cell shapes. Using the early Caenorhabditis elegans embryo as a model, we show that the stiffness of the equatorial cell surface is reduced during cytokinesis, whereas the stiffness of the polar cell surface remains stiff. The equatorial reduction of stiffness was compromised in cells with a mutation in the gene encoding the ZEN-4/kinesin-6 subunit of centralspindlin. Theoretical modeling showed that the absence of the equatorial reduction of stiffness could explain the arrest of furrow ingression in the mutant. By contrast, the equatorial reduction of stiffness was sufficient to generate a cleavage furrow even without the constriction force of the contractile ring. In this regime, the contractile ring had a supportive contribution to furrow ingression. We conclude that stiffness is reduced around the equator in a centralspindlin-dependent manner. In addition, computational modeling suggests that proper regulation of stiffness could be sufficient for cleavage furrow ingression.

Continuous alendronate treatment throughout growth, maturation, and aging in the rat results in increases in bone mass and mechanical properties.

Alendronate (4-amino-1-hydroxybutylidene bisphosphonate) is a novel amino bisphosphonate that is being developed for the treatment of osteolytic bone disorders such as osteoporosis. As part of a 2-year carcinogenicity study, we investigated the morphologic and biomechanical effects of long-term alendronate (ALN) therapy, given throughout skeletal growth, maturation, and aging, on rat vertebrae and femora. Three treatment groups, receiving either deionized water, low- (1.00 mg/kg), or high-dose (3.75 mg/kg) ALN, were given daily oral treatment for 105 weeks. Results from mechanical tests indicate that ALN therapy (in males) increased the vertebral ultimate compressive load by 96% in the high- and 51% in the low-dose groups when compared with controls. ALN similarly increased the male ultimate femoral bending load by 59% in the high- and 31% in the low-dose groups. Vertebrae and femora from female rats treated with both high- and low-dose ALN also failed at significantly higher loads than controls, but no differences were seen between low- and high-dose groups. Morphologic analysis of both male and female vertebrae revealed a dose-dependent increase in area fraction of bone. Rats receiving high-dose ALN had a greater area fraction of bone than those receiving low doses. Both groups were greater than controls. Thus, the administration of ALN resulted in increased femoral cortical bending load when compared with control animals, as well as increased vertebral ultimate compressive load commensurate with a dose-related preservation of vertebral bone.(ABSTRACT TRUNCATED AT 250 WORDS)