β1 integrin has been shown to contribute to vascular smooth muscle cell differentiation, adhesion and mechanosensation in vitro. Here we showed that deletion of β1 integrin at the onset of smooth muscle differentiation resulted in interrupted aortic arch, aneurysms and failure to assemble extracellular matrix proteins. These defects result in lethality prior to birth. Our data indicates that β1 integrin is not required for the acquisition, but it is essential for the maintenance of the smooth muscle cell phenotype, as levels of critical smooth muscle proteins are gradually reduced in mutant mice. Furthermore, while deposition of extracellular matrix was not affected, its structure was disrupted. Interestingly, defects in extracellular matrix and vascular wall assembly, were restricted to the aortic arch and its branches, compromising the brachiocephalic and carotid arteries and to the exclusion of the descending aorta. Additional analysis of β1 integrin in the pharyngeal arch smooth muscle progenitors was performed using wnt1Cre. Neural crest cells deleted for β1 integrin were able to migrate to the pharyngeal arches and associate with endothelial lined arteries; but exhibited vascular remodeling defects and early lethality. This work demonstrates that β1 integrin is dispensable for migration and initiation of the smooth muscle differentiation program, however, it is essential for remodeling of the pharyngeal arch arteries and for the assembly of the vessel wall of their derivatives. It further establishes a critical role of β1 integrin in the protection against aneurysms that is particularly confined to the ascending aorta and its branches.

Mechanical forces such as fluid shear have been shown to enhance cell growth and differentiation, but knowledge of their mechanistic effect on cells is limited because the local flow patterns and associated metrics are not precisely known. Here we present real-time, noninvasive measures of local hydrodynamics in 3D biomaterials based on nuclear magnetic resonance. Microflow maps were further used to derive pressure, shear and fluid permeability fields. Finally, remodeling of collagen gels in response to precise fluid flow parameters was correlated with structural changes. It is anticipated that accurate flow maps within 3D matrices will be a critical step towards understanding cell behavior in response to controlled flow dynamics.

Attention has recently focused on preventing arrhythmias by controlling sarcoplasmic reticulum (SR) Ca 2þ ''leak''. Increased leak in ventricular myocytes is associated with regenerative Ca 2þ waves and delayed afterdepolarizations, leading to arrhythmias. Studies that have measured SR Ca 2þ leak have not examined changes in [Ca 2þ ] SR independent of changes in [Ca 2þ ] i , causing a degree of uncertainty as to which factor plays a greater role. Our current work explores the possibility that changes in [Ca 2þ ] i have a greater effect on leak than changes in [Ca 2þ ] SR . In quiescent rat ventricular myocytes, we recorded steady-state Ca 2þ levels, then blocked the ryanodine receptors (RyRs) with a saturating concentration of tetracaine. Using the calcium indicator fluo-3, we recorded changes in [Ca 2þ ] i using a confocal microscope and analyzed the data using leak calculations that took into account underlying assumptions about cytosolic and SR buffers. When extracellular Ca 2þ ([Ca 2þ ] e ) was increased from 0.5 mM to 1.0 mM at rest, leak increased 37% (951.019 vs. 12.351.108 mM/s), [Ca 2þ ] i increased 6.6% (98.9 50.09 vs. 105.453.2 nM, and [Ca 2þ ] SR decreased 5.2% (489521 vs. 464523 mM). We also compared leak in resting cells versus leak in the same cells immediately after pacing for 10 s at 1 Hz. At 1 mM [Ca 2þ ] e , pacing increased leak by 17.9% (12.351.108 vs. 14.558.8 mM/s), increased [Ca 2þ ] i by 9.4% (105.453.2 vs. 115.355.7 nM), but increased [Ca 2þ ] SR by only 1.0% (463.7522.7 vs. 468.2526.8 mM). Qualitatively similar results were obtained after pacing in 0.5 mmol [Ca 2þ ] e. These results suggest that [Ca] i plays a larger role in determining diastolic SR Ca 2þ leak than [Ca] SR .

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