Calcium participates in feedback regulation of the oscillating ROP1 Rho GTPase in pollen tubes.

Biological oscillation occurs at various levels, from cellular signaling to organismal behaviors. Mathematical modeling has allowed a quantitative understanding of slow oscillators requiring changes in gene expression (e.g., circadian rhythms), but few theoretical studies have focused on the rapid oscillation of cellular signaling. The tobacco pollen tube, which exhibits growth bursts every 80 s or so, is an excellent system for investigating signaling oscillation. Pollen tube growth is controlled by a tip-localized ROP1 GTPase, whose activity oscillates in a phase about 90 degrees ahead of growth. We constructed a mathematical model of ROP1 activity oscillation consisting of interlinking positive and negative feedback loops involving F-actin and calcium, ROP1-signaling targets that oscillate in a phase about 20 degrees and 110 degrees behind ROP1 activity, respectively. The model simulates the observed changes in ROP1 activity caused by F-actin disruption and predicts a role for calcium in the negative feedback regulation of the ROP1 activity. Our experimental data strongly support this role of calcium in tip growth. Thus, our findings provide insight into the mechanism of pollen tube growth and the oscillation of cellular signaling.

Exocytosis-coordinated mechanisms for tip growth underlie pollen tube growth guidance.

Many tip-growing cells are capable of responding to guidance cues, during which cells precisely steer their growth toward the source of guidance signals. Though several players in signal perception have been identified, little is known about the downstream signaling that controls growth direction during guidance. Here, using combined modeling and experimental studies, we demonstrate that the growth guidance of Arabidopsis pollen tubes is regulated by the signaling network that controls tip growth. Tip-localized exocytosis plays a key role in this network by integrating guidance signals with the ROP1 Rho GTPase signaling and coordinating intracellular signaling with cell wall mechanics. This model reproduces the high robustness and responsiveness of pollen tube guidance and explains the connection between guidance efficiency and the parameters of the tip growth system. Hence, our findings establish an exocytosis-coordinated mechanism underlying the cellular pathfinding guided by signal gradients and the mechanistic linkage between tip growth and guidance.

Vascular stents with rationally-designed surface patterning.

Herein, we discuss our recent progress towards realization of next-generation vascular stents that seek to mitigate adverse physiological responses to stenting via rational design of stent surface topography at the nanoscale. Specifically, we will discuss advances in patterning of deep sub-micrometer scale features in titanium (Ti) substrates, creation of cylindrical stents from micromachined planar Ti substrates, and integration of these processes to produce devices that will eventually allow evaluation of rationally-designed nanopatterning in physiologically-relevant contexts. We will also discuss results from mechanical testing and finite element modeling of these devices to assess their mechanical performance. These efforts represent key steps towards our long-term goal of developing a new paradigm for stents in which rationally-designed surface nanopatterning provides a physical means for complementing, or replacing, current pharmacological interventions.