ALBERT

Description: Important processes of living cells, including intracellular transport, cell crawling, contraction, division, and mechanochemical signal transduction, are controlled by cytoskeletal (CSK) dynamics. CSK dynamics can be measured by tracking the motion of CSK-bound particles. Particle motion has been reported to follow a superdiffusive behavior that is believed to arise from ATP-driven intracellular stress fluctuations generated by polymerization processes and motor proteins. The power spectrum of intracellular stress fluctuations has been suggested to decay with 1/2 (Lau et al, Phys Rev Lett 91:198101). Here we report direct measurements of cellular force fluctuations that are transmitted to the extracellular matrix, and compared them with the spontaneous motion of CSK-bound beads. Fibronectin coated fluorescent beads (Ø 1 m) were bound to the CSK of confluent human vascular endothelial cells. Forces transmitted to the extracellular matrix (ECM) were quantified by plating these cells onto a collagen coated elastic polyacrylamide hydrogel, and measuring the gel deformation from the displacement of embedded fluorescent beads (Ø 0.5 m). Bead motion of both CSK-bound and ECM-bound beads were measured with nanometer-resolution and expressed as mean square displacement (MSD). The MSD of both CSK-bound and ECM-bound beads displayed a superdiffusive behavior that was well described by a power law: MSD = a*t^b. Surprisingly, we found an identical power law exponent for both CSK-bound and ECM-bound beads of b = 1.6. This finding suggests that the spontaneous motion of CSK-bound beads is driven by stress fluctuations with a 1/ b+1 power spectrum. This result is consistent with the notion that CSK dynamics and CSK stress fluctuations are closely coupled.

Description: Animals reduce their exposure to predation often at the cost of nutritional inputs. A variety of predator avoidance strategies exist, but species generally exist along a spectrum with some primarily utilizing behavioral responses to avoid predation and others relying more on morphological defenses to deter predation. North American porcupines ( Erethizon dorsatum ) possess a well-developed predator deterrent strategy (i.e., quills). During the winter, porcupines enter a nutritional bottleneck where they principally rely on endogenous stores to survive winter. We hypothesized that porcupine behavior in winter would, then, be primarily driven by nutritional demands rather than predation risk. To test this trade-off between perceived risk of predation and nutritional stress, we analyzed the length and sinuosity of movement patterns, relative to nutritional condition (urinary urea: creatinine), snow conditions (depth, density), and a risk landscape (predation risk and refuges). As predicted, the sinuosity of movement paths was unrelated to any of our variables, but we detected a strong correlation between lengths of movements with risk landscape. Specifically, the length of porcupine movement declined as the average risk of predation increased, and the presence of refuge trees in their movement path was positively related to path length. Thus, even for animals possessing extreme morphological defenses effective at deterring predation, and during a period of nutritional stress, movement and behavior are nevertheless driven by perceived predation risk.