Persistent Cell Motion in the Absence of External Signals: A Search Strategy for Eukaryotic Cells

Eukaryotic cells are large enough to detect signals and then orient to them by differentiating the signal strength across the length and breadth of the cell. Amoebae, fibroblasts, neutrophils and growth cones all behave in this way. Little is known however about cell motion and searching behavior in the absence of a signal. Is individual cell motion best characterized as a random walk? Do individual cells have a search strategy when they are beyond the range of the signal they would otherwise move toward? Here we ask if single, isolated, Dictyostelium and Polysphondylium amoebae bias their motion in the absence of external cues. We placed single well-isolated Dictyostelium and Polysphondylium cells on a nutrient-free agar surface and followed them at 10 sec intervals for ~10 hr, then analyzed their motion with respect to velocity, turning angle, persistence length, and persistence time, comparing the results to the expectation for a variety of different types of random motion. We find that amoeboid behavior is well described by a special kind of random motion: Amoebae show a long persistence time (~10 min) beyond which they start to lose their direction; they move forward in a zig-zag manner; and they make turns every 1-2 min on average. They bias their motion by remembering the last turn and turning away from it. Interpreting the motion as consisting of runs and turns, the duration of a run and the amplitude of a turn are both found to be exponentially distributed. We show that this behavior greatly improves their chances of finding a target relative to performing a random walk. We believe that other eukaryotic cells may employ a strategy similar to Dictyostelium when seeking conditions or signal sources not yet within range of their detection system.Comment: 15 pages, 11 figures, accepted for publication in PLOS On

Observation of an Exotic S=+1S=+1 Baryon in Exclusive Photoproduction from the Deuteron

In an exclusive measurement of the reaction $\gamma d \to K^+ K^- p n$, a narrow peak that can be attributed to an exotic baryon with strangeness $S=+1$ is seen in the $K^+n$ invariant mass spectrum. The peak is at $1.542\pm 0.005$ GeV/c$^2$ with a measured width of 0.021 GeV/c$^2$ FWHM, which is largely determined by experimental mass resolution. The statistical significance of the peak is $5.2 \pm 0.6 \sigma$. The mass and width of the observed peak are consistent with recent reports of a narrow $S=+1$ baryon by other experimental groups.Comment: 5 pages, 5 figure

Onset of asymptotic scaling in deuteron photodisintegration

We investigate the transition from the nucleon-meson to quark-gluon description of the strong interaction using the photon energy dependence of the $d(\gamma,p)n$ differential cross section for photon energies above 0.5 GeV and center-of-mass proton angles between $30^{\circ}$ and $150^{\circ}$. A possible signature for this transition is the onset of cross section $s^{-11}$ scaling with the total energy squared, $s$, at some proton transverse momentum, $P_T$. The results show that the scaling has been reached for proton transverse momentum above about 1.1 GeV/c. This may indicate that the quark-gluon regime is reached above this momentum.Comment: Accepted by PRL; 5 pages, 2 figure

Serum HIV-1 RNA levels and time to development of AIDS in the multicenter hemophilia cohort study

Objective.-To determine if the long-term incidence of the acquired immunodeficiency syndrome (AIDS) is related to human immunodeficiency virus type 1 (HIV-1) RNA levels measured early in HIV-1 infection. Design.-Epidemiologic cohort study. Setting.-Five hemophilia treatment centers in the United States. Subjects.-A total of 165 subjects with hemophilia and HIV-1 infection (age at HIV-1 seroconversion, 1-66 years) followed from 1979 to 1995. Methods.-The HIV-1 RNA level was measured by polymerase chain reaction over a range of 200 to 1 million or more HIV-1 RNA copies/mL. in archived serum specimens collected 12 to 36 months (median, 27 months) after the estimated date of HIV-1 seroconversion. Kaplan-Meier methods were used to examine the risk of AIDS and proportional hazards models were used to estimate relative hazards. Results.-The HIV-1 RNA values were similar in subjects younger than 17 years at seroconversion (median, 5214 copies/mL) and those 18 to 34 years old (median, 4693 copies/mL), but higher in those 35 years or older (median, 12 069 copies/mL) (P=.02 compared with each younger group). At 10 years after seroconversion, the proportions of subjects with AIDS were 72% among subjects with 100 000 or more HIV-1 RNA copies/mL measured 12 to 36 months after HIV-1 seroconversion (n=9), 52% among subjects with 10 000 to 99 999 copies/mL (n=55), 22% among subjects with 1000 to 9999 copies/mL (n=82), and 0% among subjects with fewer than 1000 copies/mL (n=19) (P Conclusions.-The HIV-1 RNA level during early chronic HIV-1 infection is a strong, age-independent predictor of clinical outcome; low revels define persons with a high probability of long-term AIDS-free survival

Serum HIV-1 RNA levels and time to development of AIDS in the multicenter hemophilia cohort study

Objective.-To determine if the long-term incidence of the acquired immunodeficiency syndrome (AIDS) is related to human immunodeficiency virus type 1 (HIV-1) RNA levels measured early in HIV-1 infection. Design.-Epidemiologic cohort study. Setting.-Five hemophilia treatment centers in the United States. Subjects.-A total of 165 subjects with hemophilia and HIV-1 infection (age at HIV-1 seroconversion, 1-66 years) followed from 1979 to 1995. Methods.-The HIV-1 RNA level was measured by polymerase chain reaction over a range of 200 to 1 million or more HIV-1 RNA copies/mL. in archived serum specimens collected 12 to 36 months (median, 27 months) after the estimated date of HIV-1 seroconversion. Kaplan-Meier methods were used to examine the risk of AIDS and proportional hazards models were used to estimate relative hazards. Results.-The HIV-1 RNA values were similar in subjects younger than 17 years at seroconversion (median, 5214 copies/mL) and those 18 to 34 years old (median, 4693 copies/mL), but higher in those 35 years or older (median, 12 069 copies/mL) (P=.02 compared with each younger group). At 10 years after seroconversion, the proportions of subjects with AIDS were 72% among subjects with 100 000 or more HIV-1 RNA copies/mL measured 12 to 36 months after HIV-1 seroconversion (n=9), 52% among subjects with 10 000 to 99 999 copies/mL (n=55), 22% among subjects with 1000 to 9999 copies/mL (n=82), and 0% among subjects with fewer than 1000 copies/mL (n=19) (P<.001). The age-adjusted relative hazard for AIDS for subjects with 10 000 or more copies/mL was 14.3 (95% confidence interval, 1.9-105.6) compared with subjects with fewer than 1000 copies/mL. Conclusions.-The HIV-1 RNA level during early chronic HIV-1 infection is a strong, age-independent predictor of clinical outcome; low revels define persons with a high probability of long-term AIDS-free survival