Effects of a double mutation in the heparin binding pocket on hFGF-1

The human acidic fibroblast growth factor (hFGF-1) is a protein that plays an important role in many body processes such as angiogenesis and tissue repair. As such, it has great potential for therapeutic application. However, hFGF-1 is relatively unstable in the body with a significant fraction being denatured at body temperature and pH. It contains a group of positively charged amino acids in close proximity that create an electrostatic strain on the molecule and can lead to denaturation and increased susceptibility to enzymatic digestion. Upon its release from the cell, hFGF-1 binds to the glycosaminoglycan heparin in the extracellular matrix. Heparin stabilizes hFGF-1 by binding to this group of positively charged amino acids, known as the heparin binding pocket. Heparin also mediates the binding of hFGF-1 to cell surface FGF Receptors (FGFRs). When activated by hFGF-1, these receptors propagate a signal downstream to initiate various pathways inside the cell. This study examines a mutation of hFGF-1 in which two negative charges are introduced into the heparin binding pocket. The introduction of these charges decreased the binding affinity of hFGF-1 for heparin by about 3-fold. This mutation also showed similar bioactivity compared to wild type hFGF-1 (wt-hFGF-1) both in the presence and absence of heparin. Overall, these results suggest that heparin binding affinity is not positively correlated with mitogenic activity. Additionally, it was shown that heparin binding is not necessary for activation of the tyrosine kinase cell surface receptor

Submillimetre-sized dust aggregate collision and growth properties

The collisional and sticking properties of sub-mm-sized aggregates composed of protoplanetary dust analogue material are measured, including the statistical threshold velocity between sticking and bouncing, their surface energy and tensile strength within aggregate clusters. We performed an experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue materials were micrometre-sized monodisperse and polydisperse SiO2 particles prepared into aggregates with sizes around 120 $\mu$m and 330 $\mu$m, respectively and volume filling factors around 0.37. During the experimental run of 150 s under reduced gravity conditions, the sticking of aggregates and the formation and fragmentation of clusters of up to a few millimetres in size was observed. The sticking probability of the sub-mm-sized dust aggregates could be derived for velocities decreasing from 22 to 3 cm/s. The transition from bouncing to sticking collisions happened at 12.7 cm/s for the smaller aggregates composed of monodisperse particles and at 11.5 and 11.7 cm/s for the larger aggregates composed of mono- and polydisperse dust particles, respectively. Using the pull-off force of sub-mm-sized dust aggregates from the clusters, the surface energy of the aggregates composed of monodisperse dust was derived to be 1.6x10-5 J/m2, which can be scaled down to 1.7x10-2 J/m2 for the micrometre-sized monomer particles and is in good agreement with previous measurements for silica particles. The tensile strengths of these aggregates within the clusters were derived to be 1.9 Pa and 1.6 Pa for the small and large dust aggregates, respectively. These values are in good agreement with recent tensile strength measurements for mm-sized silica aggregates. Using our data on the sticking-bouncing threshold, estimates of the maximum aggregate size can be given. For a minimum mass solar nebula model, aggregates can reach sizes of 1 cm.Comment: 21 pages (incl. 6 pages of appendix), 23 figure

The Suborbital Particle Aggregation and Collision Experiment (SPACE): Studying the Collision Behavior of Submillimeter-Sized Dust Aggregates on the Suborbital Rocket Flight REXUS 12

The Suborbital Particle Aggregation and Collision Experiment (SPACE) is a novel approach to study the collision properties of submillimeter-sized, highly porous dust aggregates. The experiment was designed, built and carried out to increase our knowledge about the processes dominating the first phase of planet formation. During this phase, the growth of planetary precursors occurs by agglomeration of micrometer-sized dust grains into aggregates of at least millimeters to centimeters in size. However, the formation of larger bodies from the so-formed building blocks is not yet fully understood. Recent numerical models on dust growth lack a particular support by experimental studies in the size range of submillimeters, because these particles are predicted to collide at very gentle relative velocities of below 1 cm/s that can only be achieved in a reduced-gravity environment. The SPACE experiment investigates the collision behavior of an ensemble of silicate-dust aggregates inside several evacuated glass containers which are being agitated by a shaker to induce the desired collisions at chosen velocities. The dust aggregates are being observed by a high-speed camera, allowing for the determination of the collision properties of the protoplanetary dust analog material. The data obtained from the suborbital flight with the REXUS (Rocket Experiments for University Students) 12 rocket will be directly implemented into a state-of-the-art dust growth and collision model

Low-velocity collision behaviour of clusters composed of sub-mm sized dust aggregates

The experiments presented aim to measure the outcome of collisions between sub-mm sized protoplanetary dust aggregate analogues. We also observed the clusters formed from these aggregates and their collision behaviour. The experiments were performed at the drop tower in Bremen. The protoplanetary dust analogue materials were micrometre-sized monodisperse and polydisperse SiO$_2$ particles prepared into aggregates with sizes between 120~$\mu$m and 250~$\mu$m. One of the dust samples contained aggregates that were previously compacted through repeated bouncing. During three flights of 9~s of microgravity each, individual collisions between aggregates and the formation of clusters of up to a few millimetres in size were observed. In addition, the collisions of clusters with the experiment cell walls leading to compaction or fragmentation were recorded. We observed collisions amongst dust aggregates and collisions between dust clusters and the cell aluminium walls at speeds ranging from about 0.1 cm/s to 20 cm/s. The velocities at which sticking occurred ranged from 0.18 to 5.0 cm/s for aggregates composed of monodisperse dust, with an average value of 2.1 cm/s for reduced masses ranging from 1.2x10-6 to 1.8x10-3 g with an average value of 2.2x10-4 g. From the restructuring and fragmentation of clusters composed of dust aggregates colliding with the aluminium cell walls, we derived a collision recipe for dust aggregates ($\sim$100 $\mu$m) following the model of Dominik \& Thielens (1997) developed for microscopic particles. We measured a critical rolling energy of 1.8x10-13 J and a critical breaking energy of 3.5x10-13 J for 100 $\mu$m-sized non-compacted aggregates.Comment: 12 pages, 13 figure

Erforschung des Übergangs von Haftung zu Abprallen bei Kollisionen von millimetergroßen Staubaggregaten

In the past years, systematic star surveys with dedicated instruments such as the Kepler spacecraft or the HARPS spectrometer (High Accuracy Radial Velocity Searcher) have revealed the existence of thousands of planets outside of our solar system. These findings have renewed the interest in the formation process of planets. At the same time, they provide a challenge to researchers in providing a multitude of possible orbital configurations that models must be able to explain. In the early phases of planet formation, growth is governed by intermolecular forces and not yet by gravity. Starting at micrometer sizes, dust particles collide with each other and initially stick together due to the van der Waals force to form larger aggregates. Laboratory experiments are an important means of exploring the outcome of such collisions. The results are then used as input for growth simulations which follow the evolution of the dust particles. In one such simulation, Zsom et al. (2010) found the transition from sticking to bouncing to be important for the maximum size the dust aggregates can grow to. In this work, an experimental setup suitable for generating collisions between millimeter-sized dust aggregates at velocities of millimeters to centimeters per second under microgravity conditions is presented. This is the velocity range where the sticking-bouncing transition is expected to occur. Additionally, a numerical model for these collisions is developed and used to predict the outcome of collisions for aggregates of other sizes. Furthermore, a comparison between an analysis of the experimental results using two- and three-dimensional data is made. It shows that for velocity evaluation 2D data from just one camera perspective is sufficient, while accurate values for the coefficient of restitution and the impact parameter require 3D input. The experimental results show the transition zone between sticking and bouncing to be much broader than assumed previously.In den vergangenen Jahren wurden bei der systematischen Untersuchung von Sternen mit speziellen Instrumenten wie dem Kepler-Weltraumteleskop oder dem HARPS-Spektrometer (High Accuracy Radial Velocity Searcher) tausende Planeten außerhalb unseres Sonnensystems entdeckt. Während diese Entdeckungen das Interesse an den genauen Prozessen die zur Entstehung von Planeten führen erneuert haben, stellen sie gleichermaßen Forscher vor die Herausforderung, dass ihre Modelle eine Vielzahl an möglichen Bahnparametern der so entstandenen Planetensysteme erklären können müssen. In der Frühphase der Planetenentstehung wird das Wachstum von Teilchen nicht durch Gravitation, sondern durch intermolekulare Kräfte bestimmt. Anfänglich etwa mikrometergroße Staubteilchen kollidieren miteinander und haften aufgrund von Van-der-Waals-Kräften aneinander. Laborexperimente dienen dazu, die Resultate solcher Kollisionen zu untersuchen und zu klassifizieren. Daraus kann dann ein Kollisionsmodell erstellt werden, auf Grundlage dessen Wachstumssimulationen durchgeführt werden können, die die Entwicklung der Staubteilchen vorhersagen. Zsom et al. (2010) haben ein solches Modell entwickelt und dabei herausgefunden, dass es für die maximale Größe zu denen die Staubaggregate wachsen können, von entscheidender Bedeutung ist unter welchen Voraussetzungen diese in Kollisionen aneinander haften bleiben oder voneinander abprallen. In dieser Arbeit wird ein Versuchsaufbau vorgestellt, mit dem sich Kollisionen von millimetergroßen Staubaggregaten bei Geschwindigkeiten von Millimetern bis Zentimetern pro Sekunde unter Schwerelosigkeitsbedingungen untersuchen lassen. In diesem Geschwindigkeitsbereich wird der Übergang von Haftung zu Abprallen vermutet. Weiterhin wird ein Kollisionsmodell basierend auf den Ergebnissen der Kollisionen millimetergroßer Staubagglomerate entwickelt und geprüft, ob Vorraussagen für Kollisionen von Aggregaten anderer Größe getroffen werden können. Darüber hinaus werden die Ergebnisse des Experiments einmal basierend auf zwei- und einmal basierend auf dreidimensionalen Daten ausgewertet. Ein Vergleich zeigt, dass es für die Ermittlung von Kollisionsgeschwindigkeiten ausreicht, 2D-Daten zu benutzen, während für den Restitutionskoeffizienten und den Impaktparameter 3D-Informationen benötigt werden. Die Ergebnisse der Experimente deuten darauf hin, dass dieser Übergangsbereich von haftenden zu abprallenden Stößen deutlich breiter ist als bisher angenommen