How do microtubules guide migrating cells?

Microtubules have long been implicated in the polarization of migrating cells, but how they carry out this role is unclear. Here, we propose that microtubules determine cell polarity by modulating the pattern of adhesions that a cell develops with the underlying matrix, through focal inhibitions of contractility.     

How Do Kinetochores CLASP Dynamic Microtubules?

Maintenance of genetic stability during cell division requires binding of chromosomes to the mitotic spindle, a process that involves attachment of spindle microtubules to kinetochores. This enables chromosomes to move to the metaphase plate, to satisfy the spindle checkpoint and finally to segregate during anaphase. Recent studies on the function MAST in Drosophila and its human homologue CLASP1, have revealed that these microtubule-associated proteins play an essential role for the kinetochore-microtubule interaction. CLASP1 localizesto the plus ends of growing microtubules and to the most external kinetochore domain. Depletion of CLASP1 causes abnormal chromosome congression, collapse of the mitotic spindle and attachment of kinetochores to very short microtubules that do not show dynamic behavior. These results suggest that CLASP1 is required at kinetochores to regulate the dynamic behavior of attached microtubules.     

How do proteins fold?

This article emphasizes the importance of getting students to understand the ways in which polypeptides fold to form protein molecules with complex higher-ordered structures. Modern views on how this folding occurs in vitro and in the cell are summarized and set within an appropriate biological context.     

How do spores germinate?

Spore germination, as defined as those events that result in the loss of the spore-specific properties, is an essentially biophysical process. It occurs without any need for new macromolecular synthesis, so the apparatus required is already present in the mature dormant spore. Germination in response to specific chemical nutrients requires specific receptor proteins, located at the inner membrane of the spore. After penetrating the outer layers of spore coat and cortex, germinant interacts with its receptor: one early consequence of this binding is the movement of monovalent cations from the spore core, followed by Ca2(+) and dipicolinic acid (DPA). In some species, an ion transport protein is also required for these early stages. Early events - including loss of heat resistance, ion movements and partial rehydration of the spore core - can occur without cortex hydrolysis, although the latter is required for complete core rehydration and colony formation from a spore. In Bacillus subtilis two crucial cortex lytic enzymes have been identified: one is CwlJ, which is DPA-responsive and is located at the cortex-coat junction. The second, SleB, is present both in outer layers and at the inner spore membrane, and is more resistant to wet heat than is CwlJ. Cortex hydrolysis leads to the complete rehydration of the spore core, and then enzyme activity within the spore protoplast resumes. We do not yet know what activates SleB activity in the spore, and neither do we have any information at all on how the spore coat is degraded.     

How do nutrients drive growth?

The relationship between plant nutrient concentration and relative growth rate (RGR) was studied under non-steady state conditions using data from a new N interruption experiment with young lettuce plants grown hydroponically in the glasshouse. RGRs estimated from the fit of a versatile growth model were shown to decline curvilinearly with plant N concentration as N deficiency increased. Similar curvilinear relationships were also derived when the same model was used to reanalyse data for N, P and K interruption treatments from other experiments previously published in the literature. These results clearly indicate that the rate of remobilisation of nutrient reserves varies with the nutrient status of the plant. This contrasts with the linear relationships observed where the changes in plant N concentration occurred solely as a response to increasing plant age, or when plants were grown under steady state conditions with constant relative nutrient addition rates. These differences in the pattern of response provide strong evidence to support the hypothesis that the form of the relationship between RGR and plant nutrient concentration can vary depending upon whether a plant's external supplies or internal reserves of a particular nutrient are more limiting.     

How do communication systems emerge?

Communication involves a pair of behaviours—a signal and a response—that are functionally interdependent. Consequently, the emergence of communication involves a chicken-and-egg problem: if signals and responses are dependent on one another, then how does such a relationship emerge in the first place? The empirical literature suggests two solutions to this problem: ritualization and sensory manipulation; and instances of ritualization appear to be more common. However, it is not clear from a theoretical perspective why this should be the case, nor if there are any other routes to communication. Here, we develop an analytical model to examine how communication can emerge. We show that: (i) a state of non-interaction is evolutionarily stable, and so communication will not necessarily emerge even when it is in both parties'' interest; (ii) the conditions for sensory manipulation are more stringent than for ritualization, and hence ritualization is likely to be more common; and (iii) communication can arise by a third route, when the intention to communicate can itself be communicated, but this may be limited to humans. More generally, our results demonstrate the utility of a functional approach to communication.     

How do prokaryotic cells cycle?

This issue of Current Biology features five reviews covering various key aspects of the eukaryotic cell cycle. The topics include initiation of chromosome replication, assembly of the mitotic spindle, cytokinesis, the regulation of cell-cycle progression, and cell-cycle modeling, focusing mainly on budding yeast, fission yeast and animal cell model systems. The reviews underscore common themes as well as key differences in the way these processes are carried out and regulated among the different model organisms. Consequently, an important question is how cell-cycle mechanisms and controls have evolved, particularly in the broader perspective of the three domains of life.     

How do fishes manage disease?

Disease drives the evolution of proactive and reactive mitigation behaviours in fishes as for terrestrial animals. Understanding fish self-remedy behaviours could discover algal bioactives, reveal novel strategies for disease management, identify new habitats or ecosystems critical to population health and conservation, and enhance knowledge of interspecific evolutionary relationships and communication.     

How do cross-migration models arise?

In this paper we present a general method to derive spatio-temporal population models mechanistically. We consider a system of multiple species living in a patchy habitat in which the local population of each species consists of some behavioural groups. We then formulate a continuous-time model where a small positive parameter is present, measuring the time scale of behavioural transitions relative to that of giving birth, death and migration among patches. By the singular perturbation method the model is reduced to a lower dimensional one in which the migration terms are, in general, nonlinear and related to the reaction terms describing the local dynamics. Two examples demonstrating the emergence of cross-migration models, i.e., the models in which the per-capita migration rate of one species depends on the density of some other species, are given.     

How do spinal segments move?

PurposeTo study and clarify the kinematics of spinal segments following cyclic torques causing axial rotation (T_z (t)), lateral-flexion (T_x (t)), flexion/extension (T_y (t)).MethodsA 6D--Measurement of location, alignment, and migration of the instantaneous helical axis (IHA) as a function of rotational angle in cervical, thoracic, and lumbar segments subjected to axially directed preloads.ResultsIHA retained an almost constant alignment, but migrated along distinct centrodes.Thoracic segmentsIHA was almost parallel to T_z (t), T_x (t), or T_y (t), stationary for T_x (t) or T_y (t), and migrating for T_z (t) along dorsally opened bows. IHA locations hardly depended on the position or size of axial preload.Lumbar segmentsIHA was also almost parallel to T_z (t), T_x (t), or T_y (t). In axial rotation IHA-migration along wide, ventrally or dorsally bent bows depending on segmental flexional/extensional status. Distances covered: 20–60 mm. In lateral-flexion: IHA-migration to the left/right joint and vice versa. In flexion/extension IHA-migration from the facets to the centre of the disc.Cervical segmentsIn flexion/flexion IHA was almost stationary for and parallel to T_y (t). In axial rotation or lateral-flexion IHA intersected T_z (t)/T_x (t) under approximately ?30°/+30°.ConclusionsGenerally joints alternate in guidance. Lumbar segments: in axial rotation and lateral-flexion parametrical control of IHA-position and IHA-migration by axial preload position. Cervical segments: kinematical coupling between axial rotation and lateral-flexion.The IHA-migration guided by the joints should be taken into account in the design of non-fusion implants. FE-calculations of spinal mechanics and kinematics should be based on detailed data of curvature morphology of the articulating surfaces of the joint facets.