Voltage-Gated Sodium Channels: Mechanistic Insights From Atomistic Molecular Dynamics Simulations.

The permeation of ions and other molecules across biological membranes is an inherent requirement of all cellular organisms. Ion channels, in particular, are responsible for the conduction of charged species, hence modulating the propagation of electrical signals. Despite the universal physiological implications of this property, the molecular functioning of ion channels remains ambiguous. The combination of atomistic structural data with computational methodologies, such as molecular dynamics (MD) simulations, is now considered routine to investigate structure-function relationships in biological systems. A fuller understanding of conduction, selectivity, and gating, therefore, is steadily emerging due to the applicability of these techniques to ion channels. However, because their structure is known at atomic resolution, studies have consistently been biased toward K(+) channels, thus the molecular determinants of ionic selectivity, activation, and drug blockage in Na(+) channels are often overlooked. The recent increase of available crystallographic data has eminently encouraged the investigation of voltage-gated sodium (NaV) channels via computational methods. Here, we present an overview of simulation studies that have contributed to our understanding of key principles that underlie ionic conduction and selectivity in Na(+) channels, in comparison to the K(+) channel analogs.

A potent Chk1 inhibitor is selectively cytotoxic in melanomas with high levels of replicative stress.

There are few effective treatments for metastatic melanoma. Checkpoint kinase 1 (Chk1) inhibitors are being trialled for their efficacy in enhancing conventional chemotherapeutic agents, but their effectiveness as single agents is not known. We have examined the effectiveness of two novel Chk1 selective inhibitors, AR323 and AR678, in a panel of melanoma cell lines and normal cell types. We demonstrate that these drugs display single-agent activity, with IC50s in the low nanomolar range. The drugs produce cytotoxic effects in cell lines that are most sensitive to these drugs, whereas normal cells are only sensitive to these drugs at the higher concentrations where they have cytostatic activity. The cytotoxic effect is the consequence of inhibition of S-phase Chk1, which drives cells prematurely from late S phase into an aberrant mitosis and results in either failure of cytokinesis or cell death through an apoptotic mechanism. The sensitivity to the Chk1 inhibitors was correlated with the level of endogenous DNA damage indicating replicative stress. Chk1 inhibitors are viable single-agent therapies that target melanoma cells with high levels of endogenous DNA damage. This sensitivity suggests that Chk1 is a critical component of an adaptation to replicative stress in these cells. It also suggests that markers of DNA damage may be useful in identifying the melanomas and potentially other tumour types that are more likely to be sensitive to Chk1 inhibitors as single agents.

Cyclin A/cdk2 coordinates centrosomal and nuclear mitotic events.

Cyclin A/cdk2 has a role in progression through S phase, and a large pool is also activated in G2 phase. Here we report that this G2 phase pool regulates the timing of progression into mitosis. Knock down of cyclin A by siRNA or addition of a specific cdk2 small molecule inhibitor delayed entry into mitosis by delaying cells in G2 phase. The G2 phase-delayed cells contained elevated levels of inactive cyclin B/cdk1. However, increased microtubule nucleation at the centrosomes was observed, and the centrosomes stained for markers of cyclin B/cdk1 activity. Both microtubule nucleation at the centrosomes and the phosphoprotein markers were lost with short-term treatment of the cdk1/2 inhibitor roscovitine but not the Mek1/2 inhibitor U0126. Cyclin A/cdk2 localized at the centrosomes in late G2 phase after separation of the centrosomes but before the start of prophase. Thus G2 phase cyclin A/cdk2 controls the timing of entry into mitosis by controlling the subsequent activation of cyclin B/cdk1, but also has an unexpected role in coordinating the activation of cyclin B/cdk1 at the centrosome and in the nucleus.

Behavioral responses of seven species of asteroids to the asteroid predator, Solaster dawsoni (responses of asteroids to the predator Solaster dawsoni).

Seven asteroid species common to the northern California coast were studied for their defensive responses to the predator Solaster dawsoni. The presence or absence of an escape response was used to predict whether or not these species were susceptible to predation from Solaster. Strong escape responses were displayed by Patiria miniata, Henricia leviuscula, Leptasterias hexactis, Pycnopodia helianthoides, and small Pisaster ochraceus. Subsequent capture and consumption of Patiria, Henricia, Leptasterias and small P. ochraceus were observed. Solaster attacked all Pisaster spp. tested, but Pisaster brevispinus and larger P. ochraceus protected themselves from predation by utilizing their pedicellariae against Solaster whenever contact occurred. Dermasterias imbricata appeared to be immune to predation by Solaster. Contact between these two asteroids failed to elicit a defensive response in the former or an attack by the latter asteroid.

Biomechanical and ultrastructural studies on the elastic wing tendon from the domestic fowl.

The interaction between collagen and elastin networks under conditions of load-extension has been studied in the elastic wing tendon of the domestic fowl. The load-extention curves obtained could be divided into two regions, the first region representing the ability of the tendon to undergo great extension at low tension, the second representing a limit region where the collagen of the tendon appears to become fully extended. Following removal of the elastin network with pure elastase only the second region of the curve persisted, indicating that elastin is largely responsible for the mechanical event represented by the first region of the curve. The collagen network of tendons apparently is normally held in a folded conformational state by elastin, for elastase treatment results in elongation of tendons even in the absence of loading. Complete removal of elastin, and alignment of collagen bundles were confirmed ultrastructurally in the elongated tendons. The breading load of the elastase-treated tendons was also significangly reduced, indicating that an elastase-sensitive component is a limiting factor in determining the ultimate strength of the tendon.