Does the Potential for Chaos Constrain the Embryonic Cell-Cycle Oscillator?

Although many of the core components of the embryonic cell-cycle network have been elucidated, the question of how embryos achieve robust, synchronous cellular divisions post-fertilization remains unexplored. What are the different schemes that could be implemented by the embryo to achieve synchronization? By extending a cell-cycle model previously developed for embryos of the frog Xenopus laevis to include the spatial dimensions of the embryo, we establish a novel role for the rapid, fertilization-initiated calcium wave that triggers cell-cycle oscillations. Specifically, in our simulations a fast calcium wave results in synchronized cell cycles, while a slow wave results in full-blown spatio-temporal chaos. We show that such chaos would ultimately lead to an unpredictable patchwork of cell divisions across the embryo. Given this potential for chaos, our results indicate a novel design principle whereby the fast calcium-wave trigger following embryo fertilization synchronizes cell divisions

The mechanism of CSF arrest in vertebrate oocytes

International audienceA cytoplasmic activity in mature oocytes responsible for second meiotic metaphase arrest was identified over 30 years ago in amphibian oocytes. In Xenopus oocytes cytostatic factor (CSF) activity is initiated by the progesterone-dependent synthesis of Mos, a MAPK kinase kinase that activates the MAPK pathway. CSF arrest is mediated by a sole MAPK target, the protein kinase p90(Rsk). Rsk phosphorylates and activates the Bub1 protein kinase, which may cause metaphase arrest due to inhibition of the anaphase-promoting complex (APC) by a conserved mechanism defined genetically in yeast and mammalian cells. CSF arrest in vertebrate oocytes by p90(Rsk) provides a link between the MAPK pathway and the spindle assembly checkpoint in the cell cycle

Loss of AKAP150 perturbs distinct neuronal processes in mice

A-Kinase Anchoring Proteins (AKAPs) ensure the fidelity of second messenger signaling events by directing protein kinases and phosphatases toward their preferred substrates. AKAP150 brings protein kinase A (PKA), the calcium/calmodulin dependent phosphatase PP2B and protein kinase C (PKC) to postsynaptic membranes where they facilitate the phosphorylation dependent modulation of certain ion channels. Immunofluorescence and electrophysiological recordings were combined with behavioral analyses to assess whether removal of AKAP150 by gene targeting in mice changes the signaling environment to affect excitatory and inhibitory neuronal processes. Mislocalization of PKA in AKAP150 null hippocampal neurons alters the bidirectional modulation of postsynaptic AMPA receptors with concomitant changes in synaptic transmission and memory retention. AKAP150 null mice also exhibit deficits in motor coordination and strength that are consistent with a role for the anchoring protein in the cerebellum. Loss of AKAP150 in sympathetic cervical ganglion (SCG) neurons reduces muscarinic suppression of inhibitory M currents and provides these animals with a measure of resistance to seizures induced by the non-selective muscarinic agonist pilocarpine. These studies argue that distinct AKAP150-enzyme complexes regulate context-dependent neuronal signaling events in vivo

Mammalian Emi2 mediates cytostatic arrest and transduces the signal for meiotic exit via Cdc20

Fertilizable mammalian oocytes are arrested at the second meiotic metaphase (mII) by the cyclinB-Cdc2 heterodimer, maturation promoting factor (MPF). MPF is stabilized via the activity of an unidentified cytostatic factor (CSF), thereby suspending meiotic progression until fertilization. We here present evidence that a conserved 71 kDa mammalian orthologue of Xenopus XErp1/Emi2, which we term endogenous meiotic inhibitor 2 (Emi2) is an essential CSF component. Depletion in situ of Emi2 by RNA interference elicited precocious meiotic exit in maturing mouse oocytes. Reduction of Emi2 released mature mII oocytes from cytostatic arrest, frequently inducing cytodegeneration. Mos levels autonomously declined to undetectable levels in mII oocytes. Recombinant Emi2 reduced the propensity of mII oocytes to exit meiosis in response to activating stimuli. Emi2 and Cdc20 proteins mutually interact and Cdc20 ablation negated the ability of Emi2 removal to induce metaphase release. Consistent with this, Cdc20 removal prevented parthenogenetic or sperm-induced meiotic exit. These studies show in intact oocytes that the interaction of Emi2 with Cdc20 links activating stimuli to meiotic resumption at fertilization and during parthenogenesis in mammals

The discovery and development of Eg5 inhibitors for the clinic

The mitotic kinesin Eg5 (also known as kinesin spindle protein, KSP, Kif11, a member of the kinesin-5 family) represents an attractive oncology drug target in the ongoing development of anti-mitotic drugs that selectively block mitosis through disruption to the mitotic spindle. In this state-of-the-art review, we outline the progress that has been made in the development of Eg5 inhibitors for clinical use. We evaluate the preclinical development and attributes of key Eg5 inhibitors that have undergone clinical evaluation or extensive preclinical optimisation, and discuss the medicinal chemistry strategies utilised in their design to overcome the challenges encountered during lead optimisation. We critically analyse the progress that has been made towards delivering clinical benefits, and the wider implications this has in the utility of mitotic kinesin inhibitors as prospective oncology drugs