Volume 14 Issue 9, September 2012

Lineage conversion has recently attracted increasing attention as a potential alternative to the directed differentiation of pluripotent cells to obtain cells of a given lineage. Different means allowing for cell identity switch have been reported. Lineage conversion relied initially on the discovery of specific transcription factors generally enriched and characteristic of the target cell, and their forced expression in cells of a different fate. This approach has been successful in various cases, from cells of the hematopoietic systems to neurons and cardiomyocytes. Furthermore, recent reports have suggested the possibility of establishing a general lineage conversion approach bypassing pluripotency. This requires a first phase of epigenetic erasure achieved by short overexpression of the factors used to reprogram cells to a pluripotent state (such as a combination of Sox2, Klf4, c-Myc and Oct4), followed by exposure to specific developmental cues. Here we present these different direct conversion methodologies and discuss their potential as alternatives to using induced pluripotent stem cells and differentiation protocols to generate cell populations of a given fate.

In a large-scale analysis, the effects of DNA damage on the levels and localization of almost every protein in an organism have now been tracked in living cells. It is shown that that although many proteins change their position or concentration, they rarely do both.

How cells sense and respond to physical forces is an area of intense investigation, which poses significant challenges for in vitro experiments and even greater obstacles for in vivo studies. Analyses of integrin complex dynamics in Drosophila melanogaster now provide evidence that altering mechanical force modulates the stability of integrin adhesion in vivo.

Kinesin-2 motors mediate anterograde intraflagellar transport (IFT) of IFT particles from the ciliary base to its tip, where particles are remodelled before retrograde transport by dynein 2 motors. Bardet-Biedl syndrome (BBS) and IFT-A proteins are now implicated in regulation of IFT assembly at the ciliary base and tip.

It is widely assumed that peripheral membrane proteins induce intracellular membrane curvature by the asymmetric insertion of a protein segment into the lipid bilayer, or by imposing shape by adhesion of a curved protein domain to the membrane surface. Two papers now provide convincing evidence challenging these views. The first shows that specific assembly of a clathrin protein scaffold, coupled to the membrane, seems to be the most prevalent mechanism for bending a lipid bilayer in a cell. The second reports that membrane crowding, driven by protein–protein interactions, can also drive membrane bending, even in the absence of any protein insertion into the bilayer.

The CDKL5 kinase is mutated in several neurodevelopmental disorders. Broccoli and colleagues find that CDKL5 regulates dendritic spine formation by phosphorylating the adhesion molecule NGL-1, which acts on synaptic contacts. They also show that neurons (derived from induced pluripotent stem cells) from patients carrying the CDKL5 mutation have aberrant dendritic spines, similarly to rodents with impaired CDKL5 function.

The last step in autophagy involves fusion of autophagosomes with lysosomes to generate autolysosomes. Through proteomics analysis and an RNAi screen, Yu and colleagues provide mechanistic insight into how lysosomes are regenerated from autolysosomes. Their analyses reveal a key role for clathrin and phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂) in this process.

Tanentzapf and colleagues use genetic mutations that alter tensile force at the Drosophila myotendinous junction to demonstrate that mechanical force controls in vivo turnover of integrin and intracellular adhesion complexes.

Membrane deformation is necessary to generate endocytic vesicles, but the molecular mechanisms proposed to drive membrane bending are controversial. Stachowiak and Schmid et al. report that crowding of proteins at the membrane is sufficient to induce curvature in vitro.

Intraflagellar transport (IFT) particles are essential for the biogenesis and maintenance of cilia. They assemble at the cilium base and travel up and down the cilia, turning around at the tip, but the mechanisms that regulate these processes were not clear. Hu and colleagues reveal a role for the BBSome in IFT assembly and turnaround.

Marine and colleagues use mouse models to show that loss of Dicer or its downstream microRNA cluster miR-17–92 in retinal progenitors prevents retinoblastoma formation through a synthetic lethal interaction with Rb and p53 deficiency.

Brown and colleagues take a systems-level approach to the DNA damage response by analysing the changes in localization and abundance of proteins in response to replication stress, using a budding yeast GFP fusion library and high-throughput microscopy.