JCB

New issue roundup: Nov 16th

Time for another new issue roundup, and let's start - as usual - with our cover story or, to be precise, our cover stories. Two papers from Björn Öbrink's lab in Stockholm dissect the mechanism by which a homophilic cell adhesion molecule transmits information across the plasma membrane. It wasn't actually known how molecules like CEACAM1 - a member of the Ig domain superfamily - send signals into the cytoplasm when they adhere to neighboring cells. Klaile et al. and Müller et al. show that when CEACAM1 molecules self-interact across opposing membranes, they undergo a conformational change that prompts them to form homodimers within the same membrane. CEACAM1 homodimers recruit a tyrosine phosphatase to their cytoplasmic tails at the expense of c-Src, triggering signaling events inside the cell. You can read a slightly more in-depth summary of the two papers, and learn why Öbrink thinks other cell adhesion molecules may act similarly, in this month's In Focus feature.

 Meanwhile, Hayashi et al. demonstrate that heparan sulphate glycoproteins maintain stem cell niches in Drosopila. Dally is required to keep female germ stem cells in their specialized microenvironment, while dally-like plays the same role for male germ stem cells. The image above shows how the germ cells are missing from an ovariole lacking dally. On the other hand, a fascinating paper from Wu et al. suggests that mammalian male germ stem cells may not need a specialized niche at all to retain their stemness - it might just come down to chance instead. Sperm stem cells and their differentiating progeny lie next to each other on the basement membranes of seminiferous tubules, with no discernible differences in their microenvironments. Wu et al. show that this situation can be replicated in vitro - a homogeneous culture can sustain both stem cells and their progeny and the proportion of each cell type remains constant over time. The researchers suggest that the decision to differentiate or self-renew is simply a stochastic choice and show mathematically that a 33% chance of differentiating would account for the numbers of each cell type seen in culture. However, as Zhuoru Wu explains in this summary, this doesn't mean that the environment has no impact on cell fate decisions - external stimuli might alter that 33% probability of differentiating.

Switching gears, Lleres et al. present a new method of measuring the levels of chromatin condensation in living cells. The assay measures FRET in cells expressing both GFP and mCherry tagged histone proteins. In the image to the right, different levels of FRET efficiency are pseudo-colored to distinguish regions of the nucleus with distinct degrees of compaction - the arrows indicate highly condensed chromatin (red) at the nuclear periphery.

Elsewhere in this issue, we learn from Munck et al. about a protein that coordinates peroxisome biogenesis and inheritance. Like Chang et al. a few weeks ago, Munck and colleagues show that Pex3 - a protein known for its role in peroxisome formation - also helps divide copies of the organelle between mother and daughter yeast cells. Munck et al. show that Pex3p recruits a protein called Inp1p to peroxisomal membranes, which helps to retain the organelle in the mother cell by anchoring them to the cell membrane.

New peroxisomes form from the endoplasmic reticulum. This latter organelle is the focus of Schuck et al.'s attention: they're interested in why the ER expands in response to stress. Here, they show that the increase in size is controlled by regulators of lipid synthesis and by the unfolded protein response - the standard stress response that also upregulates chaperone proteins. However, ER expansion alone can alleviate stress even in the absence of chaperone production. Sebastian Schuck suggests in this summary that the larger ER may serve to dilute the concentration of unfolded proteins, preventing them from aggregating in the ER lumen.

Sticking with the secretory pathway, we take a journey through the Golgi - a meeting report from a host of top-notch Golgi researchers who all met earlier this year to discuss the current state of the field. It's a subject that's close to my heart as I worked on the Golgi for my PhD, and this review nicely summarizes the researcher's current thoughts on 4 key questions: how are proteins transported through the Golgi?; what are the connections between Golgi cisternae?; what role do distinct membrane domains play?; and, above all, is the Golgi an independent organelle? On top of that, the review outlines key questions for the future, so be sure to read it if you're at all interested in the Golgi and secretion.

Finally for today, Cianciola and Carlin reveal how an adenoviral protein called RID-alpha induces a cholesterol trafficking pathway in its host cells. Intriguingly, the protein actually helps to correct the lipid-sorting defects seen in the lysosomal storage disease Niemann-Pick type C. But I shan't go into any more detail here, as you can listen to senior author Cathleen Carlin explain all to Eun Choi in the latest episode of our biobytes podcast. which is also out today. In the same episode, you can hear David Salomon tell me about the crosstalk his lab has discovered between the Notch and Nodal signaling pathways (Watanabe et al.) and Deepta Bhattacharya describe how hematopoietic stem cells frequently exit their niche in the bone marrow without undergoing cell division (Bhattacharya et al.). You can listen below, or subscribe to the monthly podcast in iTunes by clicking here. And don't forget to check our full table of contents for everything I didn't have time to discuss here...

New issue roundup: Nov 2nd

There's some really great papers in the latest issue of the JCB - just time for a quick roundup of some of them.

Our cover image this week shows green spots of gamma-TuRC, dotted along the length of red microtubules - a localization you might not expect for a protein complex best known for nucleating the growth of new microtubules from the centrosomes. But Bouissou et al. reveal that gamma-tubulin and its associated proteins also regulate microtubule dynamics. The dots of gamma-TuRC correspond to points at which shrinking microtubules either pause or begin to regrow. Cells lacking gamma-TuRC components show increased microtubule dynamics. As I explain in this summary, the authors think that gamma-TuRC acts as a stabilization factor to limit microtubule disassembly.

Meanwhile, Niault et al. describe a fascinating new way for kinases in different signaling pathways to interact with one another. The Ras-activated kinase Raf-1 inhibits the Rho-activated kinase Rok-alpha. Usually, such inhibitory crosstalk involves one kinase phosphorylating the other, but not in this case. Niault et al. demonstrate that when Raf-1 is turned on, its N-terminal autoinhibitory domain switches from binding its own kinase activity to directly binding and inhibiting Rok-alpha's kinase domain. The authors have previously shown that Ras-induced epidermal tumors rely on Raf-1 to keep Rok-alpha attenuated, in order to promote cell survival and migration - so senior author Manuela Baccarini tells us that she wants to develop small molecules that will block this inhibitory association.

Speaking of tumors being reliant on particular proteins for their survival: Brunelle et al. compare leukemias that depend on two related anti-apoptotic proteins - Bcl-2 and Mcl-1. The group find that the two proteins block tumor cell death by mopping up the same pro-apoptotic factors. Accordingly, mice overexpressing Bcl-2 or Mcl-1 suffer clinically identical leukemias that can only be distinguished using a technique developed by lead author Anthony Letai called BH3 profiling. However, despite their great similarity, Bcl-2 and Mcl-1 dependent leukemias vary greatly in their sensitivity to chemotherapeutic drugs - Mcl-1 protein turns over more rapidly, so provides less protection against cytotoxic agents. Letai now wants to use BH3 profiling to determine the anti-apoptotic dependencies of human tumors and to identify corresponding differences in their response to chemotherapy.

On a completely different note, Loo et al. reveal a surprising heterogeneity in differentiating fat cells. Several markers are upregulated as 3T3-L1 adipocytes differentiate in vitro, but as senior author Steve Altschuler says in this week's In Focus - there's an illusion of correlation. When the researchers looked at individual cells in the population, they realized that the different markers weren't actually increasing in the same cells. Loo et al. ended up identifying four subpopulations of differentiating adipocytes (pictured) which appear to represent distinct stages of adipogenesis. The differentiation process itself occurs asynchronously, so there's always a mix of the 4 cell types giving the false impression that certain markers are upregulated at the same time, when they're actually negatively correlated. It's really worth taking a look at this study, as it shows the limitations of using methods, such as western blotting, that only give an average readout of the population as a whole...

Elsewhere in this issue, Chikashige et al. identify two new proteins required to cluster telomeres into “bouquets” during meiosis. In fission yeast lacking Bqt4, telomeres (magenta) can’t attach to the nuclear envelope (green). Bqt3, on the other hand, protects Bqt4 from protein degradation.

Rif1 was originally identified as a telomeric protein in budding yeast, but has many other roles in the cell. Buonomo et al. delete Rif1 from mice, and discover that the protein facilitates homology-directed DNA repair, particularly at stalled replication forks. Cells lacking Rif1 are thus slower at progressing through S phase. Sticking with the DNA repair theme, Messick and Greenberg  review the many ubiquitination events that occur at double strand breaks - the authors propose that different ubiquitin structures signal different functions that coordinate the repair process.

There's plenty more in the issue too, which you can find on our table of contents, but I'll leave you today with a link to our latest biosights video podcast. This time round, we discuss how melanosomes and endosomes dance together, as Delevoye et al. use electron tomography to reveal the intimate connections between the organelles during melanogenesis. You can watch the brief video, which includes interviews with Cedric Delevoye and Graca Raposo, below or by clicking here.

New Issue: October 19th

There's a new edition of the JCB out today. We'll start our roundup with Tom Misteli's editorial on the recent JCB/New York Academy of Sciences meeting on the Cell Biology of Disease. Tom explains the rationale behind the meeting, much as he did in his introductory remarks to the conference. You can find our full coverage of the meeting by clicking here.

Elsewhere in this issue, our "In Focus" centers on the discovery by Canudas and Smith that cohesion between sister telomeres and centromeres is mediated by slightly different versions of the cohesin complex. Specifically, one of the cohesin subunits, Scc3, comes in two different flavors in vertebrate cells. Canudas and Smith show that Scc3^SA1 binds telomeres together, while Scc3^SA2 keeps sister centromeres close and, as you can read in my summary, telomere cohesion turns out to particularly important for the cell - defects in Scc3^SA2 or its binding partner TIN2 result in DNA double strand breaks and telomere loss. Meanwhile, Adelfalk et al. show that a meiosis-specific cohesin called Smc1beta is essential for maintaining telomere integrity and attaching them to the nuclear envelope.

Our cover image this issue is taken from Koto et al.'s investigation of caspase regulation during development of the Drosophila sensory organ. Caspases aren't just involved in apoptosis, they can also play constructive roles in development - promoting the elongation of sensory organ shaft cells, for example. Naturally, caspase activity needs to be tightly regulated to allow their function for only as long as it takes to promote development without causing cell death. Koto et al. reveal that temporal differences in the expression levels of a caspase inhibitor called DIAP1 help caspases walk this fine line.

Tsuruta et al. show how a lipid kinase called PIKfyve helps neurons avoid cell death. Neuronal death can arise from overexcitation of their voltage-gated calcium channels (during a stroke for example). As explained in this summary, Tsuruta et al. show that sustained activation leads to the recruitment of PIKfyve to calcium channels at the membrane, where it generates the phospholipid PtdIns(3,5)P₂. This, in turn, leads to the lysosomal targeting of the calcium channels, reducing calcium entry and cellular toxicity.

Next, we move from lysosome trafficking to melanosomal transport. Melanosomes are the specialized pigment granules of skin melanocytes that - similarly to lysosomes - arise from the endosomal system. Delevoye et al. show how two proteins, the clathrin adaptor AP-1 and the kinesin motor KIF13A, coordinate melanogenesis by positioning endosomes near to developing melanosomes and by sorting melanosome-specific cargo such as the enzymes that generate the melanin pigment. Moreover, Delevoye et al. provide some beautiful electron tomography to show that transport to the melanosomes is mediated by direct contact with tubules protruding from the endosomes - as in this image, where endosome tubules are in green and the melanosome is colored red. This paper will be the subject of our next biosights video, due in two weeks time but, in the meanwhile, you can read Lakkaraju et al.'s commentary on the article, which puts these findings into a nice perspective.

We also have a commentary (by Juliane Kellner and Pierre Coulombe) on a report by Vijayaraj et al. that keratins - the most neglected components of the cytoskeleton - have a surprising role in regulating protein synthesis. Mice lacking all their intermediate keratin filaments die in early embryogenesis due to severe growth retardation. It turns out that glucose transporters are mislocalized in the absence of keratins, leading to activation of the mTOR pathway and repression of protein synthesis. You can also learn more about this study in the latest edition of our biobytes podcast - also out today - in which Eun Choi speaks to the lead author Preethi Vijayaraj. And you can hear my interview with Greg Longmore on his lab's investigation of the antagonistic relationship between two rho GTPases in controlling cell shape that was published earlier this month. You can listen to biobytes by subscribing on iTunes, or by clicking here.

And don't forget to check out the rest of today's new articles by visiting our table of contents...

New issue: October 5th 2009

Time for a quick rundown of highlights from the latest issue of the JCB...

Our cover image this week is a section through a mouse whisker, stained in red for Merkel cells, which transmit mechanical stimuli to sensory neurons (stained in green). In other words, Merkel cells are involved in touch sensation, but the embryonic origin of these cells was unclear, since they have characteristics of both epidermal and neuronal cell types. In this issue, Van Keymeulen et al. demonstrate that they arise from epidermal progenitors during both embryogenesis and adult homeostasis. Moreover, deletion of a transcription factor called Atoh1 in these cells completely prevents Merkel cell development. You can read more in this summary, including why these findings may shed light on a particularly aggressive cancer that arises from Merkel cells.

Meanwhile, our In Focus for this issue centers on a study from Sivaramakrishnan and Spudich that reveals how multiple myosin motors coordinate their activities to transport vesicles along the actin cytoskeleton. The researchers use a combination of experimentation and computer modeling to show that even monomeric myosins (which would be completely unable to move cargo on their own) can join forces to carry the load.

Switching to mitosis, O'Connell et al. use a fascinating cell biological phenomenon called MUG to unpick the contributions that kinetochores and chromatin make to the assembly of the mitotic spindle. MUG stands for Mitosis with Unreplicated Genomes, and is induced by the prolonged treatment of cells with hydroxyurea. The fascinating thing is that these cells form a normal mitotic spindle attached to kinetochores and centromeric DNA aligned at the metaphase plate. Yet the rest of the cell's (unreplicated) chromatin is pushed out to the cell periphery. As you can read in this summary, O'Connell et al. use these cells to show that kinetochores alone can direct spindle assembly without the help of a Ran-GTP gradient generated from the bulk chromatin.

Meanwhile, McCleland et al. block DNA replication in early stage Drosophila embryos to demonstrate that S phase controls the speed at which the embryonic nuclei enter mitosis. Earlier this year, the researchers used the same experimental system to observe the contributions of cyclins to mitotic progression. I won't go into any more detail here, as Eun Choi has produced an extra special edition of biosights that covers both papers and features interviews with both Mark McCleland and Pat O'Farrell. Eun even made a trip to UCSF to film the researchers at work - you can see the video by clicking here, or by watching below:

Staying with flies, Shen and Ganetzky show how autophagy promotes Drosophila synapse development. You might think of autophagy as being a destructive pathway, but it actually plays a constructive role at neuromuscular junctions by counteracting proteosome-mediated degradation. Flies overexpressing the autophagy protein Atg1 have larger NMJs because they destroy an E3 ubiquitin ligase that would otherwise limit synaptic size. As I explain in this summary, the authors believe autophagy is a great way for neurons to regulate synaptic plasticity in response to many different environmental stimuli.

Meanwhile, Massaro et al. reveal a signaling pathway that helps neurons maintain synaptic structure despite environmental stress. MAPK-JNK-Fos signaling rescues NMJ stability to prevent disruption of the neuronal cytoskeleton from causing synapse disassembly.

Finally for today, we have two papers that investigate the structures of protein particles transported back and forth along the length of cilia and flagella. Bidirectional transport of these particle trains is essential for assembling and maintaining flagella. Pigino et al. provide a beautiful electron-tomographic analysis of the "trains" of particles heading in both the anterograde and retrograde directions. Particle trains heading to the tip of the flagellum (indicated with a black arrowheads in the image to the right) are longer and narrower than trains returning to the organelle's base (white arrowhead).

How do cells control the size of cilia and flagella? Engel et al. demonstrate that trains traveling to the flagella tip are smaller in longer cilia. As the group's modeling shows, this may help the organelle to keep a consistent length, as the assembly rate will decrease as the cilia/flagella grows larger.

Plenty more in elsewhere in this issue. As always, you can find the full table of contents by clicking here.

JCB/NYAS Meeting: Stem Cells

Thursday morning's session of the JCB/NYAS meeting focused on stem cells, and began with a talk from long-time JCB editor and recent National Medal of Science awardee Elaine Fuchs, who presented a summary of her lab’s work from recent years. If you’re unfamiliar with the Fuchs lab’s pioneering work on epidermal stem cells (Disclaimer: I’m an ex-Fuchs lab member!), there are a couple of great reviews around to bring you up to speed. Some of the more recent work discussed in this session included the identification of the embryonic stem cell population that gives rise to all the lineages of the skin epithelium (hair follicles, sebaceous glands, and epidermal cells). These stem cells are specified early in embryonic development in a manner dependent on the transcription factor Sox9, and eventually give rise to the previously-identified adult skin stem cell population (Nowak et al, 2008).

Wnt signaling plays a key role in epidermal stem cell differentiation, promoting, for example, stem cell activation or hair follicle formation, depending upon the levels of Wnt activity. Wnt signaling leads to stabilization and nuclear translocation of the transcription factor β-catenin, which acts in conjunction with members of the Lef/Tcf family of transcriptional activators and repressors. The Fuchs lab recently studied Tcf3 and Tcf4, which are co-expressed in epidermal progenitor cells and surprisingly found that these two transcription factors also have Wnt-independent functions in skin homeostasis, helping to maintain stem cells in an undifferentiated state (Nguyen et al, 2009).

Fuchs is, of course, very excited at the prevalent use of adult skin cells as the starting material for induced pluripotent stem (iPS) cell generation, so it was fitting that her presentation was followed by a talk from Alex Meissner. He’s been at the forefront of this burgeoning field as both a student/postdoc with Rudolf Jaenisch and as a group leader at Harvard University and the Broad Institute – you can read more about his background and research interests in our recent profile article. Differentiated adult cells can be reprogrammed into multipotent iPS cells with just a handful of transcription factors. Meissner is interested in understanding the mechanism by which the reprogramming occurs, in order to improve the process’ efficiency. To do this, he’s studied the epigenetic profiles of both fully and partially reprogrammed cells, identifying the DNA and histone modifications corresponding to successful reprogramming.

Meissner is also using similar approaches to determine how similar iPS cells really are to embryonic stem (ES) cells (the answer seems to be “very, but with some small differences”). The most stringent test for pluripotency is a test called tetraploid complementation, which unambiguously demonstrates whether an iPS cell can develop into a live animal. This hurdle has just been cleared for mouse iPS cells (Zhao et al., 2009) but such tests are obviously not possible for human iPS lines. Studies on the epigenomes of human iPS cells will therefore be particularly important for monitoring both the efficiency and efficacy of reprogramming. To this end, Meissner is involved in efforts to produce a reference epigenomic map of human ES cells – this is revealing a lot of variation between different ES cell lines, meaning that there is a lot of work to do before we can draw conclusions about the quality of human iPS cells.

Next came a talk from Fiona Doetsch (Columbia University), who discussed her studies on adult brain stem cells. The presentation largely covered two recently published stories from the Doetsch lab: firstly, the identification of a vascular niche for stem cells in the neural subventricular zone (SVZ). SVZ stem cells are astrocytes that directly contact blood vessels and seem to receive activating signals from the blood due to specializations in the blood brain barrier in this region of the brain (Tavazoie et al., 2008). Secondly, Doetsch described the isolation of micro RNAs that regulate the differentiation of these neural stem cells. One of these, miR-124, is highly expressed in neuroblasts (developing neurons), and overexpressing the RNA promoted neuroblast formation. Knockdown of miR-124, on the other hand, kept SVZ cells dividing, but reduced their differentiation into neurons. miR-124 regulates the timing of SVZ cell differentiation by promoting cell cycle exit and inhibiting stem cell self-renewal. It does this by downregulating a number of targets identified by the Doetsch lab, including the transcription factor Sox9 (see Cheng et al, 2009 and cf. Elaine Fuchs’ epidermal progenitor cells above!).

Irina Conboy from The Berkeley Stem Cell Center was next up to the podium. Conboy is fascinated with aging – she began her talk by comparing the lifespans of rats (3 years) and squirrels (40 years). This huge difference between two ostensibly similar animals suggests to Conboy that aging is not simply controlled by the accumulation of damage over time. Conboy works on muscle, which repairs itself easily when young but not so well as time goes by, even though the number of stem cells (aka satellite cells) remains the same. Conboy has shown that the balance between Notch and TGFβ signaling is critical to determining whether the satellite cells can activate and expand to repair tissue damage (Carlson et al., 2008) and it now seems that a similar mechanism is at play in human cells too. Old muscle can be rejuvenated by restoring the Notch/TGFβ balance, either by knocking down SMADs (the downstream effectors of TGFβ) or by elevating Notch expression.

Parabiosis experiments (in which the circulatory systems of young and old mice are connected together) have shown that systemic factors in the old animal’s blood can prematurely age young muscle stem cells. TGFβ is elevated in the sera of older mice and humans, but Conboy showed that this signaling molecule isn’t acting systemically to inhibit regeneration. TGFβ only seems to act locally in determining aging – the hunt for the systemic factor that ages the stem cell niche goes on (Carlson et al., 2009). You can find out more about Irina Conboy’s work in an upcoming profile article, which will appear in the October 5th issue of the JCB.

The final speaker in the session was Michael Buszczak, from UT Southwestern. Buszczak is interested in histone modifications that silence genes in Drosophila germline stem cells (GSCs), and has used a high throughput “protein trapping” approach to identify proteins involved in GSC maintenance and differentiation. (GFP is randomly inserted throughout the genome, and fly embryos are screened for GFP-tagged proteins with interesting localization and expression patterns). One gene that Buszczak identified in this way is a histone deubiquitinating enzyme called Scrawny (Buszczak et al., 2009). Scrawny silences genes in GSCs (maintaining them in an undifferentiated state) by removing ubiquitin from histone H2B, which would otherwise serve to recruit RNA Pol II. Intriguingly, the human homolog of Scrawny is overexpressed in human ovarian cancers.

Buszczak is now complementing this random search for chromatin modifiers by systematically knocking out enzymes such as histone demethylases and deacetylases. Targeted knockouts in Drosophila are still fairly laborious, even using the new recombineering approach developed by Hugo Bellen’s lab so, in the meantime, Buszczak has begun analyzing a few existing mutants for GSC defects. Buszczak described one histone demethylase mutant that has a non-cell autonomous function in GSC differentiation.

The five talks covered a lot of ground but the connections to the rest of the meeting were clear. Stem cells, and the transcription factors and epigenetics that regulate them, are increasingly relevant to understanding disease from aging to cancer.

JCB Meeting: The Cell Biology of Disease

Our meeting at the New York Academy of Sciences is now underway. There's been some great talks so far - look out for blog updates with more details.
In the meantime, here's a snippet of JCB editor-in-chief Tom Misteli's opening remarks from yesterday evening:

New issue: September 21st

There's a new issue of the JCB published today. Our cover features just one of the many striking images in Nowak et al.'s paper on cell shape and packing. The cells in question are lens fiber cells - fascinating epithelial cells that elongate over a thousand-fold during development to wrap around the lens in concentric layers, appearing as incredibly regular, tightly-packed hexagons in cross-section. Nowak et al. find that this packing is disrupted in mice lacking tropomodulin1, a component of the spectrin-actin network that underlies and supports the fiber cell membranes. I really recommend taking a look at this paper (if only to see the beautiful immunofluorescence and electron microscopy images!), but you can also find out more in my summary of the paper, or by listening to the latest biobytes podcast, also out today.

Elsewhere in this issue, we have two sets of companion papers. The first pair, by Edwards et al. and Sumakovic et al. both describe how the small GTPase Rab2 controls the maturation of dense core vesicles in worm motor neurons. Worms expressing mutant versions of Rab2 are virtually paralyzed because critical component(s) of the vesicles are lost before they can be secreted across the synapse. It remains to be seen precisely what gets lost from Rab2-mutant vesicles, but you can find out what each group thinks might be going on by reading our In Focus article on the subject here.

Our second pair of companion pieces are from Lo et al. and Kemmler et al. Both of these articles reveal the step by step assembly of the 60S ribosome in yeast: a phosphatase called Yvh1 displaces a subunit of the pre-ribosome complex in the nucleus and is then itself displaced by a different subunit as the translation-competent 60S ribosome forms in the cytoplasm.

Meanwhile, Li et al. describe the steps that lead from ER stress to cell death. While short-term ER stress induces the unfolded protein response to allow cells to recover, prolonged stress leads to apoptosis, a process involving expression of the transcription factor CHOP and calcium release from ER stores. Li et al. show how these events are connected: CHOP drives expression of an ER-resident oxidase called ERO1- that, in turn, activates the IP3R calcium channel and subsequent calcium release. As senior author Ira Tabas points out, ER-stress and CHOP-induced apoptosis plays a role in several diseases, including atherosclerosis and diabetes, so understanding how the process occurs is an important step forward.

Returning to yeast, DeVay et al. investigate how a protein related to the endocytic GTPase dynamin promotes mitochondrial fusion. Mgm1 comes in two isoforms: a long, membrane tethered version that is proteolytically cleaved to yield a short, soluble form. Both forms are required for the fusion of mitochondrial inner membranes (a different dynamin-related protein controls outer membrane fusion). DeVay et al. find that both the long and short versions of Mgm1 are targeted to the inner membrane by the mitochondrial-specific phospholipid cardiolipin. Moreover, as summarized here, cardiolipin stimulated the oligomerization and GTPase activity of short-Mgm1, driving membrane fusion. On the other hand, the GTPase activity of long-Mgm1 is dispensable for membrane fusion. Instead, it probably acts as a tether, linking mitochondrial membranes together before short-Mgm1 catalyzes their fusion.

All of this has to be balanced by mitochondrial fission to maintain proper organelle size and distribution. Another dynamin-related GTPase called Drp1 promotes the division of both mitochondria and peroxisomes. As Wakabayashi et al. show, the mitochondria of mice lacking Drp1 are hugely elongated compared to the short, fragmented appearance of wild type mitochondria:

Image from Wakabayashi et al., 2009

These mice die at embryonic day 11.5, while brain-specific Drp1 knockout mice have cerebellar defects, demonstrating the in vivo importance of organelle fission.

Lots of other interesting articles in this issue too, including a review from Anne Simonsen and Sharon Tooze on the crosstalk between autophagosomes and other membrane compartments. You can find the full table of contents by clicking here.

New issue and biosights: September 7th

It's a holiday here in the US, but there's still a new issue of the JCB out today. So let's take a look at some of the highlights...

Our cover image this week is taken from the paper by Slattum et al., and shows an apoptotic cell being pushed out of an epithelial sheet. Dying cells can be extruded from either the upper or lower surface of a cell layer; Slattum et al. investigate how the direction of extrusion is determined. As Mitch explains in his summary, a band of actin and myosin assembles in cells surrounding the apoptotic cell and contracts to squeeze the cell out. In order to push a cell upwards, neighboring cells point microtubules to its sides and base, delivering an activator of Rho GTPase that, in turn, activates myosin to put the squeeze on.

Meanwhile, Vale et al. reveal a similar mechanism at play in determining where a cell divides during cytokinesis. They use live TIRF microscopy to show that microtubules deliver the motor protein kinesin 6 (which carries Rho GTPase activators) to the cell equator. This seems to trigger the assembly of myosin filaments which drive cell constriction and cytokinesis. The image below cells the accumulation of kinesin 6 (green) on the plus ends of microtubules (red) at the cell equator:

 

Image from Vale et al., 2009

Also in this issue, Garcia-Cortes and McCollum study the timing of cytokinesis. A fission yeast protein called Etd1 initiates cytokinesis only after the mitotic spindle has become fully elongated during anaphase, ensuring that chromosomes are completely segregated. And Etd1 is destroyed after the cell has divided, in order to switch off further rounds of cytokinesis. You can read more about both of these papers - explaining how cells determine the where and when of cytokinesis - in this month's In Focus.

On a completely different note, Stagg et al. report how cytomegalovirus evades the immune system by triggering the destruction of the MHC class I complex. Two viral proteins fool the infected cell into ubiquitinating the MHC I complex, causing its export from the ER back into the cytoplasm, where it is subsequently degraded by the proteasome. As Mitch describes in his summary, Stagg et al. screened through 373 ubiquitin ligases by RNAi to find the cellular protein responsible for targeting MHC I, namely, an E3 ligase called TRC8.

Also in this issue is a review by Charles Bond and Archa Fox on paraspeckles: ribonucleoprotein particles that regulate gene expression by retaining RNAs in the nucleus. Covering everything from their composition to cellular function, this review tells you all you need to know about this new class of nuclear bodies.

Finally for today, we have a brand new edition of our biosights series of video podcasts. Here, we explore a recent paper from Jung et al., who find that a myosin called myoJ drives the spreading of contractile vacuoles around the actin-rich cortex of dictyostelia, helping the cells discharge excess rain water and avoid osmotic stress. As well as some great videos of this fascinating organelle, senior author John Hammer III explains why the study is an important example of a myosin motor acting as a point to point membrane motor. If you can't see the movie below, you can also watch it by clicking here.

New issue: August 24th

A little bit later than usual, but here's a quick rundown of some of the highlights from the latest issue of the JCB.

Our main cover story this time around is a study from Hall et al. investigating the regulation of a large, chromatin-binding RNA called XIST. XIST coats and inactivates one X chromosome early in female mammalian development, but little is known about how the RNA associates with the chromatin and what regulates the process. Hall et al. exploit the fact that XIST is temporarily released from the X chromosome during mitosis to manipulate its localization in vivo. Through this approach, the researchers discover that the mitotic kinase Aurora B regulates XIST's association with chromatin. You can read more about the study, and why the group's approach should help other researchers studying chromatin-RNA interactions, in my 'In Focus' article here.

Elsewhere, two papers dissect the roles that type V myosins play in transporting organelles around the cell. Jung et al. find that a myosin called myoJ drives the spreading of contractile vacuoles around the actin-rich cortex of dictyostelia, helping the cells discharge excess rain water and avoid osmotic stress. It's a really fascinating process, and Jung et al.'s videos are fantastic - look out for a forthcoming biosights video podcast that describes the story in more detail. In the meantime, you can find a short summary of the work right here. Meanwhile, a myosin called Myo2 helps yeast partition peroxisomes into their daughter cells. Fagarasanu et al. explore how this is regulated in both time and space by the cell cycle, and a feedback mechanism that halts further peroxisome transport once the bud has got its fair share. Again, you can read more in this summary piece.

Sticking with budding yeast, Madia et al. explore how Sch9 - a homologue of the mammalian oncogenes Akt and S6 kinase - promotes age-dependent mutations. The group - led by USC's Valter Longo - has shown in the past that yeast lacking Sch9 live much longer than wild-type cells and accumulate fewer mutations in their genome over time. Now they show why - Sch9 promotes oxidative DNA damage and an error-prone DNA repair mechanism. The results may help explain why the incidence of cancer increases with advancing age and, surprisingly, suggests that oncogenes like Akt may promote DNA damage in non-dividing, rather than dividing cells. No summary this time, but you can hear more from Valter Longo himself in our latest biobytes podcast, also out this week. Listen here, or on the player below.

Bhagatji et al. describe how GPI-linked membrane proteins get endocytosed via a clathrin and dynamin-independent pathway. By tagging proteins with an artificial membrane anchor, the researchers show that it's the steric bulk of the GPI-linked protein that determines its specific endocytosis, rather than any particular structural feature of the protein or the GPI group itself. In the image below, for example, an antibody tagged with a di16:0PEG lipid group (green) localizes to the same endocytic compartment as the GPI-linked folate receptor (red, right panel), but doesn't co-localize with the transmembrane transferrin receptor (red, left panel).

Image from Bhagatji et al.

Ben Nichols has a nice summary of the group's findings, and also wins this week's award for best pun in an article's title. And finally for today, I'll point you in the direction of David Razafsky and Didier Hodzic's review of the protein complexes that link the cytoskeleton to the nuclear skeleton and govern nuclear and centrosomal dynamics. You can find that, and all the other articles in the latest JCB, on our table of contents.

New issue (and new biosights!): August 10th

Lots to highlight in today's new issue, so let's crack straight on with it...

Our striking cover image this week shows the disruption to the microtubule network in mouse muscle fibers caused by a defect in the dystrophin gene (which models the mutation afflicting Duchenne Muscular Dystrophy patients). Prins et al. reveal that - in addition to its interactions with the actin and intermediate cytoskeletons - dystrophin also binds microtubules. As Mitch explains in his summary, this suggests that dystrophin acts as a cytolinker, connecting microtubules to the plasma membrane, just as it does with other cytoskeletal components.

Mitch has also written an In Focus article about the surprising report from Xu et al. that components of the histone H3 lysine 4 methyltransferase complex have an additional function in regulating endosomal trafficking. One component in particular, called mDpy-30, localizes to the trans-Golgi network. Knocking down mDpy-30 (or two other H3K4 methyltransferase subunits) causes a redistribution of an endosomal cargo protein, and an accumulation of recycling endosomes at cell protrusions. Mitch's summary explains why the authors think mDpy-30 might be an important regulator of cell adhesion and migration.

Nakanishi et al. investigate the more orthodox function for histone methyl transferases. They demonstrate that the trimethylation of histone H3 on lysines 4 and 79 is absolutely dependent on the prior ubiquitination of histone H2B. This dependency had been postulated before, but had recently been called into question. In an accompanying commentary, John Latham and Sharon Dent explain why Nakanishi et al.'s work is a timely confirmation of the importance of crosstalk between different modifications on different histones.

Elsewhere, Yamakoshi et al. study crosstalk between two tumor suppressors, p16^Ink4a and p53. The group constructed transgenic mice that express a p16^Ink4a-luciferase fusion protein under the control of p16's endogenous promoter. The group could then use bioluminescent imaging to non-invasively image the real-time expression of the tumor suppressor in living animals in response to various stresses, as you can see in the picture below.

Image from Yamakoshi et al.

The researchers found that p16^Ink4a was induced by oncogenic stresses such as DNA damage, and this response was accelerated in mice lacking p53, suggesting that p16^Ink4a expression acts as a backup to p53 and is held in check if the p53 pathway is functioning normally.

Finally for this issue, we have a couple of papers on the complex assembly of focal adhesions, by which cells attach to their surrounding extracellular matrix. Ronen Zaidel-Bar takes the "adhesome" of proteins that make up focal adhesions and examines their conservation across evolution. Zaidel-Bar finds that the adhesome largely evolved by gene duplication to produce families of related proteins that interact with different partners and therefore developed specialized functions in higher organisms. Winograd-Katz et al., meanwhile, take a systems biology approach to studying the adhesome, knocking down the different components by RNAi and conducting a multi-parametric analysis of the compromised adhesions. I shan't dwell on this any longer, as you can learn more by watching our latest biosights video podcast:

If systems biology papers usually leave you feeling a little underwhelmed, I'll be glad to point you in the direction of an article written by yours truly, that investigates how systems biology approaches are really beginning to bear fruit in terms of understanding cell biology. It's clear that you'll be seeing a lot of important and informative papers like Winograd-Katz et al.'s in the JCB in future. If you're a traditionalist though, you'll be glad to hear that good, old-fashioned cell biology still has an important role to play in guiding systems biologists and testing their predictions. Have a read, and feel free to comment below, on this or any other articles in the latest edition of the JCB.