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<jats:title>A Gene for Early Acceptance</jats:title> <jats:p> One of the fundamental properties of the immune system is the ability to distinguish self- from nonself–histocompatibility. To gain insight into the evolution and molecular basis of histocompatibility, <jats:bold> Voskoboynik <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="384" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1238036">384</jats:related-article> ) sought to determine the genetic basis for a natural transplantation reaction that occurs in <jats:italic>Botryllus schlosseri</jats:italic> , a colonial urochordate. Compatibility allows vascular fusion among individuals, whereas incompatibility results in an inflammatory rejection response. A single gene determined the outcome of the reaction. Like histocompatibility genes in higher organisms, this gene is polymorphic and is expressed in the tissues that participate in the transplantation reaction. </jats:p>

<jats:title>Can You Trust Your Memory?</jats:title> <jats:p> Being highly imaginative animals, humans constantly recall past experiences. These internally generated stimuli sometimes get associated with concurrent external stimuli, which can lead to the formation of false memories. <jats:bold> Ramirez <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="387" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1239073">387</jats:related-article> ; see the cover) identified a population of cells in the dentate gyrus of the mouse hippocampus that encoded a particular context and were able to generate a false memory and study its neural and behavioral interactions with true memories. Optogenetic reactivation of memory engram–bearing cells was not only sufficient for the behavioral recall of that memory, but could also serve as a conditioned stimulus for the formation of an associative memory. </jats:p>

<jats:title>In a FtsZ</jats:title> <jats:p> FtsZ is a guanosine triphosphatase that polymerizes into protofilaments at the bacterial division site. FtsZ recruits the accessory division proteins to the septum and also provides mechanical forces needed to constrict the membrane and reduce the cell width. However, how FtsZ generates mechanical force is unclear. While one popular model suggests that mechanical forces are generated by means of a change in FtsZ structure induced by guanosine triphosphate hydrolysis, nucleotide-dependent conformational transitions have yet to be observed in FtsZ monomer structures. Such transitions may be a feature of FtsZ only in its native protofilament-forming state. <jats:bold> Li <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="392" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1239248">392</jats:related-article> ) sought to resolve this question by obtaining high-resolution structures of guanosine diphosphate–bound FtsZ filaments. The results suggest a complex and dynamic FtsZ protofilament network with a high degree of plasticity that is capable of generating forces to drive cytokinesis, during cycles of hydrolysis, while maintaining the structural integrity of individual monomers. </jats:p>

<jats:title>PTEN Variations</jats:title> <jats:p> The product of the tumor suppressor gene <jats:italic>phosphate and tensin homolog on chromosome ten</jats:italic> ( <jats:italic>PTEN)</jats:italic> is a lipid and protein phosphatase that regulates important cellular processes, including growth, survival, and metabolism (see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="355" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1242541"> <jats:bold>Leslie and Brunton</jats:bold> </jats:related-article> ). Though PTEN is best known for effects on the phosphatidylnositol 3-kinase (PI3K) signaling pathway, the PTEN protein is also found in the nucleus. <jats:bold> Bassi <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="395" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1236188">395</jats:related-article> ) found that PTEN's presence in the nucleus was regulated in response to covalent modification of the protein by SUMOylation and phosphorylation. Cells lacking nuclear PTEN showed increased sensitivity to DNA damage and underwent cell death if the PI3K pathway was also inhibited. <jats:bold> Hopkins <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="399" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1234907">399</jats:related-article> , published online 6 June) discovered an alternative translation start site in human PTEN messenger RNA that allowed expression of a protein, PTEN-Long, with about 170 extra amino acids. The unusual enzyme was released from cells and then taken up into other cells. In a mouse tumor model, uptake of the enzyme inhibited the PI3K pathway and inhibited tumor growth. </jats:p>

<jats:title>PTEN Variations</jats:title> <jats:p> The product of the tumor suppressor gene <jats:italic>phosphate and tensin homolog on chromosome ten</jats:italic> ( <jats:italic>PTEN)</jats:italic> is a lipid and protein phosphatase that regulates important cellular processes, including growth, survival, and metabolism (see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="355" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1242541"> <jats:bold>Leslie and Brunton</jats:bold> </jats:related-article> ). Though PTEN is best known for effects on the phosphatidylnositol 3-kinase (PI3K) signaling pathway, the PTEN protein is also found in the nucleus. <jats:bold> Bassi <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="395" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1236188">395</jats:related-article> ) found that PTEN's presence in the nucleus was regulated in response to covalent modification of the protein by SUMOylation and phosphorylation. Cells lacking nuclear PTEN showed increased sensitivity to DNA damage and underwent cell death if the PI3K pathway was also inhibited. <jats:bold> Hopkins <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="399" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1234907">399</jats:related-article> , published online 6 June) discovered an alternative translation start site in human PTEN messenger RNA that allowed expression of a protein, PTEN-Long, with about 170 extra amino acids. The unusual enzyme was released from cells and then taken up into other cells. In a mouse tumor model, uptake of the enzyme inhibited the PI3K pathway and inhibited tumor growth. </jats:p>

<jats:title>Whence the “Eat Me” Signal?</jats:title> <jats:p> Cells are surrounded by a lipid bilayer, the composition of which is asymmetrical and serves as a marker of the physiological status of the cell. The phospholipid, phosphatidylserine (PtdSer), is normally found only on the inner leaflet of the membrane, but in dying cells it appears on the cell surface, thus providing the phagocytes tasked with cleaning up such cellular debris with a way to recognize cells undergoing cell death. Such movement of phospholipids within the membrane requires an elusive enzyme known as a scramblase. <jats:bold> Suzuki <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="403" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1236758">403</jats:related-article> ; published online 11 July) identified an enzyme, Xkr8, which appears to act as a scramblase that promotes exposure of PtdSer on the surface of dying mammalian cells. Consistent with such a role, Xkr8 was activated after cleavage by caspase 3, a key protease that promotes apoptotic cell death. Genetic studies with the homolog of Xkr8 expressed in <jats:italic>Caenorhabditis elegans</jats:italic> indicated that the protein played a similar role in tagging dead cells in the nematode worm during development. </jats:p>

<jats:title>Glucose Control Goes Out on a Limb</jats:title> <jats:p> Roux-en-Y gastric bypass, a surgical procedure used to induce weight loss in morbidly obese patients, often leads to permanent remission of diabetes, even when patients regain weight. Studying a rat model, <jats:bold> Saeidi <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="406" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1235103">406</jats:related-article> ; see Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="351" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1242673">Berthoud</jats:related-article> </jats:bold> ) found that the surgically reconfigured intestinal segment (the Roux limb) underwent an adaptive response characterized by increased glucose uptake and utilization, apparently triggered by exposure to undigested nutrients. As a result of this change, the intestine provided a major tissue for whole-body glucose control. Whether the same adaptive response occurs in the human intestine remains to be examined. </jats:p>

<jats:title>H7N9 Adaptation</jats:title> <jats:p> Puzzling and alarming reports of an outbreak in early 2013 of human infections by a low-pathogenicity avian influenza virus has rocked the poultry industry in central eastern China and brought fears of initiating a human pandemic. Over 130 human cases have been reported with 37 deaths until closure of poultry markets accompanied a near-cessation of human case reports. From surveillance sampling of &gt;10,000 isolates obtained during April 2013, <jats:bold> Zhang <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6144" page="410" related-article-type="in-this-issue" vol="341" xlink:href="10.1126/science.1240532">410</jats:related-article> , published online 18 July) took 37 isolates of avian origin H7N9 and compared them to human H7N9 isolates. The majority of H7N9 isolates came from live poultry markets, although some originated in pigeons. Sequence analysis indicated that the chicken isolates had retained the avian characteristics at sites on the influenza genes for PB2 and the surface hemagglutinin HA, where adaptive mutations have been observed before. Sequence analysis also showed a higher variability in the internal genes than in HA and neuraminidase NA. By using glycan arrays, it was shown that avian and human isolates bound to human, but also to some extent to avian, receptors. As expected, the virus replicated well in chickens without causing disease, whereas in mice only the human isolates were highly pathogenic. The human virus, but not the avian, transmitted between ferrets through the air. </jats:p>

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