{"id":1323,"date":"2014-08-06T09:00:04","date_gmt":"2014-08-06T07:00:04","guid":{"rendered":"http:\/\/news.embl.de\/?p=1323"},"modified":"2024-11-14T16:32:26","modified_gmt":"2024-11-14T15:32:26","slug":"clarity-cold","status":"publish","type":"post","link":"https:\/\/www.embl.org\/news\/science\/clarity-cold\/","title":{"rendered":"Clarity in the cold"},"content":{"rendered":"\n

Like all insects, the fruit fly Drosophila melanogaster<\/em> is a \u2018cold-blooded\u2019 animal. The term is a bit of a misnomer; it doesn\u2019t necessarily mean that those animals are always cold, but rather that they cannot control their body temperature. If a fruit fly is sitting in a lab at 25\u00baC \u2013 which seems to be their preferred temperature \u2013 essentially all the cells in its body will be at 25\u00baC. If room temperature drops to 18\u00baC, so does the temperature inside the fly\u2019s cells. The cells that make up its compound eyes will be at 18\u00baC, its stomach cells will be at 18\u00baC, and if it\u2019s a female, the egg cells in its ovaries will be developing at 18\u00baC. This inability to keep a constant body temperature poses problems, as many of the enzymes that carry out vital processes within those cells are sensitive to temperature fluctuations.<\/p>\n\n\n\n

Anne Ephrussi\u2019s lab at EMBL Heidelberg and colleagues at Rochester University in the USA have now discovered one way in which the fruit fly manages to lay viable eggs not only in its preferred conditions, but also at lower temperatures.<\/p>\n\n\n\n

The group\u2019s main focus is on how some RNA molecules get placed in particular positions in cells, as this can be essential for cell function and embryo development. One model used for these studies is a molecule called oskar<\/em>, which acts as a determinant in the egg cell, defining the region that will become the fly embryo\u2019s posterior. oskar<\/em> is carried to the right place in the cell in the form of an RNA template, which is then translated into protein on the spot, where it is needed. In 2011, Imre Gaspar, a postdoc in the Ephrussi lab, got to talking to Michael Welte from Rochester University, when the latter came to Heidelberg to give a talk. Welte told Gaspar his lab had found that a protein called Klar accumulates in the same place as Oskar, at the same time. When he added that they had some indications that without Klar, Oskar didn\u2019t behave \u2018normally\u2019, Gaspar\u2019s curiosity was spiked. Welte went back to the US, and what would become a four-year collaboration got under way, with Gaspar aiming to find out exactly what the connection was.\u201cIt required a lot of detailed analysis to figure out exactly what was going on, because our initial ideas were completely wrong,\u201d says Gaspar.<\/span><\/p>\n\n\n\n

It required a lot of detailed analysis to figure out exactly what was going on, because our initial ideas were completely wrong<\/p><\/blockquote>\n\n\n\n

Klar was first discovered three decades ago by EMBL alumni Christiane N\u00fcsslein-Volhard and Eric Wieschaus, as part of work that eventually earned them a Nobel prize. They named it based on the German word for \u2018clear-sightedness\u2019, since embryos without Klar are transparent. This transparency arises from an inability to transport lipid droplets properly, and the scientists reasoned that, in egg cells that lack Klar, oskar too might fail to get to where it was needed. The scientists\u2019 initial data seemed to confirm this assumption, but when Gaspar delved deeper he found that the effects were more subtle \u2013 and complex.<\/p>\n\n\n\n

\u201cSomehow, in the absence of Klar, at 25 degrees-ish, all these processes are functioning at rates that are compatible with each other \u2013 things are working in sync,\u201d says Ephrussi. \u201cBut if one then goes to 18 degrees, things go wrong.\u201d Despite originating from tropical climes, Drosophila melanogaster<\/em> can thrive at 18\u00baC. However, at this temperature, in flies engineered to have no Klar, oskar<\/em> RNA is carried to the egg cell\u2019s posterior pole, but not all that RNA stays there. The surplus oskar<\/em> RNA is taken elsewhere, but is already primed for translating into protein, so the protein ends up accumulating in other parts of the egg cell \u2013 at the risk of rendering it unviable or causing major developmental defects.<\/p>\n\n\n\n

This incoordination comes about, the scientists surmise, because the drop in temperature has a more dramatic effect on the anchoring machinery than on the transport network. \u201cIf one process slows down more than the other, you may try to live with it \u2013 but you are probably going to fail,\u201d Gaspar explains. \u201cThe alternative is you have to find a dedicated regulator that brings down the other process; and that\u2019s what Klar does.\u201d In short, when a fruit fly egg cell is subjected to low temperatures, Klar cranks oskar<\/em> transport down to almost nothing, to keep delivery of oskar at a rate that the anchoring machinery \u2013 which is considerably slowed by the temperature drop \u2013 is capable of handling.<\/p>\n\n\n\n

If one process slows down more than the other, you may try to live with it \u2013 but you are probably going to fail<\/p><\/blockquote>\n\n\n\n

Klar also coordinates the transport of other cargoes within cells, so it may very well be that this protein helps keep these processes in sync, too \u2013 and not just in Drosophila melanogaster<\/em>. For instance, a separate study published just weeks before Gaspar\u2019s<\/a> showed that the embryos of several Drosophila<\/em> species can develop at a wide range of temperatures. And, since many other events that are crucial to development also involve several steps driven by different molecular machines, it seems likely that there are other enzymes besides Klar whose role is to keep such processes in synch. Such molecules are likely key to enabling \u2018cold blooded\u2019 animals to deal with the vagaries of changing environments, and to Gaspar they raise a tantalising question: how do they sense temperature shifts?<\/p>\n\n\n\n

Although understanding what this particular mini-thermometer does when it feels the cold took years of work, elaborate experiments and complex analysis, ultimately the findings have an element of simplicity, the scientists say. \u201cFruit-flies survive even without Klar, so the ancestors of modern flies could have done without it. But when Klar evolved in some of those flies, they became able to regulate this process, and lay more embryos that eventually hatched. So they will have had more progeny, and that\u2019s how they will quickly have overgrown those that could not. So in terms of evolution, this process can be understood very simply,\u201d Gaspar humbly concludes.<\/p>\n\n\n\n

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