PodcastCell Migration

Science Signaling Podcast: 26 May 2015

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Science Signaling  26 May 2015:
Vol. 8, Issue 378, pp. pc13
DOI: 10.1126/scisignal.aac4866

Abstract

This Podcast features an interview with Min Zhao and Peter Devreotes, authors of a Research Article that appears in the 26 May 2015 issue of Science Signaling, about how cells migrate along an electrical gradient. Chemotaxis is the process by which cells move along a gradient of chemical cues. Cells can also be guided by electrical fields through a process called electrotaxis, also known as galvanotaxis. Whereas much is known about how cells sense and respond to chemical cues during chemotaxis, less is known about how cells sense and respond to electrical gradients. Electrical gradients are present in many tissues, and accumulating evidence indicates that electric fields are important for guiding cells during wound healing and regeneration. Gao et al. used a high-throughput screening technique to identify genes required for electrotaxis in the slime mold Dictyostelium discoideum, which is a model that is often used to study chemotaxis. The screen revealed that chemotaxis and electrotaxis share many of the same protein mediators, including components of the target of rapamycin complex 2 (TORC2) and phosphoinositide 3-kinase (PI3K) signaling pathways.

(Length: 14 min; file size: 8.0 MB; file format: mp3; location: http://podcasts.aaas.org/science_signaling/ScienceSignaling_150526.mp3)

Technical Details

Length: 14 min

File size: 8.0 MB

File Format: mp3

RSS Feed: http://stke.sciencemag.org/rss/podcast.xml

Listen to Podcast: http://podcasts.aaas.org/science_signaling/ScienceSignaling_150526.mp3

Educational Details

Learning Resource Type: Audio

Context: High school upper division 11-12, undergraduate lower division 13-14, undergraduate upper division 15-16, graduate, professional, general public and informal education

Intended Users: Teacher, learner

Intended Educational Use: Learn, teach

Discipline: Cell biology, developmental biology, microbiology

Keywords: Science Signaling, chemotaxis, Dictyostelium discoideum, electrotaxis, galvanotaxis, genetic screen, migration, phosphoinositide 3-kinase, PI 3-kinase, PI3K, target of rapamycin complex 2, TORC2, slime mold

Transcript

Host – Annalisa VanHookWelcome to the Science Signaling Podcast for May 26th, 2015. I’m Annalisa VanHook, and today I'm talking to Min Zhao and Peter Devreotes about how cells move along an electrical gradient (1).

Most of you are probably familiar with the process of chemotaxis, in which chemical cues present in the environment guide the migration of cells. But cells can also be guided by electric fields, a phenomenon referred to as electrotaxis or galvanotaxis. We know a lot about how cells sense and respond to chemical cues during chemotaxis, but less is known about how cells sense and respond to electrical gradients. Electrical fields are present in many tissues, and there's accumulating evidence that electric fields are important for guiding cells to sites of injury during wound healing and regeneration. Groups from the labs of Min Zhao and Peter Devreotes have conducted a screen for genes required for electrotaxis in the slime mold Dictyostelium discoideum, which is a model that's often used to study chemotaxis. Their study, published this week in Science Signaling, reveals that chemotaxis and electrotaxis share some of the same protein mediators. Min Zhao spoke to me from the Univerisity of California at Davis, and Peter Devreotes joined us from the Johns Hopkins University School of Medicine in Baltimore.

Interviewer – Annalisa VanHookDr. Zhao and Dr. Devreotes, welcome to the Science Signaling Podcast.

Interviewee – Min ZhaoHi, it’s great to speak to you.

Interviewee – Peter DevreotesIt’s good to be with you.

Interviewer – Annalisa VanHookI think everyone is pretty familiar with the phenomenon of chemotaxis, with cells migrating along chemical gradients. But cells can also orient themselves in and migrate along electrical gradients. How are electrical gradients generated in tissues or in the environment?

Interviewee – Min ZhaoCells pump ions, and this pumping of ions produce[s] electric current. One typical example is the membrane potential, and we have electrical potential across the cell membrane, and that’s very classical and hardcore science. But there are also other electrical phenomena associated with the living organism. Where a wound is generated and which disrupts transepithelial potential, so there's an electrical potential gradient across epithelia everywhere, and if a wound is generated that would generate a lateral-orientated electrical field. So that is one example. In development, people also detect large electric currents in early embryo[s]. Even for giant amoeba—migrating, polarized, giant amoeba about half millimeter [in] length—and people also detect [a] current loop around the migrating amoeba. So it seems to me that the living organism would generate this type of electric field.

Interviewer – Annalisa VanHookThose electric chemical gradients are generated by cells pumping different ions into and out of the cell, between the cell interior and the environment, but how do cells sense electrical gradients?

Interviewee – Peter DevreotesI think we don’t know specifically how that works yet; I mean, that’s what we’re trying to find out. One of the things we learned from this study is there is a great overlap in the systems that cells use for chemotaxis and for electrotaxis.

Interviewee – Min Zhao[That's] one of the reasons we work together to develop this screening technology to identify molecules, genes, signaling pathways, and hope to understand how cells sense electrical field, because the voltage across the cell is really minute. Ken Robinson, Richard Nuccitelli already reported many years ago that cells can detect electrical fields at about 10 millivolts per millimeter; that’s less than 1 millivolts across a cell. There are no known molecules are able to detect and respond to such a weak electric field. Nobody knows how the cells sense such a weak electrical field.

Interviewer – Annalisa VanHookYou’re using the slime mold Dictyostelium discoideum to study electrotaxis. Why is Dicty such a good model for studying chemotaxis and electrotaxis?

Interviewee – Peter DevreotesThis is a very fundamental response of cells. And many, many kinds of cells that Min has tested are able to carry out electrotaxis with very similar characteristics. So, that makes it very amenable to using a model organism to study this process if it’s a fundamental cell biological process. And we know a lot already about mechanisms of migration in Dictyostelium. In addition, we have very nice genetic tools to create libraries of cells with random genetic differences, and that allows us to do unbiased screens for things that might be involved in electrotaxis, which is what we’ve done.

Interviewer – Annalisa VanHookCan you give us an overview of your screening process? How did you identify genes that are required for electrotaxis?

Interviewee – Min ZhaoWe have many types of mutants, hundreds of thousands, and Peter has mutant libraries of 200,000 different mutant cells. But the problem is how can we screen them for electrotaxis phenotype. And then we developed this barcoded microplate; it’s about 500 micron by 500 micron square, and we can culture cells on the plate. So, for example, we can generate millions of different type of microplates, and we can culture all sort of cells on different plates with different barcode. And then, after culture, we can mix the plate in the electrotaxis chamber, and this is how we think we solve one of the rate-limiting steps in the electrotaxis experiment. Originally, we could only do one type of cell because we could only apply field in one chamber. But now introducing this microplate with the barcode labeled for different type of cells, we can do many type, up to 100 different strains or mutants in one chamber. So in the current experiment, we screened for 30 strains in one experiment, and then we identified from the collection 20 strains of severe defective mutant[s] and 10 strains of slightly higher response, and wild-type-like group, and moderately defective strains.

Interviewee – Peter DevreotesI should add, though, even though the screen was made easier by the microplates, it still took a lot of perseverance on the part of Runchi Gao and others who carried out the screening. And, you know, it was kind of a heroic effort to screen that many.

Interviewee – Min ZhaoYes. And the plate was developed by Tingrui Pan in biomedical engineering at UC Davis. Yes, it's a lot of efforts by developing the libraries and screening, looking at millions of cells tracked through the experiment.

Interviewer – Annalisa VanHookFrom all these mutants, you identified some that had defective electrotaxis, and so from those mutants you were able to identify genes that are required for electrotaxis. Of those genes, how many of them were unique to electrotaxis, and how many were also shared for chemotaxis?

Interviewee – Peter DevreotesMost of the genes we found overlap with the genes that are involved in chemotaxis. Now we know that’s not completely the case, that everything will overlap, and we know there are some genes that are unique to chemotaxis. For example, the chemoattractant receptors and G protein subunits that couple to those receptors, we have knockout cell lines of those, and those are able to carry out electrotaxis and they’re unable to carry out chemotaxis. So there are genes which are specific for chemotaxis, but so far all of the genes that we’ve pulled out for electrotaxis, or most of the genes we pulled out for electrotaxis, seem to also be involved in chemotaxis. So, we haven’t found, if you want, the electrotaxis “receptor,” if there is one, equivalent to the chemoattractant receptor.

Interviewee – Min ZhaoBut, Peter, would that be because of the establishment of the library, like we screen them for developmental defect?

Interviewer – Annalisa VanHookRight. So you may have missed genes that are important for electrotaxis that are also required for growth or development of the organism.

Interviewee – Min ZhaoI would think so.

Interviewee – Peter DevreotesYes.

Interviewer – Annalisa VanHookSlime molds are protists; they’re not animals. How many of the genes that you identified are shared with animals and therefore might be relevant to electrotaxis in processes like development and in wound healing?

Interviewee – Peter DevreotesMost of the genes that we identified have very strong homologs in mammalian cells, so they are things that are involved in regulation of RAS, PI3 kinase. Many of the things that are involved in migration are also conserved in mammalian cells, so that goes for these genes as well.

Interviewer – Annalisa VanHookAnd, of course, the mechanical mediators of migration, the cytoskeletal proteins and their modifiers, would be shared.

Interviewee – Peter DevreotesYes. Also many of the signal transduction components as well.

Interviewee – Min ZhaoAnd very importantly, like the PI3 kinase Peter mentioned, they are fundamental for regeneration, wound healing, and development, and also mTORC2. So this is the genes we focused on from the screening-identified mutant[s]. This is also very strongly implicated in neutrophil chemotaxis.

Interviewer – Annalisa VanHookYou mentioned two signaling pathways in particular, the PI3 kinase and the TORC2 signaling pathways, that are required for electrotaxis in Dictyostelium, and they’re also required for chemotaxis in Dictyostelium and in wound healing and repair in animal models. Is there anything different about the signaling through those pathways in the context of electrotaxis compared to the context of chemotaxis? Were you able to figure out how they’re activated differently?

Interviewee – Peter DevreotesIf we immobilize cells and we put a chemotactic gradient on them, we’re able to see that these pathways are asymmetrically localized within the immobilized cells with chemotaxis, and we’re unable to see that with electrotaxis. So, we know for chemotaxis that those pathways are upstream of the motility. In the case of electrotaxis, I don’t think we can be sure about that. It could be that the electric field sets the cell in motion, and then there’s a feedback to these signaling pathways which amplifies that signal, and therefore, if you take away the motility, then the signaling pathways are not activated. We’re still working on that. But, in other words, you could be in one case the signal transduction system with a receptor from what you call the top down, whereas in the other case you could be activating the signal transduction pathway through feedback from the cytoskeletal network. We’re still working on that, but that’s how it looks to me.

Interviewer – Annalisa VanHookWell, it sounds like you have a lot of interesting candidates to follow up on.

Interviewee – Min ZhaoYes, of course, and sometimes too many. We haven’t talked about the hypersensitive [mutants], and this is also a very interesting group of cells we identified—they have enhanced response and possibly the cytoskeleton [is] being modulated more and positive feedback somehow enhanced.

Host – Annalisa VanHookPeter and Min, thanks for speaking with me.

Interviewee – Min Zhao.It’s a pleasure.

Interviewee – Peter DevreotesYes.

Host – Annalisa VanHookThat was Min Zhao and Peter Devreotes, discussing a paper published in the May 26th issue of issue Science Signaling by Gao and colleagues (1). You can read that article online at stke.sciencemag.org.

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The Science Signaling Podcast is a production of Science Signaling and the American Association for the Advancement of Science—Advancing Science, Serving Society. If you have any comments or questions, you can write to us at sciencesignalingeditors{at}aaas.org. I'm Annalisa VanHook, and on behalf of Science Signaling and AAAS, thanks for listening.

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