Tissue-specific tagging of endogenous proteins in the fruit fly

Seeing is believing, and fluorescently tagged proteins have ushered in a major revolution in cell biology. Instead of observing the static components of dead cells fixed in plastic and reacted with dyes, tagged proteins fluorescing a variety of colors can be tracked in real time in live cells and organisms. We can peek at the previously only imaginable perpetual dynamism of life at the molecular level. In addition to turning us into spell-bound voyeurs, well-defined fluorescent tags also give us a hand-hold to isolate the binding partners of proteins of interest.

In a recent article by the Rodal lab reported in Biology Open, the authors report a new tagging methods designed to get rid of technological artifacts that can cause fluorescently tagged proteins to be expressed at the wrong time and place, and at the wrong levels. By using CRISPR mediated gene editing in fruit flies, they developed a novel approach to visualize any protein of choice in any tissue of choice at the level, localization and time that nature has intended. This method, dubbed T-STEP (for tissue-specific tagging of endogenous proteins), opens up novel experimental avenues to answer long-standing questions in several areas of neuroscience and cell biology, such as: how many different neurotransmitters are expressed in one neuronal circuit? Which tissue-type is a protein expressed in and when? What happens to a disease carrying mutant protein in a tissue of interest at endogenous levels?

tstep

As a proof of principle, two endosomal proteins, Vps35 (linked to Parkinson’s disease) and OCRL (linked to Lowe syndrome), which have never before been seen or localized in fruit flies, have now been visualized live at endogenous levels. Moreover, a Parkinson’s disease-specific mutation (D620N) in Vps35 has also been tagged with fluorescent proteins, opening up exciting new research avenues for interrogating binding partners and/or kinetics that may be altered during the diseased states.

In summary, T-STEP is an exciting novel tool that offers a simple and efficient method to tissue-specifically tag any protein at endogenous levels. Authors from the Rodal lab include Kate Koles (Research Scientist) and Anna Yeh ’16.

Lipids hit a “sweet spot” to direct cellular membrane remodeling.

Lipid membrane reshaping is critical to many common cellular processes, including cargo trafficking, cell motility, and organelle biogenesis. The Rodal lab studies how dynamic membrane remodeling is achieved by the active interplay between lipids and proteins. Recent results, published in Cell Reports, demonstrate that for the membrane remodeling protein Nervous Wreck (Nwk), intramolecular autoregulation and membrane charge work together in surprising ways to restrict remodeling to a limited range of lipid compositions.

F-BAR (Fes/Cip4 homology Bin/Amphiphysin/Rvs) domain family proteins are important mediators of membrane remodeling events. The F-BAR domain forms a crescent-shaped α-helical dimer that interacts with and deforms negatively charged membrane phospholipids by assembling into higher-order scaffolds. In this paper, Kelley et al. have shown that the neuronal F-BAR protein Nwk is autoregulated by its C-terminal SH3 domains, which interact directly with the F-BAR domain to inhibit membrane binding. Until now, the dogma in the field has been that increasing concentrations of negatively charged lipids would increase Nwk membrane binding, and thus would induce membrane deformation.

Surprisingly, Kelley et al. found that autoregulation does not mediate this kind of simple “on-off” switch for membrane remodeling. Instead, increasing the concentration of negatively charged lipids increases membrane binding, but inhibits F-BAR membrane deforming activities (see below). Using a combination of in vitro assays and single particle electron microscopy, they found that the Nwk F-BAR domain efficiently assembles into higher-order structures and deforms membranes only within “sweet spot” of negative membrane charge, and that autoregulation elevates this range. The implication of this work is that autoregulation could either reduce membrane binding or promote higher-order assembly, depending on local cellular membrane composition. This study suggests a significant role for the regulation of membrane composition in remodeling.

Brandeis authors on the study included Molecular and Cell Biology graduate students Charlotte Kelley and Shiyu Wang, staff member Tania Eskin, and undergraduate Emily Messelaar ’13 from the Rodal lab; postdoctoral fellow Kangkang Song, Associate Professor of Biology Daniela Nicastro (currently at UT Southwestern), and Associate Professor of Physics Michael Hagan.

Kelley CF, Messelaar EM, Eskin TL, Wang S, Song K, Vishnia K, Becalska AN, Shupliakov O, Hagan MF, Danino D, Sokolova OS, Nicastro D, Rodal AA. Membrane Charge Directs the Outcome of F-BAR Domain Lipid Binding and Autoregulation. Cell reports. 2015;13(11):2597-609.

Mugdha Deshpande named Blazeman Postdoctoral Fellow

Assistant Professor of Biology Avital Rodal has received a grant from the Blazeman Foundation to study the traffic of growth signals in neurons in the animal models of ALS (Amyotrophic Lateral Sclerosis).  ALS, commonly known as ‘Lou Gehrig’s disease’, is a neurodegenerative disease that causes the loss of motor neurons. The Blazeman Foundation is a non-profit organization working to increase the awareness about this terminal disease and to support research towards finding treatments. Funding to the Rodal lab has enabled creation of the Blazeman Foundation Postdoctoral Fellowship for ALS Research, awarded to Mugdha Deshpande, Ph.D., who will use live imaging to examine and manipulate membrane traffic in fruit fly models of ALS, and who will also work with Dr. Suzanne Paradis to translate her findings to mammalian ALS models.

You can read more at BrandeisNOW.

Nervous Wreck forms zig-zags to induce membrane ridges and scallops.

Em:LM Nwk F-BAR S2

Merged LM/EM images of Drosophila S2 cells featuring Nwk F-BAR induced protrusions.

Sorting and processing of the proteins that span cell membranes requires extensive membrane remodeling , including budding, tubulation, and fission. F-BAR domains form crescent-shaped dimers that bind to and deform membranes. Until now, it was thought that proteins containing these F-BAR domains induced membrane tubulation by assembling in highly ordered helical coats on lipid bilayers.

A new paper in Molecular Biology of the Cell from the Rodal lab (in collaboration with the Nicastro lab and the Sokolova Lab at Lomonosov Moscow State University) describes a novel membrane deforming activity for Nervous Wreck (Nwk), an F-BAR protein that regulates trafficking of transmembrane growth signal receptors at the Drosophila neuromuscular junction. The authors found that Nwk assembles into zig-zags on lipid monolayers, unlike the canonical F-BAR protein CIP4 which forms long filaments, even though the two proteins are predicted to be very structurally similar.  Unlike other members of the F-BAR family that tubulate the membrane, Nwk can induce the formation of membrane ridges and scallops (see figure below). These deformations can lead to dramatic cellular remodeling in cooperation with the cytoskeleton (see figure above). The work done by the Rodal lab suggests that while basic self-assembly and membrane binding properties are likely conserved between F-BAR proteins, the higher-order organization of Nwk may account for differences in membrane remodeling and its specialized role in the cell.

Nwk Model

Rodal named 2013 Pew Scholar

Assistant Professor of Biology Avital Rodal has been named a 2013 Pew Scholar in the Biomedical Sciences. The program “gives innovative scientists both the freedom to take calculated risks and the resources to pursue the most promising, but untried, avenues for scientific breakthroughs”, according to Rebecca W. Rimel, President and CEO of The Pew Charitable Trusts. Rodal has been recognized for her work in understanding the role of membrane deformation and dembrane trafficking in neurons, which evidence is starting to implicate in neurodegenerative disease (Alzheimer’s, ALS).

Rodal to Receive NIH New Innovator Award

The NIH recently announced that Assistant Professor of Biology Avital Rodal will be a recipient of the 2012 NIH Directors New Innovator Award. The award allows new, exceptionally creative and ambitious investigators to begin high impact research projects. Granted to early stage investigators, candidates are eligible for the award for up to ten years after the completion of their PhD or MD. The award emphasizes bold, new approaches, which have the potential to spur large scientific steps forward. This year’s award was made to fifty-one researchers, and provides each with 1.5 million dollars of direct research funding over five years.

The Rodal lab studies the mechanisms of membrane deformation and endosomal traffic in neurons as they relate to growth signaling and disease. Membrane deformation by a core set of conserved protein complexes leads to the creation of tubules and vesicles from the plasma membrane and internal compartments. Endocytic vesicles contain, among other cargoes, activated growth factors and receptors, which traffic to the neuronal cell body to drive transcriptional responses (see movie). These growth cues somehow coordinate with neuronal activity to dramatically alter the morphology of the neuron, and disruptions to both endocytic pathways and neuronal activity have been implicated in neurodegenerative diseases such as amyotrophic lateral sclerosis and Alzheimer’s disease.

Dr. Rodal hopes to determine how neuronal activity affects the in vivo function and biochemical composition of the membrane trafficking machinery, by examining the transport of fluorescently labeled growth factor receptors in chronically or acutely activated neurons at the Drosophila neuromuscular junction (NMJ). Her group will combine these live imaging studies with a proteomic analysis of endocytic machinery purified from hyper-activated and under-activated neurons. By investigating the interplay between neuronal activity, membrane deformation, and receptor localization in live animal NMJs, she hopes to gain a better understanding of the strategies that healthy neurons employ to regulate membrane trafficking events, and provide new insight into specific points of failure in neurodegenerative disease.

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