You are here: Home / Archives for electron microscopy

Deep inside a worm’s nose

March 29, 2014 By

In a new paper in eLIFE, a team spearheaded by Brandeis postdocs David Doroquez and Cristina Berciu provide a strikingly detailed look at key structures called cilia on neurons involved in sensory perception in the nematode C. elegans. Primary cilia are the antenna-like structures onsensory neurons that gather information about the animal’s environment, such as chemicals, temperature, humidity, and touch. The genetic tools available to manipulate individual, identifiable neurons in C. elegans make worms an excellent model organism to study the assembly and function of cilia. This study requires a description of the structure of the cilia and their immediate surrounding glial support cells, and this new paper, a collaboration of the Sengupta and Nicastro labs, provides high-resolution 3D models showing how diverse and specialized these structures are.

worm-01-2

A bouquet of sensory antennae. The 3D ultrastructure of all sensory cilia
and other neuronal projections in the head of the soil roundworm C.
elegans have been reconstructed using serial section transmission electron
microscopy. Shown are 3D isosurface-rendering models emerging from a
transmission electron microscopic cross-section of the worm.

The key techniques in this study were serial section transmission electron microscopy and electron tomography, with structures well-preserved by high-pressure freezing and freeze-substitution. With these techniques, the authors achieved the first high-resolution 3D reconstructions of 50/60 cilia from C. elegans. They describe several previously uncharacterized features — for example, there are distinct types of branching patterns – in one, the two cilia originate from independent basal bodies (as previously seen in Chlamydomonas). In the second, the cilia branch after the basal transition zone, the ciliary gatekeeper region. In the latter case, this basically means that whatever is needed for the cilia to branch has to be transported through the transition zone, suggest there might be novel mechanisms of ciliary protein trafficking. In a third pattern, the branching occurs proximally before the transition zone, and represent therefore dendritic microvilli, rather than ciliary branching. The study also showed different organizations  of microtubules in different cilia types and vesicles in regions of the cilia which have never been seen before, again pointing to new mechanisms of protein transport. They also describe new cilia-glial interactions, which might suggest that cilia and glia talk to each other.

For more about these structures (with lots of pretty pictures and movies), see:

Filed Under: Biology, Neuroscience Tagged With: , , , , , , , , ,

Making a gold studded protein ring

February 20, 2014 By

PLEASE NOTE: the paper by Anthony et al. in Structure was subsequently retracted due to the discovery of research misconduct by its first author, see http://www.cell.com/structure/abstract/S0969-2126(14)00016-1.

In economically turbulent times gold is acquired and held onto as a stable, secure commodity – it’s the “gold standard”. Gold of course has been a source of wealth as a precious metal and source of beauty. Importantly, gold is an incredibly dense and malleable transition metal that maintains its beauty and strength over long time periods, existing as a stable pure solid. Gold has also been an important subject of study and use in life science applications as well as in the physical sciences and in the clinical realm – not only as a source for fillings or a bridge after the dentist deals with your teeth issues!

Kelsey Anthony, a doctoral student in the Brandeis Biochemistry program as well at the Quantitative Biology program, has been working with gold in the Pomeranz Krummel lab to study biopolymer structure. The properties of gold most important in these applications are that it is a pure and stable solid, forms monodisperse spheroidal aggregates, is electron dense, and has the property of anomalously scattering x-rays at specific wavelengths. All these properties combined make gold an optimal metal to be “visualized”. In her most recent application of gold, in press in the journal Structure, Kelsey collaborated with a group at the University of Osnabruek in Germany in the synthesis of a reagent conjugated with monodisperse gold clusters or nanoparticles (called AuNPtris-NTA, see figure) and employed this reagent to localize protein(s) of interest in large multi-protein assemblies.

gold-tag

The experiment most visually striking to demonstrate the utility of this new “gold reagent” involved attaching it to a protein that interacts with itself to form a ring shaped structure. When visualized using the electron microscope, the gold clusters or nanoparticles site-specifically attached to the protein appear as extremely dense black spots due to their significant scattering of electrons as a consequence of the gold’s electron dense structure.

In essence, Kelsey has created a stunning golden microscopic studded ring. Next up, employing this gold conjugated reagent in other new ways.

See: Anthony et al., High-Affinity Gold Nanoparticle Pin to Label and Localize Histidine-Tagged Protein in Macromolecular Assemblies, Structure (2014)

Filed Under: Biochemistry, grad students, Quantitative BIology Tagged With: ,

DeRosier wins Distinguished Scientist Award from Microscopy Society of America

August 19, 2013 By

Professor Emeritus of Biology (and current Turrigiano lab “postdoc”) David DeRosier received the Distinguished Scientist Award (for Biological Science) at this year’s annual meeting of the Microscopy Society of America.

 

Filed Under: awards, Biology Tagged With: , , , ,

Cryo-electron tomography and the structure of doublet microtubules

September 22, 2011 By

In a new paper in PNAS entitled “Cryo-electron tomography reveals conserved features of doublet microtubules“, Assistant Professor of Biology Daniela Nicastro and coworkers describe in striking new detail the structure and organization of the doublet microtubules (DMTs), the most conserved feature of eukaryotic cilia and flagella.

Cilia and flagella are thin, hair-like appendages on the surface of most animal and lower plant cells, which use these organelles to move, and to sense the environment. Defects in cilia and flagella are known to cause disease and developmental disorders, including polycystic kidney disease, respiratory disease, and neurological disorders. An essential feature of these organelles is the presence of nine outer DMTs (hollow protein tubes) that form the cylindrical core of the structure known as the axoneme. The doublet microtubule is formed by tubulin protofilaments and other structural proteins, which provide a scaffold for the attachment of dynein motors (that drive ciliary and flagellar motility) and regulatory components in a highly specific and ordered manner.

To address long-standing questions and controversies about the assembly, stability, and detailed structure of DMTs , the Nicastro lab used a high-resolution imaging technique, cryo-electron microscope tomography (cryo-ET), to probe the structure of DMTs from Chlamydomonas (single-celled algae) and sea urchin sperm flagella. Cryo-ET involves:

  1. rapid freezing of the sample to cryo-immobilize the molecules without forming ice crystals,
  2. tilting the specimen in the electron microscope to collect ~70 different views from +65° to –65°,
  3. computational alignment of the views to calculate a tomogram (a three-dimensional reconstruction of the imaged sample), and
  4. computational averaging of repeating structures in the tomogram to reduce noise and increase resolution.

Cryo-ET provided the necessary resolution to show that the B-tubules of DMTs are composed of 10 protofilaments, not 11, and that the inner and outer junctions between the A- and B-tubules are fundamentally different (see figure). The outer junction, crucial for the initial formation of the DMT, appears to be formed by interactions between the tubulin subunits of three protofilaments with unusual tubulin interfaces, but one of these protofilaments does not fit with the conventionally accepted orientation for tubulin protofilaments. This outer junction is important physiologically, as shown by mutations affecting the usual pattern of posttranslational modifications of tubulin. In contrast, the inner junction is not formed by direct interactions between tubulin protofilaments. Instead, a ladder-like structure that is clearly thinner than tubulin connects protofilaments of the A- and B-tubules.

The level of detail also allowed the Nicastro lab to show that the recently discovered microtubule inner proteins (MIPs) located within the A- and B-tubules are more complex than previously thought. MIPs 1 and 2 are both composed of alternating small and large subunits recurring every 16 and/or 48 nm along the inner A-tubule wall. MIP 3 forms small protein arches connecting the two B-tubule protofilaments closest to the inner junction, but does not form the inner junction itself. MIP 4 is associated with the inner surface of the A-tubule along the partition protofilaments, i.e., the five protofilaments of the A-tubule bounded by the two junctions with the B-tubule.

The Nicastro lab plans to build on this foundation in future work on the molecular assembly and stability of the doublet microtubule and axoneme, and hope to use it to elucidate molecular mechanisms of ciliary and flagellar motility and signal transduction in normal and disease states.

Other authors on the paper include Brandeis postdocs Xiaofeng Fu and Thomas Heuser, Brandeis undergrad Alan Tso (’10), and collaborators Mary Porter and Richard Linck from the University of Minnesota.

Filed Under: Biology, labs Tagged With: , , , , , , , , ,

3D electron microscopy reveals: twin spokes are not twins

May 31, 2011 By

Movement of cells has fascinated scientists for centuries. Improved handcrafted light microscopes in the late 17th century allowed observations of contracting muscle fibers, single-cell organisms gliding through water drops or cells crawling across surfaces. How cell motility is generated and regulated is an ongoing question researchers at Brandeis and many other institutions are trying to answer. The single-cell green algae Chlamydomonas reinhardtii has two eukaryotic flagella (Fig. A) and is a popular genetic model system for studying these motile organelles, which are also called cilia or undulipodia. Cilia and flagella are basically the same organelles that are highly similar from single-cell algae to humans, but when a cell has many relatively short and asymmetrically beating ones they are called cilia (e.g. on the multi-ciliated epithelial cells that line our airways and are important for mucus-clearance), while a few long ones with often symmetric waveforms are called (eukaryotic) flagella (e.g. the sperm flagellum). These should not be confused with bacterial and archaeal flagella, which are very different in structure and evolutionary origin. Eukaryotic cilia and flagella consist of a microtubule-based, cylindrical core with hundreds of similar building blocks that repeat along the length of the organelle (Fig. B-D). In a single flagellum the activity of thousands of motor proteins, dyneins, has to be coordinated to generate motility, and important regulatory complexes include the radial spokes, in Chlamydomonas two spokes per building block (RS1 and RS2) (Fig. D). Recently, Dr. Thomas Heuser, a postdoc in Dr. Daniela Nicastro’s lab at Brandeis, successfully used three-dimensional electron microscopy (electron tomography) to study the structure of rapidly frozen Chlamydomonas flagella in unprecedented detail (Heuser et al. 2009).

Erin Dymek from Dr. Elizabeth Smith’s laboratory at Dartmouth College found that the concentration of Calcium ions, a known regulatory signal modulating ciliary and flagellar motility, affects dynein activity through a conserved Calmodulin and Radial Spokes associated Complex (CSC) (Dymek and Smith, 2007). Erin Dymek and Elizabeth Smith have now teamed up with Tom Heuser and Daniela Nicastro to study the 3D location of this Calcium sensing complex in flagella. In a recent paper (Dymek et al. 2011 MBoC in press) they compared the wild type structure of Chlamydomonas flagella to several artificial microRNA-interference mutants lacking parts of the CSC. They found that in all amiRNAi mutants many of the flagellar building blocks were missing one specific radial spoke, RS2, while RS1 was always present (Fig. E-G), suggesting that the Calcium sensing CSC is located at or near RS2. Interestingly, RS1 and RS2 were previously assumed to be structurally identical, their different numbering simply referred to their proximal and distal location within the repeating building block. The current study not only indicates that the CSC is required for spoke assembly and wild type motility, but as one of the most surprising outcomes it also provides evidence for heterogeneity among the radial spokes, at least at the base where the spokes are anchored to the microtubules. The same team of biologists is now continuing to study the CSC location in the flagellar building block in greater detail by improving image processing strategies to increase resolution.

Filed Under: Biology, labs, postdocs Tagged With: , , , , , , , ,

High resolution virus structures from electron cryo-microscopy

February 24, 2011 By

Professor of Biochemistry Nikolaus Grigorieff discusses recent progress in obtaining virus structures at 4 Å or better resolution from electron microscopy in a new review “Near-atomic resolution reconstructions of icosahedral viruses from electron cryo-microscopy” in Current Opinon in Structural Biology.

Filed Under: Biochemistry Tagged With: , , , , ,

Protected by Akismet
Blog with WordPress

Welcome Guest | Login (Brandeis Members Only)