Institut de Biologie StructuraleGrenoble / France


Algal Remodeling in a Ubiquitous Planktonic Photosymbiosis

Photosymbiosis between single-celled hosts and microalgae is common in oceanic plankton. However, the functioning of this ecologically important cell-cell interaction and the subcellular mechanisms allowing the host to accommodate and benefit from its microalgae remain enigmatic. Here, using a combination of quantitative single-cell structural and chemical imaging techniques, a collaboration of researchers show that the structural organization, physiology, and trophic status of the algal symbionts (the haptophyte Phaeocystis) significantly change within their acantharian hosts compared to their free-living phase in culture. In symbiosis, algal cell division is blocked, photosynthesis is enhanced, and cell volume is increased by up to 10-fold with a higher number of plastids (from 2 to up to 30) and thylakoid membranes. This study unveils an unprecedented morphological and metabolic transformation of microalgae following their integration into a host, and it suggests that this widespread symbiosis is a farming strategy wherein the host engulfs and exploits microalgae.

The IBS-ISBG electron microscopy platform was involved in the preparation of the planctonic or cultured samples for electron microscopy imaging. An optimized sample preparation was also set up for correlative imaging between structural (TEM, SEM, FIB-SEM) and chemical imaging (X-ray fluorescence microscopy, SIMS).

Algal Remodeling in a Ubiquitous Planktonic Photosymbiosis. Decelle J, Stryhanyuk H, Gallet B, Veronesi G, Schmidt M, Balzano S, Marro S, Uwizeye C, Jouneau PH, Lupette J, Jouhet J, Maréchal E, Schwab Y, Schieber NL, Tucoulou R, Richnow H, Finazzi G, Musat N. Current Biology; doi: 10.1016/j.cub.2019.01.073

Molecular decoding of a key step in the maturation process of heparan sulfate

Heparan sulfate belongs to the family of glycosaminoglycans, a group of negatively charged polysaccharides, present in large quantities on cell surfaces and in interstitial tissues. They exert their activities by interacting with a large number of proteins, controlling their mechanism of action and thus intervening in most of the major biological functions (morphogenesis, division, signalling and cell migration, inflammation and immune responses, angiogenesis and tissue repair,… etc.) as well as in their pathological dysfunctions. These polysaccharides comprise various glycan domains, constituting the recognition zones for heparan binding proteins and are therefore essential for "coding" the various biological functions of the molecule. The molecular mechanisms associated with the biogenesis of these domains remain poorly documented.
A collaborative project involving the laboratory Architecture et Fonction des Macromolécules Biologiques, the Institut de Biologie Structurale and the Institut de Chimie Moléculaire et des Matériaux d’Orsay, made it possible to describe the mode of action of a key enzyme in the biogenesis of heparan sulfates, the C5-epimerase, which converts glucuronic acids (GlcA) into iduronic acids (IdoA). This function is essential to the maturation process of heparan sulfate since iduronic acids are systematically present at polysaccharide interaction sites. By combining glycan engineering and chemistry, protein biochemistry and structural biology (X-ray crystallography) approaches, the residues forming the catalytic site were identified as well as the binding modes of the substrate and the product. The mechanism of action of the enzyme involves conformational changes of the polysaccharide associated with selective distortions of the glucuronic entity to be epimerized.
These results provide the molecular and mechanistic basis for new strategies to modify glucuronic/iduronic acid residues at the polymer level and to generate, by chemo-enzymatic synthesis, heparan sulfate analogues for biotechnological or therapeutic applications.

Substrate binding mode and catalytic mechanism of human heparan sulfate D-glucuronyl C5 epimerase. Debarnot C, Monneau Y R, Roig-Zamboni V, Delauzun V, Le Narvor C, Richard E, Hénault J, Goulet A, Fadel F, Vivès R R, Priem B, Bonnaffé D, Lortat-Jacob H, Bourne Y. Proc Natl Acad Sci USA published ahead of print March 14, 2019

Cellular binding of a virus developed in cancer therapy elucidated at the atomic level

Adenoviruses cause diseases that can sometimes be fatal. By modifying them, they can also become formidable cancer cell killers. Adenoviruses are to date the most commonly used vectors in human clinical trials. Researchers have just elucidated by cryo electron microscopy the mechanism by which adenoviruses attach themselves to the cell surface. These results, published in the journal Nature Communication on March 12, 2019, could pave the way for the development of new generation anti-tumor vectors.

More than 60 adenovirus (Ad) serotypes are known in humans. While they are able to cause different types of diseases such as gastroenteritis or conjunctivitis, most of them have respiratory tropism. From childhood or adolescence, we have all been infected with several adenovirus serotypes either symptomatically (pneunomia, pharyngitis) or sometimes asymptomatically. Although not strictly speaking a major public health problem, several serotypes such as Ad3, Ad7 Ad11 and Ad14 (the subject of this study) may have been responsible for deaths among military recruits in the United States or more recently in a rehabilitation center in New Jersey where 11 of the 35 young patients died of Ad7 infection in late 2018.
In addition to this pathogenicity, adenoviruses are the most commonly used vectors in human clinical trials. Their success lies essentially in their use as oncolytic viruses. To do this, adenoviruses are modified to replicate only in the cancer cells. This treatment is already approved in China for some indications and numerous clinical trials are underway in the United States and Europe, offering great hope for new anti-tumor strategies.
Any virus needs to enter a cell to replicate, so binding to receptors on the cell surface is a key step in infection. It had been shown that some adenoviruses (Ad3, Ad7, Ad11 and Ad14) used desmoglein 2 (DSG2) to bind and enter cells. It remained to be understood at the molecular level how the adenovirus fibre (an elongated antenna-like protein present at 12 copies per virus) interacted with DSG2.
Until recently, solving the atomic structure of a small complex (the fiber/DSG2 complex is only 96kDa) seemed unthinkable. The latest technological developments of the Krios microscope have shown that this barrier can be broken. The researchers solved the structure of this complex at the atomic scale and visualized both the fibre and DSG2 residues that are involved in the interaction. Moreover, they showed that a point mutation in a single amino acid in adenoviruses was sufficient to completely abolish its binding to this receptor.
Understanding the mechanisms of adenovirus attachment to DSG2 opens two perspectives: on the one hand, consider the rational design of inhibitors of these pathogenic viruses and on the other hand, improve the targeting of oncolytic adenoviruses to tumors.

Cryo-EM structure of adenovirus type 3 fibre with desmoglein 2 shows an unsual mode of receptor engagement. Vassal-Stermann E, Effantin G, Zubieta C, Burmeister W, Iséni F, Wang H, Lieber A, Schoehn G, Fender P. Nature Communications in press, (2019)