Unraveling the role of tyrosyl- and glycyl-tRNA synthetases in neurodegeneration - insights from Drosophila

Date: 30 April 2015

Venue: UAntwerp, Campus Drie Eiken, Building Q, Auditorium Fernand Nédée - Universiteitsplein 1 - 2610 Wilrijk (Antwerp)

Time: 4:30 PM - 6:30 PM

PhD candidate: Biljana Ermanoska

Principal investigator: Prof Albena Jordanova, Prof Vincent Timmerman

Short description: PhD defence Biljana Ermanoska - Faculty of Pharmaceutical, Biomedical and Veterinary Sciences, Department of Biomedical Sciences



Abstract

The aminoacyl‐tRNA synthetases (aaRSs) are ancient enzymes that catalyze the ligation of an amino acid to its cognate tRNA in a two‐step aminoacylation reaction. By ensuring the accurate translation of the genetic information into a functional protein, these enzymes are essential for each living cell. Nevertheless, genetic defects in six aaRSs are specifically inducing neurodegeneration of the peripheral nerves in humans, diagnosed as various subtypes of Charcot Marie Tooth (CMT) disease. Dominant mutations in YARS and GARS are causing Dominant Intermediate CMT type C (DI‐CMTC) and CMT type 2D, respectively. Importantly, not all mutations in YARS and GARS impair their enzymatic activity, making the loss of the canonical aminoacylation function unlikely to be the trigger of the peripheral neurodegeneration.  It is currently unknown which function of the aaRSs is affected by the CMT‐causing mutations and how this leads to neurodegeneration.

Recently, we have described neurotoxic effects of the DI‐CMTC mutations in Drosophila, making this organism a valid model to study the underlying disease pathomechanism. Despite already fourteen CMT‐associated variations in GARS are described, none of them was successfully studied in an animal model thus far. As part of my PhD studies, we established a CMT2D model by expressing two pathogenic GARS mutations (G240R and P234KY) in Drosophila.

Interestingly, these flies exhibited phenotypes similar to the ones described in the DI‐CMTC Drosophila model, ranging from compromised viability, climbing defects and neuronal  electrophysiological and morphological impairment, as well as mutant specific phenotypes, amenable to largescale genetic screens. Additionally, we demonstrated that the cytoplasmic isoform of GARS is sufficient to induce the observed phenotypes. In order to give an answer to the question which function/s of YARS and GARS are affected by the CMT mutations, we performed an unbiased screening for diseaserelevant genetic interactions in Drosophila.

The first genetic evidences that the two CMTassociated aaRSs could have a shared mechanism underlying the disease pathology came by identifying seven common modifiers of mutant YARS and GARS‐induced ommatidial disorganization. Four of the modifier genes encode actin‐binding or ‐regulating proteins and they served as a genetic determinant to study actin cytoskeleton. Interestingly, we were able to demonstrated actin cytoskeleton misbalance in the nervous system of the DI‐CMTC/CMT2D flies, as well as in CMT patients‐derived cells.

Altered subcellular distribution of mutant YARS and GARS is a topic studied as extensively as the canonical function of both enzymes. For the first time, we demonstrate specific sub‐synaptic localization for the two aaRSs, where they affect the synaptic actin cytoskeleton and synaptic vesicles’dynamics. Additionally, we were able to modulate the neurotoxicity observed in the DI‐CMTC/CMT2D flies by altering the expression levels of Fimbrin, a genetic modifier with key function in actin remodeling.

In summary, by exploring the CMT2D and DI‐CMTC Drosophila models we were able to identify a) a novel role for the two CMT‐causing aaRSs in actin cytoskeleton remodeling, b) specific subsynaptic localization for both mutant and wild type YARS and GARS, and thereby, c) their contribution to the synaptic function and the DICMTC/CMT2D pathogenesis, respectively.