Understanding Amyotrophic Lateral Sclerosis Using the Drosophila Tripartite Synapse.

Abstract

Understanding Amyotrophic Lateral Sclerosis using the Drosophila Tripartite synapse. Shweta Tendulkar

Amyotrophic Lateral Sclerosis (ALS) is a progressive motor neurodegenerative disorder in humans with the death of motor neurons being a consequence of the disease. ALS patients have a life expectancy of 3-5 years, after disease onset, with no known cure. Research has been focused primarily on neuronal cells, with less attention given to the other two tissue types that make up the tripartite synapse, namely muscle and glia. Increasingly, it is becoming apparent that these non-neuronal cells are important contributors to the onset and/or progression of the disease. For my Ph.D. dissertation, I am attempting to understand the molecular and cellular basis of the disease, by using Drosophila melanogaster as a model organism and focusing on contributions from non-neuronal cells. As a first step, we modulate the levels of fly orthologues of human genes that cause familial ALS independently in neurons, glia and muscles. Then, we measure changes in lifespan, and motor function in these animals. For these manipulations, we use as a sensitized genetic background, a point mutation in the fly orthologue (VAP or VAP33A) of the human ALS8 causative gene VAPB. Drosophila lines carrying the VAPP58S mutation (VAPnull;+; vap-promoter>VAPP58S) have a shortened life span, show VAPP58S positive inclusions and demonstrate increased motor dysfunction with age. In the above defined fly model, using the UAS-Gal4 system, we have modulated the activity of Drosophila orthologues of seven ALS causing genes, in a tissue specific manner, namely VAPB, FUS, TBPH, SOD1, VCP, Alsin and SETX. We find that reducing or increasing the activity of some of the ALS orthologues in muscle, glia and neurons can reduce life-span of the fly model and enhance the deterioration of motor function. Interestingly, in one scenario, overexpression of a mutant form of VCPR152H in glia leads to a distinct improvement of lifespan and motor function. Since VCP (Valsolin containing protein) is an integral partner in the degradative Ubiquitin-Proteasomal System (UPS), this suggests that that flux in protein degradative pathways may influence progression of the disease. Next, we find that over-expression of Caspar (Casp) in the glia can delay onset of disease in our fly model. Casp is an orthologue of human FAF1 and we additionally demonstrate is a physical interactor of both VAP and VCP. Additionally, we also find that modulation of canonical intercellular signaling pathways in the tripartite synapse can also extend lifespan and delay motor dysfunction in our disease model. Our data and analysis suggest a critical role for UPS in ALS, with Casp connecting two ALS causative loci - VCP/ALS11 and VAP/ALS8. A complete characterization of the functional significance of the VAP/Casp/VCP interconnectivity should lead to a better understanding of molecular pathways and mechanisms that underpin the disease. In the future, we expect to further define genetic sub-networks that contain multiple ALS loci and uncover their relationship to the UPS, as also anterograde and retrograde signals in the tripartite synapse.