Drosophila Eye Model to Study Dorso-Ventral (DV) Patterning and Neurodegenerative Disorders

An important question in developmental biology is how any three-dimensional organ develops from single monolayer sheet of cells. In multicellular organisms, organogenesis requires axial patterning to determine Antero-Posterior (AP), Dorso-Ventral (DV), and Proximo-Distal (PD) axes. DV patterning marks first lineage restriction event during eye development, any deviation during this event during development results in defective organ formation. We have used Drosophila melanogaster (a.k.a, fruit fly) eye as our model organ as 75% of genetic machinery is conserved between fruit flies and humans and have identified defective proventriculus (dve, a Homeobox gene), an ortholog of SATB-homeobox-1 (special AT-rich sequence binding protein-1 in humans), as a new member of DV- patterning genes hierarchy. We have shown that (1) dve acts downstream of pannier (pnr, a GATA-1 transcription factor), and upstream of wingless (wg), (2) Loss-of-function (LOF) of both dve or pnr results in dorsal eye enlargements, while their Gain-of-function (GOF) suppresses the eye fate, and (3) Furthermore, Wingless (Wg, WNT homolog), downstream target of evolutionarily conserved Hippo growth regulatory pathway, acts downstream of dve in the eye, and exhibits similar eye enlargement or suppression phenotypes upon LOF or GOF. It suggests that like wg, dve also plays an important role in regulating growth. To characterize the function of dve (member of DV patterning pathway) during development, we looked for its interacting partners and found that it interacts antagonistically with Hippo signaling to regulate optimum levels of expression of their common downstream target, Wg, to specify eye versus head fate, during growth and patterning in developing eye. Additionally, GOF of SATB1 (vertebrate ortholog of dve) in the eye also resulted in Wg upregulation and eye suppression. Since GOF of hippo (hpo) triggers cell death, we tested if by blocking cell death by using p35 (anti-apoptotic) exhibits similar phenotypes. We found that eye enlargement phenotype resulting from GOF of hpo in dve domain, is not due to hpo mediated cell death, but by regulating retinal differentiation. Overall, this study presents a model that shows genetic interaction between two unrelated pathways of growth regulation and axial (DV) patterning and have significant bearing on developmental mechanisms. Another focus of this study is to employ Drosophila eye model to study Amyotrophic Lateral Sclerosis (ALS), a neurodegenerative disorder characterized by loss of upper and lower motor neurons in central nervous system with no known cure to-date. Mutations in genes like human-Fused in Sarcoma (h-FUS) or cabeza (caz) in Drosophila, have been known to cause ALS in flies. Misexpression of h-FUS-WT (Wild-Type), or FUS mutants FUS-R518K or FUS-R521C in Drosophila eye using GAL4-UAS genetic tool, triggers ALS-mediated neurodegeneration. To understand the mechanism of action, we screened for genetic modifiers and found hippo (hpo), as a genetic modifier. We next tested if this neuroprotective function is exclusive to hpo gene or is dependent on Hippo pathway. We modulated Hippo pathway in FUS-WT or mutant-FUS background and found that downregulation of Hippo pathway, exhibited significant rescue in the eye, but the exact mechanism of action was still unclear. Hippo pathway has been known to activate c-Jun-N-Terminal Kinase (JNK), which is involved in neurodegeneration and cell death. To elucidate the mechanism of action, we modulated JNK signaling in FUS or mutant-FUS background and found that downregulation of JNK signaling also rescued FUS mediated neurodegeneration in eye. This study presents a new model that explains how FUS causes neurodegeneration and has significant bearing on search for future therapeutic targets that can modify neurodegenerative behavior of ALS