Genetic Modifiers of Drosophila Palmitoyl–Protein Thioesterase 1-Induced Degeneration

Abstract

Infantile neuronal ceroid lipofuscinosis (INCL) is a pediatric neurodegenerative disease caused by mutations in the human CLN1 gene. CLN1 encodes palmitoyl-protein thioesterase 1 (PPT1), suggesting an important role for the regulation of palmitoylation in normal neuronal function. To further elucidate Ppt1 function, we performed a gain-of-function modifier screen in Drosophila using a collection of enhancerpromoter transgenic lines to suppress or enhance the degeneration produced by overexpression of Ppt1 in the adult visual system. Modifier genes identified in our screen connect Ppt1 function to synaptic vesicle cycling, endo-lysosomal trafficking, synaptic development, and activity-dependent remodeling of the synapse. Furthermore, several homologs of the modifying genes are known to be regulated by palmitoylation in other systems and may be in vivo substrates for Ppt1. Our results complement recent work on mouse Ppt1À/À cells that shows a reduction in synaptic vesicle pools in primary neuronal cultures and defects in endosomal trafficking in human fibroblasts. The pathways and processes implicated by our modifier loci shed light on the normal cellular function of Ppt1. A greater understanding of Ppt1 function in these cellular processes will provide valuable insight into the molecular etiology of the neuronal dysfunction underlying the disease.

Autophagy-Dependent Rhodopsin Degradation Prevents Retinal Degeneration inDrosophila

“…However, it is not clear how the loss of Ppt1 activity causes retinal degeneration. It has been reported that Ppt1 and Psd interact genetically when they are overexpressed (Buff et al, 2007). Moreover, in this study, we have demonstrated that Ppt1 mutants show light-dependent retinal degeneration and that there is a genetic interaction between these genes when they are downregulated.…”

“…The genetic interaction between the Psd and Ppt1 loss-offunction alleles was examined because a previous study described an interaction between overexpressed Ppt1 and Psd (Buff et al, 2007). To test this, dsRNAs for Psd and Ppt1 were induced simultaneously using weak conditions under which expression of each one of the dsRNAs did not cause severe defects (Fig.…”

“…Insight into INCL, a disease caused by Ppt1 mutation INCL is caused by mutations in the Ppt1 gene (Vesa et al, 1995), which encodes the soluble lysosomal enzyme, Ppt1, and is known to interact genetically with the Psd gene (Buff et al, 2007). The pathological features of INCL manifest as normal early development in the first year followed by various neural defects, including blindness (Haltia et al, 1973).…”

Mutations in palmitoyl-protein thioesterase 1 alter exocytosis and endocytosis at synapses in Drosophila larvae

“…21 Gain-of-function genetic screens in the fly have implicated Ppt1 function in the regulation of several neuronal processes, including synaptic growth/homeostasis and synaptic vesicle endo/exocytosis. 22,23 An examination of non-neuronal larval garland cells in Ppt1 mutants showed reduced endocytosis rates, without alterations in endo-lysosomal trafficking. 23 Similarly, alterations in endocytosis have been observed in fibroblasts from INCL patients.…”

Molecular Machinery Regulating Exocytosis

“…The importance of this precise control is borne out by the many examples of defects in post-translational lipid modifications that are linked to neurodegenerative diseases, such as Schizophrenia, X-linked mental retardation, Huntington disease, as well as colorectal and bladder cancers (Buff, et al, 2007;Williams, 1991). Despite significant progress in identifying the enzymes involved in post-translational lipid modification, their specific protein targets and the relationship between lipid modifications and membrane targeting remain unclear.…”