Loss of yata, a novel gene regulating the subcellular localization of APPL, induces deterioration of neural tissues and lifespan shortening

Role of the Drosophila YATA protein in the proper subcellular localization of COPI revealed by in vivo analysis

yata mutants of Drosophila melanogaster exhibit phenotypes including progressive brain shrinkage, developmental abnormalities and shortened lifespan, whereas in mammals, null mutations of the yata ortholog Scyl1 result in motor neuron degeneration. yata mutation also causes defects in the anterograde intracellular trafficking of a subset of proteins including APPL, which is the Drosophila ortholog of mammalian APP, a causative molecule in Alzheimer's disease. SCYL1 binds and regulates the function of coat protein complex I (COPI) in secretory vesicles. Here, we reveal a role for the Drosophila YATA protein in the proper localization of COPI. Immunohistochemical analyses performed using confocal microscopy and structured illumination microscopy showed that YATA colocalizes with COPI and GM130, a cis-Golgi marker. Analyses using transgenically expressed YATA with a modified N-terminal sequence revealed that the N-terminal portion of YATA is required for the proper subcellular localization of YATA. Analysis using transgenically expressed YATA proteins in which the C-terminal sequence was modified revealed a function for the C-terminal portion of YATA in the subcellular localization of COPI. Notably, when YATA was mislocalized, it also caused the mislocalization of COPI, indicating that YATA plays a role in directing COPI to the proper subcellular site. Moreover, when both YATA and COPI were mislocalized, the staining pattern of GM130 revealed Golgi with abnormal elongated shapes. Thus, our in vivo data indicate that YATA plays a role in the proper subcellular localization of COPI.

Analysis of a cellular structure observed in the compound eyes of Drosophila white; yata mutants and white mutants

We previously identified the Drosophila yata mutant, which showed phenotypes including progressive vacuolization of the white-coloured compound eye, progressive shrinkage of the brain and a shortened lifespan. The yata gene was shown to be involved in controlling intracellular trafficking of the Amyloid precursor protein-like protein, which is an orthologue of Amyloid precursor protein, which is a causative molecule of Alzheimer's disease. In this study, we examined the phenotype of the compound eye of the yata mutant using electron microscopy and confocal microscopy. We found that abnormal cellular structures that seemed to originate from bleb-like structures and contained vesicles and organelles, such as multivesicular bodies and autophagosomes, were observed in aged white; yata mutants and aged white mutants. These structures were not observed in newly eclosed flies and the presence of the structures was suppressed in flies grown under constant dark conditions after eclosion. The structures were not observed in newly eclosed red-eyed yata mutants or wild-type flies, but were observed in very aged red-eyed wild-type flies. Thus, our data suggest that the observed structures are formed as a result of changes associated with exposure to light after eclosion in white mutants, white; yata mutants and aged flies.

Suppression of the synaptic localization of a subset of proteins including APP partially ameliorates phenotypes of the Drosophila Alzheimer's disease model

APP (amyloid precursor protein), the causative molecule of Alzheimer's disease, is synthesized in neuronal cell bodies and subsequently transported to synapses. We previously showed that the yata gene is required for the synaptic transport of the APP orthologue in Drosophila melanogaster. In this study, we examined the effect of a reduction in yata expression in the Drosophila Alzheimer's disease model, in which expression of human mutant APP was induced. The synaptic localization of APP and other synaptic proteins was differentially inhibited by yata knockdown and null mutation. Expression of APP resulted in abnormal synaptic morphology and the premature death of animals. These phenotypes were partially but significantly rescued by yata knockdown, whereas yata knockdown itself caused no abnormality. Moreover, we observed that synaptic transmission accuracy was impaired in our model, and this phenotype was improved by yata knockdown. Thus, our data suggested that the phenotypes caused by APP can be partially prevented by inhibition of the synaptic localization of a subset of synaptic proteins including APP.

Analysis of Amyloid Precursor Protein Function in Drosophila melanogaster

The Amyloid precursor protein (APP) has mainly been investigated in connection with its role in Alzheimer’s Disease (AD) due to its cleavage resulting in the production of the Aβ peptides that accumulate in the plaques characteristic for this disease. However, APP is an evolutionary conserved protein that is not only found in humans but also in many other species, including Drosophila, suggesting an important physiological function. Besides Aβ, several other fragments are produced by the cleavage of APP; large secreted fragments derived from the N-terminus and a small intracellular C-terminal fragment. Although these fragments have received much less attention than Aβ, a picture about their function is finally emerging. In contrast to mammals, which express three APP family members, Drosophila expresses only one APP protein called APP-like or APPL. Therefore APPL functions can be studied in flies without the complication that other APP family members may have redundant functions. Flies lacking APPL are viable but show defects in neuronal outgrowth in the central and peripheral nervous system (PNS) in addition to synaptic changes. Furthermore, APPL has been connected with axonal transport functions. In the adult nervous system, APPL, and more specifically its secreted fragments, can protect neurons from degeneration. APPL cleavage also prevents glial death. Lastly, APPL was found to be involved in behavioral deficits and in regulating sleep/activity patterns. This review, will describe the role of APPL in neuronal development and maintenance and briefly touch on its emerging function in circadian rhythms while an accompanying review will focus on its role in learning and memory formation.

Developmental expression of drop-dead is required for early adult survival and normal body mass in Drosophila melanogaster

In Drosophila melanogaster, mutations in the gene drop-dead (drd) result in early adult lethality, with flies dying within 2 weeks of eclosion. Additional phenotypes include neurodegeneration, tracheal defects, starvation, reduced body mass, and female sterility. The cause of early lethality and the function of the drd protein remain unknown. In the current study, the temporal profiles of drd expression required for adult survival and body mass regulation were investigated. Knockdown of drd expression by UAS-RNAi transgenes and rescue of drd expression on a drd mutant background by a UAS-drd transgene were controlled with the Heat Shock Protein 70 (Hsp70)-Gal4 driver. Flies were heat-shocked at different stages of their lifecycle, and the survival and body mass of the resulting adult flies were assayed. Surprisingly, the adult lethal phenotype did not depend upon drd expression in the adult. Rather, expression of drd during the second half of metamorphosis was both necessary and sufficient to prevent rapid adult mortality. In contrast, the attainment of normal adult body mass required a different temporal pattern of drd expression. In this case, manipulation of drd expression solely during larval development or metamorphosis had no effect on body mass, while knockdown or rescue of drd expression during all of pre-adult (embryonic, larval, and pupal) development did significantly alter body mass. Together, these results indicate that the adult-lethal gene drd is required only during development. Furthermore, the mutant phenotypes of body mass and lifespan are separable phenotypes arising from an absence of drd expression at different developmental stages.

All in the Family: How the APPs Regulate Neurogenesis

Recent intriguing evidence suggests that metabolites of amyloid precursor protein (APP), mutated in familial forms of Alzheimer’s disease (AD), play critical roles in developmental and postnatal neurogenesis. Of note is soluble APPα (sAPPα) that regulates neural progenitor cell proliferation. The APP family encompasses a group of ubiquitously expressed and evolutionarily conserved, type I transmembrane glycoproteins, whose functions have yet to be fully elucidated. APP can undergo proteolytic cleavage by mutually exclusive pathways. The subtle structural differences between metabolites generated in the different pathways, as well as their equilibrium, may be crucial for neuronal function. The implications of this new body of evidence are significant. Miscleavage of APP would readily impact developmental and postnatal neurogenesis, which might contribute to cognitive deficits characterizing Alzheimer’s disease. This review will discuss the implications of the role of the APP family in neurogenesis for neuronal development, cognitive function, and brain disorders that compromise learning and memory, such as AD.

Ku70 alleviates neurodegeneration in Drosophila models of Huntington's disease

DNA damage accumulates in genome DNA during the long life of neurons, thus DNA damage repair is indispensable to keep normal functions of neurons. We previously reported that Ku70, a critical molecule for DNA double strand break (DSB) repair, is involved in the pathology of Huntington's disease (HD). Mutant huntingtin (Htt) impaired Ku70 function via direct interaction, and Ku70 supplementation recovered phenotypes of a mouse HD model. In this study, we generate multiple Drosophila HD models that express mutant huntingtin (Htt) in eye or motor neuron by different drivers and show various phenotypes. In such fly models, Ku70 co-expression recovers lifespan, locomotive activity and eye degeneration. In contrast, Ku70 reduction by heterozygous null mutation or siRNA-mediated knock down accelerates lifespan shortening and locomotion disability. These results collectively support that Ku70 is a critical mediator of the HD pathology and a candidate therapeutic target in HD.

Analysis of amyloid precursor protein function in Drosophila melanogaster

Amyloid precursor proteins (APPs) are evolutionary conserved from nematodes to man (Jacobsen and Iverfeldt in Cell Mol Life Sci 66:2299-2318, 2009) suggesting an important physiological function of these proteins. Human APP is a key factor in the pathogenesis of Alzheimer's Disease because its proteolytic processing results in the production of the neurotoxic Aβ-peptide, which accumulates in the amyloid plaques characteristic for this disease (Selkoe in Physiol Rev 81(2):741-766, 2001). However, the processing also leads to the production of several other fragments and the role of these products, as well as the function of the full-length protein is so far not well understood. The functional analysis of APP in vertebrates has been hampered by the fact that two close relatives, APLP1 and APLP2, exist and that knock-out mice for APP only show subtle defects. In contrast, invertebrates like Caenorhabditis elegans and Drosophila express only one APP-like protein but whereas a null mutation in the C. elegans APL-1 protein is lethal, flies lacking APPL (Amyloid Precursor Protein-like) are viable but show synaptic defects and behavioral abnormalities. Together with the analyses of flies that express APP proteins ectoptically or xenotopically, these studies show that APP proteins are involved in neuronal differentiation, neuritic outgrowth, and synapse formation. In addition, they play a role in long-term memory formation and maintaining brain integrity in adult flies.

Comparison of an expanded ataxia interactome with patient medical records reveals a relationship between macular degeneration and ataxia

Spinocerebellar ataxias 6 and 7 (SCA6 and SCA7) are neurodegenerative disorders caused by expansion of CAG repeats encoding polyglutamine (polyQ) tracts in CACNA1A, the alpha1A subunit of the P/Q-type calcium channel, and ataxin-7 (ATXN7), a component of a chromatin-remodeling complex, respectively. We hypothesized that finding new protein partners for ATXN7 and CACNA1A would provide insight into the biology of their respective diseases and their relationship to other ataxia-causing proteins. We identified 118 protein interactions for CACNA1A and ATXN7 linking them to other ataxia-causing proteins and the ataxia network. To begin to understand the biological relevance of these protein interactions within the ataxia network, we used OMIM to identify diseases associated with the expanded ataxia network. We then used Medicare patient records to determine if any of these diseases co-occur with hereditary ataxia. We found that patients with ataxia are at 3.03-fold greater risk of these diseases than Medicare patients overall. One of the diseases comorbid with ataxia is macular degeneration (MD). The ataxia network is significantly (P= 7.37 × 10⁻⁵) enriched for proteins that interact with known MD-causing proteins, forming a MD subnetwork. We found that at least two of the proteins in the MD subnetwork have altered expression in the retina of Ataxin-7^266Q/+ mice suggesting an in vivo functional relationship with ATXN7. Together these data reveal novel protein interactions and suggest potential pathways that can contribute to the pathophysiology of ataxia, MD, and diseases comorbid with ataxia.

Scyl1 facilitates nuclear tRNA export in mammalian cells by acting at the nuclear pore complex

We provide evidence that Scyl1 is also a cytoplasmic component of the nuclear aminoacylation-dependent tRNA export pathway. Scyl1, like the Saccharomyces cerevisiae Cex1p, may collect aminoacyl-tRNAs from the nuclear tRNA export receptors at the cytoplasmic side of the NPC and channel them to eEF-1A for use in protein synthesis.

Scyl1 is an evolutionarily conserved N-terminal protein kinase-like domain protein that plays a role in COP1-mediated retrograde protein trafficking in mammalian cells. Furthermore, loss of Scyl1 function has been shown to result in neurodegenerative disorders in mice. Here, we report that Scyl1 is also a cytoplasmic component of the mammalian nuclear tRNA export machinery. Like exportin-t, overexpression of Scyl1 restored export of a nuclear export-defective serine amber suppressor tRNA mutant in COS-7 cells. Scyl1 binds tRNA saturably, and associates with the nuclear pore complex by interacting, in part, with Nup98. Scyl1 copurifies with the nuclear tRNA export receptors exportin-t and exportin-5, the RanGTPase, and the eukaryotic elongation factor eEF-1A, which transports aminoacyl-tRNAs to the ribosomes. Scyl1 interacts directly with exportin-t and RanGTP but not with eEF-1A or RanGDP in vitro. Moreover, exportin-t containing tRNA, Scyl1, and RanGTP form a quaternary complex in vitro. Biochemical characterization also suggests that the nuclear aminoacylation-dependent pathway is primarily responsible for tRNA export in mammalian cells. These findings together suggest that Scyl1 participates in the nuclear aminoacylation-dependent tRNA export pathway and may unload aminoacyl-tRNAs from the nuclear tRNA export receptor at the cytoplasmic side of the nuclear pore complex and channels them to eEF-1A.

The ins and outs of nuclear re-export of retrogradely transported tRNAs in Saccharomyces cerevisiae

In Saccharomyces cerevisiae intron-containing pre-tRNAs are exported from the nucleus to the cytoplasm for removal of the introns, and the spliced tRNAs are returned to the nucleus for reasons that are not understood. The re-imported spliced tRNAs are then subjected to aminoacylation in the nucleolus to ensure that they are functional prior to re-export to the cytoplasm. Previous studies have shown that re-imported spliced tRNAs and mature tRNAs made entirely in the nucleus from intronless precursors are retained in the nucleus of S. cerevisiae in response to glucose, amino acid, nitrogen or inorganic phosphate deprivation. Contrary to these studies, we recently reported that starvation of S. cerevisiae of amino acids or nitrogen results in nuclear accumulation of re-imported spliced tRNAs, but not tRNAs made from intronless precursors. This finding suggests that separate pathways are used for nuclear export of retrogradely transported spliced tRNAs and tRNAs made from intronless pre-tRNAs. In addition, the data support the conclusion that the nuclear re-export pathway for retrogradely transported spliced tRNAs, but not the pathway responsible for nuclear export of tRNAs derived from intronless precursors is regulated during amino acid or nitrogen starvation. This regulation appears to occur at a step after the re-imported spliced tRNAs have undergone aminoacylation quality assurance and, in part, involves the TORC1 signalling pathway. Moreover, it was established that Utp9p is an intranuclear component that only facilitates nuclear re-export of retrogradely transported spliced tRNAs by the β-karyopherin Msn5p. Utp9p acts in concert with Utp8p, a key player in nuclear tRNA export in S. cerevisiae, to translocate aminoacylated re-imported spliced tRNAs from the nucleolus to Msn5p and assist with formation of the Msn5p-tRNA-Gsp1p-GTP export complex. This pathway, however, is not the only one responsible for nuclear re-export of retrogradely transported spliced tRNAs.