An appetite for destruction: from self-eating to cell cannibalism as a neuronal survival strategy.

Autophagy plays an important role in cellular survival by resupplying cells with nutrients during starvation or by clearing misfolded proteins and damaged organelles and thereby preventing degenerative diseases. Conversely, the autophagic process is also recognized as a cellular death mechanism. The circumstances that determine whether autophagy has a beneficial or a detrimental role in cellular survival are currently unclear. We recently showed that autophagy induction is detrimental in neurons that lack a functional AMPK enzyme (AMP-activated protein kinase) and that suffer from severe metabolic stress. We further demonstrated that autophagy and AMPK are interconnected in a negative feedback loop that prevents excessive and destructive stimulation of the autophagic process. Finally, we uncovered a new survival mechanism in AMPK-deficient neurons--cell cannibalism.

Pediatric oncology for the general pediatrician: introduction to the Educational series.

Since its establishment as a pediatric subspecialty oncology has become the near exclusive domain of specialized tertiary centers. This has greatly aided progress in the field with formation of exemplary and highly focused international consortia that have continued to streamline clinical research efforts. Recently, the rate of progress in terms of further improvements in overall outcome has paradoxically slowed. Novel strategies are therefore needed to assure continued advances. On the one hand, international and global consortia of subspecialists will continue to focus on the development of much needed improved therapies for those disease groups that have not yet seen their prognosis brighten, and on the other hand, there is a trend to invest in the development of joint-care initiatives that assure cost-effective access to standard therapy for all. This will require closer involvement of the general pediatrician in certain aspects of cancer care in its broadest sense. We hope the upcoming series will aid this process by highlighting selected topics in pediatric oncology that are likely to gain even more relevance for the general pediatrician in the years to come. They include: screening strategies for cancer predisposition syndromes; molecular diagnosis and risk-adjustment of therapy for acute leukemia in childhood; an overview of new agents in clinical development; and the impact of cancer treatment on fertility and available preservation options.

Expanding roles for AMP-activated protein kinase in neuronal survival and autophagy.

AMP-activated protein kinase (AMPK) is an evolutionarily conserved cellular switch that activates catabolic pathways and turns off anabolic processes. In this way, AMPK activation can restore the perturbation of cellular energy levels. In physiological situations, AMPK senses energy deficiency (in the form of an increased AMP/ATP ratio), but it is also activated by metabolic insults, such as glucose or oxygen deprivation. Metformin, one of the most widely prescribed anti-diabetic drugs, exerts its actions by AMPK activation. However, while the functions of AMPK as a metabolic regulator are fairly well understood, its actions in neuronal cells only recently gained attention. This review will discuss newly emerged functions of AMPK in neuroprotection and neurodegeneration. Additionally, recent views on the role of AMPK in autophagy, an important catabolic process that is also involved in neurodegeneration and cancer, will be highlighted.

AMP-activated protein kinase (AMPK) molecular crossroad for metabolic control and survival of neurons.

AMP-activated protein kinase (AMPK) constitutes a molecular hub for cellular metabolic control, common to all eukaryotic cells. Numerous reports have established how AMPK responds to changes in the AMP:ATP ratio as a measure of cellular energy levels. In this way, it integrates control over a number of metabolic enzymes and adapts cellular processes to the current energy status in various cell types, such as muscle and liver cells. The role of AMPK in the development, function, and maintenance of the nervous system, on the other hand, has only recently gained attention. Neurons, while highly metabolically active, have poor capacity for nutrient storage and are thus sensitive to energy fluctuations. Recent reports demonstrate that AMPK may have neuroprotective properties and is activated in neurons by resveratrol but also by metabolic stress in the form of ischemia/hypoxia and glucose deprivation. Novel studies on AMPK also implicate neuronal activity as a critical factor in neurodegeneration. Here we discuss the latest advances in the knowledge of AMPK's role in the metabolic control and survival of excitable cells.

Drosophila alicorn is a neuronal maintenance factor protecting against activity-induced retinal degeneration.

Exploring mechanisms that govern neuronal responses to metabolic stress is essential for the development of therapeutic strategies aimed at treatment of neuronal injury and disease. AMP-activated protein kinase (AMPK) is a key enzyme regulating cellular energy homeostasis that responds to changes in cellular energy levels by promoting energy-restorative and inhibiting energy-consumptive processes. Recent studies have suggested that AMPK might have a neuroprotective function. However, the existing evidence is contradictory and almost exclusively derived from in vitro studies based on drug treatments and metabolic stress models. To tackle these issues in vivo, we used the Drosophila visual system. In this report, we describe a novel Drosophila mutant, alicorn (alc), encoding the single beta regulatory subunit of AMPK. Loss of alc using the eyFlp system causes severe early-onset progressive nonapoptotic neurodegeneration in the retina, the optic lobe, and the antennae, as well as behavioral and neurophysiological defects. Retinal degeneration occurs immediately after normal neuronal differentiation, can be enhanced by exposure to light, and can be prevented by blocking photoreceptor excitation. Furthermore, AMPK is required for proper viability of differentiated photoreceptors by mechanisms unrelated to polarity events that AMPK controls in epithelial tissues. In conclusion, AMPK does not affect photoreceptor development but is crucial to maintaining integrity of mature neurons under conditions of increased activity and provides protection from excitotoxicity.

Aberrant lysosomal carbohydrate storage accompanies endocytic defects and neurodegeneration in Drosophila benchwarmer.

Lysosomal storage is the most common cause of neurodegenerative brain disease in preadulthood. However, the underlying cellular mechanisms that lead to neuronal dysfunction are unknown. Here, we report that loss of Drosophila benchwarmer (bnch), a predicted lysosomal sugar carrier, leads to carbohydrate storage in yolk spheres during oogenesis and results in widespread accumulation of enlarged lysosomal and late endosomal inclusions. At the bnch larval neuromuscular junction, we observe similar inclusions and find defects in synaptic vesicle recycling at the level of endocytosis. In addition, loss of bnch slows endosome-to-lysosome trafficking in larval garland cells. In adult bnch flies, we observe age-dependent synaptic dysfunction and neuronal degeneration. Finally, we find that loss of bnch strongly enhances tau neurotoxicity in a dose-dependent manner. We hypothesize that, in bnch, defective lysosomal carbohydrate efflux leads to endocytic defects with functional consequences in synaptic strength, neuronal viability, and tau neurotoxicity.

Control of planar cell polarity by interaction of DWnt4 and four-jointed.

The Drosophila eye and the wing display specific planar cell polarity. Although Frizzled (Fz) signaling has been implicated in the establishment of ommatidial and wing hair polarity, evidence for the Wnt gene function has been limited. Here we examined the function of a Drosophila homolog of Wnt4 (DWnt4) in the control of planar polarity. We show that DWnt4 mRNA and protein are preferentially expressed in the ventral region of eye disc. DWnt4 mutant eyes show polarity reversals mostly in the ventral domain, consistent with the ventral expression of DWnt4. Ectopic expression of DWnt4 in the dorsoventral (DV) polar margins is insufficient to induce ommatidial polarity but becomes inductive when coexpressed with Four-jointed (Fj). Similarly, DWnt4 and Fj result in synergistic induction of hair polarity toward the source of expression in the wing. Consistent with genetic interaction, we provide evidence for direct interaction of DWnt4 and Fj transmembrane protein. The extracellular domain of Fj is required for direct binding to DWnt4 and for the induction of hair polarity. In contrast to the synergy between DWnt4 and Fj, DWnt4 antagonizes the polarizing effect of Fz. Our results suggest that DWnt4 is involved in ommatidial polarity signaling in the ventral region of the eye and its function is mediated by interacting with Fj.

Quantitative trait loci affecting starvation resistance in Drosophila melanogaster.

The ability to withstand periods of scarce food resources is an important fitness trait. Starvation resistance is a quantitative trait controlled by multiple interacting genes and exhibits considerable genetic variation in natural populations. This genetic variation could be maintained in the face of strong selection due to a trade-off in resource allocation between reproductive activity and individual survival. Knowledge of the genes affecting starvation tolerance and the subset of genes that affect variation in starvation resistance in natural populations would enable us to evaluate this hypothesis from a quantitative genetic perspective. We screened 933 co-isogenic P-element insertion lines to identify candidate genes affecting starvation tolerance. A total of 383 P-element insertions induced highly significant and often sex-specific mutational variance in starvation resistance. We also used deficiency complementation mapping followed by complementation to mutations to identify 12 genes contributing to variation in starvation resistance between two wild-type strains. The genes we identified are involved in oogenesis, metabolism, and feeding behaviors, indicating a possible link to reproduction and survival. However, we also found genes with cell fate specification and cell proliferation phenotypes, which implies that resource allocation during development and at the cellular level may also influence the phenotypic response to starvation.

Mapping Drosophila mutations with molecularly defined P element insertions.

The isolation of chemically induced mutations in forward genetic screens is one of the hallmarks of Drosophila genetics. However, mapping the corresponding loci and identifying the molecular lesions associated with these mutations are often difficult and labor-intensive. Two mapping methods are most often used in flies: meiotic recombination mapping with marked chromosomes and deficiency mapping. The availability of the fly genome sequence allows the establishment and usage of molecular markers. Single-nucleotide polymorphisms have therefore recently been used to map several genes. Here we show that thousands of molecularly mapped P element insertions in fly strains that are publicly available provide a powerful alternative method to single-nucleotide polymorphism mapping. We present a strategy that allows mapping of lethal mutations, as well as viable mutations with visible phenotypes, with minimal resources. The most important unknown in using recombination rates to map at high resolution is how accurately recombination data correlate with molecular maps in small intervals. We therefore surveyed distortions of recombination rates in intervals <500 kb. We document the extent of distortions between the recombination and molecular maps and describe the required steps to map with an accuracy of <50 kb. Finally, we describe a recently developed method to determine molecular lesions in 50-kb intervals by using a heteroduplex DNA mutation detection system. Our data show that this mapping approach is inexpensive, efficient, and precise, and that it significantly broadens the application of P elements in Drosophila.

Quantitative analysis of bristle number in Drosophila mutants identifies genes involved in neural development.

BACKGROUND: The identification of the function of all genes that contribute to specific biological processes and complex traits is one of the major challenges in the postgenomic era. One approach is to employ forward genetic screens in genetically tractable model organisms. In Drosophila melanogaster, P element-mediated insertional mutagenesis is a versatile tool for the dissection of molecular pathways, and there is an ongoing effort to tag every gene with a P element insertion. However, the vast majority of P element insertion lines are viable and fertile as homozygotes and do not exhibit obvious phenotypic defects, perhaps because of the tendency for P elements to insert 5' of transcription units. Quantitative genetic analysis of subtle effects of P element mutations that have been induced in an isogenic background may be a highly efficient method for functional genome annotation. RESULTS: Here, we have tested the efficacy of this strategy by assessing the extent to which screening for quantitative effects of P elements on sensory bristle number can identify genes affecting neural development. We find that such quantitative screens uncover an unusually large number of genes that are known to function in neural development, as well as genes with yet uncharacterized effects on neural development, and novel loci. CONCLUSIONS: Our findings establish the use of quantitative trait analysis for functional genome annotation through forward genetics. Similar analyses of quantitative effects of P element insertions will facilitate our understanding of the genes affecting many other complex traits in Drosophila.