Drosophila Aging 2005/06

Abstract

Drosophila continues to be a model system of choice to study the genetics of aging. It has a short lifespan and small genome size, but nevertheless contains a complex organ and endocrine system that allows studying the role of conserved signal transduction pathways with sophisticated genetic tools. Oxidative stress and metabolic changes along with intersecting signaling systems Insulin Receptor (InR), Target of Rapamycin (TOR) and Jun N-terminal Kinase (JNK) have emerged as some of the major players in aging. Sleep and organ-specific aging has also been the subject of recent progress in understanding aging.

Starvation resistance and the regulation of lifespan: InR and TOR pathways

In many species, reducing nutrient intake extends lifespan. The Insulin signaling pathway is a widely conserved mechanism implicated in the control of nutrient sensing and has been shown to regulate growth, size, as well as lifespan in higher eukaryotes. In Drosophila, the functionally conserved components of the insulin pathway, like the Insulin receptor, (InR), phosphoinositide 3-kinase (PI3K) and the forkhead transcription factor dFOXO have been shown to regulate aging. One of the known targets of dFOXO is d4E-BP, a regulator of the translation initiation factor eIF4E activity. A recent report suggests that d4E-BP is a critical downstream effector of dFOXO for surviving under dietary restriction and that it is also linked to lifespan (Tettweiler et al., 2005). Other studies examined the role of the Target Of Rapamicin (TOR) pathway, another signaling pathway that responds to changes in various energy states and amino acids, and it is also known to regulate growth and size, as well as lifespan. Furthermore, dTOR seems to act downstream of dFOXO, in addition to be otherwise interconnected with the InR pathway, in regulating metabolism as well as lifespan (Luong et al., 2006). In addition to InR and TOR pathways’ involvement in regulating lifespan, the stress-responsive JNK pathway also regulates aging dependent on the InR-associated transcription factor FOXO (Wang et al., 2005), again high-lighting the crosstalk between InR, TOR as well as JNK signaling in aging.

Oxidative Stress, Mitochondria and Aging

The free radical theory of aging posits that the accumulation of macromolecular damage induced by toxic reactive oxygen species (ROS) plays a central role in the aging process and that reducing the metabolic consumption of molecular oxygen by suppressing energy utilization would reduced ROS (mainly H₂O₂, O _2.^- and ONOO^-) generation and increased lifespan correspondingly. The mitochondria are the principal generator of ROS during the conversion of molecular oxygen to energy production where approximately 0.4% to 4% of the molecular oxygen metabolized by the mitochondrial electron transport chain is converted to ROS (Aguilaniu et al., 2005). However, numerous studies carried out in Drosophila aimed at reducing mitochondrial ROS production, which included overexpression of antioxidants such as superoxide dismutase (SOD) and catalase, failed to validate this theory (Magwere et al., 2006). In contrary, some studies even yielded opposing results, where up-regulating the levels of antioxidants led to a decline of lifespan (Bayne et al., 2005). On the other hand, targeted overexpression of human uncoupling protein 2 (hUCP2) in the mitochondria of adult fly neurons as well as human SOD in adult fly motorneurons led to a decrease in ROS generation, a decrease in oxidative damage and an extension of lifespan (Fridell et al., 2005). These findings support the notion that reducing mitochondrial oxidative damage in neurons is sufficient to increase lifespan and also revealed the advantage of using Drosophila as a system for examining the functions of human proteins. However, another study showed that functional knockout of UCP5 in Drosophila led to flies living longer on low-caloric diets but no increased respiratory rate and ATP production in their mitochondria, suggesting that mitochondrial activity is not necessarily linked to longevity (Sanchez-Blanco et al., 2006).

The correlation between the free radical theory of aging and caloric restriction is also an interesting and important aspect of senescence that has been studied. The prediction that mitochondrial production of ROS determines organismal aging would suggest that dietary restriction should promote longevity since fewer ROS are produced. Indeed, caloric restriction is one of the most successful manipulations in extending life across numerous vertebrate and invertebrate species. However, flies under caloric restriction showed no significant difference in mitochondrial ROS production and no reduction in metabolic rate compared to controls, even though their lifespan is increased (Partridge et al., 2005). Therefore, no conclusion can yet be reached as to whether dietary restriction prolongs lifespan (also) via a decline in mitochondrial ROS generation.

The vertebrate Apolipoprotein D (ApoD) protein is a lipocalin secreted from glia and neurons during neural development and is upregulated in the aging brain and under numerous nervous system pathologies. The Drosophila homolog of human ApoD, Glial Lazarillo (Glaz) is also primarily expressed in subsets of adult glial cells and in its absence, reduces the organism’s ability to counter oxidative stress and starvation and also shortens male lifespan (Sanchez et al., 2006). Conversely, overexpression of Glaz, by means of ubiquitous and tissue-specific drivers, confer resistance to hyperoxia and increases lifespan under normoxia (Walker et al., 2006). Taken together, these results support the notion that Gliaz has a protective function in stress conditions and in doing so, promotes lifespan extension. The levels of mitochondrial ROS in the Glaz deficient and overexpressing flies remain to be examined.

In addition to overall lifespan extension, other parameters of Drosophila aging could also be employed in understanding the relationship between mitochondrial oxidative stress and aging. Parkinson’s disease (PD) is an age-dependent neurodegenerative disease and is thought to be triggered, at least in part, by mitochondrial dysfunction and increased susceptibility to oxidative stress and toxins. The Drosophila PTEN-induced kinase 1 (PINK1) protein is localized to mitochondria and associated with sporadic forms of PD (Clark et al., 2006; Park et al., 2006; Wang et al., 2006; Yang et al., 2006). Removal of PINK1 results in mitochondrial fragmentation and increased sensitivity to multiple stresses, including oxidative stress, whereas treatment of PINK1 knockdown flies with antioxidants protects flies against PD-associated neurodegeneration (Wang et al., 2006). These findings underline the importance of mitochondrial oxidative stress in PD pathogenesis, which can be regarded as another indicator of the aging process.

Tissue-specific Manipulation of Aging

How the aging process within an organism is coordinated between different organs and how the decline in organ physiology is regulated continues to be one of the pressing question in aging research but has been dufficult to address. Recently, changes in sleep patterns, heart function and stem cell biology with age have received some attention in Drosophila.

Sleep and aging

In humans, sleep consolidation is a well-known physiological function that deteriorates in the elderly. This is manifested by an increase in daytime sleep, accompanied by an increase in nighttime wakefulness. Flies display remarkably similar characteristics of sleep (eg. Shaw et al., 2000). Using Drosophila further to analyze age-associated sleep-wake cycle perturbations, it has been reported that the sleep-wake cycles become less robust and that sleep is increasingly fragmented with age (Koh et al., 2006). By analyzing sleep-wake cycles at different temperatures (a parameter known to modify lifespan), this study provides evidence that the rate of sleep consolidation breakdown correlates with lifespan, in that the breakdown is accelerated under conditions that cause a shortening of life span. This suggests that irregular sleep-wake cycles are linked to physiological aging. Similar alterations of sleep consolidation were associated with increased oxidative stress, consistent with the idea that oxidative stress accumulation contributes to sleep deterioration with age. Interestingly, the adult mushroom bodies (a part of the fly CNS known to be involved in learning and memory) appears to function as a central regulator of sleep in Drosophila(Joiner et al., 2006; Koh et al., 2006). This is likely the beginning of an exiting new direction of investigation, aimed at elucidating the genetic and molecular basis of the influence of sleep on aging and its control by nervous system components.

Cardiac aging

Recent efforts in elucidating the cellular and molecular basis of cardiac function in Drosophila suggest that in addition to the evolutionary conservation of embryonic heart specification there may also be similarities in the molecular control of heart physiology and aging. Indeed, like in humans, cardiac senescence in Drosophila is characterized by a reduction of heart rate increase upon stress, an increase in rhythm disturbance and pacing stress-induced cardiac failure (Wessells et al., 2004). In addition, genetic manipulation of InR signaling cell autonomously impinges on heart performance senescence, thus providing a cardiac model for genetic studies of organspecific aging (Wessells et al., 2004). The dTOR branch of the InR pathway is also involved in regulating age-dependent decline of cardiac function (Luong et al., 2006). How exactly individual organs integrate positive or negative systemic ‘aging’ signals and what pathway endpoint effectors (such as FOXO) are going to be relevant in different organs is still largely unresolved. Drosophila is very well suited to address such questions of epistasis and tissue-specificity.

In the heart, pathway effectors that directly affect its performance with age must also impinge on components of the heart’s electrical and contractile system. Indeed, characterization of the age-related phenotype of an ATP-sensitive potassium channel encoded by the dSUR gene, revealed that its RNA levels are significantly decreased in hearts of old flies, and that RNAi mediated knockdown in young hearts causes compromised heart function reminiscent of old flies (Akasaka et al., 2006), supporting an involvement of these potassium channels in the cardiac aging phenotype.

Stem cells

Due to their potential role in tissue repair and in the treatment of degenerative diseases, stem cells research has been given much attention and Drosophila has emerged as an important model for studying stem cell biology. In addition to the well-known germ stem cells that decrease their self-renewal potential with age, a new set of self-renewing cells have been discovered in Drosophila: the intestinal stem cells (Micchelli and Perrimon, 2006; Ohlstein and Spradling, 2006). They are dispersed throughout the length of the midgut and are slowly dividing, thereby giving rise to intestinal and enteroendocrine cells. Human intestinal cells are well known to be continuously renewed by stem cells, thus the Drosophila midgut epithelium may turn over similarly. Both labs identified adult intestinal stem cells by lineage tracing experiments, and demonstrated that Notch signaling regulates the differentiation of enteroendocrine cells, again reminiscent of vertebrates. The identification of Drosophila intestinal stem cells now offers the opportunity to study intestinal stem cell renewal and aging in vivo.