Changes in protein turnover after heat shock are related to accumulation of abnormal proteins in aging Drosophila melanogaster

DAMPs, ageing, and cancer: The 'DAMP Hypothesis'

Ageing is a complex and multifactorial process characterized by the accumulation of many forms of damage at the molecular, cellular, and tissue level with advancing age. Ageing increases the risk of the onset of chronic inflammation-associated diseases such as cancer, diabetes, stroke, and neurodegenerative disease. In particular, ageing and cancer share some common origins and hallmarks such as genomic instability, epigenetic alteration, aberrant telomeres, inflammation and immune injury, reprogrammed metabolism, and degradation system impairment (including within the ubiquitin-proteasome system and the autophagic machinery). Recent advances indicate that damage-associated molecular pattern molecules (DAMPs) such as high mobility group box 1, histones, S100, and heat shock proteins play location-dependent roles inside and outside the cell. These provide interaction platforms at molecular levels linked to common hallmarks of ageing and cancer. They can act as inducers, sensors, and mediators of stress through individual plasma membrane receptors, intracellular recognition receptors (e.g., advanced glycosylation end product-specific receptors, AIM2-like receptors, RIG-I-like receptors, and NOD1-like receptors, and toll-like receptors), or following endocytic uptake. Thus, the DAMP Hypothesis is novel and complements other theories that explain the features of ageing. DAMPs represent ideal biomarkers of ageing and provide an attractive target for interventions in ageing and age-associated diseases.

Ageing increases vulnerability to aβ42 toxicity in Drosophila

Age is the major risk factor for many neurodegenerative diseases, including Alzheimer's Disease (AD), for reasons that are not clear. The association could indicate that the duration or degree of exposure to toxic proteins is important for pathology, or that age itself increases susceptibility to protein toxicity. Using an inducible Drosophila model of AD, we investigated these possibilities by varying the expression of an Aβ42 transgene in neurons at different adult ages and measuring the effects on Aβ42 levels and associated pathological phenotypes. Acute induction of Arctic Aβ42 in young adult flies resulted in rapid expression and clearance of mRNA and soluble Arctic Aβ42 protein, but in irreversible expression of insoluble Arctic Aβ42 peptide. Arctic Aβ42 peptide levels accumulated with longer durations of induction, and this led to a dose-dependent reduction in negative geotaxis and lifespan. For a standardised level of mRNA expression, older flies had higher levels of Arctic Aβ42 peptide and associated toxicity, and this correlated with an age-dependent reduction in proteasome activity. Equalising Aβ42 protein at different ages shortened lifespan in correlation with the duration of exposure to the peptide, suggesting that Aβ42 expression accumulates damage over time. However, the relative reduction in lifespan compared to controls was greater in flies first exposed to the peptide at older ages, suggesting that ageing itself also increases susceptibility to Aβ42 toxicity. Indeed older flies were more vulnerable to chronic Aβ42 toxicity even with a much lower lifetime exposure to the peptide. Finally, the persistence of insoluble Aβ42 in both young and old induced flies suggests that aggregated forms of the peptide cause toxicity in later life. Our results suggest that reduced protein turnover, increased duration of exposure and increased vulnerability to protein toxicity at later ages in combination could explain the late age-of-onset of neurodegenerative phenotypes.

Doxycycline-regulated over-expression of hsp22 has negative effects on stress resistance and life span in adult Drosophila melanogaster

Drosophila hsp22 is a member of the small heat shock proteins family (shsps). The hsp22 is expressed in a tissue-general pattern in response to heat stress and during normal aging, and localizes to the mitochondrial matrix, however, its exact function and targets are unknown. Hsp22 was found to be rapidly induced in response to oxidative stress, indicating that hsp22 is also an oxidative stress response gene. To assay for effects of hsp22, a ubiquitous pattern of hsp22 gene expression was generated in young flies using the "tet-on" doxycycline-regulated promoter system. The hsp22 over-expression made flies more sensitive to heat and oxidative stress, while resistance to coumarin poisoning was not affected. Life span was also reduced, particularly at higher culture temperatures. Members of other hsp families have been shown to feedback-inhibit their own expression by interacting with the heat shock transcription factor (HSF) and preventing binding to the HSEs. Induction of hsp22:lacZ and hsp70:lacZ reporter transgenes in response to acute stress was normal in the presence of hsp22 protein over-expression and in old flies, indicating that the negative effects of hsp22 are downstream of the HSF/HSE pathway and the transcriptional heat shock response. The data demonstrate a specific over-expression phenotype for hsp22 and suggest that hsp22 interacts with heat and oxidative stress resistance pathways.

Increased hsp22 RNA levels in Drosophila lines genetically selected for increased longevity

RNAs for the small heat shock protein (hsp) genes hsp22 and hsp23 are induced during Drosophila aging, suggesting that these genes might have specific functions at late ages. To determine if hsp22 and hsp23 gene expression might correlate with life span, RNA levels for these and additional genes were analyzed throughout the adult life span in a set of five outbred "O" lines, which have been genetically selected for increased longevity, and in five matched control "B" lines. Control ribosomal protein genes rp49 and AP3/RpPO RNA levels were similar in O and B lines. In contrast, hsp22 RNA levels were twofold-tenfold higher in all five O lines relative to all five B lines, while hsp23 exhibited a smaller but significant increase. Thus increased hsp22 and hsp23 RNA levels correlate with the increased life span and increased stress resistance of the genetically selected O lines.

Diverse Caenorhabditis elegans genes that are upregulated in dauer larvae also show elevated transcript levels in long-lived, aged, or starved adults

Under adverse conditions, the nematode Caenorhabditis elegans undergoes reversible developmental arrest as dauer larvae, an alternative third larval stage adapted for dispersal and long-term survival. Following such arrest, which may exceed three times their usual life-span, worms resume development to form reproductive adults of normal subsequent longevity. Mutations of genes in the dauer-formation (daf) pathway can extend life-span two- to fourfold, even in adults that mature without diapause. To identify transcript-level changes that might contribute to extended survival, we prepared a subtractive cDNA library of messages more abundant in dauer than in non-dauer (L3) larvae. Six genes were confirmed as three- to ninefold upregulated in dauer larvae, after correction for mRNA load: genes encoding poly(A)-binding protein (PABP), heat-shock proteins hsp70 and hsp90, and three novel genes of uncertain function. The novel genes encode a partial homologue of human activating signal cointegrator 1 (ASC-1), a GTP-binding homologue of a ribosomal protein, and an SH3-domain protein. Transcript levels for all except hsp70 increased during aging in two C. elegans strains, whereas the three novel genes (and possibly PABP) were also induced to varying degrees by starvation of adults. All six genes are expressed at higher levels in young adults of long-lived daf mutant strains than in normal-longevity controls, suggesting that increased expression of these genes may play a protective function, thus favoring survival in diverse contexts.

Aging mechanisms in fruit files

Genetic analysis of Drosophila has provided evidence in support of two proposed evolutionary genetic mechanisms of aging: mutation accumulation and antagonistic pleiotropy. Both mechanisms result from the lack of natural selection acting on old organisms. Analyses of large numbers of files have revealed that mortality rates do not continue to rise with age as previously thought, but plateau at advanced ages. This phenomenon has implications both for models and for definitions of aging, and may be explained by the evolutionary theories. The physiological processes and genes most relevant to aging are being identified using Drosophila lines selected in the laboratory for postponed senescence. Oxidative stress and insufficient metabolic reserves/capacity may be particularly important factors in limiting the fruitfly lifespan. Genes which exhibit aging-related changes in expression are now being identified. Transgenic files are being used to analyze the mechanisms of such aging-related gene expression, and to test the effects of specific genes on aging and aging-related deterioration.

Exposure to heat shock affects thermosensitivity of the locust flight system

The natural habitat of the migratory locust, Locusta migratoria, is likely to result in locusts being heat stressed during their normal adult life. It is known that locusts exhibit a heat-shock response: exposure to 45 degrees C for 3 h induces thermotolerance and the expression of heat-shock proteins. We investigated the effects of exposure to heat-shock conditions on the thermosensitivity of flight rhythm generation in tethered, intact animals and in deafferented preparations. Heat shock had no effect on wingbeat frequency measured at the start of flight sequences, nor did it affect the postimaginal maturation of this parameter. During sustained flight, heat shock slowed the characteristic asymptotic reduction of wingbeat frequency. Wingbeat frequency of heat-shocked animals was less sensitive to temperature in the range 24 degrees to 47 degrees C than that of control animals, and the upper temperature limit, above which flight rhythms could not be produced, was 6 degrees to 7 degrees C higher in heat-shocked animals. These results were mirrored in the response of deafferented preparations, indicating that modifications in the properties of the flight neuromuscular system were involved in mediating the response of the intact animal. We propose that exposure to heat shock had the adaptive consequences of reducing thermosensitivity of the neural circuits in the flight system and allowing them to operate at higher temperatures.

The expression of a reporter protein, beta-galactosidase, is preserved during maturation and aging in some cells of the adult Drosophila melanogaster

The effects of maturation and aging on cell stability and maintenance of protein expression have been examined in adult Drosophila melanogaster. Counting the number of cells present in the antenna of the adult fly revealed little loss in cell number with aging. Enhancer map-marked genes expressing beta-galactosidase (beta-gal) in the antenna and an Rh1 opsin reporter gene construct expressing beta-gal in the R1-6 photoreceptor cells of the compound eye revealed no alteration in spatial distribution or amount of beta-gal with aging. A heat shock-inducible promoter coupled to the expression of beta-gal, hsp70-lacZ, revealed that the rate and amount of induction of beta-gal after heat shock is preserved during aging but the rate of decay of beta-gal may be slightly delayed in older animals. These studies suggest that the ability to express a reporter protein, beta-galactosidase, is preserved in at least a subset of cells in the aging fly.

Heat shock induces changes in the expression and binding of ubiquitin in senescent Drosophila melanogaster

We examined the effect of aging on the expression of ubiquitin RNA and the binding of the ubiquitin polypeptide to proteins following heat shock in Drosophila melanogaster. Heat-shocked adult flies transcribe two major RNA species--one of 4.4 kb and one of about 6 kb that hybridize to the polyubiquitin-encoding probe. Several less abundant RNAs were also observed but the 4.4-kb band was present as the major RNA species in both stressed and nonstressed flies of both ages. The 6-kb fragment was more abundant in heat shocked aged flies than in younger flies. The quantitative expression of the polyubiquitin gene increased in proportion to the duration of the heat stress. Moreover, the induction of the polyubiquitin RNA was markedly elevated during aging following heat shock. Hybridization of Northern blots with the monoubiquitin gene probe revealed a band of 0.9 kb that was not significantly affected by heat stress. We also investigated the relationship between the changes in polyubiquitin gene expression and the formation of ubiquitin-protein complexes in aging heat-shocked flies. Heat shock to old flies results in a significant increase in the level of proteins immunoprecipitated by anti-ubiquitin antibodies. In the case of proteins synthesized 2 h before heat shock, most of the ubiquitinated proteins were of high molecular weight. For those proteins synthesized during a 30-min heat shock and the 2 h following heat shock, two major immunoprecipitated bands were observed: an 80-kD and a 70-kD polypeptide. The ubiquitination of a 60 kD protein was also observed in nonstressed flies, but its formation was drastically reduced following heat shock.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of oxidative stress in Drosophila aging

We review the role that oxidative damage plays in regulating the lifespan of the fruit fly, Drosophila melanogaster. Results from our laboratory show that the lifespan of Drosophila is inversely correlated to its metabolic rate. The consumption of oxygen by adult insects is related to the rate of damage induced by oxygen radicals, which are purported to be generated as by-products of respiration. Moreover, products of activated oxygen species such as hydrogen peroxide and lipofuscin are higher in animals kept under conditions of increased metabolic rate. In order to understand the in vivo relationship between oxidative damage and the production of the superoxide radical, we generated transgenic strains of Drosophila melanogaster that synthesize excess levels of enzymatically active superoxide dismutase. This was accomplished by P-element transformation of Drosophila melanogaster with the bovine cDNA for CuZn superoxide dismutase, an enzyme that catalyzes the dismutation of the superoxide radical to hydrogen peroxide and water. Adult flies that express the bovine SOD in addition to native Drosophila SOD are more resistant to oxidative stresses and have a slight but significant increase in their mean lifespan. Thus, resistance to oxidative stress and lifespan of Drosophila can be manipulated by molecular genetic intervention. In addition, we have examined the ability of adult flies to respond to various environmental stresses during senescence. Resistance to oxidative stress, such as that induced by heat shock, is drastically reduced in senescent flies. This loss of resistance is correlated with the increase in protein damage generated in old flies by thermal stress and by the insufficient protection from cellular defense systems which includes the heat shock proteins as well as the oxygen radical scavenging enzymes. Collectively, results from our laboratory demonstrate that oxidative damage plays a role in governing the lifespan of Drosophila during normal metabolism and under conditions of environmental stress.