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The vacuole becomes less acidic relatively early in a cell's lifespan. The drop in acidity hinders the vacuole’s ability to store certain nutrients which disrupts mitochondria energy source, causing them to break down. Preventing the drop in vacuole acidity preserves the mitochondria's function and shape and extends the lifespan. Mitochondrial structure and function alterations [2]. Mitochondrial dysfunction in replicatively aged cells arises from altered vacuolar pH. Vacuolar acidity declines during the early asymmetric cell divisions. Preventing the decline in vacuolar acidity suppresses mitochondrial dysfunction and extends lifespan. Changes in vacoular pH reduces the pH-dependent amino acid storage in the vacuolar lumne. DR promotes lifespan extension at least in part by incression vacuolar acidity [3]/ Acidic vacuoles are regenerated in newborn daughter cells. Age-induced mitochondrial dysfunction includes increased fragmentation, levels of ROS and loss of DNA in their progeny [6-9].
Mitochondria which are normally tubular, fragment and ultimately aggregate in aged cells. Mitochondrial fragmentation is present early in the aging process and progresses to large aggregates and small fragments that persists throughout aging. Mitochondrial function also declines in aging cells. Mitochodrial membrane potential (ΔΨ) declines (already by 7 divisions). At 18 division the ΔΨ is even lower and remains low thereafter (100% fo cells by 28 divisions). ΔΨ-dependent import of the mitochondrial matrix protein Cox4 is reduced in aged cells. Mitochondrial structure and functions are progressively altered during the aging process. VMA1 is the catalytic subunit of the peripheral-membrane-associated V1 sector of the vacuolar H+-ATPase (V-ATPase) and required for its activity [14]. Overexpression of VMA1 increases vacuolar acidity and suppresses age-induced mitochondrial dysfunction of aged cells (17 or 18 cell divisions) which requires the V-ATPase activity. VMA1 overexpression significantly increases mean, median and maximum lifespan. VPH2 is and endoplasmic-reticulum-localized integral membrane protein required for the assembly and therefore also activity of the vacuolar H+-ATPase (V-ATPase) [15]. Overexpression of VPH2 increases the levels of assembled V-ATPase at the vacuolar membrane, increases vacuolar acidity and suppresses age-induced mitochondrial dysfunction of aged cells (17 or 18 cell divsions) which requires the V-ATPase activity. VPH2 overexpression significantly increases mean, median and maximum lifespan. VMA2 deletion mutant have a reduced ΔΨ and mitochondrial morphology similar. concA blocked their ability to produce normal, tubular mitochondria. treatment of young cells causes vacuolar acidity and loss of mitochondrial depolization. Loss of ΔΨ is followed by mitochondrial fragmentation and aggreagation that resembles mitochondrial phenotypes present in aged cells. Acidity of the vacuole is decreased as newborn cells become aged mother cells. Vacuolar acidity declines progressively. Reduced vacuolar acidity in mother cells leads to subsquent development of mitochondrial dysfunction during approximately the first four cell divisions and then remain low for at least 18 divisions. Yeast cells lacking V-ATPase activity have drastically shortened lifespan [20]. AVT1 is a a neutral amino acid transporter [25]. Overexpression of AVT1 prevents mitochondrial dysfunction, prevents alterations in mitochondrial structure and ΔΨ of aged cells even through the vacuolar acidity is reduced in these cells. Deletion of AVT1 accelerates the development of age-induced mitochondrial dysfunction without effecting the kinetics of vacuolar acidity decline and prevents the suppressio of mitochondrial dysfunction by VMA1 and VPH2 overexpression without affecting vacuolar acidity. Reduced vacuolar acidity in aged cells cause mitochondrial dysfunction by preventing adequate import of netural amino acids throgh the vacuolar transporter Avt1. Overexpressing or deleting AVT1 is sufficient to extend or shorten replicative lifespan, respectively. DR (0.5% glucose, 0.05% glucose and 3% glycerol) [26,27] increase vacuolar acidity in young cells and prevents the decline of vacuolar acidity in aging cells. DR also suppresses mitochondrial dysfunciton of aged cells (21 divisions) ina V-ATPase-dependent manner. Inhibiting PKA, Sch9 or TOR by lifespan-extending mutations gpa2, shc9 or tor1 prevents the decline in vacuolar acidity and development of mitochondrial dysfunction in aging cells. Constitutively activating PKA signaling by deleting the Ras GTPase-activating protein IRA2 reduces vacuolar acidity and accelerates the development of mitochondrial dysfunction in aging cells and prevents DR-mediated enhancement of vacuolar acidity and suppression of mitochondrial dysfunction. Lifespan extension by DR is prevented in a strain lacinkn V-ATPase activity. DR does not further increase the lifespan of VMA1 overepxression strain. Although vacuolar acidity is reduced in mother cells, high acidity is restored in newborn cells, even in daughters of old mothers. This restorationr equires V-ATPase activity as it is eliminated in vma2 cells, whihc lack V-ATPase activity and have constituively reduced vacuolar acidity. Figure 2 | Vacuolar acidity is reduced in ageing cells and regulates mitochondrial function and lifespan. genotype mean median max num pvalue WT 28 28 50 50 VMA1(OE) 39 39 80 56 0.0002 Figure 3 | Reduced vacuolar acidity causes mitochondrial dysfunction by disrupting amino acid homeostasis. genotype mean median max num pvalue WT 28.95 29 51 32 - AVT1(OE) 37 62 32 0.00376 avt1 23 22.5 45 31 0.0045 Figure 4 | Calorie restriction extends lifespan by regulating vacuolar acidity. genotype treatment mean median max num pvalue WT - 31 31.5 50 32 - WT DR 45 75 31 <0.0001 vma2 - 5 5 15 60 <0.0001 vma2 DR 2 2 12 52 0.0398 WT - 29 29 50 38 - WT DR 44 43.5 76 40 <0.0001 VMA1(OE) - 44 43 36 <0.0001 VMA1(OE) DR 40 40 38 0.076 Supplementary Figure 3. Phenotypes of VPH2 overexpression mimic those observed with VMA1 overexpression. genotype mean median max num pvalue WT 26 26 50 53 - VPH2(OE) 32 32.5 67 52 0.0054 Tags: acidity aging lifespan organelle Edit this page |
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