The Great Sirtuin Debate

REFERENCE: Burnett et al. “Absence of effects of Sir2 overexpression on lifespan in C. elegans and Drosophila.” Nature (2011) 477, pgs 482 – 485.

                It all started with a report that overexpression of SIR2 in budding yeast led to increased lifespan.  Follow up studies showed similar results in C. elegans and Drosophila, which lead researchers to pursue the relationship between calorie restriction (a known way to extend lifespan) and sirtuin expression.  These results bore resveratrol, a purported activator of human Sirtuin 1 (SirT1), which most of the general public will tell you is a component of red wine.  

                The dissent on the role of resveratrol and sirtuins in lifespan extension comes from labs at the University of Washington, University of Wisconsin, Amgen Inc., and Pfizer, among others.  Their papers explicitly say resveratrol has no activating properties on SirT1, conclusions which they back up with control studies using the Fleur-de-Lys system and crystal structures.

                Another blow to the importance of sirtuins came in a recent issue of Nature.    Burnett et al. studied C. elegans and Drosophila overexpressing sir-2.1 and closely accounted for the genetic backgrounds of each.  When taking into account these parameters, longevity increase was no longer noted.  Within fruit flies, the authors further concluded that dietary restriction did increase fly lifespan but was not dependent on Drosophila Sir2.  

Gem and colleagues stress the importance of “…controlling for genetic backgrounds and for the mutagenic effects of transgene insertions in studies on genetic effects on lifespan.”  As for the importance of sirtuins, they cannot support a strong relationship between sirtuins and lifespan extension.

Leaky Protons

REFERENCE: Alavian et al. “Bcl-X_L regulated metabolic efficiency of neurons through interaction with the mitochondrial F₁F₀ ATP synthase” Nature Cell Biology (2011) 13(10) pgs 1224 – 1233.

                B-cell lymphoma-extra large (Bcl-X_L), a member of the programmed cell death Bcl-2 family proteins, is the major anti-apoptotic protein in adult neurons.  When Bcl-X_L is overexpressed, mitochondria translocate to presynaptic sites, the number and size of synpases increase, and the overall mitochondrial biomass goes up.  Synaptic strengthening requires high metabolism but the exact involvement of Bcl-X_L in these events is not clear.

                In this month’s edition of Nature Cell Biology, Alavian et al. detail a role for Bcl-X_L in binding to F₁F₀ ATP synthase.  Previous subcellular localization studies have placed Bcl-X_L within the mitochondrial outer membrane, but new immunoelectron microscopy data published here supports various other studies that suggest the protein is at the mitochondrial inner membrane, as well.  Further analysis showed that Bcl-X_L was present with the F₁F₀ ATP synthase complex and endogenous Bcl-X_L was co-immunoprecipitated with the ß subunit of ATP synthase.

                To clarify the role of Bcl-X_L binding, the authors studied ATP hydrolysis by submitochondrial vesicles enriched with F₁F₀ ATP synthase protein complexes (Figure 6.1).  The experiment used the H⁺ fluorescent indicator ACMA, which is unable to be transported into the vesicles.  Upon ATP hydrolysis, fluorescence of ACMA dropped as H⁺ is pumped out of the buffer and into the vesicles by the F₁F₀ ATPase.  Treatment of the vesicles with proton pump inhibitors or compounds that create vesicle pores (and subsequent proton leaks) resulted in higher levels of fluorescence upon ATP hydrolysis.  Interestingly, these same results were also seen when vesicles were treated with Bcl-X_L inhibitors.

                The hypothesis that Bcl-X_L is acting at F₁F₀ ATPase to prevent proton leak was further supported by patch-clamp studies where the authors measured leak conductance.  Conductance dropped dramatically in the Bcl-X_L overexpressing vesicles when either ATP or ADP was added to the buffer.  Reducing the amount of Bcl-X_L in these vesicles by way of knockdown studies showed the conductance to be higher across these membranes.

                The authors also showed that in Bcl-X_L overexpressing neurons, the uptake of oxygen is lower but ATP levels are higher than in wild type neurons and that recombinant Bcl-X_L can directly increase the rate of ATP hydrolysis by F₁F₀ ATPase.  Taken together, Jonas and colleagues conclude that Bcl-X_L reduces proton leak during ATP synthesis, which thereby increases the neurons’ ATP synthesis efficiency and improves their metabolism.