Osh6 links yeast vacuolar functions to lifespan extension and TOR.

Almost all organisms age–the aging process is both genetically determined and can be modified by the environment. Lifespan extension by dietary restriction (DR) is observed in evolutionarily distant species from yeast to mammals. Not only are the phenomena of aging and DR conserved, but at least some mechanisms and genes are evolutionarily conserved, which may pave the way to manipulate human aging. For example, TOR (target of rapamycin) mediates aging and, when suppressed, triggers anti-aging processes in many species. Moreover, identifying genes that modulate the potential for cell division is of great interest, given that changes in the number of times that cells divide have been associated with longevity manipulations in mammals (including DR).

Sterols are hydrophobic molecules present in all cellular organisms. For instance, cholesterol is an essential structural component of cellular membranes of mammals and several of its derivates have additional hormonal and signaling functions. Oxysterols are oxygenated derivates of cholesterol. Oxysterol-binding protein (OSBP)-related protein (ORP) family members are present in numerous copies from yeast to man, suggesting that this protein family has fundamental functions in eukaryotes. OSBP and ORPs regulate lipid metabolism, vesicle transport and various signaling pathways and may specifically mediate lipid exchange at membrane contact sites.

The lifespan-extending effect of DR has often been shown to be mediated by specific genes and to be accompanied by discrete changes in gene expression as well as metabolic reprogramming. Both lipid metabolism and cellular recycling activities have been demonstrated to be essential for lifespan extension in numerous species. For example, DR suppresses sterol synthesis from yeast to mammals, while it induces some form of autophagy, a mighty housekeeping mechanism utilizing lysosomes within its power to recycle various kinds of molecules and cellular structures. Vacuoles, the yeast equivalent of mammalian lysosomes, are highly dynamic organelles that fuse and divide in response to environmental or intrinsic cues. Mutants with defects in vacuolar fusion (such as ypt7Δ, nyv1Δ, vac8Δ, or erg6Δ) are either short-lived or do not appear to respond to DR.

While mammals have 12 OSBPs, the yeast genome encodes seven oxysterol-binding protein sequence homologs (OSH). Deletion of any OSH gene alone does not impact on vacuolar morphology, yet deletion of all results in highly fragmented vacuoles, a sign of defective vacuole fusion. Gebre et al. now show that overexpression of OSH family member OSH6 in yeast can complement the vacuole fusion defect of nyv1Δ but not erg6Δ or vac8Δ. Thus, Osh6 mediates vacuolar fusion, which depends on ergosterol (Erg6), and the protein anchor Vac8. In contrast, overexpression of another OSH-family member, OSH5, exacerbated fragmentation and decreased lifespan in wild-type cells. It is interesting to note that OSH5 expression progressively increases with age, and Osh6 overexpression blocked this age-dependent change in OSH5 levels. Also, elevated Osh6 maintains the enrichment of Vac8 in microdomains of vacuolar membranes with advancing age, which is required for vacuole fusion. Intriguingly, exactly at the age when the longevity protein Sir2 declines, Osh6 protein levels also decline.

Furthermore, Gebre et al. showed that PERG6-OSH6 (ERG6 promoter driving OSH6 overexpression) dramatically extends the lifespan of wild-type and nyv1Δ mutants. tor1Δ mutants are also long-lived, though not so long as PERG6-OSH6. Surprisingly, PERG6-OSH6 tor1Δ double mutant had a very short lifespan. PERG6-OSH6 mutants were more sensitive to TOR inhibitors, indicating that TOR is less active in this strain. OSH6 overexpression downregulates total cellular sterol levels, just like DR. Osh6 binds PI3P and PI(3,5)P2 which are vacuole-specific lipids. As such, Osh6 might promote vacuole fusion by regulating the transports and/or distribution of sterols to the vacuolar membranes. But where are the sterols coming from? Numerous overexpression mutants with effects in vacuolar morphology are involved in endocytosis. Similarly, Osh6’s coiled-coil domain interacts with Vps4, which is located in endosomes. TOR complex 1 (TORC1) also sits on endosomes as well as on vacuoles and actively catalyzes vacuolar scission. Osh6 may therefore (1) transport sterols from late endosomes to the vacuolar membrane (Fig. 1), which increases the homototypic fusion ability of vacuoles, and (2) averaging the lipids between late endosome and vacuoles promotes also late-endosome-to-vacuole fusion.

Figure 1. Putative mechanism of the lifespan extension conferred by Osh6 overexpression. TORC1 promotes vacuolar scission and therefore fragments vacuoles. In contrast, Osh6 enhances vacuolar fusion and might be doing this by transporting sterols from the endosomes to the vacuolar membrane. Improved vacuolar morphology then promotes autophagy. Thus, Osh6 appears to counteract TORC1 activity.

Overall, Gebre and colleagues link the vacuole to lifespan extension, perhaps via TOR, and reveal that vacuole fusion is both necessary and sufficient for lifespan extension.