This work identified key tissues, genes essential and specific for mitochondrial longevity and at least one mechanism necessary for increased longevity in response to altered mitochondrial function in a metazoan. The UPRmt
was essential for the extended longevity of ETC mutant animals and has been previously reported to be upregulated in response to RNAi of cco-1
(Yoneda et al., 2004
). Consistent with the temporal requirements of the ETC to modulate longevity during the L3/L4 larval stages, the UPRmt
could only be induced when cco-1
RNAi was administered before the L3/L4 larval stage, but not in adulthood. Therefore, induction of the UPRmt
mirrored the temporal requirements of the ETC to promote longevity when reduced. More importantly, the fact that induction of the UPRmt
can be maintained long into adulthood, well after the mitochondrial insult had been given in larval development, indicates that the animal might possess an epigenetic mechanism to ensure increased resistance to future mitochondrial perturbations.
One of the most surprising findings is the UPRmt
can be activated in a cell non-autonomous manner. Because the hsp-6
GFP reporter is primarily limited to expression in the intestine, we were well poised to ask if perturbation of cco-1
in the nervous system could induce the UPRmt
in the intestine. Therefore, neuronal limited knockdown of cco-1
could profoundly induce the hsp-6
reporter indicates that a cue from the nervous system must travel to the intestine to induce the UPRmt
(). It is not clear whether the factor is proteinacious, nucleic acid based or a small molecule, but it is clear that its production in a limited number of cells can profoundly influence the survival of the entire organism. Because this signal is the product of perceived mitochondrial stress that results in increased survival, we have termed this cell non-autonomous signal a “mitokine”.
Model for the cell non-autonomous nature of the UPRmt.
While many of these perturbations have pleiotropic effects that result in their short life span, their ability to upregulate the UPRmt
is not sufficient to overcome these potentially harmful side effects. We found that muscle-specific cco-1
RNAi could also induce the intestinal hsp-6
GFP reporter, yet these animals were not long lived (Figure S4B
). We also find that short-lived mev-1
mutant animals also induce the UPRmt
(data not shown). Finally, many of the nuclear-encoded mitochondrial genes discovered to induce the UPRmt
when inactivated using RNAi (Yoneda et al., 2004
) are not long lived (data not shown). Therefore, ectopic induction of the UPRmt
is required but is not sufficient in the establishment of the pro-longevity cue from mitochondria in these settings.
Of the currently identified UPRmt pathway members, the ubiquitin like protein, UBL-5, which provides transcriptional specificity for the homeobox transcription factor DVE-1 in response to unfolded proteins in the mitochondria, is essential for the increased longevity of ETC mutant animals. Knockdown of ubl-5 specifically in the intestine was not sufficient to block mitokine signaling from the nervous system, but was able to block induction of animals fed dsRNA of cco-1, which can not reduce cco-1 in the nervous system. Therefore, it appears that ubl-5 either functions exclusively in a cell-autonomous fashion, or the signals generated to induce the UPRmt in the nervous system are very different from the signal generated in other tissues as ubl-5 was not required for neuronal induction.
It is intriguing to speculate why reduced mitochondrial ETC in only a few tissues are able to send a pro-longevity cue, or mitokine, but others do not. Because the intestine and the sensory neurons (amphids and phasmids) are the only cells that are in direct contact with the worm's environment (the hypodermis/skin is wrapped in a protective, dense cuticle), perhaps these cells are fine-tuned to perceive mitochondrial insults that might be present in the environment. Alternatively, mitochondrial metabolism in the nervous system and intestine might have different requirements than other tissues making disruptions in these tissues more susceptible to perturbation and subsequent UPRmt
upregulation. As another possibility, there is a growing body of research emphasizing the importance of ROS, not as damaging agents, but as crucial components of cell signaling. It remains a possibility that ROS may act as signaling molecules and potentially serve as the mitokine or intermediary to elicit a nuclear response. However, animals treated with high doses of the antioxidants N-acetyl-L-cysteine (NAC) or ascorbic acid (vitamin C) were not able to block mitokine signaling (Figure S5
), a thorough investigation of mitochondrial function from each tissue will be essential to test these hypotheses.
In the future it will be important to understand how mitochondrial stress initiates the UPRmt in a cell-autonomous fashion and how this stress is then transmitted throughout the organism to induce the UPRmt in cells that have yet to possess mitochondrial stress (i.e. a cell non-autonomous fashion). Furthermore, the identity and mode of action of the mitokine will provide an avenue to explore treatment of mitochondrial diseases in a tissue and cell-type specific manner if conserved from worm to man.