Modern societies are growing old and we should be proud of such great accomplishment. Substantial improvement in hygiene, health and nutrition has allowed mankind to even surpass a three-digit age. Leaving any further social consideration aside, seems obvious that life sciences will have to make a big effort to understand the so called healthy ageing, as the current challenge is to focus in healthspan, rather than lifespan.
Ageing is a complex phenotype, being strongly influenced by lifestyle, environmental and genetic factors. This is illustrated by the unusually long life expectancy from people living in places such as Ikaria in Greece, but also by the incidence of rare genetic monogenic disorders (caused by mutations on a single gene) like premature ageing syndromes mimicking ageing but in ‘fast-forward’ mode. Moreover, epigenetic regulation has an impact on longevity too, as centenarians delay age-related methylation changes.
Even though there are several proteins (and corresponding genes) unequivocally associated to longevity, including lamin A, Zmpste24 or prolamin A (all those involved in maintaining genome integrity) 1, we are still way far from understanding the process as a whole. Hence, any conclusive study identifying proteins linked to longevity is valuable. Accordingly, a recent study published in Cell has pointed out Transient receptor potential cation channel subfamily V member 1 ion channel (TRPV1) as being involved in regulating longevity and metabolism. TRPV1 is a well-known ion channel for those studying pain, being first cloned in 1997 although formerly was known as the ‘Capsaicin receptor’ because it is responsible for the burning sensation we all have experienced after swallow a delicious and spicy chili pepper (which contains the pungent compound capsaicin).
TRPV1 is expressed in afferent sensory neurons (nociceptors) and its role in detection of high temperatures and painful stimuli from target tissues (mainly dermis and epidermis) has been extensively studied. TRPV1 activation is the reason why after eating chilies you start sweating (specially if you try habaneros). TRPV1 has also been associated with other roles but to date, not with ageing.
The first striking finding was that breeding transgenic mice with no TRPV1, resulted in mutant males living almost 100 days and females 130 days longer when compared to control animals 2. This is astonishing because inbreed mice lifespan is roughly 600-800 days on average for most laboratory strains 3. But perhaps more important was mutant TRPV1 mice displayed a youthful metabolic profile at old age. These animals had improved spatial memory and better motor coordination compared to control mice.
What is the underlying mechanism accounting for these differences? The authors started by looking at growth hormone (GH) levels and insulin growth factor 1 (IGF-1) signalling because both molecules have been tightly linked to longevity (signalling GH/IGF-1-axis dependent), but found no differences. Then, they started to look at a bunch of parameters trying to find a link. Among these, they examined body composition of aged and gender matched controls confirming no change in either organ size or morphological difference in metabolic tissue (liver, white fat or heart). They measured body temperature and assessed general blood factors (metabolic activity), food intake (looking for voluntary dietary restriction), expression of brown fat-specific genes (involved in thermogenesis) but again, no difference. Success came when evaluating the respiratory exchange ratio (RER), which measures the daily transition (night to day shift) from glucose to lipid metabolism. Old mice tend to lose their circadian shift showing a proportional substrate preference toward fat, but strikingly mutant mice kept an almost identical transition than young mice. In concordance, oxygen consumption was enhanced in TRPV1 mutants at old age. As an initial conclusion, mutant mice seemed to be more metabolic-efficient when ageing. First piece to the puzzle was on place.
But still, why do these animals show a more efficient metabolism? To address why, authors looked at the glucose metabolic profile and found no changes in fasted glucose levels (after a long period having no meals), but showed glucose higher tolerance. Injection of glucose into the mice indicated that tolerance was the result of higher insulin secretion without associated hyperinsulinemia, as fasted insulin levels were similar in both mutant and control mice. Second piece to the puzzle.
To reveal the downstream signalling pathway involved, the authors smartly moved into researchers’ favourite worm, Caenorhabditis elegans in an attempt to minimise the complexity of the mammalian neuroendocrine system. But first they had to confirm the suitability of the animal model. Luckily for them, knocking out osm-9 and ocr-2 (homologs of TRPV1 in mammals) in the nematode resulted in an extended lifespan up to 32%! After an elegant experimental design, they identified CRTC1/CREB activity as fundamental to regulate longevity of the worm. CRTC1 protein can be shuttled (or not, depending on its phosphorylation pattern) into the nucleus, where it activates the transcription factor CREB, which in turn promotes expression of different genes. Mice do have CRTC1/CREB signalling system and after doing another pile of experiments back on mice, an association between Ca2+ entry through TRPV1 into nociceptors and the pathway could be conclusively established. They were getting closer!
Under inflammatory conditions, TRPV1 activation leads to secretion of a 37 amino acid peptide called calcitonin-gene related peptide (CGRP) that promotes vasodilation to facilitate healing process. Its production is mediated by the binding of CREB to its promoter, so could CGRP be the final player on that story? The hypothesis would be then as follows: TRPV1 activation translocates CRTC1 into the nucleus activating CREB, which in turn switches on expression of CGRP. Given that nociceptors innervate beta pancreatic cells (which secrete insulin, of course), CGRP could then inhibit insulin secretion (this inhibition has been previously described, indeed).
To mimic insulin secretion from pancreatic beta cells, a mouse insulinoma (isolated cells maintained for long periods in petri dish) was used, because stimulating them with glucose produces a pronounced release of insulin and therefore are a great way to test the hypothesis. Application of a glucose challenge along with CGRP recombinant protein resulted in a sharp insulin secretion drop, up to 50%! After measuring CGRP values in old mutant mice, they found no change in concentration compared to young ones, but its concentration in old control was increased by 42%. Finally, injection of a CGRP receptor blocker into old control mice resulted in a reappearance of a more youthful metabolic RER and increased oxygen consumption after six weeks. Thus, pharmacological intervention resembles the youthful phenotype seen in mutant mice. Such an elegant work so far.
With no doubt, the present study will impact ageing field mainly because our knowledge on TRPV1 is vast already, making easier further studies. Furthermore, it raises a lot of new fascinating research questions such as how endogenous activators of TRPV1 (lipids like endovanilloids) would affect ageing. Even more interesting would be to have a look at how do societies with high chili consumption grow old on average. Do they have a smoother or instead worse healthspan?
- Nikoletopoulou V., Kyriakakis E. & Tavernarakis N. (2013). Cellular and molecular longevity pathways: the old and the new., Trends in endocrinology and metabolism: TEM, PMID: http://www.ncbi.nlm.nih.gov/pubmed/24388148 ↩
- Riera C., Patricia Follett, Mathias Leblanc, Jonathan Halloran, Roger Van Andel, Carlos Daniel de Magalhaes Filho, Carsten Merkwirth & Andrew Dillin (2014). TRPV1 Pain Receptors Regulate Longevity and Metabolism by Neuropeptide Signaling, Cell, 157 (5) 1023-1036. DOI: http://dx.doi.org/10.1016/j.cell.2014.03.051 ↩
- The Jackson Laboratory website ↩