Turning off the Wnt—a dietary restriction switch for healthy aging

Abstract

Dietary restriction (DR) with adequate nutrition (i.e., undernutrition without malnutrition) extends lifespan and improves healthspan in diverse model organisms, ranging from yeast to mammals. The molecular mechanisms by which a reduced nutrient uptake extends lifespan and delays the onset of pathologies associated with aging are only partly understood. In this issue of EMBO reports, Xu et al 1 identify a conserved microRNA named mir‐235, that is induced by DR and is required for DR‐induced longevity in Caenorhabditis elegans. mir‐235 inhibits a highly conserved developmental pathway—the Wnt signaling pathway. Consistent with the developmental roles of the Wnt pathway, the mir‐235 interferes with its activity at the onset of adulthood, and not at the embryo stage, therefore protecting the embryo from putative developmental defects due to precocious inactivation of the Wnt pathway 1. Understanding the impacts of such signals, which are induced by DR and mitigate the antagonistic activity of pleiotropic genes, could guide the design of treatments that mimic DR‐related consequences without the need for a restricted diet, to delay age‐related diseases in humans.

For several decades, scientists have shown that reducing calorie intake is a nongenetic interference that can dramatically extend lifespan and delay age‐related phenotypes in many species 2. DR has been shown to activate several nutrient‐sensing pathways such as the insulin growth factor/insulin‐like growth factor (IGF) signaling pathway, the target of rapamycin (TOR), and AMPK signaling pathway. These signaling pathways have been linked both to longevity regulation and to developmental processes. Nevertheless, at this point, it remains unclear to what extent the genetic pathways that regulate this response are conserved between species and between different regimens of DR.

microRNAs are short noncoding RNA molecules that interfere with gene expression by binding to the 3′ UTR of target mRNAs based on a complementary seed sequence and reducing the transcripts’ stability or translation. These regulatory elements play important roles in developmental processes as well as in the regulation of aging 3. Multiple studies followed the age‐associated changes in expression of miRNAs in general, and specifically in dietary‐restricted animals, and both life‐extending and life‐shortening properties have been attributed to specific miRNAs. In C. elegans, for example, mir‐71 and mir‐228 are nutrient‐regulated microRNAs required for DR‐induced longevity, whereas mir‐80 is a nutrient‐regulated microRNA whose decreased expression mimics DR‐induced longevity 4, 5.

In this issue, Xu et al 1 reveal a novel link between DR‐induced longevity and Wnt signaling in C. elegans that is controlled by a specific microRNA, named mir‐235 (Fig 1). The authors find that mir‐235 expression is induced in adult animals subject to two different DR regimens and that DR‐induced longevity is curtailed in mir‐235‐deficient animals. Curiously, similarly to the C. elegans phenotype, age‐dependent changes in the levels of two‐dozen miRNAs, including the homolog of mir‐235 (miR‐92), have been documented in the plasma of dietary‐restricted rhesus monkeys 6, indicating that the function of miRs under DR conditions might be evolutionarily conserved.

Figure 1. Dietary restriction mitigates the antagonistic effects of Wnt signaling in adults via a miRNA switch.

The Wnt signaling pathway is an antagonistic pleiotropic pathway, which drives animal development, whereas its hyper‐activation and activity during adulthood drive aging and age‐related pathologies. DR mitigates these antagonistic effects by switching off Wnt signaling as the animals transition from larval development to adulthood. This temporal inhibition of the Wnt signaling pathway in dietary‐restricted adults is achieved by the induction of a conserved microRNA named mir‐235/miR‐92 by inducing its transcription. Interestingly, two different dietary restriction (DR) regimens induce mir‐235 expression using different transcription factors. The resulting shutdown of Wnt signaling specifically in adults allows completion of development, while minimizing the antagonistic effects of the Wnt pathway in adults.

What is the target of mir‐235, whose silencing is required for lifespan extension by a variety of DR treatments? Xu et al identified the Wnt ligand cwn‐1 as a bona‐fide target of mir‐235. Wnts are growth stimulatory factors, leading to cell proliferation and serve as drivers of most types of tissue stem cells in adult mammals. cwn‐1 is an ortholog of human WNT4 and is involved in several developmental processes, including asymmetric cell division 7.

Since the Wnt signal transduction cascade is a central developmental regulator, its inhibition would likely interfere with normal development. Xu et al find that mir‐235 suppresses Wnt signaling in a temporal fashion, i.e., only in young adult worms and thus after the animals have completed their developmental program. This temporal control of the Wnt signaling pathway is achieved by inducing mir‐235 expression upon DR specifically in adulthood, by an as yet unknown mechanism. This is important as on the one hand, Wnt signaling is a critical and evolutionarily conserved developmental pathway, while on the other hand, its hyper‐activation and specifically its activity during adulthood are related to age‐related pathologies including cancer and neurodegenerative diseases 8. DR resolves this antagonistic pleiotropic paradox by the temporal shutdown of Wnt signaling. This is achieved by “switching on” mir‐235 expression in dietary‐restricted adults, which in turn “switches off” antagonistic Wnt signaling, while avoiding its disruption during the critical developmental periods (Fig 1). In this way, the trade‐off between Wnt‐associated early‐life fitness and late‐life mortality can be rebalanced. A similar temporal microRNA switch activated at the transition from larval development to adulthood has also been described in animals that are long‐lived due to depletion of their germline 9. Together, these studies suggest that microRNAs may serve as general developmental timers that control antagonistic pleiotropic genes in a temporal manner, to benefit the organism under defined circumstances such as dietary restriction or reproductive stress, and thus may be relevant in additional longevity pathways, especially those that implicate developmental genes.

Recent findings indicate that beneficial dietary restriction can be achieved by a variety of regimens. These include a decrease of ~30% in the normal caloric intake, strict control of caloric meal timing, intermittent fasting, and reduced intake of particular nutrients. 2. In C. elegans, different DR regimens have been shown to extend lifespan, albeit each makes use of a distinct molecular network 10. Xu et al used two distinct DR regimens to restrict nutrients intake and extend lifespan in C. elegans. One was by a genetic mutation (eat‐2) that reduces the pharyngeal pumping and feeding rate of the worm. The second was by diluting the food (bacteria) on solid agar plate (sDR). Both methods induced the upregulation of mir‐235 expression at the onset of adulthood, but this occurred via different mechanisms. In eat‐2 mutants, the transcription factor pha‐4/FoxA acts as an upstream regulator of mir‐235, directly inducing the transcription in adulthood. In contrast, in sDR‐treated animals, mir‐235 upregulation is controlled by the daf‐16/FoxO transcription factor. Thus, each DR regimen leads to the same effect of increasing mir‐235 level in adult worms, using different molecular signals (in this case, FoxA versus FoxO). These findings are consistent with differential requirements of these transcription factors for lifespan extension in the examined DR regimens. It will be interesting to see whether additional DR regimens also induce mir‐235 expression and whether distinct transcription factors are required for their induction. If so, this will imply that different DR regimens do ultimately integrate into a common longevity mechanism. Moreover, future studies will show whether Wnt signaling inhibition occurs also in additional longevity treatments, and if so, whether it is mediated by mir‐235 or by other means.

Does the mir‐235/Wnt signaling module limit normal adult lifespan, or does it specifically limit lifespan extension by DR? In wild‐type C. elegans, the expression of mir‐235 drops with age, and overexpression of mir‐235 mildly extends their lifespan. This suggests that this pathway affects normal aging in wild‐type animals. Interestingly, the expression of mir‐235 in wild‐type animals is not controlled by the pha‐4 or daf‐16 transcription factors, suggesting that induction of mir‐235 by these transcription factors represents a specific transcriptional response to a low nutrient status such as DR. Nevertheless, it could be that Wnt signaling is regulated by alternative means in aging wild‐type animals. These may include mir‐235‐independent regulation of cwn‐1 levels, or by regulating other components of the Wnt signaling pathway.

Finally, since mir‐235 and the Wnt signaling pathway are evolutionarily conserved, and DR treatments extend lifespan and healthspan across species, it will be interesting to determine whether a similar switch impacts longevity in mammals as well. This is likely the case as the Wnt/β‐catenin signaling pathway plays a key role in aging in mammals, and both, Wnt signaling and miR‐92/mir‐235, are regulated in mammals upon DR. If so, this may imply that Wnt signaling inhibitors could be used to mimic some aspects of DR to obtain health benefits.

EMBO Reports (2019) 20: e48114