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Reversing Aging, One Drop at a Time?

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By-line – Reviews of “Young blood reverses age-related impairments in cognitive function and synaptic plasticity in mice”, “Restoring systemic GDF11 levels reverses age-related dysfunction in mouse skeletal muscle”, and “Vascular and neurogenic rejuvenation of the aging mouse brain by young systemic factors” by Stuart P. Atkinson.

Three recent papers from the laboratories of Saul A Villeda and Tony Wyss-Coray [1], Amy J. Wagers [2], and Lida Katsimpardi and Lee L. Rubin [3] have built on previous studies showing that the blood of young animals contains factors that can improve cellular function within certain organs [4-8]. The popular press has branded these papers as uncovering the veritable fountain of youth, and we hope to give you the science behind the hype with this brief review of these important papers.

The first study from Villeda et al [1], assessed the effect of young blood on hippocampal function in the aged brain to ask if the youthful properties of young blood might counteract aging and rejuvenate cognitive processes. Gene expression analysis between paired aged (18 months:18 months - isochronic) and aged:young (18 months:3 months - heterochronic) parabionts demonstrated that genes associated with synaptic plasticity regulation and plasticity-related signaling pathways were upregulated in the aged mice during heterochronic parabiosis. Immunohistochemical analysis revealed an increase in the expression of Egr1 and c-Fos genes and phosphorylated Creb in the dentate gyrus (DG) and increased phosphorylated Creb in the CA1 region in aged mice paired as heterochronic parabionts. Structurally, aged heterochronic parabionts displayed increased dendritic spine number on granule cell neurons in the DG, but not the CA1. There was no difference in complexity, but functionally, heterochronic parabiosis enhanced long-term potentiation (LTP) in the DG, although there was no difference in synaptic strength. LTP is a putative functional correlate of learning and memory [9, 10], therefore the group investigated the possibility that young blood could enhance cognitive processes. Injections of either young or old blood plasma during the 3 weeks before cognitive testing using contextual fear conditioning and radial arm water maze (RAWM) paradigms found that aged mice receiving young blood demonstrated increased freezing in contextual, but not cued, memory testing. Similarly, aged mice receiving young blood plasma also demonstrated enhanced learning and memory for hidden platform location during the testing phase, but not the training phase. Excitingly, when Creb signalling was inhibited by the overexpression of a dominant-negative DNA-binding incompetent form of Creb (K-Creb) or shRNA against Creb, the young plasma-mediated increase in dendritic spine number was abrogated, and the improvements observed in the RAWM tests in aged mice receiving young plasma were also reduced, giving a mechanistic insight into the findings.

The second study from Sinha et al [2] concentrated on muscle stem cells known as satellite cells (SCs) which mediate regeneration after muscular injury, but which decrease in number, function and regenerative potential with age [11]. This deterioration is also associated with an increase in DNA damage and compromised DNA integrity. To assess whether these deficits could be recovered by exposure to a youthful systemic environment (i.e. young blood), the researchers compared heterochronic (22 months:2 months) parabionts to isochronic parabionts (young:young, and aged:aged). SCs from the aged heterochronic mouse demonstrated higher colony-forming activity compared to the aged isochronic mice, concurrent with the restoration of genomic integrity to the level of the young-isochronic mice. The group had previously published work on the downregulation of growth differentiation factor 11 (GDF11), a member of the TGF-b superfamily with homology to myostatin (MSTN) [12], in aged mice and that increased GDF11 levels could reverse age-related cardiac hypertrophy [8]. Daily intraperitoneal injections of recombinant GDF11 for 4 weeks led to an increase in SC frequency and muscle function and decreased DNA damage in aged mice. Furthermore, rGDF11 supplementation in aged mice also aided recovery from a cryoinjury to the tibialis anterior muscle (TA), with a more youthful profile of myofiber caliber and increased mean size of regenerating myofibers observed. GDF11 also aided the regenerative capacity of SCs transplanted into the injured muscles of aged animals, with twice as many engrafted fibres observed and larger caliber regenerated fibers when compared to vehicle injected controls. While assessing the mechanistic basis for GDF11’s effect, immunofluorescence analysis found an increase in the size of neuromuscular junctions and improvements in myofibrillar and mitochondrial morphology following rGDF11 treatment. Correlating to this, PGC-1a, a master regulator of mitochondrial biogenesis, was increased in the muscle of aged GDF11-treated mice, while the group also observed increased basal levels of autophagosome (macroautophagy) markers, which may contribute to improved cellular remodelling. Finally, GDF11 treatment enhanced physical function in exercise endurance and grip-strength analyses in aged mice, with an associated enhancement in the clearance of systemic lactate and lowering of glucose levels after exercise suggesting improved mitochondrial function.

The final study, from Katsimpardi et al [3], assessed the effects of “extrinsic young signals” on the vasculature of the brain, which influences neural stem cell (NSC) proliferation and differentiation, and deteriorates with age [13], leading to reduced NSC functionality, neuroplasticity and cognition [4, 14, 15]. As for the previous studies, the researchers compared heterochronic parabionts (2 month:15 months) with young and aged isochronic mice. Initial analysis found that the aged mice in the heterochronic parabiont had increased numbers of proliferative Ki67+ cells, Sox2+ stem cells, and Olig2+ transit amplifying progenitors in the subventricular zone (SVZ) compared to the aged isochronic mice. Additionally, NSCs isolated from aged mice and grown as neurospheres in vitro after heterochronic parabiosis were larger than those taken from aged isochronic and also generated more TuJ1+ neurons. Further in vivo analysis found that heterochronic parabiosis also led to an increase in olfactory neurogenesis in the aged mouse, which mediated a higher olfactory discrimination after exposing the mice to varying concentrations of an odorant [16]. Assessment of cerebrovascular architecture, capillary density and cerebral blood flow through creating “angiograms”, 3D reconstructions of the blood vessels, through the compilation of image sections from confocal microscopy, found that aging caused a decrease in blood vessel volume which could be reversed by heterochronic parabiosis. Additionally, heterochronic parabiosis boosted blood vessel branching and initiated the generation of new blood vessels in aged parabionts. Using magnetic resonance imaging (MRI)-mediated assessment of cerebral blood flow (CBF), the group also found that CBF in heterochronic aged mice had returned to levels seen in young animals. Finally, the group assessed which factors were mediating the improvements observed above and, as in the previous study, GDF11 was found present at higher concentrations in young and heterochronic old than in old mouse serum. Following an intensive GDF11 treatment regimen, the volume of blood vessels in old mice was 50% higher compared with the vehicle-treated mice, while the Sox2+ stem/progenitor cell number was increased by 29%. The authors found that GDF11-mediated these effects through brain capillary endothelial cells; activating the TGF-b signaling pathway and improving their proliferation.

 These three exciting studies, and those that came before, suggest that factors present in the blood of young animals can reverse some of the effects of aging observed in aged mice. Creb and GDF11-based signalling have been shown to be important for the mediation of these effects and further delineation may allow for the creation of a defined anti-aging “cocktail”. But many questions still remain – let us know what you think.

Discussion

•Can we take this forward towards an affordable and effective treatment for age-related deficits?

•Could factors from young blood reverse or inhibit the effects of degenerative diseases?

•Are there any potentially off-target unwanted side-effects of global treatment with GDF11?

•Will GDF11 play a protective role if given to younger mice?

•How might we assess this in humans?

References

  1. Villeda SA, Plambeck KE, Middeldorp J, et al. Young blood reverses age-related impairments in cognitive function and synaptic plasticity in mice. Nat Med 2014;
  2. Sinha M, Jang YC, Oh J, et al. Restoring systemic GDF11 levels reverses age-related dysfunction in mouse skeletal muscle. Science 2014;344:649-652.
  3. Katsimpardi L, Litterman NK, Schein PA, et al. Vascular and neurogenic rejuvenation of the aging mouse brain by young systemic factors. Science 2014;344:630-634.
  4. Villeda SA, Luo J, Mosher KI, et al. The aging systemic milieu negatively regulates neurogenesis and cognitive function. Nature 2011;477:90-94.
  5. Conboy IM, Conboy MJ, Wagers AJ, et al. Rejuvenation of aged progenitor cells by exposure to a young systemic environment. Nature 2005;433:760-764.
  6. Brack AS, Conboy MJ, Roy S, et al. Increased Wnt signaling during aging alters muscle stem cell fate and increases fibrosis. Science 2007;317:807-810.
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  8. Loffredo FS, Steinhauser ML, Jay SM, et al. Growth differentiation factor 11 is a circulating factor that reverses age-related cardiac hypertrophy. Cell 2013;153:828-839.
  9. Bliss TV and Collingridge GL A synaptic model of memory: long-term potentiation in the hippocampus. Nature 1993;361:31-39.
  10. Frey U and Morris RG Synaptic tagging and long-term potentiation. Nature 1997;385:533-536.
  11. Jang YC, Sinha M, Cerletti M, et al. Skeletal muscle stem cells: effects of aging and metabolism on muscle regenerative function. Cold Spring Harb Symp Quant Biol 2011;76:101-111.
  12. McPherron AC, Huynh TV, and Lee SJ Redundancy of myostatin and growth/differentiation factor 11 function. BMC Dev Biol 2009;9:24.
  13. Farkas E and Luiten PG Cerebral microvascular pathology in aging and Alzheimer's disease. Prog Neurobiol 2001;64:575-611.
  14. Kuhn HG, Dickinson-Anson H, and Gage FH Neurogenesis in the dentate gyrus of the adult rat: age-related decrease of neuronal progenitor proliferation. J Neurosci 1996;16:2027-2033.
  15. Seki T and Arai Y Age-related production of new granule cells in the adult dentate gyrus. Neuroreport 1995;6:2479-2482
  16. Witt RM, Galligan MM, Despinoy JR, et al. Olfactory behavioral testing in the adult mouse. J Vis Exp 2009