GDF11 abdicates the throne of systemic rejuvenation
As recently revealed, the key findings on diminished levels of GDF11 with age and of the positive effects of this protein on myogenesis are not reproduced [36, 40–42]. Very importantly, this would not come as a shock if one would read the relevant papers. Based on the science, GDF11 is 90% identical to myostatin, which inhibits (not activates) myogenesis. GDF11 in neurogenesis serves to limit the numbers of neural stem cells [43, 44] and this discrepancy was not cited or discussed in the pro-rejuvenation papers. GDF11 signals through the same receptor as TGF-beta1 and myostatin (more efficiently than myostatin [40] and inhibition (not activation) of this ALK5 receptor has been shown to enhance and rejuvenate myogenesis and also, neurogenesis [6, 37]. The bulk of data questions the anti-aging effects of GDF11, and makes the Cell Metabolism paper important for avoiding years of unproductive research in the wrong directions.
GDF11 was reported to enhance old muscle repair by attenuating the accumulation of DNA damage in the aged satellite cells [36]; however, a year earlier it was published that old muscle satellite cells do not accumulate DNA damage with age and that DNA damage in muscle stem cells is uncoupled from efficiency of muscle repair and likely represents a physiologically required process of terminal myogenic lineage differentiation [15], [45]. Once again, a key paper in a lesser impact journal questioning main conclusions of a high impact paper was not cited or caught in peer or editorial review.In apparent disagreement between two recent studies, GDF11 does [41] and does not [46] reduce hypertrophy of old mouse hearts. Even if there is no direct connection to aging, stem cells or heterochronic parabiosis, ectopic GDF11 may reduce the mass of both young and old hearts [47].
These controversies may be caused by the fact that antibodies to GDF11 have cross-reactivity with GDF8/myostatin. GDF11 antibody might also simply cross-react with immunoglobulins, which become elevated with age [47, 49]. A very elegant recent paper using a clean myostatin knock-out system demonstrated that GDF11 levels are 500 times less than that of myostatin or activin, thus precluding any competitiveness for the same receptor complexes and arguing against physiological modulation of pSmad2/3 signaling by GDF11 when myostatin, activin or TGF-beta1 are present [48].
Age-specific levels of GDF11 are also not without controversy: initially, it was reported that GDF11, but not GDF8/myostatin, declines with age [41]; then more recently it was determined that GDF8/myostatin does decline with age [48], an observation which was also reconfirmed by the original Cell authors, who now combine the two protein names into one, called GFD11/8 [47].
Muscle tissue produces myostatin, so the age-specific decline in myostatin protein may reflect the loss of muscle mass in the old. Consequentially people or mice with physiologically higher muscle mass (particularly in older age) are likely to be healthier, thus exhibiting an indirect link between the levels of myostatin and the health of the heart and other organs. Notably, GDF11 is associated with human colon cancers, which is likely due to the pro-angiogenic properties of GDF11 and most TGF-beta family ligands [50, 51]. Accordingly, Alk1 and Alk5 inhibitors are studied as anti-cancer anti-angiogenic blockers [52, 53].
Systemic Problems: A perspective on stem cell aging and rejuvenation
http://www.ncbi.nlm....les/PMC4637204/
Recent high-profile studies report conflicting data on the age-related change in circulating growth/differentiation factor 11 (GDF11) and myostatin as well as the former's influence on muscle regeneration. Both ligands bind and activate ActRIIB receptors with similar affinities and should therefore have similar actions, yet these studies suggest that GDF11 activates muscle regeneration whereas myostatin is well known to inhibit it. They also suggest that circulating GDF11 levels, but not those of myostatin, decline with age. We performed a careful assessment of the ELISA used to quantify circulating myostatin in these studies and determined that assay reagents significantly cross react with each protein, each of which is highly homologous. Circulating myostatin levels decreased with age and estimates of GDF11 levels using myostatin null mice indicate that they were almost 500 times lower than those for myostatin. This suggests that circulating GDF11 has little physiological relevance as it could not outcompete myostatin for ActRIIB binding sites. Together, these results further suggest that the previously reported aging muscle, heart, and brain phenotypes attributed to reduced circulating GDF11 should be reconsidered.
http://www.ncbi.nlm....pubmed/26372181
This paper doesn't seem to have circulated on these forums yet as I learned in a recent article in the news section.