Amazing aging discoveries: It’s in the blood

In a candidate for coolest experiment of the year, scientists at Stanford University surgically created conjoined-twin mice of different age pairs to assess the impact of young versus old blood on the brain.

Published this month in Nature, researchers investigated the effect of proteins in the blood to encourage or inhibit neurogenesis (new cell growth in the brain), concentrating on the dentate gyrus of the hippocampus, a key area in memory consolidation and a region known to be susceptible to environmental and age-related changes in cellular growth and death. The hippocampus is strategically located in an area rich with blood vessels, making it an ideal region to look for the effects of aging blood on the brain.

First, looking at older mice, scientists identified a decrease in neurogenesis and synaptic plasticity, as well as behavioral learning and memory, confirming deficits commonly seen in aging populations.

Next, using a procedure called parabiosis, pairs of young and old mice were surgically attached to one another, enabling the sharing of blood, plasma, and proteins between the two through their newly conjoined cardiovascular systems. In the heterochronic condition (unions of young to old mice), significant differences in the levels of new neurons and progenitor cells (similar to stem cells) were seen in each of the pair members. Older heterochronic mice had significant increases in these levels as compared to their isochronic paired counterparts (unions of old to old mice), whereas the unlucky younger mice of the heterochronic pairs saw significant reductions in the number of these new cells compared with the young isochronic mice. Additionally, extracellular recordings were taken from the heterochronic pairs, and a decrease in long-term potentiation–an indicator of the cellular plasticity essential in learning and memory–was decreased in the young heterochronic mice but increased in the older ones, further indicating the deleterious effects of old blood on the brain.

To confirm that the aging effects seen were due to proteins in the blood of these animals, a  different set of young mice were injected with the plasma from either young or old animals. Again, young mice injected with the plasma from older mice saw decreases in the number of new cells in the dentate gyrus, indicating impairment in neurogenesis, whereas no differences were seen in the animals who were given younger blood. Behaviorally, these young blood recipients showed no changes in their learning and memory abilities, however, the mice who received the plasma from older mice now exhibited impairments in their fear conditioning and spatial learning behaviors similar to those seen in aging populations.

Finally, using a method known as a proteomics, researchers attempted to identify the individual proteins that might be causing these aging effects. They measured the levels of 66 individual proteins in the plasma taken from young and old mice and then compared them to the heterochronic pairs. In normal aging mice, 17 of these proteins were identified as being negatively correlated with neurogenesis, an increase in the proteins signifying a decrease in new cell generation. Increases in these specific protein levels across aging seem to inhibit neurogenesis, and 16 of these proteins were also found to be elevated in the young heterochronic paired mice. The team then narrowed down the field to one particular protein, CCL11, which is also known to have an age-related increase in humans. Scientists performed one last experiment to confirm the role of CCL11 in aging, injecting the protein into mice. This resulted in the anticipated increase in CCL11 levels in plasma, as well as a corresponding decrease in neurogenesis and new cells in the hippocampus.

The depth of this study, including the variety of different methods the researchers used, is one of the most impressive things about it, independent of the exciting discoveries they have made. Additionally, the idea of pinpointing a specific protein to stave off aging is one that keeps popping up in science, and the fact that CCL11 has been shown to have aging effects in humans as well as mice suggests that this might be a promising one to target. Or, on the flip side, we might see an unfortunate increase in geriatric vampire literature as a result of these findings. You never know.

Questionable science

The pressure to publish in science and academia is intense, so much so that it is not uncommon to find careless mistakes in a paper or have a result pushed through to a journal before it has been thoroughly vetted. The peer review system is in place to try to prevent these rushed, over-reaching or imprecise findings, however, negligent research does get through, particularly in open-access or online-only journals. David Colquhoun recently wrote a piece for The Guardian raising this concern and citing breakdowns in peer review as the source of these flawed publications. However, the problem is not only in these small niche journals but is pervasive in scientific research and publishing as a whole.

Nature Neuroscience printed a frightening article last month about the magnitude of statistical error in psychological and neuroscience studies. The errors are rampant, biasing results to appear significant in nearly half of all studies published last year in top-tier journals, including Science, Nature, The Journal of Neuroscience, and Neuron. These faulty statistics come from a tendency to compare significance, or p-values, separately in experiment versus control conditions. On the surface, this sounds like exactly how these tests are supposed to work, but on closer inspection this practice grossly overestimates the significance of an experimental effect. For example, if mice who receive a test anti-anxiety drug significantly reduce their freezing responses (an indicator of anxiety) with an effect of p < 0.05, and mice who receive a control do not significantly produce a similar effect (p > 0.05), one would be tempted to claim that this anxiolytic drug worked, having a greater effect on the targeted symptoms than the placebo and reducing anxiety in test mice. However, the true comparison that must be made for this claim to be verified is whether the test mice had significantly fewer anxiety symptoms compared to the control mice–i.e., not drug vs. baseline compared to control vs. baseline, but drug vs. control.

The authors of this review, led by Dr. Sander Nieuwenhuis, make the point that although 0.05 is a widely accepted validation criterion for significance, it is also generally thought to be a potentially deceptive one. In the example above, the control mice could have a p-value of 0.051 while the experiment condition registered as 0.049. Technically, one condition had a significant effect while the other did not. However, if you were to compare the two effects directly, it is highly dubious that this comparison would remain significant. This false effect occurred in all types of studies, including pharmacological and behavioral group comparisons and neuroimaging research. It was also not limited to cognitive or behavioral fields, occurring in cellular and molecular neuroscience articles as well.

This discovery may help to explain a phenomenon recently described in Nature Reviews Drug Discovery by scientists at the Bayer laboratories in Germany. The authors reported that over 50% of all experimental drug trials conducted at academic research institutions were unable to be replicated in private clinical trials. They list both statistical errors and improperly reviewed research as potential sources for this irreproducibility. In addition, they cite a pressure to only report significant positive findings, rather than a null result, as an effect biasing research output. This trend has been lamented by purist researchers for years, however, it is difficult to combat the urge to publish news-worthy headlines; after all, nobody wants to read “We didn’t find anything.”

Retractions of major papers in journals are rare but do occur when enough protest has been made about over-reaching claims or questionable research methods. Unfortunately, though, the information has typically already been disseminated to the public, and there is rarely a press release for a retraction as there are for publications. Thus, the intellectual damage–particularly to non-professionals who stumble across the information through standard media outlets–is already done. Such was the case in the vaccine-autism link that was first proclaimed in the 1990s and set off panic among parent groups. The original research paper that made this claim was found to be largely fraudulent, yet the resonating impact of this paper has lasted now for nearly two decades. Fortunately, there have been recent attempts to remedy flaws like these in science by keeping researchers honest and holding them accountable to their over-zealous, inaccurate, or unproven claims. This includes blogs like Retraction Watch, which has gained notoriety for publishing posts of recently retracted findings, reminding the public of the potential for error in science and following up on errant claims made by researchers.

As so many things do, this distressing trend of questionable science stems from a push for prestige and money. Publishing in top-tier journals, or even publishing at all, is the current marker of professional success and is an expectation for researchers, particularly in academia, who want to advance their careers. In addition to professional progression, with publications comes grant funding, significant sources of money for both institutions and individuals. The commercialization of science and research is troubling and suggests that a dramatic overhaul of the system might be in order, though that is certainly easier said than done.

The healing power of the brain-gut connection

Anyone who’s ever tried to cure the blues with Ben and Jerry’s knows that there is a link between our stomachs and our moods. Foods high in fat or sugar release pleasure chemicals into the brain in much the same way that drugs do, and chocolate in particular is frequently touted as a mood-elevating treat. Now research from a team of pharmacologists in Ireland provides new support for this brain-gut connection, showing that probiotic bacteria, like those found in many strains of yogurt, can elevate mood and reduce anxiety.

Formally referred to as the microbiome-gut-brain axis, this system has been implicated in stress responding, with gut microflora affecting the HPA (hypothalamic-pituitary-adrenal) axis and altering stress and anxiety responses. In the current study, researchers gave mice Lactobacillus rhamnosus and then subjected them to various stress-inducing tests. Mice who had been fed the probiotic solution demonstrated less freezing or fear-response behaviors compared to those who were given plain broth. They were also more likely to explore exposed novel environments in an elevated maze, an indication of security and lack of anxiety. Finally, on a depression assessment, mice were placed in a forced swim test (also called the behavioral despair test), where they were submerged in water and had to struggle to stay afloat. Lack of effort and time not spent attempting to swim are seen as indicators of depression and hopelessness, and probiotic-fed mice had less immobility time than broth-fed mice. Corroborating these behavioral results, test mice also had lower levels of corticosterone after being stressed than control mice.

This interaction between the brain and the gut is facilitated by the vagus nerve, a cranial nerve that transmits sensory information from internal organs to the brain. When this nerve was cut the effects of the probiotics disappeared, and test mice had decreased exploratory behavior and greater periods of immobility, similar to the broth-fed mice.

The anxiolytic effects of L. rhamnosus seem to be tied to GABA, an inhibitory neurotransmitter involved in anxiety. Probiotic administration altered levels of GABA mRNA expression in regions of the brain, including the amygdala, hippocampus, and prefrontal cortex. In depressed individuals, GABA levels in the frontal cortex are shown to be reduced, but in the probiotic-fed animals, cortical GABA levels were elevated. This led the researchers to theorize that L. rhamnosus might help to protect against stressful or anxiety-producing events. GABA levels in the amygdala are also commonly elevated in depressed individuals, and GABA antagonists (which reduce the levels of the neurotransmitter in the brain) are sometimes used as antidepressants. In the current study, lower levels of GABA were found in the hippocampus and amygdala after probiotic consumption, suggesting an interaction between L. rhamnosus and the memory and emotional centers of the brain, potentially increasing associative learning and memory consolidation and decreasing fear responding to stressful events.

The connection between diet and behavior doesn’t just apply to stress. Certain highly specified restrictive diets have also been used to help treat and control a variety of neurological disorders, most notably epilepsy. First pioneered in the 1920s at John’s Hopkins Children’s Hospital, extreme high-fat/low-carbohydrate diets are gaining support as a possible alternative for drug-resistant epilepsy, though some physicians are still skeptical. The diet works by invoking ketosis, a process in which the body burns fat stores rather than carbohydrates for fuel. This typically occurs when the body is in a starvation state and is the premise on which low-carb diets are based. However, ketogenic diets also appear to have an antiepileptic effect, particularly in cases of severe pediatric epilepsy. Doctors are not sure why the treatment works, but one theory is that the ketone bodies produced by the liver when the body burns fat protect neurons from damage, though how or why this happens is still unknown.

A keystone paper from University College London published in 2008 was the first to empirically report the efficacy of the ketogenic diet, and a recent provocative profile in the New York Times of a family dealing with epilepsy, “keto,” and the trials it brings has brought this treatment to national attention. The diet itself is strictly regimented and incredibly difficult to follow. It requires exact caloric measurements and proportions of protein, carbohydrates, and fats, with roughly a 3-to-1 fat to carb/protein ratio. This relates to a diet of roughly 90% fat, which can be dangerous, potentially triggering kidney problems and malnutrition. However, the effectiveness of the treatment is gaining recognition, and patients who are on the ketogenic diet (mostly children) are carefully monitored for cholesterol levels and cardiovascular health.

The ketogenic diet is now being looked at to potentially treat other serious medical disorders, such as Parkinson’s disease and cancerous tumors. It is important to note, though, that individuals being treated with a prescribed diet are also frequently on concomitant medication. Diet alone will not be able to cure all ailments, but the connection between diet and mental and physical health cannot be denied, and in the very least it is a good place to start keeping yourself well and taking preventative action.