The Key to Getting More Mileage Out of the Brain: Change the Fluids

The Key to Getting More Mileage Out of the Brain: Change the Fluids

A recent study from Stanford University School of Medicine demonstrates the partial restoration of brain function in old mice with infusions of cerebrospinal fluid (CSF) from younger mice. CSF surrounds the brain and spinal cord and is critical in maintaining a healthy brain by transporting nutrients, cellular waste, and hormones in to and out of the nervous system. Maintaining an organism’s supportive fluid systems, such as cerebrospinal fluid, is gaining greater attention as a means of restoring a more youthful phenotype in research animal models. Specifically, the transfusion of blood and plasma from younger or healthier organisms to their older, more decrepit counterparts has been shown to impart restorative properties in laboratory animals. This has led to similar approaches utilizing other supportive matrices such as the microbiota of the gut and, in this study, CSF.

Older mice treated with the CSF of young mice showed improved memory on a standard laboratory test to assess memory formation and consolidation—a cognitive function known to deteriorate with age. The researchers conducting this study believe this might be due to the observed proliferation of cells within the hippocampus, a region of the brain highly involved in memory function. Further interrogation of this region revealed an increase in serum response factor (SRF), a protein that decreases with age. SRF belongs to a family known as transcription factors, which have the ability to regulate gene expression. Further tests revealed SRF to mediate the increase in hippocampal cells, specifically oligodendrocytes, observed in mice treated with the CSF of young mice.

In an attempt to further characterize the mechanisms responsible for the improvements in memory, a subsequent profiling determined a specific protein, Fgf17, to be an activator of SRF. Fgf17 is one of the proteins known as fibroblast growth factors, of which a number have been implicated in improving and/or maintaining healthspan. Further experiments were designed to assess Fgf17’s relevance in supporting the enhanced memory seen in old mice transfused with the CSF of young mice.

Experiments involving Fgf17 carried out in cultured cells produced results similar to those in mouse model experiments. Fgf17-treated cells showed both an increase in SRF activity and an increased proliferation in oligodendrocytes—the same cells found to be increased in the memory region of mouse brains treated with young CSF. In addition to the findings of the cultured cell experiments, mice treated with an antibody that inhibited the action of Fgf17 performed poorly on a series of standard memory tests.

Aging is characterized by an array of dysfunctions in multiple organ systems, e.g., cardiovascular disease, diabetes, arthritis, and kidney disease; however, in humans perhaps the most tragic are dysfunctions of the brain. Severe memory loss associated with neurodegenerative diseases such as dementia and Alzheimer’s disease places an enormous emotional and economic burden on both the families and the communities that support these individuals, and with a rapidly growing geriatric population and no known therapies, the future looks very bleak. According to the CDC, an estimated 5.8 million people over age 65 suffer from Alzheimer’s disease, a population predicted to triple by 2060. However, studies such as these may lead to adequate therapeutics desperately needed to address this impending plight.

Cool Down, Live Longer

Cool Down, Live Longer

The Second Law of Thermodynamics dictates that heat, not resulting in work, will increase a system’s state of entropy, and very simply all systems—both animate and inanimate—will proceed toward a state of disorder. This phenomenon is somewhat outside the purview of biology and is more often dealt with within the realm of physics, yet it is undeniable that as a function of both time and temperature, biological systems are fated to an increased state of entropy. From an organismal perspective, this can speed the steady accumulation of dysfunctional biomolecules exceeding the threshold of what can be adequately maintained. These changes can have a secondary effect on other systems, and this increasing systemic failure is thought by some to be the phenomenon of aging. A recent Nature Metabolism paper has sought to address this fundamental feature of aging and whether or not core body temperature can modulate lifespan.

Commonly referred to as warm-blooded, homeothermic organisms maintain a set body temperature independent of most changes in environmental conditions. Previous studies have shown that genetic alterations to the brain’s “thermostat” can lower this set point and significantly extend the lifespan of mice; however, the attribution of this effect to reductions in body temperature alone is questionable as metabolic rate and body temperature are closely intertwined, and reductions in metabolic rates are also associated with increased lifespan. To address this conundrum, experiments were designed to take advantage of a dissociation that occurs between metabolic rate and body temperature as animals approach the high end of what is known as the thermal neutral zone.

Thermograms of male and female humansThe thermal neutral zone is the ambient temperature range requiring a nominal amount of energy to maintain an organism’s prescribed core body temperature. As ambient temperatures fall below the lower limit of this range, homeotherms will generate heat through increased metabolism. At the upper limit of this thermoneutral zone, there is a point at which an animal’s body temperature will increase without a concordant change in metabolic rate. Taking advantage of this phenomenon allowed researchers to interrogate differing body temperatures independent of metabolic changes.

Animals that were kept at this critical temperature threshold experienced an average increase in body temperature of approximately 0.5°C. This temperature differential varied slightly between gender and species but was always significantly greater when compared to counterpart animals housed at temperatures below the thermoneutral zone. As a result, animals with increased body temperatures showed a reduction in lifespan ranging between 21% and 41%; lifespan reductions also varied in a manner similar to the gender- and species-dependent variations with regard to body temperature increases.

Importantly, the core body temperature of animals housed at the upper limit of the thermoneutral zone can be lowered with exposure to a continuous circulation of air. This resulted in the elimination of increased core body temperatures and completely reversed the reduced lifespan seen in their overheated counterparts, demonstrating that body temperature independent of metabolic alterations can be a potent modulator of lifespan.

Since the development of technologies that have allowed humanity to outlive the “normal” period an organism is fit to survive in the natural world, the tragedy of old age and decrepitude has frustrated researchers and driven them to seek out a root cause. Progress has been achieved, for the most part, by addressing the discrete age-related pathologies; however, taking a first-principles approach focused on temperature has allowed researchers to investigate a basic temperature-dependent facet of aging and perhaps affect an underlying driver of many, if not all, age-related phenomena.

Turning Back the Cellular Clock

Turning Back the Cellular Clock

Thinking of biological aging as analogous to a clock, research has given us two distinct avenues of lifespan-extending intervention: the clock can be either slowed down or wound back. Most research has with dealt the former, but the latter is very promising—especially for those whose clock has been winding for some time. Commonly referred to as rejuvenation therapies, these approaches typically utilize stem cells.

From a developmental perspective, stem cells can be considered neophytes of the cellular world. They are not specialized cells like liver cells, skin cells, or any variety of mature cells that provide a specific physiological utility, but foundational cells having “instructions” and capable of transforming into a variety of specialized cell types. Stem cells have the capacity to replace nearly any cell type in the body, a characteristic known as pluripotency. It has, however, been a challenge to make use of these cells as they must be harvested and then grown under very specific conditions to produce amounts sufficient for therapeutic administration without disrupting their characteristic versatility.

In 2007, Nobel Prize-winning biologist Shinya Yamanaka pioneered research devising a means with which to reprogram mature, differentiated cells back to stem cells. Forced expression of four proteins, known as Yamanaka factors, is capable of inducing a sort of “cellular amnesia”, resulting in a reversion to their original and more versatile or pluripotent state. This innovation alleviated some of the more technically challenging aspects of growing stem cells. Application of the Yamanaka factors in cell culture and advanced aging animal models results in a delayed aging phenotype; however, in animal models, long-term continuous exposure to these factors has been shown to induce cancer, specifically teratomas, resulting in premature death.

Nature Aging has published a study detailing an approach to long-term activation of these factors that might delay aging while avoiding the risk of developing cancer. The key to this novel approach is periodic expression. The study authors generated a mutant mouse in which the expression of Yamanaka factors could be controlled through the administration of the antibiotic doxycycline. This allowed for mice to be treated for 2 out of 7 days per week.

Gene expression can be regulated in a variety of ways, one of which is through modifications to the chromatin structures that DNA is spooled around—a process known as methylation. These methylated sites are collectively referred to as epigenetic marks, and they add a layer of control to gene expression. The pattern of epigenetic marks is highly correlated with biological aging, and as a result this pattern, also known as the epigenetic clock, can be used to determine the efficacy of an intervention’s ability to modulate rates of aging.

The epigenetic clock in old animals exposed to long-term, periodic expression of the Yamanaka factors was similar to those of young animals in both skin and kidney tissue and in gene expression patterns; specifically, reductions were shown to genes related to senescence and inflammation, two factors thought to be significant drivers of aging. This suggests that the treatment was effective in maintaining or possibly rejuvenating these particular tissues. Additionally, with regard to skin, treated animals showed improvements in their capacity to recover from injury as assessed by wound healing experiments, further suggesting maintenance of a youthful state. Although this method of treatment was able to exert a change in both the skin and kidneys, it did not significantly change the epigenetic status of the liver, spleen, lung, or muscle tissues.

Further analysis of metabolites was performed to assess changes beyond gene expression. In this regard, older animals treated for an extended period had similar metabolite profiles to that of younger animals, indicating that this treatment may be restoring or preserving a more youthful metabolic state. These metabolic changes were seen in a variety of tissues beyond the skin and kidneys, suggesting that periodic long-term exposure to Yamanaka factors might be facilitating a more youthful phenotype in a systemic manner without affecting the epigenetic status of all tissues.

In contrast to the prolonged treatment starting in mid-life, the authors also examined a short-term exposure in older animals. The approach resulted in gene expression reductions in stress pathways, but the majority of analysis was inconsistent with the beneficial changes produced by long-term treatments. Although the short-term application of this therapy did not recapitulate the effects of long-term interventions, there may be an earlier, yet still advanced age and period by which this approach can provide a benefit.

Importantly, this research has demonstrated that the Yamanaka factors can be applied in a seemingly safe manner through a periodically controlled method, and that it may be possible to fine tune this method to maximize its benefits.

Weight Loss Drug Shown to Reduce Obesity and Increase Muscle Mass

Weight Loss Drug Shown to Reduce Obesity and Increase Muscle Mass

In June 2021, the FDA approved semaglutide, a highly anticipated pharmaceutical treatment for obesity. This marked the first weight-loss drug to hold real promise after a decades-long history of ineffective and, at times, dangerous (fen-phen) drugs being brought to market. Now another drug, bimagrumab, seems likely to be added to the list of safe and effective weight loss drugs.

Results of a recent phase 2 clinical trial published in JAMA show overweight/obese individuals treated with bimagrumab lost 20% more fat mass, on average, as compared to individuals taking a placebo. Participants were administered bimagrumab or placebo via intravenous infusion every 4 weeks for a total of 48 weeks. Although individual results varied, over three-quarters of participants receiving bimagrumab lost at least 15% of their fat mass.

Evidence has shown that differing depots of fat have varied metabolic and inflammatory effects, playing a part in dictating overall health outcomes. In general, leaner individuals have been associated with improved health and a lower risk of disease; however, subcutaneous (below the skin) fat is considered to be more innocuous. Visceral (surrounding the organs) and hepatic (within the liver) fat are considered to be more deleterious lipid storage compartments and are associated with increased metabolic disease and risk of mortality. Importantly, this study reported fat mass reductions to be universal and not isolated to the more benign subcutaneous depot.

In addition to reduced adiposity, treatment with bimagrumab showed improvement to a variety of weight-associated health parameters. Prior to treatment, all study participants were determined to have a clinical diagnosis of type 2 diabetes. Type 2 diabetes is associated with dysregulated glucose metabolism resulting in elevated HBA1C levels. At the termination of the study, bimagrumab-treated subjects were observed to have a beneficial reduction of HBA1C levels. Although there was no significant improvement to insulin sensitivity in this trial, previous clinical studies have shown improvements ranging from 20% to 40%—assessed by either glucose tolerance test or euglycemic clamp.

Bimagrumab, being a monoclonal antibody therapeutic, acts by binding to specific proteins and altering their function. Specifically, bimagrumab binds to and inhibits the activin type II receptor and as a result stimulates muscle growth. Originally developed as a sarcopenia therapeutic, it failed to prevent muscle wasting and weakness in elderly subjects of previous clinical trials. However, in this study participants experienced a 3.7% increase in lean body mass. This is significant considering that the majority of dietary interventions to control obesity often have the undesirable effect of reducing lean body mass. Loss of lean body mass with age is highly associated with increased risk of mortality—recent studies report the preservation of lean body mass and insulin sensitivity are critical components of chronic calorie restriction’s imparted healthspan benefits.

Obesity is highly correlated with and thought to contribute to an increased incidence of heart disease, stroke, type 2 diabetes, and certain cancers and is rapidly becoming the most prevalent driver of age-related pathologies in the modern world. A 2018 analysis of obesity in the US reported a 33% percent increase in incidence as compared to the previous decade. With the exception of surgical interventions, medical treatment of obesity has been ineffective and primarily relegated to dietary interventions that are difficult to adhere to and suffer from a lack of long-term compliance. The availability of novel pharmaceutical interventions, such as bimagrumab, might provide treatment to the increasingly common condition of intractable obesity.

Can We Be Immunized Against Old Age?

Can We Be Immunized Against Old Age?

Vaccines have been on the mind of almost everyone for the last two years. Undoubtedly, vaccines have led to a near incalculable benefit to the human species by reducing or eliminating debilitating or fatal infectious diseases, but can vaccines be used to fight age-related disease and, ultimately, to extend lifespan? Research published in Nature Aging suggests that such a promising strategy could be a reality.

Cellular senescence has gained a great deal of attention as a key driver of biological aging. Thought to be in part a protective mechanism, cellular senescence prevents dysfunctional cells from dividing, limiting the replication of potentially harmful cells. However, over time these individual non-proliferating cells accumulate and secrete a number of adverse inflammatory chemical signals. This phenomenon has been dubbed the senescence-associated secretory phenotype (SASP); it results in chronic inflammation and instigates multiple age-related diseases. In addition to the hallmark inflammatory secretions, the cells also express unique protein profiles. These proteins can be utilized as an effective means of targeting senescent cells for removal, a process known as senolysis. One such protein, Gpnmb, has been found to be enriched in senescent cells.

Immuno-therapies, whereby an intervention utilizes the organism’s immune system to extirpate damaged or unwanted cells, have shown success in combating certain cancers and are being developed to treat the age-related accumulation of senescent cells. In addition to being abundant in senescent cells, Gpnmb is a transmembrane protein—a portion protrudes from the surface of the cellular membrane. Transmembrane proteins are ideal targets for immuno-therapeutic approaches as the external portion is readily recognized by the immune system. Researchers generated peptides from the extra-cellular domain of the Gpnmb protein and combined it with an immunogenic carrier protein, keyhole limpet hemocyanin, creating a seno-antigen—a protein specifically overexpressed in senescent cells that the immune system can recognize and generate antibodies against. This leads the immune system to identify Gpnmb-expressing senescent cells as “non-self” and ultimately ends in their removal through the process of antibody-dependent cellular cytotoxicity.

Having successfully generated a vaccine against the extra-cellular domain of the Gpnmb protein and demonstrating its ability to induce an antigenic response, researchers turned to animal models of discrete age-related pathologies to determine if such an approach could reduce senescent cell burden.

High-fat diet models lead to obesity and increased adiposity, resulting in multiple comorbidities and decreased lifespan. Adipose tissue has recently been shown to be a significant depot of senescent cells and a key driver of the SASP, accelerating metabolic dysfunction and an advanced aging phenotype. Mice subjected to a high-fat diet treated with the Gpnmb vaccine showed a reduction in markers of senescence in their visceral adipose depots and improvements to glucose metabolism.

As a model for atherosclerosis, ApoE mutant mice exhibit increased lipid deposition, as well as increased senescent cells, in the aortic endothelium. These lipid deposits develop into plaques, resulting in arterial hardening and constriction—cardiovascular events. The presence of senescent cells has been attributed to increased rigidity and inflammation of these blood vessels, two hallmarks of cardiovascular disease seen in aging. Similar to the high-fat diet model, treatment with the Gpnmb vaccine reduced senescent cells in the aorta, and treated animals showed reduce incidence of atherosclerotic plaques.

In a separate experiment, the physical activity of “middle-aged” wild-type mice was measured using an open field test. The mice, vaccinated at 50 weeks of age, showed significantly higher levels of activity when compared to non-vaccinated controls 20 weeks later, suggesting that a reduced senescent burden contributes to a more youthful phenotype in the context of normal aging.

In addition to these discrete models of disease, progeroid models are often used to assess an intervention’s ability to combat premature aging. Progeroid models exhibit many of the same characteristics of aging, including elevated levels of senescent cells, resulting in a phenotype with multiple age-related pathologies and extremely short lifespans. In this study, Zmpste24 KO mice (a progeroid model for Hutchinson-Gilford syndrome) were vaccinated at 10 weeks of age. Lifespans of both male and female vaccinated mice were significantly extended, demonstrating that this therapy attenuates underlying causes of accelerating aging and might have a similar effect of increasing the lifespan of normal aging organisms.

Although these results are promising, more work needs to be done to further understand what effects such an approach may have on non-senescent cells expressing Gpnmb. Historically, the majority of life-extending interventions have focused on lifestyle; however, this technique, as well as a growing number of similar approaches, offer interventions that may be delivered in a clinical setting to slow or reverse underlying impetuses of aging.

Blood from Exercised Mice Supports the Brain Health of Sedentary Mice

Blood from Exercised Mice Supports the Brain Health of Sedentary Mice

Exercise has for many years been known to be among the most robust means to support a healthy and long lifespan. In a previous post, we discussed research detailing evidence that exercise facilitates the elimination of senescent cells and might provide health benefits through the reduction of systemic inflammation. Exercise is likely to act in a pleiotropic manner, benefiting a variety of organ systems through activation of multiple biological pathways, rather than through a single manner of regulation. Researchers from Stanford University School of Medicine set out to determine if the neuroprotective effects seen in exercised laboratory mice might involve circulating factors in the blood.

When considering biological systems implicated in the outcome of any intervention, the circulatory system is an ideal candidate to interrogate. Blood, in addition to transporting nutrients and facilitating respiration, carries within it numerous signaling molecules, and it has long been speculated that circulating factors in the blood might facilitate pathways responsible for maintaining a robust healthy organism—an idea dating as far back as 1615 when physician Andreas Libavius proposed transfusing blood from the young to the elderly in an attempt to restore their vitality. This idea was further examined through the heterochronic parabiosis experiments of the 1940s in which the circulatory systems of old and young animals were surgically connected. This procedure led to observed benefits in the older animals thought to be the result of shared circulatory factors. However, through the shared circulatory system, these animals also shared organ systems. As a result, the more competent organs of the young animals could have been responsible for alleviating the burden on the older animal’s organ systems. This confounding aspect has drawn rightful criticism that such experiments cannot demonstrate that a circulating factor in the blood is solely responsible for the improvements seen in older animals.

Alternatively, collecting blood from animals and providing the resultant plasma to treatment groups intravenously does not have this disadvantage. In a recent study published in Nature, researchers utilized this method to identify an important circulating factor induced by exercise and vital to the imparted benefits of improved memory and reduced neuroinflammation.

In this study, mice were housed with or without running wheels, allowing for groups of exercised or non-exercised mice. Blood from both groups was collected and processed to acquire plasma—the fraction of blood free of cells but containing proteins. Plasma from both the “runners” and “non-runners” was subsequently administered to sedentary mice. Sedentary mice that received transfusions of plasma from the exercised “runner” group showed a significant increase in neurogenesis and improvement to both memory and cognition, as well as reduced neuroinflammation compared to those that received plasma from the “non-runner” mice.

When analyzed, the hippocampus, a region of the brain highly involved in learning and memory, showed increased neurogenesis in mice given access to running wheels for 28 days. In line with the increased cellular proliferation in the hippocampus, sedentary mice treated with “runner” plasma showed improved memory and cognition when assessed via standard behavioral tests.

Comparison of the plasma proteome collected from the “runner” and “non-runner” groups revealed increased levels of clusterin, a protein known to be involved in inflammation and aging. When neuroinflammation was induced through the administration of lipopolysaccharide, mice infused with “runner” plasma demonstrated reduced inflammatory markers in the hippocampus compared to controls. However, when clusterin was removed from “runner” plasma and administered to sedentary mice, inflammation was not reduced compared to mice treated with the intact “runner” plasma. Furthermore, mice administered recombinant clusterin showed downregulation of genes known to be elevated with pro-inflammatory lipopolysaccharide treatment.

Lipopolysaccharide treatment is an acute model of neuroinflammation and differs from conditions of chronic neuroinflammation seen in Alzheimer’s disease. To gain insight into clusterin’s role in chronic neuroinflammation, APP transgenic mice (an animal model for Alzheimer’s disease) were treated with recombinant clusterin. As a result, many of the pathologically driven genes abnormally expressed in brain endothelial cells of these APP transgenic mice were reversed.

Previous studies have demonstrated that aerobic exercise supports improved cognition and exerts an anti-inflammatory effect through multiple mechanisms. This study demonstrates that aerobic exercise in wheel-running mice provides a similar outcome and that plasma from these mice can confer the same benefit to sedentary mice. The authors believe their observations are the first to demonstrate clusterin’s role in reducing neuroinflammation, a state highly correlated with increased age and neurological disease, and hope these findings aid in combating such conditions.