In humans aging represents the accumulation of changes over time, encompassing physical, psychological, and social change. Aging is among the largest known risk factors for most human diseases: of the roughly 150,000 people who die each day across the globe, about two thirds die from age-related causes. [ --> Effects of aging ]
By 2050, the number of elderly individuals older than 80 years is projected to triple globally, many of whom will have cognitive impairment, Alzheimer disease, or 1 of several other neurodegenerative diseases for which age increases risk. As the number of aged individuals increases dramatically in the next several decades, widespread social and economic consequences of these and other aging disorders will be felt on an unprecedented scale. One strategy to meet this growing public health threat is to elucidate the mechanisms to inform therapeutic strategies aimed at ameliorating age-associated disease.
( Blood borne revitalization of the aged brain. 2015 )
Following will be presented a quick overview of some investigations that are ongoing about causes and possible therapies for aging, thus preventing the correlated diseases.
An holistic approach to aging
Aging at its core can be thought of as a systemic event.
Indeed, the effects of aging do not occur in a targeted and isolated manner, but rather functionally alter tissues throughout the body , albeit at different rates.
Even though aging was traditionally thought to be immutable, particularly evident in the loss of plasticity and cognitive abilities occurring in the aged central nervous system (CNS) , it is becoming increasingly apparent that extrinsic systemic manipulations such as exercise, caloric restriction, and changing blood composition by heterochronic parabiosis or young plasma administration can partially counteract the process of aging and this age-related loss of plasticity in the brain.
In the mid-1990s, researchers began to seriously entertain the notion that a single gene could exert significant influence over organism lifespan [ A C. elegans mutant that lives twice as long as wild type. 1993 ; The first long-lived mutants: discovery of the insulin/IGF-1 pathway for ageing. 2011 ]
Since then, numerous studies have substantiated the notion that individual genes can determine how long an organism live and opened up the possibility that a process as complex as aging could be manipulated at the molecular level.
Important discoveries have been made evidencing the relation bewteen systemic regulation of aging and brain function: many of the pro-longevity signaling pathways have more recently been shown to play important roles in higher level brain function thus opening the possibility to try new approaches to treat, beyond preventing, dementias and other related diseases (for example FoxO6 and Sirt1).
An ample body of work demonstrated that longevity genes regulate critical CNS functions and also offered the first evidence that just as organismal lifespan is malleable so too could CNS functions prove to be amenable to rejuvenation.
Therefore not only the same stimulis have effects both on the brain and on aging, but also the CNS itself plays an important role in influencing aging.
Some of the first evidence for the role of the CNS in controlling systemic aging came from cell-specific manipulations of longevity genes in model organisms such as worms, flies, and mice. For example, restoring insulin signaling in neurons, but not muscle or intestine, was sufficient to increase the lifespan of worms.
[ Regulation of C. elegans life-span by insulinlike signaling in the nervous system . 2000 ]
More recently, aging research in mice has now begun to hone in on a particular region of the brain, the hypothalamus, as the potential CNS mediator of lifespan regulation.
Reversal of aging: systemic manipulations as mediators of rejuvenation
Manipulating individual genes within the CNS may represent one approach with potential to counteract the effects of aging.
However, targeted manipulations of genes, specifically within specialized populations of neurons in regions such as the hypothalamus, prove challenging at this point.
While reversal of aging by the CNS appears distant, alternative strategies to rejuvenate aged tissues through a more systemic approach may prove equally effective in counteracting the aging process.
In this manner, broad changes in the aged systemic environment, rather than a central regulator, may provide the means for rejuvenation.
Systemic manipulations such as exercise, CR, and changes in blood composition by heterochronic parabiosis or young plasma administration have already yielded much promise in their ability to combat signs of aging in both peripheral tissues and the CNS.
Parabiosis is a surgical procedure by which two animals are physically connected. An analogous in humans could be the Siamese twins.
Chronic diseases of age such as cardiovascular disease, diabetes, osteoarthritis, or Alzheimer’s disease turn out to be of a complexity that may require transformative ideas and paradigms to understand and treat them.
Parabiosis, which mimics aspects of the naturally occurring shared blood supply in conjoined twins in humans and certain animals, may have the power to be such a transformative experimental paradigm. A forgotten procedure, it contributed to major breakthroughs in tumor biology, endocrinology, and transplantation research in the past century, and now a set of new studies in the US and Britain report stunning advances in stem cell biology and tissue regeneration using parabiosis between young and old mice.
Parabiosis is particularly apt due to its peculiarity of reproducing a whole systemic environment differently, and not just by addressing few components.
The surgical technique to physically connect two living organisms that was later termed“parabiosis” (from the Greek words, para “besides” and bios “life”) was first introduced by the French physiologist Paul Bert in the 1860’s.
In 1908 the German surgeons Sauerbruch and Heyde revived the technique and introduced the term parabiosis for the artificially established symbiosis between two animals.
From then on parabiosis was widely used to obtain various results [ Parabiosis in Physiological Studies ].
Yet the procedure had fallen out of favor with the research community with only a handful of papers using the technique in the last decades.
Nowadays parabiosis is becoming more widely used.
In the beginning, parabiosis surgeries consisted of short skin incisions and a suturing together at the flank of each animal, but the technique has evolved over the years. Nowadays, the skin incisions typically extend along the whole body flank. A detailed procedure of the surgery including reversal of the parabiotic pairing has recently been described by Conboy et al [ Heterochronic parabiosis: historical perspective and methodological considerations for studies of aging and longevity. 2013 ] .
Early parabiosis studies using adult animals reported cases in which one of the two parabionts suddenly died ( parabiotic intoxication) . While this intoxication has mainly been due to the lack of genetic uniformity resulting in tissue rejection, a survival rate similar to other invasive surgical procedures (>80%) can now be attained in mouse parabionts with appropriate precautions taken by a skilled operator, and even long-term survival seems unaffected.
The possibilities of creating eterochronic parabiotic animals opens many possibilities, setting the basis for the investigation of effects induced through exposure of an aged organism to a youthful systemic environment ( started 1972 by Ludwig and Elashoff )
Etherochronic parabiosis and extension of life span
In the early 70’s, scientists started to graft animals of different ages to each other.
Ludwig and Elashoff particularly focused on the extension of lifespan in the old heterochronic parabiont when attached to a young counterpart. Indeed, in 1972 their results provided the first evidence that the old organism in the heterochronic pairing lived longer in response to the young environment compared to the age-matched isochronic control animals.
A major hallmark of aging is that the regenerative properties significantly decline in most tissues. This has partially been attributed to impaired stem cellfunction.
However, whether these age-related effects were due to cell intrinsic changes or alterations in the microenvironment of stem cells required further investigation.
[ Parabiosis for the study of age-related chronic disease. 2014 ]
In 2005 Conboy et al. used heterochronic parabiosis experiments to address this question.
They showed that factors derived from the young systemic environment are able to activate molecular signaling pathways in hepatic or muscle stem cells of the old parabiont leading to increased proliferation and tissue regeneration. These in vivo results were furthermore confirmed ex vivo.
In 2011 another group [ the ageing systemic milieu negatively regulates neurogenesis and cognitive function ] published a similar finding suggesting an old systemic environment can be detrimental for stem cell function and negatively regulate adult neurogenesis in brains of young heterochronic parabionts.
This led to the discovery that factors in old blood are sufficient to decrease synaptic plasticity and impair contextual fear conditioning and spatial memory.
Using a systematic proteomic approach they were able to identify soluble factors that were significantly increased in blood plasma of old mice and humans. One of these factors was the chemokine CCL11, known to chemotactically attract eosinophils to tissues. Indeed, application of CCL11 was sufficient to induce impaired adult neurogenesis.
Three more recent publications using heterochronic parabiosis further support this conclusion. Ruckh et al. [ Rejuvenation of regeneration in the aging central nervous system. 2012 ] reported that recovery from experimentally induced demyelination in the CNS is enhanced in old mice that were exposed to a young systemic environment.
Salpeter and colleagues [ Systemic regulation of the age-related decline of pancreatic β-cell replication 2013 ] showed that the decline in pancreatic β-cell proliferation in old mice can be reversed in old parabionts paired with young mice.
And most recently, Loffredo et al. [ Growth differentiation factor 11 is a circulating factor that reverses age-related cardiac hypertrophy 2013 ] demonstrated that age-related loss of normal cardiac function leading to diastolic heart failure is partially due to the lack of certain circulating factors in old mice.
Indeed, it becomes increasingly evident that many diseases and biological processes,including aging, result in organism-wide, systemic changes contributing to local tissue alterations.
Aging isn’t anymore considered an irreversible process and its causes are becoming more understood. Studies on caloric restriction, exercising and especially eterochronic parabiosis are evidencing the plasticity of aging and some of its molecular regulations.
This shift in thinking has been particularly striking with respect to the brain, where decades of neuron-centric research has started to give way to studies on other brain cell types as critical regulators and factors outside the brain including gut microbiota, diet, and other systemic changes on CNS function.
An interesting world of opportunities lays before us, but with it a lot of questions that will have to be answered and problems to be solved.
Parabiosis and heterochronic parabiosis in particular could help answering some of the fundamental questions in this regard: are circulatory factors or cells in a young organism protecting against age-related disease, and vice versa?
Therapies shall be tested carefully , especially being careful for cancer since acting on stem cells might lead to more replications errors.
We just started scratching the top of the iceberg: we still have few clues on how aging works, if it’s programmed or guided by the environment or a mix of both and on where and when to act to make a good impact.
Right now the first clinical trials are ongoing to examine symptom improvements in patients with mild to moderate Alzheimer disease following the removal or addition of blood factors that impair or improve brain function ( NCT01561053 , the study will finish at the end of 2016 and NCT02256306 , finishing in October 2015 ).
A recent review on aging and investigations is available at: Aging and brain rejuvenation as systemic events. 2015