Over the last decade, anti-aging research has accelerated, becoming a dynamic field of medicine. On a practical level, a rapidly aging population demands a transformational shift from intervention to prevention and from generic treatments to personalized medicine. Anti-aging research also is driving the development of new approaches that extend the envelope of human “health span” and delay the onset of age-related disease by focusing on causal mechanisms, rather than symptoms.
The Challenges Of An Anti-Aging Approach
Why is aging so hard to reverse? The answer lies in research that has clearly refuted the notion that aging might be controlled by a single, hidden “master switch” . Our current understanding of the underlying mechanism is that it is spread across the entire system of an organism. An increased understanding of biological aging has identified nine hallmarks that are inextricably linked to aging :
Targeting aging at the cellular level is an exciting, new approach to prevent age-related disease, since cellular dysfunction directly precedes tissue disability. Over time, continuous intracellular stress leads to impairment of cellular function, stem cell exhaustion, impaired intercellular communication, and increased cellular senescence. These changes bring decline in tissue function and the manifestation of disease.
Causal Mechanisms of Cellular Senescence
As cells age they exhibit dynamic changes in gene expression . Epigenetic changes – that is, changes to how the genes express themselves rather than changes to the genes themselves – produce cumulative external stressors and replicative stress . Epigenetic alterations are biochemical modifications of DNA or DNA-associated proteins such as histones. They result in chromatin remodeling and functionally relevant changes to the genome, independent of altering the DNA sequence .
One consequence of mounting epigenetic alteration is the increased likelihood of an oncogenic, or tumor producing, event that ultimately leads to cancer development . Cells have a default protective mechanism to avert transformation through dramatic silencing of active chromatin . Once activated, this mechanism leads to formation of heterochromatin foci, causing cells to become non-proliferative, entering a state of senescence .
As organisms age, more and more cells in the body become senescent . Senescent cells exhibit a unique profile of enhanced secretory factor production, termed the senescent-associated secretory phenotype (SASP) . Many factors produced in the SASP are pro-inflammatory and/or tumor-supportive. Cellular senescence is thus tied to the progressive breakdown of tissue function with age . To this end, cell senescence is the target of new therapies aimed at combating the fundamental hallmarks of aging. Recent studies describe a new class of small molecule drugs, termed senolytics, which directly target senescent cells and lead to significant health span extension in mice . These studies are in line with other reports of lifespan extension in genetically modified mice that selectively expressed suicide genes – genes that attract tumor-killing drugs – in senescent cells . Physiologically, similar anti-aging effects are observed for endogenous miRNAs, such as miR-17, that inhibit senescence signaling .
Intercellular Communication & Stem Cell Exhaustion
Disruption of the tissue microenvironment is a hallmark of aging . Microenvironmental disruption has dramatic consequences, often leading to stem cell exhaustion and improper tissue homeostasis . Stem cell exhaustion results from an inability of stem cells to replenish a tissue with differentiated cells necessary to maintain tissue function. To sustain stem cell pools, a delicate balance of self-renewal, proliferation and quiescence is required.
When operating properly, the microenvironment houses a variety of cell types acting in concert to maintain tissue homeostasis and promote tissue function. An essential component of coordinating such activity is the proper exchange of information from cell to cell, or intercellular communication . Intercellular communication can be contact dependent or independent. Contact independent communication, termed paracrine communication, is mediated in part by the release of microvesicles, such as exosomes.
Exosomes are small membrane vesicles derived from multivesicular bodies that are released by all cell types . These vesicles contain a subset of proteins, lipids and nucleic acids that are derived from the parent cell . Extensive studies show that exosomes have important roles in intercellular communication, both locally and systemically, as they shuttle their contents, including proteins, lipids and nucleic acids, between cells . RNA that is shuttled from one cell to another, known as “exosomal shuttle RNA,” could potentially affect protein production in the recipient cell. For example, miRNAs taken up by recipient cells can change target cell behavior by classical miRNA-induced silencing of target mRNAs . This form of intercellular communication is involved in numerous physiological processes, including immune regulation .
Recent studies identify novel biomarkers within exosomes that offer diagnostic and prognostic value for a number of indications [12, 13]. In particular, exosomes are being used as a cancer diagnostic, providing information on a variety of anatomically distinct cancer types due to their transport within the systemic circulation .
Beyond diagnostics, exosomes have also gained substantial traction as a therapeutic for a number of indications ranging from cancer, to immune-related diseases, to regenerative medicine [14-17]. Within the cellular aging field, exosomes from young adult mice stem cells have been shown to be the causal mechanism of lifespan extension in a mouse model of accelerated aging .
How the cargo within exosomes mediate these therapeutic effects reveals a substantial role for intercellular shuttle of miRNA in regulating a number of aging-related signaling pathways in targeted cells [19, 20].
A growing body of scientific evidence suggests that functional “restoration” of aged tissue and cells is possible. Blood from young animals can rejuvenate old animals, and cell-free factors isolated from the stem cells of young animals can rejuvenate the stem cells of older animals [21-23]. This has led to our hypothesis that altering the expression level of many or most of these critical factors simultaneously may provide a successful intervention to promote extension of human health span.
Targeting the Hematopoetic And Immune Systems
The hematopoietic – referring to blood stem cells – and the immune systems are vital components of how we function. Blood cells suffuse most tissues of the body and serve local housekeeping and surveillance roles within the tissue microenvironment. As we age, their diminishing functions lead to compensatory increases in immune-related diseases, including cancer .
The initial goal of Rejenevie™ is to target the declining function of aging hematopoietic stem cells (HSCs) as a means to treat and delay the onset of age-related diseases. Our approach utilizes a heterochronic, or mixed age, cell culture model to stimulate production of rejuvenating factors from young blood cells. When applied to aged HSCs, youthful function is restored.
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