FOXO4 D-Retro-Inverso is identical to the protein product of the FOXO4 gene, but the normal L amino acids have been exchanged for D amino acids. The result is that FOXO4-DRI has reduced susceptibility to normal physiologic clearance mechanisms and thus remains in the body for longer periods of time. The modified protein is still capable, however, of affecting transcription and cellular pathways. In general, the FOXO4-DRI protein interferes with normal FOXO4 function.
Of greatest interest in terms of aging and senescence is the ability of FOXO4-DRI to interfere with normal FOXO4 signaling in the cell cycle by preventing the binding of FOXO4 to p53. The p53 protein is an important regulator of progression through the cell cycle as well as programmed cell death (apoptosis). When FOXO4-DRI binds to p53, it prevents FOXO4 from binding and allows p53 to bind to DNA. This, in turn, allows the cell to continue through the process of apoptosis and die. Interestingly, FOXO4-DRI appears to only have this effect in senescent cells, cells that are no longer functional or are dysfunctional as a result of aging[2]. By targeting these dysfunctional cells, FOXO4-DRI helps to rid tissue of cells that are nothing but dead weight. This, in turn, allows for better tissue functioning and helps to stimulate growth and differentiation of younger, healthier cells. The net result is better biological function and thus a decrease in “biological age”.
The relationship between FOXO4 and aging is complex and still not fully understood. There is, however, good evidence that helps to elucidate the mechanisms by which the protein has its effects. Research in the well-studied nematode C. elegans shows that FOXO4 affects insulin-like growth factor receptor signaling and thus cellular lifespan control, stress resistance, and gene regulation[3]. It also appears that FOXO4 interacts with the p53 protein to regulate the cell cycle.
Natural FOXO4 actually protects senescent cells by keeping p53, a regulator of the cell cycle, sequestered and unable to induce apoptosis. FOXO4-DRI disrupts the normal FOXO4/p53 mechanism and allows the latter protein to induce apoptosis in senescent cells. The result is an amelioration of senescence-associated loss of tissue homeostasis[4], [5]. Scientists refer to this as rejuvenation by therapeutic elimination of senescent cells. The process is not all that different from pruning a fruit tree. By removing dead and damaged branches (senescent cells), energy is redirected to healthier parts of the tree and thus to fruit production and growth. This same process happens at the organ and tissue level when cells that are contributing to overall unhealthy function are removed, allowing resources to be focused on healthy cells
Source: Journal of Cell Biology
This image displays what factors contribute to senescence and what the outcomes are senescence are. Note that eliminating a senescent cell does not alleviate stem cell exhaustion, but may slow it down. It does, however, help to reduce chronic inflammation, a well-established driver of a number of conditions like cardiac disease, stroke, etc.
Irreparable damage, which is to say cellular damage that is beyond the ability of the body to fix, is one of the primary limitations to health span. Health span, the length of time during which an organism remains healthy and functioning optimally, is generally shorter than lifespan. A decline in healthspan manifests as aging. The ability to extend health span may not result in more years lived, but it can result in living out the years allotted to us in greater health and with better functioning. In mouse models, FOXO4 has been shown to improve health span in aged mice, leading to an increase in fitness, fur density, and renal functioning. The mice do not necessarily live longer, but they have greater health, even into old age, which translates into less disability and fewer age-related conditions like heart disease, musculoskeletal dysfunction, etc[2].
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