An interesting study of mouse life span extension via a novel methodology was recently published. The researchers developed a small molecule approach to inhibition of Cdc42, a protein with numerous functions throughout the cell. This is a target for intervention because – at least in cell cultures – loss of Cdc42 activity appears to restore youthful function to aged hematopoietic stem cells. This is the cell population responsible for producing blood and immune cells, and declining immune function with age is driven at least in part by dysfunction in hematopoietic stem cells. Ways to restore immune function in older individuals should prove to be broadly beneficial to health in later life, given that the immune system has roles in tissue maintenance and function that extend far beyond merely defending against pathogens.
The effect size in mice for Cdc42 inhibition is here shown to be somewhere in the range of a 12-16% gain in median and maximum life spans, along with a reversal of age-related changes in some inflammatory cytokine levels. This gain in life span isn’t large in the grand scheme of things, given that lifelong calorie restriction can result in a 40% increase in mouse life span, but the point of interest here is that this result was achieved with a single four day treatment carried out in middle aged mice, already well on the way towards being aged. Only rapamycin and senolytics have robustly achieved similar outcomes based on short term late life treatment.
We might hypothesize that, in these aging mice, the generation of new immune cells by hematopoietic stem cells was increased for long enough via this intervention to provide the lasting benefits of a renewed and bolstered immune system. Even if raised rates of immune cell generation don’t last, the additional cells created will last. An aging immune system should be in an incrementally better state going forward as the result of any intervention capable of providing more new immune cells for a time. Unfortunately a full assessment of immune cell populations wasn’t carried out in this study; only proximate measures of immune system activity such as cytokine levels were assessed.
Cdc42 is involved in multiple and diverse functions of eukaryotic cells, including actin cytoskeleton reorganization, cell polarity, and cell growth. The activity of Cdc42 is significantly elevated in blood of elderly humans and in several tissues of aged C57BL/6 mice. We recently identified a specific small-molecule inhibitor of Cdc42 activity termed CASIN. Administration of CASIN in vivo did not show signs of toxicity. Previously, we reported that a brief ex vivo exposure of aged hematopoietic stem cells (HSCs) to CASIN that reduced the activity of Cdc42 in aged cells to the level found in young cells resulted in long-lasting youthful function of HSCs in vivo, likely due to epigenetic remodeling of aged cells upon modulation of Cdc42 activity. Consequently, we hypothesized that maybe a short-term systemic reduction of Cdc42 activity in aged animals in vivo might be also beneficial for lifespan, as an elevated activity of Cdc42 upon aging is causatively linked to a shorter lifespan in mice.
To determine whether a short-term systemic CASIN treatment of aged animals might indeed influence lifespan, we administered CASIN via intraperitoneal injection every 24 hours for 4 consecutive days to 75-week-old female C57BL/6 mice. 4 days of consecutive injections did not induce acute toxicity, and as well, none of the treated mice died within 4 weeks after CASIN injections, rendering chronic toxicity issues unlikely. Quantification of Cdc42 activity 24 hours after the last injection on day 5 demonstrated a reduction of Cdc42 activity in aged bone marrow cells to the level seen in young, confirming that CASIN is indeed reducing Cdc42 activity after a systemic in vivo treatment. Notably, aged mice treated with CASIN for only 4 consecutive days showed extension of their average and also maximum lifespan.
We performed analyses to investigate the extent to which aging-associated inflammatory cytokines in serum of aged mice were affected by CASIN treatment. Data showed a marked increase in the concentrations of INFγ, IL-1β, and IL-1α on aging and the concentrations for these cytokines were similar to concentrations in young animals upon CASIN treatment of aged mice. It is thus a possibility that a reduction in the concentrations of these cytokines upon CASIN treatment might contribute to the increase in lifespan observed in these animals.
Previously, the methylation status of CpG sites within the genes Prima1, Hsf4, and Kcns1 was shown to qualify as likely predictor of biological age of C57BL/6 mice. Applying this C57BL/6-trained DNA methylation marker panel to blood cells from aged animals treated with CASIN 9 weeks after treatment, we observed that epigenetic age predictions did not correlate anymore to the chronological age as in aged control animals, but resulted in a biological age prediction that was on average 9 weeks younger than their chronological age. These data imply that epigenetic changes underlie the extended longevity of aged CASIN-treated mice, while reinforcing the necessity to mechanistically validate tissues, cells, and biological pathways involved in the extension of longevity.