Over our lifespan, somatic cells that comprise our various organs have to contend with diverse stress-inducing processes, the consequences of which are often exacerbated by aging. Mechanisms for dealing with stress are essential for long-lived hematopoietic stem cells (HSCs) as they self-renew and differentiate to replenish the blood and immune system. While the HSC response to some stressful processes such as DNA damage has been widely studied, little is known about how stem cells deal with other detrimental processes such as the metabolic stress of nutrient deprivation. Generally, in a process called autophagy, cells faced with nutrient deficiency sequester, degrade, and recycle cellular components in order to generate the energy required to maintain key metabolic functions. Non-essential materials are isolated in vacuoles, called autophagosomes, and subsequently degraded by lysosomes. Intriguingly, autophagy shares some regulatory mechanisms with apoptosis (programed cell death) — a competing stress response process by which cells self-terminate rather than persist.
In their recent paper, Warr and colleagues from the University of California, San Francisco, sought to determine the mechanism by which HSCs survive caloric restriction in vivo and cytokine withdrawal ex vivo. Using a transgenic mouse model that allowed the visualization of autophagosomes by fluorescence microscopy, investigators isolated both HSCs and granulocyte/macrophage progenitors (GMP) based on their respective cell surface immunophenotype and cultured the cells ex vivo with or without growth factor support. As anticipated, without cytokines, both cell types showed evidence of metabolic stress. Next, the investigators made use of a drug that inhibits degradation of autolysomes, thereby allowing visualization of accumulation of a fluorescent-reporter protein fused to the autophagy protein LC3. This reporter system revealed ongoing autophagy in freshly isolated GMPs, but no further enhancement of the process was apparent during ex vivo cytokine deprivation. Unexpectedly, HSCs did not use this pathway at baseline, but autophagy was observed following cytokine deprivation or following a period of starvation. Studies in mice after a 24-hour fasting period confirmed these in vitro analyses, implying that HSCs, but not their differentiated progeny, rely on autophagy to withstand metabolic stress. An alternative and more deleterious outcome of nutrient deprivation is apoptosis. When autophagy was inhibited pharmacologically, cytokine deprivation was shown both to hasten and increase the rate of apoptosis and to compromise the repopulation efficiency of transplanted HSCs. The protective function of autophagy was further validated in a genetic model, supporting the concept that this nutrient-recycling process is an essential component of HSC homeostasis.
What is the underlying mechanism for HSC survival by “self-cannibalization”? Using gene-expression arrays, the authors demonstrated that, in comparison to GMPs and differentiated granulocytes, HSCs exhibit a more pro-autophagic signature. This difference in gene-expression profiles was shown to be mediated in part by FOXO3A, a key HSC transcription factor. These results are consistent with previous observations made in animals genetically deficient in Foxo3a function in which both the induction rate and the activity level of autophagy were found to be impaired. The authors of the current paper went on to show that the effects of FOXO3A on the pro-autophagic gene-expression profile are independent of p53, despite a shared role for these two transcription factors in both autophagy and apoptosis and the known overlap in FOXO3A and p53 target genes. The authors hypothesized that HSCs from old mice would show less utilization of autophagy, but they found, surprisingly, that aged HSCs had higher rates of autophagy at baseline compared with their younger counterparts. Indeed, autophagy proved essential to the survival of old HSCs, to progenitor clonogenicity, and to nutrient transport, findings that are in agreement with observations made by others based on experiments using Foxo3a-/- mice. Data from research into an aging syndrome (Hutchinson–Gilford progeria syndrome) have already invoked a role for autophagy in countering the progerin-associated senescence changes. The current report leaves unanswered questions about whether autophagy dependence is externally specified by the microenvironment in conjunction with aging. Nonetheless, the studies of Warr and colleagues support the concept of autophagy as an important, age-related mechanism for coping with stress in the HSC compartment.
More generally, autophagy has recently been shown to contribute to the homeostatic regulation of stemness as compared with differentiation in several experimental systems. A report based on experiments using mice with a deletion of a principal component of mammalian autophagy machinery (FIP 200) revealed an indispensible role for this process during hematopoietic development. The involvement of autophagy in resistance to chemotherapy in several types of cancers, including chronic myeloid leukemia stem cells, has been proposed. As Warr and colleagues suggest, it will be of interest to see if autophagy pathways are more generally co-opted during leukemogenesis.
In Brief
In aggregate, the study by Warr et al. reveals autophagy as a key mechanism by which HSCs endure nutrient deprivation while avoiding programmed cell death, particularly during aging. The findings challenge the prevailing paradigm that aged HSCs suffer from compromised autophagy. Because HSCs rely more heavily on autophagy than progenitors and differentiated blood cells, it will be interesting to determine whether stem cells in other tissues show a similar dependence on autophagy to cope with stress and aging.
Competing Interests
Ms. Kamimae-Lanning and Dr. Kurre indicated no relevant conflicts of interest.
