Today's open access paper outlines an investigation into how the aging of hematopoietic stem cell populations in bone marrow, responsible for producing blood and immune cells, can contribute to age-related dysfunction in the brain. The authors find that detrimental effects are mediated by circulating levels of CyPA, a signaling factor that is a part of the senescence-associated secretory phenotype (SASP), an inflammatory mix of signal molecules produced by senescent cells. The focus here is on direct inhibition of CyPA as an approach to therapy, but senolytic treatments to clear senescent cells may be the more useful approach if these errant cells are indeed the source of raised levels of CyPA. This seems reasonable, but is yet to be proven rigorously.
The aging of hematopoietic stem cells takes several forms. The population of functional stem cells declines due to damage, leading to a drop in the number of immune cells produced. This lack of reinforcements is one of the reasons why the aging immune system becomes dysfunctional, cluttered with exhausted, senescent, and malfunctioning cells. In addition, age-related changes in signaling and the stem cell niche in bone marrow cause detrimental changes in the distribution of types of immune cell produced. More myeloid cells and fewer lymphoid cells are produced, a change known as myeloid skew.
In mice and humans, the hematopoietic system undergoes many functional and structural changes during aging, characterized by myeloid expansion, decreased immunity, and chronic low-grade inflammation. We hypothesized that these cellular changes contribute to hippocampal aging through the accumulation of pro-aging immune factors in old blood. Many of the age-related changes observed in old blood have roots in hematopoietic stem cell (HSC) aging.
We employed a heterochronic HSC transplantation model to test how exposure to an aged hematopoietic system contributes to hippocampal aging. Young (2 months) recipient mice were sublethally irradiated to destroy their native HSCs and transplanted with HSCs isolated from young (2 months) or old (24 months) donors, generating isochronic (Iso) and heterochronic (Het) HSC-reconstituted young mice. Blood chimerism was assessed by measuring the proportion of CD45.2 donor cells in CD45.1 recipient mouse blood by flow cytometry. Blood derived from old HSCs exhibited characteristic age-related myeloid bias 4.5 months post-transplantation. Animals showed no signs of illness or weight loss regardless of treatment.
To gain mechanistic insight into how the old hematopoietic system exerts its deleterious effects on cognition, we assessed peripheral immune cell infiltration into the hippocampus. Immunohistochemical identification of CD45.2+ hematopoietic cells in the dentate gyrus of CD45.1 recipient mice revealed low and equivalent levels of immune cell infiltration in Het and Iso HSC-reconstituted young mice. While we cannot exclude the possible contribution of these small numbers of peripheral immune cells, we hypothesized that the pro-aging effects of the old hematopoietic system are predominantly mediated through peripheral changes in circulating blood factors.
We performed unbiased proteomic analysis on blood plasma collected from Het and Iso HSC-reconstituted young mice 4.5 months post-transplantation. Using label-free mass spectrometry, we identified 22 factors that were differentially expressed between Het and Iso HSC-reconstituted young mice. Of these, the most significantly upregulated cytokine was cyclophilin A (CyPA, encoded by Ppia) - an intracellular protein that is secreted in response to inflammatory stimuli.
To test whether increasing systemic CyPA levels are sufficient to elicit age-related cognitive or cellular impairments, young (2 months) mice were intravenously injected with overexpression constructs encoding either CyPA or GFP control. Increased systemic levels of CyPA impair cognition in young mice, while inhibition of CyPA in aged mice improves cognition. Cumulatively, our data demonstrate that age-related changes in the hematopoietic system promote molecular, cellular, and cognitive hallmarks of hippocampal aging.
Notably, inhibiting CyPA has been demonstrated to be neuroprotective in a mouse model of amyotrophic lateral sclerosis. In humans, elevated cerebrospinal fluid CyPA levels have recently been associated with cognitive impairments in Alzheimer's disease patients expressing apolipoprotein E4. Moreover, in humans elevated CyPA plasma levels accompany a number of inflammatory age-related diseases, including diabetes, and cardiovascular disease. In these studies, CyPA plasma levels were also found to be elevated with aging. While little is known about the role of CyPA in aging, recent proteomic analysis using mass spectrometry has identified CyPA as part of the senescence-associated secretory phenotype (SASP). Ultimately, our data identify the aged hematopoietic system, and downstream circulating immune factors, as potential therapeutic targets to restore cognitive function in the elderly.