The secret of youth: how is systemic rejuvenation achieved at the single cell level?

Heterochronic parabiosis (HP) refers to a condition in which two animals of different ages are surgically connected to share a common circulatory system. With the formation of microvessels at the connection site, the aged individual receives blood-borne factors from young individual, and vice versa, the young individual receives blood-borne factors from aged individual. Therefore, HP provides an opportunity to investigate how and what blood-borne factors could alter the health conditions in both young and more importantly aged organisms. In addition, the identification of rejuvenating factors in HP is a promising approach to delay the aging process and lead to therapeutic products to the reversal of aging or treatment of aging-related diseases. The development of HP has been seen as a great progress in the past century in the aging field and such advanced technologies have opened new avenue to address the long-lasting question of the molecular mechanism underlying aging and also identify new strategies for treating aging. First established in 1957 [1], the HP rodents have been served as a unique animal model to elucidate the effects of young and old blood on aging (Fig. 1). After that, aging-associated phenotype of skeletal muscle stem cells and hepatic cells were reported to be reversed in such model [2], providing proof of concept in rejuvenation of aged cells under the fused circulation system. Since then, the effects of HP on various tissues have been widely studied and to date, rejuvenative effects were reported in brain, vascular, and heart [2–5]. Meanwhile, several pro-aging or pro-rejuvenative factors have been identified in the circulation, such as growth and differentiation factor 11 (GDF11) [3, 4], C–C motif chemokine 11 (CCL11) [5] and β2 microglobulin (B2M) [6]. However, these studies have been focusing on only one or a few solid tissues, and the identified rejuvenating factors are only capable of reversing the aging-related phenotype in single cell types. As emerging studies have shown the immune system represents a key therapeutic target for slowing down aging [7], it remains largely unknown whether young blood rejuvenates aged hematopoietic stem and progenitor cells (HSPCs) that change the entire blood and immune system so as to establish a revified milieu. Similarly, it is also unclear whether there are potential systemic effects on stem cell maintenance and peripheral tissue/organ homeostasis conferred by the revitalized hematopoietic and immune system or blood-borne factors. As HSPCs and their differentiated blood and immune cell populations are highly heterogenous, and different cell types will likely be differently affected in HP, such an analysis calls for a higher resolution systemic approach. A graphical summary of the history of HP model and identification of blood-borne factors. To address these questions, Ma and colleagues took advantage of single-cell RNA sequencing (scRNA-seq) for global transcriptomic profiling to reveal the cellular heterogeneity and molecular changes in multiple tissues/organs in HP (Fig. 2A) [8]. While the molecular physiology of aging is varied across cell types and tissues and the niche environment of different cells and their communications are also dramatically different, scRNA-seq in a systematic level can largely facilitate our understanding of the rejuvenation paradigm. Similarly, in a parallel work, Róbert Pálovics et al. also applied scRNA-seq in 20 organs across the entire organism in HP and showed that young blood reversed global gene expression loss and increased the expression of genes encoding components of the electron transport chain [9]. Systemic investigation of the effects of blood-borne factors in HP model. (A) Single-cell transcriptomic sequencing analysis of 7 tissues/organs from HP depicts a systemic atlas of the effects of young blood on aging. (B) Blood-borne factors in young mice reset regulatory programs that enhances lymphoid differentiation potential of HSPCs in aged mice. MPC, myeloid progenitor cell; LPC, lymphoid progenitor cell. (C) Overexpression of identified rejuvenating factor Yy1 in aged HSPCs promoted their long-term engraftment ability, and Ccl3 overexpression increased T lymphopoietic differentiation potential of aged HSPCs. In the effort to analyze single-cell transcriptome across multiple tissues/organs in HP mice, including enriched HSPCs, bone marrow, peripheral blood, spleen, skin, liver, skeletal muscle and brain, Ma and colleagues acquired approximately 164 000 high-quality single-cell transcriptomes and annotated 108 major cell types for subsequent analyses. They focused on HSPCs because they found HSPCs are the most responsive to the changing environment (Fig. 2B). In HP, organismal exposure to young blood boosted lymphopoiesis program, rewired the continuum of HSPC differentiation to a younger state, and transcriptionally rescued hematopoietic stem cells (HSCs) compromised by aging, as manifested by the re-establishment of intercellular interactions through reactivation of cytokines and cytokine receptors upon exposure to young blood. Using the CD45 congenic mice to trace cell origins in the HP model, they also demonstrated that blood-borne factors from young mice rejuvenated HSPCs in the bone marrow of old mice while rejuvenation of the peripheral blood and spleen in aged mice is achieved mainly by replenishment of immune cells from young blood. The inclusion of experiments to distinguish aging/rejuvenation from cell crossover adds significant depth to this study, suggesting that, in HP, young mice could alter the hematopoietic and immune system of old mice through multiple mechanisms. Aging-associated progressive decline in regenerative capacity is primarily decided by tissue-resident stem cells whose overall status, in turn, relies on both the stem cells themselves and the surrounding niche cells. Therefore, this group also investigated the effects of HP on other organs and tissues bes