In addition to intrinsic changes in HSCs

In addition to intrinsic changes in HSCs, we also found a qualitative change to the BM niche, which likely contributes to the improved function of HSCs after SSA. Consistent with this, we have previously found that SSA can significantly improve hematopoietic reconstitution after HSCT (Goldberg et al., 2005, 2007, 2009), and here we could demonstrate that fewer ampa were needed for effective engraftment. There is some rationale for these findings, as (1) exposure of aged satellite stem cells to a young microenvironment reverses their age-related changes (Conboy et al., 2005), and (2) subsequently several studies have reported that circulating factors in young mice can reverse age-related defects in a number of tissues including heart, brain, and skeletal muscle (Loffredo et al., 2013; Sinha et al., 2014; Villeda et al., 2014). Pattern analysis of the changed genes within the stromal microenvironment revealed a distinct shift in gene expression after SSA from a profile characteristic of an aged mouse to that of the young by d10 after SSA. Of particular interest within this group was the gene Foxo1, which has been described in the development and differentiation of OBLs, regulating their expression of Runx2 and Bglap (Teixeira et al., 2010; Yang et al., 2011), and preventing differentiation of mesenchymal progenitor cells into fat or muscle (Nakae et al., 2003). Foxo1 has also been implicated with protection during aging (van der Horst and Burgering, 2007), with Foxo1-deficient mice displaying aberrant hematopoietic phenotypes resembling those seen with age; however, it is unclear whether these are intrinsic to HSCs or mediated through the BM microenvironment (Kim et al., 2008; Tothova et al., 2007). Consistent with these changes after SSA, the androgen receptor can directly bind the Foxo1 promoter and regulate its action, and FOXO1 can inhibit androgen receptor transcription (Liu et al., 2008; Ma et al., 2009). In addition, IGF-1, expression of which is almost completely abrogated after SSA, triggers the inactivation of FOXO1 by nuclear exclusion (Yang et al., 2011). Analysis of known hematopoietic niche interactions (Mercier et al., 2012) revealed upregulation following SSA of Spp1, Ang1, Cxcl12, Kitlg (Scf), Tgfb, Jag1, and Vcam1—many of which can be directly regulated by FOXO1 (Ferdous et al., 2011; Martinez et al., 2008; Potente et al., 2005).
Taken together, these data indicate that the widespread downstream impacts of SSA on lymphopoiesis are at least partially attributable to the considerable effects on primitive HSC function, thereby demonstrating mechanisms by which lymphopoiesis can be rejuvenated following SSA. Given that SSA can be achieved clinically, and reversibly, using currently approved agonists and antagonists of the sex steroid pathway (e.g., LHRH/GnRH), and that these improve immune function (Goldberg et al., 2009; Velardi et al., 2014), it has important implications for replenishing the diminished repertoire of lymphoid cells following damaging cytoablative treatments associated with bone marrow transplantation and cancer therapies. While the capacity for long-term BM regeneration following SSA has not yet been established, this work provides the basis for more detailed investigations into hematopoietic niche aging and SSA-induced rejuvenation, to develop more targeted strategies for HSC recovery.

Experimental Procedures

Author Contributions

Acknowledgments

Introduction
Cartilage is an avascular, alymphatic, and aneural tissue (Mankin, 1982) that, consequently, has limited repair capacity. Therefore, cartilage damage requires clinical intervention. In the last two decades, cell-based therapies have emerged as promising treatment options. Autologous chondrocyte implantation (ACI) was first applied in 1994 and is still used to treat cartilage defects in human patients (Brittberg ampa et al., 1994). In ACI, however, chondrocytes are harvested from the patient, creating an additional cartilage defect. Moreover, before use, the chondrocytes require in vitro expansion, which causes the progressive loss of cartilage matrix gene expression (Benya et al., 1978; Mayne et al., 1976).