br Introduction Hematopoietic stem cells HSCs persist

Introduction Hematopoietic stem n-acetyl-l-cysteine (HSCs) persist throughout life to produce hematopoietic progenitor cells (HPCs) and all types of blood cells. In the adult, HSCs reside in bone marrow (BM), are rare but have the unique capability for self-renewal and multilineage differentiation (Eaves, 2015). The maintenance of a steady-state HSC pool adept at dynamic change in response to stress depends on the balance of self-renewal, differentiation, survival, and proliferation (Goodell et al., 2015). Loss of this balance could lead to an overexpansion or exhaustion of HSC population, and result in an increased risk for cancer or tissue degeneration. HSCs have been used therapeutically in the clinic for several decades in life-saving treatment of malignant diseases and hematological disorders through BM transplantation protocols (Doulatov et al., 2012). However, insufficient stem cell numbers significantly limit the efficacy and success of these regimes. Expansion of HSCs while maintaining their self-renewal capability has been one of the most desired, yet elusive, goals in experimental hematology and transplantation medicine (Walasek et al., 2012). HSC fate decisions require strict control. Multiple signaling pathways regulate HSC functions through cell-intrinsic and cell-extrinsic mechanisms (Rossi et al., 2012; Gottgens, 2015). Genetic manipulation of transcription factors and signal transduction pathways enhances HSC expansion ex vivo (Andrade et al., 2010, 2011); these include the homeobox gene family (Antonchuk et al., 2002), immobilized Notch ligand (Delaney et al., 2010), Wnt-associated prostaglandin E2 (Goessling et al., 2011), the soluble growth factors angiopoietin-like 5 (Zheng et al., 2012), pleiotrophin (Himburg et al., 2010; Himburg et al., 2012), and miR-126 (Lechman et al., 2012), and the aryl hydrocarbon receptor inhibitor (Boitano et al., 2010). Moreover, employing an induced pluripotent stem cell population and targeting the HSC microenvironment hold promise for HSC expansion (Blanpain et al., 2012; Kunisaki and Frenette, 2012; Huang et al., 2013; Chen et al., 2014). However, all these attempts have had limited success clinically, due to a failure to expand sustainable and self-renewable stem cells (Walasek et al., 2012). Our understanding of the molecular pathways for HSC fate decision is insufficient to allow safe manipulation of HSCs for clinical benefit. Large natural variations in HSC number and function exist in humans (Nathan and Orkin, 2009; Sankaran and Orkin, 2013) as well as in different mouse strains (Jordan and Van Zant, 1998; Geiger et al., 2001; Henckaerts et al., 2002; Abiola et al., 2003; Hsu et al., 2007; Cahan et al., 2009; Avagyan et al., 2011). Such natural genetic diversity is an important yet largely unused tool for unraveling the genes and signaling networks associated with stem cell regulation (Van Zant and Liang, 2009). Using this genetic diversity tool, we previously identified latexin (Lxn) as a stem cell regulatory gene with expression that negatively correlates with HSC number variation in different mouse strains (de Haan, 2007; Liang et al., 2007). Lxn is a negative regulator of HSC function and works as a “brake” to constrain the HSC pool to a physiological range. In addition, the canonical function of Lxn protein is its inhibitory effect of carboxypeptidase A (Liu et al., 2000; Uratani et al., 2000; Pallares et al., 2005; Mouradov et al., 2006). Studies have also shown that LXN has high structural similarity with cystatin and tumor suppressor TIG1 gene, suggesting its potential role in inflammation and transformation (Aagaard et al., 2005). However, the in vivo function of Lxn in hematopoiesis and the underlying regulatory cellular and molecular mechanisms remain largely elusive. Particularly, when drawing upon genetic diversity to identify genes (usually multiple ones) associated with a complex trait (HSC number), all contributing genes are important (Van Zant and Liang, 2009). Therefore, it warrants to know to what extent Lxn contributes to the natural variation of the size of HSC population and how it specifically regulates HSC function and hematopoiesis.