In conclusion the present study utilized an in vivo

In conclusion, the present study utilized an in vivo piglet model of hypoxic-preconditioning-induced ischemic tolerance to demonstrate that PC induces proliferation and neurogenesis of endogenous NSPs up to 7days after the hypoxic–ischemic insult. PC appears as a broad, non-invasive and reproducible strategy to prime the endogenous NSPs to a state of “readiness” by stimulation of their survival signaling pathways before introduction into ischemic environment. The signaling pathways involved in PC can be successfully employed for developing new therapeutic interventions to enhance endogenous neurogenesis after HI glutathione s-transferase injury. Future advances in terms of identifying key molecular mediators present the possibility of improving functional outcome in hypoxic–ischemic and stroke patients. Growth factors and other proliferation-inducing molecules might be synthesized by the astroglial and the surviving neurons in the cortex and striatum and could diffuse into the neurogenic regions of the brain. This concept is strengthened by the recent observations from our laboratory showing increased vascular endothelial growth factor expression in the neurons and astrocytes throughout the newborn piglet brain subjected to hypoxic-preconditioning (Ara et al., 2011). Sun et al. (2003) showed that VEGF enhanced the delayed survival of newborn neurons in the dentate gyrus and subventricular zone after ischemia. Future studies involving the modulation of adaptive response of brain to ischemic injury by exogenous growth factor treatment or by altering the expression of growth factors will be required to determine more definitively whether above paradigms will help to increase cerebral ischemia or stroke-induced neurogenesis.
Acknowledgments This work was supported by the American Heart Association grant 0835233N, and March of Dimes Foundation grant #6-FY09-321 to J.A.
Introduction Transplantation of hematopoietic stem cells (HSCs) is an effective, sometimes the only, therapy for many human diseases. However, the rareness of HSCs, either derived from bone marrow (BM) or umbilical cord blood (UCB), has hindered wide spread application and the effectiveness of this therapy (Weissman, 2000; Kelly et al., 2010; Delaney et al., 2010a; Dahlberg et al., 2011). HSCs are normally accommodated in stem cell niches, which involve several types of cells including endothelial cells (ECs) (Wilson and Trumpp, 2006; Morrison and Spradling, 2008). These cells provide paracrine factors and direct cell–cell interaction signals in temporospatial specific manners, to maintain the stemness as well as the proliferation and differentiation of HSCs (Blank et al., 2008). Attempts have been made to employ these signaling molecules and pathways to expand HSC ex vivo and enhance HSC engraftment in vivo in transplantation (Kelly et al., 2010; Delaney et al., 2010a; Dahlberg et al., 2011). The Notch pathway plays a critical role in regulating the proliferation and differentiation of stem and progenitor cells including HSCs during development (Bigas and Espinosa, 2012; Pajcini et al., 2011). In mammals, the canonical Notch pathway is composed of five ligands (Delta-like [Dll] 1, 3, 4, Jagged 1, 2), four Notch receptors (Notch1–4), the transcription factor RBP-J, and the downstream effectors such as the Hairy and enhancer of Split (Hes) family members. The Notch ligand–receptor interaction mediated by the Delta-Serrate-Lag-2 (DSL) domain of the ligands triggers proteolytic cleavages of the receptors, resulting in the endocytosis of the ligands in signal-sending cells, and the release of Notch intracellular domain (NICD) in signal-accepting cells. NICD associates with RBP-J and transactivates downstream genes involved in development (Gridley, 2010). Notch receptors and ligands are expressed by BM stromal and hematopoietic cells (Walker et al., 2001; Singh et al., 2000). Although Notch signaling is essential for the segregation of HSCs during embryonic definitive hematopoiesis (Varnum-Finney et al., 2011; Kumano et al., 2003; Burns et al., 2005; Robert-Moreno et al., 2005), its role in adult HSCs is controversial (Mancini et al., 2005; Maillard et al., 2008; Duncan et al., 2005; Stier et al., 2002). Despite these inconsistencies, however, a large body of evidence has demonstrated that activating Notch signaling facilitates HSC expansion ex vivo (Mayani, 2010). Expression of NICD in murine hematopoietic precursors results in immortalization of HSCs and enhanced HSC self-renewal (Stier et al., 2002; Varnum-Finney et al., 2000a). Several reports have shown that soluble Notch ligands could induce HSC expansion ex vivo (Varnum-Finney et al., 2003; Suzuki et al., 2006; Ohishi et al., 2002; Karanu et al., 2001; Han et al., 2000). Meanwhile, a requirement for ligand immobilization to activate Notch signaling for HSC expansion has been demonstrated (Varnum-Finney et al., 2000b; Lauret et al., 2004). Other studies have shown the ligand selectivity, dose-dependency, and the requirement of supporting ECs in HSC expansion ex vivo (Delaney et al., 2005a; Lahmar et al., 2008; Vas et al., 2004; Butler et al., 2010). Recently, significantly reduced average time of neutropenia has been shown in a phase I clinical trial of transplantation using human CD34+ cells expanded with the immobilized Delta 1 (Delaney et al., 2010b; North and Goessling, 2010).