The extracellular matrix ECM is a network of secreted protei

The extracellular matrix (ECM) is a network of secreted proteins that function in concert to structurally support purchase Nocodazole and regulate cellular processes (Rozario and DeSimone, 2010). The ECM is an important component of the BM hematopoietic microenvironment, where it can sequester cytokines and regulate their downstream signaling (Davis and Senger, 2005); developmental niches such as the AGM and fetal liver are also ECM rich (Marshall et al., 1999). ECM structure is highly dynamic and actively remodeled by degrading proteases known as matrix metalloproteinases (MMPs) (Heissig et al., 2003). MMPs are well-characterized inflammatory mediators (Parks et al., 2004) that perform a variety of distinct and overlapping biological functions depending on their spatiotemporal localization. In particular, MMP2 and MMP9, which represent the gelatinase family, have been implicated in angiogenic ECM remodeling downstream of Notch and VEGF signaling (Funahashi et al., 2011). As Notch and VEGF are essential for HSPC formation, and given the connections between inflammatory activity and HSC production, we sought to investigate the functional requirement(s) for gelatinase activity during embryonic HSPC development.
Results
Discussion In this study, we identified distinct roles for gelatinases Mmp2 and Mmp9 in the regulation of developmental HSPC production, migration, and niche localization. We demonstrate that during zebrafish development, Mmp2 is required to remodel fibronectin-rich ECM in the VDA to facilitate HSPC egress from the AGM and subsequent migration to the CHT and thymus. In addition, we elucidate interrelated effects of Mmp9 on hematopoiesis in the CHT: via modulation of Cxcl12 activity, Mmp9 affects the structure of the vascular niche, altering HSPC localization, and promotes HSPC migration to subsequent hematopoietic niches. ECM restructuring by MMPs affects development of lungs, mammary glands, and salivary glands (Bonnans et al., 2014). Likewise, a role for MMPs in ECM remodeling and subsequent maturation occurs in the developing intestine of tadpoles (Ishizuya-Oka et al., 2000), while MMP2 in particular is required for local ECM digestion during epithelial branching (Bonnans et al., 2014). Our study reveals essential but discrete roles for Mmp2 and Mmp9 downstream of sterile inflammatory signaling during embryonic hematopoiesis, whereby each can affect HSPC migration and localization in subsequent developmental niches. As a group, MMPs are multifunctional enzymes closely regulated by a variety of upstream signaling pathways (Parks et al., 2004). Our work specifically demonstrates a relationship between inflammatory signaling in the embryo and expression of mmp2 and mmp9. Induction of these enzymes by both endogenous and exogenous PGE2 provides further insight into the mechanisms underlying the recently described effects of inflammatory signaling on hematopoiesis (Espín-Palazón et al., 2014; He et al., 2015; Li et al., 2014; Sawamiphak et al., 2014) and suggests targets for therapeutic intervention to modulate HSPC production. Our findings, taken together with these studies, suggest that Mmp2 may be constitutively expressed downstream of endogenous signaling pathways active in the AGM, while Mmp9 is the inducible enzyme transcribed in response to external stimuli, such as inflammation, to aid HSPC expansion and/or movement in the embryo. While our evidence points to Cxcl12 as the main modulator of embryonic HSPC localization in the CHT, the induction of Mmp2/9 by PGE2 and its potential downstream proteolytic activity imply that Mmp9 function during developmental hematopoiesis may affect inflammatory signaling on a larger scale. MMP2/9 are known to activate transforming growth factor β, a critical cytokine involved with HSPC regulation (Vaidya and Kale, 2015). Furthermore, gelatinases can work together with other MMPs to modulate cytokine gradients: for example, combinatorial MMP2/9, MMP1, and MMP3 inactivate interleukin-1β (Ito et al., 1996). Given this cooperativity, it will be interesting to investigate the relationships between MMP2/9 and the inflammatory mediators recently shown to regulate embryonic hematopoiesis.