In addition to supplying a source of growth factors
In addition to supplying a source of growth factors and signaling molecules, blood vessels also provide a physical framework that can support the migration of neural progenitors during development and in the adult calcium channels (Whitman et al., 2009). In the embryonic and adult rodent SVZ, newly born progenitors migrate along blood vessels toward the olfactory bulb via the rostral migratory stream (RMS) (Bovetti et al., 2007; Kokovay et al., 2010; Snapyan et al., 2009). The vasculature also provides guidance cues for the tangential migration of GABAergic progenitors from the embryonic germinal zones to the mouse cortex (Won et al., 2013). In stroke, there is an increase in angiogenesis around the infarct site (Hayashi et al., 2006), and endogenous neural stem cells and neuroblasts migrate along blood vessels to repopulate damaged regions (Madri, 2009; Thored et al., 2007; Yamashita et al., 2006).
A variety of molecular interactions promote the initial association between blood vessels and SVZ neural progenitors and the subsequent migration of the progenitors. The integrin α6β1 on the neural progenitor surface promotes binding to laminin-coated blood vessels, and this interaction is key to the subsequent exit of neural progenitors from the SVZ to join the RMS (Kokovay et al., 2010). VEGF also acts as a chemoattractant, promoting the repopulation of neural progenitors to ischemic regions damaged by stroke (Marti et al., 2000; Wittko et al., 2009). The chemokine CXCL12 on the blood vessel surface helps direct the migration of SVZ neural progenitors (Kokovay et al., 2010).
Astrocytes play a role in promoting vasculature-supported neural progenitor migration. Neural progenitors migrating along blood vessels in the SVZ are in direct contact with the astrocytes that coat these vessels (Bozoyan et al., 2012; Kaneko et al., 2010; Lois and Alvarez-Buylla, 1994). During development, astrocytes help maintain the correct organization of the vasculature in the RMS (Bozoyan et al., 2012). Lesioning the cortex can also activate endogenous SVZ neural progenitor cells and promote their migration to the site of injury, with a role for astrocyte/blood vessel-derived CXCL12 reported (Saha et al., 2013). Taken together, these studies suggest the importance of the blood vessel-astroglial environment in providing cues for directing neural progenitors to sites of injury.
We now show that grafts of mouse embryonic stem cell (ESC)-derived ESNPs in the hippocampus are well vascularized, with an increase in blood vessel area over time, likely due to an inflammatory response triggered by the injection. Following transplant to the hippocampus, ESNPs are also found in close association with blood vessels at great distances from the injection site, consistent with the vasculature providing a migratory substrate. Both blood vessels and blood vessel-associated astrocytes are coated with CXCL12, and our previous work suggests a role for this chemokine in ESNP translocation (Hartman et al., 2010). Using hippocampal slice culture, we observe a time-dependent association of ESNPs with blood vessels following deposition on the slice surface. We directly examined the interactions of ESNPs with brain endothelial cells (BECs) in vitro and observe striking morphological differences between ESNPs co-cultured with BECs and ESNPs grown alone. We demonstrate that BECs produce laminin and ESNPs express the α6β1 integrin and that a function-blocking antibody inhibits the adhesion of ESNPs to a BEC monolayer. In addition, we find that human ESNPs (hESNPs) migrate towards BECs in a Boyden chamber migration assay in response to BEC-secreted factors, including CXCL12. These data suggest that the astroglial-endothelial cell niche promotes the migration of ESNPs away from the initial site of deposition. Understanding the molecular cues that direct ESNP migration can aid investigators in accurately targeting therapeutic cell transplants to regions of damage.