Wnt9a Is Required for the Aortic Amplification of Nascent Hematopoietic Stem Cells

Abstract

All mature blood cell types in the adult animal arise from hematopoietic stem and progenitor cells (HSPCs). However, the developmental cues regulating HSPC ontogeny are incompletely understood. In particular, the details surrounding a requirement for Wnt/β-catenin signaling in the development of mature HSPCs are controversial and difficult to consolidate. Using zebrafish, we demonstrate that Wnt signaling is required to direct an amplification of HSPCs in the aorta. Wnt9a is specifically required for this process and cannot be replaced by Wnt9b or Wnt3a. This proliferative event occurs independently of initial HSPC fate specification, and the Wnt9a input is required prior to aorta formation. HSPC arterial amplification occurs prior to seeding of secondary hematopoietic tissues and proceeds, in part, through the cell cycle regulator myca (c-myc). Our results support a general paradigm, in which early signaling events, including Wnt, direct later HSPC developmental processes.

The role of Wnt signaling in hematopoietic stem cell development

“…Our recent work showed that loss of Wnt9a in the somites caused a decrease in HSC number after initiation of emergence with no discernible negative impact on HSC specification (Grainger and Richter et al 2016). This Wnt signal is unrelated to the somitic requirement for wnt16 , as it is temporally distinct and does not impact specification.…”

“…Overexpression of wnt8 , which in this context potently activates the cell fate pathway, caused an increase in HSCs during the emergence window (Goessling et al 2009). Inhibiting the secretion of all Wnt ligands using a chemical inhibitor of Porcupine (Porcn) (Chen et al 2009), an enzyme which is required for the lipid modification and subsequent secretion of Wnts, also caused a loss of HSCs during the emergence window (Biechele et al 2011, Grainger and Richter et al 2016). …”

“…Conversely, using a small molecule to increase the association between Axin and LRP6 to stimulate the Wnt pathway caused an increase in cmyb + cells during HSC emergence (Wang et al 2013). Activating the pathway by inhibiting GSK3β with lithium increased the number of flk1/cmyb – double positive HSCs emerging from the aortic floor (Grainger and Richter et al 2016). Overexpression of a constitutively active β–catenin also increased the number of cmyb+ cells within the HSC emergence window (Grainger and Richter et al 2016).…”

“…Activating the pathway by inhibiting GSK3β with lithium increased the number of flk1/cmyb – double positive HSCs emerging from the aortic floor (Grainger and Richter et al 2016). Overexpression of a constitutively active β–catenin also increased the number of cmyb+ cells within the HSC emergence window (Grainger and Richter et al 2016). Altogether, these experiments provide strong evidence that cytoplasmic components of the Wnt cell fate pathway are necessary for HSC emergence.…”

Developmental HSC Microenvironments: Lessons from Zebrafish

“…WNT (Wingless/INT) signaling is a well characterized pathway in regeneration and stem cell formation, but recent work from the Traver and Willert labs has demonstrated a new role for the WNT pathway in HSC migration to secondary sites such as the CHT (Grainger et al 2016). In their study, Grainger and colleagues depicted how early Wnt signals, specifically Wnt9a, from the developing aorta, prior to 20 hpf, are required for HSCs to undergo an expansion event at 31 hpf.…”