A familiar stranger: CD34 expression and putative functions in SVF cells of adipose tissue

Abstract

Human adipose tissue obtained by liposuction is easily accessible and an abundant potential source of autologous cells for regenerative medicine applications. After digestion of the tissue and removal of differentiated adipocytes, the so-called stromal vascular fraction (SVF) of adipose, a mix of various cell types, is obtained. SVF contains mesenchymal fibroblastic cells, able to adhere to culture plastic and to generate large colonies in vitro, that closely resemble bone marrow-derived colony forming units-fibroblastic, and whose expanded progeny, adipose mesenchymal stem/stromal cells (ASC), show strong similarities with bone marrow mesenchymal stem cells. The sialomucin CD34, which is well known as a hematopoietic stem cell marker, is also expressed by ASC in native adipose tissue but its expression is gradually lost upon standard ASC expansion in vitro. Surprisingly little is known about the functional role of CD34 in the biology and tissue forming capacity of SVF cells and ASC. The present editorial provides a short introduction to the CD34 family of sialomucins and reviews the data from the literature concerning expression and function of these proteins in SVF cells and their in vitro expanded progeny.

Keywords: CD34; Endothelial progenitors; Human adipose tissue; Mesenchymal stromal cells; Sialomucins.

Figure 1

Protein structure of the CD34 family. CD34, podocalixin and endoglycan are transmembrane proteins which display O-glycosylated and sialylated serine-,threonine- and proline-rich extracellular mucin domain, putative sites of N-glycosylation, a cysteine-containing globular domain and a juxtamembrane stalk region. NHERF1 and 2 bind to the C-terminal tail of podocalyxin and endoglycan but not CD34. Conversely the CrkL selectively binds to a distinct juxtamembrane sequence in CD34.

Figure 2

In the adipose tissue, adipose mesenchymal stem/stromal cells are localized perivascularly and are characterized by the expression of CD34. Adipose mesenchymal stem/stromal cells (ASC) are isolated upon collagenase digestion together with other cell types including endothelial and endothelial progenitor cells. Upon adhesion to plastic and active in vitro proliferation, ASC change their phenotype, lose CD34 expression and begin to express CD105.