100 I. Nazarenko et al. characteristics that are able to self-renew and propa- gate the whole tumor. In this context, we brieﬂy discuss the concept of cancer stem cells (CSCs) in glioma. PDGF/PDGFR Discovered as a serum growth factor for ﬁbroblasts, smooth muscle cells, and glial cells, the PDGF family has become one of the most extensively studied growth factor families (2-6). PDGF-B and -A were discovered ﬁrst, while PDGF-C and -D were identi- ﬁed decades later, based on gene homology searches. PDGFs are members of the evolutionary conserved family of structurally and functionally related PDGF/ VEGF growth factors. The PDGF family now includes ﬁve dimeric proteins: PDGF-AA, -AB, -BB, -CC, and -DD. The mammalian PDGF genes are situated on different chromosomes and have mostly independent transcriptional regulation and thus distinct tissue- and time-speciﬁc gene expression patterns in vivo. However, the overlapping expression patterns of PDGF-A and -C suggest the possibility of common transcriptional regulatory mechanisms (6). All PDGFs have a highly conserved growth factor domain, called the PDGF/VEGF homology domain. This domain is involved in forming inter- and intra- molecule disulphide bridges to form the PDGF dimers (7). In order to be activated, a short N-terminal extension present in PDGF-A and PDGF-B has to undergo intracellular proteolytic processing. PDGF-C and PDGF-D have a distinct protein domain as part of their N-terminal extension, called the CUB domain (7), preventing ligand– receptor binding until cleaved and are activated by extracellular proteases (8,9). The C-termini of PDGF-C and -D lack amino acid sequence exten- sions, while both PDGF-A and -B have a stretch of basic amino acids that are mainly involved in extra- cellular matrix (ECM) binding (10,11). There are two functionally distinct isoforms of the A-chain due to alternative splicing of exon 6, which encodes the C-terminal stretch: the short form of PDGF-A (PDGF-AS) lacks the positively charged retention motif and is freely diffusible, while the long form (PDGF-AL) can attach to the ECM with the help of its C-terminal tail (2,12,13). The exact role of PDGF-AL and how it functionally differs from the shorter isoform is not well understood. The ﬁve dimeric PDGF ligands act via the two cell surface tyrosine kinase (RTK) receptors, PDGFR-a and PDGFR-b (Figure 1). The PDGFRs have a common domain structure, including ﬁve extracellu- lar immunoglobulin (Ig)-like domains and a split intracellular tyrosine kinase domain (6,14). As a result CC AB BB AA DD* CC AB BB DD BB αα αβ ββ Figure 1. Receptor binding speciﬁcity of ﬁve dimeric PDGF ligands. *Ligand DD can activate ab with lower speciﬁcity. of ligands binding to the receptors, homo- and het- erodimerization of the receptors occur. This in turn leads to transphosphorylation of the intracellular domains and receptor activation. Once activated, intracellular mediators dock to phosphotyrosine resi- dues in the receptor, which leads to downstream activation of intracellular signaling pathways. The ability of each of the ﬁve dimeric PDGF ligands to bind and activate the two PDGF receptors is quite speciﬁc as summarized in Figure 1. Our early studies on PDGF in glioma The initial discoveries on the role of PDGF and PDGF receptors in cancer came substantially from studies on human brain tumors. Our efforts in this ﬁeld of research began in 1981 when Monica Nistér with Bengt Westermark was introduced to the vibrant and fruitful collaboration formed by Westermark, Wasteson, and Heldin. Carl-Henrik Heldin had puri- ﬁed and characterized human PDGF-AB from plate- lets (15). It had been shown that ﬁbroblasts, smooth muscle cells, and glial cells were responsive to PDGF (3–5), and the outstanding question was whether PDGF was involved in human cancer. Pontén and Westermark had established a large number of human glioma cell lines that were suitable to reveal if PDGF was produced by these cancer cells. One such biopsy was the U-343 MG biopsy for which a few clonal derivatives existed. Some cultures were GFAP+/astrocyte-like, while others had mesenchymal features (16,17). The strategy used by Nistér et al. was to screen conditioned media from U-343 MGa Cl2 cells using the 125I-PDGF-binding assay on ﬁbro- blasts to identify PDGF receptor competing activity. Much to our excitement, such activity was detected,