102 I. Nazarenko et al. whereas retrovirally introduced PDGF-B was able to transform ﬁbroblasts in vitro, PDGF-A could not (41). Whether this could be attributed to the presence or absence of the C-terminal stretch of basic amino acids was unclear at the time. In addition, the inhibi- tory effect of PDGF-A on PDGF-B-mediated cell proliferation and motility was puzzling, and the sig- niﬁcance of PDGF-A/a-receptor signaling in tumor development was in question. Although autocrine stimulation was extensively studied at the time, its signiﬁcance in cancer was difﬁcult to ﬁnally prove. However, the transforming effect of PDGF in glioma cells was indicated by the inhibitory effects of PDGF-BB antibodies (42) and by studies using a truncated PDGF b-receptor (43). Even now, clinical trials using TKIs fail to show signiﬁcant therapeutic effects in patients (44,45). Our recent investigations, performed in collabora- tion with Arne Östman, Johan Holmberg, and Jonas Muhr, have shed some light on the glioma cell resis- tance to TKI inhibitors. Using tumor material explanted from a number of human glioblastomas, two different subtypes of glioma cultures, type A and type B, were identiﬁed. Whereas type B cultures showed mesenchymal features and were responsive to mono-treatment with the PDGFR inhibitor imati- nib, and also to IGF-1R inhibitor, NVP-AEW541, the type A cultures displayed stem cell-like features, including in vitro self-renewal and neurosphere for- mation, and in vivo tumor growth. These type A cultures were non-responsive to mono-treatments but were growth-inhibited when exposed to a com- bination of the two inhibitors. Furthermore, the cells’ resistance to mono-treatment was dependent on the presence of transcription factor SOX2 (46). In a study by Holmberg and Muhr, SOX2, OCT4, NANOG, and KLF4 transcription factors were visualized in high-grade human glioma tissues using immunoﬂu- orescence techniques (47). These proteins were stud- ied because they are able to induce pluripotent stem cells (iPS cells) from somatic cells (48), and their presence in glioma indicates a stemness phenotype. The stem cell features may be part of glioma cells’ resistance to TKIs, and a combined treatment strat- egy targeting key stem cell regulatory mechanisms and growth factor signaling mechanisms promises to be a useful therapeutic approach. The role of PDGF during development The functional signiﬁcance of the described diversity in PDGF-A/B ligands and receptors, encoded by the different genes, became evident from the phenotypes of a number of genetically modiﬁed mice generated by the groups of Christer Betsholtz and Phil Soriano (for a review see (49)). In addition, the transcriptional regulatory mechanisms also differ, thus providing a basis for cell-, tissue-, and time-speciﬁc activities in vivo. This diversity turned out to be even higher when PDGF-C and -D were identiﬁed (8,9). A large number of gain- and loss-of-function muta- tions in Pdgf and Pdgfr genes have been created in mice (50-54). The developmental defects found in these mice underscore the importance of PDGF ligands and receptors in normal embryonic develop- ment. PDGF-B and b-receptors are essential for normal blood vessel development, while PDGF a-receptors are required for neural crest develop- ment. PDGF-B stimulates pericytes (55) and is involved in formation of glomeruli of the kidney. PDGF-A is required for the normal development of lung alveoli (50,56), intestinal villi, mesenchymal dermis, and hair follicles. Studies in genetically mod- iﬁed mice, where the cytoplasmic signaling domains of the two PDGF receptors had been swapped, dem- onstrated that PDGF receptors are partly inter- changeable during development and mediate very similar cellular responses (57). However, PDGFR-b seems to have a more important intracellular signaling capacity in the vasculature, since mice with an intro- duced PDGFR-a signaling domain exhibited vascular defects (58). PDGFs are also important in adulthood, as they are involved in wound healing, including wound healing in the CNS (59). However, excessive or aberrant expression of PDGFs can lead to patho- logical responses such as atherosclerosis, ﬁbrosis, and tumorigenesis (6). The role of PDGF in the central nervous system Some insights into the role of PDGFs in the CNS were obtained before genetic mouse models were available. Initially, our knowledge on the role of PDGF-B and PDGFR-b in the CNS was mainly based on their expression patterns in the brain. PDGF-B is present in embryonic as well as in adult neurons (60). PDGFR-b was detected in neurons, and PDGF was shown to mediate neuroprotective functions after injury (61-63). PDGFR-b is also present on pericytes (64,65). A series of in vitro studies determined that PDGF-A is produced by neurons and astrocytes (66-68) and acts as a mitogen for oligodendrocyte progenitor cells (OPCs) (67,69,70). Oligodendro- cytes differentiate postnatally from PDGFR-a– positive OPCs. Their continued proliferation and migration in the CNS depends on PDGF-A signaling through PDGFR-a (71,72). In the absence of PDGF-A, postnatally surviving mice develop tremor