Evaluation of the mutagenic effects of SV40 in mouse, hamster, and mouse-human hybrid cells

We have examined the ability of SV40 to induce changes in drug or temperature resistance in mouse, hamster, and mouse-human hybrid cells. SV40 induced a substantial increase of cells resistant to 5-bromodeoxyuridine + trifluorothymidine in Balb/c 3T3 cells and induced an increase of hybrid cells resistant to 6-thioguanine. SV40 was found to be nonmutagenic or weakly mutagenic in other test systems. The 3T3 cells were T-antigen positive, exhibited a marked reduction in TK activity, were heterogeneous for [ 3 H]BrdU incorporation by autoradiography, and exhibited instability of the drug-resistance phenotype, suggesting that SV40 may be inducing resistance by an epigenetic process. SV40-induced 6-thioguanine resistance in the hybrids appears to occur predominantly by chromosome loss.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45539/1/11188_2005_Article_BF01233058.pd

Fibroblast growth factor (FGF) homologous factors share structural but not functional homology with FGFs

Fibroblast growth factors (FGFs) interact with heparan sulfate glycosaminoglycans and the extracellular domains of FGF cell surface receptors (FGFRs) to trigger receptor activation and biological responses. FGF homologous factors (FHF1-FHF4; also known as FGF11-FGF14) are related to FGFs by substantial sequence homology, yet their only documented interactions are with an intracellular kinase scaffold protein, islet brain-2 (IB2) and with voltage-gated sodium channels. In this report, we show that recombinant FHFs can bind heparin with high affinity like classical FGFs yet fail to activate any of the seven principal FGFRs. Instead, we demonstrate that FHFs bind IB2 directly, furthering the contention that FHFs and FGFs elicit their biological effects by binding to different protein partners. To understand the molecular basis for this differential target binding specificity, we elucidated the crystal structure of FHF1b to 1.7-A resolution. The FHF1b core domain assumes a beta-trefoil fold consisting of 12 antiparallel beta strands (beta 1 through beta 12). The FHF1b beta-trefoil core is remarkably similar to that of classical FGFs and exhibits an FGF-characteristic heparin-binding surface as attested to by the number of bound sulfate ions. Using molecular modeling and structure-based mutational analysis, we identified two surface residues, Arg52 in the beta 4-beta 5 loop and Val95 in the beta 9 strand of FHF1b that are required for the interaction of FHF1b with IB2. These two residues are unique to FHFs, and mutations of the corresponding residues of FGF1 to Arg and Val diminish the capacity of FGF1 to activate FGFRs, suggesting that these two FHF residues contribute to the inability of FHFs to activate FGFRs. Hence, FHFs and FGFs bear striking structural similarity but have diverged to direct related surfaces toward interaction with distinct protein targets

Fibroblast growth factor receptors cooperate to regulate neural progenitor properties in the developing midbrain and hindbrain.

Fibroblast growth factors (FGFs) secreted from the midbrain-rhombomere 1 (r1) boundary instruct cell behavior in the surrounding neuroectoderm. For example, a combination of FGF and sonic hedgehog (SHH) can induce the development of the midbrain dopaminergic neurons, but the mechanisms behind the action and integration of these signals are unclear. We studied how FGF receptors (FGFRs) regulate cellular responses by analyzing midbrain-r1 development in mouse embryos, which carry different combinations of mutant Fgfr1, Fgfr2, and Fgfr3 alleles. Our results show that the FGFRs act redundantly to support cell survival in the dorsal neuroectoderm, promote r1 tissue identity, and regulate the production of ventral neuronal populations, including midbrain dopaminergic neurons. The compound Fgfr mutants have apparently normal WNT/SHH signaling and neurogenic gene expression in the ventral midbrain, but the number of proliferative neural progenitors is reduced as a result of precocious neuronal differentiation. Our results suggest a SoxB1 family member, Sox3, as a potential FGF-induced transcription factor promoting progenitor renewal. We propose a model for regulation of progenitor cell self-renewal and neuronal differentiation by combinatorial intercellular signals in the ventral midbrain