Immortalization of rat embryo fibroblasts by a 3'-untranslated region

Regeneration in axolotls: a model to aim for!

Urodele amphibians such as the axolotl are the champions of tissue regeneration amongst vertebrates. These animals have mastered the ability to repair and replace most of their tissues following damage or amputation even well into adulthood. In fact it seems that the ability of these organisms to regenerate perfectly is not affected by their age. In addition to being able to regenerate, these animals display a remarkable resistance to cancer. They therefore represent a unique model organism to study regeneration and cancer resistance in vertebrates. The need for this research is even more pressing at the dawn of the 21st century as we are faced with an ever aging world population which has to deal with an increase in organ failure and cancer incidence. Hopefully, this mini review will put in perspective some of the reasons why studying tissue regeneration in salamanders could yield significant knowledge to help regenerative medicine achieve the desired goal of allowing humans to repair and regenerate some of their own tissues as they age.

Macrophage-colony stimulating factor is required for the production of neutrophil-promoting activity by mouse embryo fibroblasts deficient in G-CSF and GM-CSF

G-CSF and GM-CSF play important roles in regulating neutrophil production, survival, differentiation, and function. However, we have shown previously that G-CSF/GM-CSF double-deficient [knockout (KO)] mice still develop a profound neutrophilia in bone marrow and blood after infection with Candida albicans. This finding suggests the existence of other systems, which can regulate emergency neutrophil production. We have now developed an "in vitro" technique to detect and characterize a neutrophil-promoting activity (NPA) in the media conditioned by mouse embryonic fibroblasts (MEFs) derived from G-CSF(-/-)/GM-CSF(-/-) mice. NPA is produced in vitro by the MEFs after stimulation with LPS or heat-inactivated C. albicans. Although M-CSF added directly to bone marrow cultures does not sustain granulocyte production, our studies indicate that production of NPA requires activation of the M-CSF receptor (c-fms). First, G-CSF(-/-)/GM-CSF(-/-) MEFs produce high levels of NPA after stimulation with LPS or C. albicans, and G-CSF/GM-CSF/M-CSF triple-KO MEFs do not. Second, the production of NPA by the G-CSF(-/-)/GM-CSF(-/-) MEFs is reduced significantly upon incubation with neutralizing antibodies to M-CSF or c-fms. Third, NPA production by G-CSF(-/-)/GM-CSF(-/-)/M-CSF(-/-) fibroblasts is enhanced by supplementing culture medium with M-CSF. Thus, stimulation of c-fms by M-CSF is a prerequisite for the production of NPA.

Cell death inhibiting RNA (CDIR) derived from a 3'-untranslated region binds AUF1 and heat shock protein 27

Regulators of programmed cell death were previously identified using a technical knockout genetic screen. Among the elements that inhibited interferon-gamma-induced apoptosis of HeLa cells was a 441-nucleotide fragment derived from the 3'-untranslated region (UTR) of KIAA0425, a gene of unknown function. This fragment was termed cell death inhibiting RNA (CDIR). Deletion and mutation analyses of CDIR were employed to identify the features required for its anti-apoptotic activity. Single nucleotide alterations within either copy of the duplicated U-rich motif found in the CDIR sequence abolished the anti-apoptotic activity of CDIR and altered its in vitro association with a protein complex. Further analysis of the CDIR-binding complex indicated that it contained heat shock protein 27 (Hsp27) and the regulator of mRNA turnover AUF1 (heterogeneous nuclear ribonucleoprotein D). In addition, recombinant AUF1 bound directly to CDIR. Furthermore, expression of another AUF1-binding RNA element, derived from the 3'-UTR of c-myc, inhibited apoptosis. We also demonstrate that the level and the stability of p21(waf1/Cip1/sdi1) mRNA, a target of AUF1 with anti-apoptotic activity, were increased in CDIR-transfected cells. The level of mRNA and protein of Bcl-2, another anti-apoptotic gene, containing an AUF1 binding site in its 3'-UTR was also increased in CDIR-transfected cells. Our data suggest that AUF1 regulates apoptosis by altering mRNA turnover. We propose that CDIR inhibits apoptosis by acting as a competitive inhibitor of AUF1, preventing AUF1 from binding to its targets.

BAP37 and Prohibitin are specifically recognized by an SV40 T antigen antibody

We have identified two cellular proteins that are specifically immunoprecipitated by an anti-SV40 T antigen monoclonal antibody. This antibody, PAb419, recognizes an epitope contained within a region of T antigen which we have recently demonstrated is required for the initiation of immortalization by SV40 T antigen, but is not essential for maintenance of the immortal state. The two proteins were identified as BAP37 and Prohibitin. Recent results suggest Prohibitin may enhance the transcriptional inactivation of E2F by the retinoblastoma family of pocket proteins (pRb, p107, p130). BAP37 and Prohibitin are specifically recognized by PAb419 and PAb210, another anti-SV40 T antigen monoclonal antibody, which has an overlapping epitope, but not by other anti-SV40 T antigen monoclonal antibodies, demonstrating the specificity of the interaction.

Regeneration in vertebrates

One way or another, all species possess the ability to regenerate damaged tissues. The degree of regeneration, however, varies considerably among tissues within a body and among species, with urodeles being the most spectacular. Such differences in regenerative capacity are indicative of specific mechanisms that control the different types of regeneration. In this review the different types of regeneration in vertebrates and their basic characteristics are presented. The major cellular events, such as dedifferentiation and transdifferentiation, which allow complex organ and body part regeneration, are discussed and common molecular mechanisms are pinpointed.

Genomic characterization of the lysophosphatidic acid receptor gene, lp(A2)/Edg4, and identification of a frameshift mutation in a previously characterized cDNA

To understand the regulation, evolution, and genetics of lp(A2)/Edg4, a second lysophosphatidic acid receptor gene, we characterized its complete cDNA sequence, genomic structure, and chromosomal location. The full-length mouse transcript sequence was determined using rapid amplification of cDNA ends. Southern blot and restriction fragment length polymorphism segregation analyses revealed that the mouse gene was present as a single copy and located at the middle of Chromosome 8 near the mutations for myodystrophy (myd) and "kidney-anemia-testes" (kat). This region is syntenic with human chromosome 19p12, where the human genomic clone containing the lp(A2) gene (EDG4) was mapped. Sequence analysis of genomic clones demonstrated that both mouse and human transcripts were encoded by three exons, with an intron separating the coding region for transmembrane domain VI. Reverse transcriptase-PCR demonstrated that the three exons were spliced in all mouse tissues shown to express the transcript. Finally, in a comparison of all human lp(A2) sequences present in the database, we identified several sequence variants in multiple tumors. One such variant (a G deletion) in the initially characterized Edg4 cDNA clone (derived from an ovarian tumor) results in a frameshift mutation near the 3' end of the coding region. In addition to increasing our understanding of the mechanisms underlying lysophosphatidic acid signaling and lysophospholipid receptor gene evolution, these results have important implications regarding the genomic targeting and oncogenic potential of lp(A2).

Different functions are required for initiation and maintenance of immortalization of rat embryo fibroblasts by SV40 large T antigen

We have used two different, but complementary assays to characterize functions of SV40 T antigen that are necessary for its ability to immortalize rat embryo fibroblasts. In accordance with previous work, we found that several functions were required. These include activities that map to the p53 binding domain and the amino terminal 176 amino acids which contain the J domain as well as the CR1 and CR2 domain required for binding and sequestering the RB family of pocket proteins. Moreover, we found that even though activities dependent only upon the amino terminus were sufficient for immortalization they were unable to maintain it. This suggests that immortalization by these amino terminal functions requires either additional events or immortalization of a subset of cells within the heterogeneous rat embryo fibroblast population. We further found that an activity dependent upon amino acids 17 - 27 which remove a portion of the CR1 domain and the predicted alpha-1 helix of the J domain was not necessary to maintain growth but was required for direct immortalization suggesting that at least one of the functions required initially was not required to maintain the immortal state. This represents the first demonstration that some of the functions required for maintenance of the immortal state differ from those required for initiation of immortalization.

RA regulation of keratin expression and myogenesis suggests different ways of regenerating muscle in adult amphibian limbs

Formation of a regeneration blastema following limb amputation is believed to occur through a process of dedifferentiation. It has been suggested, however, that the cells contributed to the blastema by the stump muscle are satellite-like cells, rather than cells originated by dedifferentiation. We have previously shown that simple epithelial keratins 8 and 18 are expressed in the mesenchymal progenitor cells of the regenerating amphibian limb and in cultured cells with myogenic potential, and that their expression appears to be causally related to changes in proliferation and differentiation. We show here that retinoic acid (RA) affects the expression of these keratins differently in myogenic cells originated from normal limb and limb blastema. Furthermore, we find that the effects of RA on proliferation, myogenic differentiation and adhesion of these cells also differ. In fact, whereas RA does not affect keratin expression, proliferation or myogenic differentiation in blastemal cells, it does decrease keratin levels and thymidine incorporation and increase myogenesis in cells from normal limb. Conversely, RA increases cell adhesion only in blastemal cells. Significantly, these effects of RA on cultured cells are consistent with those observed in vivo. Overall the results presented here suggest that in the urodele limb there are two distinct cell populations with myogenic potential, one originating from dedifferentiation and one equivalent to the satellite cells of the mammalian muscle, which are likely to be primarily involved in blastema formation and muscle repair, respectively.

Identification of newt connective tissue growth factor as a target of retinoid regulation in limb blastemal cells

In order to analyse target genes regulated by retinoic acid in urodele limb regeneration, we have used pseudotyped retroviruses to obtain stably transfected newt limb blastemal (progenitor) cells in culture which express chimeric retinoic acid/thyroid hormone receptors delta1 or delta2. After treatment with thyroid hormone to activate the chimeric receptors, we used a polymerase chain reaction (PCR)-based subtraction method to identify target genes which are retinoid regulated. Newt connective tissue growth factor, a secreted protein recognised in several vertebrates, has been identified in this way and found to be expressed in the limb blastema and regulated by retinoic acid. This approach should permit a systematic analysis of retinoid target genes in limb regeneration.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.