Cloning of cDNAs encoding retinoic acid receptors RAR gamma 1, RAR gamma 2, and a new splicing variant, RAR gamma 3, from Aambystoma mexicanum and characterization of their expression during early development

To analyze retinoic acid (RA) receptor (RAR) expression during early development in the urodele embryo, we have isolated cDNAs for four members of the axolotl (Ambystoma mexicanum) RAR family, namely RAR alpha (NR1B1), aRAR gamma 1 (NR1B3a), aRAR gamma 2 (NR1B3b), and a new splicing variant of aRAR gamma 2, aRAR gamma 3 (NR1B3c), which contains an insertion of five hydrophobic amino acids in the C-terminal region of the DNA binding domain. The temporal expression pattern of the RAR gamma isoforms was established by RT-PCR using total RNA from embryos of different stages. The expression of aRAR gamma 2 coincides with neurulation and is enhanced in the extremities of the embryo's anteroposterior axis. The aRAR gamma 3 is specifically expressed during gastrulation and early neurulation, whereas aRAR gamma 1 is expressed later during organogenesis. Global aRAR gamma 2 mRNA levels, as well as their spatio-temporal expression pattern in the neurula, were not affected by treatment with RA. These results show that several RARs are expressed in the axolotl embryo during early development, and reveal the existence of a new RAR gamma variant.     

Transforming growth factor: beta signaling is essential for limb regeneration in axolotls

Axolotls (urodele amphibians) have the unique ability, among vertebrates, to perfectly regenerate many parts of their body including limbs, tail, jaw and spinal cord following injury or amputation. The axolotl limb is the most widely used structure as an experimental model to study tissue regeneration. The process is well characterized, requiring multiple cellular and molecular mechanisms. The preparation phase represents the first part of the regeneration process which includes wound healing, cellular migration, dedifferentiation and proliferation. The redevelopment phase represents the second part when dedifferentiated cells stop proliferating and redifferentiate to give rise to all missing structures. In the axolotl, when a limb is amputated, the missing or wounded part is regenerated perfectly without scar formation between the stump and the regenerated structure. Multiple authors have recently highlighted the similarities between the early phases of mammalian wound healing and urodele limb regeneration. In mammals, one very important family of growth factors implicated in the control of almost all aspects of wound healing is the transforming growth factor-beta family (TGF-beta). In the present study, the full length sequence of the axolotl TGF-beta1 cDNA was isolated. The spatio-temporal expression pattern of TGF-beta1 in regenerating limbs shows that this gene is up-regulated during the preparation phase of regeneration. Our results also demonstrate the presence of multiple components of the TGF-beta signaling machinery in axolotl cells. By using a specific pharmacological inhibitor of TGF-beta type I receptor, SB-431542, we show that TGF-beta signaling is required for axolotl limb regeneration. Treatment of regenerating limbs with SB-431542 reveals that cellular proliferation during limb regeneration as well as the expression of genes directly dependent on TGF-beta signaling are down-regulated. These data directly implicate TGF-beta signaling in the initiation and control of the regeneration process in axolotls.     

Transforming Growth Factor: β Signaling Is Essential for Limb Regeneration in Axolotls

Axolotls (urodele amphibians) have the unique ability, among vertebrates, to perfectly regenerate many parts of their body including limbs, tail, jaw and spinal cord following injury or amputation. The axolotl limb is the most widely used structure as an experimental model to study tissue regeneration. The process is well characterized, requiring multiple cellular and molecular mechanisms. The preparation phase represents the first part of the regeneration process which includes wound healing, cellular migration, dedifferentiation and proliferation. The redevelopment phase represents the second part when dedifferentiated cells stop proliferating and redifferentiate to give rise to all missing structures. In the axolotl, when a limb is amputated, the missing or wounded part is regenerated perfectly without scar formation between the stump and the regenerated structure. Multiple authors have recently highlighted the similarities between the early phases of mammalian wound healing and urodele limb regeneration. In mammals, one very important family of growth factors implicated in the control of almost all aspects of wound healing is the transforming growth factor-beta family (TGF-β). In the present study, the full length sequence of the axolotl TGF-β1 cDNA was isolated. The spatio-temporal expression pattern of TGF-β1 in regenerating limbs shows that this gene is up-regulated during the preparation phase of regeneration. Our results also demonstrate the presence of multiple components of the TGF-β signaling machinery in axolotl cells. By using a specific pharmacological inhibitor of TGF-β type I receptor, SB-431542, we show that TGF-β signaling is required for axolotl limb regeneration. Treatment of regenerating limbs with SB-431542 reveals that cellular proliferation during limb regeneration as well as the expression of genes directly dependent on TGF-β signaling are down-regulated. These data directly implicate TGF-β signaling in the initiation and control of the regeneration process in axolotls.     

Expression and role of retinoic acid receptor alpha in lens regeneration

The role of retinoids in eye development has been well studied. Retinoids and their receptors regulate gene expression and morphogenesis of the eye. In this study, a highly specific antagonist of retinoic acid receptor (RAR)-alpha was used in an attempt to study its function in lens regeneration. It was found that this antagonist inhibited lens regeneration and lens fiber differentiation. It was also shown that RAR-alpha is expressed in the lens during the process of regeneration. These results indicate that different RAR might have unique as well as redundant effects and patterns of expression in the regenerating lens.     

In vitro and in vivo interactions between nuclear receptors at estrogen response elements

To study mechanisms involved in the antiestrogenic effect of retinoic acid (RA), previously described in mammalian cells, we used in vitro and in vivo approaches. One hypothesis was direct competition between nuclear receptors (ER, RAR and RXR) at the DNA level. We first showed in vitro that the RAR/RXR heterodimer could weakly bind an ERE and that retinoid receptors reduced binding of ER to an ERE. We next checked whether, in yeast, direct competition between receptors that recognize the same responsive element could be monitored in a reconstituted heterologous estrogen-responsive system, by determining the expression of a reporter gene. We then co-transformed RAR and RXR in an estrogenic responsive strain. This model demonstrated that, even though RAR/RXR was able to bind an ERE, the addition of retinoic acid had no inhibitory effect on estrogen-induced responses in this yeast system, unlike in mammalian cells. Interference between these receptors should require other factors than interactions at the ERE level. This model could be used to identify mammalian factors interacting with estrogen and retinoic acid receptors which could play a role in crosstalk between these receptors.