Urodele amphibians have the remarkable ability to re-grow lost body parts. This regenerative response after injury in urodeles involves dedifferentiation of fully differentiated cells into proliferative cells. One well-studied example of this is the dedifferentiation of multinucleated muscle cells into mononucleate cells resembling their precursors, the myoblasts. To form these mononucleate cells the differentiated myotubes in vivo must re-enter and complete the cell cycle; they again proliferate and produce progeny. A key question is what factors induce the myotubes to re-enter the cell cycle and proliferate. Early events of cell cycle re-entry can be studied in the A1 cell line, a myogenic cell line isolated from the Notophthalmus viridescens hindlimb, which traverses cell cycle until G2 in response to serum. In particular, it was found that thrombin cleavage induces a factor in serum of all animals tested so far to promote S phase re-entry in A1 myotubes. We have used this S phase re-entry of the A1 cell line to purify the serum activity and developed a 5-step purification protocol that enriches the activity almost 2 000 fold over the starting material, or 40 000 fold over serum. To conveniently produce and test potential candidates for their ability to induce S phase re-entry in A1 myotubes, we also developed an overexpression- and purification system for emerging candidates. Candidates were then tested for this activity with or without prior incubation with thrombin. We identified Bmp proteins as the first pure molecules that were found in fractions across the purification of the activity and that could also induce cell cycle re-entry in a dose-dependent manner when recombinantly added to the A1 myotubes. Furthermore, this response could be blocked in a dose-dependent manner by the known bmp-inhibitor noggin. Finally, we showed that inhibition of Bmp signaling in vivo causes defects in axolotl tail regeneration.

Urodele amphibians have the remarkable ability to re-grow lost body parts. This regenerative response after injury in urodeles involves dedifferentiation of fully differentiated cells into proliferative cells. One well-studied example of this is the dedifferentiation of multinucleated muscle cells into mononucleate cells resembling their precursors, the myoblasts. To form these mononucleate cells the differentiated myotubes in vivo must re-enter and complete the cell cycle; they again proliferate and produce progeny. A key question is what factors induce the myotubes to re-enter the cell cycle and proliferate. Early events of cell cycle re-entry can be studied in the A1 cell line, a myogenic cell line isolated from the Notophthalmus viridescens hindlimb, which traverses cell cycle until G2 in response to serum. In particular, it was found that thrombin cleavage induces a factor in serum of all animals tested so far to promote S phase re-entry in A1 myotubes. We have used this S phase re-entry of the A1 cell line to purify the serum activity and developed a 5-step purification protocol that enriches the activity almost 2 000 fold over the starting material, or 40 000 fold over serum. To conveniently produce and test potential candidates for their ability to induce S phase re-entry in A1 myotubes, we also developed an overexpression- and purification system for emerging candidates. Candidates were then tested for this activity with or without prior incubation with thrombin. We identified Bmp proteins as the first pure molecules that were found in fractions across the purification of the activity and that could also induce cell cycle re-entry in a dose-dependent manner when recombinantly added to the A1 myotubes. Furthermore, this response could be blocked in a dose-dependent manner by the known bmp-inhibitor noggin. Finally, we showed that inhibition of Bmp signaling in vivo causes defects in axolotl tail regeneration.

Les expériences suivantes testent l’hypothèse que la sérine/thréonine kinase p38α/β MAPK inhibe la rentrée dans le cycle cellulaire des cardiomyocytes ventriculaires de rats nouveau-nés (CVRNs), et induit l’expression d’un panel de cytokines/chimiokines inflammatoires. Le traitement des CVRNs par le phorbol 12,13-butyrate (PDBu), activateur de la protéine kinase C (PKC), aboutit au recrutement de l’isoforme conventionnelle (PKC-α) et des isoformes nouvelles (PKC-δ et PKC-ε) de PKC en l’absence de la rentrée dans le cycle cellulaire. Cette absence d’entrée dans le cycle cellulaire à la suite du traitement par PDBu est associée à une augmentation d’expression des ARNm des gènes qui bloquent la rentrée dans le cycle cellulaire. Les gènes comprennent Runx1(Runt-related transcription factor 1) et CDKN2a (cyclin-dependent kinase inhibitor 2A) également connu sous le nom de p16, inhibiteur du cycle cellulaire. En présence de l’inhibiteur de p38α/β MAPK, SB203580, le traitement PDBu induit une entrée dans le cycle cellulaire de deux populations distinctes de cardiomyocytes caractérisées par l’absence ou l’expression de novo de la protéine filamenteuse Nestine. En parallèle, le co-traitement PDBu/SB203580 atténue l’augmentation du niveau d’expression de l’ARNm de Runx1 et CDKN2a. L’inhibition pharmacologique du recrutement de PKC-α par GF109203X, inhibe sélectivement la rentrée dans le cycle cellulaire des CVRNs qui présentent une expression de novo de Nestine. En parallèle, le traitement par PDBu augmente le niveau d’ARNm d’un panel de cytokines inflammatoires et la co-administration de SB203580 inhibe cette réponse. Ces données révèlent que le cœur des rats nouveau-nés contient deux sous-populations distinctes de cardiomyocytes ventriculaires qui rentrent dans le cycle cellulaire à la suite d’un co-traitement PDBu / SB203580, et que la réponse proliférative est associée à une diminution des cytokines inflammatoires. Collectivement, ces résultats mettent en relief une nouvelle prémisse selon laquelle le recrutement de p38α/β MAPK médié par PKC-α joue un rôle central dans l’inhibition de l’entrée dans le cycle cellulaire et induit une réponse inflammatoire robuste par les CRVNs. / The following experiments test the hypothesis that the serine/threonine kinase p38α/β MAPK inhibits the cell cycle re-entry of neonatal rat ventricular cardiomyocytes (NNVMs) and induces the expression of a panel of inflammatory cytokines/chemokines. Treatment of NNVMs with phorbol 12,13-butyrate (PDBu), an activator of protein kinase C (PKC), results in the recruitment of the conventional isoform (PKC-α) and novel isoforms (PKC-δ and PKC-ε) of PKC in the absence of cell cycle re-entry. This lack of cell cycle re-entry following PDBu treatment is associated with an increase in the expression of mRNA of genes that inhibit cell cycle re-entry. These genes include Runx1 (Runt-related transcription factor 1) and CDKN2a (cyclin-dependent kinase inhibitor 2A), also known as p16, a cell cycle inhibitor. In the presence of the p38α/β MAPK inhibitor, SB203580, PDBu treatment induces cell cycle re-entry in two distinct populations of cardiomyocytes characterized by the absence or de novo expression of the filamentous protein Nestin. In parallel, co-treatment with PDBu/SB203580 attenuates the increase in Runx1 and CDKN2a mRNA levels. Pharmacological inhibition of PKC-α recruitment by GF109203X selectively inhibits cell cycle re-entry of NNVMs exhibiting de novo Nestin expression. Additionally, PDBu treatment increases the mRNA levels of a panel of inflammatory cytokines, and co-administration of SB203580 inhibits this response. These data reveal that the heart of neonatal rats contain two distinct subpopulations of ventricular cardiomyocytes that re-enter the cell cycle following PDBu/SB203580 co-treatment, and that the proliferative response is associated with a decrease in inflammatory cytokines. Collectively, these results highlight a novel premise whereby p38α/β MAPK recruitment mediated by PKC-α plays a central role in inhibiting cell cycle re-entry and induces a robust inflammatory response by NNVMs.

In contrast to mammals, some fish and amphibians have retained the ability to regenerate complex body structures or organs, such as the limb, the tail, the eye lens or even parts of the heart. One major difference in the response to injury is the appearance of a mesenchymal growth zone or blastema in these regenerative species instead of the scarring seen in mammals. This blastema is thought to largely derive from the dedifferentiation of various functional cell types, such as skeletal muscle, skin and cartilage. In the case of multinucleated skeletal muscle fibres, cell cycle re-entry into S-phase as well as fragmentation into mononucleated progenitors is observed both in vitro and in vivo. In order to identify molecules that initiate dedifferentiation of cells at the wound site in amphibians we have established a cellular assay with a cultured newt myogenic cell line. Using this assay we have found a serum activity that stimulates cell cycle re-entry in differentiated multinucleated newt myotubes. The activity is present in serum of all mammalian species tested so far and, interestingly, thrombin proteolysis amplifies the activity from both serum and plasma. We think this serum factor provides a link between wounding and regeneration and its identification will be a key step in understanding the remarkable differences in wound healing between mammals and amphibians. In the course of this PhD thesis we have characterized the serum factor as a thermo-labile, pH- and proteinase K-sensitive, high molecular weight protein that is resistant to denaturing conditions such as SDS, urea or organic solvents. Surprisingly, under denaturing conditions the activity behaves as a low molecular weight protein that displays charge heterogeneity on isoelectric focusing. Using these characteristics of the serum factor we have performed a systematic investigation of commonly used protein chromatography modes and separation techniques to develop a successful purification procedure. After four column chromatography steps -- cation exchange, hydrophobic interaction, heparin affinity and size exclusion chromatography under denaturing conditions -- we have achieved a 2,000-fold purification starting from a commercially available Crude Bovine Thrombin preparation. This represents about 40,000-fold purification over bovine serum. Silver stained gels of the most purified fractions revealed ten major protein bands. In order to finally identify the cell cycle re-entry factor, we are currently analyzing the purification by quantitative mass spectrometry by correlating the abundance of tryptic peptides with activity in sequential fractions across a chromatography run.

In contrast to mammals, some fish and amphibians have retained the ability to regenerate complex body structures or organs, such as the limb, the tail, the eye lens or even parts of the heart. One major difference in the response to injury is the appearance of a mesenchymal growth zone or blastema in these regenerative species instead of the scarring seen in mammals. This blastema is thought to largely derive from the dedifferentiation of various functional cell types, such as skeletal muscle, skin and cartilage. In the case of multinucleated skeletal muscle fibres, cell cycle re-entry into S-phase as well as fragmentation into mononucleated progenitors is observed both in vitro and in vivo. In order to identify molecules that initiate dedifferentiation of cells at the wound site in amphibians we have established a cellular assay with a cultured newt myogenic cell line. Using this assay we have found a serum activity that stimulates cell cycle re-entry in differentiated multinucleated newt myotubes. The activity is present in serum of all mammalian species tested so far and, interestingly, thrombin proteolysis amplifies the activity from both serum and plasma. We think this serum factor provides a link between wounding and regeneration and its identification will be a key step in understanding the remarkable differences in wound healing between mammals and amphibians. In the course of this PhD thesis we have characterized the serum factor as a thermo-labile, pH- and proteinase K-sensitive, high molecular weight protein that is resistant to denaturing conditions such as SDS, urea or organic solvents. Surprisingly, under denaturing conditions the activity behaves as a low molecular weight protein that displays charge heterogeneity on isoelectric focusing. Using these characteristics of the serum factor we have performed a systematic investigation of commonly used protein chromatography modes and separation techniques to develop a successful purification procedure. After four column chromatography steps -- cation exchange, hydrophobic interaction, heparin affinity and size exclusion chromatography under denaturing conditions -- we have achieved a 2,000-fold purification starting from a commercially available Crude Bovine Thrombin preparation. This represents about 40,000-fold purification over bovine serum. Silver stained gels of the most purified fractions revealed ten major protein bands. In order to finally identify the cell cycle re-entry factor, we are currently analyzing the purification by quantitative mass spectrometry by correlating the abundance of tryptic peptides with activity in sequential fractions across a chromatography run.