Third part. BLOOD OR YOUNG PLASMA AND REGENERATION OF THE CARDIAC MUSCLE (Factors Promoting Cardiogenesis in Vertebrates)

19.08.2018

................

Proteins of the WNT family are also known regulators of Cardiomyogenesis. WNTs can bind to Fz (Frizzled) receptors on target cells to activate different signaling pathways. Activation of the canonical pathway leads to stabilization of Ctnn-Beta (Catenin-Beta) through inactivation of GSK3Beta (Glycogen Synthase Kinase-3-Beta), to Ctnn-Beta -dependent activation of TCF (T Cell Factor)/ LEF (Lymphoid Enhancer Factor) transcription factors and induction of WNT-responsive genes. In contrast, WNT11 (Wingless-related MMTV integration site-11) signals through a Ctnn-Beta -independent non-canonical pathway involving PKC (Protein Kinase-C) and JNK (Jun-N-terminal kinase). WNTs induced Myogenic specification and mammalian Cardiac myogenesis. WNT3A (Wingless-related MMTV integration site-3A) upregulated early Cardiac markers in Mouse through Ctnn-Beta. Wnt11-induced Cardiac differentiation in Xenopus and in Murine Embryonic Cell. TAK1 and NLK (Nemo-Like Kinase) add to the list of components that can mediate a WNT signal. NLK can antagonize Ctnn-Beta dependent signaling, as well as act as a co-activator of this pathway. Different WNT signaling pathways act as an intertwined and partially cross-regulatory network. The secretion of WNT inhibitors (such as Cerberus, Dickkopf and Crescent) by the anterior endoderm prevents WNT3A and WNT8 (Wingless-related MMTV integration site-8) secreted by the neural tube from inhibiting Heart formation (Ref.11 & 12).

In Mouse, at least two lines of signaling act together to induce Cardiac differentiation: first, a Ca2+-dependent activation of Ca2+/ Calmodulin-dependent kinase, and second, a NOX4 (NADPH Oxidase-4)-dependent activation of p38MAPK(Mitogen-Activated Protein Kinase) through a moderate increase in ROS(Reactive Oxygen Species). These two signaling pathways converge at the level of MEF2C whose nuclear translocation requires the activation of both pathways. Although ROS play a critical role as intracellular signaling molecules, the molecular targets of ROS in Cardiomyocytes remain unclear, in particular during Cardiogenesis. Recently, it has been reported that NADPH-dependent ROS can also play a role in ESC during Cardiotrophin-1–induced Cardiac proliferation. NOX4-generated ROS leads to p38 phosphorylation and the subsequent MEF2C nuclear translocation. This crucial transcription factor is responsible for the activation of several Cardiac-specific embryonic genes, including MLC2v. MEF2C regulates the activity of MLC2v promoter. MEF2C is necessary for Cardiac phenotype determination, transcription of sarcomeric proteins, and myofibrillogenesis (Ref.1 & 13).

In addition to other transcription factors like GATAs , NKX2.5 and MEF2C, Tbx5 and Tbx20 also play an important role in Cardiac differentiation pathway. Tbx5 is a member of the T-box family of transcription factors, a family of proteins that are required for normal vertebrate patterning and differentiation. Human Tbx5 plays an important role in Heart development. Tbx5 is localized to the nucleus, it binds to DNA in a sequence specific fashion and it regulates the transcriptional level of its target genes. At least one of the targets of Tbx5 either directly or indirectly functions to control the progression of the embryonic cardiac cell cycle. Tbx5 could function to induce the expression of a Growth factor, for example EGF (Epidermal Growth Factor) or FGF (Fibroblast Growth Factor), which in turn is required for cell cycle progression. In the absence of this Growth factor the cell cycle may not proceed through to the completion of G1. Alternatively, Tbx5 also functions to regulate the expression of a key component of the pre-replication complex. In the absence of this key component the pre-replication complex may not assemble or may not load onto the ORCs (Origins of Replication), thus blocking DNA synthesis and hence cell cycle progression. Cell cycle progression through G1/S is regulated by members of the E2F (E2F Transcription Factor) family of transcription factors, the function of which is governed through their interaction with the Rb (Retinoblastoma) protein. In its hypophosphorylated state, Rb interacts with E2F, inhibiting its transcriptional activation activity. Upon Rb phosphorylation, the Rb-E2F interaction is disrupted and E2F is released and able to activate its downstream genes required for S-Phase entry. Thus, one function of Tbx5may be to indirectly regulate the state of Rb phosphorylation. Finally, Tbx5 may function to negatively regulate general cell cycle inhibitors such as p27Xic in Xenopus. Tbx5 depletion leads to a G1/S-phase arrest, which causes a dramatic increase in the expression of proteins associated with the Cardiac cell cycle S-phase, including CDC6 (CDC6 Cell Division Cycle-6), CcnE2 (Cyclin E2), SLBP (Stem-Loop (histone) Binding Protein) and PCNA (Proliferating Cell Nuclear Antigen). The initiation and the maintenance of a Cardiac program is the result of temporally and spatially well-orchestrated interactions between all the above factors and analyses of these interactions in animal models will be very helpful for the progress of cardiac stem cell research (Ref.14 & 15) .

References