Activation of the WNT-BMP-FGF Regulatory Network Induces the Onset of Cell Death in Anterior Mesodermal Cells to Establish the ANZ.

The spatiotemporal control of programmed cell death (PCD) plays a significant role in sculpting the limb. In the early avian limb bud, the anterior necrotic zone (ANZ) and the posterior necrotic zone are two cell death regions associated with digit number reduction. In this study, we evaluated the first events triggered by the FGF, BMP, and WNT signaling interactions to initiate cell death in the anterior margin of the limb to establish the ANZ. This study demonstrates that in a period of two to 8 h after the inhibition of WNT or FGF signaling or the activation of BMP signaling, cell death was induced in the anterior margin of the limb concomitantly with the regulation of Dkk, Fgf8, and Bmp4 expression. Comparing the gene expression profile between the ANZ and the undifferentiated zone at 22HH and 25HH and between the ANZ of 22HH and 25HH stages correlates with functional programs controlled by the regulatory network FGF, BMP, and WNT signaling in the anterior margin of the limb. This work provides novel insights to recognize a negative feedback loop between FGF8, BMP4, and DKK to control the onset of cell death in the anterior margin of the limb to the establishment of the ANZ.

Irx1 and Irx2 are coordinately expressed and regulated by retinoic acid, TGFß and FGF signaling during chick hindlimb development.

The Iroquois homeobox (Irx) genes play a crucial role in the regionalization and patterning of tissues and organs during metazoan development. The Irx1 and Irx2 gene expression pattern during hindlimb development has been investigated in different species, but its regulation during hindlimb morphogenesis has not been explored yet. The aim of this study was to evaluate the gene expression pattern of Irx1 and Irx2 as well as their regulation by important regulators of hindlimb development such as retinoic acid (RA), transforming growth factor ß (TGFß) and fibroblast growth factor (FGF) signaling during chick hindlimb development. Irx1 and Irx2 were coordinately expressed in the interdigital tissue, digital primordia, joints and in the boundary between cartilage and non-cartilage tissue. Down-regulation of Irx1 and Irx2 expression at the interdigital tissue coincided with the onset of cell death. RA was found to down-regulate their expression by a bone morphogenetic protein-independent mechanism before any evidence of cell death. Furthermore, TGFß protein regulated Irx1 and Irx2 in a stage-dependent manner at the interdigital tissue, it inhibited their expression when it was administered to the interdigital tissue at developing stages before their normal down-regulation. TGFß administered to the interdigital tissue at developing stages after normal down-regulation of Irx1 and Irx2 evidenced that expression of these genes marked the boundary between cartilage tissue and non-cartilage tissue. It was also found that at early stages of hindlimb development FGF signaling inhibited the expression of Irx2. In conclusion, the present study demonstrates that Irx1 and Irx2 are coordinately expressed and regulated during chick embryo hindlimb development as occurs in other species of vertebrates supporting the notion that the genomic architecture of Irx clusters is conserved in vertebrates.

Molecular control of cell differentiation and programmed cell death during digit development.

During the hand plate development, the processes of cell differentiation and control of cell death are relevant to ensure a correct shape of the limb. The progenitor cell pool that later will differentiate into cartilage to form the digits arises from undifferentiated mesenchymal cells beneath the apical ectodermal ridge (AER). Once these cells abandon the area of influence of signals from AER and ectoderm, some cells are committed to chondrocyte lineage forming the digital rays. However, if the cells are not committed to chondrocyte lineage, they will form the prospective interdigits that in species with free digits will subsequently die. In this work, we provide the overview of the molecular interactions between different signaling pathways responsible for the formation of digit and interdigit regions. In addition, we briefly describe some experiments concerning the most important signals responsible for promoting cell death. Finally, on the basis that the interdigital tissue has chondrogenic potential, we discuss the hypothesis that apoptotic-promoting signals might also act as antichondrogenic factors and chondrogenic factors might operate as anti-apoptotic factors.

Smad8 is expressed in the anterior necrotic zone: evidence for a role of bone morphogenetic proteins/SMAD signaling in the activation of a molecular cascade that culminates in cell death.

Bone morphogenetic proteins (BMPs) play a crucial role in programmed cell death (PCD), a biological process required for the sculpturing of the embryonic limbs. However, it is unknown if BMP signaling directly promotes cell death, or if it induces a molecular cascade that culminates in cell death. Given that Smad8, which encodes one component of BMP signaling, is expressed during the regression of interdigital tissue and responds to BMPs, we presumed that it may be expressed in other cell death areas during chick limb development such as the anterior and posterior necrotic zones (ANZ and PNZ). The present study found that the Smad8 expression pattern in the anterior mesoderm of the hindlimb is very similar to that observed in limbs stained to detect cell death. Also, BMPs and retinoic acid, which act as apoptosis-promoting factors, induced expression of Smad8 before the onset of cell death, while sonic hedgehog protein, acting as a survival factor, inhibited Smad8 expression in the ANZ. However, although there was correlation between Smad8 expression patterns and PCD in the ANZ, phosphorylated forms of SMAD1/5/8 and TUNEL staining did not co-localize in dying cells. Interestingly, a short pulse of BMP was sufficient to trigger cell death. On the other hand, most dying cells were located in the avascular region, while many cells expressing Smad8 were located in the vascular region of the ANZ. These results suggest that BMPs mediated by SMAD signaling activate a molecular cascade that culminates in PCD.

Theoretical study of [Ni (H2O)n]2+(H2O)m (n ≤ 6, m ≤ 18).

The [Ni-(H(2)O)(n)](2+)(H(2)O)(m) (n ≤ 6, m ≤ 18) complexes were studied by means of first-principles all-electron calculations performed with the BPW91 gradient corrected functional and the 6-311+G(d,p) basis sets for the H, O, and Ni atoms. Triplet states were found as low-lying states for each (n, m) combination. The estimated Ni(2+)-(H(2)O)(n) binding energies (112.8-57.4 kcal/mol for the first layer and 52.0-23.0 kcal/mol for the second one) decreases and the Ni(2+)-OH(2) bond lengths lengthen as n + m increases. With six H(2)O moieties the Ni(2+) ion furnishes its first coordination sphere of octahedral geometry. Further water addition renders the formation of the second layer. The effect of Ni(2+) on the (H(2)O)(n)···(H(2)O)(m) hydrogen bond formation for several "n" and "m" combinations was studied, revealing an enhancement of this kind of bonding, which is of key importance for the stabilization and growth of the clusters. For some n + m isomers the second layer appears before the first octahedral layer is fully formed. For example, the square planar Ni(2+)-(H(2)O)(4) core originates two-dimensional 4 + 2 and 4 + 4 isomers, where each outer water molecule accepts two H-bonds, lying 2.0 kcal/mol above the 6 and 6 + 2 ground states. The clusters were also studied by IR spectra; the OH stretching vibrational frequencies allowed us to identify the outer solvation shells by the presence of red-shifted hydrogen bond regions.

Expression of MIG-6, WNT-9A, and WNT-7B during osteoarthritis.

Although the molecular mechanisms for initiation of cartilage destruction in osteoarthritis (OA) are unknown, it has been demonstrated that disruption of mitogen-inducible gene 6 (Mig-6) in mice leads to the onset of a degenerative joint disease like OA. On this basis, we correlated gene expression of Mig-6 with Wnt-9a and Wnt-7b genes; we showed downregulation of Mig-6, Wnt-7b, and Wnt-9a during OA, while Wnt-7b was expressed also in osteoblast-like cells. Here we suggest that Aggrecan degradation occurs before the downregulation of Mig-6. It remains to be proven whether there is any relation between Wnt signaling and Aggrecan degradation.

PI3K is involved in PDGF-beta receptor upregulation post-PDGF-BB treatment in mouse HSC.

Increased expression of PDGF-beta receptors is a landmark of hepatic stellate cell activation and transdifferentiation into myofibroblasts. However, the molecular mechanisms that regulate the fate of the receptor are lacking. Recent studies suggested that N-acetylcysteine enhances the extracellular degradation of PDGF-beta receptor by cathepsin B, thus suggesting that the absence of PDGF-beta receptors in quiescent cells is due to an active process of elimination and not to a lack of expression. In this communication we investigated further molecular mechanisms involved in PDGF-beta receptor elimination and reappearance after incubation with PDGF-BB. We showed that in culture-activated hepatic stellate cells there is no internal protein pool of receptor, that the protein is maximally phosphorylated by 5 min and completely degraded after 1 h by a lysosomal-dependent mechanism. Inhibition of receptor autophosphorylation by tyrphostin 1296 prevented its degradation, but several proteasomal inhibitors had no effect. We also showed that receptor reappearance is time and dose dependent, being more delayed in cells treated with 50 ng/ml (48 h) compared with 10 ng/ml (24 h).

Evidence for epithelial-mesenchymal transitions in adult liver cells.

Both myofibroblastic hepatic stellate cells (HSC) and hepatic epithelial progenitors accumulate in damaged livers. In some injured organs, the ability to distinguish between fibroblastic and epithelial cells is sometimes difficult because cells undergo epithelial-mesenchymal transitions (EMT). During EMT, cells coexpress epithelial and mesenchymal cell markers. To determine whether EMT occurs in adult liver cells, we analyzed the expression profile of primary HSC, two HSC lines, and hepatic epithelial progenitors. As expected, all HSC expressed HSC markers. Surprisingly, these markers were also expressed by epithelial progenitors. In addition, one HSC line expressed typical epithelial progenitor mRNAs, and these epithelial markers were inducible in the second HSC line. In normal and damaged livers, small ductular-type cells stained positive for an HSC marker. In conclusion, HSC and hepatic epithelial progenitors both coexpress epithelial and mesenchymal markers, providing evidence that EMT occurs in adult liver cells.

Role for hedgehog signaling in hepatic stellate cell activation and viability.

Hepatic stellate cells (HSC) have a complex phenotype that includes both neural and myofibroblastic features. The Hedgehog (Hh) pathway has been shown to direct the fate of neural and myofibroblastic cells during embryogenesis and during tissue remodeling in adults. Therefore, we hypothesized that Hh signaling may regulate the fate of HSC in adults. In this study, we find that freshly isolated stellate cells from adult Patched-lacZ transgenic mice exhibit beta-galactosidase activity, indicating Hh pathway activity. Transcripts of Hh ligands, the Hh pathway receptor, and Hh-regulated transcription factors are expressed by stellate cells from mice, rats, and humans. Transfection experiments in a cell line using a Hh-inducible luciferase reporter demonstrate constitutive Hh pathway activity. Moreover, neutralizing antibodies to Hh increase apoptosis, while viability is restored by treatment with Hh ligand. In vitro treatment of primary stellate cells with cyclopamine (Cyc), a pharmacologic inhibitor of the Hh pathway, inhibits activation and slightly decreases cell survival, while a single injection of Cyc into healthy adult mice reduces activation of HSC by more than 50% without producing obvious liver damage. Our findings reveal a novel mechanism, namely the Hh pathway, that regulates the activation and viability of HSC.