Expression of integrins during axolotl limb regeneration

ECM-mediated positional cues are able to induce pattern, but not new positional information, during axolotl limb regeneration

The Mexican Axolotl is able to regenerate missing limb structures in any position along the limb axis throughout its life and serves as an excellent model to understand the basic mechanisms of endogenous regeneration. How the new pattern of the regenerating axolotl limb is established has not been completely resolved. An accumulating body of evidence indicates that pattern formation occurs in a hierarchical fashion, which consists of two different types of positional communications. The first type (Type 1) of communication occurs between connective tissue cells, which retain memory of their original pattern information and use this memory to generate the pattern of the regenerate. The second type (Type 2) of communication occurs from connective tissue cells to other cell types in the regenerate, which don’t retain positional memory themselves and arrange themselves according to these positional cues. Previous studies suggest that molecules within the extracellular matrix (ECM) participate in pattern formation in developing and regenerating limbs. However, it is unclear whether these molecules play a role in Type 1 or Type 2 positional communications. Utilizing the Accessory Limb Model, a regenerative assay, and transcriptomic analyses in regenerates that have been reprogrammed by treatment with Retinoic Acid, our data indicates that the ECM likely facilities Type-2 positional communications during limb regeneration.

Hierarchical pattern formation during amphibian limb regeneration

In 1901 T.H. Morgan proposed in "Regeneration" that pattern formation in amphibian limb regeneration is a stepwise process. Since, biologist have continued to piece together the molecular components of this process to better understand the "patterning code" responsible for regenerate formation. Within this context, several different models have been proposed; however, all are based on one of two underlying hypotheses. The first is the "morphogen hypothesis" that dictates that pattern emerges from localized expression of signaling molecules, which produce differing position-specific cellular responses in receptive cells depending on the intensity of the signal. The second hypothesis is that cells in the remaining tissues retain memory of their patterning information, and use this information to generate new cells with the missing positional identities. A growing body of evidence supports the possibility that these two mechanisms are not mutually exclusive. Here, we propose our theory of hierarchical pattern formation, which consists of 4 basic steps. The first is the existence of cells with positional memory. The second is the communication of positional information through cell-cell interactions in a regeneration-permissive environment. The third step is the induction of molecular signaling centers. And the last step is the interpretation of these signals by specialized cell types to ultimately restore the limb in its entirety. Biological codes are intertwined throughout this model, and we will discuss their multiple roles and mechanisms.

Blastema induction in aneurogenic state and Prrx-1 regulation by MMPs and FGFs in Ambystoma mexicanum limb regeneration

Urodele amphibians can regenerate amputated limbs. It has been considered that differentiated dermal tissues generate multipotent and undifferentiated cells called blastema cells during limb regeneration. In early phases of limb regeneration, blastema cells are induced by nerves and the apical epithelial cap (AEC). We had previously investigated the role of neurotrophic factors in blastema or blastema-like formation consisting of Prrx-1 positive cells. A new system suitable for investigating early phases of limb regeneration, called the accessory limb model (ALM), was recently developed. In this study, we performed a comparative transcriptome analysis between a blastema and wound using ALM. Matrix metalloproteinase (MMP) and fibroblast growth factor (FGF) signaling components were observed to be predominantly expressed in ALM blastema cells. Furthermore, we found that MMP activity induced a blastema marker gene, Prrx-1, in vitro, and FGF signaling pathways worked in coordination to maintain Prrx-1 expression and ALM blastema formation. Furthermore, we demonstrated that these two activities were sufficient to induce an ALM blastema in the absence of a nerve in vivo.

Cancer and the complement cascade

Despite significant research on the role of inflammation and immunosurveillance in the immunologic microenvironment of tumors, little attention has been given to the oncogenic capabilities of the complement cascade. The recent finding that complement may contribute to tumor growth suggests an insidious relationship between complement and cancer, especially in light of evidence that complement facilitates cellular proliferation and regeneration. We address the hypothesis that complement proteins promote carcinogenesis and suggest mechanisms by which complement can drive the fundamental features of cancer. Evidence shows that this diverse family of innate immune proteins facilitates dysregulation of mitogenic signaling pathways, sustained cellular proliferation, angiogenesis, insensitivity to apoptosis, invasion and migration, and escape from immunosurveillance. Given that the traditionally held functions for the complement system include innate immunity and cancer defense, our review suggests a new way of thinking about the role of complement proteins in neoplasia.

Ion imaging during axolotl tail regeneration in vivo

Several studies have reported that endogenous ion currents are involved in a wide range of biological processes from single cell and tissue behavior to regeneration. Various methods are used to assess intracellular and local ion dynamics in biological systems, e.g., patch clamping and vibrating probes. Here, we introduce an approach to detect ion kinetics in vivo using a noninvasive method that can electrophysiologically characterize an entire experimental tissue region or organism. Ion-specific vital dyes have been successfully used for live imaging of intracellular ion dynamics in vitro. Here, we demonstrate that cellular pH, cell membrane potential, calcium, sodium and potassium can be monitored in vivo during tail regeneration in the axolotl (Ambystoma mexicanum) using ion-specific vital dyes. Thus, we suggest that ion-specific vital dyes can be a powerful tool to obtain electrophysiological data during crucial biological events in vivo.

Up-regulation of oligodendrocyte precursor cell alphaV integrin and its extracellular ligands during central nervous system remyelination

To determine the role of extracellular matrix molecules and their integrin ligands in CNS remyelination, we have examined in experimentally induced focal demyelinated lesions the expression of the two classes of integrins implicated in oligodendrocyte development and myelination: alpha6 laminin-binding integrins and alphaV integrins that bind a range of extracellular matrix proteins containing the -Arg-Gly-Asp- (RGD) recognition sequence. Only alphaV integrins were up-regulated during remyelination, being expressed on oligodendrocyte precursor cells during their recruitment into the lesion. Next, therefore, we examined the expression of extracellular matrix ligands for alphaV integrins and documented increased expression of tenascin-C, tenascin-R, fibronectin, and vitronectin. Taken together with our previous discovery of high levels of expression of another alphaV ligand, osteopontin, during remyelination in these lesions, our findings suggest that alphaV integrins make an important contribution to successful repair in the CNS.

Expression of the Third Component of Complement, C3, in Regenerating Limb Blastema Cells of Urodeles

In this study we have shown that complement component C3 is expressed in the regenerating tissue during urodele limb regeneration. C3 was expressed in the dedifferentiated regeneration blastema and in the redifferentiated limb tissues in the axolotl, Amblystoma mexicanum, and in Notophthalmus viridescens. This expression was verified by immunofluorescent staining using an Ab against axolotl C3 and by in situ hybridization with an axolotl C3 cDNA probe. In the early stages of regeneration C3 appeared to be equally present in all mesenchymal cells and in the wound epithelium, whereas in the later stages it was mainly expressed in the differentiating muscle cells. Since no expression was seen in the developing limb, it appears that the C3 expression was specific to the regeneration process. We then demonstrated by hybridization experiments that a blastema cell line of myogenic origin expresses C3. All these findings implicate C3 in the dedifferentiation process and may indicate a new role for this molecule in muscle differentiation.

Retinoic acid and thyroid hormone may function through similar and competitive pathways in regenerating axolotls

The objective of this study was to determine whether thyroid hormone (TH) would interfere with retinoic acid (RA), which proximalizes axolotl larvae regenerate limb pattern. RA and TH are ligands for members of the steroid hormone thyroid hormone nuclear binding protein superfamily which form functional homodimers, but may also form stable heterodimers with the RXR protein and may recognize identical DNA sequences. TH alone does not affect limb pattern but induces metamorphosis in regenerating animals. Coinjected animals do not metamorphose, and when compared to RA controls regenerate more proximal and in some cases anteroposterior (AP) and dorsoventral (DV) duplicate limb structures. In addition, the tissues that are normally lost or changed during metamorphosis appear to be sensitized resulting in the formation of (1) new dorsal gill lamellae accompanied by bifurcation and broadening of the original gill lamellae, (2) partial resorption of the tail fin, and (3) changes in eye position and snout morphology. Bifurcation of gill lamellae tips, but not the formation of supernumerary gills, is also observed in animals treated with RA alone. These results indicate that the molecular mechanism of RA and TH function through similar and perhaps competitive pathways.