Abnormal limb regeneration in the short-toes mutant of the axolotl,Ambystoma mexicanum: Studies of age, level of amputation, and extracellular matrix

A histological study of normal and pathological limb regeneration in the Mexican axolotl Ambystoma mexicanum

Salamanders show unparalleled capacities of tissue regeneration amongst tetrapods (four-legged vertebrates), being able to repair and renew lost or damage body parts, such as tails, jaws, and limbs in a seemingly perfect fashion. Despite countless studies on axolotl (Ambystoma mexicanum) regeneration, only a few studies have thus far compared gross morphological and histological features of the original and regenerated limb skeleton. Therein, most studies have focused on nerves or muscles, while even fewer have provided detailed information about bones and cartilage. This study compares skeletal tissue structures of original and regenerated limbs with respect to tissue level histology. Histological serial sections of 55 axolotl larvae were generated, including 29 limbs that were severed by conspecifics, and 26 that were subject to targeted amputations. Amputations were executed in several larval stages (48, 52, and 53) and at different limb positions (humeral midshaft, above the mesopod). In addition, 3D reconstructions were prepared based on X-ray microtomography scans. The results demonstrate that regenerated forelimbs show a diversity of limb and digit abnormalities as a result of imperfect regeneration. Furthermore, abnormalities were more severe and more frequent in regenerated forelimbs caused by natural bites as compared with regenerated forelimbs after amputation. The results indicate that abnormalities occur frequently after regeneration in larval axolotls contradicting the notion of regeneration generally resulting in perfect limbs.

Multiplex CRISPR/Cas screen in regenerating haploid limbs of chimeric Axolotls

Axolotls and other salamanders can regenerate entire limbs after amputation as adults, and much recent effort has sought to identify the molecular programs controlling this process. While targeted mutagenesis approaches like CRISPR/Cas9 now permit gene-level investigation of these mechanisms, genetic screening in the axolotl requires an extensive commitment of time and space. Previously, we quantified CRISPR/Cas9-generated mutations in the limbs of mosaic mutant axolotls before and after regeneration and found that the regenerated limb is a highfidelity replicate of the original limb (Flowers et al. 2017). Here, we circumvent aforementioned genetic screening limitations and present methods for a multiplex CRISPR/Cas9 haploid screen in chimeric axolotls (MuCHaChA), which is a novel platform for haploid genetic screening in animals to identify genes essential for limb regeneration.

iTRAQ reveals proteomic changes during intestine regeneration in the sea cucumber Apostichopus japonicus

Sea cucumbers have a striking capacity to regenerate most of their viscera after evisceration, which has drawn the interest of many researchers. In this study, the isobaric tag for relative and absolute quantitation (iTRAQ) was utilized to investigate protein abundance changes during intestine regeneration in sea cucumbers. A total of 4073 proteins were identified, and 2321 proteins exhibited significantly differential expressions, with 1100 upregulated and 1221 downregulated proteins. Our results suggest that intestine regeneration constitutes a complex life activity regulated by the cooperation of various biological processes, including cytoskeletal changes, extracellular matrix (ECM) remodeling and ECM-receptor interactions, protein synthesis, signal recognition and transduction, energy production and conversion, and substance transport and metabolism. Additionally, real-time PCR showed mRNA expression of differentially expressed genes correlated positively with their protein levels. Our results provided a basis for studying the regulatory mechanisms associated with sea cucumber regeneration.

Scar-free wound healing and regeneration in amphibians: immunological influences on regenerative success

Salamanders and frogs are distinct orders of Amphibians with very different immune systems during adult life, exhibiting varying potential for scar free repair and regeneration. While salamanders can regenerate a range of body parts throughout all stages of life, regeneration is restricted to early stages of frog development. Comparison of these two closely related amphibian orders provides insights into the immunological influences on wound repair, and the different strategies that have evolved either to limit infection or to facilitate efficient regeneration. After injury, cells of the immune system are responsible for the removal of damaged cells and providing a cohort of important growth factors and signaling molecules. Immune cells not only regulate new vessel growth important for supplying essential nutrients to damaged tissue but, modulate the extracellular matrix environment by regulating fibroblasts and the scarring response. The profile of immune cell infiltration and their interaction with local tissue immune cells directly influences many aspects of the wound healing outcomes and can facilitate or prevent regeneration. Evidence is emerging that the transition from wound healing to regeneration is reliant on immune cell engagement and that the success of regeneration in amphibians may depend on complex interactions between stem cell progenitors and immune cell subsets. The potential immunological barriers to mammalian regeneration are discussed with implications for the successful delivery of stem cell therapeutic strategies in patients.

Glycated and carbamylated albumin are more "nephrotoxic" than unmodified albumin in the amphibian kidney

There is increasing evidence that proteins in tubular fluid are "nephrotoxic." In vivo it is difficult to study protein loading of tubular epithelial cells in isolation, i.e., without concomitant glomerular damage or changes of renal hemodynamics, etc. Recently, a unique amphibian model has been described which takes advantage of the special anatomy of the amphibian kidney in which a subset of nephrons drains the peritoneal cavity (open nephrons) so that intraperitoneal injection of protein selectively causes protein storage in and peritubular fibrosis around open but not around closed tubules. There is an ongoing debate as to what degree albumin per se is nephrotoxic and whether modification of albumin alters its nephrotoxicity. We tested the hypothesis that carbamylation and glycation render albumin more nephrotoxic compared with native albumin and alternative albumin modifications, e.g., lipid oxidation and lipid depletion. Preparations of native and modified albumin were injected into the axolotl peritoneum. The kidneys were retrieved after 10 days and studied by light microscopy as well as by immunohistochemistry [transforming growth factor (TGF)-β, PDGF, NF-κB, collagen I and IV, RAGE], nonradioactive in situ hybridization, and Western blotting. Two investigators unaware of the animal groups evaluated and scored renal histology. Compared with unmodified albumin, glycated and carbamylated albumin caused more pronounced protein storage. After no more than 10 days, selective peritubular fibrosis was seen around nephrons draining the peritoneal cavity (open nephrons), but not around closed nephrons. Additionally, more intense expression of RAGE, NF-κB, as well as PDGF, TGF-β, EGF, ET-1, and others was noted by histochemistry and confirmed by RT-PCR for fibronectin and TGF-β as well as nonradioactive in situ hybridization for TGF-β and fibronectin. The data indicate that carbamylation and glycation increase the capacity of albumin to cause tubular cell damage and peritubular fibrosis.

The short toes mutation of the axolotl

The axolotl mutant strain, short toes (s/s), can regenerate spinal cord and tail, but not limbs. This makes s/s potentially very useful for limb regeneration studies. This mutant merits a new examination that integrates the original description of the mutant, existing experimental studies, new data and current thinking about stem cells and regeneration. There are still major gaps in information about this mutant; the gene(s) causing the defects has not yet been discovered, and even the histological description is incomplete, especially regarding muscle abnormalities. In the short toes limb, MyHC (myosin heavy chain)-1, MyHC-2b and pax7 are down-regulated. In particular, all three MyHC genes and pax7 are highly expressed in the normal limb, but almost lost in the s/s limb. MyHC genes are one of the main components of skeletal muscle, and Pax7 is the skeletal muscle satellite cell marker. Histological experiments confirm that severe s/s has lost most skeletal muscle and myosin. These results suggest that skeletal muscle, which includes satellite cells, could play an important role in axolotl limb regeneration.

Extracellular matrix remodeling and metalloproteinase involvement during intestine regeneration in the sea cucumber Holothuria glaberrima

The sea cucumber, Holothuria glaberrima, has the capacity to regenerate its internal organs. Intestinal regeneration is accomplished by the thickening of the mesenteric border and the invasion of this thickening by mucosal epithelium from the esophagus and the cloaca. Extracellular matrix (ECM) remodeling has been associated with morphogenetic events during embryonic development and regeneration. We have used immunohistochemical techniques against ECM components to show that differential changes occur in the ECM during early regeneration. Labeling of fibrous collagenous components and muscle-related laminin disappear from the regenerating intestine and mesentery, while fibronectin labeling and 4G7 (an echinoderm ECM component) are continuously present. Western blots confirm a decrease in fibrous collagen content during the first 2 weeks of regeneration. We have also identified five 1,10-phenanthroline-sensitive bands in collagen gelatin zymographs. The gelatinolytic activities of these bands are enhanced during early stages of regeneration, suggesting that the metalloprotease activity is associated with ECM remodeling. Inhibition of MMPs in vivo with 1,10-phenanthroline, p-aminobenzoyl-Gly-Pro-D-Leu-D-Ala hydroxamate or N-CBZ-Pro-Leu-Gly hydroxamate produces a reversible inhibition of intestinal regeneration and ECM remodeling. Our results show that significant changes in ECM content occur during intestine regeneration in the sea cucumber and that the onset of these changes is correlated to the proteolytic activities of MMPs.

Gallium nitrate: effects on cartilage during limb regeneration in the axolotl, Ambystoma mexicanum

Gallium nitrate, a drug shown to have efficacy in Paget's disease of bone, hypercalcemia of malignancy, and a variety of experimental autoimmune diseases, also inhibits the growth of some types of cancer. We examined dose and timing of administration of gallium nitrate on limb regeneration in the Mexican axolotl, Ambystoma mexicanum. Administered by intraperitoneal injection, gallium nitrate inhibited limb regeneration in a dose-dependent manner. Gallium nitrate initially suppressed epithelial wound healing and subsequently distorted both anterior-posterior and proximo-distal chondrogenic patterns. Gallium nitrate given at three days after amputation severely inhibited regeneration at high doses (6.25 mg/axolotl) and altered the normal patterning of the regenerates at low doses (3.75 mg/axolotl). Administration of 6.25 mg of gallium nitrate at four or 14 days prior to amputation also inhibited regeneration. In amputated limbs of gallium-treated axolotls, the chondrocytes were lost from inside the radius/ulna. Limbs that regenerated after gallium treatment was terminated showed blastema formation preferentially over the ulna. New cartilage of the regenerate often attached to the sides of the existing radius/ulna proximally into the stump and less so to the distal cut ends. J. Exp. Zool. 293:384-394, 2002.

Growth control in the ontogenetic and regenerating zebrafish fin

The growth and regeneration of the zebrafish fin provide yet another opportunity to exploit genetics to study important vertebrate problems. Mutants have been identified in zebrafish that affect the development of the embryonic fin, disrupt the normal growth relationship of fin and body, or disrupt the regeneration of the fin. Analysis of a regeneration mutation suggests that the developmental checkpoints that ensure developmental integrity in normal growth are absent in the early stages of regeneration. These stages correspond to the only time in fish developmental when differentiated bone cells divide.