Hyaluronate accumulation and nerve-dependent growth during regeneration of larval Ambystoma limbs

Remarkable metabolic reorganization and altered metabolic requirements in frog metamorphic climax

Background

Metamorphic climax is the crucial stage of amphibian metamorphosis responsible for the morphological and functional changes necessary for transition to a terrestrial habitat. This developmental period is sensitive to environmental changes and pollution. Understanding its metabolic basis and requirements is significant for ecological and toxicological research. Rana omeimontis tadpoles are a useful model for investigating this stage as their liver is involved in both metabolic regulation and fat storage.

Results

We used a combined approach of transcriptomics and metabolomics to study the metabolic reorganization during natural and T3-driven metamorphic climax in the liver and tail of Rana omeimontis tadpoles. The metabolic flux from the apoptotic tail replaced hepatic fat storage as metabolic fuel, resulting in increased hepatic amino acid and fat levels. In the liver, amino acid catabolism (transamination and urea cycle) was upregulated along with energy metabolism (TCA cycle and oxidative phosphorylation), while the carbohydrate and lipid catabolism (glycolysis, pentose phosphate pathway (PPP), and β-oxidation) decreased. The hepatic glycogen phosphorylation and gluconeogenesis were upregulated, and the carbohydrate flux was used for synthesis of glycan units (e.g., UDP-glucuronate). In the tail, glycolysis, β-oxidation, and transamination were all downregulated, accompanied by synchronous downregulation of energy production and consumption. Glycogenolysis was maintained in the tail, and the carbohydrate flux likely flowed into both PPP and the synthesis of glycan units (e.g., UDP-glucuronate and UDP-glucosamine). Fatty acid elongation and desaturation, as well as the synthesis of bioactive lipid (e.g., prostaglandins) were encouraged in the tail during metamorphic climax. Protein synthesis was downregulated in both the liver and tail. The significance of these metabolic adjustments and their potential regulation mechanism are discussed.

Conclusion

The energic strategy and anabolic requirements during metamorphic climax were revealed at the molecular level. Amino acid made an increased contribution to energy metabolism during metamorphic climax. Carbohydrate anabolism was essential for the body construction of the froglets. The tail was critical in anabolism including synthesizing bioactive metabolites. These findings increase our understanding of amphibian metamorphosis and provide background information for ecological, evolutionary, conservation, and developmental studies of amphibians.

Hyaluronic Acid and Regenerative Medicine: New Insights into the Stroke Therapy

Stroke is known as one of the very important public health problems that are related to societal burden and tremendous economic losses. It has been shown that there are few therapeutic approaches for the treatment of this disease. In this regard, the present therapeutic platforms aim to obtain neuroprotection, reperfusion, and neuro recovery. Among these therapies, regenerative medicine-based therapies have appeared as new ways of stroke therapy. Hyaluronic acid (HA) is a new candidate, which could be applied as a regenerative medicine-based therapy in the treatment of stroke. HA is a glycosaminoglycan composed of disaccharide repeating elements (N-acetyl-Dglucosamine and D-glucuronic acid). Multiple lines of evidence demonstrated that HA has critical roles in normal tissues. It can be a key player in different physiological and pathophysiological conditions such as water homeostasis, multiple drug resistance, inflammatory processes, tumorigenesis, angiogenesis, and changed viscoelasticity of the extracellular matrix. HA has very important physicochemical properties i.e., availability of reactive functional groups and its solubility, which make it a biocompatible material for application in regenerative medicine. Given that HAbased bioscaffolds and biomaterials do not induce inflammation or allergies and are hydrophilic, they are used as soft tissue fillers and injectable dermal fillers. Several studies indicated that HA could be employed as a new therapeutic candidate in the treatment of stroke. These studies documented that HA and HA-based therapies exert their pharmacological effects via affecting stroke-related processes. Herein, we summarized the role of the extracellular matrix in stroke pathogenesis. Moreover, we highlighted the HA-based therapies for the treatment of stroke.

Ultrastructural immunolocalization of telomerase and hyaluronate in migrating keratinocytes in a case of oro-pharyngeal squamous cancer

The ultrastructural immunolocalization of telomerase and hyaluronate has been studied in a case of oropharyngeal squamous carcinoma. Immunofluorescence shows that telomerase immunolabeling is present in the cytoplasm and in nuclei of some keratinocytes during their migration into the underlying connective tissue. The electron microscope shows that the nuclear localization of telomerase mainly occurs in the large nucleoli and in likely Cajal bodies, the sites of assembling and maturation of proteins forming the telomerase complex. Aside ribosomes, the nucleolus has a role in the biosynthesis of this reverse transcriptase during cell proliferation in normal tissues and in tumors. The cytoplasmic labeling for telomerase is frequently associated with an irregular network of keratin bundles but the significance of this observation is unclear. Hyaluronate, detected through ultrastructural immunolocalization of a hyaluronate binding protein, is abundant mostly along the cell membrane of the detaching basal keratinocytes during epithelial mesenchymal transition. A coat of hyaluronate surrounds the free keratinocytes of the squamous epithelium and is present around the connective cells present underneath. The study supports the hypothesis that hyaluronate forms a pathway along which epithelial cells can migrate during epidermal mesenchymal transition and may also shield cancer cells from immune cells.

Inflammation and immunity in organ regeneration

The ability of vertebrates to regenerate amputated appendages is increasingly well-understood at the cellular level. Cells mediating an innate immune response and inflammation in the injured tissues are a prominent feature of the limb prior to formation of a regeneration blastema, with macrophage activity necessary for blastema growth and successful development of the new limb. Studies involving either anti-inflammatory or pro-inflammatory agents suggest that the local inflammation produced by injury and its timely resolution are both important for regeneration, with blastema patterning inhibited in the presence of unresolved inflammation. Various experiments with Xenopus larvae at stages where regenerative competence is declining show improved digit formation after treatment with certain immunosuppressive, anti-inflammatory, or antioxidant agents. Similar work with the larval Xenopus tail has implicated adaptive immunity with regenerative competence and suggests a requirement for regulatory T cells in regeneration, which also occurs in many systems of tissue regeneration. Recent analyses of the human nail organ indicate a capacity for local immune tolerance, suggesting roles for adaptive immunity in the capacity for mammalian appendage regeneration. New information and better understanding regarding the neuroendocrine-immune axis in the response to stressors, including amputation, suggest additional approaches useful for investigating effects of the immune system during repair and regeneration.

Hyaluronan, neural stem cells and tissue reconstruction after acute ischemic stroke

Focal stroke is a disabling disease with lifelong sensory, motor and cognitive impairments. Given the paucity of effective clinical treatments, basic scientists are developing novel options for protection of the affected brain and regeneration of lost tissue. Tissue bioengineering and stem/progenitor cell treatments have both been individually pursued for stroke neural repair therapies, with some benefit in tissue recovery. Emerging directions in stroke neural repair approaches combine these two therapies to use biopolymers with stem/progenitor transplants to promote greater cell survival in the transplant and directed delivery of bioactive molecules to the transplanted cells and the adjacent injured tissue. In this review the background literature on a combined use of neural stem/progenitor cells encapsulated in hyaluronan gels is discussed and the way this therapeutic approach can affect the important processes involved in brain tissue reconstruction, such as angiogenesis, axon regeneration, neural differentiation and inflammation is clarified. The glycosaminoglycan hyaluronan can optimize those processes and be employed in a successful neural tissue engineering approach.

Biochemical and mechanical environment cooperatively regulate skeletal muscle regeneration

During forelimb regeneration in the newt Notophthalmus viridescens, the dynamic expression of a transitional matrix rich in hyaluronic acid, tenascin-C, and fibronectin controls muscle cell behavior in vivo and in vitro. However, the influence of extracellular matrix (ECM) remodeling on tissue stiffness and the cellular response to mechanical variations during regeneration was unknown. By measuring the transverse stiffness of tissues in situ, we found undifferentiated regenerative blastemas were less stiff than differentiated stump muscle (13.3±1.6 vs. 16.6±1.2 kPa). To directly determine how ECM and stiffness combine to affect skeletal muscle fragmentation, migration, and fusion, we coated silicone-based substrates ranging from 2 to 100 kPa with matrices representative of transitional (tenascin-C and fibronectin) and differentiated environments (laminin and Matrigel). Using live-cell imaging, we found softer tenascin-C-coated substrates significantly enhanced migration and fragmentation of primary newt muscle cells. In contrast, stiffer substrates coated with laminin, Matrigel, or fibronectin increased differentiation while suppressing migration and fragmentation. These data support our in vivo observations that a transitional matrix of reduced stiffness regulates muscle plasticity and progenitor cell recruitment into the regenerative blastema. These new findings will enable the determination of how biochemical and mechanical cues from the ECM control genetic pathways that drive regeneration.

Wound healing and blastema formation in regenerating digit tips of adult mice

Amputation of the distal region of the terminal phalanx of mice causes an initial wound healing response followed by blastema formation and the regeneration of the digit tip. Thus far, most regeneration studies have focused in embryonic or neonatal models and few studies have examined adult digit regeneration. Here we report on studies that include morphological, immunohistological, and volumetric analyses of adult digit regeneration stages. The regenerated digit is grossly similar to the original, but is not a perfect replacement. Re-differentiation of the digit tip occurs by intramembranous ossification forming a trabecular bone network that replaces the amputated cortical bone. The digit blastema is comprised of proliferating cells that express vimentin, a general mesenchymal marker, and by comparison to mature tissues, contains fewer endothelial cells indicative of reduced vascularity. The majority of blastemal cells expressing the stem cell marker SCA-1, also co-express the endothelial marker CD31, suggesting the presence of endothelial progenitor cells. Epidermal closure during wound healing is very slow and is characterized by a failure of the wound epidermis to close across amputated bone. Instead, the wound healing phase is associated with an osteoclast response that degrades the stump bone allowing the wound epidermis to undercut the distal bone resulting in a novel re-amputation response. Thus, the regeneration process initiates from a level that is proximal to the original plane of amputation.

Amphibian regeneration and stem cells

Larval and adult urodeles and anuran tadpoles readily regenerate their limbs via a process of histolysis and dedifferentiation of mature cells local to the amputation surface that accumulate under the wound epithelium as a blastema of stem cells. These stem cells require growth and trophic factors from the apical epidermal cap (AEC) and the nerves that re-innervate the blastema for their survival and proliferation. Members of the fibroblast growth factor (FGF) family synthesized by both AEC and nerves, and glial growth factor, substance P, and transferrin of nerves are suspected survival and proliferation factors. Stem cells derived from fibroblasts and muscle cells can transdifferentiate into other cell types during regeneration. The regeneration blastema is a self-organizing system based on positional information inherited from parent limb cells. Retinoids, which act through nuclear receptors, have been used in conjunction with assays for cell adhesivity to show that positional identity of blastema cells is encoded in the cell surface. These molecules are involved in the cell-cell signaling network that re-establishes the original structural pattern of the limb. Other systems of interest that regenerate by histolysis and dedifferentiation of pigmented epithelial cells are the neural retina and lens. Members of the FGF family are also important to the regeneration of these structures. The mechanism of amphibian regeneration by dedifferentiation is of importance to the development of a regenerative medicine, since understanding this mechanism may offer insights into how we might chemically induce the regeneration of mammalian tissues.

Apoptosis in regenerating and denervated, nonregenerating urodele forelimbs

Denervated limbs of larval salamanders fail to regenerate if amputated and, unlike adult limbs, undergo regression. The cellular basis of the tissue loss is poorly understood. We used TUNEL staining of larval axolotl limbs fixed and sectioned at intervals after bilateral amputation and unilateral denervation to investigate the role of apoptosis during normal limb regeneration and denervated limb regression. In the first week after amputation a small percentage of apoptotic cells was found in both innervated and denervated limbs. During the second week the apoptotic index remained low in the mitotically active mesenchymal cells of the regenerating limbs, but increased twofold in the nondividing, dedifferentiated cells of the regressing limbs. TUNEL-positive cells resembling apoptotic bodies were restricted primarily to the dedifferentiated area beneath the wound epithelium, but were also present within the wound epithelium itself. Macrophages were identified immunohistochemically and were also found in increased numbers in distal areas of the denervated regressing limbs. The results suggest a role for apoptosis in the early phase of normal regeneration and indicate that denervated limb regression involves an increased rate of apoptosis and removal of apoptotic bodies by macrophages and the wound epithelium.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.

Lens formation from the cornea following implantation into hindlimbs of larval Xenopus laevis: the influence of limb innervation and extent of differentiation

Corneal fragments of larval Xenopus laevis at stage 48 (according to Nieuwkoop and Faber, '56), were implanted into sham denervated unamputated hindlimbs, denervated unamputated hindlimbs, amputated and sham denervated hindlimbs, and amputated and denervated hindlimbs of larvae at stages 52 and 57. The results show that unamputated limbs at stage 52, either innervated or denervated, manifest a weak capacity to promote the first lens-forming transformations of the outer cornea. This capacity is absent in both limb types at stage 57. After amputation, limbs of both early and late stages form a regenerative blastema and support lens formation from the outer cornea. Denervation of early stage limbs has no appreciable effect on blastema formation and lens-forming transformation of corneal implants. However, denervation of late stage limbs inhibits both processes. These results indicate that the limb tissues of the early stage limbs contain non-neural inductive factors at a low level and that after limb amputation and blastema formation the level of these factors becomes high enough to promote lens formation from implanted cornea, even after denervation. In contrast, the limb tissues of late stage limbs do not contain a suitable level of non-neural inductive factors.

Effect of denervation on hindlimb regeneration in Xenopus laevis larvae

Xenopus laevis larvae at stages 51-57, according to Nieuwkoop and Faber, were subjected to amputation of the right hindlimb or of both limbs at the thigh or the tarsal level, as well as to somatic denervation of the right limb. Larvae at the same stage having undergone amputation of the right limb or of both limbs and sham denervation of the right limb were used as controls. In experimental series I a single denervation of the right limb was performed at the time of amputation. In experimental series II repeated denervations were performed (before, during and after amputation). Results show that in larvae at stages 51-53 subjected to limb amputation at the proximal level (thigh) even repeated denervation of the right limb did not prevent regeneration, although giving rise to various degrees of hypotrophy. In stage-55 larvae partial inhibition of the regenerative process in the right limb was clearly visible only after repeated denervations and amputation at the proximal level. After amputation at the distal level (tarsalia) the regenerative process in the right limb underwent no significant delay with respect to the controls, although the regenerated right limb was hypotrophic. In stage-57 larvae even a single denervation at the time of amputation was enough to inhibit regeneration of the right limb after either proximal or distal amputation. Therefore, in Xenopus laevis larvae, nerve-dependence for hindlimb regeneration takes place proximodistally as the nerve fibers grow in the limb and it gradually undergoes a process of proximodistal differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)