Michael W. King

Regeneration or scarring: an immunologic perspective.

Complete regeneration of complex tissues and organs is usually precluded by fibrotic reactions that lead to scarring. Fish, salamanders, and larval anurans are among the few vertebrates capable of regenerating lost appendages, and this process seems to recapitulate ontogenic development of the structure in most respects. Recent work has revealed a capacity for excellent regeneration in certain mammalian tissues: embryonic or fetal skin and the ear of the MRL mouse. Analyses of these two systems suggest that processes of regenerative growth and patterning for the formation of new structures such as hair follicles may involve modulation of the inflammatory response to the injury in a way that reduces fibrosis and formation of scar tissue. We review evidence that this modulation includes changes in cytokine signaling and may involve properties of the extracellular matrix mediated by factors that include hyaluronic acid and “anti‐adhesive substrates” such as tenascin‐C. New studies and classic work on the capacity for limb regeneration in amphibians are then reviewed, focusing on the loss of this ability in prometamorphic anuran hindlimbs and the view that changing properties of the immune system may also underlie the declining regenerative potential in this system. Finally, we review recent work in comparative and developmental immunology, which raises the possibility that phylogenetic changes in regenerative capacity may be the result of evolutionary changes in cellular activities of the immune system. Developmental Dynamics 226:268–279, 2003.© 2003 Wiley‐Liss, Inc.

The Developing Xenopus Limb as a Model for Studies on the Balance between Inflammation and Regeneration

The roles of inflammation and immune cell reactivity triggered by amputation have only recently begun to be addressed in investigations of epimorphic regeneration, although studies of tissue repair in mammals clearly show the importance of the immune system in determining the quality of the repair process. Here, we first review inflammation‐related work in non‐mammalian systems of epimorphic regeneration which suggests that regeneration of an amputated appendage requires continuous modulation of the local immune response, from the first hours after amputation through the period of blastema patterning. We then present data on the effects of anti‐inflammatory and proinflammatory agents on regeneration of larval Xenopus hindlimbs. Treatment with the glucocorticoid beclomethasone immediately after amputation inhibits regeneration in regeneration‐complete stage 53 limbs. Other anti‐inflammatory agents, including the inhibitors of cyclooxygenase‐2 (COX‐2) activity celecoxib and diclofenac, applied similarly to larvae amputated at stage 55, when the capacity for limb regeneration is normally being lost, restore regenerative capacity. This suggests that although injury‐related events sensitive to glucocorticoids are necessary for regeneration, resolution of the inflammatory response may also be required to allow the complete regenerative response and normal blastema patterning. Conversely, if resolution of inflammation is prevented by local treatment of amputated limbs with beryllium, a strong immunoadjuvant, regeneration is inhibited, and gene expression data suggest that this inhibition results from a failure of normal blastema patterning. Both positive and negative effects of immune‐ or inflammation‐related activities occur during anuran limb regeneration and this underscores the importance of considering immune cells in studies of epimorphic regeneration. Anat Rec, 2012. ©2012 Wiley Periodicals, Inc.

Identification of genes expressed during Xenopus laevis limb regeneration by using subtractive hybridization

Suppression polymerase chain reaction–based subtractive hybridization was used to identify genes that are expressed during Xenopus laevis hindlimb regeneration. Subtractions were done by using RNAs extracted from the regeneration‐competent stage (stage 53) and regeneration‐incompetent stage (stage 59) of limb development. Forward and reverse subtractions were done between stage 53 7‐day blastema and stage 53 contralateral limb (competent stage), stage 59 7‐day pseudoblastema and stage 59 contralateral limb (incompetent stage), and stage 53 7‐day blastema and stage 59 7‐day pseudoblastema. Several thousand clones were analyzed from the various subtracted libraries, either by random selection and sequencing (1,920) or by screening subtracted cDNA clones (6,150), arrayed on nylon membranes, with tissue‐specific probes. Several hundred clones were identified from the array screens whose expression levels were at least twofold higher in experimental tissue vs. control tissue (e.g., blastema vs. limb) and selected for sequencing. In addition, primers were designed to assay several of the randomly selected clones and used to assess the level of expression of these genes during regeneration and normal limb development. Approximately half of the selected clones were differentially expressed, as expected, including several that demonstrate blastema‐specific enhancement of expression. Three distinct categories of expression were identified in our screens: (1) clones that are expressed in both regeneration‐competent blastemas and ‐incompetent pseudoblastemas, (2) clones that are expressed at highest levels in regeneration‐competent blastemas, and (3) clones that are expressed at highest levels in regeneration‐incompetent pseudoblastemas. Characterizing the role of each of these three categories of genes will be important in furthering our understanding of the process of tissue regeneration. Developmental Dynamics 226:398–409, 2003.

Global analysis of gene expression in Xenopus hindlimbs during stage-dependent complete and incomplete regeneration

Xenopus laevis tadpoles are capable of limb regeneration after amputation, in a process that initially involves the formation of a blastema. However, Xenopus has full regenerative capacity only through premetamorphic stages. We have used the Affymetrix Xenopus laevis Genome Genechip microarray to perform a large‐scale screen of gene expression in the regeneration‐complete, stage 53 (st53), and regeneration‐incomplete, stage 57 (st57), hindlimbs at 1 and 5 days postamputation. Through an exhaustive reannotation of the Genechip and a variety of comparative bioinformatic analyses, we have identified genes that are differentially expressed between the regeneration‐complete and ‐incomplete stages, detected the transcriptional changes associated with the regenerating blastema, and compared these results with those of other regeneration researchers. We focus particular attention on striking transcriptional activity observed in genes associated with patterning, stress response, and inflammation. Overall, this work provides the most comprehensive views yet of a regenerating limb and different transcriptional compositions of regeneration‐competent and deficient tissues. Developmental Dynamics 235:2667–2685, 2006.

Proteomics analysis of regenerating amphibian limbs: changes during the onset of regeneration

During amphibian epimorphic limb regeneration, local injury produces metabolic changes that lead to cellular dedifferentiation and formation of a blastema, but few details of these changes have been elucidated. Here we report the first global proteomic analysis of epimorphic regeneration comparing the profiles of abundant proteins in larval limbs of the anuran Xenopus laevis (stage 53) at the time of amputation (0dPA) and 3 days post-amputation when the regeneration blastema is developing (3dPA). We identified and quantified 1517 peptides, of which 1067 were identified with high peptide ID confidence. Of these 1067 proteins, 489 showed significant changes in quantity between the two groups. Taking into account identical peptides whose fold changes were within 20%, and not including peptides whose fold changes were below the observed fold changes of peptides for the internal standard (chicken lysozyme), we were able to identify 145 peptides elevated in 3dPA relative to 0dPA and 220 peptides in 0dPA relative to 3dPA. In this report, we focus on those proteins that were elevated in the 3dPA tissue relative to 0dPA. In this class were members of the annexin family (e.g. ANXA1, ANXA2, ANXA5) and the ANXA2-binding partner S100A10, which have important immunoregulatory roles in other systems and were also shown to be differentially expressed in stage 53 and 57 3dPA and 5dPA blastemas in our previous microarray studies. Besides elucidating the possible modulation of inflammation during amphibian limb regeneration, our proteomic study also provides insight into dedifferentiation by revealing up-regulation of proteins known to characterize many stem cells.

Changes in the Inflammatory Response to Injury and Its Resolution during the Loss of Regenerative Capacity in Developing Xenopus Limbs

Tissue and organ regeneration, unlike development, involves an injury that in postembryonic animals triggers inflammation followed by resolution. How inflammation affects epimorphic regeneration is largely uninvestigated. Here we examine inflammation and its resolution in Xenopus laevis hindlimb regeneration, which declines during larval development. During the first 5 days postamputation, both regeneration-competent stage 53 and regeneration-deficient stage 57 hindlimbs showed very rapid accumulation of leukocytes and cells expressing interleukin-1β and matrix metalloproteinase 9. Expression of genes for factors mediating inflammatory resolution appeared more persistent at stages 55 and 57 than at stage 53, suggesting changes in this process during development. FoxP3, a marker for regulatory T cells, was upregulated by amputation in limbs at all three stages but only persisted at stage 57, when it was also detected before amputation. Expression of genes for cellular reprogramming, such as SALL4, was upregulated in limbs at all 3 stages, but markers of limb patterning, such as Shh, were expressed later and less actively after amputation in regeneration-deficient limbs. Topical application of specific proinflammatory agents to freshly amputated limbs increased interleukin-1β expression locally. With aqueous solutions of the proinflammatory metal beryllium sulfate, this effect persisted through 7 days postamputation and was accompanied by inhibition of regeneration. In BeSO4-treated limbs expression of markers for both inflammation and resolution, including FoxP3, was prolonged, while genes for cellular reprogramming were relatively unaffected and those for limb patterning failed to be expressed normally. These data imply that in Xenopus hindlimbs postamputation inflammation and its resolution change during development, with little effect on cellular dedifferentiation or reprogramming, but potentially interfering with the expression of genes required for blastema patterning. The results suggest that developmental changes in the larval anuran immune system may be involved in the ontogenetic loss of epimorphic regeneration in this system.

Neural MMP-28 expression precedes myelination during development and peripheral nerve repair

Mammalian matrix metalloproteinase 28 (MMP‐28) is expressed in several normal adult tissues, and during cutaneous wound healing. We show that, in frog and mouse embryos, MMP‐28 is expressed predominantly throughout the nervous system. Xenopus expression increases during neurulation and remains elevated through early limb development where it is expressed in nerves. In the mouse, neural expression peaks at embryonic day (E) 14 but remains detectable through E17. During frog hindlimb regeneration XMMP‐28 is not initially expressed in the regenerating nerves but is detectable before myelination. Following hindlimb denervation, XMMP‐28 expression is detectable along regenerating nerves before myelination. In embryonic rat neuron–glial co‐cultures, MMP‐28 decreases after the initiation of myelination. Incubation of embryonic brain tissue with purified MMP‐28 leads to the degradation of multiple myelin proteins. These results suggest that MMP‐28 plays an evolutionarily conserved role in neural development and is likely to modulate the axonal–glial extracellular microenvironment. Developmental Dynamics 236:2852–2864, 2007.

Expression of Xenopus XlSALL4 during limb development and regeneration

The multi‐C2H2 zinc‐finger domain containing transcriptional regulators of the spalt (SAL) family plays important developmental regulatory roles. In a competitive subtractive hybridization screen of genes expressed in Xenopus laevis hindlimb regeneration blastemas, we identified a SAL family member that, by phylogenetic analysis, falls in the same clade as human SALL4 and have designated it as XlSALL4. Mutations of human SALL4 have been linked to Okihiro syndrome, which includes preaxial (anterior) limb defects. The expression pattern of XlSALL4 transcripts during normal forelimb and hindlimb development and during hindlimb regeneration at the regeneration‐competent and regeneration‐incompetent stages is temporally and regionally dynamic. We show for the first time that a SAL family member (XlSALL4) is expressed at the right place and time to play a role regulating both digit identity along the anterior/posterior axis and epimorphic limb regeneration. Developmental Dynamics 233:356–367, 2005.

Dedifferentiation and the role of sall4 in reprogramming and patterning during amphibian limb regeneration

A central feature of epimorphic regeneration during amphibian limb regeneration is cellular dedifferentiation. Two questions are discussed. First, what is the origin and nature of the soluble factors involved in triggering local cellular and tissue dedifferentiation? Secondly, what role does the key stem cell transcription factor Sall4 play in reprogramming gene expression during dedifferentiation? The pattern of Sall4 expression during Xenopus hindlimb regeneration is consistent with the hypothesis that Sall4 plays a role in dedifferentiation (reprogramming) and in maintaining limb blastema cells in an undifferentiated state. Sall4 is involved in maintenance of ESC pluripotency, is a major repressor of differentiation, plays a major role in reprogramming differentiated cells into iPSCs, and is a component of the stemness regulatory circuit of pluripotent ESCs and iPSCs. These functions suggest Sall4 as an excellent candidate to regulate reprogramming events that produce and maintain dedifferentiated blastema cells required for epimorphic regeneration. Developmental Dynamics 240:979–989, 2011.

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.