James Godwin (Online)
Immunological Control of Regeneration
17.30 – 17.50
Heart Regeneration in Pleurodeles Waltl
The contribution of the epicardium, the outermost layer of the heart, to cardiac regeneration has remained controversial due to a lack of suitable analytical tools. By combining genetic marker-independent lineage-tracing strategies with transcriptional profiling and loss-of-function methods, we report here that the epicardium of the highly regenerative salamander species Pleurodeles waltl has an intrinsic capacity to differentiate into cardiomyocytes. Following cryoinjury, CLDN6+ epicardium-derived cells appear at the lesion site, organize into honeycomb-like structures connected via focal tight junctions and undergo transcriptional reprogramming that results in concomitant differentiation into de novo cardiomyocytes. Ablation of CLDN6+ differentiation intermediates as well as disruption of their tight junctions impairs cardiac regeneration. Salamanders constitute the evolutionarily closest species to mammals with an extensive ability to regenerate heart muscle and our results highlight the epicardium and tight junctions as key targets in efforts to promote cardiac regeneration.
17.50 – 18.10
Macrophages Orchestrate Successful Lens Regeneration in Newts
Macrophage involvement during tissue regeneration has been a subject of great discussion over recent years. Lens regeneration in newts involves the transdifferentiation of iris pigmented epithelial cells (iPECs) into lens cells. The elegant simplicity of this process serves as a great platform to uncover mechanism by which macrophages promote scar-free healing in regeneration competent animals. Here we used clodronate liposomes to deplete macrophages in the eye from two newt species Notophthalmus viridescens and Pleurodeles waltl. We found that macrophages depletion inhibited lens regeneration in both species. Furthermore, early clodronate administration resulted in a significant decrease in iPECs proliferation, induced an unresolved cellular accumulation in the aqueous chamber, prolonged inflammation, generated a fibrotic like response and caused abnormalities in ECM remodeling. Even though macrophages were allowed to return after clodronate administration was stopped, ramifications caused by early macrophage depletion persisted for 100 days. Interestingly, secondary injury in the eye, alleviated the effects of macrophage depletion and restarted the regeneration process. These results add to the growing evidence on the importance of macrophages during tissue regeneration. Delineating the mechanisms by which newt macrophages promote cell cycle re-entry, resolve inflammation, and promote ECM remodeling will present a vital step towards our goal of inducing scar-free healing in other vertebrates.
Michael J Raymond (Online)
Investigation of the Mechanism Promoting Positional Plasticity in Axolotl Limb Regeneration
Following limb amputation in the Mexican Axolotl, a regenerative organ called the blastema forms at the amputation site and recreates the missing limb pattern of the regenerate. For this new pattern to form, the pattern information within cells contributing to the blastema becomes flexible, a property that we call positional plasticity. While the induction and maintenance of positional plasticity is dependent on the limb nerves, the underlying mechanisms are not well understood. To elucidate how uninjured limb cells transition to the positionally plastic state, we developed a new assay called the Plasticity-ALM (P-ALM) to characterize the underlying regulation of positional plasticity. We demonstrated that the re-expression of limb patterning genes coincides with the induction of positional plasticity and that newly generated positional information stabilizes after 3-7 days. Using the P-ALM system, we identified the nerve-dependent signaling molecules that promote positional plasticity by grafting growth factor-soaked beads into a wound site. Cooperative BMP and FGF signaling was shown to be sufficient to induce positional plasticity in mature limb cells. We also investigated the mechanisms behind the induction of positional plasticity at the genomic level. Using quantitative microscopy and custom bioimage analysis algorithms, we investigated changes to nucleus architecture and H3K27me3 modifications in limb wounds cells as they transition into positionally plastic cells. We showed that while a loss of heterochromatin correlates with positional plasticity, it is insufficient for its induction or maintenance. We detected a global loss of H3K27me3 abundance in positionally plastic cells and a reaccumulation of this mark as cells stabilize their positional information. To investigate this modification at the individual gene-level, we performed ChIP sequencing on cells from uninjured tissue, lateral wound sites, and blastemas. Surprisingly, GSEA demonstrated a significant increase in the abundance of H3K27me3 in gene sets that are associated with limb patterning in the blastema relative to wounds. In future studies, we will investigate the role of FGF and BMP signaling in mediating these modifications in wounds.