Regeneration in Echinoderms: Molecular Advancements

Which genes and gene signaling pathways mediate regenerative processes? In recent years, multiple studies, using a variety of animal models, have aimed to answer this question. Some answers have been obtained from transcriptomic and genomic studies where possible gene and gene pathway candidates thought to be involved in tissue and organ regeneration have been identified. Several of these studies have been done in echinoderms, an animal group that forms part of the deuterostomes along with vertebrates. Echinoderms, with their outstanding regenerative abilities, can provide important insights into the molecular basis of regeneration. Here we review the available data to determine the genes and signaling pathways that have been proposed to be involved in regenerative processes. Our analyses provide a curated list of genes and gene signaling pathways and match them with the different cellular processes of the regenerative response. In this way, the molecular basis of echinoderm regenerative potential is revealed, and is available for comparisons with other animal taxa.

Cell-intrinsic Aryl Hydrocarbon Receptor signalling is required for the resolution of injury-induced colonic stem cells

The aryl hydrocarbon receptor (AHR) is an environmental sensor that integrates microbial and dietary cues to influence physiological processes within the intestinal microenvironment, protecting against colitis and colitis-associated colorectal cancer development. Rapid tissue regeneration upon injury is important for the reinstatement of barrier integrity and its dysregulation promotes malignant transformation. Here we show that AHR is important for the termination of the regenerative response and the reacquisition of mature epithelial cell identity post injury in vivo and in organoid cultures in vitro. Using an integrative multi-omics approach in colon organoids, we show that AHR is required for timely termination of the regenerative response through direct regulation of transcription factors involved in epithelial cell differentiation as well as restriction of chromatin accessibility to regeneration-associated Yap/Tead transcriptional targets. Safeguarding a regulated regenerative response places AHR at a pivotal position in the delicate balance between controlled regeneration and malignant transformation.*As a note, Kathleen Shah and Muralidhara Rao Maradana are joint first authors.

TREM2-dependent lipid droplet biogenesis in phagocytes is required for remyelination

Upon demyelinating injury, microglia orchestrate a regenerative response that promotes myelin repair, thereby restoring rapid signal propagation and protecting axons from further damage. Whereas the essential phagocytic function of microglia for remyelination is well known, the underlying metabolic pathways required for myelin debris clearance are poorly understood. Here, we show that cholesterol esterification in male mouse microglia/macrophages is a necessary adaptive response to myelin debris uptake and required for the generation of lipid droplets upon demyelinating injury. When lipid droplet biogenesis is defective, innate immune cells do not resolve, and the regenerative response fails. We found that triggering receptor expressed on myeloid cells 2 (TREM2)–deficient mice are unable to adapt to excess cholesterol exposure, form fewer lipid droplets, and build up endoplasmic reticulum (ER) stress. Alleviating ER stress in TREM2-deficient mice restores lipid droplet biogenesis and resolves the innate immune response. Thus, we conclude that TREM2-dependent formation of lipid droplets constitute a protective response required for remyelination to occur.

Severe liver disease resembling PSC in mice with K5-Cre mediated deletion of Krüppel-like factor 5 (Klf5)

Chronic cholestatic liver diseases including primary sclerosing cholangitis (PSC) present a complex spectrum with regards to the cause, age of manifestation and histopathological features. Current treatment options are severely limited primarily due to a paucity of model systems mirroring the disease. Here, we describe the Keratin 5 (K5)-Cre; Klf5fl/fl mouse that spontaneously develops severe liver disease during the postnatal period with features resembling PSC including a prominent ductular reaction, fibrotic obliteration of the bile ducts and secondary degeneration/necrosis of liver parenchyma. Over time, there is an expansion of Sox9+ hepatocytes in the damaged livers suggestive of a hepatocyte-mediated regenerative response. We conclude that Klf5 is required for the normal function of the hepatobiliary system and that the K5-Cre; Klf5fl/fl mouse is an excellent model to probe the molecular events interlinking damage and regenerative response in the liver.

Notch signaling via Hey1 and Id2b regulates Müller glia’s regenerative response to retinal injury

Unlike mammals, zebrafish can regenerate a damaged retina. Key to this regenerative response are Müller glia (MG) that divide and produce progenitors for retinal repair. Although factors regulating MG’s decision to divide remain mostly unknown, a certain threshold of neuron death must be exceeded in order for MG to engage in a regenerative response. A role for Notch signaling in this process is indicated since its inhibition expands the zone of injury-responsive MG following a focal injury. Our data show that injury-dependent changes in Dll4 and Dlb control Notch signaling in MG and that Hey1 and Id2b are downstream effectors that regulate proliferation of MG and MG-derived progenitors. Although we find Hey1 and Id2b can inhibit proliferation of MG-derived progenitors, only Hey1 is able to regulate MG’s injury response threshold. Remarkably, Hey1 suppression is sufficient to recapitulate the effects of Notch inhibition on MG’s injury response threshold.

Organoid-based modeling of intestinal development, regeneration, and repair

The intestinal epithelium harbors a remarkable adaptability to undergo injury-induced repair. A key part of the regenerative response is the transient reprogramming of epithelial cells into a fetal-like state, which drives uniform proliferation, tissue remodeling, and subsequent restoration of the homeostatic state. In this review, we discuss how Wnt and YAP signaling pathways control the intestinal repair response and the transitioning of cell states, in comparison with the process of intestinal development. Furthermore, we highlight how organoid-based applications have contributed to the characterization of the mechanistic principles and key players that guide these developmental and regenerative events.

Cholinergic nerve regulation of heart regeneration

Background Cardiac nerves regulate many important physiological functions of the heart such as heart rate and contractility. The emerging role of cardiac nerves during tissue homeostasis and regeneration is beginning to be appreciated. We discovered that neonatal mice are capable of regenerating their hearts following injury within a brief period after birth by proliferation of the pre-existing cardiomyocytes. Furthermore, we have demonstrated that cholinergic nerves play an important role in guiding the neonatal heart regenerative response. However, the adult mammalian heart, including the human heart, is incapable of regeneration following injury. Thus, there is great excitement about understanding the evolutionarily conserved mechanisms of endogenous cardiac regeneration, so that we can explore potential avenues to reawaken this process in adult humans. Purpose Our overarching goal is to define the mechanisms by which cholinergic nerves regulate heart regeneration following ischemic injury by using the neonatal mouse heart regeneration model. These studies will uncover novel pathways by which cholinergic signaling promotes cardiomyocyte proliferation and heart regeneration, which holds significant therapeutic potential for treatment of adult heart disease. Methods In this project, we employed genetically engineered mouse models of the critical receptors for cholinergic signaling in the heart to define the mechanisms of cholinergic nerve regulation of heart regeneration. First, we generated a cardiomyocyte-specific deletion of the muscarinic receptor (M2), the most predominant muscarinic receptor subtype present in the heart. In addition, we utilized the α7 nicotinic receptor (Chrna7) knockout mice to study the role of Chrna7 in endogenous immune cells, which is the main mediator of the cholinergic anti-inflammatory pathway. These mouse models will address how cholinergic nerves regulate heart regeneration via the M2 muscarinic receptor signaling and the inflammatory response following injury. Results Our results demonstrate that inhibition of two different cholinergic receptors (muscarinic and nicotinic) results in a reduction in cardiomyocyte proliferation and inhibition of the neonatal cardiac regenerative response following injury. More importantly, we demonstrate that cholinergic signaling mediates the cardiac regenerative response mainly through suppression of pro-inflammatory cytokines via the cholinergic anti-inflammatory pathway. Conclusions Cholinergic nerve signaling plays an important role in mounting a robust cardiac regenerative response following injury. These results have significant therapeutic potential, which will forge new paradigms with respect to the role of cardiac nerves during mammalian cardiac regeneration and reveal potential mechanisms regarding the benefits of nerve stimulation following cardiac injury in humans. Funding Acknowledgement Type of funding source: Foundation. Main funding source(s): American Heart Association, Wisconsin Partnership Program