Extensive jejunal injury is repaired by migration and transdifferentiation of ileal enterocytes in zebrafish.

A major cause of intestinal failure (IF) is intestinal epithelium necrosis and massive loss of enterocytes, especially in the jejunum, the major intestinal segment in charge of nutrient absorption. However, mechanisms underlying jejunal epithelial regeneration after extensive loss of enterocytes remain elusive. Here, we apply a genetic ablation system to induce extensive damage to jejunal enterocytes in zebrafish, mimicking the jejunal epithelium necrosis that causes IF. In response to injury, proliferation and filopodia/lamellipodia drive anterior migration of the ileal enterocytes into the injured jejunum. The migrated fabp6+ ileal enterocytes transdifferentiate into fabp2+ jejunal enterocytes to fulfill the regeneration, consisting of dedifferentiation to precursor status followed by redifferentiation. The dedifferentiation is activated by the IL1ß-NFκB axis, whose agonist promotes regeneration. Extensive jejunal epithelial damage is repaired by the migration and transdifferentiation of ileal enterocytes, revealing an intersegmental migration mechanism of intestinal regeneration and providing potential therapeutic targets for IF caused by jejunal epithelium necrosis.

Key Developmental Regulators Suggest Multiple Origins of Pancreatic Beta Cell Regeneration.

Extensive efforts have been done to try to restore the lost ß cell mass for the cure of diabetes. Animal models have been established to provide evidences of cellular origins and contextual regulators of ß cell regeneration. Here, we used a zebrafish ß cell ablation and regeneration model to investigate ß cell neogenesis in the first few days after a near-total ß cell loss. Regeneration of ß cells first occurred within 7 h post-treatment. Developmental regulators such as neurod, pdx1, mnx1, and nkx2.2a were active in the regenerating ß cells, while at the same time suggesting different subpopulations of regenerative cellular origins. Using Cre/loxP-based lineage tracing, we showed that intrapancreatic ductal cells resisted to give rise to regenerating ß cells. Given that transdifferentiation of α cell and δ cell can regenerate ß cell, here we have provided further molecular evidence highly suggesting that the regenerating ß cells originate from multiple cellular origins.

Roles of mTOR Signaling in Tissue Regeneration.

The mammalian target of rapamycin (mTOR), is a serine/threonine protein kinase and belongs to the phosphatidylinositol 3-kinase (PI3K)-related kinase (PIKK) family. mTOR interacts with other subunits to form two distinct complexes, mTORC1 and mTORC2. mTORC1 coordinates cell growth and metabolism in response to environmental input, including growth factors, amino acid, energy and stress. mTORC2 mainly controls cell survival and migration through phosphorylating glucocorticoid-regulated kinase (SGK), protein kinase B (Akt), and protein kinase C (PKC) kinase families. The dysregulation of mTOR is involved in human diseases including cancer, cardiovascular diseases, neurodegenerative diseases, and epilepsy. Tissue damage caused by trauma, diseases or aging disrupt the tissue functions. Tissue regeneration after injuries is of significance for recovering the tissue homeostasis and functions. Mammals have very limited regenerative capacity in multiple tissues and organs, such as the heart and central nervous system (CNS). Thereby, understanding the mechanisms underlying tissue regeneration is crucial for tissue repair and regenerative medicine. mTOR is activated in multiple tissue injuries. In this review, we summarize the roles of mTOR signaling in tissue regeneration such as neurons, muscles, the liver and the intestine.

Mammalian Target of Rapamycin Complex 1 Signaling Is Required for the Dedifferentiation From Biliary Cell to Bipotential Progenitor Cell in Zebrafish Liver Regeneration.

The liver has a high regenerative capacity. Upon two-thirds partial hepatectomy, the hepatocytes proliferate and contribute to liver regeneration. After severe liver injury, when the proliferation of residual hepatocytes is blocked, the biliary epithelial cells (BECs) lose their morphology and express hepatoblast and endoderm markers, dedifferentiate into bipotential progenitor cells (BP-PCs), then proliferate and redifferentiate into mature hepatocytes. Little is known about the mechanisms involved in the formation of BP-PCs after extreme liver injury. Using a zebrafish liver extreme injury model, we found that mammalian target of rapamycin complex 1 (mTORC1) signaling regulated dedifferentiation of BECs and proliferation of BP-PCs. mTORC1 signaling was up-regulated in BECs during extreme hepatocyte ablation and continuously expressed in later liver regeneration. Inhibition of mTORC1 by early chemical treatment before hepatocyte ablation blocked the dedifferentiation from BECs into BP-PCs. Late mTORC1 inhibition after liver injury reduced the proliferation of BP-PC-derived hepatocytes and BECs but did not affect BP-PC redifferentiation. mTOR and raptor mutants exhibited defects in BEC transdifferentiation including dedifferentiation, BP-PC proliferation, and redifferentiation, similar to the chemical inhibition. Conclusion: mTORC1 signaling governs BEC-driven liver regeneration by regulating the dedifferentiation of BECs and the proliferation of BP-PC-derived hepatocytes and BECs.

Ribosome biogenesis protein Urb1 acts downstream of mTOR complex 1 to modulate digestive organ development in zebrafish.

Ribosome biogenesis is essential for the cell growth and division. Disruptions in ribosome biogenesis result in developmental defects and a group of diseases, known as ribosomopathies. Here, we report a mutation in zebrafish urb1, which encodes an essential ribosome biogenesis protein. The urb1cq31 mutant exhibits hypoplastic digestive organs, which is caused by impaired cell proliferation with the differentiation of digestive organ progenitors unaffected. Knockdown of mtor or raptor leads to similar hypoplastic phenotypes and reduced expression of urb1 in the digestive organs. Overexpression of Urb1 results in overgrowth of digestive organs, and can efficiently rescue the hypoplastic liver and pancreas in the mtor and raptor morphants. Reduced syntheses of free ribosomal subunits and impaired assembly of polysomes are observed in the urb1 mutant as well as in the mtor and raptor morphants, which can be rescued by the Urb1 overexpression. These data demonstrate that Urb1 plays an important role in governing ribosome biogenesis and protein synthesis downstream of mammalian/mechanistic target of rapamycin complex 1 (mTORC1), thus regulating the development of digestive organs. Our study indicates the requirement of hyperactive protein synthesis for the digestive organ development.