Expression of the novel stem/progenitor cell marker Neighbor of Punc E 11 after acute and chronic liver injury in the adult mouse liver

Background: Regeneration of the liver can be obtained through different cell types depending upon the type and extent of liver injury. While acute liver injury is usually followed by proliferation of differentiated hepatocytes, activation of the stem/progenitor cell compartment occurs only if hepatocytic regeneration is insufficient. We here further investigated the expression pattern of the novel oval cell marker Neighbor of Punc E 11 (Nope) in liver regeneration after chronic (Mdr2 -/- mice) and/or acute liver injury (partial hepatectomy) in adult mice.

Methods: Liver tissue was extracted from Balb/c (10 weeks) and Mdr2-/- mice of different age and stage of fibrosis. The impact of acute liver injury by partial hepatectomy was analyzed with or without pretreatment with DNA-alkylating reagents to block hepatocytic proliferation. 24 hours to 6 months postoperatively, mice were sacrificed and the liver tissue was tested for expression levels of Nope via quantitative RT-PCR. Costainings were performed for Nope in combination with the biliary (CK19) or epithelial-specific markers (E-cadherin), markers of hepatic stem/progenitor cells (A6, EpCAM) and the canalicular membrane marker dipeptidylpeptidase (DPP) IV.

Results: While normal adult liver tissue shows only negligible expression of Nope, chronic liver injury in Mdr2-/- mice leads to a considerably increased expression level of Nope. Co-stainings with CK19 demonstrated a bile-duct-specific expression of Nope in these mice. While acute liver injury in normal adult liver has no effect, an additional partial hepatectomy in the Mdr2-/- mouse model results in the detection of Nope-positive cell clusters especially if hepatocyte proliferation is blocked. These Nope-positive clusters are negative for CK19 but positive for E-cadherin and DPPIV. Oval cell markers A6 and EpCAM both show coexpression with Nope in proliferating ductular cells, but only A6 also detects a minor cell fraction within Nope-positive clusters.

Conclusion: We here report the expression of the novel oval cell marker Nope in liver regeneration. The increase of Nope in chronic liver injury is mainly limited to bile ducts, but especially after additional acute liver injury and blocking of hepatocytic regeneration a small population of Nope-positive hepatocytic progenitor cells arises. We conclude that Nope is a marker for a novel progenitor cell population in the regenerating adult liver.