The many faces of Hedgehog signalling in the liver: Recent progress reveals striking cellular diversity and the importance of microenvironments
Authors: Madlen Matz-Soja, Rolf Gebhardt
Journal: Journal of Hepatology (2014)

We have read with great interest the work of Grzelak and colleagues. The authors report the occurrence of primary cilia-positive cell populations and have characterised detailed features of Hedgehog (Hh) signalling during chronic liver injury, induced by thioacetamide (TAA) [[1]Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar]. They describe an attractive scenario for driving fibrosis and repair in response to TAA treatment based on local niches and two signalling routes: (i) the canonical pathway involving primary cilia and SMO and (ii) an apparently SMO- and primary cilium-independent pathway. The question of whether and when primary cilia-positive cells exist or occur in the liver, is important because cellular Hh signalling may qualitatively differ in cells equipped with or bare of primary cilia at their surface. Grzelak and colleagues properly approached this question by using specific staining methods for detecting this organelle. They demonstrate that the majority, if not all hepatocytes in normal and TAA-treated livers, do not express a primary cilium, whereas primary cilia-positive cell populations, identified as liver progenitor cells (LPCs), occur in injured liver. Thus, they confirm earlier results from various groups, inferred from electron and confocal microscopic images (for discussion see [[2]Matz-Soja M. Hovhannisyan A. Gebhardt R. Hedgehog signalling pathway in adult liver: a major new player in hepatocyte metabolism and zonation?.Med Hypotheses. 2013; 80: 589-594Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar]). Indeed, the presence of primary cilia may considerably enhance the strength of GLI factor activation as evidenced by Grzelak and colleagues using the BMOL1.2 LPC cell line. However, the generalisation of the results on the contribution of SMO-dependent and -independent GLI activation to the functions of this cell line should be made with due caution, because the spectrum of Hh-related signalling appears to be rather broad in different (liver) cell types as depicted in Fig. 1. Besides canonical (cilia- and SMO-dependent) signalling in endothelial cells, cholangiocytes [[3]Omenetti A. Diehl A.M. Hedgehog signaling in cholangiocytes.Curr Opin Gastroenterol. 2011; 27: 268-275Crossref PubMed Scopus (62) Google Scholar], activated hepatic stellate cells (HSC) [[4]Michelotti G.A. Xie G. Swiderska M. Choi S.S. Karaca G. Kruger L. et al.Smoothened is a master regulator of adult liver repair.J Clin Invest. 2013; 123: 2380-2394PubMed Google Scholar] and progenitor cells [1Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 5Sicklick J.K. Li Y.X. Melhem A. Schmelzer E. Zdanowicz M. Huang J. et al.Hedgehog signaling maintains resident hepatic progenitors throughout life.Am J Physiol Gastrointest Liver Physiol. 2006; 290: G859-G870Crossref PubMed Scopus (176) Google Scholar], there is obviously cilia-independent but SMO-dependent signalling in mature healthy hepatocytes [[6]Matz-Soja M. Aleithe S. Marbach E. Böttger J. Arnold K. Schmidt-Heck W. et al.Hepatic Hedgehog signaling contributes to the regulation of IGF1 and IGFBP1 serum levels.Cell Commun Signal. 2014; 12: 11Crossref PubMed Scopus (37) Google Scholar], Kupffer cells and, most probably, in quiescent HSC. The latter cells, at least their majority, do not express cilia [[1]Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar], in contrast to activated HSC. The existence of this unusual type of Hh signalling (cyclopamine responsive) is most evident for healthy hepatocytes [[6]Matz-Soja M. Aleithe S. Marbach E. Böttger J. Arnold K. Schmidt-Heck W. et al.Hepatic Hedgehog signaling contributes to the regulation of IGF1 and IGFBP1 serum levels.Cell Commun Signal. 2014; 12: 11Crossref PubMed Scopus (37) Google Scholar]. In addition, SMO-independent, truly non-canonical signalling, has been observed at least in part of the cell types (Fig. 1). Thus, each major cell type in the liver appears to exhibit its own, specific combination of different types of Hh signalling (Fig. 1) [1Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 3Omenetti A. Diehl A.M. Hedgehog signaling in cholangiocytes.Curr Opin Gastroenterol. 2011; 27: 268-275Crossref PubMed Scopus (62) Google Scholar, 4Michelotti G.A. Xie G. Swiderska M. Choi S.S. Karaca G. Kruger L. et al.Smoothened is a master regulator of adult liver repair.J Clin Invest. 2013; 123: 2380-2394PubMed Google Scholar, 5Sicklick J.K. Li Y.X. Melhem A. Schmelzer E. Zdanowicz M. Huang J. et al.Hedgehog signaling maintains resident hepatic progenitors throughout life.Am J Physiol Gastrointest Liver Physiol. 2006; 290: G859-G870Crossref PubMed Scopus (176) Google Scholar, 6Matz-Soja M. Aleithe S. Marbach E. Böttger J. Arnold K. Schmidt-Heck W. et al.Hepatic Hedgehog signaling contributes to the regulation of IGF1 and IGFBP1 serum levels.Cell Commun Signal. 2014; 12: 11Crossref PubMed Scopus (37) Google Scholar, 7Omenetti A. Choi S. Michelotti G. Diehl A.M. Hedgehog signaling in the liver.J Hepatol. 2011; 54: 366-373Abstract Full Text Full Text PDF PubMed Scopus (196) Google Scholar, 8Pereira T.A. Xie G. Choi S.S. Syn W.K. Voieta I. Lu J. et al.Macrophage-derived Hedgehog ligands promotes fibrogenic and angiogenic responses in human schistosomiasis mansoni.Liver Int. 2013; 33: 149-161Crossref PubMed Scopus (45) Google Scholar]. These differences may be highly flexible due to various activated states of these cells under pathological conditions. Since most of these cell types are not only Hh responding but also secreting cells, depending on those activated states, the concept of intrahepatic signalling niches of Hedgehog suggested by Anna Mae Diehl's group [cf. [[5]Sicklick J.K. Li Y.X. Melhem A. Schmelzer E. Zdanowicz M. Huang J. et al.Hedgehog signaling maintains resident hepatic progenitors throughout life.Am J Physiol Gastrointest Liver Physiol. 2006; 290: G859-G870Crossref PubMed Scopus (176) Google Scholar]] and Grzelak and colleagues [[1]Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar] acquires more attention. Thus, even the normal sinusoid (or possibly the space of Dissé) must be considered as a Hh signalling niche of high complexity, forcing us in the future to focus on the microanatomical environments of cells, local sources and concentrations or gradients of morphogens, and on the different sensitivities of Hh-responsive cells to understand their physiological impact. Given the cellular diversity, questions regarding which type of signals (Hh ligands and other factors modulating Hh signalling) are transmitted within different niches and what type of message they are transmitting become important. To address these questions, comparison with other organs may be helpful [[9]Swierczynska M.M. Lamounier-Zepter V. Bornstein S.R. Eaton S. Lipoproteins and Hedgehog signalling–possible implications for the adrenal gland function.Eur J Clin Invest. 2013; 43: 1178-1183PubMed Google Scholar]. Concerning liver disease, another aspect of great importance is to elucidate how the normal Hh niche becomes transformed into (locally or globally) a disease-associated niche. With respect to TAA, Grzelak and colleagues have provided us with valuable insights of what occurs in terms of the Hh-driven transient expansion of cellular diversity and the local reconstruction of niches [[1]Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar]. Concerning the signals involved and their proper cellular origin, however, their paper is less clear. Because in situ hybridisation was performed only for Shh [[1]Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar], it remains unclear whether and where Ihh is also expressed in higher amounts in response to TAA. At the protein level the use of pan Hh antibodies leaves us with a similar uncertainty: IHH immunostaining in hepatocytes was shown in disturbed areas in close proximity of LPCs only in a few pictures [[1]Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar]. Although the authors refer to the IHH-positive hepatocytes as "injured cells" [[1]Grzelak C.A. Martelotto L.G. Sigglekow N.D. Patkunanathan B. Ajami K. Calabro S.R. et al.The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury.J Hepatol. 2014; 60: 143-151Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar], proof of the actual injury is lacking. Within this context, it should be emphasised that Ihh in normal liver has been recognised as a target gene of Wnt/ß-catenin signalling [[10]Reed K.R. Athineos D. Meniel V.S. Wilkins J.A. Ridgway R.A. Burke Z.D. et al.B-catenin deficiency, but not Myc deletion, suppresses the immediate phenotypes of APC loss in the liver.Proc Natl Acad Sci U S A. 2008; 105: 18919-18923Crossref PubMed Scopus (59) Google Scholar], and, correspondingly, expression could be demonstrated in healthy hepatocytes around the central veins [6Matz-Soja M. Aleithe S. Marbach E. Böttger J. Arnold K. Schmidt-Heck W. et al.Hepatic Hedgehog signaling contributes to the regulation of IGF1 and IGFBP1 serum levels.Cell Commun Signal. 2014; 12: 11Crossref PubMed Scopus (37) Google Scholar, 10Reed K.R. Athineos D. Meniel V.S. Wilkins J.A. Ridgway R.A. Burke Z.D. et al.B-catenin deficiency, but not Myc deletion, suppresses the immediate phenotypes of APC loss in the liver.Proc Natl Acad Sci U S A. 2008; 105: 18919-18923Crossref PubMed Scopus (59) Google Scholar] where Wnt signalling is highest (cf. [[2]Matz-Soja M. Hovhannisyan A. Gebhardt R. Hedgehog signalling pathway in adult liver: a major new player in hepatocyte metabolism and zonation?.Med Hypotheses. 2013; 80: 589-594Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar]). Thus, it would be informative if more details of Hh ligand expression in control livers and in response to TAA were available. In summary, (intra)hepatic Hh signalling emerges as a major player in health and disease, the proper impact of which can only be elucidated if we manage to narrow down our research to microenvironments and mutual cell-cell interactions. The work was supported by a grant from the Bundesministerium für Forschung und Technologie ( BMBF ) within the framework of the Systems Biology initiative "Virtual Liver Network" (grant: 0315735 ). The authors declared that they do not have anything to disclose regarding funding or conflicts of interest with respect to this manuscript. The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injuryJournal of HepatologyVol. 60Issue 1PreviewIn vertebrates, canonical Hedgehog (Hh) pathway activation requires Smoothened (SMO) translocation to the primary cilium (Pc), followed by a GLI-mediated transcriptional response. In addition, a similar gene regulation occurs in response to growth factors/cytokines, although independently of SMO signalling. The Hh pathway plays a critical role in liver fibrosis/regeneration, however, the mechanism of activation in chronic liver injury is poorly understood. This study aimed to characterise Hh pathway activation upon thioacetamide (TAA)-induced chronic liver injury in vivo by defining Hh-responsive cells, namely cells harbouring Pc and Pc-localised SMO. Full-Text PDF Reply to: "The many faces of Hedgehog signalling in the liver: Recent progress reveals striking cellular diversity and the importance of microenvironments"Journal of HepatologyVol. 61Issue 6PreviewWe thank Matz-Soja and colleagues for their interest and critique of our article [1]. We agree with Matz-Soja et al. that generalisations regarding the contribution of Smoothened (SMO)-dependent and SMO-independent GLI-mediated signals during liver injury processes should be made with caution. We concur that more work is required to delineate how, within various liver cell niches, Hedgehog (Hh) pathway signalling components elicit canonical or 'non-canonical' Hh signalling responses during liver injury. Full-Text PDF Open Access