In a fascinating study, Cai et al.1 examined how the sea lamprey adapts to a programmed disappearance of the gallbladder, intra- and extrahepatic bile ducts, and bile canaliculi. The investigators studied bile acid and xenobiotic homoeostasis, and used molecular biological profiling to define the expression of transporters in the liver and kidney of lamprey larvae and adults. Adult livers were severely cholestatic as assessed by high bile salt levels but had no evidence of cytological damage such is the necrosis, fibrosis, or inflammation. In both larvae and adults plasma bile acid levels were maintained at a low level, even though the adult livers lack a biliary system. One mechanism for adaptation in the adults is to transform C 24 bile acids to C 27 bile acids. The authors found that petromyzonol sulfate, the major bile salt in lamprey larvae is cytotoxic, but is converted to the less toxic 3-keto-petromyzonol sulfate in the adult. Interestingly, apical canalicular transporters could be detected by immunochemical methods only in the livers of larvae. Additional experiments showed that the main route of excretion in the adult for bromsulfaphein (BSP) and bile acids was through the urine. In keeping with this observation they found through gene expression studies that there was marked up-regulation of orthologs for organic anion and bile acid transporters in the kidneys.
Atresia is commonly defined as the congenital absence or pathological closure of an opening, passage, or cavity. In all organisms, save the sea lamprey, the process is pathological in organs such as the esophagus, intestine, and biliary tract caused by a failure of normal development or by acquired destruction usually via inflammatory or vascular mechanisms. In the current study the authors compare the process in sea lamprey to biliary atresia in human neonates. Thus, it is instructive to contrast the pathophysiological features of biliary atresia with the normal programmed loss of entire biliary apparatus in sea lamprey larvae.
Biliary atresia in human infants is characterized by the complete obstruction of bile flow as a result of the destruction or absence of all or a portion of the extrahepatic bile ducts.2–4 As part of the underlying disease process or as a result of biliary obstruction, concomitant injury and fibrosis of the intrahepatic bile ducts also occurs to a variable extent. The disorder occurs in 1 in 10,000 to 15,000 live births in the United States, and accounts for approximately one-third of cases of neonatal cholestatic jaundice. It is the most frequent cause of death from liver disease and accounts for about 50% of all liver transplants in children. There is some evidence for two forms of biliary atresia, a fetal or embryonic form and a peri- or postnatal form. In infants with the less common fetal variant (∼10%-25% of cases) cholestasis with acholic stools is present from birth with no jaundice-free interval after resolution of normal physiological hyperbilirubinemia. At the time of exploratory laparotomy, little or none of the extrahepatic biliary structures can be found in the hepatic hilum, and there are often associated malformations such as the polysplenia syndrome and abdominal situs inversus. In contrast, in the postnatal form there is progressive inflammatory destruction of the extrahepatic biliary tract in a baby appearing healthy in the first weeks of life. Clinical features support the concept that in most cases injury to the biliary tract occurs after biliary morphogenesis usually after birth. In practice, differentiation of these clinical forms on the basis of the onset of liver dysfunction and occurrence of congenital malformations is inexact. Indeed, a recent study showed that over half of patients with biliary atresia have elevated direct/conjugated bilirubin levels shortly after birth.5
The cause of biliary atresia is unknown. Several mechanisms have been proposed to account for the progressive obliteration of the extrahepatic biliary tree. There is no evidence that biliary atresia results from a failure in morphogenesis in the majority of affected infants or from an ischemic or toxic injury to the bile ducts. There is emerging, convincing evidence for initiation of the process probably in response to a common viral infection or unknown environmental factor in a genetically susceptible host. A dysregulated cellular, humoral, and innate immune response all seem to be involved based on studies in humans and a mouse model of the disease.6, 7 In keeping with an immune basis for injury to the biliary tract, two recent genome-wide association studies identified two susceptibility genes that encode proteins involved in regulating inflammation, glypican 1-a heparan sulfate proteoglycan and X-prolyl aminopeptidase P.8, 9
In a microarray analysis of liver tissue from infants with a so-called embryonic form of biliary atresia in which extrahepatic malformations and early onset of cholestatic jaundice occur, a unique pattern of expression of genes involved in chromatin integrity and function and overexpression of five imprinted genes was found, implying a failure to down-regulate embryonic gene programs that influence the development of the liver and other organs.10 Heterozygous CFC1 (encoding the cryptic protein) mutations have been rarely associated with biliary atresia and polysplenia, and therefore may represent a genetic predisposition to this pattern of malformations.11
So what can the sea lamprey tell us about the human condition? It is important to emphasize that the entire biliary apparatus of larvae including bile canaliculi disappears completely during normal metamorphosis in contrast to the pathological state of human biliary atresia.12, 13 However, it would be interesting and informative with regard to the embryonic form of human biliary atresia to understand the genetic programming underlying disappearance of biliary apparatus in the sea lamprey. As might be expected, the degeneration of bile ducts occurs via programmed cell death or apoptosis in a related, nonparasitic lamprey, Lethenteron reissner, and in the sea lamprey, Petromyzon marinus.15, 16 Similar to the lamprey, several reports have documented cholangiocyte apoptosis as evidenced by positive transferase-mediated dUTP nick end labeling (TUNEL) staining of cholangiocytes in the livers of patients with biliary atresia. Although apoptosis is a normal process in remodeling of the mammalian ductal plate during liver development, it would not be expected in the extrahepatic biliary system except as part of the process of immunologic ductal injury.16 In the lamprey the order of degeneration of bile ducts, i.e., intrahepatic versus extrahepatic, is variable between species. In the sea lamprey the degenerative process is asynchronous, and occurs more rapidly in small peripheral biliary components than in larger, medial ducts.12, 13 There is gradual disruption of tight junctions at the bile canaliculi and relocalization of membrane enzymes including alkaline phosphatase, adenosinetriphosphatase, and 5′-nucleotidase from apical to lateral membranes. In the human the most severe focus of injury is in the extrahepatic biliary system. The intrahepatic bile ducts respond initially with ductular proliferation and may be obstructed by bile plugs. Further and irreversible injury results from the noxious effects of biliary obstruction and probable ongoing immunologic damage that is variably relieved by the Kasai hepato-portoenterostomy operation.17 In adult lampreys, there seem to be no immediate consequences from the absence of a biliary system for the elimination of bile products.
It is unknown why the lamprey needs a biliary system during the larval stage, only to allow its programmed degeneration later. The lamprey has evolved effective adaptive strategies to compensate for the lack of a biliary apparatus in the adult by up-regulation of organic anion and bile acid transporter orthologs in the kidney and by altering bile acid composition.1 The authors in the current study found that petromyzonol sulfate, the major bile salts in lamprey larva, is cytotoxic, but is converted to the markedly less toxic 3-keto-petromyzonol sulfate in the adult. Humans with obstructive cholestasis mirror some of these adaptations by up-regulation of exporters such as MRP4, MRP3, and OSTα/OSTβ on the liver basolateral membrane and by down-regulating bile acid biosynthesis. There is also up-regulation of pathways for bile acid detoxification through hydroxylation (phase I) and conjugation (phase II) via sulfation and glucuronidation, and at least in animal models up-regulating renal organic anion transporters.18 These strategies are beneficial and probably attenuate liver injury, but in most cases of biliary atresia are not effective in preventing progressive liver disease. It is possible that agonists could be developed that specifically target adaptive pathways for biotransformation and transport, possibly acting through nuclear receptors such as FXR, VDR, CAR, and PXR.