Microvilli as markers of disordered apical-membrane trafficking and assembly: Bowel and liver
Authors: Richard J. Thompson, A.S. Knisely
Journal: Hepatology (2014)

Microvillous inclusion disease (MVID), also known as microvillous atrophy, is a rare autosomal-recessive enteropathy due to mutation in MYO5B,1 encoding a myosin implicated in intracellular trafficking.2 First described in 1978,3 it has since been reported in many populations.4-6 Long-term management of MVID-associated intestinal failure has only been achieved with parenteral nutrition (PN)7 and, more recently, intestinal transplantation.8 Long-term PN from birth is frequently associated with development of cholestasis.8 That patients with MVID receiving PN develop liver disease thus is no surprise. Clear for some time, however, has been that the frequency and degree of cholestasis are greater in MVID than in other forms of intestinal failure supported with PN, such as tufting enteropathy. Girard et al.9 now present data indicating involvement of the bile salt export pump (BSEP) in MVID-associated cholestasis. That liver disease in MVID is intrinsic was always evident. In fact, hepatobiliary disease strikingly similar to that of MVID is seen with familial intrahepatic cholestasis 1 (FIC1) deficiency (ATP8B1 disease). Therefore, ATP8B1 was a strong candidate as the MVID disease gene. Mutations were not found in ATP8B1, despite all consanguine MVID patients then studied being homozygous in the region of chromosome 18 that harbors ATP8B1. That region included MYO5B, only 8 Mb away from ATP8B1. That these similar liver diseases are caused by mutations in quite different genes that lie close together in the genome might be coincidental. It might also, however, reflect close relationships between the products of the genes or between those products' functions. Among the other key players in apical membrane assembly are the RAB proteins, which act as lipid anchors and permit vesicular transport. Functionally, therefore, these three classes of protein in common subserve the generation of apical, and apically destined, membranes and are used throughout epithelia. During the course of evolution the archetypal atypical myosin has duplicated several times and many FIC1-like proteins and RAB proteins have come to exist. Humans have three members of the MYO5 family and four members of the ATP8B family. The RAB family is huge, containing at least 70 members with varied functions. Genes with closely allied functions have sometimes stayed together on the same chromosome during evolution. In humans, ATP8B1, MYO5B, and RAB27B lie together on chromosome 18. The other best preserved such cluster, of ATP8B4, MYO5A, MYO5C, and RAB27A, including a duplicated and modified myosin, is on chromosome 15. Within this cluster, mutation in either MYO5A or RAB27A leads to Griscelli syndrome (albinism and immunodeficiency).10, 11 Furthermore, the ATP8B1/MYO5B/RAB27B cluster is preserved across mammals and even in a monotreme, the platypus, where the syntenic region is on chromosome 3. Conservation of synteny in this way does not prove that products of syntenic genes cooperate in function, but it certainly supports that hypothesis, as shown by Griscelli syndrome. The histological—and, by extension, the clinical—similarities seen when either ATP8B1 or MYO5B malfunctions are further evidence of a common purpose. The work of Girard et al.9 also highlights the interaction of liver and bowel, in this case a transplanted bowel, making the disease worse in a genetically abnormal liver. In FIC1 deficiency it is often the mirror image, where a transplanted liver worsens the function of the genetically predisposed gut. As shown by Girard et al.9 and others,12-14 a principal problem in MVID is one of apical membrane assembly and maintenance. MYO5B contributes directly to this process. FIC1 is an aminophospholipid flippase that appears important in maintenance of intramembrane lipid asymmetry. When FIC1-mediated maintenance fails, mature microvilli (the vermiform extensions of cell surfaces that expand absorptive and secretory capacity) are lacking—both at the bile canaliculus15, 16 and elsewhere, as supported by in vitro experiments.17 The same lack of microvilli affects bile canaliculi in MVID. Of interest in MVID is that in enterocytes, at least, microvilli form, but line vesicles within apical cytoplasm rather than being inserted into the apical membrane.18 Such inclusion-body-like vesicles have not been reported in hepatocytes in MVID. Deficiency of microvilli at the bile canaliculus in cholestasis, however, is encountered in many different cholestatic disorders19 and may be considered an effect as well as, perhaps, a potential cause. Loss of microvilli results in substantial loss of canalicular surface area. This alone might be expected to promote cholestasis. Many forms of cholestasis may entail a secondary loss of canalicular membrane.19 However, both in MVID and in ATP8B1 deficiency canalicular-membrane assembly and maintenance seem primarily impaired. Shedding of membrane into bile occurs in ATP8B1 deficiency20—might this also be found in MVID, with ultrastructural features of bile resembling the loose, coarse granularity of "Byler bile"? Another feature of ATP8B1 deficiency is lack of gamma-glutamyl transpeptidase (GGT) expression at canalicular membranes20—is the expression of this and other canalicular antigens deficient in MVID as well? Unlike transporters such as BSEP, ectoenzymes such as GGT are not trafficked directly to cell apices, moving instead by microtubule-dependent transcytosis from the basolateral membrane.21 MYO5B mutation reportedly affects enterocyte polarity,1 with disordered expression of basolateral as well as apical transporters.22 Are basolateral microvilli of hepatocytes lacking or disorganized in MVID? It is still not clear. Girard et al. report, however, some intracellular redistribution of BSEP in MVID with cholestasis.9 Most BSEP apparently lies towards the apex of the cell, perhaps in a sub-canalicular zone. Shuttling between this zone and the canalicular membrane depends on both RAB11A and MYO5B.23, 24 They also describe intracellular accumulation of RAB11A, as might be expected in the absence of MYO5B, with intracellular accumulation of MYO5B itself (likely reflecting a nonprotein-truncating mutation). Nonetheless, it seems evident that abnormal handling of BSEP alone is not responsible for cholestasis in MVID. This leads to a larger question: Why do some patients with MVID develop hepatobiliary disease while others are spared? Girard et al. tell us that clinical disease is not associated with particular MYO5B genotypes, although the actual mutations are not presented.9 We are told that disease-causing mutations in neither ATP8B1 nor ABCB11 were present in these patients. Instead, other contributors to the machinery of membrane assembly and maintenance, perhaps in basolateral as well as in canalicular domains, such as members of the RAB family, seem good candidates. This should obviously be a focus for future work. As usual when identifying correlations between genetic mutations, histopathologic findings, and clinical manifestations of disease, any one set of observations raises more questions than it dispels. In MVID, it seems, loss or displacement of bile-canaliculus microvilli is a marker for a disruption in membrane trafficking rather than the etiology of cholestasis per se. Richard J. Thompson, B.M.B.Ch.1-3 A.S. Knisely, M.D.2 1Institute of Liver Studies Division of Transplantation Immunology and Mucosal Biology King's College London School of Medicine London, UK 2Institute of Liver Studies King's College Hospital London, UK 3Paediatric Liver GI and Nutrition Centre King's College Hospital London, UK