Abnormal expression of brush-border membrane transporters in the duodenal mucosa of two patients with microvillus inclusion disease

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Intestinal epithelial cell polarity defects in disease: lessons from microvillus inclusion disease

The intestinal epithelium is a highly organized tissue. The establishment of epithelial cell polarity, with distinct apical and basolateral plasma membrane domains, is pivotal for both barrier formation and for the uptake and vectorial transport of nutrients. The establishment of cell polarity requires a specialized subcellular machinery to transport and recycle proteins to their appropriate location. In order to understand and treat polarity-associated diseases, it is necessary to understand epithelial cell-specific trafficking mechanisms. In this Review, we focus on cell polarity in the adult mammalian intestine. We discuss how intestinal epithelial polarity is established and maintained, and how disturbances in the trafficking machinery can lead to a polarity-associated disorder, microvillus inclusion disease (MVID). Furthermore, we discuss the recent developments in studying MVID, including the creation of genetically manipulated cell lines, mouse models and intestinal organoids, and their uses in basic and applied research.

Summary: Microvillus inclusion disease serves as a useful model to enhance our understanding of the intestinal trafficking and polarity machinery in health and disease.

Microvillus inclusion disease: loss of Myosin vb disrupts intracellular traffic and cell polarity

Microvillus inclusion disease (MVID) is a congenital enteropathy characterized by loss of apical microvilli and formation of cytoplasmic inclusions lined by microvilli in enterocytes. MVID is caused by mutations in the MYO5B gene, coding for the myosin Vb motor protein. Although myosin Vb is implicated in the organization of intracellular transport and cell surface polarity in epithelial cells, its precise role in the pathogenesis of MVID is unknown. We performed correlative immunohistochemistry analyses of sections from duodenal biopsies of a MVID patient, compound heterozygous for two novel MYO5B mutations, predicting loss of function of myosin Vb in duodenal enterocytes together with a stable MYO5B CaCo2 RNAi cell system. Our findings show that myosin Vb-deficient enterocytes display disruption of cell polarity as reflected by mislocalized apical and basolateral transporter proteins, altered distribution of certain endosomal/lysosomal constituents including Rab GTPases. Together, this severe disturbance of epithelial cell function could shed light on the pathology and symptoms of MVID.

Persistent defective membrane trafficking in epithelial cells of patients with familial hemophagocytic lymphohistiocytosis type 5 due to STXBP2/MUNC18-2 mutations

BACKGROUND

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare primary immune disorder defined by mutations in the syntaxin binding protein 2 (STXBP2) alias MUNC18-2. Despite defective immunity and a hyper-inflammatory state, clinical findings such as neurological, gastrointestinal, and bleeding disorders are present in a significant number of patients and suggest an impaired expression and function of STXBP2 in cells other than cytotoxic lymphocytes.

PROCEDURE

We investigated four patients with FHL5 suffering from severe enteropathy and one of whom also had renal tubular dysfunction despite successful hematopoietic stem cell transplantation (HSCT). Gastrointestinal and renal biopsy specimens were analyzed by immunohistochemistry and electron microscopy.

RESULTS

Histopathology revealed an intracytoplasmatic accumulation of PAS-positive granules and an enlarged intracytoplasmatic CD10-positive band along the apical pole of enterocytes. Electron microscopy revealed short microvilli and granules filled with electro lucent material. In addition, we described mildly dilated renal tubules and electron micrographs displayed a higher number of cytoplasmic inclusions, electrodense lysosomal and electrolucent endosomal vesicles.

CONCLUSION

Mutations in STXBP2 do not only affect cytotoxic T lymphocytes but also cause changes in the intestinal and renal epithelium resulting in severe, osmotic diarrhea and renal proximal tubular dysfunction. These defects persist after successful treatment of hemophagocytic lymphohistocytosis by HSCT. Clinical manifestations in FHL5 patients despite successful HSCT may therefore be related to defective membrane trafficking in the gut and kidney.

Microvillous inclusion disease--an ultrastructural diagnosis: with a review of the literature

Microvillous inclusion disease is a rare disorder of infancy associated with protracted diarrhea. This malady reveals distinct ultrastructural changes. The surface enterocytes of the duodenum show vesicles lined with microvilli and the surface microvilli are poorly formed. The authors present one case of microvillous inclusion disease with a review of the literature.

Functional characterization of mutations in the myosin Vb gene associated with microvillus inclusion disease

OBJECTIVES

Microvillus inclusion disease (MVID) is a rare autosomal recessive enteropathy characterized by intractable diarrhea and malabsorption. Recently, various MYO5B gene mutations have been identified in patients with MVID. Interestingly, several patients with MVID showed only a MYO5B mutation in 1 allele (heterozygous) or no mutations in the MYO5B gene, illustrating the need to further functionally characterize the cell biological effects of the MYO5B mutations.

PATIENTS AND METHODS

The genomic DNA of 9 patients diagnosed as having MVID was screened for MYO5B mutations, and quantitative polymerase chain reaction and immunohistochemistry on the material of 2 patients was performed to investigate resultant cellular consequences.

RESULTS

We demonstrate for the first time that MYO5B mutations can be correlated with altered myosin Vb messenger RNA expression and with an aberrant subcellular distribution of the myosin Vb protein. Moreover, we demonstrate that the typical and myosin Vb-controlled accumulation of Rab11a- and FIP5-positive recycling endosomes in the apical cytoplasm of the cells is abolished in MVID enterocytes, which is indicative of altered myosin Vb function. Moreover, we report 8 novel MYO5B mutations in 9 patients of various ethnic backgrounds with MVID, including compound heterozygous mutations.

CONCLUSIONS

Our functional analysis indicates that MYO5B mutations can be correlated with an aberrant subcellular distribution of the myosin Vb protein, and apical recycling endosomes, which, together with the additional compound heterozygous mutations, significantly strengthen the link between MYO5B and MVID.

Autosomal recessive diseases among the Athabaskans of the southwestern United States: recent advances and implications for the future

Genetic and linguistic data suggest that the Na-Dene, of which the Athabaskans are the largest group, are part of a later immigration into the Americas than the first Amerind immigration. Whether a second and third immigration can be separated seems unlikely but continued cross-Bering Strait exchanges may have masked what was a greater separation in the past. The movement of tribes into Siberia appears to have involved a genetic bottleneck leading to at least one disease allele shared by Eskimo/Aleuts and Navajos and a second possibly shared by the Navajo and a Siberian population, but not the same Siberian population that share deep linguistic affinities with the Navajo. A second bottleneck appears to have occurred with the migration of Athabaskans from Northwest North America to the Southwestern United States along the Rocky Mountains. This bottleneck is reflected in several rare recessive diseases shared by the Navajo and Apache. Finally, the Navajo were captured and imprisoned under conditions which led to severe population loss. This, and the "hiding away" of a small number of Navajos in what is now the Western portion of the reservation, led to a Navajo-specific bottleneck(s) resulting in an increased frequency of several rare recessive diseases among the Navajo. Prejudice against human genetic research is high among the Southwestern Athabaskans but attempts to bridge the gap are now occurring. The involvement of Navajo scientists in this process is especially encouraging.

Navajo microvillous inclusion disease is due to a mutation in MYO5B

Microvillous Inclusion Disease (MID) is a rare, autosomal recessive gastrointestinal disease of increased frequency among the Navajos. Previous work has shown a deficiency of RAB8 in one Japanese patient, while homozygous mutations in MYO5B were found in 7 of 10 mostly Middle Eastern families. We have identified a shared homozygous mutation in MYO5B in seven affected Navajos with the expected heterozygosity in five parents. We have developed a simple restriction enzyme based assay that allows for rapid screening for this mutation.

Microvillous inclusion disease (microvillous atrophy)

Microvillous inclusion disease (MVID) or microvillous atrophy is a congenital disorder of the intestinal epithelial cells that presents with persistent life-threatening watery diarrhea and is characterized by morphological enterocyte abnormalities. MVID manifests either in the first days of life (early-onset form) or in the first two months (late-onset form) of life. MVID is a very rare disorder of unknown origin, probably transmitted as an autosomal recessive trait. To date, no prevalence data are available. Ultrastructural analyses reveal: 1) a partial to total atrophy of microvilli on mature enterocytes with apical accumulation of numerous secretory granules in immature enterocytes; 2) the highly characteristic inclusion bodies containing rudimentary or fully differentiated microvilli in mature enterocytes. Light microscopy shows accumulation of PAS-positive granules at the apical pole of immature enterocytes, together with atrophic band indicating microvillus atrophy and, in parallel, an intracellular PAS or CD10 positive line (marking the microvillous inclusion bodies seen on electron microscopy). Intestinal failure secondary to diarrhea is definitive. To date, no curative therapy exists and children with MVID are totally dependent on parenteral nutrition. Long-term outcome is generally poor, due to metabolic decompensation, repeated states of dehydration, infectious and liver complications related to the parenteral nutrition. As MVID is a very rare disorder, which is extremely difficult to diagnose and manage, children with MVID should be transferred to specialized pediatric gastro-intestinal centers, if possible, a center equipped to perform small bowel transplantation. Early small bowel transplantation resulting in intestinal autonomy gives new hope for disease management and outcome.

Unusual ultrastructural features in microvillous inclusion disease: A report of two cases

Microvillous Inclusion Disease (MID) is an inherited disorder characterized by intractable diarrhea in infancy. Ultrastructural detection of pathognomonic microvillous inclusions in the enterocytes is essential for diagnosis. The aim of this research is to contribute to the knowledge of MID studying enterocytes and goblet cells (gc). Samples of duodenal mucosa from two young infants with MID (aged 75 days and 3 months, respectively) were studied by light and electron microscopy. Detection in the intestinal villi of immature gc (with microvilli) in one of the cases led us to seek them in control samples. The total number of gc with microvilli (immature) and without microvilli (mature) were counted. In both MID specimens, light microscopy showed atrophy of villi and PAS-positive material in the enterocyte cytoplasm. The ultrastructure of villous enterocytes was characterized by brush-border abnormalities, microvillous inclusions, dense apical granules, and lysosomes. Intermediate structures between microvillous inclusions and lysosomes were also detected within a cell, as were rare microvilli on the lateral membrane of the enterocytes. In one MID specimen, immature gc were also identified in the absorptive compartment. Only mature gc were observed in the controls. The significance of the latter finding requires further studies.

Microvilli defects in retinas of ezrin knockout mice

Ezrin, a member of the ezrin/moesin/radixin (ERM) family, localizes to microvilli of epithelia in vivo, where it functions as a bridge between actin filaments and plasma membrane proteins. In the eye, ezrin has been localized to both apical microvilli of Müller cells and retinal pigment epithelium (RPE) apical microvilli and basal infoldings. In the present study, we analyze these structures in the eyes of early postnatal ezrin knockout mice. This analysis indicates that the loss of ezrin leads to substantial reductions in the apical microvilli and basal infoldings in RPE cells and in the Müller cell apical microvilli. The absence of apical microvilli in the RPE is accompanied by the presence of microvilli-like inclusions (MIs) in the RPE cytoplasm. Finally, photoreceptors in the ezrin knockout animals show substantial retardation in development as compared to their wild type littermates.

[CD10 expression in a case of microvillous inclusion disease]

All over the causes of intractable diarrhea of infancy, microvillous inclusion disease is a rare congenital defect of intestinal brush border of unknown aetiology. An autosomal recessive inheritance is suggested by cases occurring in siblings and high incidence of consanguinity. The prognosis of the disease is extremely poor, as life can be sustained only by total parenteral nutrition. Combined bowel-liver or bowel transplantation is regarded as the only potentially life-saving therapy. We report a case of microvillous atrophy who undergone a combined bowel, colonic and liver transplantation, and discuss the tools allowing the light microscopic diagnosis.

Proteomic characterization of isolated retinal pigment epithelium microvilli

Polarized epithelial cells are characterized by displaying compartmentalized functions associated with differential distribution of transporters, structural proteins, and signaling molecules on their apical and basolateral surfaces. Their apical surfaces frequently elaborate microvilli, which vary in structure according to the specific type and function of each epithelium. The molecular basis of this heterogeneity is poorly understood. However, differences in function will undoubtedly be reflected in the specific molecular composition of the apical surface in each epithelial subtype. We have exploited a method for isolating microvilli from the mouse eye using wheat germ agglutinin (WGA)-agarose beads to begin to understand the specific molecular composition of apical microvilli of the retinal pigment epithelium (RPE) and expand our knowledge of the potential function of this interface. Initially, apical RPE plasma membranes bound to WGA beads were processed for morphological analysis using known apical and basolateral surface markers. The protein composition of the apical microvilli was then established using proteomic analysis. Over 200 proteins were identified, including a number of proteins previously known to be localized to RPE microvilli, as well as others not known to be present at this surface. Localization of novel proteins identified with proteomics was confirmed by immunohistochemistry in both mouse and rat eye tissue. The data generated provides new information on the protein composition of the RPE apical microvilli. The isolation technique used should be amenable for isolating microvilli in other epithelia as well, allowing new insights into additional functions of this important epithelial compartment.

Neonatal congenital microvillus atrophy

Congenital microvillous atrophy (CMVA) is the leading cause of neonatal secretory diarrhoea with onset either in the first 72 hours of life (early onset) or at 6-8 weeks after birth (late onset). To date over 30 cases have been reported worldwide. The prognosis for this life threatening condition continues to be poor. Therapeutic agents like somatostatin and epidermal growth factor are either ineffective or of marginal benefit. Overall five year survival after small bowel transplantation is currently approximately 50%. The following brief review is aimed towards helping neonatologists/perinatologists in the early diagnosis, and management of CMVA and in counselling the parents appropriately.

Neonatal enteropathies: defining the causes of protracted diarrhea of infancy

The underlying causes of chronic diarrhea beginning early in life are increasingly well defined. Infectious and post-infectious enteropathies and food sensitive/allergic enteropathy account for the majority of cases. Recent attention has focused on characterizing defined entities, which cause protracted diarrhea in infants and young children. Disorders of intestinal ion transport usually present at birth following a pregnancy complicated by polyhydramnios. Intestinal mucosal biopsies show normal architect with intact villus-crypt axis. Neonatal enteropathies, by contrast, are characterized by blunting of the villi. These include microvillus inclusion disease, tufting enteropathy, autoimmune enteropathy and IPEX syndrome - and it is these conditions that are the subject of the current review.