Dynamic Formation of Microvillus Inclusions During Enterocyte Differentiation in Munc18-2-Deficient Intestinal Organoids

Uncovering the Relationship Between Genes and Phenotypes Beyond the Gut in Microvillus Inclusion Disease

Microvillus inclusion disease (MVID) is a rare condition that is present from birth and affects the digestive system. People with MVID experience severe diarrhea that is difficult to control, cannot absorb dietary nutrients, and struggle to grow and thrive. In addition, diverse clinical manifestations, some of which are life-threatening, have been reported in cases of MVID. MVID can be caused by variants in the MYO5B, STX3, STXBP2, or UNC45A gene. These genes produce proteins that have been functionally linked to each other in intestinal epithelial cells. MVID associated with STXBP2 variants presents in a subset of patients diagnosed with familial hemophagocytic lymphohistiocytosis type 5. MVID associated with UNC45A variants presents in most patients diagnosed with osteo-oto-hepato-enteric syndrome. Furthermore, variants in MYO5B or STX3 can also cause other diseases that are characterized by phenotypes that can co-occur in subsets of patients diagnosed with MVID. Recent studies involving clinical data and experiments with cells and animals revealed connections between specific phenotypes occurring outside of the digestive system and the type of gene variants that cause MVID. Here, we have reviewed these patterns and correlations, which are expected to be valuable for healthcare professionals in managing the disease and providing personalized care for patients and their families.

Intestinal brush border formation requires a TMIGD1-based intermicrovillar adhesion complex

Intestinal epithelial cells absorb nutrients through the brush border, composed of dense arrays of highly ordered microvilli at their apical membranes. A protocadherin-based intermicrovillar adhesion complex localized at microvilli tips mediates microvilli packing and organization. Here, we identified a second adhesion complex localized at the proximal base region of microvilli. This complex contained the immunoglobulin superfamily member TMIGD1, which directly interacted with the microvillar scaffolding proteins EBP50 and E3KARP. Complex formation with EBP50 required the activation of EBP50 by the actin-binding protein ezrin and was enhanced by the dephosphorylation of Ser¹⁶² in the PDZ2 domain of EBP50 by the phosphatase PP1α. Binding of the EBP50-ezrin complex to TMIGD1 enhanced the dynamic turnover of EBP50 at microvilli. Enterocyte-specific inactivation of Tmigd1 in mice resulted in microvillar blebbing, loss of intermicrovillar adhesion, and perturbed brush border formation. Thus, we identified a second adhesion complex in microvilli and propose a mechanism that promotes microvillar formation and dynamics.

Loss of Serum Glucocorticoid-Inducible Kinase 1 SGK1 Worsens Malabsorption and Diarrhea in Microvillus Inclusion Disease (MVID)

Microvillus inclusion disease (MVID), a lethal congenital diarrheal disease, results from loss of function mutations in the apical actin motor myosin VB (MYO5B). How loss of MYO5B leads to both malabsorption and fluid secretion is not well understood. Serum glucocorticoid-inducible kinase 1 (SGK1) regulates intestinal carbohydrate and ion transporters including cystic fibrosis transmembrane conductance regulator (CFTR). We hypothesized that loss of SGK1 could reduce CFTR fluid secretion and MVID diarrhea. Using CRISPR-Cas9 approaches, we generated R26^CreER;MYO5B^f/f conditional single knockout (cMYO5BKO) and R26^CreER;MYO5B^f/f;SGK1^f/f double knockout (cSGK1/MYO5B-DKO) mice. Tamoxifen-treated cMYO5BKO mice resulted in characteristic features of human MVID including severe diarrhea, microvillus inclusions (MIs) in enterocytes, defective apical traffic, and depolarization of transporters. However, apical CFTR distribution was preserved in crypts and depolarized in villus enterocytes, and CFTR high expresser (CHE) cells were observed. cMYO5BKO mice displayed increased phosphorylation of SGK1, PDK1, and the PDK1 target PKCι in the intestine. Surprisingly, tamoxifen-treated cSGK1/MYO5B-DKO mice displayed more severe diarrhea than cMYO5BKO, with preservation of apical CFTR and CHE cells, greater fecal glucose and reduced SGLT1 and GLUT2 in the intestine. We conclude that loss of SGK1 worsens carbohydrate malabsorption and diarrhea in MVID.

A Functional Relationship Between UNC45A and MYO5B Connects Two Rare Diseases With Shared Enteropathy

BACKGROUND & AIMS

UNC45A is a myosin (co-)chaperone, and mutations in the UNC45A gene were recently identified in osteo-oto-hepato-enteric (O2HE) syndrome patients presenting with congenital diarrhea and intrahepatic cholestasis. Congenital diarrhea and intrahepatic cholestasis are also the prime symptoms in patients with microvillus inclusion disease (MVID) and mutations in MYO5B, encoding the recycling endosome-associated myosin Vb. The aim of this study was to determine whether UNC45A and myosin Vb are functionally linked.

METHODS

CRISPR-Cas9 gene editing and site-directed mutagenesis were performed with intestinal epithelial and hepatocellular cell lines, followed by Western blotting, quantitative polymerase chain reaction, and scanning electron and/or confocal fluorescence microscopy to determine the relationship between (mutants of) UNC45A and myosin Vb.

RESULTS

UNC45A depletion in intestinal and hepatic cells reduced myosin Vb protein expression, and in intestinal epithelial cells, it affected 2 myosin Vb-dependent processes that underlie MVID pathogenesis: rat sarcoma-associated binding protein (RAB)11A-positve recycling endosome positioning and microvilli development. Reintroduction of UNC45A in UNC45A-depleted cells restored myosin Vb expression, and reintroduction of UNC45A or myosin Vb, but not the O2HE patient UNC45A-c.1268T>A variant, restored recycling endosome positioning and microvilli development. The O2HE patient-associated p.V423D substitution, encoded by the UNC45A-c.1268T>A variant, impaired UNC45A protein stability but as such not the ability of UNC45A to promote myosin Vb expression and microvilli development.

CONCLUSIONS

A functional relationship exists between UNC45A and myosin Vb, thereby connecting 2 rare congenital diseases with overlapping enteropathy at the molecular level. Protein instability rather than functional impairment underlies the pathogenicity of the O2HE syndrome-associated UNC45A-p.V423D mutation.

Strategies for genetic manipulation of adult stem cell-derived organoids

Organoid technology allows the expansion of primary epithelial cells from normal and diseased tissues, providing a unique model for human (patho)biology. In a three-dimensional environment, adult stem cells self-organize and differentiate to gain tissue-specific features. Accessibility to genetic manipulation enables the investigation of the molecular mechanisms underlying cell fate regulation, cell differentiation and cell interactions. In recent years, powerful methodologies using lentiviral transgenesis, CRISPR/Cas9 gene editing, and single-cell readouts have been developed to study gene function and carry out genetic screens in organoids. However, the multicellularity and dynamic nature of stem cell-derived organoids also present challenges for genetic experimentation. In this review, we focus on adult gastrointestinal organoids and summarize the state-of-the-art protocols for successful transgenesis. We provide an outlook on emerging genetic techniques that could further increase the applicability of organoids and enhance the potential of organoid-based techniques to deepen our understanding of gene function in tissue biology.

Intestinal immunoregulation: lessons from human mendelian diseases

The mechanisms that maintain intestinal homeostasis despite constant exposure of the gut surface to multiple environmental antigens and to billions of microbes have been scrutinized over the past 20 years with the goals to gain basic knowledge, but also to elucidate the pathogenesis of inflammatory bowel diseases (IBD) and to identify therapeutic targets for these severe diseases. Considerable insight has been obtained from studies based on gene inactivation in mice as well as from genome wide screens for genetic variants predisposing to human IBD. These studies are, however, not sufficient to delineate which pathways play key nonredundant role in the human intestinal barrier and to hierarchize their respective contribution. Here, we intend to illustrate how such insight can be derived from the study of human Mendelian diseases, in which severe intestinal pathology results from single gene defects that impair epithelial and or hematopoietic immune cell functions. We suggest that these diseases offer the unique opportunity to study in depth the pathogenic mechanisms leading to perturbation of intestinal homeostasis in humans. Furthermore, molecular dissection of monogenic intestinal diseases highlights key pathways that might be druggable and therapeutically targeted in common forms of IBD.

Advanced Microscopy for Liver and Gut Ultrastructural Pathology in Patients with MVID and PFIC Caused by MYO5B Mutations

Mutations in the actin motor protein myosinVb (myo5b) cause aberrant apical cargo transport and the congenital enteropathy microvillus inclusion disease (MVID). Recently, missense mutations in myo5b were also associated with progressive familial intrahepatic cholestasis (MYO5B-PFIC). Here, we thoroughly characterized the ultrastructural and immuno-cytochemical phenotype of hepatocytes and duodenal enterocytes from a unique case of an adult MYO5B-PFIC patient who showed constant hepatopathy but only periodic enteric symptoms. Selected data from two other patients supported the findings. Advanced methods such as cryo-fixation, freeze-substitution, immuno-gold labeling, electron tomography and immuno-fluorescence microscopy complemented the standard procedures. Liver biopsies showed mislocalization of Rab11 and bile canalicular membrane proteins. Rab11-positive vesicles clustered around bile canaliculi and resembled subapical clusters of aberrant recycling endosomes in enterocytes from MVID patients. The adult patient studied in detail showed a severe, MVID-specific enterocyte phenotype, despite only a mild clinical intestinal presentation. This included mislocalization of numerous proteins essential for apical cargo transport and morphological alterations. We characterized the heterogeneous population of large catabolic organelles regarding their complex ultrastructure and differential distribution of autophagic and lysosomal marker proteins. Finally, we generated duodenal organoids/enteroids from biopsies that recapitulated all MVID hallmarks, demonstrating the potential of this disease model for personalized medicine.

Recruitment of Polarity Complexes and Tight Junction Proteins to the Site of Apical Bulk Endocytosis

BACKGROUND & AIMS

The molecular motor, Myosin Vb (MYO5B), is well documented for its role in trafficking cargo to the apical membrane of epithelial cells. Despite its involvement in regulating apical proteins, the role of MYO5B in cell polarity is less clear. Inactivating mutations in MYO5B result in microvillus inclusion disease (MVID), a disorder characterized by loss of key apical transporters and the presence of intracellular inclusions in enterocytes. We previously identified that inclusions in Myo5b knockout (KO) mice form from invagination of the apical brush border via apical bulk endocytosis. Herein, we sought to elucidate the role of polarity complexes and tight junction proteins during the formation of inclusions.

METHODS

Intestinal tissue from neonatal control and Myo5b KO littermates was analyzed by immunofluorescence to determine the localization of polarity complexes and tight junction proteins.

RESULTS

Proteins that make up the apical polarity complexes-Crumbs3 and Pars complexes-were associated with inclusions in Myo5b KO mice. In addition, tight junction proteins were observed to be concentrated over inclusions that were present at the apical membrane of Myo5b-deficient enterocytes in vivo and in vitro. Our mouse findings are complemented by immunostaining in a large animal swine model of MVID genetically engineered to express a human MVID-associated mutation that shows an accumulation of Claudin-2 over forming inclusions. The findings from our swine model of MVID suggest that a similar mechanism of tight junction accumulation occurs in patients with MVID.

CONCLUSIONS

These data show that apical bulk endocytosis involves the altered localization of apical polarity proteins and tight junction proteins after loss of Myo5b.

Distinct roles for luminal acidification in apical protein sorting and trafficking in zebrafish

Epithelial cell physiology critically depends on the asymmetric distribution of channels and transporters. However, the mechanisms targeting membrane proteins to the apical surface are still poorly understood. Here, we performed a visual forward genetic screen in the zebrafish intestine and identified mutants with defective apical targeting of membrane proteins. One of these mutants, affecting the vacuolar H⁺-ATPase gene atp6ap1b, revealed specific requirements for luminal acidification in apical, but not basolateral, membrane protein sorting and transport. Using a low temperature block assay combined with genetic and pharmacologic perturbation of luminal pH, we monitored transport of newly synthesized membrane proteins from the TGN to apical membrane in live zebrafish. We show that vacuolar H⁺-ATPase activity regulates sorting of O-glycosylated proteins at the TGN, as well as Rab8-dependent post-Golgi trafficking of different classes of apical membrane proteins. Thus, luminal acidification plays distinct and specific roles in apical membrane biogenesis.

The Role of Metal Oxide Nanoparticles, Escherichia coli, and Lactobacillus rhamnosus on Small Intestinal Enzyme Activity

Engineered nanomaterials (ENMs) have become common in the food industry, which motivates the need to evaluate ENM effects on human health. Gastrointestinal (GI) in vitro models (e.g. Caco-2, Caco-2/HT29-MTX) have been used in nanotoxicology research. However, the human gut environment is composed of both human cells and the gut microbiota. The goal of this study is to increase the complexity of the Caco-2/HT29-MTX in vitro model by co-culturing human cells with the Gram-positive, commensal Lactobacillus rhamnosus or the Gram-negative, opportunistic Escherichia coli; with the hypothesis that the presence of bacteria would ameliorate the effects of exposure to metal oxide nanoparticles (NPs) such as iron oxide (Fe2O3), silicone dioxide (SiO2), titanium dioxide (TiO2), or zinc oxide (ZnO). To understand this relationship, Caco-2/HT29-MTX cell barriers were acutely co-exposed (4 hours) to bacteria and/or NPs (pristine or in vitro digested). The activity of the brush border membrane (BBM) enzymes intestinal alkaline phosphatase (IAP), aminopeptidase-N (APN), sucrase isomaltase (SI) and the basolateral membrane enzyme (BLM) Na+/K+ ATPase were assessed. Findings show that (i) the human digestion process alters the physicochemical properties of NPs, (ii) large agglomerates of NPs remain entrapped on the apical side of the intestinal barrier, which (iii) affects the activity of BBM enzymes. Interestingly, some NPs effects were attenuated in the presence of either bacterial strains. Confocal microscopy detected bacteria-NPs interactions, which may impede the NP-intestinal cell contact. These results highlight the importance of improving in vitro models to closely mimic the complexities of the human body.

SNAREs and developmental disorders

Members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family mediate membrane fusion processes associated with vesicular trafficking and autophagy. SNAREs mediate core membrane fusion processes essential for all cells, but some SNAREs serve cell/tissue type-specific exocytic/endocytic functions, and are therefore critical for various aspects of embryonic development. Mutations or variants of their encoding genes could give rise to developmental disorders, such as those affecting the nervous system and immune system in humans. Mutations to components in the canonical synaptic vesicle fusion SNARE complex (VAMP2, STX1A/B, and SNAP25) and a key regulator of SNARE complex formation MUNC18-1, produce variant phenotypes of autism, intellectual disability, movement disorders, and epilepsy. STX11 and MUNC18-2 mutations underlie 2 subtypes of familial hemophagocytic lymphohistiocytosis. STX3 mutations contribute to variant microvillus inclusion disease. Chromosomal microdeletions involving STX16 play a role in pseudohypoparathyroidism type IB associated with abnormal imprinting of the GNAS complex locus. In this short review, I discuss these and other SNARE gene mutations and variants that are known to be associated with a variety developmental disorders, with a focus on their underlying cellular and molecular pathological basis deciphered through disease modeling. Possible pathogenic potentials of other SNAREs whose variants could be disease predisposing are also speculated upon.

Human Microphysiological Models of Intestinal Tissue and Gut Microbiome

The gastrointestinal (GI) tract is a complex system responsible for nutrient absorption, digestion, secretion, and elimination of waste products that also hosts immune surveillance, the intestinal microbiome, and interfaces with the nervous system. Traditional in vitro systems cannot harness the architectural and functional complexity of the GI tract. Recent advances in organoid engineering, microfluidic organs-on-a-chip technology, and microfabrication allows us to create better in vitro models of human organs/tissues. These micro-physiological systems could integrate the numerous cell types involved in GI development and physiology, including intestinal epithelium, endothelium (vascular), nerve cells, immune cells, and their interplay/cooperativity with the microbiome. In this review, we report recent progress in developing micro-physiological models of the GI systems. We also discuss how these models could be used to study normal intestinal physiology such as nutrient absorption, digestion, and secretion as well as GI infection, inflammation, cancer, and metabolism.

Benefit of Anakinra in Treating Pediatric Secondary Hemophagocytic Lymphohistiocytosis

OBJECTIVE

To assess the benefit of the recombinant human interleukin-1 receptor antagonist anakinra in treating pediatric patients with secondary hemophagocytic lymphohistiocytosis (HLH)/macrophage activation syndrome (MAS) associated with rheumatic and nonrheumatic conditions.

METHODS

A retrospective chart review of all anakinra-treated patients with secondary HLH/MAS was performed at Children's of Alabama from January 2008 through December 2016. Demographic, clinical, laboratory, and genetic characteristics, outcomes data, and information on concurrent treatments were collected from the records and analyzed using appropriate univariate statistical approaches to assess changes following treatment and associations between patient variables and outcomes.

RESULTS

Forty-four patients with secondary HLH/MAS being treated with anakinra were identified in the electronic medical records. The median duration of hospitalization was 15 days. The mean pretreatment serum ferritin level was 33,316 ng/ml and dropped to 14,435 ng/ml (57% decrease) within 15 days of the start of anakinra treatment. The overall mortality rate in the cohort was 27%. Earlier initiation of anakinra (within 5 days of hospitalization) was associated with reduced mortality (P = 0.046), whereas thrombocytopenia (platelet count <100,000/μl) and STXBP2 mutations were both associated with increased mortality (P = 0.008 and P = 0.012, respectively). In considering patients according to their underlying diagnosis, those with systemic juvenile idiopathic arthritis (JIA) had the lowest mortality rate, with no deaths among the 13 systemic JIA patients included in the study (P = 0.006). In contrast, those with an underlying hematologic malignancy had the highest mortality rate, at 100% (n = 3).

CONCLUSION

These findings suggest that anakinra appears to be effective in treating pediatric patients with non-malignancy-associated secondary HLH/MAS, especially when it is given early in the disease course and when administered to patients who have an underlying rheumatic disease.

Recent advances in understanding and managing malabsorption: focus on microvillus inclusion disease

Microvillus inclusion disease (MVID) is a rare congenital severe malabsorptive and secretory diarrheal disease characterized by blunted or absent microvilli with accumulation of secretory granules and inclusion bodies in enterocytes. The typical clinical presentation of the disease is severe chronic diarrhea that rapidly leads to dehydration and metabolic acidosis. Despite significant advances in our understanding of the causative factors, to date, no curative therapy for MVID and associated diarrhea exists. Prognosis mainly relies on life-long total parenteral nutrition (TPN) and eventual small bowel and/or liver transplantation. Both TPN and intestinal transplantation are challenging and present with many side effects. A breakthrough in the understanding of MVID emanated from seminal findings revealing mutations in MYO5B as a cause for MVID. During the last decade, many studies have thus utilized cell lines and animal models with knockdown of MYO5B to closely recapitulate the human disease and investigate potential therapeutic options in disease management. We will review the most recent advances made in the research pertaining to MVID. We will also highlight the tools and models developed that can be utilized for basic and applied research to increase our understanding of MVID and develop novel and effective targeted therapies.

Crumbs organizes the transport machinery by regulating apical levels of PI(4,5)P2 in Drosophila

An efficient vectorial intracellular transport machinery depends on a well-established apico-basal polarity and is a prerequisite for the function of secretory epithelia. Despite extensive knowledge on individual trafficking pathways, little is known about the mechanisms coordinating their temporal and spatial regulation. Here, we report that the polarity protein Crumbs is essential for apical plasma membrane phospholipid-homeostasis and efficient apical secretion. Through recruiting βHeavy-Spectrin and MyosinV to the apical membrane, Crumbs maintains the Rab6-, Rab11- and Rab30-dependent trafficking and regulates the lipid phosphatases Pten and Ocrl. Crumbs knock-down results in increased apical levels of PI(4,5)P2 and formation of a novel, Moesin- and PI(4,5)P2-enriched apical membrane sac containing microvilli-like structures. Our results identify Crumbs as an essential hub required to maintain the organization of the apical membrane and the physiological activity of the larval salivary gland.

Loss of myosin Vb promotes apical bulk endocytosis in neonatal enterocytes

In patients with inactivating mutations in myosin Vb (Myo5B), enterocytes show large inclusions lined by microvilli. The origin of inclusions in small-intestinal enterocytes in microvillus inclusion disease is currently unclear. We postulated that inclusions in Myo5b KO mouse enterocytes form through invagination of the apical brush border membrane. 70-kD FITC-dextran added apically to Myo5b KO intestinal explants accumulated in intracellular inclusions. Live imaging of Myo5b KO-derived enteroids confirmed the formation of inclusions from the apical membrane. Treatment of intestinal explants and enteroids with Dyngo resulted in accumulation of inclusions at the apical membrane. Inclusions in Myo5b KO enterocytes contained VAMP4 and Pacsin 2 (Syndapin 2). Myo5b;Pacsin 2 double-KO mice showed a significant decrease in inclusion formation. Our results suggest that apical bulk endocytosis in Myo5b KO enterocytes resembles activity-dependent bulk endocytosis, the primary mechanism for synaptic vesicle uptake during intense neuronal stimulation. Thus, apical bulk endocytosis mediates the formation of inclusions in neonatal Myo5b KO enterocytes.

A V0-ATPase-dependent apical trafficking pathway maintains the polarity of the intestinal absorptive membrane

Intestine function relies on the strong polarity of intestinal epithelial cells and the array of microvilli forming a brush border at their luminal pole. Combining a genetic RNA interference (RNAi) screen with in vivo super-resolution imaging in the Caenorhabditis elegans intestine, we found that the V0 sector of the vacuolar ATPase (V0-ATPase) controls a late apical trafficking step, involving Ras-related protein 11 (RAB-11)⁺ endosomes and the N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) synaptosome-associated protein 29 (SNAP-29), and is necessary to maintain the polarized localization of both apical polarity modules and brush border proteins. We show that the V0-ATPase pathway also genetically interacts with glycosphingolipids and clathrin in enterocyte polarity maintenance. Finally, we demonstrate that silencing of the V0-ATPase fully recapitulates the severe structural, polarity and trafficking defects observed in enterocytes from individuals with microvillus inclusion disease (MVID) and use this new in vivo MVID model to follow the dynamics of microvillus inclusions. Thus, we describe a new function for V0-ATPase in apical trafficking and epithelial polarity maintenance and the promising use of the C. elegans intestine as an in vivo model to better understand the molecular mechanisms of rare genetic enteropathies.

Haematopoietic Stem Cell Transplantation for Primary Haemophagocytic Lymphohistiocytosis

Haematopoietic stem cell transplantation currently remains the only curative treatment of primary forms of haemophagocytic lymphohistiocytosis (HLH). Rapid diagnosis, efficient primary treatment of hyperinflammation, and conditioning regimens tailored to this demanding condition have substantially improved prognosis in the past 40 years. However, refractory hyperinflammation, central nervous system (CNS) involvement, unavailability of matched donors, susceptibility to conditioning-related toxicities, and a high frequency of mixed chimaerism remain a challenge in a substantial proportion of patients. Gene therapeutic approaches for several genetic defects of primary HLH are being developed at pre-clinical and translational levels.