Myosin Vb uncoupling from RAB8A and RAB11A elicits microvillus inclusion disease

Intracellular Trafficking Defects in Congenital Intestinal and Hepatic Diseases

Enterocytes and liver cells fulfill important metabolic and barrier functions and are responsible for crucial vectorial secretive and absorptive processes. To date, genetic diseases affecting metabolic enzymes or transmembrane transporters in the intestine and the liver are better comprehended than mutations affecting intracellular trafficking. In this review, we explore the emerging knowledge on intracellular trafficking defects and their clinical manifestations in both the intestine and the liver. We provide a detailed overview including more investigated diseases such as the canonical, variant and associated forms of microvillus inclusion disease, as well as recently described pathologies, highlighting the complexity and disease relevance of several trafficking pathways. We give examples of how intracellular trafficking hubs, such as the apical recycling endosome system, the trans-Golgi network, lysosomes, or the Golgi-to-endoplasmic reticulum transport are involved in the pathomechanism and lead to disease. Ultimately, understanding these processes could spark novel therapeutic approaches, which would greatly improve the quality of life of the affected patients.

MYO5B gene mutations may promote the occurrence of very early onset inflammatory bowel disease: a case report

BACKGROUND

With recent advances in gene sequencing technology, more than 60 genetic mutations associated with very early onset inflammatory bowel disease (VEO-IBD) have been reported. Most of the genes are associated with immune deficiencies. The Myosin 5B (MYO5B) gene is primarily involved in cell motility and material transport which is associated with congenital intractable diarrhea and cholestasis. No studies have examined the relationship between the MYO5B gene and VEO-IBD. We report a case of a child with a mutation in the MYO5B gene who was diagnosed with VEO-IBD, then we investigated the association between the MYO5B gene and VEO-IBD.

CASE PRESENTATION

A 7-month-old baby girl with a chief complaint of "blood in the stool for more than 4 months and vaginal pus and blood discharge for 3 weeks" was diagnosed with VEO-IBD, and her symptoms improved after treatment with mesalazine. The whole-exome sequencing was performed with peripheral blood. Immunohistochemistry was performed on the terminal ileal tissue. Western blotting, quantitative polymerase chain reaction (Q-PCR) and immunofluorescence were performed with cultured organoid tissue from the terminal ileum. Whole-exome sequencing identified heterozygous missense of MYO5B variant of unknown significance (p. [I769N]; [T1546M]). Immunohistochemistry revealed a significant decrease in the expression of MYO5B protein in the terminal ileum of the child with MYO5B mutation; Q-PCR revealed a decrease in the mRNA levels of occludin and ZO-1 and both the mRNA levels and protein levels of MYO5B was downregulated in the patient. Immunofluorescence images showed that MYO5B gene mutation disrupted the apical delivery of transporters SGLT1, NHE3 and AQP7.

CONCLUSIONS

MYO5B gene mutation leading to the downregulation of MYO5B protein may promote the occurrence of VEO-IBD by decreasing mRNA and protein levels of intestinal tight junction genes and dislocating the apical transporters.

Modeling the cell biology of monogenetic intestinal epithelial disorders

Monogenetic variants are responsible for a range of congenital human diseases. Variants in genes that are important for intestinal epithelial function cause a group of disorders characterized by severe diarrhea and loss of nutrient absorption called congenital diarrheas and enteropathies (CODEs). CODE-causing genes include nutrient transporters, enzymes, structural proteins, and vesicular trafficking proteins in intestinal epithelial cells. Several severe CODE disorders result from the loss-of-function in key regulators of polarized endocytic trafficking such as the motor protein, Myosin VB (MYO5B), as well as STX3, STXBP2, and UNC45A. Investigations of the cell biology and pathophysiology following loss-of-function in these genes have led to an increased understanding of both homeostatic and pathological vesicular trafficking in intestinal epithelial cells. Modeling different CODEs through investigation of changes in patient tissues, coupled with the development of animal models and patient-derived enteroids, has provided critical insights into the enterocyte differentiation and function. Linking basic knowledge of cell biology with the phenotype of specific patient variants is a key step in developing effective treatments for rare monogenetic diseases. This knowledge can also be applied more broadly to our understanding of common epithelial disorders.

Function and regulation of Rab GTPases in cancers

The Rab small GTPases are characterized by the distinct intracellular localization and modulate various endocytic, transcytic and exocytic transport pathways. Rab proteins function as scaffolds that connect signaling pathways and intracellular membrane trafficking processes through the recruitment of effectors, such as tethering factors, phosphatases, motors and kinases. In different cancers, Rabs play as either an onco-protein or a tumor suppressor role, highly dependending on the context. The molecular mechanistic research has revealed that Rab proteins are involved in cancer progression through influences on migration, invasion, metabolism, exosome secretion, autophagy, and drug resistance of cancer cells. Therefore, targeting Rab GTPases to recover the dysregulated vesicle transport systems may provide potential strategy to restrain cancer progression. In this review, we discuss the regulation of Rab protein level and activity in modulating pathways involved in tumor progression, and propose that Rab proteins may serve as a prognostic factor in different cancers.

Inverted apicobasal polarity in health and disease

Apicobasal epithelial polarity controls the functional properties of most organs. Thus, there has been extensive research on the molecular intricacies governing the establishment and maintenance of cell polarity. Whereas loss of apicobasal polarity is a well-documented phenomenon associated with multiple diseases, less is known regarding another type of apicobasal polarity alteration - the inversion of polarity. In this Review, we provide a unifying definition of inverted polarity and discuss multiple scenarios in mammalian systems and human health and disease in which apical and basolateral membrane domains are interchanged. This includes mammalian embryo implantation, monogenic diseases and dissemination of cancer cell clusters. For each example, the functional consequences of polarity inversion are assessed, revealing shared outcomes, including modifications in immune surveillance, altered drug sensitivity and changes in adhesions to neighboring cells. Finally, we highlight the molecular alterations associated with inverted apicobasal polarity and provide a molecular framework to connect these changes with the core cell polarity machinery and to explain roles of polarity inversion in health and disease. Based on the current state of the field, failure to respond to extracellular matrix (ECM) cues, increased cellular contractility and membrane trafficking defects are likely to account for most cases of inverted apicobasal polarity.

Reduced LHFPL3-AS2 lncRNA expression is linked to altered epithelial polarity and proliferation, and to ileal ulceration in Crohn disease

Disruption of intestinal epithelial functions is linked to Crohn disease (CD) pathogenesis. We identified a widespread reduction in the expression of long non-coding RNAs (lncRNAs) including LHFPL3-AS2 in the treatment-naïve CD ileum of the RISK pediatric cohort. We validated the reduction of LHFPL3-AS2 in adult CD and noted a further reduction in patients with more severe CD from the RISK cohort. LHFPL3-AS2 knockdown in Caco-2 cells robustly affected epithelial monolayer morphogenesis with markedly reduced confluency and spreading, showing atypical rounding, and clumping. mRNA-seq analysis of LHFPL3-AS2 knockdown cells highlighted the reduction of genes and pathways linked with apical polarity, actin bundles, morphogenesis, and the b-catenin-TCF4 complex. LHFPL3-AS2 knockdown significantly reduced the ability of cells to form an internal lumen within the 3-dimensional (3D) cyst model, with mislocalization of actin and adherent and tight junction proteins, affecting epithelial polarity. LHFPL3-AS2 knockdown also resulted in defective mitotic spindle formation and consequent reduction in epithelial proliferation. Altogether, we show that LHFPL3-AS2 reduction affects epithelial morphogenesis, polarity, mitotic spindle formation, and proliferation, which are key processes in maintaining epithelial homeostasis in CD. Reduced expression of LHFPL3-AS2 in CD patients and its further reduction with ileal ulceration outcome, emphasizes its significance in this context.

Patient-derived enteroids provide a platform for the development of therapeutic approaches in microvillus inclusion disease

Microvillus inclusion disease (MVID), caused by loss-of-function mutations in the motor protein myosin Vb (MYO5B), is a severe infantile disease characterized by diarrhea, malabsorption, and acid/base instability, requiring intensive parenteral support for nutritional and fluid management. Human patient-derived enteroids represent a model for investigation of monogenic epithelial disorders but are a rare resource from MVID patients. We developed human enteroids with different loss-of function MYO5B variants and showed that they recapitulated the structural changes found in native MVID enterocytes. Multiplex immunofluorescence imaging of patient duodenal tissues revealed patient-specific changes in localization of brush border transporters. Functional analysis of electrolyte transport revealed profound loss of Na+/H+ exchange (NHE) activity in MVID patient enteroids with near-normal chloride secretion. The chloride channel-blocking antidiarrheal drug crofelemer dose-dependently inhibited agonist-mediated fluid secretion. MVID enteroids exhibited altered differentiation and maturation versus healthy enteroids. γ-Secretase inhibition with DAPT recovered apical brush border structure and functional Na+/H+ exchange activity in MVID enteroids. Transcriptomic analysis revealed potential pathways involved in the rescue of MVID cells including serum/glucocorticoid-regulated kinase 2 (SGK2) and NHE regulatory factor 3 (NHERF3). These results demonstrate the utility of patient-derived enteroids for developing therapeutic approaches to MVID.

Antibacterial polypeptide-bioparticle for oral administration: Powder formulation, palatability and in vivo toxicity approach

The upsurge of bacterial resistance to conventional antibiotics turned a well-recognized public health threat. The need of developing new biomaterials of effective practical use in order to tackle bacterial resistance became urgent. In this study, a submicrometric bioparticle of known antibacterial activity was produced in powder form with suitable texture and appealing characteristics for effective oral administration. Through complex coacervating a natural-source antimicrobial polypeptide with chitosan-N-arginine and alginate, the bioactive polypeptide was physically incorporated to the bioparticle whose structure positively responds to the pH variations found in gastrointestinal tract. The powder formulation presented high palatability that was evaluated using fish as in vivo animal model. A thorough survey of the fish intestinal tissues, following a systematic oral administration, revealed high penetration potential of the biomaterial through epithelial cells and deeper intestine layers. Despite, no cytotoxic effect was observed in analyzing the tissues through different histology methods. The absence of intestinal damage was corroborated by immune histochemistry, being the integrity of epithelial motor myosin Vb and related traffic proteins preserved. Hematology further endorsed absence of toxicity in blood cells whose morphology was evaluated in detail. The study evidenced the applicability potential of a new biomaterial of appealing and safe oral administration of antibacterial polypeptide.

Protocadherin 20 maintains intestinal barrier function to protect against Crohn's disease by targeting ATF6

BACKGROUND

Intestinal barrier dysfunction plays a central role in the pathological onset of Crohn's disease. We identify the cadherin superfamily member protocadherin 20 (PCDH20) as a crucial factor in Crohn's disease. Here we describe the function of PCDH20 and its mechanisms in gut homeostasis, barrier integrity, and Crohn's disease development.

RESULTS

PCDH20 mRNA and protein expression is significantly downregulated in the colonic epithelium of Crohn's disease patients and mice with induced colitis compared with controls. In mice, intestinal-specific Pcdh20 knockout causes defects in enterocyte proliferation and differentiation, while causing morphological abnormalities. Specifically, the deletion disrupts barrier integrity by unzipping adherens junctions via β-catenin regulation and p120-catenin phosphorylation, thus aggravating colitis in DSS- and TNBS-induced colitis mouse models. Furthermore, we identify activating transcription factor 6 (ATF6), a key chaperone of endoplasmic reticulum stress, as a functional downstream effector of PCDH20. By administering a selective ATF6 activator, the impairment of intestinal barrier integrity and dysregulation of CHOP/β-catenin/p-p120-catenin pathway was reversed in Pcdh20-ablated mice with colitis and PCDH20-deficient colonic cell lines.

CONCLUSIONS

PCDH20 is an essential factor in maintaining intestinal epithelial homeostasis and barrier integrity. Specifically, PCDH20 helps to protect against colitis by tightening adherens junctions through the ATF6/CHOP/β-catenin/p-p120-catenin axis.

The genetics of monogenic intestinal epithelial disorders

Monogenic intestinal epithelial disorders, also known as congenital diarrheas and enteropathies (CoDEs), are a group of rare diseases that result from mutations in genes that primarily affect intestinal epithelial cell function. Patients with CoDE disorders generally present with infantile-onset diarrhea and poor growth, and often require intensive fluid and nutritional management. CoDE disorders can be classified into several categories that relate to broad areas of epithelial function, structure, and development. The advent of accessible and low-cost genetic sequencing has accelerated discovery in the field with over 45 different genes now associated with CoDE disorders. Despite this increasing knowledge in the causal genetics of disease, the underlying cellular pathophysiology remains incompletely understood for many disorders. Consequently, clinical management options for CoDE disorders are currently limited and there is an urgent need for new and disorder-specific therapies. In this review, we provide a general overview of CoDE disorders, including a historical perspective of the field and relationship to other monogenic disorders of the intestine. We describe the genetics, clinical presentation, and known pathophysiology for specific disorders. Lastly, we describe the major challenges relating to CoDE disorders, briefly outline key areas that need further study, and provide a perspective on the future genetic and therapeutic landscape.

Therapy Development for Microvillus Inclusion Disease using Patient-derived Enteroids

Microvillus Inclusion Disease (MVID), caused by loss-of-function mutations in the motor protein Myosin Vb (MYO5B), is a severe infantile disease characterized by diarrhea, malabsorption, and acid-base instability, requiring intensive parenteral support for nutritional and fluid management. Human patient-derived enteroids represent a model for investigation of monogenic epithelial disorders but are a rare resource from MVID patients. We developed human enteroids with different loss-of function MYO5B variants and showed that they recapitulated the structural changes found in native MVID enterocytes. Multiplex Immunofluorescence imaging of patient duodenal tissues revealed patient-specific changes in localization of brush border transporters. Functional analysis of electrolyte transport revealed profound loss of Na + /H + exchange (NHE) activity in MVID patient enteroids with near-normal chloride secretion. The chloride channel-blocking anti-diarrheal drug, Crofelemer, dose-dependently inhibited agonist-mediated fluid secretion. MVID enteroids exhibited altered differentiation and maturation versus healthy enteroids. Inhibition of Notch signaling with the γ-secretase inhibitor, DAPT, recovered apical brush border structure and functional Na + /H + exchange activity in MVID enteroids. Transcriptomic analysis revealed potential pathways involved in the rescue of MVID cells including serum- and glucocorticoid-induced protein kinase 2 (SGK2), and NHE regulatory factor 3 (NHERF3). These results demonstrate the utility of patient-derived enteroids for developing therapeutic approaches to MVID.

Conflict-of-interest statement

The authors have declared that no conflict of interest exists.

Compound Heterozygous Myosin 5B (Myo5b) Mutation with Early Onset Progressive Cholestasis and No Intestinal Failure

Background: Exome sequencing studies have recently identified novel genes implicated in normal or low GGT pediatric cholestasis including myosin 5B (MYO5B). Case report: We identified novel compound heterozygote mutations in exon 14 and exon 19 of the MYO5B gene in an 18-month-old Indian child with history of fluctuating jaundice and severe pruritus. His liver biopsy showed portal and perivenular fibrosis with focal bridging septa and mild activity. He is currently on UDCA, cholestyramine and vitamin supplements. There is no history of diarrhea. His asymptomatic mother showed heterozygous mutation in exon 19 of the MYO5B gene and his asymptomatic father showed heterozygous mutation in exon 14 of the MYO5B gene. Conclusion: Our report confirms that patients with compound heterozygote mutations in MYO5B develop progressive cholestasis with no intestinal disease.

Myosins and membrane trafficking in intestinal brush border assembly

Myosins are a class of motors that participate in a wide variety of cellular functions including organelle transport, cell adhesion, endocytosis and exocytosis, movement of RNA, and cell motility. Among the emerging roles for myosins is regulation of the assembly, morphology, and function of actin protrusions such as microvilli. The intestine harbors an elaborate apical membrane composed of highly organized microvilli. Microvilli assembly and function are intricately tied to several myosins including Myosin 1a, non-muscle Myosin 2c, Myosin 5b, Myosin 6, and Myosin 7b. Here, we review the research progress made in our understanding of myosin mediated apical assembly.

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.

Altered MYO5B Function Underlies Microvillus Inclusion Disease: Opportunities for Intervention at a Cellular Level

Microvillus inclusion disease (MVID) is a congenital diarrheal disorder resulting in life-threatening secretory diarrhea in newborns. Inactivating and nonsense mutations in myosin Vb (MYO5B) have been identified in MVID patients. Work using patient tissues, cell lines, mice, and pigs has led to critical insights into the pathology of MVID and a better understanding of both apical trafficking in intestinal enterocytes and intestinal stem cell differentiation. These studies have demonstrated that loss of MYO5B or inactivating mutations lead to loss of apical sodium and water transporters, without loss of apical CFTR, accounting for the major pathology of the disease. In addition, loss of MYO5B expression induces the formation of microvillus inclusions through apical bulk endocytosis that utilizes dynamin and PACSIN2 and recruits tight junction proteins to the sites of bulk endosome formation. Importantly, formation of microvillus inclusions is not required for the induction of diarrhea. Recent investigations have demonstrated that administration of lysophosphatidic acid (LPA) can partially reestablish apical ion transporters in enterocytes of MYO5B KO mice. In addition, further studies have shown that MYO5B loss induces an imbalance in Wnt/Notch signaling pathways that can lead to alterations in enterocyte maturation and tuft cell lineage differentiation. Inhibition of Notch signaling leads to improvements in those cell differentiation deficits. These studies demonstrate that directed strategies through LPA receptor activation and Notch inhibition can bypass the inhibitory effects of MYO5B loss. Thus, effective strategies may be successful in MVID patients and other congenital diarrhea syndromes to reestablish proper apical membrane absorption of sodium and water in enterocytes and ameliorate life-threatening congenital diarrhea.

UNC45A deficiency causes microvillus inclusion disease-like phenotype by impairing myosin VB-dependent apical trafficking

Variants in the UNC45A cochaperone have been recently associated with a syndrome combining diarrhea, cholestasis, deafness, and bone fragility. Yet the mechanism underlying intestinal failure in UNC45A deficiency remains unclear. Here, biallelic variants in UNC45A were identified by next-generation sequencing in 6 patients with congenital diarrhea. Corroborating in silico prediction, variants either abolished UNC45A expression or altered protein conformation. Myosin VB was identified by mass spectrometry as client of the UNC45A chaperone and was found misfolded in UNC45AKO Caco-2 cells. In keeping with impaired myosin VB function, UNC45AKO Caco-2 cells showed abnormal epithelial morphogenesis that was restored by full-length UNC45A, but not by mutant alleles. Patients and UNC45AKO 3D organoids displayed altered luminal development and microvillus inclusions, while 2D cultures revealed Rab11 and apical transporter mislocalization as well as sparse and disorganized microvilli. All those features resembled the subcellular abnormalities observed in duodenal biopsies from patients with microvillus inclusion disease. Finally, microvillus inclusions and shortened microvilli were evidenced in enterocytes from unc45a-deficient zebrafish. Taken together, our results provide evidence that UNC45A plays an essential role in epithelial morphogenesis through its cochaperone function of myosin VB and that UNC45A loss causes a variant of microvillus inclusion disease.

MYO5B Gene Mutations: A Not Negligible Cause of Intrahepatic Cholestasis of Infancy With Normal Gamma-Glutamyl Transferase Phenotype

OBJECTIVES

Progressive familial intrahepatic cholestasis is an expanding group of autosomal recessive intrahepatic cholestatic disorders. Recently, next-generation sequencing allowed identifying new genes responsible for new specific disorders. Two biochemical phenotypes have been identified according to gamma-glutamyltransferase (GGT) activity. Mutations of the myosin 5B gene (MYO5B) are known to cause microvillus inclusion disease. Recently, different mutations in MYO5B gene have been reported in patients with low-GGT cholestasis.

METHODS

A multicenter retrospective and prospective study was conducted in 32 children with cryptogenic intrahepatic cholestasis. Clinical, biochemical, histological, and treatment data were analyzed in these patients. DNA from peripheral blood was extracted, and all patients were studied by whole exome sequencing followed by Sanger sequencing.

RESULTS

Six patients out of 32 had mutations in the MYO5B gene. Of these six patients, the median age at disease onset was 0.8 years, and the median length of follow-up was 4.2 years. The most common signs were pruritus, poor growth, hepatomegaly, jaundice, and hypocholic stools. Two patients also showed intestinal involvement. Transaminases and conjugated bilirubin were moderately increased, serum bile acids elevated, and GGT persistently normal. At anti-Myo5B immunostaining, performed in liver biopsy of two patients, coarse granules were evident within the cytoplasm of hepatocytes while bile salt export pump was normally expressed at the canalicular membrane. Six variants in homozygosity or compound heterozygosity in the MYO5B gene were identified, and three of them have never been described before. All nucleotide alterations were located on the myosin motor domain except one missense variant found in the isoleucine-glutamine calmodulin-binding motif.

CONCLUSIONS

We identified causative mutations in MYO5B in 18.7% of a selected cohort of patients with intrahepatic cholestasis confirming a relevant role for the MYO5B gene in low-GGT cholestasis.

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.

Coordinated activities of Myosin Vb isoforms and mTOR signaling regulate epithelial cell morphology during development

The maintenance of epithelial architecture necessitates tight regulation of cell size and shape. However, mechanisms underlying epithelial cell size regulation remain poorly understood. We show that the interaction of Myosin Vb with Rab11 prevents the accumulation of apically derived endosomes to maintain cell-size, whereas that with Rab10 regulates vesicular transport from the trans-Golgi. These interactions are required for the fine-tuning of the epithelial cell morphology during zebrafish development. Furthermore, the compensatory cell growth upon cell-proliferation inhibition involves a preferential expansion of the apical domain, leading to flatter epithelial cells, an efficient strategy to cover the surface with fewer cells. This apical domain growth requires post-trans-Golgi transport mediated by the Rab10-interacting Myosin Vb isoform, downstream of the mTOR-Fatty Acid Synthase (FASN) axis. Changes in trans-Golgi morphology indicate that the Golgi synchronizes mTOR-FASN-regulated biosynthetic input and Myosin Vb-Rab10 dependent output. Our study unravels the mechanism of polarized growth in epithelial cells and delineates functions of Myosin Vb isoforms in cell size regulation during development.

Variants in STXBP3 are Associated with Very Early Onset Inflammatory Bowel Disease, Bilateral Sensorineural Hearing Loss and Immune Dysregulation

BACKGROUND AND AIMS

Very early onset inflammatory bowel disease [VEOIBD] is characterized by intestinal inflammation affecting infants and children less than 6 years of age. To date, over 60 monogenic aetiologies of VEOIBD have been identified, many characterized by highly penetrant recessive or dominant variants in underlying immune and/or epithelial pathways. We sought to identify the genetic cause of VEOIBD in a subset of patients with a unique clinical presentation.

METHODS

Whole exome sequencing was performed on five families with ten patients who presented with a similar constellation of symptoms including medically refractory infantile-onset IBD, bilateral sensorineural hearing loss and, in the majority, recurrent infections. Genetic aetiologies of VEOIBD were assessed and Sanger sequencing was performed to confirm novel genetic findings. Western analysis on peripheral blood mononuclear cells and functional studies with epithelial cell lines were employed.

RESULTS

In each of the ten patients, we identified damaging heterozygous or biallelic variants in the Syntaxin-Binding Protein 3 gene [STXBP3], a protein known to regulate intracellular vesicular trafficking in the syntaxin-binding protein family of molecules, but not associated to date with either VEOIBD or sensorineural hearing loss. These mutations interfere with either intron splicing or protein stability and lead to reduced STXBP3 protein expression. Knock-down of STXBP3 in CaCo2 cells resulted in defects in cell polarity.

CONCLUSION

Overall, we describe a novel genetic syndrome and identify a critical role for STXBP3 in VEOIBD, sensorineural hearing loss and immune dysregulation.

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.

Autosomal recessive diseases among the Athabaskans of the southwestern United States: anthropological, medical, and scientific aspects

The peopling of the Americas by Native Americans occurred in 4 waves of which the last was Nadene language speakers of whom Athabaskans are the largest group. As the Europeans were entering the Southwestern states of the USA, Athabaskan hunting-gathering tribes were migrating South from Canada along the Rocky Mountains and undergoing potential bottlenecks reflected in autosomal recessive diseases shared by Apaches and Navajos. About 300 years ago, the Navajo developing a sedentary culture learned from Pueblo Indians while the Apache remained hunter-gathers. Although most of the tribe was rounded up and forced to relocate to Bosque Redondo, the adult breeding population was large enough to prevent a genetic bottleneck. However, some Navajo underwent further population bottlenecks while hiding from the brutal US Army action (under Kit Carson’s guidance). This led to an increased frequency of other autosomal recessive diseases. Recent advances in population genetics, pathophysiology of the diseases, and social/ethical issues concerning their study are reviewed.

The Role of Organelles in Intestinal Function, Physiology, and Disease

The intestine maintains homeostasis by coordinating internal biological processes to adjust to fluctuating external conditions. The intestinal epithelium is continuously renewed and comprises multiple cell types, including absorptive cells, secretory cells, and resident stem cells. An important feature of this organ is its ability to coordinate many processes including cell proliferation, differentiation, regeneration, damage/stress response, immune activity, feeding behavior, and age-related changes by using conserved signaling pathways. However, the subcellular spatial organization of these signaling events and the organelles involved has only recently been studied in detail. Here we discuss how organelles of intestinal cells serve to initiate, mediate, and terminate signals, that are vital for homeostasis.

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.

Pharmacological and Parenteral Nutrition-Based Interventions in Microvillus Inclusion Disease

Microvillus inclusion disease (MVID) is a rare inherited and invariably fatal enteropathy, characterized by severe intractable secretory diarrhea and nutrient malabsorption. No cure exists, and patients typically die during infancy because of treatment-related complications. The need for alternative treatment strategies is evident. Several pharmacological interventions with variable successes have been tried and reported for individual patients as part of their clinical care. Unfortunately, these interventions and their outcomes have remained hidden in case reports and have not been reviewed. Further, recent advances regarding MVID pathogenesis have shed new light on the outcomes of these pharmacological interventions and offer suggestions for future clinical research and trials. Hence, an inventory of reported pharmacological interventions in MVID, their rationales and outcomes, and a discussion of these in the light of current knowledge is opportune. Together with a discussion on MVID-specific pharmacokinetic, -dynamic, and -genetic concerns that pose unique challenges regarding pharmacological strategies, we envision that this paper will aid researchers and clinicians in their efforts to develop pharmacological interventions to combat this devastating disease.

The Rab11 effectors Fip5 and Fip1 regulate zebrafish intestinal development

The Rab11 apical recycling endosome pathway is a well-established regulator of polarity and lumen formation; however, Rab11-vesicular trafficking also directs a diverse array of other cellular processes, raising the question of how Rab11 vesicles achieve specificity in space, time and content of cargo delivery. In part, this specificity is achieved through effector proteins, yet the role of Rab11 effector proteins in vivo remains vague. Here, we use CRISPR/Cas9 gene editing to study the role of the Rab11 effector Fip5 during zebrafish intestinal development. Zebrafish contain two paralogous genes, fip5a and fip5b, that are orthologs of human FIP5 We find that fip5a- and fip5b-mutant fish show phenotypes characteristic of microvillus inclusion disease, including microvilli defects and lysosomal accumulation. Single and double mutant analyses suggest that fip5a and fip5b function in parallel and regulate trafficking pathways required for assembly of keratin at the terminal web. Remarkably, in some genetic backgrounds, the absence of Fip5 triggers protein upregulation of a closely related family member, Fip1. This compensation mechanism occurs both during zebrafish intestinal development and in tissue culture models of lumenogenesis. In conclusion, our data implicate the Rab11 effectors Fip5 and Fip1 in a trafficking pathway required for apical microvilli formation.

The Endosomal Recycling Pathway-At the Crossroads of the Cell

The endosomal recycling pathway lies at the heart of the membrane trafficking machinery in the cell. It plays a central role in determining the composition of the plasma membrane and is thus critical for normal cellular homeostasis. However, defective endosomal recycling has been linked to a wide range of diseases, including cancer and some of the most common neurological disorders. It is also frequently subverted by many diverse human pathogens in order to successfully infect cells. Despite its importance, endosomal recycling remains relatively understudied in comparison to the endocytic and secretory transport pathways. A greater understanding of the molecular mechanisms that support transport through the endosomal recycling pathway will provide deeper insights into the pathophysiology of disease and will likely identify new approaches for their detection and treatment. This review will provide an overview of the normal physiological role of the endosomal recycling pathway, describe the consequences when it malfunctions, and discuss potential strategies for modulating its activity.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Weaving the Strands of Life (Iiná Bitł'ool): History of Genetic Research Involving Navajo People

To date, some genetic studies offer medical benefits but lack a clear pathway to benefit for people from underrepresented backgrounds. Historically, Indigenous people, including the Diné (Navajo people), have raised concerns about the lack of benefits, misuse of DNA samples, lack of consultation, and ignoring of cultural and traditional ways of knowing. Shortly after the Navajo Nation Human Research Review Board was established in 1996, the Navajo Nation recognized growing concerns about genetic research, and in 2002 they established a moratorium on human genetic research studies. The moratorium effectively has protected their citizens from potential genetic research harms. Despite the placement of the moratorium, some genetic research studies have continued using blood and DNA samples from Navajo people. To understand the history of genetic research involving Navajo people, the authors conducted a literature review of genetic or genetics-related research publications that involved Navajo people, identifying 79 articles from the years 1926 to 2018. To their knowledge, no known literature review has comprehensively examined the history of genetic research in the Navajo community. This review divides the genetic research articles into the following general classifications: bacteria or virus genetics, blood and human leukocyte antigens, complex diseases, forensics, hereditary diseases, and population genetics and migration. The authors evaluated the methods reported in each article, described the number of Navajo individuals reported, recorded the academic and tribal approval statements, and noted whether the study considered Diné cultural values. Several studies focused on severe combined immunodeficiency disease, population history, neuropathy, albinism, and eye and skin disorders that affect Navajo people. The authors contextualize Diné ways of knowing related to genetics and health with Western scientific concepts to acknowledge the complex philosophy and belief system that guides Diné people and recognizes Indigenous science. They also encourage researchers to consider cultural perspectives and traditional knowledge that has the potential to create stronger conclusions and better-informed, ethical, and respectful science.

MYO5B mutations in pheochromocytoma/paraganglioma promote cancer progression

Identification of additional cancer-associated genes and secondary mutations driving the metastatic progression in pheochromocytoma and paraganglioma (PPGL) is important for subtyping, and may provide optimization of therapeutic regimens. We recently reported novel recurrent nonsynonymous mutations in the MYO5B gene in metastatic PPGL. Here, we explored the functional impact of these MYO5B mutations, and analyzed MYO5B expression in primary PPGL tumor cases in relation to mutation status. Immunohistochemistry and mRNA expression analysis in 30 PPGL tumors revealed an increased MYO5B expression in metastatic compared to non-metastatic cases. In addition, subcellular localization of MYO5B protein was altered from cytoplasmic to membranous in some metastatic tumors, and the strongest and most abnormal expression pattern was observed in a paraganglioma harboring a somatic MYO5B:p.G1611S mutation. In addition to five previously discovered MYO5B mutations, the present study of 30 PPGL (8 previous and 22 new samples) also revealed two, and hence recurrent, mutations in the gene paralog MYO5A. The three MYO5B missense mutations with the highest prediction scores (p.L587P, p.G1611S and p.R1641C) were selected and functionally validated using site directed mutagenesis and stable transfection into human neuroblastoma cells (SK-N-AS) and embryonic kidney cells (HEK293). In vitro analysis showed a significant increased proliferation rate in all three MYO5B mutated clones. The two somatically derived mutations, p.L587P and p.G1611S, were also found to increase the migration rate. Expression analysis of MYO5B mutants compared to wild type clones, demonstrated a significant enrichment of genes involved in migration, proliferation, cell adhesion, glucose metabolism, and cellular homeostasis. Our study validates the functional role of novel MYO5B mutations in proliferation and migration, and suggest the MYO5-pathway to be involved in the malignant progression in some PPGL tumors.

Up to 25% of pheochromocytoma/paraganglioma (PPGL) cases develop metastatic disease with poor outcome and few treatment options. The disease mechanism is not fully understood, and to date there are no reliable markers to predict malignancy. We have recently discovered novel missense mutations in the non-conventional myosin 5 gene (MYO5B), an endosomal transport protein, which we now show enhances progression and migration in PPGLs. MYO5B mutations were preferentially found in patients with metastatic disease and SDH deficiency (germline SDHB-mutations). Abolished SDH activity result in a metabolic switch to aerobic glycolysis requiring increased glucose consumption. Since the MYO5B mutations were found to drive progression through downstream up-regulation of glucose metabolism genes, e.g. glucagon, we hypothesize that these mutations may fuel the pseudohypoxic state by altering glucose uptake in cancer cells. Our result is the first to link the myosin 5 genes to PPGL tumorigenesis. Further, it shows that the tumor progression route in PPGL is complex, with contribution from several genetic factors. An increasing number of studies show dysregulation and importance of the MYO5-proteins in cancer, but little is still known about the precise role and mechanism of mutations, hence more research in this area is needed.

Editing Myosin VB Gene to Create Porcine Model of Microvillus Inclusion Disease, With Microvillus-Lined Inclusions and Alterations in Sodium Transporters

BACKGROUND & AIMS

Microvillus inclusion disease (MVID) is caused by inactivating mutations in the myosin VB gene (MYO5B). MVID is a complex disorder characterized by chronic, watery, life-threatening diarrhea that usually begins in the first hours to days of life. We developed a large animal model of MVID to better understand its pathophysiology.

METHODS

Pigs were cloned by transfer of chromatin from swine primary fetal fibroblasts, which were edited with TALENs and single-strand oligonucleotide to introduce a P663-L663 substitution in the endogenous swine MYO5B (corresponding to the P660L mutation in human MYO5B, associated with MVID) to fertilized oocytes. We analyzed duodenal tissues from patients with MVID (with the MYO5B P660L mutation) and without (controls), and from pigs using immunohistochemistry. Enteroids were generated from pigs with MYO5B(P663L) and without the substitution (control pigs).

RESULTS

Duodenal tissues from patients with MVID lacked MYO5B at the base of the apical membrane of intestinal cells; instead MYO5B was intracellular. Intestinal tissues and derived enteroids from MYO5B(P663L) piglets had reduced apical levels and diffuse subapical levels of sodium hydrogen exchanger 3 and SGLT1, which regulate transport of sodium, glucose, and water, compared with tissues from control piglets. However, intestinal tissues and derived enteroids from MYO5B(P663L) piglets maintained CFTR on apical membranes, like tissues from control pigs. Liver tissues from MYO5B(P663L) piglets had alterations in bile salt export pump, a transporter that facilitates bile flow, which is normally expressed in the bile canaliculi in the liver.

CONCLUSIONS

We developed a large animal model of MVID that has many features of the human disease. Studies of this model could provide information about the functions of MYO5B and MVID pathogenesis, and might lead to new treatments.

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.

The challenge of personalized cell biology: The example of microvillus inclusion disease

Whole exome sequencing now provides a tool for rapid analysis of patients manifesting congenital diseases. Congenital diarrheal diseases provide a critical example of the challenges of combining identification of genetic mutations responsible for disease with characterization of the cell biological and cell physiological deficits observed in patients. Recent studies exploring the cellular events associated with loss of functional Myosin 5B (MYO5B) have demonstrated the importance of cell biological and physiological analyses to provide a greater understanding of the implications of pathological mutations. Development of enteroids derived from biopsies of patients with complex congenital diarrheal diseases provides a critical resource for evaluation of the cell biological impact of specific monogenic mutations on enterocyte function. The ability to identify putative causative mutations for congenital disease now provides an opportunity to coordinate the efforts of physicians and cell biologists in an effort to provide patients with personalized cell biology analysis to improve patient diagnosis and treatment.

Glucocorticoids and myosin5b loss of function induce heightened PKA signaling in addition to membrane traffic defects

Loss-of-function mutations in the nonconventional myosin Vb (Myo5b) result in microvillus inclusion disease (MVID) and massive secretory diarrhea that often begins at birth. Myo5b mutations disrupt the apical recycling endosome (ARE) and membrane traffic, resulting in reduced surface expression of apical membrane proteins. ARE disruption also results in constitutive phosphoinositide-dependent kinase 1 gain of function. In MVID, decreased surface expression of apical anion channels involved in Cl^- extrusion, such as cystic fibrosis transmembrane conductance regulator (CFTR), should reduce fluid secretion into the intestinal lumen. But the opposite phenotype is observed. To explain this contradiction and the onset of diarrhea, we hypothesized that signaling effects downstream from Myo5b loss of function synergize with higher levels of glucocorticoids to activate PKA and CFTR. Data from intestinal cell lines, human MVID, and Myo5b KO mouse intestine revealed changes in the subcellular redistribution of PKA activity to the apical pole, increased CFTR phosphorylation, and establishment of apical cAMP gradients in Myo5b-defective cells exposed to physiological levels of glucocorticoids. These cells also displayed net secretory fluid fluxes and transepithelial currents mainly from PKA-dependent Cl^- secretion. We conclude that Myo5b defects result in PKA stimulation that activates residual channels on the surface when intestinal epithelia are exposed to glucocorticoids at birth.

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 Caenorhabditiselegans 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.

The Actin-Based Motor Myosin Vb Is Crucial to Maintain Epidermal Barrier Integrity

Myosin Vb (Myo5b) is an unconventional myosin involved in the actin-dependent transport and tethering of intracellular organelles. In the epidermis, granular keratinocytes accumulate cytoplasmic lamellar bodies (LBs), secretory vesicles released at the junction with the stratum corneum that participate actively in the maintenance of the epidermal barrier. We have previously demonstrated that LB biogenesis is controlled by the Rab11a guanosine triphosphate hydrolase, known for its ability to recruit the Myo5b motor. In order to better characterize the molecular pathway that controls LB trafficking, we analyzed the role of F-actin and Myo5b in the epidermis. We demonstrated that LB distribution in granular keratinocytes was dependent on a dynamic F-actin cytoskeleton. Myo5b was shown to be highly expressed in granular keratinocytes and associated with corneodesmosin-loaded LB. In reconstructed human epidermis, Myo5b silencing led to epidermal barrier defects associated with structural alterations of the stratum corneum and a reduced pool of LB showing signs of disordered maturation. Myo5b depletion also disturbed the expression and distribution of both LB cargoes and junctional components, such as claudin-1, which demonstrates its action on both LB trafficking and junctional complex composition. Together, our data reveal the essential role of Myo5b in maintaining the epidermal barrier integrity.

Loss of MYO5B Leads to Reductions in Na+ Absorption With Maintenance of CFTR-Dependent Cl- Secretion in Enterocytes

BACKGROUND & AIMS

Inactivating mutations in MYO5B cause microvillus inclusion disease (MVID), but the physiological cause of the diarrhea associated with this disease is unclear. We investigated whether loss of MYO5B results in aberrant expression of apical enterocyte transporters.

METHODS

We studied alterations in apical membrane transporters in MYO5B-knockout mice, as well as mice with tamoxifen-inducible, intestine-specific disruption of Myo5b (VilCreERT2;Myo5bflox/flox mice) or those not given tamoxifen (controls). Intestinal tissues were collected from mice and analyzed by immunostaining, immunoelectron microscopy, or cultured enteroids were derived. Functions of brush border transporters in intestinal mucosa were measured in Ussing chambers. We obtained duodenal biopsy specimens from individuals with MVID and individuals without MVID (controls) and compared transporter distribution by immunocytochemistry.

RESULTS

Compared to intestinal tissues from littermate controls, intestinal tissues from MYO5B-knockout mice had decreased apical localization of SLC9A3 (also called NHE3), SLC5A1 (also called SGLT1), aquaporin (AQP) 7, and sucrase isomaltase, and subapical localization of intestinal alkaline phosphatase and CDC42. However, CFTR was present on apical membranes of enterocytes from MYO5B knockout and control mice. Intestinal biopsies from patients with MVID had subapical localization of NHE3, SGLT1, and AQP7, but maintained apical CFTR. After tamoxifen administration, VilCreERT2;Myo5bflox/flox mice lost apical NHE3, SGLT1, DRA, and AQP7, similar to germline MYO5B knockout mice. Intestinal tissues from VilCreERT2;Myo5bflox/flox mice had increased CFTR in crypts and CFTR localized to the apical membranes of enterocytes. Intestinal mucosa from VilCreERT2;Myo5bflox/flox mice given tamoxifen did not have an intestinal barrier defect, based on Ussing chamber analysis, but did have decreased SGLT1 activity and increased CFTR activity.

CONCLUSIONS

Although trafficking of many apical transporters is regulated by MYO5B, trafficking of CFTR is largely independent of MYO5B. Decreased apical localization of NHE3, SGLT1, DRA, and AQP7 might be responsible for dysfunctional water absorption in enterocytes of patients with MVID. Maintenance of apical CFTR might exacerbate water loss by active secretion of chloride into the intestinal lumen.

Progressive Familial Intrahepatic Cholestasis

Genetic cholestasis has been dissected through genetic investigation. The major PFIC genes are now described. ATP8B1 encodes FIC1, ABCB11 encodes BSEP, ABCB4 encodes MDR3, TJP2 encodes TJP2, NR1H4 encodes FXR, and MYO5B encodes MYO5B. The full spectra of phenotypes associated with mutations in each gene are discussed, along with our understanding of the disease mechanisms. Differences in treatment response and targets for future treatment are emerging.

Reversible deficits in apical transporter trafficking associated with deficiency in diacylglycerol acyltransferase

Deficiency in diacylglycerol acyltransferase (DGAT1) is a rare cause of neonatal diarrhea, without a known mechanism or in vitro model. A patient presenting at our institution at 7 weeks of life with failure to thrive and diarrhea was found by whole-exome sequencing to have a homozygous DGAT1 truncation mutation. Duodenal biopsies showed loss of DGAT1 and deficits in apical membrane transporters and junctional proteins in enterocytes. When placed on a very low-fat diet, the patient's diarrhea resolved with normalization of brush border transporter localization in endoscopic biopsies. DGAT1 knockdown in Caco2-BBe cells modeled the deficits in apical trafficking, with loss of apical DPPIV and junctional occludin. Elevation in cellular lipid levels, including diacylglycerol (DAG) and phospholipid metabolites of DAG, was documented by lipid analysis in DGAT1 knockdown cells. Culture of the DGAT1 knockdown cells in lipid-depleted media led to re-establishment of occludin and return of apical DPPIV. DGAT1 loss appears to elicit global changes in enterocyte polarized trafficking that could account for deficits in absorption seen in the patient. The in vitro modeling of this disease should allow for investigation of possible therapeutic targets.

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

BACKGROUND & AIMS

Microvillus inclusion disease (MVID) is a congenital intestinal malabsorption disorder caused by defective apical vesicular transport. Existing cellular models do not fully recapitulate this heterogeneous pathology. The aim of this study was to characterize 3-dimensional intestinal organoids that continuously generate polarized absorptive cells as an accessible and relevant model to investigate MVID.

METHODS

Intestinal organoids from Munc18-2/Stxbp2-null mice that are deficient for apical vesicular transport were subjected to enterocyte-specific differentiation protocols. Lentiviral rescue experiments were performed using human MUNC18-2 variants. Apical trafficking and microvillus formation were characterized by confocal and transmission electron microscopy. Spinning disc time-lapse microscopy was used to document the lifecycle of microvillus inclusions.

RESULTS

Loss of Munc18-2/Stxbp2 recapitulated the pathologic features observed in patients with MUNC18-2 deficiency. The defects were fully restored by transgenic wild-type human MUNC18-2 protein, but not the patient variant (P477L). Importantly, we discovered that the MVID phenotype was correlated with the degree of enterocyte differentiation: secretory vesicles accumulated already in crypt progenitors, while differentiated enterocytes showed an apical tubulovesicular network and enlarged lysosomes. Upon prolonged enterocyte differentiation, cytoplasmic F-actin-positive foci were observed that further progressed into classic microvillus inclusions. Time-lapse microscopy showed their dynamic formation by intracellular maturation or invagination of the apical or basolateral plasma membrane.

CONCLUSIONS

We show that prolonged enterocyte-specific differentiation is required to recapitulate the entire spectrum of MVID. Primary organoids can provide a powerful model for this heterogeneous pathology. Formation of microvillus inclusions from multiple membrane sources showed an unexpected dynamic of the enterocyte brush border.

Advances in Evaluation of Chronic Diarrhea in Infants

Diarrhea is common in infants (children less than 2 years of age), usually acute, and, if chronic, commonly caused by allergies and occasionally by infectious agents. Congenital diarrheas and enteropathies (CODEs) are rare causes of devastating chronic diarrhea in infants. Evaluation of CODEs is a lengthy process and infrequently leads to a clear diagnosis. However, genomic analyses and the development of model systems have increased our understanding of CODE pathogenesis. With these advances, a new diagnostic approach is needed. We propose a revised approach to determine causes of diarrhea in infants, including CODEs, based on stool analysis, histologic features, responses to dietary modifications, and genetic tests. After exclusion of common causes of diarrhea in infants, the evaluation proceeds through analyses of stool characteristics (watery, fatty, or bloody) and histologic features, such as the villus to crypt ratio in intestinal biopsies. Infants with CODEs resulting from defects in digestion, absorption, transport of nutrients and electrolytes, or enteroendocrine cell development or function have normal villi to crypt ratios; defects in enterocyte structure or immune-mediated conditions result in an abnormal villus to crypt ratios and morphology. Whole-exome and genome sequencing in the early stages of evaluation can reduce the time required for a definitive diagnosis of CODEs, or lead to identification of new variants associated with these enteropathies. The functional effects of gene mutations can be analyzed in model systems such as enteroids or induced pluripotent stem cells and are facilitated by recent advances in gene editing procedures. Characterization and investigation of new CODE disorders will improve management of patients and advance our understanding of epithelial cells and other cells in the intestinal mucosa.

Rab GTPases and their interacting protein partners: Structural insights into Rab functional diversity

Rab molecular switches are key players in defining membrane identity and regulating intracellular trafficking events in eukaryotic cells. In spite of their global structural similarity, Rab-family members acquired particular features that allow them to perform specific cellular functions. The overall fold and local sequence conservations enable them to utilize a common machinery for prenylation and recycling; while individual Rab structural differences determine interactions with specific partners such as GEFs, GAPs and effector proteins. These interactions orchestrate the spatiotemporal regulation of Rab localization and their turning ON and OFF, leading to tightly controlled Rab-specific functionalities such as membrane composition modifications, recruitment of molecular motors for intracellular trafficking, or recruitment of scaffold proteins that mediate interactions with downstream partners, as well as actin cytoskeleton regulation. In this review we summarize structural information on Rab GTPases and their complexes with protein partners in the context of partner binding specificity and functional outcomes of their interactions in the cell.

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.

Apical Membrane Alterations in Non-intestinal Organs in Microvillus Inclusion Disease

OBJECTIVES

Microvillus inclusion disease (MVID) is a severe form of neonatal diarrhea, caused mainly by mutations in MYO5B. Inactivating mutations in MYO5B causes depolarization of enterocytes in the small intestine, which gives rise to chronic, unremitting secretory diarrhea. While the pathology of the small intestine in MVID patients is well described, little is known about extraintestinal effects of MYO5B mutation.

METHODS

We examined stomach, liver, pancreas, colon, and kidney in Navajo MVID patients, who share a single homozygous MYO5B-P660L (1979C>T p.Pro660Leu, exon 16). Sections were stained for markers of the apical membrane to assess polarized trafficking.

RESULTS

Navajo MVID patients showed notable changes in H/K-ATPase-containing tubulovesicle structure in the stomach parietal cells. Colonic mucosa was morphologically normal, but did show losses in apical ezrin and Syntaxin 3. Hepatocytes in the MVID patients displayed aberrant canalicular expression of the essential transporters MRP2 and BSEP. The pancreas showed small fragmented islets and a decrease in apical ezrin in pancreatic ducts. Kidney showed normal primary cilia.

CONCLUSIONS

These findings indicate that the effects of the P660L mutation in MYO5B in Navajo MVID patients are not limited to the small intestine, but that certain tissues may be able to compensate functionally for alterations in apical trafficking.

MYO5B, STX3, and STXBP2 mutations reveal a common disease mechanism that unifies a subset of congenital diarrheal disorders: A mutation update

Microvillus inclusion disease (MVID) is a rare but fatal autosomal recessive congenital diarrheal disorder caused by MYO5B mutations. In 2013, we launched an open‐access registry for MVID patients and their MYO5B mutations (www.mvid-central.org). Since then, additional unique MYO5B mutations have been identified in MVID patients, but also in non‐MVID patients. Animal models have been generated that formally prove the causality between MYO5B and MVID. Importantly, mutations in two other genes, STXBP2 and STX3, have since been associated with variants of MVID, shedding new light on the pathogenesis of this congenital diarrheal disorder. Here, we review these additional genes and their mutations. Furthermore, we discuss recent data from cell studies that indicate that the three genes are functionally linked and, therefore, may constitute a common disease mechanism that unifies a subset of phenotypically linked congenital diarrheal disorders. We present new data based on patient material to support this. To congregate existing and future information on MVID geno‐/phenotypes, we have updated and expanded the MVID registry to include all currently known MVID‐associated gene mutations, their demonstrated or predicted functional consequences, and associated clinical information.