Development of the chicken intestinal epithelium

Identification of intestinal ion transport defects in microvillus inclusion disease

Loss of function mutations in the actin motor myosin Vb (Myo5b) lead to microvillus inclusion disease (MVID) and death in newborns and children. MVID results in secretory diarrhea, brush border (BB) defects, villus atrophy, and microvillus inclusions (MVIs) in enterocytes. How loss of Myo5b results in increased stool loss of chloride (Cl(-)) and sodium (Na(+)) is unknown. The present study used Myo5b loss-of-function human MVID intestine, polarized intestinal cell models of secretory crypt (T84) and villus resembling (CaCo2BBe, C2BBe) enterocytes lacking Myo5b in conjunction with immunofluorescence confocal stimulated emission depletion (gSTED) imaging, immunohistochemical staining, transmission electron microscopy, shRNA silencing, immunoblots, and electrophysiological approaches to examine the distribution, expression, and function of the major BB ion transporters NHE3 (Na(+)), CFTR (Cl(-)), and SLC26A3 (DRA) (Cl(-)/HCO3 (-)) that control intestinal fluid transport. We hypothesized that enterocyte maturation defects lead villus atrophy with immature secretory cryptlike enterocytes in the MVID epithelium. We investigated the role of Myo5b in enterocyte maturation. NHE3 and DRA localization and function were markedly reduced on the BB membrane of human MVID enterocytes and Myo5bKD C2BBe cells, while CFTR localization was preserved. Forskolin-stimulated CFTR ion transport in Myo5bKD T84 cells resembled that of control. Loss of Myo5b led to YAP1 nuclear retention, retarded enterocyte maturation, and a cryptlike phenotype. We conclude that preservation of functional CFTR in immature enterocytes, reduced functional expression of NHE3, and DRA contribute to Cl(-) and Na(+) stool loss in MVID diarrhea.

Microvilli appear to represent the first step in actin bundle formation in Drosophila bristles

During bristle development the emerging bristle shaft, socket cell, and the apical surface of thoracic epithelial cells form tiny protuberances or pimples that contain electron-dense material located on the cytoplasmic surface of the pimple tip. In a few cases short actin filaments extend from this material into the cortical cytoplasm. When cultured in the presence of jasplakinolide, an agent that prevents filament disassembly, pimples elongate to form microvilli containing a core of crosslinked filaments. Emerging-bristle mutants delay cortical bundle formation and are aggregated by forked protein crossbridges. Using these mutants and enhancing core bundle formation with jasplakinolide we found that microvillar formation represents the first stage in the morphogenesis of much larger actin bundles in Drosophila bristle shaft cells. Evidence is presented showing that socket cells do not contain forked protein crossbridges, a fact that may explain why cortical bundles only appear in bristle shaft cells. Furthermore, as pimples and microvilli form in the absence of both forked and fascin crossbridges, we also conclude that neither of these crossbridges account for core bundle formation in microvilli, but there must exist a third, as yet unidentified crossbridge in this system. Immunocytochemisty suggested that this new crossbridge is not Drosophila villin. Finally, ultrastructural comparisons suggest that microspikes and microvilli form very differently.

The structure, function, and assembly of actin filament bundles

The cellular organization, function, and molecular composition of selected biological systems with prominent actin filament bundles are reviewed. An overall picture of the great variety of functions served by actin bundles emerges from this overview. A unifying theme is that the actin cross-linking proteins are conserved throughout the eukaryotic kingdom and yet assembled in a variety of combinations to produce actin bundles of differing functions. Mechanisms of actin bundle formation in vitro are considered illustrating the variety of physical and chemical driving forces in this exceedingly complex process. Our limited knowledge regarding the formation of actin filament bundles in vivo is contrasted with the elegant biophysical studies performed in vitro but nonetheless reveals that interactions with membranes, nucleation sites, and other organizational components must contribute to formation of actin bundles in vivo.

Biphasic intestinal development in amphibians: embryogenesis and remodeling during metamorphosis

Intestinal development in anurans is a biphasic process. The embryogenesis of intestine resembles that in higher vertebrates. The subsequent remodeling process during metamorphosis to produce an adult organ is controlled by TH. Recent progress in studying TH action and its application to amphibian metamorphosis has provided considerable insights into the remodeling process. One possible model for the TH-induced gene regulation cascade of intestinal remodeling is presented in Fig. 9. It is assumed that TRs function as heterodimers with RXRs. In the absence of TH, the TR-RXR heterodimers can bind to TH response elements present in the target genes and repress the basal transcription of these genes (Fondell et al., 1993; Damm et al., 1989; Sap et al., 1989; Baniahmad et al., 1992; Ranjan et al., 1994). The binding of TH leads to conformational changes in the receptor complexes that in turn activate gene transcription. The products of these early response genes then participate in the activation of the remaining gene regulation cascade. Exactly how this occurs remains unknown. Interestingly, the early response genes include not only transcription factors but also other proteins such as metalloproteinases. The transcription factors could activate or repress downstream TH response genes directly. Other proteins are likely to assert their effect indirectly. For example, they could modify the ECM or cell surface. In addition, they could regulate and/or participate in signal transduction by growth factors. The cooperation between these complex intra- and extracellular processes eventually results in the degeneration of the larval organ and formation of the adult tissue. This simplified scheme immediately raises many questions. Although the mRNAs for TRs and RXRs are present in the intestine and the other tissues during metamorphosis (Yaoita and Brown, 1990; Kawahara et al., 1991; Y.-B. Shi, unpublished observations), it is unknown whether the mRNA levels reflect the protein levels. It also remains to be tested whether TR-RXR is indeed the functional complex in vivo and whether the heterodimer is responsible for the activation of the early response genes isolated to date. The majority of the early response genes are ubiquitous. Of the few intestine-specific genes, none of them have yet been identified by sequence analysis. It is of great interest to understand how the same genes expressed in tissues undergoing drastically different changes can exert their biological effects. It is likely that together with existing proteins in the intestine, these early genes regulate tissue-specific downstream genes, which in turn determine the tissue-specific transformation. An important issue is to establish the identity of these downstream genes.

Phospholipid membrane-associated brush border myosin-I activity

Brush border myosin-I (BBMI) is associated with the membrane of intestinal epithelial cells where it probably plays a structural role. BBMI also has been identified on Golgi-derived vesicles in intestinal epithelial cells where it may translocate vesicles into the brush border. However, the mechanochemical activity of BBMI bound to a phospholipid membrane has not been described. This study reports that phospholipid membrane-associated BBMI displays ATPase activity when bound to phospholipids, but does not move actin filaments when associated with a phospholipid bilayer. BBMI does not bind significantly to brush border membrane lipids, which contain about 16% phosphatidylserine (PS), in either a pelleting or planar membrane assay. Similarly, planar membranes containing 20% PS do not bind a significant amount of BBMI. Increasing the concentration of PS to 40% does result in the binding of BBMI to both vesicles and planar membranes. This binding is enhanced with increased Ca2+ concentrations. BBMI retains its ATPase activity when bound to phospholipid vesicles containing 40% PS. However, BBMI attached to a phospholipid bilayer surface does not move actin filaments, even though the amount of BBMI bound to the lipid surface, as reflected by the number of actin filaments associated with bilayer-bound BBMI, is sufficient to observe motility in control experiments. When membrane fluidity is reduced by adding cholesterol to the membrane lipids containing 40% PS, BBMI still binds to the membrane, but again no actin filament motility is observed. The lack of binding by BBMI to brush border membrane lipids and the absence of membrane-associated BBMI mechanical activity suggest that factors in addition to membrane lipids are necessary for membrane-associated myosin-I motility.

Cell-cell and cell-ECM interactions in epithelial apoptosis and cell renewal during frog intestinal development

Amphibian intestinal remodeling during metamorphosis is a developmental system that is entirely controlled by thyroid hormone. It transforms a simple tubular organ into a complex multiply folded frog intestine similar to that in higher vertebrates. This process involves the degeneration of the larval epithelium through programmed cell death (apoptosis) and concurrent proliferation and differentiation of adult cell types. Earlier morphological and cellular studies have provided strong evidence implicating the importance of cell-cell and cell-ECM (extracellular matrix) interactions in this process. The recent molecular characterization of the genes that are regulated by thyroid hormone has begun to reveal some molecular clues underlying such interactions. In particular, the Xenopus putative morphogen hedgehog appears to be involved in regulating/mediating cell-cell interactions during adult epithelial proliferation, differentiation, and/or intestinal morphogenesis. On the other hand, several matrix metalloproteinases (MMPs) may be involved in remodeling the ECM. Of special interest is stromelysin-3, whose spatial and temporal expression profile during intestinal metamorphosis implicates a role in ECM remodeling, which in turn facilitates cell fate determination, i.e., apoptosis vs proliferation and differentiation. Understanding the mechanisms of action for those extracellular molecules will present a future challenge in developmental research.

An in vitro model for the analysis of intestinal brush border assembly. I. Ultrastructural analysis of cell contact-induced brush border assembly in Caco-2BBe cells

Intestinal epithelial cells assemble and maintain a polarized, highly organized membrane-cytoskeleton array, the brush border. We describe an in vitro, cell contact-induced brush border assembly model using the Caco-2BBe clones. Subconfluent cells were ‘depolarized’ by brief passage through suspension culture in the presence of cytochalasin D and re-plated on filters at high density in low-Ca2+ medium. Upon return to regular medium, these small, rounded cells with bleb-like protrusions formed, over the course of 19 days, a polarized monolayer of tall, columnar cells with a well-defined brush border. Ultrastructural changes were documented by both transmission and scanning electron microscopy. The earliest events of microvillar assembly coincided with a short period of cell aggregation. Intercellular cysts were occasionally observed within these aggregates, and junction formation between cells which had no contact with the filter was also observed. Monolayer formation was completed within 48 hours, and cell height steadily increased approximately 3.5-fold over 19 days. Concurrent with monolayer formation and the increase in cell height, sparse microvilli with a few actin core filaments gradually became more dense and better organized. By the third day, the actin core bundles had begun to extend into the subjacent cytoplasm, while terminal web assembly was underway at five days. The mature morphology of the brush border was first observed at nine days, although cell height and microvillar density continued to increase during the subsequent ten days. Microvillar density rose approximately nine-fold throughout brush border assembly in the Caco-2BBe cells. With the exception of the formation of cellular aggregates at the onset of the time course, this sequence of morphological changes is comparable to that observed during brush border assembly in embryonic intestinal epithelial cells. The Caco-2BBe assembly model provides a useful system in which to investigate various molecular aspects of brush border assembly.

An in vitro model for the analysis of intestinal brush border assembly. II. Changes in expression and localization of brush border proteins during cell contact-induced brush border assembly in Caco-2BBe cells

In the companion paper (M. D. Peterson and M. S. Mooseker (1993). J. Cell Sci. 105, 445–460) we describe a method for modeling brush border assembly in the Caco-2BBe clones. In this study we have examined the molecular changes accompanying cell contact-induced brush border assembly. A subset of brush border proteins was tracked throughout brush border assembly by immunoblotting and by immunofluorescent localization using laser scanning confocal microscopy. Actin, fodrin, villin and presumptive unconventional myosin immunogens were distributed at the periphery of depolarized cells. All proteins partitioned primarily with the membrane fraction upon differential sedimentation of depolarized cell lysates; the fractionation patterns were comparable to those of confluent cells. After a monolayer had formed, each protein showed a redistribution to the apical domain in a discrete sequence. Actin and villin began to shift apically at 2 d, while fodrin and the unconventional myosin immunogens did not redistribute until 3 d. Enterocyte-like localization was observed by 5 d for all proteins. Sucrase-isomaltase was not reliably detectable until 9 d by immunofluorescence, after brush border assembly was complete. Quantitative immunoblot analysis of total cell extracts demonstrated an average 10-fold increase in villin levels, while fodrin levels appeared to remain unchanged. Three putative unconventional myosin immunogens of 140 kDa, 130 kDa, and 110 kDa have been detected previously in the C2BBe cells with a head-specific monoclonal antibody to avian brush border myosin I (M. D. Peterson and M. S. Mooseker (1992) J. Cell Sci. 102, 581–600). Each of these immunogens displayed distinct expression patterns during brush border assembly. The 140 kDa species decreased by half, while the 130 kDa immunogen(s) did not change in any consistent fashion. The 110 kDa protein, presumed to be human brush border myosin I, rose on average 8-fold. A ribonuclease protection assay was also performed using a probe for human brush border myosin I. Equal amounts of total RNA from depolarized and confluent cells were assayed; the level of protected product was approximately 9-fold greater in the confluent cells. The expression patterns of the brush border proteins, coupled with the correlation to the ultrastructural features during brush border assembly in C2BBe cells, show that differentiation of the C2BBe cells closely resembles the changes that occur during human fetal intestinal differentiation.