Transfection reagent artefact likely accounts for some reports of extracellular vesicle function

Extracellular vesicles (EV) are important mediators of cell communication and physiology. EVs are frequently investigated by transiently transfecting cells with plasmid DNA to produce EVs modified with protein(s) or nucleic acid(s) of interest. DNA-transfection reagent complexes (DTC) are approximately the same size as EVs, raising the possibility that some purification procedures may fail to separate these two species and activity arising from carryover DTC may be improperly attributed to EVs. We find that differential ultracentrifugation, a commonly employed EV isolation procedure, does not separate EVs from DTC present in the cell culture supernatant of transiently transfected cells. We demonstrate that the biological activity of an EV-directed Cre recombinase is due to contaminating plasmid DNA and not EV-mediated delivery of Cre protein. Moreover, steps commonly taken to remove plasmid DNA from EV samples, such as media exchanges and treatment with nucleases, are ineffective at avoiding this artefact. Due to the pernicious nature of plasmid DNA in these cellular assays, some reports of EV function are likely artefacts produced by contaminating DTC. EVs and DTC can be separated by density gradient ultracentrifugation, highlighting the importance of validating elimination of DTC when using transient transfection of EV-producing cells to interrogate EV function.

An alternative N-terminal fold of the intestine-specific annexin A13a induces dimerization and regulates membrane-binding

Annexin proteins function as Ca²⁺-dependent regulators of membrane trafficking and repair that may also modulate membrane curvature. Here, using high-resolution confocal imaging, we report that the intestine-specific annexin A13 (ANX A13) localizes to the tips of intestinal microvilli and determined the crystal structure of the ANX A13a isoform to 2.6 Å resolution. The structure revealed that the N terminus exhibits an alternative fold that converts the first two helices and the associated helix-loop-helix motif into a continuous α-helix, as stabilized by a domain-swapped dimer. We also found that the dimer is present in solution and partially occludes the membrane-binding surfaces of annexin, suggesting that dimerization may function as a means for regulating membrane binding. Accordingly, as revealed by in vitro binding and cellular localization assays, ANX A13a variants that favor a monomeric state exhibited increased membrane association relative to variants that favor the dimeric form. Together, our findings support a mechanism for how the association of the ANX A13a isoform with the membrane is regulated.

A requirement for filopodia extension toward Slit during Robo-mediated axon repulsion

Axons navigate long distances through complex 3D environments to interconnect the nervous system during development. Although the precise spatiotemporal effects of most axon guidance cues remain poorly characterized, a prevailing model posits that attractive guidance cues stimulate actin polymerization in neuronal growth cones whereas repulsive cues induce actin disassembly. Contrary to this model, we find that the repulsive guidance cue Slit stimulates the formation and elongation of actin-based filopodia from mouse dorsal root ganglion growth cones. Surprisingly, filopodia form and elongate toward sources of Slit, a response that we find is required for subsequent axonal repulsion away from Slit. Mechanistically, Slit evokes changes in filopodium dynamics by increasing direct binding of its receptor, Robo, to members of the actin-regulatory Ena/VASP family. Perturbing filopodium dynamics pharmacologically or genetically disrupts Slit-mediated repulsion and produces severe axon guidance defects in vivo. Thus, Slit locally stimulates directional filopodial extension, a process that is required for subsequent axonal repulsion downstream of the Robo receptor.

Lamellipodin promotes invasive 3D cancer cell migration via regulated interactions with Ena/VASP and SCAR/WAVE

Cancer invasion is a hallmark of metastasis. The mesenchymal mode of cancer cell invasion is mediated by elongated membrane protrusions driven by the assembly of branched F-actin networks. How deregulation of actin regulators promotes cancer cell invasion is still enigmatic. We report that increased expression and membrane localization of the actin regulator Lamellipodin correlate with reduced metastasis-free survival and poor prognosis in breast cancer patients. In agreement, we find that Lamellipodin depletion reduced lung metastasis in an orthotopic mouse breast cancer model. Invasive 3D cancer cell migration as well as invadopodia formation and matrix degradation was impaired upon Lamellipodin depletion. Mechanistically, we show that Lamellipodin promotes invasive 3D cancer cell migration via both actin-elongating Ena/VASP proteins and the Scar/WAVE complex, which stimulates actin branching. In contrast, Lamellipodin interaction with Scar/WAVE but not with Ena/VASP is required for random 2D cell migration. We identified a phosphorylation-dependent mechanism that regulates selective recruitment of these effectors to Lamellipodin: Abl-mediated Lamellipodin phosphorylation promotes its association with both Scar/WAVE and Ena/VASP, whereas Src-dependent phosphorylation enhances binding to Scar/WAVE but not to Ena/VASP. Through these selective, regulated interactions Lamellipodin mediates directional sensing of epidermal growth factor (EGF) gradients and invasive 3D migration of breast cancer cells. Our findings imply that increased Lamellipodin levels enhance Ena/VASP and Scar/WAVE activities at the plasma membrane to promote 3D invasion and metastasis.

Intestinal brush border assembly driven by protocadherin-based intermicrovillar adhesion

Transporting epithelial cells build apical microvilli to increase membrane surface area and enhance absorptive capacity. The intestinal brush border provides an elaborate example with tightly packed microvilli that function in nutrient absorption and host defense. Although the brush border is essential for physiological homeostasis, its assembly is poorly understood. We found that brush border assembly is driven by the formation of Ca(2+)-dependent adhesion links between adjacent microvilli. Intermicrovillar links are composed of protocadherin-24 and mucin-like protocadherin, which target to microvillar tips and interact to form a trans-heterophilic complex. The cytoplasmic domains of microvillar protocadherins interact with the scaffolding protein, harmonin, and myosin-7b, which promote localization to microvillar tips. Finally, a mouse model of Usher syndrome lacking harmonin exhibits microvillar protocadherin mislocalization and severe defects in brush border morphology. These data reveal an adhesion-based mechanism for brush border assembly and illuminate the basis of intestinal pathology in patients with Usher syndrome. PAPERFLICK:

Enterocyte microvillus-derived vesicles detoxify bacterial products and regulate epithelial-microbial interactions

The continuous monolayer of intestinal epithelial cells (IECs) lining the gut lumen functions as the site of nutrient absorption and as a physical barrier to prevent the translocation of microbes and associated toxic compounds into the peripheral vasculature. IECs also express host defense proteins such as intestinal alkaline phosphatase (IAP), which detoxify bacterial products and prevent intestinal inflammation. Our laboratory recently showed that IAP is enriched on vesicles that are released from the tips of IEC microvilli and accumulate in the intestinal lumen. Here, we show that these native "lumenal vesicles" (LVs) (1) contain catalytically active IAP that can dephosphorylate lipopolysaccharide (LPS), (2) cluster on the surface of native lumenal bacteria, (3) prevent the adherence of enteropathogenic E. coli (EPEC) to epithelial monolayers, and (4) limit bacterial population growth. We also find that IECs upregulate LV production in response to EPEC and other Gram-negative pathogens. Together, these results suggest that microvillar vesicle shedding represents a novel mechanism for distributing host defense machinery into the intestinal lumen and that microvillus-derived LVs modulate epithelial-microbial interactions.

Proteomic analysis of the enterocyte brush border

The brush border domain at the apex of intestinal epithelial cells is the primary site of nutrient absorption in the intestinal tract and the primary surface of interaction with microbes that reside in the lumen. Because the brush border is positioned at such a critical physiological interface, we set out to create a comprehensive list of the proteins that reside in this domain using shotgun mass spectrometry. The resulting proteome contains 646 proteins with diverse functions. In addition to the expected collection of nutrient processing and transport components, we also identified molecules expected to function in the regulation of actin dynamics, membrane bending, and extracellular adhesion. These results provide a foundation for future studies aimed at defining the molecular mechanisms underpinning brush border assembly and function.

Leveraging the membrane - cytoskeleton interface with myosin-1

Class 1 myosins are small motor proteins with the ability to simultaneously bind to actin filaments and cellular membranes. Given their ability to generate mechanical force, and their high prevalence in many cell types, these molecules are well positioned to carry out several important biological functions at the interface of membrane and the actin cytoskeleton. Indeed, recent studies implicate these motors in endocytosis, exocytosis, release of extracellular vesicles, and the regulation of tension between membrane and the cytoskeleton. Many class 1 myosins also exhibit a load-dependent mechano-chemical cycle that enables them to maintain tension for long periods of time without hydrolyzing ATP. These properties put myosins-1 in a unique position to regulate dynamic membrane-cytoskeleton interactions and respond to physical forces during these events.

Differential localization and dynamics of class I myosins in the enterocyte microvillus

These data establish myosin-1d as a component of the brush border cytoskeleton that demonstrates microvillar tip localization.

Epithelial cells lining the intestinal tract build an apical array of microvilli known as the brush border. Each microvillus is a cylindrical membrane protrusion that is linked to a supporting actin bundle by myosin-1a (Myo1a). Mice lacking Myo1a demonstrate no overt physiological symptoms, suggesting that other myosins may compensate for the loss of Myo1a in these animals. To investigate changes in the microvillar myosin population that may limit the Myo1a KO phenotype, we performed proteomic analysis on WT and Myo1a KO brush borders. These studies revealed that WT brush borders also contain the short-tailed class I myosin, myosin-1d (Myo1d). Myo1d localizes to the terminal web and striking puncta at the tips of microvilli. In the absence of Myo1a, Myo1d peptide counts increase twofold; this motor also redistributes along the length of microvilli, into compartments normally occupied by Myo1a. FRAP studies demonstrate that Myo1a is less dynamic than Myo1d, providing a mechanistic explanation for the observed differential localization. These data suggest that Myo1d may be the primary compensating class I myosin in the Myo1a KO model; they also suggest that dynamics govern the localization and function of different yet closely related myosins that target common actin structures.

The enterocyte microvillus is a vesicle-generating organelle

For decades, enterocyte brush border microvilli have been viewed as passive cytoskeletal scaffolds that serve to increase apical membrane surface area. However, recent studies revealed that in the in vitro context of isolated brush borders, myosin-1a (myo1a) powers the sliding of microvillar membrane along core actin bundles. This activity also leads to the shedding of small vesicles from microvillar tips, suggesting that microvilli may function as vesicle-generating organelles in vivo. In this study, we present data in support of this hypothesis, showing that enterocyte microvilli release unilamellar vesicles into the intestinal lumen; these vesicles retain the right side out orientation of microvillar membrane, contain catalytically active brush border enzymes, and are specifically enriched in intestinal alkaline phosphatase. Moreover, myo1a knockout mice demonstrate striking perturbations in vesicle production, clearly implicating this motor in the in vivo regulation of this novel activity. In combination, these data show that microvilli function as vesicle-generating organelles, which enable enterocytes to deploy catalytic activities into the intestinal lumen.

Myosin-1a powers the sliding of apical membrane along microvillar actin bundles

Microvilli are actin-rich membrane protrusions common to a variety of epithelial cell types. Within microvilli of the enterocyte brush border (BB), myosin-1a (Myo1a) forms an ordered ensemble of bridges that link the plasma membrane to the underlying polarized actin bundle. Despite decades of investigation, the function of this unique actomyosin array has remained unclear. Here, we show that addition of ATP to isolated BBs induces a plus end–directed translation of apical membrane along microvillar actin bundles. Upon reaching microvillar tips, membrane is “shed” into solution in the form of small vesicles. Because this movement demonstrates the polarity, velocity, and nucleotide dependence expected for a Myo1a-driven process, and BBs lacking Myo1a fail to undergo membrane translation, we conclude that Myo1a powers this novel form of motility. Thus, in addition to providing a means for amplifying apical surface area, we propose that microvilli function as actomyosin contractile arrays that power the release of BB membrane vesicles into the intestinal lumen.

Isotonic nasogastric tube feedings: do they cause diarrhea?

NG tube feedings in hospitalized patients, whether in a ward or ICU, are considered a common etiology of diarrhea. To evaluate the accuracy of this assumption, 13 hospitalized postoperative patients with head and neck cancer, 11 ICU patients, and five healthy volunteers were given isotonic, low-residue, lactose-free tube feedings starting at 30 kcal/kg.day. There was no prior history of diarrhea in any patient studied. There was a significant difference in both albumin levels and diarrhea incidence in the three groups (analysis of variance, p less than .05). Diarrhea occurred in four of 11 ICU patients while receiving feedings, but not in the healthy volunteers or non-ICU patients. The four patients with diarrhea had an average albumin level of 2.8 g/dl, while the other ICU patients had an average albumin of 2.6 g/dl. We conclude that isotonic NG tube feedings do not cause diarrhea in healthy volunteers or postoperative head and neck cancer patients. However, in some mechanically ventilated ICU patients, this regimen may cause diarrhea even though no risk factors can be clearly identified.

Enumeration of absolute numbers of T lymphocyte subsets in B-chronic lymphocytic leukaemia using an immunoperoxidase technique: relation to clinical stage

An immunoperoxidase technique has been used to identify and enumerate helper and suppressor T-cell subsets, as defined by reactivity with Coulter T4 and OKT8 monoclonal antibodies in 54 patients with B chronic lymphocytic leukaemia (B-CLL) and in the same number of matched controls. The ratio of T4+ to T8+ cells was significantly reduced in the B-CLL group as a whole (P less than 0.001) and in each stage of the three clinical staging systems. There was an increase in the median absolute level of T8+ cells in the whole CLL group (P less than 0.001). However, subdivision of the CLL group by clinical staging systems revealed a large group (28 patients) in which median T8+ cell levels were not raised and median T4+ cell levels were low (P less than 0.01). There was no significant decrease in T4+:T8+ ratio, increase in T8+ cells or decrease in T4+ cells with progression of clinical stage. Absolute numbers of E+ cells were significantly raised in all staging systems as were E+ T4- T8- cells (P less than 0.001). A significant alteration in either of these populations with progression of clinical stage was not present.

Acid hydrolase activities in B cell chronic lymphocytic leukaemia lymphocytes: correlation of cytochemical reactions with immunological phenotype

The cytochemical reactions of 5 acid hydrolases, alpha-naphthyl acetate esterase (ANAE), acid phosphatase (AP), beta-glucuronidase, beta-glucosaminidase and dipeptidylaminopeptidase IV (DAP IV) were investigated in lymphocytes from 30 patients with B cell chronic lymphocytic leukaemia (B-CLL). Based on ANAE and AP reactivities, 4 cytochemically distinctive subgroups were identified: Group 1: AP and ANAE less than 50% positive lymphocytes (5 cases); Group 2: AP greater than 50%, ANAE less than 50% positive lymphocytes (11 cases); Group 3: AP less than 50%, ANAE greater than 50% positive lymphocytes (7 cases); Group 4: AP and ANAE greater than 50% positive lymphocytes (7 cases). beta-Glucuronidase displayed similar patterns of reactivity to AP. beta-Glucosaminidase activity was observed in the majority of lymphocytes in most patients, whereas DAP IV activity was present in less than 20% of lymphoid cells. The study failed to establish any relationship between cytochemical grouping and patients' clinical status, peripheral lymphocyte counts, E or mouse rosette values, light or heavy chain cellular immunoglobulin (Ig) class. Attempts to correlate acid hydrolase and Ig heavy chain isotype expression, putative markers of B cell maturation, were unsuccessful and indicate that within the narrow spectrum of B cell differentiation seen in B-CLL these characteristics are unrelated.

Identification of cellular immunoglobulins in chronic lymphocytic leukaemia by immunoperoxidase staining

An indirect immunoperoxidase technique has been used for visualisation of cellular immunoglobulins in chronic lymphocytic leukaemia. Baker's formol calcium was used as fixative. Monoclonal light and heavy chain patterns were demonstrated in 24 out of 27 cases. Only one case did not have any demonstrable immunoglobulins. The presence of alpha or gamma heavy chain immunoglobulin isotypes in leukaemic lymphocytes was found to be related to low mouse rosetting capacity (p less than 0.05).