Pivotal to brain development and function is an intact blood-brain barrier (BBB), which acts as a gatekeeper to control the passage and exchange of molecules and nutrients between the circulatory system and the brain parenchyma. The BBB also ensures homeostasis of the central nervous system (CNS). We report that germ-free mice, beginning with intrauterine life, displayed increased BBB permeability compared to pathogen-free mice with a normal gut flora. The increased BBB permeability was maintained in germ-free mice after birth and during adulthood and was associated with reduced expression of the tight junction proteins occludin and claudin-5, which are known to regulate barrier function in endothelial tissues. Exposure of germ-free adult mice to a pathogen-free gut microbiota decreased BBB permeability and up-regulated the expression of tight junction proteins. Our results suggest that gut microbiota–BBB communication is initiated during gestation and propagated throughout life.

MUC2, the major colonic mucin, forms large polymers by N-terminal trimerization and C-terminal dimerization. Although the assembly process for MUC2 is established, it is not known how MUC2 is packed in the regulated secretory granulae of the goblet cell. When the N-terminal VWD1-D2-D′D3 domains (MUC2-N) were expressed in a goblet-like cell line, the protein was stored together with fulllength MUC2. By mimicking the pH and calcium conditions of the secretory pathway we analyzed purified MUC2-N by gel filtration, density gradient centrifugation, and transmission electron microscopy. At pH 7.4 the MUC2-N trimer eluted as a single peak by gel filtration. At pH 6.2 with Ca 2+ it formed large aggregates that did not enter the gel filtration column but were made visible after density gradient centrifugation. Electron microscopy studies revealed that the aggregates were composed of rings also observed in secretory granulae of colon tissue sections. The MUC2-N aggregates were dissolved by removing Ca 2+ and raising pH. After release from goblet cells, the unfolded full-length MUC2 formed stratified layers. These findings suggest a model for mucin packing in the granulae and the mechanism for mucin release, unfolding, and expansion.ucins are large glycoproteins that coat the surface of cells in the respiratory, digestive, and urogenital tracts (1, 2). Their main function is protection of epithelial cells from infection and physical injury. Mucins are characterized by mucin domains that are heavily O-glycosylated on the protein sequence rich in proline, threonine, and serine, therefore called PTS domains (3). These domains have little interspecies sequence conservation but often have tandemly repeated amino acid sequences that vary in number and length (3). There are several mucin types; the gelforming mucins are the only ones that form large polymers. In humans there are four gel-forming mucin genes that are known to be expressed, MUC2 in the intestine (4), MUC5AC in lungs and stomach, MUC5B in lungs and saliva, and MUC6 in stomach (1).MUC2 mucin is the major component of the mucus (mixture of mucins and other associated proteins) in the small and large intestine (2). In colon this is organized into two layers: an inner, densely packed layer that is attached to the epithelium that is impermeable to bacteria, and an outer, easily removable loose layer that is the habitat for the commensal bacteria (5). Human MUC2 mucin has 5,179 amino acids and contains multiple domains arranged in the following order (Fig. 1A): von Willebrand D1 domain (VWD1), VWD2, VWD′D3, (VWD1-D2-D′D3), first CysD, small PTS, second CysD, large PTS (tandemly repeated), C-terminal VWD4 followed by VWB, VWC, and a cystine-knot domain (CK) (4). The primary translational product of full-length MUC2 is quickly dimerized in the endoplasmic reticulum (ER) via disulfide bonds in the CK domain (6). The dimers pass into the Golgi apparatus, where the two PTS domains become O-glycosylated to form the two mucin domains. In the trans-Golgi network the glycosylated ...

Ileal mucus in CftrΔ508 mice is more adherent, denser, and less penetrable than that of WT mice, but addition of bicarbonate normalizes the properties of CftrΔ508 mucus.