323 Advances in using membrane vesicles for studying intestinal functions

Abstract Enterocytes are the major cell lineage of the small intestinal epithelia along the crypt-villus axis in gut mucosa. Enterocytes participate in the gut terminal nutrient digestion and absorption; and have essential roles in nutrient and metabolite recycling and efflux, nutrient and hormonal sensing, interactions with symbiotic microbiome and binding to infectious viruses, thus influencing the host homeostasis and whole-body physiological status. Intestinal membrane vesicles have been widely used for studying various gut functions in the past several decades. Villus enterocytic brush border membrane and the apical membrane vesicles along the crypt-villus axis in gut mucosa can be prepared and isolated by Mg2+- and Ca2+-precipitation and differential centrifugation procedures. Whereas, gut mucosal basolateral membrane vesicles can be prepared, fractionated and differentiated from other intracellular organelles such as the mitochondrial inner membrane by the Percoll gradient ultracentrifugation. Nutrient transport and exchange activities in the membrane vesicles in vitro are conducted often by rapid hand filtration, and in excess, in an automated fast sampling apparatus. Nutrient transport and exchange activity kinetics are derived according to the well-established classic Michaelis-Menten and the tracer inhibitory kinetic models with a simple linear diffusional component. Physiological and biochemical properties associated with various nutrient uptake systems such as pH optimum, requirements for co-transported ions (e.g., Na+, K+, Cl-), transporter affinity and substrate specificity can be effectively investigated in vitro with the membrane vesicles by altering the compositions of uptake buffers and intra-vesicular pre-loading buffers. Some of the fundamental gut functions are established via using the membrane vesicles in different mammalian species. And these include H+-peptide cotransport, Na+-cotransport and Na+-independent systems of hexose uptake across the apical membrane and the low-affinity Na+-independent hexose bidirectional uptake across the basolateral membrane that were characterized in rodents; Na+-dependent cotransport and Na+-independent systems of amino acid (AA) transport across the apical membrane that were revealed in rodents and pigs; as well as the Na+-hexose and AA cotransport and the co-expression of the coronavirus SARS-CoV-2 host epithelial apical membrane binding receptor, i.e., the angiotensin-converting enzyme-2 (ACE2), along the gut crypt-villus axis that were identified in liquid formula-fed piglets. In the post genomic and metagenomic sequencing era, further research efforts need to pursue understanding of expression and various functionality of the gut terminal hydrolases, nutrient transporters, nutrient exchangers, nutrient sensors, substrate and particle receptors, taste receptors, viral binding receptors such as ACE2 and hormonal receptors and their interactions with luminal factors such as nutrient, metabolites, symbiotic bacterial outer membrane vesicles etc., along the longitudinal and crypt-villus axes, through using the mammalian primary enterocytic apical and the basolateral membrane vesicles under various research settings.