Intestinal cells surprisingly active in pursuit of nutrition and defense

Jun 29, 2009
Intestinal cells release enzyme-laden vesicles into the gut lumen from the tips of their microvilli (red circles) through the action of the motor protein myosin-1a. These lumenal vesicles may function to process nutrients and protect against bacterial infection. Credit: McConnell, R.E., et al. 2009. J. Cell Biol. doi:10.1083/jcb.200902147.

Every cell lining the small intestine bristles with thousands of tightly packed microvilli that project into the gut lumen, forming a brush border that absorbs nutrients and protects the body from intestinal bacteria. In the June 29, 2009 issue of the Journal of Cell Biology, Matthew McConnell, Matthew Tyska, and colleagues now find that microvilli extend their functional reach even further using a molecular motor to send vesicles packed with gut enzymes out into the lumen to get a head start on breaking down their substrates.

Microvilli have traditionally been viewed as passive scaffolds that increase the surface area of the gut wall. The apical plasma membrane tightly wraps around each protrusive bundle of actin, providing more space for nutrient processing and absorption. The motor protein myosin-1a (myo1a) maintains this structure by connecting the plasma membrane to the actin filaments.

In 2007, Tyska and colleagues found that myo1a functions in isolated brush borders to actively move membrane along the length of the microvilli, like a "membrane escalator." To their surprise, at the top of these escalators—the tips of the microvilli—the membrane pinched off to form small vesicles that were released into the surrounding medium. According to Tyska, when they showed their data to gastroenterologists, they immediately asked "Why would brush borders do that? They're wasting perfectly good apical membrane!" Tyska therefore wanted to see if vesicle shedding was a bona fide physiological function for microvilli.

Sure enough, scanning electron micrographs of rat intestines showed protrusions at the tips of microvilli that looked similar to budding vesicles. And a look at the gut's contents revealed vesicles enriched in the brush border intestinal alkaline phosphatase (IAP). The vesicles were packed with classical brush border membrane proteins such as aminopeptidases and sugar-processing enzymes, suggesting that the vesicles were derived from microvilli. The vesicles also contained several proteins such as annexin A13 that bend cell membranes and could form part of the vesicle budding machinery.

One protein definitely involved in vesicle formation is myo1a. Myo1a knockout mice still produce lumenal vesicles but they are irregularly sized and no longer enriched in specific proteins like IAP. Tyska thinks that these knockout vesicles are actually chunks of microvillar that are nonspecifically shed when myo1a isn't present to keep them attached to the actin core.

Returning to the gastroenterologists' question: Why would brush borders do that? McConnell et al. showed that the packaged enzymes were exposed on the vesicles' outer surface and were catalytically active. Releasing the enzymes in vesicles might increase their mixing with substrates in the gut's contents. Tyska is particularly interested in IAP, which has recently been shown to detoxify the bacterial outer-membrane component lipopolysaccharide. Releasing IAP in lumenal vesicles could be an important defense mechanism against intestinal pathogens.

More information: McConnell, R.E., et al. 2009. J. Cell Biol. doi:10.1083/jcb.200902147. www.jcb.org

Source: Rockefeller University (news : web)

Explore further: Hot-spring bacteria reveal ability to use far-red light for photosynthesis

add to favorites email to friend print save as pdf

Related Stories

Molecular motors may speed nutrient processing

May 30, 2007

Matthew Tyska, Ph.D., recalls being intrigued, from the first day of his postdoctoral fellowship in 1999, with a nearly 30-year-old photograph. It was an electron micrograph that showed the internal structures of an intestinal ...

Finding microscopic motors in the gut

Jun 28, 2007

Digestion has a previously unsuspected mechanical dimension: Vanderbilt researchers have discovered that the tiny, hair-like protrusions that line the gut are filled with millions of molecular motors that produce streams ...

Biologists prove critical step in membrane fusion

Apr 17, 2007

Cells constantly swap cargo bound in vesicles, miniscule membrane-enclosed packages of proteins and other chemicals. Before the swap can take place, the vesicle membrane must fuse with another membrane, creating channels ...

Neurotransmitter current not flowing through ion channels

Aug 29, 2007

In studying how neurotransmitters travel between cells -- by analysis of events in the dimensions of nanometers -- Cornell researchers have discovered that an electrical current thought to be present during that process does ...

Recommended for you

How plant cell compartments change with cell growth

4 hours ago

A research team led by Kiminori Toyooka from the RIKEN Center for Sustainable Resource Science has developed a sophisticated microscopy technique that for the first time captures the detailed movement of ...

Plants can 'switch off' virus DNA

4 hours ago

A team of virologists and plant geneticists at Wageningen UR has demonstrated that when tomato plants contain Ty-1 resistance to the important Tomato yellow leaf curl virus (TYLCV), parts of the virus DNA ...

A better understanding of cell to cell communication

5 hours ago

Researchers of the ISREC Institute at the School of Life Sciences, EPFL, have deciphered the mechanism whereby some microRNAs are retained in the cell while others are secreted and delivered to neighboring ...

A glimpse at the rings that make cell division possible

5 hours ago

Forming like a blown smoke ring does, a "contractile ring" similar to a tiny muscle pinches yeast cells in two. The division of cells makes life possible, but the actual mechanics of this fundamental process ...

User comments : 0