Contracting lacteals send lipids down the drain

Villi are small, finger-like projections of the small intestine that facilitate the absorption of dietary molecules, including lipids, from the intestinal lumen. Lacteals, lymphatic microvessels that extend into the center of each villus, are involved in shuttling accumulated subepithelial lipids into the lymphatic system. Although it is clear that lacteals play a vital role in dietary lipid transport, the precise physiological mechanics of the in vivo transport process are not known. Kibaek Choe, Jeon Yeob Jang, Intae Park and colleagues at the Korea Advanced Institute of Science and Technology (KAIST) and the Pusan National University School of Medicine utilized an intravital imaging approach to gain a better understanding of lacteal function.  Using a video-rate imaging system, the authors were able to visualize the movement of fluorescently tagged lipids in individual villi of the mouse small intestine.  Lacteals were observed to drain higher molecular weight lipids and other large molecules that were not efficiently taken up by capillaries in the villi.  In addition, the high-frame rate of this system allowed the authors to determine that lacteals undergo cycles of rapid contraction and relaxation, which help move lipids into the lymphatic system.  This rhythmic cycling was mediated by smooth muscles surrounding the lacteal and through neural inputs from the autonomic nervous system.  Together, the data demonstrate the power of studying physiological systems in vivo with high-powered imaging systems.  Moreover, these tools should lead to a more complete view of lipid and therapeutic drug uptake from the intestinal lumen.  The accompanying movies show real-time imaging of murine intestinal lacteals on a custom-built video-rate laser scanning confocal microscope. Video 1 shows that the contractile movement of intestinal lacteals (green;Prox-GFP) drains dietary lipids (red). Video 2 shows smooth muscles (red; actin-dsRed) surrounding a contracting lacteal (green; Prox-GFP).  

Published October 5, 2015, by Brian P. Head

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Intravital imaging of intestinal lacteals unveils lipid drainage through contractility
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Published October 5, 2015
Citation Information: J Clin Invest. 2015;125(11):4042-4052. https://doi.org/10.1172/JCI76509.
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Intravital imaging of intestinal lacteals unveils lipid drainage through contractility

Abstract

Lacteals are lymphatic vessels located at the center of each intestinal villus and provide essential transport routes for lipids and other lipophilic molecules. However, it is unclear how absorbed molecules are transported through the lacteal. Here, we used reporter mice that express GFP under the control of the lymphatic-specific promoter Prox1 and a custom-built confocal microscope and performed intravital real-time visualization of the absorption and transport dynamics of fluorescence-tagged fatty acids (FAs) and various exogenous molecules in the intestinal villi in vivo. These analyses clearly revealed transepithelial absorption of these molecules via enterocytes, diffusive distribution over the lamina propria, and subsequent transport through lacteals. Moreover, we observed active contraction of lacteals, which seemed to be directly involved in dietary lipid drainage. Our analysis revealed that the smooth muscles that surround each lacteal are responsible for contractile dynamics and that lacteal contraction is ultimately controlled by the autonomic nervous system. These results indicate that the lacteal is a unique organ-specific lymphatic system and does not merely serve as a passive conduit but as an active pump that transports lipids. Collectively, using this efficient imaging method, we uncovered drainage of absorbed molecules in small intestinal villus lacteals and the involvement of lacteal contractibility.

Authors

Kibaek Choe, Jeon Yeob Jang, Intae Park, Yeseul Kim, Soyeon Ahn, Dae-Young Park, Young-Kwon Hong, Kari Alitalo, Gou Young Koh, Pilhan Kim

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