Dietary interactions with the bacterial sensing machinery in the intestine: the plant polyphenol case

Abstract

There are millions of microbes that live in the human gut. These are important in digestion as well as defense. The host immune system needs to be able to distinguish between the harmless bacteria and pathogens. The initial interaction between bacteria and the host happen through the pattern recognition receptors (PRRs). As these receptors are in direct contact with the external environment, this makes them important candidates for regulation by dietary components and therefore potential targets for therapy. In this review, we introduce some of the main PRRs including a cellular process known as autophagy, and how they function. Additionally we review dietary phytochemicals from plants which are believed to be beneficial for humans. The purpose of this review was to give a better understanding of how these components work in order to create better awareness on how they could be explored in the future.

Keywords: Nod-like receptors; Toll-like receptors; autophagy; microbiota; phytochemicals.

FIGURE 1

Host cells including epithelial cells, macrophages, and dendritic cells contain pattern recognition receptors (PRRs) that can recognize molecules released from pathogens during infection (PAMPs) or stressed cells during tissue damage (DAMPs). This activates downstream pathways to resolve infection or repair damaged tissue.

FIGURE 2

Microautophagy occurs when bulk cytosolic components are directly engulfed by lysosomes through the invaginations of the lysosomal membrane where they are rapidly degraded by hydrolase enzymes. This process has been studied in yeast and is still poorly characterized in mammals (Cuervo and Macian, 2012). Chaperone-mediated autophagy (CMA) is a very complex and specific pathway which is initiated when a chaperone recognizes a targeting motif in the cytosolic protein to be degraded. The chaperone/substrate complex reaches the lysosome and the substrate is internalized through the translocation complex in the lysosomal membrane (Cuervo and Macian, 2012). CMA is considerably different from the other types of autophagy because it does not directly engulf the protein material but selectively transfers it individually into the lysosome (Kadian and Garg, 2012). The most extensively described type of autophagy in the literature is macroautophagy which is also referred to as autophagy in the literature. A review on macroautophagy (which will be referred to as autophagy from this point) is provided in this review.