**1. Introduction**

The gastrointestinal system is, together with the skin and the respiratory system, the habitat most exposed to the external environment. Every day, thousands of microorganisms and compounds derived from digestion come into contact with it. This condition requires a complex defense system capable of separating the intestinal contents from the host tissues, regulating the absorption of nutrients and allowing the interaction between the resident microbial flora and the mucosal immune system, inhibiting the translocation of pathogens in the underlying tissues. All these functions are performed by the intestinal barrier.

The intestinal barrier is a functional unit, organized as a multi-layered system, in which it is possible to recognize two main parts: a physical surface barrier, which prevents bacterial adhesion and regulates the paracellular diffusion towards the underlying host tissues and a deeper functional barrier, which is able to discriminate between commensal and pathogenic microorganisms, organizing the immunological tolerance towards the commensal bacteria and the immune response towards the pathogens [1].

The intestinal epithelium is organized into a monolayer of cells with a thickness of only 20 μm and is composed of 5 different cell types: enterocytes (IECs), mucus-producing goblet cells (GCs), endocrine cells, "M" cells, "G" cells, and defensin-producing Paneth cells, all of which differentiate from intestinal epithelial Lgr5+ stem cells [2–4]. Lgr5+ cells are crypt base columnar (CBCSs) stem cells, a population of rapidly dividing cells at the crypt base expressing leucine-rich-repeat containing G-protein coupled receptor 5 (Lgr5), giving rise to all terminally differentiated intestinal epithelial cell (IEC) types [5] CBCSs divide into progenitor cells which move upward within the crypt into the transit amplifying zone [6]. It is here that the cells differentiate further and travel to the villus where their functions are required. At the villus tip, senescent IECs slough off through *anoikis*, a specific type of programmed cell death for anchorage-dependent cells, and make room for newly formed cells to take their place [6]. Paneth cells are the exception as these cells are long-lived secretory cells that migrate to the crypt base and reside between Lgr5+ CBCSs where they produce and secrete antimicrobial peptides and stem cell factors such as epidermal growth factor (EGF), and other factors that sustain the stem cell niche [7].

IECs are the most represented cell type. They act as a physical barrier that inhibits the translocation of the luminal content into the innermost tissues; IECs form a seamless structure. In fact, they are connected by particular inter-cellular binding structures called adherent junctions (AJs) and tight junctions (TJs), characterized by trans-membrane proteins that interact with adjacent cells and with intracellular proteins, intimately connected with the enterocyte cytoskeleton. The fundamental elements on which the integrity of the "*intestinal barrier*" depends are, therefore, the IECs and the intercellular junctions.

### **1.1 Intestinal epithelial cells (IECs) and their metabolism**

The main function of enterocytes is the absorption of nutrients, and this function is performed by the mature or "*absorptive*" IECs, which are differentiated from the intestinal stem cells, CBCSs, residing at the bottom of the crypt. Nutrients such as glucose and amino acids are transported and absorbed by various transporters embedded on the membranes of these enterocytes. Metabolism occurs in each cell along the crypt-villus axis (CVA). The intestinal epithelial cells are the most vigorous, self-renewing cells, regenerating from the crypt bottom to the villus tip in only 3–5 days. Intestinal epithelial cells continuously migrate and mature along the CVA; the energy metabolism in intestinal epithelial cells increases from the bottom of the crypt to the top of the villi. Moreover, the expression of proteins related to the metabolism of glucose, most amino acids, and fatty acids increases in intestinal epithelial cells during maturation along the CVA, while the expression of proteins related to glutamine metabolism decreases from crypt to villus tip. The expression of proteins involved in the citrate cycle is also increased in IECs during maturation along CVA [8].

L-Glutamate is one of the most abundant amino acids in alimentary proteins, but its concentration in blood is among the lowest. This is largely because L-glutamate is extensively oxidized in small intestine epithelial cells during its transcellular journey
