Ion channels and transporters regulate nutrient absorption in health and disease

Abstract Ion channels and transporters are ubiquitously expressed on cell membrane, which involve in a plethora of physiological process such as contraction, neurotransmission, secretion and so on. Ion channels and transporters is of great importance to maintaining membrane potential homeostasis, which is essential to absorption of nutrients in gastrointestinal tract. Most of nutrients are electrogenic and require ion channels and transporters to absorb. This review summarizes the latest research on the role of ion channels and transporters in regulating nutrient uptake such as K+ channels, Ca2+ channels and ion exchangers. Revealing the mechanism of ion channels and transporters associated with nutrient uptake will be helpful to provide new methods to diagnosis and find potential targets for diseases like diabetes, inflammatory bowel diseases, etc. Even though some of study still remain ambiguous and in early stage, we believe that ion channels and transporters will be novel therapeutic targets in the future.


| INTRODUC TI ON
The basic physiological function of the gastrointestinal (GI) tract is to digest and absorb nutrients from the diet and to excrete unabsorbed or harmful substances. Meals are digested enzymatically into absorbable nutrients such as monosaccharides, dipeptides, free amino acids and fatty acids. Absorption of the above nutrients requires specific transporters accompanied by cotransport or reverse transport of ions (Na + , H + , Cl − or HCO 3 − ). Establishing and maintaining the driving forces (membrane potential and transmembrane ion gradients) is essential for maximum uptake. Firstly, it was found that changes in membrane potential (Em) affecting nutrient absorption. Membrane potential is primarily uptake constructing a mouse model for short-circuit current measurement. The peptide transporter1 (PEPT1) uses a proton electrochemical gradient as the driving force, and its acid-loading activity requires an apical Na + /H + exchangers or an anion exchangers to reduce epithelial acidification. 3,4 In addition, as an important intracellular secondary messenger, researchers have confirmed that Ca 2+ regulates many intracellular physiological activities and plays an important role in regulating intestinal nutrient absorption through perfusion techniques in the small intestine of rabbits and mice. [5][6][7] Intracellular Ca 2+ ([Ca 2+ ] i ) homeostasis is finely controlled by Ca 2+ channels and transporters which may ultimately affect nutrient absorption (Table 1).
Nutritional disorders are more or less present in many diseases, which may exacerbate the condition and affect the progress of recovery. Given that enteral nutrition (EN) remains the preferred option, findings from epidemiology study, animal study and clinical study have proved that the treatments targeted at IEC malabsorption may be more effective and safer. [8][9][10] Most recent studies of controlling nutrient absorption by ion transport in the gut have focused on neural and hormonal regulation 11,12 ; there are few studies on regulating nutrient absorption by IEC ion channels and transporters. This review does not address the role of intestinal neuroendocrine and paracrine factors on nutrient absorption; we focus on the functional and molecular mechanisms of IEC ion channels and transporters involved in the regulation of nutrient absorption in physiological and pathophysiological states, particularly in diabetes and inflammatory bowel disease (IBD).

| Absorption of sugars in the intestine
Dietary carbohydrates must be broken down into monosaccharides (such as glucose and galactose) before they can be transported and absorbed. Intestinal monosaccharide uptake mainly depends on sodium-glucose cotransporter 1 (SGLT1) and glucose transporter 2 (GLUT2). SGLT1 and GLUT2 were detected primarily in the small intestine (duodenum, jejunum and ileum) but not in the oesophagus, stomach, colon or rectum. 13 Take glucose absorption, for example; low-concentration glucose (< 30 mM) in the lumen is cotransported with 2 Na + to epithelial cells through SGLT1 of brush border membrane (BBM). GLUT2 of the basolateral membrane (BLM) transports glucose from IEC to mesenteric veins. In addition to the classical pathways described above, active glucose transport saturates in the presence of high concentrations of glucose (> 30 mM) in the intestinal lumen, possibly involving intercellular glucose transport and GLUT2 can rapidly bind to the brush border membrane of enterocytes and participate in promoting glucose diffusion across the membrane. show that PEPT1 (SLC15A1), a high-capacity/low-affinity peptide transporter, can achieve proton coupling absorption of more than 8000 different dipeptides and tripeptides, a variety of peptide-like compounds are also PEPT1 transport substrates, including specific immune stimulants, angiotensin-converting enzyme inhibitors and β-lactam antibiotics. 14,15 The peptides in IECs are further hydrolyzed to FAA, and a small part of the absorbed oligopeptides are transported out by the H + -dependent peptide transport system on the basement membrane. At the same time, FAA is mediated by several specific amino acid transporters. Most of the FAA in the lumen are absorbed by Na + -dependent cotransporters (such as system A, ASC, B 0 ), while a small portion of the FAA enters intestinal cells with other ions such as H + , protein associated with topoisomerase (PAT), Cl − or other amino acids. 16 IECs could convert or metabolize a portion of amino acids (e.g. glutamine, glutamate and aspartate) for their own use. The other part is transported out through the transport system of the basolateral membrane into the portal circulation.     showed similar results in the small intestine of rats verifying the expression and location of Kv subtypes by RT-PCR, Western blotting and immunohistochemistry to suggest that basolateral K + channels maintain glucose and amino acid absorption but did not identify specific K + channels. 34 Chao Du et al. studied the role of K v channel subtypes in the mouse jejunum, demonstrating that K v 1.1 and K v 1.3 channels are functionally expressed on the serosal and mucosal sides, respectively, through in vivo experiments, such as immunofluorescence, PCR and protein imprinting. 35 They regulate intestinal glucose absorption, and inhibitors of these channels attenuate glucose transport. In addition to Na + coupled nutrient transport, recent studies have found that the basolateral K Ca 3.1 (KCNN4) plays a major role in regulating H + /dipeptide absorption. K + induces membrane hyperpolarization, thereby increasing the driving force of H + -coupled electrogenic dipeptide absorption. Currently, there is no direct evidence that K + channels in IEC regulate lipids absorption; it is still worth studying.  37 In addition to the above classical pathways, more serosal channels in IEC were discovered to mediate Ca 2+ influx, such as TRPV1/4 and store-operated Ca 2+ channels (SOC). [38][39][40] Extracellular calcium-sensitive receptor (CaSR) is one of the important nutrient-sensing receptors. 41  activation stimulates the secretion of intestinal endocrine cells in a Ca 2+ -dependent manner. 52 When TRPV1 as nonselective cation channels are activated by their well known agonist capsaicin, sodium and calcium ions would enter through TRPV1 channels to depolarize nociceptive neurons, leading to action potential firing and finally the sensation of spiciness. 53 However, Wan et al. recently found that capsaicin enhanced glucose absorption in the jejunum of wild-type (WT) mice. Applying a TRPV1 blocker did not alter this response, but the TRPV4 agonist did. In addition, TRPV4 KO mice absorbed glucose more strongly than WT mice. Therefore, it is suggested that capsaicin can promote Na + -glucose absorption by inhibiting TRPV4 channel. 54 Studies in Caco-2 cells showed that reducing Ca 2+ levels stimulated Na(+)/H(+) exchanger 3 (NHE3) activity, thereby maintaining the high electrochemical driving force of PEPT1 to drive peptide transport. 55 Similarly, Yukiko et al. found that capsaicin interacts with TRPV1 to reduce PEPT1-mediated transport in rat intestines; the inhibitory effect was attenuated by intravenous administration of ruthenium red, a nonselective inhibitor of TRP channels. 56 However, a recent study shows that [Ca 2+ ] i activated Na + -glutamine absorption in ileal epithelial cells via several Ca 2+ permeable channels and transporters, including mucosal Cav1.3, serosal TRPV1/2, SOC channels and NCX. 57 And it is generally accepted that peptides and L-AA activate Ca 2+ -sensing receptors (CaSR) and serve as allosteric modifiers to nutrient signal supply to intestinal epithelial cells (IEC). 58 the jejunal membrane of rats, and nifedipine as an L-type voltagegated calcium channels (VGCC) blocker could attenuate the effect of glucose using perfused rat jejunum in vivo. 66 We considered another possible Ca 2+ signalling pathway linking nutrients uptake. [Ca 2+ ] i binds CaM as a compound that activates calmodulin-dependent protein kinase β (CaMKK β). Previous literature has shown that AMP-activated protein kinase (AMPK) is a substrate for CaMKK2 in mammalian cells from rat brain or expressed in E. coli phosphorylates, 67 and AMPK is a key sensor of intracellular energy status and affects the expression and transport function of almost all intestinal nutrient transporters (such as SGLT1, GLUT2, PEPT1 and CD36). [68][69][70][71] In summary, Ca 2+ signalling could regulate the activities of NHE, K + channels and targeted transporters, which are involved in nutrient transport. It can be seen that the mechanism is complex and still needs to be further studied.

| INTE S TINAL I ON TR AN S P ORTER S MED IATE N UTRIENT TR AN S P ORT
The importance of [Ca 2+ ] i and [Na + ] i in nutrient absorption is recognized; in addition to the above channels, there are also transporters to maintain ion homeostasis. There are currently three subtypes of Na + -Ca 2+ exchangers (NCX) expressed in various cells, including IEC. The transport ratio of NCX is 3:1; in forward transport mode: 3 Na + are transferred in and 1 Ca 2+ is moved out of the cell, while in reverse transfer mode is the opposite. The direction depends on the gradient of Na + and Ca 2+ concentrations and membrane potential showed that intracellular carbonic anhydrase (CA) activity promotes H + motility, thereby maintaining the transmembrane ion gradient for maximum PEPT1 uptake using intracellular carboxy-SNARF-1 fluorescence in combination with whole-cell microspectrofluorimetry or confocal microscopy. 86 In the study of expression and function colocalization, Pat-1 was found to colocalize with PEPT1 in the apical epithelium membrane. 87  Gly-Sar-induced I sc was reduced in the jejunum of NBCe1 KO mice.
NBCe1 is a key steady-state mechanism that ensures peptide uptake. 88 From this, the acid load generated by most modes of nutrient absorption in the small intestine could activate NBCe1, which in turn plays a role in maintaining pH and Em during all nutrient absorption.

| Diabetes mellitus
Diabetes mellitus is a metabolic disease characterized by hyperglycaemia, which can lead to chronic damage and dysfunction of various

| Cystic fibrosis (CF)
Cystic fibrosis is an autosomal recessive disease caused by muta- firstly, the activity of K + channels or NHE can be controlled to affect the transmembrane potential, and secondly, the movement of nutrient transporters can be directly affected. Therefore, the therapeutic strategy of selecting Ca 2+ channels or transporters' drugs to adjust nutrient absorption must be reconsidered. A series of ion exchangers (such as NHE3, NBCe1 and PAT-1) regulate nutrient uptake, especially proteins and lipids hydrolysates, mainly by influencing pH i . Taken together, although, in recent years, research on the intestinal ion channels and transporters regulating nutrient absorption to achieve considerable progress, targeting ion channels in disease treatment has promising prospects. In the development of ion channel targeting allows it to be more effective personalized therapy before, we still need to dig deeper into the mechanics.

DATA AVA I L A B I L I T Y S TAT E M E N T
Not applicable.

CO N S E NT FO R PU B LI C ATI O N
We have obtained consents to publish this paper from all the participants of this study.