Crypt and Villus Enterochromaffin Cells are Distinct Stress Sensors in the Gut.

The crypt-villus structure of the small intestine serves as an essential protective barrier, with its integrity monitored by the gut's sensory system. Enterochromaffin (EC) cells, which are rare sensory epithelial cells that release serotonin (5-HT), surveil the mucosal environment and signal both within and outside the gut. However, it remains unclear whether EC cells in intestinal crypts and villi respond to different stimuli and elicit distinct responses. In this study, we introduce a new reporter mouse model to observe the release and propagation of serotonin in live intestines. Using this system, we show that crypt EC cells exhibit two modes of serotonin release: transient receptor potential A1 (TRPA1)-dependent tonic serotonin release that controls basal ionic secretion, and irritant-evoked serotonin release that activates gut sensory neurons. Furthermore, we find that a thick protective mucus layer prevents TRPA1 receptors on crypt EC cells from responding to luminal irritants such as reactive electrophiles; if this mucus layer is compromised, then crypt EC cells become susceptible to activation by luminal irritants. On the other hand, villus EC cells detect oxidative stress through TRPM2 channels and co-release serotonin and ATP to activate nearby gut sensory fibers. Our work highlights the physiological importance of intestinal architecture and differential TRP channel expression in sensing noxious stimuli that elicit nausea and/or pain sensations in the gut.

Gut enterochromaffin cells drive visceral pain and anxiety.

Gastrointestinal (GI) discomfort is a hallmark of most gut disorders and represents an important component of chronic visceral pain1. For the growing population afflicted by irritable bowel syndrome, GI hypersensitivity and pain persist long after tissue injury has resolved2. Irritable bowel syndrome also exhibits a strong sex bias, afflicting women three times more than men1. Here, we focus on enterochromaffin (EC) cells, which are rare excitable, serotonergic neuroendocrine cells in the gut epithelium3-5. EC cells detect and transduce noxious stimuli to nearby mucosal nerve endings3,6 but involvement of this signalling pathway in visceral pain and attendant sex differences has not been assessed. By enhancing or suppressing EC cell function in vivo, we show that these cells are sufficient to elicit hypersensitivity to gut distension and necessary for the sensitizing actions of isovalerate, a bacterial short-chain fatty acid associated with GI inflammation7,8. Remarkably, prolonged EC cell activation produced persistent visceral hypersensitivity, even in the absence of an instigating inflammatory episode. Furthermore, perturbing EC cell activity promoted anxiety-like behaviours which normalized after blockade of serotonergic signalling. Sex differences were noted across a range of paradigms, indicating that the EC cell-mucosal afferent circuit is tonically engaged in females. Our findings validate a critical role for EC cell-mucosal afferent signalling in acute and persistent GI pain, in addition to highlighting genetic models for studying visceral hypersensitivity and the sex bias of gut pain.

State-selective modulation of heterotrimeric Gαs signaling with macrocyclic peptides.

The G protein-coupled receptor cascade leading to production of the second messenger cAMP is replete with pharmacologically targetable proteins, with the exception of the Gα subunit, Gαs. GTPases remain largely undruggable given the difficulty of displacing high-affinity guanine nucleotides and the lack of other drug binding sites. We explored a chemical library of 1012 cyclic peptides to expand the chemical search for inhibitors of this enzyme class. We identified two macrocyclic peptides, GN13 and GD20, that antagonize the active and inactive states of Gαs, respectively. Both macrocyclic peptides fine-tune Gαs activity with high nucleotide-binding-state selectivity and G protein class-specificity. Co-crystal structures reveal that GN13 and GD20 distinguish the conformational differences within the switch II/α3 pocket. Cell-permeable analogs of GN13 and GD20 modulate Gαs/Gßγ signaling in cells through binding to crystallographically defined pockets. The discovery of cyclic peptide inhibitors targeting Gαs provides a path for further development of state-dependent GTPase inhibitors.

Cryo-EM analysis of PIP2 regulation in mammalian GIRK channels.

G-protein-gated inward rectifier potassium (GIRK) channels are regulated by G proteins and PIP2. Here, using cryo-EM single particle analysis we describe the equilibrium ensemble of structures of neuronal GIRK2 as a function of the C8-PIP2 concentration. We find that PIP2 shifts the equilibrium between two distinguishable structures of neuronal GIRK (GIRK2), extended and docked, towards the docked form. In the docked form the cytoplasmic domain, to which Gßγ binds, becomes accessible to the cytoplasmic membrane surface where Gßγ resides. Furthermore, PIP2 binding reshapes the Gßγ binding surface on the cytoplasmic domain, preparing it to receive Gßγ. We find that cardiac GIRK (GIRK1/4) can also exist in both extended and docked conformations. These findings lead us to conclude that PIP2 influences GIRK channels in a structurally similar manner to Kir2.2 channels. In Kir2.2 channels, the PIP2-induced conformational changes open the pore. In GIRK channels, they prepare the channel for activation by Gßγ.

Structural identification of a hotspot on CFTR for potentiation.

Cystic fibrosis is a fatal disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). Two main categories of drugs are being developed: correctors that improve folding of CFTR and potentiators that recover the function of CFTR. Here, we report two cryo-electron microscopy structures of human CFTR in complex with potentiators: one with the U.S. Food and Drug Administration (FDA)-approved drug ivacaftor at 3.3-angstrom resolution and the other with an investigational drug, GLPG1837, at 3.2-angstrom resolution. These two drugs, although chemically dissimilar, bind to the same site within the transmembrane region. Mutagenesis suggests that in both cases, hydrogen bonds provided by the protein are important for drug recognition. The molecular details of how ivacaftor and GLPG1837 interact with CFTR may facilitate structure-based optimization of therapeutic compounds.

Molecular basis of signaling specificity between GIRK channels and GPCRs.

Stimulated muscarinic acetylcholine receptors (M2Rs) release Gßγ subunits, which slow heart rate by activating a G protein-gated K+ channel (GIRK). Stimulated ß2 adrenergic receptors (ß2ARs) also release Gßγ subunits, but GIRK is not activated. This study addresses the mechanism underlying this specificity of GIRK activation by M2Rs. K+ currents and bioluminescence resonance energy transfer between labelled G proteins and GIRK show that M2Rs catalyze Gßγ subunit release at higher rates than ß2ARs, generating higher Gßγ concentrations that activate GIRK and regulate other targets of Gßγ. The higher rate of Gßγ release is attributable to a faster G protein coupled receptor - G protein trimer association rate in M2R compared to ß2AR. Thus, a rate difference in a single kinetic step accounts for specificity.

Piezo1 forms a slowly-inactivating mechanosensory channel in mouse embryonic stem cells.

Piezo1 is a mechanosensitive (MS) ion channel with characteristic fast-inactivation kinetics. We found a slowly-inactivating MS current in mouse embryonic stem (mES) cells and characterized it throughout their differentiation into motor-neurons to investigate its components. MS currents were large and slowly-inactivating in the stem-cell stage, and became smaller and faster-inactivating throughout the differentiation. We found that Piezo1 is expressed in mES cells, and its knockout abolishes MS currents, indicating that the slowly-inactivating current in mES cells is carried by Piezo1. To further investigate its slow inactivation in these cells, we cloned Piezo1 cDNA from mES cells and found that it displays fast-inactivation kinetics in heterologous expression, indicating that sources of modulation other than the aminoacid sequence determine its slow kinetics in mES cells. Finally, we report that Piezo1 knockout ES cells showed a reduced rate of proliferation but no significant differences in other markers of pluripotency and differentiation.

Cooperative regulation by G proteins and Na(+) of neuronal GIRK2 K(+) channels.

G protein gated inward rectifier K(+) (GIRK) channels open and thereby silence cellular electrical activity when inhibitory G protein coupled receptors (GPCRs) are stimulated. Here we describe an assay to measure neuronal GIRK2 activity as a function of membrane-anchored G protein concentration. Using this assay we show that four Gßγ subunits bind cooperatively to open GIRK2, and that intracellular Na(+) - which enters neurons during action potentials - further amplifies opening mostly by increasing Gßγ affinity. A Na(+) amplification function is characterized and used to estimate the concentration of Gßγ subunits that appear in the membrane of mouse dopamine neurons when GABAB receptors are stimulated. We conclude that GIRK2, through its dual responsiveness to Gßγ and Na(+), mediates a form of neuronal inhibition that is amplifiable in the setting of excess electrical activity.

The GIRK1 subunit potentiates G protein activation of cardiac GIRK1/4 hetero-tetramers.

G protein gated inward rectifier potassium (GIRK) channels are gated by direct binding of G protein beta-gamma subunits (Gßγ), signaling lipids, and intracellular Na(+). In cardiac pacemaker cells, hetero-tetramer GIRK1/4 channels and homo-tetramer GIRK4 channels play a central role in parasympathetic slowing of heart rate. It is known that the Na(+) binding site of the GIRK1 subunit is defective, but the functional difference between GIRK1/4 hetero-tetramers and GIRK4 homo-tetramers remains unclear. Here, using purified proteins and the lipid bilayer system, we characterize Gßγ and Na(+) regulation of GIRK1/4 hetero-tetramers and GIRK4 homo-tetramers. We find in GIRK4 homo-tetramers that Na(+) binding increases Gßγ affinity and thereby increases the GIRK4 responsiveness to G protein stimulation. GIRK1/4 hetero-tetramers are not activated by Na(+), but rather are in a permanent state of high responsiveness to Gßγ, suggesting that the GIRK1 subunit functions like a GIRK4 subunit with Na(+) permanently bound.

Structural basis for reversible phosphorolysis and hydrolysis reactions of 2-O-α-glucosylglycerol phosphorylase.

2-O-α-Glucosylglycerol phosphorylase (GGP) from Bacillus selenitireducens catalyzes both the reversible phosphorolysis of 2-O-α-glucosylglycerol (GG) and the hydrolysis of ß-d-glucose 1-phosphate (ßGlc1P). GGP belongs to the glycoside hydrolase (GH) family 65 and can efficiently and specifically produce GG. However, its structural basis has remained unclear. In this study, the crystal structures of GGP complexed with glucose and the glucose analog isofagomine and glycerol were determined. Subsite -1 of GGP is similar to those of other GH65 enzymes, maltose phosphorylase and kojibiose phosphorylase, whereas subsite +1 is largely different and is well designed for GG recognition. An automated docking analysis was performed to complement these crystal structures, ßGlc1P being docked at an appropriate position. To investigate the importance of residues at subsite +1 in the bifunctionality of GGP, we constructed mutants at these residues. Y327F and K587A did not show detectable activities for either reverse phosphorolysis or ßGlc1P hydrolysis. Y572F also showed significantly reduced activities for both of these reactions. In contrast, W381F showed significantly reduced reverse phosphorolytic activity but retained ßGlc1P hydrolysis. The mode of substrate recognition and the reaction mechanisms of GGP were proposed based on these analyses. Specifically, an extensive hydrogen bond network formed by Tyr-327, Tyr-572, Lys-587, and water molecules contributes to fixing the acceptor molecule in both reverse phosphorolysis (glycerol) and ßGlc1P hydrolysis (water) for a glycosyl transfer reaction. This study will contribute to the development of a large scale production system of GG by facilitating the rational engineering of GGP.

Structural and biochemical analyses of glycoside hydrolase family 26 ß-mannanase from a symbiotic protist of the termite Reticulitermes speratus.

Termites and their symbiotic protists have established a prominent dual lignocellulolytic system, which can be applied to the biorefinery process. One of the major components of lignocellulose from conifers is glucomannan, which comprises a heterogeneous combination of ß-1,4-linked mannose and glucose. Mannanases are known to hydrolyze the internal linkage of the glucomannan backbone, but the specific mechanism by which they recognize and accommodate heteropolysaccharides is currently unclear. Here, we report biochemical and structural analyses of glycoside hydrolase family 26 mannanase C (RsMan26C) from a symbiotic protist of the termite Reticulitermes speratus. RsMan26C was characterized based on its catalytic efficiency toward glucomannan, compared with pure mannan. The crystal structure of RsMan26C complexed with gluco-manno-oligosaccharide(s) explained its specificities for glucose and mannose at subsites -5 and -2, respectively, in addition to accommodation of both glucose and mannose at subsites -3 and -4. RsMan26C has a long open cleft with a hydrophobic platform of Trp(94) at subsite -5, facilitating enzyme binding to polysaccharides. Notably, a unique oxidized Met(85) specifically interacts with the equatorial O-2 of glucose at subsite -3. Our results collectively indicate that specific recognition and accommodation of glucose at the distal negative subsites confers efficient degradation of the heteropolysaccharide by mannanase.