Bitter taste receptors (T2Rs) in the human airway detect harmful compounds, including secreted bacterial products. Here, using human primary sinonasal air-liquid interface cultures and tissue explants, we determined that activation of a subset of airway T2Rs expressed in nasal solitary chemosensory cells activates a calcium wave that propagates through gap junctions to the surrounding respiratory epithelial cells. The T2R-dependent calcium wave stimulated robust secretion of antimicrobial peptides into the mucus that was capable of killing a variety of respiratory pathogens. Furthermore, sweet taste receptor (T1R2/3) activation suppressed T2R-mediated antimicrobial peptide secretion, suggesting that T1R2/3-mediated inhibition of T2Rs prevents full antimicrobial peptide release during times of relative health. In contrast, during acute bacterial infection, T1R2/3 is likely deactivated in response to bacterial consumption of airway surface liquid glucose, alleviating T2R inhibition and resulting in antimicrobial peptide secretion. We found that patients with chronic rhinosinusitis have elevated glucose concentrations in their nasal secretions, and other reports have shown that patients with hyperglycemia likewise have elevated nasal glucose levels. These data suggest that increased glucose in respiratory secretions in pathologic states, such as chronic rhinosinusitis or hyperglycemia, promotes tonic activation of T1R2/3 and suppresses T2R-mediated innate defense. Furthermore, targeting T1R2/3-dependent suppression of T2Rs may have therapeutic potential for upper respiratory tract infections.

Alzheimer's disease (AD) is a genetically heterogeneous disorder characterized by early hippocampal atrophy and cerebral amyloid-beta (Abeta) peptide deposition. Using TissueInfo to screen for genes preferentially expressed in the hippocampus and located in AD linkage regions, we identified a gene on 10q24.33 that we call CALHM1. We show that CALHM1 encodes a multipass transmembrane glycoprotein that controls cytosolic Ca(2+) concentrations and Abeta levels. CALHM1 homomultimerizes, shares strong sequence similarities with the selectivity filter of the NMDA receptor, and generates a large Ca(2+) conductance across the plasma membrane. Importantly, we determined that the CALHM1 P86L polymorphism (rs2986017) is significantly associated with AD in independent case-control studies of 3404 participants (allele-specific OR = 1.44, p = 2 x 10(-10)). We further found that the P86L polymorphism increases Abeta levels by interfering with CALHM1-mediated Ca(2+) permeability. We propose that CALHM1 encodes an essential component of a previously uncharacterized cerebral Ca(2+) channel that controls Abeta levels and susceptibility to late-onset AD.

Alzheimer's disease | mutagenesis | selectivity | neurodegeneration | polymorphism O riginally identified as a possible modifier of the age of onset of Alzheimer's disease (1, 2), calcium homeostasis modulator 1 (CALHM1) encodes a glycosylated membrane protein expressed throughout the brain that lacks homology to other proteins. Six human CALHM homologs have been identified, with alternatively spliced variants and different expression patterns throughout the body, and CALHM1 is conserved across >20 species. Expression of recombinant human CALHM1 in mammalian cells was found to strongly influence processing of amyloid precursor protein to amyloid beta (Aβ) under an experimental protocol that involved removal of Ca 2+ o for several minutes and its subsequent restoration to the bathing medium (1). This procedure resulted in a large rise of [Ca 2+ ] i . Accordingly, it was speculated that CALHM1 influences Aβ production by altering cellular Ca 2+ homeostasis. CALHM1 was found to homo-multimerize and it was speculated that it might function as an ion channel component or regulator of membrane ion conductances (1 Results CALHM1 Expression Induces a Voltage-Dependent Plasma MembraneConductance. Previously, outwardly rectified ion currents were observed in CALHM1-expressing Xenopus oocytes and CHO cells by a slow voltage ramp protocol (1). However, those experiments did not establish whether CALHM1 is an essential component of an underlying ion channel(s), accessory protein, or actual pore-forming subunit. Furthermore the detailed permeation and gating properties of the conductance were not determined. In addition, precautions were not taken to fully ensure lack of contribution of endogenous conductances. To distinguish whether CALHM1 is a unique ion channel or a regulator of endogenous channels, plasma membrane currents were recorded in Xenopus oocytes under conditions that minimized contributions of endogenous conductances (Fig. S1) (3, 4). Membrane depolarization in solutions containing 2 mM Ca 2+ and 1 mM Mg 2+ generated large outward currents that activated slowly (τ ∼ 3.11 ± 0.28 s at +60 mV; n = 10) and deactivated at hyperpolarized voltages (τ = 0.204 ± 0.022 s at −80 mV; n = 10) specifically in CALHM1-expressing oocytes (Fig. 1A). Expression of CALHM1-EGFP localized to the plasma membrane (Fig. 1B). Similar results were obtained in transiently transfected N2A mammalian neuroblastoma cells (Fig. 1C). Thus, expression of CALHM1 induced a voltage-dependent plasma membrane conductance.The monovalent ion permeabilities of this conductance were estimated by changing-bath [NaCl] in a nominally 0-Ca 2+ solution (free [Ca 2+ ] ∼10 μM) and measuring shifts of reversal potential, ΔE rev (Fig. 1D). Using the Goldman-Hodgkin-Katz (GHK) constant field equation, the relative permeabilities were estimated as P Na :P K :P Cl = 1:1.17:0.56. Similar results were obtained with bath Na + replaced by K + in either 0 or 2 mM Ca 2+ o .

Background: CALHM1 is an ion channel for which structural information is lacking. Results: CALHM1 has poor ion selectivity and a wide (ϳ14 Å) pore and is a hexamer, with monomers having four transmembrane domains with cytoplasmic termini. Conclusion: CALHM1 shares structural features with pannexins, connexins, and innexins. Significance: CALHMs, connexins, and pannexins/innexins are three structurally related protein families with shared and distinct functional properties.

The cellular mechanisms by which contractile activity stimulates skeletal muscle hypertrophy are beginning to be elucidated and appear to include activation of the phosphatidylinositol 3-kinase signaling substrate mammalian target of rapamycin (mTOR). We examined the time course and location of mTOR phosphorylation in response to an acute bout of contractile activity. Rat hindlimb muscle contractile activity was elicited by high-frequency electrical stimulation (HFES) of the sciatic nerve. Plantaris (Pla), tibialis anterior (TA), and soleus (Sol) muscles from stimulated and control limbs were collected immediately or 6 h after stimulation. HFES resulted in mTOR phosphorylation immediately after (3.4 +/- 0.9-fold, P < 0.01) contractile activity in Pla, whereas TA was unchanged compared with controls. mTOR phosphorylation remained elevated in Pla (3.6 +/- 0.6-fold) and increased in TA (4.6 +/- 0.9-fold, P < 0.05) 6 h after HFES. Interestingly, mTOR activation occurred predominantly in fibers expressing type IIa but not type I myosin heavy chain isoform. Furthermore, HFES induced modest ribosomal protein S6 kinase phosphorylation immediately after exercise in Pla (0.4 +/- 0.1-fold, P < 0.05) but not TA and more markedly 6 h after in both Pla and TA (1.4 +/- 0.4-fold vs. 2.4 +/- 0.3-fold, respectively, P < 0.01). Akt/PKB phosphorylation was similar to controls at both time points. These results suggest that mTOR signaling is increased after a single bout of muscle contractile activity. Despite reports that mTOR is activated downstream of Akt/PKB, in this study, HFES induced mTOR signaling independent of Akt/PKB phosphorylation. Fiber type-dependent mTOR phosphorylation may be a molecular basis by which some fiber types are more susceptible to contraction-induced hypertrophy.