ANO2

ANO2 (TMEM16B) is a calcium-activated chloride channel that shapes neuronal excitability and sensory signaling across multiple cell types including olfactory sensory neurons, inferior olivary neurons, lateral septum neurons, and vagal afferents. The channel is directly gated by intracellular Ca²⁺ through its third intracellular loop, with voltage dependence controlled by glutamate residues in the first intracellular loop, ion selectivity determined by pore residues R573/K540, and kinetics governed by the C-terminus and TMD7-8 region (PMID:19475416, PMID:22412191, PMID:23570556, PMID:28046119). In olfactory sensory neurons, ANO2 localizes to cilia and controls the time-to-peak and termination of odorant-evoked receptor currents, limits action potential firing, and is required for normal glomerular targeting (PMID:27619419, PMID:39167717). Beyond olfaction, ANO2 mediates CCK-induced satiety signaling in nodose neurons, regulates spike frequency adaptation and aggression-related behavior in lateral septum neurons, drives membrane repolarization and cerebellar motor learning in inferior olivary neurons, and forms heteromeric channels with TMEM16A in pinealocytes to regulate melatonin secretion (PMID:30843875, PMID:31320449, PMID:28858616, PMID:29187602).

1. 2009 High

   Establishing ANO2 as a calcium-activated chloride channel resolved the long-sought molecular identity of the native CaCC in olfactory cilia, placing it in the TMEM16 family and enabling all subsequent structure-function dissection.

   Evidence Proteomic screen of olfactory cilial membranes followed by heterologous expression and patch-clamp in HEK-293 cells; independently confirmed by inside-out patch recording showing direct Ca²⁺ gating and retinal synapse localization via PDZ-domain scaffolding

   PMID:19474308 PMID:19475416 PMID:19561302

   Open questions at the time

   • Stoichiometry of the native channel complex in cilia was not determined
   • Whether post-translational modifications affect trafficking to cilia was not addressed
2. 2010 High

   Direct comparison of recombinant TMEM16B with native OSN currents confirmed that TMEM16B accounts for the olfactory CaCC, validating the proteomic identification and establishing a benchmark for biophysical characterization.

   Evidence Whole-cell voltage-clamp with caged Ca²⁺ photorelease and pharmacological profiling in OSNs versus HEK 293T cells

   PMID:20837642

   Open questions at the time

   • Contribution of other CaCC subunits or splice variants to the native current was not excluded
   • Quantitative match of single-channel conductance between native and recombinant channels was lacking
3. 2012 High

   Identification of glutamate residues E367 and 386EEEEE390 in the first intracellular loop as determinants of voltage-dependent gating — distinct from Ca²⁺ sensitivity — established the first structure-function map of ANO2 gating domains.

   Evidence Site-directed mutagenesis with whole-cell voltage-clamp at varying Ca²⁺ concentrations in HEK 293T cells

   PMID:22412191

   Open questions at the time

   • No structural model to explain how these residues sense or couple voltage to the gate
   • Whether these residues participate in inter-subunit interactions was unknown
4. 2013 High

   Chimera-based domain mapping separated the molecular determinants of Ca²⁺ sensitivity (third intracellular loop) from those of activation/deactivation kinetics (TMD7-8 and C-terminus), explaining why TMEM16B has lower Ca²⁺ affinity and faster kinetics than TMEM16A.

   Evidence TMEM16A-TMEM16B domain-swap chimeras with systematic patch-clamp electrophysiology

   PMID:23570556

   Open questions at the time

   • Precise Ca²⁺-binding residues within the third intracellular loop were not identified
   • Whether these domains operate independently or allosterically was not resolved
5. 2014 High

   Demonstration that permeant anions modulate TMEM16B gating and Ca²⁺ sensitivity revealed an intrinsic coupling between the permeation pathway and the gating apparatus, a principle later pinned to specific pore residues.

   Evidence Systematic ion substitution with Ca²⁺ dose-response analysis in whole-cell and inside-out patches

   PMID:24863931

   Open questions at the time

   • Structural basis of permeation-gating coupling was unresolved at this stage
6. 2015 High

   Identification of dual (fast and slow) voltage-dependent gating modes and the role of residues 480RSQ482 in slow gating refined the gating model, showing that ANO2 possesses multiple kinetically distinct conformational transitions.

   Evidence Long depolarizing pulse protocols with first intracellular loop mutagenesis in HEK-293 inside-out and whole-cell patches

   PMID:26728431

   Open questions at the time

   • Whether dual gating modes operate in native neurons was not tested
   • Relationship between slow gating and physiological firing patterns was not established
7. 2016 High

   Knockout of ANO2 in mice eliminated CaCCs in OSNs and revealed that the channel limits action potential firing duration and is required for normal olfactory glomerular targeting, establishing its first in vivo physiological role.

   Evidence TMEM16B KO mouse with suction electrode and on-cell recordings from OSNs, olfactory bulb glomerular mapping

   PMID:27619419

   Open questions at the time

   • Mechanism linking CaCC loss to supernumerary glomeruli was not identified
   • Whether behavioral olfactory discrimination is impaired was not definitively shown
8. 2017 High

   Mapping pore residues R573 and K540 as determinants of ion selectivity and permeation-gating coupling, and identifying PIP2 as an inhibitory modulator of channel gating, extended the molecular architecture of ANO2 from gating domains to the permeation pathway and lipid regulation.

   Evidence Site-directed mutagenesis of pore residues with electrophysiology; genetically encoded DrVSP and PI4P5K co-expression to manipulate PIP2 levels

   PMID:28046119 PMID:28616863

   Open questions at the time

   • Direct PIP2-binding site on ANO2 was not identified
   • No cryo-EM or X-ray structure of ANO2 pore to validate residue positions
9. 2017 High

   Discovery of ANO2 function in inferior olivary neurons — where it mediates repolarization and is essential for cerebellar motor learning — and in pinealocytes — where it forms heteromers with TMEM16A to regulate melatonin secretion — broadened the channel's physiological scope far beyond olfaction.

   Evidence TMEM16B KO mouse electrophysiology in IO neurons with motor learning behavioral assays; BiFC, FRET, co-IP, and melatonin secretion assays in pinealocytes

   PMID:28858616 PMID:29187602

   Open questions at the time

   • Stoichiometry and subunit arrangement of TMEM16A-TMEM16B heteromers were not determined
   • Whether heteromerization occurs in neurons beyond pinealocytes was unknown
10. 2019 High

    ANO2 was shown to mediate CCK-induced satiety signaling in vagal afferent nodose neurons and to regulate spike frequency adaptation and aggression in lateral septum neurons, establishing the channel as a broadly deployed neuronal excitability modulator with behavioral consequences.

    Evidence Heterozygous TMEM16B KO with patch-clamp in nodose neurons and metabolic phenotyping; TMEM16B KO with LS neuron electrophysiology and resident-intruder aggression assays

    PMID:30843875 PMID:31320449

    Open questions at the time

    • Downstream signaling from ANO2 in nodose neurons to satiety circuits was not mapped
    • Mechanism by which ANO2 loss promotes prolonged aggression was not dissected at the circuit level
11. 2024 High

    CLCA4 was identified as an extrinsic potentiator of ANO2 through its VWA domain MIDAS motif, and refined KO analysis showed that ANO2 specifically shapes the time-to-peak and termination of odorant responses rather than baseline membrane properties, completing a detailed dissection of its temporal role in olfactory transduction.

    Evidence Domain deletion/mutation of CLCA4 with patch-clamp in HEK293T cells; systematic electrophysiology in TMEM16B KO OSNs with odorant stimulation

    PMID:38825009 PMID:39167717

    Open questions at the time

    • Whether CLCA4 regulates ANO2 in vivo in native tissues is untested
    • The mechanism by which CLCA4's VWA domain potentiates ANO2 gating is structurally unresolved

• No high-resolution structure of ANO2 exists, leaving the atomic basis of its Ca²⁺-binding site, pore architecture, and lipid/CLCA4 regulatory interfaces unresolved; the mechanism linking ANO2 loss to aberrant olfactory glomerular targeting also remains unknown.
• No cryo-EM or crystal structure of ANO2
• Structural basis of Ca²⁺ binding and selectivity filter geometry unknown
• Developmental mechanism of glomerular targeting defect in KO mice uncharacterized