TACR2

TACR2 (NK2R) is a G protein-coupled receptor for neurokinin A that functions as a pleiotropic regulator of gastrointestinal motility, energy homeostasis, immune cell effector function, and neuroendocrine signaling. The receptor adopts at least two distinct active conformations—A1L (Gq/Ca²⁺) and A2L (Gs/cAMP)—that are differentially stabilized by allosteric modulators, providing a molecular basis for functional selectivity (PMID:17371796). In the gut, NK2R negatively regulates nNOS/VIP-NO signaling via CREB and NF-κB pathways to maintain gastric motility, and controls intestinal lipid mobilization by suppressing chylomicron output (PMID:28585346, PMID:39537932). In the immune system, IFN-γ–STAT1 signaling induces NK2R expression on CD8⁺ T cells, where NKA–NK2R activation drives ERK1/2 phosphorylation, IκBα degradation, and enhanced IFN-γ and granzyme B production required for antitumor effector function (PMID:36715504).

1. 2000 Medium

   Establishing that NK2R mediates substance P-induced Ca²⁺ signaling and IL-1α cytokine production in keratinocytes answered whether tachykinin receptors other than NK1R drive innate immune responses in epithelial cells.

   Evidence RT-PCR, Ca²⁺ measurement, and NK2R-selective antagonist blockade in murine PAM 212 keratinocytes

   PMID:10688374

   Open questions at the time

   • Single cell line; relevance to primary human keratinocytes not shown
   • Downstream transcription factor pathway linking Ca²⁺ to IL-1α induction not identified
2. 2007 High

   Demonstrating that NK2R exists in two pharmacologically and functionally distinct active conformations (A1L/Gq/Ca²⁺ and A2L/Gs/cAMP) established a mechanistic framework for biased agonism and allosteric modulation at this receptor.

   Evidence Radioligand dissociation kinetics, Ca²⁺ and cAMP assays, and conformation-selective allosteric inhibitor (LPI805) pharmacology

   PMID:17371796

   Open questions at the time

   • Structural basis for A1L vs A2L conformations not resolved
   • Physiological contexts in which one conformation predominates not determined
3. 2017 High

   Genetic ablation of Tacr2 revealed that NK2R is required to suppress nNOS/VIP-NO signaling in the enteric nervous system and maintain normal gastric motility, answering how tachykinin signaling shapes inhibitory neurotransmission in the gut.

   Evidence Tacr2 knockout mice with gastric emptying assays, EFS, MMC recording, Western blot, immunohistochemistry, and NOS-inhibitor rescue

   PMID:28585346

   Open questions at the time

   • Cell-type-specific contribution (smooth muscle vs enteric neuron) not dissected with conditional knockouts
   • Whether A1L or A2L conformation mediates nNOS suppression is unknown
4. 2019 Medium

   Showing that the NK2R C-terminal domain is a key determinant of receptor folding and signaling competence addressed a longstanding difficulty in heterologous expression of this GPCR.

   Evidence Human–rat NK2R chimeras tested by radioligand binding and signaling in yeast and mammalian cells

   PMID:32400863

   Open questions at the time

   • Structural mechanism by which the C-terminus facilitates folding not resolved
   • Single-lab finding without independent replication
5. 2019 Medium

   Identifying stress-induced hypomethylation of Tacr2 CpG sites correlated with hypothalamic NK2R upregulation established epigenetic regulation as a mechanism modulating NK2R expression in stress-responsive brain circuits.

   Evidence CUMS rat model with RT-PCR, Western blot, MeDIP-seq, and bisulfite sequencing

   PMID:30711443

   Open questions at the time

   • Correlational design; causal role of methylation changes not confirmed by targeted demethylation or CRISPRi
   • Functional consequences of hypothalamic NK2R upregulation on behavior not directly tested
6. 2021 Medium

   Showing that NK2R contributes to tachykinin-mediated LH secretion with functional redundancy with NK3R clarified why isolated TACR2 loss may not cause overt hypogonadism.

   Evidence Tacr2 knockout mice with LH pulsatility measurement, NK2R agonist challenge, and NK3R antagonist epistasis

   PMID:33427049

   Open questions at the time

   • Whether NK2R acts on GnRH neurons or KNDy neurons directly not resolved
   • Female reproductive phenotype not fully characterized
7. 2022 Medium

   Discovering that IFN-α/β–JAK1/2 signaling upregulates NK2R in colon cancer cells, where NKA stimulation drives proliferation via ERK1/2, revealed a tumor-intrinsic pro-proliferative role for NK2R in the inflammatory microenvironment.

   Evidence IFN-α/β stimulation with JAK inhibitor, ERK1/2 phosphorylation, proliferation assays, and NK2R antagonist treatment in a syngeneic mouse tumor model

   PMID:35561088

   Open questions at the time

   • Relative contribution of tumor-cell-intrinsic vs immune-cell NK2R signaling to net tumor outcome not dissected
   • Downstream ERK1/2 targets mediating proliferation not identified
8. 2023 High

   Establishing that IFN-γ–STAT1 induces NK2R on CD8⁺ T cells and that NKA–NK2R signaling augments granzyme B and IFN-γ production via ERK1/2 and NF-κB resolved how tachykinin signaling enhances antitumor T cell effector function.

   Evidence IFN-γ stimulation, STAT1-dependence assay, NK2R knockout mice with increased tumor growth, ERK1/2/IκBα assays, syngeneic liver cancer model

   PMID:36715504

   Open questions at the time

   • Source of NKA in the tumor microenvironment not identified
   • Whether NK2R signaling affects T cell exhaustion or memory differentiation is unknown
9. 2024 High

   Demonstrating that selective NK2R agonism suppresses appetite centrally, increases energy expenditure peripherally, and improves insulin sensitivity—through leptin-independent pathways—positioned NK2R as a therapeutic target for obesity and metabolic disease.

   Evidence Long-acting NK2R agonists in diet-induced obese mice and diabetic macaques, hyperinsulinemic-euglycemic clamp, metabolic phenotyping

   PMID:39537932

   Open questions at the time

   • Central neuronal populations mediating appetite suppression not identified
   • Mechanism by which NK2R agonism enhances peripheral energy expenditure is unknown
   • Long-term safety and efficacy data beyond the study period not available

• Key unresolved questions include the structural basis for A1L/A2L conformational selectivity, the identity of the central circuits mediating NK2R-driven appetite suppression, and how tumor-cell-intrinsic versus immune-cell NK2R signaling integrates to determine net tumor outcomes.
• No high-resolution structure of NK2R in either active conformation
• Central neuronal targets of NK2R for appetite and energy expenditure not mapped
• Conditional knockout studies needed to separate cell-type-specific roles in cancer