Papers of the Week

Chronic visceral pain (CVP) is a prevalent and debilitating syndrome with limited treatments. Guanylyl cyclase C (GUCY2C) is the receptor for uroguanylin (GUCA2B) in small intestine and guanylin (GUCA2A) in colorectum. This hormone-receptor axis produces cyclic (c)GMP accumulation, inducing intestinal fluid secretion. This forms the basis for use of GUCY2C agonists linaclotide and plecanatide to treat constipation-type irritable bowel syndrome (IBS-C) and chronic idiopathic constipation (CIC). Unexpectedly, GUCY2C agonists relieve CVP in these patients, and in mouse models of visceral pain. Interestingly, these patients are deficient in uroguanylin, suggesting that constipation and CVP may reflect hormone insufficiency silencing GUCY2C. Indeed, eliminating GUCY2C in mice (GUCY2C ) produced CVP, evoked by cyclic rectal distension, quantified by the abdominal withdrawal reflex and by phospho-ERK signaling in the spinal cord dorsal horn. CVP severity in GUCY2C mice recapitulated TNBS-induced inflammatory bowel disease in wild type (GUCY2 ) mice. Oral linaclotide relieved TNBS-induced CVP in wild type mice, but was without effect on CVP in GUCY2C mice. The mechanistic basis of GUCY2C-dependent visceral analgesia remains obscure. While GUCY2C is expressed by all epithelial cells in intestine, a GUCY2C promoter-driven GFP reporter revealed a novel rare population of cells enriched in GUCY2C mRNA, protein, and activity. GUCY2C cells, concentrated in duodenum but rare in rectum, morphologically resemble enteroendocrine cells with a basal neuropod potentially synapsing on visceral afferents in the lamina propria, offering a cellular substrate for GUCY2C visceral nociceptive signaling. RNAseq analysis revealed that GUCY2C neuropod cells were deficient in gene products canonically associated with GUCY2C-driven secretion, including GUCA2A, GUCA2B, CFTR and NHE3. Rather, these cells were enriched in gene sets characteristic of neurons. Importantly, dorsal root ganglia (DRG) cells formed functional connections with GUCY2C neuropod cells in co-culture. Thus, DRG cells with GUCY2C neuropod cells alone (GUCA2B-deficient, GUCY2C silenced) were hyperexcitable following current injection, with a reduced rheobase and repetitive action potentials (APs). In contrast, adding linaclotide (hormone replete, GUCY2C activated) silenced DRG neuron excitability, raising the rheobase and eliminating repetitive APs. DRG neuron excitability was not affected by linaclotide in co-cultures with neuropod cells from GUCY2C mice. Moreover, the effects of linaclotide on DRG neuron excitability was not recapitulated by extracellular cGMP. These observations suggest that GUCY2C neuropod cells synapse with DRG visceral afferents and modulate their excitability. They support a model in which GUCA2B sufficiency activates GUCY2C to suppress DRG neuron excitability, while GUCA2B insufficiency silences GUCY2C to produce DRG hyperexcitability and CVP. Finally, they suggest that linaclotide relieves CVP by stimulating GUCY2C neuropod cells to inhibit DRG excitability and nociceptive signaling.