(206) Investigating Colonic pH in Cystic Fibrosis: Wireless Motility Capsule to Single-Cell Sequencing

Background: Gastrointestinal pH is crucial for physiological gut functioning. Previous wireless motility capsule (WMC) studies demonstrated decreased small intestinal pH in people with CF (PwCF). Data is lacking on the colonic pH profile of PwCF. Our aim was to use WMC to determine the colonic pH/bicarbonate concentration in PwCF and single-cell RNA sequencing (sc-RNAseq) to examine the normal expression of acid-base transporters in the colon/rectum to better understand the regulation of pH in the colon.

Methods: We re-analyzed WMC data previously collected on CF and matched non-CF controls (age ±3 years, sex, and BMI ±2) (Dig Dis Sci 2013;58:2275-81). The study included 10 CF and 10 non-CF patients. All subjects were ³18 years old. All CF patients had exocrine pancreatic insufficiency. Each CF subject underwent two independent WMC studies, ~8 weeks apart. CF subjects were at baseline health with no hospitalization for ³2 months prior to the study. None were on CFTR modulators. All patients were off acid-blocking medications and inhaled antibiotics during the study period. pH profiles were extracted using GIMS software. We defined 2-hour windows for each subject prior to capsule exit and measured 4 x 30-minute segments. Bicarbonate concentration was calculated using the Henderson-Hasselbach equation with previously published sigmoid pCO₂ values (Acta Anaesthesiol Scand 1999;43:702-7). Published sc-RNAseq metadata annotations of the human colon (4,472 cells) and rectum (3,898 cells) from 2 non-CF patients (J Exp Med 2020 Feb;217) were re-analyzed using Seurat R toolkit. Significance was determined by a non-parametric unpaired t-test.

Results: Proximal to distal regional WMC from non-CF patients showed mean (standard deviation) pH values of 7.3 (0.9), 7.4 (0.9), 7.4 (0.7), 7.9 (0.8) and mean (standard deviation) bicarbonate concentrations (in mM) of 55.1 (159.2), 63.8 (145.2), 46.2 (145.2), 121.9 (610.9). CF patients showed significantly lower pH and bicarbonate concentration values (P< 0.05), particularly in the most distal region: 6.7 (0.9), 6.7 (0.9), 6.7 (0.9), 6.7 (0.8), and 6.2 (306.4), 5.6 (653.5), 6.1 (158.7), 5.5 (86.0), respectively. There was no difference in colonic contractions per minute, motility index, or pressure area under the curve between CF and non-CF subjects (P >0.05). Sc-RNAseq showed colonic expression profiles of acid-base transporters in enterocytes (greatest to least) as: SLC26A3 > >CFTR»SLC16A1 >SLC26A6. For rectum: SLC26A3 > >SLC16A1»SLC26A6 >CFTR. SLC9A3 was not detected in either colon or rectum.

Conclusions: Like the small intestine, colonic pH is impaired in PwCF. Decreased pH and bicarbonate concentration are most profound in the distal rectosigmoid. While dysmotility can alter intraluminal pH, our observed changes likely originate from alterations in intestinal ion transport rather than colonic dysmotility. SLC26A3 is abundantly expressed in the human colon and rectum and may be a potential therapeutic target for the restoration of bicarbonate transport in PwCF. Ongoing work is examining the effect of CFTR modulators on colonic pH, bicarbonate, and acid-base transporter expression profiles. These findings may contribute to our understanding of the gastrointestinal symptoms in PwCF.

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