ACE2 and TMPRSS2 co-express with nAChRs in stem cells, goblet cells, and enterocytes
We first investigated which cells express ACE2 and TMPRSS2 in the human intestinal context. To this end, we obtained the data from GSE134809 including single-cell RNA-sequencing on inflamed and uninflamed ileal biopsies from patients with Crohn’s disease(20). Unsupervised clustering analysis identified 22 clusters (FIG. 2A), with clusters 8, 11, 13, 14, and 18 likely representing the epithelial cells based on their expression of E-cadherin (CDH1) and Villin-1 (VIL1) (FIG. 2B and 2C). Expectedly, both ACE2 and TMPRSS2 were expressed in the same clusters confirming specific gene expression in epithelial cells in ileum. Subsequent clustering analysis of the epithelial cells yielded 15 subclusters (FIG. 2D), in which we identified stem cells (LGR5, ASCL2, OLFM4, GKN3, SLC12A2, AXIN2), goblet cells (MUC2, TFF3, CLCA3, AGR2), enterocytes (FABP1, ALPI, APOA1, APOA4), enteroendocrine cells (CHGA, CHGB, TAC1, TPH1, NEUROG3), and tuft cells (DCLK1, TRPM5, GFI1B, IL25) (FIG. 2D)(27, 28). Visualizing the expression of ACE2 or TMPRSS2 alongside the expression of the nAChR genes revealed co-expression in stem cells, goblet cells, and enterocytes for CHRNA5, CHRNA7, CHRNA10, CHRNB1, and CHRNE (encoding nAChR subunits α5, α7, α10, β1, and ε, respectively).
Notably, ACE2 is clearly expressed in goblet cells, although it has been demonstrated that SARS-CoV and SARS-CoV-2 cannot infect goblet cells in both airway and intestinal epithelia(15). Moreover, the nAChR that was mostly co-expressed with ACE2 and TMPRSS2 was α7 nAChR (encoded by CHRNA7). This is an essential regulator of inflammation by inhibiting cytokine release through the aforementioned cholinergic anti-inflammatory pathway(32). This is relevant because Russo and Leung reported on the role of α7 nAChR in SARS-CoV-2 and demonstrated that smoking results in an upregulation of α7 nAChR leading to an increase of ACE2(33-35).
Chronic electrical VNS does not alter intestinal ACE2 expression in mice and humans
As a positive correlation of ACE2 and CHRNA7 was previously observed, and chronic cholinergic stimulation typically leads to upregulation of nAChRs, we investigated whether chronic VNS affected the expression of ACE2(35, 36). Accordingly, intestinal ileal tissues of mice that were subjected to electrical chronic VNS, applied via an implantable device, were obtained and analyzed for Ace2 mRNA expression. Adequate VNS was confirmed by behavioral changes of the mice, impedance measurements, and HE stainings not showing any damage of the vagus nerve caused by the cuff electrodes (FIG. 3). Our data show no effect on Ace2 expression levels. Since samples contained bulk RNA of intestine, values were corrected for epithelial markers Cdh1 and Vil1 to determine the epithelial fraction. Also after correction, no differences were observed (FIG. 4A).
Next, we assessed ACE2 mRNA expression in human intestinal tissues of patients treated with chronic VNS as was described previously(18, 19). In line with the mouse data, no differential expression was observed between samples collected prior to VNS and 6 and 12 months after start VNS. Correction for CDH1 and VIL1, markers that have shown to be stable under inflammatory conditions (data not shown), did not alter the results (FIG. 4B). Though not significant, a trend towards higher levels of ACE2 at 12 months VNS could be detected. Noticeably, ACE2 expression levels in colon samples were relatively low when compared with ileal samples, which is corroborated by earlier literature showing low basal expression levels of ACE2 in colonic tissue(11).
It should be mentioned that in this study we made use of already available tissues. Thus, stimulation parameters such as duration of stimulation, pulse width, and frequency were not optimized for the current research question. In addition, both VNS experiments have been performed in diseased subjects; mice were exposed to DSS inducing colitis, and all humans had active Crohn’s disease. ACE2 has been shown to be upregulated in inflammatory bowel diseases (IBD), and in ileum in particular(37-39). For that reason, in our experiments, basal ACE2 expression levels might have been higher when compared with healthy subjects because of the colitis background of the tissues, possibly mitigating the results. The considerable divergence between ileal and colonic expression levels substantiates this confounding effect. Though, our analyses did not show significant differences between uninflamed and inflamed samples (FIG. 4B).
Data on the role of α7 nAChRs in colitis are conflicting. Sun et al. demonstrated increased CHRNA7 expression upon 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis, whereas Baird et al. found decreased CHRNA7 expression in IBD(40, 41). Further, in an experimental colitis model, selective α7 nAChR agonists worsened disease(42). These data imply that colitis is associated with CHRNA7 expression, again modifying the ACE2 expression levels perchance confusing the current results.
Importantly, when in this manuscript we focus on nicotinic receptors and α7 nAChR specifically, it is important to note that other receptors might play a role in regulating ACE2 as well since VNS unequivocally activates other acetylcholine-binding receptor types. This was not examined in this study.
In addition, we show that VNS does not alter intestinal ACE2 mRNA expression levels, protein expression levels, which might be of more importance in this context, have not been assessed. Hence, an effect on SARS-CoV-2 infection could still be observed. This is substantiated by Lamers et al. who demonstrated that SARS-CoV-2 infection of gut enterocytes is independent from the ACE2 expression level(15). Conversely, since SARS-CoV-2 is known to downregulate ACE2 expression, it is plausible that VNS could restore ACE2 levels in infected patients(43, 44). To this end, VNS studies including SARS-CoV-2 infected patients should be performed.
Regardless of the ACE2 expression level, the antagonizing effect of VNS on the devastating systemic cytokine storm might strengthen this intervention as potential treatment for COVID-19. This is greatly underlined by the positive outcomes of the RECOVERY trial studying the effect of low-dose steroid treatment with dexamethasone on SARS-CoV-2(45). Alike VNS, dexamethasone is acknowledged as inhibitor of the production of pro-inflammatory cytokines(46, 47).
Concerning the effect of VNS on systemic cytokine responses, a critical note is that these responses depend on the stimulation parameters such as frequency, amplitude, and pulse width as was demonstrated by Tsaava et al., and recently reviewed(48, 49). Although low frequency stimulation (as was used in our studies) was not examined, stimulation with high amplitudes resulted in an increase in IL-6 and decrease in IL-5 and TNF-α. When refining these parameters in VNS, the subsequent positive regulation of cytokines could substantiate the role of VNS in treating COVID-19.
Finally, granting the potential role for VNS in SARS-CoV-2 infection, one must be aware that parasympathetic neuronal activity through the vagus nerve can induce bronchoconstriction, an absolutely undesirable adverse effect in COVID-19 patients(50). Even though results on this matter are conflicting, application of VNS in these patients must be performed with great caution(51, 52). To elucidate and overcome this issue, future studies should focus on more specific stimulation techniques exclusively targeting the cytokine-producing organ of interest.