Background: Muscularis macrophages (MMs) reside in the gut muscularis propria. A population of MMs share a phenotype with microglia and produce a large amount of Complement component 1qA (C1qa). Recently, we and others showed that absence of C1qa in macrophages associates with faster whole gut transit and increased small intestine and colonic motility. In the CNS, changes in C1qa expression are linked to brain disorders during aging. However, the mechanism and the impact of C1qa-MMs on age-related gut motility is unknown.
Aim: To elucidate the underlying mechanism regulating C1qa MMs-ENS functional interaction and to assess their involvement during aging. Methods: Young (4 months) and old (24 months) C57BL/6 (WT) and LysMCre * C1qafl/fl; B6(SJL) - C1qatm1c(EUCOMM)Wtsi/TennJ (C1qa^CKO) mice were examined for GI transit, including whole gut transit, gastric emptying of solids and colon transit; gene expression by single-cell RNA sequencing and real-time PCR and protein expression by immunohistochemistry.
Results: Age matched young C1qa^CKO mice compared to WT had a higher of pre-synaptic markers such as Syp in the stomach (1.6±0.19-fold change; p<0.05), small intestine (2.6±0.42-fold change; p<0.01) and colon (2.0±0.16-fold change; p<0.01). Post-synaptic markers such as Psd95 were also increased in small intestine (2.8±0.40; p<0.01) and colon (1.6±0.12; p<0.05) but not in the stomach. Single cell data (Fig 1) identified a total of 705 genes differential expressed between young WT and C1qa^CKO. Macrophage’s clusters in C1qa^CKO were particularly enriched in genes associated with the antiinflammatory M2 phenotype such as Hmox, Lyve1, Ccl6 and Wfdc7. On the other hand, neuronal clusters in C1qa^CKO were particularly enriched in genes related to synapses formation and maintenance such as Ubc and Rgs6. Increased C1qa expression levels between young and old WT mice were observed in the stomach (2.1±0.24-fold change; p<0.001), whereas a decreased expression level was observed in the small intestine (0.5±0.04-fold change: p<0.01) and in the colon (0.7±0.13-fold change; p=ns). Accordingly, age matched old C1qa^CKO and WT mice showed no differences in whole-gut transit time (C1qa^CKO:380±35minutes; WT:360±40 minutes, n=3). Gastric emptying of solids was faster for old C1qa^CKO mice (p<0.05) compared to age matched WT, but there were no differences in colon transit time (C1qa^CKO:233±46 seconds; WT: 228±33 seconds, respectively, n=5). Conclusion: C1qa MMs regulate GI motility by maintaining a well-organized synapse organization. In young mice, their impact is predominantly on the small intestine and colon, while during aging, C1qa MMs mainly influence the stomach by affecting gastric emptying.
Funding: DK129297; DK127992-1; ANMS Young Investigator grant; AGA grant#36; P30DK084567; Robert and Arlene Kogod Center on Aging Fundamental Mechanisms of Aging Award.

<b>Figure 1.</b> UMAP representation of the 22-cluster population (different colors) identified in WT and C1qa<sup>CKO</sup> colon muscle layer.

Figure 1. UMAP representation of the 22-cluster population (different colors) identified in WT and C1qa^CKO colon muscle layer.

Mohamed Eldesouki

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