Are lymphatic contractions initiated and/or coordinated by ICC‐like cells in the lymphatic vessel wall?

Abstract

Lymph propulsion relies critically on the spontaneous contractions of collecting lymphatic vessels, which are surrounded by smooth muscle cells (LMCs). Twitch‐like contractions are generated by an intrinsic ionic pacemaker that is assumed to originate in the LMC; however there are reasons to consider that pacemaking might be initiated and/or coordinated by a network of specialized cells. Precedents include interstitial cells of Cajal (ICC) in the GI tract and ICC‐like cells (ICC‐LC) in other smooth muscle‐invested organs, which are known to initiate pacemaking. ICCs express the receptor tyrosine kinase c‐Kit, and c‐Kit(+) cells have been identified in the lymphatic wall, although their function is unknown. We used existing and newly‐generated mouse models to test the hypothesis that ICC‐LCs control lymphatic pacemaking. In addition to c‐Kit, we considered PDGFRa, a receptor expressed by some ICC‐LCs. Our strategy was to use Cre lines specific for four cell types (c‐Kit‐ or PDGFRa‐expressing cells, pericytes, LMCs) and: 1) cross them with reporter mice to identify the recombined cell populations in the lymphatic wall; 2) use them to express the endogenous Ca2+ indicator GCaMP6f in the respective cell types and correlate Ca2+ signals with the lymphatic contraction cycle; 3) express the photo‐activated cation channel ChR2 in the respective cell types and test the consequences of triggering ChR2 activation. Rounded c‐Kit(+) cells were associated with the outer adventitia and co‐stained for mast cell markers. GCaMP expression revealed Ca2+ signals that did not oscillate with the contraction cycle, but responded robustly to a mast cell degranulator. PDGFRa cells formed a partial sheath of oak‐leaf shaped cells outside of the medial layer—characteristics of adventitial fibroblasts; they did not exhibit spontaneous Ca2+ oscillations, but occasional cells responded to ACh. A reputed pericyte‐specific Cre, Ng2CreERT2, produced recombination in about 50% of LMCs, some neurons, some spindle‐shaped cells in the wall, and a subpopulation of PDGFRa(+) cells. Expression of GCaMP under this Cre revealed Ca2+ oscillations in LMCs and spindle‐shaped cells at the same frequency as spontaneous contractions. Expression of GCaMP only in LMCs using SmmhcCreERT2 revealed synchronized Ca2+ signals in synchrony with and preceding spontaneous lymphatic contractions. Expression of ChR2 using the same Cre, followed by photo‐stimulation of single LMCs, triggered contractions that were consistently entrained over the entire vessel. Although we cannot exclude a role for pericytes due to the promiscuous recombination by Ng2CreERT2, our results suggest that LMCs are the only cell type responsible for lymphatic pacemaking in the mouse.Support or Funding InformationNIH HL‐122578This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.