PROJECT SUMMARY/ABSTRACT
Obesity and type-2 diabetes are both considered major risk factors for the development of multiple
cardiovascular diseases worldwide. In the US alone, about 36% of the adult population are obese and over 25
million suffer from diabetes. Importantly, the prevalence of type-2 diabetes has been shown to dramatically
increase with obesity. Surprisingly, while signs of lymphatic dysfunction have been reported in obesity and
diabetes, the mechanisms underlying lymphatic dysfunction are unknown. Efficient lymph transport relies
critically on the intrinsic spontaneous contractions of lymphatic muscle cells (LMCs). These contractions are
initiated by action potentials that originate at a pacemaking site and then rapidly propagate between the strongly-
coupled LMCs. In contrast to arterioles, electrical coupling between lymphatic endothelial cells (LECs) and LMCs
through MEJs (myoendothelial junctions) appears to be quite limited; while this may be essential for the focal
generation of pacemaking signals, preventing leak of depolarizing current into the endothelium, the idea has not
been investigated. Thus, it is completely unknown whether there are any conducted signals in the lymphatic
endothelium that can regulate LMC contractility and to what extent heterocellular cross-talk between LECs and
LMCs plays a role in lymphatic function. The preliminary data obtained for this proposal show that: 1) signals
can be conducted as Ca2+ waves in the lymphatic endothelium, and these waves can regulate the contractile
function of LMCs; 2) in obese mice, the spontaneous contractions of isolated lymphatic vessels are impaired (i.e.
decreased ejection fraction); 3) the serine protease inhibitor PAI-1, which regulates the formation of vascular
MEJs, is an important biomarker for metabolic syndrome, diabetes, and obesity and is upregulated in lymphatic
vessels from obese mice compared to those from control healthy mice; and 4) genetic overexpression of PAI-1
(tgPAI-1) in non-obese mice results in lymphatic vessels with impaired contractility, while vessels from age-
matched PAI-1 KO mice (PAI-1-/-) show improved lymphatic function. These results suggest novel endothelium-
dependent mechanisms through which lymphatic contractions can be regulated, associate obesity with lymphatic
contractile dysfunction, point to a novel, critical role for PAI-1 in regulating lymphatic function, and suggest its
potential use as therapeutic target to ameliorate lymphatic dysfunction associated with obesity. Therefore, the
hypotheses to be tested are: a) Conducted signals in the lymphatic endothelium can regulate LMC contractility
through either direct LEC-LMC coupling via MEJs and/or diffusion of released factors; b) increased levels of PAI-
1 result in abnormal cellular interactions, upregulating the formation of MEJs; c) PAI-1 upregulation associated
with obesity results in impaired lymphatic function through endothelium-dependent mechanisms; and, d) PAI-1
inhibition can ameliorate lymphatic function in obesity. The studies here proposed and their outcome will expand
our understanding on the pathophysiological mechanisms leading to lymphatic dysfunction in metabolic
syndrome and diabetes associated with obesity.