Lymphatic Connexins and Pannexins in Health and Disease

TRPML1 acts as a predisposing factor in lymphedema development by regulating the subcellular localization of aquaporin-3, -5

An imbalance in lymphatic fluid, whether it is caused by generation, transport, outflow, or dysfunctional vessels, can lead to lymphedema; however, the exact pathogenesis of this disease remains unclear. To explore the mechanism, we focused on the association among TRPML1, aquaporin-3 (AQP3), and aquaporin-5 (AQP5) in human lymphatic endothelial cells (HLECs). We explored the role of TRPML1 in altering the permeability of HLECs in lymphedema. Meanwhile, we constructed a disease model using gene-knockout mice to observe the effect of TRPML1 on inflammation and fibrosis in lymphedema sites. Our results indicate that TRPML1 not only regulates the localization of AQP3, -5 to the cell membrane but also increases HLEC permeability, disrupts lymphatic fluid transport, and mediates the development of chronic inflammation at the site of lymphedema. Our study suggests that TRPML1 is a precipitating factor in lymphedema. Our findings improve the understanding of TRPML1 and aquaporins in secondary lymphedema, providing valuable insights for future research.

Pannexin1 deletion in lymphatic endothelium affects lymphatic function in a sex-dependent manner

The lymphatic network of capillaries and collecting vessels ensures tissue fluid homeostasis, absorption of dietary fats and trafficking of immune cells. Pannexin1 (Panx1) channels allow for the passage of ions and small metabolites between the cytosol and extracellular environment. Panx1 channels regulate the pathophysiological function of several tissues in a sex-dependent manner. Here, we studied the role of Panx1 in lymphatic function, and potential sex-dependent differences therein, in Prox1-CreERT2Panx1fl/fl and Panx1fl/fl control mice. Panx1 expression was higher in lymphatic endothelial cells (LECs) of male mice. Lymphatic vessel morphology was not affected in Prox1-CreERT2Panx1fl/fl male and female mice. Lymphatic drainage was decreased by 25% in male Prox1-CreERT2Panx1fl/fl mice, but was similar in females of both genotypes. Accordingly, only male Prox1-CreERT2Panx1fl/fl mice exhibited tail swelling, pointing to interstitial fluid accumulation in males upon Panx1 deletion in LECs. Moreover, serum triglyceride and free fatty acid levels raised less in Prox1-CreERT2Panx1fl/fl mice of both sexes in an oral lipid tolerance test. Finally, the percentage of migratory dendritic cells arriving in draining lymph nodes was increased in Prox1-CreERT2Panx1fl/fl female mice, but was comparable between male mice of both genotypes. Our results point to a LEC-specific role for Panx1 in the functions of the lymphatic system.

A 1D model characterizing the role of spatiotemporal contraction distributions on lymph transport

Lymphedema is a condition in which lymph transport is compromised. The factors that govern the timing of lymphatic contractions are largely unknown; however, these factors likely play a central role in lymphatic health. Computational models have proven useful in quantifying changes in lymph transport; nevertheless, there is still much unknown regarding the regulation of contractions. The purpose of this paper is to utilize computational modeling to examine the role of pacemaking activity in lymph transport. A 1D fluid-solid modeling framework was utilized to describe the interaction between the contracting vessel and the lymph flow. The distribution of contractions along a three-lymphangion chain in time and space was determined by specifying the pacemaking sites and parameters obtained from experimentation. The model effectively replicates the contractility patterns in experiments. Quantitatively, the flow rates were measured at 5.44 and 2.29 [Formula: see text], and the EF values were 78% and less than 33% in the WT and KO models, respectively, which are consistent with the literature. Applying pacemaking parameters in this modeling framework effectively captures lymphatic contractile wave propagations and their relation to lymph transport. It can serve as a motivation for conducting novel studies to evaluate lymphatic pumping function during the development of lymphedema.

Mechanisms of Diseases Associated with Mutation in GJC2/Connexin 47

Connexins are members of a family of integral membrane proteins that provide a pathway for both electrical and metabolic coupling between cells. Astroglia express connexin 30 (Cx30)-GJB6 and Cx43-GJA1, while oligodendroglia express Cx29/Cx31.3-GJC3, Cx32-GJB1, and Cx47-GJC2. Connexins organize into hexameric hemichannels (homomeric if all subunits are identical or heteromeric if one or more differs). Hemichannels from one cell then form cell-cell channels with a hemichannel from an apposed cell. (These are termed homotypic if the hemichannels are identical and heterotypic if the hemichannels differ). Oligodendrocytes couple to each other through Cx32/Cx32 or Cx47/Cx47 homotypic channels and they couple to astrocytes via Cx32/Cx30 or Cx47/Cx43 heterotypic channels. Astrocytes couple via Cx30/Cx30 and Cx43/Cx43 homotypic channels. Though Cx32 and Cx47 may be expressed in the same cells, all available data suggest that Cx32 and Cx47 cannot interact heteromerically. Animal models wherein one or in some cases two different CNS glial connexins have been deleted have helped to clarify the role of these molecules in CNS function. Mutations in a number of different CNS glial connexin genes cause human disease. Mutations in GJC2 lead to three distinct phenotypes, Pelizaeus Merzbacher like disease, hereditary spastic paraparesis (SPG44) and subclinical leukodystrophy.

Cataract-linked serine mutations in the gap junction protein connexin50 expose a sorting signal that promotes its lysosomal degradation

Many human connexin50 (Cx50) mutants have been linked to cataracts including two carboxyl terminus serine mutants that are known phosphorylation sites in the lens (Cx50S258F and Cx50S259Y). To examine the behavior of these mutants and the role of phosphorylation at these positions, we stably transfected HeLa cells with cataract-linked and phosphorylation-mimicking (Cx50S258D and Cx50S259D) Cx50 mutants. We observed that gap junctional plaques were rarely detected in Cx50S258F- and Cx50S259Y-expressing cells compared with wild-type cells. In addition, gap junction abundance and size were greatly increased for Cx50S258D and Cx50S259D mutants. Cx50S258F and Cx50S259Y supported very low levels of gap junctional coupling, whereas Cx50S258D and Cx50S259D supported extensive intercellular communication. Furthermore, Cx50 levels as detected by immunoblotting were lower in Cx50S258F and Cx50S259Y mutants than in the wild type or the aspartate substitution mutants, and chloroquine or ammonium chloride treatment significantly increased Cx50S258F and Cx50S259Y protein levels, implying participation of the lysosome in their increased degradation. Alanine substitution of amino acids within a predicted tyrosine-based sorting signal in Cx50S258F and Cx50S259Y increased levels of gap junctional plaques and intercellular transfer of Neurobiotin. These results suggest that absence of phosphorylatable serines at these positions exposes a sorting signal leading to lysosomal degradation of Cx50, whereas phosphorylation at these sites conceals this signal and allows targeting of Cx50 to the plasma membrane and stabilization of gap junction plaques. We propose that in the lens degradation of Cx50S258F and Cx50S259Y decreases Cx50 levels at the plasma membrane and consequently Cx50 function, leading to cataracts.

Plastic Bronchitis—A Serious Rare Complication Affecting Children Only after Fontan Procedure?

Background: Plastic bronchitis (PB) may occur not only in children following palliative Fontan procedure but also in those without underlying heart disease. We aim to assess the clinical course, therapeutic measures, outcome, and follow-up of PB in children with congenital heart disease (CHD) and children without cardiac problems. Methods: This retrospective case series assessed children with PB admitted to hospital between 2015 and 2019. Parents or guardians of patients were contacted by e-mail or telephone between September 2017 and June 2019 to enquiry about recurrence of PB and strategy of treatment. The diagnosis of PB was based on the expectoration (spontaneous or during bronchoscopy) of endobronchial plugs. Results: This study delineated the clinical, histological, and laboratory features of plastic bronchitis in children following Fontan procedure (Group A) and in those without heart defects (Group B, non-CHD children). The main symptoms were cough accompanied by dyspnea, and hypoxemia with a decrease in oxygen saturation, often leading to acute respiratory failure. In children with CHD, the first episode of PB occurred at a relatively young age. Although chronic, i.e., lasting more than 3 weeks, inhaled therapy was implemented in both groups of patients, the recurrences of PB were observed. The mean time to PB recurrence after the first episode in Group A was longer than that in Group B (1.47 vs. 0.265 years, p = 0.2035). There was no re-episode with recurrence of PB in 3 cases out of 10 in total in Group A (30%) and 1 case out of 4 in total in Group B (25%). While the majority of children in Group A usually developed bronchial casts on the right side, the patients in Group B (without CHD) suffered from bronchial casts located only on the left side. Conclusions: Despite many similarities, clinical, histological, and laboratory studies in the children with plastic bronchitis after Fontan’s surgery and in children without heart defects suggest that there are differences in the course of the disease in patients without CHD, such as a more advanced age of the first episode of PB, the location of plastic casts on the left side, and a stronger role of inflammatory factors and mechanisms. Further research is needed to understand the pathophysiology of PB and choose the most appropriate therapy.