Phosphorylation-Dependent Intra-Domain Interaction of the Cx37 Carboxyl-Terminus Controls Cell Survival

Connexin 37 sequestering of activated-ERK in the cytoplasm promotes p27-mediated endothelial cell cycle arrest

Connexin37-mediated regulation of cell cycle modulators and, consequently, growth arrest lack mechanistic understanding. We previously showed that arterial shear stress up-regulates Cx37 in endothelial cells and activates a Notch/Cx37/p27 signaling axis to promote G1 cell cycle arrest, and this is required to enable arterial gene expression. However, how induced expression of a gap junction protein, Cx37, up-regulates cyclin-dependent kinase inhibitor p27 to enable endothelial growth suppression and arterial specification is unclear. Herein, we fill this knowledge gap by expressing wild-type and regulatory domain mutants of Cx37 in cultured endothelial cells expressing the Fucci cell cycle reporter. We determined that both the channel-forming and cytoplasmic tail domains of Cx37 are required for p27 up-regulation and late G1 arrest. Mechanistically, the cytoplasmic tail domain of Cx37 interacts with, and sequesters, activated ERK in the cytoplasm. This then stabilizes pERK nuclear target Foxo3a, which up-regulates p27 transcription. Consistent with previous studies, we found this Cx37/pERK/Foxo3a/p27 signaling axis functions downstream of arterial shear stress to promote endothelial late G1 state and enable up-regulation of arterial genes.

Connexin37 Regulates Cell Cycle in the Vasculature

Control of vascular cell growth responses is critical for development and maintenance of a healthy vasculature. Connexins - the proteins comprising gap junction channels - are key regulators of cell growth in diseases such as cancer, but their involvement in controlling cell growth in the vasculature is less well appreciated. Connexin37 (Cx37) is one of four connexin isotypes expressed in the vessel wall. Its primary role in blood vessels relies on its unique ability to transduce flow-sensitive signals into changes in cell cycle status of endothelial (and perhaps, mural) cells. Here, we review available evidence for Cx37's role in the regulation of vascular growth, vessel organization, and vascular tone in healthy and diseased vasculature. We propose a novel mechanism whereby Cx37 accomplishes this with a phosphorylation-dependent transition between closed (growth-suppressive) and multiple open (growth-permissive) channel conformations that result from interactions of the C-terminus with cell-cycle regulators to limit or support cell cycle progression. Lastly, we discuss Cx37 and its downstream signaling as a novel potential target in the treatment of cardiovascular disease, and we address outstanding research questions that still challenge the development of such therapies.

Endothelial cells promote smooth muscle cell resilience to H2 O2 -induced cell death in mouse cerebral arteries

AIM

Brain injury produces reactive oxygen species (ROS). However, little is known of how acute oxidative stress affects cell survival in the cerebral vascular supply. We hypothesized that endothelial cells (ECs) are more resilient to H₂ O₂ and protect vascular smooth muscle cells (SMCs) during acute oxidative stress.

METHODS

Mouse posterior cerebral arteries (PCAs; diameter, ~80 µm) were exposed to H₂ O₂ (200 µM, 50 min, 37°C). Nuclear staining identified dead and live cells of intact and endothelium-disrupted vessels. SMC [Ca²⁺ ]_i was assessed with Fura-2 fluorescence, and superoxide production was assessed by dihydroethidium and MitoSOX fluorescence.

RESULTS

In response to H₂ O₂ : SMC death (21%) exceeded EC death (5%) and increased following endothelial disruption (to 48%) with a corresponding increase in SMC Ca²⁺ entry through transient receptor potential (TRP) channels. Whereas pharmacological inhibition of TRPV4 channels prevented SMC death and reduced Ca²⁺ entry for intact vessels, both remained elevated following endothelial disruption. In contrast, pharmacological inhibition or genetic deletion of TRPC3 prevented SMC death and attenuated Ca²⁺ entry for both intact and endothelium-disrupted vessels. Inhibiting gap junctions increased EC death (to 22%) while SMC death and [Ca²⁺ ]_i responses were attenuated by inhibiting nitric oxide synthesis or scavenging superoxide/peroxynitrite. Inhibiting NADPH oxidases also prevented SMC Ca²⁺ entry and death. H₂ O₂ increased mitochondrial ROS production while scavenging mitochondria-derived superoxide prevented SMC death but not Ca²⁺ entry.

CONCLUSIONS

During acute exposure of cerebral arteries to acute oxidative stress, ECs are more resilient than SMCs and the endothelium may protect SMCs by reducing Ca²⁺ entry through TRPC3 channels.

Apoptosis in resistance arteries induced by hydrogen peroxide: greater resilience of endothelium versus smooth muscle

Reactive oxygen species (ROS) are implicated in cardiovascular and neurologic disorders including atherosclerosis, heart attack, stroke, and traumatic brain injury. Although oxidative stress can lead to apoptosis of vascular cells, such findings are largely based upon isolated vascular smooth muscle cells (SMCs) and endothelial cells (ECs) studied in culture. Studying intact resistance arteries, we have focused on understanding how SMCs and ECs in the blood vessel wall respond to acute oxidative stress induced by hydrogen peroxide, a ubiquitous, membrane-permeant ROS. We find that apoptosis induced by H₂O₂ is far greater in SMCs compared to ECs. For both cell types, apoptosis is associated with a rise in intracellular calcium concentration ([Ca²⁺]_i) during H₂O₂ exposure. Consistent with their greater death, the rise in [Ca²⁺]_i for SMCs exceeds that in ECs. Finding that disruption of the endothelium increases SMC death, we address how myoendothelial coupling and paracrine signaling attenuate apoptosis. Remarkably, conditions associated with chronic oxidative stress (advanced age, Western-style diet) protect SMCs during H₂O₂ exposure, as does female sex. In light of intracellular Ca²⁺ handling, we consider how glycolytic versus oxidative pathways for ATP production and changes in mitochondrial structure and function impact cellular resilience to H₂O₂-induced apoptosis. Gaining new insight into protective signaling within and between SMCs and ECs of the arterial wall can be applied to promote vascular cell survival (and recovery of blood flow) in tissues subjected to acute oxidative stress as occurs during reperfusion following myocardial infarction and thrombotic stroke.

Connexins in Cancer: Jekyll or Hyde?

The expression, localization, and function of connexins, the protein subunits that comprise gap junctions, are often altered in cancer. In addition to cell-cell coupling through gap junction channels, connexins also form hemichannels that allow communication between the cell and the extracellular space and perform non-junctional intracellular activities. Historically, connexins have been considered tumor suppressors; however, they can also serve tumor-promoting functions in some contexts. Here, we review the literature surrounding connexins in cancer cells in terms of specific connexin functions and propose that connexins function upstream of most, if not all, of the hallmarks of cancer. The development of advanced connexin targeting approaches remains an opportunity for the field to further interrogate the role of connexins in cancer phenotypes, particularly through the use of in vivo models. More specific modulators of connexin function will both help elucidate the functions of connexins in cancer and advance connexin-specific therapies in the clinic.

Serine 319 phosphorylation is necessary and sufficient to induce a Cx37 conformation that leads to arrested cell cycling

Connexin 37 (Cx37; protein product of GJA4) expression profoundly suppresses proliferation of rat insulinoma (Rin) cells in a manner dependent on gap junction channel (GJCh) functionality and the presence and phosphorylation status of its C-terminus (CT). In Rin cells, growth is arrested upon induced Cx37 expression and serine 319 (S319) is frequently phosphorylated. Here, we show that preventing phosphorylation at this site (alanine substitution; S319A) relieved Cx37 of its growth-suppressive effect whereas mimicking phosphorylation at this site (aspartate substitution; S319D) enhanced the growth-suppressive properties of Cx37. Like wild-type Cx37 (Cx37-WT), Cx37-S319D GJChs and hemichannels (HChs) preferred the closed state, rarely opening fully, and gated slowly. In contrast, Cx37-S319A channels preferred open states, opened fully and gated rapidly. These data indicate that phosphorylation-dependent conformational differences in Cx37 protein and channel function underlie Cx37-induced growth arrest versus growth-permissive phenotypes. That the closed state of Cx37-WT and Cx37-S319D GJChs and HChs favors growth arrest suggests that rather than specific permeants mediating cell cycle arrest, the closed conformation instead supports interaction of Cx37 with growth regulatory proteins that result in growth arrest.