Silvia Dragoni

Diabetic retinopathy is a sight-threatening complication of diabetes mellitus, affecting millions of people worldwide. From a vascular perspective, diabetic retinopathy compromises the structure and function of the blood–retinal barrier, leading to aberrant angiogenesis and vascular leakage, with consequent loss of vision. This review will delve into the vascular abnormalities caused by diabetic retinopathy in the inner blood–retinal barrier, focusing primarily on retinal endothelial cells. It will then discuss how calcium signalling regulates inner blood–retina barrier function and dysfunction, how calcium channels contribute to the development of diabetic retinopathy, and how studying the components of the calcium toolkit may identify new therapeutic targets.

The inner blood–retinal barrier (iBRB) protects the retinal vasculature from the peripheral circulation. Endothelial cells (ECs) are the core component of the iBRB; their close apposition and linkage via tight junctions limit the passage of fluids, proteins, and cells from the bloodstream to the parenchyma. Dysfunction of the iBRB is a hallmark of many retinal disorders. Vascular endothelial growth factor (VEGF) has been identified as the primary driver leading to a dysfunctional iBRB, thereby becoming the main target for therapy. However, a complete understanding of the molecular mechanisms underlying iBRB dysfunction is elusive and alternative therapeutic targets remain unexplored. Calcium (Ca²⁺) is a universal intracellular messenger whose homeostasis and dynamics are dysregulated in many pathological disorders. Among the extensive components of the cellular Ca²⁺-signaling toolkit, cation-selective transient receptor potential (TRP) channels are broadly involved in cell physiology and disease and, therefore, are widely studied as possible targets for therapy. Albeit that TRP channels have been discovered in the photoreceptors of Drosophila and have been studied in the neuroretina, their presence and function in the iBRB have only recently emerged. Within this article, we discuss the structure and functions of the iBRB with a particular focus on Ca²⁺ signaling in retinal ECs and highlight the potential of TRP channels as new targets for retinal diseases.

NRP1 is a therapeutic target for inhibiting vascular endothelial growth factor (VEGF)-induced blood vessel dysfunction. The small molecule EG00229 was designed to inhibit VEGF binding to NRP1 and reduce pathological blood vessel growth. However, it is unknown whether EG00229 could also be used to reduce VEGF164-induced vascular leakage, which often exacerbates ischemic diseases due to VEGF upregulation. Here, we show that prior treatment with EG00229 prevents VEGF164-induced vascular permeability signalling, but, unexpectedly, also find that EG00229 increased rather than inhibited vascular leakage. Thus, EG00229 increased vascular leakage either when added alone or concurrently with VEGF164, both in perfused retinal explants and across primary brain EC monolayers. This EG00229-induced vascular leakage was not an off-target effect, because it relied on endothelial NRP1 expression and NRP1’s VEGF164 binding pocket, yet was independent of VEGFR1 and VEGFR2. Moreover, EG00229 activated molecular events typical of VEGF164-induced paracellular permeability, including p38 MAP kinase (p38) and SRC family kinase (SFK) phosphorylation as well as CDH5 rearrangement in endothelial junctions. Investigating EG00229-induced signalling therefore helps elucidate NRP1-dependent mechanisms of paracellular permeability induction and might help identifying new approaches to modulate the neurovascular barrier.

In all vertebrates, closed blood and open lymph circulatory systems are essential for the delivery of nutrients and oxygen to tissues, waste clearance, and immune function

Lymphocyte transendothelial migration (TEM) relies on ICAM-1 engagement on the luminal surface of the endothelial cells (ECs). In blood–brain barrier (BBB) ECs, ICAM-1 triggers TEM signalling, including through JNK MAP kinase and AMP-activated protein kinase (AMPK), which lead to the phosphorylation and internalisation of the adherens junction protein VE-cadherin. In addition to ICAM-1, G protein-coupled receptors (GPCRs) are also required for lymphocytes TEM across BBB ECs. Here, we investigated the role of protease activated GPCRs (PARs) and found a specific role for PAR1 in support of lymphocyte TEM across BBB ECs in vitro. PAR1 requirement for TEM was confirmed using protease inhibitors, specific small molecule and peptide antagonists, function blocking antibodies and siRNA-mediated knockdown. In BBB ECs, PAR1 stimulation led to activation of signalling pathways essential to TEM; notably involving JNK and endothelial nitric oxide synthase (eNOS), with the latter downstream of AMPK. In turn, nitric oxide production through eNOS was essential for TEM by modulating VE-cadherin on Y731. Collectively, our data showed that non-canonical PAR1 activation by a lymphocyte-released serine protease is required for lymphocyte TEM across the BBB in vitro, and that this feeds into previously established ICAM-1-mediated endothelial TEM signalling pathways.

At blood–neural barriers, endothelial VEGFA signalling is highly polarised, with entirely different responses being triggered by luminal or abluminal stimulation. These recent findings were made in a field which is still in its mechanistic infancy. For a long time, endothelial polarity has intuitively been presumed, and likened to that of epithelial cells, but rarely demonstrated. In the cerebral and the retinal microvasculature, the uneven distribution of VEGF receptors 1 and 2, with the former predominant on the luminal and the latter on the abluminal face of the endothelium, leads to a completely polarised signalling response to VEGFA. Luminal VEGFA activates VEGFR1 homodimers and AKT, leading to a cytoprotective response, whilst abluminal VEGFA induces vascular leakage via VEGFR2 homodimers and p38. Whilst these findings do not provide a complete picture of VEGFA signalling in the microvasculature—there are still unclear roles for heterodimeric receptor complexes as well as co-receptors—they provide essential insight into the adaptation of vascular systems to environmental cues that are naturally different, depending on whether they are present on the blood or tissue side. Importantly, sided responses are not only restricted to VEGFA, but exist for other important vasoactive agents.

Lymphocyte transendothelial migration (TEM) is critically dependent on intraendothelial signaling triggered by adhesion to ICAM-1. Here we show that endothelial MAPKs ERK, p38, and JNK mediate diapedesis-related and diapedesis-unrelated functions of ICAM-1 in cerebral and dermal microvascular endothelial cells (MVECs). All three MAPKs were activated by ICAM-1 engagement, either through lymphocyte adhesion or Ab-mediated clustering. MAPKs were involved in ICAM-1–dependent expression of TNF-α in cerebral and dermal MVECs, and CXCL8, CCL3, CCL4, VCAM-1, and cyclooxygenase 2 (COX-2) in cerebral MVECs. Endothelial JNK and to a much lesser degree p38 were the principal MAPKs involved in facilitating diapedesis of CD4⁺ lymphocytes across both types of MVECs, whereas ERK was additionally required for TEM across dermal MVECs. JNK activity was critical for ICAM-1–induced F-actin rearrangements. Furthermore, activation of endothelial ICAM-1/JNK led to phosphorylation of paxillin, its association with VE-cadherin, and internalization of the latter. Importantly ICAM-1–induced phosphorylation of paxillin was required for lymphocyte TEM and converged functionally with VE-cadherin phosphorylation. Taken together we conclude that during lymphocyte TEM, ICAM-1 signaling diverges into pathways regulating lymphocyte diapedesis, and other pathways modulating gene expression thereby contributing to the long-term inflammatory response of the endothelium.

Store-operated Ca²⁺entry (SOCE) is activated following depletion of the inositol-1,4,5-trisphosphate (InsP₃)-sensitive Ca²⁺pool to regulate proliferation in immortalized cell lines established from either primary or metastatic lesions. The molecular nature of SOCE may involve both Stim1, which senses Ca²⁺levels within the endoplasmic reticulum (ER) Ca²⁺reservoir, and a number of a Ca²⁺-permeable channels on the plasma membrane, including Orai1, Orai3, and members of the canonical transient receptor (TRPC1–7) family of ion channels. The present study was undertaken to assess whether SOCE is expressed and controls proliferation in primary cultures isolated from secondary lesions of heavily pretreated metastatic renal cell carcinoma (mRCC) patients. SOCE was induced following pharmacological depletion of the ER Ca²⁺store, but not by InsP₃-dependent Ca²⁺release. Metastatic RCC cells express Stim1-2, Orai1–3, and TRPC1–7 transcripts and proteins. In these cells, SOCE was insensitive to BTP-2, 10 µM Gd³⁺and Pyr6, while it was inhibited by 100 µM Gd³⁺, 2-APB, and carboxyamidotriazole (CAI). Neither Gd³⁺nor 2-APB or CAI impaired mRCC cell proliferation. Consistently, no detectable Ca²⁺signal was elicited by growth factor stimulation. Therefore, a functional SOCE is expressed but does not control proliferation of mRCC cells isolated from patients resistant to multikinase inhibitors.