Rac1 mediates laminar shear stress-induced vascular endothelial cell migration

Organoid-on-a-chip model of human ARPKD reveals mechanosensing pathomechanisms for drug discovery

Organoids serve as a novel tool for disease modeling in three-dimensional multicellular contexts. Static organoids, however, lack the requisite biophysical microenvironment such as fluid flow, limiting their ability to faithfully recapitulate disease pathology. Here, we unite organoids with organ-on-a-chip technology to unravel disease pathology and develop therapies for autosomal recessive polycystic kidney disease. PKHD1-mutant organoids-on-a-chip are subjected to flow that induces clinically relevant phenotypes of distal nephron dilatation. Transcriptomics discover 229 signal pathways that are not identified by static models. Mechanosensing molecules, RAC1 and FOS, are identified as potential therapeutic targets and validated by patient kidney samples. On the basis of this insight, we tested two U.S. Food and Drug Administration-approved and one investigational new drugs that target RAC1 and FOS in our organoid-on-a-chip model, which suppressed cyst formation. Our observations highlight the vast potential of organoid-on-a-chip models to elucidate complex disease mechanisms for therapeutic testing and discovery.

Stress-Sensitive Protein Rac1 and Its Involvement in Neurodevelopmental Disorders

Ras-related C3 botulinum toxin substrate 1 (Rac1) is a small GTPase that is well known for its sensitivity to the environmental stress of a cell or an organism. It senses the external signals which are transmitted from membrane-bound receptors and induces downstream signaling cascades to exert its physiological functions. Rac1 is an important regulator of a variety of cellular processes, such as cytoskeletal organization, generation of oxidative products, and gene expression. In particular, Rac1 has a significant influence on certain brain functions like neuronal migration, synaptic plasticity, and memory formation via regulation of actin dynamics in neurons. Abnormal Rac1 expression and activity have been observed in multiple neurological diseases. Here, we review recent findings to delineate the role of Rac1 signaling in neurodevelopmental disorders associated with abnormal spine morphology, synaptogenesis, and synaptic plasticity. Moreover, certain novel inhibitors of Rac1 and related pathways are discussed as potential avenues toward future treatment for these diseases.

Myo1e modulates the recruitment of activated B cells to inguinal lymph nodes

The inclusion of lymphocytes in high endothelial venules and their migration to the lymph nodes are critical steps in the immune response. Cell migration is regulated by the actin cytoskeleton and myosins. Myo1e is a long-tailed class I myosin and is highly expressed in B cells, which have not been studied in the context of cell migration. By using intravital microscopy in an in vivo model and performing in vitro experiments, we studied the relevance of Myo1e for the adhesion and inclusion of activated B cells in high endothelial venules. We observed reduced expression of integrins and F-actin in the membrane protrusions of B lymphocytes, which might be explained by deficiencies in vesicular trafficking. Interestingly, the lack of Myo1e reduced the phosphorylation of focal adhesion kinase (FAK; also known as PTK2), AKT (also known as AKT1) and RAC-1, disturbing the FAK-PI3K-RAC-1 signaling pathway. Taken together, our results indicate a critical role of Myo1e in the mechanism of B-cell adhesion and migration.

Adgrf5 contributes to patterning of the endothelial deep layer in retina

Neovascularization of the inner retinal space is a major cause of vision loss. In retinal angiomatous proliferation (RAP) syndrome, newly formed vessels originate from the retinal plexus and invade the inner retinal space. However, the molecular pathways preventing subretinal vascularization remain largely unknown. In most murine models of RAP, pathological neovascularization occurs concomitantly with the development of the retinal vasculature. Here, we demonstrate that disturbing the sequence of morphogenetic events that shape the three-layered retinal vascular network leads to subretinal vascularization. Sprouts emerging from the perivenous region after the first postnatal week extended toward the retinal space where they merged into the deep layer. The small GTPase Rac1 was required for the formation of these vascular extensions and the vascular inner plexus is formed coaxially to the overarching veins. The adhesion receptor Adgrf5 was highly expressed in the endothelium of the central nervous system, where it regulates blood–brain barrier formation. The vascular superficial plexus of Adgrf5 mutant mouse retinae exhibited an increased vascular density in the perivenous areas with increased projections toward the inner plexus where they subsequently created hyper-dense endothelial cells (EC) clusters. Disturbing the perivenous pool of EC thus significantly altered the inner plexus formation. These abnormalities culminated in transient vascular protrusions in the inner retinal space. Taken together, these results reveal a previously unobserved vascular morphogenetic defect in Adgrf5 knockout mice, implicating a role for ADGRF5 in the initiation of subretinal vascularization. Our findings also illustrate how vein-derived EC shape the inner retinal layer formation and could control the appearance of angiomatous malformations.

Small GTPases and Their Role in Vascular Disease

Over eighty million people in the United States have cardiovascular disease that can affect the heart causing myocardial infarction; the carotid arteries causing stroke; and the lower extremities leading to amputation. The treatment for end-stage cardiovascular disease is surgical—either endovascular therapy with balloons and stents—or open reconstruction to reestablish blood flow. All interventions damage or destroy the protective inner lining of the blood vessel—the endothelium. An intact endothelium is essential to provide a protective; antithrombotic lining of a blood vessel. Currently; there are no agents used in the clinical setting that promote reendothelialization. This process requires migration of endothelial cells to the denuded vessel; proliferation of endothelial cells on the denuded vessel surface; and the reconstitution of the tight adherence junctions responsible for the formation of an impermeable surface. These processes are all regulated in part and are dependent on small GTPases. As important as the small GTPases are for reendothelialization, dysregulation of these molecules can result in various vascular pathologies including aneurysm formation, atherosclerosis, diabetes, angiogenesis, and hypertension. A better understanding of the role of small GTPases in endothelial cell migration is essential to the development for novel agents to treat vascular disease.

Altered DNA methylation indicates an oscillatory flow mediated epithelial-to-mesenchymal transition signature in ascending aorta of patients with bicuspid aortic valve

Disturbed flow has been suggested to contribute to aneurysm susceptibility in bicuspid aortic valve (BAV) patients. Lately, flow has emerged as an important modulator of DNA methylation. Hear we combined global methylation analysis with in vitro studies of flow-sensitive methylation to identify biological processes associated with BAV-aortopathy and the potential contribution of flow. Biopsies from non-dilated and dilated ascending aortas were collected from BAV (n = 21) and tricuspid aortic valve (TAV) patients (n = 23). DNA methylation and gene expression was measured in aortic intima-media tissue samples, and in EA.hy926 and primary aortic endothelial cells (ECs) isolated from BAV and TAV exposed to oscillatory (±12 dynes/cm²) or laminar (12 dynes/cm²) flow. We show methylation changes related to epithelial-mesenchymal-transition (EMT) in the non-dilated BAV aorta, associated with oscillatory flow related to endocytosis. The results indicate that the flow-response in BAV ECs involves hypomethylation and increased expression of WNT/β-catenin genes, as opposed to an angiogenic profile in TAV ECs. The EMT-signature was exasperated in dilated BAV aortas. Aberrant EMT in BAV aortic walls could contribute to increased aneurysm susceptibility, and may be due to disturbed flow-exposure. Perturbations during the spatiotemporally related embryonic development of ascending aorta and semilunar valves can however not be excluded.