Regulating the regulators of angiogenesis by CCN1 and taking it up a Notch

Treatment of critical bone defects using calcium phosphate cement and mesoporous bioactive glass providing spatiotemporal drug delivery

Calcium phosphate cements (CPC) are currently widely used bone replacement materials with excellent bioactivity, but have considerable disadvantages like slow degradation. For critical-sized defects, however, an improved degradation is essential to match the tissue regeneration, especially in younger patients who are still growing. We demonstrate that a combination of CPC with mesoporous bioactive glass (MBG) particles led to an enhanced degradation in vitro and in a critical alveolar cleft defect in rats. Additionally, to support new bone formation the MBG was functionalized with hypoxia conditioned medium (HCM) derived from rat bone marrow stromal cells. HCM-functionalized scaffolds showed an improved cell proliferation and the highest formation of new bone volume. This highly flexible material system together with the drug delivery capacity is adaptable to patient specific needs and has great potential for clinical translation.

Atomic Force Microscopy-Based Measurements of Retinal Microvessel Stiffness in Mice with Endothelial-Specific Deletion of CCN1

Vascular stiffness is an independent predictor of human vascular diseases and is linked to ischemia, diabetes, high blood pressure, hyperlipidemia, and/or aging. Blood vessel stiffening increases owing to changes in the microscale architecture and/or content of extracellular, cytoskeletal, and nuclear matrix proteins. These alterations, while best appreciated in large blood vessels, also gradually occur in the microvasculature and play an important role in the initiation and progression of numerous microangiopathies including diabetic retinopathy. Although macroscopic measurements of arterial stiffness by pulse wave velocity are often used for clinical diagnosis, stiffness changes of intact microvessels and their causative factors have not been characterized. Herein, we describe the use of atomic force microscopy (AFM) to determine stiffness of mouse retinal capillaries and assess its regulation by the cellular communication network (CCN) 1, a stiffness-sensitive gene-encoded matricellular protein. AFM yields reproducible measurements of retinal capillary stiffness in lightly fixed freshly isolated retinal flat mounts. AFM measurements also show significant changes in compliance properties of the retinal microvasculature of mice with endothelial-specific deletion of CCN1, indicating that CCN1 expression, or lack thereof, affects the mechanical properties of microvascular cells in vivo. Thus, AFM has the force sensitivity and the spatial resolution necessary to measure the local modulus of retinal capillaries in situ and eventually to investigate microvascular compliance heterogeneities as key components of disease pathogenesis.

Role of Matricellular CCN Proteins in Skeletal Muscle: Focus on CCN2/CTGF and Its Regulation by Vasoactive Peptides

The Cellular Communication Network (CCN) family of matricellular proteins comprises six proteins that share conserved structural features and play numerous biological roles. These proteins can interact with several receptors or soluble proteins, regulating cell signaling pathways in various tissues under physiological and pathological conditions. In the skeletal muscle of mammals, most of the six CCN family members are expressed during embryonic development or in adulthood. Their roles during the adult stage are related to the regulation of muscle mass and regeneration, maintaining vascularization, and the modulation of skeletal muscle fibrosis. This work reviews the CCNs proteins' role in skeletal muscle physiology and disease, focusing on skeletal muscle fibrosis and its regulation by Connective Tissue Growth factor (CCN2/CTGF). Furthermore, we review evidence on the modulation of fibrosis and CCN2/CTGF by the renin-angiotensin system and the kallikrein-kinin system of vasoactive peptides.

Aberrant regulation of retinoic acid signaling genes in cerebral arterio venous malformation nidus and neighboring astrocytes

BACKGROUND

Cerebral arterio venous malformations (AVM) are a major causal factor for intracranial hemorrhage, which result in permanent disability or death. The molecular mechanisms of AVM are complex, and their pathogenesis remains an enigma. Current research on cerebral AVM is focused on characterizing the molecular features of AVM nidus to elucidate the aberrant signaling pathways. The initial stimuli that lead to the development of AVM nidus structures between a dilated artery and a vein are however not known.

METHODS

In order to understand the molecular basis of development of cerebral AVM, we used in-depth RNA sequencing with the total RNA isolated from cerebral AVM nidus. Immunoblot and qRT-PCR assays were used to study the differential gene expression in AVM nidus, and immunofluorescence staining was used to study the expression pattern of aberrant proteins in AVM nidus and control tissues. Immunohistochemistry was used to study the expression pattern of aberrant proteins in AVM nidus and control tissues.

RESULTS

The transcriptome study has identified 38 differentially expressed genes in cerebral AVM nidus, of which 35 genes were upregulated and 3 genes were downregulated. A final modular analysis identified an upregulation of ALDH1A2, a key rate-limiting enzyme of retinoic acid signaling pathway. Further analysis revealed that CYR61, a regulator of angiogenesis, and the target gene for retinoic acid signaling is upregulated in AVM nidus. We observed that astrocytes associated with AVM nidus are abnormal with increased expression of GFAP and Vimentin. Triple immunofluorescence staining of the AVM nidus revealed that CYR61 was also overexpressed in the abnormal astrocytes associated with AVM tissue.

CONCLUSION

Using high-throughput RNA sequencing analysis and immunostaining, we report deregulated expression of retinoic acid signaling genes in AVM nidus and its associated astrocytes and speculate that this might trigger the abnormal angiogenesis and the development of cerebral AVM in humans.

CCN1-Yes-Associated Protein Feedback Loop Regulates Physiological and Pathological Angiogenesis

Cellular communication network factor 1 (CCN1) is a dynamically expressed, matricellular protein required for vascular development and tissue repair. The CCN1 gene is a presumed target of Yes-associated protein (YAP), a transcriptional coactivator that regulates cell growth and organ size. Herein, we demonstrate that the CCN1 promoter is indeed a direct genomic target of YAP in endothelial cells (ECs) of new blood vessel sprouts and that YAP deficiency in mice downregulates CCN1 and alters cytoskeletal and mitogenic gene expression. Interestingly, CCN1 overexpression in cultured ECs inactivates YAP in a negative feedback and causes its nuclear exclusion. Accordingly, EC-specific deletion of the CCN1 gene in mice mimics a YAP gain-of-function phenotype, characterized by EC hyperproliferation and blood vessel enlargement. CCN1 brings about its effect by providing cells with a soft compliant matrix that creates YAP-repressive cytoskeletal states. Concordantly, pharmacological inhibition of cell stiffness recapitulates the CCN1 deletion vascular phenotype. Furthermore, adeno-associated virus-mediated expression of CCN1 reversed the pathology of YAP hyperactivation and the subsequent aberrant growth of blood vessels in mice with ischemic retinopathy. Our studies unravel a new paradigm of functional interaction between CCN1 and YAP and underscore the significance of their interplay in the pathogenesis of neovascular diseases.

Caught between a "Rho" and a hard place: are CCN1/CYR61 and CCN2/CTGF the arbiters of microvascular stiffness?

The extracellular matrix (ECM) is a deformable dynamic structure that dictates the behavior, function and integrity of blood vessels. The composition, density, chemistry and architecture of major globular and fibrillar proteins of the matrisome regulate the mechanical properties of the vasculature (i.e., stiffness/compliance). ECM proteins are linked via integrins to a protein adhesome directly connected to the actin cytoskeleton and various downstream signaling pathways that enable the cells to respond to external stimuli in a coordinated manner and maintain optimal tissue stiffness. However, cardiovascular risk factors such as diabetes, dyslipidemia, hypertension, ischemia and aging compromise the mechanical balance of the vascular wall. Stiffening of large blood vessels is associated with well-known qualitative and quantitative changes of fibrillar and fibrous macromolecules of the vascular matrisome. However, the mechanical properties of the thin-walled microvasculature are essentially defined by components of the subendothelial matrix. Cellular communication network (CCN) 1 and 2 proteins (aka Cyr61 and CTGF, respectively) of the CCN protein family localize in and act on the pericellular matrix of microvessels and constitute primary candidate markers and regulators of microvascular compliance. CCN1 and CCN2 bind various integrin and non-integrin receptors and initiate signaling pathways that regulate connective tissue remodeling and response to injury, the associated mechanoresponse of vascular cells, and the subsequent inflammatory response. The CCN1 and CCN2 genes are themselves responsive to mechanical stimuli in vascular cells, wherein mechanotransduction signaling converges into the common Rho GTPase pathway, which promotes actomyosin-based contractility and cellular stiffening. However, CCN1 and CCN2 each exhibit unique functional attributes in these processes. A better understanding of their synergistic or antagonistic effects on the maintenance (or loss) of microvascular compliance in physiological and pathological situations will assist more broadly based studies of their functional properties and translational value.

Abscisic acid - an anti-angiogenic phytohormone that modulates the phenotypical plasticity of endothelial cells and macrophages

Abscisic acid (ABA) has shown anti-inflammatory and immunoregulatory properties in preclinical models of diabetes and inflammation. Herein, we studied the effects of ABA on angiogenesis, a strictly controlled process that, when dysregulated, leads to severe angiogenic disorders including vascular overgrowth, exudation, cellular inflammation and organ dysfunction. By using a 3D sprouting assay, we show that ABA effectively inhibits migration, growth and expansion of endothelial tubes without affecting cell viability. Analyses of the retinal vasculature in developing normoxic and hyperoxic mice challenged by oxygen toxicity reveal that exogenously administered ABA stunts the development and regeneration of blood vessels. In these models, ABA downregulates endothelial cell (EC)-specific growth and migratory genes, interferes with tip and stalk cell specification, and hinders the function of filopodial protrusions required for precise guidance of vascular sprouts. In addition, ABA skews macrophage polarization towards the M1 phenotype characterized by anti-angiogenic marker expression. In accordance with this, ABA treatment accelerates macrophage-induced programmed regression of fetal blood vessels. These findings reveal protective functions of ABA against neovascular growth through modulation of EC and macrophage plasticity, suggesting the potential utility of ABA as a treatment in vasoproliferative diseases.

Interplay between CCN1 and Wnt5a in endothelial cells and pericytes determines the angiogenic outcome in a model of ischemic retinopathy

CYR61-CTGF-NOV (CCN)1 is a dynamically expressed extracellular matrix (ECM) protein with critical functions in cardiovascular development and tissue repair. Angiogenic endothelial cells (ECs) are a major cellular source of CCN1 which, once secreted, associates with the ECM and the cell surface and tightly controls the bidirectional flow of information between cells and the surrounding matrix. Endothelium-specific CCN1 deletion in mice using a cre/lox strategy induces EC hyperplasia and causes blood vessels to coalesce into large flat hyperplastic sinuses with no distinctive hierarchical organization. This is consistent with the role of CCN1 as a negative feedback regulator of vascular endothelial growth factor (VEGF) receptor activation. In the mouse model of oxygen-induced retinopathy (OIR), pericytes become the predominant CCN1 producing cells. Pericyte-specific deletion of CCN1 significantly decreases pathological retinal neovascularization following OIR. CCN1 induces the expression of the non-canonical Wnt5a in pericyte but not in EC cultures. In turn, exogenous Wnt5a inhibits CCN1 gene expression, induces EC proliferation and increases hypersprouting. Concordantly, treatment of mice with TNP470, a non-canonical Wnt5a inhibitor, reestablishes endothelial expression of CCN1 and significantly decreases pathological neovascular growth in OIR. Our data highlight the significance of CCN1-EC and CCN1-pericyte communication signals in driving physiological and pathological angiogenesis.