Notch Signaling Regulates Platelet-Derived Growth Factor Receptor-β Expression in Vascular Smooth Muscle Cells

Sonic Hedgehog induces Notch target gene expression in vascular smooth muscle cells via VEGF-A

OBJECTIVE

Notch, VEGF, and components of the Hedgehog (Hh) signaling pathway have been implicated in vascular morphogenesis. The role of Notch in mediating hedgehog control of adult vascular smooth muscle cell (SMC) growth and survival remains unexplored.

METHODS AND RESULTS

In cultured SMCs, activation of Hh signaling with recombinant rShh (3.5 mug/mL) or plasmid encoded Shh increased Ptc1 expression, enhanced SMC growth and survival and promoted Hairy-related transcription factor (Hrt) expression while concomitantly increasing VEGF-A levels. These effects were significantly reversed after Hh inhibition with cyclopamine. Shh-induced stimulation of Hrt-3 mRNA and SMC growth and survival was attenuated after inhibition of Notch-mediated CBF-1/RBP-Jk-dependent signaling with RPMS-1 while siRNA knockdown of Hrt-3 inhibited SMC growth and survival. Recombinant VEGF-A increased Hrt-3 mRNA levels while siRNA knockdown abolished rShh stimulated VEGF-A expression while concomitantly inhibiting Shh-induced increases in Hrt-3 mRNA levels, proliferating cell nuclear antigen (PCNA), and Notch 1 IC expression, respectively. Hedgehog components were expressed within intimal SMCs of murine carotid arteries after vascular injury concomitant with a significant increase in mRNA for Ptc1, Gli(2), VEGF-A, Notch 1, and Hrts.

CONCLUSIONS

Hedgehog promotes a coordinate regulation of Notch target genes in adult SMCs via VEGF-A.

Notch signaling regulates the differentiation of bone marrow-derived cells into smooth muscle-like cells during arterial lesion formation

Bone marrow- (BM-) derived cells can differentiate into smooth muscle-like cells (SMLC), resulting in vascular pathogenesis. However, the molecular mechanism of the differentiation remains unknown. We have recently reported that Notch signaling promotes while a Notch target HERP1 inhibit the differentiation of mesenchymal cells to SMC. During the differentiation of BM-derived mononuclear cells into smooth muscle alpha-actin (SMA)-positive cells, expression of Jagged1 and SMC-specific Notch3 was increased. Blocking Notch with gamma-secretase inhibitor prevented the induction of SMA. Wire-mediated vascular injury was produced in femoral arteries in mice transplanted with green fluorescent protein (GFP)-positive cells. Many double-positive cells for GFP/Jagged1 or GFP/Notch3 were detected in the thickened neointima. In contrast, only a few SMA-positive cells were positive for GFP in neointima where HERP1, a suppressor for Notch, were abundantly expressed. In conclusion, Notch-HERP1 pathway plays an important role in differentiation of BM-derived mononuclear cells into SMLC.

Endothelial-mural cell signaling in vascular development and angiogenesis

Mural cells are essential components of blood vessels and are necessary for normal development, homeostasis, and organ function. Alterations in mural cell density or the stable attachment of mural cells to the endothelium is associated with several human diseases such as diabetic retinopathy, venous malformation, and hereditary stroke. In addition mural cells are implicated in regulating tumor growth and have thus been suggested as potential antiangiogenic targets in tumor therapy. In recent years our knowledge of mural cell function and endothelial-mural cell signaling has increased dramatically, and we now begin to understand the mechanistic basis of the key signaling pathways involved. This is mainly thanks to sophisticated in vivo experiments using a broad repertoire of genetic technologies. In this review, we summarize the five currently best understood signaling pathways implicated in mural cell biology. We discuss PDGFB/PDGFRbeta- dependent pericyte recruitment, as well as the role of angiopoietins and Tie receptors in vascular maturation. In addition, we highlight the effects of sphingosine-1-phosphate signaling on adherens junction assembly and vascular stability, as well as the role of TGF-beta-signaling in mural cell differentiation. We further reflect recent data suggesting an important function for Notch3 signaling in mural cell maturation.

Notch and Vascular Smooth Muscle Cell Phenotype

The Notch signaling pathway is critical for cell fate determination during embryonic development, including many aspects of vascular development. An emerging paradigm suggests that the Notch gene regulatory network is often recapitulated in the context of phenotypic modulation of vascular smooth muscle cells (VSMC), vascular remodeling, and repair in adult vascular disease following injury. Notch ligand receptor interactions lead to cleavage of receptor, translocation of the intracellular receptor (Notch IC), activation of transcriptional CBF-1/RBP-Jκ–dependent and –independent pathways, and transduction of downstream Notch target gene expression. Hereditary mutations of Notch components are associated with congenital defects of the cardiovascular system in humans such as Alagille syndrome and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Recent loss- or gain-of-function studies have provided insight into novel Notch-mediated CBF-1/RBP-Jκ–dependent and –independent signaling and cross-regulation to other molecules that may play a critical role in VSMC phenotypic switching. Notch receptors are critical for controlling VSMC differentiation and dictating the phenotypic response following vascular injury through interaction with a triad of transcription factors that act synergistically to regulate VSMC differentiation. This review focuses on the role of Notch receptor ligand interactions in dictating VSMC behavior and phenotype and presents recent findings on the molecular interactions between the Notch components and VSMC-specific genes to further understand the function of Notch signaling in vascular tissue and disease.

Differential gene expression in a coculture model of angiogenesis reveals modulation of select pathways and a role for Notch signaling

Communication between endothelial and mural cells (smooth muscle cells, pericytes, and fibroblasts) can dictate blood vessel size and shape during angiogenesis, and control the functional aspects of mature blood vessels, by determining things such as contractile properties. The ability of these different cell types to regulate each other's activities led us to ask how their interactions directly modulate gene expression. To address this, we utilized a three-dimensional model of angiogenesis and screened for genes whose expression was altered under coculture conditions. Using a BeadChip array, we identified 323 genes that were uniquely regulated when endothelial cells and mural cells (fibroblasts) were cultured together. Data mining tools revealed that differential expression of genes from the integrin, blood coagulation, and angiogenesis pathways were overrepresented in coculture conditions. Scans of the promoters of these differentially modulated genes identified a multitude of conserved C promoter binding factor (CBF)1/CSL elements, implicating Notch signaling in their regulation. Accordingly, inhibition of the Notch pathway with gamma-secretase inhibitor DAPT or NOTCH3-specific small interfering RNA blocked the coculture-induced regulation of several of these genes in fibroblasts. These data show that coculturing of endothelial cells and fibroblasts causes profound changes in gene expression and suggest that Notch signaling is a critical mediator of the resultant transcription.

CADASIL: the most common hereditary subcortical vascular dementia

Cerebral autosomal dominant arteriopathy with subcortical infarct and leukoencephalopathy (CADASIL) is the most common hereditary subcortical vascular dementia. It is caused by the defective NOTCH3 gene, which encodes a transmembrane receptor; over 170 different mutations are known. The main clinical features are migraine with aura (often atypical or isolated), strokes, cognitive decline/dementia and psychiatric symptoms. Executive and organizing cognitive functions are impaired first, memory is affected late. Typical MRI findings are T2 weighted hyperintensities in temporopolar white matter and the capsula externa. Smooth muscle cells in small arteries throughout the body degenerate and vessel walls become fibrotic. In the brain, this results in circulatory disturbances and lacunar infarcts, mainly in cerebral white matter and deep gray matter. The exact pathogenesis is still open: a dominant-negative toxic effect is suggested, possibly related to Notch3 misfolding. Diagnosis is reached either by identifying a pathogenic NOTCH3 mutation or by electron microscopic demonstration of granular osmiophilic material in a (skin) biopsy. Only symptomatic treatment is available at present.