"Yin and Yang" for Notch signaling in the mature vasculature.

Notch signaling is central to a number of vital developmental processes, as evidenced by the lethality of global knockout of a number of Notch proteins (5). Notch signaling requires both a membrane-bound ligand [e.g., Jagger1, Jagger2, delta-like protein (Dll)1, Dll3, or Dll4] and membrane-bound receptor (e.g., Notch1, Notch2, Notch3, or Notch4) on a neighboring cell. The role of these signaling cascades has long been understood to regulate the proliferation, migration, and differentiation of these cells during both development and pathophysiological states, such as injury-induced angiogenesis. Indeed, angiogenesis can be alternatively regulated, depending on whether Jagger1 or DII4 is the dominant ligand (7). However, we are just beginning to understand the vital role for Notch signaling independent of developmental requirements, e.g., in vascular tone of mature arteries. In arteries, Notch signaling between smooth muscle cells (SMCs) and endothelial cells (ECs), where ligands and receptors from homotypic and heterotypic cells can interact and induce both canonical and noncanonical Notch-dependent signaling cascades, may augment or limit the contractility of SMCs (1). Once more, the expression patterns of Notch receptors and ligands differ between SMCs and ECs, enabling unique signaling mechanisms in a directional manner. This could conceivably allow for specific pathways to constrict or dilate an artery. The vasoreactivity of arteries has been found to be altered in mice with defective Notch signaling. In Notch3 knockout mice, myogenic tone was impaired in numerous vascular beds, with a concomitant decrease in RhoA/Rho kinase-dependent phosphorylation of myosin light chain (MLC) (4, 6). Interestingly, global loss of Notch3 imparts resistance to increases in vascular resistance and blood pressure, including angiotensin IIinduced hypertension and hypoxia-induced pulmonary arterial hypertension (5, 8). These data suggest a potential role of Notch3 signaling in regulating vascular resistance and thus blood pressure. Although global Notch3 knockout is one of the few nonconditional Notch protein knockout mice that are not lethal, developmental effects such as changes in structural composition of arteries may contribute to the blood pressure phenotypes. To examine the role of smooth muscle Notch signaling in vascular tone regulation, expression of dominant negative MAML1 (DNMAML1) protein was used to inhibit CSL-dependent canonical Notch signaling (SMC-DNMAML1 mice) (3). Arteries from these mice failed to vasodilate or vasoconstrict in response to receptorand nonreceptor-mediated vasoactive agents. Although baseline blood pressure was unaffected, in vivo pressor responses were blunted when canonical Notch signaling was inhibited. Notch signaling can be induced by ligands expressed in either ECs or SMCs and initiate canonical and noncanonical signaling mechanisms. In a study recently published in the American Journal of Physiology-Heart and Circulatory Physiology, Basu et al. (2) have provided convincing evidence for differential regulation of vascular tone by Jagged1 and Dll4 through the use of cell type-specific and temporal conditional deletion mice. Smooth muscle cell-specific deletion of Jagged1 resulted in an overall decrease in myosin light chain kinase (MLCK) protein content, with no changes in other proteins vital to regulation and development of SMC constriction [including myosin phosphatase target subunit 1 (MYPT1), smooth muscle myosin heavy chain, and smooth muscle actin]. This decrease in MLCK, which phosphorylates MLC (pMLC), resulted in a reduced agonist and depolarization-induced vasoconstriction due to blunted p-MLC levels. Alternatively, deletion of Dll4 from ECs resulted in reduced MYPT1 protein levels, the myosin phosphatase regulatory protein, which led to both increased vasoconstriction and impaired vasodilation. As predicted with the changes in regulatory protein levels in each of these mice, Ca sensitivity levels were differentially altered, with loss of SMC-Jagged1 reducing and loss of EC-Dll4 increasing Ca sensitivity. In isolated SMCs, Jagged1 induced a Rho kinase-dependent increase in phosphorylation of MYPT1 at sites that enable inhibition of myosin phosphatase, which shifted the sensitivity of SMCs to more vasoconstriction. Conversely, Dll4 treatment on isolated SMCs increased total levels of MYPT1 without increasing myosin phosphatase-inhibitory phosphorylation sites, thus shifting the cells toward a less vasoconstrictive status. These data indicate a possible “Yin and Yang” role for Jagged1 and Dll4 on SMC function in vascular function independent of their roles in development (Fig. 1). SMC-DNMAML1 mice, which have impaired Notch signaling in SMCs, have no reported change in overall baseline blood pressure (3). Basu et al. (2) showed that, indeed, both MLCK and MYPT1 protein levels were reduced in SMCs from SMC-DNMAML1 mice. The question remains as to whether the loss of just one of the Notch ligands results in changes in baseline blood pressure. Based on the blunted pressure changes observed in SMC-DNMAML1 mice after treatment with vasoconstrictors and changes in vasoreactivity, it would be predicted that loss of Jagged1 would result in a hypotensive baseline blood pressure, whereas loss of Dll4 would be predicted to cause an increase in baseline blood pressure. This regulation of blood pressure may represent a novel mechanism of hypertension or pulmonary hypertension development. InAddress for reprint requests and other correspondence: B. E. Isakson, Univ. of Virginia School of Medicine, PO Box 801394, Charlottesville, VA 22908 (e-mail: brant@virginia.edu). Am J Physiol Heart Circ Physiol 315: H1789–H1790, 2018; First published September 28, 2018; doi:10.1152/ajpheart.00626. 2018.