doi: 10.1161/CIRCRESAHA.107.157446
© 2007 American Heart Association, Inc.
Editorials |
Cell Polarization Defects in Early Heart Development
From the McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine, Baltimore Md.
Correspondence to Professor Nicholas Katsanis, Johns Hopkins University School of Medicine, McKusick-Nathans Institute of Genetic Medicine, 733 N. Broadway, BRB Suite 439, Baltimore, MD 21205. E-mail katsanis{at}jhmi.edu
See related article, pages 137–145
Key Words: PCP • cardiac tube • N-cadherin • congenital heart defects
Planar Cell Polarity defects have been associated with a range of phenotypes in vertebrates, most notably the closure of the neural tube, the orientation of stereociliary bundles in the inner ear, and the orientation of hair and fur. In a new study in this issue of Circulation Research, Phillips and colleagues present data that implicate PCP signaling in the developing mouse heart.
Characterized originally as the mechanism governing the polarity of cells in Drosophila, 25 years of genetic and molecular dissection has established Planar Cell Polarity (PCP) as a morphogenetic mechanism in embryonic development that is indispensable for the formation of the body plan. PCP can be defined broadly as a signaling cascade that involves cell/tissue polarity within the plane of the epithelium, which, in Drosophila and to a certain extent in vertebrates, is typically mediated by a small set of "core" proteins. In flies, the best known examples of PCP have been described in relation to the organization of 2 structures: the hexagonal wing cells and their connected hairs, and the omatidia of the compound eye. In each case, the asymmetric distribution of key proteins, such as Strabismus, Prickle, and Flamingo initiates a signaling cascade that passes polarizing information to cells undergoing remodeling.1 In vertebrates, classic examples of PCP events include convergent extension in zebrafish and xenopus, and neural tube closure, stereociliary bundle organization, hair follicle orientation in the skin, and eyelid closure in mammals (see ref. 2 for a recent representative review of the topic; Figure). Notably, PCP studies in vertebrates (primarily mouse and zebrafish) have unveiled a host of PCP proteins, some of which are functionally conserved with the Drosophila PCP pathway, such as members of the Frizzled (Fz) serpentine receptor family and the Disheveled family of proteins (Dvl), while others seem to have acquired novel PCP functions in the vertebrate lineage, such as the ciliary-associated BBS proteins, Inversin, Inturned and Fuzzy.2
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Circletail (Crc) and Looptail (Lp) are two mouse mutants noted for manifesting the most severe forms of neural tube defects (NTDs). Positional cloning identified the underlying genes for each of these two models as Scribble (Scrib)3 and Van gogh-like 2 (Vangl2),4,5 respectively. Importantly, the proteins encoded by both genes have been established as PCP proteins.6 Although Vangl2 has a Drosophila ortholog that is a core PCP protein, Drosophila Scribble regulates apical-basal polarity and acts as a tumor suppressor, but does not give rise to PCP phenotypes in flies.7 Both genetic and molecular lines of evidence support an interaction between Scrib and Vangl2 in vertebrates based on the observations that: (1) +/Crc,+/Lp mice give rise to NTD phenotypes equal in severity to either Crc/Crc or Lp/Lp mice5; and (2) yeast 2 hybrid and coimmunoprecipitation data indicate that both proteins interact physically through discrete PDZ binding domains.8
In this issue of Circulation Research, Phillips et al, motivated by previous studies of the same group that showed that Lp/Lp mutants display aberrant cardiac cell polarization,9 expand on the role of Scrib and PCP in promoting correct cardiac development in the Crc mouse.10 During early embryonic development (E8.5) when Scrib is first expressed in wild-type animals and localizes to the cell membrane, Crc/Crc animals lacking Scrib display disorganization of cardiomyocytes, consistent with its role as a regulator of cell adhesion as evidenced by displaced N-cadherin and ß-catenin. Importantly, the proliferation of cardiomyocytes appears normal, excluding the possibility that Scrib is acting to regulate cell proliferation. The authors go on to show that although specification of the chambers of the heart is normal in Crc/Crc mice, heart looping is incomplete, and the ventricles are smaller with abnormal trabeculation. By E13.5–E17.5, Crc/Crc mice display a constellation of additional heart defects that include abnormal arterial junctions, transposition of the great arteries, and septal defects. Intriguingly, at later gestational stages, the authors note that on analysis of cell junction markers such as laminin, N-cadherin, and alpha-actinin, cardiomyocytes recover somewhat from early cell adhesion abnormalities, suggesting that Scrib is necessary during a discrete window of time for the establishment of correct cell polarity or migration in the developing heart.
Building on the intrinsic genetic and functional link between Scrib and Vangl2 described previously in neural tube formation and cochlear organization, the authors next queried the interaction and codependence of the 2 proteins during cardiac development. Exhibiting overlapping spatiotemporal expression in wild-type E9.5 cardiomyocytes, Scrib and Vangl2 display a nonreciprocal relationship in terms of cellular localization in mutant lines; Vangl2 is not necessary for normal membrane localization of Scrib, however the absence of Scrib perturbs normal Vangl2 targeting to the cell membrane. Still, the comprehensive phenotypic analysis of +/Crc,+/Lp mice at E13.5-E17.5 provides compelling evidence for the interplay between Scrib and Vangl2 in cardiac development. Ninety percent of double heterozygotes displayed heart and arterial defects that mirrored their Crc/Crc counterparts, and of direct relevance to human cardiomyopathies, heterozygous mutations in the two PCP proteins is sufficient to produce cardiac defects independent of NTD.
The observations reported in this article raise several intriguing questions both at the cellular level, as well as at the level of tissue organization. The observations that Crc/Crc embryos exhibit abnormal size and shape of cardiomyocytes concomitant with disrupted N-cadherin localization before heart looping is suggestive of cell migration defects. These data, together with the previously known involvement of Scrib in PCP in the neural tube,3 argues favorably toward the observed cardiac tube phenotypes being causally related to PCP defects. However, the formal possibility remains that Scrib contributes to the phenotype in an apicobasal polarization capacity, interacting with PCP molecules, such as Vangl2, which is then required for PCP in the mouse heart, probably acting via RhoA/Rock1 noncanonical Wnt signaling.11 In this regard, it will be important to evaluate the behavior of downstream PCP signaling components, such as Rho-k, and c-Jun. The challenge will be to determine whether either of two signaling cascades is perturbed in a Vangl2-independent manner, thereby associating Scrib directly with PCP.12 At the same time, it will also be important to determine the expression and localization of other Scrib-interacting polarity proteins such as Lethal giant larvae (Lgl)2 and Discs large (Dlg)13 in both Lp/Lp and Crc/Crc or double heterozygotes to help determine the potential hierarchal order of recruitment of proteins to the basolateral membrane. This might in turn help us understand how the organization of that region of the cell may contribute to the cellular and morphological phenotype observed in these classes of mutants.
Likewise, organization of the apical membrane will also need to be investigated. The abnormal distribution of Z0–1 in Crc/Crc cardiomyocytes is consistent with the notion that the broad polarity of the cell is perturbed; it is intriguing to speculate that this might be causally related to a disorganized cytoskeletal network which would also be envisaged to contribute to the early heart looping defects of these animals, but further studies will be required to examine this issue.
Unanswered questions aside, the authors highlight the increasing potential for PCP proteins as candidate genes for congenital heart disease. In addition to the cardiac and outflow tract defects in Crc and Lp mice, cardiac development defects have been observed in mutants of other PCP molecules, potentially highlighting a unifying theme: Dvl2–/– mice display up to 50% embryonic lethality attributed primarily to severe cardiovascular outflow tract abnormalities.14 It will be intriguing to explore the roles, if any, of other vertebrate PCP genes and proteins in the developing heart by examining both expression of the former and the subcellular distribution of the latter during early cardiogenesis.
It is intriguing to note that Vangl2 has been localized recently to the cilium of mouse renal and respiratory epithelial cells.15 Further, some ciliary mouse mutants, including Ofd1–/–16 and Alms–/–,17 exhibit congenital heart defects that overlap with Crc/Crc mice, while at the same time some ciliopathy patients manifest a number of cardiac phenotypes that include a variety of septal defects.18 Although the initial view was that some heart defects found in ciliopathy patients might be the result of defective left-right axis specification, in light of the present findings, that notion might need to be revisited. Given that ciliary defects have now been associated with defective PCP, and that genetic interaction has been demonstrated between basal body/ciliary proteins such as bbs1, bbs4, and bbs6, and vangl2 in both mouse and zebrafish,15 the intriguing possibility arises that some of the cardiac abnormalities in ciliary mouse mutants and patients might be the outcome of a combination of both LR and PCP defects.
Finally, as the authors note, the early developmental cardiac tube phenotype of Crc/Crc mice manifests as far milder structural heart defects, probably because of the onset of rescuing pathways and processes later in heart development. These factors will be important to uncover for their intrinsic therapeutic value. Moreover, the a priori expectation that PCP defects in humans might be too severe to be compatible with life might require revision. In the same manner that mutations in the VANGL2 paralog VANGL1 have been associated with sporadic neural tube defects in humans,19 it is laudable to hypothesize that a similar phenotypic overlap will exist between genetic lesions that compromise heart PCP processes and sporadic congenital heart defects.
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Acknowledgments |
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Sources of Funding
This work was supported by a Ruth L. Kirschstein NRSA F32 DK0799541 (to E.E.D.) and NIH grants HD042601, DK072301, and DK075972 (to N.K.).
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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Related Article:
Circ. Res. 2007 101: 137-145.
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