Heterophilic and homophilic cadherin interactions in intestinal intermicrovillar links are species dependent

Enterocytes are specialized epithelial cells lining the luminal surface of the small intestine that build densely packed arrays of microvilli known as brush borders. These microvilli drive nutrient absorption and are arranged in a hexagonal pattern maintained by intermicrovillar links formed by 2 nonclassical members of the cadherin superfamily of calcium-dependent cell adhesion proteins: protocadherin-24 (PCDH24, also known as CDHR2) and the mucin-like protocadherin (CDHR5). The extracellular domains of these proteins are involved in heterophilic and homophilic interactions important for intermicrovillar function, yet the structural determinants of these interactions remain unresolved. Here, we present X-ray crystal structures of the PCDH24 and CDHR5 extracellular tips and analyze their species-specific features relevant for adhesive interactions. In parallel, we use binding assays to identify the PCDH24 and CDHR5 domains involved in both heterophilic and homophilic adhesion for human and mouse proteins. Our results suggest that homophilic and heterophilic interactions involving PCDH24 and CDHR5 are species dependent with unique and distinct minimal adhesive units.

Protocadherin-7 Regulates Osteoclast Differentiation through Intracellular SET-Binding Domain-Mediated RhoA and Rac1 Activation

Protocadherin-7 (Pcdh7) is a member of the non-clustered protocadherin δ1 subgroup of the cadherin superfamily. Although the cell-intrinsic role of Pcdh7 in osteoclast differentiation has been demonstrated, the molecular mechanisms of Pcdh7 regulating osteoclast differentiation remain to be determined. Here, we demonstrate that Pcdh7 contributes to osteoclast differentiation by regulating small GTPases, RhoA and Rac1, through its SET oncoprotein binding domain. Pcdh7 is associated with SET along with RhoA and Rac1 during osteoclast differentiation. Pcdh7-deficient (Pcdh7−/−) cells showed abolished RANKL-induced RhoA and Rac1 activation, and impaired osteoclast differentiation. Impaired osteoclast differentiation in Pcdh7−/− cells was restored by retroviral transduction of full-length Pcdh7 but not by a Pcdh7 mutant that lacks SET binding domain. The direct crosslink of the Pcdh7 intracellular region induced the activation of RhoA and Rac1, which was not observed when Pcdh7 lacks the SET binding domain. Additionally, retroviral transduction of the constitutively active form of RhoA and Rac1 completely restored the impaired osteoclast differentiation in Pcdh7−/− cells. Collectively, these results demonstrate that Pcdh7 controls osteoclast differentiation by regulating RhoA and Rac1 activation through the SET binding domain.

Species-Dependent Heterophilic and Homophilic Cadherin Interactions in Intestinal Intermicrovillar Links

ABSTRACTEnterocytes are specialized epithelial cells lining the luminal surface of the small intestine that build densely packed arrays of microvilli known as brush borders. These microvilli drive nutrient absorption and are arranged in a hexagonal pattern maintained by intermicrovillar links formed by two non-classical members of the cadherin superfamily of calcium-dependent cell adhesion proteins: protocadherin-24 (PCDH24, also known as CDHR2) and the mucin-like protocadherin (CDHR5). The extracellular domains of these proteins are involved in heterophilic and homophilic interactions important for intermicrovillar function, yet the structural determinants of these interactions remain unresolved. Here we present X-ray crystal structures of the PCDH24 and CDHR5 extracellular tips and analyze their species-specific features relevant for adhesive interactions. In parallel, we use binding assays to identify the PCDH24 and CDHR5 domains involved in both heterophilic and homophilic adhesion for human and mouse proteins. Our results suggest that homophilic and heterophilic interactions involving PCDH24 and CDHR5 are species dependent with unique and distinct minimal adhesive units.

HSF2 protects against proteotoxicity by maintaining cell-cell adhesion

ABSTRACTCellular ability to maintain proper protein homeostasis (proteostasis) is essential for survival upon protein-damaging conditions. Heat shock transcription factor 2 (HSF2) is one of the human HSFs activated in response to proteotoxic stress. HSF2 is dispensable for cell survival during acute heat stress, but its amount and DNA-binding activity increase under prolonged proteotoxic stress conditions, such as proteasome inhibition. Nevertheless, the specific role(s) of HSF2 and the global HSF2-dependent gene expression profile during sustained stress have remained elusive. We found that HSF2 is required for cell survival during prolonged proteotoxicity, as shown by treating wild-type and HSF2-deficient human osteosarcoma U2OS cells with the proteasome inhibitor Bortezomib. Strikingly, our RNA-seq analyses revealed that HSF2 disruption leads to marked downregulation of cadherin superfamily genes and subsequent functional impairment of cadherin-mediated cell-cell adhesion. We propose HSF2 as a key regulator of genes belonging to the cadherin superfamily. We also demonstrate that HSF2-dependent downregulation of cadherin-mediated cell-cell adhesion predisposes U2OS cells to Bortezomib-induced proteotoxic stress. In conclusion, we show that by maintaining cell-cell adhesion HSF2 is essential for cell survival and thereby we describe a novel regime in the HSF-mediated protection against stress-induced protein damage.

DSCAM promotes self-avoidance in the developing mouse retina by masking the functions of cadherin superfamily members

During neural development, self-avoidance ensures that a neuron’s processes arborize to evenly fill a particular spatial domain. At the individual cell level, self-avoidance is promoted by genes encoding cell-surface molecules capable of generating thousands of diverse isoforms, such as Dscam1 (Down syndrome cell adhesion molecule 1) in Drosophila. Isoform choice differs between neighboring cells, allowing neurons to distinguish “self” from “nonself”. In the mouse retina, Dscam promotes self-avoidance at the level of cell types, but without extreme isoform diversity. Therefore, we hypothesize that DSCAM is a general self-avoidance cue that “masks” other cell type-specific adhesion systems to prevent overly exuberant adhesion. Here, we provide in vivo and in vitro evidence that DSCAM masks the functions of members of the cadherin superfamily, supporting this hypothesis. Thus, unlike the isoform-rich molecules tasked with self-avoidance at the individual cell level, here the diversity resides on the adhesive side, positioning DSCAM as a generalized modulator of cell adhesion during neural development.

Gene expression profile analysis of aortic vascular smooth muscle cells reveals upregulation of cadherin genes in myocardial infarction patients

Myocardial infarction (MI) induced by acute coronary arterial occlusion is usually secondary to atherosclerotic plaque rupture. Dysregulated response of vascular smooth muscle cells (VSMCs) in atherosclerotic plaques may promote plaque rupture. Cadherins (CDHs) form adherens junctions and are known stabilizers of atherosclerotic plaques. To date, the expression patterns of cadherin have not been well investigated in MI aortic VSMCs. We aimed to investigate the expression of cadherin genes in the aortic wall of patients with and without MI. Laser capture microdissected VSMCs were obtained from aortic tissue samples of patients undergoing coronary artery bypass graft surgery. Integrative bioinformatic analysis of the microarray profiles of the VSMCs revealed that MI is discriminated at the whole transcriptome level by hundreds of differentially expressed genes, including genes involved in cell adhesion, of which the cadherin superfamily genes were among the top structural category. Eleven significantly deregulated candidates of the cadherin superfamily were chosen and formed a new classifier that collectively discriminated MI vs. non-MI with ~95% accuracy. Significance validation was performed with an independent cohort by quantitative RT-quantitative PCR, confirming overexpression of CDH2, CDH12, PCDH17, and PCDH18 in MI VSMCs. The dysregulation of these cadherin superfamily genes might be related to an MI-induced remote effect on aortic wall VSMCs and to imbalances in signaling pathways and myocardial repair mechanisms. Although pathophysiological significance of our findings requires functional studies, mRNA upregulation of the identified cadherin superfamily members in VSMCs might be associated with the progression of atherosclerosis and angiogenesis activation in MI.

Pcdh18a-positive tip cells instruct notochord formation in zebrafish

The notochord defines the axial structure of all vertebrates during development. Notogenesis is a result of major cell reorganization in the mesoderm, the convergence and the extension of the axial cells. However, it is currently not known how these processes act together in a coordinated way during notochord formation. Analysing the tissue flow, we determined the displacement of the axial mesoderm and identified, relative to the ectoderm, an actively-migrating notochord tip cell population and a group of trailing notochordal plate cells. Molecularly, these tip cells express Protocadherin18a, a member of the cadherin superfamily. We show that Pcdh18a-mediated recycling of E-cadherin adhesion complexes transforms these tip cells into a cohesive and fast migrating cell group. In turn, these tip cells subsequently instruct the trailing mesoderm. We simulated cell migration during early mesoderm formation using a lattice-based mathematical framework, and predicted that the requirement for an anterior, local motile cell cluster could guide the intercalation of the posterior, axial cells. Indeed, grafting experiments validated the predictions and induced ectopic notochord-like rods. Our findings indicate that the tip cells influence the trailing mesodermal cell sheet by inducing the formation of the notochord.

Rotating for elongation: Fat2 whips for the race

Dynamic rearrangements of the actin cytoskeleton are crucial for cell shape and migration. In this issue, Squarr et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201508081) show that the cadherin superfamily protein Fat2 regulates actin-rich protrusions driving collective cell migration during Drosophila melanogaster egg morphogenesis through its interaction with the WAVE regulatory complex.