Cadherin-mediated cell-cell adhesion and signaling in the skeleton

Abstract

Direct cell-to-cell interactions via cell adhesion molecules, in particular cadherins, are critical for morphogenesis, tissue architecture, and cell sorting and differentiation. Partially overlapping, yet distinct roles of N-cadherin (cadherin-2) and cadherin-11 in the skeletal system have emerged from mouse genetics and in vitro studies. Both cadherins are important for precursor commitment to the osteogenic lineage, and genetic ablation of Cdh2 and Cdh11 results in skeletal growth defects and impaired bone formation. While Cdh11 defines the osteogenic lineage, persistence of Cdh2 in osteoblasts in vivo actually inhibits their terminal differentiation and impairs bone formation. The action of cadherins involves both cell-cell adhesion and interference with intracellular signaling, and in particular the Wnt/β-catenin pathway. Both cadherin-2 and cadherin-11 bind to β-catenin, thus modulating its cytoplasmic pools and transcriptional activity. Recent data demonstrate that cadherin-2 also interferes with Lrp5/6 signaling by sequestering these receptors in inactive pools via axin binding. These data extend the biologic action of cadherins in bone forming cells, and provide novel mechanisms for development of therapeutic strategies aimed at enhancing bone formation.

Figure 1. Homophilic cadherin interactions promote cell-cell adhesion and osteoblast differentiation during the early stages of bone formation

Mesenchymal stem cells express many cadherins at low abundance. Both Cdh2 and Cdh11 increase with commitment to the osteogenic lineage, but as differentiation progresses Cdh2 expression decreases whereas Cdh11 persits. Terminally differentiated bone forming cells loose cadherin-2 and cell-cell adhesion; many undergo apoptosis, a few become osteocytes, matrix embedded cells with very limited physical contacts with other cells. Cdh2 is also up-regulated during early steps of chondrogenic differentiation, whereas Cdh11 is not present in condensed growth plate chondrocytes. Differentiated chondrocyte do not express either cadherin. Adipogenic differentiation is associated with loss of both Cdh2 and Cdh11. Indeed, Cdh11 inhibits adipogenesis, while favoring chondro-osteogenesis.

Figure 2. Interaction between cadherins and Wnt/β-catenin signaling

Cadherin cis-dimers are connected to the actin cytoskeleton via binding to β-catenin and γ-catenin (plakoglobin), which in turn bind α-catenin and throough it, actin. Cytoplasmic β-catenin – not bound to cadherins – can translocate to the nucleus to induce gene transcription, and its levels are kept in homeostatic balance by glucose synthase kinase-3β (GSK3β) phosphorylation and targeting for ubiquitination and proteosomal degradation. GSK3β is inhibited by binding to an APC/axin/GSK3β complex, which forms in response to Wnt activation of Frizzled receptors. Since β-catenin bound to cadherins is kept away from transcriptionally active pools, increased cadherin expression results in β-catenin sequestration at the plasma membrane and reduced β-catenin availability for transcriptional activation.

Figure 3. Cadherin-2 is a negative regulator of full osteoblast differentiation

Cadherin-2 (via the 62 amino-acids at the C-terminus) interacts with the Wnt co-receptors LRP5/6 via binding to axin through its C-terminus domain. Such interaction prevents activation of β-catenin, ERK1/2 and PI3K/Akt signalling, resulting in inhibition of cell proliferation, reduced osteoblast differentiation and survival (left panel). Disruption of cadherin-2-axin-LRP5/6 interaction by a small competitor peptide (28AA) removes this inhibitory interaction on LRP5/6, thus allowing activation of signalling cascades leading to cell proliferation, osteogenic differentiation and bone formation (right panel).