Crystal structure of the nonclassical cadherin-17 N-terminus and implications for its adhesive binding mechanism

The cadherin superfamily of calcium-dependent cell-adhesion proteins has over 100 members in the human genome. All members of the superfamily feature at least a pair of extracellular cadherin (EC) repeats with calcium-binding sites in the EC linker region. The EC repeats across family members form distinct complexes that mediate cellular adhesion. For instance, classical cadherins (five EC repeats) strand-swap their N-termini and exchange tryptophan residues in EC1, while the clustered protocadherins (six EC repeats) use an extended antiparallel `forearm handshake' involving repeats EC1-EC4. The 7D-cadherins, cadherin-16 (CDH16) and cadherin-17 (CDH17), are the most similar to classical cadherins and have seven EC repeats, two of which are likely to have arisen from gene duplication of EC1-2 from a classical ancestor. However, CDH16 and CDH17 lack the EC1 tryptophan residue used by classical cadherins to mediate adhesion. The structure of human CDH17 EC1-2 presented here reveals features that are not seen in classical cadherins and that are incompatible with the EC1 strand-swap mechanism for adhesion. Analyses of crystal contacts, predicted glycosylation and disease-related mutations are presented along with sequence alignments suggesting that the novel features in the CDH17 EC1-2 structure are well conserved. These results hint at distinct adhesive properties for 7D-cadherins.

Diagnostic value and prognostic significance of LI-cadherin and miR-378e in colorectal cancer

Expression levels of LI-cadherin and miR-378e in the serum of patients with colorectal cancer, and the diagnostic value and prognostic significance in colorectal cancer were investigated. A total of 110 patients who were diagnosed with colorectal cancer in Weihai Central Hospital, from January 2012 to November 2014, were selected and enrolled in the experimental group, and 90 healthy subjects who underwent physical examination were enrolled in the control group. The expression level of miR-378e in serum was detected by reverse transcription-quantitative PCR and the expression of LI-cadherin in serum was detected by ELISA. ROC curves of LI-cadherin and miR-378e were drawn and the sensitivity and specificity of the diagnosis were estimated. The association of the expression levels of LI-cadherin and miR-378e with the survival of the patients was analyzed. LI-cadherin and miR-378e expression levels were significantly higher in the control group than those in the experimental group (P<0.001). LI-cadherin was significantly associated with the pathogenic site, the lymphatic metastasis, depth of infiltration, degree of differentiation and clinical stage (P<0.05). The sensitivity and specificity of the LI-cadherin combined with miR-378e detection were respectively 86 and 94%; the sensitivity of miR-378e detection was the highest, as well as the specificity of the combined detection. At the end of the follow-up period, the survival rates of the patients in the LI-cadherin high-expression group and miR-378e high-expression group were significantly higher than those in the low-expression groups (P<0.05). There was a significant positive correlation between the LI-cadherin and miR-378e expression levels in both the experimental and control group (r=0.5845 and 0.6356, respectively; P<0.05). In conclusion, LI-cadherin and miR-378e are expressed at low levels in colorectal cancer, suggesting that they have a good diagnostic value for colorectal cancer and can be used as biomarkers for colorectal cancer prognosis.

Water transport through the intestinal epithelial barrier under different osmotic conditions is dependent on LI-cadherin trans-interaction

In the intestine water has to be reabsorbed from the chymus across the intestinal epithelium. The osmolarity within the lumen is subjected to high variations meaning that water transport often has to take place against osmotic gradients. It has been hypothesized that LI-cadherin is important in this process by keeping the intercellular cleft narrow facilitating the buildup of an osmotic gradient allowing water reabsorption. LI-cadherin is exceptional among the cadherin superfamily with respect to its localization along the lateral plasma membrane of epithelial cells being excluded from adherens junction. Furthermore it has 7 but not 5 extracellular cadherin repeats (EC1-EC7) and a small cytosolic domain. In this study we identified the peptide VAALD as an inhibitor of LI-cadherin trans-interaction by modeling the structure of LI-cadherin and comparison with the known adhesive interfaces of E-cadherin. This inhibitory peptide was used to measure LI-cadherin dependency of water transport through a monolayer of epithelial CACO2 cells under various osmotic conditions. If LI-cadherin trans-interaction was inhibited by use of the peptide, water transport from the luminal to the basolateral side was impaired and even reversed in the case of hypertonic conditions whereas no effect could be observed at isotonic conditions. These data are in line with a recently published model predicting LI-cadherin to keep the width of the lateral intercellular cleft small. In this narrow cleft a high osmolarity can be achieved due to ion pumps yielding a standing osmotic gradient allowing water absorption from the gut even if the faeces is highly hypertonic.

Possible roles of LI-Cadherin in the formation and maintenance of the intestinal epithelial barrier

LI-cadherin belongs to the so called 7D-cadherins, exceptional members of the cadherin superfamily which are characterized by seven extracellular cadherin repeats and a small cytosolic domain. Under physiological conditions LI-cadherin is expressed in the intestine and colon in human and mouse and in the rat also in hepatocytes. LI-cadherin was shown to act as a functional Ca²⁺-dependent adhesion molecule, linking neighboring cells and a lot of biophysical and biochemical parameters were determined in the last time. It is also known that dysregulated LI-cadherin expression can be found in a variety of diseases. Although there are several hypothesis and theoretical models concerning the function of LI-cadherin, the physiological role of LI-cadherin is still enigmatic.