Clustered protocadherin cis-interactions are required for combinatorial cell-cell recognition underlying neuronal self-avoidance.

In the developing human brain, only 53 stochastically expressed clustered protocadherin (cPcdh) isoforms enable neurites from individual neurons to recognize and self-avoid while simultaneously maintaining contact with neurites from other neurons. Cell assays have demonstrated that self-recognition occurs only when all cPcdh isoforms perfectly match across the cell boundary, with a single mismatch in the cPcdh expression profile interfering with recognition. It remains unclear, however, how a single mismatched isoform between neighboring cells is sufficient to block erroneous recognitions. Using systematic cell aggregation experiments, we show that abolishing cPcdh interactions on the same membrane (cis) results in a complete loss of specific combinatorial binding between cells (trans). Our computer simulations demonstrate that the organization of cPcdh in linear array oligomers, composed of cis and trans interactions, enhances self-recognition by increasing the concentration and stability of cPcdh trans complexes between the homotypic membranes. Importantly, we show that the presence of mismatched isoforms between cells drastically diminishes the concentration and stability of the trans complexes. Overall, we provide an explanation for the role of the cPcdh assembly arrangements in neuronal self/non-self-discrimination underlying neuronal self-avoidance.

On the formation of ordered protein assemblies in cell-cell interfaces.

Crystal structures of many cell-cell adhesion receptors reveal the formation of linear "molecular zippers" comprising an ordered one-dimensional array of proteins that form both intercellular (trans) and intracellular (cis) interactions. The clustered protocadherins (cPcdhs) provide an exemplar of this phenomenon and use it as a basis of barcoding of vertebrate neurons. Here, we report both Metropolis and kinetic Monte Carlo simulations of cPcdh zipper formation using simplified models of cPcdhs that nevertheless capture essential features of their three-dimensional structure. The simulations reveal that the formation of long zippers is an implicit feature of cPcdh structure and is driven by their cis and trans interactions that have been quantitatively characterized in previous work. Moreover, in agreement with cryo-electron tomography studies, the zippers are found to organize into two-dimensional arrays even in the absence of attractive interactions between individual zippers. Our results suggest that the formation of ordered two-dimensional arrays of linear zippers of adhesion proteins is a common feature of cell-cell interfaces. From the perspective of simulations, they demonstrate the importance of a realistic depiction of adhesion protein structure and interactions if important biological phenomena are to be properly captured.

Honey increased saliva, plasma, and urine content of total nitrite concentrations in normal individuals.

This study investigated effects of oral honey solution on total nitrite, a stable nitric oxide metabolite, in saliva, plasma, and urine samples collected from normal subjects. Fourteen adult healthy volunteers, 25-50 years old, nine males and three females, were enrolled in the study. Total nitrite was estimated in saliva, plasma, and urine after 14 hours of food fasting. Each subject was then asked to drink honey solution (80 g of raw honey dissolved in 250 mL of water). Saliva and blood samples were collected at 1, 2, and 3 hours after ingestion of honey solution for total nitrite assay, while urine samples were collected after 3 hours for total nitrite assay. The mean total fasting nitrite in saliva was 108 +/- 61.3 micromol/L, which was increased to 130 +/- 62.9, 131.2 +/- 59, and 135.1 +/- 64.3 micromol/L at 1, 2, and 3 hours, respectively. Plasma total nitrite was 22.41 +/- 16.22 micromol/L before drinking honey, which was increased to 34.71 +/- 18.13, 29.38 +/- 14.29, and 33 +/- 13.09 micromol/L at 1, 2, and 3 hours, respectively, after drinking honey. Urine total nitrite before drinking honey was 75.8 +/- 54.79 micromol/L, which was increased to 107.8 +/- 70.83 micromol/L 3 hours after ingestion of honey solution. Although not statistically significant, honey solution showed a tendency to increase total nitrite concentration in different biological fluids from humans, including saliva, plasma, and urine.