Calreticulin (CRT) is a lectin-like molecular chaperon localized in the endoplasmic reticulum. Recently, it is reported that CRT highly expressed on surface of cancer cells and it involved in promoting immunogenic cell death. We focused on possibility of cell surface CRT as a cancer marker towards immunogenic cell death, whereas there is no specific labeling compound for cell surface CRT. In this study, we report synthetic study of fluorescent labeling compound for cell surface CRT, constructing with tetrasaccharide (Glc1Man3) and cell membrane impermeable fluorescent tag.
We first synthesized trisaccharide (Glc1Man2) conjugated with hydrophobic Fmoc-group as a 1st generation compound for CRT labeling (Figure 1). The compound was designed to bind both lectin- and chaperon-domains of CRT by constructing with Glc1Man2 and the Fmoc group, respectively. The binding constant of the compound to CRT was found to show 1,000 times higher than that of the Glc1Man2 alone. This affinity is comparable to that of the natural type G1M9 glycan. However, the compact trisaccharide structure and the hydrophobic Fmoc-group might allow cell penetration. Thus, we changed the trisaccharide to tetrasaccharide (Glc1Man3) having more steric bulkiness, hydrophilicity and stronger affinity to CRT. And we also changed Fmoc group to Alexa 488 having stronger fluorescence intensity and cell impermeable feature. For 2nd generation compound synthesis, we successfully achieved preparation of the tetrasaccharide. We also examined introduction of the fluorescent tag into the tetrasaccharide.
- Y.C. Lu et al., Biomed. Res. Int., 2015, 526524.