Mannose receptor targeting by mannosylated liposomes has been demonstrated for a variety of mannosylated lipid conjugates in a variety of liposome morphologies and compositions in several different in vitro and in vivo models. A very large number of publications is about using a hydrophobic derivative of mannose (4-aminophenyl alpha-D-mannopyranoside) rather than using a mannosylated lipid in clodronate liposomes. This is mainly due to the high cost and complexity of synthesizing and conjugating mannose to lipid. 4-aminophenyl alpha-D-mannopyranoside is commercially available and far less expensive than synthesizing mannose conjugated lipid.

Why mannose? Mannose is one of the carbohydrate components of many bacterial and viral cell surfaces; therefore, the ever-efficient, highly redundant immune system has evolved multiple mechanisms for identifying pathogens based on mannose recognition. The animal and plant kingdoms likewise utilize carbohydrate recognition signaling mechanisms including mannose residues. Many publications evaluate other carbohydrates as targeting mechanisms for various cell types, however mannose targeting to phagocytes appears to be one of the more specific mechanisms identified to date. Mammalian cell surface identification molecules based on mannose binding, such as the ICAM family of leukocyte adhesion molecules, target the SIGN family of mannose receptors to accomplish self-recognition in vivo.

A well-known and cited study by Umezawa & Eto [1] demonstrates that liposomes containing aminophenyl mannoside were most efficiently incorporated into the mouse brain across the blood brain barrier. The radiolabeled liposomes bearing aminophenyl-alpha-D-mannopyranoside were maximally incorporated into the mouse brain after 48 hours, whereas in the spleen and liver, these radioactivities were maximum after 12 hours. The studies also showed that liposomes were most incorporated was glial cells rather than neuronal cell. The subcellular fractionation study indicates that mannose labeled liposomes are incorporated into lysosomes rich fraction both in liver and brain.

There are five mannosylated fluorescent control liposome products (m-Fluoroliposome®) for m-Clodrosome® (mannosylated clodronate liposomes). All five mannosylated fluorescent liposomes incorporate a lipophilic dye inside their membranes. They are insoluble in water; however, their fluorescence is easily detected when incorporated into membranes. DiI, DiO, DiD, DiR and DiA cover a wide range of excitation and emission wavelengths from 300s to 900s. DiI and DiO have fluorescence excitation and emission maxima separated by about 65 nm, facilitating two-color labeling. The emission spectrum of DiA is very broad, allowing it to be detected as green, orange, or even red fluorescence depending on the optical filter used. DiI, DiO, DiD and DiR belong to the dialkylcarbocyanines family of compounds. The spectral properties of the dialkylcarbocyanines are largely independent of the lengths of the alkyl chains but are instead determined by the heteroatoms in the terminal ring systems and the length of the connecting bridge. They have extremely high extinction coefficients, moderate fluorescence quantum yields, and short excited state lifetimes in lipid environments (~1 ns). The fluorescence spectrum of each dye is shown below.

You can choose the m-Fluoroliposome® based on the type of the fluorescent equipment and filters that you use in your lab. Mannosylated clodronate liposomes cannot be made fluorescent simply due to the potential for inaccurate and/or uninterpretable data being generated by labelled m-Clodrosome®. For more information, please refer to the technical note section.