9.B.247. The Mannose 6-Phosphate Receptor (M6PR) Family 

The two members of the P-type lectin family, the 46 kDa cation-dependent mannose 6-phosphate receptor (CD-MPR) and the 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR), are ubiquitously expressed throughout the animal kingdom and are distinguished from all other lectins by their ability to recognize phosphorylated mannose residues (Dahms et al. 2008). The best-characterized function of the MPRs is their ability to direct the delivery of approximately 60 different newly synthesized soluble lysosomal enzymes bearing mannose 6-phosphate (Man-6-P) on their N-linked oligosaccharides to the lysosome. In addition to its intracellular role in lysosome biogenesis, the CI-MPR, but not the CD-MPR, participates in a number of other biological processes by interacting with various molecules at the cell surface. The list of extracellular ligands recognized by this multifunctional receptor has grown to include a diverse spectrum of Man-6-P-containing proteins as well as several non-Man-6-P-containing ligands. Structural studies have provided a clearer view of how these two receptors use related, but yet distinct, approaches in the recognition of phosphomannosyl residues (Dahms et al. 2008).

The cation-dependent mannose 6-phosphate receptor (CD-MPR) is a two-pass type I membrane protein that functions to transport lysosomal enzymes displaying phosphomannosyl residues from the Golgi complex and the cell surface to the lysosome. This glycosylated protein contains three disulfide bridges in its 159-residue extracytoplasmic domain (Olson and Dahms 2018). The first TMS is N-terminal and is cleaved off, and the second is near the C-terminus of the protein. CD-M6PR is a P-type lectin that plays a crucial role in lysosomal enzyme transport, bacterial resistance, and viral entry. Liu et al. 2023 cloned and analyzed the ORF of the CD-M6PR gene from Crassostrea hongkongensis and named it ChCD-M6PR. Expression of the ChCD-M6PR gene was significantly up-regulated for a short time in response to Vibrio alginolyticus infection in the gill and hemocytes, while it was down-regulated in the gonads. The expression patterns of ChCD-M6PR also varied in other tissues. The 96 h cumulative mortality rate of Crassostrea hongkongensis infected with Vibrio alginolyticus after knockdown the ChCD-M6PR gene was significantly higher. Thus, ChCD-M6PR plays a crucial role in the immune response to Vibrio alginolyticus infection, and its tissue-specific expression patterns may be indicatitive of varied immune responses across tissues (Liu et al. 2023).