TARGETED DISRUPTION OF THE MOUSE CATION-DEPENDENT MANNOSE 6-PHOSPHATERECEPTOR RESULTS IN PARTIAL MISSORTING OF MULTIPLE LYSOSOMAL-ENZYMES

In mammalian cells two mannose 6-phosphate receptors (MPRs) are involv ed in lysosomal enzyme transport. To understand the precise function o f the cation-dependent mannose 6-phosphate receptor (CD-MPR), one alle le of the corresponding gene has been disrupted in mouse embryonic ste m cells and homozygous mice lacking this receptor have been generated. The homozygous mice appear normal, suggesting that other targeting me chanisms can partially compensate for the loss of the CD-MPR in vivo. However, homozygous receptor-deficient cells and animals clearly exhib it defects in targeting of multiple lysosomal enzymes when compared wi th wild-types. Increased levels of phosphorylated lysosomal enzymes we re present in body fluids of homozygous animals. In thymocytes from ho mozygous mice or in primary cultures of fibroblasts from homozygous em bryos, there is a marked increase in the amount of phosphorylated lyso somal enzymes that are secreted into the extracellular medium. The cul tured fibroblasts have decreased intracellular levels of multiple lyso somal enzymes and accumulate macromolecules within their endosomal/lys osomal system. Taken together, these results clearly indicate that the CD-MPR is required for efficient intracellular targeting of multiple lysosomal enzymes.