Roles of Specialized Mucolipin-Endowed LysosomesPublic Deposited
Mucolipins are lysosomal cation channels, with high permeability to calcium ions, that are thought to regulate endolysosomal trafficking. While mucolipin 1 is expressed in all cells, mucolipin 3 is expressed in a small subset of cells (i.e. neonatal intestinal enterocytes, cochlear hair cells and marginal cells of stria vascularis). This selective co-expression of mucolipins suggests that they may contribute to these cells' unique characteristics. For example, we found that mucolipins in fact facilitated endolysosomal trafficking in the neonatal intestinal enterocytes. In adults, protein from ingested food are digested by proteases in the acidic stomach, and the resulting free amino acids are absorbed by the enterocytes. However, during suckling period, the neonatal enterocytes are unique in that they undergo high level of endocytosis to uptake maternal nutrients. In neonates, stomach is not acidic yet and intact proteins pass into the intestine where they are endocytosed by the enterocytes for intracellular digestion through lysosomes. We found mucolipins 3 and 1 were expressed at high level in the neonatal enterocytes, and subsided by weaning. Mice lacking both mucolipins, but not either one alone, had severely vacuolated enterocytes. We demonstrated that these enlarged vacuoles resulted from the endolysosomal hybridorganelles’ failure to undergo lysosomal scission after the completion of intracellular digestion of maternal milk. These enlarged vacuoles led to reduced endocytosis of nutrients which explained their growth defect, previously reported by our laboratory. We also found for the first time that human infant enterocytes also utilized this specialized mucolipin-endowed lysosomes. We reported evidence that the disruption of this specialized endolysosomal system may be the cause of necrotizing enterocolitis, a devastating pediatric intestinal disease that causes up to 80% infant mortality rate. Unlike the enterocytes in which they exhibit high level of endocytosis and lysosomal degradation, the auditory hair cells have lower level of endocytosis compared to the neonatal intestinal enterocytes. Nevertheless, hair cells’ unique feature is that they are remarkably resilient as they can survive for decades despite constant mechanical and chemical insults. The restricted co-expression in both hair cells and stria vascularis of the inner ear suggests that mucolipins may contribute to the maintenance of hearing. In fact, we found that mice lacking mucolipins 3 and 1, but not either one alone, suffered early-onset age-related hearing loss due to auditory outer hair cell death. Using mice conditionally lacking mucolipins 3 and 1 in only hair cells, but not in the stria vascularis, we found that mucolipins primarily played a role in maintaining long-term hearing in hair cells. Similar to the neonatal intestinal enterocytes lacking mucolipins, outer hair cells from these mutant mice contained vacuolated organelles. However, these enlarged organelles were lysosomes, unlike the endolysosomal hybridorganelles observed in the enterocytes of mice lacking mucolipins. Because of mucolipins co-expression in hair cells, it is reasonable to assume that the specialized mucolipin-endowed lysosomes could participate in a self-repair mechanism to recycle damaged organelles through autophagy and, therefore, promote their long-term survival. However, we found that this was not the case as shown by the normal level and distribution of autophagosomes, mitochondria and peroxisomes (organelles whose defects can lead to hair cell death and hearing loss). We found that lysosomes of outer hair cells lacking mucolipins were actually damaged themselves, becoming permeabilized, and, thus allowing toxic hydrolases, such as cathepsin D, into the cytoplasm—a known trigger of multiple cell death pathways. Here, we implicated for the first time hair cell lysosomes in age-related hearing loss, and proposed a mechanism by which toxic lysosomes could lead to outer hair cell death.