Protein recycling through the endolysosomal system relies on molecular assemblies that interact with cargo proteins, membranes, and effector molecules. Among them, the COMMD/CCDC22/CCDC93 (CCC) complex plays a critical role in recycling events. While CCC is closely associated with retriever, a cargo recognition complex, its mechanism of action remains unexplained. Herein we show that CCC and retriever are closely linked through sharing a common subunit (VPS35L), yet the integrity of CCC, but not retriever, is required to maintain normal endosomal levels of phosphatidylinositol-3-phosphate (PI(3)P). CCC complex depletion leads to elevated PI(3)P levels, enhanced recruitment and activation of WASH (an actin nucleation promoting factor), excess endosomal F-actin and trapping of internalized receptors. Mechanistically, we find that CCC regulates the phosphorylation and endosomal recruitment of the PI(3)P phosphatase MTMR2. Taken together, we show that the regulation of PI(3)P levels by the CCC complex is critical to protein recycling in the endosomal compartment.

Cholesteryl ester storage disease (CESD) results from loss-of-function mutations in LIPA, the gene that encodes lysosomal acid lipase (LAL). Hepatomegaly and deposition of esterified cholesterol (EC) in multiple organs ensue. The present studies quantitated rates of synthesis, absorption, and disposition of cholesterol, and whole body cholesterol pool size in a mouse model of CESD. In 50-day-old lal(-/-) and matching lal(+/+) mice fed a low-cholesterol diet, whole animal cholesterol content equalled 210 and 50 mg, respectively, indicating that since birth the lal(-/-) mice sequestered cholesterol at an average rate of 3.2 mg·day(-1)·animal(-1). The proportion of the body sterol pool contained in the liver of the lal(-/-) mice was 64 vs. 6.3% in their lal(+/+) controls. EC concentrations in the liver, spleen, small intestine, and lungs of the lal(-/-) mice were elevated 100-, 35-, 15-, and 6-fold, respectively. In the lal(-/-) mice, whole liver cholesterol synthesis increased 10.2-fold, resulting in a 3.2-fold greater rate of whole animal sterol synthesis compared with their lal(+/+) controls. The rate of cholesterol synthesis in the lal(-/-) mice exceeded that in the lal(+/+) controls by 3.7 mg·day(-1)·animal(-1). Fractional cholesterol absorption and fecal bile acid excretion were unchanged in the lal(-/-) mice, but their rate of neutral sterol excretion was 59% higher than in their lal(+/+) controls. Thus, in this model, the continual expansion of the body sterol pool is driven by the synthesis of excess cholesterol, primarily in the liver. Despite the severity of their disease, the median life span of the lal(-/-) mice was 355 days.

Mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2) are the principal cause of Rett syndrome, a progressive neurodevelopmental disorder afflicting 1 in 10,000 to 15,000 females. Studies using hemizygous Mecp2 mouse models have revealed disruptions to some aspects of their lipid metabolism including a partial suppression of cholesterol synthesis in the brains of mature Mecp2 mutants. The present studies investigated whether this suppression is evident from early neonatal life, or becomes manifest at a later stage of development. We measured the rate of cholesterol synthesis, in vivo, in the brains of male Mecp2−/y and their Mecp2+/y littermates at 7, 14, 21, 28, 42 and 56 days of age. Brain weight was consistently lower in the Mecp2−/y mice than in their Mecp2+/y controls except at 7 days of age. In the 7- and 14-day-old mice there was no genotypic difference in the rate of brain cholesterol synthesis but, from 21 days and later, it was always marginally lower in the Mecp2−/y mice than in age-matched Mecp2+/y littermates. At no age was a genotypic difference detected in either the rate of fatty acid synthesis or cholesterol concentration in the brain. Cholesterol synthesis rates in the liver and lungs of 56-day-old Mecp2−/y mice were normal. The onset of lower rates of brain cholesterol synthesis at about the time closure of the blood brain barrier purportedly occurs might signify a disruption to mechanism(s) that dictate intracellular levels of cholesterol metabolites including oxysterols known to exert a regulatory influence on the cholesterol biosynthetic pathway.

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Copper is an essential transition metal for all eukaryotes. In mammals, intestinal copper absorption is mediated by the ATP7A copper transporter, whereas copper excretion occurs predominatly through the biliary route and is mediated by the paralog ATP7B. Both transporters have been shown to be actively recycled between the endosomal network and the plasma membrane by a molecular machinery known as the COMMD/CCDC22/CCDC93 or CCC complex. In fact, mutations in COMMD1 can lead to impaired biliary copper excretion and liver pathology in dogs and mice with liver-specific Commd1 deficiency recapitulating aspects of this phenotype as well. Nonetheless, the role of the CCC complex in intestinal copper absorption in vivo has not been studied, and the potential redundancy of various COMMD family members has not been tested. In this study, we examined copper homeostasis in enterocyte-specific and hepatocyte-specific Commd-deficient mice. We find that in contrast to effects in cell lines in culture, COMMD protein deficiency induces minimal changes in ATP7A in enterocytes and does not lead to altered copper levels under low or high copper diets, suggesting that regulation of ATP7A in enterocytes is not of physiologic consequence. In contrast, deficiency of any of 3 Commd genes (Commd1, 6, and 9) all result in hepatic copper accumulation under high copper diets. We find that each of these deficiencies cause destabilization of the entire CCC complex, and suggest that this might explain their shared phenotype. Overall, we conclude that the CCC complex plays an important role in ATP7B endosomal recycling and function.