ATP7B regulates vesicular storage of copper and buffers its levels in enterocytes to maintain a range necessary for formation of lipid-transporting vesicles, researchers report in the January issue of Gastroenterology. The authors show that a misbalance of copper and lipid in the intestine could account for gastrointestinal manifestations of Wilson disease.
Wilson disease is caused by mutations in the copper transporter ATP7B. Normally, the liver filters extra copper and releases it into bile. In patients with Wilson disease, the liver does not filter copper correctly so it accumulates in the liver, brain, eyes, and other organs, causing life-threatening organ damage.
However, patients with Wilson Disease can also have defects in lipid metabolism and 41% report abdominal pain that decreases with therapy. The intestine expresses ATP7B, but little is known about intestinal copper homeostasis or to Wilson disease manifestations.
Hannah Pierson et al characterized the roles of ATP7B in intestinal organoids and tissues from control and ATP7B-knockout mice—a model of Wilson disease. Like patients, these mice accumulate copper in the liver and also have defects in lipid metabolism.
Using immunostaining, the authors found ATP7B to be highly expressed in the duodenum and jejunum, the primary site of dietary copper uptake, but not the ileum or colon. ATP7B was significantly more abundant in crypts (especially in Paneth and transient amplification cells) compared to villi. In villi, ATP7B was detected in absorptive cells (enterocytes) and was much less abundant in the mucus-producing goblet cells. In contrast, ATP7A, was uniformly expressed in the crypts and villi. These results indicated differences in copper transport by specialized intestinal cells.
In the liver, ATP7B activates the copper-dependent enzyme ceruloplasmin by transporting copper from the cytosol into the lumen of the trans-Golgi network and facilitates export of excess copper across the apical membrane by packaging excess copper into apical vesicles. Hepatic ATP7B is detected predominantly at the trans-Golgi network and traffics toward the apical membrane when levels of copper increase. Pierson et al found that in the intestine, the intracellular localization of ATP7B was significantly different. In intestinal cells expressing ATP7B, the distribution was mostly vesicular. The ATP7B-containing vesicles were spread throughout the cytosol.
Pierson et al found copper to correlate with patterns of ATP7B distribution—levels of ATP7B and copper were higher in crypts than in villi. In small intestine of Atp7b−/− mice, this uneven distribution of copper between crypt and villus was lost.
In mice with intestine-specific deletion of copper transporter CTR1, copper accumulated inenterocytes. The authors proposed that the excess copper was trapped in ATP7B-containing vesicles, and found that enlarged ATP7B vesicles did contain copper, but were located the apical side of the enterocyte villi. This indicated an altered localization of intestinal intracellular ATP7B-containing vesicles to parallel sites of copper accumulation.
Pierson et al observed the same expression patterns of ATP7A and ATP7B from tissues in 3-dimensional enteroid cultures, and again showed copper to regulate ATP7B abundance and the size of ATP7B-containing vesicles in enterocytes.
In an editorial that accompanies the article, Karl Heinz Weiss and Hans Zischka state that these findings provide evidence for a copper storage function of ATP7B in intestinal cells.
Pierson et al found the copper-dependent regulation of enteric ATP7B to differ from that in hepatocytes, consistent with the proposed role for enteric ATP7B in vesicular copper storage.
Atp7b−/− enterocytes have an altered morphology. Transmission electron microscopy experiments revealed a significant increase in the length of microvilli of Atp7b−/− enterocytes (∼2-fold longer) and increased numbers of mitochondria. Atp7b−/− enterocytes were crowded with small triglyceride-filled vesicles. The combination of increased microvilli length and mitochondrial ballooning (both associated with systemic fat deficiency), along with accumulation of lipids in enterocytes, were consistent with disruption of dietary fat acquisition.
Analyses of mice with deletion of ATP7B specifically from hepatocytes (Atp7bΔtp7) revealed that loss of ATP7B from the intestine was required for the observed lipid accumulation in enterocytes.
Chylomicrons are are lipoprotein particles that transport dietary lipids from the intestines to other locations in the body. Pierson et al measured the size of lipid vesicles that accumulated in the Atp7b−/− enterocytes and found the diameter of the triglyceride-rich vesicles was consistent with the size of endoplasmic reticulum luminal lipid droplets and pre-chylomicron transport vesicles. Furthermore, levels of triglycerides were approximately 30% higher in Atp7b−/− intestine than in controls, so there seemed to be a backup in processing of triglycerides into chylomicrons. The authors detected many chylomicrons in control duodenum and enteroids, but found none in Atp7b−/− tissues. In addition, Atp7b−/−mice had lower serum levels of triglycerides and very-low-density lipoprotein, providing evidence of disruptions in chylomicron synthesis or export.
Pierson et al propose a model for the role of ATP7B in enterocytes: after copper is transfered from the lumen of intestine into enterocytes, it is exported into the blood by ATP7A. Excess copper is stored by ATP7B in vesicles to buffer and maintain a steady concentration in the cytosol. Inactivation of ATP7B causes loss of copper homeostasis, preventing maturation nascent chylomicrons and leading to accumulation of triglycerides in the cytoplasm of enterocytes.
Peirson et al conclude that in addition to demonstrating the roles for ATP7B in enterocytes, their findings reveal important differences between the properties of hepatic and intestinal ATP7B. The observed lipid misbalance in patients with Wilson disease might originate in the intestine and then be exacerbated by copper overload in the liver. Treatments (the chelator d-penicillamine and zinc) have small inhibitory effects on chylomicron formation in enteroids. The observed interactions between copper level and lipids indicate the importance of monitoring dietary copper levels during pregnancy, levels in community water supplies, and its possible role in obesity.
Weiss and Zischka write that Pierson et al have provided a sophisticated demonstration of the tight linkage between ATP7B-dependent intestinal copper metabolism and lipid turnover. Further studies are needed of impaired lipid metabolism in Wilson disease and in patients with nutritional disorders.