Loss of lysosomal protein NCU-G1 results in spontaneous liver fibrosis with accumulation of lipofuscin and iron in Kupffer cells

Abstract

Human kidney predominant protein, NCU-G1, is a highly conserved protein with an unknown biological function. Initially described as a nuclear protein, it was later shown to be a bona fide lysosomal integral membrane protein. To gain insight into the physiological function of NCU-G1, mice with no detectable expression of this gene were created using a gene-trap strategy, and Ncu-g1gt/gt mice were successfully characterized. Lysosomal disorders are mainly caused by lack of or malfunctioning of proteins in the endosomal-lysosomal pathway. The clinical symptoms vary, but often include liver dysfunction. Persistent liver damage activates fibrogenesis and, if unremedied, eventually leads to liver fibrosis/cirrhosis and death. We demonstrate that the disruption of Ncu-g1 results in spontaneous liver fibrosis in mice as the predominant phenotype. Evidence for an increased rate of hepatic cell death, oxidative stress and active fibrogenesis were detected in Ncu-g1gt/gt liver. In addition to collagen deposition, microscopic examination of liver sections revealed accumulation of autofluorescent lipofuscin and iron in Ncu-g1gt/gt Kupffer cells. Because only a few transgenic mouse models have been identified with chronic liver injury and spontaneous liver fibrosis development, we propose that the Ncu-g1gt/gt mouse could be a valuable new tool in the development of novel treatments for the attenuation of fibrosis due to chronic liver damage.

Lack of the Lysosomal Membrane Protein, GLMP, in Mice Results in Metabolic Dysregulation in Liver

“…Abnormal accumulation of lipids in the liver is a known consequence of dysregulation of metabolism [ 41 ]. However, in our initial report, histological analysis failed to reveal obvious steatosis in Glmp gt/gt liver [ 5 ]. Here, using a biochemical approach, we showed that Glmp gt/gt mice accumulated slightly more liver TAG than WT animals.…”

“…We have recently established that the Glmp gt/gt mice suffer from a mild chronic liver injury, as indicated by the modest increase in serum transaminase levels. However, by the age of 6 months, the sustained liver injury led to a well-established fibrosis with increased inflammation and oxidative stress [ 5 ], conditions known to disturb the liver's capacity for regulating glucose, lipid and protein homeostasis [ 18 ]. As the liver is an important organ for glucose homeostasis [ 27 ], the small decrease in resting blood glucose may be caused by increased glucose flow to the liver.…”

“…All animal experiments were reviewed and approved by the Norwegian Animal Research Authority, and performed according to national laws and regulations. Wild type (WT) and Glmp gt/gt mice [ 5 ] were housed in an approved animal facility with access to standard rodent chow and water ad libitum unless otherwise stated. Mice were 3 weeks old at the beginning of the feeding experiment.…”

“…The biological function of GLMP is unknown, but a high degree of conservation of the amino acid sequence indicates an important function [ 1 ]. Recently, we created and characterized a novel mouse model with no detectable expression of GLMP ( Glmp gt/gt mouse, formerly known as Ncu-g1 gt/gt mouse), and showed that the predominant phenotype is chronic liver injury which had developed into a well-established fibrosis by the age of 6 months [ 5 ]. Further analyses identified accumulation of iron in Kupffer cells [ 5 ], which has been shown to be associated with metabolic dysregulation [ 6 ].…”

“…Recently, we created and characterized a novel mouse model with no detectable expression of GLMP ( Glmp gt/gt mouse, formerly known as Ncu-g1 gt/gt mouse), and showed that the predominant phenotype is chronic liver injury which had developed into a well-established fibrosis by the age of 6 months [ 5 ]. Further analyses identified accumulation of iron in Kupffer cells [ 5 ], which has been shown to be associated with metabolic dysregulation [ 6 ]. Specific accumulation of iron in Kupffer cells has been shown to correlate with the severity of metabolic liver injury [ 7 ].…”