Reveal new mechanisms for disposing of misfolded proteins

About 30 years ago, Dr. Richard Schiffers embarked on a journey to discover why people with a rare condition known as alpha-1 antitrypsin (AAT) deficiency vary widely in the severity of liver disease. His journey led him to discover the basic basis of this condition, and unexpectedly uncovered new cellular mechanisms for disposing of misfolded proteins. The latter affects not only AAT deficiency, but also other more common conditions associated with defective protein accumulation, including neurological disorders such as Alzheimer’s disease.

AAT deficiency can occur in people who carry the AAT gene with a mutation called Z.

“I was interested in the wide range of severity of this condition, so I started studying AAT deficiency. Some people who had two copies of the Z mutation developed lung disease in their later years. Some people have liver disease, and interestingly, this condition can also occur very early. Some newborns and infants develop severe liver disease and have to live. A transplant was needed, “said Schiffers, a member of the Dan L Duncan Comprehensive Cancer Center at Baylor Medical College and a professor of pathoimmunology.

Other groups have shown that about 1 in 1,700 people have two copies of the AAT-Z gene. However, only about 17% of these newborns with AAT-Z have clinically significant liver disease, and less than 3% have progressed to life-threatening end-stage disease as infants.

One of Sifers’ first contributions was to help develop the first screening test to determine if a newborn is at risk of developing severe liver disease.

“By developing a screening method, I realized that my child was at high risk for liver disease, but I still didn’t know what was causing it,” Schiffers said.

Understanding AAT deficiency

AAT is a protein produced in the liver that is transported to the lungs through the blood and protects the lungs from damage caused by other enzymes that break down proteins in the lungs. The Z mutation produces a defective AAT protein that cannot be folded into the proper 3D conformation. Improperly folded AAT-Z proteins cannot leave the liver and therefore do not move to the lungs to protect them from destruction. This can lead to lung damage that contributes to emphysema and other lung conditions.

“While studying the disease, I noticed that AAT-Z, which should be released from the liver, is actually accumulating,” says Sifers. “This suggested that the natural placement mechanism of cells may not be working.”

Sifers and others have delved deeper into how cells process misfolded proteins. They found that cells shuttle defective proteins from the endoplasmic reticulum (ER), where they are synthesized, to the cytosol, where they are broken down into cell structures called the proteasome. The key to this process is to tag the protein for destruction.

“We have shown that removing certain sugars from proteins can flag them for degradation,” Schiffers said. “Specifically, we found that the human enzyme mannosidase Man1b1 acts like a quality control factor that mediates the removal of sugar mannose from misfolded AAT-Z proteins and promotes their degradation. . “

The AAT deficiency model has been used by many other researchers studying conditions associated with the toxic accumulation of misfolded proteins in cells. This approach accelerates the understanding of the underlying causes of these conditions and offers new opportunities for potential treatment.

Association of Man1b1 and liver disease associated with AAT deficiency in infants

Researchers knew that liver damage associated with AAT deficiency was associated with the accumulation of misfolded AAT-Z protein in the liver, but there is still no explanation for severe liver disease in infants. There was not.

In a 2009 paper, Sifers and his colleagues studied liver tissue samples from unrelated babies or children over 2 years of age who underwent liver transplantation for end-stage liver disease. They also performed genetic linkage and functional laboratory experiments with other cells cultured in the laboratory.

They showed that certain genetic modifications, single nucleotide polymorphisms that result in altered expression of the Man1b1 gene, reduce levels of the Man1b1 protein in the endoplasmic reticulum of hepatocytes.

Sifers et al. Suggested that low levels of Man1b1 impaired the liver’s ability to cope with the accumulation of misfolded AAT-Z. This accelerates reaching the permissible threshold for protein accumulation and can lead to premature liver failure.

Having a gene mutation that delays the disposal of AAT-Z and the misfolded AAT-Z protein can explain the condition of the youngest patient.

“After years of research, I found an explanation for the mystery of liver disease associated with AAT deficiency in infants, and wondered if my lab would make another major contribution in the future,” Sifers said. Says.

Man1b1 has multiple roles

As Sifers and colleagues continued to study Man1b1, they unexpectedly came across a previously unexplained role for this protein.

“By enzymatically removing the mannose group, in addition to tagging and degrading misfolded proteins, Man1b1 can also facilitate proteolysis by a separate mechanism independent of the original mechanism. I see, “said Sifers.

Sifers and his co-authors Dr. Ashlee H. Sun (now Polypus-transfection Biotechnology) and Dr. John R. Collette (postdoc in his lab) Minutes of the National Academy of Sciences, USA, the traditional enzyme lyase system is on one side of the C-terminal domain, Man1b1. In contrast, the new unconventional system is controlled by the contralateral N-terminal domain of Man1b1. Further research will reveal whether and how both systems work synergistically.

Researchers suggest that a new unconventional system may be involved in the removal of soluble protein aggregates associated with conformational diseases. For example, human Man1b1 is associated with multiple birth defects such as intellectual disability and HIV infection, and a poor prognosis for bladder cancer patients.

“Our study is a clear example of how rare disease studies can provide solutions to more common conditions.”

By investigating rare liver diseases in babies, we came across a potentially targeted pathway to prevent more common neurological disorders that occur in later years, “said Sifers.

See also: Sun AH, Collette JR, Sifers RN. The cytoplasmic tail of the human mannosidase Man1b1 contributes to quality control that is independent of the catalytic action of misfolded alpha1-antitrypsin. Proc Natl Acad Sci USA.. 2020; 117 (40): 24825. doi: 10.1073 /pnas.1919013117.

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