Nature Sub-Journal: Lysosomal Deletion Pathway

In a rare disease called mucolipid deposition type II, the heart and abdomen of the patient are swollen and skeletal deformities
.

Mucolipid deposition type II is a lysosomal storage disorder that can cause edema and skeletal dysplasia
in internal organs.
Children diagnosed with this genetic disorder usually die
before the age of 7.
Now, researchers at the University of Michigan have discovered a new gene linked to the disease, TMEM251, which is necessary for lysosomes
to function properly.

Lysosomes are organelles within all cells of the body (except red blood cells) that are responsible for absorbing and recycling the garbage produced by the
cells.
When the lysosome is not working properly, it cannot recycle the garbage and simply store them in
the organelles.

The team, led by Ming Li, an assistant professor of molecular, cellular, and developmental biology, found that if TMEM251 is defective, it cannot encode the pathways
in which enzymes necessary for the correct function of lysosomes move inside the lysosome.
The study was published in the journal Nature Communications
.

There are about 50 to 60 enzymes in the lysosome, which digest broken parts inside the cell and waste products
outside the cell.
Lysosomes also recycle these wastes — proteins, nucleic acids, carbohydrates and lipids — into usable substances
.
But for these enzymes to move within the lysosome, they need a signal
called the mannose-6-phosphate biosynthesis pathway (M6P).

"It's like a stamp
.
Enzymes must have this signal to enter the lysosome
.
If they don't have M6P, they can't get into the lysosome," Ming Li said
.
"So you still have lysosomes, but none of them are functional because they lack these enzymes
.
"

Ming Li's lab studies the composition of lysosomes, especially lysosomal membrane proteins
.
Lysosomes have the ability to regulate their own membrane proteins, triggering the degradation
of these proteins through a process called ubiquitination.
This process allows proteins to move from the membrane of lysosomes within the organelle for degradation
.
The researchers also wanted to understand which genes were responsible for lysosomal function and what
happens when those genes are defective.

To do this, the team used CRISPR knockout screening, knocking out every gene
in the human genome one by one at the cellular level.
The researchers were then able to study the response
in lysosomes to each gene deletion.
Specifically, the researchers are looking for genes
that may cause lysosomal degradation.

The experimental result is TMEM251
.

"So the game becomes, why is this gene so important for human health?" Why is it so important for lysosomal function?" Ming Li said
.

The team found that the TMEM251 gene encodes an enzyme that activates M6P, a pathway needed for 50 to 60 digestive enzymes in lysosomes
.
In a literature review, the researchers also found a 2021 paper describing human type II mucose-like symptoms
due to a defect in the TMEM251 gene.

"Our findings answer the molecular mechanisms of this new type of human disease
.
"

The gene TMEM251 encodes the protein that needs to activate another enzyme called GNPT, which catalyzes the M6P pathway
.
The researchers also demonstrated that TMEM251 is localized in the Golgi apparatus, a structure
that forms lysosomes.
Li said the two enzymes are localized to the Golgi apparatus, which is in line with the idea that proteins must work together to add M6P to lysosomal enzymes
.
The researchers named TMEM251 GNPT Lysis and Active Factor, or GCAF for
short.

The researchers then examined what
would happen if they knocked out the TMEM251 gene in zebrafish.
Comparing wild zebrafish to zebrafish with TMEM251 gene deletions, the researchers could see defects
in the abdomen, bone and cartilage development and heart of the zebrafish.

Co-author Xi Yang said the team also proposed a therapeutic strategy to combat this disease in
humans.
The therapy is still in a very early stage, based on what they call "enzyme replacement therapy
.
" The researchers demonstrated that if they offered an enzyme containing M6P modifications to cells lacking tmem251, the enzyme was able to filter into the cell through a process called endocytosis and be delivered to a dysfunctional lysosome
.

"We know the pathogenesis of this disease because you don't have a functional lysosome, and this knockout cell can actually use these endocytic enzymes to rebuild their lysosomes and make them function
again.
" You can make up for deficiencies, at least at the cellular level
.
”

The team recently received a grant from the National Institutes of Health to further investigate the TMEM251 gene, specifically how the TMEM251 enzyme interacts with the GNPT enzyme to promote the occurrence
of M6P.
The team also intends to describe what TMEM251 looks like
at the structural level.

Original:

GCAF(TMEM251) regulates lysosome biogenesis by activating the mannose-6-phosphate pathway