Lysosomes and LDL Receptor

Lysosomes and Endocytosis
Lysosomes are membrane-enclosed cytoplasmic
organelles with a diameter of 0.05–0.5
μm. They contain awide variety of active hydrolytic
enzymes (hydrolases) such as glycosidases,
sulfatases, phosphatases, lipases,
phospholipases, proteases, and nucleases (lysosomal
enzymes) in an acid milieu (pH about 5).
Lysosomal enzymes enter a lysosome by means
of a recognition signal (mannose 6-phosphate)
and a corresponding receptor.

Receptor-mediated endocytosis and lysosome formation

Extracellular molecules to be degraded are
taken into the cell by endocytosis. First, the
molecules are bound to specific cell surface receptors
(receptor-mediated endocytosis). The
loaded receptors are concentrated in an invagination
of the plasma membrane (coated pit).
This separates from the plasma membrane and
forms a membrane-enclosed cytoplasmic compartment
(coated vesicle). Hormones, growth
factors, energy-delivering proteins, and numerous
viruses and toxins also enter cells by receptor-
mediated endocytosis (see p. 360). The cytoplasmic
lining of the vesicle consists of a network
of a trimeric protein, clathrin. The clathrin
coat is quickly lost within the cell, and an endosome
forms, which fuses with membrane vesicles
fromthe Golgi apparatus to form larger endosomal
compartments. Here, the receptors are
separated from the ligands and are returned to
the cell surface in membrane vesicles (receptor
recycling). Parts of the membrane are also reused.
The ligands are nowwithin amultivesicular
body (endolysosomes). Hydrolases (lysosomal
enzymes) are transported from the Golgi
apparatus to an endolysosome in clathrin-enclosed
vesicles after they become equipped
with a recognition signal (mannose-6-
phosphate receptor), required for uptake into
the endolysosome and for normal functioning
of the lysosome.

Mannose-6-phosphate receptors

There are two types of mannose-6-phosphate
receptor molecules, which differ in their binding
properties and their cation dependence.
They consist of either 2 or 16 extracellular
domains with different numbers of amino
acids. The cDNA of Ci-MPR (cation-independent
mannose-6-phosphate receptor) is identical
with insulinlike growth factor II (IGF-2). Thus,
Ci-MPR is a multifunctional binding protein.

Biosynthesis of the recognition signal

Two enzymes are essential for the formation of
mannose-6-phosphate recognition signals: a
phosphate transferase and a phosphoglycosidase.
The phosphate is delivered by uridine-
diphosphate-N-acetylglucosamine (UDPGlcNAc)
(uridine-5!-diphosphate-N-acetylglucosamine-
glycoprotein-N-acetylglucosaminylphosphotransferase).
A second enzyme, (Nacetylglucosamine-
1-phosphodiester-N-acetylglucosaminidase)
cleaves off the N-acetylglucosamine,
leaving the phosphate residue at
position 6 of the mannose.

Diseases Due to Lysosomal Enzyme Defects

Mutations in genes for enzymes that degrade
complex macromolecules in lysosomes (lysosomal
enzymes) lead to disease. Clinical signs
and biochemical and cellular manifestations
depend on the altered enzyme’s specificity in
lysosomal metabolism. With such an enzyme
defect, macromolecules that normallywould be
degraded are stored (storage disease). This occurs
at different rates, so that each disease has
its own characteristic course. Twelve groups of
diseases due to genetically determined disorders
of specific lysosomal function are
known, each with about three to ten individually
defined diseases.

Defective uptake of enzymes into lysosomes: I-cell disease (mucolipidosis type II)

Due to a mutation of the gene on chromosome 4
for the phosphotransferase needed to form the
mannose-6-phosphate recognition signal (see
p. 352), hydrolases cannot be taken up into lysosomes.
Unlike normal cultured fibroblasts (1),
those of patients (2) contain numerous densely
packed cytoplasmic inclusion bodies (thus, the
name I-cell disease). The vesicular inclusions
consist of hydrolases that cannot enter the lysosomes
because the mannose-6-phosphate recognition
signal is absent. Numerous enzymes
are missing from the lysosomes, while their
concentration in other parts of the cells and in
body fluids is increased. Patients (3) show a
severe progressive clinical picture, with the first
signs usually apparent in the first half-year of
life.

Degradation of heparan sulfate by eight lysosomal enzymes

Heparan sulfate is an example of a macromolecule
that is degraded stepwise by different
lysosomal enzymes. Lysosomal enzymes are
bond-specific, not substrate-specific. Thus, they
also degrade other glycosaminoglycans, such as
dermatan sulfate, keratan sulfate, and chondroitin
sulfate (mucopolysaccharides). Specific
enzyme defects cause the mucopolysaccharide
storage diseases (see next page).
The first step in mucopolysaccharide degradation
is the removal of sulfate from the terminal
iduronate group by an iduronate sulfatase. A defect
in the gene that codes for this enzyme leads
to mucopolysaccharide storage disease type II
(Hunter). The gene is located on the X chromosome,
so that Hunter disease is transmitted by
X-chromosomal inheritance. All other mucopolysaccharidoses
are autosomal recessive. In
the next step (2), the terminal iduronate is split
off by an !-L-iduronidase. A mutation of the
gene coding for this enzyme in the homozygous
state leads to mucopolysaccharidosis (MPS)
type I (Hurler/Scheie). In the next three steps a
mutation (in the homozygous state) of a gene
coding for one of the enzymes causes mucopolysaccharidosis
type III (Sanfilippo). The
four genetically and enzymatically different
types (III-A to III-D) cannot be distinguished
clinically. MPS type VII (Sly), due to a defect of
"-glucuronidase, has a further characteristic
clinical picture.