Prion Disease Pathogenesis - Recent Advances in Microbiology - page 70

DISCUSSION
The results presented in this report demonstrate an unprecedented role of PrP in facili-
tating iron uptake by cells and its transport to cellular ferritin. Using a combination
of neuroblastoma cell lines expressing normal and mutant PrP forms, we demonstrate
that over-expression of PrP C increases intracellular LIP and the amount of iron depos-
ited in ferritin. Pathogenic and non-pathogenic mutations of PrP over-expressed to the
same extent as PrP C alter cellular LIP and ferritin iron content differentially, specific to
the mutation. Certain cell lines, especially cells expressing anchorless PrP 231stop , dem-
onstrate increased LIP in the presence of decreased ferritin iron, while PrP 102L -cells
display low LIP in the presence of adequate ferritin iron. Furthermore, stimulation of
endocytosis by PrP specific antibody increases ferritin iron, while cross-linking at the
plasma membrane increases LIP but has minimal effect on ferritin iron, indicating that
alteration of PrP function or cellular localization disturbs the homeostasis between
ferritin iron and cellular LIP. The differential incorporation of iron by mutant cell lines
is maintained in the presence of excess extra-cellular iron, demonstrating a dominant
role of PrP C in iron uptake and transport. The positive effect of PrP C on cellular iron is
mainly due to enhanced uptake since the amount released into the culture medium is
not altered in any of the cell lines tested. Together, these observations suggest a role for
PrP C in mediating iron uptake and transport to ferritin directly, or by interacting with
other iron modulating proteins. Below we discuss these data with reference to possible
functions of PrP C in cellular iron metabolism, and the implications thereof in inducing
imbalance in iron homeostasis observed in prion disease affected brains [15, 16, 45].
It is surprising that a GPI-linked protein such as PrP C is involved in iron trans-
port to ferritin since PrP C is a membrane protein that undergoes vesicular transport
while ferritin is cytosolic [29]. Normally, cellular iron uptake is mediated by the Tf/
TfR dependent and independent pathways, the former being most prominent and well
characterized especially in neuroblastoma cells. In the Tf/TfR dependent pathway,
ferric iron captured by Tf is taken up by the cells through TfR-mediated uptake via
clathrin coated pits. Tf-bound ferric iron is released in the acidic environment of the
endosomes, reduced to ferrous iron by an endosomal ferric reductase Steap3, and
transported across the endosomal membrane by DMT1 to cytosolic ferritin where it is
oxidized to the fairly inert ferric form by ferritin H-chain and stored [18, 19, 29]. In the
Tf-independent pathway, iron is taken up by an unknown transport mechanism, possi-
bly non-specifi cally by fl uid phase of endocytosis, and stored in ferritin. Ferritin regu-
lates the biologically available LIP in the cell, and is itself regulated by iron regulatory
proteins (IRPs) 1 and 2 [18, 19, 30, 31]. In neuroblastoma cells, the LIP is a function
of total cellular iron, and an increase in cellular iron is accompanied by increased fer-
ritin content to maintain the LIP within safe limits [32, 33]. Where might PrP intersect
with this tightly orchestrated mechanism of iron uptake, transport, and storage? Three
potential mechanisms are plausible: (1) modulation of uptake at the plasma membrane
independently or by interacting with the Tf/TfR dependent pathway, (2) facilitation
of iron transport to cytosolic ferritin across the endosomal membrane by promoting
ferric iron release from Tf and/or its reduction for transfer through DMT1 [19], or
(3) assistance in deposition into ferritin by oxidizing ferrous iron to the ferric form.