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Proteolytic shedding of the prion
protein: Uncovering “new” biological
implications of a conserved cleavage
event
Feizhi Song, Valerija Kovac, Behnam Mohammadi, Josephine E. Pippi, Vladka Curin Serbec, Markus Glatzel, Hermann C. Altmeppen
2026, 21 (6):
2329-2330.
doi: 10.4103/NRR.NRR-D-25-00013
Novel insights into complex biological processes
very often critically depend on the establishment
of new potent read-out tools and improved
protocols. A lot has been learned over the past
four decades on physiological functions and,
importantly, disease-related roles of the prion
protein (PrP), a relatively broadly expressed
membrane-anchored glycoprotein with high levels
in several cell types of the nervous and immune
system and with well-established key roles in
different progressive and fatal neurodegenerative
protein misfolding diseases (proteopathies).
However, while several controversies and
unclarities remain even for these widely accepted
involvements, currently unexplored (and
unexpected) facets and functions may still wait to
be discovered. New light might be shed into these
aspects by a better understanding of potential
intrinsic roles of previously largely unconsidered
post-translationally generated forms or fragments
of PrP, for instance those resulting from
endogenous proteolytic cleavage (Mohammadi et
al., 2022; Vanni et al., 2022). In fact, membranebound
full-length (FL) PrP, the form most research
of the past has focused on, may not even represent
the majority of total PrP in the brain (Vanni et
al., 2022). A nearly FL form released from cells
by a constitutive and very membrane-proximate
proteolytic cleavage event (“shedding”) makes up
for a rather small yet relevant fraction and is of
emerging interest. This physiological, anchorless,
and predominantly double-glycosylated form,
now called “shed PrP” (sPrP), has repeatedly been
reported in the past (e.g., Parizek et al., 2001),
for instance, in cell culture media supernatants
or body fluids, yet its mechanistic origin and
biological relevance remained obscure for a
long time. The latter, to a great deal, is due to
technical challenges differentiating this fragment
from excess FL-PrP present in most biological
specimens (e.g., tissue homogenates; but even
in body fluids or cell culture supernatants, FLPrP
is present on cellular membrane debris and
physiologically released extracellular vesicles
(EVs)). Both forms are of similar molecular weight
and share structure and sequence and, hence,
epitopes for most available antibodies used for
detection in standard laboratory techniques
(Mohammadi et al., 2022). Besides recently
improved protocols to differentiate and quantify
the abundance of different PrP “proteoforms”
by immunoblotting (Vanni et al., 2022), cleavage
site-directed antibodies previously presented
for the reliable detection of rodent sPrP have
become a convenient tool to systematically, highly
specifically, and comparably comfortably assess
sPrP with various methods, as these antibodies
are “blind” for the just few amino acids (plus the
C-terminally attached glycosylphosphatidylinositol
anchor) longer FL form (Linsenmeier et al., 2021;
Mohammadi et al., 2022). However, given that
many key aspects regarding PrP shedding, such as
cleavage site and responsible protease, remained
uncharacterized for the human body, our groups
recently set out to unravel many unknowns in this
regard. We eventually succeeded in identifying
the shedding site (Y226↓Q227) in human PrP and
recently presented an in-depth characterization
of respective antibodies exclusively detecting sPrP
(Song et al., 2024). As expected from previous
mouse data, we revealed that the shedding of
human PrP is likewise strictly dependent on the
metalloprotease ADAM10, and we did not find any
evidence for alternative proteolytic cleavages in the vicinity of Y226 (Song et al., 2024).
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