MINIREVIEW
Therapeutic approaches for prion and Alzheimer’s diseases
Thomas Wisniewski
1,2,3
and Einar M. Sigurdsson
2,3
1 Department of Neurology, New York University School of Medicine, NY, USA
2 Department of Pathology, New York University School of Medicine, NY, USA
3 Department of Psychiatry, New York University School of Medicine, NY, USA
Introduction
Alzheimer’s disease (AD) and prion disease belong to
a category of conformational disorders showing sub-
stantial overlap in pathologic mechanism [1–3]. The
basic pathomechanism in both disorders is related to a
conformational change of normally expressed proteins:
amyloid-b (Ab) in AD and the prion protein (PrP) in
Keywords
Alzheimer’s disease; metals; mucosal
vaccination; prion; vaccine
Correspondence
T. Wisniewski, New York University School
of Medicine, Departments of Neurology,
Psychiatry and Pathology, Millhauser
Laboratories, Room HN419, 560 First
Avenue, New York, NY 10016, USA
Fax: +1 212 263 7528
Tel: +1 212 263 7993
E-mail:
(Received 9 March 2007, revised 3 May
2007, accepted 4 May 2007)
doi:10.1111/j.1742-4658.2007.05919.x

ACT, a1-antichymotrypsin; AD, Alzheimer’s disease; Ab, amyloid-b; apoE, apolipoprotein E; BBB, blood–brain barrier; BSE, bovine
spongiform encephalopathy; CAA, congophilic amyloid angiopathy; CNS, central nervous system; CWD, chronic wasting disease;
DC, dendritic cell; GSSS, Gerstmann–Stra
¨
usler–Scheinker syndrome; PrP, prion protein; sAb , soluble Ab; sCJD, sporadic CJD;
Tg, transgenic; vCJD, variant Creutzfeld–Jakob disease.
3784 FEBS Journal 274 (2007) 3784–3798 ª 2007 The Authors Journal compilation ª 2007 FEBS
prion disease (Fig. 1) [4,5]. This occurs without an
alteration in the amino-acid sequence of the proteins.
Ab is a 40–43 amino acid peptide, which, in AD, self-
assembles into toxic oligomers and fibrils that accumu-
late in the brain, forming plaques and deposits in the
walls of meningocephalic vessels [6,7]. The same pep-
tide can be detected in most physiological fluids, such
as serum or cerebrospinal fluid, where it is called sol-
uble Ab (sAb) [7]. PrP
C
(C-cellular) is a 209 amino
acid, cell membrane anchored protein expressed at
highest levels by neurons and follicular dendritic cells
of the immune system. In the setting of prion disease,
this protein undergoes a transformation to toxic PrP
Sc
(Sc-scrapie) [8–10]. Fibrillar A b and PrP
Sc
have a high
b-sheet content which renders them insoluble, resistant
to proteolytic degradation and toxic to neurons. Neu-
rological symptoms in AD and prion disease are
directly related to loss of neurons and synaptic connec-

gates, which are not associated with neuronal loss [19].
Similarly, in prion disease, extensive data points toward
the existence of an unidentified protein X actively
involved in the conversion of PrP
C
into PrP
Sc
[20].
AD and prion diseases exist as sporadic and inher-
ited illnesses. In addition, prion disease can be trans-
mitted from one subject to another. In experimental
model settings, some evidence also exists for the infec-
tivity of AD [21,22]. An important event in the patho-
mechanism of AD is thought to be reaching a critical
concentration of sAb and ⁄ or chaperone proteins in the
brain, at which point the conformational change
occurs [23]. This leads to the formation of Ab aggre-
gates, initiating a neurodegenerative cascade. In
sporadic AD, this occurs due to an age-associated
overproduction of Ab, impaired clearance from the
brain, and ⁄ or influx into the central nervous system
(CNS) of sAb circulating in the serum [24]. Inherited
forms of AD are associated with various genetic
defects, resulting in overproduction of total sAb,or
more fibrillogenic Ab 1–42 species [25].
Sporadic prionoses like sCJD are thought to result
from the spontaneous conversion of PrP
C
into PrP
Sc

CONFORMATIONAL DISORDERS
PrP
C
PrP
Sc
Mainly
Random Coil
Monomers
Non-Toxic
Alzheimer’s Disease
A
β
Plaque
Neurofibrillary
Tangle
Prionoses
Neuronal loss
Spongiform changes
Pathological
Chaperones
Metals
Fig. 1. Conversion of sAb peptide or PrP
C
to their pathological
b-sheet conformers is a key step in the pathogenesis of AD and pri-
onoses, respectively. In AD, these b-sheet rich structures consist
of oligomers, protofibrils and fibrils that form plaques within the
brain parenchyma or deposit in the cerebrovasculature. A compar-
able entity in prion diseases consists of the proteinase K resistant
scrapie form of the prion protein (PrP

organism, PrP
Sc
initially replicates within the lym-
phoreticular organs, including the spleen, lymph nodes
and tonsils, for months to years prior to neuroinvasion
and the onset of neurological symptoms. Therefore,
infected but asymptomatic individuals are a reservoir
of infectious material. This occurs because PrP
C
is
expressed by follicular dendritic cells and other lym-
phoid cells [27]. Accumulation of PrP
Sc
in the lympha-
tic organs of presymptomatic humans infected with
BSE has been demonstrated by immunohistochemistry
[28]. PrP
Sc
replication is possible because it does not
elicit an immune response [29]. This is related to the
inability of the immune system to distinguish between
PrP
C
and PrP
Sc
.
Vaccination approaches for AD
Vaccination was the first treatment approach demon-
strated to have genuine impact on disease process, at
least in animal models of AD. Vaccination of AD

cytotoxicity [41]. Not all patients who received
AN1792 responded with antibody production. The
majority mounted a humoral response and showed a
modest but statistically significant cognitive benefit
demonstrated as an improvement on some cognitive
testing scales compared to baseline and a slowed rate
of disease progression compared to patients who did
not form antibodies [42]. The follow-up data from the
‘Zurich’s cohort’, who are a subset of the Elan ⁄ Wyeth
trial followed by Dr Nitsch’s group [42,43], indicated
that the vaccination approach may be beneficial for
human AD patients but that the concept of the vaccine
has to be redesigned.
It appears that a humoral response elicited by the
vaccine has at least two mechanisms of action and both
of these are thought to be involved in amyloid clearance
[44,45]. Conformational selective anti-Ab serum may
target Ab deposits in the brain [43] leading to their
disassembly [46,47] and elicit Fc mediated phagocytosis
by microglia cells. The second mechanism by which
anti-Ab serum likely prevents Ab deposition is the cre-
ation of a ‘peripheral sink’ effect, where the removal of
excess sAb circulating in the blood stream leads to sAb
being drawn out from the brain [31,34, 47,48]. This per-
ipheral sink mechanism is likely to be the dominant
means of reducing Ab peptides in the brain.
The cause(s) for the toxicity in 6% of the Elan
trial patients are not entirely known; however, from
the available clinical and limited autopsy data, it is
thought that an excessive Th-1 cell-mediated response

(congophilic amyloid angiopathy; CAA) that weakens
the blood vessel wall. Several reports have shown an
increase in microhemorrhages in different AD mouse
models following passive intraperitoneal immunization
with different monoclonal antibodies with high affinity
for Ab plaques and CAA [50–52]. The risk of micro-
hemorrhage following active immunization in animal
models has not been fully assessed. It has not been
a problem in our own active immunization studies
[34,35], but has been reported in one study [53].
Furthermore, the clinical trial data from the limited
number of autopsied cases suggests that vascular
amyloid was not being cleared and that hemorrhage
may have been increased [54–56]. In one of these
autopsies, numerous cortical bleeds, which are
typically rare in AD patients, were evident [55]. In
addition, the association of T lymphocytosis and
cuffing with the cerebral vessel Ab in these autopsies
suggests a potential role of CAA and an excessive
Th-1 response in the genesis of the inflammatory side-
effects [57]. This is an important issue because CAA is
present in virtually all AD cases, with approximately
20% of AD patients having ‘severe’ CAA [58].
Furthermore CAA is present in approximately 33% of
cognitively normal elderly, control populations [59–61].
Understanding the antigenic profile of Ab peptide,
allows engineering of modifications that favor a
humoral response and reduce the potential for a Th-1
mediated response. This approach has been termed
altered peptide ligands. Computer models have predic-

method may potentially be safer than typical active
immunization.
Mucosal vaccination can be an alternative way to
achieve a primarily humoral response. This mechanism
is based on the presence of lymphocytes in the mucosa
of the nasal cavity and of the gastrointestinal tract. This
type of response produces primarily S-IgA antibodies
but, when the antigen is coadministrated with adjuvants
such as cholera toxin subunit B or heat-labile Escheri-
chia coli enterotoxin, significant IgG titer in the serum
may be achieved [65,66]. A marked reduction of Ab bur-
den in AD Tg mice immunized this way using Ab as an
antigen has been already demonstrated [66,67]. Interest-
ingly, this type of mucosal immunization has recently
been shown to be highly effective for prion infection
[68,69,70]. This promising approach requires further
exploration, especially using nonfibrillar and nontoxic
Ab homologous peptides as an antigen. Mucosal
immunization offers a great potential advantage in that
a more limited humoral immune response can be
obtained, with little or no cell-mediated immunity.
Inhibition of Ab fibrillization
Formation of Ab fibrils and deposition of Ab in the
brain parenchyma or in the brain’s vessels occurs in
the setting of increased local Ab peptide concentra-
tions [71]. Initially, conditions do not favor aggrega-
tion of fibrils; however, once a critical nucleus has
been formed, aggregation with fast kinetics is favored.
Any available monomer can then become entrapped in
an aggregate or fibril. Several compounds, such as

pathological chaperones. This group of proteins
promotes conformational transformation at certain
concentrations by increasing the b-sheet content of
these disease specific proteins and stabilizes their
abnormal structure [89,90]. Examples of such proteins
in AD include apoE, especially its E
4
isoform [18,91],
ACT [20] or C1q complement factor [21,22]. In
their presence, the formation of Ab fibrils in a solution
of sAb monomers becomes much more efficient
[18,20]. These ‘pathological chaperone’ proteins have
been found histologically and biochemically in associ-
ation with fibrillar Ab deposits [23,89,92,93] but not in
preamyloid aggregates that are not associated with
neuronal toxicity [24,94]. Inheritance of the apoE
4
isoform has been identified as the major identified
genetic risk factor for sporadic, late-onset AD [95] and
correlates with an earlier age of onset and greater Ab
deposition, in an allele-dose-dependent manner
[19,95,96]. In vitro, all apoE isoforms can propagate
the b-sheet content of Ab peptides promoting fibril
formation [92], with apoE
4
being the most efficient
[18]. The critical dependence of Ab deposition in
plaques on the presence of apoE has also been
confirmed in AD Tg APP
V717F

half-life, have allowed us to use this peptide therapeu-
tically in the APP
K670N ⁄ M671L
⁄ PS1
M146L
double Tg
mice model. Tg mice treated with Ab12–28P for
1 month demonstrated a 63.3% reduction in Ab load
in the cortex (P ¼ 0.0043) and a 59.5% (P ¼ 0.0087)
reduction in the hippocampus comparing to age-
matched control Tg mice that received placebo
[105,106]. The treated Tg mice also had a cognitive
benefit [105,106]. No antibodies against Ab were detec-
ted in sera of treated mice; therefore, the observed
therapeutic effect of Ab12–28P cannot be attributed to
an antibody clearance response. This experiment
demonstrates that compounds blocking the interaction
between Ab and its pathological chaperones may be
beneficial for treatment of Ab accumulation in AD
[14,105,106]. Whether similar approaches can be used
for prion disease remains to be determined.
Prion disease
Interest in prion disease has greatly increased subse-
quent to the emergence of BSE in England and the
resulting appearance of vCJD in human populations.
BSE arose from the feeding of cattle with prion con-
taminated meat and bone meal products, whereas
vCJD developed following entry of BSE into the
human food chain [107,108]. Since the original report
in 1995, a total of 201 probable or confirmed cases of

cases in the UK has declined from a peak of 28 in
2000 to five cases in 2006 [107]. A complicating factor
for estimating future numbers of vCJD is the docu-
mentation of several transfusion associated cases.
These occurred after incubation periods of 6–8 years.
One of these disease associated donations was made
more than 3 years before the donor became sympto-
matic, suggesting that vCJD can be transmitted from
silently infected individuals [110]. The estimated risk
for new cases of vCJD in other European countries
looks more optimistic. In the UK, 200 000 cases of
BSE were reported (it is estimated that four times this
number entered the food chain), compared to approxi-
mately 5600 BSE cases in other European countries
(with the highest numbers being 1590, 1030 and 986 in
Ireland, Portugal and Frances, respectively). This sug-
gests a significantly lower exposure of these popula-
tions to BSE prions. A few cases of BSE have also
been reported in other parts of the world, such as
Japan, the USA and Canada.
Of greater concern in North America is chronic wast-
ing disease (CWD). This disease is now endemic in
Colorado, Wyoming and Nebraska and continues to
spread to other parts in the USA, initially in the Mid-
west but now detected as far East as New York State
[111,112]. Most vulnerable to CWD infection are white
tailed deer and the disease is now found in areas with a
large population of these animals, which indicates that
its prevalence can be expected to increase substantially
in the future. The occurrence of CJD among three

Lymphatic organs such as the spleen, tonsils, lymph
nodes or gut associated lymphoid tissue contain high
concentrations of PrP
Sc
long before PrP
Sc
replication
starts in the brain [27,120,121]. Cells found to be par-
ticularly important for peripheral PrP
Sc
replication are
the follicular dendritic cells (DC) and the migratory
bone-marrow derived DC [121,122]. DC from infected
animals are capable of spreading the disease [122]. An
emerging therapeutic approach for prion infection is
immunomodulation [68,70,123].
Currently, there is no treatment that would arrest
and ⁄ or reverse progression of prion disease in non-
experimental settings, although many approaches have
been tried [124]. Partly due to the success in AD
models discussed above, similar experiments with
anti-PrP serum were initiated in prion infectivity cul-
ture models as well as active and passive immuniza-
tion studies in rodent models. Earlier in vivo studies
showed that infection with a slow strain of PrP
Sc
blocked expression of a more virulent fast strain of
PrP, mimicking vaccination with a live attenuated
organism [125]. In tissue culture studies, anti-PrP
serum and antigen binding fragments directed

the clinically symptomatic stages of prion infection.
Also, passive immunization would be an approach
that is too costly for animal prion diseases.
In the development of immunotherapeutic approa-
ches targeting a self-antigen, designing a vaccine avoid-
ing auto-immune related toxicity is a major concern.
The emerging data from AD targeting immunization is
that toxicity is due to excessive cell-mediated immunity
within the CNS, whereas the therapeutic response is
linked to humoral immunity. In addition, toxicity
could be partially related to the immunogen and ⁄ or to
the adjuvant used; in the human AD vaccination trial,
fibrillar Ab1–42 was used as an immunogen. This pep-
tide is well characterized to be toxic. Hence, we have
been promoting the use of nonamyloidogenic deriva-
tives as immunogens for protein conformational disor-
ders, including AD and prion disease [31,34,38]. How
significant an issue direct toxicity of the immunogen
may be for prion vaccination remains unclear. Unlike
the Ab peptide used for vaccination in AD models,
direct application of recombinant PrP has not been
shown to be toxic. However, this issue has not been
investigated as thoroughly as in the Alzheimer’s field
and remains controversial. Several lines of evidence
suggest that intracellular accumulations of PrP
Sc
pro-
mote neurodegeneration [133].
A potential ideal means of using immunomodulation
to prevent prion infection is by mucosal immunization.

live attenuated Salmonella has been extensively con-
firmed in humans and animals [136,137]. Ruminants
and other veterinary species can be effectively immun-
ized by the oral route using attenuated Salmonella,to
induce humoral mucosal responses [138,139]. We are
currently exploring ways to increase the efficacy even
further. In these studies, the mucosal IgA anti-PrP titer
correlates well with the delay or prevention of prion
infection, further supporting the importance of the
humoral response for the therapeutic effect. Salmonella
target M-cells, antigen sampling cells in the intestines,
which may also be important for uptake of PrP
Sc
[27,68,121]. Hence, this approach is more targeted
than prior vaccination studies, likely explaining the
improved efficacy. By exploring other strains of attenu-
ated Salmonella, using different bacteria or oral
adjuvants, and ⁄ or by altering the expression levels or
sequence of the PrP antigen, it is likely that the
percentage of uninfected animals can be improved.
Our recent work utilizing this approach indicates that
complete protection to clinical prion infection via an
oral route is possible. Overall, this approach holds
great promise as an inexpensive prophylactic immuno-
therapy to prevent the spread of prion disease, partic-
ularly in animals at risk and perhaps eventually in
certain high risk human populations.
Metal chelation for prion and AD
Metal chelation is emerging as an important therapeu-
tic approach for AD, which is currently in clinical trial

conversion is
complex [10,149]. We were the first to show that, sim-
ilar to studies in AD Tg models, metal chelation can
be used therapeutically [150] in prion infection. Our
studies indicated that penicillamine, a copper chelator,
prolongs the incubation period of scrapie in mice
[150]. Consistent with this observation, the presence of
copper has also been shown to stabilize the PrP
Sc
con-
formation using preformed fibrils [151–158], as well as
to induce aggregation of the prion peptide 106–126
[159]. Some tissue culture studies of prion infection
have also suggested that copper chelators are suitable
candidates for antiprion drugs [160]. However, there
are conflicting reports indicating that the interaction
between copper and PrP is likely to be quite complex.
For example, copper has been shown to inhibit the
in vitro conversion of recombinant PrP into amyloid
fibrils but, also in contrast, to enhance the protein-
ase K resistance of preformed fibrils [157]. These
findings indicate that copper may have a dual and
opposite effect on prion propagation. It may both
inhibit prion replication and prevent clearance of
potentially infectious forms of the prion protein.
Furthermore, copper treatment has also been shown to
inhibit PrP
Sc
amplification in reactions where brain
derived PrP

appears that the deleterious or beneficial role of copper
in prion infection might vary depending on which
function predominates under the distinct experimental
conditions being used. Nevertheless, it is clear that a
greater understanding of the role of metal binding in
prion infection presents a therapeutic opportunity.
Conclusions
Immunization appears to be an effective therapeutic
method for prevention of Ab deposition and cognitive
decline in AD, provided that cell-mediated auto-
immune toxicity can be avoided. The second genera-
tion AD vaccines, which are under development, are
based on nontoxic and nonfibrillar Ab homologous
peptides that are modified to eliminate the potential
for inducing cellular immunity, and elicit primarily a
humoral response. Other related approaches include
direct administration of antibodies that target Ab.
These interventions would likely favor a peripheral
sink effect, clearing soluble Ab from the blood stream
and inducing efflux of Ab from the brain. Additional
potentially synergistic therapeutic approaches for AD
would include blocking the interaction of Ab with its
‘pathological chaperones’ such as apoE, as well as use
of b-sheet breaker compounds. Immunization approa-
ches could be used for sporadic AD, familial AD, and
AD associated with Down’s syndrome. The effective-
ness of treatment would depend on its initiation early
in the disease course. Therefore, such a treatment
needs to coincide with the development of a procedure
for the detection and monitoring of Ab deposits.

T. Wisniewski and E. M Sigurdsson Therapy for prion disease and AD
FEBS Journal 274 (2007) 3784–3798 ª 2007 The Authors Journal compilation ª 2007 FEBS 3791
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