Current
Volume 4, Number 4, 2004
Contents
The Role of PrP in Health and Disease Pp.337-353
The Peripheral Nervous System and the
Pathogenesis of Prion Diseases Pp.355-359
Markus
Glatzel, Olivier Giger, Nathalie Braun and Adriano Aguzzi
Impacts and Concerns for vCJD in Blood
Transfusion: Current Status Pp.361-373
I.R.
MacGregor and C.V. Prowse
Genetic Studies in Relation to Kuru: An
Overview Pp.375-384
L.G.
Goldfarb, L. Cervenakova and D.C. Gajdusek
The Genetics of Scrapie in Sheep and Goats Pp.385-396
M.
Baylis and W. Goldmann
Molecular Mechanisms of Neurotoxicity of
Pathological Prion Protein Pp.397-403
Joaquin
Castilla, Claudio Hetz and Claudio Soto
Sphingolipids in Inflammation: Roles and
Implications Pp.405-418
B.J.
Pettus, C.E. Chalfant and Y.A. Hannun
Fluorescence Molecular Imaging of Small
Animal Tumor Models Pp.419-430
E.E.
Graves, R. Weissleder and V. Ntziachristos
Trafficking of Natural Killer Cells Pp.431-438
M.A.
Morris and K. Ley
Differential TNF-Signaling in Chronic
Inflammatory Disorders Pp.439-444
Martin
H. Holtmann and Markus F. Neurath
Abstracts
[Back to top] The Role of PrP in Health and Disease
E.
Flechsig and C. Weissmann
Transmissible
spongiform encephalopathies (TSEs) such as scrapie in sheep, bovine spongiform
encephalopathy (BSE) in cattle or Creutzfeldt-Jacob disease (CJD) and Gerstmann-Sträussler-Scheinker
syndrome (GSS) in humans, are caused by an infectious agent designated
prion.The "protein only" hypothesis states that the prion
consists partly or entirely of a conformational isoform of the normal host
protein PrPC and that the abnormal conformer, when introduced into
the organism, causes the conversion of PrPC into a likeness of
itself. Since the proposal of the “protein only” hypothesis more than
three decades ago, cloning of the PrP gene, studies on PrP knockout mice and on
mice transgenic for mutant PrP genes allowed deep insights into prion biology.
Reverse genetics on PrP knockout mice containing modified PrP transgenes was
used to address a variety of problems: mapping PrP regions required for prion
replication, studying PrP mutations affecting the species barrier, modeling
familial forms of human prion disease, analysing the cell specificity of prion
propagation and investigating the physiological role of PrP by
structure-function studies. Many questions regarding the role of PrP in
susceptibility to prions have been elucidated, however the physiological role
of PrP and the pathological mechanisms of neurodegeneration in prion diseases
are still elusive.
[Back to top] The Peripheral Nervous System and the
Pathogenesis of Prion Diseases
Markus
Glatzel, Olivier Giger, Nathalie Braun and Adriano Aguzzi
Prion diseases are
inevitably fatal neurodegenerative conditions which affect humans and a wide
variety of animals. Unlike other protein aggregation diseases such as
Alzheimer’s, Parkinson’s, and polyglutamine repeat diseases, prion diseases are
unique in that they are transmissible.Therefore, prion diseases are also called
transmissible spongiform encephalopathies.
A number of prion
diseases are caused by peripheral uptake of the infectious agent. In order to
reach their target, the central nervous system, prions enter their host,
accumulate and replicate in lymphoid organs, and eventually spread to the
central nervous system via peripheral nerves. Once the agent has reached the
central nervous system, disease progression is rapid, resulting in
neurodegeneration and death. In this article, we review the state of knowledge
on the routes of neuroinvasion used by the infectious agent in order to gain
access to the central nervous system upon entry into extracerebral sites.
[Back to top] Impacts and Concerns for vCJD in Blood
Transfusion: Current Status
I.R.
MacGregor and C.V. Prowse
The impact of vCJD
upon blood transfusion practice hinges on its lymphoreticular involvement. B
lymphocytes play a key supporting role for the capture and replication of
infectivity by follicular dendritic cells of the lymphoid tissue in animal
models of transmissible spongiform encephalopathies (TSE) and tonsils, spleen
and appendix in man can harbour vCJD infectivity, a situation not seen with the
other human TSEs. Leucodepletion of blood donations in the UK was implemented
to reduce possible vCJD transmission and preliminary data suggests that white
cell associated infectivity will be effectively removed although plasma
infectivity will not.
Blood screening
assays are under development but none yet are ready for application. The
conformation dependant immunoassay, based on differences in secondary and
tertiary structure between normal and TSE-associated abnormal prion protein,
has a sensitivity now approaching the best bioassay. Even so further
development is needed to detect the fg/ml levels likely in the event that vCJD
blood does contain abnormal prion, which is as yet unproven. Surrogate assays,
such as for erythroid associated factor, may provide additional means of
identifying donors harbouring vCJD.
Validation of
clearance of TSEs from pooled plasma products consistently demonstrates
effective removal of the agents in downscaled systems and studies comparing
vCJD, BSE and scrapie agents yield similar results.
Many approaches to
therapy are under investigation, in cell culture and animal models, targeted to
normal or abnormal prion metabolism, including chemical and immunological
interventions. Efficacy of quinacrine/chlorpromazine and pentosan polysulphate
in a clinical setting, and agents yet to be used, will be more accurately known
following recent agreement of clinical drug evaluation protocols
[Back to top] Genetic Studies in Relation to Kuru: An
Overview
L.G.
Goldfarb, L. Cervenakova and D.C. Gajdusek
Kuru is a subacute
neurodegenerative disease presenting with limb ataxia, dysarthria, and a
shivering tremor. The disease progress to complete motor and mental incapacity
and death within 6 to 24 months. Neuropathologically, a typical pattern of
neuronal loss, astrocytic and microglial proliferation, characteristic
“kuru-type” amyloid plaques, and PrP deposits in the cerebral cortex and
cerebellum are observed. Kuru is the prototype of a group of human
transmissible spongiform encephalopathies (TSEs), or “prion” diseases, that
include hereditary, sporadic and infectious forms. The latest member of this
group, the variant Creutzfeldt-Jakob disease (vCJD), linked to transmission of
bovine spongiform encephalopathy (BSE) to humans, shows features similar to
kuru. Kuru has emerged at the beginning of the 1900s in a small indigenous
population of New-Guinean Eastern Highlands, reached epidemic proportions in
the mid-1950s and disappeared progressively in the latter half of the century
to complete absence at the end of the 1990s. Early studies made infection, the
first etiologic assumption, seem unlikely and led to a hypothesis that kuru
might be a genetically determined or genetically mediated illness. After
transmissibility of kuru had been discovered and all major epidemiologic
phenomena adequately explained by the spread of an infectious agent with long
incubation period through the practice of cannibalism, the pattern of
occurrence still continued to suggest a role for genetic predisposition. Recent
studies indicate that individuals homozygous for Methionine at a polymorphic
position 129 of the prion protein were preferentially affected during the kuru
epidemic. The carriers of the alternative 129Met/Val and 129Val/Val genotypes
had a longer incubation period and thus developed disease at a later age and at
a later stage of the epidemic. Observations made during the kuru epidemic are
helpful in the understanding of the current vCJD outbreak, and vice versa
clinical and experimental data accumulated in studies of other TSE disorders
contribute to better understanding of the documented kuru phenomena.
[Back to top] The Genetics of Scrapie in Sheep and Goats
M. Baylis and W. Goldmann
Scrapie, an
invariably fatal disease of sheep and goats, is a transmissible spongiform
encephalopathy (TSE). The putative infectious agent is the host-encoded prion
protein, PrP. The development of scrapie is closely linked to polymorphisms in
the host PrP gene. The pathogenesis of most TSEs involves conversion of normal,
cellular PrP into a protease-resistant, pathogenic isoform called PrPSc.
The conversion to PrPSc involves change in secondary structure; it
is impacts on these structural changes that may link polymorphisms to disease.
Within the structured C-terminal part of PrP polymorphisms have been reported
at 15 and 10 codons of the sheep and goat PrP genes respectively. Three
polymorphisms in sheep are acutely linked to the occurrence of scrapie: A136V,
R154H and Q171R/H. These generate five commonly observed alleles: ARQ, ARR,
AHQ, ARH and VRQ. ARR and AHQ are associated with resistance; ARQ, ARH and VRQ
are associated with susceptibility. There are subtle effects of specific allele
pairings (genotypes). Generally, more susceptible genotypes have younger ages
at death from scrapie. Different strains of scrapie occur which may attack
genotypes differently. Different sheep breeds vary in the assortment of the
five alleles that they predominantly encode. The reason for this variation is
not known. Furthermore,certain genotypes may be susceptible to scrapie in some
breeds and resistant in others. The explanation is not known, but may relate to
different scrapie strains circulating in different breeds, or there may be
effects of other genes which modulate the effect of PrP.
[Back to top] Molecular Mechanisms of Neurotoxicity of
Pathological Prion Protein
Joaquin
Castilla, Claudio Hetz and Claudio Soto
Transmissible
Spongiform Encephalopathies or prion related disorders are fatal and infectious
neurodegenerative diseases characterized by extensive neuronal apoptosis and
accumulation of a misfolded form of the cellular prion protein (PrP), denoted
PrPSc. Although the mechanism of neurodegeneration and the
involvement of PrPSc is far from clear, data indicates that neuronal
apoptosis might be related to activation of several signaling pathways,
including proteasome dysfunction, alterations in prion maturation pathway and
endoplasmic reticulum (ER) stress. In this article we describe recent studies
investigating the molecular mechanism of PrPSc neurotoxicity. We
propose a model in which the key step in the pathogenesis of prion disorders,
independent on their etiology, is the alteration of ER-homeostasis due to
drastic modifications of the physicochemical properties of PrP, leading to the
activation of ER-dependent signaling pathways that controls cellular survival.
[Back to top] Sphingolipids in Inflammation: Roles and
Implications
B.J.
Pettus, C.E. Chalfant and Y.A. Hannun
Sphingolipids,
historically described as potential reservoirs for bioactive lipids, presently
define a new family of cellular mediators, joining the well-established
glycerolipid-derived mediators of signal transduction such as diacylglycerol,
phosphatidylinositides, and eicosanoids. Sphingolipid metabolism is clearly involved
in the regulation of cell growth, differentiation, and programmed cell death.
Indeed, a majority of the greater than four thousand studies conducted on
sphingolipids during the past five years were investigations of the role of
sphingolipids as cellular bioregulators. Studies spanning more than a decade
have shown multiple interactions and intersections of the sphingolipidmediated
pathways and the eicosanoid pathway. This review will discuss the emerging
mechanisms by which sphingolipids induce inflammatory responses via the
eicosanoid pathway in addition to linking previous literature on sphingolipids
and inflammation with newer findings of distinct roles for
sphingosine-1-phosphate in regulating cyclooygenase-2 and ceramide-1-phosphate
in the regulation of cytosolic phospholipase A2a.
Finally, the relationship between bioactive sphingolipids and inflammation is
discussed.
[Back to top] Fluorescence Molecular Imaging of Small
Animal Tumor Models
E.E.
Graves, R. Weissleder and V. Ntziachristos
In vivo imaging of molecular events in small animals
has great potential to impact basic science and drug development. For this
reason, several imaging technologies have been adapted to small animal
research, including X-ray, magnetic resonance, and radioisotope imaging.
Despite this plethora of visualization techniques, fluorescence imaging is
emerging as an important alternative because of its operational simplicity,
safety, and cost-effectiveness. Fluorescence imaging has recently become
particularly interesting because of advances in fluorescent probe technology,
including targeted fluorochromes as well as fluorescent “switches” sensitive to
specific biochemical events. While past biological investigations using
fluorescence have focused on microscopic examination of ex vivo, in vitro,
or intravital specimens, techniques for macroscopic fluorescence imaging are
now emerging for in vivo molecular imaging applications. This review
illuminates fluorescence imaging technologies that hold promise for small
animal imaging. In particular we focus on planar illumination techniques, also
known as Fluorescence Reflectance Imaging (FRI), and discuss its performance
and current use. We then discuss fluorescence molecular tomography (FMT), an evolving
technique for quantitative three-dimensional imaging of fluorescence in vivo.
This technique offers the promise of non-invasively quantifying and visualizing
specific molecular activity in living subjects in three dimensions.
[Back to top] Trafficking of Natural Killer Cells
M.A.
Morris and K. Ley
Natural killer
(NK) cells comprise a set of lymphocytes that is capable of mediating innate
immune responses to viral infections, malignancies, and allogeneic bone marrow grafts.
This review summarizes what is known about the mechanisms NK cells use to
arrive at their sites of action. NK cells express a wide array of adhesion
molecules including aLb2, aMb2, aXb2, and a4b1 integrins, ICAM-1, PSGL-1, and L-selectin.
Like other immune and inflammatory cells, NK cells use the blood circulation to
enter tissues and organs, which requires that they interact with the vessel
wall under flow conditions, arrest, and transmigrate. NK cells are able to
chemotax to a variety of cytokines and chemokines, including IL-12, IFN-a/b, CCL2, 3,
4, 5, 7, 8, CXCL8, and CX3CL1. In many cases, NK cells appear to
migrate towards these soluble factors without any kind of priming. These cells
also appear to distribute in secondary and tertiary lymphoid sites (i.e.,
spleen, bone marrow, liver, lung, and lymph nodes) both with and without
stimulation. In addition to their ability to move throughout the body in an
unprimed state, activated NK cells may have increased specificity in homing to
sites of inflammation. NK cells not only react to, but also produce IFN-g, TNF- a, GM-CSF, CCL3, CCL4, and CCL5, enabling them
to recruit various immune cells to sites of immune response.
[Back to top] Differential TNF-Signaling in Chronic Inflammatory Disorders
Martin H. Holtmann and Markus F. Neurath
TNF-a is a pleiotropic cytokine with strong
proinflammatory and immunomodulatory properties. TNF- a plays a critical role in many acute or
chronic inflammatory diseases and anti-TNFstrategies have proven to be
clinically effective. Two TNF-specific cell surface receptors TNF-R1 and TNF-R2
have been identified and the function of these receptors and the downstream
intracellular signal transduction pathways have been extensively studied in
vitro. For a long time TNF-R1 was considered to be the predominant mediator
of TNF-signaling, whereas TNF-R2 was ascribed only auxilliary function.
However, there is increasing clinical and experimental evidence for an
important independent role of p80 signaling in chronic inflammatory conditions.
It is conceivable that the multiple TNF-mediated chronic inflammatory disorders
differ in terms of the ligand form (soluble TNF- a
versus membrane bound TNF- a), the
receptor (TNF-R1 versus TNF-R2) and the downstream signaling cascades utilized.
The elucidation of the specific characteristics of TNF-signaling in distinct
inflammatory disorders will lead to a better understanding ot the pathogenesis
of these diseases and will be the basis for the development of more specific
and more efficient therapeutic approaches.