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Trail:
Ecosystems
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 Scientific
papers - Ecosystems - page 2
TSEs and the misfolded metallo-glycoprotein, prion protein
Metalloproteins require specific complementary metals which
assist in their folding, catalytic and/or metabolic activities
(12). Alternative ‘foreign’ metals can sometimes substitute
for the correct metal partner during periods of deficiency. For
instance, Mn2+ or Fe2+ can also ligate onto the nitrogen of the
histidine imidazole rings of certain Cu metalloproteins (12),
resulting in the formation of misfolded isoforms which can no
longer perform their correct metabolic functions
TSEs are considered
to result from the accumulation of an abnormal protease
resistant misfolded isoform of a nerve membrane
metalloglycoprotein called prion protein (PrP) (5). PrP is
largely expressed in the CNS and lymphatic systems (5).
It is proposed that
TSE pathogenesis is initiated (Fig. 1) once Mn2+ or Mn3+
substitutes for Cu at the ‘vacated’ histidine sites of Cu
domains on PrP (15) and other cuproproteins such as the beta
amyloid precursor protein. This could occur during periods of Cu
deficiency, when PrP subsequently looses its correct
conformation, ‘metamorphosing’ into a misfolded PrP-foreign
transition metal complex, whereby Mn2+, or its oxidised Mn3+
species, serves as the ‘so called’ infectious transmissible
agent whose potentially lethal auto-oxidative capacity (7,11) is
carried along with PrP like a ‘trojan horse’ until the
introduction of oxygen re-activates its latent oxidative
capacity. At this point Mn3+ initiates a more aggressive
auto-oxidative cascade of free radical chain reaction, involving
the auto-oxidation of dopamine, serotonin and adrenaline; it is
recognised that Mn3+ readily oxidises these catecholamines
(10,7,11) forming 6-hydroxy-dopamine, etc, which, in turn,
oxidise more rapidly, forming quinones and superoxide/hydroxyl
radicals which modify the catecholamine terminals, which could
theoretically modify various amino acids on PrP and other
membrane proteins. Such radical chain reactions would be prone
to proliferate in mammals living in environments that are
depleted in Cu, Fe, Zn, Se. Such deficiencies cause a down
regulation in the turnover of the SOD, catalase, peroxidase and
vitamin E scavengers (10).).
Fig. 1
The multifactorial aetiological template underpinning the
pathogenesis of sporadic TSEs, where an Mn3+ initiated chain
reaction of auto oxidation invokes a multi site radical attack
on PrP and other CNS membrane/cytoskeletal proteins.
The pathogenic
scenario involving the cycling of the stable Mn2+ and Mn3+
species can be likened to the redox cycling of the paraquat
herbicide molecule and its role in initiating a Parkinsons-like
pathogenesis (10,13,14,16). Alternating double bonds and
‘resonance energy’ within paraquat provides a stable
‘dormant’ radical which auto oxidises once oxygen is
introduced. After reduction within the cell it reacts with
oxygen to generate the lethal superoxide radical.(17)
Mn is also recognised to activate some species of lectin, such
as phytohemagglutinins and concanavalin A (4), where high levels
of free Mn can accelerate the normal rate of concanavalin A
interaction with cell surface glycoproteins, such as PrP.
Interestingly, the action of concanavalin A and
phytohemagglutins mediates a 3.5 fold increase in the surface
abundance of PrP (18). Furthermore, increases in both the
surface expression of PrP and CNS membrane receptor capping with
concanavalin A are consistently recorded in the early stages of
the TSE disease process (19), suggesting that an excess of
available Mn in the CNS could putatively perform a key role (via
lectin activation) in initiating these facets of TSE
pathogenesis.
CU CHELATING CHEMICALS INDUCE SPONGIFORM ENCEPHALOPATHIES
Cuprizone, neocuproine, mercaptoethanol, dithiophosphates and
triethyltin are ‘true’ Cu chelating chemicals (20-22) which
deplete Cu supplies in the CNS producing a
‘non-transmissible’, reversible type of spongiform
encephalopathy. Cuprizone was actually utilised as a research
tool for inducing a scrapie-like spongiosis in mice at Compton
laboratories during the 1970s (23). CNS Cu deficiency could
therefore represent one of the key prerequisites underlying TSE
pathogenesis - albeit one which fails to account for the
transmissible facet of TSEs.
On the other hand,
treatment with the ‘pseudo’ types of Cu chelating chemical
such as diethyldithiocarbamate (DEDTC) (24) which chelate and
redistribute Cu around the body rather than chelating and
excreting it, do not produce spongiform encephalopathy (25);
Allain et al. (24) treated rats with DEDTC which resulted in a
77% increase of Cu levels in the brain. Furthermore, DEDTC
redistributes Cu into regions of the CNS that do not normally
exhibit Cu binding ligands, thereby depositing free Cu in the
extracellular space so it can initiate lipid peroxidation and
chain reactions of hydroxyl and Cu 3 radical formation (10).
Exposures to DEDTC and its parent compound sulfiram has been
associated with the development of Parkinson’s disease and
other extrapyramidal disorders (24, 26-28), supporting the
proposal that CNS Cu deficiency rather than CNS Cu overloading
is associated with TSE aetiology.
Is CNS Cu deficiency a prerequisite for TSEs?
Certain facets of TSE pathogenesis, such as tissue increase of
Fe store, found as Ferritin (29), indicate a state of Cu/Fe
deficiency.
Furthermore, Lung
tissues of copper deficient chicks have demonstrated abnormal
variations in the concentration of various types of
glycoaminoglycans (30). The possibility of an ‘in vivo’
metabolic relationship between PrP reduction within the cell, it
reacts with oxygen to generate the lethal superoxide radical
(17) and the glycoaminoglycans becomes evident after studying
the TSE disease process (5); where concentrations of the
sulphated glycoaminoglycans are raised in the CNS of diseased
animals, whilst dramatic therapeutic benefits are witnessed
‘in vitro’ when TSE affected cell cultures are treated with
these molecules. It seems likely that the normal cellular form
of PrP (PrPc) binds with a specific co-species of
glycoaminoglycans ‘in vivo’, which might serve as a means of
protecting PrPc against conversion into its abnormal TSE isoform.
This hypothesis proposes that CNS Cu deficiency is integral to
the aetiology of TSEs, where the depleted supply of Cu to
PrP’s Cu domain renders the protein vulnerable to invasion
with alternative cations, such as Mn, which could ligate to the
Cu domain and lead to the development of the misfolded,
pathogenic prion associated with TSE.
CNS Cu deficiency can be invoked via various naturally occurring
or artificially invoked mechanisms, or combinations thereof:
-
due
to indigenous copper deficiency in the external food chain
(influenced by seasonal, climatic and/or other geological
(3, 4, 31) characteristics such as high soil molybdenum
levels).
-
due
to an oxidant mediated upregulation of the expression of Cu-metalloproteins,
such as PrP, placing demands on the supply of available Cu
in the CNS (32, 15, 33, 34).
-
due
to chelation of available Cu in the CNS by certain foreign
organo pollutants (21).
-
due
to the inhibitory effects placed on Cu absorption by
excesses of Ca or estrogens (35, 36) as in feeds such as
alfalfa/soya respectively.
-
due
to the accelerated excretion of Cu resulting from
therapeutic treatment with steroids, etc (37). due to a
foreign organic pollutant induced covalent modification of
(or binding to) the active histidine/tyrosine residues (10
p45) (12) (38) (39) on PrP’s Cu domain; thereby preventing
Cu from accessing its binding domain on PrP (15, 34).
Experimental
evidence indicates a role for PrP's Cu domain in protecting the
cell against oxidative stress; via a PrP-mediated regulation of
SOD1 activity.
D Brown and others have provided strong ‘in vitro’ and ‘in
vivo’ experimental evidence that supports a functional role
for PrPc in protecting CNS cerebellar cells against the
deleterious impact of oxidative stress (33). Treatment with the
antioxidant vitamin E has also been shown to protect cells
lacking PrP expression against oxidant mediated cell death (33).
Brown proposes that
PrPc influences the activity of SOD1 (40) and points to the Cu
domain that has been identified at the N terminal octapeptide
repeat region of PrP (15, 34) , suggesting that PrP may perform
a role in the transportation of Cu to the sites of SOD 1
synthesis, Cu and Zn act as co factors in the synthesis of the
SOD 1 superoxide scavenger. Mice devoid of PrP due to genetic
ablation demonstrate a reduced resistance to oxidative stress.
Cu and Zn act as co factors in the synthesis of the SOD 1
superoxide scavenger. Mice devoid of PrP due to genetic ablation
demonstrate a reduced resistance to oxidative stress.
The key
biochemical and pathological facets of TSE suggest a pivotal
role of oxidative stress in TSE pathogenesis.
The biochemistry, pathology and distribution of CNS
abnormalities associated with the pathogenesis of TSEs suggests
that oxidative stress plays a major aetiological role in TSEs;
Decreased amounts of
phospholipids and gangliosides/increased levels of cholesterol
in membranes (41, p.100), an abundance of lipofuscin inclusions
in neurones (42), an upregulation of the signal transduction
cycle and kinase C phosphorylation, increases in intraneuronal
free calcium (43, 44), reduction in monamine oxidase/NAD
diaphorase (45,46), an increase in citrulline/ornithine in blood
sera (47), a decrease in membrane fluidity (41, p.106),
rupturing of lysosomal membranes (46), an excessive CNS
accumulation of iron in its Ferritin form (29), and a marked
increase in multimeric mitochondrial DNA/DNA strand breakage
(48). All of these abnormalities are characteristic of free
radical disturbances (10, 49-52) and are shared in the
pathogenesis of other neurodegenerative diseases similar to TSEs
such as Parkinsons (PD), Alzheimer’s (AD) and Motor Neurone
Disease (MND) - diseases which are now recognised to stem from a
free radical-mediated pathogenesis (52, 53).
It is proposed that
the different ‘strains’ of TSEs may be caused by different
species of radical-generating divalent cation (Mn, Nickel, Fe or
cobalt, etc.) that can successfully compete and ligate at
PrP’s Cu domain during states of Cu deficiency. The greater
the oxidative capacity of the metal species involved (e.g. Mn3+,
Mn4+ or even radioactive species), the more virulent strain of
TSE to emerge. This aetiological model operates in combination
with various other multifactorial criteria:
- The duration
and intensity of exposure to the different classes of
divalent cation/organic chemical pollutant in the
environment, (more directly related to the particular
species of radical that they generate).
- The TSE-susceptibility
of the exposed individual; relating to their PRP (41)/SOD
1,2,3/ceruloplasmin/cytochrome P450 genotype.
- Other
external environmental variables which can ultimately
influence the CMS uptake of Mn, e.g.:
- low Fe,
- levels
of stress or environmental pollutants which mediate ACTH
turnover, blood-brain barrier homeostatis and Mn uptake
in CNS.
- Daylight
interval in relation to the untra violet mediated
regulation of melatonin turn over, which, in turn,
mediates estrogen/corticosteroid turn over and Mn
uptake.
- levels
of estrogenic/steroid pollutants in the environment
(4,54) and their ability to upregulate the expression of
caeruloplasmin (55) which can lead to oxidation of
increased amounts of Mn2+ into its more lethal Mn3+
species (7), particularly in the absence of its normal
oxidation target, Fe2+.
- The
developmental stage of the victim during intoxication (56).
SPORADIC
TSEs - AN ANALYTICAL SURVEY INTO THE LEVELS OF METALS IN
ECOSYSTEMS SUPPORTING CLUSTERS OF TSE: DESIGN
Foodchains
supporting isolated, long standing clusters of sporadic TSEs
were sampled by the author for analysis of mineral status in
order to ascertain whether any elemental deficiencies or
toxic excesses are common idiosncratic characteristics of
these TSE foci, Adjoining TSE-free regions populated by
significant numbers of the respective species associated
with thee study were sampled as controls.
The TSE clusters
of chronic wasting disease (CWD) in wild deer/elk in N.
Central Colorado (57,58), sheep scrapie in N Iceland (59,60)
and CJD in Slovakia (61-64) were selected for this survey
for various reasons:
- A long
history of TSE being confined to specific well-defined
regions.
- The
dependence of TSE affected populations on the local
foodchain.
- The
relative freedom of those foodchains (excluding the
Slovakia TSE foci) from excessive contamination by
synthetic pro-oxidant industrial/agricultural pollutants
which would complicate the study.
The repeated failure of various Government TSE eradication
programmes executed in the TSE endemic regions of
Colorado/Iceland (57,59) - involving blanket slaughter, 4
year fallow, then restocking - suggests the persistent
presence of a hitherto unrecognized environmental causal
factor common to these regions.
It is worth
noting that high altitude, snow covered peaked mountain
ranges of volcanic origin - with an abundance of coniferous
trees - characterize the environments where sporadic TSE
foci have traditionally arisen. This could be linked to the
annual spring snow thaw and the resulting waterlogging of
soils; where the main minerals utilized as antioxidant
cofactors in biological systems are leached out of the soil,
whilst others, like manganese/aluminium, readily accumulate
in the plant horizon as a result of the temporary
acidification of the soils due to increases in anaerobic
conditions during the wet season.
Furthermore, the
hypoxia of high-altitude lining increases susceptibility to
oxidative stress and increases permeability of the brain
barrier to cations, etc.
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