Fig 1.
Mercury in the locus ceruleus, substantia nigra, cerebral cortex and striatum in Parkinson’s disease.
(A) Black mercury grains (eg, open arrowheads) are present in the cytoplasm of most locus ceruleus neurons in PD1 (main image) and PD2 (inset). Lewy bodies with haloes (filled arrowheads) are seen in some mercury-containing neurons. Numerous oligodendrocytes have small mercury deposits (eg, arrows) adjacent to their nuclei. Autometallography/Luxol fast blue. (B) Magenta immunostaining of a-synuclein shows co-localisation of Lewy bodies (open arrowheads) with black-staining mercury in locus ceruleus neurons of PD1. An extra-neuronal a-synuclein aggregate (open arrow) has no associated mercury. No Lewy bodies are present in a locus ceruleus neuron not containing mercury (filled arrowhead). The right upper inset shows a nearby locus ceruleus neuron at higher magnification with Lewy body/mercury co-localisation. The left lower inset shows mercury within a Lewy body. Autometallography/a-synuclein Magenta/hematoxylin. (C) Brown DAB immunostaining of a-synuclein shows co-localisation of a Lewy body with mercury (filled arrow) in a remaining substantia nigra neuron of PD1. A nearby Lewy neurite (right upper inset) shows associated mercury grains (open arrowhead). Some intraneuronal (filled arrowhead) and extraneuronal (*) Lewy bodies do not appear to contain mercury, possibly because of masking by the dense brown DAB staining. Scattered oligodendrocytes have small mercury deposits (eg, thin arrows). Autometallography/a-synuclein DAB/hematoxylin. Left lower inset: Magenta a-synuclein staining shows black-staining mercury co-localised with a Lewy body in a nearby substantia nigra neuron (autometallography/a-synuclein Magenta/hematoxylin). Right lower inset: black-staining mercury is also present in a few remaining substantia nigra neurons in PD2 (autometallography/Luxol fast blue). (D) Dense mercury grains (filled arrowheads) are present in two corticomotoneuron cell bodies (dashed outlines) of PD2. Scattered oligodendrocytes (arrows) have small mercury deposits (one magnified in the left lower inset). Autometallography/Luxol fast blue. Two nearby pericytes contain mercury deposits (open arrowheads, right upper inset, autometallography/CD31/hematoxylin). (E) Numerous oligodendrocytes in the parietal white matter of PD1 have mercury deposits (eg, arrows) either attached to the nuclear membrane or adjacent to the nucleus. A magnified view is shown in the left lower inset. Autometallography/a-synuclein/hematoxylin. (F) Mercury grains (eg, open arrowheads) are present in the paranuclear region of medium-sized neurons in the putamen (one enlarged in the upper inset) of PD1. A large neuron (open arrow) contains no mercury. Small mercury deposits are present adjacent to nuclei of scattered oligodendrocytes (thin arrow) and pericytes (closed arrowhead). In the left lower inset, a similar distribution of mercury is seen in PD2 in medium-sized (right) but not large (left) neurons in the putamen. Autometallography/Luxol fast blue. PD: case numbers. Bars = 20 μm.
Fig 2.
Mercury in the thalamus, cerebellum, other CNS regions, and non-CNS organs in Parkinson’s disease.
(A) Black mercury grains are present in the cytoplasm of most thalamic neurons (eg, open arrowheads), in both PD1 (main image) and PD2 (right upper inset). Numerous oligodendrocytes have small mercury deposits (eg, arrows) adjacent to their nuclei. Autometallography/Luxol fast blue. A PD1 thalamic neuron with mercury co-localised with aggregated a-synuclein is seen (left lower inset, autometallography/a-synuclein DAB/hematoxylin). (B) Cerebellar Purkinje cells of PD1 contain small scattered cytoplasmic mercury grains (arrowheads). Cerebellar granule cells contain single paranuclear mercury grains (enlarged in the left lower inset). Scattered oligodendrocytes in the adjacent cerebellar white matter contain paranuclear mercury grains (eg, arrows, right lower inset). No mercury is present in the cerebellar cortex of PD2 (right upper inset). Autometallography/Luxol fast blue. (C) Black mercury grains (arrowheads) are present in the cytoplasm of cerebellar dentate neurons of PD2 (main image) and PD1 (right lower inset). Numerous oligodendrocytes in the nearby dentate internal white matter have paranuclear mercury deposits (arrows, left lower inset). Autometallography/Luxol fast blue. (D) Scattered neurons in the inferior olivary nucleus in the medulla oblongata of PD1 contain mercury grains (arrowheads). White matter internal to the olive contains oligodendrocytes with paranuclear mercury deposits (arrows, right upper inset). Autometallography/Luxol fast blue. Left lower inset: one nearby neuron in the inferior olivary nucleus has co-localised black mercury grains and brown aggregated a-synuclein (autometallography/a-synuclein DAB/hematoxylin). (E) Mercury grains are present in the juxtanuclear region of oligodendrocytes (arrows), but not of neurons (arrowhead), in the red nucleus (RE). Black mercury deposits are seen in the cytoplasm of neurons in the dorsal raphe nucleus (RA), lateral geniculate nucleus (GE), dorsal vagal nucleus (DV), the facial motor nucleus (7n) and the amygdala (AM). Autometallography/Luxol fast blue. (F) Mercury grains are present in cells of the adrenal medulla (AD), anterior pituitary (AP), kidney Henle loop (KI), and thyroid follicles (TH). Autometallography/hematoxylin. PD: case numbers. Bars = 20 μm.
Table 1.
Mercury in neurons of people (1) with PD, (2) with mercury exposure but without PD, and (3) without known mercury exposure or PD.
Fig 3.
Mercury staining after exposure to mercury and in advanced age.
(A) In ME1, black mercury grains are seen in the frontal motor cortex in two corticomotoneurons (open arrowheads), in scattered oligodendrocytes (eg, thin arrows), in endothelial cells (eg, closed arrowheads), but not in smaller cortical neurons (eg, open arrow). Autometallography/hematoxylin. Inset: a mercury-containing oligodendrocyte (arrow) is surrounded by GFAP-positive processes of interlaminar astrocytes (autometallography/GFAP DAB/hematoxylin). (B) In the frontal lobe white matter of ME1, mercury is present in endothelial cells (open arrows), and perivascular astrocyte cells bodies (eg, arrowheads) and processes (eg, thin arrows). Autometallography/GFAP DAB/hematoxylin. (C) Mercury is present in numerous pinealocytes in the pineal gland of ME1. Autometallography/hematoxylin. (D) In ME2, mercury grains are present in the cytoplasm of numerous lateral geniculate nucleus neurons (open arrowheads), scattered oligodendrocytes (eg, arrows), and in endothelial cells (filled arrowhead). Autometallography/hematoxylin. (E) In the lateral geniculate nucleus of ME2 a perivascular astrocyte (open arrow) has processes connecting a blood vessel (*) with mercury-containing neurons (open arrowheads) and oligodendrocytes (thin arrow). Inset: a nearby astrocyte process (closed arrowhead) contains mercury grains and connects with a mercury-containing oligodendrocyte (thin arrow). Autometallography/GFAP DAB/hematoxylin. (F) In the 95 years-old non-PD male control, some neurons (open arrowheads) and small blood vessels (filled arrowheads) in the dorsal vagal nucleus contain mercury. Autometallography/hematoxylin. ME: case numbers. Bars = 20 μm.
Fig 4.
LA-ICP-MS in Parkinson’s disease (PD2).
(A) Particulate mercury is seen in both locus ceruleus nuclei (outlined) where numerous neurons were autometallography-positive. (B) Particulate iron deposits are seen in both locus ceruleus nuclei (outlined) in the posterior pons. Linear deposits of iron in the pontine white matter (eg, arrow) are probably from red blood cells in blood vessels. (C) Particulate aluminium is seen in both locus ceruleus nuclei (outlined). (D) A normal high nuclear density shown by the phosphorus image is present in the hippocampal white matter (top, outlined) as well as in the dentate gyrus (arrow). (E) A large amount of iron is present in the hippocampal white matter (outlined), and in grey matter adjacent to the dentate gyrus (arrow, dark blue line). (F) Particulate mercury is seen in the hippocampal white matter (eg, arrow) where oligodendrocytes were autometallography-positive. (G) The normal high nuclear density of the frontal white matter (outlined) is shown in the phosphorus image. (H) The frontal white matter contains more iron than the frontal cortex, where most iron is in the deeper cortical layers (*) adjacent to the white matter. (I) Particulate mercury is present in the frontal white matter where oligodendrocytes were autometallography-positive. (J) Speckled mercury is present in the lateral geniculate nucleus where neurons were autometallography-positive. (K) Mercury is seen in pontine facial motor neurons, which were autometallography-positive. (L) Chromium is widespread in the posterior pons, without particular accumulation in the locus ceruleus (outlined). Scale = counts per second (proportional to abundance).
Fig 5.
Hypothetical model of mercury-induced Parkinson’s disease.
In this model, an early-life exposure to methylmercury (MeHg), aided by genetic susceptibility to brain mercury uptake, seeds susceptible cells, with later toxic effects modified by further genetic susceptibilities. During the prodromal period, mercury in colonic and sleep cycle-related neurons causes constipation and sleep disorders. Early uptake of mercury by the locus ceruleus places the substantia nigra at risk of toxicant damage, and permits toxicants to pass through the blood-brain-barrier. Inorganic mercury (iHg) slowly accumulates in neurons and oligodendrocytes, both from demethylation of methylmercury and from further exposures to mercury. When cellular mercury concentrations reach a critical level in neurons and oligodendrocytes, aided by synergistic effects in iron-containing cells, interactions with other toxicants, and decreasing aging-related natural defenses, mercury damages membranous organelles in these cells and promotes a-synuclein aggregation. This results in symptomatic PD, with motor, non-motor, and non-CNS associated symptoms. Hg°: mercury vapor.
Fig 6.
Mercury in neurons that could contribute to bradykinesia, rigidity and tremor.
Regions of the brain implicated in PD motor disorders whose neurons contain mercury are shown in red. The classical pathway of PD motor pathophysiology is included in the upper half of the diagram. Neurons in the substantia nigra compacta (SNc), striatum, thalamus and motor cortex contain mercury, but not those in the subthalamic nucleus (STN), or the globus pallidus interna (GPi) and externa (GPe). Mercury in the locus ceruleus could decrease noradrenaline output to the substantia nigra, leaving it more susceptible to toxicants, and to blood vessels, making the blood-brain-barrier more permeable to circulating toxicants. Mercury in neurons of the cerebellar cortex, dentate nucleus (DN) and inferior olivary nucleus could underlie types of tremor in PD (lower left in diagram). Also shown (bottom left) is a stimulatory pathway from the subthalamic nucleus via the anterior pontine nuclei to the cerebellar cortex and dentate nucleus (both containing mercury), that could cause a resting tremor. BV: blood vessel, DIRECT: direct pathway, INDIRECT: indirect pathway, DOP: dopamine, GABA: gamma-aminobutyric acid, NOR: noradrenaline.