Vulnerability of monoaminergic neurons in the brainstem of the zitter rat in oxidative stress

Parkinson's Disease: Cells Succumbing to Lifelong Dopamine-Related Oxidative Stress and Other Bioenergetic Challenges

The core pathological event in Parkinson's disease (PD) is the specific dying of dopamine (DA) neurons of the substantia nigra pars compacta (SNc). The reasons why SNc DA neurons are especially vulnerable and why idiopathic PD has only been found in humans are still puzzling. The two main underlying factors of SNc DA neuron vulnerability appear related to high DA production, namely (i) the toxic effects of cytoplasmic DA metabolism and (ii) continuous cytosolic Ca2+ oscillations in the absence of the Ca2+-buffer protein calbindin. Both factors cause oxidative stress by producing highly reactive quinones and increasing intra-mitochondrial Ca2+ concentrations, respectively. High DA expression in human SNc DA neuron cell bodies is suggested by the abundant presence of the DA-derived pigment neuromelanin, which is not found in such abundance in other species and has been associated with toxicity at higher levels. The oxidative stress created by their DA production system, despite the fact that the SN does not use unusually high amounts of energy, explains why SNc DA neurons are sensitive to various genetic and environmental factors that create mitochondrial damage and thereby promote PD. Aging increases multiple risk factors for PD, and, to a large extent, PD is accelerated aging. To prevent PD neurodegeneration, possible approaches that are discussed here are (1) reducing cytoplasmic DA accumulation, (2) blocking cytoplasmic Ca2+ oscillations, and (3) providing bioenergetic support.

Attractin deficiency causes metabolic and morphological abnormalities in slow-twitch muscle

Skeletal muscle fibers are classified as slow-twitch and fast-twitch fibers, which have different reactive oxygen species (ROS) metabolism and mitochondrial biogenesis. Recently, Attractin (Atrn), which encodes secreted (sAtrn) and transmembrane (mAtrn)-type proteins, has been shown to be involved in free radical scavenging. Although Atrn has been found in skeletal muscle, little is known about the expression levels and function of Atrn in each muscle fiber type. Therefore, we investigate sAtrn and mAtrn expression levels in the slow-twitch soleus (sol) and fast-twitch extensor digitorum longus (EDL) muscles as well as the morphology and expression levels of antioxidant enzymes and functional mitochondrial markers using Atrn-deficient muscles. Both types of Atrn were expressed in the sol and EDL. mAtrn was mainly expressed in the adult sol, whereas sAtrn expression levels did not differ between muscle types. Moreover, mAtrn in the sol was abundantly localized in the subsarcolemmal area, especially in the myoplasm near mitochondria. Atrn-deficient Zitter rats showed muscle fiber atrophy, myofibril misalignment, mitochondrial swelling and vacuolation in the sol but not EDL. Furthermore, the Atrn-deficient sol exhibited a marked reduction in antioxidant enzyme SOD1, GPx1, catalase and Prx6 and mitochondrial functional protein, UCP2, expression. Even Atrn-deficient EDL showed a significant reduction in Prx3, Prx6, UCP2 and UCP3 expression. These data indicate that Atrn-deficiency disturbs ROS metabolism in skeletal muscles. In particular, mAtrn is involved in metabolism in the slow-twitch sol muscle and mAtrn-deficiency may cause ROS imbalance, resulting in morphological abnormalities in the muscle.

Minocycline Alleviates Cluster Formation of Activated Microglia and Age-dependent Dopaminergic Cell Death in the Substantia Nigra of Zitter Mutant Rat

Microglial activation is a component of neurodegenerative pathology. Here, we examine whether activated microglia participate in age-related dopaminergic (DA) cell death in the substantia nigra pars compacta (SNc) of the zitter (zi/zi) rat, a mutant characterized by deletion of the attractin gene. Confocal microscopy with double-immunohistochemical staining revealed activated microglia-formed cell-clusters surrounding DA neurons in the SNc from 2 weeks after birth. An immunoelectron microscopic study showed that the cytoplasm of activated microglia usually contains phagosome-like vacuoles and lamellar inclusions. Expression levels of the pro-inflammatory cytokines interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and inducible nitric oxide synthase (iNOS) were increased in the midbrain of 2-month-old zi/zi rats. Chronic treatment with the anti-inflammatory agent minocycline altered the morphology of the microglia, reduced cluster formation by the microglia, and attenuated DA cell death in the SNc, and reduced the expression of IL-1β in the midbrain. These results indicate that activated microglia, at least in part and especially at the initial phase, contribute to DA cell death in the SNc of the zi/zi rat.

Microglia pre-activation and neurodegeneration precipitate neuroinflammation without exacerbating tissue injury in experimental autoimmune encephalomyelitis

Human inflammatory or neurodegenerative diseases, such as progressive multiple sclerosis (MS), occur on a background of age-related microglia activation and iron accumulation as well as pre-existing neurodegeneration. Most experimental models for CNS diseases, however, are induced in rodents, which are naturally characterized by a homeostatic microglia phenotype, low cellular iron load and absence of neurodegeneration. Here, we show that naïve LEWzizi rats – Lewis rats with a zitter rat background – show a spontaneous phenotype partly mimicking the changes seen in human aging and particularly in the normal-appearing white and grey matter of patients with progressive MS. Using this model system, we further aimed to investigate (i) whether the acute monophasic MS model experimental autoimmune encephalomyelitis (EAE) transforms into chronic progressive disease and (ii) whether EAE-induced neuroinflammation and tissue damage aggravate on the LEWzizi background. We found that the pre-existing LEWzizi-specific pathology precipitated EAE-related neuroinflammation into forebrain areas, which are devoid of EAE lesions in normal Lewis rats. However, EAE-related tissue damage was neither modified by the LEWzizi-specific pathology nor did EAE-induced neuroinflammation modify the LEWzizi-related pathological process. Our data indicate that the interaction between pre-activated microglia and CD4⁺ autoreactive T cells during the induction and propagation of tissue damage in the CNS is limited.

Tyrosine hydroxylase afferents to the interstitial nucleus of the posterior limb of the anterior commissure are neurochemically distinct from those projecting to neighboring nuclei

The interstitial nucleus of the posterior limb of the anterior commissure (IPAC) is exclusively innervated by tyrosine hydroxylase-immunoreactive (TH-IR) fibers as observed in the other nuclei of the rat forebrain such as the striatum and nucleus accumbens. Distinguishing TH-IR afferents to the IPAC from those projecting to neighboring nuclei has been difficult. However, we previously showed that the TH-IR fibers projecting to the IPAC were invulnerable to neurodegeneration in zitter mutant rats, whereas almost all TH-IR afferents fibers to the dorsolateral striatum were lost, indicating that these two groups of TH-IR afferents have distinct neurochemical properties. Here, to explore this observation further, we injected Fluorogold (FG) retrograde tracers to identify neurons projecting to the IPAC or dorsal striatum. We also determined the distribution of attractin mRNA and protein, causative factors for the pathological phenotypes of zitter mutant rats, within the normal rat midbrain. In rats injected with FG into the dorsal striatum, we detected many FG-positive neurons in the ventral aspect of the substantia nigra pars compacta (SNC). In contrast, many FG-positive neurons were observed in the dorsal aspect of the SNC of rats injected with FG into the IPAC. Immunohistochemistry and in situ hybridization studies of intact animals revealed that both attractin mRNA and protein were expressed at higher levels in the ventral aspect of the SNC, whereas both attractin mRNA and protein were expressed at lower levels in the dorsal aspect of the SNC. Taken together, these results indicate that TH-IR afferents to the IPAC have distinct neurochemical properties from those to the striatum and may account for the differential vulnerability to neurodegeneration observed in zitter mutant rats.

Mechanisms of neuronal death in synucleinopathy

α-synuclein is a key molecule in the pathogenesis of synucleinopathy including Parkinson's disease and multiple system atrophy. In this mini-review, we mainly focus on recent data obtained from cellular models of synucleinopathy and discuss the possible mechanisms of neurodegeneration. Recent progress suggests that the aggregate formation of α-synuclein is cytoprotective and that its precursor oligomer (protofibril) may be cytotoxic. The catechol-derived quinones are the candidate molecules that facilitate the oligomer formation of α-synuclein. Furthermore, the cellular membranes are shown to be the primary targets injured by mutant α-synucleins, and the mitochondrial dysfunction seems to be an initial step in the neuronal death.

Application of Fluoro-Jade C in acute and chronic neurodegeneration models: utilities and staining differences

Recent neuropathological studies have shown that Fluoro-Jade C (FJC), an anionic fluorescent dye, is a good marker of degenerating neurons. However, those studies have mostly examined acute rather than chronic models of neurodegeneration. We therefore compared FJC staining using the intrastriatal 6-hydroxydopamine (6-OHDA)-injected rat as an acute model and the zitter rat as a chronic model, as both show dopaminergic (DA) neurodegeneration. In the 6-OHDA-injected rat, FJC-positive neurons were found in the substantia nigra pars compacta (SNc) before the loss of tyrosine hydroxylase (TH)-positive DA neurons. In the zitter rat, FJC-labeled fibers were first detected at 1 month old (1M) and were considerably increased in the striatum at 4M, whereas FJC-labeled cell bodies were found at 4M, but not at 1M in the SNc. Furthermore, FJC-labeled neurons of the zitter rat showed TH-immunoreactivity in fibers, but little in cell bodies, while those from the 6-OHDA-injected rat showed TH-immunoreactivity even in the cell bodies. These results demonstrate that FJC is a useful tool for detecting chronically degenerating neurons, and suggest that intracellular substances bound to FJC may accumulate in the cell bodies from fibers at a slower rate in the chronic model than in the acute model.

Chronic treatment with melatonin attenuates serotonergic degeneration in the striatum and olfactory tubercle of zitter mutant rats

The effects of chronic treatment with the antioxidant hormone melatonin on degeneration of serotonergic fibers were studied in the striatum and olfactory tubercle of the zitter rat, which shows a loss-of-function mutation of the glycosylated transmembrane protein attractin. In these animals, serotonergic fibers in the striatum and olfactory tubercle undergo spontaneous and progressive degeneration as a result of abnormal metabolism of reactive oxygen species. Homozygous zitter (zi/zi) rats were provided ad libitum access to drinking water containing melatonin for 9 months (M) after weaning. High-performance liquid chromatography analysis revealed that melatonin treatment significantly increased serotonin in the caudate-putamen, (CPU), nucleus accumbens (NA) and olfactory tubercle (OT). Immunohistochemical staining for serotonin was consistent with the neurochemical data and further demonstrated substantially increased numbers of serotonergic nerve terminals in these areas. Aberrant serotonergic fibers characterized by swollen varicosities (>1 microm in diameter) were observed in the CPU and NA of 10 M zi/zi rats. The number of these fibers decreased after melatonin treatment ended. Furthermore, hyperinnervation of serotonergic fibers was observed in the OT of melatonin-treated zi/zi rats. These results suggest that melatonin protects serotonergic fibers and terminals in zitter rats and/or promotes their neuroplasticity.

Non-cell autonomous impairment of oligodendrocyte differentiation precedes CNS degeneration in the Zitter rat: implications of macrophage/microglial activation in the pathogenesis

BACKGROUND

The zitter (zi/zi) rat, a loss-of-function mutant of the glycosylated transmembrane protein attractin (atrn), exhibits widespread age-dependent spongiform degeneration, hypomyelination, and abnormal metabolism of reactive oxygen species (ROS) in the brain. To date, the mechanisms underlying these phenotypes have remained unclear.

RESULTS

Here, we show differentiation defects in zi/zi oligodendrocytes, accompanied by aberrant extension of cell-processes and hypomyelination. Axonal bundles were relatively preserved during postnatal development. With increasing in age, the injured oligodendrocytes in zi/zi rats become pathological, as evidenced by the accumulation of iron in their cell bodies. Immunohistochemical analysis revealed that atrn expression was absent from an oligodendrocyte lineage, including A2B5-positive progenitors and CNPase-positive differentiated cells. The number and distribution of Olig2-positive oligodendrocyte progenitors was unchanged in the zi/zi brain. Furthermore, an in vitro differentiation assay of cultured oligodendrocyte progenitors prepared from zi/zi brains revealed their normal competence for proliferation and differentiation into mature oligodendrocytes. Interestingly, we demonstrated the accelerated recruitment of ED1-positive macrophages/microglia to the developing zi/zi brain parenchyma prior to the onset of hypomyelination. Semiquantitative RT-PCR analysis revealed a significant up-regulation of CD26 and IL1-beta in the zi/zi brain during this early postnatal stage.

CONCLUSION

We demonstrated that the onset of the impairment of oligodendrocyte differentiation occurs in a non-cell autonomous manner in zi/zi rats. Hypomyelination of oligodendrocytes was not due to a failure of the intrinsic program of oligodendrocytes, but rather, was caused by extrinsic factors that interrupt oligodendrocyte development. It is likely that macrophage/microglial activation in the zi/zi CNS leads to disturbances in oligodendrocyte differentiation via deleterious extrinsic factors, such as the cytokine IL1-beta or ROS. Atrn might be involved in the activation of brain macrophages/microglia by suppressing excessive migration of monocytes into the CNS, or by accelerating the transformation of brain monocytes into resting microglia. Understanding the pathogenesis of the zi/zi rat may provide novel insights into the developmental interaction betweens macrophages/microglia and cells of an oligodendrocyte lineage.

Progressive dopaminergic neurodegeneration of substantia nigra in the zitter mutant rat

Zitter mutant rats exhibit abnormal metabolism of superoxide species and demonstrate progressive degeneration of dopamine (DA) neurons in the substantia nigra (SN). Furthermore, long-term intake of vitamin E, an effective free radical scavenger, prevents the loss of DA neurons caused by free radicals. However, it is unclear how this degeneration progresses. In this study, we ultrastructurally examined cell death in the zitter mutant rat SN. Conventional electron-microscopic examination revealed two different types of neurons in the SN pars compacta. In zitter mutant rats, although the first type (clear neurons) exhibited no obvious ultrastructural changes with aging, the second type (dark neurons) demonstrated age-related damage from 2 months. Immunoelectron-microscopic analysis clarified that the second-type neurons were dopaminergic neurons. In the dopaminergic neuronal somata, many lipofuscin granules and abnormal endoplasmic reticula were observed from 2 months of age, and these dopaminergic neurons showed progressive degeneration with age. Moreover, in zitter mutant rats, abnormally enlarged myelinated axons with dense bodies and splitting myelin with dense material were observed in the SN at 2, 4, and 12 months, and oligodendrocytes with numerous lipofuscin, multivesicular bodies, multilamellar bodies, and dense bodies were frequently observed at 4 and 12 months. These findings clarified that dopaminergic neurons in zitter mutant rats had degenerated with age, and that myelinated axons also exhibited age-related injury. Moreover, ubiquitin-immunohistochemical analysis indicated that the accumulation of products of the endosomal-lysosomal system may be involved in this degeneration.

Degeneration of dopaminergic neurons in the substantia nigra of zitter mutant rat and protection by chronic intake of Vitamin E

Dopaminergic cell death in the ventral and dorsal tiers of substantia nigra pars copmacta (SNc) and their prevention by anti-oxidant diet was immunohistochemically studied in the zitter mutant rats, which are characterized by abnormal metabolism of superoxide. Similar to previous reports, the number of SNc neurons in Nissl-stained section decreased with age. Tyrosine hydroxylase (TH) immunohistochemistry demonstrated that the dopaminergic neurons in the ventral tier of SNc degenerated early, whereas the dorsal tier gradually degenerated with age. Thus, the ventral tier dopaminergic neurons are affected first, but the dorsal tier neurons do become impact by the zi/zi mutation. Following 9-month period after weaning, zitter rats supplemented with 500 mg D,L-alpha-tocophenol (VE(+))/kg diet exhibited a significant increased of surviving TH-immunoreactive neurons in both the tiers of SNc as compared with the zi/zi rats with control and VE(-) diets. These results suggest that VE supplement may slow the dopaminergic cell loss in zitter mutant rat, and further support that degeneration of the dopaminergic neurons in this mutant rat is caused by oxidant stress. Thus, the zitter rat may represent a good model for studying the dopaminergic cell death by superoxide species.

Loss of D 3 receptors in the zitter mutant rat is not reversed by l -dopa treatment

In Parkinson's disease (PD) and animal models of parkinsonism the destruction of nigrostriatal (NSB) system results in a marked loss of the dopamine D(3) receptor and mRNA in the islands of Calleja (ICj) and the nucleus accumbens shell (NAS). In animal models, it has been reported that both measures are elevated by repeated intermittent administration of L-dopa. However, a large proportion of PD cases are resistant to L-dopa-induced elevation of D(3) receptor number. The zitter mutant (Zi/Zi) rat replicates the slow progressive degeneration of the NSB observed in PD and also exhibits a loss of D(3) receptor number in the NAS or ICj. To test if this could be reversed with subchronic L-dopa treatment, injections of carbidopa (10 mg/kg i.p.) were followed an hour later with injection of L-dopa (100 mg/kg i.p.) twice a day for 10 days. In control Sprague-Dawley (SD) and zitter heterozygote (Zi/-) rats that do not show a loss of D(3) receptors with vehicle treatment, L-dopa produced no change in D(3) receptor number or in DA terminal density as measured by dopamine transporter (DAT) binding and tyrosine hydroxylase immunoautoradiography (TH-IR). There was a marked loss of DAT and TH-IR in caudate-putamen (CPu) and NA, as well as D(3) receptors in NAS and ICj in Zi/Zi rats but no further change with L-dopa treatment. To determine if the resistance to L-dopa-induced increase in D(3) receptor was due to a deficiency in expression of cortical BDNF or its receptor, TrkB, in CPu and NAS, we examined BDNF mRNA by ISHH in frontal cortex and TrkB mRNA in frontal cortex, CPu, and NA. The loss of the NSB in the Zi/Zi did not alter levels of BDNF or TrkB mRNA, nor did L-dopa administration alter levels BDNF or TrkB mRNA. Thus, unlike in 6-hydroxydopamine-treated rats, in Zi/Zi rats administered L-dopa does not reverse the loss of BDNF mRNA or lead to an elevation of D(3) receptor number.