Amanda L. Woerman

Evidence for α-synuclein prions causing multiple system atrophy in humans with parkinsonism

Significance Prions are proteins that assume alternate shapes that become self-propagating, and while some prions perform normal physiological functions, others cause disease. Prions were discovered while studying the cause of rare neurodegenerative diseases of animals and humans called scrapie and Creutzfeldt–Jakob disease, respectively. We report here the discovery of α-synuclein prions that cause a more common neurodegenerative disease in humans called multiple system atrophy (MSA). In contrast to MSA, brain extracts from Parkinson’s disease (PD) patients were not transmissible to genetically engineered cells or mice, although much evidence argues that PD is also caused by α-synuclein, suggesting that this strain (or variant) is different from those that cause MSA. Prions are proteins that adopt alternative conformations that become self-propagating; the PrPSc prion causes the rare human disorder Creutzfeldt–Jakob disease (CJD). We report here that multiple system atrophy (MSA) is caused by a different human prion composed of the α-synuclein protein. MSA is a slowly evolving disorder characterized by progressive loss of autonomic nervous system function and often signs of parkinsonism; the neuropathological hallmark of MSA is glial cytoplasmic inclusions consisting of filaments of α-synuclein. To determine whether human α-synuclein forms prions, we examined 14 human brain homogenates for transmission to cultured human embryonic kidney (HEK) cells expressing full-length, mutant human α-synuclein fused to yellow fluorescent protein (α-syn140*A53T–YFP) and TgM83+/− mice expressing α-synuclein (A53T). The TgM83+/− mice that were hemizygous for the mutant transgene did not develop spontaneous illness; in contrast, the TgM83+/+ mice that were homozygous developed neurological dysfunction. Brain extracts from 14 MSA cases all transmitted neurodegeneration to TgM83+/− mice after incubation periods of ∼120 d, which was accompanied by deposition of α-synuclein within neuronal cell bodies and axons. All of the MSA extracts also induced aggregation of α-syn*A53T–YFP in cultured cells, whereas none of six Parkinson’s disease (PD) extracts or a control sample did so. Our findings argue that MSA is caused by a unique strain of α-synuclein prions, which is different from the putative prions causing PD and from those causing spontaneous neurodegeneration in TgM83+/+ mice. Remarkably, α-synuclein is the first new human prion to be identified, to our knowledge, since the discovery a half century ago that CJD was transmissible.

Propagation of prions causing synucleinopathies in cultured cells.

Significance Progressive supranuclear palsy (PSP) and multiple system atrophy (MSA) are neurodegenerative diseases caused by tau and α-synuclein prions, respectively. Prions, purified from human brains of deceased patients with PSP and MSA using phosphotungstic acid, were applied to cultured cell models that selectively form aggregates in the presence of tau or α-synuclein prions, respectively. Whereas brain homogenates prepared from two PSP and six MSA patients infected cultured cells, the same approach was unsuccessful with brain samples from three Parkinson’s disease patients. Our findings provide compelling evidence that PSP and MSA are prion diseases, and that MSA is caused by several distinct prion strains. Increasingly, evidence argues that many neurodegenerative diseases, including progressive supranuclear palsy (PSP), are caused by prions, which are alternatively folded proteins undergoing self-propagation. In earlier studies, PSP prions were detected by infecting human embryonic kidney (HEK) cells expressing a tau fragment [TauRD(LM)] fused to yellow fluorescent protein (YFP). Here, we report on an improved bioassay using selective precipitation of tau prions from human PSP brain homogenates before infection of the HEK cells. Tau prions were measured by counting the number of cells with TauRD(LM)–YFP aggregates using confocal fluorescence microscopy. In parallel studies, we fused α-synuclein to YFP to bioassay α-synuclein prions in the brains of patients who died of multiple system atrophy (MSA). Previously, MSA prion detection required ∼120 d for transmission into transgenic mice, whereas our cultured cell assay needed only 4 d. Variation in MSA prion levels in four different brain regions from three patients provided evidence for three different MSA prion strains. Attempts to demonstrate α-synuclein prions in brain homogenates from Parkinson’s disease patients were unsuccessful, identifying an important biological difference between the two synucleinopathies. Partial purification of tau and α-synuclein prions facilitated measuring the levels of these protein pathogens in human brains. Our studies should facilitate investigations of the pathogenesis of both tau and α-synuclein prion disorders as well as help decipher the basic biology of those prions that attack the CNS.

Tau prions from Alzheimer’s disease and chronic traumatic encephalopathy patients propagate in cultured cells

Significance The progressive nature of neurodegenerative diseases is due to the spread of prions, misfolded infectious proteins, in the brain. In tauopathies, the protein tau misfolds, causing several diseases, including Alzheimer’s disease (AD) and chronic traumatic encephalopathy (CTE). Here we created a panel of mammalian cell lines expressing a fragment of tau fused to yellow fluorescent protein. Each cell line selectively detects tau prions that are misfolded into self-propagating conformations; such cells permit identification of minute differences among tauopathies. For example, tau prions in AD and CTE are distinct from prions in other tauopathies such as Pick’s disease and progressive supranuclear palsy. These insights are likely to contribute to the development of future therapeutics. Tau prions are thought to aggregate in the central nervous system, resulting in neurodegeneration. Among the tauopathies, Alzheimer’s disease (AD) is the most common, whereas argyrophilic grain disease (AGD), corticobasal degeneration (CBD), chronic traumatic encephalopathy (CTE), Pick’s disease (PiD), and progressive supranuclear palsy (PSP) are less prevalent. Brain extracts from deceased individuals with PiD, a neurodegenerative disorder characterized by three-repeat (3R) tau prions, were used to infect HEK293T cells expressing 3R tau fused to yellow fluorescent protein (YFP). Extracts from AGD, CBD, and PSP patient samples, which contain four-repeat (4R) tau prions, were transmitted to HEK293 cells expressing 4R tau fused to YFP. These studies demonstrated that prion propagation in HEK cells requires isoform pairing between the infecting prion and the recipient substrate. Interestingly, tau aggregates in AD and CTE, containing both 3R and 4R isoforms, were unable to robustly infect either 3R- or 4R-expressing cells. However, AD and CTE prions were able to replicate in HEK293T cells expressing both 3R and 4R tau. Unexpectedly, increasing the level of 4R isoform expression alone supported the propagation of both AD and CTE prions. These results allowed us to determine the levels of tau prions in AD and CTE brain extracts.

α-Synuclein: Multiple System Atrophy Prions.

Multiple system atrophy (MSA) is a rapidly progressive neurodegenerative disease arising from the misfolding and accumulation of the protein α-synuclein in oligodendrocytes, where it forms glial cytoplasmic inclusions (GCIs). Several years of studying synthetic α-synuclein fibrils has provided critical insight into the ability of α-synuclein to template endogenous protein misfolding, giving rise to fibrillar structures capable of propagating from cell to cell. However, more recent studies with MSA-derived α-synuclein aggregates have shown that they have a similar ability to undergo template-directed propagation, like PrP prions. Almost 20 years after α-synuclein was discovered as the primary component of GCIs, α-synuclein aggregates isolated from MSA patient samples were shown to infect cultured mammalian cells and also to transmit neurological disease to transgenic mice. These findings argue that α-synuclein becomes a prion in MSA patients. In this review, we discuss the in vitro and in vivo data supporting the recent classification of MSA as a prion disease.

Prenatal nicotine exposure enhances the trigeminocardiac reflex via serotonin receptor facilitation in brainstem pathways

In this study we used a rat model for prenatal nicotine exposure to test whether clinically relevant concentrations of brain nicotine and cotinine are passed from dams exposed to nicotine to her pups, whether this changes the trigeminocardiac reflex (TCR), and whether serotonergic function in the TCR brainstem circuitry is altered. Pregnant Sprague-Dawley dams were exposed to 6 mg·kg(-1)·day(-1) of nicotine via osmotic minipumps for the duration of pregnancy. Following birth dams and pups were killed, blood was collected, and brain nicotine and cotinine levels were measured. A separate group of prenatal nicotine-exposed pups was used for electrophysiological recordings. A horizontal brainstem slice was obtained by carefully preserving the trigeminal nerve with fluorescent identification of cardiac vagal neurons (CVNs) in the nucleus ambiguus. Stimulation of the trigeminal nerve evoked excitatory postsynaptic current in CVNs. Our data demonstrate that prenatal nicotine exposure significantly exaggerates both the TCR-evoked changes in heart rate in conscious unrestrained pups, and the excitatory neurotransmission to CVNs upon trigeminal afferent nerve stimulation within this brainstem reflex circuit. Application of the 5-HT1A receptor antagonist WAY 100635 (100 μM) and 5-HT2A/C receptor antagonist ketanserin (10 μM)significantly decreased neurotransmission, indicating an increased facilitation of 5-HT function in prenatal nicotine-exposed animals. Prenatal nicotine exposure enhances activation of 5-HT receptors and exaggerates the trigeminocardiac reflex.

Modulation of bulbospinal rostral ventral lateral medulla neurons by hypoxia/hypercapnia but not medullary respiratory activity.

Although sympathetic vasomotor discharge has respiratory modulation, the site(s) responsible for this cardiorespiratory interaction is unknown. One likely source for this coupling is the rostral ventral lateral medulla (RVLM), where presympathetic neurons originate in close apposition to respiratory neurons. The current study tested the hypothesis that RVLM bulbospinal neurons are modulated by medullary respiratory network activity using whole-cell patch-clamp electrophysiological recordings of RVLM neurons while simultaneously recording fictive respiratory bursting activity from the hypoglossal rootlet. Additionally, we examined whether challenges to cardiorespiratory function, mainly hypoxia/hypercapnia, alter the activity of bulbospinal neurons and, secondarily, whether changes in synaptic input mediate these responses. Surprisingly, our results indicate that inspiratory-related activity did not modulate glutamatergic, &ggr;-aminobutyric acid-ergic, or glycinergic synaptic events or spontaneous action potential firing in these RVLM neurons. However, hypoxia/hypercapnia reversibly decreased the frequency of &ggr;-aminobutyric acid and glycine inhibitory postsynaptic currents. Glycinergic inhibitory postsynaptic current frequency was depressed from the fifth through the 10th minute, whereas the depression of &ggr;-aminobutyric acid-ergic events became significant only at the 10th minute of hypoxia/hypercapnia. On the basis of spontaneous firing activity, there were 2 populations of RVLM bulbospinal neurons. The firing frequency of low-discharging RVLM neurons was facilitated by hypoxia/hypercapnia, and this increase depended on reduced inhibitory neurotransmission. The firing frequency in RVLM neurons with high-discharge rates was inhibited, independent of synaptic input, by hypoxia/hypercapnia. This article demonstrates that sympathetic-respiratory coupling is not active in the neonatal brain stem slice, and reductions in inhibitory neurotransmission to low spontaneously active bulbospinal RVLM neurons are responsible for hypoxia/hypercapnia-elicited increases in activity.

Bioassays and Inactivation of Prions

The experimental study of prions requires a model for their propagation. However, because prions lack nucleic acids, the simple techniques used to replicate bacteria and viruses are not applicable. For much of the history of prion research, time-consuming bioassays in animals were the only option for measuring infectivity. Although cell models and other in vitro tools for the propagation of prions have been developed, they all suffer limitations, and animal bioassays remain the gold standard for measuring infectivity. A wealth of recent data argues that both β-amyloid (Aβ) and tau proteins form prions that cause Alzheimers disease, and α-synuclein forms prions that cause multiple system atrophy and Parkinsons disease. Cell and animal models that recapitulate some of the key features of cell-to-cell spreading and distinct strains of prions can now be measured.

Postnatal Sulfur Dioxide Exposure Reversibly Alters Parasympathetic Regulation of Heart Rate

Perinatal sulfur dioxide exposure disrupts parasympathetic regulation of cardiovascular activity. Here, we examine the relative risks of prenatal versus postnatal exposure to the air pollutant and the reversibility of the cardiovascular effects. Two groups of animals were used for this study. For prenatal exposure, pregnant Sprague–Dawley dams were exposed to 5 parts per million sulfur dioxide for 1 hour daily throughout gestation and with their pups after birth to medical-grade air through 6 days postnatal. For postnatal exposure, dams were exposed to air, and after delivery along with their pups to 5 parts per million sulfur dioxide through postnatal day 6. ECGs were recorded from pups on postnatal day 5 to examine changes in heart rate. Whole-cell patch-clamp electrophysiology was used to examine changes in neurotransmission to cardiac vagal neurons in the nucleus ambiguus on sulfur dioxide exposure. Postnatal sulfur dioxide exposure diminished glutamatergic neurotransmission to cardiac vagal neurons by 40.9% and increased heart rate, whereas prenatal exposure altered neither of these properties. When postnatal exposure concluded on postnatal day 5, excitatory neurotransmission remained decreased through day 6 and returned to basal levels by day 7. ECGs showed that heart rate remained elevated through day 6 and recovered by day 7. On activation of the parasympathetic diving reflex, the response was significantly blunted by postnatal sulfur dioxide exposure through day 7 but recovered by day 8. Postnatal, but not prenatal, exposure to sulfur dioxide can disrupt parasympathetic regulation of cardiovascular activity. Neonates can recover from these effects within 2 to 3 days of discontinued exposure.

Perinatal sulfur dioxide exposure alters brainstem parasympathetic control of heart rate

AIMS Sulfur dioxide (SO₂) is an air pollutant that impedes neonatal development and induces adverse cardiorespiratory health effects, including tachycardia. Here, an animal model was developed that enabled characterization of (i) in vivo alterations in heart rate and (ii) altered activity in brainstem neurons that control heart rate after perinatal SO₂ exposure. METHODS AND RESULTS Pregnant Sprague-Dawley dams and their pups were exposed to 5 parts per million SO₂ for 1 h daily throughout gestation and 6 days postnatal. Electrocardiograms were recorded from pups at 5 days postnatal to examine changes in basal and diving reflex-evoked changes in heart rate following perinatal SO₂ exposure. In vitro studies employed whole-cell patch-clamp electrophysiology to examine changes in neurotransmission to cardiac vagal neurons within the nucleus ambiguus upon SO₂ exposure using a preparation that maintains fictive inspiratory activity recorded from the hypoglossal rootlet. Perinatal SO₂ exposure increased heart rate and blunted the parasympathetic-mediated diving reflex-evoked changes in heart rate. Neither spontaneous nor inspiratory-related inhibitory GABAergic or glycinergic neurotransmission to cardiac vagal neurons was altered by SO₂ exposure. However, excitatory glutamatergic neurotransmission was decreased by 51.2% upon SO₂ exposure. This diminished excitatory neurotransmission was tetrodotoxin-sensitive, indicating SO₂ exposure impaired the activity of preceding glutamatergic neurons that synapse upon cardiac vagal neurons. CONCLUSIONS Diminished glutamatergic, but unaltered inhibitory neurotransmission to cardiac vagal neurons provides a mechanism for the observed SO₂-induced elevated heart rate via an impairment of brainstem cardioinhibitory parasympathetic activity to the heart.

Kinetics of Human Mutant Tau Prion Formation in the Brains of 2 Transgenic Mouse Lines

Importance Accumulation of the protein tau is a defining characteristic of several neurodegenerative diseases. Thorough assessment of transgenic (Tg) mouse lines that replicate this process is critical for establishing the models used for testing anti-tau therapeutics in vivo. Objective To define a consistent mouse model of disease for use in future compound efficacy studies. Design, Setting, and Participants In this time course study, cohorts of Tg and control mice were euthanized at defined intervals. Collected brains were bisected down the midline. One half was frozen and used to measure the tau prion content, while the other half was fixed for immunostaining with anti-tau antibodies. All mice were maintained at the Hunters Point Animal Facility at the University of California, San Francisco, and all experiments were performed at the Mission Bay Campus of the University of California, San Francisco. Study animals were PS19, homozygous and hemizygous Tg(MAPT*P301S), and B6/J mice. The study dates were August 9, 2010, to October 3, 2016. Main Outcomes and Measures Tau prions were measured using a cell-based assay. Neuropathology was measured by determining the percentage area positive for immunostaining in defined brain regions. A separate cohort of mice was aged until each mouse developed neurological signs as determined by trained animal technicians to assess mortality. Results A total of 1035 mice were used in this time course study. These included PS19 mice (51.2% [126 of 246] male and 48.8% [120 of 246] female), Tg(MAPT*P301S+/+) mice (52.3% [216 of 413] male, 43.8% [181 of 413] female, and 3.9% [16 of 413] undetermined), Tg(MAPT*P301S+/−) mice (51.8% [101 of 195] male and 48.2% [94 of 195] female), and B6/J mice (49.7% [90 of 181] male and 50.3% [91 of 181] female). While considerable interanimal variability in neuropathology, disease onset, and tau prion formation in the PS19 mice was observed, all 3 measures of disease were more uniform in the Tg(MAPT*P301S+/+) mice. Comparing tau prion formation in Tg(MAPT*P301S+/+) mice with B6/J controls, the 95% CIs for the 2 mouse lines diverged before age 5 weeks, and significant (P < .05) neuropathology in the hindbrain of 24-week-old mice was quantifiable. Conclusions and Relevance The assessment of disease progression using 3 criteria showed that disease onset in PS19 mice is too variable to obtain reliable measurements for drug discovery research. However, the reproducibility of tau prion formation in young Tg(MAPT*P301S+/+) mice establishes a rapid assay for compound efficacy in vivo.