The Gut-Brain Axis in Parkinson's Disease.

Parkinson's disease (PD) involves both the central nervous system (CNS) and enteric nervous system (ENS), and their interaction is important for understanding both the clinical manifestations of the disease and the underlying disease pathophysiology. Although the neuroanatomical distribution of pathology strongly suggests that the ENS is involved in disease pathophysiology, there are significant gaps in knowledge about the underlying mechanisms. In this article, we review the clinical presentation and management of gastrointestinal dysfunction in PD. In addition, we discuss the current understanding of disease pathophysiology in the gut, including controversies about early involvement of the gut in disease pathogenesis. We also review current knowledge about gut α-synuclein and the microbiome, discuss experimental models of PD-linked gastrointestinal pathophysiology, and highlight areas for further research. Finally, we discuss opportunities to use the gut-brain axis for the development of biomarkers and disease-modifying treatments.

Perspectives of People At-Risk on Parkinson's Prevention Research.

 The movement toward prevention trials in people at-risk for Parkinson's disease (PD) is rapidly becoming a reality. The authors of this article include a genetically at-risk advocate with the LRRK2 G2019 S variant and two patients with rapid eye movement sleep behavior disorder (RBD), one of whom has now been diagnosed with PD. These authors participated as speakers, panelists, and moderators in the "Planning for Prevention of Parkinson's: A Trial Design Forum" hosted by Massachusetts General Hospital in 2021 and 2022. Other authors include a young onset person with Parkinson's (PwP) and retired family physician, an expert in patient engagement in Parkinson's, and early career and veteran movement disorders clinician researchers. Several themes emerged from the at-risk participant voice concerning the importance of early intervention, the legitimacy of their input in decision-making, and the desire for transparent communication and feedback throughout the entire research study process. Challenges and opportunities in the current environment include lack of awareness among primary care physicians and general neurologists about PD risk, legal and psychological implications of risk disclosure, limited return of individual research study results, and undefined engagement and integration of individuals at-risk into the broader Parkinson's community. Incorporating the perspectives of individuals at-risk as well as those living with PD at this early stage of prevention trial development is crucial to success.

Functional and Pathological Effects of α-Synuclein on Synaptic SNARE Complexes.

α-Synuclein is an abundant protein at the neuronal synapse that has been implicated in Parkinson's disease for over 25 years and characterizes the hallmark pathology of a group of neurodegenerative diseases now known as the synucleinopathies. Physiologically, α-synuclein exists in an equilibrium between a synaptic vesicle membrane-bound α-helical multimer and a cytosolic largely unstructured monomer. Through its membrane-bound state, α-synuclein functions in neurotransmitter release by modulating several steps in the synaptic vesicle cycle, including synaptic vesicle clustering and docking, SNARE complex assembly, and homeostasis of synaptic vesicle pools. These functions have been ascribed to α-synuclein's interactions with the synaptic vesicle SNARE protein VAMP2/synaptobrevin-2, the synaptic vesicle-attached synapsins, and the synaptic vesicle membrane itself. How α-synuclein affects these processes, and whether disease is due to loss-of-function or gain-of-toxic-function of α-synuclein remains unclear. In this review, we provide an in-depth summary of the existing literature, discuss possible reasons for the discrepancies in the field, and propose a working model that reconciles the findings in the literature.

Synaptic vesicle binding of α-synuclein is modulated by ß- and γ-synucleins.

α-synuclein, ß-synuclein, and γ-synuclein are abundantly expressed proteins in the vertebrate nervous system. α-synuclein functions in neurotransmitter release by binding to and clustering synaptic vesicles and chaperoning SNARE-complex assembly. Pathologically, aggregates originating from soluble pools of α-synuclein are deposited into Lewy bodies in Parkinson's disease and related synucleinopathies. The functions of ß-synuclein and γ-synuclein in presynaptic terminals remain poorly studied. Using in vitro liposome binding studies, circular dichroism spectroscopy, immunoprecipitation, and fluorescence resonance energy transfer (FRET) experiments on isolated synaptic vesicles in combination with subcellular fractionation of brains from synuclein mouse models, we show that ß-synuclein and γ-synuclein have a reduced affinity toward synaptic vesicles compared with α-synuclein, and that heteromerization of ß-synuclein or γ-synuclein with α-synuclein results in reduced synaptic vesicle binding of α-synuclein in a concentration-dependent manner. Our data suggest that ß-synuclein and γ-synuclein are modulators of synaptic vesicle binding of α-synuclein and thereby reduce α-synuclein's physiological activity at the neuronal synapse.

Posterior Reversible Encephalopathy Syndrome.

PURPOSE OF REVIEW: This review provides an updated discussion on the clinical presentation, diagnosis and radiographic features, mechanisms, associations and epidemiology, treatment, and prognosis of posterior reversible encephalopathy syndrome (PRES). Headache is common in PRES, though headache associated with PRES was not identified as a separate entity in the 2018 International Classification of Headache Disorders. Here, we review the relevant literature and suggest criteria for consideration of its inclusion. RECENT FINDINGS: COVID-19 has been identified as a potential risk factor for PRES, with a prevalence of 1-4% in patients with SARS-CoV-2 infection undergoing neuroimaging, thus making a discussion of its identification and treatment particularly timely given the ongoing global pandemic at the time of this writing. PRES is a neuro-clinical syndrome with specific imaging findings. The clinical manifestations of PRES include headache, seizures, encephalopathy, visual disturbances, and focal neurologic deficits. Associations with PRES include renal failure, preeclampsia and eclampsia, autoimmune conditions, and immunosuppression. PRES is theorized to be a syndrome of disordered autoregulation and endothelial dysfunction resulting in preferential hyperperfusion of the posterior circulation. Treatment typically focuses on treating the underlying cause and removal of the offending agents.

Inpatient Child and Adolescent Psychiatry as an Extracurricular Venue for Medical Student and Resident Education and Professional Development: Story Time and Teen Talk.

OBJECTIVE: Comparatively little systematic data exists concerning medical student education from the inpatient child and adolescent psychiatry venue. Training in this venue is vulnerable to pressures including increasingly reduced lengths of stay and greater emphasis on clinical productivity. An extracurricular psychosocial opportunity during evening hours may be a productive means through which to provide meaningful patient exposure, training, and mentorship to trainees. We sought to evaluate the impact upon student beliefs and attitudes through participation in an initiative titled "Story Time and Teen Talk." METHODS: Under the direct supervision of postgraduate resident physicians in psychiatry, students read fairy tales and children's literature to children and conducted group discussions with adolescents during weekly held evening hours. Students were invited to complete a 15-item questionnaire that surveyed the impact of their participation on their beliefs and attitudes concerning general medical education and patient care, the field of psychiatry, and training in child and adolescent psychiatry. A subset of students underwent a semi-structured interview which was evaluated via grounded theory analysis to determine qualitative themes related to impact of program participation. RESULTS: Thirty students (N = 30) completed the survey and five students (N = 5) completed the interview. The majority of students reported strongly agreeing or agreeing that participation impacted their attitudes and beliefs about general medical education and patient care in twelve of the thirteen assayed items. Themes from qualitative analysis supported these findings. CONCLUSIONS: Extracurricular opportunities may be a productive venue for graduate medical education and, specifically, child and adolescent psychiatry.

Posterior reversible encephalopathy syndrome in patients with COVID-19.

OBJECTIVE: To report four patients with coronavirus disease 2019 (COVID-19) who developed posterior reversible encephalopathy syndrome (PRES). METHODS: Patient data was abstracted from medical records at Weill Cornell Medical Center. RESULTS: Four patients with SARS-CoV-2 infection and PRES were identified. The patients' ages ranged from 64 to 74 years, and two were women. All four patients were admitted to the hospital with acute respiratory distress syndrome requiring intensive care unit admission and mechanical ventilation. PRES was diagnosed after persistent confusion, lethargy, new focal neurological deficits, or seizures were noted, with evidence of seizures on electroencephalogram for two of the patients. Imaging confirmed the presence of cerebral vasogenic edema. All four patients had elevated blood pressure and renal injury in the days preceding PRES diagnosis, as well as evidence of systemic inflammation and systemic hypercoagulability. Symptoms of PRES improved with blood pressure control. CONCLUSIONS: Our four cases demonstrate the occurrence of PRES in critically-ill patients with COVID-19. PRES should be considered in the differential for acute neurological deficits and seizures in this setting.

Wilm's tumor 1 promotes memory flexibility.

Under physiological conditions, strength and persistence of memory must be regulated in order to produce behavioral flexibility. In fact, impairments in memory flexibility are associated with pathologies such as post-traumatic stress disorder or autism; however, the underlying mechanisms that enable memory flexibility are still poorly understood. Here, we identify transcriptional repressor Wilm's Tumor 1 (WT1) as a critical synaptic plasticity regulator that decreases memory strength, promoting memory flexibility. WT1 is activated in the hippocampus following induction of long-term potentiation (LTP) or learning. WT1 knockdown enhances CA1 neuronal excitability, LTP and long-term memory whereas its overexpression weakens memory retention. Moreover, forebrain WT1-deficient mice show deficits in both reversal, sequential learning tasks and contextual fear extinction, exhibiting impaired memory flexibility. We conclude that WT1 limits memory strength or promotes memory weakening, thus enabling memory flexibility, a process that is critical for learning from new experiences.

Lactate from astrocytes fuels learning-induced mRNA translation in excitatory and inhibitory neurons.

Glycogenolysis and lactate transport from astrocytes to neurons is required for long-term memory formation, but the role of this lactate is poorly understood. Here we show that the Krebs cycle substrates pyruvate and ketone body B3HB can functionally replace lactate in rescuing memory impairment caused by inhibition of glycogenolysis or expression knockdown of glia monocarboxylate transporters (MCTs) 1 and 4 in the dorsal hippocampus of rats. In contrast, either metabolite is unable to rescue memory impairment produced by expression knockdown of MCT2, which is selectively expressed by neurons, indicating that a critical role of astrocytic lactate is to provide energy for neuronal responses required for long-term memory. These responses include learning-induced mRNA translation in both excitatory and inhibitory neurons, as well as expression of Arc/Arg3.1. Thus, astrocytic lactate acts as an energy substrate to fuel learning-induced de novo neuronal translation critical for long-term memory.

Astrocyte glycogen and lactate: New insights into learning and memory mechanisms.

Memory, the ability to retain learned information, is necessary for survival. Thus far, molecular and cellular investigations of memory formation and storage have mainly focused on neuronal mechanisms. In addition to neurons, however, the brain comprises other types of cells and systems, including glia and vasculature. Accordingly, recent experimental work has begun to ask questions about the roles of non-neuronal cells in memory formation. These studies provide evidence that all types of glial cells (astrocytes, oligodendrocytes, and microglia) make important contributions to the processing of encoded information and storing memories. In this review, we summarize and discuss recent findings on the critical role of astrocytes as providers of energy for the long-lasting neuronal changes that are necessary for long-term memory formation. We focus on three main findings: first, the role of glucose metabolism and the learning- and activity-dependent metabolic coupling between astrocytes and neurons in the service of long-term memory formation; second, the role of astrocytic glucose metabolism in arousal, a state that contributes to the formation of very long-lasting and detailed memories; and finally, in light of the high energy demands of the brain during early development, we will discuss the possible role of astrocytic and neuronal glucose metabolisms in the formation of early-life memories. We conclude by proposing future directions and discussing the implications of these findings for brain health and disease. Astrocyte glycogenolysis and lactate play a critical role in memory formation. Emotionally salient experiences form strong memories by recruiting astrocytic ß2 adrenergic receptors and astrocyte-generated lactate. Glycogenolysis and astrocyte-neuron metabolic coupling may also play critical roles in memory formation during development, when the energy requirements of brain metabolism are at their peak.

Astrocytic ß2-adrenergic receptors mediate hippocampal long-term memory consolidation.

Emotionally relevant experiences form strong and long-lasting memories by critically engaging the stress hormone/neurotransmitter noradrenaline, which mediates and modulates the consolidation of these memories. Noradrenaline acts through adrenergic receptors (ARs), of which ß2-adrenergic receptors (ßARs) are of particular importance. The differential anatomical and cellular distribution of ßAR subtypes in the brain suggests that they play distinct roles in memory processing, although much about their specific contributions and mechanisms of action remains to be understood. Here we show that astrocytic rather than neuronal ß2ARs in the hippocampus play a key role in the consolidation of a fear-based contextual memory. These hippocampal ß2ARs, but not ß1ARs, are coupled to the training-dependent release of lactate from astrocytes, which is necessary for long-term memory formation and for underlying molecular changes. This key metabolic role of astrocytic ß2ARs may represent a novel target mechanism for stress-related psychopathologies and neurodegeneration.

The Role of Lactate-Mediated Metabolic Coupling between Astrocytes and Neurons in Long-Term Memory Formation.

Long-term memory formation, the ability to retain information over time about an experience, is a complex function that affects multiple behaviors, and is an integral part of an individual's identity. In the last 50 years many scientists have focused their work on understanding the biological mechanisms underlying memory formation and processing. Molecular studies over the last three decades have mostly investigated, or given attention to, neuronal mechanisms. However, the brain is composed of different cell types that, by concerted actions, cooperate to mediate brain functions. Here, we consider some new insights that emerged from recent studies implicating astrocytic glycogen and glucose metabolisms, and particularly their coupling to neuronal functions via lactate, as an essential mechanism for long-term memory formation.

Dishevelled-2 regulates cocaine-induced structural plasticity and Rac1 activity in the nucleus accumbens.

Chronic cocaine exposure increases the density of dendritic spines on medium spiny neurons (MSNs), the predominant neuronal cell type of the nucleus accumbens (NAc), a key brain reward region. We recently showed that suppression of Rac1, a small GTPase, is a critical mediator of this structural plasticity, but the upstream determinants of Rac1 activity in this context remain to be elucidated. In this study we examined whether isoforms of Dishevelled, a key hub protein of multiple branches of Wnt signaling, including Rac1, are regulated in the NAc by chronic cocaine, and whether these Dishevelled isoforms control Rac1 activity in this brain region in vivo. We found that chronic cocaine administration decreased expression of Dishevelled-2, and several other Wnt signaling components, in the NAc, and that overexpression of Dishevelled-2, but not Dishevelled-1, conversely upregulated Rac1 activity and prevented the cocaine induction of dendritic spines on NAc MSNs. We posit that the cocaine-induced downregulation of Dishevelled-2 in the NAc is an upstream regulator of Rac1 activity and plays an important role in the dynamic structural plasticity of NAc MSNs seen in response to chronic cocaine exposure.

Rac1 is essential in cocaine-induced structural plasticity of nucleus accumbens neurons.

Repeated cocaine administration increases the dendritic arborization of nucleus accumbens neurons, but the underlying signaling events remain unknown. Here we show that repeated exposure to cocaine negatively regulates the active form of Rac1, a small GTPase that controls actin remodeling in other systems. Further, we show, using viral-mediated gene transfer, that overexpression of a dominant negative mutant of Rac1 or local knockout of Rac1 is sufficient to increase the density of immature dendritic spines on nucleus accumbens neurons, whereas overexpression of a constitutively active Rac1 or light activation of a photoactivatable form of Rac1 blocks the ability of repeated cocaine exposure to produce this effect. Downregulation of Rac1 activity likewise promotes behavioral responses to cocaine exposure, with activation of Rac1 producing the opposite effect. These findings establish that Rac1 signaling mediates structural and behavioral plasticity in response to cocaine exposure.