Donato A. Di Monte

Rapid ATP Loss Caused by Methamphetamine in the Mouse Striatum: Relationship Between Energy Impairment and Dopaminergic Neurotoxicity

Abstract: To study the relationship between energy impairment and the effects of α‐methamphetamine (METH) on dopaminergic neurons, ATP and dopamine levels were measured in the brain of C57BL/6 mice treated with either a single or four injections of METH (10 mg/kg, i.p.) at 2‐h intervals. Neither striatal ATP nor dopamine concentrations changed after a single injection of METH, but both were significantly decreased 1.5 h after the multiple‐dose regimen. The effects of METH on ATP levels appear to be selective for the striatum, as ATP concentrations were not affected in the cerebellar cortex and hippocampus after either a single or multiple injections of METH. In a second set of experiments, an intraperitoneal injection of 2‐deoxyglucose (2‐DG; 1 g/kg), an inhibitor of glucose uptake and utilization, was given 30 min before the third and fourth injections of METH. 2‐DG significantly potentiated METH‐induced striatal ATP loss at 1.5 h and dopamine depletions at 1.5 h and 1 week. These results indicate that a toxic regimen of METH selectively causes striatal energy impairment and raise the possibility that perturbations of energy metabolism play a role in METH‐induced dopaminergic neurotoxicity.

Caudo-rostral brain spreading of α-synuclein through vagal connections.

α‐Synuclein accumulation and pathology in Parkinsons disease typically display a caudo‐rostral pattern of progression, involving neuronal nuclei in the medulla oblongata at the earliest stages. In this study, selective expression and accumulation of human α‐synuclein within medullary neurons was achieved via retrograde transport of adeno‐associated viral vectors unilaterally injected into the vagus nerve in the rat neck. The exogenous protein progressively spread toward more rostral brain regions where it could be detected within axonal projections. Propagation to the pons, midbrain and forebrain followed a stereotypical pattern of topographical distribution. It affected areas such as the coeruleus–subcoeruleus complex, dorsal raphae, hypothalamus and amygdala ipsilateral and, to a lesser extent, contralateral to the injection side. Spreading was accompanied by evidence of neuritic pathology in the form of axonal varicosities intensely immunoreactive for human α‐synuclein and containing Thioflavin‐S‐positive fibrils. Thus, overexpression of human α‐synuclein in the lower brainstem is sufficient to induce its long‐distance caudo‐rostral propagation, recapitulating features of Parkinsons disease and mechanisms of disease progression.

Oxidative and nitrative alpha-synuclein modifications and proteostatic stress: implications for disease mechanisms and interventions in synucleinopathies.

Alpha‐synuclein (ASYN) is a major constituent of the typical protein aggregates observed in several neurodegenerative diseases that are collectively referred to as synucleinopathies. A causal involvement of ASYN in the initiation and progression of neurological diseases is suggested by observations indicating that single‐point (e.g., A30P, A53T) or multiplication mutations of the gene encoding for ASYN cause early onset forms of Parkinsons disease (PD). The relative regional specificity of ASYN pathology is still a riddle that cannot be simply explained by its expression pattern. Also, transgenic over‐expression of ASYN in mice does not recapitulate the typical dopaminergic neuronal death observed in PD. Thus, additional factors must contribute to ASYN‐related toxicity. For instance, synucleinopathies are usually associated with inflammation and elevated levels of oxidative stress in affected brain areas. In turn, these conditions favor oxidative modifications of ASYN. Among these modifications, nitration of tyrosine residues, formation of covalent ASYN dimers, as well as methionine sulfoxidations are prominent examples that are observed in post‐mortem PD brain sections. Oxidative modifications can affect ASYN aggregation, as well as its binding to biological membranes. This would affect neurotransmitter recycling, mitochondrial function and dynamics (fission/fusion), ASYNs degradation within a cell and, possibly, the transfer of modified ASYN to adjacent cells. Here, we propose a model on how covalent modifications of ASYN link energy stress, altered proteostasis, and oxidative stress, three major pathogenic processes involved in PD progression. Moreover, we hypothesize that ASYN may act physiologically as a catalytically regenerated scavenger of oxidants in healthy cells, thus performing an important protective role prior to the onset of disease or during aging.

Neurodegeneration by activation of the microglial complement-phagosome pathway

Systemic inflammatory reactions have been postulated to exacerbate neurodegenerative diseases via microglial activation. We now demonstrate in vivo that repeated systemic challenge of mice over four consecutive days with bacterial LPS maintained an elevated microglial inflammatory phenotype and induced loss of dopaminergic neurons in the substantia nigra. The same total cumulative LPS dose given within a single application did not induce neurodegeneration. Whole-genome transcriptome analysis of the brain demonstrated that repeated systemic LPS application induced an activation pattern involving the classical complement system and its associated phagosome pathway. Loss of dopaminergic neurons induced by repeated systemic LPS application was rescued in complement C3-deficient mice, confirming the involvement of the complement system in neurodegeneration. Our data demonstrate that a phagosomal inflammatory response of microglia is leading to complement-mediated loss of dopaminergic neurons.

Brain-to-stomach transfer of α-synuclein via vagal preganglionic projections

Detection of α-synuclein lesions in peripheral tissues is a feature of human synucleinopathies of likely pathogenetic relevance and bearing important clinical implications. Experiments were carried out to elucidate the relationship between α-synuclein accumulation in the brain and in peripheral organs, and to identify potential pathways involved in long-distance protein transfer. Results of this in vivo study revealed a route-specific transmission of α-synuclein from the rat brain to the stomach. Following targeted midbrain overexpression of human α-synuclein, the exogenous protein was capable of reaching the gastric wall where it was accumulated into preganglionic vagal terminals. This brain-to-stomach connection likely involved intra- and inter-neuronal transfer of non-fibrillar α-synuclein that first reached the medulla oblongata, then gained access into cholinergic neurons of the dorsal motor nucleus of the vagus nerve and finally traveled via efferent fibers of these neurons contained within the vagus nerve. Data also showed a particular propensity of vagal motor neurons and efferents to accrue α-synuclein and deliver it to peripheral tissues; indeed, following its midbrain overexpression, human α-synuclein was detected within gastric nerve endings of visceromotor but not viscerosensory vagal projections. Thus, the dorsal motor nucleus of the vagus nerve represents a key relay center for central-to-peripheral α-synuclein transmission, and efferent vagal fibers may act as unique conduits for protein transfer. The presence of α-synuclein in peripheral tissues could reflect, at least in some synucleinopathy patients, an ongoing pathological process that originates within the brain and, from there, reaches distant organs innervated by motor vagal projections.

Brain propagation of transduced α-synuclein involves non-fibrillar protein species and is enhanced in α-synuclein null mice

Aggregation and neuron-to-neuron transmission are attributes of α-synuclein relevant to its pathogenetic role in human synucleinopathies such as Parkinsons disease. Intraparenchymal injections of fibrillar α-synuclein trigger widespread propagation of amyloidogenic protein species via mechanisms that require expression of endogenous α-synuclein and, possibly, its structural corruption by misfolded conformers acting as pathological seeds. Here we describe another paradigm of long-distance brain diffusion of α-synuclein that involves inter-neuronal transfer of monomeric and/or oligomeric species and is independent of recruitment of the endogenous protein. Targeted expression of human α-synuclein was induced in the mouse medulla oblongata through an injection of viral vectors into the vagus nerve. Enhanced levels of intra-neuronal α-synuclein were sufficient to initiate its caudo-rostral diffusion that likely involved at least one synaptic transfer and progressively reached specific brain regions such as the locus coeruleus, dorsal raphae and amygdala in the pons, midbrain and forebrain. Transfer of human α-synuclein was compared in two separate lines of α-synuclein-deficient mice versus their respective wild-type controls and, interestingly, lack of endogenous α-synuclein expression did not counteract diffusion but actually resulted in a more pronounced and advanced propagation of exogenous α-synuclein. Self-interaction of adjacent molecules of human α-synuclein was detected in both wild-type and mutant mice. In the former, interaction of human α-synuclein with mouse α-synuclein was also observed and might have contributed to differences in protein transmission. In wild-type and α-synuclein-deficient mice, accumulation of human α-synuclein within recipient axons in the pons, midbrain and forebrain caused morphological evidence of neuritic pathology. Tissue sections from the medulla oblongata and pons were stained with different antibodies recognizing oligomeric, fibrillar and/or total (monomeric and aggregated) α-synuclein. Following viral vector transduction, monomeric, oligomeric and fibrillar protein was detected within donor neurons in the medulla oblongata. In contrast, recipient axons in the pons were devoid of immunoreactivity for fibrillar α-synuclein, indicating that non-fibrillar forms of α-synuclein were primarily transferred from one neuron to the other, diffused within the brain and led to initial neuronal injury. This study elucidates a paradigm of α-synuclein propagation that may play a particularly important role under pathophysiological conditions associated with enhanced α-synuclein expression. Rapid long-distance diffusion and accumulation of monomeric and oligomeric α-synuclein does not necessarily involve pathological seeding but could still result in a significant neuronal burden during the pathogenesis of neurodegenerative diseases.

α-Synuclein elevation in human neurodegenerative diseases: experimental, pathogenetic, and therapeutic implications.

The discovery of α-synuclein has had profound implications concerning our understanding of Parkinson’s disease (PD) and other neurodegenerative disorders characterized by α-synuclein accumulation. In fact, as compared with pre-α-synuclein times, a “new” PD can now be described as a whole-body disease in which a progressive spreading of α-synuclein pathology underlies a wide spectrum of motor as well as nonmotor clinical manifestations. Not only is α-synuclein accumulation a pathological hallmark of human α-synucleinopathies but increased protein levels are sufficient to trigger neurodegenerative processes. α-Synuclein elevations could also be a mechanism by which disease risk factors (e.g., aging) increase neuronal vulnerability to degeneration. An important corollary to the role of enhanced α-synuclein in PD pathogenesis is the possibility of developing α-synuclein-based biomarkers and new therapeutics aimed at suppressing α-synuclein expression. The use of in vitro and in vivo experimental models, including transgenic mice overexpressing α-synuclein and animals with viral vector-mediated α-synuclein transduction, has helped clarify pathogenetic mechanisms and therapeutic strategies involving α-synuclein. These models are not devoid of significant limitations, however. Therefore, further pursuit of new clues on the cause and treatment of PD in this post-α-synuclein era would benefit substantially from the development of improved research paradigms of α-synuclein elevation.

Function and developmental origin of a mesocortical inhibitory circuit

Midbrain ventral tegmental neurons project to the prefrontal cortex and modulate cognitive functions. Using viral tracing, optogenetics and electrophysiology, we found that mesocortical neurons in the mouse ventrotegmental area provide fast glutamatergic excitation of GABAergic interneurons in the prefrontal cortex and inhibit prefrontal cortical pyramidal neurons in a robust and reliable manner. These mesocortical neurons were derived from a subset of dopaminergic progenitors, which were dependent on prolonged Sonic Hedgehog signaling for their induction. Loss of these progenitors resulted in the loss of the mesocortical inhibitory circuit and an increase in perseverative behavior, whereas mesolimbic and mesostriatal dopaminergic projections, as well as impulsivity and attentional function, were largely spared. Thus, we identified a previously uncharacterized mesocortical circuit contributing to perseverative behaviors and found that the diversity of dopaminergic neurons begins to be established during their progenitor phase.

Tipping Points and Endogenous Determinants of Nigrostriatal Degeneration by MPTP

The neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes a Parkinsons disease (PD)-like syndrome by inducing degeneration of nigrostriatal dopaminergic neurons. Studies of the MPTP model have revealed the pathomechanisms underlying dopaminergic neurodegeneration and facilitated the development of drug treatments for PD. In this review, we provide an update on MPTP bioactivation and biodistribution, reconcile the distinct views on energetic failure versus reactive oxygen species (ROS) formation as main drivers of MPTP-induced neurodegeneration, and describe recently identified intrinsic features of the nigrostriatal system that make it particularly vulnerable to MPTP. We discuss these new perspectives on the endogenous tipping points of tissue homeostasis and the drivers responsible for vicious cycles in relation to their relevance for the development of novel intervention strategies for PD.

The L444P Gba1 mutation enhances alpha-synuclein induced loss of nigral dopaminergic neurons in mice

Mutations in glucocerebrosidase 1 (GBA1) are the commonest risk factor for Parkinson’s disease. Using Gba1 mouse models, Migdalska-Richards et al. reveal an interaction between glucocerebrosidase activity and alpha-synuclein accumulation. Gba1 deficiency increases vulnerability of nigral dopaminergic neurons to degeneration triggered by increased alpha-synuclein expression.