Brief mitochondrial inhibition causes lasting changes in motor behavior and corticostriatal synaptic physiology in the Fischer 344 rat

Analysis of Mitochondrial Dysfunction by Microplate Reader in hiPSC-Derived Neuronal Cell Models of Neurodegenerative Disorders

Mitochondria are responsible for many vital pathways governing cellular homeostasis, including cellular energy management, heme biosynthesis, lipid metabolism, cellular proliferation and differentiation, cell cycle regulation, and cellular viability. Electron transport and ADP phosphorylation coupled with proton pumping through the mitochondrial complexes contribute to the preservation of mitochondrial membrane potential (ΔΨ_m). Importantly, mitochondrial polarization is essential for reactive oxygen species (ROS) production and cytosolic calcium (Ca²⁺) handling. Thus, changes in mitochondrial oxidative phosphorylation (OXPHOS), ΔΨ_m, and ATP/ADP may occur in parallel or stimulate each other. Brain cells like neurons are heavily reliant on mitochondrial OXPHOS for its high-energy demands, and hence improper mitochondrial function is detrimental for neuronal survival. Indeed, several neurodegenerative disorders are associated with mitochondrial dysfunction. Modeling this disease-relevant phenotype in neuronal cells differentiated from patient-derived human induced pluripotent stem cells (hiPSCs) provide an appropriate cellular platform for studying the disease pathology and drug discovery. In this review, we describe high-throughput analysis of crucial parameters related to mitochondrial function in hiPSC-derived neurons. These methodologies include measurement of ΔΨ_m, intracellular Ca²⁺, oxidative stress, and ATP/ADP levels using fluorescence probes via a microplate reader. Benefits of such an approach include analysis of mitochondrial parameters on a large population of cells, simultaneous analysis of different cell lines and experimental conditions, and for drug screening to identify compounds restoring mitochondrial function.

Neonatal Rotenone Administration Induces Psychiatric Disorder-Like Behavior and Changes in Mitochondrial Biogenesis and Synaptic Proteins in Adulthood

Since psychiatric disorders are associated with changes in the development of the nervous system, an energy-dependent mechanism, we investigated whether mitochondrial inhibition during the critical neurodevelopment window in rodents would be able to induce metabolic alterations culminating in psychiatric-like behavior. We treated male Wistar rat puppies (P) with rotenone (Rot), an inhibitor of mitochondrial complex I, from postnatal days 5 to 11 (P5-P11). We demonstrated that at P60 and P120, Rot-treated animals showed hyperlocomotion and deficits in social interaction and aversive contextual memory, features observed in animal models of schizophrenia, autism spectrum disorder, and attention deficit hyperactivity disorder. During adulthood, Rot-treated rodents also presented modifications in CBP and CREB levels in addition to a decrease in mitochondrial biogenesis and Nrf1 expression. Additionally, NFE2L2-activation was not altered in Rot-treated P60 and P120 animals; an upregulation of pNFE2L2/ NFE2L2 was only observed in P12 cortices. Curiously, ATP/ADP levels did not change in all ages evaluated. Rot administration in newborn rodents also promoted modification in Rest and Mecp2 expression, and in synaptic protein levels, named PSD-95, Synaptotagmin-1, and Synaptophysin in the adult rats. Altogether, our data indicate that behavioral abnormalities and changes in synaptic proteins in adulthood induced by neonatal Rot administration might be a result of adjustments in CREB pathways and alterations in mitochondrial biogenesis and Nrf1 expression, rather than a direct deficiency of energy supply, as previously speculated. Consequently, Rot-induced psychiatric-like behavior would be an outcome of alterations in neuronal paths due to mitochondrial deregulation.

Physiological aging at striatal synapses

Mike Levine's body of work guides thinking on how the basal ganglia process information to create coordinated movements and skill learning throughout the life span and in disease. This special issue is a nod to Mike's career and a well-deserved gesture by the neuroscience community thanking him for the impact he has made on many people's careers and the field of basal ganglia physiology. This paper reviews how aging impacts basal ganglia processing with a focus on single cell and synaptic physiology. This review begins with the work Mike did with his collaborators Nat Buchwald, Chester Hull and Jay Schneider. These early studies paved the way for subsequent studies on changes in synaptic processing that occur with aging in the basal ganglia. The primary focus of this review is aging at corticostriatal synapses. Corticostriatal synapses show reduced expression of both short-term and long-term synaptic potentiation. The roles of age-related changes in calcium homeostasis, vesicle cycling, dopamine modulation, and NMDA receptor function in aging's effect on synaptic plasticity are discussed. The article ends with a review of mitochondrial aging theory as it applies to age-induced changes in corticostriatal synaptic function.

Ribosomal Protein S6 Phosphorylation Is Involved in Novelty-Induced Locomotion, Synaptic Plasticity and mRNA Translation

The phosphorylation of the ribosomal protein S6 (rpS6) is widely used to track neuronal activity. Although it is generally assumed that rpS6 phosphorylation has a stimulatory effect on global protein synthesis in neurons, its exact biological function remains unknown. By using a phospho-deficient rpS6 knockin mouse model, we directly tested the role of phospho-rpS6 in mRNA translation, plasticity and behavior. The analysis of multiple brain areas shows for the first time that, in neurons, phospho-rpS6 is dispensable for overall protein synthesis. Instead, we found that phospho-rpS6 controls the translation of a subset of mRNAs in a specific brain region, the nucleus accumbens (Acb), but not in the dorsal striatum. We further show that rpS6 phospho-mutant mice display altered long-term potentiation (LTP) in the Acb and enhanced novelty-induced locomotion. Collectively, our findings suggest a previously unappreciated role of phospho-rpS6 in the physiology of the Acb, through the translation of a selective subclass of mRNAs, rather than the regulation of general protein synthesis.

Spodoptera littoralis detoxifies neurotoxic 3-nitropropanoic acid by conjugation with amino acids

Spodoptera littoralis is a phytophagous generalist. Its host range includes more than 40 plant species, some of which produce 3-nitropropanoic acid (3-NPA), an irreversible inhibitor of mitochondrial succinate dehydrogenase. Growth in larvae fed an artificial diet with a sublethal admixture of 3-NPA (4.2 μmol per g) was slowed significantly, but larvae experienced no increase in mortality. In contrast, larvae injected with 25.2 μmol/g (bodyweight) 3-NPA experienced acute toxicity and death. To study the detoxification mechanism of 3-NPA in S. littoralis, the insect frass was analyzed by HPLC-MS. Comparative analysis of 3-NPA-treated and -untreated control samples using HR-MS(2) revealed a group of differential signals that were identified as amino acid amides of 3-NPA with glycine, alanine, serine, and threonine. When sublethal amounts of stable isotope-labeled 3-NPA were injected into a larva's hemolymph, 3-NPA amino acid conjugates were identified as putative detoxification products. Bioassays with synthetic standards confirmed that the toxicity of the amides was negligible in comparison to the toxicity of free 3-NPA, demonstrating that amino acid conjugation in S. littoralis represents an efficient way to detoxify 3-NPA. Furthermore, biosynthetic studies using crude fractions of the gut tissue indicated that conjugation of 3-NPA with amino acids occurs in epithelial cells of the insect's gut. Taken together, these results suggest that the detoxification of 3-NPA in S. littoralis proceeds via conjugation to specific amino acids within the epithelial cells followed by export of the nontoxic amino acid conjugates to the hemolymph via as yet uncharacterized mechanisms, most likely involving the Malpighian tubules.

Age-related alterations in the expression of genes and synaptic plasticity associated with nitric oxide signaling in the mouse dorsal striatum

Age-related alterations in the expression of genes and corticostriatal synaptic plasticity were studied in the dorsal striatum of mice of four age groups from young (2-3 months old) to old (18-24 months of age) animals. A significant decrease in transcripts encoding neuronal nitric oxide (NO) synthase and receptors involved in its activation (NR1 subunit of the glutamate NMDA receptor and D1 dopamine receptor) was found in the striatum of old mice using gene array and real-time RT-PCR analysis. The old striatum showed also a significantly higher number of GFAP-expressing astrocytes and an increased expression of astroglial, inflammatory, and oxidative stress markers. Field potential recordings from striatal slices revealed age-related alterations in the magnitude and dynamics of electrically induced long-term depression (LTD) and significant enhancement of electrically induced long-term potentiation in the middle-aged striatum (6-7 and 12-13 months of age). Corticostriatal NO-dependent LTD induced by pharmacological activation of group I metabotropic glutamate receptors underwent significant reduction with aging and could be restored by inhibition of cGMP hydrolysis indicating that its age-related deficit is caused by an altered NO-cGMP signaling cascade. It is suggested that age-related alterations in corticostriatal synaptic plasticity may result from functional alterations in receptor-activated signaling cascades associated with increasing neuroinflammation and a prooxidant state.

Mechanisms for axon maintenance and plasticity in motoneurons: alterations in motoneuron disease

In motoneuron disease and other neurodegenerative disorders, the loss of synapses and axon branches occurs early but is compensated by sprouting of neighboring axon terminals. Defective local axonal signaling for maintenance and dynamics of the axonal microtubule and actin cytoskeleton plays a central role in this context. The molecular mechanisms that lead to defective cytoskeleton architecture in two mouse models of motoneuron disease are summarized and discussed in this manuscript. In the progressive motor neuropathy (pmn) mouse model of motoneuron disease that is caused by a mutation in the tubulin-specific chaperone E gene, death of motoneuron cell bodies appears as a consequence of axonal degeneration. Treatment with bcl-2 overexpression or with glial-derived neurotrophic factor prevents loss of motoneuron cell bodies but does not influence the course of disease. In contrast, treatment with ciliary neurotrophic factor (CNTF) significantly delays disease onset and prolongs survival of pmn mice. This difference is due to the activation of Stat-3 via the CNTF receptor complex in axons of pmn mutant motoneurons. Most of the activated Stat-3 protein is not transported to the nucleus to activate transcription, but interacts locally in axons with stathmin, a protein that destabilizes microtubules. This interaction plays a major role in CNTF signaling for microtubule dynamics in axons. In Smn-deficient mice, a model of spinal muscular atrophy, defects in axonal translocation of β-actin mRNA and possibly other mRNA species have been observed. Moreover, the regulation of local protein synthesis in response to signals from neurotrophic factors and extracellular matrix proteins is altered in motoneurons from this model of motoneuron disease. These findings indicate that local signals are important for maintenance and plasticity of axonal branches and neuromuscular endplates, and that disturbances in these signaling mechanisms could contribute to the pathophysiology of motoneuron diseases.

Metal dyshomeostasis and inflammation in Alzheimer's and Parkinson's diseases: possible impact of environmental exposures

A dysregulated metal homeostasis is associated with both Alzheimer's (AD) and Parkinson's (PD) diseases; AD patients have decreased cortex and elevated serum copper levels along with extracellular amyloid-beta plaques containing copper, iron, and zinc. For AD, a putative hepcidin-mediated lowering of cortex copper mechanism is suggested. An age-related mild chronic inflammation and/or elevated intracellular iron can trigger hepcidin production followed by its binding to ferroportin which is the only neuronal iron exporter, thereby subjecting it to lysosomal degradation. Subsequently raised neuronal iron levels can induce translation of the ferroportin assisting and copper binding amyloid precursor protein (APP); constitutive APP transmembrane passage lowers the copper pool which is important for many enzymes. Using in silico gene expression analyses, we here show significantly decreased expression of copper-dependent enzymes in AD brain and metallothioneins were upregulated in both diseases. Although few AD exposure risk factors are known, AD-related tauopathies can result from cyanobacterial microcystin and β-methylamino-L-alanine (BMAA) intake. Several environmental exposures may represent risk factors for PD; for this disease neurodegeneration is likely to involve mitochondrial dysfunction, microglial activation, and neuroinflammation. Administration of metal chelators and anti-inflammatory agents could affect disease outcomes.