Subanesthetic ketamine reverses neuronal and astroglial metabolic activity deficits in a social defeat model of depression

Depression is one of the most debilitating neuropsychiatric disorders. Most of the current antidepressants have long remission time and low recovery rate. This study explores the impact of ketamine on neuronal and astroglial metabolic activity in prefrontal cortex in a social defeat (SD) model of depression. C57BL/6 mice were subjected to a social defeat paradigm for 5 min a day for 10 consecutive days. Ketamine (10 mg/kg, intraperitoneal) was administered to mice for two consecutive days following the last defeat stress. Mice were infused with [1,6‐¹³C₂]glucose or [2‐¹³C]acetate to assess neuronal and astroglial metabolic activity, respectively, together with proton‐observed carbon‐edited nuclear magnetic resonance spectroscopy in prefrontal cortex tissue extract. The ¹³C labeling of amino acids from glucose and acetate was decreased in SD mice. Ketamine treatment in SD mice restored sucrose preference, social interaction and immobility time to control values. Acute subanesthetic ketamine restored the ¹³C labeling of brain amino acids from glucose as well as acetate in SD mice to the respective control values, suggesting that rates of neuronal and astroglial tricarboxylic acid (TCA) cycle and neurotransmitter cycling were re‐established to normal levels. The finding of improved energy metabolism in SD mice suggests that fast anti‐depressant action of ketamine is linked with improved neurotransmitter cycling.

     

Genetic removal of eIF2α kinase PERK in mice enables hippocampal L‐LTP independent of mTORC1 activity

Characterization of the molecular signaling pathways underlying protein synthesis‐dependent forms of synaptic plasticity, such as late long‐term potentiation (L‐LTP ), can provide insights not only into memory expression/maintenance under physiological conditions but also potential mechanisms associated with the pathogenesis of memory disorders. Here, we report in mice that L‐LTP failure induced by the mammalian (mechanistic) target of rapamycin complex 1 (mTORC 1) inhibitor rapamycin is reversed by brain‐specific genetic deletion of PKR ‐like ER kinase, PERK (PERK KO ), a kinase for eukaryotic initiation factor 2α (eIF 2α). In contrast, genetic removal of general control non‐derepressible‐2, GCN 2 (GCN 2 KO ), another eIF 2α kinase, or treatment of hippocampal slices with the PERK inhibitor GSK 2606414, does not rescue rapamycin‐induced L‐LTP failure, suggesting mechanisms independent of eIF 2α phosphorylation. Moreover, we demonstrate that phosphorylation of eukaryotic elongation factor 2 (eEF 2) is significantly decreased in PERK KO mice but unaltered in GCN 2 KO mice or slices treated with the PERK inhibitor. Reduction in eEF 2 phosphorylation results in increased general protein synthesis, and thus could contribute to the mTORC 1‐independent L‐LTP in PERK KO mice. We further performed experiments on mutant mice with genetic removal of eEF 2K (eEF 2K KO ), the only known kinase for eEF 2, and found that L‐LTP in eEF 2K KO mice is insensitive to rapamycin. These data, for the first time, connect reduction in PERK activity with the regulation of translation elongation in enabling L‐LTP independent of mTORC 1. Thus, our findings indicate previously unrecognized levels of complexity in the regulation of protein synthesis‐dependent synaptic plasticity.

Read the Editorial Highlight for this article on page 119 . Cover Image for this issue: doi: 10.1111/jnc.14185 .

     

Transcriptional co‐repressor SIN3A silencing rescues decline in memory consolidation during scopolamine‐induced amnesia

Epigenetic modifications through methylation of DNA and acetylation of histones modulate neuronal gene expression and regulate long‐term memory. Earlier we demonstrated that scopolamine‐induced decrease in memory consolidation is correlated with enhanced expression of hippocampal DNA methyltransferase 1 (DNMT 1) and histone deacetylase 2 (HDAC 2) in mice. DNMT 1 and HDAC 2 act together by recruiting a co‐repressor complex and deacetylating the chromatin. The catalytic activity of HDAC s is mainly dependent on its incorporation into multiprotein co‐repressor complexes, among which SIN 3A‐HDAC 2 co‐repressor is widely studied to regulate synaptic plasticity. However, the involvement of co‐repressor complex in regulating memory loss or amnesia is unexplored. This study examines the role of co‐repressor SIN 3A in scopolamine‐induced amnesia through epigenetic changes in the hippocampus. Scopolamine treatment remarkably enhanced hippocampal SIN 3A expression in mice. To prevent such increase in SIN 3A expression, we used hippocampal infusion of SIN 3A‐siRNA and assessed the effect of SIN 3A silencing on scopolamine‐induced amnesia. Silencing of SIN 3A in amnesic mice reduced the binding of HDAC 2 at neuronal immediate early genes (IEG s) promoter, but did not change the expression of HDAC 2. Furthermore, it increased acetylation of H3K9 and H3K14 at neuronal IEG s (Arc, Egr1, Homer1 and Narp) promoter, prevented scopolamine‐induced down‐regulation of IEG s and improved consolidation of memory during novel object recognition task. These findings together suggest that SIN 3A has a critical role in regulation of synaptic plasticity and might act as a potential therapeutic target to rescue memory decline during amnesia and other neuropsychiatric pathologies.

     

A dual role for Integrin α6β4 in modulating hereditary neuropathy with liability to pressure palsies

Peripheral myelin protein 22 (PMP 22) is a component of compact myelin in the peripheral nervous system. The amount of PMP 22 in myelin is tightly regulated, and PMP 22 over or under‐expression cause Charcot‐Marie‐Tooth 1A (CMT 1A) and Hereditary Neuropathy with Pressure Palsies (HNPP ). Despite the importance of PMP 22 , its function remains largely unknown. It was reported that PMP 22 interacts with the β4 subunit of the laminin receptor α6β4 integrin, suggesting that α6β4 integrin and laminins may contribute to the pathogenesis of CMT 1A or HNPP . Here we asked if the lack of α6β4 integrin in Schwann cells influences myelin stability in the HNPP mouse model. Our data indicate that PMP 22 and β4 integrin may not interact directly in myelinating Schwann cells, however, ablating β4 integrin delays the formation of tomacula, a characteristic feature of HNPP . In contrast, ablation of integrin β4 worsens nerve conduction velocities and non‐compact myelin organization in HNPP animals. This study demonstrates that indirect interactions between an extracellular matrix receptor and a myelin protein influence the stability and function of myelinated fibers.

     

Defective trafficking of Kv2.1 channels in MPTP‐induced nigrostriatal degeneration

Intracellular protein trafficking is tightly regulated, and improper trafficking might be the fundamental provocateur for human diseases including neurodegeneration. In neurons, protein trafficking to and from the plasma membrane affects synaptic plasticity. Voltage‐gated potassium channel 2.1 (Kv2.1) is a predominant delayed rectifier potassium (K⁺) current, and electrical activity patterns of dopamine (DA) neurons within the substantia nigra are generated and modulated by the orchestrated function of different ion channels. The pathological hallmark of Parkinson's disease (PD) is the progressive loss of these DA neurons, resulting in the degeneration of striatal dopaminergic terminals. However, whether trafficking of Kv2.1 channels contributes to PD remains unclear. In this study, we demonstrated that MPTP/MPP⁺ increases the surface expression of the Kv2.1 channel and causes nigrostriatal degeneration by using a subchronic MPTP mouse model. The inhibition of the Kv2.1 channel by using a specific blocker, guangxitoxin‐1E, protected nigrostriatal projections against MPTP/MPP⁺ insult and thus facilitated the recovery of motor coordination. These findings highlight the importance of trafficking of Kv2.1 channels in the pathogenesis of PD.

     

Cyclophilin D regulates neuronal activity‐induced filopodiagenesis by fine‐tuning dendritic mitochondrial calcium dynamics

Recent studies have highlighted the role of mitochondria in dendritic protrusion growth and plasticity. However, the detailed mechanisms that mitochondria regulate dendritic filopodia morphogenesis remain elusive. Cyclophilin D (CypD, gene name: Ppif ) controls the opening of mitochondrial permeability transition pore. Although the pathological relevance of CypD has been intensively investigated, little is known about its physiological function in neurons. Here, we have found that genetic depletion of or pharmaceutical inhibition of CypD blunts the outgrowth of dendritic filopodia in response to KC l‐stimulated neuronal depolarization. Further cell biological studies suggest that such inhibitory effect of CypD loss‐of‐function is closely associated with compromised flexibility of dendritic mitochondrial calcium regulation during neuronal depolarization, as well as the resultant changes in intradendritic calcium homeostasis, calcium signaling activation, dendritic mitochondrial motility and redistribution. Interestingly, loss of CypD attenuates oxidative stress‐induced mitochondrial calcium perturbations and dendritic protrusion injury. Therefore, our study has revealed the physiological function of CypD in dendritic plasticity by acting as a fine‐tuner of mitochondrial calcium homeostasis. Moreover, CypD plays distinct roles in neuronal physiology and pathology.

Cover Image for this issue: doi: 10.1111/jnc.14189 .

     

Differential toxicity of TAR DNA‐binding protein 43 isoforms depends on their submitochondrial localization in neuronal cells

TAR DNA ‐binding protein 43 (TDP ‐43) is an RNA ‐binding protein and a major component of protein aggregates found in amyotrophic lateral sclerosis and several other neurodegenerative diseases. TDP ‐43 exists as a full‐length protein and as two shorter forms of 25 and 35 kD a. Full‐length mutant TDP ‐43s found in amyotrophic lateral sclerosis patients re‐localize from the nucleus to the cytoplasm and in part to mitochondria, where they exert a toxic role associated with neurodegeneration. However, induction of mitochondrial damage by TDP ‐43 fragments is yet to be clarified. In this work, we show that the mitochondrial 35 kD a truncated form of TDP ‐43 is restricted to the intermembrane space, while the full‐length forms also localize in the mitochondrial matrix in cultured neuronal NSC ‐34 cells. Interestingly, the full‐length forms clearly affect mitochondrial metabolism and morphology, possibly via their ability to inhibit the expression of Complex I subunits encoded by the mitochondrial‐transcribed mRNA s, while the 35 kD a form does not. In the light of the known differential contribution of the full‐length and short isoforms to generate toxic aggregates, we propose that the presence of full‐length TDP ‐43s in the matrix is a primary cause of mitochondrial damage. This in turn may cause oxidative stress inducing toxic oligomers formation, in which short TDP ‐43 forms play a major role.

     

p75 neurotrophin receptor interacts with and promotes BACE1 localization in endosomes aggravating amyloidogenesis

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive deposition of amyloid beta (Aβ) and dysregulation of neurotrophic signaling, causing synaptic dysfunction, loss of memory, and cell death. The expression of p75 neurotrophin receptor is elevated in the brain of AD patients, suggesting its involvement in this disease. However, the exact mechanism of its action is not yet clear. Here, we show that p75 interacts with beta‐site amyloid precursor protein cleaving enzyme‐1 (BACE1), and this interaction is enhanced in the presence of Aβ. Our results suggest that the colocalization of BACE1 and amyloid precursor protein (APP) is increased in the presence of both Aβ and p75 in cortical neurons. In addition, the localization of APP and BACE1 in early endosomes is increased in the presence of Aβ and p75. An increased phosphorylation of APP‐Thr668 and BACE1‐Ser498 by c‐Jun N‐terminal kinase (JNK) in the presence of Aβ and p75 could be responsible for this localization. In conclusion, our study proposes a potential involvement in amyloidogenesis for p75, which may represent a future therapeutic target for AD.

Cover Image for this Issue: doi. 10.1111/jnc.14163 .

     

Microglia modulation through external vagus nerve stimulation in a murine model of Alzheimer's disease

Chronically activated microglia contribute to the development of neurodegenerative diseases such as Alzheimer's disease (AD ) by the release of pro‐inflammatory mediators that compromise neuronal function and structure. Modulating microglia functions could be instrumental to interfere with disease pathogenesis. Previous studies have shown anti‐inflammatory effects of acetylcholine (AC h) or norepinephrine (NE ), which mainly activates the β‐receptors on microglial cells. Non‐invasive vagus nerve stimulation (nVNS ) is used in treatment of drug‐resistant depression, which is a risk factor for developing AD . The vagus nerve projects to the brainstem's locus coeruleus from which noradrenergic fibers reach to the Nucleus Basalis of Meynert (NBM ) and widely throughout the brain. Pilot studies showed first signs of cognitive‐enhancing effects of nVNS in AD patients. In this study, the effects of nVNS on mouse microglia cell morphology were analyzed over a period of 280 min by 2‐photon laser scanning in vivo microscopy. Total branch length, average branch order and number of branches, which are commonly used indicators for the microglial activation state were determined and compared between young and old wild‐type and amyloid precursor protein/presenilin‐1 (APP/PS1) transgenic mice. Overall, these experiments show strong morphological changes in microglia, from a neurodestructive to a neuroprotective phenotype, following a brief nVNS in aged animals, especially in APP/PS 1 animals, whereas microglia from young animals were morphologically unaffected.

     

Nimodipine confers clinical improvement in two models of experimental autoimmune encephalomyelitis

Multiple sclerosis is characterised by inflammatory neurodegeneration, with axonal injury and neuronal cell death occurring in parallel to demyelination. Regarding the molecular mechanisms responsible for demyelination and axonopathy, energy failure, aberrant expression of ion channels and excitotoxicity have been suggested to lead to Ca²⁺ overload and subsequent activation of calcium‐dependent damage pathways. Thus, the inhibition of Ca²⁺ influx by pharmacological modulation of Ca²⁺ channels may represent a novel neuroprotective strategy in the treatment of secondary axonopathy. We therefore investigated the effects of the L‐type voltage‐gated calcium channel blocker nimodipine in two different models of mouse experimental autoimmune encephalomyelitis (EAE ), an established experimental paradigm for multiple sclerosis. We show that preventive application of nimodipine (10 mg/kg per day) starting on the day of induction had ameliorating effects on EAE in SJL /J mice immunised with encephalitic myelin peptide PLP _139–151, specifically in late‐stage disease. Furthermore, supporting these data, administration of nimodipine to MOG _35–55‐immunised C57BL /6 mice starting at the peak of pre‐established disease, also led to a significant decrease in disease score, indicating a protective effect on secondary CNS damage. Histological analysis confirmed that nimodipine attenuated demyelination, axonal loss and pathological axonal β‐amyloid precursor protein accumulation in the cerebellum and spinal cord in the chronic phase of disease. Of note, we observed no effects of nimodipine on the peripheral immune response in EAE mice with regard to distribution, antigen‐specific proliferation or activation patterns of lymphocytes. Taken together, our data suggest a CNS ‐specific effect of L‐type voltage‐gated calcium channel blockade to inflammation‐induced neurodegeneration.

     

G5G2.5 core‐shell tecto‐dendrimer specifically targets reactive glia in brain ischemia

Secondary neuronal death is a serious stroke complication. This process is facilitated by the conversion of glial cells to the reactive pro‐inflammatory phenotype that induces neurodegeneration. Therefore, regulation of glial activation is a compelling strategy to reduce brain damage after stroke. However, drugs have difficulties to access the CNS , and to specifically target glial cells. In the present work, we explored the use core‐shell polyamidoamine tecto‐dendrimer (G5G2.5 PAMAM ) and studied its ability to target distinct populations of stroke‐activated glial cells. We found that G5G2.5 tecto‐dendrimer is actively engulfed by primary glial cells in a time‐ and dose‐dependent manner showing high cellular selectivity and lysosomal localization. In addition, oxygen‐glucose deprivation or lipopolysaccharides exposure in vitro and brain ischemia in vivo increase glial G5G2.5 uptake; not being incorporated by neurons or other cell types. We conclude that G5G2.5 tecto‐dendrimer is a highly suitable carrier for targeted drug delivery to reactive glial cells in vitro and in vivo after brain ischemia.