DAPK1 Promotes Extrasynaptic GluN2B Phosphorylation and Striatal Spine Instability in the YAC128 Mouse Model of Huntington Disease

Synaptic Vesicle-Related Proteins and Ubiquilin 2 in Cortical Synaptosomes Mediate Cognitive Impairment in Vascular Dementia Rats

Synaptic dysfunction is considered the best neuropathological correlate of cognitive decline in vascular dementia (VaD). However, the alterations of synaptic proteins at the synaptosomal level in VaD remain unclear. In this study, a VaD model was established in male rats using bilateral common carotid artery occlusion (2VO). We performed a novel object recognition task to evaluate cognitive impairment. Immunohistochemistry was used to assess the expression of neuron-specific nuclear binding protein (NeuN). Brain synaptosomes were isolated and subjected to label-free proteomic analysis to quantify and identify the synaptic features of differentially expressed proteins (DEPs). Synaptic and hub protein expression was detected in synaptosomes using western blotting. We found that male rats with VaD presented impaired memory and decreased NeuN protein expression in the cortex. Synaptosome proteomic analysis revealed 604 DEPs, with 493 and 111 markedly downregulated and upregulated proteins, respectively. KEGG analysis and SynGO annotation revealed that the synaptic vesicle (SV) cycle may be a key signaling pathway in VaD. Hub protein analysis of the main nodes in the protein network identified UBQLN2 and SV-related proteins, including CLTC, SNAP91, AP2S1, CLTA, VAMP2, EPN1, UBQLN2, AP2B1, AP2A2, and AP2M1. Western blotting showed that the levels of SV2A, CLTC, AP2S1, and VAMP2 decreased in the synaptosomes of 2VO rats, while UBQLN2 expression significantly increased. Our results suggest that the disruption in the presynaptic SV cycle is a key event in male rats with VaD, which could be characterized by the aberrant SV2A expression. SV-related proteins and UBQLN2 may be essential in synaptopathy. Thus, targeting the specific molecular markers in synaptosomes may be critical for the development of mechanism-directed therapies against VaD.

Mechanism of DAPK1 for Regulating Cancer Stem Cells in Thyroid Cancer

Death-associated protein kinase 1 (DAPK1) is a calcium/calmodulin (Ca2+/CaM)-dependent serine/threonine (Ser/Thr) protein kinase and is characteristically downregulated in metastatic cancer. Several studies showed that DAPK1 is involved in both the early and late stages of cancer. DAPK1 downregulation is elaborately controlled by epigenetic, transcriptional, posttranscriptional, and posttranslational processes. DAPK1 is known to regulate not only cancer cells but also stromal cells. Recent studies showed that DAPK1 was involved not only in tumor suppression but also in epithelial-mesenchymal transition (EMT) and cancer stem cell (CSC) formation in colon and thyroid cancers. CSCs are major factors in determining cancer aggressiveness in cancer metastasis and treatment prognosis by influencing EMT. However, the molecular mechanism involved in the regulation of cancer cells by DAPK1 remains unclear. In particular, little is known about the existence of CSCs and how they are regulated in papillary thyroid carcinoma (PTC) among thyroid cancers. In this review, we describe the molecular mechanism of CSC regulation by DAPK1 in PTC progression.

Death-associated protein kinase 1 as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly and represents a major clinical challenge in the ageing society. Neuropathological hallmarks of AD include neurofibrillary tangles composed of hyperphosphorylated tau, senile plaques derived from the deposition of amyloid-β (Aβ) peptides, brain atrophy induced by neuronal loss, and synaptic dysfunctions. Death-associated protein kinase 1 (DAPK1) is ubiquitously expressed in the central nervous system. Dysregulation of DAPK1 has been shown to contribute to various neurological diseases including AD, ischemic stroke and Parkinson's disease (PD). We have established an upstream effect of DAPK1 on Aβ and tau pathologies and neuronal apoptosis through kinase-mediated protein phosphorylation, supporting a causal role of DAPK1 in the pathophysiology of AD. In this review, we summarize current knowledge about how DAPK1 is involved in various AD pathological changes including tau hyperphosphorylation, Aβ deposition, neuronal cell death and synaptic degeneration. The underlying molecular mechanisms of DAPK1 dysregulation in AD are discussed. We also review the recent progress regarding the development of novel DAPK1 modulators and their potential applications in AD intervention. These findings substantiate DAPK1 as a novel therapeutic target for the development of multifunctional disease-modifying treatments for AD and other neurological disorders.

Ablation of Death-Associated Protein Kinase 1 Changes the Transcriptomic Profile and Alters Neural-Related Pathways in the Brain

Death-associated protein kinase 1 (DAPK1), a Ca2+/calmodulin-dependent serine/threonine kinase, mediates various neuronal functions, including cell death. Abnormal upregulation of DAPK1 is observed in human patients with neurological diseases, such as Alzheimer’s disease (AD) and epilepsy. Ablation of DAPK1 expression and suppression of DAPK1 activity attenuates neuropathology and behavior impairments. However, whether DAPK1 regulates gene expression in the brain, and whether its gene profile is implicated in neuronal disorders, remains elusive. To reveal the function and pathogenic role of DAPK1 in neurological diseases in the brain, differential transcriptional profiling was performed in the brains of DAPK1 knockout (DAPK1-KO) mice compared with those of wild-type (WT) mice by RNA sequencing. We showed significantly altered genes in the cerebral cortex, hippocampus, brain stem, and cerebellum of both male and female DAPK1-KO mice compared to those in WT mice, respectively. The genes are implicated in multiple neural-related pathways, including: AD, Parkinson’s disease (PD), Huntington’s disease (HD), neurodegeneration, glutamatergic synapse, and GABAergic synapse pathways. Moreover, our findings imply that the potassium voltage-gated channel subfamily A member 1 (Kcna1) may be involved in the modulation of DAPK1 in epilepsy. Our study provides insight into the pathological role of DAPK1 in the regulatory networks in the brain and new therapeutic strategies for the treatment of neurological diseases.

NMDA Receptor GluN2B Subunit Is Involved in Excitotoxicity Mediated by Death-Associated Protein Kinase 1 in Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia among the elderly. Excitotoxicity has been implicated as playing a dominant role in AD, especially related to the hyperactivation of excitatory neurons. Death-associated protein kinase 1 (DAPK1) is a calcium/calmodulin-dependent kinase and involved in the pathogenesis of AD, but the roles and mechanisms of DAPK1 in excitotoxicity in AD are still uncertain.

OBJECTIVE

We mainly explored the underlying mechanisms of DAPK1 involved in the excitotoxicity of AD and its clinical relevance.

METHODS

Differentiated SH-SY5Y human neuroblastoma cells, PS1 V97 L transgenic mice, and human plasma samples were used. Protein expression was assayed by immunoblotting, and intracellular calcium and neuronal damage were analyzed by flow cytometry. Plasma DAPK1 was measured by ELISA.

RESULTS

We found that DAPK1 was activated after amyloid-β oligomers (AβOs) exposure in differentiated SH-SY5Y cells. Besides, we found the phosphorylation of GluN2B subunit at Ser1303 was increased, which contributing to excitotoxicity and Ca2+ overload in SH-SY5Y cells. Inhibiting DAPK1 activity, knockdown of DAPK1 expression, and antagonizing GluN2B subunits could effectively prevent AβOs-induced activation of GluN2B subunit, Ca2+ overload, and neuronal apoptosis. Additionally, we found that DAPK1 was elevated in the brain of AD transgenic mouse and in the plasma of AD patients.

CONCLUSION

Our finding will help to understand the mechanism of DAPK1 in the excitotoxicity in AD and provide a reference for the diagnosis and therapy of AD.

Phosphoproteomic dysregulation in Huntington's disease mice is rescued by environmental enrichment

Huntington's disease is a fatal autosomal-dominant neurodegenerative disorder, characterized by neuronal cell dysfunction and loss, primarily in the striatum, cortex and hippocampus, causing motor, cognitive and psychiatric impairments. Unfortunately, no treatments are yet available to modify the progression of the disease. Recent evidence from Huntington's disease mouse models suggests that protein phosphorylation (catalysed by kinases and hydrolysed by phosphatases) might be dysregulated, making this major post-translational modification a potential area of interest to find novel therapeutic targets. Furthermore, environmental enrichment, used to model an active lifestyle in preclinical models, has been shown to alleviate Huntington's disease-related motor and cognitive symptoms. However, the molecular mechanisms leading to these therapeutic effects are still largely unknown. In this study, we applied a phosphoproteomics approach combined with proteomic analyses on brain samples from pre-motor symptomatic R6/1 Huntington's disease male mice and their wild-type littermates, after being housed either in environmental enrichment conditions, or in standard housing conditions from 4 to 8 weeks of age (n = 6 per group). We hypothesized that protein phosphorylation dysregulations occur prior to motor onset in this mouse model, in two highly affected brain regions, the striatum and hippocampus. Furthermore, we hypothesized that these phosphoproteome alterations are rescued by environmental enrichment. When comparing 8-week-old Huntington's disease mice and wild-type mice in standard housing conditions, our analysis revealed 229 differentially phosphorylated peptides in the striatum, compared with only 15 differentially phosphorylated peptides in the hippocampus (statistical thresholds fold discovery rate 0.05, fold change 1.5). At the same disease stage, minor differences were found in protein levels, with 24 and 22 proteins dysregulated in the striatum and hippocampus, respectively. Notably, we found no differences in striatal protein phosphorylation and protein expression when comparing Huntington's disease mice and their wild-type littermates in environmentally enriched conditions. In the hippocampus, only four peptides were differentially phosphorylated between the two genotypes under environmentally enriched conditions, and 22 proteins were differentially expressed. Together, our data indicates that protein phosphorylation dysregulations occur in the striatum of Huntington's disease mice, prior to motor symptoms, and that the kinases and phosphatases leading to these changes in protein phosphorylation might be viable drug targets to consider for this disorder. Furthermore, we show that an early environmental intervention was able to rescue the changes observed in protein expression and phosphorylation in the striatum of Huntington's disease mice and might underlie the beneficial effects of environmental enrichment, thus identifying novel therapeutic targets.

Neurodegenerative Diseases: From Dysproteostasis, Altered Calcium Signalosome to Selective Neuronal Vulnerability to AAV-Mediated Gene Therapy

Despite intense research into the multifaceted etiology of neurodegenerative diseases (ND), they remain incurable. Here we provide a brief overview of several major ND and explore novel therapeutic approaches. Although the cause (s) of ND are not fully understood, the accumulation of misfolded/aggregated proteins in the brain is a common pathological feature. This aggregation may initiate disruption of Ca⁺⁺ signaling, which is an early pathological event leading to altered dendritic structure, neuronal dysfunction, and cell death. Presently, ND gene therapies remain unidimensional, elusive, and limited to modifying one pathological feature while ignoring others. Considering the complexity of signaling cascades in ND, we discuss emerging therapeutic concepts and suggest that deciphering the molecular mechanisms involved in dendritic pathology may broaden the phenotypic spectrum of ND treatment. An innovative multiplexed gene transfer strategy that employs silencing and/or over-expressing multiple effectors could preserve vulnerable neurons before they are lost. Such therapeutic approaches may extend brain health span and ameliorate burdensome chronic disease states.

Inhibition of Synaptic Glutamate Exocytosis and Prevention of Glutamate Neurotoxicity by Eupatilin from Artemisia argyi in the Rat Cortex

The inhibition of synaptic glutamate release to maintain glutamate homeostasis contributes to the alleviation of neuronal cell injury, and accumulating evidence suggests that natural products can repress glutamate levels and associated excitotoxicity. In this study, we investigated whether eupatilin, a constituent of Artemisia argyi, affected glutamate release in rat cortical nerve terminals (synaptosomes). Additionally, we evaluated the effect of eupatilin in an animal model of kainic acid (KA) excitotoxicity, particularly on the levels of glutamate and N-methyl-D-aspartate (NMDA) receptor subunits (GluN2A and GluN2B). We found that eupatilin decreased depolarization-evoked glutamate release from rat cortical synaptosomes and that this effect was accompanied by a reduction in cytosolic Ca2+ elevation, inhibition of P/Q-type Ca2+ channels, decreased synapsin I Ca2+-dependent phosphorylation and no detectable effect on the membrane potential. In a KA-induced glutamate excitotoxicity rat model, the administration of eupatilin before KA administration prevented neuronal cell degeneration, glutamate elevation, glutamate-generating enzyme glutaminase increase, excitatory amino acid transporter (EAAT) decrease, GluN2A protein decrease and GluN2B protein increase in the rat cortex. Taken together, the results suggest that eupatilin depresses glutamate exocytosis from cerebrocortical synaptosomes by decreasing P/Q-type Ca2+ channels and synapsin I phosphorylation and alleviates glutamate excitotoxicity caused by KA by preventing glutamatergic alterations in the rat cortex. Thus, this study suggests that eupatilin can be considered a potential therapeutic agent in the treatment of brain impairment associated with glutamate excitotoxicity.

Postsynaptic signaling at glutamatergic synapses as therapeutic targets

Dysregulation of glutamatergic synapses plays an important role in the pathogenesis of neurological diseases. In addition to mediating excitatory synaptic transmission, postsynaptic glutamate receptors interact with various membrane and intracellular proteins. They form structural and/or signaling synaptic protein complexes and thereby play diverse postsynaptic functions. Recently, several postsynaptic protein complexes have been associated with various neurological diseases and hence, have been characterized as important therapeutic targets. Moreover, novel small molecules and therapeutic peptides targeting and modulating the activities of these protein complexes have been discovered, some of which have advanced through preclinical translational research and/or clinical studies. This article describes the recent investigation of eight key protein complexes associated with the postsynaptic ionotropic and metabotropic glutamate receptors as therapeutic targets for central nervous system diseases.

Inhibition of glial D-serine release rescues synaptic damage after brain injury

Synaptic damage is one of the most prevalent pathophysiological responses to traumatic CNS injury and underlies much of the associated cognitive dysfunction; however, it is poorly understood. The D-amino acid, D-serine, serves as the primary co-agonist at synaptic NMDA receptors (NDMARs) and is a critical mediator of NMDAR-dependent transmission and synaptic plasticity. In physiological conditions, D-serine is produced and released by neurons from the enzymatic conversion of L-serine by serine racemase (SRR). However, under inflammatory conditions, glial cells become a major source of D-serine. Here, we report that D-serine synthesized by reactive glia plays a critical role in synaptic damage after traumatic brain injury (TBI) and identify the therapeutic potential of inhibiting glial D-serine release though the transporter Slc1a4 (ASCT1). Furthermore, using cell-specific genetic strategies and pharmacology, we demonstrate that TBI-induced synaptic damage and memory impairment requires D-serine synthesis and release from both reactive astrocytes and microglia. Analysis of the murine cortex and acutely resected human TBI brain also show increased SRR and Slc1a4 levels. Together, these findings support a novel role for glial D-serine in acute pathological dysfunction following brain trauma, whereby these reactive cells provide the excess co-agonist levels necessary to initiate NMDAR-mediated synaptic damage.

Quantitative Phosphoproteomics Reveals Extensive Protein Phosphorylation Dysregulation in the Cerebral Cortex of Huntington's Disease Mice Prior to Onset of Symptoms

Protein phosphorylation plays a role in many important cellular functions such as cellular plasticity, gene expression, and intracellular trafficking. All of these are dysregulated in Huntington's disease (HD), a devastating neurodegenerative disorder caused by an expanded CAG repeat in exon 1 of the huntingtin gene. However, no studies have yet found protein phosphorylation differences in preclinical HD mouse models. Our current study investigated changes occurring in the cortical phosphoproteome of 8-week-old (prior to motor deficits) and 20-week-old (fully symptomatic) R6/1 transgenic HD mice. When comparing 8-week-old HD mice with their wild-type (WT) littermates, we found 660 peptides differentially phosphorylated, which were mapped to 227 phosphoproteins. These proteins were mainly involved in synaptogenesis, cytoskeleton organization, axon development, and nervous system development. Tau protein, found hyperphosphorylated at multiple sites in early symptomatic HD mice, also appeared as a main upstream regulator for the changes observed. Surprisingly, we found fewer changes in the phosphorylation profile of HD mice at the fully symptomatic stage, with 29 peptides differentially phosphorylated compared to WT mice, mapped to 25 phosphoproteins. These proteins were involved in cAMP signaling, dendrite development, and microtubule binding. Furthermore, huntingtin protein appeared as an upstream regulator for the changes observed at the fully symptomatic stage, suggesting impacts on kinases and phosphatases that extend beyond the mutated polyglutamine tract. In summary, our findings show that the most extensive changes in the phosphorylation machinery appear at an early presymptomatic stage in HD pathogenesis and might constitute a new target for the development of treatments.

Roles of N-Methyl-D-Aspartate Receptors (NMDARs) in Epilepsy

Epilepsy is one of the most common neurological disorders characterized by recurrent seizures. The mechanism of epilepsy remains unclear and previous studies suggest that N-methyl-D-aspartate receptors (NMDARs) play an important role in abnormal discharges, nerve conduction, neuron injury and inflammation, thereby they may participate in epileptogenesis. NMDARs belong to a family of ionotropic glutamate receptors that play essential roles in excitatory neurotransmission and synaptic plasticity in the mammalian CNS. Despite numerous studies focusing on the role of NMDAR in epilepsy, the relationship appeared to be elusive. In this article, we reviewed the regulation of NMDAR and possible mechanisms of NMDAR in epilepsy and in respect of onset, development, and treatment, trying to provide more evidence for future studies.

C57BL/6 Background Attenuates mHTT Toxicity in the Striatum of YAC128 Mice

Mouse models are frequently used to study Huntington’s disease (HD). The onset and severity of neuronal and behavioral pathologies vary greatly between HD mouse models, which results from different huntingtin expression levels and different CAG repeat length. HD pathology appears to depend also on the strain background of mouse models. Thus, behavioral deficits of HD mice are more severe in the FVB than in the C57BL/6 background. Alterations in medium spiny neuron (MSN) morphology and function have been well documented in young YAC128 mice in the FVB background. Here, we tested the relevance of strain background for mutant huntingtin (mHTT) toxicity on the cellular level by investigating HD pathologies in YAC128 mice in the C57BL/6 background (YAC128/BL6). Morphology, spine density, synapse function and membrane properties were not or only subtly altered in MSNs of 12-month-old YAC128/BL6 mice. Despite the mild cellular phenotype, YAC128/BL6 mice showed deficits in motor performance. More pronounced alterations in MSN function were found in the HdhQ150 mouse model in the C57BL/6 background (HdhQ150/BL6). Consistent with the differences in HD pathology, the number of inclusion bodies was considerably lower in YAC128/BL6 mice than HdhQ150/BL6 mice. This study highlights the relevance of strain background for mHTT toxicity in HD mouse models.

Non-Cell Autonomous and Epigenetic Mechanisms of Huntington's Disease

Huntington's disease (HD) is a rare neurodegenerative disorder caused by an expansion of CAG trinucleotide repeat located in the exon 1 of Huntingtin (HTT) gene in human chromosome 4. The HTT protein is ubiquitously expressed in the brain. Specifically, mutant HTT (mHTT) protein-mediated toxicity leads to a dramatic degeneration of the striatum among many regions of the brain. HD symptoms exhibit a major involuntary movement followed by cognitive and psychiatric dysfunctions. In this review, we address the conventional role of wild type HTT (wtHTT) and how mHTT protein disrupts the function of medium spiny neurons (MSNs). We also discuss how mHTT modulates epigenetic modifications and transcriptional pathways in MSNs. In addition, we define how non-cell autonomous pathways lead to damage and death of MSNs under HD pathological conditions. Lastly, we overview therapeutic approaches for HD. Together, understanding of precise neuropathological mechanisms of HD may improve therapeutic approaches to treat the onset and progression of HD.

Rescue of aberrant huntingtin palmitoylation ameliorates mutant huntingtin-induced toxicity

Huntington disease (HD) is a neurodegenerative disorder caused by a CAG expansion in the HTT gene that codes for an elongated polyglutamine tract in the huntingtin (HTT) protein. HTT is subject to multiple post-translational modifications (PTMs) that regulate its cellular function. Mutating specific PTM sites within mutant HTT (mHTT) in HD mouse models can modulate disease phenotypes, highlighting the key role of HTT PTMs in the pathogenesis of HD. These findings have led to increased interest in developing small molecules to modulate HTT PTMs in order to decrease mHTT toxicity. However, the therapeutic efficacy of pharmacological modulation of HTT PTMs in preclinical HD models remains largely unknown. HTT is palmitoylated at cysteine 214 by the huntingtin-interacting protein 14 (HIP14 or ZDHHC17) and 14-like (HIP14L or ZDHHC13) acyltransferases. Here, we assessed if HTT palmitoylation should be regarded as a therapeutic target to treat HD by (1) investigating palmitoylation dysregulation in rodent and human HD model systems, (2) measuring the impact of mHTT-lowering therapy on brain palmitoylation, and (3) evaluating if HTT palmitoylation can be pharmacologically modulated. We show that palmitoylation of mHTT and some HIP14/HIP14L-substrates is decreased early in multiple HD mouse models, and that mHTT palmitoylation decreases further with aging. Lowering mHTT in the brain of YAC128 mice is not sufficient to rescue aberrant palmitoylation. However, we demonstrate that mHTT palmitoylation can be normalized in COS-7 cells, in YAC128 cortico-striatal primary neurons and HD patient-derived lymphoblasts using an acyl-protein thioesterase (APT) inhibitor. Moreover, we show that modulating palmitoylation reduces mHTT aggregation and mHTT-induced cytotoxicity in COS-7 cells and YAC128 neurons.