Down-regulation of brain-derived neurotrophic factor and its signaling components in the brain tissues of scrapie experimental animals

Molecular signatures in prion disease: altered death receptor pathways in a mouse model

BACKGROUND

Prion diseases are transmissible and fatal neurodegenerative diseases characterized by accumulation of misfolded prion protein isoform (PrPSc), astrocytosis, microgliosis, spongiosis, and neurodegeneration. Elevated levels of cell membrane associated PrPSc protein and inflammatory cytokines hint towards the activation of death receptor (DR) pathway/s in prion diseases. Activation of DRs regulate, either cell survival or apoptosis, autophagy and necroptosis based on the adaptors they interact. Very little is known about the DR pathways activation in prion disease. DR3 and DR5 that are expressed in normal mouse brain were never studied in prion disease, so also their ligands and any DR adaptors. This research gap is notable and investigated in the present study.

METHODS

C57BL/6J mice were infected with Rocky Mountain Laboratory scrapie mouse prion strain. The progression of prion disease was examined by observing morphological and behavioural abnormalities. The levels of PrP isoforms and GFAP were measured as the marker of PrPSc accumulation and astrocytosis respectively using antibody-based techniques that detect proteins on blot and brain section. The levels of DRs, their glycosylation and ectodomain shedding, and associated factors warrant their examination at protein level, hence western blot analysis was employed in this study.

RESULTS

Prion-infected mice developed motor deficits and neuropathology like PrPSc accumulation and astrocytosis similar to other prion diseases. Results from this research show higher expression of all DR ligands, TNFR1, Fas and p75NTR but decreased levels DR3 and DR5. The levels of DR adaptor proteins like TRADD and TRAF2 (primarily regulate pro-survival pathways) are reduced. FADD, which primarily regulate cell death, its level remains unchanged. RIPK1, which regulate pro-survival, apoptosis and necroptosis, its expression and proteolysis (inhibits necroptosis but activates apoptosis) are increased.

CONCLUSIONS

The findings from the present study provide evidence towards the involvement of DR3, DR5, DR6, TL1A, TRAIL, TRADD, TRAF2, FADD and RIPK1 for the first time in prion diseases. The knowledge obtained from this research discuss the possible impacts of these 16 differentially expressed DR factors on our understanding towards the multifaceted neuropathology of prion diseases and towards future explorations into potential targeted therapeutic interventions for prion disease specific neuropathology.

Extensive Disturbances of Intracellular Components and Dysfunctions of Biological Pathways in the Brain Tissues During Prion Infection - China's Studies

The study describes some of the major findings of changes in intracellular components and biological pathways in the brain during prion infection and hypothesizes some important physiological and pathological approaches mainly based on our studies. Omics techniques analysis of messenger RNA (mRNA) and proteins were carried out in the study. Meanwhile, Western blot, immunohistochemistry, and immunofluorescence were used for protein analysis in different signaling pathways. Statistical analyses were used to describe the protein differences in signaling pathways of infected and normal samples. This report reviewed and summarized our studies on the aberrant changes in intracellular components and biological functions in the brains of prion disease (PrD). Omics analyses proposed extensive abnormal alterations of brain mRNAs transcriptions, protein expressions, and post-translational modifications. The molecular disturbances for microtubule instability and depolymerization, the dysregulations of different signals related with neuron loss and synaptic plasticity, the abnormalities of mitochondrial and endoplasmic reticulum stress, and disturbance of intracellular reactive oxygen species homeostasis during prion infection were precisely analyzed and reviewed. Aberrant disturbances of numerous biological molecules and signals in brain tissues were found during prion infection.

Different Aberrant Changes of mGluR5 and Its Downstream Signaling Pathways in the Scrapie-Infected Cell Line and the Brains of Scrapie-Infected Experimental Rodents

Metabotropic glutamate receptor subtype 5 (mGluR5) is a G-protein-coupled receptor found widely in the central nervous system. It has been involved in the development and progression of some neurodegenerative diseases, but its role in prion diseases is rarely described. In this study, the changes of mGluR5 and its downstream signaling pathways in prion-infected cell line SMB-S15 and the brains of scrapie-infected experimental rodents were evaluated by various methodologies. We found the levels of mGluR5 were significantly increased in a prion-infected cell line SMB-S15 and the cultured cells transiently express an abnormal form PrP (Cyto-PrP). Using immunoprecipitation tests and immunofluorescent assays (IFA), molecular interaction and morphological colocalization between PrP and mGluR5 were observed in the cultured cells. We identified that the (GPCRs)-IP3-IP3R-Ca2+ pathway was activated and the levels of the downstream kinases p38, ERK, and JNK were increased in SMB-S15 cells. After treated with mGluR5 antagonist (MTEP) or the removal of prion replication by resveratrol in SMB-S15 cells, the upregulations of mGluR5 and the downstream kinases were restored in a certain degree. Moreover, increased mGluR5 contributes to the cell damage in prion-infected cells. Contrarily, the levels of mGluR5 in the brains of several scrapie-infected rodent models were decreased at terminal stage. IFA of the brain sections of scrapie-infected rodents demonstrated that the signals of mGluR5 were preferentially colocalized with the NeuN-positive cells, accompanying with severe neuron losses in Nissl staining, which might be a reason for the decrease of mGluR5. Our data indicate the different aberrant alterations of mGluR5 and the downstream signaling pathways during prion infection in vivo and in vitro.

Towards a comprehensive understanding of p75 neurotrophin receptor functions and interactions in the brain

The role of neurotrophins in neuronal plasticity has recently become a strong focus in neuroregeneration research field to elucidate the biological mechanisms by which these molecules modulate synapses, modify the response to injury, and alter the adaptation response. Intriguingly, the prior studies highlight the role of p75 neurotrophin receptor (p75^NTR) in various injuries and diseases such as central nervous system injuries, Alzheimer's disease and amyotrophic lateral sclerosis. More comprehensive elucidation of the mechanisms, and therapies targeting these molecular signaling networks may allow for neuronal tissue regeneration following an injury. Due to a diverse role of the p75^NTR in biology, the body of evidence comprising its biological role is diffusely spread out over numerous fields. This review condenses the main evidence of p75^NTR for clinical applications and presents new findings from published literature how data mining approach combined with bioinformatic analyses can be utilized to gain new hypotheses in a molecular and network level.

Evidence of p75 Neurotrophin Receptor Involvement in the Central Nervous System Pathogenesis of Classical Scrapie in Sheep and a Transgenic Mouse Model

Neurotrophins constitute a group of growth factor that exerts important functions in the nervous system of vertebrates. They act through two classes of transmembrane receptors: tyrosine-kinase receptors and the p75 neurotrophin receptor (p75^NTR). The activation of p75^NTR can favor cell survival or apoptosis depending on diverse factors. Several studies evidenced a link between p75^NTR and the pathogenesis of prion diseases. In this study, we investigated the distribution of several neurotrophins and their receptors, including p75^NTR, in the brain of naturally scrapie-affected sheep and experimentally infected ovinized transgenic mice and its correlation with other markers of prion disease. No evident changes in infected mice or sheep were observed regarding neurotrophins and their receptors except for the immunohistochemistry against p75^NTR. Infected mice showed higher abundance of p75^NTR immunostained cells than their non-infected counterparts. The astrocytic labeling correlated with other neuropathological alterations of prion disease. Confocal microscopy demonstrated the co-localization of p75^NTR and the astrocytic marker GFAP, suggesting an involvement of astrocytes in p75^NTR-mediated neurodegeneration. In contrast, p75^NTR staining in sheep lacked astrocytic labeling. However, digital image analyses revealed increased labeling intensities in preclinical sheep compared with non-infected and terminal sheep in several brain nuclei. This suggests that this receptor is overexpressed in early stages of prion-related neurodegeneration in sheep. Our results confirm a role of p75^NTR in the pathogenesis of classical ovine scrapie in both the natural host and in an experimental transgenic mouse model.

A comprehensive p75 neurotrophin receptor gene network and pathway analyses identifying new target genes

P75 neurotrophic receptor (p75NTR) is an important receptor for the role of neurotrophins in modulating brain plasticity and apoptosis. The current understanding of the role of p75NTR in cellular adaptation following pathological insults remains blurred, which makes p75NTR’s related signaling networks an interesting and challenging initial point of investigation. We identified p75NTR and related genes through extensive data mining of a PubMed literature search including published works related to p75NTR from the past 20 years. Bioinformatic network and pathway analyses of identified genes (n = 235) were performed using ReactomeFIViz in Cytoscape based on the highly reliable Reactome functional interaction network algorithm. This approach merges interactions extracted from human curated pathways with predicted interactions from machine learning. Genome-wide pathway analysis showed total of 16 enriched hierarchical clusters. A total of 278 enriched single pathways were also identified (p < 0.05, false discovery rate corrected). Gene network analyses showed multiple known and new targets in the p75NTR gene network. This study provides a comprehensive analysis and investigation into the current knowledge of p75NTR signaling networks and pathways. These results also identify several genes and their respective protein products as involved in the p75NTR network, which have not previously been clearly studied in this pathway. These results can be used to generate novel hypotheses to gain a greater understanding of p75NTR in acute brain injuries, neurodegenerative diseases and general response to cellular damage.

The low levels of nerve growth factor and its upstream regulatory kinases in prion infection is reversed by resveratrol

Resveratrol shows ability to eliminate prion replication, but the exact mechanism for prion eradication was not clear yet. Our previous studies demonstrate a downregulation of brain-derived nerve growth factor (BDNF) during prion infection, meanwhile recovery of cerebral nerve growth factor (NGF) level by resveratrol treatment has been reported in other neurodegenerative models. To obtain the possible changes of brain NGF and its upstream regulatory cascade during prion infection and after removal of prion propagation, the levels of NGF and its upstream regulatory factors in various prion-infected and prion-eradicated SMB cell lines and mice brains inoculated with various SMB cellular lysates were assessed with various methodologies. The levels of NGF were significantly decreased during prion replication, while recovered after removal of PrP^Sc by resveratrol in vitro. Morphological assays revealed that the NGF signals mainly colocalized within neurons, but not in the proliferative astrocytes and microglia. The upstream positive regulatory kinases, such as p-CREB, p-CaMKIV, CaMKK2 were decreased in the prion infected cells and mice brains, whereas the negative regulatory one, p-CaMKK2, was increased. The aberrant situations of those kinases in prion infected cell lines or mice brains could be also partially reversed by removal of prion agent. Moreover, we demonstrated that the signals of CaMKK2 and p-CaMKK2 were also distributed predominately in neurons in the brain tissues. The data illustrate a direct linkage of abnormally repressive NGF and its upstream regulatory kinases with prion infection. Resveratrol has not only the ability to inhibit prion replication, but also to improve the expression of NGF via CaMKK2/CaMKIV cascade, which might benefit the microenvironment in brains.

Decrease of RyR2 in the prion infected cell line and in the brains of the scrapie infected mice models and the patients of human prion diseases

The levels of ryanodine receptors (RyRs) are usually increased in the brains of human Alzheimer disease (AD) and AD animal models. To evaluate the underlying alteration of brain RyRs in prion disease, scrapie infected cell line SMB-S15 and its infected mice were tested. RyR2 specific Western blots revealed markedly decreased RyR2 levels both in the cells and in the brains of infected mice. Assays of the brain samples of other scrapie (agents 139A and ME7) infected mice collected at different time-points during incubation period showed time-dependent decreases of RyR2. Immunofluorescent assays (IFA) verified that the expression of RyR2 locates predominantly in cytoplasm of SMB cells and overlapped with the neurons in the brain slices of mice. Furthermore, significant down-regulation of RyR2 was also detected in the postmortem cortical brains of the patients of various types of human prion diseases, including sporadic Creutzfeldt-Jakob disease (sCJD), fatal familial insomnia (FFI) and G114V-genetic CJD. Our data here propose the evidences of remarkably decreased brain RyR2 at terminal stages of both human prion diseases and prion infected rodent models. It also highlights that the therapeutic strategy with antagonist of RyRs in AD may not be suitable for prion disease.

Aberrant alterations of the expressions and S-nitrosylation of calmodulin and the downstream factors in the brains of the rodents during scrapie infection

The aberrant alterations of calmodulin (CaM) and its downstream substrates have been reported in some neurodegenerative diseases, but rarely described in prion disease. In this study, the potential changes of Ca²⁺/CaM and its associated agents in the brains of scrapie agent 263K-infected hamsters and the prion infected cell line SMB-S15 were evaluated by various methodologies. We found that the level of CaM in the brains of 263K-infected hamsters started to increase at early stage and maintained at high level till terminal stage. The increased CaM mainly accumulated in the regions of cortex, thalamus and cerebellum of 263K-infected hamsters and well localization of CaM with NeuN positive cells. However, the related kinases such as total and phosphorylated forms of CaMKII and CaMKIV, as well as the downstream proteins such as CREB and BDNF in the brain of 263K-infected hamsters were decreased. Further analysis showed a remarkable increase of S-nitrosylated (SNO) form of CaM in the brains of 263K-infected hamsters. Dynamic analysis of S-nitrosylated CaM showed the SNO form of CaM abnormally increases in a time-dependent manner during prion infection. Compared with that of the normal partner cell line SMB-PS, the CaM level in SMB-S15 cells was increased, meanwhile, the downstream proteins, such as CaMKII, p-CaMKII, CREB, as well as BDNF, were also increased, especially in the nucleic fraction. No SNO-CaM was detected in the cell lines SMB-S15 and SMB-PS. Our data indicate an aberrant increase of CaM during prion infection in vivo and in vitro.

Reduction of NF-κB (p65) in Scrapie-Infected Cultured Cells and in the Brains of Scrapie-Infected Rodents

Transcription factor NF-κB functions as a pleiotropic regulator of target genes controlling physiological function as well as pathological processes of many different diseases, including some neurodegenerative diseases. However, the role of NF-κB in the pathogenesis of prion disease remains ambiguous. In this study, the status of NF-κB (p65) in a prion-infected cell line SMB-S15 was first evaluated. Significantly lower levels of p65 and the phosphorylated form of p65 (p-p65) were detected in SMB-S15 cells, compared with its normal partner cell line SMB-PS. Markedly slower responses of the NF-κB system to the stimulation of TNF-α were observed in SMB-S15 cells. Removal of PrP^Sc replication in SMB-S15 cells rescued the expression and activity of NF-κB. However, overexpression of p65 in SMB-S15 cells did not influence the propagation of PrP^Sc. Moreover, significant decline of p65 level was also observed in the brain tissues of mice infected with the lysates of SMB-S15 cells and hamsters infected with scrapie agent 263K at terminal stage. Immunofluorescence assays (IFAs) on brain sections from either normal or scrapie-infected rodents revealed colocalization of p65 with neuronal nuclear (NeuN) protein positive cells but not with glial fibrillary acidic protein (GFAP) positive cells. Assays of the agents involving in the regulation of NF-κB showed down-regulated phosphoinositide 3-kinase (PI3K) and protein kinase B (PKB/Akt) both in SMB-S15 cells and in the brains of scrapie-infected rodents. Those data indicate a remarkable repression of the classical NF-κB pathway during prion infection both in vitro and in vivo. The alteration of NF-κB (p65) shows close association with the replication and accumulation of PrP^Sc in the cells.