Cellular prion protein protects from inflammatory and neuropathic pain

The role of cellular prion protein in immune system

Numerous studies have investigated the cellular prion protein (PrPC) since its discovery. These investigations have explained that its structure is predominantly composed of alpha helices and short beta sheet segments, and when its abnormal scrapie isoform (PrPSc) is infected, PrPSc transforms the PrPC, leading to prion diseases, including Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cattle. Given its ubiquitous distribution across a variety of cellular types, the PrPC manifests a diverse range of biological functions, including cell-cell adhesion, neuroprotection, signalings, and oxidative stress response. PrPC is also expressed in immune tissues, and its functions in these tissues include the activation of immune cells and the formation of secondary lymphoid tissues, such as the spleen and lymph nodes. Moreover, high expression of PrPC in immune cells plays a crucial role in the pathogenesis of prion diseases. In addition, it affects inflammation and the development and progression of cancer via various mechanisms. In this review, we discuss the studies on the role of PrPC from various immunological perspectives. [BMB Reports 2023; 56(12): 645-650].

Machine-learned analysis of the association of next-generation sequencing-based genotypes with persistent pain after breast cancer surgery

Cancer and its surgical treatment are among the most important triggering events for persistent pain, but additional factors need to be present for the clinical manifestation, such as variants in pain-relevant genes. In a cohort of 140 women undergoing breast cancer surgery, assigned based on a 3-year follow-up to either a persistent or nonpersistent pain phenotype, next-generation sequencing was performed for 77 genes selected for known functional involvement in persistent pain. Applying machine-learning and item categorization techniques, 21 variants in 13 different genes were found to be relevant to the assignment of a patient to either the persistent pain or the nonpersistent pain phenotype group. In descending order of importance for correct group assignment, the relevant genes comprised DRD1, FAAH, GCH1, GPR132, OPRM1, DRD3, RELN, GABRA5, NF1, COMT, TRPA1, ABHD6, and DRD4, of which one in the DRD4 gene was a novel discovery. Particularly relevant variants were found in the DRD1 and GPR132 genes, or in a cis-eCTL position of the OPRM1 gene. Supervised machine-learning-based classifiers, trained with 2/3 of the data, identified the correct pain phenotype group in the remaining 1/3 of the patients at accuracies and areas under the receiver operator characteristic curves of 65% to 72%. When using conservative classical statistical approaches, none of the variants passed α-corrected testing. The present data analysis approach, using machine learning and training artificial intelligences, provided biologically plausible results and outperformed classical approaches to genotype-phenotype association.

Development of an AmpliSeqTM Panel for Next-Generation Sequencing of a Set of Genetic Predictors of Persisting Pain

Background: Many gene variants modulate the individual perception of pain and possibly also its persistence. The limited selection of single functional variants is increasingly being replaced by analyses of the full coding and regulatory sequences of pain-relevant genes accessible by means of next generation sequencing (NGS).

Methods: An NGS panel was created for a set of 77 human genes selected following different lines of evidence supporting their role in persisting pain. To address the role of these candidate genes, we established a sequencing assay based on a custom AmpliSeq^TM panel to assess the exomic sequences in 72 subjects of Caucasian ethnicity. To identify the systems biology of the genes, the biological functions associated with these genes were assessed by means of a computational over-representation analysis.

Results: Sequencing generated a median of 2.85 ⋅ 10⁶ reads per run with a mean depth close to 200 reads, mean read length of 205 called bases and an average chip loading of 71%. A total of 3,185 genetic variants were called. A computational functional genomics analysis indicated that the proposed NGS gene panel covers biological processes identified previously as characterizing the functional genomics of persisting pain.

Conclusion: Results of the NGS assay suggested that the produced nucleotide sequences are comparable to those earned with the classical Sanger sequencing technique. The assay is applicable for small to large-scale experimental setups to target the accessing of information about any nucleotide within the addressed genes in a study cohort.

Functions of the Prion Protein

Although initially disregarded compared to prion pathogenesis, the functions exerted by the cellular prion protein PrP^C have gained much interest over the past two decades. Research aiming at unraveling PrP^C functions started to intensify when it became appreciated that it would give clues as to how it is subverted in the context of prion infection and, more recently, in the context of Alzheimer's disease. It must now be admitted that PrP^C is implicated in an incredible variety of biological processes, including neuronal homeostasis, stem cell fate, protection against stress, or cell adhesion. It appears that these diverse roles can all be fulfilled through the involvement of PrP^C in cell signaling events. Our aim here is to provide an overview of our current understanding of PrP^C functions from the animal to the molecular scale and to highlight some of the remaining gaps that should be addressed in future research.

The prion protein regulates glutamate-mediated Ca2+ entry and mitochondrial Ca2+ accumulation in neurons

The cellular prion protein (PrP^C) whose conformational misfolding leads to the production of deadly prions, has a still-unclarified cellular function despite decades of intensive research. Following our recent finding that PrP^C limits Ca²⁺ entry via store-operated Ca²⁺ channels in neurons, we investigated whether the protein could also control the activity of ionotropic glutamate receptors (iGluRs). To this end, we compared local Ca²⁺ movements in primary cerebellar granule neurons and cortical neurons transduced with genetically encoded Ca²⁺ probes and expressing, or not expressing, PrP^C Our investigation demonstrated that PrP^C downregulates Ca²⁺ entry through each specific agonist-stimulated iGluR and after stimulation by glutamate. We found that, although PrP-knockout (KO) mitochondria were displaced from the plasma membrane, glutamate addition resulted in a higher mitochondrial Ca²⁺ uptake in PrP-KO neurons than in their PrP^C-expressing counterpart. This was because the increased Ca²⁺ entry through iGluRs in PrP-KO neurons led to a parallel increase in Ca²⁺-induced Ca²⁺ release via ryanodine receptor channels. These data thus suggest that PrP^C takes part in the cell apparatus controlling Ca²⁺ homeostasis, and that PrP^C is involved in protecting neurons from toxic Ca²⁺ overloads.

The biological function of the cellular prion protein: an update

The misfolding of the cellular prion protein (PrP^C) causes fatal neurodegenerative diseases. Yet PrP^C is highly conserved in mammals, suggesting that it exerts beneficial functions preventing its evolutionary elimination. Ablation of PrP^C in mice results in well-defined structural and functional alterations in the peripheral nervous system. Many additional phenotypes were ascribed to the lack of PrP^C, but some of these were found to arise from genetic artifacts of the underlying mouse models. Here, we revisit the proposed physiological roles of PrP^C in the central and peripheral nervous systems and highlight the need for their critical reassessment using new, rigorously controlled animal models.

Physiological Functions of the Cellular Prion Protein

The prion protein, PrP^C, is a small, cell-surface glycoprotein notable primarily for its critical role in pathogenesis of the neurodegenerative disorders known as prion diseases. A hallmark of prion diseases is the conversion of PrP^C into an abnormally folded isoform, which provides a template for further pathogenic conversion of PrP^C, allowing disease to spread from cell to cell and, in some circumstances, to transfer to a new host. In addition to the putative neurotoxicity caused by the misfolded form(s), loss of normal PrP^C function could be an integral part of the neurodegenerative processes and, consequently, significant research efforts have been directed toward determining the physiological functions of PrP^C. In this review, we first summarise important aspects of the biochemistry of PrP^C before moving on to address the current understanding of the various proposed functions of the protein, including details of the underlying molecular mechanisms potentially involved in these functions. Over years of study, PrP^C has been associated with a wide array of different cellular processes and many interacting partners have been suggested. However, recent studies have cast doubt on the previously well-established links between PrP^C and processes such as stress-protection, copper homeostasis and neuronal excitability. Instead, the functions best-supported by the current literature include regulation of myelin maintenance and of processes linked to cellular differentiation, including proliferation, adhesion, and control of cell morphology. Intriguing connections have also been made between PrP^C and the modulation of circadian rhythm, glucose homeostasis, immune function and cellular iron uptake, all of which warrant further investigation.

Almost a century of prion protein(s): From pathology to physiology, and back to pathology

Prions are one of the few pathogens whose name is renowned at all population levels, after the dramatic years pervaded by the fear of eating prion-infected food. If now this, somehow irrational, scare of bovine meat inexorably transmitting devastating brain disorders is largely subdued, several prion-related issues are still unsolved, precluding the design of therapeutic approaches that could slow, if not halt, prion diseases. One unsolved issue is, for example, the role of the prion protein (PrP^C), whole conformational misfolding originates the prion but whose physiologic reason d'etre in neurons, and in cells at large, remains enigmatic. Preceded by a historical outline, the present review will discuss the functional pleiotropicity ascribed to PrP^C, and whether this aspect could fall, at least in part, into a more concise framework. It will also be devoted to radically different perspectives for PrP^C, which have been recently brought to the attention of the scientific world with unexpected force. Finally, it will discuss the possible reasons allowing an evolutionary conserved and benign protein, as PrP^C is, to turn into a high affinity receptor for pathologic misfolded oligomers, and to transmit their toxic message into neurons.

Synthesis and characterization of a disubstituted piperazine derivative with T-type channel blocking action and analgesic properties

Background

T-type calcium channels are important contributors to signaling in the primary afferent pain pathway and are thus important targets for the development of analgesics. It has been previously reported that certain piperazine-based compounds such as flunarizine are able to inhibit T-type calcium channels. Thus, we hypothesized that novel piperazine compounds could potentially act as analgesics.

Results

Here, we have created a series of 14 compound derivatives around a diphenyl methyl-piperazine core pharmacophore. Testing their effects on transiently expressed Cav3.2 calcium channels revealed one derivative (3-((4-(bis(4-fluorophenyl)methyl)piperazin-1-yl)methyl)-4-(2-methoxyphenyl)-1,2,5-oxadiazole 2-oxide, compound 10e) as a potent blocker. 10e mediate tonic block of these channels with an IC50 of around 4 micromolar. 10e also blocked Cav3.1 and Cav3.3 channels, but only weakly affected high-voltage-activated Cav1.2 and Cav2.2 channels. Intrathecal delivery of 10e mediated relief from formalin and complete Freund’s adjuvant induced inflammatory pain that was ablated by genetic knockout of Cav3.2 channels.

Conclusions

Altogether, our data identify a novel T-type calcium channel blocker with tight structure activity relationship (SAR) and relevant in vivo efficacy in inflammatory pain conditions.

Behavioral abnormalities in prion protein knockout mice and the potential relevance of PrP(C) for the cytoskeleton

The cellular prion protein (PrP(C)) is a highly conserved protein, which is anchored to the outer surface of the plasma membrane. Even though its physiological function has already been investigated in different cell or mouse models where PrP(C) expression is either upregulated or depleted, its exact physiological role in a mammalian organism remains elusive. Recent studies indicate that PrP(C) has multiple functions and is involved in cognition, learning, anxiety, locomotion, depression, offensive aggression and nest building behavior. While young animals (3 months of age) show only marginal abnormalities, most of the deficits become apparent as the animals age, which might indicate its role in neurodegeneration or neuroprotection. However, the exact biochemical mechanism and signal transduction pathways involving PrP(C) are only gradually becoming clearer. We report the observations made in different studies using different Prnp0/0 mouse models and propose that PrP(C) plays an important role in the regulation of the cytoskeleton and associated proteins. In particular, we showed a nocodazole treatment influenced colocalization of PrP(C) and α tubulin 1. In addition, we confirmed the observed deficits in nest building using a different backcrossed Prnp0/0 mouse line.

Effect of the T-type channel blocker KYS-05090S in mouse models of acute and neuropathic pain

T-type channels are important contributors to the initiation and the maintenance of chronic pain states. Blocking T-type channels is therefore a possible therapeutic strategy for relieving pain. Here, we report the Cav3.2 T-type channel blocking action of a previously reported small organic molecule, KYS-05090S. This compound was able to reduce transiently expressed Cav3.2 currents with low micromolar affinity and mediated a hyperpolarizing shift in half-inactivation potential. KYS-05090S was then tested in models of acute and neuropathic pain. KYS-05090S (10 μg/10 μl delivered intrathecally) significantly reduced acute pain induced by formalin in both the tonic and inflammatory phases. Its antinociceptive effect was not observed when delivered to Cav3.2 null-mice revealing a Cav3.2-dependent mechanism. KYS-05090S also reduced neuropathic pain in a model of partial sciatic nerve injury. Those results indicate that KYS-05090S mediates a potent analgesic effect in inflammatory and neuropathic pain through T-type channel modulation, suggesting that its scaffold could be explored as a new class of analgesic compounds.

Analgesic effect of a broad-spectrum dihydropyridine inhibitor of voltage-gated calcium channels

Voltage-activated calcium channels are important facilitators of nociceptive transmission in the primary afferent pathway. Consequently, molecules that block these channels are of potential use as pain therapeutics. Our group has recently reported on the identification of a novel class of dihydropyridines (DHPs) that included compounds with preferential selectivity for T-type over L-type channels. Among those compounds, M4 was found to be an equipotent inhibitor of both Cav1.2 L- and Cav3.2 T-type calcium channels. Here, we have further characterized the effects of this compound on other types of calcium channels and examined its analgesic effect when delivered either spinally (i.t.) or systemically (i.p.) to mice. Both delivery routes resulted in antinociception in a model of acute pain. Furthermore, M4 was able to reverse mechanical hyperalgesia produced by nerve injury when delivered intrathecally. M4 retained partial activity when delivered to Cav3.2 null mice, indicating that this compound acts on multiple targets. Additional whole-cell patch clamp experiments in transfected tsA-201 cells revealed that M4 also effectively blocks Cav3.3 (T-type) and Cav2.2 (N-type) currents. Altogether, our data indicate that broad-spectrum inhibition of multiple calcium channel subtypes can lead to potent analgesia in rodents.

Cellular prion protein (PrP(C)) and its role in stress responses

Investigation of the physiological function of cellular prion protein (PrP(C)) has been developed by the generation of transgenic mice, however, the pathological mechanisms related to PrP(C) in prion diseases such as transmissible spongiform encephalopathies (TSEs) are still abstruse. Regardless of some differences, most studies describe the neuroprotective role of PrP(C) in environmental stresses. In this review, we will update the current knowledge on the responses of PrP(C) to various stresses, especially those correlated with cell signaling and neural degeneration, including ischemia, oxidative stress, inflammation and autophagy.

Review of studies that have used knockout mice to assess normal function of prion protein under immunological or pathophysiological stress

Deletion of cellular isoform of prion protein (PrP(C)) increases neuronal predisposition to damage by modulating apoptosis and the negative consequences of oxidative stress. In vivo studies have demonstrated that PrP(C)-deficient mice are more prone to seizure, depression, and induction of epilepsy and experience extensive cerebral damage following ischemic challenge or viral infection. In addition, adenovirus-mediated overexpression of PrP(C) reduces brain damage in rat models of cerebral ischemia. In experimental autoimmune encephalomyelitis, PrP(C)-deficient mice reportedly have a more aggressive disease onset and less clinical improvement during the chronic phase than wild-type mice mice. In mice given oral dextran sulfate, PrP(C) has a potential protective role against inflammatory bowel disease. PrP(C)-deficient mice demonstrate significantly greater increases in blood glucose concentrations after intraperitoneal injection of glucose than wild-type mice. Further in vivo challenges to PrP gene-deficient models and conditional knockout models with siRNA and in vivo administration of PrP-ligating agents may assist in refining knowledge of the lymphoid function of PrP(C) and predicting the effects of anti-PrP treatment on the immune system. Together, these findings indicate that PrP(C) may have multiple neuroprotective and anti-inflammatory roles, which explains why this protein is so widely expressed.

Glutamate receptors function as scaffolds for the regulation of β-amyloid and cellular prion protein signaling complexes

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects 36 million people worldwide, but currently has no effective treatment options. One of the original hallmarks of AD are plaques comprised of beta amyloid (Aβ) and neurofibrillary tangles comprised of phosphorylated Tau protein. However, it is soluble oligomeric Aβ which is more closely correlated with cognitive decline and is therefore considered to be the neurotoxic species. Oligomeric Aβ has recently been shown to form complexes with the glycosylphosphatidylinositol (GPI)-anchored membrane protein, cellular prion protein (PrP(c)), and these complexes are believed to play an important role in the progression of AD pathogenesis. Glutamate, the major excitatory neurotransmitter is responsible for mediating learning and memory under normal physiological conditions. However, the dysregulation of glutamatergic signaling has also been implicated in a number of neurodegenerative diseases including AD. Glutamate acts via both ionotropic glutamate receptors (iGluR) and metabotropic glutamate receptors (mGluR), each of which have been implicated in AD. There is now growing evidence to suggest that mGluR5 may contribute the AD pathogenesis by acting as scaffolds for the PrP(c)/Aβ oligomer complex, enabling the propagation of neurotoxic signaling in AD. In addition, PrP(c) and Aβ oligomer signaling via NMDARs may also contribute to AD pathology. The current review overviews our current understanding of the role of PrP(c) and Aβ oligomers in regulating glutamate receptor signaling, as well as highlights the importance of understanding these signaling complexes to develop more effective therapeutic strategies to treat AD.

Characterization of novel cannabinoid based T-type calcium channel blockers with analgesic effects

Low-voltage-activated (T-type) calcium channels are important regulators of the transmission of nociceptive information in the primary afferent pathway and finding ligands that modulate these channels is a key focus of the drug discovery field. Recently, we characterized a set of novel compounds with mixed cannabinoid receptor/T-type channel blocking activity and examined their analgesic effects in animal models of pain. Here, we have built on these previous findings and synthesized a new series of small organic compounds. We then screened them using whole-cell voltage clamp techniques to identify the most potent T-type calcium channel inhibitors. The two most potent blockers (compounds 9 and 10) were then characterized using radioligand binding assays to determine their affinity for CB1 and CB2 receptors. The structure–activity relationship and optimization studies have led to the discovery of a new T-type calcium channel blocker, compound 9. Compound 9 was efficacious in mediating analgesia in mouse models of acute inflammatory pain and in reducing tactile allodynia in the partial nerve ligation model. This compound was shown to be ineffective in Cav3.2 T-type calcium channel null mice at therapeutically relevant concentrations, and it caused no significant motor deficits in open field tests. Taken together, our data reveal a novel class of compounds whose physiological and therapeutic actions are mediated through block of Cav3.2 calcium channels.

Cellular prion protein and NMDA receptor modulation: protecting against excitotoxicity

Although it is well established that misfolding of the cellular prion protein (PrP^C) into the β-sheet-rich, aggregated scrapie conformation (PrP^Sc) causes a variety of transmissible spongiform encephalopathies (TSEs), the physiological roles of PrP^C are still incompletely understood. There is accumulating evidence describing the roles of PrP^C in neurodegeneration and neuroinflammation. Recently, we identified a functional regulation of NMDA receptors by PrP^C that involves formation of a physical protein complex between these proteins. Excessive NMDA receptor activity during conditions such as ischemia mediates enhanced Ca²⁺ entry into cells and contributes to excitotoxic neuronal death. In addition, NMDA receptors and/or PrP^C play critical roles in neuroinflammation and glial cell toxicity. Inhibition of NMDA receptor activity protects against PrP^Sc-induced neuronal death. Moreover, in mice lacking PrP^C, infarct size is increased after focal cerebral ischemia, and absence of PrP^C increases susceptibility of neurons to NMDA receptor-dependent death. Recently, PrP^C was found to be a receptor for oligomeric beta-amyloid (Aβ) peptides, suggesting a role for PrP^C in Alzheimer's disease (AD). Our recent findings suggest that Aβ peptides enhance NMDA receptor current by perturbing the normal copper- and PrP^C-dependent regulation of these receptors. Here, we review evidence highlighting a role for PrP^C in preventing NMDA receptor-mediated excitotoxicity and inflammation. There is a need for more detailed molecular characterization of PrP^C-mediated regulation of NMDA receptors, such as determining which NMDA receptor subunits mediate pathogenic effects upon loss of PrP^C-mediated regulation and identifying PrP^C binding site(s) on the receptor. This knowledge will allow development of novel therapeutic interventions for not only TSEs, but also for AD and other neurodegenerative disorders involving dysfunction of PrP^C.

Analgesic effect of a mixed T-type channel inhibitor/CB2 receptor agonist

BACKGROUND

Cannabinoid receptors and T-type calcium channels are potential targets for treating pain. Here we report on the design, synthesis and analgesic properties of a new mixed cannabinoid/T-type channel ligand, NMP-181.

RESULTS

NMP-181 action on CB1 and CB2 receptors was characterized in radioligand binding and in vitro GTPγ[35S] functional assays, and block of transiently expressed human Cav3.2 T-type channels by NMP-181 was analyzed by patch clamp. The analgesic effects and in vivo mechanism of action of NMP-181 delivered spinally or systemically were analyzed in formalin and CFA mouse models of pain. NMP-181 inhibited peak CaV3.2 currents with IC50 values in the low micromolar range and acted as a CB2 agonist. Inactivated state dependence further augmented the inhibitory action of NMP-181. NMP-181 produced a dose-dependent antinociceptive effect when administered either spinally or systemically in both phases of the formalin test. Both i.t. and i.p. treatment of mice with NMP-181 reversed the mechanical hyperalgesia induced by CFA injection. NMP-181 showed no antinocieptive effect in CaV3.2 null mice. The antinociceptive effect of intrathecally delivered NMP-181 in the formalin test was reversed by i.t. treatment of mice with AM-630 (CB2 antagonist). In contrast, the NMP-181-induced antinociception was not affected by treatment of mice with AM-281 (CB1 antagonist).

CONCLUSIONS

Our work shows that both T-type channels as well as CB2 receptors play a role in the antinociceptive action of NMP-181, and also provides a novel avenue for suppressing chronic pain through novel mixed T-type/cannabinoid receptor ligands.

The diverse roles of mononuclear phagocytes in prion disease pathogenesis

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are neurological diseases that can be transmitted through a number of different routes. A wide range of mammalian species are affected by the disease. After peripheral exposure, some TSE agents accumulate in lymphoid tissues at an early stage of disease prior to spreading to the nerves and the brain. Much research has focused on identifying the cells and molecules involved in the transmission of TSE agents from the site of exposure to the brain and several crucial cell types have been associated with this process. The identification of the key cells that influence the different stages of disease transmission might identify targets for therapeutic intervention. This review highlights the involvement of mononuclear phagocytes in TSE disease. Current data suggest these cells may exhibit a diverse range of roles in TSE disease from the transport or destruction of TSE agents in lymphoid tissues, to mediators or protectors of neuropathology in the brain.

Copper-dependent regulation of NMDA receptors by cellular prion protein: implications for neurodegenerative disorders

N-Methyl-D-aspartate (NMDA) receptors mediate a wide range of important nervous system functions. Conversely, excessive NMDA receptor activity leads to cytotoxic calcium overload and neuronal damage in a wide variety of CNS disorders. It is well established that NMDA receptors are tightly regulated by a number of cell signalling pathways. Recently, it has been shown that NMDA receptor activity is modulated by cellular prion protein (PrP(C)) in a copper-dependent manner. Here we give an overview of the current state of knowledge concerning the novel concept of potent modulation of this receptor's kinetics by copper ions, and the interplay between NMDA receptors and PrP(C) in the context of neurological diseases such as Alzheimer's disease, epilepsy, pain and depression.