Threshold for epileptiform activity is elevated in prion knockout mice

Analysis of co-isogenic prion protein deficient mice reveals behavioral deficits, learning impairment, and enhanced hippocampal excitability

Background

Cellular prion protein (PrP^C) is a cell surface GPI-anchored protein, usually known for its role in the pathogenesis of human and animal prionopathies. However, increasing knowledge about the participation of PrP^C in prion pathogenesis contrasts with puzzling data regarding its natural physiological role. PrP^C is expressed in a number of tissues, including at high levels in the nervous system, especially in neurons and glial cells, and while previous studies have established a neuroprotective role, conflicting evidence for a synaptic function has revealed both reduced and enhanced long-term potentiation, and variable observations on memory, learning, and behavior. Such evidence has been confounded by the absence of an appropriate knock-out mouse model to dissect the biological relevance of PrP^C, with some functions recently shown to be misattributed to PrP^C due to the presence of genetic artifacts in mouse models. Here we elucidate the role of PrP^C in the hippocampal circuitry and its related functions, such as learning and memory, using a recently available strictly co-isogenic Prnp^0/0 mouse model (Prnp^ZH3/ZH3).

Results

We performed behavioral and operant conditioning tests to evaluate memory and learning capabilities, with results showing decreased motility, impaired operant conditioning learning, and anxiety-related behavior in Prnp^ZH3/ZH3 animals. We also carried in vivo electrophysiological recordings on CA3-CA1 synapses in living behaving mice and monitored spontaneous neuronal firing and network formation in primary neuronal cultures of Prnp^ZH3/ZH3 vs wildtype mice. PrP^C absence enhanced susceptibility to high-intensity stimulations and kainate-induced seizures. However, long-term potentiation (LTP) was not enhanced in the Prnp^ZH3/ZH3 hippocampus. In addition, we observed a delay in neuronal maturation and network formation in Prnp^ZH3/ZH3 cultures.

Conclusion

Our results demonstrate that PrP^C promotes neuronal network formation and connectivity. PrP^C mediates synaptic function and protects the synapse from excitotoxic insults. Its deletion may underlie an epileptogenic-susceptible brain that fails to perform highly cognitive-demanding tasks such as associative learning and anxiety-like behaviors.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01203-0.

Harnessing the Physiological Functions of Cellular Prion Protein in the Kidneys: Applications for Treating Renal Diseases

A cellular prion protein (PrP^C) is a ubiquitous cell surface glycoprotein, and its physiological functions have been receiving increased attention. Endogenous PrP^C is present in various kidney tissues and undergoes glomerular filtration. In prion diseases, abnormal prion proteins are found to accumulate in renal tissues and filtered into urine. Urinary prion protein could serve as a diagnostic biomarker. PrP^C plays a role in cellular signaling pathways, reno-protective effects, and kidney iron uptake. PrP^C signaling affects mitochondrial function via the ERK pathway and is affected by the regulatory influence of microRNAs, small molecules, and signaling proteins. Targeting PrP^C in acute and chronic kidney disease could help improve iron homeostasis, ameliorate damage from ischemia/reperfusion injury, and enhance the efficacy of mesenchymal stem/stromal cell or extracellular vesicle-based therapeutic strategies. PrP^C may also be under the influence of BMP/Smad signaling and affect the progression of TGF-β-related renal fibrosis. PrP^C conveys TNF-α resistance in some renal cancers, and therefore, the coadministration of anti-PrP^C antibodies improves chemotherapy. PrP^C can be used to design antibody-drug conjugates, aptamer-drug conjugates, and customized tissue inhibitors of metalloproteinases to suppress cancer. With preclinical studies demonstrating promising results, further research on PrP^C in the kidney may lead to innovative PrP^C-based therapeutic strategies for renal disease.

Increased risk of post-stroke epilepsy in Chinese patients with a TRPM6 polymorphism

OBJECTIVES

We attempted to determine whether a functional polymorphism of TRPM6 (rs2274924) is associated with susceptibility to epilepsy following ischemic stroke, and to further explore the effect of this polymorphism on serum levels of Mg²⁺ in post-stroke patients.

METHODS

We carried out a case-control study on 378 post-stroke epilepsy patients and 420 controls (stroke patients without secondary epilepsy). We used DNA sequencing to determine the genotypes of the TRPM6 rs2274924 polymorphism, and used the ion selective electrode method to measure serum levels of Mg²⁺.

RESULTS

The distribution of the CC genotype and the frequency of the C allele were significantly higher in the post-stroke epilepsy patients than in the controls (P < 0.01). With regard to the post-stroke epilepsy patients, the serum levels of Mg²⁺ decreased significantly in the TRPM6 rs2274924 C allele carriers compared to the rs2274924 T allele carriers.

CONCLUSION

The TRPM6 rs2274924 polymorphism may be associated with susceptibility to epilepsy following stroke, and the C allele may be associated with increased risk of post-stroke epilepsy. The TRPM6 rs2274924 polymorphism may also influence serum levels of Mg²⁺ in post-stroke epilepsy patients.

Mechanism and consequence of abnormal calcium homeostasis in Rett syndrome astrocytes

Astrocytes play an important role in Rett syndrome (RTT) disease progression. Although the non-cell-autonomous effect of RTT astrocytes on neurons was documented, cell-autonomous phenotypes and mechanisms within RTT astrocytes are not well understood. We report that spontaneous calcium activity is abnormal in RTT astrocytes in vitro, in situ, and in vivo. Such abnormal calcium activity is mediated by calcium overload in the endoplasmic reticulum caused by abnormal store operated calcium entry, which is in part dependent on elevated expression of TRPC4. Furthermore, the abnormal calcium activity leads to excessive activation of extrasynaptic NMDA receptors (eNMDARs) on neighboring neurons and increased network excitability in Mecp2 knockout mice. Finally, both the abnormal astrocytic calcium activity and the excessive activation of eNMDARs are caused by Mecp2 deletion in astrocytes in vivo. Our findings provide evidence that abnormal calcium homeostasis is a key cell-autonomous phenotype in RTT astrocytes, and reveal its mechanism and consequence.

Protein Misfolding in Prion and Prion-Like Diseases: Reconsidering a Required Role for Protein Loss-of-Function

Prion disease research has contributed much toward understanding other neurodegenerative diseases, including recent demonstrations that Alzheimer's disease (AD) and other neurodegenerative diseases are prion-like. Prion-like diseases involve the spread of degeneration between individuals and/or among cells or tissues via template directed misfolding, wherein misfolded protein conformers propagate disease by causing normal proteins to misfold. Here we use the premise that AD, amyotrophic lateral sclerosis, Huntington's disease, and other similar diseases are prion-like and ask: Can we apply knowledge gained from studies of these prion-like diseases to resolve debates about classical prion diseases? We focus on controversies about what role(s) protein loss-of-function might have in prion diseases because this has therapeutic implications, including for AD. We examine which loss-of-function events are recognizable in prion-like diseases by considering the normal functions of the proteins before their misfolding and aggregation. We then delineate scenarios wherein gain-of-function and/or loss-of-function would be necessary or sufficient for neurodegeneration. We consider roles of PrPC loss-of-function in prion diseases and in AD, and conclude that the conventional wisdom that prion diseases are 'toxic gain-of-function diseases' has limitations. While prion diseases certainly have required gain-of-function components, we propose that disease phenotypes are predominantly caused by deficits in the normal physiology of PrPC and its interaction partners as PrPC converts to PrPSc. In this model, gain-of-function serves mainly to spread disease, and loss-of-function directly mediates neuron dysfunction. We propose experiments and predictions to assess our conclusion. Further study on the normal physiological roles of these key proteins is warranted.

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.

Functions of the cellular prion protein, the end of Moore's law, and Ockham's razor theory

Since its discovery the cellular prion protein (encoded by the Prnp gene) has been associated with a large number of functions. The proposed functions rank from basic cellular processes such as cell cycle and survival to neural functions such as behavior and neuroprotection, following a pattern similar to that of Moore's law for electronics. In addition, particular interest is increasing in the participation of Prnp in neurodegeneration. However, in recent years a redefinition of these functions has begun, since examples of previously attributed functions were increasingly re-associated with other proteins. Most of these functions are linked to so-called "Prnp-flanking genes" that are close to the genomic locus of Prnp and which are present in the genome of some Prnp mouse models. In addition, their role in neuroprotection against convulsive insults has been confirmed in recent studies. Lastly, in recent years a large number of models indicating the participation of different domains of the protein in apoptosis have been uncovered. However, after more than 10 years of molecular dissection our view is that the simplest mechanistic model in PrP(C)-mediated cell death should be considered, as Ockham's razor theory suggested.

Involvement of PrP(C) in kainate-induced excitotoxicity in several mouse strains

The cellular prion protein (PrP(C)) has been associated with a plethora of cellular functions ranging from cell cycle to neuroprotection. Mice lacking PrP(C) show an increased susceptibility to epileptic seizures; the protein, then, is neuroprotective. However, lack of experimental reproducibility has led to considering the possibility that other factors besides PrP(C) deletion, such as the genetic background of mice or the presence of so-called "Prnp flanking genes", might contribute to the reported susceptibility. Here, we performed a comparative analysis of seizure-susceptibility using characterized Prnp(+/+) and Prnp(0/0) mice of B6129, B6.129, 129/Ola or FVB/N genetic backgrounds. Our study indicates that PrP(C) plays a role in neuroprotection in KA-treated cells and mice. For this function, PrP(C) should contain the aa32-93 region and needs to be linked to the membrane. In addition, some unidentified "Prnp-flanking genes" play a role parallel to PrP(C) in the KA-mediated responses in B6129 and B6.129 Prnp(0/0) mice.

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.

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.

SIRPα polymorphisms, but not the prion protein, control phagocytosis of apoptotic cells

Prnp(-/-) mice lack the prion protein PrP(C) and are resistant to prion infections, but variable phenotypes have been reported in Prnp(-/-) mice and the physiological function of PrP(C) remains poorly understood. Here we examined a cell-autonomous phenotype, inhibition of macrophage phagocytosis of apoptotic cells, previously reported in Prnp(-/-) mice. Using formal genetic, genomic, and immunological analyses, we found that the regulation of phagocytosis previously ascribed to PrP(C) is instead controlled by a linked locus encoding the signal regulatory protein α (Sirpa). These findings indicate that control of phagocytosis was previously misattributed to the prion protein and illustrate the requirement for stringent approaches to eliminate confounding effects of flanking genes in studies modeling human disease in gene-targeted mice. The plethora of seemingly unrelated functions attributed to PrP(C) suggests that additional phenotypes reported in Prnp(-/-) mice may actually relate to Sirpa or other genetic confounders.

Prion protein and susceptibility to kainate-induced seizures: genetic pitfalls in the use of PrP knockout mice

Prion protein (PrP) is a cell surface glycoprotein which is required for susceptibility to prion infection and disease. However, PrP is expressed in many different cell types located in numerous organs. Therefore, in addition to its role in prion diseases, PrP may have a large variety of other biological functions involving the nervous system and other systems. We recently showed that susceptibility to kainate-induced seizures differed in Prnp^−/− and Prnp^+/+ mice on the C57BL/10SnJ background. However, in a genetic complementation experiment a PrP expressing transgene was not able to rescue the Prnp^+/+ phenotype. Thus the apparent effect of PrP on seizures was actually due to genes flanking the Prnp^−/− gene rather that the Prnp deletion itself. We discuss here several pitfalls in the use of Prnp^−/− genotypes expressed in various mouse genetic backgrounds to determine the functions of PrP. In particular, the use of Prnp^−/− mice with heterogeneous mixed genetic backgrounds may have weakened the conclusions of many previous experiments. Use of either co-isogenic mice or congenic mice with more homogeneous genetic backgrounds is now feasible. For congenic mice, the potential problem of flanking genes can be mitigated by the use of appropriate transgene rescue experiments to confirm the conclusions.

Lack of influence of prion protein gene expression on kainate-induced seizures in mice: studies using congenic, coisogenic and transgenic strains

Prion protein (PrP) is a glycosylphosphatidylinositol (GPI) anchored cell surface protein expressed by many cells, including those of the mammalian nervous system. At present the physiologic functions of PrP remain unclear. Deletion of Prnp, the gene encoding PrP in mice, has been shown to alter normal synaptic and electrophysiologic activities, indicating a potential role in seizure susceptibility. However, published efforts to link PrP with seizures, using both in vivo and in vitro models, are conflicting and difficult to interpret due to use of various mouse backgrounds and seizure induction techniques. Here we investigated the role of PrP in kainic acid (KA)-induced seizure sensitivity, using three types of mice. In contrast to previous published results, Prnp-/- mice on the C57BL/10SnJ background had a significant decrease in KA-induced seizure susceptibility. In genetic complementation experiments using a PrP-expressing transgene, genes derived from strain 129/Ola, which flanked the Prnp-/- locus in C57BL/10SnJ mice, rather than Prnp itself, appeared to account for this effect. Furthermore, using coisogenic 129/Ola mice differing only at Prnp, this difference was not reproduced when comparing PrP-negative and PrP-positive mice. In contrast, substrains of PrP-expressing C57BL mice, showed large variations in KA-induced seizure sensitivity. The magnitude of these differences in susceptibility was larger than that associated with the presence of the Prnp gene, suggesting extensive influence of genes other than Prnp on seizure sensitivity in this system.

Unraveling the neuroprotective mechanisms of PrP (C) in excitotoxicity

Knowledge of the natural roles of cellular prion protein (PrP (C) ) is essential to an understanding of the molecular basis of prion pathologies. This GPI-anchored protein has been described in synaptic contacts, and loss of its synaptic function in complex systems may contribute to the synaptic loss and neuronal degeneration observed in prionopathy. In addition, Prnp knockout mice show enhanced susceptibility to several excitotoxic insults, GABAA receptor-mediated fast inhibition was weakened, LTP was modified and cellular stress increased. Although little is known about how PrP (C) exerts its function at the synapse or the downstream events leading to PrP (C) -mediated neuroprotection against excitotoxic insults, PrP (C) has recently been reported to interact with two glutamate receptor subunits (NR2D and GluR6/7). In both cases the presence of PrP (C) blocks the neurotoxicity induced by NMDA and Kainate respectively. Furthermore, signals for seizure and neuronal cell death in response to Kainate in Prnp knockout mouse are associated with JNK3 activity, through enhancing the interaction of GluR6 with PSD-95. In combination with previous data, these results shed light on the molecular mechanisms behind the role of PrP (C) in excitotoxicity. Future experimental approaches are suggested and discussed.

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.

Ablation of cellular prion protein does not ameliorate abnormal neural network activity or cognitive dysfunction in the J20 line of human amyloid precursor protein transgenic mice

Previous studies suggested that the cellular prion protein (PrP(c)) plays a critical role in the pathogenesis of Alzheimer's disease (AD). Specifically, amyloid-β (Aβ) oligomers were proposed to cause synaptic and cognitive dysfunction by binding to PrP(c). To test this hypothesis, we crossed human amyloid precursor protein (hAPP) transgenic mice from line J20 onto a PrP(c)-deficient background. Ablation of PrP(c) did not prevent the premature mortality and abnormal neural network activity typically seen in hAPPJ20 mice. Furthermore, hAPPJ20 mice with or without PrP(c) expression showed comparably robust abnormalities in learning and memory and in other behavioral domains at 6-8 months of age. Notably, these abnormalities are not refractory to therapeutic manipulations in general: they can be effectively prevented by interventions that prevent Aβ-dependent neuronal dysfunction also in other lines of hAPP transgenic mice. Thus, at least in this model, PrP(c) is not an important mediator of Aβ-induced neurological impairments.

Cellular prion protein protects from inflammatory and neuropathic pain

Cellular prion protein (PrP^C) inhibits N-Methyl-D-Aspartate (NMDA) receptors. Since NMDA receptors play an important role in the transmission of pain signals in the dorsal horn of spinal cord, we thus wanted to determine if PrP^C null mice show a reduced threshold for various pain behaviours.

We compared nociceptive thresholds between wild type and PrP^C null mice in models of inflammatory and neuropathic pain, in the presence and the absence of a NMDA receptor antagonist. 2-3 months old male PrP^C null mice exhibited an MK-801 sensitive decrease in the paw withdrawal threshold in response both mechanical and thermal stimuli. PrP^C null mice also exhibited significantly longer licking/biting time during both the first and second phases of formalin-induced inflammation of the paw, which was again prevented by treatment of the mice with MK-801, and responded more strongly to glutamate injection into the paw. Compared to wild type animals, PrP^C null mice also exhibited a significantly greater nociceptive response (licking/biting) after intrathecal injection of NMDA. Sciatic nerve ligation resulted in MK-801 sensitive neuropathic pain in wild-type mice, but did not further augment the basal increase in pain behaviour observed in the null mice, suggesting that mice lacking PrP^C may already be in a state of tonic central sensitization. Altogether, our data indicate that PrP^C exerts a critical role in modulating nociceptive transmission at the spinal cord level, and fit with the concept of NMDA receptor hyperfunction in the absence of PrP^C.