Altered neuron excitability and synaptic plasticity in the cerebellar granular layer of juvenile prion protein knock-out mice with impaired motor control

Exploring pathological link between antimicrobial and amyloid peptides

Amyloid peptides (AMYs) and antimicrobial peptides (AMPs) are considered as the two distinct families of peptides, characterized by their unique sequences, structures, biological functions, and specific pathological targets. However, accumulating evidence has revealed intriguing pathological connections between these peptide families in the context of microbial infection and neurodegenerative diseases. Some AMYs and AMPs share certain structural and functional characteristics, including the ability to self-assemble, the presence of β-sheet-rich structures, and membrane-disrupting mechanisms. These shared features enable AMYs to possess antimicrobial activity and AMPs to acquire amyloidogenic properties. Despite limited studies on AMYs-AMPs systems, the cross-seeding phenomenon between AMYs and AMPs has emerged as a crucial factor in the bidirectional communication between the pathogenesis of neurodegenerative diseases and host defense against microbial infections. In this review, we examine recent developments in the potential interplay between AMYs and AMPs, as well as their pathological implications for both infectious and neurodegenerative diseases. By discussing the current progress and challenges in this emerging field, this account aims to inspire further research and investments to enhance our understanding of the intricate molecular crosstalk between AMYs and AMPs. This knowledge holds great promise for the development of innovative therapies to combat both microbial infections and neurodegenerative disorders.

The Role of Cellular Prion Protein in Promoting Stemness and Differentiation in Cancer

Cellular prion protein (PrPC) is seminal to modulate a variety of baseline cell functions to grant homeostasis. The classic role of such a protein was defined as a chaperone-like molecule being able to rescue cell survival. Nonetheless, PrPC also represents the precursor of the deleterious misfolded variant known as scrapie prion protein (PrPSc). This variant is detrimental in a variety of prion disorders. This multi-faceted role of PrP is greatly increased by recent findings showing how PrPC in its folded conformation may foster tumor progression by acting at multiple levels. The present review focuses on such a cancer-promoting effect. The manuscript analyzes recent findings on the occurrence of PrPC in various cancers and discusses the multiple effects, which sustain cancer progression. Within this frame, the effects of PrPC on stemness and differentiation are discussed. A special emphasis is provided on the spreading of PrPC and the epigenetic effects, which are induced in neighboring cells to activate cancer-related genes. These detrimental effects are further discussed in relation to the aberrancy of its physiological and beneficial role on cell homeostasis. A specific paragraph is dedicated to the role of PrPC beyond its effects in the biology of cancer to represent a potential biomarker in the follow up of patients following surgical resection.

The cellular prion protein promotes neuronal regeneration after acute nasotoxic injury

Adult neurogenesis, analogous to early development, is comprised of several, often concomitant, processes including proliferation, differentiation, and formation of synaptic connections. However, due to continual, asynchronous turn-over, newly-born adult olfactory sensory neurons (OSNs) must integrate into existing circuitry. Additionally, OSNs express high levels of cellular prion protein (PrPC), particularly in the axon, which implies a role in this cell type. The cellular prion has been shown to be important for proper adult OSN neurogenesis primarily by stabilizing mature olfactory neurons within this circuitry. However, the role of PrPC on each specific adult neurogenic processes remains to be investigated in detail. To tease out the subtle effects of prion protein expression level, a large population of regenerating neurons must be investigated. The thyroid drug methimazole (MTZ) causes nearly complete OSN loss in rodents and is used as a model of acute olfactory injury, providing a mechanism to induce synchronized OSN regeneration. This study investigated the effect of PrPC on adult neurogenesis after acute nasotoxic injury. Altered PrPC levels affected olfactory sensory epithelial (OSE) regeneration, cell proliferation, and differentiation. Attempts to investigate the role of PrPC level on axon regeneration did not support previous studies, and glomerular targeting did not recover to vehicle-treated levels, even by 20 weeks. Together, these studies demonstrate that the cellular prion protein is critical for regeneration of neurons, whereby increased PrPC levels promote early neurogenesis, and that lack of PrPC delays the regeneration of this tissue after acute injury.

Optimization of Efficient Neuron Models With Realistic Firing Dynamics. The Case of the Cerebellar Granule Cell

Biologically relevant large-scale computational models currently represent one of the main methods in neuroscience for studying information processing primitives of brain areas. However, biologically realistic neuron models tend to be computationally heavy and thus prevent these models from being part of brain-area models including thousands or even millions of neurons. The cerebellar input layer represents a canonical example of large scale networks. In particular, the cerebellar granule cells, the most numerous cells in the whole mammalian brain, have been proposed as playing a pivotal role in the creation of somato-sensorial information representations. Enhanced burst frequency (spiking resonance) in the granule cells has been proposed as facilitating the input signal transmission at the theta-frequency band (4–12 Hz), but the functional role of this cell feature in the operation of the granular layer remains largely unclear. This study aims to develop a methodological pipeline for creating neuron models that maintain biological realism and computational efficiency whilst capturing essential aspects of single-neuron processing. Therefore, we selected a light computational neuron model template (the adaptive-exponential integrate-and-fire model), whose parameters were progressively refined using an automatic parameter tuning with evolutionary algorithms (EAs). The resulting point-neuron models are suitable for reproducing the main firing properties of a realistic granule cell from electrophysiological measurements, including the spiking resonance at the theta-frequency band, repetitive firing according to a specified intensity-frequency (I-F) curve and delayed firing under current-pulse stimulation. Interestingly, the proposed model also reproduced some other emergent properties (namely, silent at rest, rheobase and negligible adaptation under depolarizing currents) even though these properties were not set in the EA as a target in the fitness function (FF), proving that these features are compatible even in computationally simple models. The proposed methodology represents a valuable tool for adjusting AdEx models according to a FF defined in the spiking regime and based on biological data. These models are appropriate for future research of the functional implication of bursting resonance at the theta band in large-scale granular layer network models.

Emerging Role of Cellular Prion Protein in the Maintenance and Expansion of Glioma Stem Cells

Cellular prion protein (PrP^C) is a membrane-anchored glycoprotein representing the physiological counterpart of PrP scrapie (PrP^Sc), which plays a pathogenetic role in prion diseases. Relatively little information is however available about physiological role of PrP^C. Although PrP^C ablation in mice does not induce lethal phenotypes, impairment of neuronal and bone marrow plasticity was reported in embryos and adult animals. In neurons, PrP^C stimulates neurite growth, prevents oxidative stress-dependent cell death, and favors antiapoptotic signaling. However, PrP^C activity is not restricted to post-mitotic neurons, but promotes cell proliferation and migration during embryogenesis and tissue regeneration in adult. PrP^C acts as scaffold to stabilize the binding between different membrane receptors, growth factors, and basement proteins, contributing to tumorigenesis. Indeed, ablation of PrP^C expression reduces cancer cell proliferation and migration and restores cell sensitivity to chemotherapy. Conversely, PrP^C overexpression in cancer stem cells (CSCs) from different tumors, including gliomas—the most malignant brain tumors—is predictive for poor prognosis, and correlates with relapses. The mechanisms of the PrP^C role in tumorigenesis and its molecular partners in this activity are the topic of the present review, with a particular focus on PrP^C contribution to glioma CSCs multipotency, invasiveness, and tumorigenicity.

Prion Protein in Glioblastoma Multiforme

The cellular prion protein (PrPc) is an evolutionarily conserved cell surface protein encoded by the PRNP gene. PrPc is ubiquitously expressed within nearly all mammalian cells, though most abundantly within the CNS. Besides being implicated in the pathogenesis and transmission of prion diseases, recent studies have demonstrated that PrPc contributes to tumorigenesis by regulating tumor growth, differentiation, and resistance to conventional therapies. In particular, PrPc over-expression has been related to the acquisition of a malignant phenotype of cancer stem cells (CSCs) in a variety of solid tumors, encompassing pancreatic ductal adenocarcinoma (PDAC), osteosarcoma, breast cancer, gastric cancer, and primary brain tumors, mostly glioblastoma multiforme (GBM). Thus, PrPc is emerging as a key in maintaining glioblastoma cancer stem cells’ (GSCs) phenotype, thereby strongly affecting GBM infiltration and relapse. In fact, PrPc contributes to GSCs niche’s maintenance by modulating GSCs’ stem cell-like properties while restraining them from differentiation. This is the first review that discusses the role of PrPc in GBM. The manuscript focuses on how PrPc may act on GSCs to modify their expression and translational profile while making the micro-environment surrounding the GSCs niche more favorable to GBM growth and infiltration.

Response Dynamics in an Olivocerebellar Spiking Neural Network With Non-linear Neuron Properties

Sensorimotor signals are integrated and processed by the cerebellar circuit to predict accurate control of actions. In order to investigate how single neuron dynamics and geometrical modular connectivity affect cerebellar processing, we have built an olivocerebellar Spiking Neural Network (SNN) based on a novel simplification algorithm for single point models (Extended Generalized Leaky Integrate and Fire, EGLIF) capturing essential non-linear neuronal dynamics (e.g., pacemaking, bursting, adaptation, oscillation and resonance). EGLIF models specifically tuned for each neuron type were embedded into an olivocerebellar scaffold reproducing realistic spatial organization and physiological convergence and divergence ratios of connections. In order to emulate the circuit involved in an eye blink response to two associated stimuli, we modeled two adjacent olivocerebellar microcomplexes with a common mossy fiber input but different climbing fiber inputs (either on or off). EGLIF-SNN model simulations revealed the emergence of fundamental response properties in Purkinje cells (burst-pause) and deep nuclei cells (pause-burst) similar to those reported in vivo. The expression of these properties depended on the specific activation of climbing fibers in the microcomplexes and did not emerge with scaffold models using simplified point neurons. This result supports the importance of embedding SNNs with realistic neuronal dynamics and appropriate connectivity and anticipates the scale-up of EGLIF-SNN and the embedding of plasticity rules required to investigate cerebellar functioning at multiple scales.

Prions Strongly Reduce NMDA Receptor S-Nitrosylation Levels at Pre-symptomatic and Terminal Stages of Prion Diseases

Prion diseases are fatal neurodegenerative disorders characterized by the cellular prion protein (PrP^C) conversion into a misfolded and infectious isoform termed prion or PrP^Sc. The neuropathological mechanism underlying prion toxicity is still unclear, and the debate on prion protein gain- or loss-of-function is still open. PrP^C participates to a plethora of physiological mechanisms. For instance, PrP^C and copper cooperatively modulate N-methyl-D-aspartate receptor (NMDAR) activity by mediating S-nitrosylation, an inhibitory post-translational modification, hence protecting neurons from excitotoxicity. Here, NMDAR S-nitrosylation levels were biochemically investigated at pre- and post-symptomatic stages of mice intracerebrally inoculated with RML, 139A, and ME7 prion strains. Neuropathological aspects of prion disease were studied by histological analysis and proteinase K digestion. We report that hippocampal NMDAR S-nitrosylation is greatly reduced in all three prion strain infections in both pre-symptomatic and terminal stages of mouse disease. Indeed, we show that NMDAR S-nitrosylation dysregulation affecting prion-inoculated animals precedes the appearance of clinical signs of disease and visible neuropathological changes, such as PrP^Sc accumulation and deposition. The pre-symptomatic reduction of NMDAR S-nitrosylation in prion-infected mice may be a possible cause of neuronal death in prion pathology, and it might contribute to the pathology progression opening new therapeutic strategies against prion disorders.

Hyperexcitability and Hyperplasticity Disrupt Cerebellar Signal Transfer in the IB2 KO Mouse Model of Autism

Autism spectrum disorders (ASDs) are pervasive neurodevelopmental conditions that often involve mutations affecting synaptic mechanisms. Recently, the involvement of cerebellum in ASDs has been suggested, but the underlying functional alterations remained obscure. We investigated single-neuron and microcircuit properties in IB2 (Islet Brain-2) KO mice of either sex. The IB2 gene (chr22q13.3 terminal region) deletion occurs in virtually all cases of Phelan-McDermid syndrome, causing autistic symptoms and a severe delay in motor skill acquisition. IB2 KO granule cells showed a larger NMDA receptor-mediated current and enhanced intrinsic excitability, raising the excitatory/inhibitory balance. Furthermore, the spatial organization of granular layer responses to mossy fibers shifted from a "Mexican hat" to a "stovepipe hat" profile, with stronger excitation in the core and weaker inhibition in the surround. Finally, the size and extension of long-term synaptic plasticity were remarkably increased. These results show for the first time that hyperexcitability and hyperplasticity disrupt signal transfer in the granular layer of IB2 KO mice, supporting cerebellar involvement in the pathogenesis of ASD.SIGNIFICANCE STATEMENT This article shows for the first time a complex set of alterations in the cerebellum granular layer of a mouse model [IB2 (Islet Brain-2) KO] of autism spectrum disorders. The IB2 KO in mice mimics the deletion of the corresponding gene in the Phelan-McDermid syndrome in humans. The changes reported here are centered on NMDA receptor hyperactivity, hyperplasticity, and hyperexcitability. These, in turn, increase the excitatory/inhibitory balance and alter the shape of center/surround structures that emerge in the granular layer in response to mossy fiber activity. These results support recent theories suggesting the involvement of cerebellum in autism spectrum disorders.

The Prion Protein Regulates Synaptic Transmission by Controlling the Expression of Proteins Key to Synaptic Vesicle Recycling and Exocytosis

The cellular prion protein (PrP^C), whose misfolded conformers are implicated in prion diseases, localizes to both the presynaptic membrane and postsynaptic density. To explore possible molecular contributions of PrP^C to synaptic transmission, we utilized a mass spectrometry approach to quantify the release of glutamate from primary cerebellar granule neurons (CGN) expressing, or deprived of (PrP-KO), PrP^C, following a depolarizing stimulus. Under the same conditions, we also tracked recycling of synaptic vesicles (SVs) in the two neuronal populations. We found that in PrP-KO CGN these processes decreased by 40 and 60%, respectively, compared to PrP^C-expressing neurons. Unbiased quantitative mass spectrometry was then employed to compare the whole proteome of CGN with the two PrP genotypes. This approach allowed us to assess that, relative to the PrP^C-expressing counterpart, the absence of PrP^C modified the protein expression profile, including diminution of some components of SV recycling and fusion machinery. Subsequent quantitative RT-PCR closely reproduced proteomic data, indicating that PrP^C is committed to ensuring optimal synaptic transmission by regulating genes involved in SV dynamics and neurotransmitter release. These novel molecular and cellular aspects of PrP^C add insight into the underlying mechanisms for synaptic dysfunctions occurring in neurodegenerative disorders in which a compromised PrP^C is likely to intervene.

Cerebellar Learning Properties Are Modulated by the CRF Receptor

Corticotropin-releasing factor (CRF) and its type 1 receptor (CRFR₁) play an important role in the responses to stressful challenges. Despite the well established expression of CRFR₁ in granular cells (GrCs), its role in procedural motor performance and memory formation remains elusive. To investigate the role of CRFR₁ expression in cerebellar GrCs, we used a mouse model depleted of CRFR₁ in these cells. We detected changes in the cellular learning mechanisms in GrCs depleted of CRFR₁ in that they showed changes in intrinsic excitability and long-term synaptic plasticity. Analysis of cerebella transcriptome obtained from KO and control mice detected prominent alterations in the expression of calcium signaling pathways components. Moreover, male mice depleted of CRFR₁ specifically in GrCs showed accelerated Pavlovian associative eye-blink conditioning, but no differences in baseline motor performance, locomotion, or fear and anxiety-related behaviors. Our findings shed light on the interplay between stress-related central mechanisms and cerebellar motor conditioning, highlighting the role of the CRF system in regulating particular forms of cerebellar learning.SIGNIFICANCE STATEMENT Although it is known that the corticotropin-releasing factor type 1 receptor (CRFR₁) is highly expressed in the cerebellum, little attention has been given to its role in cerebellar functions in the behaving animal. Moreover, most of the attention was directed at the effect of CRF on Purkinje cells at the cellular level and, to this date, almost no data exist on the role of this stress-related receptor in other cerebellar structures. Here, we explored the behavioral and cellular effect of granular cell-specific ablation of CRFR₁ We found a profound effect on learning both at the cellular and behavioral levels without an effect on baseline motor skills.

The cellular prion protein promotes olfactory sensory neuron survival and axon targeting during adult neurogenesis

The cellular prion protein (PrP^C) has been associated with diverse biological processes including cell signaling, neurogenesis, and neuroprotection, but its physiological function(s) remain ambiguous. Here we determine the role of PrP^C in adult neurogenesis using the olfactory system model in transgenic mice. Olfactory sensory neurons (OSNs) within the olfactory sensory epithelium (OSE) undergo neurogenesis, integration, and turnover even into adulthood. The neurogenic processes of proliferation, differentiation/maturation, and axon targeting were evaluated in wild type, PrP-overexpressing, and PrP-null transgenic mice. Our results indicate that PrP^C plays a role in maintaining mature OSNs within the epithelium: overexpression of PrP^C resulted in greater survival of mitotically active cells within the OSE, whereas absence of prion protein resulted in fewer cells being maintained over time. These results are supported by both quantitative PCR analysis of gene expression and protein analysis characteristic of OSN differentiation. Finally, evaluation of axon migration determined that OSN axon targeting in the olfactory bulb is PrP^C dose-dependent. Together, these findings provide new mechanistic insight into the neuroprotective role for PrP^C in adult OSE neurogenesis, whereby more mature neurons are stably maintained in animals expressing PrP^C.

Non-synaptic signaling from cerebellar climbing fibers modulates Golgi cell activity

Golgi cells are the principal inhibitory neurons at the input stage of the cerebellum, providing feedforward and feedback inhibition through mossy fiber and parallel fiber synapses. In vivo studies have shown that Golgi cell activity is regulated by climbing fiber stimulation, yet there is little functional or anatomical evidence for synapses between climbing fibers and Golgi cells. Here, we show that glutamate released from climbing fibers activates ionotropic and metabotropic receptors on Golgi cells through spillover-mediated transmission. The interplay of excitatory and inhibitory conductances provides flexible control over Golgi cell spiking, allowing either excitation or a biphasic sequence of excitation and inhibition following single climbing fiber stimulation. Together with prior studies of spillover transmission to molecular layer interneurons, these results reveal that climbing fibers exert control over inhibition at both the input and output layers of the cerebellar cortex.

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.

Dual role of cellular prion protein in normal host and Alzheimer's disease

Using PrPC-knockout cell lines, it has been shown that the inhibition of apoptosis through STI1 is mediated by PrPC-dependent SOD activation. Antioxidant PrPC may contribute to suppression of inflammasome activation. PrPC is functionally involved in copper metabolism, signal transduction, neuroprotection, and cell maturation. Recently several reports have shown that PrPC participates in trans-membrane signaling processes associated with hematopoietic stem cell replication and neuronal differentiation. In another role, PrPC also tends to function as a neurotoxic protein. Aβ oligomer, which is associated with neurodegeneration in Alzheimer's disease (AD), has also been reported to act as a ligand of PrPC. However, the physiological role of PrPC as an Aβ42-binding protein is not clear. Actually, PrPC is critical in Aβ42-mediated autophagy in neurons. PrPC shows a beneficial role in lipid rafts to promote autophagy. Further search for PrPC-interaction molecules using Prnp-/- mice and various types of Prnp-/- cell lines under various conditions may elucidate other important PrPC important functions.

Developmental delay in motor skill acquisition in Niemann-Pick C1 mice reveals abnormal cerebellar morphogenesis

Niemann-Pick type C1 (NPC1) disease is a lysosomal storage disorder caused by defective intracellular trafficking of exogenous cholesterol. Purkinje cell (PC) degeneration is the main sign of cerebellar dysfunction in both NPC1 patients and animal models. It has been recently shown that a significant decrease in Sonic hedgehog (Shh) expression reduces the proliferative potential of granule neuron precursors in the developing cerebellum of Npc1^−/− mice. Pursuing the hypothesis that this developmental defect translates into functional impairments, we have assayed Npc1-deficient pups belonging to the milder mutant mouse strain Npc1^nmf164 for sensorimotor development from postnatal day (PN) 3 to PN21. Npc1^nmf164/ Npc1^nmf164 pups displayed a 2.5-day delay in the acquisition of complex motor abilities compared to wild-type (wt) littermates, in agreement with the significant disorganization of cerebellar cortex cytoarchitecture observed between PN11 and PN15. Compared to wt, Npc1^nmf164 homozygous mice exhibited a poorer morphological differentiation of Bergmann glia (BG), as indicated by thicker radial shafts and less elaborate reticular pattern of lateral processes. Also BG functional development was defective, as indicated by the significant reduction in GLAST and Glutamine synthetase expression. A reduced VGluT2 and GAD65 expression also indicated an overall derangement of the glutamatergic/GABAergic stimulation that PCs receive by climbing/parallel fibers and basket/stellate cells, respectively. Lastly, Npc1-deficiency also affected oligodendrocyte differentiation as indicated by the strong reduction of myelin basic protein. Two sequential 2-hydroxypropyl-β-cyclodextrin administrations at PN4 and PN7 counteract these defects, partially preventing functional impairment of BG and fully restoring the normal patterns of glutamatergic/GABAergic stimulation to PCs.

These findings indicate that in Npc1^nmf164 homozygous mice the derangement of synaptic connectivity and dysmyelination during cerebellar morphogenesis largely anticipate motor deficits that are typically observed during adulthood.

Prion protein as a mediator of synaptic transmission

Neurodegenerative disorders are characterized by synaptic and neuronal dysfunction which precedes general neuronal loss and subsequent cognitive or behavioral anomalies. Although the exact early cellular signaling mechanisms involved in neurodegenerative diseases are largely unknown, a view is emerging that compromised synaptic function may underlie the initial steps in disease progression. Much recent research has been aimed at understanding these early underlying processes leading to dysfunctional synaptic signaling, as this knowledge could identify putative sites of interventions, which could potentially slow progression and delay onset of disease. We have recently reported that synaptic function in a Drosophila melanogaster model can be modulated by the presence of native mouse prion protein and this modulation is negatively affected by a mutation within the protein which is associated with the Gerstmann-Sträussler-Scheinker syndrome, a human form of prion disease. Indeed, wild-type prion protein facilitates synaptic release, whereas the mutated form induced diminished phenotypes. It is believed that together with the gain-of-function of neurotoxic misfolded prion signaling, the lack of prion protein contributes to the pathology in prion diseases. Therefore, our study investigated a potential endogenous role of prion protein in synaptic signaling, the lack of which could resemble a lack-of-function phenotype in prion disease.

Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23

Spinocerebellar ataxia type 23 is caused by mutations in PDYN, which encodes the opioid neuropeptide precursor protein, prodynorphin. Prodynorphin is processed into the opioid peptides, α-neoendorphin, and dynorphins A and B, that normally exhibit opioid-receptor mediated actions in pain signalling and addiction. Dynorphin A is likely a mutational hotspot for spinocerebellar ataxia type 23 mutations, and in vitro data suggested that dynorphin A mutations lead to persistently elevated mutant peptide levels that are cytotoxic and may thus play a crucial role in the pathogenesis of spinocerebellar ataxia type 23. To further test this and study spinocerebellar ataxia type 23 in more detail, we generated a mouse carrying the spinocerebellar ataxia type 23 mutation R212W in PDYN. Analysis of peptide levels using a radioimmunoassay shows that these PDYN(R212W) mice display markedly elevated levels of mutant dynorphin A, which are associated with climber fibre retraction and Purkinje cell loss, visualized with immunohistochemical stainings. The PDYN(R212W) mice reproduced many of the clinical features of spinocerebellar ataxia type 23, with gait deficits starting at 3 months of age revealed by footprint pattern analysis, and progressive loss of motor coordination and balance at the age of 12 months demonstrated by declining performances on the accelerating Rotarod. The pathologically elevated mutant dynorphin A levels in the cerebellum coincided with transcriptionally dysregulated ionotropic and metabotropic glutamate receptors and glutamate transporters, and altered neuronal excitability. In conclusion, the PDYN(R212W) mouse is the first animal model of spinocerebellar ataxia type 23 and our work indicates that the elevated mutant dynorphin A peptide levels are likely responsible for the initiation and progression of the disease, affecting glutamatergic signalling, neuronal excitability, and motor performance. Our novel mouse model defines a critical role for opioid neuropeptides in spinocerebellar ataxia, and suggests that restoring the elevated mutant neuropeptide levels can be explored as a therapeutic intervention.

Prion Protein Signaling in the Nervous System—A Review and Perspective

Prion protein (PrPC) was originally known as the causative agent of transmissible spongiform encephalopathy (TSE) but with recent research, its true function in cells is becoming clearer. It is known to act as a scaffolding protein, binding multiple ligands at the cell membrane and to be involved in signal transduction, passing information from the extracellular matrix (ECM) to the cytoplasm. Its role in the coordination of transmitters at the synapse, glyapse, and gap junction and in short- and long-range neurotrophic signaling gives PrPC a major part in neural transmission and nervous system signaling. It acts to regulate cellular function in multiple targets through its role as a controller of redox status and calcium ion flux. Given the importance of PrPC in cell physiology, this review considers its potential role in disease apart from TSE. The putative functions of PrPC point to involvement in neurodegenerative disease, neuropathic pain, chronic headache, and inflammatory disease including neuroinflammatory disease of the nervous system. Potential targets for the treatment of disease influenced by PrPC are discussed.

To develop with or without the prion protein

The deletion of the cellular form of the prion protein (PrP^C) in mouse, goat, and cattle has no drastic phenotypic consequence. This stands in apparent contradiction with PrP^C quasi-ubiquitous expression and conserved primary and tertiary structures in mammals, and its pivotal role in neurodegenerative diseases such as prion and Alzheimer's diseases. In zebrafish embryos, depletion of PrP ortholog leads to a severe loss-of-function phenotype. This raises the question of a potential role of PrP^C in the development of all vertebrates. This view is further supported by the early expression of the PrP^C encoding gene (Prnp) in many tissues of the mouse embryo, the transient disruption of a broad number of cellular pathways in early Prnp^−/− mouse embryos, and a growing body of evidence for PrP^C involvement in the regulation of cell proliferation and differentiation in various types of mammalian stem cells and progenitors. Finally, several studies in both zebrafish embryos and in mammalian cells and tissues in formation support a role for PrP^C in cell adhesion, extra-cellular matrix interactions and cytoskeleton. In this review, we summarize and compare the different models used to decipher PrP^C functions at early developmental stages during embryo- and organo-genesis and discuss their relevance.

Deficiency of prion protein induces impaired autophagic flux in neurons

Normal cellular prion protein (PrP^C) is highly expressed in the central nervous system. The Zürich I Prnp-deficient mouse strain did not show an abnormal phenotype in initial studies, however, in later studies, deficits in exploratory behavior and short- and long-term memory have been revealed. In the present study, numerous autophagic vacuoles were found in neurons from Zürich I Prnp-deficient mice. The autophagic accumulation in the soma of cortical neurons in Zürich I Prnp-deficient mice was observed as early as 3 months of age, and in the hippocampal neurons at 6 months of age. Specifically, there is accumulation of electron dense pigments associated with autophagy in the neurons of Zürich I Prnp-deficient mice. Furthermore, autophagic accumulations were observed as early as 3 months of age in the CA3 region of hippocampal and cerebral cortical neuropils. The autophagic vacuoles increased with age in the hippocampus of Zürich I Prnp-deficient mice at a faster rate and to a greater extent than in normal C57BL/6J mice, whereas the cortex exhibited high levels that were maintained from 3 months old in Zürich I Prnp-deficient mice. The pigmented autophagic accumulation is due to the incompletely digested material from autophagic vacuoles. Furthermore, a deficiency in PrP^C may disrupt the autophagic flux by inhibiting autophagosome-lysosomal fusion. Overall, our results provide insight into the protective role of PrP^C in neurons, which may play a role in normal behavior and other brain functions.

Computational modeling predicts the ionic mechanism of late-onset responses in unipolar brush cells

Unipolar Brush Cells (UBCs) have been suggested to play a critical role in cerebellar functioning, yet the corresponding cellular mechanisms remain poorly understood. UBCs have recently been reported to generate, in addition to early-onset glutamate receptor-dependent synaptic responses, a late-onset response (LOR) composed of a slow depolarizing ramp followed by a spike burst (Locatelli et al., 2013). The LOR activates as a consequence of synaptic activity and involves an intracellular cascade modulating H- and TRP-current gating. In order to assess the LOR mechanisms, we have developed a UBC multi-compartmental model (including soma, dendrite, initial segment, and axon) incorporating biologically realistic representations of ionic currents and a cytoplasmic coupling mechanism regulating TRP and H channel gating. The model finely reproduced UBC responses to current injection, including a burst triggered by a low-threshold spike (LTS) sustained by CaLVA currents, a persistent discharge sustained by CaHVA currents, and a rebound burst following hyperpolarization sustained by H- and CaLVA-currents. Moreover, the model predicted that H- and TRP-current regulation was necessary and sufficient to generate the LOR and its dependence on the intensity and duration of mossy fiber activity. Therefore, the model showed that, using a basic set of ionic channels, UBCs generate a rich repertoire of bursts, which could effectively implement tunable delay-lines in the local microcircuit.

Prion protein facilitates synaptic vesicle release by enhancing release probability

The cellular prion protein (PrP(C)) has been implicated in several neurodegenerative diseases as a result of protein misfolding. In humans, prion disease occurs typically with a sporadic origin where uncharacterized mechanisms induce spontaneous PrP(C) misfolding leading to neurotoxic PrP-scrapie formation (PrP(SC)). The consequences of misfolded PrP(C) signalling are well characterized but little is known about the physiological roles of PrP(C) and its involvement in disease. Here we investigated wild-type PrP(C) signalling in synaptic function as well as the effects of a disease-relevant mutation within PrP(C) (proline-to-leucine mutation at codon 101). Expression of wild-type PrP(C) at the Drosophila neuromuscular junction leads to enhanced synaptic responses as detected in larger miniature synaptic currents which are caused by enlarged presynaptic vesicles. The expression of the mutated PrP(C) leads to reduction of both parameters compared with wild-type PrP(C). Wild-type PrP(C) enhances synaptic release probability and quantal content but reduces the size of the ready-releasable vesicle pool. Partially, these changes are not detectable following expression of the mutant PrP(C). A behavioural test revealed that expression of either protein caused an increase in locomotor activities consistent with enhanced synaptic release and stronger muscle contractions. Both proteins were sensitive to proteinase digestion. These data uncover new functions of wild-type PrP(C) at the synapse with a disease-relevant mutation in PrP(C) leading to diminished functional phenotypes. Thus, our data present essential new information possibly related to prion pathogenesis in which a functional synaptic role of PrP(C) is compromised due to its advanced conversion into PrP(SC) thereby creating a lack-of-function scenario.

Modulatory mechanisms and multiple functions of somatodendritic A-type K (+) channel auxiliary subunits

Auxiliary subunits are non-conducting, modulatory components of the multi-protein ion channel complexes that underlie normal neuronal signaling. They interact with the pore-forming α-subunits to modulate surface distribution, ion conductance, and channel gating properties. For the somatodendritic subthreshold A-type potassium (ISA) channel based on Kv4 α-subunits, two types of auxiliary subunits have been extensively studied: Kv channel-interacting proteins (KChIPs) and dipeptidyl peptidase-like proteins (DPLPs). KChIPs are cytoplasmic calcium-binding proteins that interact with intracellular portions of the Kv4 subunits, whereas DPLPs are type II transmembrane proteins that associate with the Kv4 channel core. Both KChIPs and DPLPs genes contain multiple start sites that are used by various neuronal populations to drive the differential expression of functionally distinct N-terminal variants. In turn, these N-terminal variants generate tremendous functional diversity across the nervous system. Here, we focus our review on (1) the molecular mechanism underlying the unique properties of different N-terminal variants, (2) the shaping of native ISA properties by the concerted actions of KChIPs and DPLP variants, and (3) the surprising ways that KChIPs and DPLPs coordinate the activity of multiple channels to fine-tune neuronal excitability. Unlocking the unique contributions of different auxiliary subunit N-terminal variants may provide an important opportunity to develop novel targeted therapeutics to treat numerous neurological disorders.

Neuropathogenesis of prion disease

ABSTRACT: Although much is known about prion diseases (characterized by a post-translational misfolding of the prion protein [PrP]) and their neuropathology and molecular pathology, the fundamental cause of illness, the basic neuropathogenesis, remains uncertain. There are three broad considerations discussed in this review: the possible loss of normal PrP function, the possible direct toxicity of the abnormally folded PrP and a harmful interaction between the normal and abnormal protein. In considering these possibilities, there are difficulties, including the facts that the relevant normal functions of the PrP are somewhat uncertain and that there are a number of possible toxic species of abnormal protein. In addition to the possible interactions of normal and abnormal PrP in prion disease, PrP may play a role in the neuropathogenesis of other diseases (such as Alzheimer’s disease).

The prion protein modulates A-type K+ currents mediated by Kv4.2 complexes through dipeptidyl aminopeptidase-like protein 6

Widely expressed in the adult central nervous system, the cellular prion protein (PrP(C)) is implicated in a variety of processes, including neuronal excitability. Dipeptidyl aminopeptidase-like protein 6 (DPP6) was first identified as a PrP(C) interactor using in vivo formaldehyde cross-linking of wild type (WT) mouse brain. This finding was confirmed in three cell lines and, because DPP6 directs the functional assembly of K(+) channels, we assessed the impact of WT and mutant PrP(C) upon Kv4.2-based cell surface macromolecular complexes. Whereas a Gerstmann-Sträussler-Scheinker disease version of PrP with eight extra octarepeats was a loss of function both for complex formation and for modulation of Kv4.2 channels, WT PrP(C), in a DPP6-dependent manner, modulated Kv4.2 channel properties, causing an increase in peak amplitude, a rightward shift of the voltage-dependent steady-state inactivation curve, a slower inactivation, and a faster recovery from steady-state inactivation. Thus, the net impact of wt PrP(C) was one of enhancement, which plays a critical role in the down-regulation of neuronal membrane excitability and is associated with a decreased susceptibility to seizures. Insofar as previous work has established a requirement for WT PrP(C) in the Aβ-dependent modulation of excitability in cholinergic basal forebrain neurons, our findings implicate PrP(C) regulation of Kv4.2 channels as a mechanism contributing to the effects of oligomeric Aβ upon neuronal excitability and viability.

The role of prion protein in stem cell regulation

Cellular prion protein (PrP(C)) has been well described as an essential partner of prion diseases due to the existence of a pathological conformation (PrP(Sc)). Recently, it has also been demonstrated that PrP(C) is an important element of the pluripotency and self-renewal matrix, with an increasing amount of evidence pointing in this direction. Here, we review the data that demonstrate its role in the transcriptional regulation of pluripotency, in the differentiation of stem cells into different lineages (e.g. muscle and neurons), in embryonic development, and its involvement in reproductive cells. Also highlighted are recent results from our laboratory that describe an important regulation by PrP(C) of the major pluripotency gene Nanog. Together, these data support the appearance of new strategies to control stemness, which could represent an important advance in the field of regenerative medicine.

The Functional Role of Prion Protein (PrPC) on Autophagy

Cellular prion protein (PrP^C) plays an important role in the cellular defense against oxidative stress. However, the exact protective mechanism of PrP^C is unclear. Autophagy is essential for survival, differentiation, development, and homeostasis in several organisms. Although the role that autophagy plays in neurodegenerative disease has yet to be established, it is clear that autophagy-induced cell death is observed in neurodegenerative disorders that exhibit protein aggregations. Moreover, autophagy can promote cell survival and cell death under various conditions. In this review, we describe the involvement of autophagy in prion disease and the effects of PrP^C.

PrPC controls via protein kinase A the direction of synaptic plasticity in the immature hippocampus

The cellular form of prion protein PrP(C) is highly expressed in the brain, where it can be converted into its abnormally folded isoform PrP(Sc) to cause neurodegenerative diseases. Its predominant synaptic localization suggests a crucial role in synaptic signaling. Interestingly, PrP(C) is developmentally regulated and its high expression in the immature brain could be instrumental in regulating neurogenesis and cell proliferation. Here, PrP(C)-deficient (Prnp(0/0)) mice were used to assess whether the prion protein is involved in synaptic plasticity processes in the neonatal hippocampus. To this aim, calcium transients associated with giant depolarizing potentials, a hallmark of developmental networks, were transiently paired with mossy fiber activation in such a way that the two events were coincident. While this procedure caused long-term potentiation (LTP) in wild-type (WT) animals, it caused long-term depression (LTD) in Prnp(0/0) mice. Induction of LTP was postsynaptic and required the activation of cAMP-dependent protein kinase A (PKA) signaling. The induction of LTD was presynaptic and relied on G-protein-coupled GluK1 receptor and protein lipase C. In addition, at emerging CA3-CA1 synapses in WT mice, but not in Prnp(0/0) mice, pairing Schaffer collateral stimulation with depolarization of CA1 principal cells induced LTP, known to be PKA dependent. Postsynaptic infusion of a constitutively active isoform of PKA catalytic subunit Cα into CA1 and CA3 principal cells in the hippocampus of Prnp(0/0) mice caused a persistent synaptic facilitation that was occluded by subsequent pairing. These data suggest that PrP(C) plays a crucial role in regulating via PKA synaptic plasticity in the developing hippocampus.

Age-dependent impairment of eyeblink conditioning in prion protein-deficient mice

Mice lacking the prion protein (PrP(C)) gene (Prnp), Ngsk Prnp (0/0) mice, show late-onset cerebellar Purkinje cell (PC) degeneration because of ectopic overexpression of PrP(C)-like protein (PrPLP/Dpl). Because PrP(C) is highly expressed in cerebellar neurons (including PCs and granule cells), it may be involved in cerebellar synaptic function and cerebellar cognitive function. However, no studies have been conducted to investigate the possible involvement of PrP(C) and/or PrPLP/Dpl in cerebellum-dependent discrete motor learning. Therefore, the present cross-sectional study was designed to examine cerebellum-dependent delay eyeblink conditioning in Ngsk Prnp (0/0) mice in adulthood (16, 40, and 60 weeks of age). The aims of the present study were two-fold: (1) to examine the role of PrP(C) and/or PrPLP/Dpl in cerebellum-dependent motor learning and (2) to confirm the age-related deterioration of eyeblink conditioning in Ngsk Prnp (0/0) mice as an animal model of progressive cerebellar degeneration. Ngsk Prnp (0/0) mice aged 16 weeks exhibited intact acquisition of conditioned eyeblink responses (CRs), although the CR timing was altered. The same result was observed in another line of PrP(c)-deficient mice, ZrchI PrnP (0/0) mice. However, at 40 weeks of age, CR incidence impairment was observed in Ngsk Prnp (0/0) mice. Furthermore, Ngsk Prnp (0/0) mice aged 60 weeks showed more significantly impaired CR acquisition than Ngsk Prnp (0/0) mice aged 40 weeks, indicating the temporal correlation between cerebellar PC degeneration and motor learning deficits. Our findings indicate the importance of the cerebellar cortex in delay eyeblink conditioning and suggest an important physiological role of prion protein in cerebellar motor learning.

Modeling spike-train processing in the cerebellum granular layer and changes in plasticity reveal single neuron effects in neural ensembles

The cerebellum input stage has been known to perform combinatorial operations on input signals. In this paper, two types of mathematical models were used to reproduce the role of feed-forward inhibition and computation in the granular layer microcircuitry to investigate spike train processing. A simple spiking model and a biophysically-detailed model of the network were used to study signal recoding in the granular layer and to test observations like center-surround organization and time-window hypothesis in addition to effects of induced plasticity. Simulations suggest that simple neuron models may be used to abstract timing phenomenon in large networks, however detailed models were needed to reconstruct population coding via evoked local field potentials (LFP) and for simulating changes in synaptic plasticity. Our results also indicated that spatio-temporal code of the granular network is mainly controlled by the feed-forward inhibition from the Golgi cell synapses. Spike amplitude and total number of spikes were modulated by LTP and LTD. Reconstructing granular layer evoked-LFP suggests that granular layer propagates the nonlinearities of individual neurons. Simulations indicate that granular layer network operates a robust population code for a wide range of intervals, controlled by the Golgi cell inhibition and is regulated by the post-synaptic excitability.

Late-onset bursts evoked by mossy fibre bundle stimulation in unipolar brush cells: evidence for the involvement of H- and TRP-currents

Synaptic transmission at central synapses has usually short latency and graded amplitude, thereby regulating threshold crossing and the probability of action potential generation. In the granular layer of the vestibulo-cerebellum, unipolar brush cells (UBCs) receive a giant synapse generating a stereotyped excitatory postsynaptic potential (EPSP)-burst complex with early-onset (∼2 ms) and high reliability. By using patch-clamp recordings in cerebellar slices of the rat vestibulo-cerebellum, we found that mossy fibre bundle stimulation also evoked (in ∼80% of cases) a late-onset burst (after tens to hundreds of milliseconds) independent of EPSP generation. Different from the early-onset, the late-onset burst delay decreased and its duration increased by raising stimulation intensity or the number of impulses. Although depending on synaptic activity, the late-onset response was insensitive to perfusion of APV ((2R)-5-amino-phosphonopentanoate), NBQX (2,3-dioxo-6-nitro-tetrahydrobenzo(f)quinoxaline-7-sulfonamide) and MCPG ((RS)-α-methyl-4-carboxyphenylglycine) and did not therefore depend on conventional glutamatergic transmission mechanisms. The late-onset response was initiated by a slow depolarizing ramp driven by activation of an H-current (sensitive to ZD7288 and Cs(+)) and of a TRP- (transient receptor potential) current (sensitive to SKF96365), while the high voltage-activated and high voltage-activated Ca(2+) currents (sensitive to nimodipine and mibefradil, respectively) played a negligible role. The late-onset burst was occluded by intracellular cAMP. These results indicate that afferent activity can regulate H- and TRP-current gating in UBCs generating synaptically driven EPSP-independent responses, in which the delay rather than amplitude is graded with the intensity of the input pattern. This modality of synaptic transmission may play an important role in regulating UBC activation and granular layer functions in the vestibulo-cerebellum.

Role of the cellular prion protein in oligodendrocyte precursor cell proliferation and differentiation in the developing and adult mouse CNS

There are numerous studies describing the signaling mechanisms that mediate oligodendrocyte precursor cell (OPC) proliferation and differentiation, although the contribution of the cellular prion protein (PrP^c) to this process remains unclear. PrP^c is a glycosyl-phosphatidylinositol (GPI)-anchored glycoprotein involved in diverse cellular processes during the development and maturation of the mammalian central nervous system (CNS). Here we describe how PrP^c influences oligodendrocyte proliferation in the developing and adult CNS. OPCs that lack PrP^c proliferate more vigorously at the expense of a delay in differentiation, which correlates with changes in the expression of oligodendrocyte lineage markers. In addition, numerous NG2-positive cells were observed in cortical regions of adult PrP^c knockout mice, although no significant changes in myelination can be seen, probably due to the death of surplus cells.

Prion potency in stem cells biology

Prion protein (PrP) can be considered a pivotal molecule because it interacts with several partners to perform a diverse range of critical biological functions that might differ in embryonic and adult cells. In recent years, there have been major advances in elucidating the putative role of PrP in the basic biology of stem cells in many different systems. Here, we review the evidence indicating that PrP is a key molecule involved in driving different aspects of the potency of embryonic and tissue-specific stem cells in self-perpetuation and differentiation in many cell types. It has been shown that PrP is involved in stem cell self-renewal, controlling pluripotency gene expression, proliferation, and neural and cardiomyocyte differentiation. PrP also has essential roles in distinct processes that regulate tissue-specific stem cell biology in nervous and hematopoietic systems and during muscle regeneration. Results from our own investigations have shown that PrP is able to modulate self-renewal and proliferation in neural stem cells, processes that are enhanced by PrP interactions with stress inducible protein 1 (STI1). Thus, the available data reveal the influence of PrP in acting upon the maintenance of pluripotent status or the differentiation of stem cells from the early embryogenesis through adulthood.

Exacerbation of experimental autoimmune encephalomyelitis in prion protein (PrPc)-null mice: evidence for a critical role of the central nervous system

BACKGROUND

The cellular prion protein (PrPc) is a host-encoded glycoprotein whose transconformation into PrP scrapie (PrPSc) initiates prion diseases. The role of PrPc in health is still obscure, but many candidate functions have been attributed to the protein, both in the immune and the nervous systems. Recent data show that experimental autoimmune encephalomyelitis (EAE) is worsened in mice lacking PrPc. Disease exacerbation has been attributed to T cells that would differentiate into more aggressive effectors when deprived of PrPc. However, alternative interpretations such as reduced resistance of neurons to autoimmune insult and exacerbated gliosis leading to neuronal deficits were not considered.

METHOD

To better discriminate the contribution of immune cells versus neural cells, reciprocal bone marrow chimeras with differential expression of PrPc in the lymphoid or in the central nervous system (CNS) were generated. Mice were subsequently challenged with MOG35-55 peptide and clinical disease as well as histopathology were compared in both groups. Furthermore, to test directly the T cell hypothesis, we compared the encephalitogenicity of adoptively transferred PrPc-deficient versus PrPc-sufficient, anti-MOG T cells.

RESULTS

First, EAE exacerbation in PrPc-deficient mice was confirmed. Irradiation exacerbated EAE in all the chimeras and controls, but disease was more severe in mice with a PrPc-deleted CNS and a normal immune system than in the reciprocal construction. Moreover, there was no indication that anti-MOG responses were different in PrPc-sufficient and PrPc-deficient mice. Paradoxically, PrPc-deficient anti-MOG 2D2 T cells were less pathogenic than PrPc-expressing 2D2 T cells.

CONCLUSIONS

In view of the present data, it can be concluded that the origin of EAE exacerbation in PrPc-ablated mice resides in the absence of the prion protein in the CNS. Furthermore, the absence of PrPc on both neural and immune cells does not synergize for disease worsening. These conclusions highlight the critical role of PrPc in maintaining the integrity of the CNS in situations of stress, especially during a neuroinflammatory insult.

Prion protein at the crossroads of physiology and disease

The presence of the cellular prion protein (PrP(C)) on the cell surface is critical for the neurotoxicity of prions. Although several biological activities have been attributed to PrP(C), a definitive demonstration of its physiological function remains elusive. In this review, we discuss some of the proposed functions of PrP(C), focusing on recently suggested roles in cell adhesion, regulation of ionic currents at the cell membrane and neuroprotection. We also discuss recent evidence supporting the idea that PrP(C) may function as a receptor for soluble oligomers of the amyloid β peptide and possibly other toxic protein aggregates. These data suggest surprising new connections between the physiological function of PrP(C) and its role in neurodegenerative diseases beyond those caused by prions.

Neuroprotective role of PrPC against kainate-induced epileptic seizures and cell death depends on the modulation of JNK3 activation by GluR6/7-PSD-95 binding

Cellular prion protein (PrP(C)) is a glycosyl-phosphatidylinositol-anchored glycoprotein. When mutated or misfolded, the pathogenic form (PrP(SC)) induces transmissible spongiform encephalopathies. In contrast, PrP(C) has a number of physiological functions in several neural processes. Several lines of evidence implicate PrP(C) in synaptic transmission and neuroprotection since its absence results in an increase in neuronal excitability and enhanced excitotoxicity in vitro and in vivo. Furthermore, PrP(C) has been implicated in the inhibition of N-methyl-d-aspartic acid (NMDA)-mediated neurotransmission, and prion protein gene (Prnp) knockout mice show enhanced neuronal death in response to NMDA and kainate (KA). In this study, we demonstrate that neurotoxicity induced by KA in Prnp knockout mice depends on the c-Jun N-terminal kinase 3 (JNK3) pathway since Prnp(o/o)Jnk3(o/o) mice were not affected by KA. Pharmacological blockage of JNK3 activity impaired PrP(C)-dependent neurotoxicity. Furthermore, our results indicate that JNK3 activation depends on the interaction of PrP(C) with postsynaptic density 95 protein (PSD-95) and glutamate receptor 6/7 (GluR6/7). Indeed, GluR6-PSD-95 interaction after KA injections was favored by the absence of PrP(C). Finally, neurotoxicity in Prnp knockout mice was reversed by an AMPA/KA inhibitor (6,7-dinitroquinoxaline-2,3-dione) and the GluR6 antagonist NS-102. We conclude that the protection afforded by PrP(C) against KA is due to its ability to modulate GluR6/7-mediated neurotransmission and hence JNK3 activation.

Possible role for Ca2+ in the pathophysiology of the prion protein?

Transmissible spongiform encephalopathies, or prion diseases, are lethal neurodegenerative disorders caused by the infectious agent named prion, whose main constituent is an aberrant conformational isoform of the cellular prion protein, PrP(C) . The mechanisms of prion-associated neurodegeneration and the physiologic function of PrP(C) are still unclear, although it is now increasingly acknowledged that PrP(C) plays a role in cell differentiation and survival. PrP(C) thus exhibits dichotomic attributes, as it can switch from a benign function under normal conditions to the triggering of neuronal death during disease. By reviewing data from models of prion infection and PrP-knockout paradigms, here we discuss the possibility that Ca(2+) is the hidden factor behind the multifaceted behavior of PrP(C) . By featuring in almost all processes of cell signaling, Ca(2+) might explain diverse aspects of PrP(C) pathophysiology, including the recently proposed one in which PrP(C) acts as a mediator of synaptic degeneration in Alzheimer's disease.

Behavioral and cerebellar transmission deficits in mice lacking the autism-linked gene islet brain-2

Deletion of the human SHANK3 gene near the terminus of chromosome 22q is associated with Phelan-McDermid syndrome and autism spectrum disorders. Nearly all such deletions also span the tightly linked IB2 gene. We show here that IB2 protein is broadly expressed in the brain and is highly enriched within postsynaptic densities. Experimental disruption of the IB2 gene in mice reduces AMPA and enhances NMDA receptor-mediated glutamatergic transmission in cerebellum, changes the morphology of Purkinje cell dendritic arbors, and induces motor and cognitive deficits suggesting an autism phenotype. These findings support a role for human IB2 mutation as a contributing genetic factor in Chr22qter-associated cognitive disorders.

Cellular prion protein promotes regeneration of adult muscle tissue

It is now well established that the conversion of the cellular prion protein, PrP(C), into its anomalous conformer, PrP(Sc), is central to the onset of prion disease. However, both the mechanism of prion-related neurodegeneration and the physiologic role of PrP(C) are still unknown. The use of animal and cell models has suggested a number of putative functions for the protein, including cell signaling, adhesion, proliferation, and differentiation. Given that skeletal muscles express significant amounts of PrP(C) and have been related to PrP(C) pathophysiology, in the present study, we used skeletal muscles to analyze whether the protein plays a role in adult morphogenesis. We employed an in vivo paradigm that allowed us to compare the regeneration of acutely damaged hind-limb tibialis anterior muscles of mice expressing, or not expressing, PrP(C). Using morphometric and biochemical parameters, we provide compelling evidence that the absence of PrP(C) significantly slows the regeneration process compared to wild-type muscles by attenuating the stress-activated p38 pathway, and the consequent exit from the cell cycle, of myogenic precursor cells. Demonstrating the specificity of this finding, restoring PrP(C) expression completely rescued the muscle phenotype evidenced in the absence of PrP(C).

Cellular form of prion protein inhibits Reelin-mediated shedding of Caspr from the neuronal cell surface to potentiate Caspr-mediated inhibition of neurite outgrowth

Extension of axonal and dendritic processes in the CNS is tightly regulated by outgrowth-promoting and -inhibitory cues to assure precision of synaptic connections. We identify a novel role for contactin-associated protein (Caspr) as an inhibitory cue that reduces neurite outgrowth from CNS neurons. We show that proteolysis of Caspr at the cell surface is regulated by the cellular form of prion protein (PrP), which directly binds to Caspr. PrP inhibits Reelin-mediated shedding of Caspr from the cell surface, thereby increasing surface levels of Caspr and potentiating the inhibitory effect of Caspr on neurite outgrowth. PrP deficiency results in reduced levels of Caspr at the cell surface, enhanced neurite outgrowth in vitro, and more efficient regeneration of axons in vivo following spinal cord injury. Thus, we reveal a previously unrecognized role for Caspr and PrP in inhibitory modulation of neurite outgrowth in CNS neurons, which is counterbalanced by the proteolytic activity of Reelin.

Reduced blockade by extracellular Mg(2+) is permissive to NMDA receptor activation in cerebellar granule neurons that model a migratory phenotype

NMDA receptors (NMDAR) contribute to neuronal development throughout the CNS. However, their mode(s) of activation preceding synaptic maturation is unclear, as they are not co-localized with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) which normally provide sufficient depolarization to relieve voltage-dependent blockade by Mg(2+). We used cerebellar granule neurons (CGNs) cultured at a near-physiological KCl concentration to examine maturation-dependent changes in NMDAR responses. In contrast, most studies use KCl-supplemented medium to promote survival. At 2-4 days in vitro CGNs: (i) express developmental markers resembling the in vivo migratory phenotype; (ii) maintain a basal amount of calcium responsive element-binding protein phosphorylation that requires NMDARs and calcium/calmodulin-dependent kinases, but not AMPARs; (iii) exhibit NMDA-mediated Ca(2+) influx not effectively blocked by ambient Mg(2+) (0.75 mM) or AMPARs; (iv) maintain a more depolarized resting membrane potential and increased resistance compared to synaptically-connected CGNs. Moreover, migrating CGNs in explant cultures demonstrate NMDA-mediated Ca(2+) influx not effectively blocked by 0.75 mM Mg(2+), and NMDAR but not AMPAR antagonists slow migration. These data suggest the biophysical properties of immature CGNs render NMDARs less sensitive to Mg(2+) blockade, enhancing the likelihood of activation in the absence of AMPAR depolarization.

Features of sequential learning in hemicerebellectomized rats

Because the sequencing property is one of the functions in which cerebellar circuits are involved, it is important to analyze the features of sequential learning in the presence of cerebellar damage. Hemicerebellectomized and control rats were tested in a four-choice visuomotor learning task that required both the detection of a specific sequence of correct choices and the acquisition of procedural rules about how to perform the task. The findings indicate that the presence of the hemicerebellectomy did not affect the first phases of detection and acquisition of the sequential visuomotor task, delayed but did not prevent the learning of the sequential task, slowed down speed-up and proceduralization phases, and loosened the reward-response associative structure. The performances of hemicerebellectomized animals in the serial learning task as well as in the open field task demonstrated that the delayed sequential learning task could not be ascribed to impairment of motor functions or discriminative abilities or to low levels of motivation. The delay in sequential learning observed in the presence of a cerebellar lesion appeared to be related mainly to a delay of the automatization of the response. In conclusion, it may be advanced that, through cortical and subcortical connections, the cerebellum provides the acquisition of rapid and accurate sensory-guided sequence of responses.

Differential induction of bidirectional long-term changes in neurotransmitter release by frequency-coded patterns at the cerebellar input

Sensory stimulation conveys spike discharges of variable frequency and duration along the mossy fibres of cerebellum raising the question of whether and how these patterns determine plastic changes at the mossy fibre-granule cell synapse. Although various combinations of high-frequency bursts and membrane depolarization can induce NMDA receptor-dependent long-term depression (LTD) and long-term potentiation (LTP), the effect of different discharge frequencies remained unknown. Here we show that low-frequency mossy fibre stimulation (100 impulses1 Hz) induces mGlu receptor-dependent LTD. For various burst frequencies, the plasticity-[Ca(2+)](i) relationship was U-shaped resembling the Bienenstok-Cooper-Munro (BCM) learning rule. Moreover, LTD expression was associated with increased paired-pulse ratio, coefficient of variation and failure rate, and with a decrease in release probability, therefore showing changes opposite to those characterizing LTP. The plasticity-[Ca(2+)](i) relationship and the changes in neurotransmitter release measured by varying induction frequencies were indistinguishable from those obtained by varying high-frequency burst duration. These results suggest that different glutamate receptors converge onto a final common mechanism translating the frequency and duration of mossy fibre discharges into a regulation of the LTP/LTD balance, which may play an important role in adapting spatio-temporal signal transformations at the cerebellar input stage.