Prions prevent brain damage after experimental brain injury: a preliminary report

Antiviral activity of prion protein against Japanese encephalitis virus infection in vitro and in vivo

Flaviviruses are a major cause of viral diseases worldwide, for which effective treatments have yet to be discovered. The prion protein (PrPc) is abundantly expressed in brain cells and has been shown to play a variety of roles, including neuroprotection, cell homeostasis, and regulation of cellular signaling. However, it is still unclear whether PrPc can protect against flaviviruses. In this study, we investigated the role of PrPc in regulating autophagy flux and its potential antiviral activity during Japanese encephalitis virus (JEV) infection. Our in vivo experiment showed that JEV was more lethal to the PrPc knocked out mice which was further supported by histological analysis, western blot and rtPCR results from infected mice brain samples. Role of PrPc against viral propagation in vitro was verified through cell survival study, protein expression and RNA replication analysis, and adenoviral vector assay by overexpressing PrPc. Further analysis indicated that after virus entry, PrPc inhibited autophagic flux that prevented JEV replication inside the host cell. Our results from in vivo and in vitro investigations demonstrate that prion protein effectively inhibited JEV propagation by regulating autophagy flux which is used by JEV to release its genetic material and replication after entering the host cell, suggesting that prion protein may be a promising therapeutic target for flavivirus infection.

Plasma PRPC Levels Correlate With Severity and Prognosis of Intracerebral Hemorrhage

Background

Cellular prion protein (PRPC) exerts brain-protective effects. We determined the relationship between plasma PRPC levels and disease severity plus clinical outcome after acute intracerebral hemorrhage (ICH).

Methods

A total of 138 ICH patients and 138 healthy controls were included in this prospective, observational study. Hematoma volume and Glasgow coma scale (GCS) score were used to assess disease severity. Glasgow outcome scale (GOS) scores of 1–3 and 4–5 at 90 days after stroke were defined as a poor outcome and good outcome, respectively. Using multivariate analysis, we discerned the relation of plasma PRPC levels to disease severity and poor outcome. The receiver operating characteristic (ROC) curve was built to evaluate the prognostic predictive capability.

Results

Plasma PRPC levels in ICH patients were significantly higher than those in healthy controls (median, 4.20 vs. 2.02 ng/ml; P < 0.001), and were independently correlated with GCS score (r = −0.645, P < 0.001) and hematoma volume (r = 0.627, P < 0.001). Plasma PRPC levels were highly correlated with GOS score (r = −0.762, P < 0.001), and were substantially higher in patients with poor outcomes than in those with the good outcomes. Using maximum Youden index, plasma PRPC levels >3.893 ng/ml distinguished the risk of poor outcome at 90 days, with a sensitivity of 86.4% and a specificity of 65.8% (area under the curve, 0.809; 95% confidence interval (CI), 0.737–0.881, P < 0.001). Plasma PRPC levels >3.893 ng/ml were independently associated with a poor 90-day outcome with an odds ratio of 12.278 (95% CI, 5.101–29.554).

Conclusion

Elevated plasma PRPC levels are significantly associated with disease severity and poor 90-day outcome in ICH patients, indicating that plasma PRPC may be used as a potential prognostic biomarker after ICH.

Plasma cellular prion protein concentrations correlate with severity and prognosis of aneurysmal subarachnoid hemorrhage

BACKGROUND

Cellular prion protein (PrPc) is greatly expressed in injured brain tissues. We investigates correlation of plasma PrPc concentrations with severity, delayed cerebral ischemia (DCI) plus prognosis following aneurysmal subarachnoid hemorrhage (aSAH).

METHODS

Plasma PrPc concentrations were measured in 110 aSAH patients and 110 healthy controls. The World Federation of Neurological Surgeons scale (WFNS) score, Glasgow coma scale (GCS) score, Hunt-Hess score and modified Fisher score were utilized to assess hemorrhagic severity. Relations of plasma PrPc concentrations to DCI and 90-day poor outcome (Glasgow outcome scale score of 1-3) were analyzed using multivariate analysis. Prognostic predictive capabilities were determined under receiver operating characteristic curve.

RESULTS

Plasma PrPc concentrations were significantly higher in patients than in controls. Plasma PrPc concentrations were tightly correlated with WFNS score, GCS score, Hunt-Hess score and modified Fisher score. Plasma PrPc emerged as an independent predictor for 90-day poor outcome, but not for DCI. Plasma PrPc concentrations exhibited similar prognostic predictive abilities, as compared to WFNS score, GCS score, Hunt-Hess score and modified Fisher score.

CONCLUSIONS

Plasma PrPc concentrations are highly associated with severity and poor outcome after hemorrhagic stroke, indicating that plasma PrPc may serve as a useful prognostic biomarker for aSAH.

Show Me Your Friends and I Tell You Who You Are: The Many Facets of Prion Protein in Stroke

Ischemic stroke belongs to the leading causes of mortality and disability worldwide. Although treatments for the acute phase of stroke are available, not all patients are eligible. There is a need to search for therapeutic options to promote neurological recovery after stroke. The cellular prion protein (PrPC) has been consistently linked to a neuroprotective role after ischemic damage: it is upregulated in the penumbra area following stroke in humans, and animal models of stroke have shown that lack of PrPC aggravates the ischemic damage and lessens the functional outcome. Mechanistically, these effects can be linked to numerous functions attributed to PrPC: (1) as a signaling partner of the PI3K/Akt and MAPK pathways, (2) as a regulator of glutamate receptors, and (3) promoting stem cell homing mechanisms, leading to angio- and neurogenesis. PrPC can be cleaved at different sites and the proteolytic fragments can account for the manifold functions. Moreover, PrPC is present on extracellular vesicles (EVs), released membrane particles originating from all types of cells that have drawn attention as potential therapeutic tools in stroke and many other diseases. Thus, identification of the many mechanisms underlying PrPC-induced neuroprotection will not only provide further understanding of the physiological functions of PrPC but also new ideas for possible treatment options after ischemic stroke.

The Quest for Cellular Prion Protein Functions in the Aged and Neurodegenerating Brain

Cellular (also termed ‘natural’) prion protein has been extensively studied for many years for its pathogenic role in prionopathies after misfolding. However, neuroprotective properties of the protein have been demonstrated under various scenarios. In this line, the involvement of the cellular prion protein in neurodegenerative diseases other than prionopathies continues to be widely debated by the scientific community. In fact, studies on knock-out mice show a vast range of physiological functions for the protein that can be supported by its ability as a cell surface scaffold protein. In this review, we first summarize the most commonly described roles of cellular prion protein in neuroprotection, including antioxidant and antiapoptotic activities and modulation of glutamate receptors. Second, in light of recently described interaction between cellular prion protein and some amyloid misfolded proteins, we will also discuss the molecular mechanisms potentially involved in protection against neurodegeneration in pathologies such as Alzheimer’s, Parkinson’s, and Huntington’s diseases.

Low-Field Magnetic Stimulation Restores Cognitive and Motor Functions in the Mouse Model of Repeated Traumatic Brain Injury: Role of Cellular Prion Protein

Traumatic brain injury (TBI)/concussion is a growing epidemic throughout the world. Memory and neurobehavioral dysfunctions are among the sequelae of TBI. Dislodgement of cellular prion protein (PrPc) and disruption of circadian rhythm have been linked to TBI. Low-field magnetic stimulation (LFMS) is a new noninvasive repetitive transcranial magnetic stimulation (rTMS) technique that generates diffused and low-intensity magnetic stimulation to deep cortical and subcortical areas. The role of LFMS on PrPc, proteins related to the circadian rhythm, and behavior alterations in a repeated TBI mouse model were studied in the present study. TBI was induced to the mice (right hemisphere) using weight-drop method, once daily for 3 days. LFMS treatment was given for 20 min once daily for 4 days (immediately after each TBI induction). The results showed that LFMS-treated TBI mice significantly improved cognitive and motor function as evidenced by open field exploration, rotarod, and novel location recognition tasks. In addition, a significant increase in PrPc and decreased glial fibrillary acidic protein levels were observed in cortical and hippocampal regions of LFMS-treated TBI mice brain compared with sham-treated TBI mice, while neuronal nuclei level was significantly increased in cortical region. In LFMS-treated mice, a decrease in proteins related to circadian rhythm were observed, compared with sham-treated TBI mice. The results obtained from the study demonstrated the neuroprotective effect of LFMS, which may be through regulating PrPc and/or proteins related to circadian rhythm. Thus, the present study suggests that LFMS may improve the subject's neurological condition following TBI.

The prion protein in neuroimmune crosstalk

The cellular prion protein (PrP^C) is a medium-sized glycoprotein, attached to the cell surface by a glycosylphosphatidylinositol anchor. PrP^C is encoded by a single-copy gene, PRNP, which is abundantly expressed in the central nervous system and at lower levels in non-neuronal cells, including those of the immune system. Evidence from experimental knockout of PRNP in rodents, goats, and cattle and the occurrence of a nonsense mutation in goat that prevents synthesis of PrP^C, have shown that the molecule is non-essential for life. Indeed, no easily recognizable phenotypes are associate with a lack of PrP^C, except the potentially advantageous trait that animals without PrP^C cannot develop prion disease. This is because, in prion diseases, PrP^C converts to a pathogenic "scrapie" conformer, PrP^Sc, which aggregates and eventually induces neurodegeneration. In addition, endogenous neuronal PrP^C serves as a toxic receptor to mediate prion-induced neurotoxicity. Thus, PrP^C is an interesting target for treatment of prion diseases. Although loss of PrP^C has no discernable effect, alteration of its normal physiological function can have very harmful consequences. It is therefore important to understand cellular processes involving PrP^C, and research of this topic has advanced considerably in the past decade. Here, we summarize data that indicate the role of PrP^C in modulating immune signaling, with emphasis on neuroimmune crosstalk both under basal conditions and during inflammatory stress.

LPS-induced systemic inflammation reveals an immunomodulatory role for the prion protein at the blood-brain interface

Background

The cellular prion protein (PrP^C) is an evolutionary conserved protein abundantly expressed not only in the central nervous system but also peripherally including the immune system. A line of Norwegian dairy goats naturally devoid of PrP^C (PRNP^Ter/Ter) provides a novel model for studying PrP^C physiology.

Methods

In order to explore putative roles for PrP^C in acute inflammatory responses, we performed a lipopolysaccharide (LPS, Escherichia coli O26:B6) challenge of 16 goats (8 PRNP^+/+ and 8 PRNP^Ter/Ter) and included 10 saline-treated controls (5 of each PRNP genotype). Clinical examinations were performed continuously, and blood samples were collected throughout the trial. Genome-wide transcription profiles of the choroid plexus, which is at the blood-brain interface, and the hippocampus were analyzed by RNA sequencing, and the same tissues were histologically evaluated.

Results

All LPS-treated goats displayed clinical signs of sickness behavior, which were of significantly (p < 0.01) longer duration in animals without PrP^C. In the choroid plexus, a substantial alteration of the transcriptome and activation of Iba1-positive cells were observed. This response included genotype-dependent differential expression of several genes associated with the immune response, such as ISG15, CXCL12, CXCL14, and acute phase proteins, among others. Activation of cytokine-responsive genes was skewed towards a more profound type I interferon response, and a less obvious type II response, in PrP^C-deficient goats. The magnitude of gene expression in response to LPS was smaller in the hippocampus than in the choroid plexus. Resting state expression profiles revealed a few differences between the PRNP genotypes.

Conclusions

Our data suggest that PrP^C acts as a modulator of certain pathways of innate immunity signaling, particularly downstream of interferons, and probably contributes to protection of vulnerable tissues against inflammatory damage.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0879-5) contains supplementary material, which is available to authorized users.

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.

Examining the Neural and Astroglial Protective Effects of Cellular Prion Protein Expression and Cell Death Protease Inhibition in Mouse Cerebrocortical Mixed Cultures

Overexpression of cellular prion protein, PrP(C), has cytoprotective effects against neuronal injuries. Inhibition of cell death-associated proteases such as necrosis-linked calpain and apoptosis-linked caspase are also neuroprotective. Here, we systematically studied how PrP(C) expression levels and cell death protease inhibition affect cytotoxic challenges to both neuronal and glial cells in mouse cerebrocortical mixed cultures (CCM). Primary CCM derived from three mouse lines expressing no (PrP(C) knockout mice (PrPKO)), normal (wild-type (wt)), or high (tga20) levels of PrP(C) were subjected to necrotic challenge (calcium ionophore A23187) and apoptotic challenge (staurosporine (STS)). CCM which originated from tga20 mice provided the most robust neuron-astroglia protective effects against necrotic and early apoptotic cell death (lactate dehydrogenase (LDH) release) at 6 h but subsequently lost its cytoprotective effects. In contrast, PrPKO-derived cultures displayed elevated A23187- and STS-induced cell death at 24 h. Calpain inhibitor SNJ-1945 protected against A23187 challenge at 6 h in CCM from all three mouse lines but protected only against A23187 and STS treatments by 24 h in the PrPKO line. In parallel, caspase inhibitor Z-D-DCB protected against pro-apoptotic STS challenge at 6 and 24 h. Furthermore, we also examined αII-spectrin breakdown products (primarily from neurons) and glial fibrillary acidic protein (GFAP) breakdown products (from astroglia) as cytoskeletal proteolytic biomarkers. Overall, it appeared that both neurons and astroglial cells were less vulnerable to proteolytic attack during A23187 and STS challenges in tga20-derived cultures but more vulnerable in PrPKO-derived cultures. In addition, calpain and caspase inhibitors provide further protection against respective protease attacks on these neuronal and glial cytoskeletal proteins in CCM regardless of mouse-line origin. Lastly, some synergistic cytoprotective effects between PrP(C) expression and addition of cell death-linked protease inhibitors were also observed.

The Cellular Prion Protein: A Player in Immunological Quiescence

Despite intensive studies since the 1990s, the physiological role of the cellular prion protein (PrP(C)) remains elusive. Here, we present a novel concept suggesting that PrP(C) contributes to immunological quiescence in addition to cell protection. PrP(C) is highly expressed in diverse organs that by multiple means are particularly protected from inflammation, such as the brain, eye, placenta, pregnant uterus, and testes, while at the same time it is expressed in most cells of the lymphoreticular system. In this paradigm, PrP(C) serves two principal roles: to modulate the inflammatory potential of immune cells and to protect vulnerable parenchymal cells against noxious insults generated through inflammation. Here, we review studies of PrP(C) physiology in view of this concept.

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.

Primary blast-induced traumatic brain injury in rats leads to increased prion protein in plasma: a potential biomarker for blast-induced traumatic brain injury

Traumatic brain injury (TBI) is deemed the "signature injury" of recent military conflicts in Afghanistan and Iraq, largely because of increased blast exposure. Injuries to the brain can often be misdiagnosed, leading to further complications in the future. Therefore, the use of protein biomarkers for the screening and diagnosis of TBI is urgently needed. In the present study, we have investigated the plasma levels of soluble cellular prion protein (PrPC) as a novel biomarker for the diagnosis of primary blast-induced TBI (bTBI). We hypothesize that the primary blast wave can disrupt the brain and dislodge extracellular localized PrPC, leading to a rise in concentration within the systemic circulation. Adult male Sprague-Dawley rats were exposed to single pulse shockwave overpressures of varying intensities (15-30 psi or 103.4-206.8 kPa] using an advanced blast simulator. Blood plasma was collected 24 h after insult, and PrPC concentration was determined with a modified commercial enzyme-linked immunosorbent assay (ELISA) specific for PrPC. We provide the first report that mean PrPC concentration in primary blast exposed rats (3.97 ng/mL ± 0.13 SE) is significantly increased compared with controls (2.46 ng/mL ± 0.14 SE; two tailed test p < 0.0001). Furthermore, we report a mild positive rank correlation between PrPC concentration and increasing blast intensity (psi) reflecting a plateaued response at higher pressure magnitudes, which may have implications for all military service members exposed to blast events. In conclusion, it appears that plasma levels of PrPC may be a novel biomarker for the detection of primary bTBI.

Cobalamin and normal prions: a new horizon for cobalamin neurotrophism

It is known that cobalamin (Cbl) deficiency damages myelin by increasing tumor necrosis factor (TNF)-α and decreasing epidermal growth factor (EGF) levels in rat central nervous system (CNS), and affects the peripheral nervous system (PNS) morphologically and functionally. It is also known that some polyneuropathies not due to Cbl deficiency are connected with increased TNF-α levels, and that various cytokines (including TNF-α) and growth factors regulate the in vitro synthesis of normal prions (PrP(C)s). Given that there is extensive evidence that PrP(C)s play a key role in the maintenance of CNS and PNS myelin, we investigated whether the PrP(C) octapeptide repeat (OR) region is involved in the pathogenesis of rat Cbl-deficient (Cbl-D) polyneuropathy. After intracerebroventricularly administering antibodies (Abs) against the OR region (OR-Abs) to Cbl-D rats to prevent myelin damage and maximum nerve conduction velocity (MNCV) abnormalities, and PrP(C)s to otherwise normal rats to reproduce PNS Cbl-D-like lesions, we measured PrP(C) levels and MNCV of the sciatic and tibial nerves. PrP(C) and TNF-α levels were increased in sciatic and tibial nerves of Cbl-D and saline-treated rats, and the OR-Abs normalized the myelin ultrastructure, TNF-α levels, and MNCV values of the sciatic and tibial nerves of Cbl-D rats. The same peripheral nerves of the otherwise normal PrP(C)-treated rats showed typical Cbl-D myelin lesions, significantly increased TNF-α levels, and significantly decreased MNCV values. These findings demonstrate that Cbl deficiency induces excess PrP(C)s and thereby excess OR regions, which seem to be responsible for the PNS myelin damage, as has recently been found in the case of CNS myelin damage [66]. Furthermore, excess TNF-α is also involved in the pathogenesis of Cbl-D polyneuropathy. In conclusion, we have extended the list of prion diseases by adding one caused by excess PrP(C)s and the polyneuropathies related to excess TNF-α.

Neuroprotective effects of the cellular prion protein in autoimmune optic neuritis

Although the pathologic role of the prion protein in transmissible spongiform encephalopathic diseases has been widely investigated, the physiologic role of the cellular prion protein (PrP(C)) is not known. Among the many functions attributed to PrP(C), there is increasing evidence that it is involved in cell survival and mediates neuroprotection. A potential role in the immune response has also been suggested. However, how these two functions interplay in autoimmune disease is unclear. To address this, autoimmune optic neuritis, a model of multiple sclerosis, was induced in C57Bl/6 mice, and up-regulation of PrP(C) was observed throughout the disease course. In addition, compared with wild-type mice, in PrP(C)-deficient mice and mice overexpressing PrP(C), histopathologic analysis demonstrated that optic neuritis was exacerbated, as indicated by axonal degeneration, inflammatory infiltration, and demyelination. However, significant neuroprotection of retinal ganglion cells, the axons of which form the optic nerve, was observed in mice that overexpressed PrP(C). Conversely, mice lacking PrP(C) demonstrated significantly more neurodegeneration. This suggests that PrP(C) may have a neuroprotective function independent of its role in regulating the immune response.

Prion protein: From physiology to cancer biology

Prion protein (PrPc) was originally viewed solely as being involved in prion disease, but now several intriguing lines of evidence have emerged indicating that it plays a fundamental role not only in the nervous system, but also throughout the human body. PrPc is expressed most abundantly in the brain, but has also been detected in other non-neuronal tissues as diverse as lymphoid cells, lung, heart, kidney, gastrointestinal tract, muscle, and mammary glands. Recent data indicate that PrPc may be implicated in biology of glioblastoma, breast cancer, prostate and gastric cancer. Over expression of PrPc is correlated to the acquisition by tumor cells of a phenotype for resistance to cell death induced by TNF alpha and TRAIL or antitumor drugs such as paclitaxel and anthracyclines. PrPc may promote tumorigenesis, proliferation and G1/S transition in gastric cancer cells. This review revisits the physiological functions of PrPc, and its possible implications for cancer biology.

Prion protein lacks robust cytoprotective activity in cultured cells

BACKGROUND

The physiological function of the cellular prion protein (PrPC) remains unknown. However, PrPC has been reported to possess a cytoprotective activity that prevents death of neurons and other cells after a toxic stimulus. To explore this effect further, we attempted to reproduce several of the assays in which a protective activity of PrP had been previously demonstrated in mammalian cells.

RESULTS

In the first set of experiments, we found that PrP over-expression had a minimal effect on the death of MCF-7 breast carcinoma cells treated with TNF-alpha and Prn-p0/0 immortalized hippocampal neurons (HpL3-4 cells) subjected to serum deprivation. In the second set of assays, we observed only a small difference in viability between cerebellar granule neurons cultured from PrP-null and control mice in response to activation of endogenous or exogenous Bax.

CONCLUSION

Taken together, our results suggest either that cytoprotection is not a physiologically relevant activity of PrPC, or that PrPC-dependent protective pathways operative in vivo are not adequately modeled by these cell culture systems. We suggest that cell systems capable of mimicking the neurotoxic effects produced in transgenic mice by N-terminally deleted forms of PrP or Doppel may represent more useful tools for analyzing the cytoprotective function of PrPC.

Antagonistic roles of the N-terminal domain of prion protein to doppel

Prion protein (PrP)-like molecule, doppel (Dpl), is neurotoxic in mice, causing Purkinje cell degeneration. In contrast, PrP antagonizes Dpl in trans, rescuing mice from Purkinje cell death. We have previously shown that PrP with deletion of the N-terminal residues 23-88 failed to neutralize Dpl in mice, indicating that the N-terminal region, particularly that including residues 23-88, may have trans-protective activity against Dpl. Interestingly, PrP with deletion elongated to residues 121 or 134 in the N-terminal region was shown to be similarly neurotoxic to Dpl, indicating that the PrP C-terminal region may have toxicity which is normally prevented by the N-terminal domain in cis. We recently investigated further roles for the N-terminal region of PrP in antagonistic interactions with Dpl by producing three different types of transgenic mice. These mice expressed PrP with deletion of residues 25-50 or 51-90, or a fusion protein of the N-terminal region of PrP with Dpl. Here, we discuss a possible model for the antagonistic interaction between PrP and Dpl.

Dominant-negative effects of the N-terminal half of prion protein on neurotoxicity of prion protein-like protein/doppel in mice

Prion protein-like protein/doppel is neurotoxic, causing ataxia and Purkinje cell degeneration in mice, whereas prion protein antagonizes doppel-induced neurodegeneration. Doppel is homologous to the C-terminal half of prion protein but lacks the amino acid sequences corresponding to the N-terminal half of prion protein. We show here that transgenic mice expressing a fusion protein consisting of the N-terminal half, corresponding to residues 1-124, of prion protein and doppel in neurons failed to develop any neurological signs for up to 730 days in a background devoid of prion protein. In addition, the fusion protein prolonged the onset of ataxia in mice expressing exogenous doppel. These results suggested that the N-terminal part of prion protein has a neuroprotective potential acting both cis and trans on doppel. We also show that prion protein lacking the pre-octapeptide repeat (Delta25-50) or octapeptide repeat (Delta51-90) region alone could not impair the antagonistic function against doppel.

Canadian Association of Neurosciences Review: prion protein and prion diseases: the good and the bad

In the 1700's a strange new disease affecting sheep was recognized in Europe. The disease later became known as "Scrapie" and was the first of a family of similar diseases affecting a number of species that are now known as the Transmissible Spongiform Encephalopathies (TSEs). The appearance of a new disease in humans linked to the consumption of meat products from infected cattle has stimulated widespread public concern and scientific interest in the prion protein and related diseases. Nearly 300 years after the first report, these diseases still merit the descriptor "strange". This family of diseases is characterized by a unique profile of histological changes, can be transmitted as inherited or acquired diseases, as well as apparent sporadic spontaneous generation of the disease. These diseases are believed by many, to be caused by a unique protein only infectious agent. The "prion protein" (PrPC), a term first coined by Stanley Prusiner in 1982 is crucial to the development of these diseases, apparently by acting as a substrate for an abnormal disease associated form. However, aside from being critical to the pathogenesis of the disease, the function of PrPC, which is expressed in all mammals, has defied definitive description. Several roles have been proposed on the basis of in vitro studies, however, thus far, in vivo confirmation has not been forthcoming. The biological features of PrPC also seem to be unusual. Numerous mouse models have been generated in an attempt to understand the pathogenesis of these diseases. This review summarizes the current state of histological features, the etiologic agent, the normal metabolism and the function of the prion protein, as well as the limitations of the mouse models.

The role of the octarepeat region in neuroprotective function of the cellular prion protein

Structural alterations of the cellular prion protein (PrP^C) seem to be the core of the pathogenesis of prion diseases. However, the physiological function of PrP^C remains an enigma. Cell culture experiments have indicated that PrP^C and in particular its N‐terminal octarepeat region together with the phosphatidylinositol 3‐kinase (PI3K)/Akt signaling pathways have a fundamental involvement in neuroprotection and oxidative stress reactions. We used wild‐type mice, PrP knockout (Prnp^−/−) animals and transgenic mice that lack the octarepeat region (C4/−) and subjected them to controlled ischemia. We identified an increased cleavage and synthesis of PrP^C in ischemic brain areas of wild‐type mice compared with sham controls. The infarct size in Prnp^−/− animals was increased threefold when compared with wild‐type mice. The infarct size in C4/− animals was identical to Prnp^−/− mice, that is, around three times larger than in wild‐type mice. We showed that the PrP in C4/− mice does not functionally rescue the Prnp^−/− phenotype; furthermore it is unable to undergo β cleavage, although an increased amount of C1 fragments was found in ischemic brain areas compared with sham controls. We demonstrated that the N‐terminal octarepeat region has a lead function in PrP^C physiology and neuroprotection against oxidative stress in vivo.

The prion protein knockout mouse: a phenotype under challenge

The key pathogenic event in prion disease involves misfolding and aggregation of the cellular prion protein (PrP). Beyond this fundamental observation, the mechanism by which PrP misfolding in neurons leads to injury and death remains enigmatic. Prion toxicity may come about by perverting the normal function of PrP. If so, understanding the normal function of PrP may help to elucidate the molecular mechansim of prion disease. Ablation of the Prnp gene, which encodes PrP, was instrumental for determining that the continuous production of PrP is essential for replicating prion infectivity. Since the structure of PrP has not provided any hints to its possible function, and there is no obvious phenotype in PrP KO mice, studies of PrP function have often relied on intuition and serendipity. Here, we enumerate the multitude of phenotypes described in PrP deficient mice, many of which manifest themselves only upon physiological challenge. We discuss the pleiotropic phenotypes of PrP deficient mice in relation to the possible normal function of PrP. The critical question remains open: which of these phenotypes are primary effects of PrP deletion and what do they tell us about the function of PrP?

Neonatal lethality in transgenic mice expressing prion protein with a deletion of residues 105-125

To identify sequence domains important for the neurotoxic and neuroprotective activities of the prion protein (PrP), we have engineered transgenic mice that express a form of murine PrP deleted for a conserved block of 21 amino acids (residues 105-125) in the unstructured, N-terminal tail of the protein. These mice spontaneously developed a severe neurodegenerative illness that was lethal within 1 week of birth in the absence of endogenous PrP. This phenotype was reversed in a dose-dependent fashion by coexpression of wild-type PrP, with five-fold overexpression delaying death beyond 1 year. The phenotype of Tg(PrPDelta105-125) mice is reminiscent of, but much more severe than, those described in mice that express PrP harboring larger deletions of the N-terminus, and in mice that ectopically express Doppel, a PrP paralog, in the CNS. The dramatically increased toxicity of PrPDelta105-125 is most consistent with a model in which this protein has greatly enhanced affinity for a hypothetical receptor that serves to transduce the toxic signal. We speculate that altered binding interactions involving the 105-125 region of PrP may also play a role in generating neurotoxic signals during prion infection.

Cellular prion protein modulates the intracellular calcium response to hydrogen peroxide

The physiological function of the cellular prion protein (PrP(C)) is still under intense investigation. It has been suggested that PrP(C) has a protective role in neuronal cells, particularly against environmental stress caused by reactive oxygen species (ROS). Here we analysed the acute effect of a major ROS, hydrogen peroxide (H(2)O(2)), on intracellular calcium homeostasis in cultured cerebellar granule cells and immortalized hippocampal neuronal cells. Both neuronal cell culture models showed that the rise in intracellular calcium following application of H(2)O(2) was strongly dependent on the presence of PrP(C). Moreover, the N-terminal octapeptide repeats of PrP(C) were required for this effect, because neuronal cells expressing a PrP(C) lacking the N-terminus resembled the PrP(C)-deficient phenotype. Neurones deficient of fyn kinase, or pharmacological inhibition of fyn, also abrogated the calcium response to H(2)O(2) treatment, indicating that fyn activation is a critical step within the PrP(C) signalling cascade. Finally, we identified a possible role of this PrP(C) signalling pathway in the neuroprotective response of PrP(C) to oxidative stress. In conclusion, we put forward the hypothesis that PrP(C) functions as a sensor for H(2)O(2), thereby activating a protective signalling cascade involving fyn kinase that leads to calcium release from intracellular stores.

Role of cellular prion protein on LTP expression in aged mice

Cellular prion protein (PrP(c)) has been associated with some physiological functions in the last few years. In a previous paper, we have demonstrated an increased hippocampal synaptic transmission in adult mice lacking this protein. In the present study, we investigate the impact of aging on the generation and maintenance of hippocampal long-term Potentiation (LTP) in 9-month-old mice devoid of PrP(c) protein (Prnp(0/0)). We observed a lower threshold for inducing LTP in 9-month-old Prnp(0/0) mice compared to wild-type ones at the same age. The maintenance of dentate gyrus LTP was more persistent in hippocampal slices from Prnp(0/0) mice. Furthermore, the expression of mRNA for NR2A and NR2B subunits of the NMDA glutamatergic receptor in hippocampus of aged Prnp(0/0) animals showed an increase compared to the wild type. We propose that increased hippocampal glutamatergic transmission in Prnp(0/0) mice is related to the enhanced plasticity and persistence of the dentate LTP.

Deletion of Cellular Prion Protein Results in Reduced Akt Activation, Enhanced Postischemic Caspase-3 Activation, and Exacerbation of Ischemic Brain Injury

Background and Purpose— The physiological function of cellular prion protein (PrP c ) is not yet understood. Recent findings suggest that PrP c may have neuroprotective properties, and its absence increases susceptibility to neuronal injury. The purpose of this study was to elucidate the role of PrP c in ischemic brain injury in vivo.

Methods— PrP knockout (Prnp 0/0 ) and Prnp +/+ wild-type (WT) mice were subjected to 60-minute transient or permanent focal cerebral ischemia followed by infarct volume analysis 24 hours after lesion. To identify effects of PrP c deletion on mechanisms regulating ischemic cell death, expression analysis of several proapoptotic and antiapoptotic proteins was performed at 6 and 24 hours after transient ischemia and in nonischemic controls using Western blot or immunohistochemistry.

Results— Prnp 0/0 mice displayed significantly increased infarct volumes after both transient or permanent ischemia when compared with WT animals (70.2±23 versus 13.3±4 mm 3 ; 119.8±24 versus 86.4±25 mm 3 ). Expression of phospho-Akt (Ser473) was significantly reduced in Prnp 0/0 compared with WT animals both early after ischemia and in sham controls. Furthermore, postischemic caspase-3 activation was significantly enhanced in the basal ganglia and the parietal cortex of Prnp 0/0 mice. In contrast, expression of total Akt, Bax, and Bcl-2 did not differ between both groups.

Conclusions— These results demonstrate that PrP c deletion impairs the antiapoptotic phosphatidylinositol 3-kinase/Akt pathway by resulting in reduced postischemic phospho-Akt expression, followed by enhanced postischemic caspase-3 activation, and aggravated neuronal injury after transient and permanent cerebral ischemia.

Female-specific neuroprotection against transient brain ischemia observed in mice devoid of prion protein is abolished by ectopic expression of prion protein-like protein

This study was designed to examine the function of cellular prion protein and prion protein-like protein/Doppel, in transient ischemia-related neuronal death in the hippocampus. Two different lines of mice devoid of cellular prion protein, Zrch I Prnp(0/0) and Ngsk Prnp(0/0), were used. The former lacks cellular prion protein whereas the latter ectopically expresses prion protein-like protein/Doppel in the brain in the absence of cellular prion protein. Mice were subjected to 10 min-occlusion of the bilateral common carotid arteries with recovery for 14 days. Less than 10% of the pyramidal neurons in the CA1 subfield were degenerated in male and female wild-type mice. In contrast, more than half of the neurons were lost in male Zrch I Prnp(0/0) and Ngsk Prnp(0/0) mice. Such severe neuronal loss was also observed in female Ngsk Prnp(0/0) mice. However, female Zrch I Prnp(0/0) mice showed mild neuronal loss similar to wild-type mice. Flunarizine, a T- and L-type Ca(2+)-channel antagonist, significantly reduced the neuronal loss in female but not in male Ngsk Prnp(0/0) mice. These results indicate that loss of cellular prion protein renders hippocampal neurons susceptible to ischemic insult specifically in male but not female mice and the ectopic expression of prion protein-like protein/Doppel aggravates the ischemic neuronal death in female prion protein-null mice probably via overloading of Ca(2+)-dependent signaling.

Altered behavioural response to acute stress in mice lacking cellular prion protein

Although many studies have investigated the function of cellular prion protein (PrPc), its physiologic role remains elusive. PrPc null mice have been reported to develop normally and to show normal performance in most behavioural tests. In the present study we investigated whether this also holds true after episodes of acute stress. PrPc gene ablated (Prnp0/0) and wild-type mice were subjected to restraint stress, electric foot shock, or swimming and compared with non-stressed animals. Immediately after the stressful situation, the anxiety levels and locomotion of the animals were measured using plus-maze and open-field tests. Among non-stressed animals, there was no significant difference in performance between Prnp0/0 and wild type animals in either test. However, after acute stress provoked by a foot shock or a swimming trial, Prnp0/0 animals showed a significant decrease in anxiety levels when compared with control animals. Moreover, after the swimming test, knockout mice presented decreased locomotion when compared to wild-type mice. Because of this observation, we also assessed both types of mice in a forced swimming test with the objective of better evaluating muscle function and found that Prnp0/0 animals presented reduced forced swimming capacity when compared to controls. As far as we know, this is the first report suggesting that cellular prion protein is involved in modulation of anxiety or muscular activity after acute psychic or physical stress.

Mammalian prion protein suppresses Bax-induced cell death in yeast

Several lines of evidence suggest that PrP(C), the non-infectious form of the prion protein, may function to protect neurons and other cells from stress or toxicity. In this paper, we report on the use of the yeast Saccharomyces cerevisiae as a model system to assay the cytoprotective activity of PrP(C). The mammalian pro-apoptotic protein, Bax, confers a lethal phenotype when expressed in yeast. Since overexpression of PrP(C) has been found to prevent Bax-mediated cell death in cultured human neurons, we explored whether PrP could also suppress Bax-induced cell death in yeast. We utilized a form of mouse PrP containing a modified signal peptide that we had previously shown is efficiently targeted to the secretory pathway in yeast. We found that this PrP potently suppressed the death of yeast cells expressing mammalian Bax under control of a galactose-inducible promoter. In contrast, cytosolic PrP-(23-231) failed to rescue growth of Bax-expressing yeast, indicating that protective activity requires targeting of PrP to the secretory pathway. Deletion of the octapeptide repeat region did not affect the rescuing activity of PrP, but deletion of a charged region encompassing residues 23-31 partially eliminated activity. We also tested several PrP mutants associated with human familial prion diseases and found that only a mutant containing nine extra octapeptide repeats failed to suppress Bax-induced cell death. These findings establish a simple and genetically tractable system for assaying a putative biological activity of PrP(C).

Cellular prion protein neuroprotective function: implications in prion diseases

Prion protein can display two conformations: a normal cellular conformation (PrP) and a pathological conformation associated with prion diseases (PrP(Sc)). Three complementary strategies are used by researchers investigating how PrP is involved in the pathogenesis of prion diseases: elucidation of the normal function of PrP, determination of how PrP(Sc) is toxic to neurons, and unraveling the mechanism for the conversion of PrP to PrP(Sc). We review the normal function of PrP as an antioxidant and an antiapoptotic protein in vivo and in vitro. This review also addresses contrasting evidence that PrP is cytotoxic. Finally, we discuss the implication of the neuroprotective role of PrP in prion diseases.

Upregulation of cellular prion protein (PrPc) after focal cerebral ischemia and influence of lesion severity

The pathological isoform of the prion protein (PrP(Sc)) has been identified to mediate transmissible spongiform encephalopathies like Creutzfeldt-Jakob disease (CJD). In contrast, the physiological function of the normal cellular prion protein (PrP(c)) is not yet understood. Recent findings suggest that PrP(c) may have neuroprotective properties and that its absence increases susceptibility to oxidative stress and neuronal injury. To determine whether PrP(c) is part of the cellular response to neuronal injury in vivo, we investigated PrP(c) regulation after severe and mild focal ischemic brain injury in mice using the thread occlusion stroke model. Western Blot and ELISA analysis showed a significant upregulation of PrP(c) in the ischemic hemisphere at 4 and 8h after onset of permanent focal ischemia, which was no longer detectable at 24h after lesion induction when compared to control animals. In contrast, transient focal ischemia (60 min) did only lead to slightly but not significantly elevated PrP(c) levels in the ischemic hemisphere when compared to controls. These results demonstrate that cerebral PrP(c) is upregulated early in response to focal cerebral ischemia. The extent of upregulation, however, seems to depend on the severity of ischemia and may therefore reflect the extent of ischemia induced neuronal damage. Given the known neuroprotective effects of PrP(c) in vitro, ischemia-induced upregulation of cerebral PrP(c) supports the hypothesis that, as part of an early adaptive cellular response to ischemic brain injury, PrP(c) may be involved in the regulation of ischemia-induced neuronal cell death in vivo.