Prion protein N1 cleavage peptides stimulate microglial interaction with surrounding cells

A PrP EGFR signaling axis controls neural stem cell senescence through modulating cellular energy pathways

Mis-folding of the prion protein (PrP) is known to cause neurodegenerative disease; however, the native function of this protein remains poorly defined. PrP has been linked with many cellular functions, including cellular proliferation and senescence. It is also known to influence epidermal growth factor receptor (EGFR) signaling, a pathway that is itself linked with both cell growth and senescence. Adult neural stem cells (NSCs) persist at low levels in the brain throughout life and retain the ability to proliferate and differentiate into new neural lineage cells. KO of PrP has previously been shown to reduce NSC proliferative capacity. We used PrP KO and WT NSCs from adult mouse brain to examine the influence of PrP on cellular senescence, EGFR signaling, and the downstream cellular processes. PrP KO NSCs showed decreased cell proliferation and increased senescence in in vitro cultures. Expression of EGFR was decreased in PrP KO NSCs compared with WT NSCs and additional supplementation of EGF was sufficient to reduce senescence. RNA-seq analysis confirmed that significant changes were occurring at the mRNA level within the EGFR signaling pathway and these were associated with reduced expression of mitochondrial components and correspondingly reduced mitochondrial function. Metabolomic analysis of cellular energy pathways showed that blockages were occurring at critical sites for production of energy and biomass, including catabolism of pyruvate. We conclude that, in the absence of PrP, NSC growth pathways are downregulated as a consequence of insufficient energy and growth intermediates.

Temporary alteration of neuronal network communication is a protective response to redox imbalance that requires GPI-anchored prion protein

Cellular prion protein (PrP^C) protects neurons against oxidative stress damage. This role is lost upon its misfolding into insoluble prions in prion diseases, and correlated with cytoskeletal breakdown and neurophysiological deficits. Here we used mouse neuronal models to assess how PrP^C protects the neuronal cytoskeleton, and its role in network communication, from oxidative stress damage. Oxidative stress was induced extrinsically by potassium superoxide (KO₂) or intrinsically by Mito-Paraquat (MtPQ), targeting the mitochondria. In mouse neural lineage cells, KO₂ was damaging to the cytoskeleton, with cells lacking PrP^C (PrP^-/-) damaged more than wild-type (WT) cells. In hippocampal slices, KO₂ acutely inhibited neuronal communication in WT controls without damaging the cytoskeleton. This inhibition was not observed in PrP^-/- slices. Neuronal communication and the cytoskeleton of PrP^-/- slices became progressively disrupted and degenerated post-recovery, whereas the dysfunction in WT slices recovered in 5 days. This suggests that the acute inhibition of neuronal activity in WT slices in response to KO₂ was a neuroprotective role of PrP^C, which PrP^-/- slices lacked. Heterozygous expression of PrP^C was sufficient for this neuroprotection. Further, hippocampal slices from mice expressing PrP^C without its GPI anchor (PrP^GPI-/-) displayed acute inhibition of neuronal activity by KO₂. However, they failed to restore normal activity and cytoskeletal formation post-recovery. This suggests that PrP^C facilitates the depressive response to KO₂ and its GPI anchoring is required to restore KO₂-induced damages. Immuno spin-trapping showed increased radicals formed on the filamentous actin of PrP^-/- and PrP^GPI-/- slices, but not WT and PrP^+/- slices, post-recovery suggesting ongoing dysregulation of redox balance in the slices lacking GPI-anchored PrP^C. The MtPQ treatment of hippocampal slices temporarily inhibited neuronal communication independent of PrP^C expression. Overall, GPI-anchored PrP^C alters synapses and neurotransmission to protect and repair the neuronal cytoskeleton, and neuronal communication, from extrinsically induced oxidative stress damages.

What is the role of lipids in prion conversion and disease?

The molecular cause of transmissible spongiform encephalopathies (TSEs) involves the conversion of the cellular prion protein (PrP^C) into its pathogenic form, called prion scrapie (PrP^Sc), which is prone to the formation of amorphous and amyloid aggregates found in TSE patients. Although the mechanisms of conversion of PrP^C into PrP^Sc are not entirely understood, two key points are currently accepted: (i) PrP^Sc acts as a seed for the recruitment of native PrP^C, inducing the latter's conversion to PrP^Sc; and (ii) other biomolecules, such as DNA, RNA, or lipids, can act as cofactors, mediating the conversion from PrP^C to PrP^Sc. Interestingly, PrP^C is anchored by a glycosylphosphatidylinositol molecule in the outer cell membrane. Therefore, interactions with lipid membranes or alterations in the membranes themselves have been widely investigated as possible factors for conversion. Alone or in combination with RNA molecules, lipids can induce the formation of PrP in vitro-produced aggregates capable of infecting animal models. Here, we discuss the role of lipids in prion conversion and infectivity, highlighting the structural and cytotoxic aspects of lipid-prion interactions. Strikingly, disorders like Alzheimer's and Parkinson's disease also seem to be caused by changes in protein structure and share pathogenic mechanisms with TSEs. Thus, we posit that comprehending the process of PrP conversion is relevant to understanding critical events involved in a variety of neurodegenerative disorders and will contribute to developing future therapeutic strategies for these devastating conditions.

Anchorless risk or released benefit? An updated view on the ADAM10-mediated shedding of the prion protein

The prion protein (PrP) is a broadly expressed glycoprotein linked with a multitude of (suggested) biological and pathological implications. Some of these roles seem to be due to constitutively generated proteolytic fragments of the protein. Among them is a soluble PrP form, which is released from the surface of neurons and other cell types by action of the metalloprotease ADAM10 in a process termed 'shedding'. The latter aspect is the focus of this review, which aims to provide a comprehensive overview on (i) the relevance of proteolytic processing in regulating cellular PrP functions, (ii) currently described involvement of shed PrP in neurodegenerative diseases (including prion diseases and Alzheimer's disease), (iii) shed PrP's expected roles in intercellular communication in many more (patho)physiological conditions (such as stroke, cancer or immune responses), (iv) and the need for improved research tools in respective (future) studies. Deeper mechanistic insight into roles played by PrP shedding and its resulting fragment may pave the way for improved diagnostics and future therapeutic approaches in diseases of the brain and beyond.

Prion Protein: The Molecule of Many Forms and Faces

Cellular prion protein (PrP^C) is a glycosylphosphatidylinositol (GPI)-anchored protein most abundantly found in the outer membrane of neurons. Due to structural characteristics (a flexible tail and structured core), PrP^C interacts with a wide range of partners. Although PrP^C has been proposed to be involved in many physiological functions, only peripheral nerve myelination homeostasis has been confirmed as a bona fide function thus far. PrP^C misfolding causes prion diseases and PrP^C has been shown to mediate β-rich oligomer-induced neurotoxicity in Alzheimer’s and Parkinson’s disease as well as neuroprotection in ischemia. Upon proteolytic cleavage, PrP^C is transformed into released and attached forms of PrP that can, depending on the contained structural characteristics of PrP^C, display protective or toxic properties. In this review, we will outline prion protein and prion protein fragment properties as well as overview their involvement with interacting partners and signal pathways in myelination, neuroprotection and neurodegenerative diseases.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

Dissecting the copper bioinorganic chemistry of the functional and pathological roles of the prion protein: Relevance in Alzheimer's disease and cancer

The cellular prion protein (PrP^C) is a metal-binding biomolecule that can interact with different protein partners involved in pivotal physiological processes, such as neurogenesis and neuronal plasticity. Recent studies profile copper and PrP^C as important players in the pathological mechanisms of Alzheimer's disease and cancer. Although the copper-PrP^C interaction has been characterized extensively, the role of the metal ion in the physiological and pathological roles of PrP^C has been barely explored. In this article, we discuss how copper binding and proteolytic processing may impact the ability of PrP^C to recruit protein partners for its functional roles. The importance to dissect the role of copper-PrP^C interactions in health and disease is also underscored.

Innate immune responses after stimulation with Toll-like receptor agonists in ex vivo microglial cultures and an in vivo model using mice with reduced microglia

Background

Past experiments studying innate immunity in the central nervous system (CNS) utilized microglia obtained from neonatal mouse brain, which differ developmentally from adult microglia. These differences might impact our current understanding of the role of microglia in CNS development, function, and disease.

Methods

Cytokine protein secretion was compared in ex vivo P3 and adult microglial cultures after exposure to agonists for three different toll-like receptors (TLR4, lipopolysaccharide [LPS]; TLR7, imiquimod [IMQ]; and TLR9, CpG Oligodeoxynucleotide [CpG-ODN] 1585). In addition, changes in inflammatory gene expression in ex vivo adult microglia in response to the TLR agonists was assessed. Furthermore, in vivo experiments evaluated changes in gene expression associated with inflammation and TLR signaling in brains of mice with or without treatment with PLX5622 to reduce microglia.

Results

Ex vivo adult and P3 microglia increased cytokine secretion when exposed to TLR4 agonist LPS and to TLR7 agonist IMQ. However, adult microglia decreased expression of numerous genes after exposure to TLR 9 agonist CpG-ODN 1585. In contrast, in vivo studies indicated a core group of inflammatory and TLR signaling genes increased when each of the TLR agonists was introduced into the CNS. Reducing microglia in the brain led to decreased expression of various inflammatory and TLR signaling genes. Mice with reduced microglia showed extreme impairment in upregulation of genes after exposure to TLR7 agonist IMQ.

Conclusions

Cultured adult microglia were more reactive than P3 microglia to LPS or IMQ exposure. In vivo results indicated microglial influences on neuroinflammation were agonist specific, with responses to TLR7 agonist IMQ more dysregulated in mice with reduced microglia. Thus, TLR7-mediated innate immune responses in the CNS appeared more dependent on the presence of microglia. Furthermore, partial responses to TLR4 and TLR9 agonists in mice with reduced microglia suggested other cell types in the CNS can compensate for their absence.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02240-w.

Neuroprotective effect and potential of cellular prion protein and its cleavage products for treatment of neurodegenerative disorders part I. a literature review

INTRODUCTION

The cellular prion protein (PrP^C) is well known for its pathogenic roles in prion diseases, several other neurodegenerative diseases (such as Alzheimer's disease), and multiple types of cancer, but the beneficial aspects of PrP^C and its cleavage products received much less attention.

AREAS COVERED

Here the authors will systematically review the literatures on the negative as well as protective aspects of PrP^C and its derivatives (especially PrP N-terminal N1 peptide and shed PrP). The authors will dissect the current findings on N1 and shed PrP, including evidence for their neuroprotective effects, the categories of PrP^C cleavage, and numerous cleavage enzymes involved. The authors will also discuss the protective effects and therapeutic potentials of PrP^C-rich exosomes. The cited articles were obtained from extensive PubMed searches of recent literature, including peer-reviewed original articles and review articles.

EXPERT OPINION

PrP and its N-terminal fragments have strong neuroprotective activities that should be explored for therapeutics and prophylactics development against prion disease, Alzheimer's disease and a few other neurodegenerative diseases. The strategies to develop PrP-based therapeutics and prophylactics for these neurodegenerative diseases will be discussed in a companion article (Part II).

A 3D cell culture approach for studying neuroinflammation

BACKGROUND

Neurodegenerative diseases are highly complex making them challenging to model in cell culture. All cell types of the brain have been implicated as exerting an effect on pathogenesis, and disease progression is likely influenced by the cross-talk between the different cell types. Sophisticated investigation of the cellular level consequences of cross-talk between different cells types requires three-dimensional (3D) co-culture systems.

NEW METHOD

Murine neural stem cells were differentiated into mixed-neuronal lineage populations in 3D culture. By seeding these differentiated cultures with microglia from adult brain, we have generated a 3D ex-vivo model of murine brain tissue populated with microglia.

RESULTS

Monitoring the infiltration of GFP-expressing microglia into the 3D neuronal lineage cultures showed population throughout the tissue and assumption of ramified homeostatic morphology by the microglia. The co-cultures showed good longevity and were functionally responsive to external stimuli.

COMPARISON WITH EXISTING METHODS

We have previously used 2-dimensional adhered cultures to model cell-cell interactions between microglia and neuronal lineage cells. While the microglia integrate well into these cultures and demonstrate inter-cellular cross-talk, it is known that adhered culture can change their activation state and therefore a 3D system better represents communication throughout a network of neuronal and support cells.

CONCLUSIONS

Our system offers a straight-forward and time effective way to model 3D mouse brain tissue that is responsive to external neuroinflammatory stimulus. It not only allows inter-cellular interactions to be studied in live tissue but additionally permits study of changes within any available mouse genotype.

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.