Co-localization of CD3 and prion protein in Jurkat lymphocytes after hypothermal stimulation

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.

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.

Wide-scale quantitative phosphoproteomic analysis reveals that cold treatment of T cells closely mimics soluble antibody stimulation

The activation of T lymphocytes through antigen-mediated T cell receptor (TCR) clustering is vital in regulating the adaptive immune response. Although T cell receptor signaling has been extensively studied, the fundamental mechanisms for signal initiation are not fully understood. Reduced temperatures have initiated some of the hallmarks of TCR signaling, such as increased phosphorylation and activation on ERK and calcium release from the endoplasmic reticulum, as well as coalesced the T cell membrane microdomains. The precise mechanism of the TCR signaling initiation due to temperature change remains obscure. One critical question is whether the signaling initiated by the cold treatment of T cells differs from the signaling initiated by the cross-linking of the T cell receptor. To address this uncertainty, we performed a wide-scale, quantitative mass-spectrometry-based phosphoproteomic analysis on T cells stimulated either by temperature shifts or through the cross-linking of the TCR. Careful statistical comparisons between the two stimulations revealed a striking level of identity among the subset of 339 sites that changed significantly with both stimulations. This study demonstrates for the first time, in unprecedented detail, that T cell cold treatment was sufficient to initiate signaling patterns that were nearly identical to those of soluble antibody stimulation, shedding new light on the mechanism of activation of these critically important immune cells.

Enhanced susceptibility of T lymphocytes to oxidative stress in the absence of the cellular prion protein

The cellular prion glycoprotein (PrP(C)) is ubiquitously expressed but its physiologic functions remain enigmatic, particularly in the immune system. Here, we demonstrate in vitro and in vivo that PrP(C) is involved in T lymphocytes response to oxidative stress. By monitoring the intracellular level of reduced glutathione, we show that PrP(-/-) thymocytes display a higher susceptibility to H(2)O(2) exposure than PrP(+/+) cells. Furthermore, we find that in mice fed with a restricted diet, a regimen known to increase the intracellular level of ROS, PrP(-/-) thymocytes are more sensitive to oxidative stress. PrP(C) function appears to be specific for oxidative stress, since no significant differences are observed between PrP(-/-) and PrP(+/+) mice exposed to other kinds of stress. We also show a marked evolution of the redox status of T cells throughout differentiation in the thymus. Taken together, our results clearly ascribe to PrP(C) a protective function in thymocytes against oxidative stress.

Prion protein facilitates hormone-induced differentiation of mammary gland epithelial cells

Expression of prion protein has been reported for a variety of cell types including neuronal cells, haematopoietic stem cells, lymphocytes, fibroblasts, and epithelial cells. However, the characterization of the physiological roles exhibited by this protein is still in progress and multiple biological functions have been described to date. In this study we have characterized the contribution of prion protein during hormone-induced differentiation of mouse mammary gland epithelial cells. We present evidence that prion expression enhances the differentiation-capabilities of these cells indicating novel physiological roles during mammary gland development. In addition we were able to demonstrate the presence of prion molecules resistant to mild proteinase digestion in differentiated mammary gland epithelial cells. This represents the first report of proteinase-resistant prion proteins in a physiological, non-pathogenic context.

Bovine lactoferricin causes apoptosis in Jurkat T-leukemia cells by sequential permeabilization of the cell membrane and targeting of mitochondria

Bovine lactoferricin (LfcinB) is a cationic antimicrobial peptide that kills Jurkat T-leukemia cells by the mitochondrial pathway of apoptosis. However, the process by which LfcinB triggers mitochondria-dependent apoptosis is not well understood. Here, we show that LfcinB-induced apoptosis in Jurkat T-leukemia cells was preceded by LfcinB binding to, and progressive permeabilization of the cell membrane. Colloidal gold electron microscopy revealed that LfcinB entered the cytoplasm of Jurkat T-leukemia cells prior to the onset of mitochondrial depolarization. LfcinB was not internalized by endocytosis because endocytosis inhibitors did not prevent LfcinB-induced cytotoxicity. Furthermore, intracellular delivery of LfcinB via fusogenic liposomes caused the death of Jurkat T-leukemia cells, as well as normal human fibroblasts. Collectively, these findings suggest that LfcinB caused damage to the cell membrane that allowed LfcinB to enter the cytoplasm of Jurkat T-leukemia cells and mediate cytotoxicity. In addition, confocal microscopy showed that intracellular LfcinB co-localized with mitochondria in Jurkat T-leukemia cells, while flow cytometry and colloidal gold electron microscopy showed that LfcinB rapidly associated with purified mitochondria. Furthermore, purified mitochondria treated with LfcinB rapidly lost transmembrane potential and released cytochrome c. We conclude that LfcinB-induced apoptosis in Jurkat T-leukemia cells resulted from cell membrane damage and the subsequent disruption of mitochondrial membranes by internalized LfcinB.

Prion protein resides in membrane microclusters of the immunological synapse during lymphocyte activation

Expression of prion protein (PrP) has been reported for a variety of cell types including neuronal cells, haematopoietic stem cells, antigen-presenting cells, as well as lymphocytes. However, besides this widespread occurrence little is known about the physiological roles exhibited by this enigmatic protein. In this study, the contribution of PrP to the classical T-lymphocyte activation process was characterized by clustering the T-cell receptor component CD3epsilon as well as PrP with soluble and surface-immobilized antibodies, respectively. We present evidence that PrP is a component of signaling structures recently described as plasma membrane microclusters established during T-lymphocyte activation. The formation of immunological synapses, however, did not depend on the presence of PrP as proven by siRNA knockdown experiments, indicating very subtle physiological roles of PrP in vivo within the immune system.

The role of the cellular prion protein in the immune system

Prion protein (PrP) plays a key role in the pathogenesis of prion diseases. However, the normal function of the protein remains unclear. The cellular isoform (PrP(C)) is expressed widely in the immune system, in haematopoietic stem cells and mature lymphoid and myeloid compartments in addition to cells of the central nervous system. It is up-regulated in T cell activation and may be expressed at higher levels by specialized classes of lymphocyte. Furthermore, antibody cross-linking of surface PrP modulates T cell activation and leads to rearrangements of lipid raft constituents and increased phosphorylation of signalling proteins. These findings appear to indicate an important but, as yet, ill-defined role in T cell function. Although PrP(-/-) mice have been reported to have only minor alterations in immune function, recent work has suggested that PrP is required for self-renewal of haematopoietic stem cells. Here, we consider the evidence for a distinctive role for PrP(C) in the immune system and what the effects of anti-prion therapeutics may be on immune function.

Prion Protein Expression by Mouse Dendritic Cells Is Restricted to the Nonplasmacytoid Subsets and Correlates with the Maturation State

Expression of the physiological cellular prion protein (PrP(C)) is remarkably regulated during differentiation and activation of cells of the immune system. Among these, dendritic cells (DCs) display particularly high levels of membrane PrP(C), which increase upon maturation, in parallel with that of molecules involved in Ag presentation to T cells. Freshly isolated mouse Langerhans cells, dermal DCs, and DCs from thymus, spleen, and mesenteric lymph nodes expressed low to intermediate levels of PrP(C). Highest levels of both PrP(C) and MHC class II molecules were displayed by lymph node CD8alpha(int) DCs, which represent fully mature cells having migrated from peripheral tissues. Maturation induced by overnight culture resulted in increased levels of surface PrP(C), as did in vivo DC activation by bacterial LPS. Studies on Fms-like tyrosine kinase 3 ligand bone marrow-differentiated B220(-) DCs confirmed that PrP(C) expression followed that of MHC class II and costimulatory molecules, and correlated with IL-12 production in response to TLR-9 engagement by CpG. However, at variance with conventional DCs, B220(+) plasmacytoid DCs isolated from the spleen, or in vitro differentiated, did not significantly express PrP(C), both before and after activation by TLR-9 engagement. PrP knockout mice displayed higher numbers of spleen CD8alpha(+) DCs, but no significant differences in their maturation response to stimulation through TLR-4 and TLR-9 were noticed. Results are discussed in relation to the functional relevance of PrP(C) expression by DCs in the induction of T cell responses, and to the pathophysiology of prion diseases.

Prion protein modifies TGF-β induced signal transduction

Members of the transforming growth factor-beta (TGF-beta) superfamily regulate a multitude of cellular processes as well as the expression of various proteins such as, e.g., matrix metalloproteinases (MMPs). These endopeptidases selectively degrade components of the extracellular matrix as well as non-matrix substrates like growth factors and cell surface receptors. MMPs are activated during embryonic development, morphogenesis, and tissue resorption/remodeling as well as in pathological conditions such as deranged wound healing and cancer metastasis. In this report we demonstrate that over-expression of cellular prion protein in mouse mammary gland epithelial cells is able to modulate TGF-beta induced signal transduction leading to a synergistic increase of secreted MMP-2 activity. This correlates with elevated substrate detachment of cells grown as an epithelial monolayer as well as interfering with morphogenesis of cells cultured in a three-dimensional collagen type I matrix.

Functional Implication of Cellular Prion Protein in Antigen-Driven Interactions between T Cells and Dendritic Cells

The cellular prion protein (PrPC) is a host-encoded, GPI-anchored cell surface protein, expressed on a wide range of tissues including neuronal and lymphoreticular cells. PrPC may undergo posttranslational conversion, giving rise to scrapie PrP, the pathogenic conformer considered as responsible for prion diseases. Despite intensive studies, the normal function of PrPC is still enigmatic. Starting from microscope observations showing an accumulation of PrPC at the sites of contact between T cells and Ag-loaded dendritic cells (DC), we have studied the contribution of PrPC in alloantigen and peptide-MHC-driven T/DC interactions. Whereas the absence of PrPC on the DC results in a reduced allogeneic T cell response, its absence on the T cell partner has no apparent effect upon this response. Therefore, PrPC seems to fulfill different functions on the two cell partners forming the synapse. In contrast, PrPC mobilization by Ab reduces the stimulatory properties of DC and the proliferative potential of responding T cells. The contrasted consequences, regarding T cell function, between PrPC deletion and PrPC coating by Abs, suggests that the prion protein acts as a signaling molecule on T cells. Furthermore, our results show that the absence of PrPC has consequences in vivo also, upon the ability of APCs to stimulate proliferative T cell responses. Thus, independent of neurological considerations, some of the evolutionary constraints that may have contributed to the conservation of the Prnp gene in mammalians, could be of immunological origin.

Modulation of TGF-β signaling by EGF-CFC proteins

Members of the transforming growth factor-beta (TGF-beta) family of ligands exhibit potent growth-suppressive and/or apoptosis-inducing effects on different types of cells. They perform essential roles in the elimination of damaged or abnormal cells from healthy tissues. On the other hand, TGF-betas have also been shown to act as tumor-promoting cytokines in a number of malignancies that are capable of stimulating extracellular matrix production, cell migration, invasion, angiogenesis, and immune suppression. Dissecting the complex, multifaceted roles of different TGF-beta-related peptides especially during the development of pathological conditions and in carcinogenesis is an area of continuous research and development. The characterization of EGF-CFC proteins as essential co-receptors that contribute to the modulation of the physiological activities of some of the TGF-beta ligands will be beneficial for future medical research and the adaptation and possible readjustment of currently applied therapeutic regimes.