Calreticulin comes of age

Lysosomal biogenesis and function is critical for necrotic cell death in Caenorhabditis elegans

Necrotic cell death is defined by distinctive morphological characteristics that are displayed by dying cells (Walker, N.I., B.V. Harmon, G.C. Gobe, and J.F. Kerr. 1988. Methods Achiev. Exp. Pathol. 13:18–54). The cellular events that transpire during necrosis to generate these necrotic traits are poorly understood. Recent studies in the nematode Caenorhabditis elegans show that cytoplasmic acidification develops during necrosis and is required for cell death (Syntichaki, P., C. Samara, and N. Tavernarakis. 2005. Curr. Biol. 15:1249–1254). However, the origin of cytoplasmic acidification remains elusive. We show that the alkalization of endosomal and lysosomal compartments ameliorates necrotic cell death triggered by diverse stimuli. In addition, mutations in genes that result in altered lysosomal biogenesis and function markedly affect neuronal necrosis. We used a genetically encoded fluorescent marker to follow lysosome fate during neurodegeneration in vivo. Strikingly, we found that lysosomes fuse and localize exclusively around a swollen nucleus. In the advanced stages of cell death, the nucleus condenses and migrates toward the periphery of the cell, whereas green fluorescent protein–labeled lysosomal membranes fade, indicating lysosomal rupture. Our findings demonstrate a prominent role for lysosomes in cellular destruction during necrotic cell death, which is likely conserved in metazoans.

A novel role for an insect apolipoprotein (apolipophorin III) in beta-1,3-glucan pattern recognition and cellular encapsulation reactions

Lipoproteins and molecules for pattern recognition are centrally important in the innate immune response of both vertebrates and invertebrates. Mammalian apolipoproteins such as apolipoprotein E (apoE) are involved in LPS detoxification, phagocytosis, and possibly pattern recognition. The multifunctional insect protein, apolipophorin III (apoLp-III), is homologous to apoE. In this study we describe novel roles for apoLp-III in pattern recognition and multicellular encapsulation reactions in the innate immune response, which may be of direct relevance to mammalian systems. It is known that apoLp-III stimulates antimicrobial peptide production in insect blood, enhances phagocytosis by insect blood cells (hemocytes), and binds and detoxifies LPS and lipoteichoic acid. In the present study we show that apoLp-III from the greater wax moth, Galleria mellonella, also binds to fungal conidia and beta-1,3-glucan and therefore may act as a pattern recognition molecule for multiple microbial and parasitic invaders. This protein also stimulates increases in cellular encapsulation of nonself particles by the blood cells and exerts shorter term, time-dependent, modulatory effects on cell attachment and spreading. All these responses are dose dependent, occur within physiological levels, and, with the notable exception of beta-glucan binding, are only observed with the lipid-associated form of apoLp-III. Preliminary studies also established a beneficial role for apoLp-III in the in vivo response to an entomopathogenic fungus. These data suggest a wide range of immune functions for a multiple specificity pattern recognition molecule and may provide a useful model for identifying further potential roles for homologous proteins in mammalian immunology, particularly in terms of fungal infections, pneumoconiosis, and granulomatous reactions.

Calreticulin--an endoplasmic reticulum protein with calcium-binding activity is also found in the extracellular matrix

Previous studies have reported that calreticulin (CRT), a calcium-binding and chaperoning protein, is expressed only in the endoplasmatic reticulum, nucleus and at the cell surface. In this study we clearly show that odontoblasts and predentin matrix contain CRT. To our knowledge, this is the first time CRT has been described in the extracellular matrix. The expression of CRT was studied by immunohistochemistry, ultrastructural immunocytochemistry and in situ hybridization in developing rat teeth. CRT was detected as a 59-kDa protein in rat pulp cell culture medium and dentin extracellular matrix extract by Western blotting. The presence of the protein was shown in rat odontoblasts and predentin with immunohistochemistry. At the ultrastructural level, the labeling was distributed in the rat odontoblasts, ameloblasts and predentin. Northern blotting showed the presence of CRT mRNA in rat molars, which was confirmed by in situ hybridization in odontoblasts and ameloblasts. We now present the first convincing evidence that CRT is found in extracellular matrix where it may play an important role in mineralization.

Death by necrosis. Uncontrollable catastrophe, or is there order behind the chaos?

Cells suffer necrotic death when exposed to extreme environmental conditions, adverse and excessive stimuli, or when deleterious mutations are encoded in their genetic material. Unlike apoptosis, which involves a highly regulated and elaborate network of biochemical events and cascades, necrosis has been considered generally to be a chaotic decadence process that effects the inexorable demise of cells otherwise not destined to die. This grim prospect is now slowly being overturned, mostly by exciting new findings in two simple model organisms, Caenorhabditis elegans and Drosophila melanogaster. Despite the wide spectrum of necrosis-initiating conditions, evidence is accumulating that execution of necrotic or neurodegenerative cell death may be carried out by a finite common set of mechanisms.

Necrotic cell death in C. elegans requires the function of calreticulin and regulators of Ca(2+) release from the endoplasmic reticulum

In C. elegans, a hyperactivated MEC-4(d) ion channel induces necrotic-like neuronal death that is distinct from apoptosis. We report that null mutations in calreticulin suppress both mec-4(d)-induced cell death and the necrotic cell death induced by expression of a constitutively activated Galpha(S) subunit. RNAi-mediated knockdown of calnexin, mutations in the ER Ca(2+) release channels unc-68 (ryanodine receptor) or itr-1 (inositol 1,4,5 triphosphate receptor), and pharmacological manipulations that block ER Ca(2+) release also suppress death. Conversely, thapsigargin-induced ER Ca(2+) release can restore mec-4(d)-induced cell death when calreticulin is absent. We conclude that high [Ca(2+)](i) is a requirement for necrosis in C. elegans and suggest that an essential step in the death mechanism is release of ER-based Ca(2+) stores. ER-driven Ca(2+) release has previously been implicated in mammalian necrosis, suggesting necrotic death mechanisms may be conserved.

Glycoprotein folding in the endoplasmic reticulum

Our understanding of eukaryotic protein folding in the endoplasmic reticulum has increased enormously over the last 5 years. In this review, we summarize some of the major research themes that have captivated researchers in this field during the last years of the 20th century. We follow the path of a typical protein as it emerges from the ribosome and enters the reticular environment. While many of these events are shared between different polypeptide chains, we highlight some of the numerous differences between proteins, between cell types, and between the chaperones utilized by different ER glycoproteins. Finally, we consider the likely advances in this field as the new century unfolds and we address the prospect of a unified understanding of how protein folding, degradation, and translation are coordinated within a cell.