Morbillivirus Glycoprotein Expression Induces ER Stress, Alters Ca2+ Homeostasis and Results in the Release of Vasostatin

Although the pathology of Morbillivirus in the central nervous system (CNS) is well described, the molecular basis of neurodegenerative events still remains poorly understood. As a model to explore Morbillivirus-mediated CNS dysfunctions, we used canine distemper virus (CDV) that we inoculated into two different cell systems: a monkey cell line (Vero) and rat primary hippocampal neurons. Importantly, the recombinant CDV used in these studies not only efficiently infects both cell types but recapitulates the uncommon, non-cytolytic cell-to-cell spread mediated by virulent CDVs in brain of dogs. Here, we demonstrated that both CDV surface glycoproteins (F and H) markedly accumulated in the endoplasmic reticulum (ER). This accumulation triggered an ER stress, characterized by increased expression of the ER resident chaperon calnexin and the proapoptotic transcription factor CHOP/GADD 153. The expression of calreticulin (CRT), another ER resident chaperon critically involved in the response to misfolded proteins and in Ca2+ homeostasis, was also upregulated. Transient expression of recombinant CDV F and H surface glycoproteins in Vero cells and primary hippocampal neurons further confirmed a correlation between their accumulation in the ER, CRT upregulation, ER stress and disruption of ER Ca2+ homeostasis. Furthermore, CDV infection induced CRT fragmentation with re-localisation of a CRT amino-terminal fragment, also known as vasostatin, on the surface of infected and neighbouring non-infected cells. Altogether, these results suggest that ER stress, CRT fragmentation and re-localization on the cell surface may contribute to cytotoxic effects and ensuing cell dysfunctions triggered by Morbillivirus, a mechanism that might potentially be relevant for other neurotropic viruses

Serum-withdrawal-dependent apoptosis of hippocampal neuroblasts involves Ca(++) release by endoplasmic reticulum and caspase-12 activation

Apoptotic death caused by diseases or toxic insults is preceded and determined by endoplasmic reticulum dysfunction and altered intraluminar calcium homeostasis in many different cell types. With the present study we have explored the possibility that the ER stress could be involved also in apoptotic death induced by serum deprivation in neuronal cells. We have chosen as a model of study the cell line HN9.10e, constituted by immortalized hippocampal neuroblasts. The Ca(++) concentration in the lumen of the ER has been evaluated by using the low affinity Ca(++) probe Mag-fluo-4. We show that serum deprivation lowers the ER Ca(++) concentration with a time course closely related to the increase of apoptosis incidence. Serum deprivation also enhances the expression of a well-known marker of ER stress, the glucose-regulated protein-78 (GRP-78), a member of the heat shock/stress response protein family. Moreover, in serum-deprived neuroblasts, following GRP-78 up-regulation, the ER-associated procaspase-12 is cleaved with a time course which parallels the ER calcium loss while activation of caspase-3 is a later event. Depletion of ER Ca(++) by thapsigargin, a specific inhibitor of the ER-associated Ca(++) ATPase, also produces caspase-12 processing and apoptotic cell death, whereas agents capable of reducing the ER calcium loss protect the cells from serum-deprivation-induced apoptosis. These findings indicate that, in hippocampal neuroblasts, Ca(++) mobilization from ER and caspase-12 activation are components of the molecular pathway that leads to apoptosis triggered by serum deprivation and may constitute an amplifying loop of the mitochondrial pathway

Role of extracellular calcium and mitochondrial oxygen species in psychosine-induced oligodendrocyte cell death

Globoid cell leukodystrophy (GLD) is a metabolic disease caused by mutations in the galactocerebrosidase (GALC) gene. GALC is a lysosomal enzyme whose function is to degrade galacto-lipids, including galactosyl-ceramide and galactosyl-sphingosine (psychosine, PSY). GALC loss of function causes progressive intracellular accumulation of PSY. It is widely held that PSY is the main trigger for the degeneration of myelinating cells and progressive white-matter loss. However, still little is known about the molecular mechanisms by which PSY imparts toxicity. Here, we address the role of calcium dynamics during PSY-induced cell death. Using the human oligodendrocyte cell line MO3.13, we report that cell death by PSY is accompanied by robust cytosolic and mitochondrial calcium (Ca2+) elevations, and by mitochondrial reactive oxygen species (ROS) production. Importantly, we demonstrate that the reduction of extracellular calcium content by the chelating agent ethylenediaminete-traacetic acid can decrease intra-mitochondrial ROS production and enhance cell viability. Antioxidant administration also reduces mitochondrial ROS production and cell loss, but this treatment does not synergize with Ca2+ chelation. Our results disclose novel intracellular pathways involved in PSY-induced death that may be exploited for therapeutic purposes to delay GLD onset and/or slow down its progression

5’-Amino-4-imidazolecarboxamide riboside induces apoptosis in human neuroblastoma cells via the mitochondrial pathway

5'-Amino-4-imidazolecarboxamide (AICA) riboside induces apoptosis in neuronal cell models. In order to exert its effect, AICA riboside must enter the cell and be phosphorylated to the ribotide. In the present work, we have further studied the mechanism of apoptosis induced by AICA riboside. The results demonstrate that AICA riboside activates AMP-dependent protein kinase (AMPK), induces release of cytochrome c from mitochondria and activation of caspase 9. The role of AMPK in determining cell fate is controversial. In fact, AICA riboside has been reported to be neuroprotective or to induce apoptosis depending on its concentration, cell type or apoptotic stimuli used. In order to clarify whether the activation of AMPK is related to apoptosis in our model, we have used another AMPK stimulator, metformin, and we have analysed its effects on cell viability, nuclear morphology and AMPK activity. Five mM metformin increased AMPK activity, inhibited viability, and increased the number of apoptotic nuclei. AICA riboside, which can be generated from the ribotide (an intermediate of the purine de novo synthesis) by the action of the ubiquitous cytosolic 5'-nucleotidase (cN-II), may accumulate in those individuals in which an inborn error of purine metabolism causes both a building up of intermediates and/or an increase of the rate of de novo synthesis, and/or an overexpression of cN-II. Therefore, our results suggest that the toxic effect of AICA riboside on some types of neurons may participate in the neurological manifestations of syndromes related to purine dismetabolisms

Effect of 1alpha,25-dihydroxyvitamin D3 in embryonic hippocampal cells

Although the role of 1alpha,25-dihydroxyvitamin D3 in calcium homeostasis of bone tissue is clear, evidence of the involvement of vitamin D3 in the central nervous system functions is increasing. In fact, vitamin D3 regulates vitamin D receptor and nerve growth factor expression, modulates brain development, and reverses experimental autoimmune encephalomyelitis. Only few studies, however, address vitamin D3 effect on embryonic hippocampal cell differentiation. In this investigation, the HN9.10e cell line was used as experimental model; these cells, that are a somatic fusion product of hippocampal cells from embryonic day-18 C57BL/6 mice and N18TG2 neuroblastoma cells, show morphological and cytoskeletal features similar to their neuronal precursors. By this model, we have studied the time course of vitamin D3 localization in the nucleus and its effect on proteins involved in proliferation and/or differentiation. We found that the translocation of vitamin D3 from cytoplasm to the nucleus is transient, as the maximal nuclear concentration is reached after 10 h of incubation with (3)H-vitamin D3 and decreases to control values by 12 h. The appearance of differentiation markers such as Bcl2, NGF, STAT3, and the decrease of proliferation markers such as cyclin-1 and PCNA are late events. Moreover, physiological concentrations of vitamin D3 delay cell proliferation and induce cell differentiation of embryonic cells characterized by modification of soma lengthening and formation of axons and dendrites

Novel metabolic aspects related to adenosine deaminase inhibition in a human astrocytoma cell line

Adenosine deaminase, which catalyzes the deamination of adenosine and deoxyadenosine, plays a central role in purine metabolism. Indeed, its deficiency is associated with severe immunodeficiency and abnormalities in the functioning of many organs, including nervous system. We have mimicked an adenosine deaminase-deficient situation by incubating a human astrocytoma cell line in the presence of deoxycoformycin, a strong adenosine deaminase inhibitor, and deoxyadenosine, which accumulates in vivo when the enzyme is deficient, and have monitored the effect of the combination on cell viability, mitochondrial functions, and other metabolic features. Astrocytomas are the most common neoplastic transformations occurring in glial cell types, often characterized by a poor prognosis. Our experimental approach may provide evidence both for the response to a treatment affecting purine metabolism of a tumor reported to be particularly resistant to chemotherapeutic approaches and for the understanding of the molecular basis of neurological manifestations related to errors in purine metabolism. Cells incubated in the presence of the combination, but not those incubated with deoxyadenosine or deoxycoformycin alone, underwent apoptotic death, which appears to proceed through a mitochondrial pathway, since release of cytochrome c has been observed. The inhibition of adenosine deaminase increases both mitochondrial reactive oxygen species level and mitochondrial mass. A surprising effect of the combination is the significant reduction in lactate production, suggestive of a reduced glycolytic capacity, not ascribable to alterations in NAD+/NADH ratio, nor to a consumption of inorganic phosphate. This is a hitherto unknown effect presenting early during the incubation with deoxyadenosine and deoxycoformycin, which precedes their effect on cell viability