MATERIALS AND METHODS

Introduction of delayed neuronal cell death in gerbil brain

Delayed neuronal cell death was introduced in Mongolian gerbils (20–24 weeks old) as described previously (Matsuyama et al., 1993). Briefly, after the animals were lightly anesthetized with inhaled ether, a bilateral cerebral ischemia was produced by bilateral occlusion of the common carotid arteries. Body temperature was maintained at 36.5°C to 37°C during the ischemic insult, and for at least 1 hour following the ischemic period. Animals (n = 5 per group) were divided into a sham-operated group (control), a group that received a single ischemic insult for either 2 or 5 minutes, and a group that received a 2-minute ischemic insult followed by 5-minute ischemia, which was performed 24 hours after the first insult.

Histochemistry

Twenty-four hours after the final ischemic insult, the animals were perfused with a 4% paraformaldehyde solution under deep anesthesia, and 20-mm-thick coronal brain sections were cut using a vibratome. Sections were processed for immunohistochemistry using either monoclonal mouse anti–microtubule-associated protein 2 (MAP2; 1:1,000 dilution; Sigma, St. Louis, MO, U.S.A.) or rabbit anti-ORP150 antibody (5 g/mL) (Matsushita et al., 1998). For detection of the administered adenovirus containing LacZ, immunocytochemistry for -galactosidase was performed using a polyclonal rabbit antibody (1:1,000 dilution; ICN-Cappel, Aurora, OH, U.S.A.). The binding site of the primary antibody was visualized by avidin-biotin-peroxidase complex method, followed by the semiquantitative analysis (Matsuyama et al., 1993). In brief, the number of ORP150 immunoreactive neurons was counted in the CA1 pyramidal layer by a blind observer (12 slices per animal, n = 5 per group). Results were presented as percentage of total pyramidal neurons.

In situ hybridization

Animals were divided into control, 2-minute ischemia, and 5-minute ischemia groups (n = 5 per group). In situ hybridization analysis was performed on the ischemic gerbil brain during the 24-hour reperfusion period as described previously (Matsushita et al., 1998). In brief, a rat ORP150 cDNA fragment (151–381) was subcloned into the EcoRV site of the pBluescript KS vector (Stratagene Inc., La Jolla, CA, U.S.A.). After linearization of the plasmid, digoxigenin-labeled single-stranded RNA was synthesized by T7 and T3 RNA polymerase with digoxigenin-UTP as well as unlabeled ATP, GTP, and CTP. Hybridization of ORP150 mRNA was performed at 50°C for 16 hours, and signals were localized using the Nucleic Acid Detection Kit (Roche Molecular Biochemicals, Mannheim, Germany).

Cell culture of hippocampal neurons

Cultured hippocampal neurons were prepared from newborn C57BL/6 mice (Japan SLC, Inc.) within 1 day of birth, as described previously (Kitao et al., 2001). In brief, the hippocampus was separated and incubated in calcium- and magnesium-free Hank's balanced salt solution containing papain (0.2%). Cells were plated in plastic wells precoated with poly-L-lysine (10 g/mL), and the cultures were then maintained for 3 days in minimum essential medium supplemented with heat-inactivated fetal calf serum (10%), glucose (30 mmol/L), L-glutamine (2 mmol/L), pyruvate (1 mmol/L), KCl (20 mmol/L), sodium bicarbonate (10 mmol/L), and HEPES (1 mmol/L, pH = 7.2).

Construction of adenovirus and their applications

A replication-deficient recombinant adenovirus containing the cDNA for ORP150 was constructed according to the COS/TPC method as described elsewhere (Miyake et al., 1996). Control adenovirus carrying -galactosidase (Ad/LacZ) and GFP (Ad/GFP) were obtained from The Institute of Physical and Chemical Research (Tokyo, Japan).

In primary hippocampal neuron culture, infection was carried out by adding recombinant Ad(s) to serum-containing medium. The cells were incubated at 37°C for 60 minutes with constant agitation. The medium was changed, and the cells were further incubated at 37°C for 48 hours before exposure to hypoxia or treatment with chemical materials (Tamatani et al., 1999).

In the gerbil ischemic model, a mixture of Ad/ORP150S and Ad/LacZ (5 10⁷ pfu each) was directly administered into the gerbil CA1 region as described previously (Yagi et al., 2000) 5 days prior to the ischemic insult. As the control experiment, Ad/LacZ alone (10⁸ pfu) was administered in the CA1 of the other hemisphere in the same animal.

Assessment of cell death in vitro

Cell viability after hypoxic or chemical challenge was assessed with morphologic criteria (Yu et al., 1999). In brief, a neuron with intact neurites and cell body that was smooth and round-to-oval was considered viable. A neuron with fragmented neurites and a cell body that was shrunken and rough in appearance was considered nonviable.

Assessment of cell death in vivo

Neuronal cell viability was assessed in five adenovirus-treated animals according to a previously reported method, with some modification (Hoehn et al., 2001). Seven days after the ischemic onset, gerbil brain was perfusion-fixed with 4% PFA and postfixed with 4% PFA for 1 to 2 days. Twenty-micrometer frozen sections in the coronal plane were taken at 100-m increments 1 mm anterior and posterior to the needle track. Slices were first subjected to immunohistochemistry for -galactosidase to select the slice that showed the maximal immunoreactivity in CA1 pyramidal layer, and the region of interest (ROI)—a 1-mm span in the pyramidal layer of the slice where -galactosidase showed the maximal spread—was determined. The adjacent section was stained with MAP2 antibody, and MAP2-positive neurons were counted by a blinded observer in 100-m spans from the center to the lateral or central direction in the ROI. Data are shown as both peak and mean number of MAP2-positive neurons in the ROIs.

Western blot

Levels of ORP150 antigen in brain tissue or primary neuron culture were determined by immunoblotting as described previously (Kuwabara et al., 1996;Matsushita et al., 1998). Hippocampal tissue obtained from the four groups of animals (three independent experiments in duplicate) was separated 24 hours after the last ischemic insult, homogenated in 1 mL phosphate-buffered saline (PBS) containing NP-40 (1%), and subjected to Western blot analysis. Where indicated, Western blotting with anti-KDEL (StressGen biotechnologies) or anti–-actin (Sigma, St. Louis) monoclonal antibody was used to assess the levels of GRP78, GRP94, or -actin, as described previously (Ozawa et al., 2001;Tamatani et al., 1999). The ORP150 level in brain tissue was semiquantitatively determined by densitometry analysis. In each case, total protein content in the mixture was determined by the method of Lowry et al. (1951).

Measurement of intracellular calcium level

Measurement of the intracellular calcium in cultured neurons was performed as described previously (Wahl et al., 1989). After infection with the adenovirus, cultured hippocampal neurons in 96-well plates (3 10⁵ cells) were washed three times in Locke's buffer and then incubated at 20°C for 15 minutes with 25 L Fluo-3/AM (final concentration, 20 mol/L) dissolved in HEPES-buffered saline (20 mol/L, pH = 7.4). Cells were then washed with HEPES-buffered saline and the fluorescence was recorded at 37°C with a Fluoroscan II (Labsystems) microtiter fluorometer using fluoroscan filter pairs (excitation wavelength, 485 nm; emission wavelength, 533 nm).

The calibration was performed on cells incubated with the ionophore A-23187 (10 mol/L) to obtain the Fmax and with CaCl₂ (2 mmol/L) and A-23187 (10 mol/L) dissolved in NaCl (0.9%) to obtain the Fmin. Cytosolic-free Ca²⁺ was calculated by the following formula:

Activation of cathepsin B activity

Activation of Ca2⁺-dependent proteinase was assessed as described elsewhere (Tsuchiya et al., 1999). To evaluate cathepsin B activity, the culture homogenate in PBS (10 L) was incubated in sodium acetate buffer (2.5 L, 400 mmol/L, pH = 5.5) containing cysteine (8 mmol/L) at 37°C for 5 minutes. Synthetic peptide substrate, Z-Arg-Arg-AMC (1 mmol/L; Peptide Research Institute, Japan), was added and the mixture was incubated for a further 5 minutes at 37°C. The reaction was terminated by the addition of SDS (5% final concentration) to the sample and dilution in Tris-HCl (2 mL, 100 mmol/L). Release of 7-amino-4-methylcoumarin (AMC) was measured by fluorescence spectrophotometry using the wavelengths of 370 nm (excitation) and 460 nm (emission). Where indicated, an inhibitor of -calpain (ZLLLal, 10 mol/L;Tsubuki et al., 1996) was added to the mixture prior to the assay.

Data analysis

Statistical analysis was performed using the nonpaired t-test or ANOVA followed by multiple comparison analysis using the Newman-Keuls equation. Where indicated, the data were analyzed by a two-way ANOVA followed by the multiple contrast analysis. For nonparametric data, Kruskal-Wallis analysis or ² analysis was applied.

RESULTS

Expression of ORP150 in gerbil hippocampus

The basal expression of ORP150 in hippocampus of control mice was low, and its expression in animals subjected to a 2-minute temporal occlusion became evident (an approximate fivefold increase compared with controls). In animals treated with a 5-minute ischemia, the expression was decreased compared with 2-minute ischemia, whereas pretreatment of the animals with a nonlethal ischemia (2 minutes, in this case) enhanced the expression of ORP150 antigen after a 5-minute occlusion (an approximate eightfold increase compared with controls) (Fig. 1A and Fig. 1B). Immunohistochemical analysis using an anti-ORP150 antibody revealed the enhanced expression of ORP150 antigen in pyramidal cells (Fig. 1C–Fig. 1G), and this was in parallel with the observations by Western blot analysis. In situ hybridization analysis using a cRNA probe demonstrated the expression of ORP150 transcripts following the ischemic insult. At the basal level, the ORP150 transcript was only slightly expressed in pyramidal cells of the CA1 region (Fig. 1H and Fig. 1K). The expression of ORP150 message was more clearly detected after a 2-minute occlusion of the carotid artery (Fig. 1I and Fig. 1L), and it was most remarkable after a 5-minute occlusion (Fig. 1J and Fig. 1M).

Figure 1.

Expression of ORP150 in the gerbil hippocampus after an ischemic insult. (A and B) Hippocampus tissues of male mongolian gerbils were prepared 24 hours after the ischemic insult for 0 minutes (control lanes), 2 minutes (2-minute lanes), 5 minutes (5-minute lanes) and 2 minutes followed by 5-minute occlusion (2-minute + 5-minute lanes), and protein extract was then subjected to Western blot analysis using anti-ORP150 antibody or anti–-actin antibody. A typical example of the blot is shown in panel A (two animals at each time point). Semiquantitative analysis is shown in panel B, where the levels of ORP150 intensity are represented as percent increase against control (N = 6, three independent experiments in duplicate, mean + SD; * P < 0.05, ** P < 0.01 by multiple comparison analysis compared with control). (C–M) Male gerbils were subjected to temporal brain ischemia by a bilateral occlusion of the carotid artery for either 0 minutes (D, H, K), 2 minutes (E, I, L), 5 minutes (F, J, M), or 2 minutes followed by 5-minute occlusion (G). Brain sections were subjected to immunohistochemical analysis and ORP150-positive neurons are demonstrated as percentage of total pyramidal cells in CA1 (C, N = 5, mean + SD; * P < 0.05, ** P < 0.01 by multiple comparison analysis compared with control). (D–G) Typical examples of immunostaining in CA1. (H–M) brain sections were also subjected to in situ hybridization using an antisense-ORP150 probe. Typical example of five repeated experiments is shown for the entire hippocampus (H–J) and for CA1 (K–M). Open and closed bars in each panel represent 1 mm and 0.1 mm, respectively. ORP150, 150-kd oxygen-regulated protein.

Full figure and legend (298K)

Expression of ORP150 in cultured hippocampal neurons

These in vivo observations suggest a possible relationship between ORP150 expression and the tolerance of neurons in DND. In cultured neurons, both exposure to hypoxia and treatment with glutamate induced the expression of ORP150 (Fig. 2A). In each case, pretreatment with cycloheximide (0.1 g/mL), which suppresses protein synthesis as assessed by the incorporation of ³H-leuscine (by 30% compared with control cultures), inhibited the expression of ORP150 (Fig. 2A, lanes Cx + Hypoxia and Cx + Glutamate). At the same time, pretreatment of cultures with cycloheximide accelerated the cell death under both glutamate and hypoxia (Fig. 2B and Fig. 2C), suggesting a central role for protein synthesis in the stress response of neurons to hypoxia and glutamate.

Figure 2.

Inhibition of protein synthesis and neuronal cell death in vitro. Hippocampal neurons were prepared from newborn mice and cultured on six-well plates (10⁴ cells). (A) Cells were exposed to hypoxia for 24 hours or to glutamate (20 mol/L), either in the presence or absence of cycloheximide (0.1 g/mL; Cx+) and harvested. The protein extract (5 g/lane) was subjected to Western blotting followed by staining with anti-ORP150 antibody. Cultures were exposed either to hypoxia (B) or to 0 to 100 mol/L glutamate (C) in the presence (closed bars) or absence (open bars) of cycloheximide (0.1 g/mL). Neuronal cell viability was assessed morphologically 24 hours after glutamate treatment (B) or at the indicated time points in the hypoxic culture (C). In each case, N = 6 (three independent experiments performed in duplicate), and the mean + SD is shown. ** P < 0.01 by multiple contrast analysis compared with noncycloheximide-treated cultures.

Full figure and legend (118K)

To focus on the mechanism of ORP150 induction in cultured neurons, the expression of ORP150 was assessed after the sublethal chemical stimuli. Exposure of cultured hippocampal neurons to both hypoxia and glutamate induced the expression of ORP150, which reached a maximum 8 to 12 hours after the onset of stimuli (Fig. 3A to Fig. 3I, III). Exposure of cultures to other chemical stimuli including breferdin A (5 ng/mL;Fig. 3Ato Fig. 3I, Fig. 3III) and thapsigargin (0.3 g/mL;Fig. 3A–V), which cause ER-stress (Low et al., 1991;Yu et al., 1999), also induced the expression of ORP150. In contrast, other chemical stressors including NaAs (100 mol/L;Uto et al., 1995), staurosporine (1 mol/L;Deshmukh and Johnson, 2000), and hydrogen peroxide (10 mol/L;See and Loeffler, 2001), failed to induce ORP150 in cultured neurons (Fig. 3A–V, Fig. 3VI, Fig. 3VII). Several doses of chemical materials were added to hippocampal neurons and the similar results were obtained (Fig. 3B). These data suggest that expression of ORP150 in cultured neurons represents their response to maintain the function of the ER. The expression of ORP150 in the ischemic hippocampus, therefore, represents the existence of environmental alterations in ischemic hippocampus, which, at least in part, cause ER stress.

Figure 3.

Expression of ORP150 in cultures neurons. (A) Hippocampal neurons were exposed to hypoxia (A-I), or incubated at normoxic condition in the presence of either breferdin A (5 ng/mL, A-II), glutamate (50 mol/L, A-III), thapsigardin (0.3 g/mL, A-IV), sodium arsenate (NaAs, 100 mol/L, A-V), staurosporine (1 mol/L, A-VI), or hydrogen peroxide (H₂O₂, 10 mol/L, A-VII) for indicated time. (B) Hippocampal neurons (about 10⁵ cells) were incubated at normoxic condition in the presence of either breferdin A (0–10 ng/mL) for 4.5 hours (B-I), glutamate (0–100 mol/L) for 6 hours (B-II), thapsigardin (0–1.0 g/mL) for 6 hours (B-III), sodium arsenate (0–200 mol/L) for 6 hours (B-IV), staurosporine (0–2.0 mol/L) for 1.5 hours (B-V), or hydrogen peroxide (0–20 mol/L) for 3 hours (B-VI). Cells were harvested and protein extract was subjected to Western blot analysis by using anti-ORP150 or anti–-actin antibody. ORP150, 150-kd oxygen-regulated protein.

Full figure and legend (199K)

Role of ORP150 in cultured neurons under endoplasmic reticulum stress

To further identify the role of ORP150 in ER stress of cultured neurons, cultures were infected with adenovirus to overexpress sense ORP150 (Ad/ORP150S), antisense ORP150 (Ad/ORP150AS) (Tamatani et al., 2001), -galactosidase (Ad/LacZ), or GFP (Ad/GFP). Immunostaining of MAP2 in infected cultures showed that more than 90% of neuronal cells were GFP positive (data not shown) at the concentration of 50 multiplicities of infection (moi). Infection of neurons with Ad/ORP150S increased the ORP150 antigen in cultured neurons, while treatment of cultures with Ad/ORP150AS decreased the ORP150 antigen level compared with control (Fig. 4A, upper panel). In each case, the levels of other molecular chaperons in the ER (78-kd and 94-kd GRP78 and GRP94, respectively) remained unchanged (Fig. 4A, lower panel). Further, treatment of the culture with either LacZ or GFP adenovirus did not alter the levels of ORP150, GRP94, or GRP78 (Fig. 4A, right panel), suggesting that the ORP150 sense/antisense structure can specifically alter the level of ORP150 antigen in cultured neurons.

Figure 4.

Overexpression of ORP150 rescues neurons from ER stress. (A) Cultured hippocampal neurons (10⁵ cells) were incubated with the indicated amounts of Ad/ORP150S, Ad/ORP150AS, Ad/LacZ, or Ad/GFP. Protein extract (5 g/lane) was subjected with Western blotting using either an anti-ORP150 antibody (upper panel) or an anti-KDEL antibody (lower panel), the latter of which recognizes GRP78 and GRP94. (B and C) Cultures were infected with Ad/ORP150S (open bars), Ad/LacZ (shaded bars), or Ad/ORP150AS (closed bars; 50 multiplicities of infection [moi] in each case). (B) Hippocampal neurons (10⁵ cells) were exposed to hypoxia, or incubated at normoxic condition in the presence of breferdin A (5 ng/mL), glutamate (5 mol/L), thapsigardin (0.3 g/mL), sodium arsenate (NaAs, 100 mol/L), staurosporine (1 mol/L), or hydrogen peroxide (H₂O₂, 10 mol/L) for the indicated time. (C) Cells (10⁵) were incubated at normoxic condition in the presence of either breferdin A (0–10 ng/mL) for 9 hours, glutamate (0–100 mol/L) for 12 hours, thapsigardin (0–1.0 g/mL) for 12 hours, sodium arsenate (0–200 mol/L) for 12 hours, staurosporine (0–2.0 mol/L) for 3 hours, or hydrogen peroxide (0–20 mol/L) for 6 hours. After incubation, cell viability was assessed by morphologic criteria. In each case, N = 6 (three independent experiments performed in duplicate), and the mean + SD is shown. * P < 0.05 and ** P < 0.01 by multiple contrast analysis compared with Ad/LacZ-treated cultures. (D and E) After neurons were infected with Ad/ORP150S (open diamond), Ad/LacZ (closed circle, 100 moi), or Ad/ORP150AS (open circle), the intracellular Ca2⁺ ([Ca²⁺]i) was measured by fluorescence of Fluo-3 (D). Proteolytic activity was also measured 8 hours after the treatment with phosphate-buffered saline (control), glutamate (5 mol/L), or glutamate/ZLLLal (5/10 mol/L) in each infected culture (E). In each case, N = 6 and the mean + SD is shown. ** P < 0.01 by multiple comparison analysis compared with Ad/LacZ-treated cultures. ORP150, 150-kd oxygen-regulated protein; ER, endoplasmic reticulum.

Full figure and legend (233K)

Exposure of cultured neurons to hypoxia, breferdin A, glutamate, or thapsigargin, all of which induced ORP150, caused neuronal cell death. Overexpression of ORP150 with Ad/ORP150S suppressed the neuronal cell death, whereas suppression of ORP150 with Ad/ORP150AS accelerated neuronal cell death in these paradigms (Fig. 4B I–IV, Fig. 4C I–III). In contrast, these adenoviruses had no effect on the neuronal cell death induced by other chemical stimuli (NaAs, staurosporine, and hydrogen peroxide) (Fig. 4B V–VII, Fig. 4C IV–VI), suggesting that cell death caused by other chemical stresses occurs independent of ORP150. On the other hand, overexpression of ORP150 suppressed the glutamate-mediated increase of intracellular Ca²⁺ in a dose-dependent manner, whereas the antisense adenovirus treatment caused a reverse effect (Fig. 4D). Furthermore, treatment with glutamate resulted in a rise in cathepsin B activity most prominently in neurons infected with Ad/ORP150AS, whereas neurons infected with Ad/LacZ showed a diminished response, and neurons infected with Ad/ORP150S showed only an approximate threefold increase in cathepsin B activity (Fig. 4E). These data suggest a link between the protective role of ORP150 under stress and the maintenance of ER function.

Overexpression of ORP150 rescues neurons from delayed neuronal cell death

To further investigate the role of ORP150, neural viability in the hippocampus after infection of the ORP150 adenovirus (Ad/ORP150S) was assessed in delayed neuronal death. Immunostaining with anti–-galactosidase revealed expression of infected gene in hippocampus CA1 region in both cases (i.e., treatment with Ad/LacZ alone;Fig. 5A and Fig. 5B) and with a combination of Ad/LacZ and Ad/ORP150S (Fig. 5F and Fig. 5G), suggesting the successful infection with adenovirus vector in both cases. Immunostaining of adjacent section using an anti-MAP2 antibody revealed the almost complete loss of MAP2 signals of hippocampus pretreated with Ad/LacZ (Fig. 5C–Fig. 5E), whereas treatment of Ad/ORP150S resulted in survival of pyramidal neurons after the ischemic insult (Fig. 5H–Fig. 5J). Quantitative analysis of repeated experiments revealed that this improvement of neuronal cell viability by the overexpression of ORP150 was statistically significant (Fig. 5K). Taken together, these data demonstrate that the overexpression of ORP150 in hippocampal neurons by local administration of adenovirus can rescue neurons from DND.

Figure 5.

Overexpression of ORP150 rescues neuronal cell death in the hippocampus. Ad/LacZ alone (10⁸ pfu in 2 L phosphate-buffered saline (PBS;A–E), or a combination of Ad/LacZ and Ad/ORP150S (5 10⁷ pfu each in 2 L PBS;F–J) were administrated directly into the each hemisphere of the hippocampi. Five days after the infection, animals were subjected to bilateral occlusion of the common carotid artery for 5 minutes. One week after the ischemic insult, brain sections were subjected to -galactosidase immunostaining (A, B, F, G) to determine regions of interest (ROIs; 1-mm wide in CA1 region) as described in text. Adjacent sections were then stained with anti-MAP2 antibody (C, D, H, I) and MAP2-positive neurons were counted from the center of the ROIs by a 100-m width in both the medial and lateral directions (E and J). Note that areas indicated by open box in panels C and H are magnified in panels D and I, respectively. (K) Results of statistical analysis after five repeated experiments. Neuronal cell viability is shown as a peak number (K-I) or mean number (K-II) of MAP2-immunoreactive cells within the ROIs in gerbils treated either with Ad/LacZ only (open bars) or with a combination of Ad/LacZ and Ad/ORP150S (closed bars). Mean + SD is shown (N = 5). * P < 0.01 by nonpaired t-test. ORP150, 150-kd oxygen-regulated protein; MAP2, microtubule-associated protein 2.

Full figure and legend (298K)

DISCUSSION

Delayed neuronal cell death and ischemic tolerance in hippocampus CA1 of gerbil brain are well-known phenomena; however, the molecular mechanisms involved are not yet fully delineated. To determine the function of ORP150 in DND of gerbil brain, we performed a line of experiments. We showed that ORP150 is induced by transient ischemia in CA1 pyramidal neurons. However, the expression of ORP150 antigen was less induced by 5-minute ischemic insult than by 2-minute ischemia, whereas ORP150 message is strongly induced. This finding is consistent with those of previous reports that an overall suppression of protein synthesis occurs in DND (Hossmann, 1993) and that some stress proteins, including ubiquitin (Nowak, 1991), HSP27, HSP70, HSP90 (Wagstaff et al., 1996), and HSP40 (Tanaka et al., 2002), are upregulated only at the message level. Protein synthesis of ORP150 was restored in CA1 by 2-minute ischemia followed by 5-minute occlusion (Fig. 1) in a manner similar to that shown in the cases of other stress responders. ORP150 may rescue neurons from DND by relieving ER stress, which plays a pivotal role in the suppression of protein synthesis (DeGracia et al., 2002). These data suggest the possibility that ORP150 is one of the factors responsible for the acquisition of ischemic tolerance.

We demonstrated that the exposure of hippocampal neurons to hypoxia, glutamate, and other reagents that cause ER stress resulted in the expression of ORP150, whereas no such response was observed with other stressors. Further, the overexpression of ORP150 by adenovirus improved the cellular viability in neurons to glutamate and other ER stressors, whereas its suppression accelerated neuronal cell death, suggesting the protective role of ORP150 in neurons following glutamate exposure. These results are consistent with those of our previous study (Kitao et al., 2001;Tamatani et al., 2001), and indicate that the neuroprotective effect of ORP150 is not directly involved in reactive oxygen species or caspase activation. Moreover, we demonstrated that overexpression of ORP150 by adenovirus injection to hippocampus also suppressed the DND of CA1 pyramidal neuron. Taking these findings into consideration, it is possible that transient ischemia might damage the ER and that the ER dysfunction may, at least in part, lead to DND.

We have already demonstrated that the chaperone function of ORP150 can rescue neurons from hypoxia-mediated cell death, by an eventual suppression of caspase-3–like activity (Tamatani et al., 2001). On the other hand, our present data indicate the possible ability of ORP150 to maintain Ca²⁺ metabolism in the ER under glutamate excitotoxicity, which may be more prolonged in ischemic conditions (Mitani and Kataoka, 1991), by suppressing Ca²⁺ dependent proteolysis activity (Fig. 4C and Fig. 4D). ORP150 shares several components similar to GRP78, the 78-kd GRP, which is a molecular chaperon located in the ER. GRP78 is also induced by glutamate stress in neurons, and suppression of GRP78 increased the vulnerability of neurons to glutamate (Yu et al., 1999). GRP78 protects neurons through its ability to maintain Ca²⁺ homeostasis, which is consistent with the finding that GRP78 can bind to Ca²⁺, although the mechanism has not yet been clearly demonstrated (Lievremont et al., 1997). Furthermore, ORP150 is a homologue of CPB140, a 140-kd calcium-binding protein (Naved et al., 1995), which was identified as an ER protein with the capacity to bind Ca²⁺. These reports are consistent with our finding that ORP150 is involved in calcium homeostasis in neurons exposed to glutamate.

Although increasing evidence suggests that an active process similar to programmed cell death contributes to DND in gerbil, the mechanism of neuronal death still remains controversial. It has been suggested, mainly based on biochemical criteria, that postischemic hippocampal CA1 damage is classified as apoptosis (Honkaniemi et al., 1996), though the typical patterns of apoptotic morphologic change often do not appear in DND (Colbourne et al., 1999). Even in the cultured neurons, cell death caused by hypoxia or chemical stresses may include both apoptotic and necrotic pathways. In this report, therefore, viability of hippocampal neuron culture was assessed by morphologic criteria, and apoptotic cell death was not evaluated.

Although there still exists a gap between the function of ORP150 in cultured neurons and the role of this stress protein in the brain ischemia, our data suggest that the maintenance of ER function may be a novel target for the strategies of intervention medicine against ischemic neuronal cell death.

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