Cloning of rat grp75, an hsp70-family member, and its expression in normal and ischemic brain

Cell-cycle dependent tyrosine phosphorylation on mortalin regulates its interaction with fibroblast growth factor-1

We previously reported that endogenously expressed, intracellularly localized fibroblast growth factor (FGF)-1 interacts with mortalin. Here we report that FGF-1 added to the culture medium of quiescent BALB/c3T3 cells is taken up by the cells and interacts with mortalin in the cells in a regulated manner. Although both the internalized FGF-1 and mortalin were present at high levels throughout the FGF-1-initiated cell cycle, their interaction became apparent only in late G1 phase. Interestingly, mortalin was preferentially tyrosine phosphorylated at the same time, and when its normally weak phosphorylation in early G1 phase was augmented by treating the cells with vanadate, a strong interaction between mortalin and FGF-1 was established. Conversely, when phosphorylated mortalin was treated with tyrosine phosphatase, its interaction with FGF-1 was abrogated. These results indicate that FGF-1 taken up by cells preferentially interacts with mortalin in late G1 phase of the cell cycle, and that tyrosine phosphorylation of mortalin regulates this interaction.

Sustained induction of heme oxygenase-1 in the traumatized spinal cord

Oxidative stress contributes to secondary injury after spinal cord trauma. Among the consequences of oxidative stress is the induction of heme oxygenase-1 (HO-1), an inducible isozyme that metabolizes heme to iron, biliverdin, and carbon monoxide. Here we examine the induction of HO-1 in the hemisected spinal cord, a model that results in reproducible degeneration in the ipsilateral white matter. HO-1 was induced in microglia and macrophages from 24 h to at least 42 days after injury. Within the first week after injury, HO-1 was induced in both the gray and the white matter. Thereafter, HO-1 expression was limited to degenerating fiber tracts. HSP70, a heat shock protein induced mainly by the presence of denatured proteins, was consistently colocalized with HO-1 in the microglia and macrophages. This study to demonstrates long-term induction of HO-1 and HSP70 in microglia and macrophages after traumatic injury and an association between induction of HO-1 and Wallerian degeneration. White matter degeneration is characterized by phagocytosis of cellular debris and remodeling of surviving tissue. This results in the metabolism, synthesis, and turnover of heme and heme proteins. Thus, sustained induction of HO-1 and HSP70 in microglia and macrophages suggests that tissue degeneration is an ongoing process, lasting 6 weeks and perhaps even longer.

Extramitochondrial localization of mortalin/mthsp70/PBP74/GRP75

Subcellular fractionation and immunofluorescence microscopy were used to identify the specific sites of intracellular residence of mortalin, also called a mitochondrial homologue of the hsp70 family, in immortal human cell lines previously assigned to four distinct complementation groups (A-D) for indefinite cell division. In addition to the mitochondria it was seen in the endoplasmic reticulum (ER) fractions of all the cell lines analyzed. Interestingly, three of the group A cells lines (EJ, GM639, and HT1080), in addition to the mitochondria and ER, exhibited cytosolically (extra-organelle) localized pool of mortalin. These findings demonstrate that mortalin is not present exclusively in mitochondria. Its residence in different organelles may be the basis of differential distribution observed previously in different human cell lines.

Clostridium difficile toxin A causes early damage to mitochondria in cultured cells

BACKGROUND & AIMS

The mechanism by which Clostridium difficile toxin A causes actin depolymerization and cell rounding involves toxin internalization and subsequent monoglucosylation of the Rho family of proteins. This study explored toxin internalization and effects on mitochondrial function before cell rounding.

METHODS

Chinese hamster ovary (CHO) cells were exposed to toxin A, and mitochondrial localization was assayed by confocal microscopy. Mitochondrial function was measured by adenosine triphosphate (ATP) concentration, mitochondrial permeability, and leakage of cytochrome c.

RESULTS

Confocal microscopy showed toxin A colocalization with the mitochondrial protein GRP 75 at 5 minutes after toxin exposure. Between 5 and 15 minutes, toxin A caused an 80% diminution in cellular ATP levels; cell rounding and Rho glucosylation commenced between 15 and 30 minutes. Toxin A also resulted in reduction of mitochondrial membrane potential and a 2-3-fold increase in reactive oxygen radicals. Preincubation of CHO cells with the antioxidants butylated hydroxyanisole or butylated hydroxytoluene blocked the toxin A-induced increase in oxygen radicals and diminished cell rounding. Western blot analysis of toxin A-exposed isolated mitochondria showed a direct effect of toxin A on leakage of cytochrome c.

CONCLUSIONS

The results show that extensive mitochondrial damage occurs within 15 minutes in CHO cells exposed to toxin A. Diminished ATP concentrations and increased oxygen radicals are likely to contribute to cytotoxicity from this bacterial toxin.

Stress-associated endoplasmic reticulum protein 1 (SERP1)/Ribosome-associated membrane protein 4 (RAMP4) stabilizes membrane proteins during stress and facilitates subsequent glycosylation

Application of differential display to cultured rat astrocytes subjected to hypoxia allowed cloning of a novel cDNA, termed stress-associated endoplasmic reticulum protein 1 (SERP1). Expression of SERP1 was enhanced in vitro by hypoxia and/or reoxygenation or other forms of stress, causing accumulation of unfolded proteins in endoplasmic reticulum (ER) stress, and in vivo by middle cerebral artery occlusion in rats. The SERP1 cDNA encodes a 66-amino acid polypeptide which was found to be identical to ribosome-associated membrane protein 4 (RAMP4) and bearing 29% identity to yeast suppressor of SecY 6 protein (YSY6p), suggesting participation in pathways controlling membrane protein biogenesis at ER. In cultured 293 cells subjected to ER stress, overexpression of SERP1/RAMP4 suppressed aggregation and/or degradation of newly synthesized integral membrane proteins, and subsequently, facilitated their glycosylation when the stress was removed. SERP1/RAMP4 interacted with Sec61alpha and Sec61beta, which are subunits of translocon, and a molecular chaperon calnexin. Furthermore, Sec61alpha and Sec61beta, but not SERP1/RAMP4, were found to associate with newly synthesized integral membrane proteins under stress. These results suggest that stabilization of membrane proteins in response to stress involves the concerted action of a rescue unit in the ER membrane comprised of SERP1/RAMP4, other components of translocon, and molecular chaperons in ER.

Involvement of c-Jun NH(2)-terminal kinase pathway in differential regulation of heat shock proteins by anticancer drugs

In the present study, we examined the modulation of heat shock factor 1 (HSF1) activity and expression of heat shock proteins (HSPs) after exposure to anticancer drugs. Anticancer drugs induced HSF1 DNA-binding activity, and this was followed by an increase of mitochondrial HSP75 and HSP60 levels and concurrent decrease of cytoplasmic HSP70 levels. Unlike heat shock-induced full phosphorylation, HSF1 was partially phosphorylated after exposure to vincristine, and this result was tightly correlated with the kinetics of JNK/SAPK activation, and up-regulation of mitochondrial HSP75 level and concurrent down-regulation of HSP70. Furthermore, the dominant-negative mutant of SEK1 blocked the phosphorylation of HSF1 and up-regulation of mitochondrial HSP75 in response to vincristine or vinblastine. These data suggest that anticancer drugs regulate the HSF1 transcriptional activity differently from heat shock, and JNK/SAPK pathway appears to be involved in anticancer drug-induced HSF1 phosphorylation and consequently differential regulation of mitochondrial HSP75 and HSP60 and cytoplasmic HSP70.

Transient MRI-detected water apparent diffusion coefficient reduction correlates with c-fos mRNA but not hsp70 mRNA induction during focal cerebral ischemia in rats

Cerebral ischemia induces immediate early genes such as c-fos and stress genes such as hsp70. In this study, the spatial relationships between c-fos and hsp70 mRNA expression and changes detectable with diffusion and perfusion magnetic resonance (MR) imaging were examined. The middle cerebral artery (MCA) of young adult rats was occluded for 30 or 60 min. Diffusion MR (D-MR) images were acquired continuously during the ischemic period and dysprosium-contrast perfusion (P-MR) images were acquired at the end of the ischemic period. C-fos and hsp70 mRNA expression were examined with in situ hybridization. The most significant finding of this work was that for both durations of ischemia, c-fos induction was observed in cortical and sub-cortical regions exhibiting a transient reduction in the apparent diffusion coefficient of water (ADC). Transients which occurred on a time scale of 3 min may have been caused by spreading depression. Those occurring on a 10-min time scale may have been caused by an initial reduction in blood flow with occlusion that was followed by an ischemia-induced increase in collateral blood flow. P-MR imaging showed that perfusion in c-fos positive regions was higher than in regions with persistently reduced ADC. Hsp70 induction did not correlate with transient ADC reduction. It was induced in the MCA territory in regions showing persistent ADC changes, with induction being greatest at the periphery of these regions. It was also induced in regions that exhibited both spontaneous reversal of the diffusion changes and decreased perfusion.

Identification of novel 70-kDa heat shock protein-encoding cDNAs from Schistosoma japonicum

A diverse range of organisms respond to a variety of chemical, physiological and temperature-associated stresses by a rapid and transient increase in the synthesis of heat shock proteins. We immunoscreened a Uni-ZAP XR cDNA library, prepared from mRNA isolated from the Philippine strain of the Asian bloodfluke, Schistosoma japonicum, using hyperimmune rabbit sera raised against soluble adult S. japonicum proteins. Six 70-kDa heat shock protein-encoding cDNA clones were identified which, upon further analysis, were separated into two distinct protein groups within the 70-kDa heat shock protein family, the 70-kDa heat shock proteins and the immunoglobulin heavy chain-binding proteins/glucose-related proteins (Grp78). A representative from both groups was fully sequenced and compared with homologous sequences available in the GenBank/EMBL database as the first stage in determining the role of their expression products in the regulation of S. japonicum development, in the induction of immunity, and whether they act as molecular chaperones capable of modulating the correct folding or repair of proteins within this species of schistosome.

Induction of PBP74/mortalin/Grp75, a member of the hsp70 family, by low doses of ionizing radiation: a possible role in induced radioresistance

The identification of genes whose expression is altered following exposure to a low dose of ionizing radiation (IR) is an important step in understanding the phenomenon of the adaptive response. Using the differential mRNA display method we have identified a gene whose expression is up-regulated following exposure to 0.25 Gy IR. Partial DNA sequence and restriction endonuclease analysis of this gene showed that it is identical to the gene encoding for the human peptide-binding protein 74 (PBP74/mortalin/Grp75), a member of the heat shock 70 protein family. Time-course measurement of the PBP74/mortalin/Grp75 mRNA showed that its level was elevated after a lag of at least 15 min. The maximum induction appears to be at 30 min following gamma-irradiation and there is then a steady decline to control levels within 5 h in the HT29 cell line. On the other hand, the level of the PBP74/mortalin/Grp75 mRNA in the human breast adenocarcinoma cell line MCF-7 is consistently elevated after gamma-irradiation for up to 6 h post-irradiation. Furthermore, a cell line that does not demonstrate the induced radioresistance phenomenon (SW48) shows no induction of the PBP74/mortalin/Grp75 mRNA in contrast with HT29 or MCF-7. Treatment of the HT29 cells with antisense oligonucleotide directed towards the initiation codon of PBP74 sensitized cells to ionizing radiation.

Elevated levels of mortalin expression in human brain tumors

We have performed immunohistochemical studies of mortalin in normal and tumor human brain sections. In normal brain sections, the expression was seen mainly as being confined to neurons. Normal astrocytes showed undetectable expression of this unique member of the heat shock 70 protein family. Three grades of astrocyte tumors (low-grade astrocytoma, anaplastic astrocytoma, and glioblastoma), however, showed an increasing number of mortalin-positive cells. Other types of brain tumors, such as meningiomas, neurinomas, pituitary adenomas, and metastases, also showed elevated levels of mortalin expression compared to those in the normal brain. Mortalin has earlier been reported to have differential intracellular distribution in normal and transformed cells in vitro. Therefore, we substantiated the present study with immunofluorescence localization of the protein in normal and glioblastoma cells. The observations indicated that the tumors might be expressing a nonpancytosolic mortalin. An increase in number of mortalin-positive cells with malignant progression of brain tumors and its correlation with Ki-67 (a cell proliferation marker)-positive cells further suggested an involvement of nonpancytosolic mortalin(s) in malignant transformation of cells in vivo.

Induction of the stress protein Grp75 by amino acid deprivation in CHO cells does not involve an increase in Grp75 mRNA levels

The induction of the stress protein Grp75 in response to amino acid deprivation of Chinese Hamster Ovary cells was characterised using a specific monoclonal antibody. A 2-fold increase in the Grp75 protein content occurred over a period of 5-10 h after incubation of the cells in amino acid-free medium. A partial induction was obtained when either all non-essential amino acids or all essential amino acids were omitted from the medium indicating a broad-specificity response. Deletion of the single amino acids tryptophan, histidine or phenylalanine from otherwise complete medium also produced a partial induction of the protein. The increase in the level of Grp75 was completely blocked by cycloheximide, but only partially blocked by the inhibitors of mRNA synthesis actinomycin D and alpha-amanitin. A specific cDNA probe for Grp75 was generated by PCR and used to quantify mRNA levels. No increase in Grp75 mRNA was observed during the induction of the protein indicating that the primary regulation of Grp75 expression was not at the transcriptional level. These results contrast with the large increase in asparagine synthetase mRNA which has been shown to occur during amino acid deprivation, and indicate that cells respond to this form of stress by more than one mechanism.

Expression analysis of mortalin, a unique member of the Hsp70 family of proteins, in rat tissues

We have investigated the expression of mortalin in rat tissues by Northern analysis, RNA in situ hybridization, and immunohistochemical studies. By Northern assay, the highest level of expression was detected in brain, heart, and skeletal muscle followed by lung, liver, and kidney, and the least level of expression was detected in testis and spleen. RNA in situ and immunohistochemical studies showed that mortalin expression is significantly higher in upper nondividing layers than in the lower basal layers of skin, in neurons and nerve fibers than in surrounding glial cells in brain, in cardiomyocytes than in nonmyocytes in heart, and in interstitial secretory tissue than in germinating follicles in ovary. Such tissue- and cell-specific expression patterns of mortalin coordinates with its earlier reported antiproliferative function in normal cells. However, a deregulation of the expression is observed in rat brain tumor along with the detection of nonpancytosolic mortalin in rat glioma cell line C6. The study suggests that mortalin is involved in pathways that regulate division capacities of cells in vivo.

p75, a member of the heat shock protein family, undergoes tyrosine phosphorylation in response to oxidative stress

The combination of H2O2 and vanadate generates aqueous peroxovanadium (pV) species, which are effective cell-permeable oxidants, and potent inhibitors of protein-tyrosine phosphatases. As a result, treatment of intact cells with pV compounds significantly enhances protein Tyr phosphorylation. Here we demonstrate that treatment of intact rat hepatoma Fao cells with pV markedly enhances Tyr phosphorylation of a 75-kDa protein, termed pp75. Amino-terminal sequencing of pp75 revealed that this protein is a member of the 70-75-kDa heat shock protein family, which includes PBP-74, glucose-related protein (GRP)-75, and mortalin. Tyr phosphorylation of pp75 is selective, because other proteins that belong to the heat shock protein 70 family, such as GRP-72, Bip (GRP-78), and HSC-70 fail to undergo Tyr phosphorylation when cells are treated with pV. Our findings suggest that heat shock proteins such as pp75 may undergo tyrosine phosphorylation when intact cells are subjected to oxidative stress induced by pV compounds.

Methylmalonyl-CoA mutase induction by cerebral ischemia and neurotoxicity of the mitochondrial toxin methylmalonic acid

Differential screening of gerbil brain hippocampal cDNA libraries was used to search for genes expressed in ischemic, but not normal, brain. The methylmalonyl-CoA mutase (MCM) cDNA was highly expressed after ischemia and showed a 95% similarity to mouse and 91% similarity to the human MCM cDNAs. Transient global ischemia induced a fourfold increase in MCM mRNA on Northern blots from both hippocampus and whole forebrain. MCM protein exhibited a similar induction on Western blots of gerbil cerebral cortex 8 and 24 hr after ischemia. Treatment of primary brain astrocytes with either the branched-chain amino acid (BCAA) isoleucine or the BCAA metabolite, propionate, induced MCM mRNA fourfold. Increased concentrations of BCAAs and odd-chain fatty acids, both of which are metabolized to propionate, may contribute to inducing the MCM gene during ischemia. Methylmalonic acid, which is formed from the MCM substrate methylmalonyl-CoA and which inhibits succinate dehydrogenase (SDH), produced dose-related cell death when injected into the basal ganglia of adult rat brain. This neurotoxicity is similar to that of structurally related mitochondrial SDH inhibitors, malonate and 3-nitropropionic acid. Methylmalonic acid may contribute to neuronal injury in human conditions in which it accumulates, including MCM mutations and B12 deficiency. This study shows that methylmalonyl-CoA mutase is induced by several stresses, including ischemia, and would serve to decrease the accumulation of an endogenous cellular mitochondrial inhibitor and neurotoxin, methylmalonic acid.

DNA fragmentation and HSP70 protein induction in hippocampus and cortex occurs in separate neurons following permanent middle cerebral artery occlusions

DNA nick end-labeling (TUNEL) and heat shock protein (HSP)70 immunocytochemistry were performed on the same brain sections 1 (n = 6), 3 (n = 12), and 7 (n = 7) days following permanent middle cerebral artery (MCA) occlusions produced in adult rats using the endovascular carotid suture method. In the cortex at 1 and 3 days following MCA occlusions, HSP70 immunoreactive neurons were located outside areas of infarction and showed little evidence of DNA fragmentation. HSP70-stained cortical neurons were intermingled with TUNEL cells near the infarct, but extended for greater distances away from the infarct. DNA fragmentation occurred in CA1 hippocampal neurons in 39% of the animals at 1 and 3 days following ipsilateral MCA occlusion. Bilateral DNA fragmentation occurred in CA1 neurons in one subject. HSP70 protein was expressed in CA1 hippocampal neurons in nine of 18 (50%) animals at 1 and 3 days following MCA occlusions, including all animals that exhibited hippocampal DNA fragmentation. Three animals had bilateral expression of HSP70 in CA1 neurons. Cells that stained for either HSP70 protein or DNA fragmentation existed in close proximity to one another. Approximately 5-7% of HSP70-stained cells were TUNEL stained and 3% of TUNEL-positive cells also stained for HSP70. There was no HSP70 staining or DNA fragmentation in the brains of sham-operated controls (n = 4) or in the brains of animals 7 days following MCA occlusions. These data suggest that ischemic cells capable of translating HSP70 protein generally do not undergo DNA fragmentation. These data support the concept that most HSP70 protein-containing neurons in the cortical "penumbra" and hippocampus survive ischemic injury and are "reversibly injured." It is shown that CA1 hippocampal pyramidal neurons die or are reversibly injured in approximately 50% of animals following permanent MCA occlusions. Although the mechanism of this hippocampal injury is unknown, it could relate to transynaptic activation of N-methyl-D-aspartate (NMDA) receptors that mediate induction of early genes in hippocampus.

Focal hyperexpression of hemeoxygenase-1 protein and messenger RNA in rat brain caused by cellular stress following subarachnoid injections of lysed blood

Induction of the hemeoxygenase-1 (ho-1) stress gene is of importance for rapid heme metabolism and protection against oxidative injury in vitro and in vivo. Although ho-1 expression is observed in glia following exposure to whole blood and oxyhemoglobin, expression is mild, and other stress genes are not induced simultaneously in this setting. Hemeoxygenase-1 can be induced by several other physiological stresses in addition to heme. In the brain, ho-1 induction has been observed in the penumbra following focal cerebral ischemia. Because lysed blood is a spasmogen, the authors studied focal hyperexpression of the ho-1 gene after injection of lysed blood, whole blood, or saline into the cisterna magna of adult rats. Immunocytochemical analysis of HO-1 was performed at 1, 2, 3, and 4 days after the injections. Because the 70-kD inducible heat shock protein (HSP70) is induced by cellular stress, alternate sections were immunostained for HSP70 to assess whether focal hyperexpression was a stress phenomenon. An oligonucleotide probe was also used for in situ hybridization to demonstrate that ho-1 messenger (m)RNA was present. Focal HO-1 immunostained areas were observed after lysed blood injection only and were located mainly in the basal cortex and cerebellar hemisphere, although focal hyperexpression was also found in many other regions. The intensity of staining and the number of regions were maximum at 1 day. Double-labeled immunofluorescence revealed that many HO-1-immunoreactive cells were microglia. The HSP70 immunostaining of adjacent sections from the same animals demonstrated focal regions of immunoreactivity whose topography corresponded exactly with the topography of the HO-1-immunostained areas. Conventional histology in regions of HO-1 hyperexpression was often normal. In situ hybridization using the same oligonucleotide demonstrated that ho-1 mRNA was induced in focal areas of forebrain and in large regions of cerebellum within 6 hours of injection. These results demonstrate that focal hyperexpression of the ho-1 stress gene occurs after lysed blood injection and appears to be an indicator of cellular stress and injury in regions in which infarction does not occur. These results also suggest that cellular injury that occurs after injection of lysed blood may go undetected using conventional histology. Although direct heme metabolism was not investigated, our results indicate that rapid metabolism of heme, both intracellular and extracellular, may prove to be beneficial after subarachnoid hemorrhage.

Induction of glucose-regulated protein (glucose-regulated protein 78/BiP and glucose-regulated protein 94) and heat shock protein 70 transcripts in the immature rat brain following status epilepticus

Prior to 21 days of age, the immature rat brain is relatively resistant to excitotoxicity caused by the glutamate analogue, kainate. As stress-inducible proteins (GRP78, GRP94 and HSP70) have been proposed to possess molecular chaperone activity and protect cells from the deleterious effects of damaged proteins, we examined the pattern of expression of their respective messenger RNAs following systemic kainate at different postnatal ages. In untreated rats between seven and 21 days old, there was a higher basal level of grp78 and grp94 expression compared to hsp70. Unlike hsp70, which was inducible only in 21-day-old rats, kainate-mediated grp94 induction occurred in several regions of the brain as early as postnatal day 7. Grp78 messenger RNA expression was also increased by kainate treatment in 14-day-old rats, and the induction was most pronounced in the kainate-resistant dentate gyrus. With increasing age, longer lasting expression of both grp78 and grp94 messenger RNAs was observed in kainate-vulnerable regions, similar to observations in the adult rat brain. These results demonstrate non-overlapping expression patterns of glucose-regulated proteins and HSP70 in the immature central nervous system, suggesting that they serve different functions. While hsp70 induction could be a marker for potential cell injury and death, increased expression of grp78 and grp94 could play a neuroprotective role in the developing rat brain.

Stress proteins and tolerance to focal cerebral ischemia

Stress proteins are induced after a variety of neuronal injuries. The inducible 72-kDa heat shock protein (hsp70) is a stress protein that protects neurons from glutamate toxicity in vitro. Hsp70 has also been proposed to underlie the phenomenon of ischemic tolerance whereby brief sublethal intervals of global ischemia protect the hippocampus from subsequent lethal prolonged ischemia. To determine if the phenomenon of tolerance occurs in cortex after focal ischemia, the rat middle cerebral artery (MCA) was occluded by the suture method. Three 10-min intervals of transient ischemia (3 x 10-isc) separated by 45-min periods of reperfusion made up the most effective paradigm of preconditioning ischemia studied, and substantially reduced the volume of infarction 72 h after subsequent 100-min MCA occlusion. This approach induced protection if the interval between the 3 x 10-isc and the 100-min ischemia was 2, 3, or 5 days but not 1 or 7 days. Three 10-min intervals of transient ischemia alone produced minimal histological changes in the cortex at 72 h. Moreover, there were no significant changes in regional cerebral blood flow in the tolerant regions at 72 h after 3 x 10-isc before or during MCA occlusion. To explore the role of stress proteins in the induction of tolerance, expression of hsp70 and the glucose-regulated proteins grp75 and grp78 were studied. Samples from tolerant regions of the brain that had undergone preconditioning ischemia were evaluated at 1, 2, 3, 5, 7, and 14 days after 3 x 10-isc by Western blot analysis. The time course of hsp70 expression most closely correlated with tolerance. Hsp70 protein expression increased during times when tolerance was present (at 2-5 days) but did not increase thereafter (at 7 and 14 days). However, hsp70 was also increased before tolerance was present (at 1 day). Immunocytochemistry showed that hsp70 protein was expressed in neurons in the tolerant regions 24 h after 3 x 10-isc and was expressed in both neurons and glia after 72 h. Although immunocytochemistry suggested that there was increased neuronal expression of grp75 and grp78, no significant differences were found in protein expression as determined by Western blot before (at 1 day), during (at 2-5 days), and after (at 7 days and thereafter) tolerance. Thus, the time course of grp75 and grp78 expression did not correlate with that of tolerance. This model of ischemic tolerance is a useful method by which mechanisms of endogenous neuroprotection may be explored.

Deficient heat shock protein expression: a potential mechanism for the sudden infant death syndrome

Induction of heat shock proteins follows a metabolic stress and protects from subsequent stresses. Stressors proposed for the sudden infant death syndrome include infection, environmentally-induced hyperthermia and hypoxia. Failure to express heat shock proteins to such stressful conditions may lead to reduced tolerance, and enhance inappropriate physiologic responses and vulnerability which ultimately may lead to infant death.