Heat-shock cognate 70 messenger RNA expression in postmortem human hippocampus: regional differences and age-related changes

Heat shock protein responses to aging and proteotoxicity in the olfactory bulb

The olfactory bulb is one of the most vulnerable brain regions in age-related proteinopathies. Proteinopathic stress is mitigated by the heat shock protein (Hsp) family of chaperones. Here, we describe age-related decreases in Hsc70 in the olfactory bulb of the female rat and higher levels of Hsp70 and Hsp25 in middle and old age than at 2-4 months. To model proteotoxic and oxidative stress in the olfactory bulb, primary olfactory bulb cultures were treated with the proteasome inhibitors lactacystin and MG132 or the pro-oxidant paraquat. Toxin-induced increases were observed in Hsp70, Hsp25, and Hsp32. To determine the functional consequences of the increase in Hsp70, we attenuated Hsp70 activity with two mechanistically distinct inhibitors. The Hsp70 inhibitors greatly potentiated the toxicity of sublethal lactacystin or MG132 but not of paraquat. Although ubiquitinated protein levels were unchanged with aging in vivo or with sublethal MG132 in vitro, there was a large, synergistic increase in ubiquitinated proteins when proteasome and Hsp70 functions were simultaneously inhibited. Our study suggests that olfactory bulb cells rely heavily on Hsp70 chaperones to maintain homeostasis during mild proteotoxic, but not oxidative insults, and that Hsp70 prevents the accrual of ubiquitinated proteins in these cells. The olfactory bulb is affected in the early phases of many age-related neurodegenerative disorders. Here, we described the impact of aging on multiple heat shock proteins (Hsps), such as Hsp70, in the female rat olfactory bulb in vivo. Using multiple proteasome and Hsp70 inhibitors (see schematic), we found that proteotoxicity elicited a compensatory increase in Hsp70 in primary olfactory bulb cells in vitro. Hsp70 then reduced the proteotoxic buildup of ubiquitinated proteins and robustly protected against cell death according to three independent viability assays. Thus, olfactory bulb neurons can mount impressive natural adaptations to proteotoxic injury, perhaps explaining why neurodegenerative disorders are so delayed in onset and so slow to progress.

Heat shock proteins in neurodegenerative disorders and aging

Many members of the heat shock protein family act in unison to refold or degrade misfolded proteins. Some heat shock proteins also directly interfere with apoptosis. These homeostatic functions are especially important in proteinopathic neurodegenerative diseases, in which specific proteins misfold, aggregate, and kill cells through proteotoxic stress. Heat shock protein levels may be increased or decreased in these disorders, with the direction of the response depending on the individual heat shock protein, the disease, cell type, and brain region. Aging is also associated with an accrual of proteotoxic stress and modulates expression of several heat shock proteins. We speculate that the increase in some heat shock proteins in neurodegenerative conditions may be partly responsible for the slow progression of these disorders, whereas the increase in some heat shock proteins with aging may help delay senescence. The protective nature of many heat shock proteins in experimental models of neurodegeneration supports these hypotheses. Furthermore, some heat shock proteins appear to be expressed at higher levels in longer-lived species. However, increases in heat shock proteins may be insufficient to override overwhelming proteotoxic stress or reverse the course of these conditions, because the expression of several other heat shock proteins and endogenous defense systems is lowered. In this review we describe a number of stress-induced changes in heat shock proteins as a function of age and neurodegenerative pathology, with an emphasis on the heat shock protein 70 (Hsp70) family and the two most common proteinopathic disorders of the brain, Alzheimer's and Parkinson's disease.

Hsc70 rapidly engages tau after microtubule destabilization

The microtubule-associated protein Tau plays a crucial role in regulating the dynamic stability of microtubules during neuronal development and synaptic transmission. In a group of neurodegenerative diseases, such as Alzheimer disease and other tauopathies, conformational changes in Tau are associated with the initial stages of disease pathology. Folding of Tau into the MC1 conformation, where the amino acids at residues 7-9 interact with residues 312-342, is one of the earliest pathological alterations of Tau in Alzheimer disease. The mechanism of this conformational change in Tau and the subsequent effect on function and association to microtubules is largely unknown. Recent work by our group and others suggests that members of the Hsp70 family play a significant role in Tau regulation. Our new findings suggest that heat shock cognate (Hsc) 70 facilitates Tau-mediated microtubule polymerization. The association of Hsc70 with Tau was rapidly enhanced following treatment with microtubule-destabilizing agents. The fate of Tau released from the microtubule was found to be dependent on ATPase activity of Hsc70. Microtubule destabilization also rapidly increased the MC1 folded conformation of Tau. An in vitro assay suggests that Hsc70 facilitates formation of MC1 Tau. However, in a hyperphosphorylating environment, the formation of MC1 was abrogated, but Hsc70 binding to Tau was enhanced. Thus, under normal circumstances, MC1 formation may be a protective conformation facilitated by Hsc70. However, in a diseased environment, Hsc70 may preserve Tau in a more unstructured state, perhaps facilitating its pathogenicity.

Measurement of hippocampal subfields and age-related changes with high resolution MRI at 4T

Histological studies suggest that hippocampal subfields are differently affected by aging and Alzheimer's disease (AD). The aims of this study were: (1) To test if hippocampal subfields can be identified and marked using anatomical landmarks on high resolution MR images obtained on a 4T magnet. (2) To test if age-specific volume changes of subfields can be detected. Forty-two healthy controls (21-85 years) and three AD subjects (76-86 years) were studied with a high resolution T2 weighted fast spin echo sequence. The entorhinal cortex (ERC), subiculum, CA1, CA2 and CA3/4 and dentate were marked. A significant correlation between age and CA1 (r=-0.51, p=0.0002) which was most pronounced in the seventh decade of life was found in healthy controls. In AD subjects, CA1 and subiculum were smaller than in age-matched controls. These preliminary findings suggest that measurement of hippocampal subfields may be helpful to distinguish between normal aging and AD.

Age-associated increases in heme oxygenase-1 and ferritin immunoreactivity in the autopsied brain

Heme oxygenase-1 (HO-1) is a 32 kDa heat shock protein (HSP) that catalyzes heme to biliverdin, free iron and carbon monoxide in the brain. Furthermore, the release of free ferrous ion by HO-1 plays an essential role in ferritin synthesis, and ferritin stores iron either for intracellular utilization, or for detoxification. It is well known that HO-1 immunoreactivity is enhanced greatly in neurons and glia of the hippocampus and cerebral cortex in various pathophysiological conditions. The expression of HSP 70 is well known for the age-associated increase, but the expression modalities of HO-1 and ferritin associated with aging are still unknown. A study was therefore performed to examine the correlations in the expression of HO-1 and ferritin with age using immunohistochemistry. We investigated 31 autopsied brains (3-84-year-olds) without traumatic brain injury and neurodegenerative disease. The specimens were taken from the cerebral cortex and hippocampus. In the cerebral cortex, age (aging) had a statistically significant positive correlation with HO-1 (r=0.894, P<0.01) and ferritin (r=0.731, P<0.01). In the hippocampus, age had a significant positive correlation with only HO-1 (r=0.660, P<0.01). These results showed that HO-1 and ferritin underwent an age-related increase in human brain, especially in the cerebral cortex. Our results also indicate that various stress responses may persist in the aged human brain.

Immunohistochemical localization of heat shock protein 70 in the human medulla oblongata in forensic autopsies

Heat shock protein 70 (hsp70) can be induced under various stresses in experimental animals. We investigated hsp70 immunoreactivity in the human medulla oblongata in forensic autopsies. Hsp70 immunoreactivity was observed in the cytoplasm of some neurons in the hypoglossal nucleus (XII), the dorsal motor nucleus of the vagal nerve (X), the lateral cuneate nucleus (Cun), and the inferior olive (Oli). Neurons with positive hsp70 immunoreactivity were statistically significantly fewer in the Oli than in the XII, X, and Cun. There was no statistically significant correlation between the AMI (the antemortem interval between the onset of injury and death) or PMI (the postmortem interval between death and autopsy), and the percentage of positive cytoplasmic hsp70 immunoreactivity in any of the nuclei studied. Age had a statistically significant negative correlation with the percentage of positive hsp70 immunoreactivity in the Oli. The percentages of positive hsp70 immunoreactivity in the XII and Cun were statistically significantly lower in burn cases than in other cases. Therefore, the induction of hsp70 immunoreactivity in the medulla oblongata may not reflect the duration of stress in the AMI, but may reflect the regional (nuclei) and conditional (burns) differences in autopsy specimens.