Neuronal expression of constitutive heat shock proteins: implications for neurodegenerative diseases

Abstract

Neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis have been termed "protein misfolding disorders." These diseases differ widely in frequency and impact different classes of neurons. Heat shock proteins provide a line of defense against misfolded, aggregation-prone proteins and are among the most potent suppressors of neurodegeneration in animal models. Analysis of constitutively expressed heat shock proteins revealed variable levels of Hsc70 and Hsp27 in different classes of neurons in the adult rat brain. The differing levels of these constitutively expressed heat shock proteins in neuronal cell populations correlated with the relative frequencies of the previously mentioned neurodegenerative diseases.

Fig 1.

Relative frequencies of Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Alzheimer's disease is 4-fold more frequent than Parkinson's disease and 133-fold more frequent than ALS, while Parkinson's disease exhibits a frequency that is 33-fold greater than ALS. Calculations are based on the prevalence of these neurodegenerative diseases in the US population

Fig 3.

Quantitative analysis of constitutively expressed heat shock proteins in neuronal populations. Motor neurons, that are impacted in ALS, have a higher defense capacity against protein-misfolding disorders because of their elevated levels of Hsc70 and Hsp27 compared to dopaminergic neurons of the substantia nigra that are affected during Parkinson's disease, which in turn have higher levels of these constitutively expressed Hsps than entorhinal cortical and hippocampal neurons that are impacted in Alzheimer's disease. Fluorescence intensity is shown relative to a 100% value for entorhinal cortical and hippocampal neurons. Mean and standard deviation are shown. (n = 50; *, P < 0.05, compared to entorhinal cortical and hippocampal neurons; #, P < 0.05, compared to dopaminergic neurons)

Fig 4.

Western blot analysis of the effects of heat stress on the expression of heat shock proteins in regions of rat nervous system. Hsp110, Hsp90, Hsc70, Hsp60, and Hsp40 were constitutively expressed in the cerebral cortex, hippocampus, midbrain, and spinal cord. Thermal stress did not alter levels of expression of these constitutive Hsps; however, it did induce expression of Hsp70 and Hsp27. C, control; H, heat shock (24-hour recovery)

Fig 5.

Cellular expression pattern of stress-inducible Hsp70 and Hsp27 in the entorhinal cortex. Thermal stress induced expression of stress-inducible Hsp70 (upper panel) and Hsp27 (lower panel) in glial cells (indicated by arrows) but not in neuronal cells (indicated by arrowheads) in the entorhinal cortex. C, control; H, heat shock (24-hour recovery). Bar = 16 μm

Fig 2.

Constitutive expression of heat shock proteins in different classes of neurons. (a) Constitutive Hsc70 expression pattern. As indicated by the arrowheads, motor neurons in the spinal cord, labeled with the neuron-specific marker NeuN, exhibited a high level of constitutively expressed Hsc70. Dopaminergic neurons of the substantia nigra, labeled with tyrosine hydroxylase (TH), showed moderate Hsc70 expression, whereas neurons in the entorhinal cortex and hippocampus exhibited low levels of Hsc70. Glial cells in these regions (indicated by the arrows) did not show a detectable signal for expression of constitutive Hsc70

Fig 2.

Continued. (b) Constitutive Hsp27 expression patterns. As indicated by the arrowheads, motor neurons of the spinal cord demonstrated a high level of constitutive Hsp27 expression. An Hsp27 signal was not detectable in dopaminergic neurons of the substantia nigra and neurons of the entorhinal cortex and hippocampus. Glial cells (indicated by arrows) did not show detectable constitutive Hsp27 in any of these regions. Bar = 16 μm