Abstract
BackgroundThe proteasome is a multisubunit enzyme complex involved in protein degradation, which is essential for many cellular processes. During inflammation, the constitutive subunits are replaced by their inducible counterparts, resulting in the formation of the immunoproteasome.MethodsWe investigated the expression pattern of constitutive (β1, β5) and immunoproteasome (β1i, β5i) subunits using immunohistochemistry in malformations of cortical development (MCD; focal cortical dysplasia (FCD) IIa and b, cortical tubers from patients with tuberous sclerosis complex (TSC), and mild MCD (mMCD)). Glial cells in culture were used to elucidate the mechanisms regulating immunoproteasome subunit expression.ResultsIncreased expression was observed in both FCD II and TSC; β1, β1i, β5, and β5i were detected (within cytosol and nucleus) in dysmorphic neurons, balloon/giant cells, and reactive astrocytes. Glial and neuronal nuclear expression positively correlated with seizure frequency. Positive correlation was also observed between the glial expression of constitutive and immunoproteasome subunits and IL-1β. Accordingly, the proteasome subunit expression was modulated by IL-1β in human astrocytes in vitro. Expression of both constitutive and immunoproteasome subunits in FCD II-derived astroglial cultures was negatively regulated by treatment with the immunomodulatory drug rapamycin (inhibitor of the mammalian target of rapamycin (mTOR) pathway, which is activated in both TSC and FCD II).ConclusionsThese observations support the dysregulation of the proteasome system in both FCD and TSC and provide new insights on the mechanism of regulation the (immuno)proteasome in astrocytes and the molecular links between inflammation, mTOR activation, and epilepsy.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0662-z) contains supplementary material, which is available to authorized users.
Highlights
The proteasome is a multisubunit enzyme complex involved in protein degradation, which is essential for many cellular processes
To provide better insights into the mechanisms underlying the astroglial regulation of immunoproteasome subunits, we studied their expression in response to IL-1β stimulation in both human fetal astrocytes and focal cortical dysplasia (FCD)-derived cells
Regulation of immunoproteasome subunit expression in human glial cells in culture Since IL-1β is known to be strongly upregulated in FCD and tuberous sclerosis complex (TSC) human brain specimens [27, 34, 35] and to play a key pathogenic role in human epilepsy (for review, see [20, 36]; we investigated whether this inflammatory cytokine could play a role in the regulation of the expression and cellular localization of immunoproteasome subunits. qPCR analysis of astrocyte-enriched human fetal cell cultures demonstrated that exposure to IL-1β did not modify the expression of the constitutive subunits (Fig. 5a, d) but did consistently increase the expression of both immunoproteasome subunits β1i and β5i (Fig. 5b, e), increasing the β1/ β1i and β5/β5i ratios (Fig. 5c, f )
Summary
The proteasome is a multisubunit enzyme complex involved in protein degradation, which is essential for many cellular processes. The constitutive subunits are replaced by their inducible counterparts, resulting in the formation of the immunoproteasome. The proteasome is an evolutionarily conserved multicatalytic proteinase complex representing a major protein degradation system, present in the nucleus and cytoplasm of eukaryotic cells, that regulates diverse biological processes essential for cell survival [1,2,3,4]. Under certain conditions associated with the release of immune-modulatory cytokines (i.e., interferon-γ, IFN-γ), a specialized type of proteasome called the immunoproteasome can be generated by the incorporation of the inducible subunits, β1i (PSMB9; LMP2, low molecular weight protein 2), β2i (PSMB10; LMP10, MECL-1, multicatalytic endopeptidase complex-like 1), and β5i (PSMB8; LMP7, low molecular weight protein 7) [7, 8]. Recent experimental data support a role for the β5i subunit in modulating seizure generation in epileptic tissue, and interestingly, this subunit was not upregulated in rats exposed to pilocarpine but not developing SE and spontaneous seizures [18]
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