Browsing by Subject "ER stress"

Endoplasmic reticulum (ER) stress is caused by the accumulation of unfolded proteins in the ER, which leads to the activation of unfolded protein response (UPR) through three transmembrane protein sensors located in the ER membrane. The sensors correspond to three branches of the UPR, namely protein kinase RNA-like endoplasmic reticulum kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme 1 (IRE1) branches. Upon ER stress, IRE1 dimerizes and oligomerizes, and its endonuclease domain is activated. It specifically targets X-box-binding protein 1 (XBP1) mRNA, from which a 26 nt intron is spliced. This allows a complete translation of spliced XBP1 mRNA into a functional protein that acts as a transcription factor. Together with the other pathways, the UPR leads to a decrease in the protein folding load by causing a reduction in the general level of protein translation, and by inducing the expression of protein folding machinery. However, if the UPR is activated continuously for a long time, the apoptotic pathway will be triggered, and the cell will die. ER stress and UPR are associated with various disorders, such as some types of cancer, diabetes, chronic inflammatory syndromes, and particularly neurodegeneration. For example, in Parkinson’s disease, it was suggested that prolonged ER stress induces the extensive apoptosis of dopaminergic neurons in substantia nigra pars compacta region of the midbrain. This hinders the normal functioning of the nigrostriatal pathway, and hence results in the progressive development of Parkinson’s motor symptoms. In order to study the regulation or IRE1 branch of the UPR, and to identify the ER-stress-modulating compounds, a human luciferase reporter cell line (XBP1-NLuc) was created in this work. The reporter was expressed when IRE1 splicing was activated, since the XBP1 intron fragment was fused to the Nano luciferase gene. The expression of the reporter was observed with luciferase assay at several time points during treatments. The treatments were done with ER stress inducers thapsigargin and tunicamycin, and with IRE1 inhibitors KIRA6 and 4μ8c, or the combination of those. Quantitative PCR (qPCR) was used to validate the expression of the reporter and to monitor the expression of the other branches of the UPR. Additionally, the oligomerization of IRE1 was observed with IRE1-GFP cell line that was treated identically to the XBP1-NLuc cell line, fixed, stained for nuclei, and imaged with fluorescent microscopy. After imaging, the IRE1-GFP clusters were analysed and quantified with CellProfiller and CellAnalyst softwares. Both cell lines were used to test the effect of neurotrophic factors CDNF, MANF, and MANF mutant isomers on the UPR with and without tunicamycin treatment. Collectively, the experiments confirmed that XBP1-NLuc cell line was created successfully and that it accurately reports IRE1 splicing activity. As expected, ER stress treatment increased the reporter expression, while IRE1 inhibitors decreased the expression of the reporter. qPCR revealed that the other observed UPR markers were activated as well upon thapsigargin treatment, however, they were not decreased with the treatment with IRE1 specific inhibitors. In line with XBP1-NLuc cell line, the IRE1-GFP cell line demonstrated an increased oligomerization of IRE1 upon ER stress induction. The KIRA6 inhibitor of IRE1, which prevents IRE1 oligomerization, decreased the formation of IRE1-GFP clusters. Additionally, the IRE1-endonuclease-activity inhibitor 4μ8c induced the formation of IRE1-GFP clusters. Curiously, the distribution of the intensity of IRE1-GFP clusters was bimodal and could point to two manners of IRE1 clustering and/or activation. Together, the experiments done with cells transfected with CDNF, MANF or MANF mutants, suggested that the tested neurotrophic factors decreased IRE1 oligomerization and its activation. However, there were substantial problems in the quantification of viable cells, which should be considered in the interpretation of these results. No significant difference among the tested neurotrophic factors was observed. In conclusion, the XBP1-NLuc reporter cell line provided a reliable reporter of IRE1 endonuclease activity, whose expression is increased during the ER stress. Together with IRE1-GFP cell line, it revealed the amount of IRE1 oligomerization and activation under various treatments and at different time points relative to treatments. Due to the effectiveness and accuracy, the XBP1-NLuc cell line can be further used in studying the regulation and activation of IRE1, as well as for the identification of ER-stress modulating molecules, which can be used for development of novel treatments for ER stress associated diseases, such as Parkinson’s disease.

Our hearing perception is based on the ability to discriminate mechanical sound waves and to amplify and transduce them into electrical stimuli.This function is based on the complex cellular organization of the cochlea, the hearing organ. The sensory epithelium in the organ of Corti spirals along the cochlear duct in a tonotopic arrangement: every sound frequency elicits the strongest response at allocation along this duct. Sound stimulus is detected by three rows of outer hair cells (OHCs) which amplify- and tone-discriminate the sound stimulus, and by one row of inner hair cells (IHCs), which transduce the mechanical stimulus into electric impulses. Basal regions of the cochlea detect high- frequency sounds and apical regions detect low- frequency sounds. The complexity and sensitivity of the cochlea is linked with its vulnerability to various traumas. Most kinds of damage to the mammalian hair cells is irreversible, because these cells are not capable of regeneration. Hearing impairment has many etiologies. Common to them is that damage is permanent and no pharmacotherapy is available. Hearing impairment is often a disabling condition and it has vast societal consequences. The number of hearing impaired people is constantly increasing and the WHO has estimated that 10% of the world`s population will suffer from disabling hearing loss in 2050. Mesencephalic astrocyte- derived neurotrophic factor (MANF) is an unconventional, ER-resident protein that promotes ER- homeostasis. It has been associated with cytoprotective functions in many neurodegenerative disease- models and shown to promote recovery after ischemic trauma. MANF expression has been previously found in many cell-types in the cochlea, including OHCs and IHCs. Its deficiency in a mouse model led to upregulation of ER-stress markers and a robust, tonotopic base –to apex gradient loss of outer hair cells and severe hearing loss. This study examines the role of MANF in noise-induced trauma in the hair cells of the cochlea. In a conditionally inactivated (Manf -/- cKO) mouse model in the C57BL/6J – background, where Manf has been inactivated from most of the cochlear cells, I studied, if Manf -deficiency sensitizes the cells to noise-induced cell death in two age-groups. I also examined the basic and noise- induced MANF expression, using two mouse- strains, C57BL/6J and CBA/Ca. I also examined OHC stereociliary bundle morphology to find out if noise induces morphological changes in Manf cKO-mice that differ from noise-exposed C57BL/6j wild type mice. This study found that OHCs have a low MANF- expression, whereas in IHCs the expression is strong. MANF is expressed in a base- to apex gradient in the OHCs of the two mouse-strains examined, in a uniform pattern, that correlates with vulnerability, implicating that low levels of MANF predispose basal OHCs to vulnerability. MANF expression in the IHCs was non-gradiental. Noise did not induce upregulation, as was expected, but instead noise induced downregulation of MANF in the basal region of the OHCs by an unknown mechanism in both mouse-strains.This suggests that noise-induced trauma induces ER dyshomeostasis, possibly independent of ER stress response pathways ,unfold protein response (UPR). This study also demonstrates that MANF deficiency sensitizes the OHCs to noise- induced trauma, resulting in more elevated OHC loss and hearing thresholds. This sensitization is mainly caused by a progressive degenerative changes seen in the OHC stereociliary bundles of Manf cKO-mice, and is associated with more severe noise-induced hearing loss. The results of my study suggest that MANF has an important, yet unknown, protective role in noise-induced trauma in OHCs. These results support the possible role of MANF as a therapeutic agent in a noise-induced trauma.