A role for microRNA-29a mediating ER stress-induced apoptosis in neuronal cells in the mouse central nervous system.
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A role for microRNA-29a mediating ER stress-induced apoptosis in neuronal cells in the mouse central nervous system
Disturbances in the folding of proteins within the endoplasmic reticulum (ER) lead to the accumulation of mis-folded and aggregated proteins and the activation of the ER stress response and has been implicated in neurodegenerativo diseases such as amyotrophic lateral sclerosis. MicroRNAs are small ribonucleic acids which can modulate the expression of proteins post-translationally and in recent years have been implicated in major disease pathologies. The miR-29 family, of which miR-29a is a member, has been implicated in cancer and neurodegeneration through its effects on apoptosis.
Firstly we investigated the role of miR-29a and its proposed target MCL-1 during ER stress-induced apoptosis. We show that miR-29a is significantly upregulated compared to other miRNAs following ER stress-induced apoptosis in primary cortical neurons. Increased miR-29a expression coincided with a decrease in MCL-1 protein expression. Contrasting this we identified a neuroprotective role for miR-29a antagomir in cortical neurons during ER stress. miR-29a knockdown was shown to increase Mcl-1 mRNA and we hypothesised that this could be mediating the cytoproptective effect. We identified the importance of MCL-1 in mediating ER stress-induced apoptosis, identifying protection of cortical neurons overexpressing MCL-1 during tunicamycin-induced ER stress. Furthermore we showed loss of MCL-1 sensitised cortical neurons to ER stress-induced cell death and demonstrated a cytoprotective role for MCL-1 in cortical neurons. In addition, cortical neurons were shown to be protected against tunicamycin-induced ER stress in the absence of PUMA or BAX, however we could not abrogate apoptosis and suggest that other mediators of apoptosis compensate for loss of members of the BCL-2 family. Indeed, MCL-1 overexpression was shown to decrease LC3II protein levels suggesting a further level of regulation within cortical neurons involving other apoptosis-mediating pathways such as autophagy.
Having identified miR-29a as upregulated in the central nervous system (CNS) of C57 BL/6 mice, and reduced expression of Mcl-1 in the same CNS tissue, we confirmed miR-29a expression in lumbar spinal cord from SOD1 transgenic mice. While we could localise miR-29a expression to motoneurons in the lumbar spinal cord, miR- 29a knockdown in an SOD1G93A mouse model of ALS did not reduce motor dysfunction nor significantly increase lifespan.