MicroRNAs are post-transcriptional regulators of gene expression, crucial for neuronal differentiation, survival, and activity. Age-related dysregulation of microRNA biogenesis increases neuronal vulnerability to cellular stress and may contribute to the development and progression of neurodegenerative diseases. All major neurodegenerative disorders are also associated with oxidative stress, which is widely recognized as a potential target for protective therapies. Albeit often considered separately, microRNA networks and oxidative stress are inextricably entwined in neurodegenerative processes. Oxidative stress affects expression levels of multiple microRNAs and, conversely, microRNAs regulate many genes involved in an oxidative stress response. Both oxidative stress and microRNA regulatory networks also influence other processes linked to neurodegeneration, such as mitochondrial dysfunction, deregulation of proteostasis, and increased neuroinflammation, which ultimately lead to neuronal death. Modulating the levels of a relatively small number of microRNAs may therefore alleviate pathological oxidative damage and have neuroprotective activity. Here, we review the role of individual microRNAs in oxidative stress and related pathways in four neurodegenerative conditions: Alzheimer’s (AD), Parkinson’s (PD), Huntington’s (HD) disease, and amyotrophic lateral sclerosis (ALS). We also discuss the problems associated with the use of oversimplified cellular models and highlight perspectives of studying microRNA regulation and oxidative stress in human stem cell-derived neurons.

Mesencephalic astrocyte derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) are novel evolutionary conserved trophic factors, which exhibit cytoprotective activity via negative regulation of unfolded protein response (UPR) and inflammation. Despite multiple reports demonstrating detrimental effect of MANF/CDNF downregulation, little is known about the control of their expression. miRNAs—small non-coding RNAs—are important regulators of gene expression. Their dysregulation was demonstrated in multiple pathological processes and their ability to modulate levels of other neurotrophic factors, glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF), was previously reported. Here, for the first time we demonstrated direct regulation of MANF and CDNF by miRNAs. Using bioinformatic tools, reporter assay and analysis of endogenous MANF and CDNF, we identified that miR-144 controls MANF expression, and miR-134 and miR-141 downregulate CDNF levels. We also demonstrated that this effect is human-specific and is executed via predicted binding sites of corresponding miRNAs. Finally, we found that miR-382 suppressed hCDNF expression indirectly. In conclusion, we demonstrate for the first time direct regulation of MANF and CDNF expression by specific miRNAs, despite the fact their binding sites are not strongly evolutionary conserved. Furthermore, we demonstrate a functional effect of miR-144 mediated regulation of MANF on ER stress response markers. These findings emphasize that (1) prediction of miRNA targets based on evolutionary conservation may miss biologically meaningful regulatory pairs; and (2) interpretation of miRNA regulatory effects in animal models should be cautiously validated.

Neurodegenerative diseases are associated with proteostasis disturbances and accumulation of fibrillar proteins into insoluble aggregates. Progressive age-related degeneration of dopamine neurons is a primary cause of motor dysfunctions in Parkinson’s disease (PD) and substantial evidence supports critical involvement of α-synuclein (α-syn) in the etiology of PD. α-syn is a cytosolic protein present in high concentrations in pre-synaptic neuronal terminals and a primary constituent of intracellular protein aggregates known as Lewy Neurites or Lewy Bodies. Progression of Lewy pathology is a characteristic feature in the PD brains caused by the prion-like self-templating properties of misfolded α-syn. Modelling Lewy pathology progression with application of exogenously prepared α-syn preformed fibrils, we discovered that glial cell line-derived neurotrophic factor (GDNF) prevented formation of α-syn aggregates in dopamine neurons in culture and in vivo after viral vector expression of GDNF. These effects were abolished by CRISPR/Cas9-mediated deletion of receptor tyrosine kinase Ret , the major GDNF signaling pathway. Similar to GDNF, expression of mutated constitutively active RET (RET_MEN2B) was able to protect dopamine neurons. GDNF protection against α-syn pathology progression was abolished by Src and attenuated by Akt pathway inhibitors. For the first time, we have shown the neurotrophic factor-mediated protection against the misfolded α-syn propagation in dopamine neurons, uncovered underlying receptor and intracellular signaling pathways. These results for the first time demonstrate that activation of GDNF/RET signaling can be an effective therapeutic approach to prevent Lewy pathology spread at early stages of PD.

Motor symptoms of Parkinson’s disease (PD) are caused by the loss of dopamine neurons in the substantia nigra pars compacta. Current PD therapies offer only symptomatic relief, and the research on novel ways to stop or slow down the degeneration of dopamine neurons in PD attracts considerable efforts. A characteristic feature of PD is the abnormal accumulation and spread of misfolded α-synuclein (α-syn) protein leading to the formation of Lewy neurites and bodies in different neuronal populations, including dopamine neurons. The process of Lewy body formation compromises neuronal functions and contributes to the progression of the disease and neurodegeneration in PD patients. Treatments preventing α-syn misfolding, aggregation and/or spread may therefore provide much-needed disease-modifying therapies for PD. We and others have demonstrated that α-syn aggregation and Lewy body formation can be modelled in primary neuronal cultures treated with α-syn pre-formed fibrils (PFFs). Using this approach, we have identified several factors preventing the accumulation of phosphorylated α-syn in dopamine neurons in vitro and in vivo . In particular, we and others have demonstrated a prominent role of PKB/AKT pathway activation in promoting the survival of dopamine neurons. Here, we utilize lentiviral vectors (LVs) to express constitutively active myristoylated serine-threonine protein kinase AKT1 (mAKT1) in primary dopamine neurons. We show that lentivirally-delivered mAKT1 promoted survival of dopamine neurons after thapsigargin-induced endoplasmic reticulum (ER) stress and prevented fibril-induced accumulation of phosphorylated α-syn. Furthermore, using pharmacological tools, we demonstrate that this activity of mAKT1 is dependent on the functional Cathepsins B and D. These proof-of-principle results demonstrate the critical importance of lysosomal enzymes for processing misfolded α-syn and establish a basis for the use of LVs to develop neuroprotective gene therapy strategies for dopamine neurons in PD.

Under physiological conditions in vivo astrocytes internalize and degrade neuronal mitochondria in a process called transmitophagy. Mitophagy is widely reported to be impaired in neurodegeneration but it is unknown whether and how transmitophagy is altered in Alzheimer’s disease (AD). Here we report that the internalization and degradation of neuronal mitochondria are significantly increased in astrocytes isolated from aged AD mouse brains. We also demonstrate for the first time a similar phenomenon between human neurons and AD astrocytes, and in murine hippocampi in vivo . The results suggest the involvement of S100a4 in impaired mitochondrial transfer between neurons and aged AD astrocytes. Significant increases in the mitophagy regulator Ambra1 were observed in the aged AD astrocytes. These findings demonstrate altered neuron-supporting functions of aged AD astrocytes and provide a starting point for studying the molecular mechanisms of transmitophagy in AD.