Non-genetic transgenerational transmission of bipolar disorder: targeting DNA methyltransferases

MicroRNA Alterations in Induced Pluripotent Stem Cell-Derived Neurons from Bipolar Disorder Patients: Pathways Involved in Neuronal Differentiation, Axon Guidance, and Plasticity

Bipolar disorder (BP) is a complex psychiatric condition characterized by severe fluctuations in mood for which underlying pathological mechanisms remain unclear. Family and twin studies have identified a hereditary component to the disorder, but a single causative gene (or set of genes) has not been identified. MicroRNAs (miRNAs) are small, noncoding RNAs ∼20 nucleotides in length, that are responsible for the posttranslational regulation of multiple genes. They have been shown to play important roles in neural development as well as in the adult brain, and several miRNAs have been reported to be dysregulated in postmortem brain tissue isolated from bipolar patients. Because there are no viable cellular models to study BP, we have taken advantage of the recent discovery that somatic cells can be reprogrammed to pluripotency then directed to form the full complement of neural cells. Analysis of RNAs extracted from Control and BP patient-derived neurons identified 58 miRNAs that were differentially expressed between the two groups. Using quantitative polymerase chain reaction we validated six miRNAs that were elevated and two miRNAs that were expressed at lower levels in BP-derived neurons. Analysis of the targets of the miRNAs indicate that they may regulate a number of cellular pathways, including axon guidance, Mapk, Ras, Hippo, Neurotrophin, and Wnt signaling. Many are involved in processes previously implicated in BP, such as cell migration, axon guidance, dendrite and synapse development, and function. We have validated targets of several different miRNAs, including AXIN2, BDNF, RELN, and ANK3 as direct targets of differentially expressed miRNAs using luciferase assays. Identification of pathways altered in patient-derived neurons suggests that disruption of these regulatory networks that may contribute to the complex phenotypes in BP.

Preliminary investigation of peripheral extracellular vesicles' microRNAs in bipolar disorder

BACKGROUND

The search for biomarkers of bipolar disorder (BD), including epigenetic alterations, has heavily relied on peripheral investigations that do not necessarily reflect brain-specific mechanisms. In this study we aimed to assess peripheral extracellular vesicles (EVs)' microRNAs and determine their use as a novel source of biomarkers in BD.

METHODS

We assessed peripheral blood EVs' microRNAs from 20 patients with BD type I and 21 age- and sex-matched healthy controls by microarray, and further explored the predicted biological functions of significantly differentially expressed microRNAs.

RESULTS

Our results identified 33 nominally significant microRNAs (p < 0.05 and fold-change >1.5) altered in BD patients, including miRNAs previously reported to be altered in post-mortem tissues of patients. Pathway analyses identified some brain-relevant mechanisms enriched in these miRNAs, including axon guidance by netrin and the serotonin receptor pathway.

LIMITATIONS

Relatively small sample size, potential confounding effects of mood states, medication use, and comorbidities, analysis of total rather than brain-specific EVs, and lack of validation of significant miRNAs by other methods.

CONCLUSIONS

This study provides important preliminary evidence of the potential use of EVs as a novel source of biomarkers in BD. Overall, our findings of brain-relevant mechanisms in these vesicles suggest their potential use in living patients as a peripheral window to the brain.

The miRNome of bipolar disorder

BACKGROUND

Epigenetic mechanisms have been suggested to play a key role in the pathophysiology of bipolar disorder (BD), among which microRNAs (miRNAs) may be of particular significance according to recent studies. We aimed to summarize miRNA studies in BD to identify consistent findings, limitations, and future directions of this emerging field.

METHODS

We performed a comprehensive search on PUBMED and Medline for studies investigating an association between BD and miRNAs. The included studies report miRNA alterations in postmortem brain tissues and in the periphery, cell culture and preclinical findings, genetic associations, and the effects of medications.

RESULTS

Several studies report changes in miRNA expression levels in postmortem brain and in the periphery of patients, although most of the results so far have not been replicated and are not concordant between different populations. Genetic studies also suggest that miRNA genes are located within susceptibility loci of BD, and also a putative role of miRNAs in modulating genes previously shown to confer risk of BD.

LIMITATIONS

We did not perform a systematic review of the literature, and miRNAs represent only one facet of the plethora of epigenetic mechanisms that might be involved in BD's pathophysiology.

CONCLUSIONS

miRNA findings in BD significantly vary between studies, but are consistent to suggest a key role for these molecules in BD's pathophysiology and treatment, particularly miR-34a and miR-137. Accordingly, miRNA might represent important biomarkers of illness to be used in the clinical settings, and potentially also for the development of novel therapeutics for BD in the near future.

Exosomal MicroRNAs as Potential Biomarkers in Neuropsychiatric Disorders

This chapter will discuss the potential use of microRNAs, particularly those located in peripherally-isolated exosomes, as biomarkers in neuropsychiatric disorders. These extracellular vesicles are released as a form of cell-to-cell communication and may mediate the soma-to-germline transmission of brain-relevant information, thereby potentially contributing to the inter- or transgenerational transmission of behavioral traits. Recent novel methods allow for the enrichment of peripheral exosomes specifically released by neurons and astrocytes and may provide valuable brain-relevant biosignatures of disease.