MicroRNA-34a as a Novel Mediator of Alzheimer’s Disease-associated Mitochondrial Dysfunction

Human Health Relevance

Late-onset Alzheimer’s disease (AD), the most common form of dementia (accounting for >90% of cases), is a devastating age-related neurodegenerative condition for which there is no cure (Querfurth and LaFerla, 2010). Nationally, while only 3% of persons aged 65-74 years suffer from AD, among those greater than 84 years, 1 in 3 has AD (Herbet et al., 2013). Historically, AD mouse models have depended on constitutive expression of mutant human amyloid precursor protein, presenilin or tau to drive the emergence of cognitive dysfunction in mid-adulthood and several potential treatments have been developed based on discoveries in these models. Yet, 150+ AD clinical trials of novel pharmacological agents directed against the prevailing theorized pathologies, amyloid-beta (Aβ) plaques and neurofibrillary tangles, have failed thus far (PhRMA, 2018). There is clearly a current need for alternative preclinical AD models if we are to develop novel therapeutic targets.

Scientific Premise

Recently, research attention has turned to other processes disrupted in AD, namely altered cellular energy availability from mitochondrial. Indeed, AD patients have mitochondrial DNA alterations (Chen et al., 2013; Swerdlow, 2018). Mitochondrial dysfunction is known in transgenic (3xTg) and inducible (streptozocin) preclinical models of AD (Chen et al., 2013; Correia et al., 2013; Yao et al., 2009). More broadly, genome-wide association studies have indicated that many genes necessary for energy metabolism and synapse activity are downregulated in AD (Liang et al., 2008). Given the likely polygenic mechanisms that mediate the development of late onset AD, identifying factors that control multiple aspects of the pathological process could offer novel approaches to treat this devastating neurodegenerative condition.

MicroRNAs (miRNAs) are small, endogenous, non-coding, and highly conserved RNAs that regulate post-transcriptional gene expression via degradation of the target mRNA and inhibition of translation 13 which could be one of these factors. We and others have taken initial steps to identify a role of miR-34a in AD (Jian et al., 2017; Sarkar et al., 2016; Sarkar et al., 2019; Schipper et al., 2007; Xu et al., 2018). Indeed, we noted, 1) elevated miR-34a levels in post-mortem brain tissue from AD patients correlated with disease severity, 2) elevated miR-34a in brains of 3x Tg mice, 3) levels of proteins related energy metabolism were reduced in brains of patients with high miR-34a levels or repressed by overexpression of miR-34a in primary neurons, 4) miR-34a construct transfection into primary neurons resulted in reduced levels of mitochondrial electron transport chain proteins, 5) the tentative promotor of the miR-34a gene showed the presence of NFκB, STAT1, c-Fos, CREB and p53 response elements, all of which are implicated in pathological brain aging, and 6) induced transient miR-34a overexpression resulted in cognitive decline and accumulation of AD-like pathologies. Further, Cre-lox miR-34a knock out prevented cognitive deficits typically observed in transgenic AD mice.

Approach

To methodically explore the impact of mitochondrial dysfunction on accumulation of AD neuropathology and cognitive dysfunction, we generated a novel inducible miR-34a overexpressing mouse that successfully recapitulates several key features of the AD-related pathological cascade. The next steps in this project line is to further characterize the inducible miR-34a overexpressing preclinical model we developed. Aim 1 of the submitted administrative supplement will determine if the consequences of miR-34a overexpression are progressive with increasing dose/duration of miR-34a exposure.

Aim 2 will address whether chronic miR-34a exposure is necessary to drive these effects or if these effects can spontaneously resolve following cessation of miR-34a exposure. Importantly, the innovation of this potential new inducible AD animal model is that it will be the first time that the field will be able to determine whether AD neuropathology can be halted or reversed, once seeded. This lack of this knowledge represents a critical barrier to scientific progress. Our compelling preliminary data and novel animal model uniquely qualify us to execute the proposed aims.

Outcomes

Characterization of our novel animal model will lay foundations for a competitive R01 proposal in which we leverage brain region and cell-type specific overexpression models to address mechanisms of miR-34a control over mitochondrial dysfunction and probe the consequences for miR-34a overexpression-induced mitochondrial dysfunction when accumulation of AD pathological hallmarks is methodically controlled.