MicroRNA in neurodegenerative drug discovery: the way forward?

Figures & data

Figure 1. Methods of miRNA manipulation in neurodegeneration. (A) Upon loss of expression of neuroprotective miRNA, miRNA mimics or synthetic miRNAs can be utilized for functional replacement. (B) Loss of expression of neuroprotective mRNA during disease due to repression by miRNA favoring neurodegeneration can be ameliorated by using anti-miRs (antisense inhibitors), multiple-target anti-miRNAs or miRNA sponges.

Figure 2. miRNA implicated in neurodegenerative disease. (A) Use of an miR-128 mimic in Parkinson’s-like mice resulted in improved motor function and downregulation of genes involved in the ERK1/2 signaling network which is implicated in excitotoxicity as well as downregulation of ion channels [7]. (B) miR-29a is upregulated in SOD1^G93A transgenic mice, a familial model of ALS. Amelioration of upregulation using anti-miR-29a resulted in alleviation of repression of target genes, and increased lifespan [9]. (C) In a mouse model of Huntington’s disease, upregulation of miR-196a is protective against neurodegeneration and suppressed expression of HTT in the brain. Studies using patient-derived induced pluripotent stem cells overexpressing miR-196a also showed reduction of HTT levels as well as improvement in pathological aggregates [14].

Table 1. Delivery methods for miRNA administration to the CNS.

	Method	Description	Benefit	Caveat
Non-viral mediated delivery	Synthetic RNA duplexes	Synthetic miRNA complementarily bound to protective passenger strand and a carrier molecule (ex) cholesterol) to facilitate uptake into tissues [7]	Simple delivery vehicles	Not tissue specific Does not allow constitutive expression of the miRNA Repeat administration necessary Passenger strand may have targets as well
	Systemic injection of targeted exosomes	Using genetically engineered dentritic cells to produce and exosomal membrane protein conjugated to a neuron-directed peptide, based on the rabies virus glycoprotein, which facilitates receptor mediated endocytosis	Easily loaded with genetic material via electroporation Can be administered easily IV Endogenous natural transporters of RNA and proteins Previously shown to transport miRNA between cells naturally [39] Can be produced from patient cells, therefore low immunogenicity potential	Proof of concept studies have all been done with siRNA not miRNA May be complications to using dendritic cells to produce exosomes for administration to the brain [40] Repeat administration necessary
	Chemical modification of oligonucleotide: locked nucleic acid (LNA)	Creating a methylene bridge between the 2′-O-oxygen to the 4′ position to lock the molecule in a stable ring conformation	Simple delivery vehicles Can be administered subcutaneously Designed for high affinity and specificity Can be modified with polymers for increased cellular uptake	Not tissue specific Does not allow constitutive expression of the miRNA Repeat administration necessary
	Cyclodextrins	Naturally occurring oligosaccharides that can be chemically modified to be amphiphilic, with polar groups on one face and lipophilic groups on the other [41]	< 200 nm in size Stable in serum Able to effectively deliver genetic material to hippocampal and hypothalamic neurons in vitro [41] Low toxicity	Studies are currently limited to siRNA In vivo testing has not been done
Viral mediated miRNA delivery	Lenti-associated virus (LAV) and AAV	Production of pseudoviral particles using cotransformation of miRNA/antimiR containing vector with packaging plasmids to produce replication incompetent virus	Tissue-specific option (different AAV serotypes can target different cellular receptors) [7] Ability to use tissue-specific promoters Allows constitutive miRNA expression Viral vectors can successfully transverse the blood brain barrier	Complications from LAV integration into the genome (could stimulate oncogene expression) Long-term effects of gene integration are unknown

IV: Intravenously; miRNA: MicroRNA.

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