Analysis of the Gap Junction-dependent Transfer of miRNA with 3D-FRAP Microscopy

Horizontal MicroRNA Transfer by Platelets - Evidence and Implications

For decades, platelets have been known for their central role in hemostasis and their ability to release bioactive molecules, allowing inter-platelet communication and crosstalk with the immune system and vascular cells. However, with the detection of microRNAs in platelets and platelet-derived microvesicles (MVs), a new level of inter-cellular regulation was revealed. By shedding MVs from their plasma membrane, platelets are able to release functional microRNA complexes that are protected from plasma RNases. Upon contact with macrophages, endothelial cells and smooth muscle cells platelet microRNAs are rapidly internalized and fine-tune the functionality of the recipient cell by post-transcriptional reprogramming. Moreover, microRNA transfer by platelet MVs allows infiltration into tissues with limited cellular access such as solid tumors, thereby they not only modulate tumor progression but also provide a potential route for drug delivery. Understanding the precise mechanisms of horizontal transfer of platelet microRNAs under physiological and pathological conditions allows to design side-specific therapeutic (micro)RNA delivery systems. This review summarizes the current knowledge and the scientific evidence of horizontal microRNA transfer by platelets and platelet-derived MVs into vascular and non-vascular cells and its physiological consequences.

How Does a Tumor Get Its Shape? MicroRNAs Act as Morphogens at the Cancer Invasion Front

The generation and organization of the invasion front shape of neoplasms is an intriguing problem. The intimate mechanism is not yet understood, but the prevailing theory is that it represents an example of morphogenesis. Morphogenesis requires the presence of specific molecules, known as morphogens (activators and inhibitors), which can diffuse and elicit dose-dependent responses in their target cells. Due to their ability to modulate most of the coding transcriptome, their well-established role in embryogenesis, and their capacity to rapidly move between neighboring and distant cells, we propose microRNAs as inhibitors that could shape the cancer invasion front. In order to explain the genesis of the tumor border, we use Alan Turing’s reaction diffusion model, refined by Meinhardt and Gierer. This assumes the existence of an activator called a, and an inhibitor called h, which we hypothesize could be a freely moving microRNA. We used the fractal dimension as a measure of tumor border irregularity. We observed that the change in fractal dimension associates with variations in the diffusion coefficient of the activator (D_a) or the inhibitor (D_h). We determined that the fractal dimension remains constant (i.e., the irregularity of the tumor border does not change) across a D_h interval, which becomes narrower as D_a rises. We therefore conclude that a change in fractal dimension occurs when the balance between D_a and D_h is disrupted. Biologically, this could be explained by a faulty distribution of the inhibitor caused by an abnormal density of the intercellular connection network. From a translational perspective, if experimentally confirmed, our observations can be used for a better diagnosis of cancer aggressiveness.

Potential mechanisms of microRNA mobility

microRNAs (miRNAs) are important epigenetic modulators of gene expression that control cellular physiology as well as tissue homeostasis, and development. In addition to the temporal aspects of miRNA-mediated gene regulation, the intracellular localization of miRNA is crucial for its silencing activity. Recent studies indicated that miRNA is even translocated between cells via gap junctional cell-cell contacts, allowing spatiotemporal modulation of gene expression within multicellular systems. Although non coding RNA remains a focus of intense research, studies regarding the intra-and intercellular mobility of small RNAs are still largely missing. Emerging data from experimental and computational work suggest the involvement of transport mechanisms governing proper localization of miRNA in single cells and cellular syncytia. Based on these data, we discuss a model of miRNA translocation that could help to address the spatial aspects of miRNA function and the impact of miRNA molecules on the intercellular signaling network.