Chemical Imaging of RNA-Tau Amyloid Fibrils at the Nanoscale Using Tip-Enhanced Raman Spectroscopy

In the presence of cofactors, tau protein can form amyloid deposits in the brain which are implicated in many neurodegenerative disorders. Heparin, lipids, and RNA are used to recreate tau aggregates in vitro from recombinant protein. However, the mechanism of interaction of these cofactors and the interactions between cofactors and tau are poorly understood. Herein, we use tip-enhanced Raman spectroscopy (TERS) to visualize the spatial distribution of adenine, protein secondary structure, and amino acids (arginine, lysine and histidine) in single polyadenosine (polyA)-induced tau fibrils with nanoscale spatial resolution (<10-20 nm). Based on reference unenhanced and surface-enhanced Raman spectra, we show that the polyA anionic cofactor is incorporated in the fibril structure and seems to be superficial to the β-sheet core, but nonetheless enveloped within the random-coiled fuzzy coat. TERS images also prove the colocalization of positively charged arginine, lysine, and histidine amino acids and negatively charged polyA, which constitutes an important step forward to better comprehend the action of RNA cofactors in the mechanism of formation of toxic tau fibrils. TERS appears as a powerful technique for the identification of cofactors in individual tau fibrils and their mode of interaction.

Total Internal Reflection Tip-Enhanced Raman Spectroscopy of Tau Fibrils

Total internal reflection tip-enhanced Raman spectroscopy (TIR-TERS) has recently emerged as a promising technique for noninvasive nanoscale chemical characterization of biomolecules. We demonstrate that the TERS enhancement achieved in this experimental configuration is nearly 30 times higher than that in linear polarization and 8 times higher than that in radial polarization using traditional bottom-illumination geometry. TIR-TERS is applied to the study of Tau amyloid fibrils formed with the human full-length Tau protein mixed with heparin. This technique reveals the possibility to perform TERS imaging with 1-4 nm axial and 5-10 nm lateral spatial optical resolution. In these Tau/heparin fibrils, spectral signatures assigned to aromatic amino acid residues (phenylalanine, histidine, and tyrosine) and nonaromatic ones (e.g., cysteine, lysine, arginine, asparagine, and glutamine) are distinctly observed. Amide I and amide III bands can also be detected. In a fibril portion, it is shown that antiparallel β-sheets and fibril core β-sheets are abundant and are often localized in amino acid-rich regions where parallel β-sheets and random coils are present in lower proportions. This first TIR-TERS study on a nonresonant biological sample paves the way for future nanoscale chemical and structural characterization of biomolecules using this performant and original technique.

Small RNA-Seq reveals novel miRNAs shaping the transcriptomic identity of rat brain structures

Small RNA-Seq of the rat central nervous system reveals known and novel miRNAs specifically regulated in brain structures and correlated with the expression of their predicted target genes, suggesting a critical role in the transcriptomic identity of brain structures.

In the central nervous system (CNS), miRNAs are involved in key functions, such as neurogenesis and synaptic plasticity. Moreover, they are essential to define specific transcriptomes in tissues and cells. However, few studies were performed to determine the miRNome of the different structures of the rat CNS, although a major model in neuroscience. Here, we determined by small RNA-Seq, the miRNome of the olfactory bulb, the hippocampus, the cortex, the striatum, and the spinal cord and showed the expression of 365 known miRNAs and 90 novel miRNAs. Differential expression analysis showed that several miRNAs were specifically enriched/depleted in these CNS structures. Transcriptome analysis by mRNA-Seq and correlation based on miRNA target predictions suggest that the specifically enriched/depleted miRNAs have a strong impact on the transcriptomic identity of the CNS structures. Altogether, these results suggest the critical role played by these enriched/depleted miRNAs, in particular the novel miRNAs, in the functional identities of CNS structures.