Fiber-specific age-related differences in the white matter of healthy adults uncovered by fixel-based analysis.

Diffusion magnetic resonance imaging studies often investigate white matter (WM) microstructural degeneration in aging by probing WM regions that exhibit negative age associations of fractional anisotropy (FA). However, WM regions in which FA is unassociated with age are not necessarily "spared" in aging. Besides the confound of inter-participant heterogeneity, FA conflates all intravoxel fiber populations and does not allow the detection of individual fiber-specific age associations. In this study of 541 healthy adults aged 36-100 years, we use fixel-based analysis to investigate age associations among each "fixel" within a voxel, representing individual fiber populations. We find age associations of fixel-based measures that indicate age-related differences in individual fiber populations amid complex fiber architectures. Different crossing fiber populations exhibit different slopes of age associations. Our findings may provide evidence of selective degeneration of intravoxel WM fibers in aging, which does not necessarily manifest as a change in FA and therefore escapes notice if conventional voxel-based analyses are relied upon alone.

Structure-based view of the druggable genome.

International efforts are underway to develop chemical probes for specific protein families, and a 'Target 2035' call to expand these efforts towards a comprehensive chemical coverage of the druggable human genome was recently announced. But what is the druggable genome? Here, we systematically review structures of proteins bound to drug-like ligands available from the Protein Data Bank (PDB) and use ligand desolvation upon binding as a druggability metric to draw a landscape of the human druggable genome. The vast majority of druggable protein families, including some highly populated and disease-associated families, are almost orphan of small-molecule ligands. We propose a list of 46 druggable domains representing 3440 human proteins that could be the focus of large chemical probe discovery efforts.