Disintegrating brain networks: from syndromes to molecular nexopathies

Abstract

In this issue of Neuron, Raj et al. (2012) and Zhou et al. (2012) use graph theory to suggest that neurodegenerative diseases spread diffusively via intrinsic brain networks. These studies provide a powerful model for understanding and predicting disease-specific profiles of neurodegeneration.

Figure 1

From Syndromes to Molecular Nexopathies The figure attempts to reconcile the transneuronal model of network degeneration suggested by the data of Raj et al. (2012) and Zhou et al. (2012) with empirical data on distinctive brain atrophy profiles associated with specific genetic proteinopathies (e.g., Rohrer et al., 2011). In the central brain schematic, circles represent local network elements and lines represent transneuronal anatomical connections (these need not correspond closely to the “edges” of graph theoretical representations: Zhou et al., 2012). The effects of proteinopathies (colored) on intrinsic network architecture (gray) in the frontal (FL), temporal (TL), and parietal (PL) lobes in each cerebral hemisphere are shown. The side panels show coronal (left) and axial (right) brain MRI sections representing typical atrophy profiles produced by mutations in the microtubule-associated protein tau gene (MAPT: relatively symmetrical, relatively localized anterior temporal and inferior frontal atrophy) and the progranulin gene (GRN: strongly asymmetric fronto-temporo-parietal atrophy). Behavioral variant frontotemporal dementia was the presenting syndrome in each case. Initial involvement of an epicenter (ep) in right orbitofrontal cortex propagates over common transneuronal pathways in the target network (green); we hypothesize that, in addition, MAPT mutations may preferentially damage shorter anatomical pathways within the target network (dark blue) and connected off-target network elements (light blue), while GRN mutations may preferentially damage longer intrahemispheric pathways within the target network (red) and connected off-target pathways (magenta) via a catastrophic reamplification process. This modulation of the basic diffusive template for network disintegration anticipates the distinctive atrophy profiles observed.