Cortical circuit alterations precede motor impairments in Huntington\u2019s disease mice

Johanna Burgold,Fabian Hosp,Rüdiger Klein,Sabine Liebscher,Irina Dudanova,Sara Gutiérrez-Ángel,Jakob Maximilian Bader,Elena Katharina Schulz-Trieglaff,Matthias Mann,Thomas Arzberger,Kerstin Voelkl

doi:10.1038/s41598-019-43024-w

Abstract

Huntington’s disease (HD) is a devastating hereditary movement disorder, characterized by degeneration of neurons in the striatum and cortex. Studies in human patients and mouse HD models suggest that disturbances of neuronal function in the neocortex play an important role in disease onset and progression. However, the precise nature and time course of cortical alterations in HD have remained elusive. Here, we use chronic in vivo two-photon calcium imaging to longitudinally monitor the activity of identified single neurons in layer 2/3 of the primary motor cortex in awake, behaving R6/2 transgenic HD mice and wildtype littermates. R6/2 mice show age-dependent changes in cortical network function, with an increase in activity that affects a large fraction of cells and occurs rather abruptly within one week, preceeding the onset of motor defects. Furthermore, quantitative proteomics demonstrate a pronounced downregulation of synaptic proteins in the cortex, and histological analyses in R6/2 mice and human HD autopsy cases reveal a reduction in perisomatic inhibitory synaptic contacts on layer 2/3 pyramidal cells. Taken together, our study provides a time-resolved description of cortical network dysfunction in behaving HD mice and points to disturbed excitation/inhibition balance as an important pathomechanism in HD.

Highlights

IntroductionMediated expression of mRuby[2] (red) and GCaMP6s (green) in layer 2/3 (L2/3) neurons in M1. (e) Imaging setup
Mediated expression of mRuby[2] and GCaMP6s in layer 2/3 (L2/3) neurons in M1. (e) Imaging setup
The exact age of onset of motor deficits varies between different R6/2 colonies and is dependent on the CAG repeat length, which can increase over generations due to genetic instability of the repeats

Summary

Introduction

Mediated expression of mRuby[2] (red) and GCaMP6s (green) in L2/3 neurons in M1. (e) Imaging setup. Reduced dendritic arborizations and a decline in the density and stability of dendritic spines on PCs were observed in the somatosensory cortex[22,23,24] These defects were paralleled by lower levels of several synaptic proteins and a decrease in excitatory synapse density, detected at an advanced disease stage[24]. Electrophysiological recordings in various HD mice revealed changes in both excitatory and inhibitory inputs onto layer 2/3 (L2/3) PCs, with a consistent reduction in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs)[19,25,26] Despite these insights into the cortical circuit impairments in HD, it has remained unclear how cortical network function is affected in vivo during the presymptomatic phase and at disease onset, and which molecular and circuit mechanisms underlie these functional alterations. Proteomic analyses show a pronounced downregulation of synaptic proteins in the cortex, whereas histological findings in HD mouse brains and in human postmortem tissue point to a loss of inhibitory synapses on PCs, suggesting that excitation/inhibition dysbalance plays a role in the cortical dysfunction in HD

Methods

Results

Conclusion