Alterations in striatal synaptic transmission are consistent across genetic mouse models of Huntington's disease

Abstract

Since the identification of the gene responsible for HD (Huntington's disease), many genetic mouse models have been generated. Each employs a unique approach for delivery of the mutated gene and has a different CAG repeat length and background strain. The resultant diversity in the genetic context and phenotypes of these models has led to extensive debate regarding the relevance of each model to the human disorder. Here, we compare and contrast the striatal synaptic phenotypes of two models of HD, namely the YAC128 mouse, which carries the full-length huntingtin gene on a yeast artificial chromosome, and the CAG140 KI (knock-in) mouse, which carries a human/mouse chimaeric gene that is expressed in the context of the mouse genome, with our previously published data obtained from the R6/2 mouse, which is transgenic for exon 1 mutant huntingtin. We show that striatal MSNs (medium-sized spiny neurons) in YAC128 and CAG140 KI mice have similar electrophysiological phenotypes to that of the R6/2 mouse. These include a progressive increase in membrane input resistance, a reduction in membrane capacitance, a lower frequency of spontaneous excitatory postsynaptic currents and a greater frequency of spontaneous inhibitory postsynaptic currents in a subpopulation of striatal neurons. Thus, despite differences in the context of the inserted gene between these three models of HD, the primary electrophysiological changes observed in striatal MSNs are consistent. The outcomes suggest that the changes are due to the expression of mutant huntingtin and such alterations can be extended to the human condition.

Keywords: ACSF, artificial cerebrospinal fluid; AP5, dl-2-amino-5-phosphonovaleric acid; BIC, bicuculline methobromide; CAG 140 knock-in mouse model; CNQX, 6-cyano-7-nitroquinoxaline-2,3-dione; EPSC, excitatory postsynaptic current; GABAA, γ-aminobutyric acid type A; HD, Huntington's disease; HF, high frequency; Huntington's disease; IPSC, inhibitory postsynaptic current; KI, knock-in; LF, low frequency; MSN, medium-sized spiny neuron; WT, wild-type; YAC128 mouse model; electrophysiology; striatum.

Figure 1. Spontaneous EPSCs from MSNs of YAC128 and CAG140 KI mice

(A) Typical recordings from 6-month YAC128 and 12-month CAG140 KI mice in ACSF. Mean frequencies±S.E.M. are plotted for each age. (B) Mean frequencies±S.E.M. for spontaneous EPSCs isolated by BIC. The variation in frequency between the respective WTs at 6 and 12 months is likely due to the difference in background strain. (C) Percentage change in EPSC frequency after BIC application. Sample sizes are indicated inside the bars. Sample sizes in (C) are the same as (B). Statistical significance with respect to WTs is shown by *P<0.05, **P<0.01 and ***P<0.001.

Figure 2. Spontaneous IPSCs from MSNs of YAC128 and CAG140 KI mice

Typical recordings from YAC128 at 6 months and CAG140 KI at 12 months. Two populations of cells were identified in mutant animals only. One displayed LF IPSCs and the other HF IPSCs. IPSCs were isolated at a holding potential of +10 mV and in the presence of CNQX and AP5. Results show mean frequencies±S.E.M. for each age. Statistical significance with respect to WTs is indicated by **P<0.01 and ***P<0.001.

Figure 3. Decay kinetics of spontaneous postsynaptic currents in MSNs of YAC128 and CAG140 KI mice

(A) Top: examples of averaged spontaneous EPSCs from WT (grey) and mutant (black) mice (YAC128, 6 months; CAG140 KI, 12 months). Middle: means±S.E.M. of decay times for each age. Bottom: means±S.E.M. of half-amplitude durations for each age. (B) Top: examples of averaged spontaneous IPSCs from WT (grey) and mutant (black) mice (YAC128, 6 months; CAG140 KI, 12 months). Middle: means±S.E.M. of decay times for each age. Bottom: means±S.E.M. of half-amplitude durations for each age. Sample sizes are shown in Figure 1(A) for EPSCs and Figure 2 for IPSCs. Statistical significance is indicated by *P<0.05, **P<0.01 and ***P<0.001 with respect to WT.

Figure 4. Timeline of key phenotypes including striatal and cortical electrophysiology

Age in days and months are indicated within the arrows. Each phenotype is indicated such that the labels begin at the earliest age detected. Timelines are shown for R6/2 (top), YAC128 (middle) and CAG140 KI (bottom) with key behavioural, cognitive and morphological phenotypes (green bars), as well as cortical (beige bars) and striatal (blue bars) electrophysiological phenotypes. Upward arrows indicate increases or improvements; downward arrows indicate decreases or deficits. Electrophysiological striatal and cortical phenotypes are summarized from the present paper and from Cummings et al. (2009) and Cepeda et al. (2003, . Behavioural, cognitive and morphological phenotypes are from Lione et al. (1999), Meade et al. (2002), Hickey et al. (2008), Menalled et al. (2003), Van Raamsdonk et al. (2005) and Slow et al. (2003).