Corticostriatal synaptic function in mouse models of Huntington's disease: early effects of huntingtin repeat length and protein load

Abstract

Huntington's disease (HD) is an autosomal dominant, late onset, neurodegenerative disease characterized by motor deficits and dementia that is caused by expansion of a CAG repeat in the HD gene. Clinical manifestations result from selective neuronal degeneration of predominantly GABAergic striatal medium-sized spiny neurons (MSNs). A growing number of studies demonstrate that personality, mood and cognitive disturbances are some of the earliest signs of HD and may reflect synaptic dysfunction prior to neuronal loss. Previous studies in striatal MSNs demonstrated early alterations in NMDA-type glutamate receptor currents in several HD mouse models, as well as evidence for presynaptic dysfunction prior to disease manifestations in the R6/2 HD fragment mouse model. We have compared corticostriatal synaptic function in full-length, human HD gene-carrying YAC transgenic mice expressing a non-pathogenic CAG repeat (YAC18; control) with three increasingly severe variants of pathogenic HD gene-expressing mice (YAC72 and two different lines of YAC128), at ages that precede any detectable disease phenotype. We report presynaptic dysfunction and a propensity towards synaptic depression in YAC72 and YAC128 compared to YAC18 mice, and, in the most severe model, we also observed altered AMPA receptor function. When normalized to evoked AMPAR currents, postsynaptic NMDAR currents are augmented in all three pathogenic HD YAC variants. These findings demonstrate multiple perturbations to corticostriatal synaptic function in HD mice, furthering our understanding of the early effects of the HD mutation that may contribute to cognitive dysfunction, mood disorders and later development of more serious dysfunction. Furthermore, this study provides a set of neurophysiological sequelae against which to test and compare other mouse models and potential therapies in HD.

Figure 1. Spontaneous glutamatergic transmission and AMPAR function

A: top, a typical example of AMPAR-mediated spontaneous EPSCs (sEPSCs) obtained by WC recording of striatal MSNs in the absence of GABA_A transmission (100 μm PTX, V_h−70 mV, YAC18); bottom, average peak-scaled events, for clarity only three genotypes are shown. B, sEPSC mean cell frequencies are not significantly altered across genotypes. C, sEPSC interevent intervals were also not significantly different, although a trend is apparent, in YAC128(53) MSNs with respect to YAC18. D, sEPSC decay kinetics are significantly faster in YAC128(53) MSNs than YAC18 (also see A). E, sEPSC mean cell amplitudes are not significantly different between genotypes (inset), whereas mean YAC128(53) sEPSC amplitude cumulative probability plots demonstrate a significantly higher proportion of smaller events with respect to YAC18. F, sEPSC amplitude bins similarly demonstrate a higher frequency of smaller AMPAR-mediated events, in addition to a lower frequency of medium-sized events in YAC128(53) MSNs with respect to YAC18. *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 2. Altered paired-pulse facilitation (PPF)

Left, representative examples (traces are averages of 3 sweeps). Pairing two evoked EPSCs (interstimulus interval 100 ms) results in facilitation of the second response. Right: the degree of facilitation is significantly greater at all three mhtt-expressing mouse corticostriatal synapses, suggesting decreased probability of initial presynaptic release (*P < 0.05, ***P < 0.001).

Figure 3. Reduction of evoked current magnitude is elevated in mhtt-expressing MSNs

A, 10 min of afferent stimulation resulted in a reduction in the amplitude of eEPSCs over time. Representative example (top) and grouped data (bottom) demonstrating that YAC72 MSN AMPAR-mediated current (V_h−70 mV) reduced with repetitive stimulation, in the absence of any alteration to access resistance (R_a). B, these reductions were significant (*P < 0.05, ***P < 0.001) in YAC18 MSNs (although modest) and both YAC72 and YAC128(53) MSNs. eEPSC amplitude reductions were significantly greater in YAC72 and YAC128(53) MSNs than those in YAC18 MSNs (†P < 0.05, ††P < 0.01).

Figure 4. Stimulus-evoked currents and altered NMDAR transmission in mhtt-expressing MSNs

Representative traces of predominantly AMPAR-mediated eEPSCs (I_AMPA) produced by 50 and 150 μA stimulation in YAC mice are shown (top, averages of 3 sweeps V_h−70 mV). A, at near-maximal stimulation, AMPAR-mediated eEPSC amplitudes are significantly reduced in YAC72 and YAC128(53) MSNs. B, eEPSC AMPAR current decay kinetics are not significantly different across genotypes. C, isolated NMDAR eEPSC amplitudes are similar across genotypes (inset, same cells as shown for I_AMPA, V_h+60 mV, 10 μm CNQX). D, the ratio of isolated NMDAR eEPSC peak amplitude to AMPAR eEPSC peak amplitude is significantly higher in all mhtt-expressing MSNs than that observed in YAC18 (inset, NMDAR currents shown in C peak scaled to I_AMPA peak amplitudes, above respective genotypes). E, isolated NMDAR eEPSC slow component decay time constants are significantly longer in YAC128(53) than YAC18 MSNs. F, normalized current–voltage relationships demonstrate that Mg²⁺ sensitivity is equivalent across genotypes. *P < 0.05, **P < 0.01, ***P < 0.001.