Synaptic RTP801 contributes to motor-learning dysfunction in Huntington’s disease

Abstract

RTP801/REDD1 is a stress-responsive protein that mediates mutant huntingtin (mhtt) toxicity in cellular models and is up regulated in Huntington’s disease (HD) patients’ putamen. Here, we investigated whether RTP801 is involved in motor impairment in HD by affecting striatal synaptic plasticity. To explore this hypothesis, ectopic mhtt was over expressed in cultured rat primary neurons. Moreover, the protein levels of RTP801 were assessed in homogenates and crude synaptic fractions from human postmortem HD brains and mouse models of HD. Finally, striatal RTP801 expression was knocked down with adeno-associated viral particles containing a shRNA in the R6/1 mouse model of HD and motor learning was then tested. Ectopic mhtt elevated RTP801 in synapses of cultured neurons. RTP801 was also up regulated in striatal synapses from HD patients and mouse models. Knocking down RTP801 in the R6/1 mouse striatum prevented motor-learning impairment. RTP801 silencing normalized the Ser473 Akt hyperphosphorylation by downregulating Rictor and it induced synaptic elevation of calcium permeable GluA1 subunit and TrkB receptor levels, suggesting an enhancement in synaptic plasticity. These results indicate that mhtt-induced RTP801 mediates motor dysfunction in a HD murine model, revealing a potential role in the human disease. These findings open a new therapeutic framework focused on the RTP801/Akt/mTOR axis.

Rtp801 Regulates Motor Cortex Synaptic Transmission and Learning

“…Hence, we first isolated cortical and striatal crude synaptosomes from adult postmortem human brain, adult rat and mouse brains and from cultured rat cortical neurons. In all samples we observed the presence of RTP801 or its enrichment in crude isolated synaptic terminals in comparison to the initial homogenates ( Fig 1 A ), corroborating our own previous results (Martín-Flores et al , 2020). Interestingly, in cultured cortical neurons, we observed that RTP801 was expressed in the soma, dendrites and dendritic spines ( Fig 1 B ).…”

“…Hence, we initially confirmed that RTP801 was present in the synapses from a wide range of human and murine samples, as we previously described in HD murine models and HD human postmortem samples (Martín-Flores et al , 2020). Interestingly, RTP801 was highly enriched in human and rat crude synaptosomes but not that elevated in synaptic WT mice samples.…”

“…Moreover, RTP801 downregulation alleviated stress-induced neurodegeneration in a mouse model of genetic PD (Zhang et al , 2018). More recently, our group described that synaptic RTP801 mediated motor-learning dysfunction in the R6/1 mouse model of HD (Martín-Flores et al , 2020). However, its potential physiological synaptic role has never been investigated in depth.…”

“…RTP801 induces neuron death by a sequential inactivation of mTOR and the survival kinase Akt (Malagelada et al , 2008) via the tuberous sclerosis complex 1/2 (TSC1/2). Regarding HD, RTP801 levels are highly increased in HD human brains, in differentiated neurons derived from induced pluripotent stem cells (iPSC) from HD patients (Martín-Flores et al , 2016) and in striatal synapses from HD mouse models (Martín-Flores et al , 2020). Besides, in neuronal models of the disease, RTP801 mediates mutant huntingtin (mhtt)-induced toxicity (Martin-Flores et al , 2015).…”

“…Besides, in neuronal models of the disease, RTP801 mediates mutant huntingtin (mhtt)-induced toxicity (Martin-Flores et al , 2015). Importantly, RTP801 contributes to motor-learning dysfunction in HD since RTP801 knockdown prevents from the appearance of motor learning deficits in the R6/1 model of the disease (Martín-Flores et al , 2020). This suggests that synaptic RTP801 deregulation is a common hallmark in neurodegeneration.…”