Final Report Summary - ASTRO-HD (Role of astrocytes in Huntington's Disease: characterization of a novel mouse model with targeted expression of mutant huntingtin in the striatum)
Huntington's disease (HD) is an inherited neurodegenerative disease, which has no cure, resulting from the mutation of the gene coding for huntingtin protein (htt). Clinically, HD patients present with progressive involuntary spasmodic movements, and cognitive impairments. Post-mortem studies of HD patients reveal severe atrophy of the striatum, reflecting major neuronal death specifically of medium spiny neurons (MSNs). The etiology of HD and the path leading to MSNs death remains unknown. Mutant htt is ubiquitously expressed in various types of cells, including glia, but causes selective neurodegeneration. It remains largely unknown whether and how mutant huntingtin in glia can contribute to the neurological symptoms of HD.
The main objective of this project was to determine whether astrocytes contribute to the pathogenesis of Huntington Disease. This objective has been achieved using two different experimental models.
(1) A novel double transgenic mice generated by crossing Glial Fibrillary Acidic Protein-Cre (GFAP-Cre) mice with BACHD mice. In BACHD mice, mutated huntingtin (mhtt) is overexpressed in both neurons and astrocytes. Since exon 1 is flanked by loxP sites in BACHD mice, the expression of mhtt can be switched off in astrocytes by crossing these animals with GFAP-Cre mice. In our new model, mhtt would not be expressed in astrocytes. Using this mouse model, we aimed to characterize both the function of neurons using electrophysiological recordings and their morphology during the course of the disease and determine whether the absence of mhtt into astrocytes significantly change their two indexes.
(2) A model we recently developed in which the expression of the mutant huntingtin, responsible for the disease, is restricted to striatal astrocytes using a lentiviral approach. Using these mouse models, we aimed to characterize both the function of neurons using electrophysiological recordings and their morphology during the course of the disease and determine whether the absence of mhtt into astrocytes significantly change their two indexes. We ultimately aimed to characterize whether the expression of mhtt only into astrocytes is sufficient to alter the function and/or the morphology of striatal neurons.
The results demonstrate that long-term effects of ubiquitous mhtt expression in BACHD mice lead to 'pre-degenerative' functional and morphological changes in MSNs. The vulnerability of this cell population mainly consists in dramatic spine loss, likely indicative of major decrease of excitatory input. However, MSNs in BACHD mice simultaneously showed significant hyperexcitability compared to WT and a maintained, if not increased, excitatory synaptic signaling. Thus, despite compensatory mechanisms represented by the loss of dendritic spines, BACHD mice still show increased glutamatergic activity in the striatum that might result in excitotoxic phenomena, which could underlie the neuropathological features of this mouse model. Preliminary results show that mutant huntingtin restricted to striatal astrocytes might lead to morphological changes in the MSNs in terms of spine loss with no change in passive nor active properties.
The efforts devoted to this project will allow the scientific community to better understand how the expression of pathologic proteins can lead to astrocyte dysfunction and initiate neurodegeneration that are the hallmark of diseases and disorders including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), other tauopathies, HD, and Parkinson's disease.
Our results provided insight into:
- fundamental aspects of the investigation of both functional and structural functions of the neurons in pathological context. Because neuronal structure is a key determinant of function, our results show that plasticity reserve and compensatory mechanisms are to be taken into consideration in disease conditions and in potential therapeutic strategies.
-Reinforcing the pivotal role of astrocytes into the pathogenesis and therefore give ground to find potential therapeutic targets and treatments that would benefit a widespread patient population.
The main objective of this project was to determine whether astrocytes contribute to the pathogenesis of Huntington Disease. This objective has been achieved using two different experimental models.
(1) A novel double transgenic mice generated by crossing Glial Fibrillary Acidic Protein-Cre (GFAP-Cre) mice with BACHD mice. In BACHD mice, mutated huntingtin (mhtt) is overexpressed in both neurons and astrocytes. Since exon 1 is flanked by loxP sites in BACHD mice, the expression of mhtt can be switched off in astrocytes by crossing these animals with GFAP-Cre mice. In our new model, mhtt would not be expressed in astrocytes. Using this mouse model, we aimed to characterize both the function of neurons using electrophysiological recordings and their morphology during the course of the disease and determine whether the absence of mhtt into astrocytes significantly change their two indexes.
(2) A model we recently developed in which the expression of the mutant huntingtin, responsible for the disease, is restricted to striatal astrocytes using a lentiviral approach. Using these mouse models, we aimed to characterize both the function of neurons using electrophysiological recordings and their morphology during the course of the disease and determine whether the absence of mhtt into astrocytes significantly change their two indexes. We ultimately aimed to characterize whether the expression of mhtt only into astrocytes is sufficient to alter the function and/or the morphology of striatal neurons.
The results demonstrate that long-term effects of ubiquitous mhtt expression in BACHD mice lead to 'pre-degenerative' functional and morphological changes in MSNs. The vulnerability of this cell population mainly consists in dramatic spine loss, likely indicative of major decrease of excitatory input. However, MSNs in BACHD mice simultaneously showed significant hyperexcitability compared to WT and a maintained, if not increased, excitatory synaptic signaling. Thus, despite compensatory mechanisms represented by the loss of dendritic spines, BACHD mice still show increased glutamatergic activity in the striatum that might result in excitotoxic phenomena, which could underlie the neuropathological features of this mouse model. Preliminary results show that mutant huntingtin restricted to striatal astrocytes might lead to morphological changes in the MSNs in terms of spine loss with no change in passive nor active properties.
The efforts devoted to this project will allow the scientific community to better understand how the expression of pathologic proteins can lead to astrocyte dysfunction and initiate neurodegeneration that are the hallmark of diseases and disorders including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), other tauopathies, HD, and Parkinson's disease.
Our results provided insight into:
- fundamental aspects of the investigation of both functional and structural functions of the neurons in pathological context. Because neuronal structure is a key determinant of function, our results show that plasticity reserve and compensatory mechanisms are to be taken into consideration in disease conditions and in potential therapeutic strategies.
-Reinforcing the pivotal role of astrocytes into the pathogenesis and therefore give ground to find potential therapeutic targets and treatments that would benefit a widespread patient population.