The possible molecular basis for the previously described antagonistic interactions between adenosine A ₁ receptors (A ₁ R) and dopamine D ₁ receptors (D ₁ R) in the brain have been studied in mouse fibroblast Ltk ⁻ cells cotransfected with human A ₁ R and D ₁ R cDNAs or with human A ₁ R and dopamine D ₂ receptor (long-form) (D ₂ R) cDNAs and in cortical neurons in culture. A ₁ R and D ₁ R, but not A ₁ R and D ₂ R, were found to coimmunoprecipitate in cotransfected fibroblasts. This selective A ₁ R/D ₁ R heteromerization disappeared after pretreatment with the D ₁ R agonist, but not after combined pretreatment with D ₁ R and A ₁ R agonists. A high degree of A ₁ R and D ₁ R colocalization, demonstrated in double immunofluorescence experiments with confocal laser microscopy, was found in both cotransfected fibroblast cells and cortical neurons in culture. On the other hand, a low degree of A ₁ R and D ₂ R colocalization was observed in cotransfected fibroblasts. Pretreatment with the A ₁ R agonist caused coclustering (coaggregation) of A ₁ R and D ₁ R, which was blocked by combined pretreatment with the D ₁ R and A ₁ R agonists in both fibroblast cells and in cortical neurons in culture. Combined pretreatment with D ₁ R and A ₁ R agonists, but not with either one alone, substantially reduced the D ₁ R agonist-induced accumulation of cAMP. The A ₁ R/D ₁ R heteromerization may be one molecular basis for the demonstrated antagonistic modulation of A ₁ R of D ₁ R receptor signaling in the brain. The persistence of A ₁ R/D ₁ R heteromerization seems to be essential for the blockade of A ₁ R agonist-induced A ₁ R/D ₁ R coclustering and for the desensitization of the D ₁ R agonist-induced cAMP accumulation seen on combined pretreatment with D ₁ R and A ₁ R agonists, which indicates a potential role of A ₁ R/D ₁ R heteromers also in desensitization mechanisms and receptor trafficking.

Endocannabinoids act as neuromodulatory and neuroprotective cues by engaging type 1 cannabinoid receptors. These receptors are highly abundant in the basal ganglia and play a pivotal role in the control of motor behaviour. An early downregulation of type 1 cannabinoid receptors has been documented in the basal ganglia of patients with Huntington’s disease and animal models. However, the pathophysiological impact of this loss of receptors in Huntington’s disease is as yet unknown. Here, we generated a double-mutant mouse model that expresses human mutant huntingtin exon 1 in a type 1 cannabinoid receptor-null background, and found that receptor deletion aggravates the symptoms, neuropathology and molecular pathology of the disease. Moreover, pharmacological administration of the cannabinoid Δ⁹-tetrahydrocannabinol to mice expressing human mutant huntingtin exon 1 exerted a therapeutic effect and ameliorated those parameters. Experiments conducted in striatal cells show that the mutant huntingtin-dependent downregulation of the receptors involves the control of the type 1 cannabinoid receptor gene promoter by repressor element 1 silencing transcription factor and sensitizes cells to excitotoxic damage. We also provide in vitro and in vivo evidence that supports type 1 cannabinoid receptor control of striatal brain-derived neurotrophic factor expression and the decrease in brain-derived neurotrophic factor levels concomitant with type 1 cannabinoid receptor loss, which may contribute significantly to striatal damage in Huntington’s disease. Altogether, these results support the notion that downregulation of type 1 cannabinoid receptors is a key pathogenic event in Huntington’s disease, and suggest that activation of these receptors in patients with Huntington’s disease may attenuate disease progression.

It has been well documented that neurotrophins, including brain-derived neurotrophic factor (BDNF), are severely affected in Alzheimer9s disease (AD) but their administration faces a myriad of technical challenges. Here we took advantage of the early astrogliosis observed in an amyloid mouse model of AD (5xFAD) and used it as an internal sensor to administer BDNF conditionally and locally. We first demonstrate the relevance of BDNF release from astrocytes by evaluating the effects of co-culturing wild type neurons and BDNF-deficient astrocytes. Next, we crossed 5xFAD mice with pGFAP:BDNF mice (only males were used) to create 5xFAD mice that over-express BDNF when and where astrogliosis is initiated (5xF:pGB mice). We evaluated the behavioral phenotype of these mice. We first found that BDNF from astrocytes is crucial for dendrite outgrowth and spine number in cultured wild type neurons. Double mutant 5xF:pGB mice displayed improvements in cognitive tasks as compared to 5xFAD littermates. In these mice, there was a rescue of BDNF/TrkB downstream signaling activity associated with an improvement of dendritic spine density and morphology. Clusters of synaptic markers, PSD-95 and synaptophysin, were also recovered in 5xF:pGB as compared to 5xFAD mice as well as the number of pre-synaptic vesicles at excitatory synapses. Additionally, experimentally-evoked LTP in vivo was increased in 5xF:pGB mice. The beneficial effects of conditional BDNF production and local delivery at the location of active neuropathology highlight the potential to use endogenous biomarkers with early onset such as astrogliosis as regulators of neurotrophic therapy in AD. Significance statement Recent evidence places astrocytes as pivotal players during synaptic plasticity and memory processes. In the present work we first provide evidence that astrocytes are essential for neuronal morphology via BDNF release. We then crossed transgenic mice (5xFAD mice) with the transgenic pGFAP-BDNF mice which express BDNF under the glial fibrillary astrocytic protein (GFAP) promoter. The resultant double mutant mice 5xF:pGB mice displayed a full rescue of hippocampal BDNF loss and related signaling compared with 5xFAD mice and a significant and specific improvement in all the evaluated cognitive tasks. These improvements did not correlate with amelioration of beta amyloid load or hippocampal adult neurogenesis rate but were accompanied by a dramatic recovery of structural and functional synaptic plasticity.

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder caused by an expanded CAG/polyglutamine repeat in the coding region of the huntingtin (htt) gene. Although HD is classically considered a motor disorder, there is now considerable evidence that early cognitive deficits appear in patients before the onset of motor disturbances. Here we demonstrate early impairment of long-term spatial and recognition memory in heterozygous HD knock-in mutant mice (Hdh^Q7/Q111), a genetically accurate HD mouse model. Cognitive deficits are associated with reduced hippocampal expression of CREB-binding protein (CBP) and diminished levels of histone H3 acetylation. In agreement with reduced CBP, the expression of CREB/CBP target genes related to memory, such c-fos, Arc and Nr4a2, was significantly reduced in the hippocampus of Hdh^Q7/Q111 mice compared with wild-type mice. Finally, and consistent with a role of CBP in cognitive impairment in Hdh^Q7/Q111 mice, administration of the histone deacetylase inhibitor trichostatin A rescues recognition memory deficits and transcription of selective CREB/CBP target genes in Hdh^Q7/Q111 mice. These findings demonstrate an important role for CBP in cognitive dysfunction in HD and suggest the use of histone deacetylase inhibitors as a novel therapeutic strategy for the treatment of memory deficits in this disease.

Mitochondria-associated membranes (MAMs) are dynamic structures that communicate endoplasmic reticulum (ER) and mitochondria allowing calcium transfer between these two organelles. Since calcium dysregulation is an important hallmark of several neurodegenerative diseases, disruption of MAMs has been speculated to contribute to pathological features associated with these neurodegenerative processes. In Huntington's disease (HD), mutant huntingtin induces the selective loss of medium spiny neurons within the striatum. The cause of this specific susceptibility remain unclear. However, defects on mitochondrial dynamics and bioenergetics have been proposed as critical contributors, causing accumulation of fragmented mitochondria and subsequent Ca2+ homeostasis alterations. In the present work, we show that aberrant Drp1-mediated mitochondrial fragmentation within the striatum of HD mutant mice, forces mitochondria to place far away from the ER disrupting the ER-mitochondria association and therefore causing drawbacks in Ca2+ efflux and an excessive production of mitochondria superoxide species. Accordingly, inhibition of Drp1 activity by Mdivi-1 treatment restored ER-mitochondria contacts, mitochondria dysfunction and Ca2+ homeostasis. In sum, our results give new insight on how defects on mitochondria dynamics may contribute to striatal vulnerability in HD and highlights MAMs dysfunction as an important factor involved in HD striatal pathology.

Background: Huntington's disease (HD) is an inherited neurodegenerative disorder triggered by an expanded polyglutamine tract in huntingtin that is thought to confer a new conformational property on this large protein. The propensity of small amino-terminal fragments with mutant, but not wild-type, glutamine tracts to self-aggregate is consistent with an altered conformation but such fragments occur relatively late in the disease process in human patients and mouse models expressing full-length mutant protein. This suggests that the altered conformational property may act within the full-length mutant huntingtin to initially trigger pathogenesis. Indeed, genotype-phenotype studies in HD have defined genetic criteria for the disease initiating mechanism, and these are all fulfilled by phenotypes associated with expression of full-length mutant huntingtin, but not amino-terminal fragment, in mouse models. As the in vitro aggregation of amino-terminal mutant huntingtin fragment offers a ready assay to identify small compounds that interfere with the conformation of the polyglutamine tract, we have identified a number of aggregation inhibitors, and tested whether these are also capable of reversing a phenotype caused by endogenous expression of mutant huntingtin in a striatal cell line from the Hdh ^Q111/Q111 knock-in mouse. Results: We screened the NINDS Custom Collection of 1,040 FDA approved drugs and bioactive compounds for their ability to prevent in vitro aggregation of Q58-htn 1–171 amino terminal fragment. Ten compounds were identified that inhibited aggregation with IC₅₀ < 15 μM, including gossypol, gambogic acid, juglone, celastrol, sanguinarine and anthralin. Of these, both juglone and celastrol were effective in reversing the abnormal cellular localization of full-length mutant huntingtin observed in mutant Hdh ^Q111/Q111 striatal cells. Conclusions: At least some compounds identified as aggregation inhibitors also prevent a neuronal cellular phenotype caused by full-length mutant huntingtin, suggesting that in vitro fragment aggregation can act as a proxy for monitoring the disease-producing conformational property in HD. Thus, identification and testing of compounds that alter in vitro aggregation is a viable approach for defining potential therapeutic compounds that may act on the deleterious conformational property of full-length mutant huntingtin.