RTP801/REDD1 contributes to neuroinflammation severity and memory impairments in Alzheimer's disease

RTP801/REDD1 is a stress-regulated protein whose upregulation is necessary and sufficient to trigger neuronal death. Its downregulation in Parkinson's and Huntington's disease models ameliorates the pathological phenotypes. In the context of Alzheimer's disease (AD), the coding gene for RTP801, DDIT4, is responsive to Aβ and modulates its cytotoxicity in vitro. Also, RTP801 mRNA levels are increased in AD patients' lymphocytes. However, the involvement of RTP801 in the pathophysiology of AD has not been yet tested. Here, we demonstrate that RTP801 levels are increased in postmortem hippocampal samples from AD patients. Interestingly, RTP801 protein levels correlated with both Braak and Thal stages of the disease and with GFAP expression. RTP801 levels are also upregulated in hippocampal synaptosomal fractions obtained from murine 5xFAD and rTg4510 mice models of the disease. A local RTP801 knockdown in the 5xFAD hippocampal neurons with shRNA-containing AAV particles ameliorates cognitive deficits in 7-month-old animals. Upon RTP801 silencing in the 5xFAD mice, no major changes were detected in hippocampal synaptic markers or spine density. Importantly, we found an unanticipated recovery of several gliosis hallmarks and inflammasome key proteins upon neuronal RTP801 downregulation in the 5xFAD mice. Altogether our results suggest that RTP801 could be a potential future target for theranostic studies since it could be a biomarker of neuroinflammation and neurotoxicity severity of the disease and, at the same time, a promising therapeutic target in the treatment of AD.

Longitudinal evaluation of a novel BChE PET tracer as an early in vivo biomarker in the brain of a mouse model for Alzheimer disease

Purpose: The increase in butyrylcholinesterase (BChE) activity in the brain of Alzheimer disease (AD) patients and animal models of AD position this enzyme as a potential biomarker of the disease. However, the information on the ability of BChE to serve as AD biomarker is contradicting, also due to scarce longitudinal studies of BChE activity abundance. Here, we report ¹¹C-labeling, in vivo stability, biodistribution, and longitudinal study on BChE abundance in the brains of control and 5xFAD (AD model) animals, using a potent BChE selective inhibitor, [¹¹C]4, and positron emission tomography (PET) in combination with computerised tomography (CT). We correlate the results with in vivo amyloid beta (Aβ) deposition, longitudinally assessed by [¹⁸F]florbetaben-PET imaging.

Methods: [¹¹C]4 was radiolabelled through ¹¹C-methylation. Metabolism studies were performed on blood and brain samples of female wild type (WT) mice. Biodistribution studies were performed in female WT mice using dynamic PET-CT imaging. Specific binding was demonstrated by ex vivo and in vivo PET imaging blocking studies in female WT and 5xFAD mice at the age of 7 months. Longitudinal PET imaging of BChE was conducted in female 5xFAD mice at 4, 6, 8, 10 and 12 months of age and compared to age-matched control animals. Additionally, Aβ plaque distribution was assessed in the same mice using [¹⁸F]florbetaben at the ages of 2, 5, 7 and 11 months. The results were validated by ex vivo staining of BChE at 4, 8, and 12 months and Aβ at 12 months on brain samples.

Results: [¹¹C]4 was produced in sufficient radiochemical yield and molar activity for the use in PET imaging. Metabolism and biodistribution studies confirmed sufficient stability in vivo, the ability of [¹¹C]4 to cross the blood brain barrier (BBB) and rapid washout from the brain. Blocking studies confirmed specificity of the binding. Longitudinal PET studies showed increased levels of BChE in the cerebral cortex, hippocampus, striatum, thalamus, cerebellum and brain stem in aged AD mice compared to WT littermates. [¹⁸F]Florbetaben-PET imaging showed similar trend of Aβ plaques accumulation in the cerebral cortex and the hippocampus of AD animals as the one observed for BChE at ages 4 to 8 months. Contrarily to the results obtained by ex vivo staining, lower abundance of BChE was observed in vivo at 10 and 12 months than at 8 months of age.

Conclusions: The BChE inhibitor [¹¹C]4 crosses the BBB and is quickly washed out of the brain of WT mice. Comparison between AD and WT mice shows accumulation of the radiotracer in the AD-affected areas of the brain over time during the early disease progression. The results correspond well with Aβ accumulation, suggesting that BChE is a promising early biomarker for incipient AD.

Comparison of the biological impact of aerosol of e-vapor device with MESH® technology and cigarette smoke on human bronchial and alveolar cultures

Exposure to aerosol from electronic vapor (e-vapor) products has been suggested to result in less risk of harm to smokers than cigarette smoke (CS) exposure. Although many studies on e-vapor products have tested the effects of liquid formulations on cell cultures, few have evaluated the effects of aerosolized formulations. We examined the effects of acute exposure to the aerosol of an e-vapor device that uses the MESH® technology (IQOS® MESH, Philip Morris International) and to CS from the 3R4F reference cigarette on human organotypic bronchial epithelial culture and alveolar triculture models. In contrast to 3R4F CS exposure, exposure to the IQOS MESH aerosol (Classic Tobacco flavor) did not cause cytotoxicity in bronchial epithelial cultures or alveolar tricultures despite its greater concentrations of deposited nicotine (3- and 4-fold, respectively). CS exposure caused a marked decrease in the frequency and active area of ciliary beating in bronchial cultures, whereas IQOS MESH aerosol exposure did not. Global mRNA expression and secreted protein profiles revealed a significantly lower impact of IQOS MESH aerosol exposure than 3R4F CS exposure. Overall, our whole aerosol exposure study shows a clearly reduced impact of IQOS MESH aerosol relative to CS in bronchial and alveolar cultures, even at greater nicotine doses.

Comparison of the basic morphology and function of 3D lung epithelial cultures derived from several donors

In vitro models of the human lung play an essential role in evaluating the toxicity of inhaled compounds and understanding the development of respiratory diseases. Three-dimensional (3D) organotypic models derived from lung basal epithelial cells and grown at the air–liquid interface resemble human airway epithelium in multiple aspects, including morphology, cell composition, transcriptional profile, and xenobiotic metabolism. Whether the different characteristics of basal cell donors have an impact on model characteristics and responses remains unknown. In addition, studies are often conducted with 3D cultures from one donor, assuming a representative response on the population level. Whether this assumption is correct requires further investigation. In this study, we compared the morphology and functionality of 3D organotypic bronchial and small airway cultures from different donors at different weeks after air-lift to assess the interdonor variability in these parameters. The thickness, cell type composition, and transepithelial electrical resistance varied among the donors and over time after air-lift. Cilia beating frequency increased in response to isoproterenol treatment in both culture types, independent of the donor. The cultures presented low basal cytochrome P450 (CYP) 1A1/1B1 activity, but 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment induced CYP1A1/1B1 activity regardless of the donor. In conclusion, lung epithelial cultures prepared from different donors present diverse morphology but similar functionality and metabolic activity, with certain variability in their response to stimulation.

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3D lung cultures derived from various donors differed mostly at the morphological level.

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Epithelial thickness, presence of cysts, ciliation, and goblet cell number are donor dependent.

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Cilia beating frequency varied across donors but the response to isoproterenol was similar.

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CYP450 activity in response to xenobiotics was preserved across donors.

Astrocytic BDNF and TrkB regulate severity and neuronal activity in mouse models of temporal lobe epilepsy

Astrocytes have emerged as crucial regulators of neuronal network activity, synapse formation, and underlying behavioral and cognitive processes. Despite some pathways have been identified, the communication between astrocytes and neurons remains to be completely elucidated. Unraveling this communication is crucial to design potential treatments for neurological disorders like temporal lobe epilepsy (TLE). The BDNF and TrkB molecules have emerged as very promising therapeutic targets. However, their modulation can be accompanied by several off-target effects such as excitotoxicity in case of uncontrolled upregulation or dementia, amnesia, and other memory disorders in case of downregulation. Here, we show that BDNF and TrkB from astrocytes modulate neuronal dysfunction in TLE models. First, conditional overexpression of BDNF from astrocytes worsened the phenotype in the lithium-pilocarpine mouse model. Our evidences pointed out to the astrocytic pro-BDNF isoform as a major player of this altered phenotype. Conversely, specific genetic deletion of BDNF in astrocytes prevented the increase in the number of firing neurons and the global firing rate in an in vitro model of TLE. Regarding to the TrkB, we generated mice with a genetic deletion of TrkB specifically in hippocampal neurons or astrocytes. Interestingly, both lines displayed neuroprotection in the lithium-pilocarpine model but only the mice with genetic deletion of TrkB in astrocytes showed significantly preserved spatial learning skills. These data identify the astrocytic BDNF and TrkB molecules as promising therapeutic targets for the treatment of TLE.

Lack of Helios During Neural Development Induces Adult Schizophrenia-Like Behaviors Associated With Aberrant Levels of the TRIF-Recruiter Protein WDFY1

The role of the WDFY1 protein has been studied as a TLR3/4 scaffold/recruiting protein in the immune system and in different oncogenic conditions. However, its function in brain remains poorly understood. We have found that in mice devoid of Helios (He^-/- mice), a transcription factor specifically expressed during the development of the immune cells and the central nervous system, there is a permanent and sustained increase of Wdfy1 gene expression in the striatum and hippocampus. Interestingly, we observed that WDFY1 protein levels were also increased in the hippocampus and dorsolateral prefrontal cortex of schizophrenic patients, but not in the hippocampus of Alzheimer's disease patients with an associated psychotic disorder. Accordingly, young He^-/- mice displayed several schizophrenic-like behaviors related to dysfunctions in the striatum and hippocampus. These changes were associated with an increase in spine density in medium spiny neurons (MSNs) and with a decrease in the number and size of PSD-95-positive clusters in the stratum radiatum of the CA1. Moreover, these alterations in structural synaptic plasticity were associated with a strong reduction of neuronal NF-κB in the pyramidal layer of the CA1 in He^-/- mice. Altogether, our data indicate that alterations involving the molecular axis Helios-WDFY1 in neurons during the development of core brain regions could be relevant for the pathophysiology of neuropsychiatric disorders such as schizophrenia.

Meridianins and Lignarenone B as Potential GSK3β Inhibitors and Inductors of Structural Neuronal Plasticity

Glycogen Synthase Kinase 3 (GSK3) is an essential protein, with a relevant role in many diseases such as diabetes, cancer and neurodegenerative disorders. Particularly, the isoform GSK3β is related to pathologies such as Alzheimer's disease (AD). This enzyme constitutes a very interesting target for the discovery and/or design of new therapeutic agents against AD due to its relation to the hyperphosphorylation of the microtubule-associated protein tau (MAPT), and therefore, its contribution to neurofibrillary tangles (NFT) formation. An in silico target profiling study identified two marine molecular families, the indole alkaloids meridianins from the tunicate genus Aplidium, and lignarenones, the secondary metabolites of the shelled cephalaspidean mollusc Scaphander lignarius, as possible GSK3β inhibitors. The analysis of the surface of GSK3β, aimed to find possible binding regions, and the subsequent in silico binding studies revealed that both marine molecular families can act over the ATP and/or substrate binding regions. The predicted inhibitory potential of the molecules from these two chemical families was experimentally validated in vitro by showing a ~50% of increased Ser9 phosphorylation levels of the GSK3β protein. Furthermore, we determined that molecules from both molecular families potentiate structural neuronal plasticity in vitro. These results allow us to suggest that meridianins and lignarenone B could be used as possible therapeutic candidates for the treatment of GSK3β involved pathologies, such as AD.

The non-receptor tyrosine kinase Pyk2 modulates acute locomotor effects of cocaine in D1 receptor-expressing neurons of the nucleus accumbens

The striatum is critical for cocaine-induced locomotor responses. Although the role of D1 receptor-expressing neurons is established, underlying molecular pathways are not fully understood. We studied the role of Pyk2, a non-receptor, calcium-dependent protein-tyrosine kinase. The locomotor coordination and basal activity of Pyk2 knock-out mice were not altered and major striatal protein markers were normal. Cocaine injection increased Pyk2 tyrosine phosphorylation in mouse striatum. Pyk2-deficient mice displayed decreased locomotor response to acute cocaine injection. In contrast, locomotor sensitization and conditioned place preference were normal. Cocaine-activated ERK phosphorylation, a signaling pathway essential for these late responses, was unaltered. Conditional deletion of Pyk2 in the nucleus accumbens or in D1 neurons reproduced decreased locomotor response to cocaine, whereas deletion of Pyk2 in the dorsal striatum or in A2A receptor-expressing neurons did not. In mice lacking Pyk2 in D1-neurons locomotor response to D1 agonist SKF-81297, but not to an anticholinergic drug, was blunted. Our results identify Pyk2 as a regulator of acute locomotor responses to psychostimulants. They highlight the role of tyrosine phosphorylation pathways in striatal neurons and suggest that changes in Pyk2 expression or activation may alter specific responses to drugs of abuse, or possibly other behavioral responses linked to dopamine action.

CDK4 Regulates Lysosomal Function and mTORC1 Activation to Promote Cancer Cell Survival

Cyclin-dependent kinase 4 (CDK4) is well-known for its role in regulating the cell cycle, however, its role in cancer metabolism, especially mTOR signaling, is undefined. In this study, we established a connection between CDK4 and lysosomes, an emerging metabolic organelle crucial for mTORC1 activation. On the one hand, CDK4 phosphorylated the tumor suppressor folliculin (FLCN), regulating mTORC1 recruitment to the lysosomal surface in response to amino acids. On the other hand, CDK4 directly regulated lysosomal function and was essential for lysosomal degradation, ultimately regulating mTORC1 activity. Pharmacologic inhibition or genetic inactivation of CDK4, other than retaining FLCN at the lysosomal surface, led to the accumulation of undigested material inside lysosomes, which impaired the autophagic flux and induced cancer cell senescence in vitro and in xenograft models. Importantly, the use of CDK4 inhibitors in therapy is known to cause senescence but not cell death. To overcome this phenomenon and based on our findings, we increased the autophagic flux in cancer cells by using an AMPK activator in combination with a CDK4 inhibitor. The cotreatment induced autophagy (AMPK activation) and impaired lysosomal function (CDK4 inhibition), resulting in cell death and tumor regression. Altogether, we uncovered a previously unknown role for CDK4 in lysosomal biology and propose a novel therapeutic strategy to target cancer cells. SIGNIFICANCE: These findings uncover a novel function of CDK4 in lysosomal biology, which promotes cancer progression by activating mTORC1; targeting this function offers a new therapeutic strategy for cancer treatment.

Cyclin-Dependent Kinase 5 Dysfunction Contributes to Depressive-like Behaviors in Huntington's Disease by Altering the DARPP-32 Phosphorylation Status in the Nucleus Accumbens

BACKGROUND

Depression is the most common psychiatric condition in Huntington's disease (HD), with rates more than twice those found in the general population. At the present time, there is no established molecular evidence to use as a basis for depression treatment in HD. Indeed, in some patients, classic antidepressant drugs exacerbate chorea or anxiety. Cyclin-dependent kinase 5 (Cdk5) has been involved in processes associated with anxiety and depression. This study evaluated the involvement of Cdk5 in the development and prevalence of depressive-like behaviors in HD and aimed to validate Cdk5 as a target for depression treatment.

METHODS

We evaluated the impact of pharmacological inhibition of Cdk5 in depressive-like and anxiety-like behaviors in Hdh^+/Q111 knock-in mutant mice by using a battery of behavioral tests. Biochemical and morphological studies were performed to define the molecular mechanisms acting downstream of Cdk5 activation. A double huntingtin/DARPP-32 (dopamine- and cAMP-regulated phosphoprotein 32) knock-in mutant mouse was generated to analyze the role of DARPP-32 in HD depression.

RESULTS

We found that Hdh^+/Q111 mutant mice exhibited depressive-like, but not anxiety-like, behaviors starting at 2 months of age. Cdk5 inhibition by roscovitine infusion prevented depressive-like behavior and reduced DARPP-32 phosphorylation at Thr75 in the nucleus accumbens. Hdh^+/Q111 mice heterozygous for DARPP-32 Thr75Ala point mutation were resistant to depressive-like behaviors. We identified β-adducin phosphorylation as a Cdk5 downstream mechanism potentially mediating structural spine plasticity changes in the nucleus accumbens and depressive-like behavior.

CONCLUSIONS

These results point to Cdk5 in the nucleus accumbens as a critical contributor to depressive-like behaviors in HD mice by altering DARPP-32/β-adducin signaling and disrupting the dendritic spine cytoskeleton.

PKR knockout in the 5xFAD model of Alzheimer's disease reveals beneficial effects on spatial memory and brain lesions

Brain lesions in Alzheimer's disease (AD) include amyloid plaques made of Aβ peptides and neurofibrillary tangles composed of hyperphosphorylated tau protein with synaptic and neuronal loss and neuroinflammation. Aβ oligomers can trigger tau phosphorylation and neuronal alterations through activation of neuronal kinases leading to progressive cognitive decline. PKR is a ubiquitous pro-apoptotic serine/threonine kinase, and levels of activated PKR are increased in AD brains and AD CSF. In addition, PKR regulates negatively memory formation in mice. To assess the role of PKR in an AD in vivo model, we crossed 5xFAD transgenic mice with PKR knockout (PKRKO) mice and we explored the contribution of PKR on cognition and brain lesions in the 5xFAD mouse model of AD as well as in neuron-microglia co-cultures exposed to the innate immunity activator lipopolysaccharide (LPS). Nine-month-old double-mutant mice revealed significantly improved memory consolidation with the new object location test, starmaze test, and elevated plus maze test as compared to 5xFAD mice. Brain amyloid accumulation and BACE1 levels were statistically decreased in double-mutant mice. Apoptosis, neurodegeneration markers, and synaptic alterations were significantly reduced in double-mutant mice as well as neuroinflammation markers such as microglial load and brain cytokine levels. Using cocultures, we found that PKR in neurons was essential for LPS microglia-induced neuronal death. Our results demonstrate the clear involvement of PKR in abnormal spatial memory and brain lesions in the 5xFAD model and underline its interest as a target for neuroprotection in AD.

Early Downregulation of p75NTR by Genetic and Pharmacological Approaches Delays the Onset of Motor Deficits and Striatal Dysfunction in Huntington's Disease Mice

Deficits in striatal brain-derived neurotrophic factor (BDNF) delivery and/or BDNF/tropomyosin receptor kinase B (TrkB) signaling may contribute to neurotrophic support reduction and selective early degeneration of striatal medium spiny neurons in Huntington's disease (HD). Furthermore, we and others have demonstrated that TrkB/p75NTR imbalance in vitro increases the vulnerability of striatal neurons to excitotoxic insults and induces corticostriatal synaptic alterations. We have now expanded these studies by analyzing the consequences of BDNF/TrkB/p75NTR imbalance in the onset of motor behavior and striatal neuropathology in HD mice. Our findings demonstrate for the first time that the onset of motor coordination abnormalities, in a full-length knock-in HD mouse model (KI), correlates with the reduction of BDNF and TrkB levels, along with an increase in p75NTR expression. Genetic normalization of p75NTR expression in KI mutant mice delayed the onset of motor deficits and striatal neuropathology, as shown by restored levels of striatal-enriched proteins and dendritic spine density and reduced huntingtin aggregation. We found that the BDNF/TrkB/p75NTR imbalance led to abnormal BDNF signaling, manifested as a diminished activation of TrkB-phospholipase C-gamma pathway but upregulation of c-Jun kinase pathway. Moreover, we confirmed the contribution of the proper balance of BDNF/TrkB/p75NTR on HD pathology by a pharmacological approach using fingolimod. We observed that chronic infusion of fingolimod normalizes p75NTR levels, which is likely to improve motor coordination and striatal neuropathology in HD transgenic mice. We conclude that downregulation of p75NTR expression can delay disease progression suggesting that therapeutic approaches aimed to restore the balance between BDNF, TrkB, and p75NTR could be promising to prevent motor deficits in HD.

E2F1 promotes hepatic gluconeogenesis and contributes to hyperglycemia during diabetes

OBJECTIVE

Aberrant hepatic glucose production contributes to the development of hyperglycemia and is a hallmark of type 2 diabetes. In a recent study, we showed that the transcription factor E2F1, a component of the cell cycle machinery, contributes to hepatic steatosis through the transcriptional regulation of key lipogenic enzymes. Here, we investigate if E2F1 contributes to hyperglycemia by regulating hepatic gluconeogenesis.

METHODS

We use different genetic models to investigate if E2F1 regulates gluconeogenesis in primary hepatocytes and in vivo. We study the impact of depleting E2F1 or inhibiting E2F1 activity in diabetic mouse models to evaluate if this transcription factor contributes to hyperglycemia during insulin resistance. We analyze E2F1 mRNA levels in the livers of human diabetic patients to assess the relevance of E2F1 in human pathophysiology.

RESULTS

Lack of E2F1 impaired gluconeogenesis in primary hepatocytes. Conversely, E2F1 overexpression increased glucose production in hepatocytes and in mice. Several genetic models showed that the canonical CDK4-RB1-E2F1 pathway is directly involved in this regulation. E2F1 mRNA levels were increased in the livers from human diabetic patients and correlated with the expression of the gluconeogenic enzyme Pck1. Genetic invalidation or pharmacological inhibition of E2F1 improved glucose homeostasis in diabetic mouse models.

CONCLUSIONS

Our study unveils that the transcription factor E2F1 contributes to mammalian glucose homeostasis by directly controlling hepatic gluconeogenesis. Together with our previous finding that E2F1 promotes hepatic steatosis, the data presented here show that E2F1 contributes to both hyperlipidemia and hyperglycemia in diabetes, suggesting that specifically targeting E2F1 in the liver could be an interesting strategy for therapies against type 2 diabetes.

7,8-dihydroxyflavone ameliorates cognitive and motor deficits in a Huntington's disease mouse model through specific activation of the PLCγ1 pathway

Huntington's disease (HD) is a fatal neurodegenerative disease with motor, cognitive and psychiatric impairment. Dysfunctions in HD models have been related to reduced levels of striatal brain-derived neurotrophic factor (BDNF) and imbalance between its receptors TrkB and p75(NTR). Thus, molecules with activity on the BDNF/TrkB/p75 system can have therapeutic potential. 7,8-Dihydroxyflavone (7,8-DHF) was described as a TrkB agonist in several models of neuro-degenerative diseases, however, its TrkB activation profile needs further investigation due to its pleiotropic properties and divergence from BDNF effect. To investigate this, we used in vitro and in vivo models of HD to dissect TrkB activation upon 7,8-DHF treatment. 7,8-DHF treatment in primary cultures showed phosphorylation of TrkBY816 but not TrkBY515 with activation of the PLCγ1 pathway leading to morphological and functional improvements. Chronic administration of 7,8-DHF delayed motor deficits in R6/1 mice and reversed deficits on the Novel Object Recognition Test (NORT) at 17 weeks. Morphological and biochemical analyses revealed improved striatal levels of enkephalin, and prevention of striatal volume loss. We found a TrkBY816 but not TrkBY515 phosphorylation recovery in striatum concordant with in vitro results. Additionally, 7,8-DHF normalized striatal levels of induced and neuronal nitric oxide synthase (iNOS and nNOS, respectively) and ameliorated the imbalance of p75/TrkB. Our results provide new insights into the mechanism of action of 7,8-DHF suggesting that its effect through the TrkB receptor in striatum is via selective phosphorylation of its Y816 residue and activation of PLCγ1 pathway, but pleiotropic effects of the drug also contribute to its therapeutic potential.

CDK4 Phosphorylates AMPKα2 to Inhibit Its Activity and Repress Fatty Acid Oxidation

The roles of CDK4 in the cell cycle have been extensively studied, but less is known about the mechanisms underlying the metabolic regulation by CDK4. Here, we report that CDK4 promotes anaerobic glycolysis and represses fatty acid oxidation in mouse embryonic fibroblasts (MEFs) by targeting the AMP-activated protein kinase (AMPK). We also show that fatty acid oxidation (FAO) is specifically induced by AMPK complexes containing the α2 subunit. Moreover, we report that CDK4 represses FAO through direct phosphorylation and inhibition of AMPKα2. The expression of non-phosphorylatable AMPKα2 mutants, or the use of a CDK4 inhibitor, increased FAO rates in MEFs and myotubes. In addition, Cdk4-/- mice have increased oxidative metabolism and exercise capacity. Inhibition of CDK4 mimicked these alterations in normal mice, but not when skeletal muscle was AMPK deficient. This novel mechanism explains how CDK4 promotes anabolism by blocking catabolic processes (FAO) that are activated by AMPK.

E2F1 inhibits circulating cholesterol clearance by regulating Pcsk9 expression in the liver

Cholesterol accumulation in the liver is an early event in nonalcoholic fatty liver disease (NAFLD). Here, we demonstrate that E2F1 plays a crucial role in maintaining cellular cholesterol homeostasis by regulating cholesterol uptake via proprotein convertase subtilisin/kexin 9 (PCSK9), an enzyme that promotes low-density lipoprotein receptor (LDLR) degradation upon activation. E2f1-/- mice display reduced total plasma cholesterol levels and increased cholesterol content in the liver. In this study, we show that E2f1 deletion in cellular and mouse models leads to a marked decrease in Pcsk9 expression and an increase in LDLR expression. In addition to the upregulation of LDLR, we report that E2f1-/- hepatocytes exhibit increased LDL uptake. ChIP-Seq and PCSK9 promoter reporter experiments confirmed that E2F1 binds to and transactivates the PCSK9 promoter. Interestingly, E2f1-/- mice fed a high-cholesterol diet (HCD) display a fatty liver phenotype and liver fibrosis, which is reversed by reexpression of PCSK9 in the liver. Collectively, these data indicate that E2F1 regulates cholesterol uptake and that the loss of E2F1 leads to abnormal cholesterol accumulation in the liver and the development of fibrosis in response to an HCD.

Helios expression coordinates the development of a subset of striatopallidal medium spiny neurons

Here, we unravel the mechanism of action of the Ikaros family zinc finger protein Helios (He) during the development of striatal medium spiny neurons (MSNs). He regulates the second wave of striatal neurogenesis involved in the generation of striatopallidal neurons, which express dopamine 2 receptor and enkephalin. To exert this effect, He is expressed in neural progenitor cells (NPCs) keeping them in the G₁/G₀ phase of the cell cycle. Thus, a lack of He results in an increase of S-phase entry and S-phase length of NPCs, which in turn impairs striatal neurogenesis and produces an accumulation of the number of cycling NPCs in the germinal zone (GZ), which end up dying at postnatal stages. Therefore, He^−/− mice show a reduction in the number of dorso-medial striatal MSNs in the adult that produces deficits in motor skills acquisition. In addition, overexpression of He in NPCs induces misexpression of DARPP-32 when transplanted in mouse striatum. These findings demonstrate that He is involved in the correct development of a subset of striatopallidal MSNs and reveal new cellular mechanisms for neuronal development.

Summary: The transcription factor Helios regulates G1-S transition to promote neuronal differentiation of a striatopallidal neuronal subpopulation involved in motor skill acquisition.

E2F1, a Novel Regulator of Metabolism

In the past years, several lines of evidence have shown that cell cycle regulatory proteins also can modulate metabolic processes. The transcription factor E2F1 is a central player involved in cell cycle progression, DNA-damage response, and apoptosis. Its crucial role in the control of cell fate has been extensively studied and reviewed before; however, here, we focus on the participation of E2F1 in the regulation of metabolism. We summarize recent findings about the cell cycle-independent roles of E2F1 in various tissues that contribute to global metabolic homeostasis and highlight that E2F1 activity is increased during obesity. Finally, coming back to the pivotal role of E2F1 in cancer development, we discuss how E2F1 links cell cycle progression with different metabolic adaptations required for cell growth and survival.

Targeting CAG repeat RNAs reduces Huntington's disease phenotype independently of huntingtin levels

Huntington's disease (HD) is a polyglutamine disorder caused by a CAG expansion in the Huntingtin (HTT) gene exon 1. This expansion encodes a mutant protein whose abnormal function is traditionally associated with HD pathogenesis; however, recent evidence has also linked HD pathogenesis to RNA stable hairpins formed by the mutant HTT expansion. Here, we have shown that a locked nucleic acid-modified antisense oligonucleotide complementary to the CAG repeat (LNA-CTG) preferentially binds to mutant HTT without affecting HTT mRNA or protein levels. LNA-CTGs produced rapid and sustained improvement of motor deficits in an R6/2 mouse HD model that was paralleled by persistent binding of LNA-CTG to the expanded HTT exon 1 transgene. Motor improvement was accompanied by a pronounced recovery in the levels of several striatal neuronal markers severely impaired in R6/2 mice. Furthermore, in R6/2 mice, LNA-CTG blocked several pathogenic mechanisms caused by expanded CAG RNA, including small RNA toxicity and decreased Rn45s expression levels. These results suggest that LNA-CTGs promote neuroprotection by blocking the detrimental activity of CAG repeats within HTT mRNA. The present data emphasize the relevance of expanded CAG RNA to HD pathogenesis, indicate that inhibition of HTT expression is not required to reverse motor deficits, and further suggest a therapeutic potential for LNA-CTG in polyglutamine disorders.

Acute drug-induced spine changes in the nucleus accumbens are dependent on β-adducin

Chronic modifications of dopamine transmission alter striatal dendritic spines. Here, we show that spine density and length are increased in the nucleus accumbens 24 h after a single injection of caffeine or quinpirole, a dopamine D2/D3 dopamine receptors agonist, whereas the dopamine antagonist haloperidol has opposite effects. These effects are absent in mice lacking β-adducin, a protein that stabilizes actin/spectrin cortical cytoskeleton and modulates synaptic plasticity. Phosphorylation of adducin (Ser713 in β-adducin), which disrupts actin/spectrin interaction, is increased by quinpirole, haloperidol, or caffeine. We previously demonstrated that DARPP-32 interacts with β-adducin and facilitates its phosphorylation. Quinpirole increased DARPP-32 phosphorylation at Thr75 and haloperidol at Ser97, two modifications that can have similar consequences on adducin phosphorylation through distinct mechanisms. Experiments in DARPP-32 mutant mice confirmed that the apparently paradoxical similar effects of quinpirole and haloperidol on adducin phosphorylation may result from differential effects of these drugs on DARPP-32 phosphorylation at Thr75 and Ser97. Our data provide novel insights on how a single dose of widely used psychoactive drugs can affect spine plasticity in the nucleus accumbens, a component of the reward system.

Cognitive Dysfunction in Huntington's Disease: Humans, Mouse Models and Molecular Mechanisms

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder due to an expanded CAG/polyglutamine repeat in the coding region of the huntingtin (htt) gene that causes the preferential degeneration of striatal neurons. Although HD is classically considered a motor disorder, cognitive decline manifests even before the appearance of motor symptoms, and reflects the impairment of additional neuronal populations, such as cortical and hippocampal neurons, in the presence of mutant htt (mhtt). Studies on cognitive dysfunction in HD patients have focused on the cortico-striatal pathway. Here we will describe that HD patients and mouse models share many cognitive defects. Alterations in hippocampal synaptic plasticity and function found in HD mouse models highlight that changes in the functioning of the hippocampal formation contribute to cognitive dysfunction in humans. The similarity between the cognitive dysfunction in HD patients and mouse models has helped to understand better how cognitive dysfunction takes place. Moreover, it validates the use of HD mice to study the molecular mechanisms involved in HD cognitive decline. Several studies in HD mouse models indicate that altered synaptic composition/function, deficient neurotrophic support, kinase/phosphatase imbalance, and transcription dysregulation play an important role in cognitive impairment. This knowledge opens the possibility of identifying relevant therapeutic targets to fight cognitive decline in HD. The finding that in HD many mechanisms are similarly altered in hippocampal and striatal neurons suggests the possibility of a common therapeutic strategy to ameliorate both cognitive and motor dysfunction.

CDK4 is an essential insulin effector in adipocytes

Insulin resistance is a fundamental pathogenic factor that characterizes various metabolic disorders, including obesity and type 2 diabetes. Adipose tissue contributes to the development of obesity-related insulin resistance through increased release of fatty acids, altered adipokine secretion, and/or macrophage infiltration and cytokine release. Here, we aimed to analyze the participation of the cyclin-dependent kinase 4 (CDK4) in adipose tissue biology. We determined that white adipose tissue (WAT) from CDK4-deficient mice exhibits impaired lipogenesis and increased lipolysis. Conversely, lipolysis was decreased and lipogenesis was increased in mice expressing a mutant hyperactive form of CDK4 (CDK4(R24C)). A global kinome analysis of CDK4-deficient mice following insulin stimulation revealed that insulin signaling is impaired in these animals. We determined that insulin activates the CCND3-CDK4 complex, which in turn phosphorylates insulin receptor substrate 2 (IRS2) at serine 388, thereby creating a positive feedback loop that maintains adipocyte insulin signaling. Furthermore, we found that CCND3 expression and IRS2 serine 388 phosphorylation are increased in human obese subjects. Together, our results demonstrate that CDK4 is a major regulator of insulin signaling in WAT.