Caveolins (CAV) are essential components of caveolae; plasma membrane invaginations with reduced fluidity, reflecting cholesterol accumulation [1]. CAV proteins bind cholesterol, and CAV’s ability to move between cellular compartments helps control intracellular cholesterol fluxes [1–3]. In humans, CAV1 mutations result in lipodystrophy, cell transformation, and cancer [4–7]. CAV1 gene-disrupted mice exhibit cardiovascular diseases, diabetes, cancer, atherosclerosis, and pulmonary fibrosis [8, 9]. The mechanism(s) underlying these disparate effects are unknown, but our past work suggested CAV1 deficiency might alter metabolism: CAV1−/− mice exhibit impaired liver regeneration unless supplemented with glucose, suggesting systemic inefficiencies requiring additional metabolic intermediates [10]. Establishing a functional link between CAV1 and metabolism would provide a unifying theme to explain these myriad pathologies [11]. Here, we demonstrate that impaired proliferation and low survival with glucose restriction is a shortcoming of CAV1 deficient cells, caused by impaired mitochondrial function. Without CAV1, free cholesterol accumulates in mitochondrial membranes, increasing membrane condensation and reducing efficiency of the respiratory chain and intrinsic anti-oxidant defence. Upon activation of oxidative phosphorylation, this promotes accumulation of reactive oxygen species resulting in cell death. We confirm that this mitochondrial dysfunction predisposes CAV1 deficient animals to mitochondrial related diseases such as steatohepatitis and neurodegeneration.

Intrastriatal injection of quinolinate has been proven to be a very useful animal model to study the pathogenesis and treatment of Huntington's disease. To determine whether growth factors of the neurotrophin family are able to prevent the degeneration of striatal projection neurons, cell lines expressing brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or neurotrophin-4/5 (NT-4/5) were grafted in the adult rat striatum before quinolinate injection. Three days after lesioning, ongoing cell death was assessed by in situ detection of DNA fragmentation. In animals grafted with the control cell line, quinolinate injection induced a gradual cell loss that was differentially prevented by intrastriatal grafting of BDNF-, NT-3-, or NT-4/5-secreting cells. Seven days after lesioning, we characterized striatal projection neurons that were protected by neurotrophins. Quinolinate injection, alone or in combination with the control cell line, induced a selective loss of striatal projection neurons. Grafting of a BDNF-secreting cell line prevented the loss of all types of striatal projection neurons analyzed. Glutamic acid decarboxylase 67-, preproenkephalin-, and preprotachykinin A-but not prodynorphinexpressing neurons were protected by grafting of NT-3-or NT-4/5-secreting cells but with less efficiency than the BDNF-secreting cells. Our findings show that neurotrophins are able to promote the survival of striatal projection neurons in vivo and suggest that BDNF might be beneficial for the treatment of striatonigral degenerative disorders, including Huntington's disease. Key Words: Rat striatum-Quinolinate-Grafting-Survival. J. Neurochem. 75, 2190 -2199 (2000).Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expanded polyglutamine repeat in the huntingtin gene (MacDonald and Gusella, 1996;Mangiarini et al., 1996). Its predominant pathological feature is a massive and progressive degeneration of striatal output neurons without substantial loss of striatal interneurons and afferents (for review, see DiFiglia, 1990). Intrastriatal injection of quinolinate (QUIN), an NMDA receptor agonist, replicates many neurochemical, histological, and behavioral features of HD (Beal et al., 1986;DiFiglia, 1990). Striatal projection neurons containing enkephalin are affected to a greater extent than substance P-containing neurons, and those neurons surviving the lesion express reduced levels of their mRNAs both in HD (Reiner et al., 1988;Richfield et al., 1995) and after QUIN injections (Pérez-Navarro et al., 1999a,b). Excitotoxicity and apoptosis have been suggested to be involved in the degeneration of neurons in HD (Thomas et al., 1995;Petersén et al., 1999) and after QUIN injection (Ferrer et al., 1995;Portera-Cailliau et al., 1995;Hughes et al., 1996). Furthermore, intrastriatal QUIN injections have been found to induce huntingtin mRNA (Carlock et al., 1995) and protein (Tatter et al., 1995), providing a possible link between the QUIN model and HD.Members of the neurotr...

Huntington's disease (HD) patients and mouse models show learning and memory impairment even before the onset of motor symptoms. However, the molecular events involved in this cognitive decline are still poorly understood. Here, using three different paradigms, the novel object recognition test, the T-maze spontaneous alternation task and the Morris water maze, we detected severe cognitive deficits in the R6/1 mouse model of HD before the onset of motor symptoms. When we examined the putative molecular pathways involved in these alterations, we observed hippocampal cAMP-dependent protein kinase (PKA) hyper-activation in naïve R6/1 mice compared with wild-type (WT) mice, whereas extracellular signal-regulated kinase 1/2 and calcineurin activities were not modified. Increased PKA activity resulted in hyper-phosphorylation of its substrates N-methyl-D-aspartate receptor subunit 1, Ras-guanine nucleotide releasing factor-1 and striatal-enriched protein tyrosine phosphatase, but not cAMP-responsive element binding protein or the microtubule-associated protein tau. In correlation with the over-activation of the PKA pathway, we found a down-regulation of the protein levels of some phosphodiesterase (PDE) 4 family members. Similar molecular changes were found in the hippocampus of R6/2 mice and HD patients. Furthermore, chronic treatment of WT mice with the PDE4 inhibitor rolipram up-regulated PKA activity, and induced learning and memory deficits similar to those seen in R6 mice, but had no effect on R6/1 mice cognitive impairment. Importantly, hippocampal PKA inhibition by infusion of Rp-cAMPS restored long-term memory in R6/2 mice. Thus, our results suggest that occlusion of PKA-dependent processes is one of the molecular mechanisms underlying cognitive decline in R6 animals.

STriatal-Enriched protein tyrosine Phosphatase (STEP) is highly expressed in striatal projection neurons, the neuronal population most affected in Huntington's disease. Here, we examined STEP expression and phosphorylation, which regulates its activity, in N-terminal exon-1 and full-length mutant huntingtin mouse models. R6/1 mice displayed reduced STEP protein levels in the striatum and cortex whereas its phosphorylation was increased in the striatum, cortex and hippocampus. The early increase in striatal STEP phosphorylation levels correlated with a deregulation of the protein kinase A pathway, and decreased calcineurin activity at later stages further contributes to an enhancement of STEP phosphorylation and inactivation. Accordingly, we detected an accumulation of phosphorylated ERK2 and p38, two targets of STEP, in R6/1 mice striatum at advanced stages of the disease. Activation of STEP participates in excitotoxic-induced cell death. Since Huntington's disease mouse models develop resistance to excitotoxicity, we analyzed whether decreased STEP activity was involved in this process. After intrastriatal quinolinic acid (QUIN) injection, we detected higher phosphorylated STEP levels in R6/1 than in wild-type mice suggesting that STEP inactivation could mediate neuroprotection in R6/1 striatum. In agreement, intrastriatal injection of TAT-STEP increased QUIN-induced cell death. R6/2, Tet/HD94 and HdhQ7/Q111 mice striatum also displayed decreased STEP protein and increased phosphorylation levels. In Tet/HD94 mice striatum mutant huntingtin transgene shut-down reestablished STEP expression. In conclusion, the STEP pathway is severely down-regulated in the presence of mutant huntingtin and may participate in compensatory mechanisms activated by striatal neurons that lead to the resistance to excitotoxicity.

Adenosine A2A receptor (A2AR) is a G-protein-coupled receptor highly expressed in basal ganglia. Its expression levels are severely reduced in Huntington's disease (HD), and several pharmacological therapies have shown its implication in this neurodegenerative disorder. The main goal of this study was to gain insight into the molecular mechanisms that regulate A2AR gene (ADORA2A) expression in HD. Based on previous data reported by our group, we measured the methylcytosine (5mC) and hydroxymethylcytosine (5hmC) content in the 5'UTR region of ADORA2A in the putamen of HD patients and in the striatum of R6/1 and R6/2 mice at late stages of the disease. In this genomic region, 5mC and 5hmC remained unchanged in both mice strains, although low striatal A2AR levels were associated with reduced 5mC levels in 30-week-old R6/1 mice and reduced 5hmC levels in 12-week-old R6/2 mice in exon m2. In order to analyze when this mechanism appears during the progression of the disease, a time course for A2AR protein levels was carried out in R6/1 mice striatum (8, 12, and 20 weeks of age). A2AR levels were reduced from 12 weeks of age onwards, and this downregulation was concomitant with reduced 5hmC levels in the 5'UTR region of ADORA2A. Interestingly, increased 5mC levels and reduced 5hmC were found in the 5'UTR region of ADORA2A in the putamen of HD patients with respect to age-matched controls. Therefore, an altered DNA methylation pattern in ADORA2A seems to play a role in the pathologically decreased A2AR expression levels found in HD.

Changes in BDNF expression after different types of brain insults are related to neuroprotection, stimulation of sprouting, and synaptic reorganization. In the cerebral cortex, an autocrine-paracrine mechanism for BDNF has been proposed because the distribution patterns of BDNF and TrkB expression are almost identical. Moreover, cortical BDNF is anterogradely transported to the striatum, suggesting a role of BDNF in the functional interaction between the two brain regions. Here we have examined the expression of this neurotrophin in the cerebral cortex after various striatal lesions. Intrastriatal injection of quinolinate, kainate, 3-nitropropionic acid, or colchicine increased BDNF mRNA levels in cerebral cortex. In contrast, stimulation of neuronal activity in the striatum did not change cortical BDNF expression. Both excitatory amino acids increased BDNF expression in neurons of cortical layers II/III, V, and VI that project to the striatum. Moreover, grafting a BDNFsecreting cell line prevented both the loss of striatal neurons and the cortical upregulation of BDNF induced by excitotoxins. Because retrograde transport in the corticostriatal pathway was intact after striatal lesions, our results suggest that striatal damage upregulates endogenous BDNF in corticostriatal neurons by a transneuronal mechanism, which may constitute a protective mechanism for striatal and/or cortical cells.

Dysregulation of gene expression is one of the mechanisms involved in the pathophysiology of Huntington's disease (HD). Here, we examined whether mutant huntingtin regulates the levels of PH domain leucine-rich repeat protein phosphatase 1 (PHLPP1), a phosphatase that specifically dephosphorylates Akt at Ser473. Our results show decreased PHLPP1 protein levels in knock-in models (Hdh Q111/Q111 mouse striatum and STHdh Q111/Q111 cells), in the striatum of N-terminal exon-1 mutant huntingtin transgenic mouse models (R6/1; R6/1 : BDNF þ /À, R6/2 and Tet/HD94) and in the putamen of HD patients. Quantitative PCR analysis revealed a reduction in PHLPP1 mRNA levels in the striatum of R6/1 compared with wild-type mice. Coincident with reduced PHLPP1 protein levels, we observed increased phosphorylated Akt (Ser473) levels specifically in the striatum. The analysis of the conditional mouse model Tet/HD94 disclosed that after mutant huntingtin shutdown PHLPP1 levels returned to wild-type levels whereas phospho-Akt levels were partially reduced. In conclusion, our results show that mutant huntingtin downregulates PHLPP1 expression. In the striatum, these reduced levels of PHLPP1 can contribute to maintain high levels of activated Akt that may delay cell death and allow the recovery of neuronal viability after mutant huntingtin silencing.

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