Clinically Relevant KCNQ1 Variants Causing KCNQ1-KCNE2 Gain-of-Function Affect the Ca2+ Sensitivity of the Channel.

Dominant KCNQ1 variants are well-known for underlying cardiac arrhythmia syndromes. The two heterozygous KCNQ1 missense variants, R116L and P369L, cause an allelic disorder characterized by pituitary hormone deficiency and maternally inherited gingival fibromatosis. Increased K+ conductance upon co-expression of KCNQ1 mutant channels with the beta subunit KCNE2 is suggested to underlie the phenotype; however, the reason for KCNQ1-KCNE2 (Q1E2) channel gain-of-function is unknown. We aimed to discover the genetic defect in a single individual and three family members with gingival overgrowth and identified the KCNQ1 variants P369L and V185M, respectively. Patch-clamp experiments demonstrated increased constitutive K+ conductance of V185M-Q1E2 channels, confirming the pathogenicity of the novel variant. To gain insight into the pathomechanism, we examined all three disease-causing KCNQ1 mutants. Manipulation of the intracellular Ca2+ concentration prior to and during whole-cell recordings identified an impaired Ca2+ sensitivity of the mutant KCNQ1 channels. With low Ca2+, wild-type KCNQ1 currents were efficiently reduced and exhibited a pre-pulse-dependent cross-over of current traces and a high-voltage-activated component. These features were absent in mutant KCNQ1 channels and in wild-type channels co-expressed with calmodulin and exposed to high intracellular Ca2+. Moreover, co-expression of calmodulin with wild-type Q1E2 channels and loading the cells with high Ca2+ drastically increased Q1E2 current amplitudes, suggesting that KCNE2 normally limits the resting Q1E2 conductance by an increased demand for calcified calmodulin to achieve effective channel opening. Our data link impaired Ca2+ sensitivity of the KCNQ1 mutants R116L, V185M and P369L to Q1E2 gain-of-function that is associated with a particular KCNQ1 channelopathy.

Mitochondrial and redox modifications in early stages of Huntington's disease.

Deficits in mitochondrial function and redox deregulation have been attributed to Huntington's disease (HD), a genetic neurodegenerative disorder largely affecting the striatum. However, whether these changes occur in early stages of the disease and can be detected in vivo is still unclear. In the present study, we analysed changes in mitochondrial function and production of reactive oxygen species (ROS) at early stages and with disease progression. Studies were performed in vivo in human brain by PET using [64Cu]-ATSM and ex vivo in human skin fibroblasts of premanifest and prodromal (Pre-M) and manifest HD carriers. In vivo brain [64Cu]-ATSM PET in YAC128 transgenic mouse and striatal and cortical isolated mitochondria were assessed at presymptomatic (3 month-old, mo) and symptomatic (6-12 mo) stages. Pre-M HD carriers exhibited enhanced whole-brain (with exception of caudate) [64Cu]-ATSM labelling, correlating with CAG repeat number. Fibroblasts from Pre-M showed enhanced basal and maximal respiration, proton leak and increased hydrogen peroxide (H2O2) levels, later progressing in manifest HD. Mitochondria from fibroblasts of Pre-M HD carriers also showed reduced circularity, while higher number of mitochondrial DNA copies correlated with maximal respiratory capacity. In vivo animal PET analysis showed increased accumulation of [64Cu]-ATSM in YAC128 mouse striatum. YAC128 mouse (at 3 months) striatal isolated mitochondria exhibited a rise in basal and maximal mitochondrial respiration and in ATP production, and increased complex II and III activities. YAC128 mouse striatal mitochondria also showed enhanced mitochondrial H2O2 levels and circularity, revealed by brain ultrastructure analysis, and defects in Ca2+ handling, supporting increased striatal susceptibility. Data demonstrate both human and mouse mitochondrial overactivity and altered morphology at early HD stages, facilitating redox unbalance, the latter progressing with manifest disease.

A De Novo BSCL2 Gene S90L Mutation in a Progressive Tetraparesis with Urinary Dysfunction and Corpus Callosum Involvement.

A Silver syndrome is a rare autosomal dominant spastic paraparesis in which spasticity of the lower limbs is accompanied by amyotrophy of the small hand muscles. The causative gene is the Berardinelli-Seip congenital lipodystrophy 2 ( BSCL2) , which is related to a spectrum of neurological phenotypes. In the current study, we presented a 14-year-old male with a slowly progressive spastic paraparesis with urinary incontinence that later on exhibited atrophy and weakness in the thenar and dorsal interosseous muscles. Magnetic resonance imaging (MRI) revealed discrete atrophy of the corpus callosum isthmus and an extended next-generation sequencing panel identified a de novo heterozygous mutation in BSCL2 gene, c.269C > T p.(S90L). Various clinical expression and incomplete penetrance of BSCL2 gene mutations complicate the establishment of a genetic etiology for these cases. Therefore, Silver syndrome should be included in the differential diagnosis if the initial presentation is a spastic paraparesis by urinary involvement with childhood-onset, even with MRI atypical findings. This report described the first Iberian Silver syndrome case carrying a de novo c.269C > T p. (S90L) BSCL2 gene mutation.

A whole brain longitudinal study in the YAC128 mouse model of Huntington's disease shows distinct trajectories of neurochemical, structural connectivity and volumetric changes.

Huntington's disease (HD) is a neurodegenerative disorder causing cognitive and motor impairments, evolving to death within 15-20 years after symptom onset. We previously established a mouse model with the entire human HD gene containing 128 CAG repeats (YAC128) which accurately recapitulates the natural history of the human disease. Defined time points in this natural history enable the understanding of longitudinal trajectories from the neurochemical and structural points of view using non-invasive high-resolution multi-modal imaging. Accordingly, we designed a longitudinal structural imaging (MRI and DTI) and spectroscopy (1H-MRS) study in YAC128, at 3, 6, 9 and 12 months of age, at 9.4 T. Structural analysis (MRI/DTI), confirmed that the striatum is the earliest affected brain region, but other regions were also identified through connectivity analysis (pre-frontal cortex, hippocampus, globus pallidus and thalamus), suggesting a striking homology with the human disease. Importantly, we found for the first time, a negative correlation between striatal and hippocampal changes only in YAC128. In fact, the striatum showed accelerated volumetric decay in HD, as opposed to the hippocampus. Neurochemical analysis of the HD striatum suggested early neurometabolic alterations in neurotransmission and metabolism, with a significant increase in striatal GABA levels, and specifically anticorrelated levels of N-acetyl aspartate and taurine, suggesting that the later is homeostatically adjusted for neuroprotection, as neural loss, indicated by the former, is progressing. These results provide novel insights into the natural history of HD and prove a valuable role for longitudinal multi-modal panels of structural and metabolite/neurotransmission in the YAC128 model.

Expanded and Wild-type Ataxin-3 Modify the Redox Status of SH-SY5Y Cells Overexpressing α-Synuclein.

Neurodegenerative diseases are considered to be distinct clinical entities, although they share the formation of proteinaceous aggregates and several neuropathological mechanisms. Increasing evidence suggest a possible interaction between proteins that have been classically associated to distinct neurodegenerative diseases. Thus, common molecular and cellular pathways might explain similarities between disease phenotypes. Interestingly, the characteristic Parkinson's disease (PD) phenotype linked to bradykinesia is also a clinical presentation of other neurodegenerative diseases. An example is Machado-Joseph disease (MJD), with some patients presenting parkinsonism and a positive response to levodopa (L-DOPA). Protein aggregates positive for α-synuclein (α-Syn), a protein associated with PD, in the substantia nigra of MJD models made us hypothesize a putative additive biological effect induced by expression of α-Syn and ataxin-3 (Atx3), the protein affected in MJD. Hence, in this study we analysed the influence of these two proteins (α-Syn and wild-type or mutant Atx3) on modified redox signaling, a pathological process potentially linked to both diseases, and also the impact of exposure to iron and rotenone in SH-SY5Y neuroblastoma cells. Our results show that both α-Syn and mutant Atx3 overexpression per se increased oxidation of dichlorodihydrofluorescein (DCFH2), and co-expression of these proteins exhibited additive effect on intracellular oxidation, with no correlation with apoptotic features. Mutant Atx3 and α-Syn also potentiated altered redox status induced by iron and rotenone, a hint to how these proteins might influence neuronal dysfunction under pro-oxidant conditions. We further show that overexpression of wild-type Atx3 decreased intracellular DCFH2 oxidation, possibly exerting a neuroprotective role.

Comparative Mitochondrial-Based Protective Effects of Resveratrol and Nicotinamide in Huntington's Disease Models.

Sirtuin 1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD+)-dependent lysine deacetylase that regulates longevity and enhances mitochondrial metabolism. Both activation and inhibition of SIRT1 were previously shown to ameliorate neuropathological mechanisms in Huntington's disease (HD), a neurodegenerative disease that selectively affects the striatum and cortex and is commonly linked to mitochondrial dysfunction. Thus, in this study, we tested the influence of resveratrol (RESV, a SIRT1 activator) versus nicotinamide (NAM, a SIRT1 inhibitor) in counteracting mitochondrial dysfunction in HD models, namely striatal and cortical neurons isolated from YAC128 transgenic mice embryos, HD human lymphoblasts, and an in vivo HD model. HD cell models displayed a deregulation in mitochondrial membrane potential and respiration, implicating a decline in mitochondrial function. Further studies revealed decreased PGC-1α and TFAM protein levels, linked to mitochondrial DNA loss in HD lymphoblasts. Remarkably, RESV completely restored these parameters, while NAM increased NAD+ levels, providing a positive add on mitochondrial function in in vitro HD models. In general, RESV decreased while NAM increased H3 acetylation at lysine 9. In agreement with in vitro data, continuous RESV treatment for 28 days significantly improved motor coordination and learning and enhanced expression of mitochondrial-encoded electron transport chain genes in YAC128 mice. In contrast, high concentrations of NAM blocked mitochondrial-related transcription, worsening motor phenotype. Overall, data indicate that activation of deacetylase activity by RESV improved gene transcription associated to mitochondrial function in HD, which may partially control HD-related motor disturbances.

Activation of IGF-1 and insulin signaling pathways ameliorate mitochondrial function and energy metabolism in Huntington's Disease human lymphoblasts.

Huntington's disease (HD) is an inherited neurodegenerative disease caused by a polyglutamine repeat expansion in the huntingtin protein. Mitochondrial dysfunction associated with energy failure plays an important role in this untreated pathology. In the present work, we used lymphoblasts obtained from HD patients or unaffected parentally related individuals to study the protective role of insulin-like growth factor 1 (IGF-1) versus insulin (at low nM) on signaling and metabolic and mitochondrial functions. Deregulation of intracellular signaling pathways linked to activation of insulin and IGF-1 receptors (IR,IGF-1R), Akt, and ERK was largely restored by IGF-1 and, at a less extent, by insulin in HD human lymphoblasts. Importantly, both neurotrophic factors stimulated huntingtin phosphorylation at Ser421 in HD cells. IGF-1 and insulin also rescued energy levels in HD peripheral cells, as evaluated by increased ATP and phosphocreatine, and decreased lactate levels. Moreover, IGF-1 effectively ameliorated O2 consumption and mitochondrial membrane potential (Δψm) in HD lymphoblasts, which occurred concomitantly with increased levels of cytochrome c. Indeed, constitutive phosphorylation of huntingtin was able to restore the Δψm in lymphoblasts expressing an abnormal expansion of polyglutamines. HD lymphoblasts further exhibited increased intracellular Ca(2+) levels before and after exposure to hydrogen peroxide (H2O2), and decreased mitochondrial Ca(2+) accumulation, being the later recovered by IGF-1 and insulin in HD lymphoblasts pre-exposed to H2O2. In summary, the data support an important role for IR/IGF-1R mediated activation of signaling pathways and improved mitochondrial and metabolic function in HD human lymphoblasts.

Mitochondrial respiratory chain complex activity and bioenergetic alterations in human platelets derived from pre-symptomatic and symptomatic Huntington's disease carriers.

Mitochondrial dysfunction has been implicated in Huntington's disease (HD) pathogenesis. We analyzed the activity of mitochondrial complexes (Cx) I-IV, protein levels of selected Cx subunits and adenine nucleotides in platelet mitochondria from pre-symptomatic versus symptomatic HD human carriers and age-matched control individuals. Mitochondrial platelets exhibited reduced activity of citrate synthase in pre-symptomatic and Cx-I in pre-symptomatic and symptomatic HD carriers. Positive correlation between Cx activity and protein subunits was observed for Cx-I in symptomatic HD patient's mitochondria. Moreover, AMP increased in mitochondria from pre-symptomatic HD carriers. Results highlight mitochondrial changes occurring before the onset of HD clinical symptoms.

Valosin-containing protein (VCP/p97) is an activator of wild-type ataxin-3.

Alterations in the ubiquitin-proteasome system (UPS) have been reported in several neurodegenerative disorders characterized by protein misfolding and aggregation, including the polylgutamine diseases. Machado-Joseph disease (MJD) or Spinocerebellar Ataxia type 3 is caused by a polyglutamine-encoding CAG expansion in the ATXN3 gene, which encodes a 42 kDa deubiquitinating enzyme (DUB), ataxin-3. We investigated ataxin-3 deubiquitinating activity and the functional relevance of ataxin-3 interactions with two proteins previously described to interact with ataxin-3, hHR23A and valosin-containing protein (VCP/p97). We confirmed ataxin-3 affinity for both hHR23A and VCP/p97. hHR23A and ataxin-3 were shown to co-localize in discrete nuclear foci, while VCP/p97 was primarily cytoplasmic. hHR23A and VCP/p97 recombinant proteins were added, separately or together, to normal and expanded ataxin-3 in in vitro deubiquitination assays to evaluate their influence on ataxin-3 activity. VCP/p97 was shown to be an activator specifically of wild-type ataxin-3, exhibiting no effect on expanded ataxin-3, In contrast, we observed no significant alterations in ataxin-3 enzyme kinetics or substrate preference in the presence of hHR23A alone or in combination with VCP. Based on our results we propose a model where ataxin-3 normally functions with its interactors to specify the cellular fate of ubiquitinated proteins.

Compromised mitochondrial complex II in models of Machado-Joseph disease.

Machado-Joseph disease (MJD), also known as Spinocerebellar Ataxia type 3, is an inherited dominant autosomal neurodegenerative disorder. An expansion of Cytosine-Adenine-Guanine (CAG) repeats in the ATXN3 gene is translated as an expanded polyglutamine domain in the disease protein, ataxin-3. Selective neurodegeneration in MJD is evident in several subcortical brain regions including the cerebellum. Mitochondrial dysfunction has been proposed as a mechanism of neurodegeneration in polyglutamine disorders. In this study, we used different cell models and transgenic mice to assess the importance of mitochondria on cytotoxicity observed in MJD. Transiently transfected HEK cell lines with expanded (Q84) ataxin-3 exhibited a higher susceptibility to 3-nitropropionic acid (3-NP), an irreversible inhibitor of mitochondrial complex II. Increased susceptibility to 3-NP was also detected in stably transfected PC6-3 cells that inducibly express expanded (Q108) ataxin-3 in a tetracycline-regulated manner. Moreover, cerebellar granule cells from MJD transgenic mice were more sensitive to 3-NP inhibition than wild-type cerebellar neurons. PC6-3 (Q108) cells differentiated into a neuronal-like phenotype with nerve growth factor (NGF) exhibited a significant decrease in mitochondrial complex II activity. Mitochondria from MJD transgenic mouse model and lymphoblast cell lines derived from MJD patients also showed a trend toward reduced complex II activity. Our results suggest that mitochondrial complex II activity is moderately compromised in MJD, which may designate a common feature in polyglutamine toxicity.

Dysregulation of CREB activation and histone acetylation in 3-nitropropionic acid-treated cortical neurons: prevention by BDNF and NGF.

3-Nitropropionic acid (3-NP), an inhibitor of mitochondrial complex II, leads to metabolic impairment and neurodegeneration. In this study, we investigated the roles of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in the dysregulation of transcription factors and histone modifying enzymes induced by 3-NP in primary cortical neurons. BDNF prevented the 3-NP-induced decrease in cAMP response-element binding protein (CREB) phosphorylation and CREB-binding protein levels. Both NGF and BDNF counteracted the increase in the levels of histone H3 and H4 acetylations and reduced histone deacetylase (HDAC) activity induced by 3-NP. BDNF further led to hyperphosphorylation of HDAC2. Our results support an important role for neurotrophins, particularly BDNF, in preventing detrimental changes in transcription factors and histone acetylation states in cortical neurons that have been subjected to selective mitochondrial inhibition.

BDNF regulates BIM expression levels in 3-nitropropionic acid-treated cortical neurons.

3-Nitropropionic acid (3-NP) is an irreversible inhibitor of succinate dehydrogenase that has been used to explore the primary mechanisms of cell death associated with mitochondrial dysfunction and neurodegeneration in Huntington's disease. In this study we investigated the ability of brain-derived neurotrophic factor (BDNF) to suppress mitochondrial-dependent cell death induced by 3-NP in primary cortical neurons. This neurotrophin prevented 3-NP-induced release of cytochrome c and Smac/Diablo, caspase-3-like activity and nuclear condensation/fragmentation. Furthermore, it greatly increased phosphorylation of Akt and MAPK, suggesting the involvement of these signalling pathways in BDNF neuroprotection. Interestingly, BDNF decreased the levels of the pro-apoptotic protein Bim in mitochondrial and total cell lysates through the activation of the MEK1/2 pathway. This effect was due to an increase in the degradation rates of Bim. Our data support an important role for BDNF, in protecting cortical neurons against apoptotic cell death caused by inhibition of mitochondrial complex II.

Cellular turnover of the polyglutamine disease protein ataxin-3 is regulated by its catalytic activity.

Ataxin-3, a deubiquitinating enzyme, is the disease protein in spinocerebellar ataxia type 3, one of many neurodegenerative disorders caused by polyglutamine expansion. Little is known about the cellular regulation of ataxin-3. This is an important issue, since growing evidence links disease protein context to pathogenesis in polyglutamine disorders. Expanded ataxin-3, for example, is more neurotoxic in fruit fly models when its active site cysteine is mutated (1). We therefore sought to determine the influence of ataxin-3 enzymatic activity on various cellular properties. Here we present evidence that the catalytic activity of ataxin-3 regulates its cellular turnover, ubiquitination, and subcellular distribution. Cellular protein levels of catalytically inactive ataxin-3 were much higher than those of active ataxin-3, in part reflecting slower degradation. In vitro studies revealed that inactive ataxin-3 was more slowly degraded by the proteasome and that this degradation occurred independent of ubiquitination. Slower degradation of inactive ataxin-3 correlated with reduced interaction with the proteasome shuttle protein, VCP/p97. Enzymatically active ataxin-3 also showed a greater tendency to concentrate in the nucleus, where it colocalized with the proteasome in subnuclear foci. Taken together, these and other findings suggest that the catalytic activity of this disease-linked deubiquitinating enzyme regulates several of its cellular properties, which in turn may influence disease pathogenesis.

Expression of NR1/NR2B N-methyl-D-aspartate receptors enhances heroin toxicity in HEK293 cells.

Repeated use of drugs of abuse, namely opiates, has been shown to affect glutamate-releasing neurons. Moreover, blockade of N-methyl-D-aspartate (NMDA) receptors (NMDAR) prevents cell death by apoptosis induced by morphine, a heroin metabolite. Thus, in this article we investigated the involvement of different NMDAR subunits in heroin cytotoxicity. Human embryonic kidney (HEK293) cells, which do not express native NMDAR, were transfected with NR1/NR2A or NR1/NR2B subunits. As a control, cells were transfected with NR1 alone, which does not form functional channels. Incubation with heroin for 24 h induced a dose-dependent decrease in cell viability both in NR1-transfected and nontransfected cells. The loss of membrane integrity induced by heroin was more evident in cells transfected with NR1/NR2B than in cells transfected with NR1 alone or NR1/NR2A. This decrease in cell viability was blocked by MK-801, a selective and noncompetitive antagonist of NMDAR. Nevertheless, no significant changes in intracellular adenosine 5'-triphosphate (ATP) were observed in cells treated with heroin. These data implicate NR2B-composed NMDAR as important mediators of heroin neurotoxicity.