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Biomedical subjects

Christopher M Gomez

Publications and source records attributed to Christopher M Gomez.

At least 19 recordsLinked to original sources
PubMed2006

Dominant-negative suppression of Cav2.1 currents by alpha(1)2.1 truncations requires the conserved interaction domain for beta subunits.

Episodic ataxia type 2 (EA2) is an autosomal dominant disorder arising from CACNA1A mutations, which commonly predict heterozygous expression of Ca(v)2.1 calcium channels with truncated alpha(1)2.1 pore subunits. We hypothesized that alpha(1)2.1 truncations in EA2 exert dominant-negative effects on the function of wild-type subunits. Wild-type and truncated alpha(1)2.1 subunits with fluorescent protein tags were transiently co-expressed in cells stably expressing Ca(v) auxiliary beta subunits, which facilitate alpha1 subunit functional expression through high-affinity interactions with the alpha interaction domain (AID). Co-expression of wild-type subunits with truncations often resulted in severely reduced whole-cell currents compared to expression of wild-type subunits alone. Cellular image analyses revealed that current suppression was not due to reduced wild-type expression levels. Instead, the current suppression depended on truncations terminating distal to the AID. Moreover, only AID-bearing alpha(1)2.1 proteins co-immunoprecipitated with Ca(v) beta subunits. These results indicate that Ca(v) beta subunits may play a prominent role in EA2 disease pathogenesis.

Analysis of Variance↗
PubMed2006

Inositol-1,4,5-triphosphate receptors mediate activity-induced synaptic Ca2+ signals in muscle fibers and Ca2+ overload in slow-channel syndrome.

Strict control of calcium entry through excitatory synaptic receptors is important for shaping synaptic responses, gene expression, and cell survival. Disruption of this control may lead to pathological accumulation of Ca2+. The slow-channel congenital myasthenic syndrome (SCS), due to mutations in muscle acetylcholine receptor (AChR), perturbs the kinetics of synaptic currents, leading to post-synaptic Ca2+ accumulation. To understand the regulation of calcium signaling at the neuromuscular junction (NMJ) and the etiology of Ca2+ overload in SCS we studied the role of sarcoplasmic Ca2+ stores in SCS. Using fura-2 loaded dissociated fibers activated with acetylcholine puffs, we confirmed that Ca2+ accumulates around wild type NMJ and discovered that Ca2+ accumulates significantly faster around the NMJ of SCS transgenic dissociated muscle fibers. Additionally, we determined that this process is dependant on the activation, altered kinetics, and movement of Ca2+ ions through the AChR, although, surprisingly, depletion of intracellular stores also prevents the accumulation of this cation around the NMJ. Finally, we concluded that the sarcoplasmic reticulum is the main source of Ca2+ and that inositol-1,4,5-triphosphate receptors (IP3R), and to a lesser degree L-type voltage gated Ca2+ channels, are responsible for the efflux of this cation from intracellular stores. These results suggest that a signaling system mediated by the activation of AChR, Ca2+, and IP3R is responsible for localized Ca2+ signals observed in muscle fibers and the Ca2+ overload observed in SCS.

Animals↗
PubMed2006

Activation of apoptotic pathways at muscle fiber synapses is circumscribed and reversible in a slow-channel syndrome model.

In the slow-channel syndrome (SCS) mutant acetylcholine receptors elicit calcium overload and myonuclear degeneration at the neuromuscular junction (NMJ), without muscle fiber death. Activated caspases are present at SCS motor endplates. We hypothesized that SCS represents a limited form of apoptosis. We found condensed chromatin and occasional single-strand DNA nicks in degenerating synaptic nuclei. Cleaved forms of caspases-3 and -9 were present in mouse SCS muscle homogenates and were specifically localized to NMJs. Finally, interruption of cholinergic activity by axotomy markedly reduced NMJ caspase activity and improved the morphological features of apoptosis at NMJs. These results demonstrate that in SCS processes leading to apoptosis may remain compartmentalized and reversible. Use of cysteine protease inhibitors may aid in treatment of this and other dystrophic muscle and excitotoxic disorders. Identification of extrasynaptic factors that prevent the spread of apoptosis in SCS muscle fibers may aid in developing treatments for neurological disorders characterized by excitotoxicity or apoptosis.

Animals↗
PubMed2006

Depth perception in cerebellar and basal ganglia disease.

There is increasing evidence that the cerebellum and the basal ganglia serve not only a role in motor control but also in visual perception. Patients with Parkinson's disease (PD) as well as patients with cerebellar lesions exhibit impairments of vision that are not fully explained by ocular motor deficits. It is less clear to which extent these visual deficits contribute to an impaired control of visually guided movements. This study examined whether a dysfunction of the cerebellum or the basal ganglia induces impairments in depth perception, which affect action. We employed an illusionary display, the Ames trapezoidal window, to determine the ability of PD patients (n=10) and patients with spinocerebellar ataxia (SCA) (n=6) to process depth cues when estimating object slant. Participants either pointed to the edges of the window (motor judgement) or verbally indicated the perceived orientation of the display (verbal judgement). To control for ocular and limb motor deficits, participants judged the slant of a non-illusionary display in a second task. Slant estimation of the non-illusionary window was not impaired in either patient group when compared to control subjects (all P>0.2). In contrast, SCA as well as PD patients exhibited significantly greater slant estimation errors than controls when pointing to the illusionary window (P=0.005). In addition, both patient groups made larger errors than controls in their verbal judgements during binocular viewing of the illusion (P=0.005), but not during monocular viewing (P>0.2). In sum, the present findings point towards a role for both the basal ganglia and cerebellum for the processing of visual information about depth. Since the deficits were seen both in the context of action and perception and were only partially reconciled by the availability of binocular depth cues, we conclude that basal ganglia as well as cerebellar disease may affect the visual perception of depth.

Adult↗
PubMed2006

Evaluation of sleep and daytime somnolence in spinocerebellar ataxia type 6 (SCA6).

To determine the frequency of subjective sleep problems in spinocerebellar ataxia type 6 (SCA6), the authors surveyed 25 patients and 25 age-matched controls with the Epworth Sleepiness Scale (ESS) and Pittsburgh Sleep Quality Index (PSQI). The ESS was higher in patients with SCA6 (9.12 +/- 5.80; CI: 2.28) than in controls (4.96 +/- 3.01; CI: 1.18) (p = 0.003, t test). The PSQI was higher in patients with SCA6 (7.96 +/- 4.65; CI: 1.86) than in controls (5.08 +/- 3.39; CI: 1.36) (p = 0.018, t test).

Aged↗
PubMed2006

Novel beta subunit mutation causes a slow-channel syndrome by enhancing activation and decreasing the rate of agonist dissociation.

We traced the cause of a slow-channel syndrome (SCS) in a patient with progressive muscle weakness, repetitive compound muscle action potential and prolonged low amplitude synaptic currents to a V --> F substitution in the M1 domain of the beta subunit (betaV229F) of the muscle acetylcholine receptor (AChR). In vitro expression studies in Xenopus oocytes indicated that the novel mutation betaV229F expressed normal amounts of AChRs and decreased the ACh EC50 by 10-fold compared to wild type. Kinetic analysis indicated that the mutation displayed prolonged mean open duration and repeated openings during activation. Prolonged openings caused by the betaV229F mutation were due to a reduction in the channel closing rate and an increase in the effective channel opening rate. Repeated openings of the channel during activation were caused by a significant reduction in the agonist dissociation constant. In addition, the betaV229F mutation produced an increase in calcium permeability. The kinetic and permeation studies presented in this work are sufficient to explain the consequences of the betaV229F mutation on the miniature endplate currents and thus are direct evidence that the betaV229F mutation is responsible for compromising the safety margin of neuromuscular transmission in the patient.

Acetylcholine↗
PubMed2006

C-termini of P/Q-type Ca2+ channel alpha1A subunits translocate to nuclei and promote polyglutamine-mediated toxicity.

P/Q-type voltage-gated calcium channels are regulated, in part, through the cytoplasmic C-terminus of their alpha1A subunit. Genetic absence or alteration of the C-terminus leads to abnormal channel function and neurological disease. Here, we show that the terminal 60-75 kDa of the endogenous alpha1A C-terminus is cleaved from the full-length protein and is present in cell nuclei. Antiserum to the C-terminus (CT-2) labels both wild-type mouse and human Purkinje cell nuclei, but not leaner mouse cerebellum. Human embryonic kidney cells stably expressing beta3 and alpha2delta subunits and transiently transfected with full-length human alpha1A contain a 75 kDa CT-2 reactive peptide in their nuclear fraction. Primary granule cells transfected with C-terminally Green fluorescent protein (GFP)-tagged alpha1A exhibit GFP nuclear labeling. Nuclear translocation depends partly on the presence of three nuclear localization signals within the C-terminus. The C-terminal fragment bears a polyglutamine tract which, when expanded (Q33) as in spinocerebellar ataxia type 6 (SCA6), is toxic to cells. Moreover, polyglutamine-mediated toxicity is dependent on nuclear localization. Finally, in the absence of flanking sequence, the Q33 expansion alone does not kill cells. These results suggest a novel processing of the P/Q-type calcium channel and a potential mechanism for the pathogenesis of SCA6.

Active Transport, Cell Nucleus↗
PubMed2006

Macroscopic properties of spontaneous mutations in slow-channel syndrome: correlation by domain and disease severity.

The slow-channel syndrome (SCS) is a neuromuscular disorder characterized by fatigability, progressive weakness, and degeneration of the neuromuscular junction. The SCS is caused by missense mutations in the four subunits of the skeletal muscle acetylcholine receptor (AChR), which leads to altered channel gating, prolonged neuromuscular postsynaptic currents, and impaired neuromuscular transmission. Although a diverse set of mutations in different functional domains of the AChR appear to be associated with symptoms of widely ranging severity, there is as yet no mutant channel property or combination that explains the variations in disease severity. By observing the recovery time of AChR from desensitization, the authors determined that this process is significantly enhanced in SCS channels. In addition, as expected, the authors found that SCS macroscopic decay currents in transfected HEK293 cells are slower than wild type currents. While slight differences in relative Ca(2+) permeability between some SCS mutations were identified, they did not correlate with apparent disease severity. These results suggest that of the different AChR kinetic features studied, only recovery from desensitization and slow postsynaptic currents correlate with the severity observed in the different mutations of this syndrome.

Age of Onset↗
PubMed2005

Spinocerebellar ataxia type 26 maps to chromosome 19p13.3 adjacent to SCA6.

The dominantly inherited spinocerebellar ataxias (SCA) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by progressive gait ataxia, upper limb incoordination, and dysarthria. We studied a six-generation kindred of Norwegian ancestry with pure cerebellar ataxia inherited in an autosomal dominant pattern. All affected family members had a slowly progressive cerebellar ataxia, with an age of onset range from 26 to 60 years. Brain magnetic resonance imaging study of 11 affected patients showed that atrophy was confined to the cerebellum. After excluding all the known SCAs using linkage analysis or direct mutation screen, we conducted a genomewide genetic linkage scan. With the aid of a novel linkage analysis strategy, we found linkage between the disease locus and marker D19S591 and D19S1034. Subsequent genetic and clinical analysis identified a critical region of 15.55cM interval on chromosome 19p13.3, flanked by markers D19S886 and D19S894, and have established a new genetic locus designated SCA26. The SCA26 locus is adjacent to the genes for Cayman ataxia and SCA6. The region consists of 3.3 million base pairs (Mb) of DNA sequences with approximately 100 known and predicted genes. Identification of the responsible gene for SCA26 ataxia will provide further insight into mechanisms of neurodegeneration.

Adult↗
PubMed2005

Clinical feature profile of spinocerebellar ataxia type 1-8 predicts genetically defined subtypes.

An increasing number of genetically defined types of spinocerebellar ataxia (SCA) have been reported in the past decade. Phenotype--genotype correlation studies have suggested a broad overlap between SCA types. The aim of the present study was to identify patterns of clinical features that were likely to distinguish between SCA types and to test the specificity and sensitivity of these signs and symptoms using a Bayesian classifier. In total, 127 patients from 50 families with SCA types 1 to 8 were examined using a worksheet with a panel of 33 symptoms and signs. By computing the probabilities of each trait for each SCA type, we rated the predictive value of each feature for each form of ataxia and then combined the probabilities for the entire panel of traits to construct a Bayesian classifier. Results of this analysis were summarized in a simpler, more operator-based algorithm. Patients with SCA5, SCA6, and SCA8 demonstrated a predominant cerebellar syndrome, whereas patients with SCA1, SCA2, SCA3, SCA4, and SCA7 frequently had clinical features indicating an extracerebellar involvement. The Bayesian classifier predicted the SCA type in 78% of patients with sensitivities between 60 and 100% and specificities between 94 and 98.2%. The highest sensitivity to correctly predict the true SCA type was found for SCA5, SCA7, and SCA8. Sensitivities and specificities found in the present study validate the use of algorithms to help to prioritize specific SCA gene testing, which will help to reduce costs for gene testing.

Age of Onset↗
PubMed2004

Antigliadin antibodies in Huntington's disease.

The relevance of gluten sensitivity in sporadic and hereditary ataxia pathogenesis is unclear. The authors found high antigliadin antibody titers in 23 of 52 (44%) patients with Huntington's disease (HD), suggesting a previously unrecognized association between HD and gluten sensitivity. The results further question "gluten ataxia" as a distinct disease entity and raise the possibility that antigliadin antibodies in ataxia and other neurodegenerative diseases may be an epiphenomenon, the mechanisms of which remain to be investigated.

Antibodies↗
PubMed2004

Focal caspase activation underlies the endplate myopathy in slow-channel syndrome.

Slow-channel syndrome (SCS) is a progressive neuromuscular disorder caused by abnormal gating of mutant acetylcholine receptors (AChRs) in the neuromuscular junction (NMJ). The pathological hallmark is selective degeneration of the NMJ termed endplate myopathy. Endplate myopathy consists of a combination of ultrastructural abnormalities, including degenerating subsynaptic nuclei, mitochondria, and postsynaptic folds, caused by localized cation overload through mutant AChRs. Because some of these changes resemble those seen in programmed cell death, we evaluated SCS muscle for evidence of focal activation of apoptotic pathways. Using antisera specific for the activated forms of caspases, the family of cysteine proteases that underlies apoptosis, we demonstrated that active forms of initiator and effector caspases are selectively localized at the NMJ in SCS. In comparison with an electron microscopic assessment of the abnormalities seen in endplate myopathy, we found that activated caspases were present at between 15 and 57% of endplates, similar to the proportion of endplates with degenerating mitochondria or vacuoles. This greatly exceeds the number of NMJs exhibiting nuclear degeneration. These findings provide the first evidence supporting the view that caspase activation in human disease can play a prominent role in localized cellular degenerative processes without causing nuclear or cell death.

Age of Onset↗
PubMed2004

Deficits in ocular and manual tracking due to episodic ataxia type 2.

Four patients with a novel mutation leading to episodic ataxia type 2 were studied in a task that required them to track target motion either with the eyes or with the index finger of the right hand. The target initially moved in a straight line and then changed direction at an unpredictable time by an unpredictable amount. On the day of testing, 3 of the patients were evaluated as normal on a neurological exam, whereas the fourth was severely ataxic. Nevertheless, all 4 showed deficits in tracking behavior with common features. Ocular tracking tended to result in hypermetric saccades at longer than normal latencies. Smooth pursuit tracking was absent in 1 patient and had lower than normal gain in the others. Deficits in manual tracking showed similarities to the deficits in ocular tracking, with hypermetric compensations for changes in target direction. The similarities in the deficits in manual and ocular tracking suggest that they are subject to similar control by the cerebellar structures.

Adult↗
PubMed2003

Hereditary cerebellar ataxia progressively impairs force adaptation during goal-directed arm movements.

We investigated how humans with hereditary cerebellar degeneration [spinocerebellar ataxia (SCA) type 6 and 8, n = 9] and age- and sex-matched healthy controls (n = 9) adapted goal-directed arm movements to an unknown external force field. We tested whether learning could be generalized to untrained regions in the workspace, an aspect central to the idea of an internal model, and if any learning could be retained. After removal of the force field, SCA patients showed little or no learning-related aftereffects indicating that repeated force-field exposure never led to successful force compensation. In contrast, healthy control subjects quickly adapted their movements to the new force field. The difference in force adaptation was significant for movements to targets that required both the shoulder and elbow joint (P < 0.001). Moreover, the generalization of learned movements to targets outside the learned workspace was prevented by the cerebellar degeneration (P < 0.01). Retention of force adaptation was significantly lower in SCA patients (P = 0.003). The severity of ataxia in SCA patients correlated negatively with the extent of learning (r = -0.84, P = 0.004). Our findings imply that progressive loss of cerebellar function gradually impairs force adaptation. The failure to generalize learning suggests that cerebellar degeneration prevents the formation of an internal representation of the limb dynamics.

Adaptation, Physiological↗
PubMed2003

Dysfunction of the basal ganglia, but not the cerebellum, impairs kinaesthesia.

Precise knowledge about limb position and orientation is essential for the ability of the nervous system to plan and control voluntary movement. While it is well established that proprioceptive signals from peripheral receptors are necessary for sensing limb position and motion, it is less clear which supraspinal structures mediate the signals that ultimately lead to the conscious awareness of limb position (kinaesthesia). Recent functional imaging studies have revealed that the cerebellum, but not the basal ganglia, are involved in sensory processing of proprioceptive information induced by passive and active movements. Yet psychophysical studies have suggested a prominent role of the basal ganglia in kinaesthesia. This study addresses this apparent dichotomy by investigating the contributions of the cerebellum and the basal ganglia to the perception of limb position. Using a passive movement task, we examined the elbow position sense in patients with a dysfunction of the basal ganglia (Parkinson's disease, n = 9), patients with cerebellar degeneration [spinocerebellar ataxia (SCA) types 6 and 8, n = 6] and age-matched healthy control subjects (n = 11). In comparison with healthy control subjects, Parkinson's disease patients, but not SCA patients, were significantly impaired in the ability to detect displacements correctly. A 1 degrees forearm displacement was correctly recognized in >75% of trials by control subjects and SCA patients, but only in 55% of Parkinson's disease patients. Only at 6 degrees displacement did Parkinson's disease patients exhibit a response rate similar to those of the two other groups. Thresholds for 75% correct responses were 1.03 degrees for controls, 1.15 degrees for cerebellar patients and 2.10 degrees for Parkinson's disease patients. This kinaesthetic impairment significantly correlated with the severity of disease in Parkinson's disease patients, as determined by the Unified Parkinson's Disease Rating Scale (r = -0.7, P = 0.03) and duration of disease (r = -0.7, P = 0.05). In contrast, there was no significant correlation between performance and the daily levodopa equivalent dose. These results imply that an intact cerebro-basal ganglia loop is essential for awareness of limb position and suggest a selective role of the basal ganglia but not the cerebellum in kinaesthesia.

Adult↗
PubMed2003

Novel CACNA1A mutation causes febrile episodic ataxia with interictal cerebellar deficits.

Episodic ataxia type 2 (EA2) is a dominantly inherited disorder, characterized by spells of ataxia, dysarthria, vertigo, and migraines, associated with mutations in the neuronal calcium-channel gene CACNA1A. Ataxic spells lasting minutes to hours are provoked by stress, exercise, or alcohol. Some patients exhibit nystagmus between spells and some develop progressive ataxia later in life. At least 21 distinct CACNA1A mutations have been identified in EA2. The clinical and genetic complexities of EA2 have offered few insights into the underlying pathogenic mechanisms for this disorder. We identified a novel EA2 kindred in which members had ataxic spells induced by fevers or high environmental temperature. We identified a novel CACNA1A mutation (nucleotides 1253+1 G-->A) that was present in all subjects with febrile spells or ataxia. Moreover, we found that, regardless of age or interictal clinical status, all affected subjects had objective evidence of abnormal saccades, ocular fixation, and postural stability. These findings suggest that early cerebellar dysfunction in EA2 results from the intrinsically abnormal properties of the CACNA1A channel rather than a degenerative process.

Adolescent↗