Mutations in the Na+/K+ -ATPase alpha3 gene ATP1A3 are associated with rapid-onset dystonia parkinsonism

Mutations Phe785Leu and Thr618Met in Na+,K+-ATPase, associated with familial rapid-onset dystonia parkinsonism, interfere with Na+ interaction by distinct mechanisms

The Na(+),K(+)-ATPase plays key roles in brain function. Recently, missense mutations in the Na(+),K(+)-ATPase were found associated with familial rapid-onset dystonia parkinsonism (FRDP). Here, we have characterized the functional consequences of FRDP mutations Phe785Leu and Thr618Met. Both mutations lead to functionally altered, but active, Na(+),K(+)-pumps, that display reduced apparent affinity for cytoplasmic Na(+), but the underlying mechanism differs between the mutants. In Phe785Leu, the interaction of the E(1) form with Na(+) is defective, and the E(1)-E(2) equilibrium is not displaced. In Thr618Met, the Na(+) affinity is reduced because of displacement of the conformational equilibrium in favor of the K(+)-occluded E(2)(K(2)) form. In both mutants, K(+) interaction at the external activating sites of the E(2)P phosphoenzyme is normal. The change of cellular Na(+) homeostasis is likely a major factor contributing to the development of FRDP in patients carrying the Phe785Leu or Thr618Met mutation. Phe785Leu moreover interferes with Na(+) interaction on the extracellular side and reduces the affinity for ouabain significantly. Analysis of two additional Phe(785) mutants, Phe785Leu/Leu786Phe and Phe785Tyr, demonstrated that the aromatic function of the side chain, as well as its exact position, is critical for Na(+) and ouabain binding. The effects of substituting Phe(785) could be explained by structural modeling, demonstrating that Phe(785) participates in a hydrophobic network between three transmembrane segments. Thr(618) is located in the cytoplasmic part of the molecule near the catalytic site, and the structural modeling indicates that the Thr618Met mutation interferes with the bonding pattern in the catalytic site in the E(1) form, thereby destabilizing E(1) relative to E(2)(K(2)).

Unbalanced whole arm translocation resulting in loss of 18p in dystonia

Dystonia represents a genetically and clinically heterogeneous disorder, characterized by abnormal and sustained muscle contractions and rigid postures. At least 15 different loci (DYT1-DYT15) have been identified in dystonia. Adult-onset idiopathic focal dystonia affecting specific parts of the body, such as the eye (blepharospasm), neck (cervical dystonia), and hand (writer's cramp), is mostly associated with the DYT7 locus, which was originally mapped to chromosome 18p by genomewide linkage analysis in a large family showing autosomal dominant inheritance. We have identified a family in which the mother is affected with dystonia and the son shows signs of dystonia. Using fluorescent BAC probes spanning 18p, we were able to identify a deletion in these two individuals, spanning the entire short arm of 18p. This deletion is accompanied by a centric fusion involving chromosome 14. The 18p deleted region spans 15 megabases of DNA, with a number of interesting DYT7 candidate genes, including genes involved in G-protein-coupled signaling (GNAL), cell death (CIDEA), muscle development (MYOM1 and MRLM), mitochondrial activity (NDUFV2), and neuronal function (ADYCAP1, TGIF, DAP-1, and AFG3L2).

FXYD proteins: new regulators of Na-K-ATPase

FXYD proteins belong to a family of small-membrane proteins. Recent experimental evidence suggests that at least five of the seven members of this family, FXYD1 (phospholemman), FXYD2 (gamma-subunit of Na-K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), and FXYD7, are auxiliary subunits of Na-K-ATPase and regulate Na-K-ATPase activity in a tissue- and isoform-specific way. These results highlight the complexity of the regulation of Na+ and K+ handling by Na-K-ATPase, which is necessary to ensure appropriate tissue functions such as renal Na+ reabsorption, muscle contractility, and neuronal excitability. Moreover, a mutation in FXYD2 has been linked to cases of human hypomagnesemia, indicating that perturbations in the regulation of Na-K-ATPase by FXYD proteins may be critically involved in pathophysiological states. A better understanding of this novel regulatory mechanism of Na-K-ATPase should help in learning more about its role in pathophysiological states. This review summarizes the present knowledge of the role of FXYD proteins in the modulation of Na-K-ATPase as well as of other proteins, their regulation, and their structure-function relationship.

Kufor Rakeb disease: autosomal recessive, levodopa-responsive parkinsonism with pyramidal degeneration, supranuclear gaze palsy, and dementia

Kufor Rakeb disease is an autosomal recessive disorder characterized by subacute, juvenile-onset, levodopa-responsive parkinsonism, pyramidal signs, dementia, and a supranuclear gaze palsy. It was originally described more than a decade ago, and linkage analysis identified a locus on chromosome 1p36 that was previously assigned PARK9. We have further characterized the clinical picture and specifically re-assessed the response to levodopa in the original family, in the northern highlands of Jordan. In the 4 surviving patients, there has been a narrowing of the therapeutic window for levodopa with the emergence of peak-dose dyskinesias with increased spasticity and cognitive decline. Several new features were identified, including facial-faucial-finger mini-myoclonus, visual hallucinations, and oculogyric dystonic spasms.

Modulation of Na, K-ATPase activity by prostaglandin E1 and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin

Adenylyl cyclase is activated by prostaglandin E and inhibited by mu-opioids. Since cAMP-related events influence the activity of the Na Pump and its biochemical correlate Na,K-ATPase in many systems, we tested the hypothesis that prostaglandin E1 and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), a mu-opioid agonist, have opposing actions on Na,K-ATPase activity. Studies were conducted with alamethicin-permeabilized SH-SY5Y human neuroblastoma cells. Prostaglandin E1 (1 microM) transiently inhibited Na,K-ATPase activity for 10-15 min. A direct activator of protein kinase A, 8-Br-cAMP (150 and 500 microM), also inhibited, but more rapidly and for a shorter duration. Both DAMGO (1 microM) and Rp-adenosine 3',5'-cyclic monophosphorothioate (500 microM), a protein kinase A-inhibitor, reversed the inhibitory effect of prostaglandin E1. DAMGO alone (1 microM) stimulated Na,K-ATPase activity up to nearly three-fold control activity. The stimulatory action of DAMGO was blocked by cyclosporine A (2 microM), an inhibitor of calcineurin, and was dependent on Ca2+ entry through nifedipine-sensitive Ca2+ channels. In the presence of 1 mM EGTA, DAMGO inhibited Na,K-ATPase activity. DAMGO-induced inhibition was blocked by the inositol 1,4,5-trisphosphate receptor antagonist xestospongin C (1 microM). Na,K-ATPase is poised to modulate neuronal excitability through its roles in maintaining the membrane potential and transmembrane ion gradients. The differential effects of prostaglandin E1 and opioids on Na,K-ATPase activity may be related to their actions in hyperalgesia.

Sporadic rapid-onset dystonia-parkinsonism presenting as Parkinson's disease

We report on a 38-year-old patient with rapid-onset dystonia-parkinsonism (RDP) with a missense mutation in the Na/K-ATPase alpha3 subunit (ATP1A3). Asymmetrical parkinsonian symptoms evolved over a year. After a stable episode of another 2.5 years, overnight he developed oromandibular dystonia and more severe parkinsonian symptoms. We conclude that RDP should be considered as a rare cause of levodopa-unresponsive parkinsonism even if there is no family history and the classic presentation is lacking.

Neuropathology in Drosophila membrane excitability mutants

Mutations affecting ion channels and neuronal membrane excitability have been identified in Drosophila as well as in other organisms and characterized for their acute effects on behavior and neuronal function. However, the long-term effect of these perturbations on the maintenance of neuronal viability has not been studied in detail. Here we perform an initial survey of mutations affecting Na+ channels and K+ channels in Drosophila to investigate their effects on life span and neuronal viability as a function of age. We find that mutations that decrease membrane excitability as well as those that increase excitability can trigger neurodegeneration to varying degrees. Results of double-mutant interactions with dominant Na+/K+ ATPase mutations, which themselves cause severe neurodegeneration, suggest that excitotoxicity owing to hyperexcitability is insufficient to explain the resultant phenotype. Although the exact mechanisms remain unclear, our results suggest that there is an important link between maintenance of proper neuronal signaling and maintenance of long-term neuronal viability. Disruption of these signaling mechanisms in any of a variety of ways increases the incidence of neurodegeneration.

Movement disorder emergencies

For the past 4 years, Dr. Stanley Fahn and I have given a course at the American Academy of Neurology annual meeting on the topic of movement disorder emergencies. The purpose of this review article is to summarize the topic and to present it to readers of this journal. The text of this article has appeared in nearly the same form as the Academy syllabus accompanying our course. It is being presented here so that readers of the journal may review the material.

Target-determined expression of ?3 isoform of the Na+,K+-ATPase in the somatic nervous system of rat

Factors that determine the differential expression of isoforms of Na(+),K(+)-ATPase in the nervous system of vertebrates are not understood. To address this question we studied the expression of alpha(3) Na(+),K(+)-ATPase in the L5 dorsal root ganglia (DRG) of developing rat, the normal adult rat, and the adult rat after peripheral axotomy. During development, the first alpha(3) Na(+),K(+)-ATPase-positive DRG neurons appear by embryonic day 21. At birth, the L5 DRG have a full complement (14 +/- 2%) of these neurons. By 15 days after sciatic nerve transection in adult rat, the number of alpha(3) Na(+),K(+)-ATPase-positive DRG neurons and small myelinated L5 ventral root axons decreases to about 35% of control counts. These results combined with data from the literature suggest that the expression of alpha(3) Na(+),K(+)-ATPase by rat somatic neurons is determined by target-muscle spindle-derived factors.

PINK, PANK, or PARK? A clinicians' guide to familial parkinsonism

We provide a pragmatic guide for clinicians, and detail the recent developments in the genetics of Parkinson's disease that have shaped our current understanding and management of this disease and other parkinsonian disorders. These developments have been rapid, and in total over 20 genes have been identified, three of which were discovered in the past year. Although there are undoubtedly more genes to be found, the major challenge for the future is to determine how they function and whether they interact. These genes help us to understand the heterogeneity of parkinsonism, and also inform on the molecular and clinical features of individual parkinsonisms. However, their discovery also requires us to raise issues about genetic testing and genetic counselling.

Paying the price at the pump: dystonia from mutations in a Na+/K+ -ATPase

Dystonia is a disorder of involuntary sustained muscle contraction, which usually affects a focal region of the body but may be generalized and results in twisting contorted movements or abnormal postures. Several clinical subtypes of dystonia have been delineated and many have a strong inherited basis. In this issue of Neuron, de Carvalho Aguiar and colleagues report the identification of missense mutations in the gene for the Na+/K+ -ATPase alpha3 subunit (ATP1A3) as a cause of rapid-onset dystonia-parkinsonism (RDP, DYT12).