5 November 2012. Human bones may benefit from lots of dietary calcium, but brain cells are another matter. Too much calcium in certain neurons—or too little—can lead to neurological disorders such as ataxia, epilepsy, and certain extreme migraines. Now, three recent papers suggest ways to restore normal synaptic activity to neurons with too much calcium. Two studies, one led by Freek Hoebeek, Erasmus Medical Centre, Rotterdam, the Netherlands, and the other by Ilya Bezprozvanny, University of Texas Southwestern Medical Center, Dallas, propose opening up calcium-activated potassium channels to let positive potassium ions out of the cell and thus restore the membrane potential and normal neuronal firing. Another report, from the lab of Richard Silverman, Northwestern University in Evanston, Illinois, details a compound that selectively blocks incoming calcium to substantia nigra cells—a potential treatment for Parkinson’s.
“In neurodegenerative diseases, there are excessive calcium signals that cells can’t handle,” said Bezprozvanny. “You want to pharmacologically bring that down.”
Significant calcium influx occurs during action potentials in Purkinje cells in the cerebellum. These cells generally fire in a steady, tonic rhythm that helps encode information from the cortical cerebellum to deep cerebellar nuclei and other motor coordination areas (see Ito, 2002). However, sickly Purkinje cells can fire in bursts (see Kasumu et al., 2012), which can lead to too much calcium in the cells and precipitate ataxic disorders marked by poor muscle coordination. Certain human mutations also cause calcium imbalances in Purkinje cells. For instance, polymorphisms in the CACNA1A gene, coding for the pore-forming unit of CaV2.1 voltage-gated calcium channels, allow either too much or too little calcium into a neuron. How to correct that imbalance?
One effective strategy in mice compensates for calcium shortage by activating small-conductance calcium-activated potassium (SK) channels (see Walter et al., 2006) that play a role in tonic Purkinje cell firing (see Womack and Khodakhah, 2002). These channels open in the presence of calcium to let potassium out of the cell. In the case of reduced calcium, SK channels do not open as easily and tonic firing falls. However, activators stimulate SK channel opening even in the presence of reduced calcium, restoring firing to a more normal rate. In the October 31 Journal of Neuroscience, Hoebeek and colleagues report that these same SK channel activators, somewhat counterintuitively, can also correct for a calcium overload.
First author Zhenyu Gao and colleagues applied SK channel blockers and activators to cerebellar slices from mice with a mutation (S218L) in the Cacna1a gene (van den Maagdenberg et al., 2010). This mutation enhances calcium channel opening, boosts calcium influx, and in humans causes ataxia, epilepsy, and familial hemiplegic migraine type 1 (migraine headaches marked by temporary stroke-like symptoms, such as tingling, numbness, or paralysis). Since SK activators correct for a drop in calcium, the research team thought that SK blockers would fix the calcium overload. However, irregular Purkinje cell bursts returned to a steady pace only with SK channel activators such as 1-EBIO and chlorzoxazone. It appeared that a stronger current of potassium out of the cell helped to balance the excess of calcium coming in. “You need this extra outflux of potassium to overrule the ‘burst’ firing pattern of Purkinje cells that originates from too much calcium influx,” said Hoebeek.
The strategy worked in vivo as well. When the team applied 1-EBIO directly to the cerebellum in awake, immobilized S218L mice, Purkinje cell firing became more regular. Mice given chlorzoxazone in their drinking water for seven days performed better than their untreated counterparts in an accelerating rotarod test of motor coordination and balance.
These results indicate that, in addition to compensating for inadequate calcium levels, SK activators may be able to counteract genetic mutations that cause excessive neuronal calcium. “It suggests the possibility of a much broader implementation for the same drugs,” Hoebeek told Alzforum. SK activators are not yet clinically approved for treating ataxia, but chlorzoxazone is approved as a muscle relaxant in humans. This drug could produce muscle-weakening side effects if used for ataxia, Hoebeek pointed out. Next, he wants to study what happens downstream of the Purkinje cell firing alterations, in the deep cerebellar nuclei and beyond.
Bezprozvanny and colleagues employed a similar strategy in treating a different ataxic animal, as described in their October 26 Chemistry & Biology paper. Previously, the researchers genetically reduced release from Purkinje cell calcium stores in aging mouse models of spinocerebellar ataxia type 2 (SCA2)—an autosomal dominant polyglutamine expansion disorder marked by worsening movement coordination (see Huynh et al., 2000). Corralling the calcium rescued both firing dysfunction and problems with motor coordination (see Kasumu et al., 2012). Because genetic solutions are not easily translatable to therapies in humans, Bezprozvanny and colleagues sought a pharmacological way to rebalance calcium in these Purkinje cells.
First authors Adebimpe Kasumu and Charlotte Hougaard targeted SK channels. By pumping out potassium, they hoped to maintain a more negative membrane potential so that the neuron would fire less frequently, just as Hoebeek and colleagues had done, “basically, putting the brake on neuronal firing,” said Bezprozvanny.
The researchers bathed brain slices from SCA2 mice with both the general SK activator NS309 and the more specific and brain-penetrant SK3/SK2 modulator CyPPA (SK2-type channels predominate in the Purkinje cells). Both restored bursts to a steadier rhythm.
Collaborators led by Palle Christophersen at Aniona—a drug development company in Ballerup, Denmark—then optimized the chemistry of the SK activators, developing the much more potent NS13001, which penetrated the brain and had higher affinity than the other compounds for SK3/SK2 channels. Three weeks of oral NS13001 improved nine-month-old SCA2 mice’s performance on both rotarod and balance beam tasks. Two months after treatment, fewer Purkinje cells had degenerated in treated mice than in controls. By reducing burst frequency, the compound likely relieves Purkinje cells of a strenuous metabolic burden and protects them from death, Bezprozvanny told Alzforum.
SK channels are also important for synaptic plasticity and memory (see Kuiper et al., 2012). Could drugs that target these channels for ataxia adversely affect cognitive functions? Possibly, said Bezprozvanny. Treatment would probably have to involve a careful balance of these positive and negative effects, either by limiting treatment duration or dose, or targeting treatment to specific cells, he told Alzforum. “There’s a lot of evidence that excitotoxicity and hyperexcitability of neurons contributes to degeneration in a number of different disorders,” said Mark Mattson, National Institute on Aging, Baltimore, Maryland, who was not involved in either study. However, excitatory glutamatergic synaptic transmission is critical for normal cognitive function, he said. “One just has to do the studies to see to what extent these drugs compromise learning and memory.”
“Together, these papers suggest that multiple forms of ataxia could be targeted using these SK channel drugs,” said Harry Orr, University of Minnesota, Minneapolis. These therapies may not be limited to genetic forms of these diseases, he added. “If the SK channels are a common nodal point in Purkinje cell pathophysiology as it relates to ataxia, these [drugs] could potentially also be applicable to the sporadic forms.
A third paper on calcium control appeared in the October 23 Nature Communications. Researchers led by Richard Silverman and James Surmeir of Northwestern University in Evanston and Chicago, Illinois, respectively, wanted to regulate calcium influx into substantia nigra cells for the potential treatment of Parkinson’s disease. No compound yet has been able to selectively block CaV1.3 channels in those neurons without interfering with CaV1.2, channels, which are important for cardiovascular function. Using chemical screens followed by some medicinal chemistry, first author Soosung Kang and colleagues made 1-(3-chlorophenethyl)-3-cyclopentylpyrimidine-2,4,6-(1H,3H,5H)-trione, which selectively antagonized CaV1.3 channels with high potency. Surmeir previously reported that CaV1.3 channel activity explains the selective vulnerability of dopaminergic substantia nigra neurons in Parkinson’s (see ARF related news story).
“It’s a big step toward developing a selective compound for an important target,” said Bezprozvanny. Though the selectivity is sufficient, he added, improving the compound’s potency even further would mean it could eventually be given in lower doses and lead to fewer off-target effects.—Gwyneth Dickey Zakaib.
Gao Z, Todorov B, Barrett CF, van Dorp S, Ferrari MD, van den Maagdenberg AM, De Zeeuw CI, Hoebeek FE. Cerebellar Ataxia by Enhanced CaV2.1 Currents Is Alleviated by Ca2+-Dependent K+-Channel Activators in Cacna1aS218L Mutant Mice.
J Neurosci. 2012 Oct 31;32(44):15533-46. Abstract
Kasumu AW, Hougaard C, Rode F, Jacobsen TA, Sabatier JM, Eriksen BL, Strøbæk D, Liang X, Egorova P, Vorontsova D, Christophersen P, Rønn LC, Bezprozvanny I. Selective positive modulator of calcium-activated potassium channels exerts beneficial effects in a mouse model of spinocerebellar ataxia type 2. Chem Biol. 2012 Oct 26;19(10):1340-53. Abstract
Kang S, Cooper G, Dunne SF, Dusel B, Luan CH, Surmeier DJ, Silverman RB. Ca(V)1.3-selective L-type calcium channel antagonists as potential new therapeutics for Parkinson's disease. Nat Commun. 2012 Oct 23;3:1146. Abstract