A dearth of calcium appears to silence nerves attempting to communicate with muscles in a zebrafish model expressing a mutant version of the amyotrophic lateral sclerosis gene TDP-43. Because of this, scientists propose, motor neurons shrivel away from their muscular connections. Drugs that open up calcium channels in those neurons restore both this communication and the fishes' ability to swim properly. The study appears in the January 23 Journal of Neuroscience. Similar medicines might benefit people in the early stages of ALS, suggested first author Gary Armstrong, who performed the study in the laboratory of Pierre Drapeau at the University of Montréal in Canada.

Armstrong and Drapeau studied the mechanisms of TDP-43 pathology in zebrafish larvae because their neurons make it possible to examine hundreds of fish in easy patch-clamp studies in one day. The experiments would have been much more complicated in a mammal, Armstrong told Alzforum. He worked on non-transgenic fish and on others expressing either wild-type human TDP-43 or a glycine-368-cysteine mutation found in human ALS (Kühnlein et al., 2008). The larvae expressing wild-type TDP-43 were fairly normal, but the mutants swam slowly and only for short distances when nudged (Kabashi et al., 2010).

To find out what crippled the fish, Armstrong examined how their motor neurons and muscles handled action potentials. Individually, each cell type carried current well, although the motor neurons of the mutant fish were easy to overexcite. The defect became clear when Armstrong provided current to neurons and recorded the resulting signal, called endplate current, in the adjacent muscle cells. “There was poor communication, and when they did communicate, these endplate currents were reduced,” Armstrong said. “That suggests there is something wrong … at the neuromuscular junction.”

A healthy junction maintains itself by way of regular signaling. Armstrong theorized that the faulty communication, over time, could lead to the dying back of axons from the synapse. Such pulling back is among the earliest events known in ALS, both in people and in animal models; it is detectable even before symptoms arise (Fischer et al., 2004; Murray et al., 2010; Dadon-Nachum et al., 2011). Muscle endplates become denervated early in disease (see Maselli et al., 1993) and in rodents expressing mutant TDP-43 or superoxide dismutase 1, another ALS gene (Zhou et al., 2010; Swarup et al., 2011; Frey et al., 2000).

What might go wrong at the neuromuscular junction of fish with mutant TDP-43? In frogs, calcium channels are important players in the release of acetylcholine at that junction. In response to an action potential, calcium rushes into axon tips, setting loose the neurotransmitter, Armstrong said (Thaler et al., 2001; Arenson and Evans, 2001; Nurullin et al., 2011).

Trying various drugs to affect the calcium channels in the fish, Armstrong discovered that opening L-type voltage-dependent calcium channels restored movement. The experimental channel agonists FPL 64176 and Bay K 8644 both returned swimming speed and distance to normal in the mutant fish. Neither of these compounds has been approved for human use. In patch-clamp experiments, Armstrong found that the agonists returned neuron-muscle communication back to normal. He suspects that opening up the channels leads to greater calcium influx during action potentials. “That allows for a greater number of these acetylcholine vesicles to be … dumped into the neuromuscular junction,” he said. In an e-mail to Alzforum, Hongxia Zhou of Thomas Jefferson University in Philadelphia, Pennsylvania, commented that “the study provides one more piece of evidence supporting the involvement of L-type calcium channels in ALS pathogenesis.”

Similar compounds might help people with ALS, Armstrong suggested. However, he cautioned that high calcium levels can be toxic to neurons. In fact, some scientists are pursuing calcium channel blockers, not openers, to treat Parkinson’s (see ARF related news story on Kang et al., 2012; ARF related news story on Guzman et al., 2010). For people in the end stages of ALS, boosting calcium influx might make things worse, Armstrong suggested. He envisions a peripheral treatment targeting neuromuscular junctions early in disease to preserve those synapses as long as possible. A drug that does not cross the blood-brain barrier would be ideal, he said, because it could reach the junctions but might not affect calcium levels very much in the nerve cell bodies.

Unfortunately, that vision would be difficult to enact today even if researchers had a compound they could try safely in humans. Plenty of neural damage occurs before symptoms of ALS manifest, and diagnosis can take months. “There are no clinical or preclinical markers of ALS,” noted Nizar Souayah, of the University of Medicine and Dentistry of New Jersey in Newark, in an e-mail to Alzforum. “Preclinical intervention right now is not realistic.” In the meantime, Armstrong is testing the channel-opening compounds in a mouse model of TDP-43 pathology.—Amber Dance.

Reference:
Armstrong GA, Drapeau P. Calcium channel agonists protect against neuromuscular dysfunction in a genetic model of TDP-43 mutation in ALS. J Neurosci. 2013 Jan 23;33(4):1741-52. Abstract

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References

News Citations

  1. Controlling Calcium: Potential Treatments for Ataxia, Parkinson’s
  2. Can Calcium Channel Blockers Save Stressed-Out Dopaminergic Neurons?

Paper Citations

  1. . Two German kindreds with familial amyotrophic lateral sclerosis due to TARDBP mutations. Arch Neurol. 2008 Sep;65(9):1185-9. PubMed.
  2. . Gain and loss of function of ALS-related mutations of TARDBP (TDP-43) cause motor deficits in vivo. Hum Mol Genet. 2010 Feb 15;19(4):671-83. PubMed.
  3. . Amyotrophic lateral sclerosis is a distal axonopathy: evidence in mice and man. Exp Neurol. 2004 Feb;185(2):232-40. PubMed.
  4. . Review: neuromuscular synaptic vulnerability in motor neurone disease: amyotrophic lateral sclerosis and spinal muscular atrophy. Neuropathol Appl Neurobiol. 2010 Apr;36(2):133-56. PubMed.
  5. . The "dying-back" phenomenon of motor neurons in ALS. J Mol Neurosci. 2011 Mar;43(3):470-7. PubMed.
  6. . Neuromuscular transmission in amyotrophic lateral sclerosis. Muscle Nerve. 1993 Nov;16(11):1193-203. PubMed.
  7. . Transgenic rat model of neurodegeneration caused by mutation in the TDP gene. PLoS Genet. 2010 Mar;6(3):e1000887. PubMed.
  8. . Pathological hallmarks of amyotrophic lateral sclerosis/frontotemporal lobar degeneration in transgenic mice produced with TDP-43 genomic fragments. Brain. 2011 Sep;134(Pt 9):2610-26. PubMed.
  9. . Early and selective loss of neuromuscular synapse subtypes with low sprouting competence in motoneuron diseases. J Neurosci. 2000 Apr 1;20(7):2534-42. PubMed.
  10. . Calcium channel isoforms underlying synaptic transmission at embryonic Xenopus neuromuscular junctions. J Neurosci. 2001 Jan 15;21(2):412-22. PubMed.
  11. . Activation of protein kinase C increases acetylcholine release from frog motor nerves by a direct action on L-type Ca(2+) channels and apparently not by depolarisation of the terminal. Neuroscience. 2001;104(4):1157-64. PubMed.
  12. . Voltage-dependent P/Q-type calcium channels at the frog neuromuscular junction. Physiol Res. 2011;60(5):815-23. PubMed.
  13. . 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. PubMed.
  14. . Oxidant stress evoked by pacemaking in dopaminergic neurons is attenuated by DJ-1. Nature. 2010 Dec 2;468(7324):696-700. PubMed.
  15. . Calcium Channel Agonists Protect against Neuromuscular Dysfunction in a Genetic Model of TDP-43 Mutation in ALS. J Neurosci. 2013 Jan 23;33(4):1741-52. PubMed.

Further Reading

Papers

  1. . Calcium Channel Agonists Protect against Neuromuscular Dysfunction in a Genetic Model of TDP-43 Mutation in ALS. J Neurosci. 2013 Jan 23;33(4):1741-52. PubMed.
  2. . Defective neuromuscular transmission in the SOD1 G93A transgenic mouse improves after administration of human umbilical cord blood cells. Stem Cell Rev. 2012 Mar;8(1):224-8. PubMed.
  3. . Methylene blue protects against TDP-43 and FUS neuronal toxicity in C. elegans and D. rerio. PLoS One. 2012;7(7):e42117. PubMed.
  4. . FUS and TARDBP but not SOD1 interact in genetic models of amyotrophic lateral sclerosis. PLoS Genet. 2011 Aug;7(8):e1002214. PubMed.
  5. . In the swim of things: recent insights to neurogenetic disorders from zebrafish. Trends Genet. 2010 Aug;26(8):373-81. PubMed.
  6. . Zebrafish models for the functional genomics of neurogenetic disorders. Biochim Biophys Acta. 2011 Mar;1812(3):335-45. PubMed.

Primary Papers

  1. . Calcium Channel Agonists Protect against Neuromuscular Dysfunction in a Genetic Model of TDP-43 Mutation in ALS. J Neurosci. 2013 Jan 23;33(4):1741-52. PubMed.