The role of persistent currents generated by sodium channel splicing in neurological disorders
Lead Research Organisation:
University College London
Department Name: UNLISTED
Abstract
Neurons in the brain communicate using electrical signals. These signals are generated by a particular set of molecules called sodium channels. Many drugs used to treat neurological illnesses, such as epilepsy and chronic pain, bind to sodium channels. These drugs do not simply block the channels (that would be fatal to the patient). Instead they allow the channels to make the electrical signals, but they ensure that in between the signals the channels are firmly closed. We have discovered that the genes that code for sodium channels can often be read in two alternate ways in order to make two slightly different channels from each gene. These different channels are called splice variants, because they are made by splicing the genes differently. We have shown that the splice variants from one gene are much more likely to open in between electrical signals, and consequently they may be the most important target of many drugs.
We have also found that some diseases, such as epilepsy, cause the sodium channel genes to generate more of the splice variants that open between electrical signals. This means the disease itself may lead to neurons in the brain having more of these channels that require drugs to keep closed between signals. Many diseases are thought to change the number of channels that open between signals, including Parkinson s disease, motor neurone disease and neuropathic pain, as well as epilepsy. We ask whether any or all of these diseases change the amount of sodium channel splice variants, and whether these sodium channel splice variants may represent a more specific target for drugs that would produce fewer side effects than drugs that affect all sodium channel splice variants.
By looking at the behaviour of neuronal cells grown in petri dishes, and examining how drugs bind to the sodium channels and how the genes are being spliced we will determine whether the splice variants are specifically targeted by drugs and how removing the splice variants will change the neurons. We will also investigate whether conditions that mimic diseases lead to changes in the splicing. It is possible that by preventing the neurons from making the splice variants that open between electrical signals, we will be able to directly alleviate some of the changes in neuronal behaviour linked to neurological disorders. This may lead to a new approach to treating some of these difficult and persistent disorders.
We have also found that some diseases, such as epilepsy, cause the sodium channel genes to generate more of the splice variants that open between electrical signals. This means the disease itself may lead to neurons in the brain having more of these channels that require drugs to keep closed between signals. Many diseases are thought to change the number of channels that open between signals, including Parkinson s disease, motor neurone disease and neuropathic pain, as well as epilepsy. We ask whether any or all of these diseases change the amount of sodium channel splice variants, and whether these sodium channel splice variants may represent a more specific target for drugs that would produce fewer side effects than drugs that affect all sodium channel splice variants.
By looking at the behaviour of neuronal cells grown in petri dishes, and examining how drugs bind to the sodium channels and how the genes are being spliced we will determine whether the splice variants are specifically targeted by drugs and how removing the splice variants will change the neurons. We will also investigate whether conditions that mimic diseases lead to changes in the splicing. It is possible that by preventing the neurons from making the splice variants that open between electrical signals, we will be able to directly alleviate some of the changes in neuronal behaviour linked to neurological disorders. This may lead to a new approach to treating some of these difficult and persistent disorders.
Technical Summary
Voltage-gated sodium channels generate the action potentials that define virtually all excitable cells. The rapidly inactivating transient sodium currents from these channels controls the duration of action potentials, but a second type of sodium current, the persistent current, modulates the timing, spacing and threshold of action potentials. Although the persistent current is much smaller in amplitude than the transient current, it has profound effects on neuronal properties. In fact, subtle changes in persistent currents are associated with many mutations in sodium channels that cause inherited epilepsies, and seem to be sufficient to lead to seizures.
The molecular basis of persistent currents remains mysterious. They are modulated by G-protein pathways, but may be generated by the same physical channels that generate transient sodium currents. We have recently shown that a conserved site of alternative splicing in TTX-sensitive sodium channels has dramatic effects on the amount of persistent current one type of channel (SCN1A) produces when heterologously expressed in mammalian cell lines. This alternative splicing is dynamically regulated during development, and also in epilepsy, most likely leading to changes in persistent currents in neurons and their behaviour.
We will ask how this splicing affects closely-related sodium channels, whether the changes in persistent currents seen in other neurological disorders are also associated with changes in splicing, and how directly manipulating the levels of the splice variants in vitro affects neuronal behaviour. We have also recently shown that certain sodium channel beta subunits are capable of masking the effects of alternative splicing in SCN1A, however there is evidence that these beta subunits are down-regulated in the same conditions that promote the splicing that leads to persistent currents. Consequently we will also investigate how the regulation of the beta subunits complements the changes in splicing in the alpha subunits, and whether directly down-regulating the beta subunits will lead to unmasking of low levels of alternatively spliced alpha subunits in otherwise healthy cells.
We are combining our biophysical, physiological and molecular expertise to approach the question of the regulation, functional impact and consequences of alternative splicing for these important channels. Finally, because the persistent currents are an important target of many drugs that target sodium currents, understanding the nature, generation and regulation of these currents and their link to alternative splicing should provide new insights into drug effects and may provide a more specific target for future drug design.
The molecular basis of persistent currents remains mysterious. They are modulated by G-protein pathways, but may be generated by the same physical channels that generate transient sodium currents. We have recently shown that a conserved site of alternative splicing in TTX-sensitive sodium channels has dramatic effects on the amount of persistent current one type of channel (SCN1A) produces when heterologously expressed in mammalian cell lines. This alternative splicing is dynamically regulated during development, and also in epilepsy, most likely leading to changes in persistent currents in neurons and their behaviour.
We will ask how this splicing affects closely-related sodium channels, whether the changes in persistent currents seen in other neurological disorders are also associated with changes in splicing, and how directly manipulating the levels of the splice variants in vitro affects neuronal behaviour. We have also recently shown that certain sodium channel beta subunits are capable of masking the effects of alternative splicing in SCN1A, however there is evidence that these beta subunits are down-regulated in the same conditions that promote the splicing that leads to persistent currents. Consequently we will also investigate how the regulation of the beta subunits complements the changes in splicing in the alpha subunits, and whether directly down-regulating the beta subunits will lead to unmasking of low levels of alternatively spliced alpha subunits in otherwise healthy cells.
We are combining our biophysical, physiological and molecular expertise to approach the question of the regulation, functional impact and consequences of alternative splicing for these important channels. Finally, because the persistent currents are an important target of many drugs that target sodium currents, understanding the nature, generation and regulation of these currents and their link to alternative splicing should provide new insights into drug effects and may provide a more specific target for future drug design.
