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N-type Calcium Channel

Crystal Structure of N-Type Channel

N-type calcium channels are voltage gated calcium channels that are distributed throughout the entire body. These channels are high voltage activated channels composed of alpha-1B subunits. The alpha subunit forms the pore through which the calcium enters and helps to determine most of the channel's properties. The alpha subunit is also known as the calcium channel/voltage dependent/N type, alpha 1 subunit (CACNA1B), or Cav2.2 which is used in therapeutic processes), which in humans is encoded by the CACNA1B gene, or calcium voltage-gated channel subunit alpha1 B. The subunit is essential to modulate neurotransmitter release. They also contain associated subunits such as β1, β3, β4, α2δ, and possibly γ. These channels are known for their importance in the nervous system. They play a small role in the migration of immature neurons before the establishment of their mature synapses, and they are critically involved in the release of neurotransmitters, which is also similar to another type of calcium channels, known as P-type calcium channels. N-type calcium channels are targets for the development of drugs to relieve chronic and neuropathic pain. They are also used for the treatment of hypertension, Autism Spectrum Disorder, Osteoarthritis, and other medical diagnoses. There are many known N-type calcium channel blockers that function to inhibit channel activity, but the most notable blocker are being ω-Conotoxins.

N-type-calcium-channel

Structure

N-type Calcium channels are categorized as high threshold-activated channels and seen in the Cav2 gene family. The structure of the N-type Calcium Channel is very similar to other voltage-dependent calcium channels. Alpha, beta, and gamma subunits show that those subunits are substrates for cAMP-dependent protein phosphorylation. The most important part of the channel is the actual pore that is formed by the alpha-1 subunit. This pore is the location of the import of the extracellular ions. The alpha 1 subunit has as many as 2000 amino acid residues within an amino acid sequence with the transmembrane structure with a pore. This is organized into 6 six segments(S1-S6). S1, S2, S3, S5, and S6 are hydrophobic while S4 serves as the voltage-sensor. In addition there is a membrane-associated loop in between S5 and S6. The activity of the pore is modulated by 4 subunits: an intracellular β-subunit, a transmembrane gamma subunit, and complex of alpha-2 and delta subunits.

In addition to the α1 subunit, the following subunits are present in the N-type calcium channel:

• α2δ – CACNA2D1, CACNA2D2
• β3 – CACNB3
• β4 – CACNB4

Function

N-type calcium channels are highly known for their function in the nervous system, but they are also involved with the function of the heart and kidneys. They are important in neurotransmitter release because they are localized at the synaptic terminals. When calcium flows into N-type calcium channels due to an action potential, it triggers the fusion of the secretory vesicles. Studies on the cardiovascular system reveal that ω-Conotoxin is introduced causing there to be no more release of norepinephrine, and this shows that only the N-type calcium channel, and not the P/Q/L type calcium channels are involved in the release of norepinephrine. In the Kidneys, N-type calcium channels reduce glomerular pressure through dilation of arterioles when the channel is blocked. The inhibition of this channel by calcium channel blockers can lead to renal microcirculation. N-type calcium channels have been shown to play a part in the localization of neurite growth in the sympathetic nervous system and the skin and spinal cord. The neurite outgrowth was shown to be inhibited through an interaction between laminin and the 11th loop of the n-type calcium channel structure. It has been suggested that neuritis outgrowth is inhibited by the influx of calcium through the growth cone, and this happens when the Cav2.2 subunit comes in contact with laminin 2, and in response can induce a stretch activation of the N-type calcium channel.

Blockers

Ziconotide 1DW5

• ω-Conotoxins
• Cadmium
• Caroverine
• Cilnidipine
• Gabapentin
• Levetiracetam
• Lamotrigine
• Nicardipine
• NP078585
• Pregabalin
• TROX-1
• Ziconotide

Mutation Studies

The N-type calcium channel when mutated can lead to problems in its function, and can also lead to clinical problems. Most mutations occur within the Alpha subunit of the channel, the CACNA1B gene. Mutations of said gene have been known to directly correlate with several neuropsychiatric diseases. For example, a missense mutation can lead to Myoclonus-Dystonia syndrome, where as a duplication can lead to Autism. Mutation in the N-type channel can also cause bipolar disorder and schizophrenia, two heritable diseases with overlapping genetic components. Studies have also shown that deletion of the α2δ subunit can alleviate neuropathic inflammatory pain, specifically.

Clinical Significance

N-type calcium channels have been connected to a variety of different clinical diagnoses. Alteration of N-type calcium channels for therapeutic processes occurs in four major ways; through the blockage of N-type calcium channel peptides, interference of the flow of ions through the channel itself, activation of G-protein coupled signaling, and interference of the G-protein pathways. They are most commonly linked to therapeutic treatment of chronic pain. Studies have shown that the intrathecal injection of calcium channel inhibitors such as Ziconotide, to block the N-type calcium channels, have produced alleviation of intractable pain. Administering N-type peptide blockers via injection into the spinal canal (intrathecal injection) allows the drugs to reach the Cerebral Spinal Fluid where the channels are located. The use of blockers to inhibit the N-type calcium channels have produced alleviation of chronic pain for a variety of different diseases. For example, blockade of the N-type calcium channel is a potential therapeutic strategy for the treatment of alcoholism using N-type channel antagonists. Because prolonged alcohol exposure over time has been known to increase N-type channel function, experiments have been performed to determine the effect inhibiting them has on alcohol addiction. Studies have shown that using N-type antagonists to decrease channel activity has shown reduced voluntary consumption of alcohol.

Studies have also shown that N-type peptide blockers, like Ziconotide, have been used to relieve pain that results from osteoarthritis, hypertension, and diabetic neuropathy. It has also been known to be used to aid in relief of pain associated with cancer, and can be used for patients actively battling cancer, and studies are being done to see if the treatment may also be used for those survivors still experiencing chronic pain. Also, because the N-type calcium channel is intrinsic to neurotransmitter release and excitability, they can be regulated via GPCR signaling using analgesic opioid drugs to relieve pain. This is done by modulating the sensory transmitter release and subsequently lessening the feelings of pain. However, the use of intrathecally injected N-type channel blockers has proven to be more beneficial than common opioid remedies since there are less negative side effects associated. The Cav2.2 channel inhibitor, Ziconotide, for example, is the only drug on the market as of now that directly targets the n-type channel and its conotoxin peptide.

N-type calcium channels have also been associated with several known diseases as well.For example, mutations in the CACNA1B gene have been associated with Myoclonus-Dystonia syndrome, an unusual hyperkinetic movement disorder with muscular contractile symptoms. This can be explained by the fact that the channel is mutated with an increase in current flow, subsequently causing hyperexcitability. It has also been linked to cardiac arrhythmia, as increased excitability can cause tachycardia. Issues with the alpha subunit of the N-type calcium channel has also been linked to cases of the Autism Spectrum Disorders as well as psychiatric diseases such as bipolar disorder and schizophrenia.

Role in Pain Regulation

N-type calcium voltage gated ion channels are permeable to calcium and are known to regulate neuronal excitability and the firing of action potentials in the neurons. These actions affect the transmission of neurotransmitters in nociceptive pathways. Nociceptive pathways are the neurons that are involved in pain perception, which typically arises from damage of the neurons or disease occurring in the nerves themselves. When action potentials in this pathway make it to the central terminals of the sensory neurons that are in the spinal cord, there is an influx of calcium that enters through the voltage-gated ion channels. This influx of calcium then triggers the release of the pro-nociceptive neurotransmitters. These neurotransmitters then bind to the receptors on the sensory neurons that cause a person to feel pain. Since these channels are able to influence the neurotransmitters in the body that cause a person to sense pain, voltage-gated ion channels that are permeable to calcium have also been found to be good targets for regulating chronic pain in patients who have difficulty finding effective treatment. A few of the conditions involving pain that are difficult to treat are acute pain, chronic inflammatory pain, and chronic europathic pain.
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These conditions are treated with medicine that has either been prone to becoming addictive to the patient, or can cause serious side effects, making the medicine controversial. Studies have shown that N-type channels are particularly of interest in developing new medicine to help in treating chronic pain. N-type channels play a key role in being able to control the neurotransmission of pain in the spinal cord. Studies have shown that N-type channels are located in high amounts at the presynaptic terminals of neurons. They have a complex of proteins that form a pore with the ɑ1Bsubunit (also known as CaV2.2) and an auxiliary ɑ2δδ and β subunits. When there is tissue damage, these subunits are upregulated in the neuron, which helps to prove that these channels are involved in nociceptive transmission. When looking at the spinal cord, the distribution of N-type channels also coincides with the evidence of these channels having a role in the transmission of nociceptive information in the body. A new medicine, with the general name of Ziconotide, is being tested to alter N-type channels in order to treat these conditions of chronic pain. One of the newest medicines is an analgesic drug that works through targeting N-type calcium channels specifically in order to lessen the transmission of pain. Peptide ɷɷ-conotoxins are what bind to the N-type calcium channels in order to block ion permeation, which then blocks the influx of calcium and the transmission of the nociceptive responses. Neurons located in the dorsal horn of the spinal cord experience ɷɷ-conotoxins that, when binding to the receptors, can inhibit the release of the pro-nociceptive neurotransmitters and neuromodulators that come from the central nerve terminals of the afferent neurons. This inhibition is what ultimately can stop the transmission of pain to the person from damaged cells. Ziconotide is an inorganic version of the ɷɷ-conotoxin, MVIIA. This medicine is able to inhibit the release of pro-nociceptive receptors, which will then block the transmission of pain.