Purkinje cells, referring to the channel's initial site of discovery.[2][3] P-type calcium channels play a similar role to the N-type calcium channel
in neurotransmitter release at the presynaptic terminal and in neuronal integration in many neuronal types.
History
The calcium channel experiments that led to the discovery of P-type calcium channels were initially completed by
Purkinje neurons.[3] They were able to use an in vitro preparation to examine the ionic currents that account for Purkinje cells' electrophysiological properties. They found that there are calcium dependent action potentials which rise slowly and fall quickly then undergo hyperpolarization. The action potentials were voltage dependent and the afterhyperpolarizing potentials were connected to the spike bursts, located within the dendrites of the Purkinje cells. Without calcium flux in the Purkinje cells, action potentials fire sporadically at a high frequency.[2]
Voltage-gated P-type calcium channels consist of a main pore-forming α1 subunit (which is more specifically referred to as
S6 region are thought to be responsible for the channel's inactivation, the S4 region serves as the voltage sensor and S5-S6 loop forms the pore.[4] There are seven subunits within the α1 subunit. The A subunit, called α1ACa2+, corresponds to what is functionally defined as the P-type and Q-type isoforms. P-type and Q-type calcium channels are closely related as they are produced from the same gene via alternative splicing. As a complication of the alternative splicing, P-type and Q-type channels may have different subunit compositions.[6] The β subunit regulates the kinetics and expression of the channel, along with the α2δ subunit.[1]
Channel distribution
Antibody labeling is the primary method used to identify channel location.[7]
Areas of high expression in mammalian systems include:
P-type calcium channel blockers act to impede the flow of calcium. The blocking of calcium currents may cause the organism to experience impaired functioning and viability. These effects can lead to various diseases which are described in more detail in the section below.
The pore of P-type calcium channels are sensitive to compounds that can be divided into three groups:
There are only two peptide toxins that selectively block P-type channels: ω-agatoxin IVA and ω-agatoxin IVB. The other blockers mentioned, such as the low molecular weight and therapeutic blockers, are nonselective. This means they act can act on P-type channels as well as other channels.[1]
Selective peptide toxin ω-agatoxin
Venom of the Agelenopsis spider is a specific P-type calcium channel blocker
The two known blockers which are specific to P-type calcium channels are peptides derived from the spider venom of
disulfide bonds. Although ω-agatoxin IVA and ω-agatoxin IVB have the same affinity and selectivity for P-type channels, their kinetics are different. The ω-agatoxin IVA effects the gating mechanism of the P-type channel. When there is a strong depolarization to activate the channel, ω-agatoxin IVA can no longer block the channel. Therefore, ω-agatoxin IVA has a very low affinity for the channel when it is open. It binds to the α1A subunit on the outside of the pore. The ω-agatoxin IVA receptor on the P-type channel is located at the S3-S4 linker. On the other hand, channel blocking by ω-agatoxin IVB occurs much more slowly. Yet, similar to ω-agatoxin IVA, ω-agatoxin IVB cannot bind to the channel upon a strong depolarization.[1]
Non-selective peptide toxins
Grammostola spatulata. It acts to modify the P-type channel gating
.
ω-PnTx3-3, PnTx3-3, and phonetoxin IIA are all toxins from the spider Phonoetrica nigriventer which act to block the current through the P-type calcium channels.
DW13.3 is a peptide toxin from the spider Filistata hibernalis and it is composed of 74 amino acids. It also functions to block the current through P-type calcium channels.
ω-
pyramidal neurons to block the P-type channels. Also, within the hippocampal CA3 neurons, this toxin blocks synaptic transmission
. Its effects are slow.
Calcicludine is from venom of Dendroaspis angusticeps, which is a green mamba. It has the ability to voltage-dependently block P-type channels.
Low molecular weight channel blockers have advantages over peptide blockers in drug development. One advantage of low molecular weight channel blockers is that they can penetrate tissue, which is important for crossing the blood–brain barrier. There is no specific low molecular weight channel blocker for P-type channels. However, there are a number of these blocker compounds which can effect the activity of the P-type channels.[1] These include:
neostriatal interneurons by slowing the deactivation of the channel. Also, roscovitine can either act as an agonist or antagonist
for the P-type calcium channels in the presynaptic membrane.
NMDA receptor antagonists and act to block P-type channels. Eliprodil can decrease P-type channel currents in the Purkinje neurons in the cerebellum
.
Dodecylamine can only block P-type channels when they are in the open state.
Ethanol can block P-type channels when at a high enough concentration. The blocking of the P-type channels could be the reason for ataxia when drinking alcohol.[1]
Therapeutics
There are therapeutics used clinically which can effect the activity of P-type calcium channels. However, the primary
angina pectoris and Alzheimer's disease. Although many of the therapeutic compounds' main target is not P-type channels, further research needs to determine if the clinical effects of these compounds are also influenced by the P-type channel blockage.[1]
Related diseases
How neurotransmitters are released from a presynaptic neuron(A). B is post synaptic neuron. 1. Mitochondria; 2. Synaptic vesicle full of neurotransmitter; 3. Autoreceptor; 4. Synaptic cleft; 5. Neurotransmitter receptor; 6. Calcium Channel; 7. Fused vesicle releasing neurotransmitter; 8. Neurotransmitter re-uptake pump
There are a number of neurological diseases that have been attributed to malfunctioning or mutated P/Q type channels.[6]
Alzheimer's disease
In
β-amyloid protein (Aβ) in brain. Amyloid plaques develop which result in the key symptoms of Alzheimer Disease. Aβ globulomer protein is an artificial substance used in research experiments that has similar properties to Aβ oligomer which is present in the body. Aβ oligomer directly regulates P/Q type calcium channels. The α1A subunit is the responsible for the conduction of calcium current. When only P/Q type calcium channels are present with Aβ globulomer protein, there is a direct effect on the α1A subunit and results in an increased calcium current through the P/Q type calcium channel. The response is dose dependent as concentrations of 20nM and 200nM of Aβ globulomer are necessary for significant increase of calcium current through channel in Xenopus oocytes, showing that a certain buildup of Aβ globulomer is necessary before the effects are seen. When the calcium current is increased, neurotransmitter release also rises, offering a possible cause for the toxicity in Alzheimer's disease patients.[9]
Migraine headaches
The
knockin experiment, this mutation could be expressed in mice so research could be conducted.[5][10] The mutant mouse has a significantly higher P2X3 receptor activity than the wild type mouse[5] due to increased channel open probability and channel activation at lower voltages.[10] This increased receptor activity results in a higher flux of calcium through the P/Q type calcium channel. The increased intracellular calcium concentration may contribute to the acute trigeminal pain that typically results in a headache.[5] Evidence supports that migraines are a disorder of brain excitability characterized by deficient regulation of the cortical excitatory–inhibitory balance.[10]
Seizures
generalized seizures. Levetiracetam inhibits P/Q channel-mediated glutamate release and decreases the excitatory post synaptic currents of both AMPA and NMDA receptors in the hippocampus, specifically the dentate gyrus, which is known to propagate seizure activities. The inhibition of glutamate release results in an anti-epileptic response in patients because it decreases the excitatory postsynaptic current. There are many different types of calcium channels, so to prove that the P/Q type calcium channels are directly involved, a P/Q type voltage gated calcium channel inhibitor, omega-agatoxin TK, was used to block the channel. When blocked, patients no longer benefited from the anti-epileptic effects from the drugs. When blockers for L type and N type calcium channels were used, the effects of Levetiracetam were still seen. This is strong evidence that the P/Q type calcium channels are involved in the Levetiracetam treatment which allow for relief from seizures.[12]
Mutation studies
Many P-type calcium channels mutations result in a decreased level of intracellular free calcium. Maintaining calcium homeostasis is essential for normally functioning neurons. Changing the cellular calcium ion concentration acts as a trigger for multiple diseases, in severe cases these diseases can result in mass neuronal death.[6]
Mutation studies allow experimenters to study genetically inherited channelopathies. A channelopathy is any disease that results from an ion channel with malfunctioning subunits or regulatory proteins.[6] One example of a P-type calcium channel channelopathy is shown in homozygous ataxic mice, who are recessive for both the tottering and leaner genes. These mice present with mutations in the alpha1A subunit of their P/Q type channels. Mutations in these channels result in deficiencies within the cerebellar Purkinje cells that dramatically reduce the channels current density.[6]
The tottering mutations within mice result from a
pre-Bötzinger Complex, a cluster of interneurons in the brainstem which help to regulate breathing.[5]
