Voltage-Gated Calcium Channels

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Voltage-Gated Calcium Channels

• Daniel Blackman, Zhihui Zhou, Thomas Arnold

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Calcium Ion Channel Family

• Cav1 initiate contraction, secretion, and
  regulation of gene expression, integration of
  synaptic input in neurons, and synaptic
  transmission at ribbon synapses of specialized
  sensory cells
• Cav2 synaptic transmission of fast synapses
• Cav3 important for repetitive or rhythmic
  firing of Aps (cardiac, thalamic)

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Physiology of Voltage-Gated Ca2 Channels
Image taken from Caterall 2011
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Image taken from Caterall 2011
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Image taken from Caterall 2011
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Images taken from Caterall 2011
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Images taken from Caterall 2011
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Cav1 channel

• Excitation-contraction coupling
• Excitation-transcription coupling
• Excitation-secretion coupling

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Excitation-contraction coupling
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ardiovascular-physiology/deck/9845939
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Regulation of excitation-contraction coupling

• PKA phosphorylation and its anchoring via a
  kinase anchoring protein (APAK).
• An autoinhibited Ca2 channel complex with
  noncovalently bound distal carboxyl-terminus.
• Ca2/ calmodulin-dependent inactivation

Image taken from Caterall 2011
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Excitation-transcription coupling

• Calmodulin binds to the proximal caboxy-terminal
  domain, the Ca2 /calmodulin complex moves to the
  nucleus
• The distal carboxy-terminal domain is regulated
  by Ca2 in neurons.

Image taken from Caterall 2011
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Excitation-secretion coupling

• Initialization of the secretion of hormones from
  endocrine cells and release of neurotransmitters.
• The distal carboxy-terminal domain plays an
  autoregulatory role in some Cav 1 channel, such
  as Cav1.3, Cav1.4.

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Cav2 Channels

• http//physrev.physiology.org/content/90/4/1461

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Image taken from Caterall 2011
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Cav2 specific information

• Initiate fast release of glutamate, GABA, and
  acetylcholine
• SNARE proteins
• G Protein subunits are responsible for modulation
• Additional binding proteins

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Cav3 Channels

• Molecular structure
• Negative potential activation (fast inactivation)
• Similar to Cav1 and 2 by 25
• Functional
• Present in rhythmic structures
• SA node (pacemaker), relay neurons of thalamus
  (sleep), adrenal cortex (aldosterone)
• Mutations can cause absence epilepsy (sleep-like
  state)
• Regulation
• Dopamine NTMs
• Angiotensin II

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Conlcusion

• Ca2 channel complexes effector and regulator
• Four cases effectors enhance Cav1 Cav2
• Skeletal muscle
• SNARE proteins
• Ca2/CaM-dependent protein kinase II
• RIM
• Common Theme Effector Checkpoint

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Point of interest

• LOF for Nav1.7 causes anosmia
• Cav2.2 is involved with the first synapse of the
  olfactory system
• Cacna1b LOF mutation causes an absence of Cav2.2
  channels
• Effects of Lacking Cav2.2 on Olfactory Sensory
  Neurons (OSN) in the Main Olfactory Bulb (MOB)
  and on Vomeronasal Sensory Neurons (VSN) in the
  Accessory Olfactory Bulb (AOB)

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Summary

• N-type Cav Channels are main contributors to
  presynaptic release
• MOB and AOB respond differently to Cav2.2
  mutation
• Presence of unknown Cav channel type in MOB
• Lack of Cav2.2 does not cause anosmia
• Mutation causes hyperaggressive behavior

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Question?

• Ca2 and Na own nearly identical diameters (2A)
• The extracellular concentration of Na is 70-fold
  higher than Ca2
• The conductance of Na is more than 500-fold
  lower than Ca2 via Cav channel
• How the Cav channel keeps the high selectivity of
  Ca2 ?

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Selectivity filter
NavAb 175TLESWSM181, outward sodium
current CavAb 175TLDDWSD181 , inward calcium
current
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No significant alteration in backbone structure
between NavAb and CavAb------the selectivity is
mainly determined by the side chains.
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• Three Ca2 - binding sites
• Site 1 the carboxyl groups of D178.
• Site 2 four carboxylate oxygen atoms from D177
  and four backbone carbonyl oxygen atoms from
  L176.
• Site 3 a plane of four carbonyls from T175
• The bound Ca2 ion is continuously stabilized in
  a fully hydrated state through the pore.

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• D178 VS S178
• Site 1
• Over 100-fold change in PCaPNa.
• D178 forms the first hydrated Ca2 - binding
  site
• S178 blocks the conduction of Ca2 by directly
  binding Ca2 and displacing the hydration shell

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• D177 VS E177
• Site 2
• 5.5-fold change in PCaPNa.
• D177 interacts with Ca2
• E177 swings away from the selectivity filter

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• D181 VS N181 VS M181
• Site 1
• 4- to 5-fold change in PCaPNa.
• D181 an N181 constrains the side-chain of the
  D178 ring by forming a hydrogen bond.
• M181 unconstrains the side-chain of the D178 and
  results in a blocking Ca2 tightly bound at
  Site1.

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• Binding forces
• Site 2gt Site 1 gt Site 3
• Ca2 cant occupy adjacent sites simultaneously
  due to electrostatic repulsive interactions.
• High extracellular concentration of Ca2 and weak
  binding of Ca2 to Site 3 generate a
  unidirectional flux of Ca2 .

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Direct Recording and molecular identity of the
calcium channel of primary cilia

• Daniel Blackman, Zhihui Zhou, Thomas Arnold

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Primary Cilia

• Specialized compartments
• Calcium signaling
• Hedgehog pathways
• Human retina pigmented epithelium cells tagged
  with GFP

Image taken from DeCaen et al
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Image taken from DeCaen et al
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Image taken from DeCaen et al
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Polycystin proteins (PC/PKD)

• Identified in polycystic kidney disease
• Form ion channels at high densities in multiple
  cell types
• Two structural classes (PKD1s and PKD2s)
• Hypothesis PKD1L1-PKD2L1 heteromultermerize to
  form calcium-permeant ciliary channels

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Image taken from DeCaen et al
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Conclusion

• No Ca2 current with just PKD1L1 (current
  observed with PKD2L1)
• Only with both PKD1L1 and PKD2L1 was current
  observed matching human