Molecular sensory physiology group (Head: Alexandra Koschak)   

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Staff


Researchers:
- Hartwig Seitter
- Kathrin Kähler
- Lucia Zanetti
- Irem Kilicarslan
- Alexandra Koschak
Technician:
- Bettina Tschugg
Collaborators:
- Simone Sartori
- Jörg Striessnig
- Nicolas Singewald
- SFB-F44


This research group is interested in L-type calcium channels in molecular sensory physiology and pharmacology and focuses on the following three topics


  1. Contribution of L-type calcium channels in sensory physiology of vision
  2. Role of L-type calcium channels in visual disorders
  3. Pharmacotherapeutic potential of protein-protein interactions in congenital stationary night blindness
 

1.       Contribution of L-type calcium channels in sensory physiology of vision


Photoreceptors are morphologically and physiological highly specialised light sensing cells of the retina. In darkness their membrane potential is depolarized up to -36 mV due to the activity of cGMP activated cation channels. Sustained release of glutamate from photoreceptor ribbon synapses is Ca2+ dependent and L-type calcium channels (LTCCs) serve as the predominant source for Ca2+ entry. Cav1.4 LTCCs channels – which are encoded by the CACNA1F gene - are located in close vicinity to the typical horseshoe shaped ribbon synapses.

 

We 1 showed that heterologously expressed Cav1.4 currents activate rapidly, open at negative membrane potentials and thereby allow the channel to conduct Ca2+ at potentials negative to -40 mV. Cav1.4 channels also show slow voltage-dependent inactivation accompanied by complete absence of Ca2+-dependent inactivation making them perfectly suited to mediate continuous neurotransmitter release. Cav1.4 expression is suggested also in retinal bipolar cells – which receive input from photoreceptors and relay directly or indirectly to ganglion cells – but information about morphological and downstream pathways is rare. We are currently generating a mouse model designed to unequivocally identify Cav1.4 localisation in retinal cell types as well as Cav1.4 expression in other parts of the central nervous system. Our approach will include immunohistochemical and electrophysiological analyses.

 

Data that emphasize the role of LTCC subtypes different from Cav1.4 come from studies in the non-human retina. Expression of the Cav1.3 in different retinal cells/layers has been described in the literature but so far detailed functional studies are lacking. We will probe the relationship of retinal activity and Cav1.3 expression by recording the discharge pattern of ganglion cells in different Cav1.3 mouse models in multi-electrode array studies. We are further interested to study the role of retinal Cav1.3 LTCCs in other processes than transmission which we will approach in biochemical and immunohistochemical studies.

 

2.       Role of L-type calcium channels in visual disorders



A number of channel dysfunctions 2 have been described that are caused by mutations in the CACNA1F gene, which encodes for the Cav1.4 LTCC alpha1 subunits. A majority of mutations was, however, identified among patients originally diagnosed with congenital night blindness type 2 (CSNB2; OMIM: 30071). Many Cav1.4 mutations are predicted to cause severe structural changes 3that they are unlikely to form functional channels, often due to premature truncation forming a large group of ‘loss-of-function’ mutations. ‘Gain-of-function’ mutations would promote pronounced Ca2+ entry through the channel. Truncation mutations in the C-terminus of Cav1.4 (and also Cav1.3) channels form functional channels but they lack the distal part of a C-terminal modulator (CTM) domain that determines the properties of calcium-dependent inactivation 4-6. Mutations with ‘impaired Cav1.4-CTM’ are of particular interest because they also allow insight into intramolecular regulatory processes of Cav1.4 LTCCs. We still wish to better understand how the different functional channel phenotypes can all result in defective retinal synaptic transmission underlying CSNB2 symptoms. We have just recently published our investigations on the functional dysregulation and morphological consequences observed in retinas from mice carrying the gain-of-function mutation Cav1.4-IT in mice 6. Our data show that this mouse line, in contrast to other mouse models characterized so far, very well reflects the functional phenotype described in a family with the Cav1.4 I745T point mutation.

 

3.       Pharmacotherapeutic potential of protein-protein interactions in congenital stationary night blindness



The concept of an intrinsic C-terminal modulator (CTM) in Cav1.4 channels underlies normal retinal function. Although partial blockade of mutated Cav1.4 channels seems to be a way to modulate these channels, such approach might not practicable in vivo because antagonist drugs, as the DHP isradipine, are 10- to 20-fold more potent in blocking Cav1.2 than Cav1.4 LTCCs 1. The higher concentration required to block CaV1.4 channels can be expected to cause unwanted side effects from vasodilatation. The discovery of the gating modulator in the C-terminus of Cav1.4 channels raised the important question about its potential as a novel concept for (pharmacological) control of Cav1.4 channels 2. A “CTM rescue approach” would provide an alternative strategy for Cav1.4 channel modulation that avoids complete concentration-dependent inhibition of these channels. We therefore develop a new mutant CTM mouse model to quantify the functional role of the CTM for Cav1.4-dependent signalling in the retina.


 

Selected literature


1 Koschak A, Reimer D, Walter D, Hoda JC, Heinzle T, Grabner M, Striessnig J. Cav1.4alpha1 subunits can form slowly inactivating dihydropyridine-sensitive L-type Ca2+ channels lacking Ca2+-dependent inactivation (2003). J. Neurosci., 23(14):6041-9.

 

2 Burtscher V, Schicker K, Novikova E, Pöhn B, Stockner T, Kugler C, Singh A, Zeitz C, Lancelot ME, Audo I, Leroy BP, Freissmuth M, Herzig S, Matthes J, Koschak A. Spectrum of Cav1.4 dysfunction in congenital stationary night blindness type 2 (2014). Biochim Biophys Acta., 1838(8):2053-65.

 

3 Stockner T, Koschak A. What can naturally occurring mutations tell us about Ca(v)1.x channel function? (2013). Biochim Biophys Acta., 1828(7):1598-607.

 

4 Singh A, Hamedinger D, Hoda JC, Gebhart M, Koschak A, Romanin C, Striessnig J. C-terminal modulator controls Ca2+-dependent gating of Cav1.4 L-type Ca2+ channels (2006). Nat Neurosci., 29(9):1108-16.

 

5 Singh A, Gebhart M, Fritsch R, Sinnegger-Brauns MJ, Poggiani C, Hoda JC, Engel J, Romanin C, Striessnig J, Koschak A. Modulation of voltage- and Ca2+-dependent gating of Cav1.3 L-type calcium channels by alternative splicing of a C-terminal regulatory domain (2008). J Biol Chem., 283(30):20733-44.

 

6 Bock G, Gebhart M, Scharinger A, Jangsangthong W, Busquet P, Poggiani C, Sartori S, Mangoni ME, Sinnegger-Brauns MJ, Herzig S, Striessnig J, Koschak A. Functional properties of a newly identified C-terminal splice variant of Cav1.3 L-type Ca2+ channels (2011). J Biol Chem., 286(49):42736-48.

 

7 Knoflach D, Kerov V, Sartori SB, Obermair GJ, Schmuckermair C, Liu X, Sothilingam V, Garrido MG, Baker SA, Glösmann M, Schicker K, Seeliger M, Lee A, Koschak A. Cav1.4 IT mouse as model for vision impairment in human congenital stationary night blindness type 2 (2013). Channels (Austin)., 7(6):503-13.


 

If you are interested in more general information about:
‘The Organisation of the Retina and the Visual System’ – click here

 
‘Pathologies of Calcium Channels’– click here
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