Cav1.4 L-Type Calcium Channels in Retinal Function- Lessons from Channelopathies

Voltage-gated Ca2+ channels mediate the entry of Ca2+ ions into electrically excitable cells during membrane depolarization. In sensory cells the Ca2+ signal triggers neurotransmitter release. CaV1.4 L-type calcium channels (LTCCs) are particularly important at the photoreceptor to second-order neuron synapse. This has become evident from the fact that mutations in the CACNA1F gene - which encodes for the Cav1.4 channel protein result in defect neurotransmission in the human retina causing an X-linked from of night blindness in the affected patient (congenital stationary night blindness type 2, CSNB2). The aim of this project is to assess the different modes of functional dysregulation in Cav1.4 CSNB2 mutations because changes in gating are of immediate relevance for the unique physiological and pathophysiological role of this channel. We will also assess the potential differential role of other LTCCs (in particular Cav1.3) in the natural environment of an isolated retinal cell or a retinal network. Moreover we will evaluate the modulatory potential of a regulatory mechanism in the C-terminus of LTCCs that tightly controls their gating (C-terminal modulator). We attempt to develop a CaV1.4- specific rescue approach for human Cav1.4 channel mutations with impaired C-terminal modulator function in a proof-of-concept study.

In this project we assessed different modes of functional dysregulation in CaV1.4 L-type calcium channel mutations causing congenital stationary night blindness type 2. Changes in the gating properties are of immediate relevance for the unique physiological and pathophysiological role of this channel. Our deep knowledge about the functional properties of CaV1.4, also allowed us to contribute also to a study that investigated the role of splice variants of accessory 24 subunit on the function of CaV1.4 channels. In addition to heterologously expressed CaV1.4 channel mutants, we also investigated the effect of a CaV1.4 gain-of-function point mutation - originally described in a New Zealand CSNB2 family - in a mouse model in comparison to CaV1.4 deficient animals. This approach helps to answer the question of how gain- and loss-of-function mutations can both result in impaired retinal synaptic transmission in patients that carry these mutations. To assess the modulatory potential of a regulatory mechanism in the C-terminus of CaV1.4 channels that tightly controls their gating and is also relevant in the generation of the disease, we developed a new mouse model. This model will not only allow insights to the functional role of the CaV1.4 C-terminus in vivo but also to test its pharmaco-therapeutic potential.