Multiple Calcium Channel Types with Unique Expression Patterns Mediate Retinal Signaling at Bipolar Cell Ribbon Synapses

Abstract

Retinal bipolar cells (BCs) compose the canonical vertical excitatory pathway that conveys photoreceptor output to inner retinal neurons. Although synaptic transmission from BC terminals is thought to rely almost exclusively on Ca 21 influx through voltage-gated Ca 21 (Ca V ) channels mediating L-type currents, the molecular identity of Ca V channels in BCs is uncertain. Therefore, we combined molecular and functional analyses to determine the expression profiles of Ca V a 1 , b, and a 2 d subunits in mouse rod bipolar (RB) cells, BCs from which the dynamics of synaptic transmission are relatively well-characterized. We found significant heterogeneity in Ca V subunit expression within the RB population from mice of either sex, and significantly, we discovered that transmission from RB synapses was mediated by Ca 21 influx through P/Q-type (Ca V 2.1) and N-type (Ca V 2.2) conductances as well as the previouslydescribed L-type (Ca V 1) and T-type (Ca V 3) conductances. Furthermore, we found both Ca V 1.3 and Ca V 1.4 proteins located near presynaptic ribbon-type active zones in RB axon terminals, indicating that the L-type conductance is mediated by multiple Ca V 1 subtypes. Similarly, Ca V 3 a 1 , b, and a 2 d subunits also appear to obey a "multisubtype" rule, i.e., we observed a combination of multiple subtypes, rather than a single subtype as previously thought, for each Ca V subunit in individual cells.

Large-scale interrogation of retinal cell functions by 1-photon light-sheet microscopy

“…For this, we used retinas from Ai148; PCP2Cre transgenic mice that express GCaMP6f primarily in rod BCs but also exhibit some Cre expression in type 2 and type 6 BCs. 14 , 15 , 40 Depolarization of BCs causes calcium influx at the axon terminal release sites, prior to glutamate release, 22 making these release sites ideal for calcium imaging. Light-sheet excitation was first targeted to the inner portion of the IPL adjacent to the ganglion cell layer, where the majority of release sites were observed.…”

“…We were able to resolve hundreds of individual synaptic release sites in BC axon terminals, measure calcium responses to visual stimuli, and functionally classify BCs into distinct ON and OFF types. 14 , 15 These results demonstrate that 1-photon light-sheet imaging is a relatively affordable and viable platform for monitoring and functionally characterizing neural activity across large populations of retinal neurons with high spatial resolutions.…”

Synapse Dysfunctions in Multiple Sclerosis

“…Retinal and inner ear ribbon synapses contain presynaptic L-type Cav channels [ 116 , 117 ]. Some synapses even contain multiple types of Cav channels [ 118 ]. Following synaptic vesicle fusion, retrieval of fused synaptic vesicle membrane and vesicle proteins are recovered via different types of endocytosis [ 119 ].…”

Calcium Homeostasis, Transporters, and Blockers in Health and Diseases of the Cardiovascular System

“…The single channel conductance of both types of channels is 10-20 pS (Table 1) (for review, please see ref. [88]). These types of calcium-selective VOCCs are mainly found in synaptic terminals, are high-voltage dependent, and are implicated in vesicle release and neurological diseases such as ataxia, migraine, and Alzheimer's [84].…”