Unconjugated
Synaptic deficit has emerged as a key early hallmark for neurodegeneration in the visual pathway. The molecular pathway connecting local synaptic deficit with global cell dysfunction and death remains unclear. We have previously shown that a2d4, an auxiliary subunit of the voltage-gated calcium channel, is targeted to photoreceptor synapses and required for their formation and function. Notably, a2d4 mutations have been identified in patients with retinal dystrophy. However, how loss of synaptic a2d4 leads to overall photoreceptor degeneration remains unknown. Here, we showed that a2d4 loss in mice leads to a late onset photoreceptor degeneration around 7 months. Consistent with clinical observation, the progression of degeneration is minimal until 17 months, as supported by ERG, OCT imaging and histology. We found that Cav1.4 KO mice, where the calcium channel is missing, display an earlier degeneration onset than a2d4 KO mice, where calcium channel is partially preserved. Proteomic studies revealed that tricarboxylic acid (TCA) cycle is significantly downregulated in the young a2d4 KO retinas prior to degeneration. Transmission electron microscopy study demonstrated significant reduction in mitochondrial size and number in photoreceptor synaptic terminals, but not in the inner segment (IS), of the young a2d4 KO retinas. Consistently, immunohistochemistry (IHC) studies showed significant reduction of mitochondrial proteins in the outer plexiform layer (OPL). IHC studies on ER and mitochondrial proteins revealed that ryanodine receptor (RyR2) and mitochondrial calcium uniporter (MCU) are downregulated in the OPL, but not in the IS. Together, our results propose a model where a2d4 dysfunction impairs Cav1.4 channel activity, leading to disrupted calcium crosstalk among the plasma membrane, ER, and mitochondria, as well as mitochondrial damage and metabolic deficits. Importantly, our study underscores the critical role of synaptic calcium homeostasis and mitochondrial integrity in connecting the early stages of synaptic dysfunction with the later stages of cell degeneration.
