Reduced Presynaptic Signaling in Retinal Neurons of a Transgenic Mouse Model for Familial Hemiplegic Migraine

Abstract

Purpose: : Migraine is a severe neurovascular disorder that affects up to 12% of the general Western population. Besides the characteristic migraine headache, one third of patients experiences migraine aura, which typically manifests as visual disturbances. Gain-of-function mutations in the Ca_v2.1 (P/Q-type) voltage-gated Ca²⁺ channel cause the severe subtype of familial hemiplegic migraine type 1 (FHM1), an established model for migraine with aura. Given the tightly controlled Ca²⁺ homeostasis in the retina, we hypothesized that the Ca_v2.1 mutation may cause disturbed Ca²⁺ signaling in retinal neurons.

Methods: : Studies were performed in FHM1 knock-in mice with the R192Q mutation in Ca_v2.1 Ca²⁺ channels. Localization and expression of voltage-gated Ca²⁺ channels and intracellular Ca²⁺ release channels was investigated using quantitative polymerase chain reaction (qPCR) and immunohistochemistry. Changes in intracellular Ca²⁺ signaling including changes in release of Ca²⁺ from intracellular stores were measured with optical imaging techniques.

Results: : In contrast to findings in cerebellar and cortical neurons, Ca_v2.1-specific immunoreactivity was reduced significantly in all layers of the R192Q knock-in neural retina, most significantly in the nuclear layers and most notably in the ganglion cell layer. qPCR revealed no change in the expression of Ca_v channel subtypes, however, mRNA levels of all four isoforms of auxiliary α₂Δ subunits were significantly reduced. Ca²⁺ release from intracellular stores was significantly lower in R192Q knock-in retinal neurons.

Conclusions: : Data from the present study suggest that the FHM1 gain-of-function mutation R192Q in Ca_v2.1 severely disrupts Ca²⁺ homeostasis in retinal neurons by reducing the expression of plasma membrane voltage-gated Ca²⁺ channels and by suppressing intracellular Ca²⁺ release. This represents a novel compensatory mechanism associated with this FHM1 mutation. While it is unclear whether the ensuing abnormal Ca²⁺ signaling contributes to the visual disturbances occurring during migraine aura, individual components of the affected cellular signaling pathways may represent suitable therapeutic targets for ophthalmological diseases resulting from Ca²⁺ dyshomeostasis.

Keywords: retina • ion channels • immunohistochemistry