Altered calcium levels and poorly regulated activity in specific calcium channels in nerve cells could underlie fragile X syndrome (FXS) and be used as a diagnostic and treatment biomarker, according to a new study.
The research, “New Insights Into the Role of Cav2 Protein Family in Calcium Flux Deregulation in Fmr1-KO Neurons,” appeared in the journal Frontiers in Molecular Neuroscience.
Fragile X, the leading genetic cause of autism, is characterized by a mutation in the FMR1 gene that affects production of the FMRP protein. This protein binds to multiple RNA molecules, including those encoding proteins that serve as ion channels — through which calcium ions flow in and out of a cell.
In particular, FMRP directly interacts with two voltage-gated calcium channels (VGCCs) named Cav2.1 and Cav2.2.
Upon stimulation, VGCCs allow calcium ions to enter the cells, where they can activate various signaling pathways that result in muscle contraction, neuronal communication, and protein phosphorylation (a hallmark chemical alteration in protein activation) and secretion.
Different VGCCs are classified based on which particular subunit forms the channel pore — its central “entry” gate. They can be classified as L-, N-, R- and P/Q-type channels. L- and T-type VGCCs are found in a great variety of cells, while N-, P/Q- and R-types are mostly expressed in nerve cells.
Cav2.1, a P/Q-type VGCC, plays a key role in the release of neurotransmitters — chemical messengers that allow nerve cells to communicate — and is mainly produced in the cerebellum, a brain area with a major role in motor coordination, balance, and speech.
Mutations in the gene encoding Cav2.1, known as CACNA1A, were recently associated with autism spectrum disorder, suggesting a broader role for Cav2.1 in cognition, memory, and social interaction.
Researchers used an imaging technique, called ratiometric calcium imaging, to analyze calcium dynamics in mouse-derived neurons in which the Fmr1 gene (Fmr1-KO) had been deleted.
The results showed higher baseline, or steady state, calcium concentration and lower stimulation-induced calcium entry in these neurons.
By using VGCC blockers specific to the various channel types, researchers were able to reduce calcium entry into the neurons. However, while omega-Agatoxin Iva, a P/Q-type VGCC blocker, had a milder effect in neurons lacking Fmr1 than in those that had a working Fmr1 gene, omega-Conotoxin-GVIa — a N-type VGCC blocker — had the opposite effect.
“These findings strongly suggest that N- and P/Q-type channels are deregulated in Fmr1-KO neurons,” researchers wrote.
Matching the reduced sensitivity to P/Q-type VGCC inhibition, the data also showed reduced CACNA1A gene expression in Fmr1-KO neurons compared to controls, leading to lower amounts of Cav2.1 voltage-gated calcium channels in the cell membrane.
“Our findings point out the critical role that Cav2.1 plays in the altered [calcium] flux in Fmr1-KO neurons,” the researchers wrote.
Although more studies are necessary to better understand the precise molecular mechanisms causing this deregulation in fragile X syndrome, “it is interesting to underline here that an imbalance between the levels and the activities of N- and P/Q-type channels, could have some impacts on the physiopathology of FXS [fragile-X syndrome],” they added.
Overall, the alterations observed “can be considered a novel cellular biomarker” in fragile X and “useful for diagnostic purposes and particularly as a follow-up for specific therapies.”