A molecular structure for compartmentalized calcium signals in brain neuron cell bodies

Kv2.1 is a member of the Kv2 family of voltage-gated K⁺ (Kv) channels, and one of the most highly expressed Kv channels in neurons.  Kv2 channels play a particularly important role in tethering the endoplasmic reticulum (ER) with plasma membrane (PM) to form ER-PM junctions (EPJs) in the proximal dendrites and soma of mammalian brain neurons. It is known that Kv2.1 clustering at neuronal EPJs strictly regulated and is independent of Kv conductance, however the function and physiological effect of concentrating of Kv2.1 at EPJs is currently unknown. Given that important and diverse forms of Ca²⁺ signaling events occur at EPJs in many other cells (cardiomyocytes, skeletal muscle) through the spatial and functional coupling of L-type calcium channels and ryanodine receptors, an important question was whether Kv2.1 was contributing to such signaling in mammalian neurons.

To answer this, researchers Dr. Nicholas Vierra, Deborah van der List, Dr. Michael Kirmiz, Dr. Luis Fernando Santana and led by Prof. James S Trimmer at University of California, Davis investigated the subcellular Kv2.1, LTCCs and RyRs distribution in hippocampal neurons. Unbiased proteomic analysis of brain tissue was used to determine whether LTCCs and RyR   spatially localized to the clustered Kv2.1. The effect of Kv2.1 on spatial coupling and functional properties of LTCCs and RyRs was also investigated using cultured hippocampal neurons (CHNs) and heterologous cells. The investigators also used samples from Kv2.1 knockout mice to investigate how loss of Kv2.1 affected the function and localization of LTCCs and RyRs.  Together, the results supported an essential effect of Kv2.1 in enhancing neuronal LTCC activity and support a critical role for Kv2.1 in the generation of somatodendritic Ca²⁺ signaling, as well as a functional relationship between Kv2.1, LTCC and RyRs. The research work is now published in the journal eLife.

The research team found that clustering of PM LTCCs with the ER RyR Ca^{2 +} release channels is an important non-conducting function of Kv2.1 at EPJs. Surprisingly, opening of LTCCs (in this case Cav1.2)  in response to membrane depolarization increased due to Kv2.1 clustering. Localized LTCC-dependent Ca²⁺  release events or Ca²⁺ sparks occurred independent of action potentials at Kv2.1-containing EPJs. Ca²⁺ sparks were enhanced by Kv2.1-mediated co-clustering of LTCCs and RyRs, showing a role for Kv2.1 in promoting both the spatial and functional coupling of these Ca²⁺ signaling proteins.

The findings support a new model for the formation of Ca²⁺ signaling microdomains at EPJs and  local control of Ca²⁺ signaling at specific sites in neurons. In this model Kv2.1-containing neuronal EPJs are Ca²⁺ signaling microdomains, at which LTCCs and RyRs are brought together by Kv2.1-mediated clustering, and forming a specialized complex for generating localized Ca²⁺ signals on neuronal somata.

The authors proposed that a non-conducting function of PM Kv2.1 channels is to promote the organization of PM LTCCs such as Cav1.2 into clusters at which ER-localized RyRs are found in direct apposition, as well as a more general role in anchoring the PM to the ER through direct interaction of Kv2.1 with ER VAP proteins. These findings further indicated that the clustering mediated by Kv2.1 also increases the spontaneous activity of Cav1.2, which allows a small but sufficient amount of Ca²⁺ to enter the cell at Kv2.1-containing EPJs to activate nearby ER RyRs, leading to Ca²⁺-induced Ca²⁺ release that occurs independently of action potentials. The proposed model posits that Kv2.1 modulates these Ca²⁺ signaling events by simultaneously promoting the spatial association of Cav1.2 channels with RyRs and increasing their activity to trigger CICR.

In summary, the study by Dr. Nicholas Vierra and colleagues report a new mode of LTCC regulation by clustering as mediated by a Kv channel, and establish a special structural mechanism by which Kv2.1-associated EPJs provide the molecular framework for the restricted localization of somatodendritic Ca²⁺ signals in mammalian brain neurons.