Dynamic changes in β-cell electrical activity and [Ca²⁺] regulates NFATc3 activation and downstream gene transcription

Abstract

<jats:title>Abstract</jats:title><jats:p>Diabetes results from insufficient insulin secretion as a result of dysfunction to β-cells within the islet of Langerhans. Elevated glucose causes β-cell membrane depolarization and action potential generation, voltage gated Ca<jats:sup>2+</jats:sup> channel activation and oscillations in free-Ca<jats:sup>2+</jats:sup> activity ([Ca<jats:sup>2+</jats:sup>]), triggering insulin release. Nuclear Factor of Activated T-cell (NFAT) is a transcription factor that is regulated by increases in [Ca<jats:sup>2+</jats:sup>] and calceineurin (CaN) activation. NFAT regulation links cell activity with gene transcription in many systems, and within the β-cell regulates proliferation and insulin granule biogenesis. However the link between the regulation of β-cell electrical activity and oscillatory [Ca<jats:sup>2+</jats:sup>], with NFAT activation and downstream transcription is poorly understood. In this study we tested whether dynamic changes to β-cell electrical activity and [Ca<jats:sup>2+</jats:sup>] regulates NFAT activation and downstream transcription. In cell lines, mouse islets and human islets, including those from donors with type2 diabetes, we applied both agonists/antagonists of ion channels together with optogenetics to modulate β-cell electrical activity. Both glucose-induced membrane depolarization and optogenetic-stimulation triggered NFAT activation, and increased transcription of NFAT targets and intermediate early genes (IEGs). Importantly only conditions in which slow sustained [Ca<jats:sup>2+</jats:sup>] oscillations were generated led to NFAT activation and downstream transcription. In contrast in human islets from donors with type2 diabetes NFAT activation by glucose was diminished, but rescued upon pharmacological stimulation of electrical activity. Thus, we gain insight into the specific patterns of electrical activity that regulate NFAT activation and gene transcription and how this is disrupted in diabetes.</jats:p>

Functional architecture of the pancreatic islets reveals first responder cells which drive the first-phase [Ca2+] response

“…rearranging the position of the first responder cells disrupted their consistency. While technically challenging, dissociation of islets post-first responder cell identification, and association of islets into pseudo-islet structures would allow this property to be tested [44]. Nevertheless, we did observe that the first-phase response time is spatially organized within 2-3 cell layers further suggesting spatial positioning is important to generate first responder cells.…”

“…Decreases in Cx36 gap junction conductance (as is observed in first responder cells, Fig.4) would allow the more excitable first responder cells to respond earlier and impact their neighboring cells. Notably elevated electrical activity and [Ca 2+ ] increases GJD2 expression and increases Cx36 gap junction permeability[43], suggesting changes in excitability may mediate such transient activity.…”

Near-UV Light Induced ROS Production Initiates Spatial Ca2+ Spiking to Fire NFATc3 Translocation

“…Among other NFAT members, NFATc3 has the highest expression level in human and mouse pancreatic islets [ 5 ] and plays an important role in pancreatic β-cells [ 10 ]. To gain more insight into the nuclear translocation mechanism of NFATc3, we investigated the translocation kinetics of NFATc3 in two distinct phases.…”

“…In excitable cells, NFAT proteins are mainly activated via transmembrane Ca 2+ fluxes through the CaV1 family of L-type voltage-gated Ca 2+ channels [ 7 , 8 ]. However, activation of NFAT proteins via the L-type Ca 2+ channel requires a minimum threshold frequency of Ca 2+ oscillations [ 9 , 10 ]. Interestingly, not all members of the NFAT family are activated by the same frequency of Ca 2+ oscillations through the L-type Ca 2+ channel.…”

“…Interestingly, not all members of the NFAT family are activated by the same frequency of Ca 2+ oscillations through the L-type Ca 2+ channel. For instance, only NFATc2 and NFATc3 proteins were shown to be activated upon glucose-induced Ca 2+ oscillations via L-type Ca 2+ channel in β-cells, indicating the isoform-specific sensitivity for Ca 2+ oscillations [ 10 ].…”