Cav1.2 Ca²⁺ channels, a type of voltage-gated L-type Ca²⁺ channel, are ubiquitously expressed, and the predominant Ca²⁺ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca²⁺-binding protein that causes Ca²⁺-dependent facilitation (CDF) and inactivation (CDI). Ca²⁺-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca²⁺/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

Culture of hippocampal neurons in low-Mg²⁺ medium (low-Mg²⁺ neurons) results in induction of continuous seizure activity. However, the underlying mechanism of the contribution of low Mg²⁺ to hyperexcitability of neurons has not been clarified. Our data, obtained using the patch-clamp technique, show that voltage-gated Na⁺ channel (VGSC) activity, which is associated with a persistent, noninactivating Na⁺ current ( I_Na,P), was modulated by calmodulin (CaM) in a concentration-dependent manner in normal and low-Mg²⁺ neurons, but the channel activity was more sensitive to Ca²⁺/CaM regulation in low-Mg²⁺ than normal neurons. The increased sensitivity of VGSCs in low-Mg²⁺ neurons was partially retained when CaM₁₂ and CaM₃₄, CaM mutants with disabled binding sites in the N or C lobe, were used but was diminished when CaM₁₂₃₄, a CaM mutant in which all four Ca²⁺ sites are disabled, was used, indicating that functional Ca²⁺-binding sites from either lobe of CaM are required for modulation of VGSCs in low-Mg²⁺ neurons. Furthermore, the number of neurons exhibiting colocalization of CaM with the VGSC subtypes Na_V1.1, Na_V1.2, and Na_V1.3 was significantly higher in low- Mg²⁺ than normal neurons, as shown by immunofluorescence. Our main finding is that low-Mg²⁺ treatment increases sensitivity of VGSCs to Ca²⁺/CaM-mediated regulation. Our data reveal that CaM, as a core regulating factor, connects the functional roles of the three main intracellular ions, Na⁺, Ca²⁺, and Mg²⁺, by modulating VGSCs and provides a possible explanation for the seizure discharge observed in low-Mg²⁺ neurons.

Background: Communication between the vascular endothelium and peripheral leukocytes is an important process controlling inflammatory responses in the brain. Increased plasma levels of CTSD (cathepsin D) have been associated with vascular events, but whether circulating CTSD may affect the functional status of the endothelium remains unknown. Methods: To resolve this issue, two groups of transgenic hCTSD ^hi mice with high levels of circulating CTSD were subjected to traumatic and ischemic injury, respectively. Then, single‐cell RNA sequencing was performed to characterize gene expression changes in the brains of hCTSD ^hi mice. Additionally, transgenic CTSD ^{Mono knockout} mice with reduced plasma CTSD levels were generated to investigate whether CTSD downregulation confers protection against ischemic injury and traumatic brain injury‐induced brain damage. Results: We found the hCTSD ^hi mice showed increased brain neutrophil infiltration in response to the brain injury. Severe motor deficit and delayed behavioral recovery were observed in hCTSD ^hi mice after brain traumatic and ischemic injury. Mechanistically, the circulating CTSD was characterized as the nonenzymatic prototype CTSD, pro‐CTSD. We further found the circulating nonenzymatic pro‐CTSD was able to activate brain endothelium by upregulating VCAM‐1 (vascular cell adhesion molecule 1) expression, which is necessary for the transmigration of neutrophils into the brain. Furthermore, genetic reduction of CTSD effectively attenuated the ischemic injury‐induced neutrophil infiltration and motor deficits. Conclusions: Taken together, these results demonstrate that circulating prototype CTSD exacerbates the injury‐induced brain damage by upregulating brain endothelial VCAM‐1 and promoting neutrophil transmigration into the brain.