Oligomerization of Ca2+/calmodulin-dependent protein kinase kinase

Research advances on CaMKs-mediated neurodevelopmental injury

Calcium/calmodulin-dependent protein kinases (CaMKs) are important proteins in the calcium signaling cascade response pathway, which can broadly regulate biological functions in vivo. Multifunctional CaMKs play key roles in neural development, including neuronal circuit building, synaptic plasticity establishment, and neurotrophic factor secretion. Currently, four familial proteins, calcium/calmodulin-dependent protein kinase I (CaMKI), calcium/calmodulin-dependent protein kinase II (CaMKII), eukaryotic elongation factor 2 kinase (eEF2K, popularly known as CaMKIII) and calcium/calmodulin-dependent protein kinase IV (CaMKIV), are thought to have been the most extensively studied during neurodevelopment. Although their spatial structures are extremely similar, as well as the initial starting point of activation, both require the activation of calcium and calmodulin (CaM) complexes to be involved in the process, and the phosphorylation sites and modes of each member are different. Furthermore, due to the high structural similarity of CaMKs, their members may play synergistic roles in the regulation of neural development, but different CaMKs also have their own means of regulating neural development. In this review, we first describe the visualized protein structural forms of CaMKI, CaMKII, eEF2K and CaMKIV, and then describe the functions of each kinase in neurodevelopment. After that, we focus on four main mechanisms of neurodevelopmental damage caused by CaMKs: CaMKI/ERK/CREB pathway inhibition leading to dendritic spine structural damage; Ca2+/CaM/CaMKII through induction of mitochondrial kinetic disorders leading to neurodevelopmental damage; CaMKIII/eEF2 hyperphosphorylation affects the establishment of synaptic plasticity; and CaMKIV/JNK/NF-κB through induction of an inflammatory response leading to neurodevelopmental damage. In conclusion, we briefly discuss the pathophysiological significance of aberrant CaMK family expression in neurodevelopmental disorders, as well as the protective effects of conventional CaMKII and CaMKIII antagonists against neurodevelopmental injury.

Molecular Mechanisms Underlying Ca2+/Calmodulin-Dependent Protein Kinase Kinase Signal Transduction

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) is the activating kinase for multiple downstream kinases, including CaM-kinase I (CaMKI), CaM-kinase IV (CaMKIV), protein kinase B (PKB/Akt), and 5′AMP-kinase (AMPK), through the phosphorylation of their activation-loop Thr residues in response to increasing the intracellular Ca²⁺ concentration, as CaMKK itself is a Ca²⁺/CaM-dependent enzyme. The CaMKK-mediated kinase cascade plays important roles in a number of Ca²⁺-dependent pathways, such as neuronal morphogenesis and plasticity, transcriptional activation, autophagy, and metabolic regulation, as well as in pathophysiological pathways, including cancer progression, metabolic syndrome, and mental disorders. This review focuses on the molecular mechanism underlying CaMKK-mediated signal transduction in normal and pathophysiological conditions. We summarize the current knowledge of the structural, functional, and physiological properties of the regulatory kinase, CaMKK, and the development and application of its pharmacological inhibitors.

Conformation-Dependent Reversible Interaction of Ca2+/Calmodulin-Dependent Protein Kinase Kinase with an Inhibitor, TIM-063

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK), a Ca²⁺/CaM-dependent enzyme that phosphorylates and activates multifunctional kinases, including CaMKI, CaMKIV, protein kinase B/Akt, and 5'AMP-activated protein kinase, is involved in various Ca²⁺-signaling pathways in cells. Recently, we developed an ATP-competitive CaMKK inhibitor, TIM-063 (2-hydroxy-3-nitro-7H-benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one, Ohtsuka et al. Biochemistry 2020, 59, 1701-1710). To gain mechanistic insights into the interaction of CaMKK with TIM-063, we prepared TIM-063-coupled sepharose (TIM-127-sepharose) for association/dissociation analysis of the enzyme/inhibitor complex. CaMKKα/β in transfected COS-7 cells and in mouse brain extracts specifically bound to TIM-127-sepharose and dissociated following the addition of TIM-063 in a manner similar to that of recombinant GST-CaMKKα/β, which could bind to TIM-127-sepharose in a Ca²⁺/CaM-dependent fashion and dissociate from the sepharose following the addition of TIM-063 in a dose-dependent manner. In contrast to GST-CaMKKα, GST-CaMKKβ was able to weakly bind to TIM-127-sepharose in the presence of EGTA, probably due to the partially active conformation of recombinant GST-CaMKKβ without Ca²⁺/CaM-binding. These results suggested that the regulatory domain of CaMKKα prevented the inhibitor from interacting with the catalytic domain as the GST-CaMKKα mutant (residues 126-434) lacking the regulatory domain (residues 438-463) interacted with TIM-127-sepharose regardless of the presence or absence of Ca²⁺/CaM. Furthermore, CaMKKα bound to TIM-127-sepharose in the presence of Ca²⁺/CaM completely dissociated from TIM-127-sepharose following the addition of excess EGTA. These results indicated that TIM-063 interacted with and inhibited CaMKK in its active state but not in its autoinhibited state and that this interaction is likely reversible, depending on the concentration of intracellular Ca²⁺.