Functional uncoupling between Ca ²⁺ release and afterhyperpolarization in mutant hippocampal neurons lacking junctophilins

Abstract

Junctional membrane complexes (JMCs) composed of the plasma membrane and endoplasmic͞sarcoplasmic reticulum seem to be a structural platform for channel crosstalk. Junctophilins (JPs) contribute to JMC formation by spanning the sarcoplasmic reticulum membrane and binding with the plasma membrane in muscle cells. In this article, we report that mutant JP double-knockout (JP-DKO) mice lacking neural JP subtypes exhibited an irregular hindlimb reflex and impaired memory. Electrophysiological experiments indicated that the activation of small-conductance Ca 2؉ -activated K ؉ channels responsible for afterhyperpolarization in hippocampal neurons requires endoplasmic reticulum Ca 2؉ release through ryanodine receptors, triggered by NMDA receptor-mediated Ca 2؉ influx. We propose that in JP-DKO neurons lacking afterhyperpolarization, the functional communications between NMDA receptors, ryanodine receptors, and small-conductance Ca 2؉ -activated K ؉ channels are disconnected because of JMC disassembly. Moreover, JP-DKO neurons showed an impaired long-term potentiation and hyperactivation of Ca 2؉ ͞calmodulin-dependent protein kinase II. Therefore, JPs seem to have an essential role in neural excitability fundamental to plasticity and integrated functions.hippocampus ͉ learning and memory ͉ long-term potentiation ͉ ryanodine receptor ͉ SK channel F unctional communication between cell-surface and intracellular channels is an essential feature of excitable cells (1). During initiation of contraction in striated muscle cells, the activation of cell-surface dihydropyridine receptor (DHPRs) channels opens ryanodine receptors (RyRs) and triggers Ca 2ϩ release from the sarcoplasmic reticulum via either the ''Ca 2ϩ -induced Ca 2ϩ release'' or the ''voltage-induced Ca 2ϩ release'' mechanism (2). The functional couplings between the channels take place in junctional membrane complexes (JMCs), designated as the ''triad junction'' in skeletal muscle, ''diad'' in cardiac muscle, and ''peripheral coupling'' in immature striated and smooth muscles (3, 4). Recent studies indicated that junctophilin (JP) subtypes, namely JP-1-JP-4, contribute to JMC formation in muscle cells (5, 6). In JP-1 knockout mice with perinatal lethality, mutant skeletal muscle shows deficiency of triad junctions and insufficient contraction probably caused by impaired communication between DHPRs and RyRs (7). In JP-2 knockout embryos showing cardiac arrest, mutant cardiac myocytes exhibit deficiency of peripheral couplings and arrhythmic Ca 2ϩ signaling probably caused by functional uncoupling between DHPRs and RyRs (5). In the brain, both JP-3 and JP-4 are expressed in similar discrete neuronal sites and may collaboratively contribute to JMC formation (8, 9). However, the role of neural JP subtypes is largely unknown. Using knockout mice lacking both JP-3 and JP-4 (JP-DKO mice), we report their essential contributions to the tuning of excitability and plasticity in hippocampal pyramidal neurons. ResultsGeneration of JP-DKO Mice Bearing Lethality. JP-4 knockou...

Homer1a is a core brain molecular correlate of sleep loss

“…Homer1 also activates ryanodine receptors and L-type calcium channels (28,29). Interestingly, Jph3, which was identified by our mRNA-tagging strategy as being upregulated by sleep deprivation, has been shown to play a major role in ryanodine receptor-mediated, calcium-induced opening of small-conductance, calcium-activated potassium (SK) channels (28,30). SK channels are responsible for the generation of slow afterhyperpolarizations in neurons of the nucleus reticularis thalami and thus contribute to the EEG slow waves characteristic of NREM sleep (31).…”

Meichroacidin Containing the Membrane Occupation and Recognition Nexus Motif Is Essential for Spermatozoa Morphogenesis

“…JPs, which are components of the junctional complexes that are expressed abundantly in the heart and brain, contain a conserved MORN motif (3,4). The MORN motifs of JPs have tandem repeats of eight MORN motifs at the N-terminal region and a hydrophobic domain in the C-terminal region, which may function in anchoring the protein to the cellular membrane (see supplemental Fig.…”

“…Recently, junctophilins (JPs), which are a novel conserved family of proteins that are components of the junctional complexes, were reported to contain a repeat amino acid sequence similar to that of MCA (3). However, MCA consists of a hydrophilic amino acid region, whereas JPs have a C-terminal hydrophobic segment spanning the endoplasmic/sarcoplasmic reticulum and are expressed abundantly in a variety of tissues (3,4). The repeat amino acid sequence, referred to as the membrane occupation and recognition nexus (MORN) motif, is a novel protein-folding module that is shared by functionally different proteins and may have specific physiological roles (3).…”

Molecular evolution of the junctophilin gene family

“…31. level of expression in the ventrolateral, ventroposterior, and posterior thalamic nuclei and spinal gray matter, whereas JPH4 is undetectable in these regions (37). Furthermore, in knockout mouse models a phenotype was only apparent when both JPH3 and 4 where knocked out simultaneously (34).…”