AKAP79/150 interacts with the neuronal calcium-binding protein caldendrin

Calcium-Associated Proteins in Neuroregeneration

The dysregulation of intracellular calcium levels is a critical factor in neurodegeneration, leading to the aberrant activation of calcium-dependent processes and, ultimately, cell death. Ca2+ signals vary in magnitude, duration, and the type of neuron affected. A moderate Ca2+ concentration can initiate certain cellular repair pathways and promote neuroregeneration. While the peripheral nervous system exhibits an intrinsic regenerative capability, the central nervous system has limited self-repair potential. There is evidence that significant variations exist in evoked calcium responses and axonal regeneration among neurons, and individual differences in regenerative capacity are apparent even within the same type of neurons. Furthermore, some studies have shown that neuronal activity could serve as a potent regulator of this process. The spatio-temporal patterns of calcium dynamics are intricately controlled by a variety of proteins, including channels, ion pumps, enzymes, and various calcium-binding proteins, each of which can exert either positive or negative effects on neural repair, depending on the cellular context. In this concise review, we focus on several calcium-associated proteins such as CaM kinase II, GAP-43, oncomodulin, caldendrin, calneuron, and NCS-1 in order to elaborate on their roles in the intrinsic mechanisms governing neuronal regeneration following traumatic damage processes.

Caldendrin represses neurite regeneration and growth in dorsal root ganglion neurons

Caldendrin is a Ca2+ binding protein that interacts with multiple effectors, such as the Cav1 L-type Ca2+ channel, which play a prominent role in regulating the outgrowth of dendrites and axons (i.e., neurites) during development and in response to injury. Here, we investigated the role of caldendrin in Cav1-dependent pathways that impinge upon neurite growth in dorsal root ganglion neurons (DRGNs). By immunofluorescence, caldendrin was localized in medium- and large- diameter DRGNs. Compared to DRGNs cultured from WT mice, DRGNs of caldendrin knockout (KO) mice exhibited enhanced neurite regeneration and outgrowth. Strong depolarization, which normally represses neurite growth through activation of Cav1 channels, had no effect on neurite growth in DRGN cultures from female caldendrin KO mice. Remarkably, DRGNs from caldendrin KO males were no different from those of WT males in terms of depolarization-dependent neurite growth repression. We conclude that caldendrin opposes neurite regeneration and growth, and this involves coupling of Cav1 channels to growth-inhibitory pathways in DRGNs of females but not males.

Neuronal Calcium and cAMP Cross-Talk Mediated by Cannabinoid CB1 Receptor and EF-Hand Calcium Sensor Interactions

Endocannabinoids are important players in neural development and function. They act via receptors, whose activation inhibits cAMP production. The aim of the paper was to look for calcium- and cAMP-signaling cross-talk mediated by cannabinoid CB₁ receptors (CB₁R) and to assess the relevance of EF-hand CaM-like calcium sensors in this regard. Using a heterologous expression system, we demonstrated that CB₁R interacts with calneuron-1 and NCS1 but not with caldendrin. Furthermore, interaction motives were identified in both calcium binding proteins and the receptor, and we showed that the first two sensors competed for binding to the receptor in a Ca²⁺-dependent manner. Assays in neuronal primary cultures showed that, CB₁R-NCS1 complexes predominate at basal Ca²⁺ levels, whereas in the presence of ionomycin, a calcium ionophore, CB₁R-calneuron-1 complexes were more abundant. Signaling assays following forskolin-induced intracellular cAMP levels showed in mouse striatal neurons that binding of CB₁R to NCS1 is required for CB₁R-mediated signaling, while the binding of CB₁R to calneuron-1 completely blocked G_i-mediated signaling in response to a selective receptor agonist, arachidonyl-2-chloroethylamide. Calcium levels and interaction with calcium sensors may even lead to apparent Gs coupling after CB₁R agonist challenge.

Functions of CaBP1 and CaBP2 in the peripheral auditory system

CaBPs are a family of Ca²⁺ binding proteins related to calmodulin. Two CaBP family members, CaBP1 and CaBP2, are highly expressed in the cochlea. Here, we investigated the significance of CaBP1 and CaBP2 for hearing in mice lacking expression of these proteins (CaBP1 KO and CaBP2 KO) using auditory brain responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). In CaBP1 KO mice, ABR wave I was larger in amplitude, and shorter in latency and faster in decay, suggestive of enhanced synchrony of auditory nerve fibers. This interpretation was supported by the greater excitability of CaBP1 KO than WT neurons in whole-cell patch clamp recordings of spiral ganglion neurons in culture, and normal presynaptic function of CaBP1 KO IHCs. DPOAEs and ABR thresholds were normal in 4-week old CaBP1 KO mice, but elevated ABR thresholds became evident at 32 kHz at 9 weeks, and at 8 and 16 kHz by 6 months of age. In contrast, CaBP2 KO mice exhibited significant ABR threshold elevations at 4 weeks of age that became more severe in the mid-frequency range by 9 weeks. Though normal at 4 weeks, DPOAEs in CaBP2 KO mice were significantly reduced in the mid-frequency range by 9 weeks. Our results reveal requirements for CaBP1 and CaBP2 in the peripheral auditory system and highlight the diverse modes by which CaBPs influence sensory processing.

Caldendrin Directly Couples Postsynaptic Calcium Signals to Actin Remodeling in Dendritic Spines

Compartmentalization of calcium-dependent plasticity allows for rapid actin remodeling in dendritic spines. However, molecular mechanisms for the spatio-temporal regulation of filamentous actin (F-actin) dynamics by spinous Ca²⁺-transients are still poorly defined. We show that the postsynaptic Ca²⁺ sensor caldendrin orchestrates nano-domain actin dynamics that are essential for actin remodeling in the early phase of long-term potentiation (LTP). Steep elevation in spinous [Ca²⁺]_i disrupts an intramolecular interaction of caldendrin and allows cortactin binding. The fast on and slow off rate of this interaction keeps cortactin in an active conformation, and protects F-actin at the spine base against cofilin-induced severing. Caldendrin gene knockout results in higher synaptic actin turnover, altered nanoscale organization of spinous F-actin, defects in structural spine plasticity, LTP, and hippocampus-dependent learning. Collectively, the data indicate that caldendrin-cortactin directly couple [Ca²⁺]_i to preserve a minimal F-actin pool that is required for actin remodeling in the early phase of LTP.

CaMKII regulates the depalmitoylation and synaptic removal of the scaffold protein AKAP79/150 to mediate structural long-term depression

Both long-term potentiation (LTP) and depression (LTD) of excitatory synapse strength require the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) and its autonomous activity generated by Thr-286 autophosphorylation. Additionally, LTP and LTD are correlated with dendritic spine enlargement and shrinkage that are accompanied by the synaptic accumulation or removal, respectively, of the AMPA-receptor regulatory scaffold protein A-kinase anchoring protein (AKAP) 79/150. We show here that the spine shrinkage associated with LTD indeed requires synaptic AKAP79/150 removal, which in turn requires CaMKII activity. In contrast to normal CaMKII substrates, the substrate sites within the AKAP79/150 N-terminal polybasic membrane-cytoskeletal targeting domain were phosphorylated more efficiently by autonomous compared with Ca²⁺/CaM-stimulated CaMKII activity. This unusual regulation was mediated by Ca²⁺/CaM binding to the substrate sites resulting in protection from phosphorylation in the presence of Ca²⁺/CaM, a mechanism that favors phosphorylation by prolonged, weak LTD stimuli versus brief, strong LTP stimuli. Phosphorylation by CaMKII inhibited AKAP79/150 association with F-actin; it also facilitated AKAP79/150 removal from spines but was not required for it. By contrast, LTD-induced spine removal of AKAP79/150 required its depalmitoylation on two Cys residues within the N-terminal targeting domain. Notably, such LTD-induced depalmitoylation was also blocked by CaMKII inhibition. These results provide a mechanism how CaMKII can indeed mediate not only LTP but also LTD through regulated substrate selection; however, in the case of AKAP79/150, indirect CaMKII effects on palmitoylation are more important than the effects of direct phosphorylation. Additionally, our results provide the first direct evidence for a function of the well-described AKAP79/150 trafficking in regulating LTD-induced spine shrinkage.

Biophysical analysis of the dynamics of calmodulin interactions with neurogranin and Ca2+ /calmodulin-dependent kinase II

Calmodulin (CaM) functions depend on interactions with CaM‐binding proteins, regulated by Ca2+. Induced structural changes influence the affinity, kinetics, and specificities of the interactions. The dynamics of CaM interactions with neurogranin (Ng) and the CaM‐binding region of Ca2+/calmodulin‐dependent kinase II (CaMKII_290−309) have been studied using biophysical methods. These proteins have opposite Ca2+ dependencies for CaM binding. Surface plasmon resonance biosensor analysis confirmed that Ca2+ and CaM interact very rapidly, and with moderate affinity ( KDSPR=3μM). Calmodulin‐CaMKII_290−309 interactions were only detected in the presence of Ca2+, exhibiting fast kinetics and nanomolar affinity ( KDSPR=7.1nM). The CaM–Ng interaction had higher affinity under Ca2+‐depleted ( KDSPR=480nM,k1=3.4×105M−1s−1 and k₋₁ = 1.6 × 10⁻¹s⁻¹) than Ca2+‐saturated conditions ( KDSPR=19μM). The IQ motif of Ng (Ng₂₇₋₅₀) had similar affinity for CaM as Ng under Ca2+‐saturated conditions ( KDSPR=14μM), but no interaction was seen under Ca2+‐depleted conditions. Microscale thermophoresis using fluorescently labeled CaM confirmed the surface plasmon resonance results qualitatively, but estimated lower affinities for the Ng ( KDMST=890nM) and CaMKII_290−309( KDMST=190nM) interactions. Although CaMKII_290−309 showed expected interaction characteristics, they may be different for full‐length CaMKII. The data for full‐length Ng, but not Ng₂₇₋₅₀, agree with the current model on Ng regulation of Ca2+/CaM signaling.

Lack of CaBP1/Caldendrin or CaBP2 Leads to Altered Ganglion Cell Responses

Calcium-binding proteins (CaBPs) form a subfamily of calmodulin-like proteins that were cloned from the retina. CaBP4 and CaBP5 have been shown to be important for normal visual function. Although CaBP1/caldendrin and CaBP2 have been shown to modulate various targets in vitro, it is not known whether they contribute to the transmission of light responses through the retina. Therefore, we generated mice that lack CaBP2 or CaBP1/caldendrin (Cabp2^–/– and Cabp1^–/–) to test whether these CaBPs are essential for normal retinal function. By immunohistochemistry, the overall morphology of Cabp1^–/– and Cabp2^–/– retinas and the number of synaptic ribbons appear normal; transmission electron microscopy shows normal tethered ribbon synapses and synaptic vesicles as in wild-type retinas. However, whole-cell patch clamp recordings showed that light responses of retinal ganglion cells of Cabp2^–/– and Cabp1^–/– mice differ in amplitude and kinetics from those of wild-type mice. We conclude that CaBP1/caldendrin and CaBP2 are not required for normal gross retinal and synapse morphology but are necessary for the proper transmission of light responses through the retina; like other CaBPs, CaBP1/caldendrin and CaBP2 likely act by modulating presynaptic Ca²⁺-dependent signaling mechanisms.

Decalmodulation of Cav1 channels by CaBPs

Ca²⁺-dependent inactivation (CDI) is a negative feedback regulation of voltage-gated Ca_v1 and Ca_v2 channels that is mediated by the Ca²⁺ sensing protein, calmodulin (CaM), binding to the pore-forming Ca_v α₁ subunit. David Yue and his colleagues made seminal contributions to our understanding of this process, as well as factors that regulate CDI. Important in this regard are members of a family of Ca²⁺ binding proteins (CaBPs) that are related to calmodulin. CaBPs are expressed mainly in neural tissues and can antagonize CaM-dependent CDI for Ca_v1 L-type channels. This review will focus on the roles of CaBPs as Ca_v1-interacting proteins, and the significance of these interactions for vision, hearing, and neuronal Ca²⁺ signaling events.

Localization and expression of CaBP1/caldendrin in the mouse brain

Ca(2+) binding protein 1 (CaBP1) and caldendrin are alternatively spliced variants of a subfamily of CaBPs with high homology to calmodulin. Although CaBP1 and caldendrin regulate effectors including plasma membrane and intracellular Ca(2+) channels in heterologous expression systems, little is known about their functions in vivo. Therefore, we generated mice deficient in CaBP1/caldendrin expression (C-KO) and analyzed the expression and cellular localization of CaBP1 and caldendrin in the mouse brain. Immunoperoxidase labeling with antibodies recognizing both CaBP1 and caldendrin was absent in the brain of C-KO mice, but was intense in multiple brain regions of wild-type mice. By Western blot, the antibodies detected two proteins that were absent in the C-KO mouse and consistent in size with caldendrin variants originating from alternative translation initiation sites. By quantitative PCR, caldendrin transcript levels were far greater than those for CaBP1, particularly in the cerebral cortex and hippocampus. In the frontal cortex but not in the hippocampus, caldendrin expression increased steadily from birth. By double-label immunofluorescence, CaBP1/caldendrin was localized in principal neurons and parvalbumin-positive interneurons. In the cerebellum, CaBP1/caldendrin antibodies labeled interneurons in the molecular layer and in basket cell terminals surrounding the soma and axon initial segment of Purkinje neurons, but immunolabeling was absent in Purkinje neurons. We conclude that CaBP1/caldendrin is localized both pre- and postsynaptically where it may regulate Ca(2+) signaling and excitability in select groups of excitatory and inhibitory neurons.

Mapping the binding site of TRPV1 on AKAP79: implications for inflammatory hyperalgesia

Inflammation causes hyperalgesia, an enhanced sensitivity to noxious stimuli. Transient receptor potential vanilloid 1 (TRPV1), a thermo-TRP ion channel activated by painful levels of heat, is an important contributor because hyperalgesia is reduced when TRPV1 is either genetically deleted or pharmacologically blocked. Inflammatory mediators such as prostaglandin-E2 or bradykinin cause hyperalgesia by activating cellular kinases that phosphorylate TRPV1, a process that has recently been shown to rely on a scaffolding protein, AKAP79, to target the kinases to TRPV1. Here we use Förster resonance energy transfer, immunoprecipitation, and TRPV1 membrane trafficking experiments to identify a key region on AKAP79, between amino acids 326-336, which is responsible for its interaction with TRPV1. A peptide identical to this domain inhibited sensitization of TRPV1 in vitro, and when covalently linked to a TAT peptide to promote uptake across the cell membrane the peptide inhibited in vivo inflammatory hyperalgesia in mice. Critically, it did so without affecting pain thresholds in the absence of inflammation. These results suggest that antagonizing the TRPV1-AKAP79 interaction will be a useful strategy for inhibiting inflammatory hyperalgesia.

Increased density of AKAP5-expressing neurons in the anterior cingulate cortex of subjects with bipolar disorder

Brain anatomical abnormalities as well as cognitive and emotional processing deficits have been reported for the prefrontal cortex in bipolar disorder, which are in part attributable to cellular and laminar abnormalities in postsynaptic protein expression. A kinase anchoring protein (AKAP) 5/79 plays a key role in postsynaptic signalling of excitatory synapses. We aimed to reveal if the cellular expression of AKAP5/79 protein is altered in the anterior cingulate cortex and the dorsolateral prefrontal cortex in bipolar disorder. Ten subjects with bipolar disorder and ten control cases were investigated by use of immunohistochemical and morphometric techniques. Compared with controls in subjects with bipolar disorder, the numerical density of AKAP5-expressing neurons was significantly increased in the left (p = 0.002) and right (p = 0.008) anterior cingulate cortex. Layer-specific counting revealed that left side layers II (p = 0.000), III (p = 0.001) and V (p = 0.005) as well as right side layers III (p = 0.007), IV (p = 0.007) and V (p = 0.004) had significantly increased AKAP5-positive cell densities in bipolar disorder. In contrast, no statistically significant differences were found for the dorsolateral prefrontal cortex. However, we observed a more intense intraneuronal immunostaining in both prefrontal areas in bipolar disorder patients. Elevated cell numbers and increased intracellular expression of AKAP, together with the altered expression patterns of most intracellular interaction partners of this protein in bipolar disorder as known from the literature, might point to disease-related abnormalities of the AKAP-associated signalosome in prefrontal cortex neurons.

Kinetic and mechanistic differences in the interactions between caldendrin and calmodulin with AKAP79 suggest different roles in synaptic function

The kinetic and mechanistic details of the interaction between caldendrin, calmodulin and the B-domain of AKAP79 were determined using a biosensor-based approach. Caldendrin was found to compete with calmodulin for binding at AKAP79, indicating overlapping binding sites. Although the AKAP79 affinities were similar for caldendrin (K(D) = 20 nM) and calmodulin (K(D) = 30 nM), their interaction characteristics were different. The calmodulin interaction was well described by a reversible one-step model, but was only detected in the presence of Ca(2+). Caldendrin interacted with a higher level of complexity, deduced to be an induced fit mechanism with a slow relaxation back to the initial encounter complex. It interacted with AKAP79 also in the absence of Ca(2+), but with different kinetic rate constants. The data are consistent with a similar initial Ca(2+)-dependent binding step for the two proteins. For caldendrin, a second Ca(2+)-independent rearrangement step follows, resulting in a stable complex. The study shows the importance of establishing the mechanism and kinetics of protein-protein interactions and that minor differences in the interaction of two homologous proteins can have major implications in their functional characteristics. These results are important for the further elucidation of the roles of caldendrin and calmodulin in synaptic function.