Distinct roles of α- and βCaMKII in controlling long-term potentiation of GABAA-receptor mediated transmission in murine Purkinje cells

Understanding the pathogenetic mechanisms underlying altered neuronal function associated with CAMK2B mutations

'Dominant mutations in CAMK2B, encoding a subunit of the calcium/calmodulin-dependent protein kinase II (CAMK2), a serine/threonine kinase playing a key role in synaptic plasticity, learning and memory, underlie a recently characterized neurodevelopmental disorder (MRD54) characterized by delayed psychomotor development, mild to severe intellectual disability, hypotonia, and behavioral abnormalities. Targeted therapies to treat MRD54 are currently unavailable. In this review, we revise current knowledge on the molecular and cellular mechanisms underlying the altered neuronal function associated with defective CAMKIIβ function. We also summarize the identified genotype-phenotype correlations and discuss the disease models that have been generated to profile the altered neuronal phenotype and understand the pathophysiology of this disease.

The Yin and Yang of GABAergic and Glutamatergic Synaptic Plasticity: Opposites in Balance by Crosstalking Mechanisms

Synaptic plasticity is a critical process that regulates neuronal activity by allowing neurons to adjust their synaptic strength in response to changes in activity. Despite the high proximity of excitatory glutamatergic and inhibitory GABAergic postsynaptic zones and their functional integration within dendritic regions, concurrent plasticity has historically been underassessed. Growing evidence for pathological disruptions in the excitation and inhibition (E/I) balance in neurological and neurodevelopmental disorders indicates the need for an improved, more "holistic" understanding of synaptic interplay. There continues to be a long-standing focus on the persistent strengthening of excitation (excitatory long-term potentiation; eLTP) and its role in learning and memory, although the importance of inhibitory long-term potentiation (iLTP) and depression (iLTD) has become increasingly apparent. Emerging evidence further points to a dynamic dialogue between excitatory and inhibitory synapses, but much remains to be understood regarding the mechanisms and extent of this exchange. In this mini-review, we explore the role calcium signaling and synaptic crosstalk play in regulating postsynaptic plasticity and neuronal excitability. We examine current knowledge on GABAergic and glutamatergic synapse responses to perturbances in activity, with a focus on postsynaptic plasticity induced by short-term pharmacological treatments which act to either enhance or reduce neuronal excitability via ionotropic receptor regulation in neuronal culture. To delve deeper into potential mechanisms of synaptic crosstalk, we discuss the influence of synaptic activity on key regulatory proteins, including kinases, phosphatases, and synaptic structural/scaffolding proteins. Finally, we briefly suggest avenues for future research to better understand the crosstalk between glutamatergic and GABAergic synapses.

Elucidation of pathological mechanism caused by human disease mutation in CaMKIIβ

Recently, we have identified CaMKIIα and CaMKIIβ mutations in patients with neurodevelopmental disorders by whole exome sequencing study. Most CaMKII mutants have increased phosphorylation of Thr286/287, which induces autonomous activity of CaMKII, using cell culture experiments. In this study, we explored the pathological mechanism of motor dysfunction observed exclusively in a patient with Pro213Leu mutation in CaMKIIβ using a mouse model of the human disease. The homozygous CaMKIIβ Pro213Leu knockin mice showed age-dependent motor dysfunction and growth failure from 2 weeks after birth. In the cerebellum, the mutation did not alter the mRNA transcript level, but the CaMKIIβ protein level was dramatically decreased. Furthermore, in contrast to previous result from cell culture, Thr287 phosphorylation of CaMKIIβ was also reduced. CaMKIIβ Pro213Leu knockin mice showed similar motor dysfunction as CaMKIIβ knockout mice, newly providing evidence for a loss of function rather than a gain of function. Our disease model mouse showed similar phenotypes of the patient, except for epileptic seizures. We clearly demonstrated that the pathological mechanism is a reduction of mutant CaMKIIβ in the brain, and the physiological aspects of mutation were greatly different between in vivo and cell culture.

Treadmill exercise enhances synaptic plasticity in the ischemic penumbra of MCAO mice by inducing the expression of Camk2a via CYFIP1 upregulation

AIMS

Physical exercise is beneficial to the recovery of patients with ischemic stroke. However, the underlying mechanism by which exercise promotes dendritic remodeling and synaptic plasticity is still obscure. This study explored the mechanism by which treadmill exercise enhances synaptic plasticity and dendritic remodeling in the ischemic penumbra.

MAIN METHODS

A middle cerebral artery occlusion (MCAO) model was generated in C57BL/6 mice, and lentivirus-mediated cytoplasmic FMRP-associated protein 1 (CYFIP1) shRNA expression was utilized to confirm the role of CYFIP1 in the exercise-induced increase in synaptic plasticity and dendritic remodeling. Neurological deficits were measured using the Zea Longa scale. Hematoxylin-eosin (H&E) staining and Nissl staining were performed to assess cerebral ischemic injury. Golgi-Cox staining was used to observe changes in dendritic remodeling and synaptic plasticity. Transmission electron microscopy (TEM) was performed to observe the synaptic ultrastructure. Molecular mechanisms were explored using immunofluorescence staining and western blotting.

KEY FINDINGS

Treadmill training enhanced synaptic plasticity in the penumbra. Additionally, we observed significant increases in the expression of CYFIP1 and calcium/calmodulin-dependent kinase 2a (Camk2a); enhanced neurological recovery and a decreased infarct volume. However, the injection of a lentivirus containing CYFIP1 shRNA into the lateral ventricle exerted negative effects on synaptic plasticity. Moreover, the exercise-induced neuroprotective effects were abolished by lentivirus-mediated CYFIP1 shRNA expression, consistent with the downregulation of Camk2a expression and the deterioration of neurological function.

SIGNIFICANCE

Treadmill training enhances synaptic plasticity and dendritic remodeling in the ischemic penumbra by inducing the expression of Camk2a via upregulation of CYFIP1.

CaMKIIβ in Neuronal Development and Plasticity: An Emerging Candidate in Brain Diseases

The calcium/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous and central player in Ca²⁺ signaling that is best known for its functions in the brain. In particular, the α isoform of CaMKII has been the subject of intense research and it has been established as a central regulator of neuronal plasticity. In contrast, little attention has been paid to CaMKIIβ, the other predominant brain isoform that interacts directly with the actin cytoskeleton, and the functions of CaMKIIβ in this organ remain largely unexplored. However, recently, the perturbation of CaMKIIβ expression has been associated with multiple neuropsychiatric and neurodevelopmental diseases, highlighting CAMK2B as a gene of interest. Herein, after highlighting the main structural and expression differences between the α and β isoforms, we will review the specific functions of CaMKIIβ, as described so far, in neuronal development and plasticity, as well as its potential implication in brain diseases.

Analysis of the CaMKIIα and β splice-variant distribution among brain regions reveals isoform-specific differences in holoenzyme formation

Four CaMKII isoforms are encoded by distinct genes, and alternative splicing within the variable linker-region generates additional diversity. The α and β isoforms are largely brain-specific, where they mediate synaptic functions underlying learning, memory and cognition. Here, we determined the α and β splice-variant distribution among different mouse brain regions. Surprisingly, the nuclear variant αB was detected in all regions, and even dominated in hypothalamus and brain stem. For CaMKIIβ, the full-length variant dominated in most regions (with higher amounts of minor variants again seen in hypothalamus and brain stem). The mammalian but not fish CaMKIIβ gene lacks exon v3_N that encodes the nuclear localization signal in α_B, but contains three exons not found in the CaMKIIα gene (exons v1, v4, v5). While skipping of exons v1 and/or v5 generated the minor splice-variants β’, βe and βe’, essentially all transcripts contained exon v4. However, we instead detected another minor splice-variant (now termed βH), which lacks part of the hub domain that mediates formation of CaMKII holoenzymes. Surprisingly, in an optogenetic cellular assay of protein interactions, CaMKIIβH was impaired for binding to the β hub domain, but still bound CaMKIIα. This provides the first indication for isoform-specific differences in holoenzyme formation.

Non-canonical heterogeneous cellular distribution and co-localization of CaMKIIα and CaMKIIβ in the spinal superficial dorsal horn

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a key enzyme in long-term plasticity in many neurons, including in the nociceptive circuitry of the spinal dorsal horn. However, although the role of CaMKII heterooligomers in neuronal plasticity is isoform-dependent, the distribution and co-localization of CaMKII isoforms in the dorsal horn have not been comprehensively investigated. Here, quantitative immunofluorescence analysis was used to examine the distribution of the two major neuronal CaMKII isoforms, α and β, in laminae I–III of the rat dorsal horn, with reference to inhibitory interneurons and neuronal populations defined by expression of parvalbumin, calretinin, and calbindin D28k. Unexpectedly, all or nearly all inhibitory and excitatory neurons showed both CaMKIIα and CaMKIIβ immunoreactivity, although at highly variable levels. Lamina III neurons showed less CaMKIIα immunoreactivity than laminae I–II neurons. Whereas CaMKIIα immunoreactivity was found at nearly similar levels in inhibitory and excitatory neurons, CaMKIIβ generally showed considerably lower immunoreactivity in inhibitory neurons. Distinct populations of inhibitory calretinin neurons and excitatory parvalbumin neurons exhibited high CaMKIIα-to-CaMKIIβ immunoreactivity ratios. CaMKIIα and CaMKIIβ immunoreactivity showed positive correlation at GluA2⁺ puncta in pepsin-treated tissue. These results suggest that, unlike the forebrain, the dorsal horn is characterized by similar expression of CaMKIIα in excitatory and inhibitory neurons, whereas CaMKIIβ is less expressed in inhibitory neurons. Moreover, CaMKII isoform expression varies considerably within and between neuronal populations defined by laminar location, calcium-binding protein expression, and transmitter phenotype, suggesting differences in CaMKII function both between and within neuronal populations in the superficial dorsal horn.

SLC26A11 (KBAT) in Purkinje Cells Is Critical for Inhibitory Transmission and Contributes to Locomotor Coordination

Chloride homeostasis determines the impact of inhibitory synaptic transmission and thereby mediates the excitability of neurons. Even though cerebellar Purkinje cells (PCs) receive a pronounced inhibitory GABAergic input from stellate and basket cells, the role of chloride homeostasis in these neurons is largely unknown. Here we studied at both the cellular and systems physiological level the function of a recently discovered chloride channel, SLC26A11 or kidney brain anion transporter (KBAT), which is prominently expressed in PCs. Using perforated patch clamp recordings of PCs, we found that a lack of KBAT channel in PC-specific KBAT KO mice (L7-KBAT KOs) induces a negative shift in the reversal potential of chloride as reflected in the GABA_A-receptor-evoked currents, indicating a decrease in intracellular chloride concentration. Surprisingly, both in vitro and in vivo PCs in L7-KBAT KOs showed a significantly increased action potential firing frequency of simple spikes, which correlated with impaired motor performance on the Erasmus Ladder. Our findings support an important role for SLC26A11 in moderating chloride homeostasis and neuronal activity in the cerebellum.

The molecular, temporal and region-specific requirements of the beta isoform of Calcium/Calmodulin-dependent protein kinase type 2 (CAMK2B) in mouse locomotion

Genetic approaches using temporal and brain region-specific restricted gene deletions have provided a wealth of insight in the brain regions and temporal aspects underlying spatial and associative learning. However, for locomotion such extensive studies are still scarce. Previous studies demonstrated that Camk2b^–/– mice, which lack the β isoform of Calcium/Calmodulin-dependent protein kinase 2 (CAMK2B), show very severe locomotion deficits. However, where these locomotion deficits originate is unknown. Here we made use of novel Camk2b mutants (Camk2b^f/f and Camk2b^T287A), to explore the molecular, temporal and brain region-specific requirements of CAMK2B for locomotion. At the molecular level we found that normal locomotion requires Calcium/Calmodulin mediated activation of CAMK2B, but CAMK2B autonomous activity is largely dispensable. At a systems level, we found that global deletion of Camk2b in the adult mouse causes only mild locomotion deficits, suggesting that the severe locomotion deficits of Camk2b^–/– mice are largely of developmental origin. However, early onset deletion of Camk2b in cerebellum, striatum or forebrain did not recapitulate the locomotion deficits, suggesting that these deficits cannot be attributed to a single brain area. Taken together, these results provide the first insights into the molecular, temporal and region-specific role of CAMK2B in locomotion.

Motor Learning and the Cerebellum

Although our ability to store semantic declarative information can nowadays be readily surpassed by that of simple personal computers, our ability to learn and express procedural memories still outperforms that of supercomputers controlling the most advanced robots. To a large extent, our procedural memories are formed in the cerebellum, which embodies more than two-thirds of all neurons in our brain. In this review, we will focus on the emerging view that different modules of the cerebellum use different encoding schemes to form and express their respective memories. More specifically, zebrin-positive zones in the cerebellum, such as those controlling adaptation of the vestibulo-ocular reflex, appear to predominantly form their memories by potentiation mechanisms and express their memories via rate coding, whereas zebrin-negative zones, such as those controlling eyeblink conditioning, appear to predominantly form their memories by suppression mechanisms and express their memories in part by temporal coding using rebound bursting. Together, the different types of modules offer a rich repertoire to acquire and control sensorimotor processes with specific challenges in the spatiotemporal domain.

Opposite regulation of inhibitory synaptic plasticity by α and β subunits of Ca(2+)/calmodulin-dependent protein kinase II

Induction of several forms of synaptic plasticity, a cellular basis for learning and memory, depends on the activation of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII). CaMKII acts as a holoenzyme consisting of α and β subunits (α- and βCaMKII). However, it remains elusive how the subunit composition of a CaMKII holoenzyme affects its activation and hence synaptic plasticity. We addressed this issue by focusing on long-term potentiation (LTP) at inhibitory synapses on cerebellar Purkinje neurons (PNs) (called rebound potentiation, RP). The contribution of each subunit to RP was examined by selective knock-down or overexpression of that subunit. Electrophysiological recording from a rat cultured PN demonstrated that βCaMKII is essential for RP induction, whereas αCaMKII suppresses it. Thus, RP was negatively regulated due to the greater relative abundance of αCaMKII compared to βCaMKII, suggesting a critical role of CaMKII subunit composition in RP. The higher affinity of βCaMKII to Ca(2+)/CaM compared with αCaMKII was responsible for the predominant role in RP induction. Live-cell imaging of CaMKII activity based on the Förster resonance energy transfer (FRET) technique revealed that βCaMKII enrichment enhances the total CaMKII activation upon a transient conditioning depolarization. Taken together, these findings clarified that α- and βCaMKII oppositely regulate CaMKII activation, controlling the induction of inhibitory synaptic plasticity in a PN, which might contribute to the adaptive information processing of the cerebellar cortex.