Immunohistochemical localization of Ca2+/calmodulin-dependent protein kinase II in the rat retina

Overexpression of Motopsin, an Extracellular Serine Protease Related to Intellectual Disability, Promotes Adult Neurogenesis and Neuronal Responsiveness in the Dentate Gyrus

Motopsin, a serine protease encoded by PRSS12, is secreted by neuronal cells into the synaptic clefts in an activity-dependent manner, where it induces synaptogenesis by modulating Na+/K+-ATPase activity. In humans, motopsin deficiency leads to severe intellectual disability and, in mice, it disturbs spatial memory and social behavior. In this study, we investigated mice that overexpressed motopsin in the forebrain using the Tet-Off system (DTG-OE mice). The elevated agrin cleavage or the reduced Na+/K+-ATPase activity was not detected. However, motopsin overexpression led to a reduction in spine density in hippocampal CA1 basal dendrites. While motopsin overexpression decreased the ratio of mature mushroom spines in the DG, it increased the ratio of immature thin spines in CA1 apical dendrites. Female DTG-OE mice showed elevated locomotor activity in their home cages. DTG-OE mice showed aberrant behaviors, such as delayed latency to the target hole in the Barnes maze test and prolonged duration of sniffing objects in the novel object recognition test (NOR), although they retained memory comparable to that of TRE-motopsin littermates, which normally express motopsin. After NOR, c-Fos-positive cells increased in the dentate gyrus (DG) of DTG-OE mice compared with that of DTG-SO littermates, in which motopsin overexpression was suppressed by the administration of doxycycline, and TRE-motopsin littermates. Notably, the numbers of doublecortin- and 5-bromo-2'-deoxyuridine-labeled cells significantly increased in the DG of DTG-OE mice, suggesting increased adult neurogenesis. Importantly, our results revealed a new function in addition to modulating neuronal responsiveness and spine morphology in the DG: the regulation of neurogenesis.

Unveiling the role of CaMKII in retinal degeneration: from biological mechanism to therapeutic strategies

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a family of broad substrate specificity serine (Ser)/threonine (Thr) protein kinases that play a crucial role in the Ca2+-dependent signaling pathways. Its significance as an intracellular Ca2+ sensor has garnered abundant research interest in the domain of neurodegeneration. Accumulating evidences suggest that CaMKII is implicated in the pathology of degenerative retinopathies such as diabetic retinopathy (DR), age-related macular degeneration (AMD), retinitis pigmentosa (RP) and glaucoma optic neuropathy. CaMKII can induce the aberrant proliferation of retinal blood vessels, influence the synaptic signaling, and exert dual effects on the survival of retinal ganglion cells and pigment epithelial cells. Researchers have put forth multiple therapeutic agents, encompassing small molecules, peptides, and nucleotides that possess the capability to modulate CaMKII activity. Due to its broad range isoforms and splice variants therapeutic strategies seek to inhibit specifically the CaMKII are confronted with considerable challenges. Therefore, it becomes crucial to discern the detrimental and advantageous aspects of CaMKII, thereby facilitating the development of efficacious treatment. In this review, we summarize recent research findings on the cellular and molecular biology of CaMKII, with special emphasis on its metabolic and regulatory mechanisms. We delve into the involvement of CaMKII in the retinal signal transduction pathways and discuss the correlation between CaMKII and calcium overload. Furthermore, we elaborate the therapeutic trials targeting CaMKII, and introduce recent developments in the zone of CaMKII inhibitors. These findings would enrich our knowledge of CaMKII, and shed light on the development of a therapeutic target for degenerative retinopathy.

TRPV4 antagonist suppresses retinal ganglion cell apoptosis by regulating the activation of CaMKII and TNF-α expression in a chronic ocular hypertension rat model

Glaucoma is characterized by a progressive loss of retinal ganglion cells (RGCs), leading to irreversible visual function impairment. Sustained increase in intraocular pressure represents a major risk factor for glaucoma, yet the underlying mechanisms of RGC apoptosis induced by intraocular pressure remains unclear. This study aims to investigate the role of TRPV4 in RGC apoptosis in a rat model of chronic ocular hypertension (COH) and the underlying molecular mechanism. In the COH rat models, we evaluated the visual function, retinal pathological changes and RGC apoptosis. TRPV4 expression and downstream signaling molecules were also detected. We found that RGC density decreased and RGC apoptosis was induced in COH eyes compared with control eyes. TRPV4 expression increased significantly in response to elevated IOP. TRPV4 inhibition by the TRPV4 antagonist HC-067047 (HC-067) suppressed RGC apoptosis and protected visual function. HC-067 treatment upregulated the phosphorylation of CaMKII in both control and COH eyes. Finally, HC-067 treatment suppressed the production of TNF-α induced by ocular hypertension. The TRPV4 antagonist HC-067 might suppress RGC apoptosis by regulating the activation of CaMKII and inhibiting the production of TNF-α in the COH model. This indicated that TRPV4 antagonists may be a potential and novel therapeutic strategy for glaucoma.

Retinal ganglion cells expressing CaM kinase II in human and nonhuman primates

Immunoreactivity for calcium-/calmodulin-dependent protein kinase II (CaMKII) in the primate dorsal lateral geniculate nucleus (dLGN) has been attributed to geniculocortical relay neurons and has also been suggested to arise from terminals of retinal ganglion cells. Here, we combined immunostaining with single-cell injections to investigate the expression of CaMKII in retinal ganglion cells of three primate species: macaque (Macaca fascicularis, M. nemestrina), human, and marmoset (Callithrix jacchus). We found that in all species, about 2%-10% of the total ganglion cell population expressed CaMKII. In all species, CaMKII was expressed by multiple types of wide-field ganglion cell including large sparse, giant sparse (melanopsin-expressing), broad thorny, and narrow thorny cells. Three other ganglion cells types, namely, inner and outer stratifying maze cells in macaque and tufted cells in marmoset were also found. Double labeling experiments showed that CaMKII-expressing cells included inner and outer stratifying melanopsin cells. Nearly all CaMKII-expressing ganglion cell types identified here are known to project to the koniocellular layers of the dLGN as well as to the superior colliculus. The best characterized koniocellular projecting cell type-the small bistratified (blue ON/yellow OFF) cell-was, however, not CaMKII-positive in any species. Our results indicate that the pattern of CaMKII expression in retinal ganglion cells is largely conserved across different species of primate suggesting a common functional role. But the results also show that CaMKII is not a marker for all koniocellular projecting retinal ganglion cells.

CAMKII as a therapeutic target for growth factor-induced retinal and choroidal neovascularization

While anti-VEGF drugs are commonly used to inhibit pathological retinal and choroidal neovascularization, not all patients respond in an optimal manner. Mechanisms underpinning resistance to anti‑VEGF therapy include the upregulation of other proangiogenic factors. Therefore, therapeutic strategies that simultaneously target multiple growth factor signaling pathways would have significant value. Here, we show that Ca2+/calmodulin-dependent kinase II (CAMKII) mediates the angiogenic actions of a range of growth factors in human retinal endothelial cells and that this kinase acts as a key nodal point for the activation of several signal transduction cascades that are known to play a critical role in growth factor-induced angiogenesis. We also demonstrate that endothelial CAMKIIγ and -δ isoforms differentially regulate the angiogenic effects of different growth factors and that genetic deletion of these isoforms suppresses pathological retinal and choroidal neovascularization in vivo. Our studies suggest that CAMKII could provide a novel and efficacious target to inhibit multiple angiogenic signaling pathways for the treatment of vasoproliferative diseases of the eye. CAMKIIγ represents a particularly promising target, as deletion of this isoform inhibited pathological neovascularization, while enhancing reparative angiogenesis in the ischemic retina.

Optic nerve thinning and neurosensory retinal degeneration in the rTg4510 mouse model of frontotemporal dementia

Visual impairments, such as difficulties in reading and finding objects, perceiving depth and structure from motion, and impaired stereopsis, have been reported in tauopathy disorders, such as frontotemporal dementia (FTD). These impairments however have been previously attributed to cortical pathologies rather than changes in the neurosensory retina or the optic nerve. Here, we examined tau pathology in the neurosensory retina of the rTg(tauP301L)4510 mouse model of FTD. Optic nerve pathology in mice was also assessed using MRI, and corresponding measurements taken in a cohort of five FTD sufferers and five healthy controls. rTg(tauP301L)4510 mice were imaged (T2-weighted MRI) prior to being terminally anesthetized and eyes and brains removed for immunohistochemical and histological analysis. Central and peripheral retinal labelling of tau and phosphorylated tau (pTau) was quantified and retinal layer thicknesses and cell numbers assessed. MR volumetric changes of specific brain regions and the optic nerve were compared to tau accumulation and cell loss in the visual pathway. In addition, the optic nerves of a cohort of healthy controls and behavioural variant FTD patients, were segmented from T1- and T2-weighted images for volumetric study. Accumulation of tau and pTau were observed in both the central and peripheral retinal ganglion cell (RGC), inner plexiform and inner nuclear layers of the neurosensory retina of rTg(tauP301L)4510 mice. This pathology was associated with reduced nuclear density (− 24.9 ± 3.4%) of the central RGC layer, and a reduced volume (− 19.3 ± 4.6%) and elevated T2 signal (+ 27.1 ± 1.8%) in the optic nerve of the transgenic mice. Significant atrophy of the cortex (containing the visual cortex) was observed but not in other area associated with visual processing, e.g. the lateral geniculate nucleus or superior colliculus. Atrophic changes in optic nerve volume were similarly observed in FTD patients (− 36.6 ± 2.6%). The association between tau-induced changes in the neurosensory retina and reduced optic nerve volume in mice, combined with the observation of optic nerve atrophy in clinical FTD suggests that ophthalmic tau pathology may also exist in the eyes of FTD patients. If tau pathology and neurodegeneration in the retina were to reflect the degree of cortical tau burden, then cost-effective and non-invasive imaging of the neurosensory retina could provide valuable biomarkers in tauopathy. Further work should aim to validate whether these observations are fully translatable to a clinical scenario, which would recommend follow-up retinal and optic nerve examination in FTD.

Autophosphorylated CaMKII Facilitates Spike Propagation in Rat Optic Nerve

Repeated spike firing can transmit information at synapses and modulate spike timing, shape, and conduction velocity. These latter effects have been found to result from voltage-induced changes in ion currents and could alter the signals carried by axons. Here, we test whether Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) regulates spike propagation in adult rat optic nerve. We find that small-, medium-, and large-diameter axons bind anti-Thr286-phosphorylated CaMKII (pT286) antibodies and that, in isolated optic nerves, electrical stimulation reduces pT286 levels, spike propagation is hastened by CaMKII autophosphorylation and slowed by CaMKII dephosphorylation, single and multiple spikes slow propagation of subsequently activated spikes, and more frequent stimulation produces greater slowing. Likewise, exposing freely moving animals to flickering illumination reduces pT286 levels in optic nerves and electrically eliciting spikes in vivo in either the optic nerve or optic chiasm slows subsequent spike propagation in the optic nerve. By increasing the time that elapses between successive spikes as they propagate, pT286 dephosphorylation and activity-induced spike slowing reduce the frequency of propagated spikes below the frequency at which they were elicited and would thus limit the frequency at which axons synaptically drive target neurons. Consistent with this, the ability of retinal ganglion cells to drive at least some lateral geniculate neurons has been found to increase when presented with light flashes at low and moderate temporal frequencies but less so at high frequencies. Activity-induced decreases in spike frequency may also reduce the energy required to maintain normal intracellular Na⁺ and Ca²⁺ levels.SIGNIFICANCE STATEMENT By propagating along axons at constant velocities, spikes could drive synapses as frequently as they are initiated. However, the onset of spiking has been found to alter the conduction velocity of subsequent ("follower") spikes in various preparations. Here, we find that spikes reduce spike frequency in rat optic nerve by slowing follower spike propagation and that electrically stimulated spiking ex vivo and spike-generating flickering illumination in vivo produce net decreases in axonal Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation. Consistent with these effects, propagation speed increases and decreases, respectively, with CaMKII autophosphorylation and dephosphorylation. Lowering spike frequency by CaMKII dephosphorylation is a novel consequence of axonal spiking and light adaptation that could decrease synaptic gain as stimulus frequency increases and may also reduce energy use.

Increase in mitochondrial DNA mutations impairs retinal function and renders the retina vulnerable to injury

Mouse models that accumulate high levels of mitochondrial DNA (mtDNA) mutations owing to impairments in mitochondrial polymerase γ (PolG) proofreading function have been shown to develop phenotypes consistent with accelerated aging. As increase in mtDNA mutations and aging are risk factors for neurodegenerative diseases, we sought to determine whether increase in mtDNA mutations renders neurons more vulnerable to injury. We therefore examined the in vivo functional activity of retinal neurons and their ability to cope with stress in transgenic mice harboring a neural-targeted mutant PolG gene with an impaired proofreading capability (Kasahara, et al. (2006) Mol Psychiatry11(6):577-93, 523). We confirmed that the retina of these transgenic mice have increased mtDNA deletions and point mutations and decreased expression of mitochondrial oxidative phosphorylation enzymes. Associated with these changes, the PolG transgenic mice demonstrated accelerated age-related loss in retinal function as measured by dark-adapted electroretinogram, particularly in the inner and middle retina. Furthermore, the retinal ganglion cell-dominant inner retinal function in PolG transgenic mice showed greater vulnerability to injury induced by raised intraocular pressure, an insult known to produce mechanical, metabolic, and oxidative stress in the retina. These findings indicate that an accumulation of mtDNA mutations is associated with impairment in neural function and reduced capacity of neurons to resist external stress in vivo, suggesting a potential mechanism whereby aging central nervous system can become more vulnerable to neurodegeneration.

The relationship between neurotrophic factors and CaMKII in the death and survival of retinal ganglion cells

The scientific discourse relating to the causes and treatments for glaucoma are becoming reflective of the need to protect and preserve retinal neurons from degenerative changes, which result from the injurious environment associated with this disease. Knowledge, in particular, of the signal transduction pathways which affect death and survival of the retinal ganglion cells is critical to this discourse and to the development of a suitable neurotherapeutic strategy for this disease. The goal of this chapter is to review what is known of the chief suspects involved in initiating the cell death/survival pathways in these cells, and what still remains to be uncovered. The least controversial aspect of the subject relates to the potential role of neurotrophic factors in the protection of the retinal ganglion cells. On the other hand, the postulated triggers for signaling cell death in glaucoma remain controversial. Certainly, the restricted flow of neurotrophic factors has been cited as one possible trigger. However, the connections between glaucoma and other factors present in the retina, such as glutamate, long held to be a prospective culprit in retinal ganglion cell death are still being questioned. Whatever the outcome of this particular debate, it is clear that the downstream intersections between the cell death and survival pathways should provide important foci for future studies whose goal is to protect retinal neurons, situated as they are, in the stressful environment of a cell destroying disease. The evidence for CaMKII being one of these intersecting points is discussed.

Calcium/calmodulin-dependent protein kinase II regulates the phosphorylation of CREB in NMDA-induced retinal neurotoxicity

We examined the role of the phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) and cyclic AMP-response element binding protein (CREB) in N-methyl-d-aspartate (NMDA)-induced neurotoxicity in the rat retina. Western blot analysis showed early elevation of phosphorylated CaMKII (p-CaMKII) protein levels and subsequential elevation of phosphorylated CREB (p-CREB) protein after NMDA injection. Immunohistochemistry showed that p-CaMKII was colocalized with Thy-1-positive retinal ganglion cells (RGCs) after NMDA injection. The increase in the p-CaMKII protein level was significantly inhibited by the preinjection of CaMKII small interfering RNA (siRNA), whereas negative control siRNA did not affect. Moreover, the increase in the p-CREB protein level after NMDA injection was also prevented by preinjection of CaMKII siRNA. In addition, our morphometric study of neurotracer retrograde labeling and Thy-1-positive cells showed that CaMKII siRNA significantly accelerated NMDA-induced RGC loss. Furthermore, the prevention of CREB binding by CRE decoy oligonucleotide also exacerbated RGC loss. These results suggest that the activation of CaMKII may regulate CREB phosphorylation and that the transient phosphorylation of CaMKII and CREB may be a neuroprotective response against NMDA-induced neurotoxicity.

Molecular Mechanism of Learning and Memory Based on the Research for Ca2+/Calmodulin-dependent Protein Kinase II

In the central nervous system (CNS), the synapse is a specialized junctional complex by which axons and dendrites emerging from different neuron intercommunicates. Changes in the efficiency of synaptic transmission are important for a number of aspects of neural function. Much has been learned about the activity-dependent synaptic modifications that are thought to underlie memory storage, but the mechanism by which these modifications are stored remains unclear. Thus, it is important to find and characterize "memory molecules," and "memory apparatus or memory forming apparatus." A good candidate for the storage mechanism is Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II). CaM kinase II is one of the most prominent protein kinases, present in essentially every tissue but most concentrated in the brain. Neuronal CaM kinase II regulates important neuronal functions, including neurotransmitter synthesis, neurotransmitter release, modulation of ion channel activity, cellular transport, cell morphology and neurite extension, synaptic plasticity, learning and memory, and gene expression. Studies concerning this kinase open a door of the molecular basis of nerve function, especially learning and memory, and indicate one direction for the studies in the field of neuroscience. This review presents molecular structure, properties and functions of CaM kinase II, as a major component of neuron, which are mainly developed in our laboratory.

Neuronal Ca2+/calmodulin-dependent protein kinase II--discovery, progress in a quarter of a century, and perspective: implication for learning and memory

Much has been learned about the activity-dependent synaptic modifications that are thought to underlie memory storage, but the mechanism by which these modifications are stored remains unclear. A good candidate for the storage mechanism is Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). CaM kinase II is one of the most prominent protein kinases, present in essentially every tissue but most concentrated in brain. Although it has been about a quarter of a century since the finding, CaM kinase II has been of the major interest in the region of brain science. It plays a multifunctional role in many intracellular events, and the expression of the enzyme is carefully regulated in brain regions and during brain development. Neuronal CaM kinase II regulates important neuronal functions, including neurotransmitter synthesis, neurotransmitter release, modulation of ion channel activity, cellular transport, cell morphology and neurite extension, synaptic plasticity, learning and memory, and gene expression. Studies concerning this kinase have provided insight into the molecular basis of nerve functions, especially learning and memory, and indicate one direction for studies in the field of neuroscience. This review presents the molecular structure, properties and functions of CaM kinase II, as a major component of neurons, based mainly developed on findings made in our laboratory.

Calmodulin gene expression in the neural retina of the adult rat

Calmodulin (CaM) mRNAs are expressed with low abundancy in the adult rat neural retina. However, when digoxigenin (DIG)-labeled cRNA probes specific for each CaM mRNA population were hybridized at slightly alkaline pH (pH 8.0), the widespread distribution of CaM mRNA-expressing cells was revealed, with similar abundance for all three CaM genes. The CaM genes displayed a uniquely similar, layer-specific expression throughout the retina, and no significant differences were found in the distribution patterns of the CaM mRNA populations or the labeled cell types. The strongest signal for all CaM mRNAs was demonstrated in the ganglion cell layer and the inner nuclear layer, where the highest signal intensity was found within the inner sublamina. Similarly intermediate signal intensities for all CaM genes were detected in the inner and outer plexiform layers, within the vicinity of the outer limiting membrane and in the retinal pigment epithelium. A very low specific signal was characteristic in the outer nuclear layer and the photoreceptor inner segment layer, while no specific hybridization signal was observed in the photoreceptor outer segment layer. In summary, all CaM genes exhibited a similar and a characteristically layer-specific expression pattern in the adult rat retina.

Differential expression and cellular localization of doublecortin in the developing rat retina

Doublecortin is 40 kDa microtubule-associated phosphoprotein required for neuronal migration and differentiation in various regions of the developing central nervous system. We have investigated the expression and cellular localization of doublecortin in the developing rat retina using immunocytochemistry and Western blot analysis. The expression of doublecortin was high from embryonic day 18 (E18) until E20 and was low during the postnatal period. The doublecortin immunoreactivity first appeared in a few radially orientated cells in the mantle zone of the primitive retina at E15. From E16 onward, the immunoreactivity appeared in two different regions: the inner part of the retina and middle of the neuroblastic layer. In the inner part, the somata of cells in the ganglion cell layer, in the distal row of the neuroblastic layer and profiles in the inner plexiform layer showed doublecortin immunoreactivity up to postnatal day 1 (P1). Afterwards, the doublecortin immunoreactivity persisted in the inner plexiform layer until P15, although the intensity decreased gradually with the maturation of the retina. In the middle of the neuroblastic layer, doublecortin immunoreactivity appeared in the radially orientated cells. These cells transformed into horizontal cells. The doublecortin immunoreactivity persisted in these cells up to P21. Given these results, doublecortin may play an important role in the migration and differentiation of specific neuronal populations in developmental stages of the rat retina.

Developmentally regulated expression of CaMKII and iGluRs in the rat retina

Calcium/calmodulin-dependent protein kinase II (CaMKII) and the ionotropic glutamate receptors (iGluRs) have been shown to be pivotal in the maturation of synapses during development of the central nervous system. The purpose of the current study was to assay the expression profiles of these molecules during the development of the rat retina. The mRNA levels of CaMKII were determined by the semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) method. The protein levels of CaMKII were assayed in slot blots. The CaMKII enzyme activity was also measured. In addition, the protein levels of iGluRs in a retinal membrane-enriched fraction were evaluated in Western blots. The results show that the levels of CaMKII (mRNA, protein, and activity) and distinct subunits of iGluR proteins increased during the first 2 weeks after birth. The highest level of CaMKII was reached during the second postnatal week, coincident with the peak of synaptogenesis in the inner plexiform layer of the rat retina. The expressions of NMDAR-NR1 and -NR2A were relatively low in the first postnatal week but rose quickly thereafter. However, NMDAR-NR2B was relatively high at postnatal day 5 (P5) and increased steadily during the postnatal period. Thus, the subunit compositional profile of the retinal NMDARs was altered during retinal maturation. The developmental pattern of AMPAR-GluR1 was similar to that of NMDAR-NR2B, with high expression at P5, and modest increases thereafter. The patterns of CaMKII and NR1/NR2A were better correlated than were CaMKII and NR2B, or CaMKII and GluR1. The temporal differences in subunit expression of these synaptically relevant molecules suggest that they play distinct roles during the development of the retina.

Visual-mediated regulation of retinal CaMKII and its GluR1 substrate is age-dependent

Previous studies have shown that multifunctional calcium/calmodulin-dependent protein kinase II (CaMKII) and one of its substrates, the glutamate receptor, are key players in experience-driven synaptic plasticity in several areas of the central nervous system (CNS). To determine if CaMKII and the glutamate receptor are regulated by visual activity in the retina, we compared dark-reared (DR; 1 week) rats with control rats raised in a diurnal light-dark cycle (LD), at the following ages: postnatal day 12 (P12d), 2-month (2m) and 6-month (6m) old. The mRNA levels of CaMKIIalpha and beta were determined by a competitive reverse transcription polymerase chain reaction (competitive RT-PCR) method. The protein levels of these two subunits were evaluated by immunoblots. The data show that the mRNAs for CaMKIIalpha and beta were increased about 8-fold and 10-fold, respectively, in the retinae of DR P12d rats. As for the proteins, 2- and 2.6-fold elevations for CaMKIIalpha and beta, respectively, were evident. The GluR1 subunit of the AMPAR (AMPAR-GluR1) was also evaluated in antibody-treated blots and found to be increased about 2-fold after 1 week of dark rearing in the retinae of P12d rats. This type of experience-driven molecular change was age-dependent, showing less increase in 2m old rats and not present in 6m old rats. Returning DR 2m old rats to the LD environment for 1 week was sufficient to restore the dark-induced changes to the levels of the age-matched LD controls. Based on the data, a theoretical model for activity-dependent modulation of the developing retinal synapses is proposed.

Localization of calcineurin in the mature and developing retina

We studied the localization of calcineurin by immunoblotting analysis and immunohistochemistry as a first step in clarifying the role of calcineurin in the retina. Rat, bovine, and human retinal tissues were examined with subtype-nonspecific and subtype-specific antibodies for the A alpha and A beta isoforms of its catalytic subunit. In mature retinas of the three species, calcineurin was localized mainly in the cell bodies of ganglion cells and the cells in the inner nuclear layer, in which amacrine cells were distinctively positive. The calcineurin A alpha and A beta isoforms were differentially localized in the nucleus and the cytoplasm of the ganglion cell, respectively. Calcineurin was also present in developing rat retinas, in which the ganglion cells were consistently positive for it. The presence of calcineurin across mammalian species and regardless of age shown in the present study may reflect its importance in visual function and retinal development, although its function in the retina has not yet been clarified. (J Histochem Cytochem 49:187-195, 2001)

Neuroprotective effect of AIP on N-methyl-D-aspartate-induced cell death in retinal neurons

Excessive activation of glutamate receptors mediates neuronal death, but the intracellular signaling pathways that mediate this type of neuronal death are only partly understood. Previously, we have demonstrated that calcium/calmodulin-dependent protein kinase II-alpha(B) (CaMKII-alpha(B)) containing a nuclear localizing signal but not CaMKII-alpha is altered in retinal neurons exposed to N-methyl-D-aspartate (NMDA). The present study describes a prospective function of CaMKII-alpha(B) in signal transduction leading to apoptosis. The terminal deoxyribonucleotidyl transferase (TdT)-mediated biotin-16-dUTP nick-end labelling (TUNEL) method was used to detect fragmented DNA in fixed tissue sections of rat retina. The TUNEL assay confirmed that cell death occurs in the inner nuclear and ganglion cell layers following injection of 4 mM NMDA. A specific AIP (myristoylated autocamtide-2-related inhibitory peptide) with proven cell permeability inhibits CaMKII activity in vivo. Neuroprotection achieved by 500 microM AIP was complete when administered 2 h before and coincident with the NMDA application. Additionally, 100 microM of AIP protects only partially against the NMDA-induced excitotoxicity. The conformationally active fragment of caspase-3 (17 kDa), known to be involved in neuronal apoptosis was apparent within 30 min and at 2 h postinjection with NMDA. This activation was inhibited by 500 microM AIP when administered 2 h before and coincident with the NMDA application. The results suggest that CaMKII-alpha(B) isoform plays a role in excitotoxicity-induced neuronal apoptosis.

Photoreceptor regulated expression of Ca(2+)/calmodulin-dependent protein kinase II in the mouse retina

The objective of this investigation is to determine mechanisms for regulation of retinal calmodulin kinase II (CaMKII). To this end, the expression and activity of CaMKII are examined in the retina of the rdta mouse, in which rod photoreceptors have been genetically ablated [47]. CaMKII levels are compared between rdta mice and the normal, littermate control mice. It is demonstrated that retinal CaMKII protein, enzyme activity and mRNA are significantly increased in response to the genetic ablation of rod photoreceptors. The data indicate that CaMKII expression/activity in amacrine and ganglion cells is negatively regulated by the rod photoreceptor-mediated visual input. The regulation appears to occur primarily at the transcriptional level. It is shown that the cytoplasmic polyadenylation element binding protein (CPEB), a regulatory factor for translation that is known to promote CaMKIIalpha translation in dendrites [83], is also present in the mouse retina. However, the polyadenylation-mediated translational control mechanism is not activated in this experimental paradigm.

Activation of Ca2+--calmodulin kinase II induces desensitization by background light in dogfish retinal 'on' bipolar cells

Retinal 'on' bipolar cells possess a metabotropic glutamate receptor (mGluR6) linked to the control of a G-protein and cGMP-activated channels which functions to generate high synaptic amplification of rod signals under dark-adapted conditions. Desensitization of 'on' bipolar cells is initiated by a rise in Ca2+ during background light too weak to adapt rod photoreceptors. Desensitization could also be elicited by raising intracellular Ca2+ above 1 microM. In order to investigate the mechanism of desensitization, whole-cell current responses to brief flashes and to steps of light were obtained from voltage-clamped 'on' bipolar cells in dark-adapted dogfish retinal slices. The inclusion of Ca2+-calmodulin kinase II (CaMKII) inhibitor peptides in the patch pipette solutions not only blocked desensitization of 'on' bipolar cells by dim background light and by 50 microM Ca2+, but also increased their flash sensitivity. The substrate of phosphorylation by CaMKII is the 'on' bipolar cell cGMP-activated channels. Desensitization probably results from a reduction in their sensitivity to cGMP and a voltage-dependent decrease in their conductance. A role for protein kinase C (PKC) in this process was excluded since activating PKC independently of Ca2+ with the phorbol ester PMA failed to induce desensitization of 'on' bipolar cells.

Calcium/calmodulin-dependent protein kinase II containing a nuclear localizing signal is altered in retinal neurons exposed to N-methyl-D-aspartate

This study investigated N-methyl-D-aspartate (NMDA) mediated cell death and its possible regulation by calcium/calmodulin-dependent protein kinase II (CaMKII) in the adult rat retina. To investigate cell death, the terminal deoxyribonucleotidyltransferase (TdT)-mediated biotin-16-dUTP nick-end labelling (TUNEL) method was used to detect fragmented DNA in fixed tissue sections of rat retina. The TUNEL assay confirmed that apoptosis occurs in the inner nuclear layer (INL) and ganglion cell layer (GCL) following NMDA injection. The level of antibody binding to CaMKII-alpha, the activity of CaMKII, and the mRNA level for the alpha(B) subunit of CaMKII were found to be elevated for short time periods (30 min, 2 h) after a single intravitreal injection of NMDA. In contrast to this, there was a decrease in CaMKII activity and in the CaMKII-alpha(B) mRNA levels at longer time periods (24 h) following injection of NMDA. These effects were specific for the mRNA for the alpha(B) subunit, an alternatively spliced product of the CaMKII-alpha gene, that contains a nuclear localizing signal (NLS) known to target this protein to the nucleus. It is suggested that regulated expression of CaMKII-alpha(B) could be involved in the NMDA-mediated cell death in retinal neurons.

The expression of Ca2+/calmodulin-dependent protein kinase I in rat retina is regulated by light stimulation

Ca2+/calmodulin-dependent protein kinase I (CaM-kinase I) in rat retina was analyzed by immunohistochemical analysis, Western blot analysis and kinase activity assay. Western blot analysis revealed two immunoreactive bands similar to those detected in the brain. Developmental studies revealed that CaM-kinase I expression increased in accordance with postnatal development. Expression of CaM-kinase I in the retinas of rats raised in the complete darkness markedly decreased. CaM-kinase I activity assay supported these findings. Synapsin I was shown to be a possible intrinsic substrate of CaM-kinase I in rat retina. These results elucidated that CaM-kinase I is expressed in the retina and may play an important role in the retinal functions and that the expression of CaM-kinase I is regulated by light stimulation.

Regulation of calcium/calmodulin-dependent protein kinase II in the adult rat retina is mediated by ionotropic glutamate receptors

This study is concerned with the transmitter-mediated regulation of the alpha(50 kDa) and beta(60 kDa) subunits of calcium calmodulin dependent protein kinase II (CamKII) in the adult rat retina. The level of antibody binding to the CamKII and the activity of CamKII were found to be increased after intravitreal injection of glutamate. Changes in the levels of the antibody-binding to the subunits of CamKII were observed in different subcellular fractions of the retina with a maximum response observed in crude synaptic membrane fractions. The glutamate mediated increases in CamKII were specific and blocked by 3,5-Dimethyl-1 adamantanamine; 3,5-Dimethylamantadine (Memantine), (+/-) 2-Amino-5-Phosphopentonic (AP-5) and 6-Cyano-7-Nitroquinoxaline-2,3-Dione (CNQX) but not with dl -2-Amino-3-Phosphono-Propionic (AP-3). The results indicate that the retinal neurotransmitter, glutamate, can regulate retinal CamKII activity through ionotropic but not metabotropic glutamate receptors. NMDA-receptors were found to be necessary but insufficient to stimulate CamKII. A model in which cooperative interaction between NMDA and non-NMDA glutamate receptors/ion channels is presented to explain the glutamate stimulated increases in CamKII activity in the retina.

CREB-induced transcriptional activation depends on mGluR6 in rod bipolar cells

To investigate the molecular mechanisms of stimuli-induced transcriptional activation in neuronal cells, we have investigated the light-induced gene expression in the neural retina of rats. The immunoreactivity for phosphorylated cAMP responsive element binding protein (PCREB-IR) was expressed in the outer half of the inner nuclear layer (INL) and the ganglion cell layer (GCL) after 5 min exposure to steady light also in mice. In addition to these cells, PCREB-IR was also detected in the inner border of the INL after 5 min exposure to flashing light. Both steady and flashing lights induced c-fos mRNA in the same types of cells as the PCREB-IR-positive cells. Majority of PCREB immunoreactive nuclei in the outer half of the INL were also immunopositive for anti-protein kinase C alpha (PKC alpha), a marker of rod bipolar cells, while CaM kinase IV immunoreactivity was not detected in these cells. PCREB-IR and c-fos gene expression in the PKC alpha positive rod bipolar cells were lost in mice lacking metabotropic glutamate receptor 6 (mGluR6). Thus, we propose that the transcriptional response of CREB to light stimulation in rod bipolar cells is regulated via mGluR6.

Regional differences between the immunohistochemical distribution of Ca2+/calmodulin-dependent protein kinase II alpha and beta isoforms in the brainstem of the rat

The distribution of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) alpha and beta isoforms in the brainstem of adult rats was investigated using an immunohistochemical method with two monoclonal antibodies which specifically recognize the alpha and beta isoform, respectively. We found that these isoforms were differentially expressed by neurons in the substantia nigra, red nucleus, dorsal cochlear nucleus, pontine nuclei and inferior olivary nucleus. Neurons in the inferior olivary nucleus express the alpha isoform, but not the beta isoform. In contrast, neurons in the substantia nigra, red nucleus and pontine nuclei were immunostained with the beta antibody, but not with the alpha antibody. In the dorsal cochlear nucleus, neurons in layers I and II were alpha-immunopositive, whereas neurons in layers III and IV were beta-immunopositive. Therefore, the distribution of the CaM kinase II alpha-immunopositive neurons is completely different from that of CaM kinase II beta-immunopositive neurons. Next we examined the possible coexistence of CaM kinase II alpha isoform and glutamate or that of CaM kinase II beta isoform and glutamic acid decarboxylase (GAD) in the single neuron by double immunofluorescence labelling using a pair of anti-alpha and anti-glutamate antibodies, or a pair of anti-beta and anti-GAD antibodies. The results indicated that neurons expressing anti-alpha immunoreactivity were also immunopositive against anti-glutamate antibody, and neurons expressing beta isoform were also immunopositive against anti-GAD antibody, suggesting that alpha-immunopositive neurons are classified as excitatory-type neurons, and on the contrary, beta-immunopositive neurons are classified as inhibitory-type neurons. In conclusion, the present study confirmed that alpha- and beta-isoforms of CaM kinase II are differentially expressed in the nuclei of the brainstem and have different roles.

Ca(2+)-dependency of spinule plasticity at dendrites of retinal horizontal cells and its possible implication for the functional role of spinules

Calcium is involved in many aspects of synaptic plasticity and we have analyzed its involvement in spinule dynamics at retinal horizontal cell dendrites. We show here that in particular the retraction of spinules is a Ca(2+)-dependent process. Inhibiting calmodulin or CaMKII, blocked the retraction that was also impaired in low calcium Ringer. Changes of the cytosolic Ca(2+)-concentration through depletion of internal Ca(2+)-stores were without effect. This suggested that Ca(2+)-influx during dark adaption and subsequent activation of CaMKII is an important step for spinule retraction. Voltage dependent Ca(2+)-channels were not responsible for the Ca(2+)-influx, rather Ca2+ leaking through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-gated channels. This suggested a close local link between AMPA/kainate receptors and CaMKII indicating a possible postsynaptic function of spinules. The distribution of bound, omega-shaped vesicles within the cone pedicles and its dependence on artificial depolarization further supported the idea of a postsynaptic function of spinules.

Immunohistochemical localization of Ca2+/calmodulin-dependent protein kinase type IV in the mature and developing rat retina

The localization of Ca2+/calmodulin-dependent protein kinase IV (CaM kinase IV) in the mature and developing rat retina was examined by immunohistochemistry and in situ hybridization histochemistry. In immunoblotting analysis, a single band of 63 kDa was detected in the crude homogenate of the adult rat retina, indicating the presence of the alpha polypeptide of CaM kinase IV. In the adult rat retina, most of the bipolar cells and some ganglion cells exhibited CaM kinase IV-immunoreactivity. By immunoelectron microscopy, the immunoreactive product was predominantly localized to the nucleus of immunoreactive cells. In the developing rat retina, immunoreactive bipolar cells were first detected on postnatal day 10 (P10), and they were abundant on P14. All these findings suggest that CaM kinase IV may participate in some yet unknown nuclear Ca(2+)-relating visual signal-processing of the retina.

The Ca2+/calmodulin-dependent protein kinase II-associated protein complex isolated from chicken retina

Retinal cytosolic Ca2+/calmodulin-dependent protein kinase II (CaM KII) was isolated from hatched 6-wk chicken retinae by ultracentrifugation and affinity chromatography using calmodulin (CaM) and anti-CaM KII-alpha columns. Samples from different fractions were examined with SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and silver staining or immunoblotting. Comparisons were made between the final antibody affinity eluates from retina and forebrain. Silver-stained gels showed that multiple proteins were present in the antibody affinity eluates from retina, including major proteins of 178, 56, and 45 kDa and several minor proteins. Immunoblots revealed that CaM KII-alpha was present in eluates from the retina and forebrain. CaM KII-beta was present in the antibody eluate from forebrain but not retina. The latter subunit was present in the crude homogenates of the retina. Regarding the antibody eluate from retina, the possibility that the major 56 kDa protein was tubulin was ruled out, but protein tau (tau) and synapsin I were present. The presence of multiple proteins in the antibody affinity eluate indicates that these proteins were coisolated in a CaM KII-alpha-associated protein complex. The finding that protein tau and synapsin I are associated with retinal CaM KII provides further insight into the mechanisms underlying the function of the kinase in this tissue. The lack of cytosolic CaM KII-beta subunit in the antibody affinity eluate from retina is indicative of a brain region-specificity in subunit composition of the kinase.

Retraction of spinule-type neurites from carp retinal horizontal cell dendrites during dark adaptation involves the activation of Ca2+/calmodulin-dependent protein kinase II

The formation of spinules at the terminal dendrites of retinal horizontal cells with the onset of light and their subsequent retraction during darkness is a remarkable example of synaptic plasticity where sensory experience modifies reversibly, and on a time scale of minutes the ultrastructure of synaptic connectivity. The signals and the subsequent intracellular cascades underlying the prominent morphological alterations are only partially understood. We show here that lowering the external calcium concentration did prevent dark- and AMPA-induced retraction of spinules in a eyecup preparation. Furthermore, spinule retraction was prevented in vivo by the injection of calmidazolium, an inhibitor of calmodulin, into the eyeball, and also by the injection of KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase (CaMkII). We conclude that local Ca2+ influx through AMPA-gated channels followed by activation of CaMkII is an important step for spinule retraction during dark adaptation. The phosphorylation patterns of phosphoproteins derived from purified horizontal cells was affected by the inhibitors of calmodulin and CaMkII respectively. Some of the affected phosphoproteins appeared to be cytoskeleton-associated proteins, including GAP-43. Based on these observations, a putative scenario for the retraction of spinules is proposed.

Onset of expression of the alpha subunit of Ca2+/calmodulin-dependent protein kinase II and a novel related protein in the developing retina

Calcium-calmodulin-dependent protein kinase II is an abundant protein in the nervous system and has been associated with many aspects of neuronal function, including events related to synaptic transmission. The purpose of this study is to correlate the onset of expression of this kinase with a specific developmental event in retinal morphogenesis using a monoclonal antibody to the 50-kDa alpha-subunit. Microscopy showed the antigen to be associated with the plexiform layers of the retina. Western blots demonstrated that the onset of expression of the alpha-subunit coincided in time with the initial formation of the plexiform layers. However, the onset of expression of the 50-kDa alpha-subunit was preceded by the earlier embryonic appearance of a related 82.5-kDa antigen that was recognized by the antibody. The amount of this latter protein declined as the amount of the alpha-subunit increased in retinal homogenates. Although this related 82.5 kDa protein disappeared from blots of retinal homogenates after embryonic d 14, it could be detected in concentrated supernatant fractions isolated from the retinae of hatched chicks. Microscopy showed that a subset of retinal cells and their processes contained this antigen in early embryonic chicks. Finally, the 50 kDa alpha-subunit of kinase II and the 82.5 kDa novel antigen were shown to be separable by differential centrifugation.

Specific distribution of Ca2+/calmodulin-dependent protein kinase II alpha and beta isoforms in some structures of the rat forebrain

The immunohistochemical distribution of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) alpha and beta isoforms in the rat forebrain was examined by using monoclonal antibodies specific to each isoform. The present study confirmed that alpha and beta immunoreactivities are localized only in neuronal elements. At the light microscopic level, specific distribution patterns of these isoforms and staining characteristics were recognized in some regions of the forebrain as follows. Firstly, alpha-immunoreactive neurons were more homogeneously distributed throughout the cellular layers of the cerebral cortex (i.e., layers II-VI) than beta-immunoreactive ones. Secondly, neurons in the globus pallidus were immunostained by the anti-beta antibody, but not by the anti-alpha antibody. Thirdly, neurons in the medial habenular nucleus, the subthalamic nucleus and the reticular thalamic nucleus were more densely stained with the anti-beta antibody than with the anti-alpha antibody. However, marked differences were not observed in the hippocampal formation at the light microscopic level. The electron microscopic analysis of the cerebral cortex demonstrated that subcellular localizations of alpha- and beta-immunoreactive products within the cortical neurons were quite dissimilar: (i) the nucleus was stained only with the anti-alpha antibody, but not with the anti-beta antibody, and (ii) beta-immunoreactive products were more sporadically localized in the cytoplasms of the perikarya and dendrites than the alpha-immunoreactive ones. These regional and subcellular differences between the distribution patterns of alpha and beta immunoreactivities suggest the functional diversity of CaM kinase II alpha and beta isoforms in the central nervous system.

Purification and characterization of the Ca2+/calmodulin-dependent protein kinase II from chicken forebrain

CaM kinase II is known to be enriched in mammalian and avian brains. To determine the holoenzymic composition and functional characteristics of this kinase, a new approach for isolation was applied to isolate it from the chicken forebrain. Forebrains of hatched 45-d chicken were dissected, homogenized, and centrifuged. The supernatant was loaded onto a CaM-agarose affinity column and the calmodulin-binding proteins were eluted with EGTA. Selected eluates were loaded onto the antibody-agarose affinity column, which was prepared with monoclonal antibody (MAb) (6G9) to the CaM kinase II alpha subunit. Samples were subjected to SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and either silver-stained or blotted onto a nitrocellulose membrane. The protein composition and the immunoreactivity of the antibody-agarose affinity eluate fractions were analyzed with a densitometric scanner. Silver staining of gels showed that the beta subunit doublet, the beta' subunit, and a putative substrate were coeluted with the alpha subunit from the antibody affinity column although only the alpha subunit bound the 6G9 antibody. Scintillation counting showed that the autophosphorylation of the kinase was significantly reduced in the eluate from the antibody affinity column. Whereas silver staining indicated an increase in the relative amount of alpha subunit had occurred during purification, phosphorylation assays indicated an increase in the relative amount of the alpha subunit after the last purification step. A possible reason for this is discussed. The presence of beta/beta' subunits in the antibody-agarose affinity eluate indicated the existence of an alpha beta/beta' heteropolymer. The phosphorylation assay was not a good indication of the amount of purification because of the loss of enzyme activity following purification. In contrast, the immunoassay indicated a 97-fold purification from the cytosolic fraction was achieved using the method. In conclusion, the data indicate the existence of the CaM kinase II alpha beta/beta' heteropolymer in the chicken forebrain.

Immunocytochemical localization of calcium/calmodulin-dependent protein kinase II isoforms in the ganglion cells of the rat retina: immunofluorescence histochemistry combined with a fluorescent retrograde tracer

To determine whether or not calcium/calmodulin-dependent protein kinase II (CaM kinase II) is localized in the ganglion cells in the rat retina, we labeled ganglion cells by injection of Fast blue (FB) into the lateral geniculate nucleus and then stained the retina immunohistochemically with monoclonal antibodies which react specifically with the alpha and beta isoforms of CaM kinase II. Eighty and 90% of the FB-labeled ganglion cells in the ganglion cell layer were immunoreactive with the alpha and beta antibodies, respectively, suggesting that both alpha and beta isoforms of CaM kinase II are expressed in most ganglion cells which project to the lateral geniculate nucleus.