Contribution of L‐type channels to Ca 2+ regulation of neuronal properties in early developing Purkinje neurons

Promotion of Dendritic Differentiation of Cerebellar Purkinje Cells by Ca2+/calmodulin-dependent Protein Kinase IIα, IIβ and IV and Possible Involvement of CREB Phosphorylation

Cerebellar Purkinje cells develop the most elaborate dendritic trees among neurons in the brain. To examine the role of Ca²⁺/calmodulin-dependent protein kinase (CaMK) IIα, IIβ and IV in the dendritic differentiation of Purkinje cells, we introduced siRNA against these CaMKs into Purkinje cells in cerebellar cell cultures using a single-cell electroporation technique. Single-cell electroporation enables us to transfer siRNA into specific cells within a heterogeneous cell population. In addition, we can easily and reliably transfer multiple types of siRNA into a cell simply by loading them together in one micropipette. Any one of the siRNA against CaMKIIα, IIβ and IV (single knockdown) or any combinations of two of the siRNA against these CaMKs (double knockdown) had no significant effects on the dendritic differentiation of Purkinje cells. However, the combination of all three siRNA against these CaMKs (triple knockdown) inhibited the branching of Purkinje cell dendrites. Furthermore, the triple knockdown reduced the phosphorylation of CREB in Purkinje cells. These findings suggest the promotion of dendritic differentiation of Purkinje cells by CaMKIIα, IIβ and IV and the possible involvement of phosphorylation of CREB as a common substrate of these CaMKs.

Infant and adult SCA13 mutations differentially affect Purkinje cell excitability, maturation, and viability in vivo

Mutations in KCNC3, which encodes the Kv3.3 K⁺ channel, cause spinocerebellar ataxia 13 (SCA13). SCA13 exists in distinct forms with onset in infancy or adulthood. Using zebrafish, we tested the hypothesis that infant- and adult-onset mutations differentially affect the excitability and viability of Purkinje cells in vivo during cerebellar development. An infant-onset mutation dramatically and transiently increased Purkinje cell excitability, stunted process extension, impaired dendritic branching and synaptogenesis, and caused rapid cell death during cerebellar development. Reducing excitability increased early Purkinje cell survival. In contrast, an adult-onset mutation did not significantly alter basal tonic firing in Purkinje cells, but reduced excitability during evoked high frequency spiking. Purkinje cells expressing the adult-onset mutation matured normally and did not degenerate during cerebellar development. Our results suggest that differential changes in the excitability of cerebellar neurons contribute to the distinct ages of onset and timing of cerebellar degeneration in infant- and adult-onset SCA13.

In vitro maturation of dopaminergic neurons derived from mouse embryonic stem cells: implications for transplantation

The obvious motor symptoms of Parkinson's disease result from a loss of dopaminergic neurons from the substantia nigra. Embryonic stem cell-derived neural progenitor or precursor cells, adult neurons and fetal midbrain tissue have all been used to replace dying dopaminergic neurons. Transplanted cell survival is compromised by factors relating to the new environment, for example; hypoxia, mechanical trauma and excitatory amino acid toxicity. In this study we investigate, using live-cell fluorescence Ca²⁺ and Cl⁻ imaging, the functional properties of catecholaminergic neurons as they mature. We also investigate whether GABA has the capacity to act as a neurotoxin early in the development of these neurons. From day 13 to day 21 of differentiation [Cl⁻]_i progressively dropped in tyrosine hydroxylase positive (TH⁺) neurons from 56.0 (95% confidence interval, 55.1, 56.9) mM to 6.9 (6.8, 7.1) mM. At days 13 and 15 TH⁺ neurons responded to GABA (30 µM) with reductions in intracellular Cl⁻ ([Cl⁻]_i); from day 21 the majority of neurons responded to GABA (30 µM) with elevations of [Cl⁻]_i. As [Cl⁻]_i reduced, the ability of GABA (30 µM) to elevate intracellular Ca²⁺ ([Ca²⁺]_i) did also. At day 13 of differentiation a three hour exposure to GABA (30 µM) or L-glutamate (30 µM) increased the number of midbrain dopaminergic (TH⁺ and Pitx3⁺) neurons labeled with the membrane-impermeable nuclear dye TOPRO-3. By day 23 cultures were resistant to the effects of both GABA and L-glutamate. We believe that neuronal susceptibility to amino acid excitotoxicity is dependent upon neuronal maturity, and this should be considered when isolating cells for transplantation studies.

Dynamic interplay of excitatory and inhibitory coupling modes of neuronal L-type calcium channels

L-type voltage-gated calcium channels (LTCCs) have long been considered as crucial regulators of neuronal excitability. This role is thought to rely largely on coupling of LTCC-mediated Ca(2+) influx to Ca(2+)-dependent conductances, namely Ca(2+)-dependent K(+) (K(Ca)) channels and nonspecific cation (CAN) channels, which mediate afterhyperpolarizations (AHPs) and afterdepolarizations (ADPs), respectively. However, in which manner LTCCs, K(Ca) channels, and CAN channels co-operate remained scarcely known. In this study, we examined how activation of LTCCs affects neuronal depolarizations and analyzed the contribution of Ca(2+)-dependent potassium- and cation-conductances. With the use of hippocampal neurons in primary culture, pulsed current-injections were applied in the presence of tetrodotoxin (TTX) for stepwise depolarization and the availability of LTCCs was modulated by BAY K 8644 and isradipine. By varying pulse length and current strength, we found that weak depolarizing stimuli tend to be enhanced by LTCC activation, whereas in the course of stronger depolarizations LTCCs counteract excitation. Both effect modes appear to involve the same channels that mediate ADP and AHP, respectively. Indeed, ADPs were activated at lower stimulation levels than AHPs. In the absence of TTX, activation of LTCCs prolonged or shortened burst firing, depending on the initial burst duration, and invariably augmented brief unprovoked (such as excitatory postsynaptic potentials) and provoked electrical events. Hence, regulation of membrane excitability by LTCCs involves synchronous activity of both excitatory and inhibitory Ca(2+)-activated ion channels. The overall enhancing or dampening effect of LTCC stimulation on excitability does not only depend on the relative abundance of the respective coupling partner but also on the stimulus intensity.

Somatic Ca2+ signaling in cerebellar Purkinje neurons

Activity-driven Ca(2+) signaling plays an important role in a number of neuronal functions, including neuronal growth, differentiation, and plasticity. Both cytosolic and nuclear Ca(2+) has been implicated in these functions. In the current study, we investigated membrane-to-nucleus Ca(2+) signaling in cerebellar Purkinje neurons in culture to gain insight into the pathways and mechanisms that can initiate nuclear Ca(2+) signaling in this neuronal type. Purkinje neurons are known to express an abundance of Ca(2+) signaling molecules such as voltage-gated Ca(2+) channels, ryanodine receptors, and IP3 receptors. Results show that membrane depolarization evoked by brief stimulation with K(+) saline elicits a prominent Ca(2+) signal in the cytosol and nucleus of the Purkinje neurons. Ca(2+) influx through P/Q- and L-type voltage-gated Ca(2+) channels and Ca(2+)-induced Ca(2+) release (CICR) from intracellular stores contributed to the Ca(2+) signal, which spread from the plasma membrane to the nucleus. At strong K(+) stimulations, the amplitude of the nuclear Ca(2+) signal exceeded that of the cytosolic Ca(2+) signal, suggesting the involvement of a nuclear amplification mechanism and/or differences in Ca(2+) buffering in these two cellular compartments. An enhanced nuclear Ca(2+) signal was more prominent for Ca(2+) signals elicited by membrane depolarization than for Ca(2+) signals elicited by activation of the metabotropic glutamate receptor pathway (mGluR1), which is linked to Ca(2+) release from intracellular stores controlled by the IP3 receptor.

mGluR1 agonists elicit a Ca 2+ signal and membrane hyperpolarization mediated by apamin-sensitive potassium channels in immature rat purkinje neurons

The type 1 metabotropic glutamate receptor (mGluR1) plays an import role in the synaptic physiology and development of cerebellar Purkinje neurons. mGluR1 expression occurs early in the developmental program of Purkinje neurons, at an age that precedes expression of the dendritic structure. Few studies have investigated the physiological response produced by mGluR1 activation in early-developing Purkinje neurons. To address this question, simultaneous recording of membrane potential and intracellular Ca(2+) was performed in immature cultured Purkinje neurons coupled with exogenous application of mGluR1 agonists. Membrane potential was measured using the perforated patch method of whole-cell recording, and intracellular Ca(2+) was measured using fura-2-based Ca(2+) imaging. Brief, 1-sec micropressure application of the group I mGluR-selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked a prominent Ca(2+) signal and coincident fast hyperpolarization in the immature neurons. The mGluR1-selective antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester blocked the Ca(2+) signal and fast hyperpolarization, confirming the involvement of mGluR1s. Amplitude of the fast hyperpolarization varied as a function of membrane potential and intracellular Ca(2+) and was blocked by apamin, an antagonist of the small-conductance Ca(2+)-activated K(+) channel (SK), identifying this K(+) channel as an underlying mechanism. In similar experiments with mature cultured Purkinje neurons, DHPG elicited a Ca(2+) signal, but fast membrane hyperpolarization was not evident. These results suggest that mGluR1 activation and the resulting release of Ca(2+) from intracellular stores and activation of SK channels may be a mechanism through which mGluR1 can modulate neuronal excitability of Purkinje neurons during early development.

Characterization of cochlear nucleus principal cells of Meriones unguiculatus and Monodelphis domestica by use of calcium-binding protein immunolabeling

Antibodies directed against calcium-binding proteins (CaBPs) parvalbumin, calbindin-D28k and calretinin were used as neuronal markers to identify and characterize different principal cell types in the mammalian cochlear nucleus. For this purpose, double immunofluorescence labeling and the combination of CaBP-labeling with pan-neuronal markers were applied to analyze the CaBPs distribution in neurons of the cochlear nucleus (CN) of the Mongolian gerbil (Meriones unguiculatus) and the gray short-tailed opossum (Monodelphis domestica). Despite of the fact, that these two mammalian species are not closely related, principal cell types in the CN of the two species showed many corresponding morphological features and similarities in immunolabeling of the CaBPs. Parvalbumin seems not to be suited as a differential neuronal marker in the CN since it is expressed by almost all neurons. In contrast, calbindin and calretinin were more restricted to specific cell types and showed a mostly complementary labeling pattern. As one of the most interesting findings, calbindin and calretinin were predominantly found in subpopulations of globular bushy cells and octopus cells in the ventral CN. Such a neuron-specific CaBP-expression in subpopulations of morphologically defined cell types argues for a more refined classification of CN cell types in Meriones and Monodelphis. Additionally, other cell types (cartwheel cells, unipolar brush cells, fusiform cells) were marked with calbindin or calretinin as well. Calretinin staining was predominantly observed in auditory nerve fibers and their endings including endbulbs of Held in Meriones. Spherical bushy cells showed a different calretinin-immunolabeling in Meriones and Monodelphis. This species-specific difference may be related to adaptive differences in auditory function.

Altered functional expression of Purkinje cell calcium channels precedes motor dysfunction in tottering mice

In tottering mice, a point mutation in the gene encoding P-type (Ca(v)2.1) voltage-gated calcium channels results in ataxia, absence epilepsy, and motor dystonia that appear 3-4 weeks postnatally. The aberrant motor behaviors have been linked to cerebellar dysfunction, and adult Purkinje cells (PCs) of tottering mice exhibit calcium-dependent changes in gene transcription suggestive of altered calcium homeostasis. In an attempt to identify early postnatal events important for the development of the behavioral phenotype, we examined calcium channel expression in cerebellar PCs from postnatal days 6-15 (P6-15). Whole cell recording was combined with selective calcium channel antagonists to allow discrimination of the various voltage-activated calcium channels types; early age-dependent differences between tottering and wild-type PCs were found. Wild-type PCs experienced a steady increase in P current density over this period, resulting in a twofold change by P15. In tottering, by contrast, P current density remained unchanged from P6-8 and was only 25% of the wild-type level by P8. A developmental delay in functional expression was implicated in this early deficit, since ensuing gains over the subsequent week brought tottering P current density close to the wild-type level by P15. At this age, tottering PCs also exhibited a 2.2-fold higher L-type calcium current density than that expressed by wild-type PCs. Increases in N current were apparent at some ages, most strikingly within a subset of tottering PCs at P15. Functional R- and T-type calcium current densities were equivalent to wild-type levels at all ages. We conclude that the tottering mutation brings about selective changes in functional calcium channel expression 1 to 2 weeks prior to the appearance of the behavioral deficits, raising the possibility that they represent an early, primary event along the path to motor dysfunction in tottering.

Developmental expression of Na+ currents in mouse Purkinje neurons

As Purkinje neurons mature during postnatal development, they change from electrically quiescent to active and exhibit high frequency spontaneous action potentials. This change in electrical activity is determined by both alteration in ion channel expression and the acquisition of synaptic input. To gain a better understanding of the development the intrinsic electrical properties of these neurons, acutely isolated Purkinje neurons from mice aged postnatal day 4 (P4) to P18 were examined. This included recording action potential frequency, threshold, height and slope, and input resistance and capacitance. Changes in a number of these properties were observed, suggesting significant changes in voltage-gated Na(+) currents. Because voltage-gated Na(+) currents, including the transient, resurgent and persistent currents, are known to play important roles in generating spontaneous action potentials, the developmental changes in these currents were examined. A large increase in the density of transient current, resurgent current and persistent current was observed at times corresponding with changes in action potential properties. Interestingly, the developmental up-regulation of the persistent current and resurgent current occurred at rate which was faster than the up-regulation of the transient current. Moreover, the relative amplitudes of the persistent and resurgent currents increased in parallel, suggesting that they share a common basis. The data indicate that developmental up-regulation of Na(+) currents plays a key role in the acquisition of Purkinje neuron excitability.

Developmental changes in Ca2+-regulated functions of early postnatal Purkinje neurons

Ca(2+) influx through L-type Ca(2+) channels regulates several different cellular processes in developing Purkinje neurons, including activation of transcription factors and expression of cellular proteins. In the current studies, we examined the age dependence of these actions of Ca(2+) during the early developmental period. Purkinje neurons acutely isolated from postnatal day 4-8 rat pups were studied. We also examined the sensitivity of the Ca(2+)-regulated processes to a toxic environmental factor (ethanol) known to show age-dependent actions on developing Purkinje neurons. Results show that Ca(2+) activation of the transcription factor cAMP-responsive element binding protein (CREB) and Ca(2+)-induced alterations in the level of the apoptotic enzyme caspase 3 show both dose and age dependence in the early-developing Purkinje neurons. Interestingly, the age dependence was opposite for the two proteins. Ca(2+) regulation of calbindin, a Ca(2+) binding protein, was dose dependent but showed little age dependence. Exposure to ethanol altered Ca(2+) activation of pCREB in an age-dependent manner but did not alter Ca(2+) regulation of caspase 3 or calbindin levels. Taken together, these results show that the downstream effects of Ca(2+) signaling have age-dependent components during early Purkinje neuron development. This age dependence may play an important role in the normal developmental program and could contribute to the critical window of sensitivity observed for certain toxic agents during early development.

L-type Ca2+ channels contribute to current-evoked spike firing and associated Ca2+ signals in cerebellar Purkinje neurons

The physiological properties of Purkinje neurons play a central role in their ability to regulate information transfer through the cerebellum. A number of ion channels contribute to Purkinje neuron physiology including an abundance of P-type Ca2+ channels, particularly in the dendritic region. Purkinje neurons also express L-type Ca2+ channels both during development and in the mature state. However, a role for L-type channels in Purkinje neuron physiology has yet to be fully defined. In the current study we used physiological recordings from cultured Purkinje neurons and the L-type Ca2+ channel agonist S-(-)-Bay K to assess a potential role for L-type Ca2+ channels in spike firing. Results show that Bay K alters current-evoked spike firing in young, immature Purkinje neurons without dendritic structure and in older, more mature Purkinje neurons with dendritic structure. Bay K also enhanced Ca2+ signals associated with the current-evoked spike firing. The effect of Bay K was more prominent in the young Purkinje neurons than in the older Purkinje neurons, suggesting that L-type Ca2+ channels may be more important in the Purkinje neuron physiology during the early stages of development rather than at mature stages. In the older Purkinje neurons, immunohistochemical studies using antibodies to L-type Ca2+ channels showed more intense immunolabeling in the somatic region than in the dendritic region. This result suggests that L-type Ca2+ channels may play a more important role in somatic physiology than dendritic physiology, whereas P-type channels may play a more important role in dendritic physiology.

Aminoglycoside-induced degeneration of adult spiral ganglion neurons involves differential modulation of tyrosine kinase B and p75 neurotrophin receptor signaling

Aminoglycoside antibiotics induce sensorineural hearing loss by destroying hair cells of the organ of Corti, causing progressive secondary degeneration of primary auditory or spiral ganglion neurons (SGNs). Recent studies show that the p75 neurotrophin receptor (NTR) is aberrantly up-regulated under pathological conditions when the neurotrophin receptor tyrosine kinases (Trks) are presumptively down-regulated. We provide in vivo evidence demonstrating that degenerating SGNs induced an augmented p75NTR expression and a coincident reduction of TrkB expression in their peripheral processes. Nuclear transcription factors c-Jun and cyclic AMP response element-binding protein phosphorylated by p75NTR- and TrkB-activated signal pathways, respectively, also showed a corresponding differential modulation, suggesting an activation of apoptotic pathways, coupled to a loss of pro-survival neurotrophic support. Our findings identified brain-derived neurotrophic factor (BDNF) expression in hair and supporting cells of the adult cochlea, and its loss, specifically the mature form, would impair TrkB-induced signaling. The precursor of BDNF (pro-BDNF) is differentially cleaved in aminoglycoside-deafened cochleae, resulting in a predominant up-regulation of a truncated form of pro-BDNF, which colocalized with p75NTR-expressing SGN fibers. Together, these data suggest that an antagonistic interplay of p75NTR and TrkB receptor signaling, possibly modulated by selective BDNF processing, mediates SGN death in vivo.