Homeostatic compensation maintains Ca2+ signaling functions in Purkinje neurons in the leaner mutant mouse

Physiology of Dystonia: Animal Studies

Dystonia is currently ranked as the third most prevalent motor disorder. It is typically characterized by involuntary muscle over- or co-contractions that can cause painful abnormal postures and jerky movements. Dystonia is a heterogenous disorder-across patients, dystonic symptoms vary in their severity, body distribution, temporal pattern, onset, and progression. There are also a growing number of genes that are associated with hereditary dystonia. In addition, multiple brain regions are associated with dystonic symptoms in both genetic and sporadic forms of the disease. The heterogeneity of dystonia has made it difficult to fully understand its underlying pathophysiology. However, the use of animal models has been used to uncover the complex circuit mechanisms that lead to dystonic behaviors. Here, we summarize findings from animal models harboring mutations in dystonia-associated genes and phenotypic animal models with overt dystonic motor signs resulting from spontaneous mutations, neural circuit perturbations, or pharmacological manipulations. Taken together, an emerging picture depicts dystonia as a result of brain-wide network dysfunction driven by basal ganglia and cerebellar dysfunction. In the basal ganglia, changes in dopaminergic, serotonergic, noradrenergic, and cholinergic signaling are found across different animal models. In the cerebellum, abnormal burst firing activity is observed in multiple dystonia models. We are now beginning to unveil the extent to which these structures mechanistically interact with each other. Such mechanisms inspire the use of pre-clinical animal models that will be used to design new therapies including drug treatments and brain stimulation.

The dendrite arbor of Purkinje cells is altered following to tail regeneration in the leopard gecko

Purkinje cells of the cerebellum have a complex arborized arrangement of dendrites and are amongst the most distinctive cell types of the nervous system. Although the neuromorphology of Purkinje cells has been well described for some mammals and teleost fish, for most vertebrates less is known. Here we used a modified Golgi-Cox method to investigate the neuromorphology of Purkinje cells from the lizard Eublepharis macularius, the leopard gecko. Using Sholl and Branch Structure Analyses, we sought to investigate whether the neuromorphology of gecko Purkinje cells was altered is response to tail loss and regeneration. Tail loss is an evolved mechanism commonly used by geckos to escape predation. Loss of the tail represents a significant and sudden change in body length and mass, which is only partially recovered as the tail is regenerated. We predicted that tail loss and regeneration would induce a quantifiable change in Purkinje cell dendrite arborization. Post hoc comparisons of Sholl analyses data showed that geckos with regenerated tails have significant changes in dendrite diameter and the number of dendrite intersections in regions corresponding to the position of parallel fiber synapses. We propose that the neuromorphological alterations observed in gecko Purkinje cells represent a compensatory response to tail regrowth, and perhaps a role in motor learning.

Pavlovian eyeblink conditioning is severely impaired in tottering mice

Cacna1a encodes the pore-forming α_1A subunit of Ca_V2.1 voltage-dependent calcium channels, which regulate neuronal excitability and synaptic transmission. Purkinje cells in the cortex of cerebellum abundantly express these Ca_V2.1 channels. Here, we show that homozygous tottering (tg) mice, which carry a loss-of-function Cacna1a mutation, exhibit severely impaired learning in Pavlovian eyeblink conditioning, which is a cerebellar-dependent learning task. Performance of reflexive eyeblinks is unaffected in tg mice. Transient seizure activity in tg mice further corrupted the amplitude of eyeblink conditioned responses. Our results indicate that normal calcium homeostasis is imperative for cerebellar learning and that the oscillatory state of the brain can affect the expression thereof.NEW & NOTEWORTHY In this study, we confirm the importance of normal calcium homeostasis in neurons for learning and memory formation. In a mouse model with a mutation in an essential calcium channel that is abundantly expressed in the cerebellum, we found severely impaired learning in eyeblink conditioning. Eyeblink conditioning is a cerebellar-dependent learning task. During brief periods of brain-wide oscillatory activity, as a result of the mutation, the expression of conditioned eyeblinks was even further disrupted.

Integration of modeling with experimental and clinical findings synthesizes and refines the central role of inositol 1,4,5-trisphosphate receptor 1 in spinocerebellar ataxia

A suite of models was developed to study the role of inositol 1,4,5-trisphosphate receptor 1 (IP3R1) in spinocerebellar ataxias (SCAs). Several SCAs are linked to reduced abundance of IP3R1 or to supranormal sensitivity of the receptor to activation by its ligand inositol 1,4,5-trisphosphate (IP3). Detailed multidimensional models have been created to simulate biochemical calcium signaling and membrane electrophysiology in cerebellar Purkinje neurons. In these models, IP3R1-mediated calcium release is allowed to interact with ion channel response on the cell membrane. Experimental findings in mice and clinical observations in humans provide data input for the models. The SCA modeling suite helps interpret experimental results and provides suggestions to guide experiments. The models predict IP3R1 supersensitivity in SCA1 and compensatory mechanisms in SCA1, SCA2, and SCA3. Simulations explain the impact of calcium buffer proteins. Results show that IP3R1-mediated calcium release activates voltage-gated calcium-activated potassium channels in the plasma membrane. The SCA modeling suite unifies observations from experiments in a number of SCAs. The cadre of simulations demonstrates the central role of IP3R1.

Excess androgens in utero alters fetal testis development

Prenatal androgenization induces a polycystic ovary syndrome-like phenotype in adult female offspring, which is associated with alterations that can be detected in the fetal ovary, suggesting gestational origins of this condition. We therefore investigated whether increased prenatal androgen exposure also altered testicular development using ovine animal models. Biweekly maternal testosterone propionate (TP; 100 mg) from day 62 to day 70/day 90 of gestation altered male developmental trajectory. In male fetuses serum LH was decreased (P < .01), and testicular STAR, CYP11, and CYP17 abundance were reduced. Coincident with this, basal testicular T synthesis was decreased in vitro (P < .001). Leydig cell distribution was severely perturbed in all testes prenatally exposed to TP (P < .001). To examine the contribution of estrogens, fetuses were injected with TP (20 mg), the potent estrogen agonist, diethylstilbestrol (DES; 20 mg), or vehicle control at day 62 and day 82 and assessed at day 90. The effects of fetal (direct) TP treatment, but not DES, paralleled maternal (indirect) TP exposure, supporting a direct androgen effect. Cessation of maternal androgenization at day 102 returned Leydig cell distribution to normal but increased basal T output, at day 112, demonstrating Leydig cell developmental plasticity. Earlier maternal androgen exposure from day 30 similarly influenced Leydig cell development at day 90 but additionally affected the expression of Sertoli and germ cell markers. We show in this study that increased prenatal androgen exposure alters development and function of Leydig cells at a time when androgen production is paramount for male development. This supports the concept that gestational antecedents associated with polycystic ovary syndrome may have effects on the male fetus.

Enhanced synaptic inhibition disrupts the efferent code of cerebellar Purkinje neurons in leaner Cav2.1 Ca 2+ channel mutant mice

Cerebellar Purkinje cells (PCs) encode afferent information in the rate and temporal structure of their spike trains. Both spontaneous firing in these neurons and its modulation by synaptic inputs depend on Ca(2+) current carried by Ca(v)2.1 (P/Q) type channels. Previous studies have described how loss-of-function Ca(v)2.1 mutations affect intrinsic excitability and excitatory transmission in PCs. This study examines the effects of the leaner mutation on fast GABAergic transmission and its modulation of spontaneous firing in PCs. The leaner mutation enhances spontaneous synaptic inhibition of PCs, leading to transitory reductions in PC firing rate and increased spike rate variability. Enhanced inhibition is paralleled by an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) measured under voltage clamp. These differences are abolished by tetrodotoxin, implicating effects of the mutation on spike-induced GABA release. Elevated sIPSC frequency in leaner PCs is not accompanied by increased mean firing rate in molecular layer interneurons, but IPSCs evoked in PCs by direct stimulation of these neurons exhibit larger amplitude, slower decay rate, and a higher burst probability compared to wild-type PCs. Ca(2+) release from internal stores appears to be required for enhanced inhibition since differences in sIPSC frequency and amplitude in leaner and wild-type PCs are abolished by thapsigargin, an ER Ca(2+) pump inhibitor. These findings represent the first account of the functional consequences of a loss-of-function P/Q channel mutation on PC firing properties through altered GABAergic transmission. Gain in synaptic inhibition shown here would compromise the fidelity of information coding in these neurons and may contribute to impaired cerebellar function resulting from loss-of function mutations in the Ca(V)2.1 channel gene.

Correlated variations in the parameters that regulate dendritic calcium signaling in mouse retinal ganglion cells

The amplitude and time course of stimulus-evoked second messenger signals carried by intracellular changes in free calcium ([Ca](free)) depend on the total influx of Ca(2+), the fraction bound to endogenous buffer and the rate of extrusion. Estimates of the values of these three parameters in proximal dendrites of 15 mouse α retinal ganglion cells were made using the "added buffer" method and found to vary greatly from one experiment to the next. The variations in the measured parameters were strongly correlated across the sample of cells. This reduced the variability in the amplitude and time course of the dendritic Ca(2+) signal and suggests that the expression of Ca(2+) channels, binding proteins and extrusion mechanisms is homeostatically coordinated to maintain the amplitude and kinetics of the Ca(2+) signal within a physiologically appropriate range.

Cytosolic Ca2+ buffers

"Ca(2+) buffers," a class of cytosolic Ca(2+)-binding proteins, act as modulators of short-lived intracellular Ca(2+) signals; they affect both the temporal and spatial aspects of these transient increases in [Ca(2+)](i). Examples of Ca(2+) buffers include parvalbumins (α and β isoforms), calbindin-D9k, calbindin-D28k, and calretinin. Besides their proven Ca(2+) buffer function, some might additionally have Ca(2+) sensor functions. Ca(2+) buffers have to be viewed as one of the components implicated in the precise regulation of Ca(2+) signaling and Ca(2+) homeostasis. Each cell is equipped with proteins, including Ca(2+) channels, transporters, and pumps that, together with the Ca(2+) buffers, shape the intracellular Ca(2+) signals. All of these molecules are not only functionally coupled, but their expression is likely to be regulated in a Ca(2+)-dependent manner to maintain normal Ca(2+) signaling, even in the absence or malfunctioning of one of the components.

Impact of the leaner P/Q-type Ca2+ channel mutation on excitatory synaptic transmission in cerebellar Purkinje cells

Loss-of-function mutations in the gene encoding P/Q-type Ca(2+) channels cause cerebellar ataxia in mice and humans, but the underlying mechanism(s) are unknown. These Ca(2+) channels play important roles in regulating both synaptic transmission and intrinsic membrane properties, and defects in either could contribute to ataxia. Our previous work described changes in intrinsic properties and excitability of cerebellar Purkinje cells (PCs) resulting from the leaner mutation, which is known to reduce whole-cell Ca(2+) currents in PCs and cause severe ataxia. Here we describe the impact of this mutation on excitatory synaptic transmission from parallel and climbing fibres (PFs, CFs) to PCs in acute cerebellar slices. We found that in leaner PCs, PF-evoked excitatory postsynaptic currents (PF-EPSCs) are approximately 50% smaller, and CF-evoked EPSCs are approximately 80% larger, than in wild-type (WT) mice. To investigate whether reduced presynaptic Ca(2+) entry plays a role in attenuating PF-EPSCs in leaner mice, we examined paired-pulse facilitation (PPF). We found that PPF is enhanced in leaner, suggesting that reduced presynaptic Ca(2+) entry reduces neurotransmitter release at these synapses. Short-term plasticity was unchanged at CF-PC synapses, suggesting that CF-EPSCs are larger in leaner PCs because of increased synapse number or postsynaptic sensitivity, rather than enhanced presynaptic Ca(2+) entry. To investigate the functional impact of the observed EPSC changes, we also compared excitatory postsynaptic potentials (EPSPs) elicited by PF and CF stimulation in WT and leaner PCs. Importantly, we found that despite pronounced changes in PF- and CF-EPSCs, evoked EPSPs in leaner mice are very similar to those observed in WT animals. These results suggest that changes in synaptic currents and intrinsic properties of PCs produced by the leaner mutation together maintain PC responsiveness to excitatory synaptic inputs. They also implicate other consequences of the leaner mutation as causes of abnormal cerebellar motor control in mutant mice.

Analysis of calcium ion homeostasis and mitochondrial function in cerebellar granule cells of adult CaV 2.1 calcium ion channel mutant mice

CaV 2.1 voltage-gated calcium channels (VGCC) are highly expressed by cerebellar neurons, and their dysfunction is linked to human disorders including familial hemiplegic migraine, episodic ataxia type 2 and spinocerebellar ataxia type 6. Altered calcium homeostasis, due to dysfunctional Ca(V 2.1 VGCC can severely affect mitochondrial function, eventually leading to neuronal cell death. We study leaner and tottering mice, which carry autosomal recessive mutations in the gene coding for the alpha 1A pore-forming subunit of CaV 2.1 VGCC. Both leaner and tottering mice exhibit cerebellar ataxia and epilepsy. Excessive leaner cerebellar granule cell (CGC) death starts soon after postnatal day 10, but it is not known whether the degree of CGC cell death observed in adult leaner mice is significantly different from wild type mice. We used Fluoro-Jade and TUNEL staining to quantify apoptotic cell death in leaner and wild type CGC. We investigated calcium homeostasis, mitochondrial function and generation of reactive oxygen species (ROS) in isolated CGC, using indicator dyes Fura-2AM, TMRM and CMH2DCFDA, respectively. We observed a small but significant increase in number of apoptotic adult leaner CGC. Calcium homeostasis and mitochondrial function also were altered in leaner CGC. However, no significant differences in ROS levels were observed. It is possible that CGC death in leaner mice may be related to mitochondrial dysfunction but may not be directly related to decreased basal intracellular calcium.

The importance of being subtle: small changes in calcium homeostasis control cognitive decline in normal aging

Aging is a complex, multifactorial process. One of the features of normal aging of the brain is a decline in cognitive functions and much experimental attention has been devoted to understanding this process. Evidence accumulated in the last decade indicates that such functional changes are not due to gross morphological alterations, but to subtle functional modification of synaptic connectivity and intracellular signalling and metabolism. Such synaptic modifications are compatible with a normal level of activity and allow the maintenance of a certain degree of functional reserve. This is in contrast to the changes in various neurodegenerative diseases, characterized by significant neuronal loss and dramatic and irreversible functional deficit. This whole special issue has been initiated with the intention of focusing on the processes of normal brain aging. In this review, we present data that shows how subtle changes in Ca(2+) homeostasis or in the state of various Ca(2+)-dependent processes or molecules, which occur in aging can have significant functional consequences.

Functional compensation by other voltage-gated Ca2+ channels in mouse basal forebrain neurons with Ca(V)2.1 mutations

Tottering (tg/tg) and leaner (tg(la)/tg(la)) mutant mice exhibit distinct mutations in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit (CACNA1A), the pore-forming subunit of the Ca(V)2.1 (P/Q type) Ca(2+) channels. These mice exhibit absence seizures and deficiencies in motor control and other functions. Previous work in cerebellar Purkinje neurons has shown that these mutations cause dramatic reductions in calcium channel function. Because Purkinje cell somata primarily express the Ca(V)2.1 channels, the general decrease in Ca(V)2.1 channel function is observed as a profound decrease in whole-cell current. In contrast to Purkinje cells, basal forebrain (BF) neurons express all of the Ca(2+) channel alpha(1) subunits, with Ca(V)2.1 contributing approximately 30% to the whole-cell current in wild-type (+/+) mice. Here, we show that whole-cell Ba(2+) current densities in BF neurons are not reduced in the mutant genotypes despite a reduction in the Ca(V)2.1 contribution. By blocking the different Ca(2+) channel subtypes with specific pharmacological agents, we found a significant increase in the proportion of Ca(V)1 Ca(2+) current in mutant phenotypes. There was no change in tissue mRNA expression of calcium channel subtypes Ca(V)2.1, Ca(V)2.2, Ca(V)1.2, Ca(V)1.3, and Ca(V)2.3 in the tottering and leaner mutant mice. These results suggest that Ca(V)1 channels may functionally upregulate to compensate for reduced Ca(V)2.1 function in the mutants without an increase in Ca(v)1 message. Single-cell reverse transcription polymerase chain reaction (RT-PCR) experiments in a subset of sampled neurons revealed that approximately 90% of the cells could be considered cholinergic based on choline acetyltransferase (ChAT) mRNA expression.

Developmental expression of neuronal nitric oxide synthase in P/Q-type voltage-gated calcium ion channel mutant mice, leaner and tottering

Nitric oxide (NO) is a diffusible messenger molecule produced primarily by neuronal nitric oxide synthase (nNOS) in the central nervous system. Both nNOS expression and NO production are regulated by calcium ions. Leaner and tottering mice carry a mutation in the pore forming subunit (alpha1A) of P/Q-type voltage-gated calcium ion channels, which decreases calcium ion current through the affected channels and disrupts calcium homeostasis. We have previously shown that nNOS expression is altered in adult leaner and tottering cerebella. In addition, leaner and tottering mice have been shown to have abnormal cerebellar granule cell-Purkinje cell synapses and leaner cerebellar granule cells undergo abnormal apoptosis during early postnatal development. Since NO production has been linked to several developmental roles including neuronal cell death, synaptogenesis and neuronal cell survival, our objective was to evaluate the expression of nNOS in developing leaner and tottering cerebella. Our results show that nNOS is differentially expressed in leaner and tottering cerebella compared to wild type cerebella and compared to each other. In whole cerebella, Western blotting revealed a significant increase in nNOS expression at postnatal day 12 in tottering but not leaner or wild type cerebella. At the cellular level the NADPH-diaphorase marker for nNOS revealed a significant increase in nNOS expression in basket cell interneurons in both mutant mice. nNOS expression in granule cells in the internal granule cell layer in tottering mice was increased at P12, while granule cells of leaner mice exhibited decreased nNOS expression at P20. The changes in nNOS expression at P12 did not correlate with a change in overall NO production, but rather maintained wild type NO concentrations. These findings suggest that changes in nNOS expression in the leaner and tottering cerebella are compensatory in nature with NO most likely functioning as a calcium-regulated neuroprotective/neurotrophic factor in postnatal cerebellar development.

Eye movements of the murine P/Q calcium channel mutant tottering, and the impact of aging

Mice carrying mutations of the gene encoding the ion pore of the P/Q calcium channel (Cacna1a) are an instance in which cerebellar dysfunction may be attributable to altered electrophysiology and thus provide an opportunity to study how neuronal intrinsic properties dictate signal processing in the ocular motor system. P/Q channel mutations can engender multiple effects at the single neuron, circuit, and behavioral levels; correlating physiological and behavioral abnormalities in multiple allelic strains will ultimately facilitate determining which alterations of physiology are responsible for specific behavioral aberrations. We used videooculography to quantify ocular motor behavior in tottering mutants aged 3 mo to 2 yr and compared their performance to data previously obtained in the allelic mutant rocker and C57BL/6 controls. Tottering mutants shared numerous abnormalities with rocker, including upward deviation of the eyes at rest, increased vestibuloocular reflex (VOR) phase lead at low stimulus frequencies, reduced VOR gain at high stimulus frequencies, reduced gain of the horizontal and vertical optokinetic reflex, reduced time constants of the neural integrator, and reduced plasticity of the VOR as assessed in a cross-axis training paradigm. Unlike rocker, young tottering mutants exhibited normal peak velocities of nystagmus fast phases, arguing against a role for neuromuscular transmission defects in the attenuation of compensatory eye movements. Tottering also differed by exhibiting directional asymmetries of the gains of optokinetic reflexes. The data suggest at least four pathophysiological mechanisms (two congenital and two acquired) are required to explain the ocular motor deficits in the two Cacna1a mutant strains.

Synaptic plasticity in the dy2J mouse model of laminin alpha2-deficient congenital muscular dystrophy

Laminin alpha2-deficient congenital muscular dystrophy is a debilitating disease affecting both muscle and neural tissue as a result of mutations in the LAMA2 gene. It presents at or soon after birth with muscle weakness and is further characterised by clinical central nervous system involvement. Laminin alpha2 is part of the extracellular matrix, linked to the cellular cystoskeleton via dystroglycan which is an integral part of the dystrophin-glycoprotein complex (DGC). We examined both short- and long-term synaptic plasticity in the C57BL6J/dy(2J) mouse, an animal model of laminin alpha2 deficient congenital muscular dystrophy. Using a cerebellar slice preparation, we show that the pre-synaptically mediated paired-pulse facilitation (PPF) was no different between dy(2J) and littermate controls. Approximately half (7/12) the dy(2J) Purkinje cells displayed a blunted LTD compared to littermate controls, and one third (4/12) of dy(2J) Purkinje cells displayed LTP. This study demonstrates that a defective laminin alpha2 causes a disruption in long-term synaptic plasticity at the Purkinje cell-parallel fibre synapse.

S100A9 deficiency alters adenosine-5'-triphosphate induced calcium signalling but does not generally interfere with calcium and zinc homeostasis in murine neutrophils

The two calcium- and zinc-binding proteins, S100A9 and S100 A8, abundant in myeloid cells are considered to play important roles in both calcium signalling and zinc homeostasis. Polymorphonuclear neutrophils from S100A9 ko mice are also devoid of S100A8. Therefore, S100A9-deficient neutrophils were used as a model to study the role of the two S100 proteins in the neutrophils's calcium and zinc metabolism. Analysis of the intracellular zinc level upon pyrithione and (+/-)-(E)-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexeneamide (NOR-1) treatment revealed no differences between S100A9-deficient and wildtype neutrophils. Similar, the calcium signals were not distinguishable from S100A9-deficient and wildtype neutrophils upon stimulation with platelet activating factor (PAF), thapsigargin or macrophage inflammatory protein 1 alpha (MIP-1 alpha), indicating despite their massive expression S100A8/A9 do neither serve as calcium nor as zinc buffering proteins in granulocytes. In contrast, stimulation with adenosine-5'-triphosphate (ATP) induces a significant stronger increase of the intracellular free calcium level in S100A9-deficient cells compared to wildtype cells. Moreover, the ATP-induced calcium signal was still different when the cells were incubated in calcium free buffer suggesting that pirinergic receptors of the P(2Y) class could be involved in this signalling pathway.

Postnatal apoptosis in cerebellar granule cells of homozygous leaner (tg1a/tg1a) mice

Leaner mice carry a homozygous, autosomal recessive mutation in the mouse CACNA1A gene encoding the Alpha1A subunit of P/Q-type calcium channels, which results in an out-of-frame splicing event in the carboxy terminus of the Alpha1A protein. Leaner mice exhibit severe ataxia, paroxysmal dyskinesia and absence seizures. Functional studies have revealed a marked decrease in calcium currents through leaner P/Q-type channels and altered neuronal calcium ion homeostasis in cerebellar Purkinje cells. Histopathological studies of leaner mice have revealed extensive postnatal cerebellar Purkinje and granule cell loss. We examined the temporospatial pattern of cerebellar granule cell death in the leaner mouse between postnatal days (P) 10 and 40. Our observations clearly indicate that leaner cerebellar granule cells die via an apoptotic process and that the peak time of neuronal death is P20. We did not observe a significant increase in microglial and astrocytic responses at P20, suggesting that glial responses are not a cause of neuronal cell death. We propose that the leaner cerebellar granule cell represents an in vivo animal model for low intracellular [Ca2+]-induced apoptosis. Since intracellular [Ca2+] is critical in the control of gene expression, it is quite likely that reduced intracellular [Ca2+] could activate a lethal cascade of altered gene expression leading to the apoptotic granule cell death in the leaner cerebellum.