Inactivation of Calcium-Binding Protein Genes Induces 160 Hz Oscillations in the Cerebellar Cortex of Alert Mice

Abstract

Oscillations in neuronal populations may either be imposed by intrinsically oscillating pacemakers neurons or emerge from specific attributes of a distributed network of connected neurons. Calretinin and calbindin are two calcium-binding proteins involved in the shaping of intraneuronal Ca 2ϩ fluxes. However, although their physiological function has been studied extensively at the level of a single neuron, little is known about their role at the network level. Here we found that null mutations of genes encoding calretinin or calbindin induce 160 Hz local field potential oscillations in the cerebellar cortex of alert mice. These oscillations reached maximum amplitude just beneath the Purkinje cell bodies and are reinforced in the cerebellum of mice deficient in both calretinin and calbindin. Purkinje cells fired simple spikes phase locked to the oscillations and synchronized along the parallel fiber axis. The oscillations reversibly disappeared when gap junctions or either GABA A or NMDA receptors were blocked. Cutaneous stimulation of the whisker region transiently suppressed the oscillations. However, the intrinsic somatic excitability of Purkinje cells recorded in slice preparation was not significantly altered in mutant mice. Functionally, these results suggest that 160 Hz oscillation emerges from a network mechanism combining synchronization of Purkinje cell assemblies through parallel fiber excitation and the network of coupled interneurons of the molecular layer. These findings demonstrate that subtle genetically induced modifications of Ca 2ϩ homeostasis in specific neuron types can alter the observed dynamics of the global network.

Differences in locomotor behavior revealed in mice deficient for the calcium-binding proteins parvalbumin, calbindin D-28k or both

“…Also our results of the rotarod assay point in the direction that the absence of CB is the main factor for the motor impairment, while the effect of PV-deficiency is rather subtle and beyond detection in most assays. As previously demonstrated for the absence of CB and CR [37], also elimination of PV leads to changes in the firing properties of Purkinje cells: an increase in the simple spike firing rate (SS), a shortening of the complex spike duration (CSd), and a shortening of the spike pause after a complex spike (CSp) [35]. Interestingly, in animals lacking CB (CB−/− and double-KO), the differences of the above parameters (SS, CSd and CSp) compared to WT are much more pronounced than in the PV−/− mice, but are not significantly different in CB−/− and double-KO mice.…”

“…This correlates well the graded differences in motor coordination impairment: PV−/−CB−/− ≈ CB−/− PV−/− > WT. In animals deficient for either one of the CaBPs (CR, CB, PV) or the combinations CB/CR and PV/CB, in addition to altered PC firing the concomitant emergence of 160-Hz oscillations in the cerebellum of alert mice has been observed [35,37]. Such oscillations are not encountered in WT and it was proposed that these oscillations would impair the normal functioning of the cerebellar circuitry possibly causing the motor coordination impairment.…”

Cytosolic Ca2+ Buffers

“…At first glance, these mice show no phenotype related to development, the general morphology of the nervous system, the visual (Wassle et al 1998) and auditory (Airaksinen et al 2000) systems, or behavior under standard housing conditions (Airaksinen et al 1997). CB-D28k 2/2 mice show a mildhowever more severe than PV 2/2 mice-impairment in motor coordination/motor learning (Airaksinen et al 1997;Farre-Castany et al 2007), likely resulting from the 160-Hz oscillations in the cerebellum (Cheron et al 2004;Servais et al 2005). The motor coordination phenotype is due to CB-D28k's absence in Purkinje cells, since this phenotype and alterations in Purkinje cell physiology also occur in mice with Purkinje cell-specific Calb1 ablation (Barski et al 2003).…”

“…The firing properties of Purkinje cells are altered in alert CR 2/2 mice: The simple spikefiring rate increased the complex spike duration and the spike pause is shorter (Schiffmann et al 1999). As in PV 2/2 and CB-D28k 2/2 mice, alert CR 2/2 mice show 160 Hz oscillations (Cheron et al 2004) that appear phenotypically as an impairment of motor coordination. In alert "rescue" mice, where CR in CR 2/2 mice is selectively re-expressed in granule cells, granule cell excitability, as well as Purkinje cell firing, resembles that in WT mice.…”

PTPRR, Cerebellum, and Motor Coordination

“…Examples include junctophilin 3 [7], carbonic anhydrase-related protein VIII (Car8) [8], and calretinin-deficient mice [9]. These mice are ataxic because of a deregulation of calcium homeostasis which impacts not only on Purkinje cell behavior but also upon other neurons of the cerebellar cortex [9]. Junctophilins contribute to the formation of junctional membrane complexes by spanning the endoplasmic/sarcoplasmic reticulum membrane and interacting with the plasma membrane.…”