HIV-1 Tat causes cognitive deficits and selective loss of parvalbumin, somatostatin, and neuronal nitric oxide synthase expressing hippocampal CA1 interneuron subpopulations

Memory deficits are characteristic of HIV-associated neurocognitive disorders (HAND) and co-occur with hippocampal pathology. The HIV-1 transactivator of transcription (Tat), a regulatory protein, plays a significant role in these events, but the cellular mechanisms involved are poorly understood. Within the hippocampus, diverse populations of interneurons form complex networks; even subtle disruptions can drastically alter synaptic output, resulting in behavioral dysfunction. We hypothesized that HIV-1 Tat would impair cognitive behavior and injure specific hippocampal interneuron subtypes. Male transgenic mice that inducibly expressed HIV-1 Tat (or non-expressing controls) were assessed for cognitive behavior or had hippocampal CA1 subregions evaluated via interneuron subpopulation markers. Tat exposure decreased spatial memory in a Barnes maze and mnemonic performance in a novel object recognition test. Tat reduced the percentage of neurons expressing neuronal nitric oxide synthase (nNOS) without neuropeptide Y immunoreactivity in the stratum pyramidale and the stratum radiatum, parvalbumin in the stratum pyramidale, and somatostatin in the stratum oriens, which are consistent with reductions in interneuron-specific interneuron type 3 (IS3), bistratified, and oriens-lacunosum-moleculare interneurons, respectively. The findings reveal that an interconnected ensemble of CA1 nNOS-expressing interneurons, the IS3 cells, as well as subpopulations of parvalbumin- and somatostatin-expressing interneurons are preferentially vulnerable to HIV-1 Tat. Importantly, the susceptible interneurons form a microcircuit thought to be involved in feedback inhibition of CA1 pyramidal cells and gating of CA1 pyramidal cell inputs. The identification of vulnerable CA1 hippocampal interneurons may provide novel insight into the basic mechanisms underlying key functional and neurobehavioral deficits associated with HAND.

Ambient illumination toggles a neuronal circuit switch in the retina and visual perception at cone threshold

VIDEO ABSTRACT

Gradual changes in the sensory environment can lead to abrupt changes in brain computations and perception. However, mechanistic understanding of the mediating microcircuits is missing. By sliding through light levels from starlight to daylight, we identify retinal ganglion cell types in the mouse that abruptly and reversibly switch the weighting of center and surround interactions in their receptive field around cone threshold. Two-photon-targeted recordings and genetic and viral tracing experiments revealed that the circuit element responsible for the switch is a large inhibitory neuron that provides direct inhibition to ganglion cells. Our experiments suggest that weak excitatory input via electrical synapses together with the spiking threshold in inhibitory cells act as a switch. We also reveal a switch-like component in the spatial integration properties of human vision at cone threshold. This work demonstrates that circuits in the retina can quickly and reversibly switch between two distinct states, implementing distinct perceptual regimes at different light levels.

Inverse regulation of the cytosolic Ca²⁺ buffer parvalbumin and mitochondrial volume in muscle cells via SIRT1/PGC-1α axis

Skeletal muscles show a high plasticity to cope with various physiological demands. Different muscle types can be distinguished by the force, endurance, contraction/relaxation kinetics (fast-twitch vs. slow-twitch muscles), oxidative/glycolytic capacity, and also with respect to Ca²⁺-signaling components. Changes in Ca²⁺ signaling and associated Ca²⁺-dependent processes are thought to underlie the high adaptive capacity of muscle fibers. Here we investigated the consequences and the involved mechanisms caused by the ectopic expression of the Ca²⁺-binding protein parvalbumin (PV) in C2C12 myotubes in vitro, and conversely, the effects caused by its absence in in fast-twitch muscles of parvalbumin null-mutant (PV⁻/⁻) mice in vivo. The absence of PV in fast-twitch muscle tibialis anterior (TA) resulted in an increase in the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and of its positive regulator, the deacetylase sirtuin 1 (SIRT1). TA muscles from PV⁻/⁻ mice also have an increased mitochondrial volume. Mild ionophore treatment of control (PV-devoid) C2C12 myotubes causing a moderate elevation in [Ca²⁺](c) resulted in an increase in mitochondrial volume, together with elevated PGC-1α and SIRT1 expression levels, whilst it increased PV expression levels in myotubes stably transfected with PV. In PV-expressing myotubes the mitochondrial volume, PGC-1α and SIRT1 were significantly lower than in control C2C12 myotubes already at basal conditions and application of ionophore had no effect on either one. SIRT1 activation causes a down-regulation of PV in transfected myotubes, whilst SIRT1 inhibition has the opposite effect. We conclude that PV expression and mitochondrial volume in muscle cells are inversely regulated via a SIRT1/PGC-1α signaling axis.

Deficits in parvalbumin and calbindin immunoreactive cells in the hippocampus of isolation reared rats

Post-mortem studies have provided evidence for abnormalities of the gamma-aminobutyric acid (GABA)-ergic system in schizophrenia. The calcium-binding proteins (CBPs), parvalbumin (PV), calbindin (CB) and calretinin (CR) can be used as markers for specific subpopulations of GABAergic neurons in the brain. Isolation rearing of rats is a non-pharmacological, non-lesion manipulation that leads to deficits in prepulse inhibition of the startle reflex (PPI) and other behavioural and neurochemical alterations reminiscent of schizophrenia. Female rats were reared in social housing (groups of three) or singly for 11 weeks post weaning and PPI was measured. Brains were removed and hippocampal CBP- containing neurons determined following immunocytochemical staining. Compared to socially housed rats, isolated rats exhibited PPI deficits and reductions in PV and CB-immunoreactive cells in the hippocampus, with no significant change in CR. These findings demonstrate selective abnormalities of sub-populations of GABAergic interneurons in the hippocampus of isolation reared rats, which resemble the neuronal deficits seen in this region in schizophrenia.

Deficiency in parvalbumin, but not in calbindin D-28k upregulates mitochondrial volume and decreases smooth endoplasmic reticulum surface selectively in a peripheral, subplasmalemmal region in the soma of Purkinje cells

The Ca(2+)-binding proteins parvalbumin (PV) and calbindin D-28k (CB) are key players in the intracellular Ca(2+)-buffering in specific cells including neurons and have profound effects on spatiotemporal aspects of Ca(2+) transients. The previously observed increase in mitochondrial volume density in fast-twitch muscle of PV-/- mice is viewed as a specific compensation mechanism to maintain Ca(2+) homeostasis. Since cerebellar Purkinje cells (PC) are characterized by high expression levels of the Ca(2+) buffers PV and CB, the question was raised, whether homeostatic mechanisms are induced in PC lacking these buffers. Mitochondrial volume density, i.e. relative mitochondrial mass was increased by 40% in the soma of PV-/- PC. Upregulation of mitochondrial volume density was not homogenous throughout the soma, but was selectively restricted to a peripheral region of 1.5 microm width underneath the plasma membrane. Accompanied was a decreased surface of subplasmalemmal smooth endoplasmic reticulum (sPL-sER) in a shell of 0.5 microm thickness underneath the plasma membrane. These alterations were specific for the absence of the "slow-onset" buffer PV, since in CB-/- mice neither changes in peripheral mitochondria nor in sPL-sER were observed. This implicates that the morphological alterations are aimed to specifically substitute the function of the slow buffer PV. We propose a novel concept that homeostatic mechanisms of components involved in Ca(2+) homeostasis do not always occur at the level of similar or closely related molecules. Rather the cell attempts to restore spatiotemporal aspects of Ca(2+) signals prevailing in the undisturbed (wildtype) situation by subtly fine tuning existing components involved in the regulation of Ca(2+) fluxes.

Mono- and dual-frequency fast cerebellar oscillation in mice lacking parvalbumin and/or calbindin D-28k

Calbindin is a fast Ca2+-binding protein expressed by Purkinje cells and involved in their firing regulation. Its deletion produced approximately 160-Hz oscillation sustained by synchronous, rhythmic Purkinje cells in the cerebellar cortex of mice. Parvalbumin is a slow-onset Ca2+-binding protein expressed in Purkinje cells and interneurons. In order to assess its function in Purkinje cell firing regulation, we studied the firing behavior of Purkinje cells in alert mice lacking parvalbumin (PV-/-), calbindin (CB-/-) or both (PV-/- CB-/-) and in wild-type controls. The absence of either protein resulted in Purkinje cell firing alterations (decreased complex spike duration and pause, increased simple spike firing rate) that were more pronounced in CB-/- than in PV-/- mice. Cumulative effects were found in complex spike alterations in PV-/- CB-/- mice. PV-/- and CB-/- mice manifested approximately 160-Hz oscillation that was sustained by Purkinje cells firing rhythmically and synchronously along the parallel fiber axis. This oscillation was dependent on GABA(A), N-methyl-D-aspartate and gap junction transmission. PV-/- CB-/- mice exhibited a dual-frequency (110 and 240 Hz) oscillation. The instantaneous spectral densities of both components were inversely correlated. Simple and complex spikes of Purkinje cells were phase-locked to one of the two oscillation frequencies. Mono- and dual-frequency oscillations presented similar pharmacological properties. These results demonstrate that the absence of the Ca2+ buffers parvalbumin and calbindin disrupts the regulation of Purkinje cell firing rate and rhythmicity in vivo and suggest that precise Ca2+ transient control is required to maintain the normal spontaneous arrhythmic and asynchronous firing pattern of the Purkinje cells.

Parvalbumin deficiency affects network properties resulting in increased susceptibility to epileptic seizures

Networks of GABAergic interneurons are of utmost importance in generating and promoting synchronous activity and are involved in producing coherent oscillations. These neurons are characterized by their fast-spiking rate and by the expression of the Ca(2+)-binding protein parvalbumin (PV). Alteration of their inhibitory activity has been proposed as a major mechanism leading to epileptic seizures and thus the role of PV in maintaining the stability of neuronal networks was assessed in knockout (PV-/-) mice. Pentylenetetrazole induced generalized tonic-clonic seizures in all genotypes, but the severity of seizures was significantly greater in PV-/- than in PV+/+ animals. Extracellular single-unit activity recorded from over 1000 neurons in vivo in the temporal cortex revealed an increase of units firing regularly and a decrease of cells firing in bursts. In the hippocampus, PV deficiency facilitated the GABA(A)ergic current reversal induced by high-frequency stimulation, a mechanism implied in the generation of epileptic activity. We postulate that PV plays a key role in the regulation of local inhibitory effects exerted by GABAergic interneurons on pyramidal neurons. Through an increase in inhibition, the absence of PV facilitates synchronous activity in the cortex and facilitates hypersynchrony through the depolarizing action of GABA in the hippocampus.

Mutational analysis of dendritic Ca2+ kinetics in rodent Purkinje cells: role of parvalbumin and calbindin D28k

The mechanisms governing the kinetics of climbing fibre-mediated Ca2+ transients in spiny dendrites of cerebellar Purkinje cells (PCs) were quantified with high-resolution confocal Ca2+ imaging. Ca2+ dynamics in parvalbumin (PV-/-) and parvalbumin/calbindin D28k null-mutant (PV/CB-/-) mice were compared with responses in wild-type (WT) animals. In the WT, Ca2+ transients in dendritic shafts were characterised by double exponential decay kinetics that were not due to buffered Ca2+ diffusion or saturation of the indicator dye. Ca2+ transients in PV-/- PCs reached the same peak amplitude as in the WT but the biphasic nature of the decay was less pronounced, an effect that could be attributed to PV's slow binding kinetics. In contrast, peak amplitudes in PV/CB-/- PCs were about two times higher than in the WT and the decay became nearly monophasic. Numerical simulations indicate that the residual deviation from a single exponential decay in PV/CB-/- is due to saturation of the Ca2+ indicator dye. Furthermore, the simulations imply that the effect of uncharacterised endogenous Ca2+ binding proteins is negligible, that buffered diffusion and dye saturation significantly affects spineous Ca2+ transients but not those in the dendritic shafts, and that neither CB nor PV undergoes saturation in spines or dendrites during climbing fibre-evoked Ca2+ transients. Calbindin's medium-affinity binding sites are fast enough to reduce the peak amplitude of the Ca2+ signal. However, similar to PV, delayed binding by CB leads to biphasic Ca2+ decay kinetics. Our results suggest that the distinct kinetics of PV and CB underlie the biphasic kinetics of synaptically evoked Ca2+ transients in dendritic shafts of PCs.

Consequence of parvalbumin deficiency in the mdx mouse: histological, biochemical and mechanical phenotype of a new double mutant

We tested the hypothesis whether the mild dystrophy in mdx mice could result from the contribution of the cytosolic calcium buffer parvalbumin in maintaining a normal cytosolic [Ca2+]i, in spite of an increased passive Ca2+ influx. By crossing mdx mice with parvalbumin-deficient mice, double mutant mice, lacking both dystrophin and parvalbumin, were obtained. Though resting cytosolic [Ca2+]i and total calcium content were similar to that of mdx muscles, this new animal model presented a slightly more severe phenotype than the mdx mouse. Muscle pseudo-hypertrophy, the density of myotubes and of centronucleated fibres as well as the loss of IIB fibres were all increased in parvalbumin-deficient mdx mice. Many of these deficits were overcome in late adulthood, albeit fibrosis was clearly more pronounced than in mdx muscles. At 90 days, parvalbumin-deficient mdx mice showed higher levels of creatine phosphokinase and lower muscle strength, in vivo, than mdx mice. Isometric tension of isolated muscle was reduced, but the susceptibility to eccentric contraction was not increased. The slight aggravation of muscle dystrophy observed in mdx mice deprived of parvalbumin cannot explain the severity of the affection observed in xmd dogs and Duchenne dystrophy patients where parvalbumin is constitutively not expressed.

Diffusional mobility of parvalbumin in spiny dendrites of cerebellar Purkinje neurons quantified by fluorescence recovery after photobleaching

Ca(2+)-binding proteins (CaBPs) represent key factors for the modulation of cellular Ca(2+) dynamics. Especially in thin extensions of nerve cells, Ca(2+) binding and buffered diffusion of Ca(2+) by CaBPs is assumed to effectively control the spatio-temporal extend of Ca(2+) signals. However, no quantitative data about the mobility of specific CaBPs in the neuronal cytosol are available. We quantified the diffusion of the endogenous CaPB parvalbumin (PV) in spiny dendrites of cerebellar Purkinje neurons with two-photon fluorescence recovery after photobleaching. Fluorescently labeled PV diffused readily between spines and dendrites with a median time constant of 49 ms (37-61 ms, interquartile range). Based on published data on spine geometry, this value corresponds to an apparent diffusion coefficient of 43 microm(2) s(-1) (34-56 microm(2) s(-1)). The absence of large or immobile binding partners for PV was confirmed in PV null-mutant mice. Our data validate the common but so far unproven assumption that PV is highly mobile in neurons and will facilitate simulations of neuronal Ca(2+) buffering. Our experimental approach represents a versatile tool for quantifying the mobility of proteins in neuronal dendrites.

Deficiency in parvalbumin increases fatigue resistance in fast-twitch muscle and upregulates mitochondria

The soluble Ca2+-binding protein parvalbumin (PV) is expressed at high levels in fast-twitch muscles of mice. Deficiency of PV in knockout mice (PV -/-) slows down the speed of twitch relaxation, while maximum force generated during tetanic contraction is unaltered. We observed that PV-deficient fast-twitch muscles were significantly more resistant to fatigue than were the wild type. Thus components involved in Ca2+ homeostasis during the contraction-relaxation cycle were analyzed. No upregulation of another cytosolic Ca2+-binding protein was found. Mitochondria are thought to play a physiological role during muscle relaxation and were thus analyzed. The fractional volume of mitochondria in the fast-twitch muscle extensor digitorum longus (EDL) was almost doubled in PV -/- mice, and this was reflected in an increase of cytochrome c oxidase. A faster removal of intracellular Ca2+ concentration ([Ca2+]i) 200-700 ms after fast-twitch muscle stimulation observed in PV -/- muscles supports the role for mitochondria in late [Ca2+]i removal. The present results also show a significant increase of the density of capillaries in EDL muscles of PV -/- mice. Thus alterations in the dynamics of Ca2+ transients detected in fast-twitch muscles of PV -/- mice might be linked to the increase in mitochondria volume and capillary density, which contribute to the greater fatigue resistance of these muscles.

Progression of spontaneous seizures after status epilepticus is associated with mossy fibre sprouting and extensive bilateral loss of hilar parvalbumin and somatostatin-immunoreactive neurons

The development of spontaneous limbic seizures was investigated in a rat model in which electrical tetanic stimulation of the angular bundle was applied for up to 90 min. This stimulation produced behavioural and electrographic seizures that led to a status epilepticus (SE) in most rats (71%). Long-term EEG monitoring showed that the majority of the rats (67%) that underwent SE, displayed a progressive increase of seizure activity once the first seizure was recorded after a latent period of about 1 week. The other SE rats (33%) did not show this progression of seizure activity. We investigated whether these different patterns of evolution of spontaneous seizures could be related to differences in cellular or structural changes in the hippocampus. This was the case regarding the following changes. (i) Cell loss in the hilar region: in progressive SE rats this was extensive and bilateral whereas in nonprogressive SE rats it was mainly unilateral. (ii) Parvalbumin and somatostatin-immunoreactive neurons: in the hilar region these were almost completely eliminated in progressive SE rats but were still largely present unilaterally in nonprogressive SE rats. (iii) Mossy fibre sprouting: in progressive SE rats, extensive mossy fibre sprouting was prominent in the inner molecular layer. In nonprogressive SE rats, mossy fibre sprouting was also present but less prominent than in progressive SE rats. Although mossy fibre sprouting has been proposed to be a prerequisite for chronic seizure activity in experimental temporal lobe epilepsy, the extent of hilar cell death also appears to be an important factor that differentiates between whether or not seizure progression will occur.

Role of the calcium-binding protein parvalbumin in short-term synaptic plasticity

GABAergic (GABA = gamma-aminobutyric acid) neurons from different brain regions contain high levels of parvalbumin, both in their soma and in their neurites. Parvalbumin is a slow Ca(2+) buffer that may affect the amplitude and time course of intracellular Ca(2+) transients in terminals after an action potential, and hence may regulate short-term synaptic plasticity. To test this possibility, we have applied paired-pulse stimulations (with 30- to 300-ms intervals) at GABAergic synapses between interneurons and Purkinje cells, both in wild-type (PV+/+) mice and in parvalbumin knockout (PV-/-) mice. We observed paired-pulse depression in PV+/+ mice, but paired-pulse facilitation in PV-/- mice. In paired recordings of connected interneuron-Purkinje cells, dialysis of the presynaptic interneuron with the slow Ca(2+) buffer EGTA (1 mM) rescues paired-pulse depression in PV-/- mice. These data show that parvalbumin potently modulates short-term synaptic plasticity.

Tetanus relaxation of fast skeletal muscles of the mouse made parvalbumin deficient by gene inactivation

The effects of tetanus duration on the relaxation rate of extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles were studied in normal (wild-type, WT) and parvalbumin-deficient (PVKO) mice, at 20 C. In EDL of PVKO, the relaxation rate was low and unaffected by tetanus duration (< 3.2 s). In contrast, the relaxation rate of WT muscles decreased when tetanus duration increased from 0.2 to 3.2 s. In WT muscles, fast relaxation recovered as the rest interval increased. Specific effect of parvalbumin was asserted by calculating the difference in relaxation rate between WT and PVKO muscles. For EDL, the rate constant of relaxation slowing was 1.10 s-1 of tetanization; the rate constant of relaxation recovery was 0.05 s-1 of rest. In FDB, the effects of tetanus duration on WT and PVKO muscles were qualitatively similar to those observed in EDL. Relaxation slowing as tetanus duration increases, reflects the progressive saturation of parvalbumin by Ca2+, while recovery as rest interval increases reflects the return to Ca2+-free parvalbumin. At all tetanus durations, relaxation rate still remained slightly faster in WT muscles. This suggested that parvalbumin facilitates calcium traffic from myofibrils to the SR. No difference was found between WT and PVKO muscles for: (i) the expression of the fast isoforms of myosin heavy chains, (ii) the force-velocity relationship and maximal shortening velocity and (iii) the Ca2+-activated ATPase activity from isolated preparations of the sarcoplasmic reticulum (SR).

Calcium-binding protein, parvalbumin, is reduced in mutant mammalian muscle with abnormal contractile properties

To elucidate the biochemical basis of hereditary muscle diseases in an experimental mammal, we performed polypeptide analyses on skeletal muscles of neuromuscular mutants of the mouse. In one of these, "arrested development of righting response" (adr), the concentration of the soluble Ca2+-binding protein parvalbumin was drastically reduced in comparison to wild type. This reduction was not an unspecific consequence of muscle disease, as it was not observed in two other neuromuscular mouse mutants, "wobbler" (wr) and "motor endplate disease" (med or medjo). Isometric twitches of adr muscle had only slightly prolonged contraction and half-relaxation times, yet long-lasting after-contractions were observed upon repeated (20-100 Hz) direct stimulation. Thus, parvalbumin may be mainly involved in the relaxation after tetanic contraction of fast-twitch fibers.