Examination of Synaptic Vesicle Recycling Using FM Dyes During Evoked, Spontaneous, and Miniature Synaptic Activities

Synaptic vesicles in functional nerve terminals undergo exocytosis and endocytosis. This synaptic vesicle recycling can be effectively analyzed using ...

Rapid Genotyping of Animals Followed by Establishing Primary Cultures of Brain Neurons

High-resolution analysis of the morphology and function of mammalian neurons often requires the genotyping of individual animals followed by the analysis ...

University of Iowa Carver College of Medicine

Early-onset generalized dystonia, DYT1, is caused by a mutation in the gene encoding the evolutionarily conserved AAA+ ATPase torsinA. Synaptic abnormalities have been implicated in DYT1 dystonia, but the details of the synaptic pathophysiology are only partially understood. Here, we demonstrate a novel role for torsinA in synaptic vesicle recycling, using cultured hippocampal neurons from a knock-in mouse model of DYT1 dystonia (ΔE-torsinA) and live-cell imaging with styryl FM dyes. Neurons from heterozygous ΔE-torsinA mice released a larger fraction of the total recycling pool (TRP) during a single round of electrical stimulation than did wild-type neurons. Moreover, when the neurons were subjected to prior high activity, the time course of release was shortened. In neurons from homozygous mice, these enhanced exocytosis phenotypes were similar, but in addition the size of the TRP was reduced. Notably, when release was triggered by applying a calcium ionophore rather than electrical stimuli, neither a single nor two ΔE-torsinA alleles affected the time course of release. Thus, the site of action of ΔE-torsinA is at or upstream of the rise in calcium concentration in nerve terminals. Our results suggest that torsinA regulates synaptic vesicle recycling in central neurons. They also indicate that this regulation is influenced by neuronal activity, further supporting the idea that synaptic abnormalities contribute to the pathophysiology of DYT1 dystonia.

Synaptic vesicle recycling is enhanced by torsinA that harbors the DYT1 dystonia mutation.

TorsinA is an evolutionarily conserved AAA+ ATPase, and human patients with an in-frame deletion of a single glutamate (ΔE) codon from the encoding gene suffer from autosomal-dominant, early-onset generalized DYT1 dystonia. Although only 30-40% of carriers of the mutation show overt motor symptoms, most experience enhanced excitability of the central nervous system. The cellular mechanism responsible for this change in excitability is not well understood. Here we show the effects of the ΔE-torsinA mutation on miniature neurotransmitter release from neurons. Neurotransmitter release was characterized in cultured hippocampal neurons obtained from wild-type, heterozygous, and homozygous ΔE-torsinA knock-in mice using two approaches. In the first approach, patch-clamp electrophysiology was used to record glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) in the presence of the Na⁺ channel blocker tetrodotoxin (TTX) and absence of GABA(A) receptor antagonists. The intervals between mEPSC events were significantly shorter in neurons obtained from the mutant mice than in those obtained from wild-type mice. In the second approach, the miniature exocytosis of synaptic vesicles was detected by imaging the unstimulated release of FM dye from the nerve terminals in the presence of TTX. Cumulative FM dye release was higher in neurons obtained from the mutant mice than in those obtained from wild-type mice. The number of glutamatergic nerve terminals was also assessed, and we found that this number was unchanged in heterozygous relative to wild-type neurons, but slightly increased in homozygous neurons. Notably, in both heterozygous and homozygous neurons, the unitary synaptic charge during each mEPSC event was unchanged. Overall, our results suggest more frequent miniature glutamate release in neurons with ΔE-torsinA mutations. This change may be one of the underlying mechanisms by which the excitability of the central nervous system is enhanced in the context of DYT1 dystonia. Moreover, qualitative differences between heterozygous and homozygous neurons with respect to certain synaptic properties indicate that the abnormalities observed in homozygotes may reflect more than a simple gene dosage effect.

Miniature release events of glutamate from hippocampal neurons are influenced by the dystonia-associated protein torsinA.

The astrocyte is a major glial cell type of the brain, and plays key roles in the formation, maturation, stabilization and elimination of synapses. Thus, changes in astrocyte condition and age can influence information processing at synapses. However, whether and how aging astrocytes affect synaptic function and maturation have not yet been thoroughly investigated. Here, we show the effects of prolonged culture on the ability of astrocytes to induce synapse formation and to modify synaptic transmission, using cultured autaptic neurons. By 9 weeks in culture, astrocytes derived from the mouse cerebral cortex demonstrated increases in β-galactosidase activity and glial fibrillary acidic protein (GFAP) expression, both of which are characteristic of aging and glial activation in vitro. Autaptic hippocampal neurons plated on these aging astrocytes showed a smaller amount of evoked release of the excitatory neurotransmitter glutamate, and a lower frequency of miniature release of glutamate, both of which were attributable to a reduction in the pool of readily releasable synaptic vesicles. Other features of synaptogenesis and synaptic transmission were retained, for example the ability to induce structural synapses, the presynaptic release probability, the fraction of functional presynaptic nerve terminals, and the ability to recruit functional AMPA and NMDA glutamate receptors to synapses. Thus the presence of aging astrocytes affects the efficiency of synaptic transmission. Given that the pool of readily releasable vesicles is also small at immature synapses, our results are consistent with astrocytic aging leading to retarded synapse maturation.

Long-term culture of astrocytes attenuates the readily releasable pool of synaptic vesicles.

Increased activities of cytoplasmic calcium and the excitatory neurotransmitter glutamate have been independently implicated in dystonia pathophysiology. However, cellular-level evidence linking these two features is not available. Here we show that glutamate-dependent changes in neuronal calcium dynamics occur in a knock-in mouse model of DYT1 dystonia, the most common hereditary form of this disorder. Fluorescence-based analysis of the dynamics of cytoplasmic calcium concentration ([Ca(2+)]c) in cultured hippocampal neurons shows that electrical stimulation depolarizes the neurons and increases the dendritic [Ca(2+)]c, which then decays slowly to the pre-stimulus level. Whereas the peak amplitude of [Ca(2+)]c was not affected, the decay period was prolonged in neurons of heterozygous mice whose genotype reflects the human condition. We found that this effect was blocked by the antagonists of ionotropic glutamate receptors, and confirmed that glutamate receptors are present in these neurons. As the [Ca(2+)]c is readout and regulator of neuronal excitability, its abnormality represents an important cellular phenotype of dystonia.

Abnormal cytoplasmic calcium dynamics in central neurons of a dystonia mouse model.

Although recent developments in methodologies for light microscopy have enabled imaging of fine biological structures, such imaging is often accompanied by two types of problems. One is a tilting of the specimen with respect to the x-y plane (i.e. rotation around the x- or y-axis) such that the sample is not perpendicular to the optical z-axis, and the other is rotation around the z-axis that precludes optimal orientations for imaging and experimentation. These rotation problems can cause optical aberrations and hamper imaging experiments, even when the angular difference from the ideal position is small.

Light-microscope specimen holder with 3-axis rotation and small-angle control.

DYT1 dystonia is the most common hereditary form of primary torsion dystonia. This autosomal-dominant disorder is characterized by involuntary muscle contractions that cause sustained twisting and repetitive movements. It is caused by an in-frame deletion in the TOR1A gene, leading to the deletion of a glutamic acid residue in the torsinA protein. Heterozygous knock-in mice, which reproduce the genetic mutation in human patients, have abnormalities in synaptic transmission at the principal GABAergic neurons in the striatum, a brain structure that is involved in the execution and modulation of motor activity. However, whether this mutation affects the excitability of striatal GABAergic neurons has not been investigated in this animal model. Here, we examined the excitability of cultured striatal neurons obtained from heterozygous knock-in mice, using calcium imaging as indirect readout. Immunofluorescence revealed that more than 97% of these neurons are positive for a marker of GABAergic neurons, and that more than 92% are also positive for a marker of medium spiny neurons, indicating that these are mixed cultures of mostly medium spiny neurons and a few (~5%) GABAergic interneurons. When these neurons were depolarized by field stimulation, the calcium concentration in the dendrites increased rapidly and then decayed slowly. The amplitudes of calcium transients were larger in heterozygous neurons than in wild-type neurons, resulting in ~15% increase in cumulative calcium transients during a train of stimuli. However, there was no change in other parameters of calcium dynamics. Given that calcium dynamics reflect neuronal excitability, these results suggest that the mutation only slightly increases the excitability of striatal GABAergic neurons in DYT1 dystonia.

Minimal Change in the cytoplasmic calcium dynamics in striatal GABAergic neurons of a DYT1 dystonia knock-in mouse model.

Images in biomedical imaging research are often affected by non-specific background noise. This poses a serious problem when the noise overlaps with specific signals to be quantified, e.g. for their number and intensity. A simple and effective means of removing background noise is to prepare a filtered image that closely reflects background noise and to subtract it from the original unfiltered image. This approach is in common use, but its effectiveness in identifying and quantifying synaptic puncta has not been characterized in detail.

N. Charles Harata

Department of Molecular Physiology & Biophysics

University of Iowa Carver College of Medicine

Examination of Synaptic Vesicle Recycling Using FM Dyes During Evoked, Spontaneous, and Miniature Synaptic Activities

Rapid Genotyping of Animals Followed by Establishing Primary Cultures of Brain Neurons

N. Charles Harata

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Molecular Physiology & Biophysics

University of Iowa Carver College of Medicine

Examination of Synaptic Vesicle Recycling Using FM Dyes During Evoked, Spontaneous, and Miniature Synaptic Activities

Rapid Genotyping of Animals Followed by Establishing Primary Cultures of Brain Neurons

Department of Molecular Physiology & Biophysics