Exploring with [18F]UCB-H the in vivo Variations in SV2A Expression through the Kainic Acid Rat Model of Temporal Lobe Epilepsy

Association of synaptic density and cognitive performance in temporal lobe epilepsy: Humans and animals PET imaging study with [18F]SynVesT-1

AIM

Cognitive impairment is a common comorbidity in individuals with temporal lobe epilepsy (TLE), yet the underlying mechanisms remain unknown. This study explored the putative association between in vivo synaptic loss and cognitive outcomes in TLE patients by PET imaging of synaptic vesicle glycoprotein 2A (SV2A).

METHODS

We enrolled 16 TLE patients and 10 cognitively normal controls. All participants underwent SV2A PET imaging using [18F]SynVesT-1 and cognitive assessment. Lithium chloride-pilocarpine-induced rats with status epilepticus (n = 20) and controls (n = 6) rats received levetiracetam (LEV, specifically binds to SV2A), valproic acid (VPA), or saline for 14 days. Then, synaptic density was quantified by [18F]SynVesT-1 micro-PET/CT. The novel object recognition and Morris water maze tests evaluated TLE-related cognitive function. SV2A expression was examined and confirmed by immunohistochemistry.

RESULTS

Temporal lobe epilepsy patients showed significantly reduced synaptic density in hippocampus, which was associated with cognitive performance. In the rat model of TLE, the expression of SV2A and synaptic density decreased consistently in a wider range of brain regions, including the entorhinal cortex, insula, hippocampus, amygdala, thalamus, and cortex. We treated TLE animal models with LEV or VPA to explore whether synaptic loss contributes to cognitive deficits. It was found that LEV significantly exerted protective effects against brain synaptic deficits and cognitive impairment.

CONCLUSION

This is the first study to link synaptic loss to cognitive deficits in TLE, suggesting [18F]SynVesT-1 PET could be a promising biomarker for monitoring synaptic loss and cognitive dysfunction. LEV might help reverse synaptic deficits and ameliorate learning and memory impairments in TLE patients.

Decreased synaptic vesicle glycoprotein 2A binding in a rodent model of familial Alzheimer's disease detected by [18F]SDM-16

Introduction

Synapse loss is one of the hallmarks of Alzheimer's disease (AD) and is associated with cognitive decline. In this study, we tested [18F]SDM-16, a novel metabolically stable SV2A PET imaging probe, in the transgenic APPswe/PS1dE9 (APP/PS1) mouse model of AD and age-matched wild-type (WT) mice at 12 months of age.

Methods

Based on previous preclinical PET imaging studies using [11C]UCB-J and [18F]SynVesT-1 in the same strain animals, we used the simplified reference tissue model (SRTM), with brain stem as the pseudo reference region to calculate distribution volume ratios (DVRs).

Results

To simplify and streamline the quantitative analysis, we compared the standardized uptake value ratios (SUVRs) from different imaging windows to DVRs and found that the averaged SUVRs from 60-90 min post-injection (p.i.) are most consistent with the DVRs. Thus, we used averaged SUVRs from 60-90 min for group comparisons and found statistically significant differences in the tracer uptake in different brain regions, e.g., hippocampus (p = 0.001), striatum (p = 0.002), thalamus (p = 0.003), and cingulate cortex (p = 0.0003).

Conclusions

In conclusion, [18F]SDM-16 was used to detect decreased SV2A levels in the brain of APP/PS1 AD mouse model at one year old. Our data suggest that [18F]SDM-16 has similar statistical power in detecting the synapse loss in APP/PS1 mice as [11C]UCB-J and [18F]SynVesT-1, albeit later imaging window (60-90 min p.i.) is needed when SUVR is used as a surrogate for DVR for [18F]SDM-16 due to its slower brain kinetics.

Cortical abnormalities of synaptic vesicle protein 2A in focal cortical dysplasia type II identified in vivo with 18F-SynVesT-1 positron emission tomography imaging

PURPOSE

The loss of synaptic vesicle glycoprotein 2A (SV2A) is well established as the major correlate of epileptogenesis in focal cortical dysplasia type II (FCD II), but this has not been directly tested in vivo. In this positron emission tomography (PET) study with the new tracer ¹⁸F-SynVesT-1, we evaluated SV2A abnormalities in patients with FCD II and compared the pattern to ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG).

METHODS

Sixteen patients with proven FCD II and 16 healthy controls were recruited. All FCD II patients underwent magnetic resonance imaging (MRI) and static PET imaging with both ¹⁸F-SynVesT-1 and ¹⁸F-FDG, while the controls underwent MRI and PET with only ¹⁸F-SynVesT-1. Visual assessment of PET images was undertaken. The standardized uptake values (SUVs) of ¹⁸F-SynVesT-1 were computed for regions of interest (ROIs), along with SUV ratio (SUVr) between ROI and centrum semiovale (white matter). Asymmetry indices (AIs) were analyzed between the lesion and the contralateral hemisphere for intersubject comparisons.

RESULTS

Lesions in the brains of FCD II patients had significantly reduced ¹⁸F-SynVesT-1 uptake compared with contralateral regions, and brains of the controls. ¹⁸F-SynVesT-1 PET indicated low lesion uptake in 14 patients (87.5%), corresponding to hypometabolism detected by ¹⁸F-FDG PET, with higher accuracy for lesion localization than MRI (43.8%) (P < 0.05). AI analyses demonstrated that in the lesions, SUVr for each of the radiotracers were not significantly different (P > 0.05), and ¹⁸F-SynVesT-1 SUVr correlated with that of ¹⁸F-FDG across subjects (R² = 0.41, P = 0.008). Subsequent visual ratings indicated that ¹⁸F-SynVesT-1 uptake had a more restricted pattern of reduction than ¹⁸F-FDG uptake in FCD II lesions (P < 0.05).

CONCLUSION

SV2A PET with ¹⁸F-SynVesT-1 shows a higher accuracy for the localization of FCD II lesions than MRI and a more restricted pattern of abnormality than ¹⁸F-FDG PET.

Spatio-Temporal Alterations in Synaptic Density During Epileptogenesis in the Rat Brain

Synaptic vesicle glycoprotein 2A (SV2A) is a transmembrane protein that binds levetiracetam and is involved in neurotransmission via an unknown mechanism. SV2A-immunoreactivity is reduced in animal models of epilepsy, and in postmortem hippocampi from patients with temporal lobe epilepsy. It is not known if other regions outside the hippocampus are affected in epilepsy, and whether SV2A is expression permanently reduced or regulated over time. In this study, we induced a generalized status epilepticus (SE) by systemic administration of lithium-pilocarpine to adult female rats. The brains from all animals experiencing SE were collected at different time points after the treatment. The radiotracer, [¹¹C]-UCB-J, binds to SV2A with high affinity, and has been used for in vivo imaging as an a-proxy marker for synaptic density. Here we determined the level of tritiated UCB-J binding by semiquantitative autoradiography in the cerebral cortex, hippocampus, thalamus, and hypothalamus, and in subregions of these. A prominent and highly significant reduction in SV2A binding capacity was observed over the first days after SE in the cerebral cortex and the hippocampus, but not in the thalamus and hypothalamus. The magnitude in reduction was larger and occurred earlier in the hippocampus and the piriform cortex, than in other cortical subregions. Interestingly, in all areas examined, the binding capacity returned to control levels 12 weeks after the SE comparable to the chronic phase. These data show that lithium-pilocarpine-induced epileptogenesis involves both loss and gain of synapses in the in a time-dependent manner.

Neuroimaging in the Epileptic Baboon

Characterization of baboon model of genetic generalized epilepsy (GGE) is driven both electroclinically and by successful adoption of neuroimaging platforms, such as magnetic resonance imaging (MRI) and positron emission tomography (PET). Based upon its phylogenetic proximity and similar brain anatomy to humans, the epileptic baboon provides an excellent translational model. Its relatively large brain size compared to smaller nonhuman primates or rodents, a gyrencephalic structure compared to lissencephalic organization of rodent brains, and the availability of a large pedigreed colony allows exploration of neuroimaging markers of diseases. Similar to human idiopathic generalized epilepsy (IGE), structural imaging in the baboon is usually normal in individual subjects, but gray matter volume/concentration (GMV/GMC) changes are reported by statistical parametric mapping (SPM) analyses. Functional neuroimaging has been effective for mapping the photoepileptic responses, the epileptic network, altered functional connectivity of physiological networks, and the effects of anti-seizure therapies. This review will provide insights into our current understanding the baboon model of GGE through functional and structural imaging.

Synaptic Vesicle Glycoprotein 2A: Features and Functions

In recent years, the field of neuroimaging dramatically moved forward by means of the expeditious development of specific radioligands of novel targets. Among these targets, the synaptic vesicle glycoprotein 2A (SV2A) is a transmembrane protein of synaptic vesicles, present in all synaptic terminals, irrespective of neurotransmitter content. It is involved in key functions of neurons, focused on the regulation of neurotransmitter release. The ubiquitous expression in gray matter regions of the brain is the basis of its candidacy as a marker of synaptic density. Following the development of molecules derived from the structure of the anti-epileptic drug levetiracetam, which selectively binds to SV2A, several radiolabeled markers have been synthetized to allow the study of SV2A distribution with positron emission tomography (PET). These radioligands permit the evaluation of in vivo changes of SV2A distribution held to be a potential measure of synaptic density in physiological and pathological conditions. The use of SV2A as a biomarker of synaptic density raises important questions. Despite numerous studies over the last decades, the biological function and the expressional properties of SV2A remain poorly understood. Some functions of SV2A were claimed, but have not been fully elucidated. While the expression of SV2A is ubiquitous, stronger associations between SV2A and Υ amino butyric acid (GABA)-ergic rather than glutamatergic synapses were observed in some brain structures. A further issue is the unclear interaction between SV2A and its tracers, which reflects a need to clarify what really is detected with neuroimaging tools. Here, we summarize the current knowledge of the SV2A protein and we discuss uncertain aspects of SV2A biology and physiology. As SV2A expression is ubiquitous, but likely more strongly related to a certain type of neurotransmission in particular circumstances, a more extensive knowledge of the protein would greatly facilitate the analysis and interpretation of neuroimaging results by allowing the evaluation not only of an increase or decrease of the protein level, but also of the type of neurotransmission involved.

Imaging Synaptic Density: The Next Holy Grail of Neuroscience?

The brain is the central and most complex organ in the nervous system, comprising billions of neurons that constantly communicate through trillions of connections called synapses. Despite being formed mainly during prenatal and early postnatal development, synapses are continually refined and eliminated throughout life via complicated and hitherto incompletely understood mechanisms. Failure to correctly regulate the numbers and distribution of synapses has been associated with many neurological and psychiatric disorders, including autism, epilepsy, Alzheimer’s disease, and schizophrenia. Therefore, measurements of brain synaptic density, as well as early detection of synaptic dysfunction, are essential for understanding normal and abnormal brain development. To date, multiple synaptic density markers have been proposed and investigated in experimental models of brain disorders. The majority of the gold standard methodologies (e.g., electron microscopy or immunohistochemistry) visualize synapses or measure changes in pre- and postsynaptic proteins ex vivo. However, the invasive nature of these classic methodologies precludes their use in living organisms. The recent development of positron emission tomography (PET) tracers [such as (¹⁸F)UCB-H or (¹¹C)UCB-J] that bind to a putative synaptic density marker, the synaptic vesicle 2A (SV2A) protein, is heralding a likely paradigm shift in detecting synaptic alterations in patients. Despite their limited specificity, novel, non-invasive magnetic resonance (MR)-based methods also show promise in inferring synaptic information by linking to glutamate neurotransmission. Although promising, all these methods entail various advantages and limitations that must be addressed before becoming part of routine clinical practice. In this review, we summarize and discuss current ex vivo and in vivo methods of quantifying synaptic density, including an evaluation of their reliability and experimental utility. We conclude with a critical assessment of challenges that need to be overcome before successfully employing synaptic density biomarkers as diagnostic and/or prognostic tools in the study of neurological and neuropsychiatric disorders.

Longitudinal [18]UCB-H/[18F]FDG imaging depicts complex patterns of structural and functional neuroplasticity following bilateral vestibular loss in the rat

Neuronal lesions trigger mechanisms of structural and functional neuroplasticity, which can support recovery. However, the temporal and spatial appearance of structure–function changes and their interrelation remain unclear. The current study aimed to directly compare serial whole-brain in vivo measurements of functional plasticity (by [¹⁸F]FDG-PET) and structural synaptic plasticity (by [¹⁸F]UCB-H-PET) before and after bilateral labyrinthectomy in rats and investigate the effect of locomotor training. Complex structure–function changes were found after bilateral labyrinthectomy: in brainstem-cerebellar circuits, regional cerebral glucose metabolism (rCGM) decreased early, followed by reduced synaptic density. In the thalamus, increased [¹⁸F]UCB-H binding preceded a higher rCGM uptake. In frontal-basal ganglia loops, an increase in synaptic density was paralleled by a decrease in rCGM. In the group with locomotor training, thalamic rCGM and [¹⁸F]UCB-H binding increased following bilateral labyrinthectomy compared to the no training group. Rats with training had considerably fewer body rotations. In conclusion, combined [¹⁸F]FDG/[¹⁸F]UCB-H dual tracer imaging reveals that adaptive neuroplasticity after bilateral vestibular loss is not a uniform process but is composed of complex spatial and temporal patterns of structure–function coupling in networks for vestibular, multisensory, and motor control, which can be modulated by early physical training.

PET Imaging of Synaptic Density: Challenges and Opportunities of Synaptic Vesicle Glycoprotein 2A PET in Small Animal Imaging

The development of novel PET imaging agents for synaptic vesicle glycoprotein 2A (SV2A) allowed for the in vivo detection of synaptic density changes, which are correlated with the progression and severity of a variety of neuropsychiatric diseases. While multiple ongoing clinical investigations using SV2A PET are expanding its applications rapidly, preclinical SV2A PET imaging in animal models is an integral component of the translation research and provides supporting and complementary information. Herein, we overview preclinical SV2A PET studies in animal models of neurodegenerative disorders and discuss the opportunities and practical challenges in small animal SV2A PET imaging. At the Yale PET Center, we have conducted SV2A PET imaging studies in animal models of multiple diseases and longitudinal SV2A PET allowed us to evaluate synaptic density dynamics in the brains of disease animal models and to assess pharmacological effects of novel interventions. In this article, we discuss key considerations when designing preclinical SV2A PET imaging studies and strategies for data analysis. Specifically, we compare the brain imaging characteristics of available SV2A tracers, i.e., [11C]UCB-J, [18F]SynVesT-1, [18F]SynVesT-2, and [18F]SDM-16, in rodent brains. We also discuss the limited spatial resolution of PET scanners for small brains and challenges of kinetic modeling. We then compare different injection routes and estimate the maximum throughput (i.e., number of animals) per radiotracer synthesis by taking into account the injectable volume for each injection method, injected mass, and radioactivity half-lives. In summary, this article provides a perspective for designing and analyzing SV2A PET imaging studies in small animals.

Animal Models of Epilepsy: A Phenotype-oriented Review

Epilepsy is a serious neurological disorder characterized by abnormal, recurrent, and synchronous discharges in the brain. Long-term recurrent seizure attacks can cause serious damage to brain function, which is usually observed in patients with temporal lobe epilepsy. Controlling seizure attacks is vital for the treatment and prognosis of epilepsy. Animal models, such as the kindling model, which was the most widely used model in the past, allow the understanding of the potential epileptogenic mechanisms and selection of antiepileptic drugs. In recent years, various animal models of epilepsy have been established to mimic different seizure types, without clear merits and demerits. Accordingly, this review provides a summary of the views mentioned above, aiming to provide a reference for animal model selection.

TSPO PET upregulation predicts epileptic phenotype at disease onset independently from chronic TSPO expression in a rat model of temporal lobe epilepsy

Neuroinflammation is a key component of epileptogenesis, the process leading to acquired epilepsy. In recent years, with the development of non-invasive in vivo positron emission tomography (PET) imaging of translocator protein 18 kDa (TSPO), a marker of neuroinflammation, it has become possible to perform longitudinal studies to characterize neuroinflammation at different disease stages in animal models of epileptogenesis. This study aimed to utilize the prognostic capability of TSPO PET imaging at disease onset (2 weeks post-SE) to categorize epileptic rats with distinct seizure burden based on TSPO levels at disease onset and investigate their association to TSPO expression at the chronic epilepsy stage. Controls (n = 14) and kainic acid-induced status epilepticus (KASE) rats (n = 41) were scanned non-invasively with [18F]PBR111 PET imaging measuring TSPO expression. Animals were monitored using video-electroencephalography (vEEG) up to chronic disease (12 weeks post-SE), at which TSPO levels ([3H]PK11195) as well as other post-mortem abnormalities (namely synaptic density ([3H]UCB-J), neuronal loss (NeuN), and neurodegeneration (FjC)) were investigated. By applying multivariate analysis, TSPO PET imaging at disease onset identified three KASE groups with significantly different spontaneous recurrent seizures (SRS) burden (defined as rare SRS, sporadic SRS, and frequent SRS) (p = 0.003). Interestingly, TSPO levels were significantly different when comparing the three KASE groups (p < 0.0001), with the frequent SRS group characterized only by a limited focal TSPO increase at disease onset. On the contrary, TSPO measured during chronic epilepsy was found to be the highest in the frequent SRS group and correlated with seizure burden (r = 0.826, p < 0.0001). Importantly, early and chronic TSPO levels did not correlate (r = -0.05). Finally, significant pathological changes in neuronal loss, synaptic density, and neurodegeneration were found not only when compared to control animals (p < 0.01), but also between the three KASE rat categories in the hippocampus (p < 0.05). Early and chronic TSPO upregulation following epileptogenic insult appear to be driven by two superimposed dynamic processes. The former is associated with epileptogenesis as measured at disease onset, while the latter is related to seizure frequency as quantified during chronic epilepsy.

The pharmacokinetics of [18F]UCB-H revisited in the healthy non-human primate brain

Background

Positron Emission Tomography (PET) imaging of the Synaptic Vesicle glycoprotein (SV) 2A is a new tool to quantify synaptic density. [¹⁸F]UCB-H was one of the first promising SV2A-ligands to be labelled and used in vivo in rodent and human, while limited information on its pharmacokinetic properties is available in the non-human primate. Here, we evaluate the reliability of the three most commonly used modelling approaches for [¹⁸F]UCB-H in the non-human cynomolgus primate, adding the coupled fit of the non-displaceable distribution volume (V_ND) as an alternative approach to improve unstable fit. The results are discussed in the light of the current state of SV2A PET ligands.

Results

[¹⁸F]UCB-H pharmacokinetic data was optimally fitted with a two-compartment model (2TCM), although the model did not always converge (large total volume of distribution (V_T) or large uncertainty of the estimate). 2TCM with coupled fit K₁/k₂ across brain regions stabilized the quantification, and confirmed a lower specific signal of [¹⁸F]UCB-H compared to the newest SV2A-ligands. However, the measures of V_ND and the influx parameter (K₁) are similar to what has been reported for other SV2A ligands. These data were reinforced by displacement studies using [¹⁹F]UCB-H, demonstrating only 50% displacement of the total [¹⁸F]UCB-H signal at maximal occupancy of SV2A. As previously demonstrated in clinical studies, the graphical method of Logan provided a more robust estimate of V_T with only a small bias compared to 2TCM.

Conclusions

Modeling issues with a 2TCM due to a slow component have previously been reported for other SV2A ligands with low specific binding, or after blocking of specific binding. As all SV2A ligands share chemical structural similarities, we hypothesize that this slow binding component is common for all SV2A ligands, but only hampers quantification when specific binding is low.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-021-00777-8.

The Rise of Synaptic Density PET Imaging

Many neurological disorders are related to synaptic loss or pathologies. Before the boom of positrons emission tomography (PET) imaging of synapses, synaptic quantification could only be achieved in vitro on brain samples after autopsy or surgical resections. Until the mid-2010s, electron microscopy and immunohistochemical labelling of synaptic proteins were the gold-standard methods for such analyses. Over the last decade, several PET radiotracers for the synaptic vesicle 2A protein have been developed to achieve in vivo synapses visualization and quantification. Different strategies were used, namely radiolabelling with either ¹¹C or ¹⁸F, preclinical development in rodent and non-human primates, and binding quantification with different kinetic modelling methods. This review provides an overview of these PET tracers and underlines their perspectives and limitations by focusing on radiochemical aspects, as well as preclinical proof-of-concept and the main clinical outcomes described so far.