Antibody engagement with amyloid-beta does not inhibit [11C]PiB binding for PET imaging.

The elimination of amyloid-beta (Aß) plaques in Alzheimer's disease patients after treatment with anti-Aß antibodies such as lecanemab and aducanumab is supported by a substantially decreased signal in amyloid positron emission tomography (PET) imaging. However, this decreased PET signal has not been matched by a similar substantial effect on cognitive function. There may be several reasons for this, including short treatment duration and advanced disease stages among the patients. However, one aspect that has not been investigated, and the subject of this study, is whether antibody engagement with amyloid plaques inhibits the binding of amyloid-PET ligands, leading to a false impression of Aß removal from the brain. In the present study, tg-ArcSwe mice received three injections of RmAb158, the murine version of lecanemab or phosphate-buffered saline (PBS) before the administration of the amyloid-PET radioligand [11C]PiB, followed by isolation of brain tissue. Autoradiography showed that RmAb158- and PBS-treated mice displayed similar [11C]PiB binding. Moreover, the total Aß1-40 levels, representing the major Aß species of plaques in the tg-ArcSwe model, as well as soluble triggering receptor on myeloid cells 2 (sTREM2) levels, were similar in both groups. Interestingly, the concentration of soluble Aß aggregates was decreased in the RmAb158-treated group, along with a small but significant decrease in the total Aß1-42 levels. In conclusion, this study indicates that the binding of [11C]PiB to Aß accurately mirrors the load of Aß plaques in the brain, aligning with how amyloid-PET is interpreted in clinical studies of anti-Aß antibodies. However, early treatment effects on soluble Aß aggregates and Aß1-42 levels were not detected.

Synaptic density in aging mice measured by [18F]SynVesT-1 PET.

Synaptic alterations in certain brain structures are related to cognitive decline in neurodegeneration and in aging. Synaptic loss in many neurodegenerative diseases can be visualized by positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A). However, the use of SV2A PET for studying synaptic changes during aging is not particularly explored. Thus, in the present study, PET ligand [18F]SynVesT-1, which binds to SV2A, was used to investigate synaptic density at different ages in healthy mice. Wild type C57BL/6 mice divided into three age groups (4-5 months (n = 7), 12-14 months (n = 11), 17-19 months (n = 7)) were PET scanned with [18F]SynVesT-1. Brain retention of [18F]SynVesT-1 expressed as the volume of distribution (VIDIF) was calculated using an image-derived input function. Estimates of VIDIF were derived using either a one-tissue compartment model (1TCM), a two-tissue compartment model (2TCM), or the Logan plot with blood input to find the best-fit model for [18F]SynVesT-1. After the PET scans, tissue sections were immunostained for the detection of SV2A and neuronal markers. We found that [18F]SynVesT-1 data acquired 60 min post intravenously injection and analyzed with 1TCM described the brain pharmacokinetics of the radioligand in mice well. [18F]SynVesT-1 brain retention was lower in the oldest group of mice, indicating a decrease in synaptic density in this age group. However, no gradual age-dependent decrease in synaptic density at a region-specific level was observed. Immunostaining indicated that SV2A expression and neuron numbers were similar across all three age groups. In general, these data obtained in healthy aging mice are consistent with previous findings in humans where synaptic density appeared stable during aging up to a certain age, after which a small decrease is observed.

PET Imaging in Preclinical Anti-Aß Drug Development.

Positron emission tomography (PET), a medical imaging technique allowing for studies of the living human brain, has gained an important role in clinical trials of novel drugs against Alzheimer's disease (AD). For example, PET data contributed to the conditional approval in 2021 of aducanumab, an antibody directed towards amyloid-beta (Aß) aggregates, by showing a dose-dependent reduction in brain amyloid after treatment. In parallel to clinical studies, preclinical studies in animal models of Aß pathology may also benefit from PET as a tool to detect target engagement and treatment effects of anti-Aß drug candidates. PET is associated with a high level of translatability between species as similar, non-invasive protocols allow for longitudinal rather than cross-sectional studies and can be used both in a preclinical and clinical setting. This review focuses on the use of preclinical PET imaging in genetically modified animals that express human Aß, and its present and potential future role in the development of drugs aimed at reducing brain Aß levels as a therapeutic strategy to halt disease progression in AD.

The polyamine transporter Slc18b1(VPAT) is important for both short and long time memory and for regulation of polyamine content in the brain.

SLC18B1 is a sister gene to the vesicular monoamine and acetylcholine transporters, and the only known polyamine transporter, with unknown physiological role. We reveal that Slc18b1 knock out mice has significantly reduced polyamine content in the brain providing the first evidence that Slc18b1 is functionally required for regulating polyamine levels. We found that this mouse has impaired short and long term memory in novel object recognition, radial arm maze and self-administration paradigms. We also show that Slc18b1 KO mice have altered expression of genes involved in Long Term Potentiation, plasticity, calcium signalling and synaptic functions and that expression of components of GABA and glutamate signalling are changed. We further observe a partial resistance to diazepam, manifested as significantly lowered reduction in locomotion after diazepam treatment. We suggest that removal of Slc18b1 leads to reduction of polyamine contents in neurons, resulting in reduced GABA signalling due to long-term reduction in glutamatergic signalling.

In vivo imaging of synaptic density with [11C]UCB-J PET in two mouse models of neurodegenerative disease.

The positron emission tomography (PET) radioligand [11C]UCB-J binds to synaptic vesicle protein 2A (SV2A) and is used to investigate synaptic density in the living brain. Clinical studies have indicated reduced [11C]UCB-J binding in Alzheimer's disease (AD) and Parkinson's disease (PD) brains compared to healthy controls. Still, it is unknown whether [11C]UCB-J PET can visualise synaptic loss in mouse models of these disorders. Such models are essential for understanding disease pathology and for evaluating the effects of novel disease-modifying drug candidates. In the present study, synaptic density in transgenic models of AD (ArcSwe) and PD (L61) was studied using [11C]UCB-J PET. Data were acquired during 60 min after injection, and time-activity curves (TACs) in different brain regions and the left ventricle of the heart were generated based on the dynamic PET images. The [11C]UCB-J brain concentrations were expressed as standardised uptake value (SUV) over time. The area under the SUV curve (AUC), the ratio of AUC in the brain to that in the heart (AUCbrain/blood), and the volume of distribution (VT) obtained by kinetic modelling using the heart TAC as input were compared between transgenic and age-matched wild type (WT) mice. The L61 mice displayed 11-13% lower AUCbrain/blood ratio and brain VT generated by kinetic modeling compared to the control WT mice. In general, also transgenic ArcSwe mice tended to show lower [11C]UCB-J brain exposure than age-matched WT controls, but variation within the different animal groups was high. Older WT mice (18-20 months) showed lower [11C]UCB-J brain exposure than younger WT mice (8-9 months). Together, these data imply that [11C]UCB-J PET reflects synaptic density in mouse models of neurodegeneration and that inter-subject variation is large. In addition, the study suggested that model-independent AUCbrain/blood ratio can be used to evaluate [11C]UCB-J binding as an alternative to full pharmacokinetic modelling.

Histological characterization of orphan transporter MCT14 (SLC16A14) shows abundant expression in mouse CNS and kidney.

BACKGROUND: MCT14 (SLC16A14) is an orphan member of the monocarboxylate transporter (MCT) family, also known as the SLC16 family of secondary active transmembrane transporters. Available expression data for this transporter is limited, and in this paper we aim to characterize MCT14 with respect to tissue distribution and cellular localization in mouse brain. RESULTS: Using qPCR, we found that Slc16a14 mRNA was highly abundant in mouse kidney and moderately in central nervous system, testis, uterus and liver. Using immunohistochemistry and in situ hybridization, we determined that MCT14 was highly expressed in excitatory and inhibitory neurons as well as epithelial cells in the mouse brain. The expression was exclusively localized to the soma of neurons. Furthermore, we showed with our phylogenetic analysis that MCT14 most closely relate to the aromatic amino acid- and thyroid-hormone transporters MCT8 (SLC16A2) and MCT10 (SLC16A10), in addition to the carnitine transporter MCT9 (SLC16A9). CONCLUSIONS: We provide here the first histological mapping of MCT14 in the brain and our data are consistent with the hypothesis that MCT14 is a neuronal aromatic-amino-acid transporter.

Long-term effects of immunotherapy with a brain penetrating Aß antibody in a mouse model of Alzheimer's disease.

BACKGROUND: Brain-directed immunotherapy is a promising strategy to target amyloid-ß (Aß) deposits in Alzheimer's disease (AD). In the present study, we compared the therapeutic efficacy of the Aß protofibril targeting antibody RmAb158 with its bispecific variant RmAb158-scFv8D3, which enters the brain by transferrin receptor-mediated transcytosis. METHODS: AppNL-G-F knock-in mice received RmAb158, RmAb158-scFv8D3, or PBS in three treatment regimens. First, to assess the acute therapeutic effect, a single antibody dose was given to 5 months old AppNL-G-F mice, with evaluation after 3 days. Second, to assess the antibodies' ability to halt the progression of Aß pathology, 3 months old AppNL-G-F mice received three doses during a week, with evaluation after 2 months. Reduction of RmAb158-scFv8D3 immunogenicity was explored by introducing mutations in the antibody or by depletion of CD4+ T cells. Third, to study the effects of chronic treatment, 7-month-old AppNL-G-F mice were CD4+ T cell depleted and treated with weekly antibody injections for 8 weeks, including a final diagnostic dose of [125I]RmAb158-scFv8D3, to determine its brain uptake ex vivo. Soluble Aß aggregates and total Aß42 were quantified with ELISA and immunostaining. RESULTS: Neither RmAb158-scFv8D3 nor RmAb158 reduced soluble Aß protofibrils or insoluble Aß1-42 after a single injection treatment. After three successive injections, Aß1-42 was reduced in mice treated with RmAb158, with a similar trend in RmAb158-scFv8D3-treated mice. Bispecific antibody immunogenicity was somewhat reduced by directed mutations, but CD4+ T cell depletion was used for long-term therapy. CD4+ T cell-depleted mice, chronically treated with RmAb158-scFv8D3, showed a dose-dependent increase in blood concentration of the diagnostic [125I]RmAb158-scFv8D3, while concentration was low in plasma and brain. Chronic treatment did not affect soluble Aß aggregates, but a reduction in total Aß42 was seen in the cortex of mice treated with both antibodies. CONCLUSIONS: Both RmAb158 and its bispecific variant RmAb158-scFv8D3 achieved positive effects of long-term treatment. Despite its ability to efficiently enter the brain, the benefit of using the bispecific antibody in chronic treatment was limited by its reduced plasma exposure, which may be a result of interactions with TfR or the immune system. Future research will focus in new antibody formats to further improve Aß immunotherapy.

Brain pharmacokinetics of mono- and bispecific amyloid-ß antibodies in wild-type and Alzheimer's disease mice measured by high cut-off microdialysis.

BACKGROUND: Treatment with amyloid-ß (Aß) targeting antibodies is a promising approach to remove Aß brain pathology in Alzheimer's disease (AD) and possibly even slow down or stop progression of the disease. One of the main challenges of brain immunotherapy is the restricted delivery of antibodies to the brain. However, bispecific antibodies that utilize the transferrin receptor (TfR) as a shuttle for transport across the blood-brain barrier (BBB) can access the brain better than traditional monospecific antibodies. Previous studies have shown that bispecific Aß targeting antibodies have higher brain distribution, and can remove Aß pathology more efficiently than monospecific antibodies. Yet, there is only limited information available on brain pharmacokinetics, especially regarding differences between mono- and bispecific antibodies. METHODS: The aim of the study was to compare brain pharmacokinetics of Aß-targeting monospecific mAb3D6 and its bispecific version mAb3D6-scFv8D3 that also targets TfR. High cut-off microdialysis was used to measure intravenously injected radiolabelled mAb3D6 and mAb3D6-scFv8D3 antibodies in the interstitial fluid (ISF) of hippocampus in wild-type mice and the AppNL-G-F mouse model of AD. Distribution of the antibodies in the brain and the peripheral tissue was examined by ex vivo autoradiography and biodistribution studies. RESULTS: Brain concentrations of the bispecific antibody were elevated compared to the monospecific antibody in the hippocampal ISF measured by microdialysis and in the brain tissue at 4-6 h after an intravenous injection. The concentration of the bispecific antibody was approximately twofold higher in the ISF dialysate compared to the concentration of monospecific antibody and eightfold higher in brain tissue 6 h post-injection. The ISF dialysate concentrations for both antibodies were similar in both wild-type and AppNL-G-F mice 24 h post-injection, although the total brain tissue concentration of the bispecific antibody was higher than that of the monospecific antibody at this time point. Some accumulation of radioactivity around the probe area was observed especially for the monospecific antibody indicating that the probe compromised the BBB to some extent at the probe insertion site. CONCLUSION: The BBB-penetrating bispecific antibody displayed higher ISF concentrations than the monospecific antibody. The concentration difference between the two antibodies was even larger in the whole brain than in the ISF. Further, the bispecific antibody, but not the monospecific antibody, displayed higher total brain concentrations than ISF concentrations, indicating association to brain tissue.

Reduction of αSYN Pathology in a Mouse Model of PD Using a Brain-Penetrating Bispecific Antibody.

Immunotherapy targeting aggregated alpha-synuclein (αSYN) is a promising approach for the treatment of Parkinson's disease. However, brain penetration of antibodies is hampered by their large size. Here, RmAbSynO2-scFv8D3, a modified bispecific antibody that targets aggregated αSYN and binds to the transferrin receptor for facilitated brain uptake, was investigated to treat αSYN pathology in transgenic mice. Ex vivo analyses of the blood and brain distribution of RmAbSynO2-scFv8D3 and the unmodified variant RmAbSynO2, as well as in vivo analyses with microdialysis and PET, confirmed fast and efficient brain uptake of the bispecific format. In addition, intravenous administration was shown to be superior to intraperitoneal injections in terms of brain uptake and distribution. Next, aged female αSYN transgenic mice (L61) were administered either RmAbSynO2-scFv8D3, RmAbSynO2, or PBS intravenously three times over five days. Levels of TBS-T soluble aggregated αSYN in the brain following treatment with RmAbSynO2-scFv8D3 were decreased in the cortex and midbrain compared to RmAbSynO2 or PBS controls. Taken together, our results indicate that facilitated brain uptake of αSYN antibodies can improve treatment of αSYN pathology.

In vivo imaging of alpha-synuclein with antibody-based PET.

The protein alpha-synuclein (αSYN) plays a central role in synucleinopathies such as Parkinsons's disease (PD) and multiple system atrophy (MSA). Presently, there are no selective αSYN positron emission tomography (PET) radioligands that do not also show affinity to amyloid-beta (Aß). We have previously shown that radiolabeled antibodies, engineered to enter the brain via the transferrin receptor (TfR), is a promising approach for PET imaging of intrabrain targets. In this study, we used this strategy to visualize αSYN in the living mouse brain. Five bispecific antibodies, binding to both the murine TfR and αSYN were generated and radiolabeled with iodine-125 or iodine-124. All bispecific antibodies bound to αSYN and mTfR before and after radiolabelling in an ELISA assay, and bound to brain sections prepared from αSYN overexpressing mice as well as human PD- and MSA subjects, but not control tissues in autoradiography. Brain concentrations of the bispecific antibodies were between 26 and 63 times higher than the unmodified IgG format 2 h post-injection, corresponding to about 1.5% of the injected dose per gram brain tissue. Additionally, intrastriatal αSYN fibrils were visualized with PET in an αSYN deposition mouse model with one of the bispecific antibodies, [124I]RmAbSynO2-scFv8D3. However, PET images acquired in αSYN transgenic mice with verified brain pathology injected with [124I]RmAbSynO2-scFv8D3 and [124I]RmAb48-scFv8D3 showed no increase in antibody retention compared to WT mice. Despite successful imaging of deposited extracellular αSYN using a brain-penetrating antibody-based radioligand with no cross-specificity towards Aß, this proof-of-concept study demonstrates challenges in imaging intracellular αSYN inclusions present in synucleinopathies.

11C-PiB and 124I-Antibody PET Provide Differing Estimates of Brain Amyloid-ß After Therapeutic Intervention.

PET imaging of amyloid-ß (Aß) has become an important component of Alzheimer disease diagnosis. 11C-Pittsburgh compound B (11C-PiB) and analogs bind to fibrillar Aß. However, levels of nonfibrillar, soluble, aggregates of Aß appear more dynamic during disease progression and more affected by Aß-reducing treatments. The aim of this study was to compare an antibody-based PET ligand targeting nonfibrillar Aß with 11C-PiB after ß-secretase (BACE-1) inhibition in 2 Alzheimer disease mouse models at an advanced stage of Aß pathology. Methods: Transgenic ArcSwe mice (16 mo old) were treated with the BACE-1 inhibitor NB-360 for 2 mo, whereas another group was kept as controls. A third group was analyzed at the age of 16 mo as a baseline. Mice were PET-scanned with 11C-PiB to measure Aß plaque load followed by a scan with the bispecific radioligand 124I-RmAb158-scFv8D3 to investigate nonfibrillar aggregates of Aß. The same study design was then applied to another mouse model, AppNL-G-F In this case, NB-360 treatment was initiated at the age of 8 mo and animals were scanned with 11C-PiB-PET and 125I-RmAb158-scFv8D3 SPECT. Brain tissue was isolated after scanning, and Aß levels were assessed. Results: 124I-RmAb158-scFv8D3 concentrations measured with PET in hippocampus and thalamus of NB-360-treated ArcSwe mice were similar to those observed in baseline animals and significantly lower than concentrations observed in same-age untreated controls. Reduced 125I-RmAb158-scFv8D3 retention was also observed with SPECT in hippocampus, cortex, and cerebellum of NB-360-treated AppNL-G-F mice. Radioligand in vivo concentrations corresponded to postmortem brain tissue analysis of soluble Aß aggregates. For both models, mice treated with NB-360 did not display a reduced 11C-PiB signal compared with untreated controls, and further, both NB-360 and control mice tended, although not reaching significance, to show higher 11C-PiB signal than the baseline groups. Conclusion: This study demonstrated the ability of an antibody-based radioligand to detect changes in brain Aß levels after anti-Aß therapy in ArcSwe and AppNL-G-F mice with pronounced Aß pathology. In contrast, the decreased Aß levels could not be quantified with 11C-PiB PET, suggesting that these ligands detect different pools of Aß.

Age-related increase of alpha-synuclein oligomers is associated with motor disturbances in L61 transgenic mice.

The pathogenesis of Parkinson's disease involves fibrillization and deposition of alpha-synuclein (α-syn) into Lewy bodies. Accumulating evidence suggests that α-syn oligomers are particularly neurotoxic. Transgenic (tg) mice overexpressing wild-type human α-syn under the Thy-1 promoter (L61) reproduce many Parkinson's disease features, but the pathogenetic relevance of α-syn oligomers in this mouse model has not been studied in detail. Here, we report an age progressive increase of α-syn oligomers in the brain of L61 tg mice. Interestingly, more profound motor symptoms were observed in animals with higher levels of membrane-bound oligomers. As this tg model is X-linked, we also performed subset analyses, indicating that both sexes display a similar age-related increase in α-syn oligomers. However, compared with females, males featured increased brain levels of oligomers from an earlier age, in addition to a more severe behavioral phenotype with hyperactivity and thigmotaxis in the open field test. Taken together, our data indicate that α-syn oligomers are central to the development of brain pathology and behavioral deficits in the L61 tg α-syn mouse model.

Blood-brain barrier integrity in a mouse model of Alzheimer's disease with or without acute 3D6 immunotherapy.

The blood-brain barrier (BBB) is suggested to be compromised in Alzheimer's disease (AD). The concomitant presence of vascular amyloid beta (Aß) pathology, so called cerebral amyloid angiopathy (CAA), also predisposes impairment of vessel integrity. Additionally, immunotherapy against Aß may lead to further damage of the BBB. To what extent this affects the BBB passage of molecules is debated. The current study aimed to investigate BBB integrity to large molecules in transgenic mice displaying abundant Aß pathology and age matched wild type animals, with or without acute anti-Aß antibody treatment. Animals were administered a single i.v. injection of PBS or 3D6 (10 mg/kg), i.e. the murine version of the clinically investigated Aß antibody bapineuzumab, supplemented with [125I]3D6. Three days post injections, a 4 kDa FITC and a 150 kDa Antonia Red dextran were administered i.v. to all animals. After termination, fluorescent detection in brain and serum was used for the calculation of dextran brain-to-blood concentration ratios. Further characterization of antibody fate and the presence of CAA were investigated using radioactivity measurements and Congo red staining. BBB passage of large molecules was equally low in wild type and transgenic mice, suggesting an intact BBB despite Aß pathology. Neither was the BBB integrity affected by acute 3D6 treatment. However, CAA was confirmed in the transgenes and local antibody accumulations were observed in the brain, indicating CAA-antibody interactions. The current study shows that independently of Aß pathology or acute 3D6 treatment, the BBB is intact, without extensive permeability to large molecules, including the 3D6 antibody.

Antibody-Based In Vivo PET Imaging Detects Amyloid-ß Reduction in Alzheimer Transgenic Mice After BACE-1 Inhibition.

Visualization of amyloid-ß (Aß) pathology with PET has become an important tool for making a specific clinical diagnosis of Alzheimer disease (AD). However, the available amyloid PET radioligands, such as 11C-Pittsburgh compound B, reflect levels of insoluble Aß plaques but do not capture soluble and protofibrillar Aß forms. Furthermore, the plaque load appears to be fairly static during clinical stages of AD and may not be affected by Aß-reducing treatments. The aim of the present study was to investigate whether a novel PET radioligand based on an antibody directed toward soluble aggregates of Aß can be used to detect changes in Aß levels during disease progression and after treatment with a ß-secretase (BACE-1) inhibitor. Methods: One set of transgenic mice (tg-ArcSwe, a model of Aß pathology) aged between 7 and 16 mo underwent PET with the Aß protofibril-selective radioligand 124I-RmAb158-scFv8D3 (where RmAb is recombinant mouse monoclonal antibody and scFv is single-chain variable fragment) to follow progression of Aß pathology in the brain. A second set of tg-ArcSwe mice, aged 10 mo, were treated with the BACE-1 inhibitor NB-360 for 3 mo and compared with an untreated control group. A third set of tg-ArcSwe mice, also aged 10 mo, underwent PET as a baseline group. Brain tissue was isolated after PET to determine levels of Aß by ELISA and immunohistochemistry. Results: The concentration of 124I-RmAb158-scFv8D3, as measured in vivo with PET, increased with age and corresponded well with the ex vivo autoradiography and Aß immunohistochemistry results. Mice treated with NB-360 showed significantly lower in vivo PET signals than untreated animals and were similar to the baseline animals. The decreased 124I-RmAb158-scFv8D3 concentrations in NB-360-treated mice, as quantified with PET, corresponded well with the decreased Aß levels measured in postmortem brain. Conclusion: Several treatments for AD are in phase 2 and 3 clinical trials, but the possibility of studying treatment effects in vivo on the important, nonfibrillar, forms of Aß is limited. This study demonstrated the ability of the Aß protofibril-selective radioligand 124I-RmAb158-scFv8D3 to follow disease progression and detect treatment effects with PET imaging in tg-ArcSwe mice.

Leptin potentiates astrogenesis in the developing hypothalamus.

BACKGROUND: The proper establishment of hypothalamic feeding circuits during early development has a profound influence on energy homeostasis, and perturbing this process could predispose individuals to obesity and its associated consequences later in life. The maturation of hypothalamic neuronal circuitry in rodents takes place during the initial postnatal weeks, and this coincides with a dramatic surge in the circulating level of leptin, which is known to regulate the outgrowth of key neuronal projections in the maturing hypothalamus. Coincidently, this early postnatal period also marks the rapid proliferation and expansion of astrocytes in the brain. METHODS: Here we examined the effects of leptin on the proliferative capacity of astrocytes in the developing hypothalamus by treating postnatal mice with leptin. Mutant mice were also generated to conditionally remove leptin receptors from glial fibrillary acidic protein (GFAP)-expressing cells in the postnatal period. RESULTS AND CONCLUSIONS: We show that GFAP-expressing cells in the periventricular zone of the 3rd ventricle were responsive to leptin during the initial postnatal week. Leptin enhanced the proliferation of astrocytes in the postnatal hypothalamus and conditional removal of leptin receptors from GFAP-expressing cells during early postnatal period limited astrocyte proliferation. While increasing evidence demonstrates a direct role of leptin in regulating astrocytes in the adult brain, and given the essential function of astrocytes in modulating neuronal function and connectivity, our study indicates that leptin may exert its metabolic effects, in part, by promoting hypothalamic astrogenesis during early postnatal development.

PAT4 is abundantly expressed in excitatory and inhibitory neurons as well as epithelial cells.

PAT4, the fourth member of the SLC36/proton dependent amino acid transporter (PAT) family, is a high-affinity, low capacity electroneutral transporter of neutral amino acids like proline and tryptophan. It has also been associated with the function of mTORC1, a complex in the mammalian target of rapamycin (mTOR) pathway. We performed in situ hybridization and immunohistological analysis to determine the expression profile of PAT4, as well as an RT-PCR study on tissue from mice exposed to leucine. We performed a phylogenetic analysis to determine the evolutionary origin of PAT4. The in situ hybridization and the immunohistochemistry on mouse brain sections and hypothalamic cells showed abundant PAT4 expression in the mouse brain intracellularly in both inhibitory and excitatory neurons, partially co-localizing with lysosomal markers and epithelial cells lining the ventricles. Its location in epithelial cells around the ventricles indicates a transport of substrates across the blood brain barrier. Phylogenetic analysis showed that PAT4 belongs to an evolutionary old family most likely predating animals, and PAT4 is the oldest member of that family.

The G protein-coupled receptor GPR162 is widely distributed in the CNS and highly expressed in the hypothalamus and in hedonic feeding areas.

The Rhodopsin family is a class of integral membrane proteins belonging to G protein-coupled receptors (GPCRs). To date, several orphan GPCRs are still uncharacterized and in this study we present an anatomical characterization of the GPR162 protein and an attempt to describe its functional role. Our results show that GPR162 is widely expressed in GABAergic as well as other neurons within the mouse hippocampus, whereas extensive expression is observed in areas related to energy homeostasis and hedonic feeding such as hypothalamus, amygdala and ventral tegmental area, regions known to be involved in the regulation of palatable food consumption.

Evolutionary origin of amino acid transporter families SLC32, SLC36 and SLC38 and physiological, pathological and therapeutic aspects.

About 25% of all solute carriers (SLCs) are likely to transport amino acids as their primary substrate. One of the major phylogenetic clusters of amino acid transporters from the SLC family is the ß-family, which is part of the PFAM APC clan. The ß-family includes three SLC families, SLC32, SLC36 and SLC38 with one, four and eleven members in humans, respectively. The most well characterized genes within these families are the vesicular inhibitory amino acid transporter (VIAAT, SLC32A1), PAT1 (SLC36A1), PAT2 (SLC36A2), PAT4 (SLC36A4), SNAT1 (SLC38A1), SNAT2 (SLC38A2), SNAT3 (SLC38A3), and SNAT4 (SLC38A4). Here we review the structural characteristics and functional role of these transporters. We also mined the complete protein sequence datasets for nine different genomes to clarify the evolutionary history of the ß-family of transporters. We show that all three main branches of the this family are found as far back as green algae suggesting that genes from these families existed in the early eukaryote before the split of animals and plants and that they are present in most animal species. We also address the potential of further drug development within this field highlighting the important role of these transporters in neurotransmission and transport of amino acids as nutrients.

B(0)AT2 (SLC6A15) is localized to neurons and astrocytes, and is involved in mediating the effect of leucine in the brain.

The B(0)AT2 protein is a product of the SLC6A15 gene belonging to the SLC6 subfamily and has been shown to be a transporter of essential branched-chain amino acids. We aimed to further characterize the B(0)AT2 transporter in CNS, and to use Slc6a15 knock out (KO) mice to investigate whether B(0)AT2 is important for mediating the anorexigenic effect of leucine. We used the Slc6a15 KO mice to investigate the role of B(0)AT2 in brain in response to leucine and in particular the effect on food intake. Slc6a15 KO mice show lower reduction of food intake as well as lower neuronal activation in the ventromedial hypothalamic nucleus (VMH) in response to leucine injections compared to wild type mice. We also used RT-PCR on rat tissues, in situ hybridization and immunohistochemistry on mouse CNS tissues to document in detail the distribution of SLC6A15 on gene and protein levels. We showed that B(0)AT2 immunoreactivity is mainly neuronal, including localization in many GABAergic neurons and spinal cord motor neurons. B(0)AT2 immunoreactivity was also found in astrocytes close to ventricles, and co-localized with cytokeratin and diazepam binding inhibitor (DBI) in epithelial cells of the choroid plexus. The data suggest that B(0)AT2 play a role in leucine homeostasis in the brain.