Layer-specific distribution and expression pattern of AMPA- and NMDA-type glutamate receptors in the barrel field of the adult rat somatosensory cortex: a quantitative electron microscopic analysis.

AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and NMDA (N-methyl-d-aspartate) glutamate receptors are driving forces for synaptic transmission and plasticity at neocortical synapses. However, their distribution pattern in the adult rat neocortex is largely unknown and was quantified using freeze fracture replication combined with postimmunogold-labeling. Both receptors were co-localized at layer (L)4 and L5 postsynaptic densities (PSDs). At L4 dendritic shaft and spine PSDs, the number of gold grains detecting AMPA was similar, whereas at L5 shaft PSDs AMPA-receptors outnumbered those on spine PSDs. Their number was significantly higher at L5 vs. L4 PSDs. At L4 and L5 dendritic shaft PSDs, the number of gold grains detecting GluN1 was ~2-fold higher than at spine PSDs. The number of gold grains detecting the GluN1-subunit was higher for both shaft and spine PSDs in L5 vs. L4. Both receptors showed a large variability in L4 and L5. A high correlation between the number of gold grains and PSD size for both receptors and targets was observed. Both receptors were distributed over the entire PSD but showed a layer- and target-specific distribution pattern. The layer- and target-specific distribution of AMPA and GluN1 glutamate receptors partially contribute to the observed functional differences in synaptic transmission and plasticity in the neocortex.

Relationships between neurotransmitter receptor densities and expression levels of their corresponding genes in the human hippocampus.

Neurotransmitter receptors are key molecules in signal transmission, their alterations are associated with brain dysfunction. Relationships between receptors and their corresponding genes are poorly understood, especially in humans. We combined in vitro receptor autoradiography and RNA sequencing to quantify, in the same tissue samples (7 subjects), the densities of 14 receptors and expression levels of their corresponding 43 genes in the Cornu Ammonis (CA) and dentate gyrus (DG) of human hippocampus. Significant differences in receptor densities between both structures were found only for metabotropic receptors, whereas significant differences in RNA expression levels mostly pertained ionotropic receptors. Receptor fingerprints of CA and DG differ in shapes but have similar sizes; the opposite holds true for their "RNA fingerprints", which represent the expression levels of multiple genes in a single area. In addition, the correlation coefficients between receptor densities and corresponding gene expression levels vary widely and the mean correlation strength was weak-to-moderate. Our results suggest that receptor densities are not only controlled by corresponding RNA expression levels, but also by multiple regionally specific post-translational factors.

The transcriptional activity of progestins used in contraception and menopausal hormone therapy via progesterone receptor A is dependent on the density of the receptor.

Studies directly comparing the efficacies and potencies of multiple progestins used in contraception and menopausal hormone therapy (MHT) in parallel via human progesterone receptor isoform A (PR-A) in the same model system are limited, and how these parameters are influenced by the density of PR-A are unclear. This is surprising as it is known that the expression levels of PR-A vary in different tissues and diseases. We thus determined for the first time the relative efficacies and potencies for transactivation of the natural PR ligand, progesterone (P4), the PR-specific agonist promegestone (R5020), and selected progestins from all four generations in parallel via different densities of PR-A overexpressed in the MDA-MB-231 breast cancer cell line. Comparative dose-response analysis showed that P4, R5020, the 1st generation progestins medroxyprogesterone acetate and norethisterone, 2nd generation progestin levonorgestrel, 3rd generation progestin gestodene, as well as 4th generation progestins nesterone, nomegestrol acetate and drospirenone display differential agonist efficacies and potencies via PR-A. Moreover, we showed that the agonist efficacies and potencies of the progestins via PR-A were modulated in a density- and progestin-specific manner. Our finding that the potencies of the progestins via PR-A, at all densities, do not exceed reported progestin serum concentrations in women, suggest that these progestins are likely to elicit similar effects in vivo. We are the first to report that P4 and the selected progestins display similar agonist activity for transrepression via PR-A, and that the density of PR-A enhances the transrepression activity of some, but not all progestogens. Collectively, our findings provide proof of concept that the effects of the selected progestins via PR-A is progestin-specific and dependent on the density of the receptor, suggesting differential progestin responses in women using these progestins in contraception and MHT.

Dopamine D2/3 Receptor Availabilities in Striatal and Extrastriatal Regions of the Adult Human Brain: Comparison of Four Methods of Analysis.

Values of binding potentials (BPND) of dopamine D2/3 receptors differ in different regions of the brain, but we do not know with certainty how much of this difference is due either to different receptor numbers, or to different affinities of tracers to the receptors, or to both. We tested the claim that both striatal and extrastriatal dopamine D2/3 receptor availabilities vary with age in vivo in humans by determining the values of BPND of the specific radioligand [11C]raclopride. We determined values of BPND in striatal and extrastriatal volumes-of-interest (VOI) with the same specific receptor radioligand. We estimated values of BPND in individual voxels of brains of healthy volunteers in vivo, and we obtained regional averages of VOI by dynamic positron emission tomography (PET). We calculated average values of BPND in caudate nucleus and putamen of striatum, and in frontal, occipital, parietal, and temporal cortices of the forebrain, by means of four methods, including the ERLiBiRD (Estimation of Reversible Ligand Binding and Receptor Density) method, the tissue reference methods of Logan and Logan-Ichise, respectively, and the SRTM (Simplified Reference Tissue Method). Voxelwise generation of parametric maps of values of BPND used the multi-linear regression version of SRTM. Age-dependent changes of the binding potential presented with an inverted U-shape with peak binding potentials reached between the ages of 20 and 30. The estimates of BPND declined significantly with age after the peak in both striatal and extrastriatal regions, as determined by all four methods, with the greatest decline observed in posterior (occipital and parietal) cortices (14% per decade) and the lowest decline in caudate nucleus (3% per decade). The sites of the greatest declines are of particular interest because of the clinical implications.

Effects of receptor density on the tactile perception of roughness: implications for neural mechanisms of texture perception.

Aim of the study: The purpose of this study was to investigate the effects of receptor density in the glabrous skin of the hand on the perception of the roughness of a textured surface.Materials and methods: This was done by having observers make magnitude estimates of the perceived roughness of raised-dot surfaces at the fingertip, with its high receptor density, and the thenar eminence, with its much lower receptor density.Results: Judgments of perceived roughness averaged over the inter-dot spacings (0.8-5.9 mm) employed in the study did not differ significantly between the two sites, which suggested that roughness perception is not exclusively dependent upon a neural code involving variation in the activity levels of the nerve fibers of spatially distributed receptors, as is the case in spatial discrimination tasks such as spatial-gap detection, grove-orientation discrimination and letter recognition. This hypothesis was further supported by the finding that the elimination of temporal cues by preventing movement of the skin over the raised-dot surface drastically impaired judgments of perceived roughness at the thenar but had little effect on judgments of perceived roughness at the fingertip.Conclusion: These findings suggested that the neural code for perceived roughness at the fingertip is mediated primarily by spatial variation in the activity levels of spatially distributed receptors whereas the neural code for perceived roughness at the thenar is mediated primarily by temporal variation in the activity levels of individual receptors.

ß-blockers Reverse Agonist-Induced ß2-AR Downregulation Regardless of Their Signaling Profile.

Altered ß-adrenergic receptor (ß-AR) density has been reported in cells, animals, and humans receiving ß-blocker treatment. In some cases, ß-AR density is upregulated, but in others, it is unaffected or even reduced. Collectively, these results would imply that changes in ß-AR density and ß-blockade are not related. However, it has still not been clarified whether the effects of ß-blockers on receptor density are related to their ability to activate different ß-AR signaling pathways. To this aim, five clinically relevant ß-blockers endowed with inverse, partial or biased agonism at the ß2-AR were evaluated for their effects on ß2-AR density in both human embryonic kidney 293 (HEK293) cells expressing exogenous FLAG-tagged human ß2-ARs and human lymphocytes expressing endogenous ß2-ARs. Cell surface ß2-AR density was measured by enzyme-linked immunosorbent assay (ELISA) and flow cytometry. Treatment with propranolol, carvedilol, pindolol, sotalol, or timolol did not induce any significant change in surface ß2-AR density in both HEK293 cells and human lymphocytes. On the contrary, treatment with the ß-AR agonist isoproterenol reduced the number of cell surface ß2-ARs in the tested cell types without affecting ß2-AR-mRNA levels. Isoproterenol-induced effects on receptor density were completely antagonized by ß-blocker treatment. In conclusion, the agonistic activity of ß-blockers does not exert an important effect on short-term regulation of ß2-AR density.

Double-blind placebo-controlled trial of the effect of omalizumab on basophils in chronic urticaria patients.

BACKGROUND: Omalizumab has been shown to be effective in treating chronic spontaneous urticaria (CSU). The reduction in FcεRI receptor density on the surface of basophils and mast cells is thought to play a major role in its effectiveness. We conducted a double-blind, randomized, placebo-controlled trial to investigate the mode of action of omalizumab in patients with antihistamine-resistant CSU. METHODS: Thirty patients were randomized in a 2:1 ratio to receive either 300 mg omalizumab or placebo. Four monthly applications of omalizumab/placebo were followed up with a visit 2 months after the last injection. The primary endpoint was the FcεRI receptor density change on basophils. RESULTS: Omalizumab led to a significant reduction in FcεRI receptor density on basophils as soon as 1 week after the first injection: baseline omalizumab vs placebo group, 80.31 ± 47.18 × 10³ vs 78.29 ± 45.09 × 10³ receptors/basophil ± SD; 1 week, 72.89 ± 47.79 × 10³ vs 27.83 ± 20.87 × 10³, P = .001. This effect continued during the treatment phase and persisted for 2 months after the last injection: 93.81 ± 56.50 × 10³ vs 21.09 ± 15.23 × 10³, P = .002. Values for basophil "releasability" and the basophil activation test (CU-BAT) of patient serum using donor basophils were unchanged despite treatment: CU-BAT, CD63 10.75% (7.35) in the placebo group vs 8.35% (15.20) in the omalizumab group, P = .778. CONCLUSION: We demonstrated a rapid reduction of FcεRI receptor density on basophils following treatment with omalizumab. Because CU-BAT using well-characterized, omalizumab-naïve donor basophils did not change during the treatment phase, autoreactive serum factors seem to remain unaltered. This points towards a cellular effect of omalizumab on basophils. To predict the omalizumab response time and to monitor disease, FcεRI density and CU-BAT might be promising cellular-based assays.

Vision is superior to touch in shape perception even with equivalent peripheral input.

Results from previous studies suggest that two-dimensional spatial patterns are processed similarly in vision and touch when the patterns are equated for effective size or when visual stimuli are blurred to mimic the spatial filtering of the skin. In the present study, we measured subjects' ability to perceive the shape of familiar and unfamiliar visual and tactile patterns to compare form processing in the two modalities. As had been previously done, the two-dimensional tactile and visual patterns were adjusted in size to stimulate an equivalent number of receptors in the two modalities. We also distorted the visual patterns, using a filter that accurately mimics the spatial filtering effected by the skin to further equate the peripheral images in the two modalities. We found that vision consistently outperformed touch regardless of the precise perceptual task and of how familiar the patterns were. Based on an examination of both the earlier and present data, we conclude that visual processing of both familiar and unfamiliar two-dimensional patterns is superior to its tactile counterpart except under very restricted conditions.

Quantifying CD4 receptor protein in two human CD4+ lymphocyte preparations for quantitative flow cytometry.

BACKGROUND: In our previous study that characterized different human CD4+ lymphocyte preparations, it was found that both commercially available cryopreserved peripheral blood mononuclear cells (PBMC) and a commercially available lyophilized PBMC (Cyto-Trol™) preparation fulfilled a set of criteria for serving as biological calibrators for quantitative flow cytometry. However, the biomarker CD4 protein expression level measured for T helper cells from Cyto-Trol was about 16% lower than those for cryopreserved PBMC and fresh whole blood using flow cytometry and mass cytometry. A primary reason was hypothesized to be due to steric interference in anti- CD4 antibody binding to the smaller sized lyophilized control cells. METHOD: Targeted multiple reaction monitoring (MRM) mass spectrometry (MS) is used to quantify the copy number of CD4 receptor protein per CD4+ lymphocyte. Scanning electron microscopy (SEM) is utilized to assist searching the underlying reasons for the observed difference in CD4 receptor copy number per cell determined by MRM MS and CD4 expression measured previously by flow cytometry. RESULTS: The copy number of CD4 receptor proteins on the surface of the CD4+ lymphocyte in cryopreserved PBMCs and in lyophilized control cells is determined to be (1.45 ± 0.09) × 10(5) and (0.85 ± 0.11) × 10(5), respectively, averaged over four signature peptides using MRM MS. In comparison with cryopreserved PBMCs, there are more variations in the CD4 copy number in lyophilized control cells determined based on each signature peptide. SEM images of CD4+ lymphocytes from lyophilized control cells are very different when compared to the CD4+ T cells from whole blood and cryopreserved PBMC. CONCLUSION: Because of the lyophilization process applied to Cyto-Trol control cells, a lower CD4 density value, defined as the copy number of CD4 receptors per CD4+ lymphocyte, averaged over three different production lots is most likely explained by the loss of the CD4 receptors on damaged and/or broken microvilli where CD4 receptors reside. Steric hindrance of antibody binding and the association of CD4 receptors with other biomolecules likely contribute significantly to the nearly 50% lower CD4 receptor density value for cryopreserved PBMC determined from flow cytometry compared to the value obtained from MRM MS.

Co-culture platform for tuning of cancer receptor density allows for evaluation of bispecific immune cell engagers.

Cancer immunotherapy, where a patient's immune system is harnessed to eradicate cancer cells selectively, is a leading strategy for cancer treatment. However, successes with immune checkpoint inhibitors (ICI) are hampered by reported systemic and organ-specific toxicities and by two-thirds of the patients being non-responders or subsequently acquiring resistance to approved ICIs. Hence substantial efforts are invested in discovering novel targeted immunotherapies aimed at reduced side-effects and improved potency. One way is utilizing the dual targeting feature of bispecific antibodies, which have made them increasingly popular for cancer immunotherapy. Easy and predictive screening methods for activation ranking of candidate drugs in tumor contra non-tumor environments are however lacking. Herein, we present a cell-based assay mimicking the tumor microenvironment by co-culturing B cells with engineered human embryonic kidney 293 T cells (HEK293T), presenting a controllable density of platelet-derived growth factor receptor ß (PDGFRß). A target density panel with three different surface protein levels on HEK293T cells was established by genetic constructs carrying regulatory elements limiting RNA translation of PDGFRß. We employed a bispecific antibody-affibody construct called an AffiMab capable of binding PDGFRß on cancer cells and CD40 expressed by B cells as a model. Specific activation of CD40-mediated signaling of immune cells was demonstrated with the two highest receptor-expressing cell lines, Level 2/3 and Level 4, while low-to-none in the low-expressing cell lines. The concept of receptor tuning and the presented co-culture protocol may be of general utility for assessing and developing novel bi-specific antibodies for immuno-oncology applications.

Pharmacokinetic Analysis of [18F]FES PET in the Human Brain and Pituitary Gland.

PURPOSE: Estrogen receptors (ER) are implicated in psychiatric disorders. We assessed if ER availability in the human brain could be quantified using 16α-[18F]-fluoro-17ß-estradiol ([18F]FES) positron emission tomography (PET). PROCEDURES: Seven post­menopausal women underwent a dynamic [18F]FES PET scan with arterial blood sampling. A T1-weighted MRI was acquired for anatomical information. After one week, four subjects received a selective ER degrader (SERD), four hours before the PET scan. Pharmacokinetic analysis was performed using a metabolite-corrected plasma curve as the input function. The optimal kinetic model was selected based on the Akaike information criterion and standard error of estimated parameters. Accuracy of Logan graphical analysis and standardized uptake value (SUV) was determined via correlational analyses. RESULTS: The reversible two-tissue compartment model (2T4k) model with fixed K1/k2 was preferred. The total volume of distribution (VT) could be more reliably estimated than the binding potential (BPND). A high correlation of VT with Logan graphical analysis was observed, but only a moderate correlation with SUV. SERD administration resulted in a reduced VT in the pituitary gland, but not in other regions. CONCLUSIONS: The optimal quantification method for [18F]FES was the 2T4k with fixed K1/k2 or Logan graphical analysis, but specific binding was only observed in the pituitary gland.

[18F]Fluspidine-A PET Tracer for Imaging of σ1 Receptors in the Central Nervous System.

σ1 receptors play a crucial role in various neurological and neurodegenerative diseases including pain, psychosis, Alzheimer's disease, and depression. Spirocyclic piperidines represent a promising class of potent σ1 receptor ligands. The relationship between structural modifications and σ1 receptor affinity and selectivity over σ2 receptors led to the 2-fluoroethyl derivative fluspidine (2, Ki = 0.59 nM). Enantiomerically pure (S)-configured fluspidine ((S)-2) was prepared by the enantioselective reduction of the α,ß-unsaturated ester 23 with NaBH4 and the enantiomerically pure co-catalyst (S,S)-24. The pharmacokinetic properties of both fluspidine enantiomers (R)-2 and (S)-2 were analyzed in vitro. Molecular dynamics simulations revealed very similar interactions of both fluspidine enantiomers with the σ1 receptor protein, with a strong ionic interaction between the protonated amino moiety of the piperidine ring and the COO- moiety of glutamate 172. The 18F-labeled radiotracers (S)-[18F]2 and (R)-[18F]2 were synthesized in automated syntheses using a TRACERlab FX FN synthesis module. High radiochemical yields and radiochemical purity were achieved. Radiometabolites were not found in the brains of mice, piglets, and rhesus monkeys. While both enantiomers revealed similar initial brain uptake, the slow washout of (R)-[18F]2 indicated a kind of irreversible binding. In the first clinical trial, (S)-[18F]2 was used to visualize σ1 receptors in the brains of patients with major depressive disorder (MDD). This study revealed an increased density of σ1 receptors in cortico-striato-(para)limbic brain regions of MDD patients. The increased density of σ1 receptors correlated with the severity of the depressive symptoms. In an occupancy study with the PET tracer (S)-[18F]2, the selective binding of pridopidine at σ1 receptors in the brain of healthy volunteers and HD patients was shown.

Receptor Density-Dependent Motility of Influenza Virus Particles on Surface Gradients.

Influenza viruses can move across the surface of host cells while interacting with their glycocalyx. This motility may assist in finding or forming locations for cell entry and thereby promote cellular uptake. Because the binding to and cleavage of cell surface receptors forms the driving force for the process, the surface-bound motility of influenza is expected to be dependent on the receptor density. Surface gradients with gradually varying receptor densities are thus a valuable tool to study binding and motility processes of influenza and can function as a mimic for local receptor density variations at the glycocalyx that may steer the directionality of a virus particle in finding the proper site of uptake. We have tracked individual influenza virus particles moving over surfaces with receptor density gradients. We analyzed the extracted virus tracks first at a general level to verify neuraminidase activity and subsequently with increasing detail to quantify the receptor density-dependent behavior on the level of individual virus particles. While a directional bias was not observed, most likely due to limitations of the steepness of the surface gradient, the surface mobility and the probability of sticking were found to be significantly dependent on receptor density. A combination of high surface mobility and high dissociation probability of influenza was observed at low receptor densities, while the opposite occurred at higher receptor densities. These properties result in an effective mechanism for finding high-receptor density patches, which are believed to be a key feature of potential locations for cell entry.

A shared spatial topography links the functional connectome correlates of cocaine use disorder and dopamine D2/3 receptor densities.

Background: The biological mechanisms that contribute to cocaine and other substance use disorders involve an array of cortical and subcortical systems. Prior work on the development and maintenance of substance use has largely focused on cortico-striatal circuits, with relatively less attention on alterations within and across large-scale functional brain networks, and associated aspects of the dopamine system. The brain-wide pattern of temporal co-activation between distinct brain regions, referred to as the functional connectome, underpins individual differences in behavior. Critically, the functional connectome correlates of substance use and their specificity to dopamine receptor densities relative to other metabotropic receptors classes remains to be established. Methods: We comprehensively characterized brain-wide differences in functional connectivity across multiple scales, including individual connections, regions, and networks in participants with cocaine use disorder (CUD; n=69) and healthy matched controls (n=62), Further, we studied the relationship between the observed functional connectivity signatures of CUD and the spatial distribution of a broad range of normative neurotransmitter receptor and transporter bindings as assessed through 18 different normative positron emission tomography (PET) maps. Results: Our analyses identified a widespread profile of functional connectivity differences between individuals with CUD and matched healthy comparison participants (8.8% of total edges; 8,185 edges; pFWE=0.025). We largely find lower connectivity preferentially linking default network and subcortical regions, and higher within-network connectivity in the default network in participants with CUD. Furthermore, we find consistent and replicable associations between signatures of CUD and normative spatial density of dopamine D2/3 receptors. Conclusions: Our analyses revealed a widespread profile of altered connectivity in individuals with CUD that extends across the functional connectome and implicates multiple circuits. This profile is robustly coupled with normative dopamine D2/3 receptors densities. Underscoring the translational potential of connectomic approaches for the study of in vivo brain functions, CUD-linked aspects of brain function were spatially coupled to disorder relevant neurotransmitter systems.

Region-specific associations between gamma-aminobutyric acid A receptor binding and cortical thickness in high-functioning autistic adults.

The neurobiology of autism has been shown to involve alterations in cortical morphology and gamma-aminobutyric acid A (GABAA ) receptor density. We hypothesized that GABAA receptor binding potential (GABAA R BPND ) would correlate with cortical thickness, but their correlations would differ between autistic adults and typically developing (TD) controls. We studied 50 adults (23 autism, 27 TD, mean age of 27 years) using magnetic resonance imaging to measure cortical thickness, and [18 F]flumazenil positron emission tomography imaging to measure GABAA R BPND . We determined the correlations between cortical thickness and GABAA R BPND by cortical lobe, region-of-interest, and diagnosis of autism spectrum disorder (ASD). We also explored potential sex differences in the relationship between cortical thickness and autism characteristics, as measured by autism spectrum quotient (AQ) scores. Comparing autism and TD groups, no significant differences were found in cortical thickness or GABAA R BPND . In both autism and TD groups, a negative relationship between cortical thickness and GABAA R BPND was observed in the frontal and occipital cortices, but no relationship was found in the temporal or limbic cortices. A positive correlation was seen in the parietal cortex that was only significant for the autism group. Interestingly, in an exploratory analysis, we found sex differences in the relationships between cortical thickness and GABAA R BPND , and cortical thickness and AQ scores in the left postcentral gyrus. LAY SUMMARY: The thickness of the brain cortex and the density of the receptors associated with inhibitory neurotransmitter GABA have been hypothesized to underlie the neurobiology of autism. In this study, we found that these biomarkers correlate positively in the parietal cortex, but negatively in the frontal and occipital cortical regions of the brain. Furthermore, we collected preliminary evidence that the correlations between cortical thickness and GABA receptor density are sexdependent in a brain region where sensory inputs are registered.

Receptor density influences the recruitment bias of aripiprazole and brexpiprazole at the dopamine D2L receptor.

Aripiprazole, brexpiprazole, and cariprazine are dopamine D2 receptor ligands considered as effective and tolerable antipsychotics. Brain imaging studies showed that schizophrenia is characterized by elevated dopamine receptor density, which is exacerbated by antipsychotic treatments. Despite the complexity of translating in vitro studies to human neurobiology, overexpression experiments in transfected cells provide a proof-of-concept model of the influence of receptor density on antipsychotic treatments. Since receptor density was demonstrated to influence the signaling profile of dopaminergic ligands, we hypothesized that high dopamine D2 receptor expression levels could influence the recruitment of Gi1 and ß-arrestin2 in response to partial agonists used as antipsychotics. A nanoluciferase complementation assay was used to monitor ß-arrestin2 and Gi1 recruitment at the dopamine D2L receptor in response to aripiprazole, brexpiprazole, and cariprazine. This was performed in transfected cells carrying a doxycycline-inducible system allowing to manipulate the expression of the dopamine D2L receptors. Increasing D2L receptor density reoriented aripiprazole's preferential recruitment from Gi1 to ß-arrestin2. With respect to brexpiprazole, which showed inverse agonism for ß-arrestin2 recruitment at the lower receptor density tested, inverse agonism for Gi1 recruitment was observed when tested at a high receptor expression level. At variance, cariprazine evoked a potent partial agonism for ß-arrestin2 recruitment only, in all the tested conditions. D2L receptor density appears to shape the recruitment bias of aripiprazole and brexpiprazole, but not cariprazine. This suggests that changes in receptor expression level could qualitatively influence the functional response of partial agonists used in psychiatry.

Expansion and contraction of resource allocation in sensory bottlenecks.

Topographic sensory representations often do not scale proportionally to the size of their input regions, with some expanded and others contracted. In vision, the foveal representation is magnified cortically, as are the fingertips in touch. What principles drive this allocation, and how should receptor density, for example, the high innervation of the fovea or the fingertips, and stimulus statistics, for example, the higher contact frequencies on the fingertips, contribute? Building on work in efficient coding, we address this problem using linear models that optimally decorrelate the sensory signals. We introduce a sensory bottleneck to impose constraints on resource allocation and derive the optimal neural allocation. We find that bottleneck width is a crucial factor in resource allocation, inducing either expansion or contraction. Both receptor density and stimulus statistics affect allocation and jointly determine convergence for wider bottlenecks. Furthermore, we show a close match between the predicted and empirical cortical allocations in a well-studied model system, the star-nosed mole. Overall, our results suggest that the strength of cortical magnification depends on resource limits.

Transient equilibrium determination of dopamine D2/D3 receptor densities and affinities in brain.

Long-term alteration of dopaminergic neurotransmission is known to modulate the D2/D3 receptor expression in the brain. The modulation can occur as a response to pathological processes or pharmacological intervention. The receptor density can be monitored by in vivo positron emission tomography (PET) of [11C] raclopride. To obtain accurate measurements of receptor-ligand interaction, it is essential to estimate binding parameters at true (if transient) equilibrium of bound and unbound ligand quantities. We designed this study as a comparison of two quantitative approaches to transient equilibrium, the TRansient EquilibriuM BoLus Estimation (TREMBLE) method and the Transient Equilibrium Model (TEM) method, to determine binding parameters at transient equilibrium with bolus injection of the radioligand. The data demonstrates that TREMBLE unlike TEM identified the time at which equilibrium existed. TREMBLE revealed that equilibrium prevailed at one or more times after bolus injection and identified differences of receptor density among regions such as putamen and caudate nucleus. We demonstrated that TREMBLE is a quantitative approach suitable for the study of pathophysiological conditions of certain types of neurotransmission the brain.

Density Control over MBD2 Receptor-Coated Surfaces Provides Superselective Binding of Hypermethylated DNA.

Using the biomarker hypermethylated DNA (hmDNA) for cancer detection requires a pretreatment to isolate or concentrate hmDNA from nonmethylated DNA. Affinity chromatography using a methyl binding domain-2 (MBD2) protein can be used, but the relatively low enrichment selectivity of MBD2 limits its clinical applicability. Here, we developed a superselective, multivalent, MBD2-coated platform to improve the selectivity of hmDNA enrichment. The multivalent platform employs control over the MBD2 surface receptor density, which is shown to strongly affect the binding of DNA with varying degrees of methylation, improving both the selectivity and the affinity of DNAs with higher numbers of methylation sites. Histidine-10-tagged MBD2 was immobilized on gold surfaces with receptor density control by tuning the amount of nickel nitrilotriacetic acid (NiNTA)-functionalized thiols in a thiol-based self-assembled monolayer. The required MBD2 surface receptor densities for DNA surface binding decreases for DNA with higher degrees of methylation. Both higher degrees of superselectivity and surface coverages were observed upon DNA binding at increasing methylation levels. Adopting the findings of this study into hmDNA enrichment of clinical samples has the potential to become more selective and sensitive than current MBD2-based methods and, therefore, to improve cancer diagnostics.

Predicting neurite extension for varying extracellular matrix stiffness and topography.

Neurite extension is a dynamic process and is dependent on the microenvironment. The mechanical properties of the extracellular matrix (ECM), such as stiffness and topography influence the microenvironment and affects neurite extension; however, the mechanistic basis for this dynamic response of neurite extension remains elusive. In this study, we develop a computational model that predicts neurite extension dynamics process as the stiffness and patterned topography of ECM changes. The model includes the contribution of receptors integrin and neural cellular adhesion molecule toward the growth of neurite tip. We use non-linear finite element analysis (FEA) to model the neuronal cell, neurite, and the ECM, which is then coupled to the force-deformation receptor properties obtained from molecular dynamics simulations. Using an empirical relation, we develop a neurite extension algorithm that simulates the dynamic process of growth cone induced by growth cone extension, receptor density, and rupture. We investigate the dependence of neurite extension on ECM stiffness using three distinct materials, the effect of width and spacing of continuous (cylindrical) and discontinuous (pillar) patterned topography, as well as the topography steepness and stiffness gradient. We find that an increasing stiffness and width of patterned topography results in increased neurite extension, but the magnitude of the increase differs depending on the growth cone extension and receptor density between them. These findings will aid in vitro studies in determining an ECM with appropriate mechanical properties, such as stiffness and topography that will improve neurite extension, thus resulting in the formation of functional neurons.