Use-dependent corticospinal excitability is associated with resilience and physical performance during simulated military operational stress

Simulated military operational stress (SMOS) provides a useful model to better understand resilience in humans as the stress associated with caloric restriction, sleep deficits, and fatiguing exertion degrades physical and cognitive performance. Habitual physical activity may confer resilience against these stressors by promoting favorable use-dependent neuroplasticity, but it is unclear how physical activity, resilience, and corticospinal excitability (CSE) relate during SMOS. To examine associations between corticospinal excitability, physical activity, and physical performance during SMOS. Fifty-three service members (age: 26 ± 5 yr, 13 women) completed a 5-day and -night intervention composed of familiarization, baseline, SMOS (2 nights/days), and recovery days. During SMOS, participants performed rigorous physical and cognitive activities while receiving half of normal sleep (two 2-h blocks) and caloric requirements. Lower and upper limb CSE were determined with transcranial magnetic stimulation (TMS) stimulus-response curves. Self-reported resilience, physical activity, military-specific physical performance (TMT), and endocrine factors were compared in individuals with high (HIGH) and low CSE based on a median split of lower limb CSE at baseline. HIGH had greater physical activity and better TMT performance throughout SMOS. Both groups maintained physical performance despite substantial psychophysiological stress. Physical activity, resilience, and TMT performance were directly associated with lower limb CSE. Individual differences in physical activity coincide with lower (but not upper) limb CSE. Such use-dependent corticospinal excitability directly relates to resilience and physical performance during SMOS. Future studies may use noninvasive neuromodulation to clarify the interplay among CSE, physical activity, and resilience and improve physical and cognitive performance.NEW & NOTEWORTHY We demonstrate that individual differences in physical activity levels coincide with lower limb corticospinal excitability. Such use-dependent corticospinal excitability directly relates to resilience and physical performance during a 5-day simulation of military operational stress with caloric restriction, sleep restriction and disruption, and heavy physical and cognitive exertion.

Reliability of corticospinal excitability estimates for the vastus lateralis: Practical considerations for lower limb TMS task selection

Transcranial magnetic stimulation (TMS) is increasingly used to examine lower extremity corticospinal excitability (CSE) in clinical and sports research. Because CSE is task-specific, there is growing emphasis on the use of ecological tasks. Nevertheless, the comparative reliability of CSE measurements during established (e.g. knee extensions; KE) and more recent ecological (e.g. squats; SQT) lower extremity tasks has received less attention. The aim of this study was to compare the test-retest reliability of CSE, force, and muscle activity (EMG) during isometric SQT and KE. 19 right-footed men (age: 25 ± 5 yrs) with similar fitness and body composition performed SQT (N = 7) or KE (N = 12) on two consecutive days. Force and EMG were recorded during maximum voluntary isometric contractions (MVC). Corticospinal excitability was determined in the dominant leg during light (15% MVC) contractions based on motor evoked potential (MEP) stimulus-response-curves (SRC). Test-retest reliability, absolute agreement, and consistency were determined for force, EMG, and SRC MEP maximum (MEPMAX) and rising phase midpoint (V50). As a secondary analysis, all outcomes were compared between groups with mixed-methods ANCOVAs (Task × Time, covariate: body-fat-percentage). Compared with SQT, KE displayed better test-retest reliability and agreement for MEPMAX whereas V50, force, and EMG were similarly reliable. Force (p = 0.01) and MEPMAX (p = 0.02) were also greater during KE despite a similar V50 (p = 0.11). Differences in test-retest reliability, absolute agreement, and between-group comparisons highlight the need to carefully select lower limb TMS assessment tasks and encourage future efforts to balance ecological validity with statistical sensitivity.

Characterizing off-target corticospinal responses to double-cone transcranial magnetic stimulation

INTRODUCTION

The double-cone coil (D-CONE) is frequently used in transcranial magnetic stimulation (TMS) experiments that target the motor cortex (M1) lower-limb representation. Anecdotal evidence and modeling studies have shed light on the off-target effects of D-CONE TMS but the physiological extent remains undetermined.

PURPOSE

To characterize the off-target effects of D-CONE TMS based on bilateral corticospinal responses in the legs and hands.

METHODS

Thirty (N = 30) participants (9 women, age: 26 ± 5yrs) completed a stimulus-response curve procedure with D-CONE TMS applied to the dominant vastus lateralis (cVL) and motor-evoked potentials (MEPs) recorded in each active VL and resting first dorsal interosseous (FDI). As a positive control (CON), the dominant FDI was directly targeted with a figure-of-eight coil and MEPs were similarly recorded in each active FDI and resting VL. MEP_MAX, V50 and MEP latencies were compared with repeated-measures ANOVAs or mixed-effects analysis and Bonferroni-corrected pairwise comparisons.

RESULTS

Off-target responses were evident in all muscles, with similar MEP_MAX in the target (cVL) and off-target (iVL) leg (p = 0.99) and cFDI compared with CON (p = 0.99). cFDI and CON MEP_MAX were greater than iFDI (p < 0.01). A main effect of target (p < 0.001) indicated that latencies were shorter with CON but similar in all muscles with D-CONE.

DISCUSSION

Concurrent MEP recordings in bilateral upper- and lower-extremity muscles confirm that lower-limb D-CONE TMS produces substantial distance-dependent off-target effects. In addition to monitoring corticospinal responses in off-target muscles to improve targeting accuracy in real-time, future studies may incorporate off-target information into statistical models post-hoc.

A-15 Network Analysis Of Sport-Related Concussion Research During The Past Decade (2010–2019)

Objective

The purpose of this study was to apply network analyses to evaluate trends in the literature using a comprehensive search of original, peer-reviewed research articles involving human participants with sport-related concussion (SRC) published between January 1, 2010 and December 31, 2019.

Data Selection

Articles were identified in a comprehensive online search using key terms to encompass all forms of SRC or mTBI and entered into a clustering algorithm (vosViewer). Each cluster (e.g., journal, institution, author, keyword) is named for the hub, or most highly interconnected individual node.

Data Synthesis

The online search yielded 6,130 articles, 528 journals, 7,598 authors, 1,966 institutions, and 3,293 keywords. The analysis supported five thematic clusters of journals: 1. Biomechanics/Sports medicine (n = 15), 2. Pediatrics/Rehabilitation (n = 15), 3. Neurotrauma/Neurology/Neurosurgery (n = 11), 4. General Sports Medicine (n = 11), 5. Neuropsychology (n = 7). The analysis identified four institutional clusters: 1. University of North Carolina (n = 19), 2. University of Toronto (n = 19), 3. University of Michigan (n = 11), 4. University of Pittsburgh (n = 10). Five primary author clusters were identified: 1. A. Kontos (n = 32), 2. G. Iverson (n = 27), 3. M. McCrea (n = 27), 4. S. Broglio (n = 25), 5. Z. Kerr (n = 16). In regards to keywords, central topics included: 1. Epidemiology (n = 14), 2. Rehabilitation (n = 12), 3. Biomechanics (n = 11), 4. Imaging (n = 10), 5. Assessment (n = 9).

Conclusions

The findings suggest that during the past decade SRC research has: 1) been published primarily in sports medicine, pediatric, and neuro-focused journals, 2) involved a select group of researchers from several key institutions, and 3) focused on new topic areas including treatment/rehabilitation.

0242 Efficient Perception-Action Coupling Relates to More Slow Wave Sleep in Military Personnel

Introduction

The ability to adapt actions to perceptions of environmental constraints, perception-action coupling, may be compromised by military operational stress (caloric restriction, sleep disruption, physical exertion). Differences in sleep may influence susceptibility to these stressors. We investigated perception-action coupling during simulated military operational stress and the influence of sleep on perception-action coupling.

Methods

During a 5-day simulated military operational stress protocol, thirty-six (6 female) service members (25.8 ± 4.7 years) completed three trials of a perception-action coupling task (PACT) in the evening after a night of baseline sleep (BASE), two nights of sleep restriction (T1) and a night of recovery sleep (T2). Participants had 8-hr for baseline and recovery sleep (2300-0700) and 4-hr disturbed sleep on sleep restriction nights (0100-0300 and 0500-0700). Polysomnography was used to determine time spent in different sleep stages: stage 2 (N2), slow wave (SWS) and rapid-eye movement (REM). The tablet-based PACT requires participants make quick, accurate perceptual judgments and responses about the ability of virtual balls to fit through virtual apertures. Linear mixed models were used to assess interaction and main effects of study day and prior sleep on PACT response time (RT) and accuracy (ACC).

Results

No significant sleep x time interactions or significant main effect of time were found for RT or ACC. A significant main effect of SWS was found for RT (F1,88.307 = 4.331, p = .04). Higher SWS was related to lower (faster) RT. No significant main effects of other sleep stages were found.

Conclusion

Perception-action coupling was maintained during simulated military operational stress. Participants with more SWS across the study responded faster during the PACT but N2 and REM sleep did not relate to perception-action coupling performance, suggesting a specific effect of SWS on perception-action coupling abilities and behaviors.

Support

Department of Defense Award #W81XWH-17-2-0070 (PI: Nindl)

Ligation-mediated PCR: robotic liquid handling for DNA damage and repair

The investigation of in vivo DNA repair in mammalian cells at nucleotide resolution requires the quantification of break frequencies less than one per kilobase. By optimizing several parameters of the ligation-mediated PCR technique, we find that the required sensitivity can be achieved. We also report details of a one-day procedure that can be performed either with or without a robotic liquid handling workstation. The use of near-infrared fluorescent-labeled primers with detection by a LI-COR DNA sequencer provides for safe, nonradioactive detection, similar in sensitivity to the use of 32P-labeled primers but with the additional advantage that high-quality digitized data are obtained directly. Multiplexing can be performed; that is, more than one sequence can be analyzed in a single reaction, and multiple reactions can be processed robotically. Primer sets for exons 5-8 of the tumor suppressor gene, p53, were designed for simultaneous thermal cycling. The improved procedure with infrared detection was used to monitor low-frequency damage (<1 break/kb) and/or repair of UVB, UVC, and chemical methylation. Quantitative data on the linearity of response and reproducibility are described. The coefficient of variation for technical replicates was typically 10%. The methods described here will permit high sample throughput for the detection of DNA damage and repair as well as in vivo protein footprints.

PCR candidate region mismatch scanning: adaptation to quantitative, high-throughput genotyping

Linkage and association analyses were performed to identify loci affecting disease susceptibility by scoring previously characterized sequence variations such as microsatellites and single nucleotide polymorphisms. Lack of markers in regions of interest, as well as difficulty in adapting various methods to high-throughput settings, often limits the effectiveness of the analyses. We have adapted the Escherichia coli mismatch detection system, employing the factors MutS, MutL and MutH, for use in PCR-based, automated, high-throughput genotyping and mutation detection of genomic DNA. Optimal sensitivity and signal-to-noise ratios were obtained in a straightforward fashion because the detection reaction proved to be principally dependent upon monovalent cation concentration and MutL concentration. Quantitative relationships of the optimal values of these parameters with length of the DNA test fragment were demonstrated, in support of the translocation model for the mechanism of action of these enzymes, rather than the molecular switch model. Thus, rapid, sequence-independent optimization was possible for each new genomic target region. Other factors potentially limiting the flexibility of mismatch scanning, such as positioning of dam recognition sites within the target fragment, have also been investigated. We developed several strategies, which can be easily adapted to automation, for limiting the analysis to intersample heteroduplexes. Thus, the principal barriers to the use of this methodology, which we have designated PCR candidate region mismatch scanning, in cost-effective, high-throughput settings have been removed.

Ligation-mediated PCR for quantitative in vivo footprinting

Ligation-mediated polymerase chain reaction (LM-PCR) is a genomic analysis technique for determination of (1) primary DNA nucleotide sequences (2) cytosine methylation patterns (3) DNA lesion formation and repair, and (4) in vivo protein-DNA footprints. However, LM-PCR can be limited by the multiple steps required and the relatively short stretch of sequence (usually <200 bp) that can be analyzed per reaction. We report here a simplified, one-day LM-PCR protocol in which all pipetting steps can be performed by a robotic workstation and which, moreover, provides longer reads (>350 bp) and enhanced signal quality by use of nonradioactive detection and a LI-COR DNA sequencing instrument. Sensitivity comparable to radiolabeling is achieved using oligonucleotide primers that are 5'-end labeled with infrared fluorochromes. We showed that the technique could be used for sensitive and reproducible in vivo photofootprinting of the human phosphoglycerate kinase 1 (PGK1) promoter, as well as providing good Maxam-Gilbert sequence information. The methods described here should allow high-throughput, high-resolution analysis of transcription factor binding and chromatin structure, and also may be useful for sequencing gaps that are refractory to cloning.

Net secretion of furosemide is subject to indomethacin inhibition, as observed in Caco-2 monolayers and excised rat jejunum

PURPOSE

To determine if intestinal secretion occurs for the poorly bioavailable diuretic, furosemide.

METHODS

Jejunal segments of male Sprague-Dawley rats were mounted on diffusion chambers, and the permeation of furosemide was measured across the excised tissue in both directions. Studies were repeated using cultured epithelia from adenocarcinoma cells (Caco-2) grown on filter inserts mounted in 6-well plates. Temperature-dependence and chemical inhibition by indomethacin was also tested using the cell culture model.

RESULTS

Net secretion from rat intestine of over 3-fold was observed for 20 microM furosemide. Net secretion of furosemide by Caco-2 cells was over 300% greater than for intestinal segments (10-fold vs. 3-fold). For both models, a decrease in furosemide transport in the direction of secretion was observed in the presence of indomethacin (100 microM), although only results using the Caco-2 cells showed in increase in the absorptive transport. Furosemide secretion from Caco-2 cells decreased with decrease in temperature from 37 degrees C to 4 degrees C, suggesting a carrier-mediated process.

CONCLUSIONS

Furosemide appears to be secreted in the small intestine. These preliminary results indicate that furosemide bioavailability may be limited by an intestinal transporter.

The dopamine D2 receptor gene: a genetic risk factor in substance abuse

Drug abuse has grown to epidemic proportions. Dopaminergic reward pathways have frequently been implicated in the etiology of drug addiction. To examine the possible role of genetic variants of the dopamine D2 (DRD2) gene in susceptibility to drug abuse we determined the prevalence of the TaqI A1 variant of the DRD2 gene in 200 white patients hospitalized in the Addiction Treatment Unit of a Veterans Administration Hospital. While the prevalence of the D2A1 allele was not significantly increased over controls, it did increase from 21% in subjects with alcohol abuse only to 32% in subjects with alcohol dependence only, consistent with other studies showing an association with the severity of alcoholism. By contrast, of 104 subjects with a discharge diagnosis of drug and alcohol abuse/dependence, 42.3% carried the D2A1 allele versus 29.0% of the 763 white controls (representing all white controls published to date) (P = 0.006). Of those who spent more than $25/week on two or more substances, 56.9% carried the D2A1 allele versus 28.2% of those abusing a single substance (P < 0.0005). Multiple logistic regression analysis showed a highly significant association between multiple substance abuse based on money spent and the presence of the D2A1 allele (P = 0.0003) and age of onset of abuse (P < 0.0001). D2A1 carriers exceeded D2A2A2 subjects for a history of being expelled from school for fighting (P = 0.001), and of those ever jailed for violent crimes, 53.1% carried the D2A1 allele versus 28.8% of those jailed for non-violent crimes (P = 0.011).(ABSTRACT TRUNCATED AT 250 WORDS)

The dopamine D2 receptor (DRD2) as a major gene in obesity and height

Dopamine plays a major role in the regulation of appetite and growth hormone. Dopaminergic agonists suppress appetite and dopamine D2 receptor antagonists enhance it. We examined the hypothesis that allelic variants of the DRD2 locus may be associated with weight and height. Sarkar and Sommer described two DRD2 polymorphisms that could be haplotyped by PCR. For weight, the mean Z score (National Center for Health Statistics) for 208 subjects without haplotype 4 was 0.086 versus 0.557 for 280 subjects with haplotype 4, P = 0.0003. Two separate sets of subjects were studied and these results were significant for both, providing an internal replication. For height, the mean Z score for 164 subjects without haplotype 4 was 0.1677 versus 0.6885 for 219 subjects with haplotype 4, P < 0.00001. These and other data suggest that the 4 haplotype is in linkage disequilibrium with allelic variants of the DRD2 gene that play a major role in the regulation of weight (obesity) and height, and may serve as a risk factor in late-onset non-insulin-dependent diabetes mellitus (NIDDM).

Dopamine D2 receptor gene Taq I 'A' locus map including 'A4' variant: relevance for alcoholism and drug abuse

The D2 dopamine receptor gene (DRD2) displays Taq I restriction fragment length polymorphisms (RFLPs) at two different loci, termed A and B. One of the three different Taq I A 'alleles' described at this site, the A1 allele (size = 6.6 kb), has been found to be associated with alcoholism and with drug abuse in the majority of studies reported to date. A complete map of the Taq I A RFLP site has been constructed, through hybridization with different fragments of the 3' flanking region and polymerase chain reaction (PCR). When screening 432 unrelated individuals to establish possible A1 allelic association with drug abuse or dependence, we have encountered a novel Taq I A RFLP, which we have named 'A4' (size = 8.6 kb). This sequence variant displays a frequency of approximately 1% in our sample and shows a Mendelizing genetic pattern in an Italian nuclear family. Primers suitable for detecting A4 using PCR have been designed. The A4, but not the A3 'allele', displays substantial overlap with the A1. In particular, A2 and A3 share the presence of a Taq I restriction site, whose absence in A1 and A4 is apparently associated with substance abuse vulnerability. Therefore, in association studies it is proper to contrast individuals displaying A1 and A4 RFLP patterns, with individuals displaying A2 and A3 RFLPs.

A single dominant gene can account for eye tracking dysfunctions and schizophrenia in offspring of discordant twins

Eye movement dysfunctions (EMDs), detectable during smooth pursuit, occur in a majority of schizophrenics and in 45% of their first-degree relatives. Previous data suggest that they represent a biologic marker for schizophrenia. To determine the mode of transmission of the schizophrenia-EMD complex, the eye movements of offspring of monozygotic and dizygotic twins were recorded. One group of twins was discordant for schizophrenia; the other group for manic depression or reactive psychosis. The data suggest that EMDs and at least some schizophrenias can be considered expressions of a single underlying trait that is transmitted by an autosomal dominant gene.

Alkylation of free sulfhydryls fortifies electroplax subsynaptic structures

The cysteine-rich 43,000-dalton peripheral membrane protein, nu 1, is localized at the cytoplasmic face of electroplax and muscle cholinergic synapses, where it is thought to play an important role in the endplate supramolecular structure. The peripheral membrane protein properties of nu 1 are inferred by its removal from nicotinic cholinergic membranes by the action of mild alkali or lithium diiodosalicylate. An interesting property of nu 1 is its high concentration of free sulfhydryl groups, whose exact role in synaptic structure is still largely unknown. Alkylation of free sulfhydryls with N-ethylmaleimide (3 mM) has a profound effect on the association of nu 1 with synaptic membranes, rendering nu 1 unextractable by pH 11 treatment or by lithium diiodosalicylate and, concomitantly, decreasing nu 1's electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gels. Iodoacetamide and iodoacetate have similar effects, but at concentrations 10- to 100-fold higher than required for N-ethylmaleimide. Furthermore, sulfhydryl modification also stabilizes the association of nicotinic receptor subunits with the detergent-insoluble cytoskeleton. N-Ethylmaleimide treatment increases the fraction of insoluble receptor molecules on extraction with Triton X-100, sodium cholate, or octylglucoside. These results suggest an important role of sulfhydryl groups in the structural stability of the postsynaptic cholinergic membrane.

The ASYST Software for Scientific Computing

The acceptance of ASYST by the scientific community could dramatically change the way scientific data are handled and reduce the need for extensive in-house software development for many applications. However, there is no substitute for a well-conceived use of any software. We feel that the full acceptance of the ideas and concepts pioneered by Adaptable Laboratory Software and other software houses will depend on the ability of the scientific community to fully test and verify the procedures used by such products. Only then can the results produced by these software packages be subjected to confirmation which is crucial to rigorous scientific endeavor.

Creatine kinase isoenzymes in Torpedo californica: absence of the major brain isoenzyme from nicotinic acetylcholine receptor membranes

Creatine kinase, actin, and nu 1 are three proteins of Mr 43 000 associated with membranes from electric organ highly enriched in nicotinic acetylcholine receptor. High levels of creatine kinase are required to maintain adequate ATP levels, while actin may play a role in maintaining the synaptic cytoskeleton. Previous investigations have prompted the conclusion that postsynaptic specializations at the receptor-enriched membrane domains in electroplax contain the brain form of creatine kinase rather than the form of creatine kinase predominantly found in muscle. We have examined this conclusion by purifying Torpedo brain creatine kinase to virtual homogeneity in order to examine its immunochemical, molecular, and electrophoretic properties. On the basis of immunological cross-reactivity and isozyme analysis, the receptor-associated creatine kinase is identified to be of the muscle type. When the molecular characteristics of Torpedo brain and muscle creatine kinase are compared, the brain enzyme is positioned at a more basic pH during chromatofocusing and on two-dimensional gel electrophoresis (pI = 7.5-7.9). Furthermore, electrophoretic mobilities of the brain and muscle forms of creatine kinase differ in sodium dodecyl sulfate electrophoresis: the brain isozyme of creatine kinase has lower apparent molecular weight (Mr 41 000) when compared with the muscle enzyme (Mr 43 000). On the basis of the results of our current investigations, the hypothesis that the brain isozyme of creatine kinase is a component of the postsynaptic specializations of the Torpedo californica electroplax must be abandoned. Recent sequence data have established close homology between Torpedo and mammalian muscle creatine kinases. On the basis of electrophoretic criteria, our results indicate that a lower degree of homology exists between the brain isozymes.

Calmodulin-binding proteins: visualization by 125I-calmodulin overlay on blots quenched with Tween 20 or bovine serum albumin and poly(ethylene oxide)

To streamline detection of calmodulin-binding proteins, blotting techniques for the electrophoretic transfer of proteins onto nitrocellulose filters, followed by overlay with 125I-calmodulin, have been adapted. Autoradiography of the 125I-calmodulin-labeled blots allows the identification and quantitation of proteins that possess affinity for calmodulin. Five protocols for suppressing nonspecific binding and for enhancing specific interactions of 125I-calmodulin with electrophoretically separated proteins were investigated. Tween 20 and bovine serum albumin alone, as well as combinations of bovine serum albumin and poly(ethylene oxide) or hemoglobin and gelatin, were evaluated as quenching and enhancing agents. Tween 20 proved highly effective for quenching nonspecific binding and for enhancing specific 125I-calmodulin binding of a 61,000-Mr rat brain protein, which was only faintly observed on blots quenched with proteins alone. However, Tween 20 dissociated 50% of 68,000-Mr proteins and 80% of 21,000-Mr 125I-labeled protein standards from the nitrocellulose filter. An alternative, the combination of bovine serum albumin followed by incubation with 15,000- to 20,000-Mr poly(ethylene oxide), proved satisfactory for the recovery of 61,000-Mr calmodulin-binding activity and for the detection of calmodulin-binding peptides (50,000 to 14,000 Mr) produced by limited proteolysis of rat brain 51,000-Mr calmodulin-binding protein. These blotting procedures for detection of calmodulin-binding proteins are compatible with a variety of one-dimensional and two-dimensional electrophoresis systems, including a two-dimensional electrophoresis system utilizing urea and sodium dodecyl sulfate in the first dimension and nonurea sodium dodecyl sulfate electrophoresis in the second, a system which proved useful for resolving calmodulin-binding proteins displaying anomalous electrophoretic migration in the presence of urea.

Immunochemical and molecular differentiation of 43 000 molecular weight proteins associated with Torpedo neuroelectrocyte synapses

Synaptic membranes, highly enriched in nicotinic receptor, contain three 43 000 molecular weight (Mr) peripheral proteins (distinctive in their peptide mapping profiles and earlier designated v1, v2, and v 3) as well as the receptor alpha 2 beta gamma delta integral membrane subunits. Of the three proteins, only v1 is copurified with the membrane-bound receptor, while v2 and v3 are prominent cytosolic proteins, which are retained at significant levels in receptor-rich membranes during multistep centrifugation and affinity partitioning purification procedures [Gysin, R., Wirth, M., & Flanagan, S. D. (1981) J. Biol. Chem. 256, 11373-11376]. Peptide mapping analysis of Torpedo v3 and rabbit skeletal actin indicates that the two proteins are closely related. The enzymatic activity, creatine phosphokinase (EC 2.7.3.2), copurifies with v2 during chromatofocusing fractionation of the cytosol. The Torpedo electroplax form of creatine phosphokinase has an electrophoretic mobility identical with that of the mammalian skeletal muscle form of the enzyme. Upon release of the membrane-bound forms of v1, creatine phosphokinase, and actin by the action of mild alkali, v1 remains in a high molecular weight form. Dissociation of v1 into lower molecular weight species requires urea or sodium dodecyl sulfate (NaDodSO4). Preparation of essentially pure v1 was achieved by eluting the v1 protein spots directly from naDodSO4-isoelectric focusing gels loaded with alkali extracts derived from membranes highly enriched in nicotinic receptor. Amino acid compositions of the purified fractions indicate that v1 and Torpedo creatine phosphokinase have distinct amino acid compositions from each other and from that of actin.(ABSTRACT TRUNCATED AT 250 WORDS)

Putative 51,000-Mr protein marker for postsynaptic densities is virtually absent in cerebellum

Cerebrum and cerebellum contain numerous asymmetric synapses characterized by the presence of a postsynaptic thickening prominently stained by phosphotungstic acid and other electron-dense stains suitable for electron microscopy. A 51,000-Mr protein, copurified in postsynaptic density-enriched fractions from cerebrum, is considered to be a well established marker for the postsynaptic density. On the basis of two criteria, our studies demonstrate that the 51,000-Mr protein marker for postsynaptic densities is virtually absent in cerebellum, First, it is present in negligible amounts in deoxycholate-insoluble fractions from cerebellum but abundant in parallel fractions from cerebrum. Secondly, the 51,000-Mr protein, which binds 125I-calmodulin after SDS PAGE is readily visualized in membrane samples from cerebrum but is virtually undetectable in cerebellar samples. It is apparent that these results require reexamination of the role of the 51,000-Mr protein in postsynaptic density structures.

Structural heterogeneity and subcellular distribution of nicotinic synapse-associated proteins

Peripheral membrane proteins (Mr = 43,000) are associated with Torpedo membranes highly enriched in nicotinic receptor. These 43,000-dalton proteins are not required for ion translocation or other known receptor functions, but they have been implicated in constraint of the nicotinic receptor within the plane of the membrane bilayer. Sodium dodecyl sulfate-polyacrylamide electrophoresis allows partial resolution of the 43,000-dalton band into a doublet. We have carried out further analysis using two-dimensional gel electrophoresis, which reveals the existence of at least seven Coomassie blue-staining spots in the isoelectric focusing dimension. Peptide maps of the individual spots serve to elucidate the observed electrophoretic complexity. Three different membrane-bound proteins, designated v1, v2, and v3, were identified on the basis of their characteristic peptide maps which show no apparent homology in amino acid sequence. Two of these proteins, v1 and v2, are resolved into multiple spots in the isoelectric focusing dimension, but each group of isoelectric focusing variants has nearly identical peptide fingerprints. Of relevance to the putative role of these proteins in synaptic or receptor supramolecular structures is the observation that only v1 is exclusively membrane bound, and co-purifies with receptor whereas both v2 and v3 are also prominent proteins of the cytoplasm and are depleted from membrane fractions most enriched in receptor. These proteins may interact in the formation or maintenance of synaptic and nicotinic receptor supramolecular structures.

Affinity partitioning of membranes. Evidence for discrete membrane domains containing cholinergic receptor

Subsynaptic membrane domains from Torpedo californica electroplax contain nicotinic cholinergic receptor molecules at densities as high as 20,000 micrometers-2. Intense homogenization of the electroplax releases membrane fragments enriched in nicotinic receptor from basal lamina and other synaptic cleft and presynaptic elements. Ideally, preparations of membrane fragments, highly enriched in nicotinic receptor, should approach 125I-alpha-bungarotoxin-specific binding activities near the levels observed after receptor dispersal in detergents and subsequent affinity chromatography. We report the application of affinity partitioning, combined with multiple extraction techniques, to yield preparations of virtually homogeneous membranes enriched in nicotinic receptor alpha, beta, gamma, and delta subunits as well as the 43,000-dalton peripheral protein subunit. The countercurrent distribution technique serves to resolve three populations of receptor-containing membranes. One fraction is refractory to affinity partitioning and may represent aggregates of receptor-rich membranes with fragments derived from nonsynaptic membranes. The second and third fractions contain membrane fragments derived from the subsynaptic membrane and are highly enriched in nicotinic receptor (5.1 to 7.8 nmol of alpha-bungarotoxin binding sites/mg of protein). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of all three fractions indicates that alpha, beta, and gamma subunits are present in stable stoichiometric ratios, while the 43,000-dalton peripheral protein content varies by 33% between the fractions. However, removal of 90% of the 43,000-dalton component by mild alkali treatment does not result in conversion of one fraction into the other. The combination of affinity partitioning and counter-current distribution techniques utilized in this study should prove useful in the resolution of a variety of subcellular particles that contain specific binding sites.

An 125I-labeled binding probe for the muscarinic cholinergic receptor

We describe the synthesis and characterization of [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate), a binding probe for the muscarinic cholinergic receptor of high specific radioactivity. The binding isotherm of this 125I-labeled compound to a rat synaptic plasma membrane-enriched fraction consists of two components: a 'specific' component which is saturable and closely fits hyperbolic binding to a single class of sites as evaluated by Scatchard analysis (Kd = 1.5 to 3 nM), and a linear component which may be measured directly by preincubating membranes in 0.1 microM 3-quinuclidinyl-benzilate, the most potent muscarinic antagonist known. The specific binding of [3-3H]3-quinuclidinyl-benzilate and [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate) to rat brain subcellular fractions is parallel in myelin, synaptic plasma membrane and mitochondrial fractions with a 3--4-fold enrichment observed in synaptic plasma membrane over crude mitochondrial fractions. The concentrations of muscarinic antagonists required to block [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate) binding parallel that reported for tritiated binding probes and are consistent with physiological measurements of their dissociation constants. Because of the high specific radioactivity of [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate), this iodinated binding probe should prove useful in assaying low levels of muscarinic receptor in tissue culture and other biological sources.

Affinity partitioning of membranes. Cholinergic receptor-containing membranes from Torpedo californica

Affinity partitioning has been employed in the purification of membranes rich in cholinergic receptor from Torpedo californica electric organs. The procedure involves a modification of poly(ethylene oxide)-dextran aqueous phase partitioning systems where a ligand selective for the receptor is conjugated to the poly(ethylene oxide). Specific partitioning of the receptor-containing membranes into the poly(ethylene oxide)-rich phase occurs when bis-alpha,omega-trimethylamino poly(ethylene oxide) or bis-rho-tri-methylammonium phenylamino poly(ethylene oxide) was added to the phase system in low mole ratio. bis-alpha,omega-Methylamino poly(ethylene oxide), which should impart equivalent interfacial electromotive potential to the system but bind poorly to the receptor sites, was much less effective in producing phase distribution changes. The ligand-polymer-dependent phase distribution shifts were blocked by bisquaternary methonium ligands at concentrations consistent with their relative affinities for the cholinergic receptor. Titration or receptor sites with cobra alpha-toxin decreased the phase distribution changes in a linear fashion up to the point of stoichiometry. These observations are consistent with the phase distribution changes being consequent to ligand-polymer association with the pharmacologically important site on the receptor. The affinity partitioning procedure, when employed following an initial purification of the membranes by differential and density gradient centrifugation, yields membrane preparations with a high degree of morphological uniformity and a specific activity between 2.9 and 4.6 nmol of bound cobra alpha-toxin/mg of protein.

Affinity partitioning. A method for purification of proteins using specific polymer-ligands in aqueous polymer two-phase systems

We describe a method, called affinity partitioning, for the purification of proteins containing specific ligand binding receptor sites. This method adds specificity to the procedures for protein purification with aqueous polymer two-phase systems by introduction of a polymer derivative, coupled to an appropriate ligand. The addition of a polymer-ligand that partitions predominantly into one phase shifts the protein that binds this substance to the same phase. By performing countercurrent distribution in the presence of a polymer-ligand, the protein that binds the polymer-ligand can be separated from a heterogenous mixture. One example of affinity paritioning used dextran as the polymer-ligand. Dextran was chosen since it is a constituent of the most commonly used system for partitioning proteins. In a dextran-poly(ethylene oxide) system, concanavalin A bound dextran and partitioned predominantly into the dextran-rich phase. The addition of the specific competitor, D-mannose, displaced the partition coefficient toward unity, while the application of L-fucose, a noncompetitor, had little effect. Application of affinity partitioning to the purification of another protein required the synthesis of a specific polymer-ligand. To study this we synthesized dinitrophenyl-poly-(ethylene oxide), which binds specifically to S-23 myeloma protein. Addition of dinitrophenyl-poly(ethylene oxide) to the dextran-poly(ethylene oxide) phase system shifted the S-23 myeloma protein into the poly(ethylene oxide)-rich phase. epsilon-N-dinitrophenyl-L-lysine, by competing with binding of dinitrophenyl-poly(ethylene oxide), antagonized the latter's effect on the partition coefficient of S-23 myeloma protein. By adding various amounts of dinitrophenyl-poly-(ethylene oxide), we correlated the partition coefficient with concentration of polymer-ligand. A model of the action of polymer-ligand derivatives on the partition coefficient, derived from thermodynamic considerations, was found to be consistent with the experimental data relating the concentration of polymer-ligand and partition coefficient. Affinity partitioning should prove to be a useful complement to affinity chromatography in the purification of mixtures of proteins. Since cells and subcellular particles may be purified with aqueous polymer two-phase systems, affinity partitioning might be applied to their fractionation by using polymer-ligands specific for unique surface receptors.