Lymphocytes activate adhesion to intracellular adhesion molecule 1 (ICAM-1) via leukocyte function-associated antigen 1 (LFA-1), their major ␤ 2 integrin, in response to PMA (phorbol 12-myristate 13-acetate) without an increase in the number of receptors expressed. The molecular details of the mechanism are unknown. To determine the effect of PMA activation on LFA-1 movement within the plasma membrane, we used the single particle tracking technique to measure the diffusion rate of LFA-1 molecules on EBVtransformed B cells before and after PMA activation. Diffusion of LFA-1 on unactivated cells was restricted compared to CR1 (CD35), another transmembrane protein of equivalent size. PMA caused a 10-fold increase in the diffusion rate of LFA-1 without any effect on CD35. The increased LFA-1 motion induced by PMA was random, not directed, indicating that it was due to a release of constraints rather than the application of forces. The diffusion rates of LFA-1 are consistent with cytoskeletal attachment before and free diffusion after PMA. Cytochalasin D led to an equivalent increase in mobility and, at low doses, stimulated adhesion. We propose that the release of LFA-1 from cytoskeletal constraints is an important early step in activation of adhesion, implying that the nonadhesive state of LFA-1 is actively maintained by the lymphocyte cytoskeleton. ( J. Clin. Invest. 1996. 97:2139 -2144 .)

Four chinchillas were trained to respond differently to /t/ and /d/ consonant-vowel syllables produced by four talkers in three vowel contexts. This training generalized to novel instances, including synthetically produced /da/ and /ta/ (voice-on-set times of 0 and +80 milliseconds, respectively). In a second experiment, synthetic stimuli with voice-onset times between 0 and +80 milliseconds were presented for identification. The form of the labeling functions and the "phonetic boundaries" for chinchillas and English-speaking adults were similar.

The mechanism by which low affinity adhesion molecules function to produce stable cell-cell adhesion is unknown. In solution, the interaction of human CD2 with its ligand CD58 is of low affinity (500 mM ؊1 ) and the interaction of rat CD2 with its ligand CD48 is of still lower affinity (40 mM ؊1 ). At the molecular level, however, the two systems are likely to be topologically identical. Fluorescently labeled glycosylphosphatidylinositol-anchored CD48 and CD58 were prepared and incorporated into supported phospholipid bilayers, in which the ligands were capable of free lateral diffusion. Quantitative fluorescence imaging was used to study the binding of cell surface human and rat CD2 molecules to the fluorescent ligands in contact areas between Jurkat cells and the bilayers. These studies provide two major conclusions. First, CD2/ligand interactions cooperate to align membranes with nanometer precision leading to a physiologically effective two-dimensional affinity. This process does not require the intact cytoplasmic tail of CD2. Second, the degree of membrane alignment that can be achieved by topologically similar receptors deteriorates with decreasing affinity. This suggests an affinity limit for the ability of this mode of cooperativity to achieve stable cell-cell adhesion at approximately 10 mM ؊1 .Many biologically important events are initiated and sustained through low affinity binding interactions in cell-cell and cell-matrix contact areas. Several models suggest that active intracellular mechanisms are engaged to assist these low affinity interactions (1-3). This intracellular regulation can occur through the cytoplasmic domains of the adhesion molecules as in integrins and cadherins (4 -6). Other models hypothesize that the ectodomains of low affinity adhesion molecules can oligomerize to produce stable arrays that enhance the intrinsic affinity of the ectodomain interaction (7-9). Testing of these models has been limited by the absence of quantitative affinity measurements in contact areas. The fluid mosaic membrane can be viewed as a nearly two-dimensional solution in which diffusion is constrained to the plane of the membrane, with a small third dimension arising from the flexible tethering of the membrane anchored adhesion molecules (10). The contact area between two apposing membranes may have additional depth in the third dimension due to fluctuations in the distance between the two membranes. The thesis of this study is that low affinity adhesion interactions can succeed in forming many bonds between cells by organizing the two apposing membranes so that the depth of the third dimension is small and optimal for the relevant adhesion molecule pair. When the third dimension is minimized by this cooperativity, the adhesion molecule concentration is increased and bond formation is strongly favored.In this study, we tested this hypothesis using the human and rat CD2 adhesion systems. CD2 is an adhesion molecule expressed primarily on T lymphocytes in human, rat and other mammalian species. In all sp...

In an attempt to clearly differentiate perceptual effects that are attributable to "auditory" and "phonetic" levels of processing in speech perception we have undertaken a series of experiments with animal listeners. 'Four chinchillas (Chinchilla laniger) were trained to respond differently to the "endpoints" of a synthetic alveolar speech continuum (0 ms VOT and +80 ms VOT) and were then tested in a generalization paradigm with the VOT stimuli between these endpoints. The resulting identification functions were nearly identical to those obtained with adult English-speaking listeners. To test the generality of this agreement, the animals were then tested with synthetic stimuli that had labial and velar places of articulation. As a whole, the functions produced by the two species were very similar; the same relative locations of the phonetic boundaries, with lowest VOT boundaries for labial stimuli and highest for velar stimuli, were obtained for each animal and human subject. No significant differences between species on the absolute values of the phonetic boundaries were obtained, but chinchillas produced identification functions that were slightly, but significantly, less steep. These results are discussed with regard to theories of speech perception, the evolution of a speech-sound repertoire, and current interpretations of the human infant's perceptual proclivities with regard to speech-sound perception.

An account of the simple vowels of American English will be given in terms of the author's auditory-perceptual theory. Data from many sources will be used to illustrate and support this view. In addition to theoretical suggestions concerning an auditory-perceptual transformation and a segmentation rule, the perceptualized spectral patterns associated with the nonretroflex vowels will be shown to fall into regions within a slab in a three-dimensional, auditory-perceptual space. These regions form a “vowel map” that by simple rotations can be related to various “vowel charts” such as those of Jones, Pike, or Fant. While the presentheory is an elaboration of traditional formant-ratio theory, it will be shown that a concept of a perceptual reference is needed not only for talker normalization but also to disambiguate vowels not distinguished by simple formant-ratio theory. Additionally, loci for the retroflex and nasalized vowels illustrate the utility of describing vowel spectra by five numbers combined into three dimensions. Simple F1 by F2 descriptions are not adequate except for enigmatophiles. [Work supported by NINCDS and AFOSR.]