Functional organization of excitatory synaptic strength in primary visual cortex.

The strength of synaptic connections fundamentally determines how neurons influence each other's firing. Excitatory connection amplitudes between pairs of cortical neurons vary over two orders of magnitude, comprising only very few strong connections among many weaker ones. Although this highly skewed distribution of connection strengths is observed in diverse cortical areas, its functional significance remains unknown: it is not clear how connection strength relates to neuronal response properties, nor how strong and weak inputs contribute to information processing in local microcircuits. Here we reveal that the strength of connections between layer 2/3 (L2/3) pyramidal neurons in mouse primary visual cortex (V1) obeys a simple rule--the few strong connections occur between neurons with most correlated responses, while only weak connections link neurons with uncorrelated responses. Moreover, we show that strong and reciprocal connections occur between cells with similar spatial receptive field structure. Although weak connections far outnumber strong connections, each neuron receives the majority of its local excitation from a small number of strong inputs provided by the few neurons with similar responses to visual features. By dominating recurrent excitation, these infrequent yet powerful inputs disproportionately contribute to feature preference and selectivity. Therefore, our results show that the apparently complex organization of excitatory connection strength reflects the similarity of neuronal responses, and suggest that rare, strong connections mediate stimulus-specific response amplification in cortical microcircuits.

Segregated Subnetworks of Intracortical Projection Neurons in Primary Visual Cortex.

The rules by which neurons in neocortex choose their synaptic partners are not fully understood. In sensory cortex, intermingled neurons encode different attributes of sensory inputs and relay them to different long-range targets. While neurons with similar responses to sensory stimuli make connections preferentially, the relationship between synaptic connectivity within an area and long-range projection target remains unclear. We examined the local connectivity and visual responses of primary visual cortex neurons projecting to anterolateral (AL) and posteromedial (PM) higher visual areas in mice. Although the response properties of layer 2/3 neurons projecting to different targets were often similar, they avoided making connections with each other. Thus, projection target, in addition to response similarity, constrains local synaptic connectivity of AL and PM projection neurons. We propose that reduced crosstalk between different populations of projection neurons permits independent function of these output channels.

Structural model for p75(NTR)-TrkA intracellular domain interaction: a combined FRET and bioinformatics study.

Nerve growth factor (NGF) is a member of the neurotrophins, which are important regulators of embryonic development and adult function in the vertebrate nervous systems. The signaling elicited by NGF regulates diverse activities, including survival, axon growth, and synaptic plasticity. NGF action is mediated by engagement with two structurally unrelated transmembrane receptors, p75(NTR) and TrkA, which are co-expressed in a variety of cells. The functional interactions of these receptors have been widely demonstrated and include complex formation, convergence of signaling pathways, and indirect interaction through adaptor proteins. Each domain of the receptors was shown to be important for the formation of TrkA and p75(NTR) complexes, but only the intramembrane and transmembrane domains seemed to be crucial for the creation of high-affinity binding sites. However, whether these occur through a physical association of the receptors is unclear. In the present work, we demonstrate by Förster resonance energy transfer that p75(NTR) and TrkA are physically associated through their intracellular (IC) domains and that this interaction occurs predominantly at the cell membrane and prior to NGF stimulation. Our data suggest that there is a pool of receptors dimerized before NGF stimulus, which could contribute to the high-affinity binding sites. We modeled the three-dimensional structure of the TrkA IC domain by homology modeling, and with this and the NMR-resolved structure of p75(NTR), we modeled the heterodimerization of TrkA and p75(NTR) by docking methods and molecular dynamics. These models, together with the results obtained by Förster resonance energy transfer, provide structural insights into the receptors' physical association.

New algorithm to determine true colocalization in combination with image restoration and time-lapse confocal microscopy to MAP kinases in mitochondria.

The subcellular localization and physiological functions of biomolecules are closely related and thus it is crucial to precisely determine the distribution of different molecules inside the intracellular structures. This is frequently accomplished by fluorescence microscopy with well-characterized markers and posterior evaluation of the signal colocalization. Rigorous study of colocalization requires statistical analysis of the data, albeit yet no single technique has been established as a standard method. Indeed, the few methods currently available are only accurate in images with particular characteristics. Here, we introduce a new algorithm to automatically obtain the true colocalization between images that is suitable for a wide variety of biological situations. To proceed, the algorithm contemplates the individual contribution of each pixel's fluorescence intensity in a pair of images to the overall Pearsons correlation and Manders' overlap coefficients. The accuracy and reliability of the algorithm was validated on both simulated and real images that reflected the characteristics of a range of biological samples. We used this algorithm in combination with image restoration by deconvolution and time-lapse confocal microscopy to address the localization of MEK1 in the mitochondria of different cell lines. Appraising the previously described behavior of Akt1 corroborated the reliability of the combined use of these techniques. Together, the present work provides a novel statistical approach to accurately and reliably determine the colocalization in a variety of biological images.