Mammalian target of rapamycin integrates diverse inputs to guide the outcome of antigen recognition in T cells

T cells must integrate a diverse array of intrinsic and extrinsic signals upon Ag recognition. Although these signals have canonically been categorized into three distinct events--Signal 1 (TCR engagement), Signal 2 (costimulation or inhibition), and Signal 3 (cytokine exposure)--it is now appreciated that many other environmental cues also dictate the outcome of T cell activation. These include nutrient availability, the presence of growth factors and stress signals, as well as chemokine exposure. Although all of these distinct inputs initiate unique signaling cascades, they also modulate the activity of the evolutionarily conserved serine/threonine kinase mammalian target of rapamycin (mTOR). Indeed, mTOR serves to integrate these diverse environmental inputs, ultimately transmitting a signaling program that determines the fate of newly activated T cells. In this review, we highlight how diverse signals from the immune microenvironment can guide the outcome of TCR activation through the activation of the mTOR pathway.     

Dissociable reward and timing signals in human midbrain and ventral striatum

Reward prediction error (RPE) signals are central to current models of reward-learning. Temporal difference (TD) learning models posit that these signals should be modulated by predictions, not only of magnitude but also timing of reward. Here we show that BOLD activity in the VTA conforms to such TD predictions: responses to unexpected rewards are modulated by a temporal hazard function and activity between a predictive stimulus and reward is depressed in proportion to predicted reward. By contrast, BOLD activity in ventral striatum (VS) does not reflect a TD RPE, but instead encodes a signal on the variable relevant for behavior, here timing but not magnitude of reward. The results have important implications for dopaminergic models of cortico-striatal learning and suggest a modification of the conventional view that VS BOLD necessarily reflects inputs from dopaminergic VTA neurons signaling an RPE.     

Integration of rotation and piston motions in coiled-coil signal transduction

A coordinated response to a complex and dynamic environment requires an organism to simultaneously monitor and interpret multiple signaling cues. In bacteria and some eukaryotes, environmental responses depend on the histidine autokinases (HKs). For example, VirA, a large integral membrane HK from Agrobacterium tumefaciens, regulates the expression of virulence genes in response to signals from multiple molecular classes (phenol, pH, and sugar). The ability of this pathogen to perceive inputs from different known host signals within a single protein receptor provides an opportunity to understand the mechanisms of signal integration. Here we exploited the conserved domain organization of the HKs and engineered chimeric kinases to explore the signaling mechanisms of phenol sensing and pH/sugar integration. Our data implicate a piston-assisted rotation of coiled coils for integration of multiple inputs and regulation of critical responses during pathogenesis.     

Cell type specificity of plant hormonal signals: Case studies and reflections on ethylene

In the light of increasing evidence that plant growth and development depend on signals perceived in distinct cell types where hormonal inputs are transformed into orchestrated responses triggering a plethora of physiological processes, we reflect on the case of ethylene signaling. Experimental approaches to address cell type-specificity of the ethylene response are discussed and future challenges in ethylene signaling studies are outlined.     

Activity-dependent elimination of neuromuscular synapses

At developing neuromuscular synapses in vertebrates, different motor axon inputs to muscle fibers compete for maintenance of their synapses. Competition results in progressive changes in synaptic structure and strength that lead to the weakening and loss of some inputs, a process that has been called synapse elimination. At the same time, a single input is strengthened and maintained throughout adult life, consistently recruiting muscle fibers to contract even at rapid firing rates. Work over the last decade has led to an understanding of some of the cell biological mechanisms that underlie competition and how these culminate in synapse elimination. We discuss current ideas about how activity modulates neuromuscular synaptic competition, how competition leads to synapse loss, and how these processes are modulated by cell-cell signaling. A common feature of competition at neuromuscular as well as CNS synapses is that temporally correlated activity seems to slow or prevent competition, while uncorrelated activity seems to trigger or enhance competition. Important questions that remain to be addressed include how patterns of motor neuron activity affect synaptic strength, what is the temporal relationship between changes in synaptic strength and structure, and what cellular signals mediate synapse loss. Answers to these questions will expand our understanding of the mechanisms by which activity edits synaptic structure and function, writing permanent changes in neural circuitry.     

Combinatorial gene regulation by Bmp and Wnt in zebrafish posterior mesoderm formation

Combinatorial signaling is an important mechanism that allows the embryo to utilize overlapping signaling pathways to specify different territories. In zebrafish, the Wnt and Bmp pathways interact to regulate the formation of the posterior body. In order to understand how this works mechanistically, we have identified tbx6 as a posterior mesodermal gene activated by both of these signaling pathways. We isolated a genomic fragment from the tbx6 gene that recapitulates the endogenous tbx6 expression, and used this to ask how the Bmp and Wnt signaling pathways combine to regulate gene expression. We find that the tbx6 promoter utilizes distinct domains to integrate the signaling inputs from each pathway, including multiple Tcf/LEF sites and a novel Bmp-response element. Surprisingly, we found that overexpression of either signaling pathway can activate the tbx6 promoter and the endogenous gene, whereas inputs from both pathways are required for the normal pattern of expression. These results demonstrate that both Bmp and Wnt are present at submaximal levels, which allows the pathways to function combinatorially. We present a model in which overlapping Wnt and Bmp signals in the ventrolateral region activate the expression of tbx6 and other posterior mesodermal genes, leading to the formation of posterior structures.     

Metabolism and Long-distance Translocation of Cytokinins

During plant development, distantly-located organs must communicate in order to adapt morphological and physiological features in response to environmental inputs. Among the recognized signaling molecules, a class of phytohormones known as the cytokinins functions as both local and long-distance regulatory signals for the coordination of plant development. This cytokinin-dependent communication system consists of orchestrated regulation of the metabolism, translocation, and signal transduction of this phyto...     

Neuromodulation via conditional release of endocannabinoids in the spinal locomotor network

Endocannabinoids act as retrograde signals to modulate synaptic transmission. Little is known, however, about their significance in integrated network activity underlying motor behavior. We have examined the physiological effects of endocannabinoids in a neuronal network underlying locomotor behavior using the isolated lamprey spinal cord. Our results show that endocannabinoids are released during locomotor activity and participate in setting the baseline burst rate. They are released in response to mGluR1 activation and act as retrograde messengers. This conditional release of endocannabinoids can transform motoneurons and crossing interneurons into modulatory neurons by enabling them to regulate their inhibitory synaptic inputs and thus contribute to the modulation of the locomotor burst frequency. These results provide evidence that endocannabinoid retrograde signaling occurs within the locomotor network and contributes to motor pattern generation and regulation in the spinal cord.     

Metabolism and Long-distance Translocation of Cytokinins

During plant development, distantly-located organs must communicate in order to adapt morphological and physiological features in response to environmental inputs. Among the recognized signaling molecules, a class of phytohormones known as the cytokinins functions as both local and long-distance regulatory signals for the coordination of plant development. This cytokinin-dependent communication system consists of orchestrated regulation of the metabolism, translocation, and signal transduction of this phytohormone class. Here, to gain insight into this elaborate signaling system, we summarize current models of biosynthesis, trans-membrane transport, and long-distance translocation of cytokinins in higher plants.     

A role for synaptic inputs at distal dendrites: instructive signals for hippocampal long-term plasticity

Synaptic potentials originating at distal dendritic locations are severely attenuated when they reach the soma and, thus, are poor at driving somatic spikes. Nonetheless, distal inputs convey essential information, suggesting that such inputs may be important for compartmentalized dendritic signaling. Here we report a new plasticity rule in which stimulation of distal perforant path inputs to hippocampal CA1 pyramidal neurons induces long-term potentiation at the CA1 proximal Schaffer collateral synapses when the two inputs are paired at a precise interval. This subthreshold form of heterosynaptic plasticity occurs in the absence of somatic spiking but requires activation of both NMDA receptors and IP(3) receptor-dependent release of Ca(2+) from internal stores. Our results suggest that direct sensory information arriving at distal CA1 synapses through the perforant path provide compartmentalized, instructive signals that assess the saliency of mnemonic information propagated through the hippocampal circuit to proximal synapses.     

Imaging calcium in neurons

Calcium ions generate versatile intracellular signals that control key functions in all types of neurons. Imaging calcium in neurons is particularly important because calcium signals exert their highly specific functions in well-defined cellular subcompartments. In this Primer, we briefly review the general mechanisms of neuronal calcium signaling. We then introduce the calcium imaging devices, including confocal and two-photon microscopy as well as miniaturized devices that are used in freely moving animals. We provide an overview of the classical chemical fluorescent calcium indicators and of the protein-based genetically encoded calcium indicators. Using application examples, we introduce new developments in the field, such as calcium imaging in awake, behaving animals and the use of calcium imaging for mapping single spine sensory inputs in cortical neurons in vivo. We conclude by providing an outlook on the prospects of calcium imaging for the analysis of neuronal signaling and plasticity in various animal models.     

Drosophila pacemaker neurons require g protein signaling and GABAergic inputs to generate twenty-four hour behavioral rhythms

Intercellular signaling is important for accurate circadian rhythms. In Drosophila, the small ventral lateral neurons (s-LN(v)s) are the dominant pacemaker neurons and set the pace of most other clock neurons in constant darkness. Here we show that two distinct G protein signaling pathways are required in LN(v)s for 24?hr rhythms. Reducing signaling in LN(v)s via the G alpha subunit Gs, which signals via cAMP, or via the G alpha subunit Go, which we show signals via Phospholipase 21c, lengthens the period of behavioral rhythms. In contrast, constitutive Gs or Go signaling makes most flies arrhythmic. Using dissociated LN(v)s in culture, we found that Go and the metabotropic GABA(B)-R3 receptor are required for the inhibitory effects of GABA on LN(v)s and that reduced GABA(B)-R3 expression in?vivo lengthens period. Although no clock neurons produce GABA, hyperexciting GABAergic neurons disrupts behavioral rhythms and s-LN(v) molecular clocks. Therefore, s-LN(v)s require GABAergic inputs for 24?hr rhythms.     

Polarized signaling: basolateral receptor localization in epithelial cells by PDZ-containing proteins

Extracellular signals are normally presented to one surface of epithelial cells and to one end of neurons, and so neuronal and epithelial cell signaling is inherently polarized. Another aspect of signaling polarity is that receptors are often asymmetrically distributed on the surfaces of polarized cells. Recent evidence from studies of Caenorhabditis elegans shows that signaling polarity plays an important role in development. The underlying mesoderm induces the overlying ectoderm to form the vulva, and asymmetric distribution of the signal receptor on the basolateral surface of the epithelium is crucial for this signaling. In neurons, the localization of neurotransmitter receptors and ion channels at synapses allows neurons to be exquisitely sensitive to synaptic inputs. Exciting recent reports suggest that receptor localization to neuronal synapses and the basolateral membrane domains of epithelia may involve a common molecular mechanism involving localization by PDZ-containing proteins.     

Normal gait is differentially influenced by the otoliths

Postural control depends on the integration of vestibular, somatosensory and visual orientation signals. The otolith contribution to postural control is achieved by the integration of otolith inputs and peripheral afferent inputs involved in crossed reflex pathways. This study shows that a functional linkage between otolith signals and activity in lower limb muscles is detectable in normal human gait. The otolith input appears to dominate particularly the neck proprioceptive and gaze motor influences during normal gait. This is demonstrated by an increase of tibialis anterior muscle activity during retroflexion of the head/neck, leading to an increased stability and counteracting possible perturbations. It is also shown by decrease of coordination during the movement caused by larger displacement of the centre of gravity demonstrated in vector diagrams.     

Direct biologically based biosensing of dynamic physiological function

Dynamic regulation of biological systems requires real-time assessment of relevant physiological needs. Biosensors, which transduce biological actions or reactions into signals amenable to processing, are well suited for such monitoring. Typically, in vivo biosensors approximate physiological function via the measurement of surrogate signals. The alternative approach presented here would be to use biologically based biosensors for the direct measurement of physiological activity via functional integration of relevant governing inputs. We show that an implanted excitable-tissue biosensor (excitable cardiac tissue) can be used as a real-time, integrated bioprocessor to analyze the complex inputs regulating a dynamic physiological variable (heart rate). This approach offers the potential for long-term biologically tuned quantification of endogenous physiological function.