Effects of arousal and movement on secondary somatosensory and visual thalamus.

Neocortical sensory areas have associated primary and secondary thalamic nuclei. While primary nuclei transmit sensory information to cortex, secondary nuclei remain poorly understood. We recorded juxtasomally from secondary somatosensory (POm) and visual (LP) nuclei of awake mice while tracking whisking and pupil size. POm activity correlated with whisking, but not precise whisker kinematics. This coarse movement modulation persisted after facial paralysis and thus was not due to sensory reafference. This phenomenon also continued during optogenetic silencing of somatosensory and motor cortex and after lesion of superior colliculus, ruling out a motor efference copy mechanism. Whisking and pupil dilation were strongly correlated, possibly reflecting arousal. Indeed LP, which is not part of the whisker system, tracked whisking equally well, further indicating that POm activity does not encode whisker movement per se. The semblance of movement-related activity is likely instead a global effect of arousal on both nuclei. We conclude that secondary thalamus monitors behavioral state, rather than movement, and may exist to alter cortical activity accordingly.

Sensorimotor strategies and neuronal representations for shape discrimination.

Humans and other animals can identify objects by active touch, requiring the coordination of exploratory motion and tactile sensation. Both the motor strategies and neural representations employed could depend on the subject's goals. We developed a shape discrimination task that challenged head-fixed mice to discriminate concave from convex shapes. Behavioral decoding revealed that mice did this by comparing contacts across whiskers. In contrast, a separate group of mice performing a shape detection task simply summed up contacts over whiskers. We recorded populations of neurons in the barrel cortex, which processes whisker input, and found that individual neurons across the cortical layers encoded touch, whisker motion, and task-related signals. Sensory representations were task-specific: during shape discrimination, but not detection, neurons responded most to behaviorally relevant whiskers, overriding somatotopy. Thus, sensory cortex employs task-specific representations compatible with behaviorally relevant computations.

A rapid whisker-based decision underlying skilled locomotion in mice.

Skilled motor behavior requires rapidly integrating external sensory input with information about internal state to decide which movements to make next. Using machine learning approaches for high-resolution kinematic analysis, we uncover the logic of a rapid decision underlying sensory-guided locomotion in mice. After detecting obstacles with their whiskers mice select distinct kinematic strategies depending on a whisker-derived estimate of obstacle location together with the position and velocity of their body. Although mice rely on whiskers for obstacle avoidance, lesions of primary whisker sensory cortex had minimal impact. While motor cortex manipulations affected the execution of the chosen strategy, the decision-making process remained largely intact. These results highlight the potential of machine learning for reductionist analysis of naturalistic behaviors and provide a case in which subcortical brain structures appear sufficient for mediating a relatively sophisticated sensorimotor decision.

Heterogeneity of retinogeniculate axon arbors.

The retinogeniculate synapse transmits information from retinal ganglion cells (RGC) in the eye to thalamocortical relay neurons in the visual thalamus, the dorsal lateral geniculate nucleus (dLGN). Studies in mice have identified genetic markers for distinct classes of RGCs encoding different features of the visual space, facilitating the dissection of RGC subtype-specific physiology and anatomy. In this study, we examine the morphological properties of axon arbors of the BD-RGC class of ON-OFF direction selective cells that, by definition, exhibit a stereotypic dendritic arbor and termination pattern in the retina. We find that axon arbors from the same class of RGCs exhibit variations in their structure based on their target region of the dLGN. Our findings suggest that target regions may influence the morphologic and synaptic properties of their afferent inputs.

Sensation, movement and learning in the absence of barrel cortex.

For many of our senses, the role of the cerebral cortex in detecting stimuli is controversial1-17. Here we examine the effects of both acute and chronic inactivation of the primary somatosensory cortex in mice trained to move their large facial whiskers to detect an object by touch and respond with a lever to obtain a water reward. Using transgenic mice, we expressed inhibitory opsins in excitatory cortical neurons. Transient optogenetic inactivation of the primary somatosensory cortex, as well as permanent lesions, initially produced both movement and sensory deficits that impaired detection behaviour, demonstrating the link between sensory and motor systems during active sensing. Unexpectedly, lesioned mice had recovered full behavioural capabilities by the subsequent session. This rapid recovery was experience-dependent, and early re-exposure to the task after lesioning facilitated recovery. Furthermore, ablation of the primary somatosensory cortex before learning did not affect task acquisition. This combined optogenetic and lesion approach suggests that manipulations of the sensory cortex may be only temporarily disruptive to other brain structures that are themselves capable of coordinating multiple, arbitrary movements with sensation. Thus, the somatosensory cortex may be dispensable for active detection of objects in the environment.

Sidekick 2 directs formation of a retinal circuit that detects differential motion.

In the mammalian retina, processes of approximately 70 types of interneurons form specific synapses on roughly 30 types of retinal ganglion cells (RGCs) in a neuropil called the inner plexiform layer. Each RGC type extracts salient features from visual input, which are sent deeper into the brain for further processing. The specificity and stereotypy of synapses formed in the inner plexiform layer account for the feature-detecting ability of RGCs. Here we analyse the development and function of synapses on one mouse RGC type, called the W3B-RGC. These cells have the remarkable property of responding when the timing of the movement of a small object differs from that of the background, but not when they coincide. Such cells, known as local edge detectors or object motion sensors, can distinguish moving objects from a visual scene that is also moving. We show that W3B-RGCs receive strong and selective input from an unusual excitatory amacrine cell type known as VG3-AC (vesicular glutamate transporter 3). Both W3B-RGCs and VG3-ACs express the immunoglobulin superfamily recognition molecule sidekick 2 (Sdk2), and both loss- and gain-of-function studies indicate that Sdk2-dependent homophilic interactions are necessary for the selectivity of the connection. The Sdk2-specified synapse is essential for visual responses of W3B-RGCs: whereas bipolar cells relay visual input directly to most RGCs, the W3B-RGCs receive much of their input indirectly, via the VG3-ACs. This non-canonical circuit introduces a delay into the pathway from photoreceptors in the centre of the receptive field to W3B-RGCs, which could improve their ability to judge the synchrony of local and global motion.

Refinement of the retinogeniculate synapse by bouton clustering.

Mammalian sensory circuits become refined over development in an activity-dependent manner. Retinal ganglion cell (RGC) axons from each eye first map to their target in the geniculate and then segregate into eye-specific layers by the removal and addition of axon branches. Once segregation is complete, robust functional remodeling continues as the number of afferent inputs to each geniculate neuron decreases from many to a few. It is widely assumed that large-scale axon retraction underlies this later phase of circuit refinement. On the contrary, RGC axons remain stable during functional pruning. Instead, presynaptic boutons grow in size and cluster during this process. Moreover, they exhibit dynamic spatial reorganization in response to sensory experience. Surprisingly, axon complexity decreases only after the completion of the thalamic critical period. Therefore, dynamic bouton redistribution along a broad axon backbone represents an unappreciated form of plasticity underlying developmental wiring and rewiring in the CNS.

Wiring and rewiring of the retinogeniculate synapse.

The formation and refinement of synaptic circuits are areas of research that have fascinated neurobiologists for decades. A recurrent theme seen at many CNS synapses is that neuronal connections are at first imprecise, but refine and can be rearranged with time or with experience. Today, with the advent of new technologies to map and monitor neuronal circuits, it is worthwhile to revisit a powerful experimental model for examining the development and plasticity of synaptic circuits--the retinogeniculate synapse.

Stereotyped axonal arbors of retinal ganglion cell subsets in the mouse superior colliculus.

Mouse retinal ganglion cells (RGCs) have been classified into around 20 subtypes based on the shape, size, and laminar position of their dendritic arbors. In most cases tested, RGC subtypes classified in this manner also have distinct functional signatures. Here we asked whether RGC subtypes defined by dendritic morphology have stereotyped axonal arbors in their main central target, the superior colliculus (SC). We used transgenic and viral methods to sparsely label RGCs and characterized both dendritic and axonal arbors of individual RGCs. Axon arbors varied in size, shape, and laminar position. For each of 12 subtypes defined dendritically, however, axonal arbors in the contralateral SC showed considerable stereotypy. We found no systematic relationship between the laminar position of an RGC's dendrites within the inner plexiform layer and that of its axon within the retinorecipient zone of the SC, suggesting that distinct developmental mechanisms specify dendritic and axonal laminar positions. We did, however, note a significant correlation between the dendritic field sizes of RGCs and the laminar position of their axon arbors: RGCs with larger dendritic areas, and hence larger receptive fields, projected to deeper strata within the SC. Finally, combining these new results with previous physiological analyses, we find that RGC subtypes that share similar functional properties, such as directional selectivity, project to similar depths within the SC.

Experience-dependent retinogeniculate synapse remodeling is abnormal in MeCP2-deficient mice.

Mutations in MECP2 underlie the neurodevelopmental disorder Rett syndrome (RTT). One hallmark of RTT is relatively normal development followed by a later onset of symptoms. Growing evidence suggests an etiology of disrupted synaptic function, yet it is unclear how these abnormalities explain the clinical presentation of RTT. Here we investigate synapse maturation in Mecp2-deficient mice at a circuit with distinct developmental phases: the retinogeniculate synapse. We find that synapse development in mutants is comparable to that of wild-type littermates between postnatal days 9 and 21, indicating that initial phases of synapse formation, elimination, and strengthening are not significantly affected by MeCP2 absence. However, during the subsequent experience-dependent phase of synapse remodeling, the circuit becomes abnormal in mutants as retinal innervation of relay neurons increases and retinal inputs fail to strengthen further. Moreover, synaptic plasticity in response to visual deprivation is disrupted in mutants. These results suggest a crucial role for Mecp2 in experience-dependent refinement of synaptic circuits.

A new class of mutations reveals a novel function for the original phosphatidylinositol 3-kinase binding site.

Previous studies have demonstrated that the specificity of Src homology 2 (SH2) and phosphotyrosine-binding domain interactions are mediated by phosphorylated tyrosines and their neighboring amino acids. Two of the first phosphotyrosine-based binding sites were found on middle T antigen of polyoma virus. Tyr-250 acts as a binding site for ShcA, whereas Tyr-315 forms a binding site for the SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase. However, genetic analysis of a given phosphotyrosine's role in signaling can be complicated when it serves as a binding site for multiple proteins. The situation is particularly difficult when the phosphotyrosine serves as a secondary binding site for a protein with primary binding determinates elsewhere. Mutation of a tyrosine residue to phenylalanine blocks association of all bound proteins. Here we show that the mutation of the amino acids following the phosphorylated tyrosine to alanine can reveal phosphotyrosine function as a secondary binding site, while abrogating the phosphotyrosine motif's role as a primary binding site for SH2 domains. We tested this methodology by using middle T antigen. Our results suggest that Tyr-250 is a secondary binding site for phosphatidylinositol 3-kinase, whereas Tyr-315 is a secondary binding site for a yet-to-be-identified protein, which is critical for transformation.

SHP-2 is a dual-specificity phosphatase involved in Stat1 dephosphorylation at both tyrosine and serine residues in nuclei.

Signal transducer and activator of transcription (STAT) proteins are both tyrosine- and serine-phosphorylated, mediating signal transduction and gene regulation. Following gene regulation, STAT activity in the nucleus is then terminated by a nuclear protein phosphatase(s), which remains unidentified. Using novel antibody arrays to screen the Stat1-specific protein phosphatase(s), we identified a SHP-2-Stat1 interaction in the A431 cell nucleus. SHP-2 and Stat1 nuclear localization and their association in response to either epidermal growth factor or interferon-gamma (IFNgamma) were confirmed by immunofluorescent staining and affinity precipitation assays. The SHP-2 C-terminal region containing protein-tyrosine phosphatase activity interacted with the C-terminal SH2 transcriptional activation domain of Stat1. In SHP-2-/- mouse fibroblast cells, Stat1 phosphorylation at both the tyrosine residue Tyr(701) and the serine residue Ser(727) by IFNgamma was enhanced and prolonged. Consistently, purified GST-SHP-2 dephosphorylated Stat1 at both tyrosine and serine residues when immunoprecipitated phospho-Stat1 or a peptide corresponding to the sequence surrounding Tyr(P)(701) or Ser(P)(727) of Stat1 was used as the substrate. Overexpression of SHP-2 in 293T cells inhibited IFNgamma-dependent Stat1 phosphorylation and suppressed Stat1-dependent induction of luciferase activity. Our findings demonstrate that SHP-2 is a dual-specificity protein phosphatase involved in Stat1 dephosphorylation at both tyrosine and serine residues and plays an important role in modulating STAT function in gene regulation.