BDNF promotes axon branching of retinal ganglion cells via miRNA-132 and p250GAP.

A crucial step in the development of the vertebrate visual system is the branching of retinal ganglion cell (RGC) axons within their target, the superior colliculus/tectum. A major player in this process is the neurotrophin brain-derived neurotrophic factor (BDNF). However, the molecular basis for the signaling pathways mediating BDNF action is less well understood. As BDNF exerts some of its functions by controlling the expression of microRNAs (miRNAs), we investigated whether miRNAs are also involved in BDNF-mediated retinal axon branching. Here, we demonstrate that the expression pattern of miRNA-132 in the retina is consistent with its involvement in this process, and that BDNF induces the upregulation of miRNA-132 in retinal cultures. Furthermore, in vitro gain-of-function and loss-of-function approaches in retinal cultures reveal that miRNA-132 mediates axon branching downstream of BDNF. A known target of miRNA-132 is the Rho family GTPase-activating protein, p250GAP. We find that p250GAP is expressed in RGC axons and mediates the effects of miRNA-132 in BDNF-induced branching. BDNF treatment or overexpression of miRNA-132 leads to a reduction in p250GAP protein levels in retinal cultures, whereas the overexpression of p250GAP abolishes BDNF-induced branching. Finally, we used a loss-of-function approach to show that miRNA-132 affects the maturation of RGC termination zones in the mouse superior colliculus in vivo, while their topographic targeting remains intact. Together, our data indicate that BDNF promotes RGC axon branching during retinocollicular/tectal map formation via upregulation of miRNA-132, which in turn downregulates p250GAP.

Wolves adapt territory size, not pack size to local habitat quality.

1. Although local variation in territorial predator density is often correlated with habitat quality, the causal mechanism underlying this frequently observed association is poorly understood and could stem from facultative adjustment in either group size or territory size. 2. To test between these alternative hypotheses, we used a novel statistical framework to construct a winter population-level utilization distribution for wolves (Canis lupus) in northern Ontario, which we then linked to a suite of environmental variables to determine factors influencing wolf space use. Next, we compared habitat quality metrics emerging from this analysis as well as an independent measure of prey abundance, with pack size and territory size to investigate which hypothesis was most supported by the data. 3. We show that wolf space use patterns were concentrated near deciduous, mixed deciduous/coniferous and disturbed forest stands favoured by moose (Alces alces), the predominant prey species in the diet of wolves in northern Ontario, and in proximity to linear corridors, including shorelines and road networks remaining from commercial forestry activities. 4. We then demonstrate that landscape metrics of wolf habitat quality - projected wolf use, probability of moose occupancy and proportion of preferred land cover classes - were inversely related to territory size but unrelated to pack size. 5. These results suggest that wolves in boreal ecosystems alter territory size, but not pack size, in response to local variation in habitat quality. This could be an adaptive strategy to balance trade-offs between territorial defence costs and energetic gains due to resource acquisition. That pack size was not responsive to habitat quality suggests that variation in group size is influenced by other factors such as intraspecific competition between wolf packs.

Emergent properties of the optic tectum revealed by population analysis of direction and orientation selectivity.

How local circuits within the brain process visual information has classically been addressed at the single neuron level. Such reductionist approaches, however, struggle to capture the full scope of functional properties associated with even "simple" brain nuclei. Using population functional calcium imaging, we aim to describe how local circuits within the zebrafish optic tectum process visual information. Specifically, how are previously identified direction-selective (DS) and orientation-selective (OS) retinal ganglion cell (RGC) inputs (Nikolaou et al., 2012) represented in tectal cells? First, we identify an emergent population of DS tectal cell with a direction preference not explicitly present in any one of the RGC inputs. Second, this is associated with a striking shift from a tiled and triangular representation of directional space (RGC inputs) into an overlapping cardinal representation by tectal cell populations. Third, and in contrast, we find that orientation space is represented similarly in both the RGC input and tectal cell populations illustrating feature-dependent differences in how tectal circuits process their inputs. Finally, we identify OS and two populations of DS cells at the superficial border of the tectal neuropil, one of which is an emergent population. This study, together with our previous one (Nikolaou et al., 2012), demonstrate that direction-selectivity is established in both the retina and tectum.

A systems-based dissection of retinal inputs to the zebrafish tectum reveals different rules for different functional classes during development.

We have examined the form, diversity, and organization of three functional classes of retinal inputs to the zebrafish optic tectum during development. Our systems-based approach was to analyze data from populations of retinal ganglion cells labeled with a presynaptic targeted calcium indicator, synaptophysin GCaMP3 (SyGCaMP3). Collectively, our findings provide an insight as to the degree of visual encoding during retino-tectal development and how it dynamically evolves from a nascent and noisy presynaptic neural-scape to an increasingly complex and refined representation. We report five key features: (1) direction-selective inputs are developmentally invariant; (2) orientation-selective inputs exhibit highly dynamic properties over the same period, with changes in their functional characteristics and spatial organization; (3) inputs defined as anisotropic are an early dominant functional class, with heterogeneous response profiles, which progressively diminish in incidence and spatial extent; (4) dark rearing selectively affects the orientation-selective responses: both functional characteristics and relative spatial distributions; and (5) orientation-selective inputs exhibit four subtypes, two more than previously identified in any species. Our approach was to label RGC axon terminals with an indicator of activity and quantitatively characterize coherent response properties to different visual stimuli. Its application in the zebrafish, given its small size and the accessibility of the tectum, has enabled a quick yet robust assessment of multiple functional populations of responses.

Standard anatomical and visual space for the mouse retina: computational reconstruction and transformation of flattened retinae with the Retistruct package.

The concept of topographic mapping is central to the understanding of the visual system at many levels, from the developmental to the computational. It is important to be able to relate different coordinate systems, e.g. maps of the visual field and maps of the retina. Retinal maps are frequently based on flat-mount preparations. These use dissection and relaxing cuts to render the quasi-spherical retina into a 2D preparation. The variable nature of relaxing cuts and associated tears limits quantitative cross-animal comparisons. We present an algorithm, "Retistruct," that reconstructs retinal flat-mounts by mapping them into a standard, spherical retinal space. This is achieved by: stitching the marked-up cuts of the flat-mount outline; dividing the stitched outline into a mesh whose vertices then are mapped onto a curtailed sphere; and finally moving the vertices so as to minimise a physically-inspired deformation energy function. Our validation studies indicate that the algorithm can estimate the position of a point on the intact adult retina to within 8° of arc (3.6% of nasotemporal axis). The coordinates in reconstructed retinae can be transformed to visuotopic coordinates. Retistruct is used to investigate the organisation of the adult mouse visual system. We orient the retina relative to the nictitating membrane and compare this to eye muscle insertions. To align the retinotopic and visuotopic coordinate systems in the mouse, we utilised the geometry of binocular vision. In standard retinal space, the composite decussation line for the uncrossed retinal projection is located 64° away from the retinal pole. Projecting anatomically defined uncrossed retinal projections into visual space gives binocular congruence if the optical axis of the mouse eye is oriented at 64° azimuth and 22° elevation, in concordance with previous results. Moreover, using these coordinates, the dorsoventral boundary for S-opsin expressing cones closely matches the horizontal meridian.

Lichen conservation in heavily managed boreal forests.

Lichens are an important component of the boreal forest, where they are long lived, tend to accumulate in older stands, and are a major food source for the threatened woodland caribou (Rangifer tarandus caribou). To be fully sustainable, silvicultural practices in the boreal forest must include the conservation of ecological integrity. Dominant forest management practices, however, have short-term negative effects on lichen diversity, particularly the application of herbicides. To better understand the long-term effects of forest management, we examined lichen regeneration in 35 mixed black spruce (Picea mariana) and jack pine (Pinus banksiana) forest stands across northern Ontario to determine recovery following logging and postharvest silvicultural practices. Our forest stands were 25-40 years old and had undergone 3 common sivilcultural treatments that included harvested and planted; harvested, planted, and treated with N-[phosphonomethyl] glycine (glyphosate); and harvested, planted, and treated with 2,4-dichlorophenoxyacetic acid (2,4-D). Forest stands with herbicide treatments had lower lichen biomass and higher beta and gamma diversity than planted stands that were not treated chemically or control stands. In northwestern Ontario, planted stands that were not treated chemically had significantly greater (p < 0.05) alpha diversity than stands treated with herbicides or control stands. Our results show that common silvicultural practices do not emulate natural disturbances caused by wildfires in the boreal forest for the lichen community. We suggest a reduction in the amount of chemical application be considered in areas where lichen biomass is likely to be high and where the recovery of woodland caribou is an objective.

An in vitro evaluation of selective demineralised enamel removal using bio-active glass air abrasion.

Unnecessary over-preparation of carious enamel often occurs clinically during operative caries management. The working hypothesis to be investigated in this study is the potential for bio-active glass air abrasion to remove selectively only demineralised enamel in artificial enamel lesions when compared to equivalent alumina air abrasion, so potentially minimising cavity over-preparation. Bisected artificial, paired smooth surface enamel lesions on ethics-approved, extracted sound human molars were created and subsequently air abraded with 27 µm alumina (n = 19) and bio-active glass (n = 19). The difference between pre-operative lesion boundary and post-operative cavity margin was calculated following optical confocal fluorescent assessment of the lesion boundary. Data indicated mean% over-preparation (sound enamel removal) of 176% with alumina and 15.2% for bio-active glass (p = 0.005). Bio-active glass abrasion removed completely the demineralised enamel from artificial lesions with clinically insignificant over-preparation of sound tissue, indicating technique selectivity towards grossly demineralised enamel. Alumina air abrasion resulted in substantial enamel removal in both sound and demineralised tissues indicating the operator selectivity required to use the techniques effectively in clinical practice.

The sparseness of neuronal responses in ferret primary visual cortex.

Various arguments suggest that neuronal coding of natural sensory stimuli should be sparse (i.e., individual neurons should respond rarely but should respond reliably). We examined sparseness of visual cortical neurons in anesthetized ferret to flashed natural scenes. Response behavior differed widely between neurons. The median firing rate of 4.1 impulses per second was slightly higher than predicted from consideration of metabolic load. Thirteen percent of neurons (12 of 89) responded to <5% of the images, but one-half responded to >25% of images. Multivariate analysis of the range of sparseness values showed that 67% of the variance was accounted for by differing response patterns to moving gratings. Repeat presentation of images showed that response variance for natural images exaggerated sparseness measures; variance was scaled with mean response, but with a lower Fano factor than for the responses to moving gratings. This response variability and the "soft" sparse responses (Rehn and Sommer, 2007) raise the question of what constitutes a reliable neuronal response and imply parallel signaling by multiple neurons. We investigated whether the temporal structure of responses might be reliable enough to give additional information about natural scenes. Poststimulus time histogram shape was similar for "strong" and "weak" stimuli, with no systematic change in first-spike latency with stimulus strength. The variance of first-spike latency for repeat presentations of the same image was greater than the latency variance between images. In general, responses to flashed natural scenes do not seem compatible with a sparse encoding in which neurons fire rarely but reliably.

Opposing gradients of ephrin-As and EphA7 in the superior colliculus are essential for topographic mapping in the mammalian visual system.

During development of the retinocollicular projection in mouse, retinal axons initially overshoot their future termination zones (TZs) in the superior colliculus (SC). The formation of TZs is initiated by interstitial branching at topographically appropriate positions. Ephrin-As are expressed in a decreasing posterior-to-anterior gradient in the SC, and they suppress branching posterior to future TZs. Here we investigate the role of an EphA7 gradient in the SC, which has the reverse orientation to the ephrin-A gradient. We find that in EphA7 mutant mice the retinocollicular map is disrupted, with nasal and temporal axons forming additional or extended TZs, respectively. In vitro, retinal axons are repelled from growing on EphA7-containing stripes. Our data support the idea that EphA7 is involved in suppressing branching anterior to future TZs. These findings suggest that opposing ephrin-A and EphA gradients are required for the proper development of the retinocollicular projection.

Mortality risk increases with natal dispersal distance in American martens.

The assumption that mortality risk increases with dispersal distance has rarely been tested. We compared patterns of natal dispersal in the American marten (Martes americana) between a large regenerating forest landscape and an uncut landscape that was dominated by more mature forest to test whether mortality risk increased with dispersal distance, and whether variation in mortality risk influenced dispersal distance. Mortality risk increased with dispersal distance in both landscape treatments, but the distance-dependent increase in mortality in the regenerating landscape was twice that in the uncut landscape. Differences in body condition, supported by other data on foraging efficiency, suggested that juveniles from the regenerating landscape were less able to cope with the energetic demands of dispersal compared with juveniles from older forests. Juveniles travelled shorter distances in the regenerating versus uncut landscape. These results implied that dispersal was costly in terms of juvenile survival and that mean dispersal distance was shaped, in part, by mortality risk.

Visual experience before eye-opening and the development of the retinogeniculate pathway.

Visual experience before eye-opening is not usually thought to have any developmental significance. Here we show that naturalistic visual stimuli presented through unopened eyelids robustly activate neurons in the ferret dorsal lateral geniculate nucleus. Further, dark-rearing prior to natural eye-opening has striking effects upon geniculate physiology. Receptive field maps after dark-rearing show increased convergence of On- and Off-center responses, and neurons frequently respond to both bright and dark phases of drifting gratings. There is also increased selectivity for the orientation of the gratings. These abnormalities of On-Off segregation can be explained by the finding that the responses of immature On and Off cells to naturalistic stimuli are strongly anticorrelated.

Abnormal functional organization in the dorsal lateral geniculate nucleus of mice lacking the beta 2 subunit of the nicotinic acetylcholine receptor.

Spontaneous activity patterns in the developing retina appear important for the functional organization of the visual system. We show here that an absence of early retinal waves in mice lacking the beta2 subunit of the nicotinic acetylcholine receptor (nAChR) is associated with both gain and loss of functional organization in the dorsal lateral geniculate nucleus (dLGN). Anatomical studies show normal gross retinotopy in the beta2(-/-) dLGN but suggest reduced topographic precision in the retinogeniculate projection. Physiological recordings reveal normal topography in the dorsoventral visual axis but a lack of fine-scale mapping in the nasotemporal visual plane. In contrast, unlike wild-type mice, on- and off-center cells in the beta2(-/-) dLGN are spatially segregated. The presence of the beta2 subunit of the nAChR in the CNS is therefore important for normal functional organization in the retinogeniculate projection.

Habitat-mediated variation in predation risk by the American marten.

The probability of prey encounter, attack, capture, and kill are often hypothesized to depend on habitat structure, but field evidence in terrestrial systems is rare. We tested whether predation efficiency by the American marten (Martes americana) and fear of predation by their primary prey, the red-backed vole (Clethrionomys gapperi), differed between 20- to 50-year-old regenerating forest stands and older uncut stands. Our results showed that the frequency of prey encounter, prey attack, and prey kill were higher in old uncut forests, despite the fact that small-mammal density was similar to that in younger logged forests. These differences in predation efficiency were linked to higher abundance of coarse woody debris, which seems to offer sensory cues to martens, thereby increasing the odds of hunting success. Red-backed voles in regenerating forest stands exhibited increased wariness compared to voles living in old uncut forest, suggestive of a behavioral response to habitat-mediated variation in predation risk.

Woodland caribou habitat selection patterns in relation to predation risk and forage abundance depend on reproductive state.

The ideal free distribution assumes that animals select habitats that are beneficial to their fitness. When the needs of dependent offspring differ from those of the parent, ideal habitat selection patterns could vary with the presence or absence of offspring. We test whether habitat selection depends on reproductive state due to top-down or bottom-up influences on the fitness of woodland caribou (Rangifer tarandus caribou), a threatened, wide-ranging herbivore. We combined established methods of fitting resource and step selection functions derived from locations of collared animals in Ontario with newer techniques, including identifying calf status from video collar footage and seasonal habitat selection analysis through latent selection difference functions. We found that females with calves avoided predation risk and proximity to roads more strongly than females without calves within their seasonal ranges. At the local scale, females with calves avoided predation more strongly than females without calves. Females with calves increased predation avoidance but not selection for food availability upon calving, whereas females without calves increased selection for food availability across the same season. These behavioral responses suggest that habitat selection by woodland caribou is influenced by reproductive state, such that females with calves at heel use habitat selection to offset the increased vulnerability of their offspring to predation risk.

The influence of early experience on the development of sensory systems.

Once sensory stimuli become able to alter firing patterns in the developing brain, they can influence the maturation of neuronal circuits. Recent experimental studies add to our understanding of precisely which developmental events are affected by early experience. In particular, it appears that experience of the external environment can affect the brain earlier in development and at earlier stages of sensory processing than previously thought. These studies emphasise the developmental importance of the patterning of neuronal firing produced either by sensory stimuli or by spontaneous activity. The timing of action potentials is also an important aspect of several exciting studies describing the mechanisms - anatomical, synaptic, and molecular - by which early experience brings about alterations in the maturation of sensory circuitry. Importantly, this kind of approach can lead to predictions concerning the nature of sensory stimulation that is most effective in instructing brain development.

45S5 Bioglass-derived glass-ceramic scaffolds for bone tissue engineering.

Three-dimensional (3D), highly porous, mechanically competent, bioactive and biodegradable scaffolds have been fabricated for the first time by the replication technique using 45S5 Bioglass powder. Under an optimum sintering condition (1000 degrees C/1h), nearly full densification of the foam struts occurred and fine crystals of Na2Ca2Si3O9 formed, which conferred the scaffolds the highest possible compressive and flexural strength for this foam structure. Important findings are that the mechanically strong crystalline phase Na2Ca2Si3O9 can transform into an amorphous calcium phosphate phase after immersion in simulated body fluid for 28 days, and that the transformation kinetics can be tailored through controlling the crystallinity of the sintered 45S5 Bioglass. Therefore, the goal of an ideal scaffold that provides good mechanical support temporarily while maintaining bioactivity, and that can biodegrade at later stages at a tailorable rate is achievable with the developed Bioglass-based scaffolds.

Geniculo-Cortical Projection Diversity Revealed within the Mouse Visual Thalamus.

The mouse dorsal lateral geniculate nucleus (dLGN) is an intermediary between retina and primary visual cortex (V1). Recent investigations are beginning to reveal regional complexity in mouse dLGN. Using local injections of retrograde tracers into V1 of adult and neonatal mice, we examined the developing organisation of geniculate projection columns: the population of dLGN-V1 projection neurons that converge in cortex. Serial sectioning of the dLGN enabled the distribution of labelled projection neurons to be reconstructed and collated within a common standardised space. This enabled us to determine: the organisation of cells within the dLGN-V1 projection columns; their internal organisation (topology); and their order relative to V1 (topography). Here, we report parameters of projection columns that are highly variable in young animals and refined in the adult, exhibiting profiles consistent with shell and core zones of the dLGN. Additionally, such profiles are disrupted in adult animals with reduced correlated spontaneous activity during development. Assessing the variability between groups with partial least squares regression suggests that 4-6 cryptic lamina may exist along the length of the projection column. Our findings further spotlight the diversity of the mouse dLGN--an increasingly important model system for understanding the pre-cortical organisation and processing of visual information. Furthermore, our approach of using standardised spaces and pooling information across many animals will enhance future functional studies of the dLGN.

Visual response properties in the dorsal lateral geniculate nucleus of mice lacking the beta2 subunit of the nicotinic acetylcholine receptor.

We present a quantitative description of single-cell visual response properties in the dorsal lateral geniculate nucleus (dLGN) of anesthetized adult mice lacking the beta2 subunit of the nicotinic acetylcholine receptor (beta2-/-) and compare these response properties with data from wild-type animals. Some response features, including all spatial receptive field characteristics and bursting behavior, are entirely normal in beta2-/- dLGN cells. In other respects, the responses of beta2-/- dLGN cells are quantitatively abnormal: the mutation is associated with higher spontaneous and visually evoked firing rates, faster visual response latencies, a preference for higher temporal frequencies, and a trend toward greater contrast sensitivity. The normal response properties in the beta2-/- dLGN show that none of the many effects of the mutation, including disrupted geniculate functional organization and abnormal cholinergic transmission, have any effect on spatial response characteristics and bursting behavior in dLGN neurons. The abnormal response characteristics in the beta2-/- dLGN are most interesting in that they are no worse than normal; any visual processing deficits found in studies of the beta2-/- visual cortex must therefore arise solely from abnormalities in cortical processing.

Spatial and temporal properties of visual responses in the thalamus of the developing ferret.

Spatiotemporal patterning of neural activity is thought to influence the development of connections in the visual pathway. This patterning can arise spontaneously or through sensory experience. Here, we use a combination of natural and simple stimuli to investigate which elements of the visual environment modulate the earliest responses in the primary visual pathway of developing ferrets. Recordings were made during the first 2 weeks of visual responsiveness, which, in the ferret, overlaps with the period that the eyelids have not yet opened. Even when the eyelids are closed, both thalamic and cortical activity was found to be temporally modulated under conditions of natural visual stimulation. The modulations correlated with temporal changes in stimulus contrast but also reflected spatial structure in the visual scene. Simple stimuli were used to show that early responses to naturalistic stimuli are influenced by the localization and structure of through-the-eyelid receptive fields. The early visual responses were also characterized by substantial variability in the ability of the cells to detect stimuli of different duration and different intensity, in a temporally precise manner. These temporal and spatial properties should constrain how plasticity mechanisms interpret naturally patterned activity.

The receptive-field organization of simple cells in primary visual cortex of ferrets under natural scene stimulation.

The responses of simple cells in primary visual cortex to sinusoidal gratings can primarily be predicted from their spatial receptive fields, as mapped using spots or bars. Although this quasilinearity is well documented, it is not clear whether it holds for complex natural stimuli. We recorded from simple cells in the primary visual cortex of anesthetized ferrets while stimulating with flashed digitized photographs of natural scenes. We applied standard reverse-correlation methods to quantify the average natural stimulus that invokes a neuronal response. Although these maps cannot be the receptive fields, we find that they still predict the preferred orientation of grating for each cell very well (r = 0.91); they do not predict the spatial-frequency tuning. Using a novel application of the linear reconstruction method called regularized pseudoinverse, we were able to recover high-resolution receptive-field maps from the responses to a relatively small number of natural scenes. These receptive-field maps not only predict the optimum orientation of each cell (r = 0.96) but also the spatial-frequency optimum (r = 0.89); the maps also predict the tuning bandwidths of many cells. Therefore, our first conclusion is that the tuning preferences of the cells are primarily linear and constant across stimulus type. However, when we used these maps to predict the actual responses of the cells to natural scenes, we did find evidence of expansive output nonlinearity and nonlinear influences from outside the classical receptive fields, orientation tuning, and spatial-frequency tuning.