SUMMARY Cerebellar outputs take polysynaptic routes to reach the rest of the brain, impeding conventional tracing. Here, we quantify pathways between the cerebellum and forebrain by using transsynaptic tracing viruses and a whole-brain analysis pipeline. With retrograde tracing, we find that most descending paths originate from the somatomotor cortex. Anterograde tracing of ascending paths encompasses most thalamic nuclei, especially ventral posteromedial, lateral posterior, mediodorsal, and reticular nuclei. In the neocortex, sensorimotor regions contain the most labeled neurons, but we find higher densities in associative areas, including orbital, anterior cingulate, prelimbic, and infralimbic cortex. Patterns of ascending expression correlate with c-Fos expression after optogenetic inhibition of Purkinje cells. Our results reveal homologous networks linking single areas of the cerebellar cortex to diverse forebrain targets. We conclude that shared areas of the cerebellum are positioned to provide sensory-motor information to regions implicated in both movement and nonmotor function.

In this study we develop and use a gain-of-function mouse allele of the Down syndrome cell adhesion molecule (Dscam) to complement loss-of-function models. We assay the role of Dscam in promoting cell death, spacing, and laminar targeting of neurons in the developing mouse retina. We find that ectopic or overexpression of Dscam is sufficient to drive cell death. Gain-of-function studies indicate that Dscam is not sufficient to increase spatial organization, prevent cell-to-cell pairing, or promote active avoidance in the mouse retina, despite the similarity of the Dscam loss-of-function phenotype in the mouse retina to phenotypes observed in Drosophila Dscam1 mutants. Both gain-and loss-of-function studies support a role for Dscam in targeting neurites; DSCAM is necessary for precise dendrite lamination, and is sufficient to retarget neurites of outer retinal cells after ectopic expression. We further demonstrate that DSCAM guides dendrite targeting in type 2 dopaminergic amacrine cells, by restricting the stratum in which exploring retinal dendrites stabilize, in a Dscam dosage-dependent manner. Based on these results we propose a single model to account for the numerous Dscam gain-and loss-of-function phenotypes reported in the mouse retina whereby DSCAM eliminates inappropriately placed cells and connections.

16Cerebellar outputs take multisynaptic paths to reach higher brain areas, impeding tracing 17 efforts. Here we quantify pathways between cerebellum and contralateral 18 thalamic/corticostriatal structures using the anterograde transsynaptic tracer herpes 19 simplex virus type 1 (H129), the retrograde tracer pseudorabies virus (Bartha), adeno-20 associated virus, and a whole-brain pipeline for neuron-level analysis using light-sheet 21 microscopy. In ascending pathways, sensorimotor regions contained the most labeled 22 neurons, but higher densities were found in associative areas, including orbital, anterior 23 cingulate, prelimbic, and infralimbic cortex. Ascending paths passed through most 24 thalamic nuclei, especially ventral posteromedial and lateral posterior (sensorimotor), 25 mediodorsal (associative), and reticular (modulatory) nuclei. Retrograde tracing revealed 26 descending paths originating largely from somatomotor cortex. Patterns of ascending 27 influence correlated with anatomical pathway strengths, as measured by brainwide 28 mapping of c-Fos responses to optogenetic inhibition of Purkinje cells. Our results 29 reveal parallel functional networks linking cerebellum to forebrain and suggest that 30 cerebellum uses sensory-motor information to guide both movement and nonmotor 31 functions. 32 most mammalian brains 8 . The major descending corticocerebellar pathway passes through the 41 pons and the majority of returning ascending fibers pass through the thalamus 9,10 , comprising 42 two massive within-brain long-distance pathways 11 . Other polysynaptic pathways exist between 43 the cerebellum and neocortex, including a smaller ascending pathway through ventral tegmental 44 area that has attracted recent interest 12 . These descending and ascending pathways are 45 suggested to form closed loops 13 , giving each cerebellar region one or more specific neocortical 46 partners with which it exchanges information. 47This picture lacks critical information: the identity of those distant regions, which have 48 been difficult to map. Given the brain-wide nature of cerebello-cortical pathways, researchers 49 have used large-scale approaches to examine the functional significance of these pathways. 50Transcranial magnetic stimulation in humans demonstrated that the cerebellum influences 51 neocortical excitability 14 , including cognitive and affective circuits 15 . Functional MRI can attain 52 subcentimeter resolution, detect long-distance correlations 16 , and when coupled with cerebellar 53 stimulation, demonstrate causal relationships 17 . Functional imaging at cellular resolution in 54 nonhuman animals has been made possible by visualizing c-Fos, an immediate-early gene 55 product whose expression is regulated by neural activity. Although useful in demonstrating 56 communication with distant brain regions, these methods do not provide cellular-resolution 57 information about cerebello-cortical circuits. 58Pathways entering and exiting the cerebellum pass through synapses in the brainstem 59 and the cer...

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We have demonstrated in a mouse model that infection with a retrovirus can lead not only to the generation of recombinants between exogenous and endogenous gammaretrovirus, but also to the mobilization of endogenous proviruses by pseudotyping entire polytropic proviral transcripts and facilitating their infectious spread to new cells. However, the frequency of this occurrence, the kinetics, and the identity of mobilized endogenous proviruses was unclear. Here we find that these mobilized transcripts are detected after only one day of infection. They predominate over recombinant polytropic viruses early in infection, persist throughout the course of disease and are comprised of multiple different polytropic proviruses. Other endogenous retroviral elements such as intracisternal A particles (IAPs) were not detected. The integration of the endogenous transcripts into new cells could result in loss of transcriptional control and elevated expression which may facilitate pathogenesis, perhaps by contributing to the generation of polytropic recombinant viruses.