Epha4 controls the midline crossing and contralateral axonal projections of inferior olive neurons

Neurogenic timing of the inferior olive subdivisions is related to the olivocerebellar projection topography

The olivocerebellar projection is organized into an intricate topographical connection from the inferior olive (IO) subdivisions to the longitudinally-striped compartments of cerebellar Purkinje Cells (PCs), to play an essential role in cerebellar coordination and learning. However, the central mechanisms for forming topography need to be clarified. IO neurons and PCs are generated during overlapping periods of a few days in embryonic development. Therefore, we examined whether their neurogenic timing is specifically involved in the olivocerebellar topographic projection relationship. First, we mapped neurogenic timing in the entire IO by using the neurogenic-tagging system of neurog2-CreER (G2A) mice and specific labeling of IO neurons with FoxP2. IO subdivisions were classified into three groups depending on their neurogenic timing range. Then, we examined the relationships in the neurogenic-timing gradient between IO neurons and PCs by labeling topographic olivocerebellar projection patterns and PC neurogenic timing. Early, intermediate, and late groups of IO subdivisions projected to late, intermediate, and early groups of the cortical compartments, respectively, except for a few particular areas. The results indicated that the olivocerebellar topographic relationship is essentially arranged according to the reverse neurogenic-timing gradients of the origin and target.

EphA4 Is Required for Neural Circuits Controlling Skilled Reaching

Skilled forelimb movements are initiated by feedforward motor commands conveyed by supraspinal motor pathways. The accuracy of reaching and grasping relies on internal feedback pathways that update ongoing motor commands. In mice lacking the axon guidance molecule EphA4, axonal misrouting of the corticospinal tract and spinal interneurons is manifested, leading to a hopping gait in hindlimbs. Moreover, mice with a conditional forebrain deletion of EphA4, display forelimb hopping in adaptive locomotion and exploratory reaching movements. However, it remains unclear how loss of EphA4 signaling disrupts function of forelimb motor circuit and skilled reaching and grasping movements. Here we investigated how neural circuits controlling skilled reaching were affected by the loss of EphA4. Both male and female C57BL/6 wild-type, heterozygous EphA4^+/-, and homozygous EphA4^-/- mice were used in behavioral and in vivo electrophysiological investigations. We found that EphA4 knock-out (-/-) mice displayed impaired goal-directed reaching movements. In vivo intracellular recordings from forelimb motor neurons demonstrated increased corticoreticulospinal excitation, decreased direct reticulospinal excitation, and reduced direct propriospinal excitation in EphA4 knock-out mice. Cerebellar surface recordings showed a functional perturbation of the lateral reticular nucleus-cerebellum internal feedback pathway in EphA4 knock-out mice. Together, our findings provide in vivo evidence at the circuit level that loss of EphA4 disrupts the function of both feedforward and feedback motor pathways, resulting in deficits in skilled reaching.SIGNIFICANCE STATEMENT The central advances of this study are the demonstration that null mutation in the axon guidance molecule EphA4 gene impairs the ability of mice to perform skilled reaching, and identification of how these behavioral deficits correlates with discrete neurophysiological changes in central motor pathways involved in the control of reaching. Our findings provide in vivo evidence at the circuit level that loss of EphA4 disrupts both feedforward and feedback motor pathways, resulting in deficits in skilled reaching. This analysis of motor circuit function may help to understand the pathophysiological mechanisms underlying movement disorders in humans.

The effect of journal guidelines on the reporting of antibody validation

BACKGROUND

Despite the widespread use of antibodies as a research tool, problems with specificity, lot-to-lot consistency and sensitivity commonly occur and may be important contributing factors to the 'replication crisis' in biomedical research. This makes the validation of antibodies and accurate reporting of this validation in the scientific literature extremely important. Therefore, some journals now require authors to comply with antibody reporting guidelines.

METHODS

We used a quasi-experimental approach to assess the effectiveness of such journal guidelines in improving antibody reporting in the scientific literature. In a sample of 120 publications, we compared the reporting of antibody validation and identification information in two journals with guidelines (Nature and the Journal of Comparative Neurology) with two journals without guidelines (Science and Neuroscience), before and after the introduction of these guidelines.

RESULTS

Our results suggest that the implementation of antibody reporting guidelines might have some influence on the reporting of antibody validation information. The percentage of validated antibodies per article slightly increased from 39% to 57% in journals with guidelines, whereas this percentage decreased from 23% to 14% in journals without guidelines. Furthermore, the reporting of validation information of all primary antibodies increased by 23 percentage points in the journals with guidelines (OR = 2.80, 95% CI = 0.96-INF; adjusted p = 1, one-tailed), compared to a decrease of 13 percentage points in journals without guidelines. Fortunately, the guidelines seem to be more effective in improving the reporting of antibody identification information. The reporting of identification information of all primary antibodies used in a study increased by 58 percentage points (OR = 17.8, 95% CI = 4.8-INF; adjusted p = 0.0003, one-tailed) in journals with guidelines. This percentage also slightly increased in journals without guidelines (by 18 percentage points), suggesting an overall increased awareness of the importance of antibody identifiability. Moreover, this suggests that reporting guidelines mostly have an influence on the reporting of information that is relatively easy to provide. A small increase in the reporting of validation by referencing the scientific literature or the manufacturer's data also indicates this.

CONCLUSION

Combined with the results of previous studies on journal guidelines, our study suggests that the effect of journal antibody guidelines on validation practices by themselves may be limited, since they mostly seem to improve antibody identification instead of actual experimental validation. These guidelines, therefore, may require additional measures to ensure effective implementation. However, due to the explorative nature of our study and our small sample size, we must remain cautious towards other factors that might have played a role in the observed change in antibody reporting behaviour.

Eph/ephrin Function Contributes to the Patterning of Spinocerebellar Mossy Fibers Into Parasagittal Zones

Purkinje cell microcircuits perform diverse functions using widespread inputs from the brain and spinal cord. The formation of these functional circuits depends on developmental programs and molecular pathways that organize mossy fiber afferents from different sources into a complex and precisely patterned map within the granular layer of the cerebellum. During development, Purkinje cell zonal patterns are thought to guide mossy fiber terminals into zones. However, the molecular mechanisms that mediate this process remain unclear. Here, we used knockout mice to test whether Eph/ephrin signaling controls Purkinje cell-mossy fiber interactions during cerebellar circuit formation. Loss of ephrin-A2 and ephrin-A5 disrupted the patterning of spinocerebellar terminals into discrete zones. Zone territories in the granular layer that normally have limited spinocerebellar input contained ectopic terminals in ephrin-A2 ^-/-;ephrin-A5 ^-/- double knockout mice. However, the overall morphology of the cerebellum, lobule position, and Purkinje cell zonal patterns developed normally in the ephrin-A2 ^-/-;ephrin-A5 ^-/- mutant mice. This work suggests that communication between Purkinje cell zones and mossy fibers during postnatal development allows contact-dependent molecular cues to sharpen the innervation of sensory afferents into functional zones.

The zinc-finger transcription factor GLI3 is a regulator of precerebellar neuronal migration

Hindbrain precerebellar neurons arise from progenitor pools at the dorsal edge of the embryonic hindbrain: the caudal rhombic lip. These neurons follow distinct migratory routes to establish nuclei that provide climbing or mossy fiber inputs to the cerebellum. Gli3, a zinc-finger transcription factor in the Sonic hedgehog signaling pathway, is an important regulator of dorsal brain development. We demonstrate that in Gli3-null mutant mice, disrupted neuronal migratory streams lead to a disorganization of precerebellar nuclei. Precerebellar progenitors are properly established in Gli3-null embryos and, using conditional gene inactivation, we provide evidence that Gli3 does not play a cell-autonomous role in migrating precerebellar neurons. Thus, GLI3 likely regulates the development of other hindbrain structures, such as non-precerebellar nuclei or cranial ganglia and their respective projections, which may in turn influence precerebellar migration. Although the organization of non-precerebellar hindbrain nuclei appears to be largely unaffected in absence of Gli3, trigeminal ganglia and their central descending tracts are disrupted. We show that rostrally migrating precerebellar neurons are normally in close contact with these tracts, but are detached in Gli3-null embryos.

Anatomical Evidence for a Direct Projection from Purkinje Cells in the Mouse Cerebellar Vermis to Medial Parabrachial Nucleus

Cerebellar malformations cause changes to the sleep-wake cycle, resulting in sleep disturbance. However, it is unclear how the cerebellum contributes to the sleep-wake cycle. To examine the neural connections between the cerebellum and the nuclei involved in the sleep-wake cycle, we investigated the axonal projections of Purkinje cells in the mouse posterior vermis by using an adeno-associated virus (AAV) vector (serotype rh10) as an anterograde tracer. When an AAV vector expressing humanized renilla green fluorescent protein was injected into the cerebellar lobule IX, hrGFP and synaptophysin double-positive axonal terminals were observed in the region of medial parabrachial nucleus (MPB). The MPB is involved in the phase transition from rapid eye movement (REM) sleep to Non-REM sleep and vice versa, and the cardiovascular and respiratory responses. The hrGFP-positive axons from lobule IX went through the ventral spinocerebellar tract and finally reached the MPB. By contrast, when the AAV vector was injected into cerebellar lobule VI, no hrGFP-positive axons were observed in the MPB. To examine neurons projecting to the MPB, we unilaterally injected Fast Blue and AAV vector (retrograde serotype, rAAV2-retro) as retrograde tracers into the MPB. The cerebellar Purkinje cells in lobules VIII–X on the ipsilateral side of the Fast Blue-injected MPB were retrogradely labeled by Fast Blue and AAV vector (retrograde serotype), but no retrograde-labeled Purkinje cells were observed in lobules VI–VII and the cerebellar hemispheres. These results indicated that Purkinje cells in lobules VIII–X directly project their axons to the ipsilateral MPB but not lobules VI–VII. The direct connection between lobules VIII–X and the MPB suggests that the cerebellum participates in the neural network controlling the sleep-wake cycle, and cardiovascular and respiratory responses, by modulating the physiological function of the MPB.

Effect of early embryonic deletion of huntingtin from pyramidal neurons on the development and long-term survival of neurons in cerebral cortex and striatum

We evaluated the impact of early embryonic deletion of huntingtin (htt) from pyramidal neurons on cortical development, cortical neuron survival and motor behavior, using a cre-loxP strategy to inactivate the mouse htt gene (Hdh) in emx1-expressing cell lineages. Western blot confirmed substantial htt reduction in cerebral cortex of these Emx-htt^KO mice, with residual cortical htt in all likelihood restricted to cortical interneurons of the subpallial lineage and/or vascular endothelial cells. Despite the loss of htt early in development, cortical lamination was normal, as revealed by layer-specific markers. Cortical volume and neuron abundance were, however, significantly less than normal, and cortical neurons showed reduced brain-derived neurotrophic factor (BDNF) expression and reduced activation of BDNF signaling pathways. Nonetheless, cortical volume and neuron abundance did not show progressive age-related decline in Emx-htt^KO mice out to 24 months. Although striatal neurochemistry was normal, reductions in striatal volume and neuron abundance were seen in Emx-htt^KO mice, which were again not progressive. Weight maintenance was normal in Emx-htt^KO mice, but a slight rotarod deficit and persistent hyperactivity were observed throughout the lifespan. Our results show that embryonic deletion of htt from developing pallium does not substantially alter migration of cortical neurons to their correct laminar destinations, but does yield reduced cortical and striatal size and neuron numbers. The Emx-htt^KO mice were persistently hyperactive, possibly due to defects in corticostriatal development. Importantly, deletion of htt from cortical pyramidal neurons did not yield age-related progressive cortical or striatal pathology.

Spatial rearrangement of Purkinje cell subsets forms the transverse and longitudinal compartmentalization in the mouse embryonic cerebellum

Transversely oriented lobules and longitudinally arrayed stripes of Purkinje cell subsets subdivide the cerebellar cortex into multiple compartments that are involved in diverse functions. In the mammalian cerebellum, anterior, and posterior lobules, which are involved in somatosensorimotor function, show an alternation of aldolase C (zebrin II) -positive and -negative stripes, whereas the central lobules (lobules VIb-VII and crus I), which are implicated in nonmotor functions, show a laterally expanded arrangement solely of aldolase C-positive stripes. To understand the developmental process of this compartmental pattern, we identified groups of Purkinje cell subsets in the entire mouse cerebellum at embryonic day (E) 14.5 by staining Purkinje cell subset markers. We then tracked four major domains of Protocadherin 10 (Pcdh10)-positive Purkinje cell subsets (medial, dorsal, central, and mid-lateral subsets), which were clearly demarcated during E14.5-17.5. These domains of Purkinje cell subsets shifted predominantly in the longitudinal direction to be positioned in the anterior and posterior lobules. However, a particular portion of the medial and mid-lateral domains, and the whole of the central domain shift in the lateral direction to be positioned in the central lobules. The results indicate that while the longitudinal shift of domains of Purkinje cell subsets forms the longitudinally striped compartments in the anterior and posterior cerebellum, the lateral shift of particular domains of Purkinje cell subsets underlies the laterally expanded arrangement of stripes in central lobules. Thus, the rearrangement of Purkinje cell subsets in the embryonic cerebellum is critically related to the compartmental organization in the mammalian cerebellum.

Twitch-related and rhythmic activation of the developing cerebellar cortex

The cerebellum is a critical sensorimotor structure that exhibits protracted postnatal development in mammals. Many aspects of cerebellar circuit development are activity dependent, but little is known about the nature and sources of the activity. Based on previous findings in 6-day-old rats, we proposed that myoclonic twitches, the spontaneous movements that occur exclusively during active sleep (AS), provide generalized as well as topographically precise activity to the developing cerebellum. Taking advantage of known stages of cerebellar cortical development, we examined the relationship between Purkinje cell activity (including complex and simple spikes), nuchal and hindlimb EMG activity, and behavioral state in unanesthetized 4-, 8-, and 12-day-old rats. AS-dependent increases in complex and simple spike activity peaked at 8 days of age, with 60% of units exhibiting significantly more activity during AS than wakefulness. Also, at all three ages, approximately one-third of complex and simple spikes significantly increased their activity within 100 ms of twitches in one of the two muscles from which we recorded. Finally, we observed rhythmicity of complex and simple spikes that was especially prominent at 8 days of age and was greatly diminished by 12 days of age, likely due to developmental changes in climbing fiber and mossy fiber innervation patterns. All together, these results indicate that the neurophysiological activity of the developing cerebellum can be used to make inferences about changes in its microcircuitry. They also support the hypothesis that sleep-related twitches are a prominent source of discrete climbing and mossy fiber activity that could contribute to the activity-dependent development of this critical sensorimotor structure.

One hand clapping: lateralization of motor control

Lateralization of motor control refers to the ability to produce pure unilateral or asymmetric movements. It is required for a variety of coordinated activities, including skilled bimanual tasks and locomotion. Here we discuss the neuroanatomical substrates and pathophysiological underpinnings of lateralized motor outputs. Significant breakthroughs have been made in the past few years by studying the two known conditions characterized by the inability to properly produce unilateral or asymmetric movements, namely human patients with congenital “mirror movements” and model rodents with a “hopping gait”. Whereas mirror movements are associated with altered interhemispheric connectivity and abnormal corticospinal projections, abnormal spinal cord interneurons trajectory is responsible for the “hopping gait”. Proper commissural axon guidance is a critical requirement for these mechanisms. Interestingly, the analysis of these two conditions reveals that the production of asymmetric movements involves similar anatomical and functional requirements but in two different structures: (i) lateralized activation of the brain or spinal cord through contralateral silencing by cross-midline inhibition; and (ii) unilateral transmission of this activation, resulting in lateralized motor output.

Inhibitory effects of draxin on axonal outgrowth and migration of precerebellar neurons

The rhombic lip, a dorsal stripe of the neuroepithelium lining the edge of the fourth ventricle, is the site of origin of precerebellar neurons (PCN), which migrate tangentially towards the floor plate. After reaching the floor plate, they project their axons to the cerebellum. Although previous studies have shown that the guidance molecules Netrin/DCC and Slit/Robo have critical roles in PCN migration, the molecular mechanisms underlying this process remain poorly understood. Here, we report that draxin, a repulsive axon guidance protein, is involved in PCN development. We found that draxin is expressed in the rhombic lip and migratory stream of some PCN in the developing hindbrain of mice. In addition, draxin inhibited neurite outgrowth and nuclei migration from rhombic lip explants. These results suggest that draxin functions as a repulsive guidance cue for PCN migration. However, we observed no significant differences in PCN distribution between draxin(-/-) and wild type embryos. Thus, draxin and other axon guidance cues may have redundant roles in PCN migration.

Branching patterns of olivocerebellar axons in relation to the compartmental organization of the cerebellum

A single olivocerebellar (OC) axon gives rise to about seven branches that terminate as climbing fibers (CFs). Branching patterns of an OC axon, which are classified into local, transverse, and longitudinal types, are highly organized, in relation to the longitudinal molecular (aldolase C or zebrin II) compartmentalization and the transverse lobulation of the cerebellum. Local branching is involved in forming a narrow band-shaped functional subarea within a molecular compartment. On the other hand, transverse and longitudinal branchings appear to be involved in linking mediolaterally separated molecular compartments and rostrocaudally separated lobular areas, respectively. Longitudinal branching occurs frequently between equivalent molecular compartments of specific combinations of lobules. These combinations include lobule V-simple lobule and crus II-paramedian lobule in the pars intermedia and hemisphere, and lobules I–V and lobule VIII in the vermis. The longitudinal branching pattern not only fits with mirror-imaged somatosensory double representation of the body in the pars intermedia, but it also suggests a general rostrocaudal link exists for the whole cerebellum across the putative rostrocaudal boundary in lobule VIc-crus I. Molecular compartments of the cerebellar cortex originate from the Purkinje cell (PC) clusters that appear in the late embryonic stage, when the immature OC projection is formed. Some clusters split rostrocaudally across crus I during the development of cortical compartments, which would result in longitudinal branching of OC projection across crus I. Supposing that the branching pattern of OC axons represents an essential organization of the cerebellum, longitudinal branching suggests a functional and developmental links between the rostral and caudal cerebellum across lobule VIc-crus I throughout the cerebellar cortex.

Clustered fine compartmentalization of the mouse embryonic cerebellar cortex and its rearrangement into the postnatal striped configuration

Compartmentalization is essential for a brain area to be involved in different functions through topographic afferent and efferent connections that reflect this organization. The adult cerebellar cortex is compartmentalized into longitudinal stripes, in which Purkinje cells (PCs) have compartment-specific molecular expression profiles. How these compartments form during development is generally not understood. To investigate this process, we focused on the late developmental stages of the cerebellar compartmentalization that occur from embryonic day 17.5 (E17.5), when embryonic compartmentalization is evidently observed, to postnatal day 6 (P6), when adult-type compartmentalization begins to be established. The transformation between these compartmentalization patterns was analyzed by mapping expression patterns of several key molecular markers in serial cerebellar sections in the mouse. A complete set of 54 clustered PC subsets, which had different expression profiles of FoxP2, PLCβ4, EphA4, Pcdh10, and a reporter molecule of the 1NM13 transgenic mouse strain, were distinguished in three-dimensional space in the E17.5 cerebellum. Following individual PC subsets during development indicated that these subsets were rearranged from a clustered and multilayered configuration to a flattened, single-layered and striped configuration by means of transverse slide, longitudinal split, or transverse twist spatial transformations during development. The Purkinje cell-free spaces that exist between clusters at E17.5 become granule cell raphes that separate striped compartments at P6. The results indicate that the ∼50 PC clusters of the embryonic cerebellum will ultimately become the longitudinal compartments of the adult cerebellum after undergoing various peri- and postnatal transformations that alter their relative spatial relationships.

Architecture and development of olivocerebellar circuit topography

The cerebellum has a simple tri-laminar structure that is comprised of relatively few cell types. Yet, its internal micro-circuitry is anatomically, biochemically, and functionally complex. The most striking feature of cerebellar circuit complexity is its compartmentalized topography. Each cell type within the cerebellar cortex is organized into an exquisite map; molecular expression patterns, dendrite projections, and axon terminal fields divide the medial-lateral axis of the cerebellum into topographic sagittal zones. Here, we discuss the mechanisms that establish zones and highlight how gene expression and neural activity contribute to cerebellar pattern formation. We focus on the olivocerebellar system because its developmental mechanisms are becoming clear, its topographic termination patterns are very precise, and its contribution to zonal function is debated. This review deconstructs the architecture and development of the olivocerebellar pathway to provide an update on how brain circuit maps form and function.