Zebrin II: a polypeptide antigen expressed selectively by Purkinje cells reveals compartments in rat and fish cerebellum

Plasticity mechanisms of genetically distinct Purkinje cells

Despite its uniform appearance, the cerebellar cortex is highly heterogeneous in terms of structure, genetics and physiology. Purkinje cells (PCs), the principal and sole output neurons of the cerebellar cortex, can be categorized into multiple populations that differentially express molecular markers and display distinctive physiological features. Such features include action potential rate, but also their propensity for synaptic and intrinsic plasticity. However, the precise molecular and genetic factors that correlate with the differential physiological properties of PCs remain elusive. In this article, we provide a detailed overview of the cellular mechanisms that regulate PC activity and plasticity. We further perform a pathway analysis to highlight how molecular characteristics of specific PC populations may influence their physiology and plasticity mechanisms.

Cerebellum Lecture: the Cerebellar Nuclei-Core of the Cerebellum

The cerebellum is a key player in many brain functions and a major topic of neuroscience research. However, the cerebellar nuclei (CN), the main output structures of the cerebellum, are often overlooked. This neglect is because research on the cerebellum typically focuses on the cortex and tends to treat the CN as relatively simple output nuclei conveying an inverted signal from the cerebellar cortex to the rest of the brain. In this review, by adopting a nucleocentric perspective we aim to rectify this impression. First, we describe CN anatomy and modularity and comprehensively integrate CN architecture with its highly organized but complex afferent and efferent connectivity. This is followed by a novel classification of the specific neuronal classes the CN comprise and speculate on the implications of CN structure and physiology for our understanding of adult cerebellar function. Based on this thorough review of the adult literature we provide a comprehensive overview of CN embryonic development and, by comparing cerebellar structures in various chordate clades, propose an interpretation of CN evolution. Despite their critical importance in cerebellar function, from a clinical perspective intriguingly few, if any, neurological disorders appear to primarily affect the CN. To highlight this curious anomaly, and encourage future nucleocentric interpretations, we build on our review to provide a brief overview of the various syndromes in which the CN are currently implicated. Finally, we summarize the specific perspectives that a nucleocentric view of the cerebellum brings, move major outstanding issues in CN biology to the limelight, and provide a roadmap to the key questions that need to be answered in order to create a comprehensive integrated model of CN structure, function, development, and evolution.

Protein kinase Cγ negatively regulates the intrinsic excitability in zebrin-negative cerebellar Purkinje cells

Protein kinase C γ (PKCγ), a neuronal isoform present exclusively in the central nervous system, is most abundantly expressed in cerebellar Purkinje cells (PCs). Targeted deletion of PKCγ causes a climbing fiber synapse elimination in developing PCs and motor deficit. However, physiological roles of PKCγ in adult mouse PCs are little understood. In this study, we aimed to unravel the roles of PKCγ in mature mouse PCs by deleting PKCγ from adult mouse PCs of PKCγfl/fl mice via cerebellar injection of adeno-associated virus (AAV) vectors expressing Cre recombinase under the control of the PC-specific L7-6 promoter. Whole cell patch-clamp recording of PCs showed higher intrinsic excitability in PCs virally lacking PKCγ [PKCγ-conditional knockout (PKCγ-cKO) PCs] than in wild-type (WT) mouse PCs in the zebrin-negative module, but not in the zebrin-positive module. AAV-mediated PKCγ re-expression in PKCγ-deficient mouse PCs in the zebrin-negative module restored the enhanced intrinsic excitability to a level comparable to that of wild-type mouse PCs. In parallel with higher intrinsic excitability, we found larger hyperpolarization-activated cyclic nucleotide-gated (HCN) channel currents in PKCγ-cKO PCs located in the zebrin-negative module, compared with those in WT mouse PCs in the same region. However, pharmacological inhibition of the HCN currents did not restore the enhanced intrinsic excitability in PKCγ-cKO PCs in the zebrin-negative module. These results suggested that PKCγ suppresses the intrinsic excitability in zebrin-negative PCs, which is likely independent of the HCN current inhibition.

A Systematic Review of Direct Outputs from the Cerebellum to the Brainstem and Diencephalon in Mammals

The cerebellum is involved in many motor, autonomic and cognitive functions, and new tasks that have a cerebellar contribution are discovered on a regular basis. Simultaneously, our insight into the functional compartmentalization of the cerebellum has markedly improved. Additionally, studies on cerebellar output pathways have seen a renaissance due to the development of viral tracing techniques. To create an overview of the current state of our understanding of cerebellar efferents, we undertook a systematic review of all studies on monosynaptic projections from the cerebellum to the brainstem and the diencephalon in mammals. This revealed that important projections from the cerebellum, to the motor nuclei, cerebral cortex, and basal ganglia, are predominantly di- or polysynaptic, rather than monosynaptic. Strikingly, most target areas receive cerebellar input from all three cerebellar nuclei, showing a convergence of cerebellar information at the output level. Overall, there appeared to be a large level of agreement between studies on different species as well as on the use of different types of neural tracers, making the emerging picture of the cerebellar output areas a solid one. Finally, we discuss how this cerebellar output network is affected by a range of diseases and syndromes, with also non-cerebellar diseases having impact on cerebellar output areas.

Knockdown of the Non-canonical Wnt Gene Prickle2 Leads to Cerebellar Purkinje Cell Abnormalities While Cerebellar-Mediated Behaviors Remain Intact

Autism spectrum disorders (ASD) involve brain wide abnormalities that contribute to a constellation of symptoms including behavioral inflexibility, cognitive dysfunction, learning impairments, altered social interactions, and perceptive time difficulties. Although a single genetic variation does not cause ASD, genetic variations such as one involving a non-canonical Wnt signaling gene, Prickle2, has been found in individuals with ASD. Previous work looking into phenotypes of Prickle2 knock-out (Prickle2-/-) and heterozygous mice (Prickle2-/+) suggest patterns of behavior similar to individuals with ASD including altered social interaction and behavioral inflexibility. Growing evidence implicates the cerebellum in ASD. As Prickle2 is expressed in the cerebellum, this animal model presents a unique opportunity to investigate the cerebellar contribution to autism-like phenotypes. Here, we explore cerebellar structural and physiological abnormalities in animals with Prickle2 knockdown using immunohistochemistry, whole-cell patch clamp electrophysiology, and several cerebellar-associated motor and timing tasks, including interval timing and eyeblink conditioning. Histologically, Prickle2-/- mice have significantly more empty spaces or gaps between Purkinje cells in the posterior lobules and a decreased propensity for Purkinje cells to fire action potentials. These structural cerebellar abnormalities did not impair cerebellar-associated behaviors as eyeblink conditioning and interval timing remained intact. Therefore, although Prickle-/- mice show classic phenotypes of ASD, they do not recapitulate the involvement of the adult cerebellum and may not represent the pathophysiological heterogeneity of the disorder.

Single axonal characterization of trigeminocerebellar projection patterns in the mouse

The cerebellar projection from the trigeminal nuclear complex is one of the major populations of the cerebellar inputs. Although this projection is essential in cerebellar functional processing and organization, its morphological organization has not been systematically clarified. The present study addressed this issue by lobule-specific retrograde neuronal labeling and single axonal reconstruction with anterograde labeling. The cerebellar projection arose mainly from the interpolaris subdivision of the spinal trigeminal nucleus (Sp5I) and the principal trigeminal sensory nucleus (Pr5). Although crus II, paramedian lobule, lobule IX, and simple lobule were the major targets, paraflocculus, and other lobules received some projections. Reconstructed single trigeminocerebellar axons showed 77.8 mossy fiber terminals on average often in multiple lobules but no nuclear collaterals. More terminals were located in zebrin-negative or lightly-positive compartments than in zebrin-positive compartments. While Pr5 axons predominantly projected to ipsilateral crus II, Sp5I axons projected either predominantly to crus II and paramedian lobule often bilaterally, or predominantly to lobule IX always ipsilaterally. Lobule IX-predominant-type Sp5I neurons specifically expressed Gpr26. Gpr26-tagged neuronal labeling produced a peculiar mossy fiber distribution, which was dense in the dorsolateral lobule IX and extending transversely to the dorsal median apex in lobule IX. The projection to the cerebellar nuclei was observed in collaterals of ascending Sp5I axons that project to the diencephalon. In sum, multiple populations of trigeminocerebellar projections showed divergent projections to cerebellar lobules. The projection was generally complementary with the pontine projection and partly matched with the reported orofacial receptive field arrangement.

Single-cell epigenomics and spatiotemporal transcriptomics reveal human cerebellar development

Human cerebellar development is orchestrated by molecular regulatory networks to achieve cytoarchitecture and coordinate motor and cognitive functions. Here, we combined single-cell transcriptomics, spatial transcriptomics and single cell chromatin accessibility states to systematically depict an integrative spatiotemporal landscape of human fetal cerebellar development. We revealed that combinations of transcription factors and cis-regulatory elements (CREs) play roles in governing progenitor differentiation and cell fate determination along trajectories in a hierarchical manner, providing a gene expression regulatory map of cell fate and spatial information for these cells. We also illustrated that granule cells located in different regions of the cerebellar cortex showed distinct molecular signatures regulated by different signals during development. Finally, we mapped single-nucleotide polymorphisms (SNPs) of disorders related to cerebellar dysfunction and discovered that several disorder-associated genes showed spatiotemporal and cell type-specific expression patterns only in humans, indicating the cellular basis and possible mechanisms of the pathogenesis of neuropsychiatric disorders.

Purkinje cell microzones mediate distinct kinematics of a single movement

The classification of neuronal subpopulations has significantly advanced, yet its relevance for behavior remains unclear. The highly organized flocculus of the cerebellum, known to fine-tune multi-axial eye movements, is an ideal substrate for the study of potential functions of neuronal subpopulations. Here, we demonstrate that its recently identified subpopulations of 9+ and 9- Purkinje cells exhibit an intermediate Aldolase C expression and electrophysiological profile, providing evidence for a graded continuum of intrinsic properties among PC subpopulations. By identifying and utilizing two Cre-lines that genetically target these floccular domains, we show with high spatial specificity that these subpopulations of Purkinje cells participate in separate micromodules with topographically organized connections. Finally, optogenetic excitation of the respective subpopulations results in movements around the same axis in space, yet with distinct kinematic profiles. These results indicate that Purkinje cell subpopulations integrate in discrete circuits and mediate particular parameters of single movements.

Purkinje-Enriched snRNA-seq in SCA7 Cerebellum Reveals Zebrin Identity Loss as a Central Feature of Polyglutamine Ataxias

Spinocerebellar ataxia type 7 (SCA7) is an inherited neurodegenerative disorder caused by a CAG-polyglutamine repeat expansion. SCA7 patients display a striking loss of Purkinje cell (PC) neurons with disease progression; however, PCs are rare, making them difficult to characterize. We developed a PC nuclei enrichment protocol and applied it to single-nucleus RNA-seq of a SCA7 knock-in mouse model. Our results unify prior observations into a central mechanism of cell identity loss, impacting both glia and PCs, driving accumulation of inhibitory synapses and altered PC spiking. Zebrin-II subtype dysregulation is the predominant signal in PCs, leading to complete loss of zebrin-II striping at motor symptom onset in SCA7 mice. We show this zebrin-II subtype degradation is shared across Polyglutamine Ataxia mouse models and SCA7 patients. It has been speculated that PC subtype organization is critical for cerebellar function, and our results suggest that a breakdown of zebrin-II parasagittal striping is pathological.

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.

Lobule-Related Action Potential Shape- and History-Dependent Current Integration in Purkinje Cells of Adult and Developing Mice

Purkinje cells (PCs) are the principal cells of the cerebellar cortex and form a central element in the modular organization of the cerebellum. Differentiation of PCs based on gene expression profiles revealed two subpopulations with distinct connectivity, action potential firing and learning-induced activity changes. However, which basal cell physiological features underlie the differences between these subpopulations and to what extent they integrate input differentially remains largely unclear. Here, we investigate the cellular electrophysiological properties of PC subpopulation in adult and juvenile mice. We found that multiple fundamental cell physiological properties, including membrane resistance and various aspects of the action potential shape, differ between PCs from anterior and nodular lobules. Moreover, the two PC subpopulations also differed in the integration of negative and positive current steps as well as in size of the hyperpolarization-activated current. A comparative analysis in juvenile mice confirmed that most of these lobule-specific differences are already present at pre-weaning ages. Finally, we found that current integration in PCs is input history-dependent for both positive and negative currents, but this is not a distinctive feature between anterior and nodular PCs. Our results support the concept of a fundamental differentiation of PCs subpopulations in terms of cell physiological properties and current integration, yet reveals that history-dependent input processing is consistent across PC subtypes.

Histological and Molecular Characterization of the Inferior Olivary Nucleus and Climbing Fibers in the Goldfish, Carassius auratus

The cerebellum receives inputs via the climbing fibers originating from the inferior olivary nucleus in the ventral medulla. In mammals, the climbing fibers entwine and terminate onto both major and peripheral branches of dendrites of the Purkinje cells. In this study, the inferior olivary nucleus and climbing fiber in the goldfish were investigated with several histological techniques. By neural tracer application to the hemisphere of the cerebellum, labeled inferior olivary neurons were found in the ventral edge of the contralateral medulla. Kainate stimulated Co ^{+ +} uptake and gephyrin immunoreactivities were found in inferior olivary neurons, indicating, respectively, that they receive both excitatory (glutamatergic) and inhibitory (GABAergic or glycinergic) inputs. Inferior olivary neurons express vglut2.1 transcripts, suggesting they are glutamatergic. Around 85% of inferior olivary neurons were labeled with anti-calretinin antiserum. Calretinin immunoreactive (ir) climbing fiber terminal-like structures were distributed near the Purkinje cells and in the molecular layer. Double labeling immunofluorescence with anti-calretinin and zebrin II antisera revealed that the calretinin-ir climbing fibers run along and made synaptic-like contacts on the major dendrites of the zebrin II-ir Purkinje cells. In teleost fish, cerebellar efferent neurons, eurydendroid cells, also lie near the Purkinje cells and extend dendrites outward to intermingle with dendrites of the Purkinje cells within the molecular layer. Here we found no contacts between the climbing fiber terminals and the eurydendroid cell dendrites. These results support the idea that Purkinje cells, but not eurydendroid cells, receive strong inputs via the climbing fibers, similar to the mammalian situation.

Deep Brain Stimulation of the Interposed Cerebellar Nuclei in a Conditional Genetic Mouse Model with Dystonia

Dystonia is a neurological disease that is currently ranked as the third most common motor disorder. Patients exhibit repetitive and sometimes sustained muscle contractions that cause limb and body twisting and abnormal postures that impair movement. Deep brain stimulation (DBS) of the basal ganglia and thalamus can be used to improve motor function when other treatment options fail. Recently, the cerebellum has garnered interest as a DBS target for treating dystonia and other motor disorders. Here, we describe a procedure for targeting DBS electrodes to the interposed cerebellar nuclei to correct motor dysfunction in a mouse model with dystonia. Targeting cerebellar outflow pathways with neuromodulation opens new possibilities for using the expansive connectivity of the cerebellum to treat motor and non-motor diseases.

Purkinje Cell Patterning-Insights from Single-Cell Sequencing

Despite their homogeneous appearance, Purkinje cells are remarkably diverse with respect to their molecular phenotypes, physiological properties, afferent and efferent connectivity, as well as their vulnerability to insults. Heterogeneity in Purkinje cells arises early in development, with molecularly distinct embryonic cell clusters present soon after Purkinje cell specification. Traditional methods have characterized cerebellar development and cell types, including Purkinje cell subtypes, based on knowledge of selected markers. However, recent single-cell RNA sequencing studies provide vastly increased resolution of the whole cerebellar transcriptome. Here we draw together the results of multiple single-cell transcriptomic studies in developing and adult cerebellum in both mouse and human. We describe how this detailed transcriptomic data has increased our understanding of the intricate development and function of Purkinje cells and provides first clues into features specific to human cerebellar development.

The Transcription Factor Pou3f1 Sheds Light on the Development and Molecular Diversity of Glutamatergic Cerebellar Nuclear Neurons in the Mouse

The cerebellar nuclear (CN) neurons are a molecularly heterogeneous population whose specification into the different cerebellar nuclei is defined by the expression of varying sets of transcription factors. Here, we present a novel molecular marker, Pou3f1, that delineates specific sets of glutamatergic CN neurons. The glutamatergic identity of Pou3f1⁺ cells was confirmed by: (1) the co-expression of vGluT2, a cell marker of glutamatergic neurons; (2) the lack of co-expression between Pou3f1 and GAD67, a marker of GABAergic neurons; (3) the co-expression of Atoh1, the master regulator required for the production of all cerebellar glutamatergic lineages; and (4) the absence of Pou3f1-expressing cells in the Atoh1-null cerebellum. Furthermore, the lack of Pax6 and Tbr1 expression in Pou3f1⁺ cells reveals that Pou3f1-expressing CN neurons specifically settle in the interposed and dentate nuclei. In addition, the Pou3f1-labeled glutamatergic CN neurons can be further classified by the expression of Brn2 and Irx3. The results of the present study align with previous findings highlighting that the survival of the interposed and dentate CN neurons is largely independent of Pax6. More importantly, the present study extends the field’s collective knowledge of the molecular diversity of cerebellar nuclei.

Single-cell multi-omics sequencing and its applications in studying the nervous system

Single-cell sequencing has become one of the most powerful and popular techniques in dissecting molecular heterogeneity and modeling the cellular architecture of a biological system. During the past twenty years, the throughput of single-cell sequencing has increased from hundreds of cells to over tens of thousands of cells in parallel. Moreover, this technology has been developed from sequencing transcriptome to measure different omics such as DNA methylome, chromatin accessibility, and so on. Currently, multi-omics which can analyze different omics in the same cell is rapidly advancing. This work advances the study of many biosystems, including the nervous system. Here, we review current single-cell multi-omics sequencing techniques and describe how they improve our understanding of the nervous system. Finally, we discuss the open scientific questions in neural research that may be answered through further improvement of single-cell multi-omics sequencing technology.

Cerebellar neuronal dysfunction accompanies early motor symptoms in spinocerebellar ataxia type 3

Spinocerebellar ataxia type 3 (SCA3) is an adult-onset, progressive ataxia. SCA3 presents with ataxia before any gross neuropathology. A feature of many cerebellar ataxias is aberrant cerebellar output that contributes to motor dysfunction. We examined whether abnormal cerebellar output was present in the CMVMJD135 SCA3 mouse model and, if so, whether it correlated with the disease onset and progression. In vivo recordings showed that the activity of deep cerebellar nuclei neurons, the main output of the cerebellum, was altered. The aberrant activity correlated with the onset of ataxia. However, although the severity of ataxia increased with age, the severity of the aberrant cerebellar output was not progressive. The abnormal cerebellar output, however, was accompanied by non-progressive abnormal activity of their upstream synaptic inputs, the Purkinje cells. In vitro recordings indicated that alterations in intrinsic Purkinje cell pacemaking and in their synaptic inputs contributed to abnormal Purkinje cell activity. These findings implicate abnormal cerebellar physiology as an early, consistent contributor to pathophysiology in SCA3, and suggest that the aberrant cerebellar output could be an appropriate therapeutic target in SCA3.

Summary: In a mouse model of spinocerebellar ataxia type 3 (SCA3), aberrant cerebellar physiology is apparent early in disease, prior to cerebellar neuronal pathology. Aberrant cerebellar output could be a therapeutic target in SCA3.

Heterogeneity of intrinsic plasticity in cerebellar Purkinje cells linked with cortical molecular zones

In the cerebellar cortex, heterogeneous populations of Purkinje cells (PCs), classified into zebrin (aldolase C)-positive (Z+) and -negative (Z-) types, are arranged into separate longitudinal zones. They have different topographic neuronal connections and show different patterns of activity in behavior tasks. However, whether the zebrin type of PCs directly links with the physiological properties of the PC has not been well clarified. Therefore, we applied in vitro whole-cell patch-clamp recording in Z+ and Z- PCs in vermal and hemispheric neighboring zebrin zones in zebrin-visualized mice. Intrinsic excitability is significantly higher in Z- PCs than in Z+ PCs. Furthermore, intrinsic plasticity and synaptic long-term potentiation are enhanced more in Z- PCs than in Z+ PCs. The difference was mediated by different modulation of SK channel activities between Z+ and Z- PCs. The results indicate that cellular physiology differentially tunes to the functional compartmentalization of heterogeneous PCs.

Molecular Identity and Location Influence Purkinje Cell Vulnerability in Autosomal-Recessive Spastic Ataxia of Charlevoix-Saguenay Mice

Patterned cell death is a common feature of many neurodegenerative diseases. In patients with autosomal-recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) and mouse models of ARSACS, it has been observed that Purkinje cells in anterior cerebellar vermis are vulnerable to degeneration while those in posterior vermis are resilient. Purkinje cells are known to express certain molecules in a highly stereotyped, patterned manner across the cerebellum. One patterned molecule is zebrin, which is expressed in distinctive stripes across the cerebellar cortex. The different zones delineated by the expression pattern of zebrin and other patterned molecules have been implicated in the patterning of Purkinje cell death, raising the question of whether they contribute to cell death in ARSACS. We found that zebrin patterning appears normal prior to disease onset in Sacs^–/– mice, suggesting that zebrin-positive and -negative Purkinje cell zones develop normally. We next observed that zebrin-negative Purkinje cells in anterior lobule III were preferentially susceptible to cell death, while anterior zebrin-positive cells and posterior zebrin-negative and -positive cells remained resilient even at late disease stages. The patterning of Purkinje cell innervation to the target neurons in the cerebellar nuclei (CN) showed a similar pattern of loss: neurons in the anterior CN, where inputs are predominantly zebrin-negative, displayed a loss of Purkinje cell innervation. In contrast, neurons in the posterior CN, which is innervated by both zebrin-negative and -positive puncta, had normal innervation. These results suggest that the location and the molecular identity of Purkinje cells determine their susceptibility to cell death in ARSACS.

OUP accepted manuscript

Mutations of the mitochondrial DNA are an important cause of inherited diseases that can severely affect the tissue’s homeostasis and integrity. The m.3243A > G mutation is the most commonly observed across mitochondrial disorders and is linked to multisystemic complications, including cognitive deficits. In line with in vitro experiments demonstrating the m.3243A > G’s negative impact on neuronal energy production and integrity, m.3243A > G patients show cerebral grey matter tissue changes. However, its impact on the most neuron dense, and therefore energy-consuming brain structure—the cerebellum—remains elusive. In this work, we used high-resolution structural and functional data acquired using 7 T MRI to characterize the neurodegenerative and functional signatures of the cerebellar cortex in m.3243A > G patients. Our results reveal altered tissue integrity within distinct clusters across the cerebellar cortex, apparent by their significantly reduced volume and longitudinal relaxation rate compared with healthy controls, indicating macroscopic atrophy and microstructural pathology. Spatial characterization reveals that these changes occur especially in regions related to the frontoparietal brain network that is involved in information processing and selective attention. In addition, based on resting-state functional MRI data, these clusters exhibit reduced functional connectivity to frontal and parietal cortical regions, especially in patients characterized by (i) a severe disease phenotype and (ii) reduced information-processing speed and attention control. Combined with our previous work, these results provide insight into the neuropathological changes and a solid base to guide longitudinal studies aimed to track disease progression.

Cerebellar Patterning Defects in Mutant Mice

The cerebellar cortex is highly compartmentalized and serves as a remarkable model for pattern formation throughout the brain. In brief, the adult cerebellar cortex is subdivided into five anteroposterior units—transverse zones—and subsequently, each zone is divided into ∼20 parasagittal stripes. Zone-and-stripe pattern formation involves the interplay of two parallel developmental pathways—one for inhibitory neurons, the second for excitatory. In the inhibitory pathway, progenitor cells of the 4th ventricle generate the Purkinje cells and inhibitory interneurons. In the excitatory pathway, progenitor cells in the upper rhombic lip give rise to the external granular layer, and subsequently to the granular layer of the adult. Both the excitatory and inhibitory developmental pathways are spatially patterned and the interactions of the two generate the complex topography of the adult. This review briefly describes the cellular and molecular mechanisms that underly zone-and-stripe development with a particular focus on mutations known to interfere with normal cerebellar development and the light they cast on the mechanisms of pattern formation.

Distributed sensory coding by cerebellar complex spikes in units of cortical segments

Sensory processing is essential for motor control. Climbing fibers from the inferior olive transmit sensory signals to Purkinje cells, but how the signals are represented in the cerebellar cortex remains elusive. To examine the olivocerebellar organization of the mouse brain, we perform quantitative Ca²⁺ imaging to measure complex spikes (CSs) evoked by climbing fiber inputs over the entire dorsal surface of the cerebellum simultaneously. The surface is divided into approximately 200 segments, each composed of ∼100 Purkinje cells that fire CSs synchronously. Our in vivo imaging reveals that, although stimulation of four limb muscles individually elicits similar global CS responses across nearly all segments, the timing and location of a stimulus are derived by Bayesian inference from coordinated activation and inactivation of multiple segments on a single trial basis. We propose that the cerebellum performs segment-based, distributed-population coding that represents the conditional probability of sensory events.

Activity of Cerebellar Nuclei Neurons Correlates with ZebrinII Identity of Their Purkinje Cell Afferents

Purkinje cells (PCs) in the cerebellar cortex can be divided into at least two main subpopulations: one subpopulation that prominently expresses ZebrinII (Z+), and shows a relatively low simple spike firing rate, and another that hardly expresses ZebrinII (Z-) and shows higher baseline firing rates. Likewise, the complex spike responses of PCs, which are evoked by climbing fiber inputs and thus reflect the activity of the inferior olive (IO), show the same dichotomy. However, it is not known whether the target neurons of PCs in the cerebellar nuclei (CN) maintain this bimodal distribution. Electrophysiological recordings in awake adult mice show that the rate of action potential firing of CN neurons that receive input from Z+ PCs was consistently lower than that of CN neurons innervated by Z- PCs. Similar in vivo recordings in juvenile and adolescent mice indicated that the firing frequency of CN neurons correlates to the ZebrinII identity of the PC afferents in adult, but not postnatal stages. Finally, the spontaneous action potential firing pattern of adult CN neurons recorded in vitro revealed no significant differences in intrinsic pacemaking activity between ZebrinII identities. Our findings indicate that all three main components of the olivocerebellar loop, i.e., PCs, IO neurons and CN neurons, operate at a higher rate in the Z- modules.

Maturation of Purkinje cell firing properties relies on neurogenesis of excitatory neurons

Preterm infants that suffer cerebellar insults often develop motor disorders and cognitive difficulty. Excitatory granule cells, the most numerous neuron type in the brain, are especially vulnerable and likely instigate disease by impairing the function of their targets, the Purkinje cells. Here, we use regional genetic manipulations and in vivo electrophysiology to test whether excitatory neurons establish the firing properties of Purkinje cells during postnatal mouse development. We generated mutant mice that lack the majority of excitatory cerebellar neurons and tracked the structural and functional consequences on Purkinje cells. We reveal that Purkinje cells fail to acquire their typical morphology and connectivity, and that the concomitant transformation of Purkinje cell firing activity does not occur either. We also show that our mutant pups have impaired motor behaviors and vocal skills. These data argue that excitatory cerebellar neurons define the maturation time-window for postnatal Purkinje cell functions and refine cerebellar-dependent behaviors.

Sphingolipid metabolism governs Purkinje cell patterned degeneration in Atxn1[82Q]/+ mice

Neuronal subtypes are differentially affected by neuropathologies. For example, Purkinje cells, the principal neurons of the cerebellum, can be divided in subpopulations based on their sensitivity to pathological insult. However, the molecular mechanisms explaining why, among seemingly identical neurons, some will degenerate while others survive remain unknown. Here, we analyzed, in a disease model of cerebellar neurodegeneration, the metabolism of sphingolipids, complex lipids involved in cell apoptosis, and found that specific sphingolipids accumulate in the cerebellar region primarily affected by neurodegeneration. Preventing this accumulation by disrupting sphingolipid metabolism via genetic mutation caused a neuroprotective effect on subpopulations of Purkinje cells. Thus, our data indicate that sphingolipid metabolism is involved in the predisposition of neuronal subtypes to neurodegeneration.

Patterned degeneration of Purkinje cells (PCs) can be observed in a wide range of neuropathologies, but mechanisms behind nonrandom cerebellar neurodegeneration remain unclear. Sphingolipid metabolism dyshomeostasis typically leads to PC neurodegeneration; hence, we questioned whether local sphingolipid balance underlies regional sensitivity to pathological insults. Here, we investigated the regional compartmentalization of sphingolipids and their related enzymes in the cerebellar cortex in healthy and pathological conditions. Analysis in wild-type animals revealed higher sphingosine kinase 1 (Sphk1) levels in the flocculonodular cerebellum, while sphingosine-1-phosphate (S1P) levels were higher in the anterior cerebellum. Next, we investigated a model for spinocerebellar ataxia type 1 (SCA1) driven by the transgenic expression of the expanded Ataxin 1 protein with 82 glutamine (82Q), exhibiting severe PC degeneration in the anterior cerebellum while the flocculonodular region is preserved. In Atxn1[82Q]/+ mice, we found that levels of Sphk1 and Sphk2 were region-specific decreased and S1P levels increased, particularly in the anterior cerebellum. To determine if there is a causal link between sphingolipid levels and neurodegeneration, we deleted the Sphk1 gene in Atxn1[82Q]/+ mice. Analysis of Atxn1[82Q]/+; Sphk1^−/− mice confirmed a neuroprotective effect, rescuing a subset of PCs in the anterior cerebellum, in domains reminiscent of the modules defined by AldolaseC expression. Finally, we showed that Sphk1 deletion acts on the ATXN1[82Q] protein expression and prevents PC degeneration. Taken together, our results demonstrate that there are regional differences in sphingolipid metabolism and that this metabolism is directly involved in PC degeneration in Atxn1[82Q]/+ mice.

Single axonal morphology reveals high heterogeneity in spinocerebellar axons originating from the lumbar spinal cord in the mouse

Among the spinocerebellar projections vital for sensorimotor coordination of limbs and the trunk, the morphology of spinocerebellar axons originating from the lumbar cord has not been well characterized compared to those from thoracic and sacral cords. We reconstructed 26 single spinocerebellar axons labeled by biotinylated dextran injections into the gray matter of the lumbar spinal cord in mice. Axon terminals were mapped with the zebrin pattern of the cerebellar cortex. Reconstructed axons were primarily classified into ipsilaterally and contralaterally ascending axons, arising mainly from the dorsal and ventral horns, respectively. The majority of ipsilateral and contralateral axons took the dorsal-medullary and ventral-pontine pathways, respectively. The axons of both groups terminated mainly in the vermal and medial paravermal areas of lobules II-V and VIII-IXa, often bilaterally but predominantly ipsilateral to the axonal origin, with a weak preference to particular portions of zebrin stripes. The ipsilateral axons originating from the medial dorsal horn in the upper lumbar cord (n = 3) had abundant (43-147) mossy fiber terminals and no medullary collaterals. The ipsilateral axons originating from the lateral dorsal horn in the lower lumbar cord (n = 9) and the contralateral axons (n = 14) showed remarkable morphology variations. The number of their mossy fiber terminals varied from 2 to 172. Their collaterals, observed in 17 axons out of 23, terminated mainly in the medial cerebellar nucleus, nucleus X, and lateral reticular nucleus in various degrees. The results indicated that the lumbar spinocerebellar projection contains highly heterogeneous axonal populations regarding their pathway, branching, and termination patterns.

Parvalbumin subtypes of cerebellar Purkinje cells contribute to differential intrinsic firing properties

Purkinje cells (PCs) are central to cerebellar information coding and appreciation for the diversity of their firing patterns and molecular profiles is growing. Heterogeneous subpopulations of PCs have been identified that display differences in intrinsic firing properties without clear mechanistic insight into what underlies the divergence in firing parameters. Although long used as a general PC marker, we report that the calcium binding protein parvalbumin labels a subpopulation of PCs, based on high and low expression, with a conserved distribution pattern across the animals examined. We trained a convolutional neural network to recognize the parvalbumin subtypes and create maps of whole cerebellar distribution and find that PCs within these areas have differences in spontaneous firing that can be modified by altering calcium buffer content. These subtypes also show differential responses to potassium and calcium channel blockade, suggesting a mechanistic role for variability in PC intrinsic firing through differences in ion channel composition. It is proposed that ion channels drive the diversity in PC intrinsic firing phenotype and parvalbumin calcium buffering provides capacity for the highest firing rates observed. These findings open new avenues for detailed classification of PC subtypes.

mGluR1 signaling in cerebellar Purkinje cells: Subcellular organization and involvement in cerebellar function and disease

The cerebellum is essential for the control, coordination, and learning of movements, and for certain aspects of cognitive function. Purkinje cells are the sole output neurons in the cerebellar cortex and therefore play crucial roles in the diverse functions of the cerebellum. The type 1 metabotropic glutamate receptor (mGluR1) is prominently enriched in Purkinje cells and triggers downstream signaling pathways that are required for functional and structural plasticity, and for synaptic responses. To understand how mGluR1 contributes to cerebellar functions, it is important to consider not only the operational properties of this receptor, but also its spatial organization and the molecular interactions that enable its proper functioning. In this review, we highlight how mGluR1 and its related signaling molecules are organized into tightly coupled microdomains to fulfill physiological functions. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunction in ataxias of human patients and mouse models.

Differential spatiotemporal development of Purkinje cell populations and cerebellum-dependent sensorimotor behaviors

Distinct populations of Purkinje cells (PCs) with unique molecular and connectivity features are at the core of the modular organization of the cerebellum. Previously, we showed that firing activity of PCs differs between ZebrinII-positive and ZebrinII-negative cerebellar modules (Zhou et al., 2014; Wu et al., 2019). Here, we investigate the timing and extent of PC differentiation during development in mice. We found that several features of PCs, including activity levels, dendritic arborization, axonal shape and climbing fiber input, develop differentially between nodular and anterior PC populations. Although all PCs show a particularly rapid development in the second postnatal week, anterior PCs typically have a prolonged physiological and dendritic maturation. In line herewith, younger mice exhibit attenuated anterior-dependent eyeblink conditioning, but faster nodular-dependent compensatory eye movement adaptation. Our results indicate that specific cerebellar regions have unique developmental timelines which match with their related, specific forms of cerebellum-dependent behaviors.

Interactions Between Purkinje Cells and Granule Cells Coordinate the Development of Functional Cerebellar Circuits

Cerebellar development has a remarkably protracted morphogenetic timeline that is coordinated by multiple cell types. Here, we discuss the intriguing cellular consequences of interactions between inhibitory Purkinje cells and excitatory granule cells during embryonic and postnatal development. Purkinje cells are central to all cerebellar circuits, they are the first cerebellar cortical neurons to be born, and based on their cellular and molecular signaling, they are considered the master regulators of cerebellar development. Although rudimentary Purkinje cell circuits are already present at birth, their connectivity is morphologically and functionally distinct from their mature counterparts. The establishment of the Purkinje cell circuit with its mature firing properties has a temporal dependence on cues provided by granule cells. Granule cells are the latest born, yet most populous, neuronal type in the cerebellar cortex. They provide a combination of mechanical, molecular and activity-based cues that shape the maturation of Purkinje cell structure, connectivity and function. We propose that the wiring of Purkinje cells for function falls into two developmental phases: an initial phase that is guided by intrinsic mechanisms and a later phase that is guided by dynamically-acting cues, some of which are provided by granule cells. In this review, we highlight the mechanisms that granule cells use to help establish the unique properties of Purkinje cell firing.

Daytime Restricted Feeding Affects Day-Night Variations in Mouse Cerebellar Proteome

The cerebellum harbors a circadian clock that can be shifted by scheduled mealtime and participates in behavioral anticipation of food access. Large-scale two-dimensional difference gel electrophoresis (2D-DIGE) combined with mass spectrometry was used to identify day–night variations in the cerebellar proteome of mice fed either during daytime or nighttime. Experimental conditions led to modified expression of 89 cerebellar proteins contained in 63 protein spots. Five and 33 spots were changed respectively by time-of-day or feeding conditions. Strikingly, several proteins of the heat-shock protein family (i.e., Hsp90aa1, 90ab1, 90b1, and Hspa2, 4, 5, 8, 9) were down-regulated in the cerebellum of daytime food-restricted mice. This was also the case for brain fatty acid protein (Fabp7) and enzymes involved in oxidative phosphorylation (Ndufs1) or folate metabolism (Aldh1l1). In contrast, aldolase C (Aldoc or zebrin II) and pyruvate carboxylase (Pc), two enzymes involved in carbohydrate metabolism, and vesicle-fusing ATPase (Nsf) were up-regulated during daytime restricted feeding, possibly reflecting increased neuronal activity. Significant feeding × time-of-day interactions were found for changes in the intensity of 20 spots. Guanine nucleotide-binding protein G(o) subunit alpha (Gnao1) was more expressed in the cerebellum before food access. Neuronal calcium-sensor proteins [i.e., parvalbumin (Pvalb) and visinin-like protein 1 (Vsnl1)] were inversely regulated in daytime food-restricted mice, compared to control mice fed at night. Furthermore, expression of three enzymes modulating the circadian clockwork, namely heterogeneous nuclear ribonucleoprotein K (Hnrnpk), serine/threonine-protein phosphatases 1 (Ppp1cc and Ppp1cb subunits) and 5 (Ppp5), was differentially altered by daytime restricted feeding. Besides cerebellar proteins affected only by feeding conditions or daily cues, specific changes in in protein abundance before food access may be related to behavioral anticipation of food access and/or feeding-induced shift of the cerebellar clockwork.

Design-based stereological study of the guinea-pig (Cavia porcellus) cerebellum

Guinea pigs have proved useful as experimental animal models in studying cerebellar anatomical and structural alterations in human neurological disease; however, they are also currently acquiring increasing veterinary interest as companion animals. The morphometric features of the normal cerebellum in guinea pigs have not been previously investigated using stereology. The objective of the present work was to establish normal volumetric and quantitative stereological parameters for cerebellar tissues in guinea pigs, by means of unbiased design-based stereology. Cerebellar total volume, gray and white matter volume fractions, molecular and granular layers volume fractions, cerebellar surface area, Purkinje cellular and nuclear volumes, and the Purkinje cell total count were stereologically estimated. For this purpose, cerebellar hemispheres from six adult male guinea pigs were employed. Isotropic, uniform random sections were obtained by applying the orientator method, and subsequently processed for light microscopy. The cerebellar total volume, the white and grey matter volume fractions, and the molecular and granular layer volumes were estimated using the Cavalieri's principle and the point counting system. The cerebellar surface area was estimated through the use of test lines; Purkinje cellular and nuclear volumes were analysed using the nucleator technique, whereas the Purkinje cell total count was obtained by means of the optical disector technique. The mean ± standard deviation total volume of a guinea-pig cerebellar hemisphere was 0.11 ± 0.01 cm³ . The mean volumetric proportions occupied by the gray and white matters were, respectively, 78.0 ± 2.6% and 22.0 ± 2.6%, whereas their mean absolute volumes were found to be 0.21 ± 0.02 cm³ and 0.059 ± 0.006 cm³ . The volumes of the molecular and granular layers were estimated at 112.4 ± 20.6 mm³ and 104.4 ± 7.3 mm³ , whereas their mean thicknesses were calculated to be 0.184 ± 0.020 mm and 0.17 ± 0.02 mm. The molecular and granular layers accounted for 40.7 ± 3.9% and 37.4 ± 1.8% of total cerebellar volume respectively. The surface area of the cerebellum measured 611.4 ± 96.8 mm² . Purkinje cells with a cellular volume of 3210.1 µm³ and with a nuclear volume of 470.9 µm³ had a higher incidence of occurrence. The mean total number of Purkinje cells for a cerebellar hemisphere was calculated to be 253,090 ± 34,754. The morphometric data emerging from the present study provide a set of reference data which might prove valuable as basic anatomical contribution for practical applications in veterinary neurology.

Region-specific preservation of Purkinje cell morphology and motor behavior in the ATXN1[82Q] mouse model of spinocerebellar ataxia 1

Purkinje cells are the primary processing units of the cerebellar cortex and display molecular heterogeneity that aligns with differences in physiological properties, projection patterns, and susceptibility to disease. In particular, multiple mouse models that feature Purkinje cell degeneration are characterized by incomplete and patterned Purkinje cell degeneration, suggestive of relative sparing of Purkinje cell subpopulations, such as those expressing Aldolase C/zebrinII (AldoC) or residing in the vestibulo‐cerebellum. Here, we investigated a well‐characterized Purkinje cell‐specific mouse model for spinocerebellar ataxia type 1 (SCA1) that expresses human ATXN1 with a polyQ expansion (82Q). Our pathological analysis confirms previous findings that Purkinje cells of the vestibulo‐cerebellum, i.e., the flocculonodular lobes, and crus I are relatively spared from key pathological hallmarks: somatodendritic atrophy, and the appearance of p62/SQSTM1‐positive inclusions. However, immunohistological analysis of transgene expression revealed that spared Purkinje cells do not express mutant ATXN1 protein, indicating the sparing of Purkinje cells can be explained by an absence of transgene expression. Additionally, we found that Purkinje cells in other cerebellar lobules that typically express AldoC, not only display severe pathology but also show loss of AldoC expression. The relatively preserved flocculonodular lobes and crus I showed a substantial fraction of Purkinje cells that expressed the mutant protein and displayed pathology as well as loss of AldoC expression. Despite considerable pathology in these lobules, behavioral analyses demonstrated a relative sparing of related functions, suggestive of sufficient functional cerebellar reserve. Together, the data indicate that mutant ATXN1 affects both AldoC‐positive and AldoC‐negative Purkinje cells and disrupts normal parasagittal AldoC expression in Purkinje cells. Our results show that, in a mouse model otherwise characterized by widespread Purkinje cell degeneration, sparing of specific subpopulations is sufficient to maintain normal performance of specific behaviors within the context of the functional, modular map of the cerebellum.

Origins, Development, and Compartmentation of the Granule Cells of the Cerebellum

Granule cells (GCs) are the most numerous cell type in the cerebellum and indeed, in the brain: at least 99% of all cerebellar neurons are granule cells. In this review article, we first consider the formation of the upper rhombic lip, from which all granule cell precursors arise, and the way by which the upper rhombic lip generates the external granular layer, a secondary germinal epithelium that serves to amplify the upper rhombic lip precursors. Next, we review the mechanisms by which postmitotic granule cells are generated in the external granular layer and migrate radially to settle in the granular layer. In addition, we review the evidence that far from being a homogeneous population, granule cells come in multiple phenotypes with distinct topographical distributions and consider ways in which the heterogeneity of granule cells might arise during development.

Heterochronic Developmental Shifts Underlying Squamate Cerebellar Diversity Unveil the Key Features of Amniote Cerebellogenesis

Despite a remarkable conservation of architecture and function, the cerebellum of vertebrates shows extensive variation in morphology, size, and foliation pattern. These features make this brain subdivision a powerful model to investigate the evolutionary developmental mechanisms underlying neuroanatomical complexity both within and between anamniote and amniote species. Here, we fill a major evolutionary gap by characterizing the developing cerebellum in two non-avian reptile species-bearded dragon lizard and African house snake-representative of extreme cerebellar morphologies and neuronal arrangement patterns found in squamates. Our data suggest that developmental strategies regarded as exclusive hallmark of birds and mammals, including transit amplification in an external granule layer (EGL) and Sonic hedgehog expression by underlying Purkinje cells (PCs), contribute to squamate cerebellogenesis independently from foliation pattern. Furthermore, direct comparison of our models suggests the key importance of spatiotemporal patterning and dynamic interaction between granule cells and PCs in defining cortical organization. Especially, the observed heterochronic shifts in early cerebellogenesis events, including upper rhombic lip progenitor activity and EGL maintenance, are strongly expected to affect the dynamics of molecular interaction between neuronal cell types in snakes. Altogether, these findings help clarifying some of the morphogenetic and molecular underpinnings of amniote cerebellar corticogenesis, but also suggest new potential molecular mechanisms underlying cerebellar complexity in squamates. Furthermore, squamate models analyzed here are revealed as key animal models to further understand mechanisms of brain organization.

Common Origin of the Cerebellar Dual Somatotopic Areas Revealed by Tracking Embryonic Purkinje Cell Clusters with Birthdate Tagging

One of the notable characteristics of the functional localization in the cerebellar cortex is the dual representation of the body (somatotopy) on its anterior-posterior axis. This somatotopy is conspicuous in the C1/C3 module, which is demarcated as the multiple zebrin-negative and weekly-positive stripes in dual paravermal areas in anterior and posterior lobules within the cerebellar compartments. In this report, we describe the early formation process of the cerebellar compartmentalization, particularly in the C1/C3 module. As developing PCs guide formation of the module-specific proper neuronal circuits in the cerebellum, we hypothesized that the rearrangement of embryonic Purkinje cell (PC) clusters shapes the adult cerebellar compartmentalization. By identifying PC clusters with immunostaining of marker molecules and genetical birthdate-tagging with Neurog2-CreER (G2A) mice, we clarified the three-dimensional spatial organization of the PC clusters and tracked the lineage relationships among the PC clusters from embryonic day 14.5 (E14.5) till E17.5. The number of recognized clusters increased from 9 at E14.5 to 37 at E17.5. Among E14.5 PC clusters, the c-l (central-lateral) cluster which lacked E10.5-born PCs divided into six c-l lineage clusters. They separately migrated underneath other clusters and positioned far apart mediolaterally as well as rostrocaudally by E17.5. They were eventually transformed mainly into multiple separate zebrin-negative and weakly-positive stripes, which together configured the adult C1/C3 module, in the anterior and posterior paravermal lobules. The results indicate that the spatial rearrangement of embryonic PC clusters is involved in forming the dual somatotopic areas in the adult mouse paravermal cerebellar cortex.

Purkinje cell neurotransmission patterns cerebellar basket cells into zonal modules defined by distinct pinceau sizes

Ramón y Cajal proclaimed the neuron doctrine based on circuit features he exemplified using cerebellar basket cell projections. Basket cells form dense inhibitory plexuses that wrap Purkinje cell somata and terminate as pinceaux at the initial segment of axons. Here, we demonstrate that HCN1, Kv1.1, PSD95 and GAD67 unexpectedly mark patterns of basket cell pinceaux that map onto Purkinje cell functional zones. Using cell-specific genetic tracing with an Ascl1^CreERT2 mouse conditional allele, we reveal that basket cell zones comprise different sizes of pinceaux. We tested whether Purkinje cells instruct the assembly of inhibitory projections into zones, as they do for excitatory afferents. Genetically silencing Purkinje cell neurotransmission blocks the formation of sharp Purkinje cell zones and disrupts excitatory axon patterning. The distribution of pinceaux into size-specific zones is eliminated without Purkinje cell GABAergic output. Our data uncover the cellular and molecular diversity of a foundational synapse that revolutionized neuroscience.

A Patterned Architecture of Monoaminergic Afferents in the Cerebellar Cortex: Noradrenergic and Serotonergic Fibre Distributions within Lobules and Parasagittal Zones

The geometry of the glutamatergic mossy-parallel fibre and climbing fibre inputs to cerebellar cortical Purkinje cells has powerfully influenced thinking about cerebellar functions. The compartmentation of the cerebellum into parasagittal zones, identifiable in olivo-cortico-nuclear projections, and the trajectories of the parallel fibres, transverse to these zones and following the long axes of the cortical folia, are particularly important. Two monoaminergic afferent systems, the serotonergic and noradrenergic, are major inputs to the cerebellar cortex but their architecture and relationship with the cortical geometry are poorly understood. Immunohistochemistry for the serotonin transporter (SERT) and for the noradrenaline transporter (NET) revealed strong anisotropy of these afferent fibres in the molecular layer of rat cerebellar cortex. Individual serotonergic fibres travel predominantly medial-lateral, along the long axes of the cortical folia, similar to parallel fibres and Zebrin II immunohistochemistry revealed that they can influence multiple zones. In contrast, individual noradrenergic fibres run predominantly parasagittally with rostral-caudal extents significantly longer than their medial-lateral deviations. Their local area of influence has similarities in form and size to those of identified microzones. Within the molecular layer, the orthogonal trajectories of these two afferent systems suggest different information processing. An individual serotonergic fibre must influence all zones and microzones within its medial-lateral trajectory. In contrast, noradrenergic fibres can influence smaller cortical territories, potentially as limited as a microzone. Evidence is emerging that these monoaminergic systems may not supply a global signal to all of their targets and their potential for cerebellar cortical functions is discussed.

Disynaptic cerebrocerebellar pathways originating from multiple functionally distinct cortical areas

The cerebral cortex and cerebellum both play important roles in sensorimotor processing, however, precise connections between these major brain structures remain elusive. Using anterograde mono-trans-synaptic tracing, we elucidate cerebrocerebellar pathways originating from primary motor, sensory, and association cortex. We confirm a highly organized topography of corticopontine projections in mice; however, we found no corticopontine projections originating from primary auditory cortex and detail several potential extra-pontine cerebrocerebellar pathways. The cerebellar hemispheres were the major target of resulting disynaptic mossy fiber terminals, but we also found at least sparse cerebrocerebellar projections to every lobule of the cerebellum. Notably, projections originating from association cortex resulted in less laterality than primary sensory/motor cortices. Within molecularly defined cerebellar modules we found spatial overlap of mossy fiber terminals, originating from functionally distinct cortical areas, within crus I, paraflocculus, and vermal regions IV/V and VI - highlighting these regions as potential hubs for multimodal cortical influence.

Striped Distribution Pattern of Purkinje Cells of Different Birthdates in the Mouse Cerebellar Cortex Studied with the Neurog2-CreER Transgenic Line

Heterogeneity of Purkinje cells (PCs) that are arranged into discrete longitudinally-striped compartments in the cerebellar cortex is related to the timing of PC generation. To understand the cerebellar compartmental organization, we mapped the PC birthdate (or differentiation timing) in the entire cerebellar cortex. We used the birthdate-tagging system of Neurog2-CreER (G2A) mice hybridized with the AldocV strain which visualizes the zebrin (aldolase C) longitudinal striped pattern. The birthdate-specific distribution pattern of PCs was arranged into longitudinally-oriented stripes consistently throughout almost all lobules except for the nodulus, paraflocculus, and flocculus, in which distinct stripes were observed. Boundaries of the birthdate stripes coincided with the boundary of zebrin stripes or located in the middle of a zebrin stripe. Each birthdate stripe contained PCs born in a particular period between embryonic day (E) 10.0 and E 13.5. In the vermis, PCs were chronologically distributed from lateral to medial stripes. In the paravermis, PCs of early birthdates were distributed in the long lateral zebrin-positive stripe (stripe 4+//5+) and the medially neighboring narrow zebrin-negative substripe (3d-//e2-), while PCs of late birthdates were distributed in the rest of all paravermal areas. In the hemisphere, PCs of early and late birthdates were intermingled in the majority of areas. The results indicate that the birthdate of a PC is a partial determinant for the zebrin compartment in which it is located. However, the correlation between the PC birthdate and the zebrin compartmentalization is complex and distinct among the vermis, paravermis, hemisphere, nodulus, and flocculus.

Multizonal Cerebellar Influence Over Sensorimotor Areas of the Rat Cerebral Cortex

The cerebral cortex requires cerebellar input for optimizing sensorimotor processing. However, how the sensorimotor cortex uses cerebellar information is far from understood. One critical and unanswered question is how cerebellar functional entities (zones or modules) are connected to distinct parts of the sensorimotor cortices. Here, we utilized retrograde transneuronal infection of rabies virus (RABV) to study the organization of connections from the cerebellar cortex to M1, M2, and S1 of the rat cerebral cortex. RABV was co-injected with cholera toxin β-subunit (CTb) into each of these cortical regions and a survival time of 66-70 h allowed for third-order retrograde RABV infection of Purkinje cells. CTb served to identify the injection site. RABV+ Purkinje cells throughout cerebellar zones were identified by reference to the cerebellar zebrin pattern. All injections, including those into S1, resulted in multiple, zonally arranged, strips of RABV+ Purkinje cells. M1 injections were characterized by input from Purkinje cells in the vermal X-zone, medial paravermis (C1- and Cx-zones), and lateral hemisphere (D2-zone); M2 receives input from D2- and C3-zones; connections to S1 originate from X-, Cx-, C3-, and D2-zones. We hypothesize that individual domains of the sensorimotor cortex require information from a specific combination of cerebellar modules.

Cerebellar Development and Circuit Maturation: A Common Framework for Spinocerebellar Ataxias

Spinocerebellar ataxias (SCAs) affect the cerebellum and its afferent and efferent systems that degenerate during disease progression. In the cerebellum, Purkinje cells (PCs) are the most vulnerable and their prominent loss in the late phase of the pathology is the main characteristic of these neurodegenerative diseases. Despite the constant advancement in the discovery of affected molecules and cellular pathways, a comprehensive description of the events leading to the development of motor impairment and degeneration is still lacking. However, in the last years the possible causal role for altered cerebellar development and neuronal circuit wiring in SCAs has been emerging. Not only wiring and synaptic transmission deficits are a common trait of SCAs, but also preventing the expression of the mutant protein during cerebellar development seems to exert a protective role. By discussing this tight relationship between cerebellar development and SCAs, in this review, we aim to highlight the importance of cerebellar circuitry for the investigation of SCAs.

Zebrin Expression in the Cerebellum of Two Crocodilian Species

While in birds and mammals the cerebellum is a highly convoluted structure that consists of numerous transverse lobules, in most amphibians and reptiles it consists of only a single unfolded sheet. Orthogonal to the lobules, the cerebellum is comprised of sagittal zones that are revealed in the pattern of afferent inputs, the projection patterns of Purkinje cells, and Purkinje cell response properties, among other features. The expression of several molecular markers, such as aldolase C, is also parasagittally organized. Aldolase C, also known as zebrin II (ZII), is a glycolytic enzyme expressed in the cerebellar Purkinje cells of the vertebrate cerebellum. In birds, mammals, and some lizards (Ctenophoresspp.), ZII is expressed in a heterogenous fashion of alternating sagittal bands of high (ZII+) and low (ZII-) expression Purkinje cells. In contrast, turtles and snakes express ZII homogenously (ZII+) in their cerebella, but the pattern in crocodilians is unknown. Here, we examined the expression of ZII in two crocodilian species (Crocodylus niloticus and Alligator mississippiensis) to help determine the evolutionary origin of striped ZII expression in vertebrates. We expected crocodilians to express ZII in a striped (ZII+/ZII-) manner because of their close phylogenetic relationship to birds and their larger and more folded cerebellum compared to that of snakes and turtles. Contrary to our prediction, all Purkinje cells in the crocodilian cerebellum had a generally homogenous expression of ZII (ZII+) rather than clear ZII+/- stripes. Our results suggest that either ZII stripes were lost in three groups (snakes, turtles, and crocodilians) or ZII stripes evolved independently three times (lizards, birds, and mammals).

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.

Cellular Mechanisms Involved in Cerebellar Microzonation

In the last 50 years, our vision of the cerebellum has vastly evolved starting with Voogd's (1967) description of extracerebellar projections' terminations and how the projection maps transformed the presumptive homogeneity of the cerebellar cortex into a more complex center subdivided into transverse and longitudinal distinct functional zones. The picture became still more complex with Richard Hawkes and colleagues' (Gravel et al., 1987) discovery of the biochemical heterogeneity of Purkinje cells (PCs), by screening their molecular identities with monoclonal antibodies. Antigens were expressed in a parasagittal pattern with subsets of PCs either possessing or lacking the respective antigens, which divided the cerebellar cortex into precise longitudinal compartments that are congruent with the projection maps. The correlation of these two maps in adult cerebellum shows a perfect matching of developmental mechanisms. This review discusses a series of arguments in favor of the essential role played by PCs in organizing the microzonation of the cerebellum during development (the "matching" hypothesis).

Dense projection of Stilling's nucleus spinocerebellar axons that convey tail proprioception to the midline area in lobule VIII of the mouse cerebellum

The cerebellar cortex has dual somatotopic representation, broadly in the anterior lobules and narrowly in the posterior lobules. However, the somatotopy has not been well understood in vermal lobule VIII, located in the center of the posterior representation. Here, we examined the axonal projections and somatosensory representation of the midline area of vermal lobule VIII in mice, using the striped zebrin expression pattern as a landmark of intra-lobular compartmentalization. Retrograde tracer injection into this area (zebrin stripes 1+ and 1- in lobule VIII) labeled neuronal clusters, bilaterally, in the pericanal gray matter (Stilling's nucleus) in the sacral spinal cord. Spinocerebellar axons labeled by biotinylated dextran amine injection into the sacral pericanal gray matter terminated bilaterally in stripes 1+ and 1- in lobule VIII, with more than 70 terminals per axon, and the vermal stripes in lobules II-III. Dorsal flexion of the tail and electrical stimulation of the sacral spinal gray matter elicited the firing of mossy fiber terminals in stripes 1+ and 1- in lobule VIII. Anterograde labeling of Purkinje cell axons in this area showed terminals in the medial pole of the medial cerebellar nucleus. Lesioning of this area impaired locomotor performance in the rotarod test. These results demonstrated that stripes 1+ and 1- in lobule VIII receive tail proprioceptive sensation from the Stilling's nucleus as their predominant mossy fiber input. The results also suggest that locomotion-related activity is represented not only in the anterior lobule, but also in lobule VIII in the cerebellar vermis.

Identification of Novel Pathways Associated with Patterned Cerebellar Purkinje Neuron Degeneration in Niemann-Pick Disease, Type C1

Niemann-Pick disease, type C1 (NPC1) is a lysosomal disease characterized by progressive cerebellar ataxia. In NPC1, a defect in cholesterol transport leads to endolysosomal storage of cholesterol and decreased cholesterol bioavailability. Purkinje neurons are sensitive to the loss of NPC1 function. However, degeneration of Purkinje neurons is not uniform. They are typically lost in an anterior-to-posterior gradient with neurons in lobule X being resistant to neurodegeneration. To gain mechanistic insight into factors that protect or potentiate Purkinje neuron loss, we compared RNA expression in cerebellar lobules III, VI, and X from control and mutant mice. An unexpected finding was that the gene expression differences between lobules III/VI and X were more pronounced than those observed between mutant and control mice. Functional analysis of genes with anterior to posterior gene expression differences revealed an enrichment of genes related to neuronal cell survival within the posterior cerebellum. This finding is consistent with the observation, in multiple diseases, that posterior Purkinje neurons are, in general, resistant to neurodegeneration. To our knowledge, this is the first study to evaluate anterior to posterior transcriptome-wide changes in gene expression in the cerebellum. Our data can be used to not only explore potential pathological mechanisms in NPC1, but also to further understand cerebellar biology.

Modular organization of cerebellar climbing fiber inputs during goal-directed behavior

The cerebellum has a parasagittal modular architecture characterized by precisely organized climbing fiber (CF) projections that are congruent with alternating aldolase C/zebrin II expression. However, the behavioral relevance of CF inputs into individual modules remains poorly understood. Here, we used two-photon calcium imaging in the cerebellar hemisphere Crus II in mice performing an auditory go/no-go task to investigate the functional differences in CF inputs to modules. CF signals in medial modules show anticipatory decreases, early increases, secondary increases, and reward-related increases or decreases, which represent quick motor initiation, go cues, fast motor behavior, and positive reward outcomes. CF signals in lateral modules show early increases and reward-related decreases, which represent no-go and/or go cues and positive reward outcomes. The boundaries of CF functions broadly correspond to those of aldolase C patterning. These results indicate that spatially segregated CF inputs in different modules play distinct roles in the execution of goal-directed behavior.