Electron microscopic analysis of postnatal histogenesis in the cerebellar cortex of staggerer mutant mice

RAR-related orphan receptor alpha and the staggerer mice: a fine molecular story

The retinoic acid-related orphan receptor alpha (RORα) protein first came into the limelight due to a set of staggerer mice, discovered at the Jackson Laboratories in the United States of America by Sidman, Lane, and Dickie (1962) and genetically deciphered by Hamilton et al. in 1996. These staggerer mice exhibited cerebellar defects, an ataxic gait, a stagger along with several other developmental abnormalities, compensatory mechanisms, and, most importantly, a deletion of 160 kilobases (kb), encompassing the RORα ligand binding domain (LBD). The discovery of the staggerer mice and the subsequent discovery of a loss of the LBD within the RORα gene of these mice at the genetic level clearly indicated that RORα's LBD played a crucial role in patterning during embryogenesis. Moreover, a chance study by Roffler-Tarlov and Sidman (1978) noted reduced concentrations of glutamic acid levels in the staggerer mice, indicating a possible role for the essence of a nutritionally balanced diet. The sequential organisation of the building blocks of intact genes, requires the nucleotide bases of deoxyribonucleic acid (DNA): purines and pyrimidines, both of which are synthesized, upon a constant supply of glutamine, an amino acid fortified in a balanced diet and a byproduct of the carbohydrate and lipid metabolic pathways. A nutritionally balanced diet, along with a metabolic "enzymatic machinery" devoid of mutations/aberrations, was essential in the uninterrupted transcription of RORα during embryogenesis. In addition to the above, following translation, a ligand-responsive RORα acts as a "molecular circadian regulator" during embryogenesis and not only is expressed selectively and differentially, but also promotes differential activity depending on the anatomical and pathological site of its expression. RORα is highly expressed in the central nervous system (CNS) and the endocrine organs. Additionally, RORα and the clock genes are core components of the circadian rhythmicity, with the expression of RORα fluctuating in a night-day-night sigmoidal pattern and undoubtedly serves as an endocrine-like, albeit "molecular-circadian regulator". Melatonin, a circadian hormone, along with tri-iodothyronine and some steroid hormones are known to regulate RORα-mediated molecular activity, with each of these hormones themselves being regulated rhythmically by the hypothalamic-pituitary axis (HPA). The HPA regulates the circadian rhythm and cyclical release of hormones, in a self-regulatory feedback loop. Irregular sleep-wake patterns affect circadian rhythmicity and the ability of the immune system to withstand infections. The staggerer mice with their thinner bones, an altered skeletal musculature, an aberrant metabolic profile, the ataxic gait and an underdeveloped cerebellar cortex; exhibited compensatory mechanisms, that not only allowed the survival of the staggerer mice, but also enhanced protection from microbial invasions and resistance to high-fat-diet induced obesity. This review has been compiled in its present form, more than 14 years later after a chromatin immunoprecipitation (ChIP) cloning and sequencing methodology helped me identify signal transducer and activator of transcription 5 (STAT5) target sequences, one of which was mapped to the first intron of the RORα gene. The 599-base-long sequence containing one consensus TTCNNNGAA (TTCN3GAA) gamma-activated sequence (GAS) and five other non-consensus TTN5AA sequences had been identified from the clones isolated from the STAT5 target sites (fragments) in human phytohemagglutinin-activated CD8+ T lymphocytes, during my doctoral studies between 2006 and 2009. Most importantly, preliminary studies noted a unique RORα expression profile, during a time-course study on the ribonucleic acid (RNA), extracted from human phytohemagglutinin (PHA) activated CD8+ T lymphocytes stimulated with interleukin-2 (IL-2). This review mainly focuses on the "staggerer mice" with one of its first roles materialising during embryogenesis, a molecular-endocrine mediated circadian-like regulatory process.

N-WASP-Arp2/3 signaling controls multiple steps of dendrite maturation in Purkinje cells in vivo

During neural development, the actin filament network must be precisely regulated to form elaborate neurite structures. N-WASP tightly controls actin polymerization dynamics by activating an actin nucleator Arp2/3. However, the importance of N-WASP-Arp2/3 signaling in the assembly of neurite architecture in vivo has not been clarified. Here, we demonstrate that N-WASP-Arp2/3 signaling plays a crucial role in the maturation of cerebellar Purkinje cell (PC) dendrites in vivo in mice. N-WASP was expressed and activated in developing PCs. Inhibition of Arp2/3 and N-WASP from the beginning of dendrite formation severely disrupted the establishment of a single stem dendrite, which is a characteristic basic structure of PC dendrites. Inhibition of Arp2/3 after stem dendrite formation resulted in hypoplasia of the PC dendritic tree. Cdc42, an upstream activator of N-WASP, is required for N-WASP-Arp2/3 signaling-mediated PC dendrite maturation. In addition, overactivation of N-WASP is also detrimental to dendrite formation in PCs. These findings reveal that proper activation of N-WASP-Arp2/3 signaling is crucial for multiple steps of PC dendrite maturation in vivo.

Scrap and Build for Functional Neural Circuits: Spatiotemporal Regulation of Dendrite Degeneration and Regeneration in Neural Development and Disease

Dendrites are cellular structures essential for the integration of neuronal information. These elegant but complex structures are highly patterned across the nervous system but vary tremendously in their size and fine architecture, each designed to best serve specific computations within their networks. Recent in vivo imaging studies reveal that the development of mature dendrite arbors in many cases involves extensive remodeling achieved through a precisely orchestrated interplay of growth, degeneration, and regeneration of dendritic branches. Both degeneration and regeneration of dendritic branches involve precise spatiotemporal regulation for the proper wiring of functional networks. In particular, dendrite degeneration must be targeted in a compartmentalized manner to avoid neuronal death. Dysregulation of these developmental processes, in particular dendrite degeneration, is associated with certain types of pathology, injury, and aging. In this article, we review recent progress in our understanding of dendrite degeneration and regeneration, focusing on molecular and cellular mechanisms underlying spatiotemporal control of dendrite remodeling in neural development. We further discuss how developmental dendrite degeneration and regeneration are molecularly and functionally related to dendrite remodeling in pathology, disease, and aging.

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.

Cerebellar Dysfunction, Cerebro-cerebellar Connectivity and Autism Spectrum Disorders

The cerebellum has long been conceptualized to control motor learning and motor coordination. However, increasing evidence suggests its roles in cognition and emotion behaviors. In particular, the cerebellum has been recognized as one of key brain regions affected in autism spectrum disorder (ASD). To better understand the contribution of the cerebellum in ASD pathogenesis, we here discuss recent behavioral, genetic, and molecular studies from the human and mouse models. In addition, we raise several questions that need to be investigated in future studies from the point view of cerebellar dysfunction, cerebro-cerebellar connectivity and ASD.

Loss of RAR-related orphan receptor alpha (RORα) selectively lowers docosahexaenoic acid in developing cerebellum

Deficiency in retinoid acid receptor-related orphan receptor alpha (RORα) of staggerer mice results in extensive granule and Purkinje cell loss in the cerebellum as well as in learned motor deficits, cognition impairments and perseverative tendencies that are commonly observed in autistic spectrum disorder (ASD). The effects of RORα on brain lipid metabolism associated with cerebellar atrophy remain unexplored. The aim of this study is to examine the effects of RORα deficiency on brain phospholipid fatty acid concentrations and compositions. Staggerer mice (Rora^sg/sg) and wildtype littermates (Rora^+/+) were fed n-3 polyunsaturated fatty acids (PUFA) containing diets ad libitum. At 2 months and 7 or more months old, brain total phospholipid fatty acids were quantified by gas chromatography-flame ionization detection. In the cerebellum, all fatty acid concentrations were reduced in 2 months old mice. Since total fatty acid concentrations were significantly different at 2-month-old, we examined changes in fatty acid composition. The composition of ARA was not significantly different between genotypes; though DHA composition remained significantly lowered. Despite cerebellar atrophy at >7-months-old, cerebellar fatty acid concentrations had recovered comparably to wildtype control. Therefore, RORα may be necessary for fatty acid accretions during neurodevelopment. Specifically, the effects of RORα on PUFA metabolisms are region-specific and age-dependent.

Single-Cell RNA-seq Reveals Profound Alterations in Mechanosensitive Dorsal Root Ganglion Neurons with Vitamin E Deficiency

Ninety percent of Americans consume less than the estimated average requirements of dietary vitamin E (vitE). Severe vitE deficiency due to genetic mutations in the tocopherol transfer protein (TTPA) in humans results in ataxia with vitE deficiency (AVED), with proprioceptive deficits and somatosensory degeneration arising from dorsal root ganglia neurons (DRGNs). Single-cell RNA-sequencing of DRGNs was performed in Ttpa-/- mice, an established model of AVED. In stark contrast to expected changes in proprioceptive neurons, Ttpa-/- DRGNs showed marked upregulation of voltage-gated Ca2+ and K+ channels in mechanosensitive, tyrosine-hydroxylase positive (TH+) DRGNs. The ensuing significant conductance changes resulted in reduced excitability in mechanosensitive Ttpa-/- DRGNs. A highly supplemented vitE diet (600 mg dl-α-tocopheryl acetate/kg diet) prevented the cellular and molecular alterations and improved mechanosensation. VitE deficiency profoundly alters the molecular signature and functional properties of mechanosensitive TH+ DRGN, representing an intriguing shift of the prevailing paradigm from proprioception to mechanical sensation.

Caspase-3 Mediated Cell Death in the Normal Development of the Mammalian Cerebellum

Caspase-3, onto which there is a convergence of the intrinsic and extrinsic apoptotic pathways, is the main executioner of apoptosis. We here review the current literature on the intervention of the protease in the execution of naturally occurring neuronal death (NOND) during cerebellar development. We will consider data on the most common altricial species (rat, mouse and rabbit), as well as humans. Among the different types of neurons and glia in cerebellum, there is ample evidence for an intervention of caspase-3 in the regulation of NOND of the post-mitotic cerebellar granule cells (CGCs) and Purkinje neurons, as a consequence of failure to establish proper synaptic contacts with target (secondary cell death). It seems possible that the GABAergic interneurons also undergo a similar type of secondary cell death, but the intervention of caspase-3 in this case still remains to be clarified in full. Remarkably, CGCs also undergo primary cell death at the precursor/pre-migratory stage of differentiation, in this instance without the intervention of caspase-3. Glial cells, as well, undergo a process of regulated cell death, but it seems possible that expression of caspase-3, at least in the Bergmann glia, is related to differentiation rather than death.

Hydroxyurea Exposure and Development of the Cerebellar External Granular Layer: Effects on Granule Cell Precursors, Bergmann Glial and Microglial Cells

The current paper presents a histological analysis of the cell death in the cerebellar external granular layer (EGL) following the treatment with a single dose (2 mg/g) of hydroxyurea (HU). The rats were examined at postnatal days (P) 5, 10, and 15, and sacrificed at appropriate times ranging from 6 to 48 h after treatment administration. Studies were done in each cortical lobe (anterior, central, posterior, and inferior). The quantification of several parameters, such as density of 5-bromo-2'-deoxyuridine, TUNEL, vimentin, and tomato lectin-stained cells, revealed that HU compromises the viability of EGL cells. Our results indicate that P10 is a time of high vulnerability to injury. We also show here that the anterior and central lobes are the cortical regions most susceptible to the action of the HU. Additionally, our data also indicate that from 6 to 24 h after HU-exposure is a time-window of high sensibility to this agent. On the other hand, our ultrastructural analysis confirmed that HU administration produces the activation of apoptotic cellular events in the EGL, resulting in a substantial number of dying cells. Different stages of apoptosis can be observed in all cortical lobes at all investigated postnatal ages and survival times. Moreover, we observed that dying neuroblasts were covered by laminar processes of Bergmann glia, and that these unipolar astrocytes presented cytological features of phagocytes engulfing apoptotic bodies and cell debris. The electron microscopy study also revealed the participation of ameboid microglial cells in the phagocytosis of apoptotic cells in the regions of the EGL with extensive cell death.

An innate immune response and altered nuclear receptor activation defines the spinal cord transcriptome during alpha-tocopherol deficiency in Ttpa-null mice

Mice with deficiency in tocopherol (alpha) transfer protein gene develop peripheral tocopherol deficiency and sensory neurodegeneration. Ttpa^-/- mice maintained on diets with deficient α-tocopherol (α-TOH) had proprioceptive deficits by six months of age, axonal degeneration and neuronal chromatolysis within the dorsal column of the spinal cord and its projections into the medulla. Transmission electron microscopy revealed degeneration of dorsal column axons. We addressed the potential pathomechanism of α-TOH deficient neurodegeneration by global transcriptome sequencing within the spinal cord and cerebellum. RNA-sequencing of the spinal cord in Ttpa^-/- mice revealed upregulation of genes associated with the innate immune response, indicating a molecular signature of microglial activation as a result of tocopherol deficiency. For the first time, low level Ttpa expression was identified in the murine spinal cord. Further, the transcription factor liver X receptor (LXR) was strongly activated by α-TOH deficiency, triggering dysregulation of cholesterol biosynthesis. The aberrant activation of transcription factor LXR suppressed the normal induction of the transcription factor retinoic-related orphan receptor-α (RORA), which is required for neural homeostasis. Thus we find that α-TOH deficiency induces LXR, which may lead to a molecular signature of microglial activation and contribute to sensory neurodegeneration.

Deletion of the GluRδ2 Receptor in the Hotfoot Mouse Mutant Causes Granule Cell Loss, Delayed Purkinje Cell Death, and Reductions in Purkinje Cell Dendritic Tree Area

Recent studies have found that in the cerebellum, the δ2 glutamate receptor (GluRδ2) plays a key role in regulating the differentiation of parallel fiber-Purkinje synapses and mediating key physiological functions in the granule cell-Purkinje cell circuit. In the hotfoot mutant or GluRδ2 knockout mice, the absence of GluRδ2 expression results in impaired motor-related tasks, ataxia, and disruption of long-term depression at parallel fiber-Purkinje cell synapses. The goal of this study was to determine the long-term consequences of deletion of GluRδ2 expression in the hotfoot mutant (GluRδ2 ^ho/ho ) on Purkinje and granule cell survival and Purkinje cell dendritic differentiation. Quantitative estimates of Purkinje and granule cell numbers in 3-, 12-, and 20-month-old hotfoot mutants and wild-type controls showed that Purkinje cell numbers are within control values at 3 and 12 months in the hotfoot mutant but reduced by 20 % at 20 months compared with controls. In contrast, the number of granule cells is significantly reduced from 3 months onwards in GluRδ2 ^ho/ho mutant mice compared to wild-type controls. Although the overall structure of Purkinje cell dendrites does not appear to be altered, there is a significant 27 % reduction in the cross-sectional area of Purkinje cell dendritic trees in the 20-month-old GluRδ2 ^ho/ho mutants. The interpretation of the results is that the GluRδ2 receptor plays an important role in the long-term organization of the granule-Purkinje cell circuit through its involvement in the regulation of parallel fiber-Purkinje cell synaptogenesis and in the normal functioning of this critical cerebellar circuit.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

Cell death and neurodegeneration in the postnatal development of cerebellar vermis in normal and Reeler mice

Programmed cell death (PCD) was demonstrated in neurons and glia in normal brain development, plasticity, and aging, but also in neurodegeneration. (Macro)autophagy, characterized by cytoplasmic vacuolization and activation of lysosomal hydrolases, and apoptosis, typically entailing cell shrinkage, chromatin and nuclear condensation, are the two more common forms of PCD. Their underlying intracellular pathways are partly shared and neurons can die following both modalities, according to the type of death-triggering stimulus. Reelin is an extracellular protein necessary for proper neuronal migration and brain lamination. In the mutant Reeler mouse, its absence causes neuronal mispositioning, with a notable degree of cerebellar hypoplasia that was tentatively related to an increase in PCD. We have carried out an ultrastructural analysis on the occurrence and type of postnatal PCD affecting the cerebellar neurons in normal and Reeler mice. In the forming cerebellar cortex, PCD took the form of apoptosis or autophagy and mainly affected the cerebellar granule cells (CGCs). Densities of apoptotic CGCs were comparable in both mouse strains at P0-P10, while, in mutants, they increased to become significantly higher at P15. In WT mice the density of autophagic neurons did not display statistically significant differences in the time interval examined in this study, whereas it was reduced in Reeler in the P0-P10 interval, but increased at P15. Besides CGCs, the Purkinje neurons also displayed autophagic features in both WT and Reeler mice. Therefore, cerebellar neurons undergo different types of PCD and a Reelin deficiency affects the type and degree of neuronal death during postnatal development of the cerebellum.

Jan Evangelista Purkyně and the cerebellum then and now

The name of Jan Evangelista Purkyně and the cerebellum belong inseparably together. He was the first who saw and described the largest nerve cells in the brain, de facto in the cerebellum. The most distinguished researchers of the nervous system then showed him the highest recognition by naming these neurons as Purkinje cells. Through experiments by J. E. Purkyně and his followers properly functionally was attributed to the cerebellum share in precision of motor skills. Despite ongoing and fruitful research, after a relatively long time, especially in the last two decades, scientists had to constantly replenish and re-evaluate the traditional conception of the cerebellum and formulate a new one. It started in the early 1990s, when it was found that cerebellar cortex contains more neurons than the cerebral cortex. Shortly thereafter it was gradually revealed that such enormous numbers of neural cells are not without an impact on brain functions and that the cerebellum, except its traditional role in the motor skills, also participates in higher nervous activity. These new findings were obtained thanks to the introduction of modern methods of examination into the clinical praxis, and experimental procedures using animal models of cerebellar disorders described below.

RORα Regulates Multiple Aspects of Dendrite Development in Cerebellar Purkinje Cells In Vivo

The establishment of cell-type-specific dendritic arbors is fundamental for proper neural circuit formation. Here, using temporal- and cell-specific knock-down, knock-out, and overexpression approaches, we show that multiple aspects of the dendritic organization of cerebellar Purkinje cells (PCs) are controlled by a single transcriptional factor, retinoic acid-related orphan receptor-alpha (RORα), a gene defective in staggerer mutant mice. As reported earlier, RORα was required for regression of primitive dendrites before postnatal day 4 (P4). RORα was also necessary for PCs to form a single Purkinje layer from P0 to P4. The knock-down of RORα from P4 impaired the elimination of perisomatic dendrites and maturation of single stem dendrites in PCs at P8. Filopodia and spines were also absent in these PCs. The knock-down of RORα from P8 impaired the formation and maintenance of terminal dendritic branches of PCs at P14. Finally, even after dendrite formation was completed at P21, RORα was required for PCs to maintain dendritic complexity and functional synapses, but their mature innervation pattern by single climbing fibers was unaffected. Interestingly, overexpression of RORα in PCs at various developmental stages did not facilitate dendrite development, but had specific detrimental effects on PCs. Because RORα deficiency during development is closely related to the severity of spinocerebellar ataxia type 1, delineating the specific roles of RORα in PCs in vivo at different time windows during development and throughout adulthood would facilitate our understanding of the pathogenesis of cerebellar disorders. Significance statement: The genetic programs by which each neuron subtype develops and maintains dendritic arbors have remained largely unclear. This is partly because dendrite development is modulated dynamically by neuronal activities and interactions with local environmental cues in vivo. In addition, dendrites are formed and maintained by the balance between their growth and regression; the effects caused by the disruption of transcription factors during the early developmental stages could be masked by dendritic growth or regression in the later stages. Here, using temporal- and cell-specific knock-down, knock-out, and overexpression approaches in vivo, we show that multiple aspects of the dendritic organization of cerebellar Purkinje cells are controlled by a single transcriptional factor, retinoic acid-related orphan receptor alpha.

Morphometric analysis of progressive changes in hereditary cerebellar cortical degenerative disease (abiotrophy) in rabbits caused by abnormal synaptogenesis

We previously investigated rabbit hereditary cerebellar cortical degenerative disease, called cerebellar cortical abiotrophy in the veterinary field, and determined that the pathogenesis of this disease is the result of failed synaptogenesis between parallel fibers and Purkinje cells. In this study, longitudinal changes in the development and atrophy of the cerebellum of rabbits with hereditary abiotrophy after birth were morphometrically examined (postnatal day [PD] 15 and 42) using image analysis. Although development of the cerebellum in rabbits with abiotrophy was observed from PD 15 to PD 42, the growth rate of the cerebellum was less than that in normal rabbits. In rabbits with abiotrophy, the number of granular cells undergoing apoptosis was significantly higher at PD 15 and dramatically decreased at PD 42. The number of granular cells did not increase from PD 15 to 42. The synaptogenesis peak at PD 15 occurred when the largest number of apoptotic granular cells in rabbits with abiotrophy was observed. Although 26% to 36% of parallel fiber terminals formed synaptic junctions with Purkinje cell spines, the remainder did not at PD 15 and 42. The rate of failure of synaptogenesis in the present study might be specific to this case of abiotrophy. Morphometric analysis revealed detailed changes in development and atrophy in animals with postnatal cerebellar disease occurring soon after birth.

The Spontaneous Ataxic Mouse Mutant Tippy is Characterized by a Novel Purkinje Cell Morphogenesis and Degeneration Phenotype

This study represents the first detailed analysis of the spontaneous neurological mouse mutant, tippy, uncovering its unique cerebellar phenotype. Homozygous tippy mutant mice are small, ataxic, and die around weaning. Although the cerebellum shows grossly normal foliation, tippy mutants display a complex cerebellar Purkinje cell phenotype consisting of abnormal dendritic branching with immature spine features and patchy, non-apoptotic cell death that is associated with widespread dystrophy and degeneration of the Purkinje cell axons throughout the white matter, the cerebellar nuclei, and the vestibular nuclei. Moderate anatomical abnormalities of climbing fiber innervation of tippy mutant Purkinje cells were not associated with changes in climbing fiber-EPSC amplitudes. However, decreased ESPC amplitudes were observed in response to parallel fiber stimulation and correlated well with anatomical evidence for patchy dark cell degeneration of Purkinje cell dendrites in the molecular layer. The data suggest that the Purkinje neurons are a primary target of the tippy mutation. Furthermore, we hypothesize that the Purkinje cell axonal pathology together with disruptions in the balance of climbing fiber and parallel fiber-Purkinje cell input in the cerebellar cortex underlie the ataxic phenotype in these mice. The constellation of Purkinje cell dendritic malformation and degeneration phenotypes in tippy mutants is unique and has not been reported in any other neurologic mutant. Fine mapping of the tippy mutation to a 2.1 MB region of distal chromosome 9, which does not encompass any gene previously implicated in cerebellar development or neuronal degeneration, confirms that the tippy mutation identifies novel biology and gene function.

Behavioral effects of neonatal lesions on the cerebellar system

Several rodent models with spontaneous mutations causing cerebellar pathology are impaired in motor functions during the neonatal period, including Grid2(Lc), Rora(sg), Dab1(scm), Girk2(Wv), Lmx1a(dr-sst), Myo5a(dn), Inpp4a(wbl), and Cacna1a(rol) mice as well as shaker and dystonic rats. Deficits are also evident in murine null mutants such as Zic1, Fgfr1/FgFr2, and Xpa/Ercc8. Behavioral deficits are time-dependent following X-irradiated- or aspiration-induced lesions of the cerebellum in rats. In addition, motor functions are deficient after lesions in cerebellar-related pathways. As in animal subjects, sensorimotor disturbances have been described in children with cerebellar lesions. These results underline the importance of the cerebellum and its connections in the development of motor functions.

From mice to men: lessons from mutant ataxic mice

Ataxic mutant mice can be used to represent models of cerebellar degenerative disorders. They serve for investigation of cerebellar function, pathogenesis of degenerative processes as well as of therapeutic approaches. Lurcher, Hot-foot, Purkinje cell degeneration, Nervous, Staggerer, Weaver, Reeler, and Scrambler mouse models and mouse models of SCA1, SCA2, SCA3, SCA6, SCA7, SCA23, DRPLA, Niemann-Pick disease and Friedreich ataxia are reviewed with special regard to cerebellar pathology, pathogenesis, functional changes and possible therapeutic influences, if any. Finally, benefits and limitations of mouse models are discussed.

Pre-clinical models of neurodevelopmental disorders: focus on the cerebellum

Recent studies have advanced our understanding of the role of the cerebellum in non-motor behaviors. Abnormalities in the cerebellar structure have been demonstrated to produce changes in emotional, cognitive, and social behaviors resembling clinical manifestations observed in patients with autism spectrum disorders (ASD) and schizophrenia. Several animal models have been used to evaluate the effects of relevant environmental and genetic risk factors on the cerebellum development and function. However, very few models of ASD and schizophrenia selectively target the cerebellum and/or specific cell types within this structure. In this review, we critically evaluate the strength and weaknesses of these models. We will propose that the future progress in this field will require time- and cell type-specific manipulations of disease-relevant genes, not only selectively in the cerebellum, but also in frontal brain areas connected with the cerebellum. Such information can advance our knowledge of the cerebellar contribution to non-motor behaviors in mental health and disease.

Mature Purkinje cells require the retinoic acid-related orphan receptor-α (RORα) to maintain climbing fiber mono-innervation and other adult characteristics

Neuronal maturation during development is a multistep process regulated by transcription factors. The transcription factor RORα (retinoic acid-related orphan receptor α) is necessary for early Purkinje cell (PC) maturation but is also expressed throughout adulthood. To identify the role of RORα in mature PCs, we used Cre-lox mouse genetic tools in vivo that delete it specifically from PCs between postnatal days 10-21. Up to 14 d of age, differences between mutant and control PCs were not detectable: both were mono-innervated by climbing fibers (CFs) extending along their well-developed dendrites with spiny branchlets. By week 4, mutant mice were ataxic, some PCs had died, and remaining PC soma and dendrites were atrophic, with almost complete disappearance of spiny branchlets. The innervation pattern of surviving RORα-deleted PCs was abnormal with several immature characteristics. Notably, multiple functional CF innervation was reestablished on these mature PCs, simultaneously with the relocation of CF contacts to the PC soma and their stem dendrite. This morphological modification of CF contacts could be induced even later, using lentivirus-mediated depletion of rora from adult PCs. These data show that the late postnatal expression of RORα cell-autonomously regulates the maintenance of PC dendritic complexity, and the CF innervation status of the PC (dendritic vs somatic contacts, and mono-innervation vs multi-innervation). Thus, the differentiation state of adult neurons is under the control of transcription factors; and in their absence, adult neurons lose their mature characteristics and acquire some characteristics of an earlier developmental stage.

Synapse elimination in the developing cerebellum

Neural circuits in neonatal animals contain numerous redundant synapses that are functionally immature. During the postnatal period, unnecessary synapses are eliminated while functionally important synapses become stronger and mature. The climbing fiber (CF) to the Purkinje cell (PC) synapse is a representative model for the analysis of postnatal refinement of neuronal circuits in the central nervous system. PCs are initially innervated by multiple CFs with similar strengths around postnatal day 3 (P3). Only a single CF is selectively strengthened during P3–P7 (functional differentiation), and the strengthened CF undergoes translocation from soma to dendrites of PCs from P9 on (dendritic translocation). Following the functional differentiation, supernumerary CF synapses on the soma are eliminated, which proceeds in two distinct phases: the early phase from P7 to around P11 and the late phase from around P12 to P17. Here, we review our current understanding of cellular and molecular mechanisms of CF synapse elimination in the developing cerebellum.

Is autism a disease of the cerebellum? An integration of clinical and pre-clinical research

Autism spectrum disorders are a group of neurodevelopmental disorders characterized by deficits in social skills and communication, stereotyped and repetitive behavior, and a range of deficits in cognitive function. While the etiology of autism is unknown, current research indicates that abnormalities of the cerebellum, now believed to be involved in cognitive function and the prefrontal cortex (PFC), are associated with autism. The current paper proposes that impaired cerebello-cortical circuitry could, at least in part, underlie autistic symptoms. The use of animal models that allow for manipulation of genetic and environmental influences are an effective means of elucidating both distal and proximal etiological factors in autism and their potential impact on cerebello-cortical circuitry. Some existing rodent models of autism, as well as some models not previously applied to the study of the disorder, display cerebellar and behavioral abnormalities that parallel those commonly seen in autistic patients. The novel findings produced from research utilizing rodent models could provide a better understanding of the neurochemical and behavioral impact of changes in cerebello-cortical circuitry in autism.

Early morphological changes of hereditary cerebellar cortical abiotrophy in rabbits

We previously investigated the hereditary cerebellar cortical abiotrophy in littermates at postnatal day (PD) 25-31 delivered from a pair of rabbits. To estimate the onset time and incipient lesions associated with the cerebellar cortical abiotrophy of the cases, we mated the same pair again and examined early stages of the disease in F1 rabbit showing ataxia (PD 15), finding evidence that the ataxia is passed to subsequent generations via autosomal recessive inheritance. Clinical signs of the affected rabbit showed early-onset dysstasia and ataxia. The affected rabbit showed apoptotic granular cells before and after migration completion, degeneration (swelling) of parallel fiber terminals, abnormal junction (invaginated junction) of the parallel fiber-Purkinje cell synapses and irregular orientation of the Purkinje dendritic arbor in the molecular layer. Additionally, a reduced number of synaptic junctions between parallel fibers and Purkinje cells were detected, as well as at PD 25-31. Secondary changes, such as reduction or degeneration of Purkinje cells and granular cells were not yet observed at early stages. As synapse abnormality preceded the degeneration or reduction of Purkinje and granular cells at early stages, we concluded that the pathogenesis of the present cerebellar lesion was caused by failed synaptogenesis during postnatal cerebellar development.

Requirement for zebrafish ataxin-7 in differentiation of photoreceptors and cerebellar neurons

The expansion of a polyglutamine (polyQ) tract in the N-terminal region of ataxin-7 (atxn7) is the causative event in spinocerebellar ataxia type 7 (SCA7), an autosomal dominant neurodegenerative disorder mainly characterized by progressive, selective loss of rod-cone photoreceptors and cerebellar Purkinje and granule cells. The molecular and cellular processes underlying this restricted neuronal vulnerability, which contrasts with the broad expression pattern of atxn7, remains one of the most enigmatic features of SCA7, and more generally of all polyQ disorders. To gain insight into this specific neuronal vulnerability and achieve a better understanding of atxn7 function, we carried out a functional analysis of this protein in the teleost fish Danio rerio. We characterized the zebrafish atxn7 gene and its transcription pattern, and by making use of morpholino-oligonucleotide-mediated gene inactivation, we analysed the phenotypes induced following mild or severe zebrafish atxn7 depletion. Severe or nearly complete zebrafish atxn7 loss-of-function markedly impaired embryonic development, leading to both early embryonic lethality and severely deformed embryos. More importantly, in relation to SCA7, moderate depletion of the protein specifically, albeit partially, prevented the differentiation of both retina photoreceptors and cerebellar Purkinje and granule cells. In addition, [1–232] human atxn7 fragment rescued these phenotypes showing strong function conservation of this protein through evolution. The specific requirement for zebrafish atxn7 in the proper differentiation of cerebellar neurons provides, to our knowledge, the first in vivo evidence of a direct functional relationship between atxn7 and the differentiation of Purkinje and granule cells, the most crucial neurons affected in SCA7 and most other polyQ-mediated SCAs. These findings further suggest that altered protein function may play a role in the pathophysiology of the disease, an important step toward the development of future therapeutic strategies.

Cerebellum: links between development, developmental disorders and motor learning

The study of the links and interactions between development and motor learning has noticeable implications for the understanding and management of neurodevelopmental disorders. This is particularly relevant for the cerebellum which is critical for sensorimotor learning. The olivocerebellar pathway is a key pathway contributing to learning of motor skills. Its developmental maturation and remodeling are being unraveled. Advances in genetics have led to major improvements in our appraisal of the genes involved in cerebellar development, especially studies in mutant mice. Cerebellar neurogenesis is compartmentalized in relationship with neurotransmitter fate. The Engrailed-2 gene is a major actor of the specification of cerebellar cell types and late embryogenic morphogenesis. Math1, expressed by the rhombic lip, is required for the genesis of glutamatergic neurons. Mutants deficient for the transcription factor Ptf1a display a lack of Purkinje cells and gabaergic interneurons. Rora gene contributes to the developmental signaling between granule cells and Purkinje neurons. The expression profile of sonic hedgehog in postnatal stages determines the final size/shape of the cerebellum. Genes affecting the development impact upon the physiological properties of the cerebellar circuits. For instance, receptors are developmentally regulated and their action interferes directly with developmental processes. Another field of research which is expanding relates to very preterm neonates. They are at risk for cerebellar lesions, which may themselves impair the developmental events. Very preterm neonates often show sensori-motor deficits, highlighting another major link between impaired developments and learning deficiencies. Pathways playing a critical role in cerebellar development are likely to become therapeutical targets for several neurodevelopmental disorders.

Hereditary cerebellar degenerative disease (cerebellar cortical abiotrophy) in rabbits

A pair of rabbits gave birth to a set of littermates (F1) with symptoms of early-onset ataxia. Microscopic examination revealed cerebellar degenerative disease in 5 of 6 littermates. Light microscopy was used to compare the thickness of each cerebellar layer in affected animals in contrast to a normal control. Affected animals showed narrowing of the molecular layer of the vermis, reduced density of Purkinje cell dendrites and irregular thickness in their branchlets, and reduced density of granular cells and scattered pyknotic cells in the granular layer. Pyknotic cells were apoptotic granular cells, confirmed by positive staining using the TUNEL method. Electron microscopy confirmed the thinning of the molecular layer seen by light microscopy and also showed a reduced number of parallel fibers, which indicate granular cells axons, and a reduced number of synaptic junctions between Purkinje and granular cells. Purkinje cells had electron-dense, irregularly shaped cytoplasm with irregularly shaped nuclei, and some of these cells had a central chromatolysis-like region. These findings support a diagnosis of cerebellar cortical abiotrophy, a hereditary condition that causes nerve function impairment leading to early-onset progressive degeneration of the cerebellar cortex.

Disruption of metabotropic glutamate receptor signalling is a major defect at cerebellar parallel fibre-Purkinje cell synapses in staggerer mutant mice

Staggerer mutant mice have functional loss of a transcription factor, retinoid-related orphan receptor α (RORα), which is abundantly expressed in Purkinje cells (PCs) of the cerebellum.Homozygous staggerer (sg/sg)mice show cerebellar hypoplasia and congenital ataxia. Sg/sg mice serve as an important extreme mouse model of the hereditary spinocerebellar ataxia type 1 (SCA1), since it has been shown that RORα dysfunction is strongly correlated with SCA1 pathogenesis. However, synaptic abnormalities, especially at parallel fibre (PF)-PC synapses, in SCA1-related sg/sg mice have not been examined in detail electrophysiologically. In this study, we report that PFs can still establish functional synapses onto PCs in sg/sg mice in spite of reduction in the number of PF-PC synapses. Compared with PF-evoked EPSCs in the wild-type or heterozygotes, the success rate of the EPSC recordings in sg/sg was quite low (∼40%) and the EPSCs showed faster kinetics and slightly decreased paired pulse facilitation at short intervals. The prominent synaptic dysfunction is that sg/sg mice lack metabotropic glutamate receptor (mGluR)-mediated slow EPSCs completely. Neither intense PF stimulation nor an exogenously applied mGluR agonist, DHPG, could elicit mGluR-mediated responses.Western blot analysis in the sg/sg cerebellum revealed low-level expression of mGluR1 and TRPC3, both of which underlie mGluR-mediated slow currents in PCs. Immunohistochemical data demonstrated marked mislocalization of mGluR1 on sg/sg PCs.We found that mGluR-mediated retrograde suppression of PF-PC EPSCs by endocannabinoid is also impaired completely in sg/sg mice. These results suggest that disruption of mGluR signalling at PF-PC synapses is one of the major synaptic defects in sg/sg mice and may manifest itself in SCA1 pathology.

Phenotypic and genetic analysis of the cerebellar mutant tmgc26, a new ENU-induced ROR-alpha allele

ROR-alpha is an orphan nuclear receptor, inactivation of which cell-autonomously blocks differentiation of cerebellar Purkinje cells with a secondary loss of granule neurons. As part of our ENU mutagenesis screen we isolated the recessive tmgc26 mouse mutant, characterized by early-onset progressive ataxia, cerebellar degeneration and juvenile lethality. Detailed analysis of the tmgc26-/- cerebella revealed Purkinje cell and granule cell abnormalities, and defects in molecular layer interneurons and radial glia. Chimera studies suggested a cell-autonomous effect of the tmgc26 mutation in Purkinje cells and molecular layer interneurons, and a non-cell-autonomous effect in granule cells. The mutation was mapped to a 13-Mb interval on chromosome 9, a region that contains the ROR-alpha gene. Sequencing of genomic DNA revealed a T-to-A transition in exon 5 of the ROR-alpha gene, resulting in a nonsense mutation C257X and severe truncation of the ROR-alpha protein. Together, our data identify new roles for ROR-alpha in molecular layer interneurons and radial glia development and suggest tmgc26 as a novel ROR-alpha allele that may be used to further delineate the molecular mechanisms of ROR-alpha action.

CEREBELLAR PURKINJE CELL LOSS IN HETEROZYGOUSRORA+/−MICE: A LONGITUDINAL STUDY

The staggerer (sg) mutation is a spontaneous deletion in the Rora gene that prevents the translation of the ligand-binding domain (LBD), leading to the loss of RORalpha activity. The homozygous Rorasg/sg mutant mouse, whose most obvious phenotype is ataxia associated with cerebellar degeneration, also displays a variety of other phenotypes. The heterozygous Rora+/sg is able to develop a cerebellum that is qualitatively normal but which suffers a significant loss of cerebellar neuronal cells with advancing age. A truncated protein synthesized by the mutated allele may play a role both in Rorasg/sg and Rora+/sg. To determine the effects during life span of true haplo-insufficiency of the RORalpha protein, derived from the invalidation of the gene, we compared the evolution of Purkinje cell numbers in heterozygous Rora knock-out males (Rora+/-) and in their wild-type counterparts from 1 to 24 months of age. We also compared the evolution of Purkinje cell (PC) numbers in Rora+/- and Rora+/sg males from 1 to 9 months. The main finding is that in Rora+/- mice, for which only one-half the normal amount of protein is produced, the deficit was established as early as 1 month and did not change during the animals' adult lifespans. Thus, the effects of aging on PC number were apparent much earlier in Rora+/- than in Rora+/sg, although at 24 months of age the degrees of deficit were similar.

Retinoid-related orphan receptors (RORs): critical roles in development, immunity, circadian rhythm, and cellular metabolism

The last few years have witnessed a rapid increase in our knowledge of the retinoid-related orphan receptors RORα, -β, and -γ (NR1F1-3), their mechanism of action, physiological functions, and their potential role in several pathologies. The characterization of ROR-deficient mice and gene expression profiling in particular have provided great insights into the critical functions of RORs in the regulation of a variety of physiological processes. These studies revealed that RORα plays a critical role in the development of the cerebellum, that both RORα and RORβ are required for the maturation of photoreceptors in the retina, and that RORγ is essential for the development of several secondary lymphoid tissues, including lymph nodes. RORs have been further implicated in the regulation of various metabolic pathways, energy homeostasis, and thymopoiesis. Recent studies identified a critical role for RORγ in lineage specification of uncommitted CD4⁺ T helper cells into Th17 cells. In addition, RORs regulate the expression of several components of the circadian clock and may play a role in integrating the circadian clock and the rhythmic pattern of expression of downstream (metabolic) genes. Study of ROR target genes has provided insights into the mechanisms by which RORs control these processes. Moreover, several reports have presented evidence for a potential role of RORs in several pathologies, including osteoporosis, several autoimmune diseases, asthma, cancer, and obesity, and raised the possibility that RORs may serve as potential targets for chemotherapeutic intervention. This prospect was strengthened by recent evidence showing that RORs can function as ligand-dependent transcription factors.

RORalpha-mediated Purkinje cell development determines disease severity in adult SCA1 mice

Spinocerebellar ataxia type 1 (SCA1) is one of nine inherited, typically adult onset, polyglutamine neurodegenerative diseases. To examine whether development impacts SCA1, we used a conditional transgenic mouse model of SCA1 to delay the postnatal expression of mutant ATXN1 until after completion of cerebellar development. Delayed postnatal expression of mutant ATXN1 led to a substantial reduction in severity of disease in adults in comparison with early postnatal gene expression. This was linked to a destabilization of RORalpha, a transcription factor critical for cerebellar development. In SCA1 mice, there was a depletion of RORalpha and a reduction in expression of genes controlled by RORalpha. Partial loss of RORalpha enhanced mutant ATXN1 pathogenicity. Additionally, evidence points to the existence of a complex containing ATXN1, RORalpha, and the RORalpha coactivator Tip60. These studies indicate RORalpha and Tip60 have a role in SCA1 and suggest a mechanism by which compromising cerebellar development contributes to severity of neurodegeneration in an adult.

RORalpha, a pivotal nuclear receptor for Purkinje neuron survival and differentiation: from development to ageing

RORalpha (Retinoid-related Orphan Receptor) is a transcription factor belonging to the superfamily of nuclear receptors. The spontaneous staggerer (sg) mutation, which consists of a deletion in the Rora gene, has been shown to cause the loss of function of the RORalpha protein. The total loss of RORalpha expression leads to cerebellar developmental defects, particularly to a dramatic decreased survival of Purkinje cells and an early block in the differentiation process. This review focuses on recent studies which position RORalpha as a pivotal factor controlling Purkinje cell survival and differentiation, from development to ageing.

Regional Variations of 5HT Concentrations in Rora sg (staggerer) Mutants

Ataxic Rora(sg) (staggerer) mouse mutants, containing a deletion of the Rora gene which encodes a retinoid-like nuclear receptor, were compared to non-ataxic controls for concentrations of 5-hydroxytryptamine (HT), its main metabolite (5-hydroxy-indole acetic acid, 5HIAA), and its precursor (tryptophan) in cerebellum, brainstem, and forebrain. In Rora(sg) cerebellum, 5HT concentrations increased relative to controls, while tryptophan concentrations decreased. 5HIAA concentrations increased in mutant cerebellum and brainstem, but the 5HIAA/5HT ratio declined only in cerebellum. These results indicate that 5HT turnover decreased in cerebellum of an ataxic mutant, perhaps indicative of presynaptic accumulation and compromised neurotransmission and susceptible to be modified by 5HT pharmacotherapy.

Effect of NMDA antagonists on the death of cerebellar granule neurons at different ages

Cerebellar granule neurons (CGN) are the most abundant neuronal type in the cerebellum. During development, these cells migrate from the external to the internal granule layer (IGL), where they receive excitatory glutamatergic and cholinergic contacts from mossy fibers. During this period of development a large proportion of CGN are eliminated via apoptosis. In vitro studies have demonstrated that when CGN are obtained from rats at postnatal day 8 (P8), the sustained activation of N-methyl-D-aspartate (NMDA) receptor at 2-4 days in vitro rescues neurons from cell death. The NMDA action on cultured CGN could mimic the in vivo actions of the transient activation of the glutamate receptors by the transmitter released by mossy fibers by P12. However, some results suggest that glutamate stimulation could be relevant for CGN at earlier stages of development. In this study we evaluated the effect of NMDA receptor stimulation or blockade on the cell death of both in vivo and cultured CGN obtained from P2 to P8 rats. Our results showed that the blockade of NMDA receptors with the antagonists D,L-2-amino-5-phosphonovaleric acid or dizocilpine (MK-801) reduces cell survival to 20-40%, whereas NMDA treatment increases neuronal survival by approximately 50-60%. In vivo, the treatment with MK-801 reduced the number of apoptotic CGN in the molecular layer (ML) from P5 to P8. These results suggest that NMDA receptor stimulation plays a critical role in the regulation of CGN death during the first week of rat cerebellar development.

Anp32e/Cpd1 regulates protein phosphatase 2A activity at synapses during synaptogenesis

Anp32e/Cpd1, a member of the acidic nuclear phosphoprotein (Anp)32 family, is characterized by the presence of an amino terminal domain containing four leucine-rich repeats and a carboxyl-terminal low-compositional complexity acidic region. In previous studies performed to understand the biological role of Anp32e/Cpd1, we showed a predominant presence of Anp32e/Cpd1 in the nucleus. However, when Anp32e/Cpd1 is in the cytoplasm, it co-localizes spatially with protein phosphatase 2A (PP2A) near cell membranes, far from the synapses. In the present work, we show that Anp32e/Cpd1 is also present as a membrane-bound 74/76-kDa protein with a widespread distribution in the brain. We reveal that the expression, synthesis and half-life of this high-molecular-weight form of Anp32e/Cpd1 are spatially and temporally correlated with the cerebellar synaptogenesis period. We demonstrate that synaptic Anp32e/Cpd1 co-localizes, interacts and inhibits PP2A activity, and that phosphorylation of Anp32/Cpd1 is required for the Anp32e-PP2A interaction. Also, subcellular localization was shown with electronic microscopy. Finally, we examine Anp32e/Cpd1 and PP2A distribution in two ataxic mutant models, weaver and staggerer, and show that their co-localization in Purkinje cell dendrites depends on parallel fibre/Purkinje cell contacts. Based on these observations, we propose that Anp32e/Cpd1 mediates synaptogenesis process by modulating PP2A activity.

Spontaneous and induced mouse mutations with cerebellar dysfunctions: behavior and neurochemistry

Grid2(Lc) (Lurcher), Grid2(ho) (hot-foot), Rora(sg) (staggerer), nr (nervous), Agtpbp1(pcd) (Purkinje cell degeneration), Reln(rl) (reeler), and Girk2(Wv) (Weaver) are spontaneous mutations with cerebellar atrophy, ataxia, and deficits in motor coordination tasks requiring balance and equilibrium. In addition to these signs, the Dst(dt) (dystonia musculorum) spinocerebellar mutant displays dystonic postures and crawling. More recently, transgenic models with human spinocerebellar ataxia mutations and alterations in calcium homeostasis have been shown to exhibit cerebellar anomalies and motor coordination deficits. We describe neurochemical characteristics of these mutants with respect to regional brain metabolism as well as amino acid and biogenic amine concentrations, uptake sites, and receptors.

ROR alpha in genetic control of cerebellum development: 50 staggering years

The staggerer mutation was first identified at the Jackson Laboratory in 1955. In the ensuing half-century, studies of staggerer mice have provided new insights into developmental neurobiology, gene regulatory networks, and circadian behavior. Recent work has expanded the role of RORalpha, the transcription factor mutated in staggerer, to peripheral tissues, including cholesterol and lipid metabolism, immune function, and bone development. This review focuses on the role of RORalpha in neural development and behavior revealed by the staggerer mutation and subsequent molecular studies.

Retinoid-related Orphan Receptors (RORs): Roles in Cellular Differentiation and Development

Retinoid-related orphan receptors RORalpha, -beta, and -gamma are transcription factors belonging to the steroid hormone receptor superfamily. During embryonic development RORs are expressed in a spatial and temporal manner and are critical in the regulation of cellular differentiation and the development of several tissues. RORalpha plays a key role in the development of the cerebellum particularly in the regulation of the maturation and survival of Purkinje cells. In RORalpha-deficient mice, the reduced production of sonic hedgehog by these cells appears to be the major cause of the decreased proliferation of granule cell precursors and the observed cerebellar atrophy. RORalpha has been implicated in the regulation of a number of other physiological processes, including bone formation. RORbeta expression is largely restricted to several regions of the brain, the retina, and pineal gland. Mice deficient in RORbeta develop retinal degeneration that results in blindness. RORgamma is essential for lymph node organogenesis. In the intestine RORgamma is required for the formation of several other lymphoid tissues: Peyer's patches, cryptopatches, and isolated lymphoid follicles. RORgamma plays a key role in the generation of lymphoid tissue inducer (LTi) cells that are essential for the development of these lymphoid tissues. In addition, RORgamma is a critical regulator of thymopoiesis. It controls the differentiation of immature single-positive thymocytes into double-positive thymocytes and promotes the survival of double-positive thymocytes by inducing the expression of the anti-apoptotic gene Bcl-X(L). Interestingly, all three ROR receptors appear to play a role in the control of circadian rhythms. RORalpha positively regulates the expression of Bmal1, a transcription factor that is critical in the control of the circadian clock. This review intends to provide an overview of the current status of the functions RORs have in these biological processes.

Postnatal development and synapse elimination of climbing fiber to Purkinje cell projection in the cerebellum

Cerebellar climbing fiber (CF) to Purkinje cell (PC) synapses in rodents provides a good model to study mechanisms underlying postnatal development of synaptic functions and elimination of redundant synapses in the central nervous system. At birth, each PC is innervated by multiple CFs. Then, single CF input is selected, matured and strengthened, while surplus CFs are eliminated. By the end of the third postnatal week, most PCs become innervated by single CFs. This up-date article aims to provide an overview of recent studies on the mechanisms of this process.

Correlation between synaptogenesis and the PTEN phosphatase expression in dendrites during postnatal brain development

The PTEN (phosphatase and tensin homolog deleted on chromosome 10) tumor suppressor gene codifies a lipid inositol 3'-phosphatase that negatively regulates cell survival mediated by the phosphatidyl inositol 3' kinase (PIP3-kinase)--protein kinase B/Akt signaling pathway. Recently, PIP3-kinase was involved in axon polarization, but PTEN functions in dendrites are uncertain. Using amino-terminal antibodies against the catalytic domain, we found a 34 kDa fragment of PTEN protein detected only in mouse brain tissue, present in neuron dendrites and spines of cerebral cortex, cerebellum, hippocampus and olfactory bulb. The PTEN-fragment reaches the synaptic fraction with a positive temporal correlation with synaptic stabilization in postnatal cerebellum and brain. In the weaver mutant mice, PTEN was absent only in the Purkinje cells dendrites that cannot receive the granule cells synaptic input. Furthermore, the activated p-Akt/PKB was present in axons but not in dendrites of mature neuron cells. P-Akt was also altered by the weaver mutation maintaining the inverse correlation with the PTEN-fragment in Purkinje cell dendrites. In contrast, the expression of this fragment was not affected by the staggerer mutation. Together, these results suggest that synaptogenesis is a necessary process for polarization in PIP3 pathway mediated by the PTEN catalytic-fragment into dendrites of CNS neurons.

Early postnatal Purkinje cells from staggerer mice undergo aberrant development in vitro with characteristic morphologic and gene expression abnormalities

The staggerer (sg) mutant mouse shows an early block in Purkinje cell (PC) development due to a truncation of the transcription factor retinoic acid-related orphan receptor alpha (ROR alpha). Cerebellar cultures from neonatal sg mice recapitulate many in vivo postnatal changes in PCs and allow for detailed observation of sequential dendritic outgrowth and gene expression changes. In addition, this in vitro system provides a tractable basis for ROR alpha replacement studies.

Disruption of Intraneuronal Divalent Cation Regulation by Methylmercury: Are Specific Targets Involved in Altered Neuronal Development and Cytotoxicity in Methylmercury Poisoning?

Methylmercury is an environmental contaminant which causes relatively specific degeneration of the granular layer of the cerebellum, despite its ability to bind thiol groups in proteins of all cell types. The mechanisms underlying the specific targeting of cells during MeHg poisoning may depend on specific receptors and other targets related to divalent cation homeostasis, particularly intracellular calcium (Ca(2+)(i) signaling. MeHg disrupts Ca(2+)(i) homeostasis in a number of neuronal models, including cerebellar granule cells in primary culture, and contributes to MeHg-induced cell death, impaired synaptic function and disruption of neuronal development. Interestingly, the disruption of [Ca(2+)](i) regulation occurs through specific pathways which affect Ca(2+) regulation by organelles, particularly mitochondria and the smooth endoplasmic reticulum (SER). Cholinergic pathways which affect [Ca(2+)](i) signaling also appear to be critical targets, particularly muscarinic acetylcholine (ACh) receptors which are linked to Ca(2+) release through inositol-1,4,5-triphosphate (IP(3)) receptors. [Ca(2+)](i) dysregulation may also underlie observed alterations in cerebellar neuron development through interaction with specific target(s) in the developing axon. In this review, we examine the hypothesis that MeHg affects specific targets to cause disruption of neuronal development and cell death.

Evidence for an axonal localization of the type 2 corticotropin-releasing factor receptor during postnatal development of the mouse cerebellum

Previous studies have described the embryonic and postnatal development of CRF, as well as the type 1 CRF receptor in the mouse cerebellum. The present immunohistochemical study localizes the cellular distribution of the type 2 CRF receptor (CRF-R2) during postnatal development of the mouse cerebellum. Western blot analysis indicates that the antibody used in this analysis recognizes both a full-length and a truncated isoform of the type 2 receptor. We propose that each isoform has a unique cellular distribution. In the present study, the postnatal (P) development (P0-P14) and cellular localization of CRF-R2 in different cell types was analyzed using PAP and double-label fluorescent immunohistochemistry; cell-specific antibodies were used to identify cells expressing CRF-R2 at different stages of postnatal development. At P0, CRF-R2 immunoreactivity was localized within the somata of Purkinje cells and migrating GABAergic interneurons. CRF-R2 was first observed in the initial axonal segments of some Purkinje cells at P5, and was evident in many Purkinje cell axon hillocks at P8. Punctate immunoreactivity is present in the molecular layer by P5 and is interpreted to be immunolabeled parallel fibers. Between P8 and P14, CRF-R2 immunostaining is present in the initial axonal segments of Golgi cells, within the internal granule cell layer. Finally, CRF-R2 is present in both radial glia in the molecular layer as well as in astrocytes in the white matter and internal granule cell layer from P5 to P14. The present results suggest that CRF-R2, both the truncated and the full-length isoforms, are present in the developing cerebellum, each with a unique cellular distribution. The immunohistochemical evidence indicates that the truncated isoform of the type 2 CRF receptor is in the axons of several different types of cerebellar cortical neurons, and suggests that CRF could play a role in cerebellar development by modulating the release of transmitters from excitatory and/or inhibitory interneurons, which in turn could directly alter the maturation of cerebellar circuits. In contrast, the binding of a ligand to the full-length isoform of CRF-R2 or to CRF-R1, both in a postsynaptic location, may have a more direct effect on regulating the responsiveness of these cells to growth factors or neurotransmitters released from afferent axons by regulating permeability of ion channels or altering second messenger systems.

Immunohistochemical characterization of the orphan nuclear receptor ROR alpha in the mouse nervous system

ROR alpha is an orphan nuclear receptor. A deletion mutation in the ROR alpha gene leads to severe cerebellar defects, known as the staggerer mutant mouse. Although previous in situ hybridization (ISH) studies have shown that ROR alpha is highly expressed in the cerebellum, especially in Purkinje cells, and in the thalamus, sufficient immunohistochemical (IHC) study has not yet been presented. I demonstrate here the IHC analysis of ROR alpha using a specific anti-ROR alpha antibody, in adult and developing mouse nervous system. ROR alpha immunoreactivity was observed in the Purkinje cell and molecular layers of the cerebellum. The co-localization of ROR alpha with calbindin D(28K) (CaBP) and parvalbumin indicates that ROR alpha-positive cells were Purkinje cells, stellate cells, and basket cells. In addition to the cerebellum, strong to medium ROR alpha immunoreactivity was found in the thalamus, cerebral cortex (mainly in the layer IV), dorsal cochlear nucleus (DCN), suprachiasmatic nucleus (SCN), superior colliculus, spinal trigeminal nucleus, and retina. The immunostaining was restricted in nuclei of neurons. Developmentally, ROR alpha immunoreactivity was observed in the cerebellum and thalamus from embryonal day 16 (E16). The distribution of ROR alpha immunoreactivity and ROR alpha mRNA hybridization signal was almost coincident. However, the intensity of hybridization signal was not always parallel to that of immunoreactivity.

The effects of cerebellar damage on maze learning in animals

The role of the cerebellum in spatial learning has recently been investigated in genetically and non-genetically lesioned animal models, particularly in water mazes, in view of the minimal impact such lesions exert on swimming movements. A dissociation between place and cued learning in the Morris water maze has been observed in several models, including cerebellar mutant mice (Rora(sg), Nna1(pcd-1J), nervous), rats with lesions of either the lateral cerebellar cortex or the dentate nucleus, and rats with selective Purkinje cell loss caused by intracerebroventricular injections of OX-7-saporin, confirming the hypothesis that cerebellar damage may cause a cognitive deficit independently of fine motor control. In addition, the results of hemicerebellectomized rats indicate the probable involvement of the cerebellum in working memory and the procedural aspect of maze learning. The findings of impaired maze learning in cerebellar-lesioned mice and rats are concordant with those of deficient visuospatial functions in patients with cerebellar atrophy. The spatial deficits may be ascribed to altered metabolic activity in cerebellar-related pathways.

Patterned Purkinje cell death in the cerebellum

The object of this review is to assemble much of the literature concerning Purkinje cell death in cerebellar pathology and to relate this to what is now known about the complex topography of the cerebellar cortex. A brief introduction to Purkinje cells, and their regionalization is provided, and then the data on Purkinje cell death in mouse models and, where appropriate, their human counterparts, have been arranged according to several broad categories--naturally-occurring and targeted mutations leading to Purkinje cell death, Purkinje cell death due to toxins, Purkinje cell death in ischemia, Purkinje cell death in infection and in inherited disorders, etc. The data reveal that cerebellar Purkinje cell death is much more topographically complex than is usually appreciated.

Postnatal development of the cerebellum and the CNS adrenergic system is independent of norepinephrine and epinephrine

A fundamental question in the formation of the nervous system is the extent to which a neurotransmitter contributes to the development of the neurons that synthesize and release it. A complementary question is whether neurotransmitter signaling contributes to the development of postsynaptic targets. Prior studies have suggested that adrenergic signaling may promote adrenergic neuronal proliferation or survival and may be critical for the postnatal development of the cerebellum. To test these possibilities genetically, we studied mice that are unable to synthesize norepinephrine and epinephrine (NE/E), the endogenous adrenergic receptor ligands, due to a disruption the gene for dopamine beta-hydroxylase. These mice develop postnatally in the absence of NE/E. Here we report that the adrenergic neurons of these mutant mice are present in normal numbers and locations and exhibit typical innervation patterns throughout the central nervous system (CNS), as assessed by immunostaining for tyrosine hydroxylase and the NE transporter. Furthermore, cerebellar cortical development (size, foliation, layering, cell number, and position), which proceeds to a large degree postnatally, is unaltered in the mutants. These results indicate that the fate and innervation pattern of the adrenergic neurons, as well as the development of the cerebellum, do not depend on postnatal signaling by NE/E. The results also suggest that when restoration of adrenergic signaling is performed in this mutant mouse model (by administering a synthetic precursor of NE), reversal of phenotypes is due to the synthesis and release of NE/E from adrenergic terminals that are distributed normally within the CNS.

Cell cycle regulation of neuronal apoptosis in development and disease

Apoptosis of neurons is indispensable to the normal development of the nervous system and contributes to neuronal loss in neurologic injury and disease. Life and death decisions are imposed upon neurons by extracellular and intracellular stimuli including the lack of trophic support, exposure to neurotoxins, oxidative stress, and DNA damage. These stimuli induce signaling pathways that are integrated at the mitochondrial apoptotic machinery culminating in cell survival or death. Growing evidence suggests that cell cycle proteins are expressed in dying neurons in the developing and adult brain. However, the role and mechanisms by which re-activation of cell cycle pathways in postmitotic neurons propagates an apoptotic signal to the cell death machinery are just beginning to be characterized. Here, we will review the molecular mechanisms of neuronal cell death and survival with a focus on recent findings on cell cycle regulation of neuronal apoptosis in primary cultures of neurons, mouse models of neuronal diseases, and human neurodegenerative diseases.