Cerebellar Purkinje cell p75 neurotrophin receptor and autistic behavior

The mouse at the popcorn stage of development

In mice, rats, and rabbits vigorous jumping and hyperexcitability occur at the popcorn stage of postnatal development. In view of subcortical structures appearing before cortical ones, the trait is deemed to occur at the maturation time of ascending excitatory projections from the brainstem and to disappear at the maturation time of descending inhibitory projections from the forebrain. There is evidence that the popcorn stage may be due in part to the lack of a cholinergic influence on dopamine systems. Based mostly on results found in adult mice and rats, there may also be a role for cortico-subcortical systems that include the cerebellum and basal ganglia requiring the influence of biogenic amines, glutamate, and endocannabinoids.

Cerebellar Calcium-Binding Protein and Neurotrophin Receptor Defects in Down Syndrome and Alzheimer's Disease

Cerebellar hypoplasia is a major characteristic of the Down syndrome (DS) brain. However, the consequences of trisomy upon cerebellar Purkinje cells (PC) and interneurons in DS are unclear. The present study performed a quantitative and qualitative analysis of cerebellar neurons immunostained with antibodies against calbindin D-28k (Calb), parvalbumin (Parv), and calretinin (Calr), phosphorylated and non-phosphorylated intermediate neurofilaments (SMI-34 and SMI-32), and high (TrkA) and low (p75NTR) affinity nerve growth factor (NGF) receptors as well as tau and amyloid in DS (n = 12), Alzheimer's disease (AD) (n = 10), and healthy non-dementia control (HC) (n = 8) cases. Our findings revealed higher Aβ42 plaque load in DS compared to AD and HC but no differences in APP/Aβ plaque load between HC, AD, and DS. The cerebellar cortex neither displayed Aβ40 containing plaques nor pathologic phosphorylated tau in any of the cases examined. The number and optical density (OD) measurements of Calb immunoreactive (-ir) PC soma and dendrites were similar between groups, while the number of PCs positive for Parv and SMI-32 were significantly reduced in AD and DS compared to HC. By contrast, the number of SMI-34-ir PC dystrophic axonal swellings, termed torpedoes, was significantly greater in AD compared to DS. No differences in SMI-32- and Parv-ir PC OD measurements were observed between groups. Conversely, total number of Parv- (stellate/basket) and Calr (Lugaro, brush, and Golgi)-positive interneurons were significantly reduced in DS compared to AD and HC. A strong negative correlation was found between counts for Parv-ir interneurons, Calr-ir Golgi and brush cells, and Aβ42 plaque load. Number of TrkA and p75NTR positive PCs were reduced in AD compared to HC. These findings suggest that disturbances in calcium binding proteins play a critical role in cerebellar neuronal dysfunction in adults with DS.

Cerebellar modulation of the reward circuitry and social behavior

The cerebellum has been implicated in a number of nonmotor mental disorders such as autism spectrum disorder, schizophrenia, and addiction. However, its contribution to these disorders is not well understood. In mice, we found that the cerebellum sends direct excitatory projections to the ventral tegmental area (VTA), one of the brain regions that processes and encodes reward. Optogenetic activation of the cerebello-VTA projections was rewarding and, in a three-chamber social task, these projections were more active when the animal explored the social chamber. Intriguingly, activity in the cerebello-VTA pathway was required for the mice to show social preference in this task. Our data delineate a major, previously unappreciated role for the cerebellum in controlling the reward circuitry and social behavior.

Opposite Expression Patterns of Spry3 and p75NTR in Cerebellar Vermis Suggest a Male-Specific Mechanism of Autism Pathogenesis

Autism is a genetically complex neurobehavioral disorder with a population prevalence of more than 1%. Cerebellar abnormalities, including Purkinje cell deficits in the vermis, are consistently reported, and rodent models of cerebellar dysfunction exhibit features analogous to human autism. We previously analyzed the regulation and expression of the pseudoautosomal region 2 gene SPRY3, which is adjacent to X chromosome-linked TMLHE, a known autism susceptibility gene. SPRY3 is a regulator of branching morphogenesis and is strongly expressed in Purkinje cells. We previously showed that mouse Spry3 is not expressed in cerebellar vermis lobules VI-VII and X, regions which exhibit significant Purkinje cell loss or abnormalities in autism. However, these lobules have relatively high expression of p75NTR, which encodes a neurotrophin receptor implicated in autism. We propose a mechanism whereby inappropriate SPRY3 expression in these lobules could interact with TrkB and p75NTR signaling pathways resulting in Purkinje cell pathology. We report preliminary characterization of X and Y chromosome-linked regulatory sequences upstream of SPRY3, which are polymorphic in the general population. We suggest that an OREG-annotated region on chromosome Yq12 ∼60 kb from SPRY3 acts as a silencer of Y-linked SPRY3 expression. Deletion of a β-satellite repeat, or alterations in chromatin structure in this region due to trans-acting factors, could affect the proposed silencing function, leading to reactivation and inappropriate expression of Y-linked SPRY3. This proposed male-specific mechanism could contribute to the male bias in autism prevalence.

Is there an "antisocial" cerebellum? Evidence from disorders other than autism characterized by abnormal social behaviours

The cerebellum is a hindbrain structure which involvement in functions not related to motor control and planning is being increasingly recognized in the last decades. Studies on Autism Spectrum Disorders (ASD) have reported cerebellar involvement on these conditions characterized by social deficits and repetitive motor behavior patterns. Although such an involvement hints at a possible cerebellar participation in the social domain, the fact that ASD patients present both social and motor deficits impedes drawing any firm conclusion regarding cerebellar involvement in pathological social behaviours, probably influenced by the classical view of the cerebellum as a purely "motor" brain structure. Here, we suggest the cerebellum can be a key node for the production and control of normal and particularly aberrant social behaviours, as indicated by its involvement in other neuropsychiatric disorders which main symptom is deregulated social behaviour. Therefore, in this work, we briefly review cerebellar involvement in social behavior in rodent models, followed by discussing the findings linking the cerebellum to those other psychiatric conditions characterized by defective social behaviours. Finally, possible commonalities between the studies and putative underlying impaired functions will be discussed and experimental approaches both in patients and experimental animals will also be proposed, aimed at stimulating research on the role of the cerebellum in social behaviours and disorders characterized by social impairments, which, if successful, will definitely help reinforcing the proposed cerebellar involvement in the social domain.

Neuronal survival in the brain: neuron type-specific mechanisms

Neurogenic regions of mammalian brain produce many more neurons that will eventually survive and reach a mature stage. Developmental cell death affects both embryonically produced immature neurons and those immature neurons that are generated in regions of adult neurogenesis. Removal of substantial numbers of neurons that are not yet completely integrated into the local circuits helps to ensure that maturation and homeostatic function of neuronal networks in the brain proceed correctly. External signals from brain microenvironment together with intrinsic signaling pathways determine whether a particular neuron will die. To accommodate this signaling, immature neurons in the brain express a number of transmembrane factors as well as intracellular signaling molecules that will regulate the cell survival/death decision, and many of these factors cease being expressed upon neuronal maturation. Furthermore, pro-survival factors and intracellular responses depend on the type of neuron and region of the brain. Thus, in addition to some common neuronal pro-survival signaling, different types of neurons possess a variety of 'neuron type-specific' pro-survival constituents that might help them to adapt for survival in a certain brain region. This review focuses on how immature neurons survive during normal and impaired brain development, both in the embryonic/neonatal brain and in brain regions associated with adult neurogenesis, and emphasizes neuron type-specific mechanisms that help to survive for various types of immature neurons. Importantly, we mainly focus on in vivo data to describe neuronal survival specifically in the brain, without extrapolating data obtained in the PNS or spinal cord, and thus emphasize the influence of the complex brain environment on neuronal survival during development.

The Role of MicroRNAs in Cerebellar Development and Autism Spectrum Disorder During Embryogenesis

MicroRNAs (miRNAs) are a class of small non-coding RNA molecules with wide-ranging and subtle effects on protein production. Their activity during the development of the cerebellum provides a valuable exemplar of how non-coding molecules may assist the development and function of the central nervous system and drive neurodevelopmental disorders. Three distinct aspects of miRNA contribution to early cerebellar development will here be reviewed. Aspects are the establishment of the cerebellar anlage, the generation and maturation of at least two principal cell types of the developing cerebellar microcircuit, and the etiology and early progression of autism spectrum disorder. It will be argued here that the autism spectrum is an adept model to explore miRNA impact on the cognitive and affective processes that descend from the developing cerebellum.

Brain-derived neurotrophic factor signaling is altered in the forebrain of Engrailed-2 knockout mice

Engrailed-2 (En2), a homeodomain transcription factor involved in regionalization and patterning of the midbrain and hindbrain regions has been associated to autism spectrum disorders (ASDs). En2 knockout (En2(-/-)) mice show ASD-like features accompanied by a significant loss of GABAergic subpopulations in the hippocampus and neocortex. Brain-derived neurotrophic factor (BDNF) is a crucial factor for the postnatal development of forebrain GABAergic neurons, and altered GABA signaling has been hypothesized to underlie the symptoms of ASD. Here we sought to determine whether interneuron loss in the En2(-/-) forebrain might be related to altered expression of BDNF and its signaling receptors. We first evaluated the expression of different BDNF mRNA isoforms in the neocortex and hippocampus of wild-type (WT) and En2(-/-) mice. Quantitative RT-PCR showed a marked down-regulation of several splicing variants of BDNF mRNA in the neocortex but not hippocampus of adult En2(-/-) mice, as compared to WT controls. Accordingly, levels of mature BDNF protein were lower in the neocortex but not hippocampus of En2(-/-) mice, as compared to WT. Increased levels of phosphorylated TrkB and decreased levels of p75 receptor were also detected in the neocortex of mutant mice. Accordingly, the expression of low density lipoprotein receptor (LDLR) and RhoA, two genes regulated via p75 was significantly altered in forebrain areas of mutant mice. These data indicate that BDNF signaling alterations might be involved in the anatomical changes observed in the En2(-/-) forebrain and suggest a pathogenic role of altered BDNF signaling in this mouse model of ASD.

Autistic-Like Traits and Cerebellar Dysfunction in Purkinje Cell PTEN Knock-Out Mice

Autism spectrum disorders (ASDs) are neurodevelopmental disorders characterized by impaired social interaction, isolated areas of interest, and insistence on sameness. Mutations in Phosphatase and tensin homolog missing on chromosome 10 (PTEN) have been reported in individuals with ASDs. Recent evidence highlights a crucial role of the cerebellum in the etiopathogenesis of ASDs. In the present study we analyzed the specific contribution of cerebellar Purkinje cell (PC) PTEN loss to these disorders. Using the Cre-loxP recombination system, we generated conditional knockout mice in which PTEN inactivation was induced specifically in PCs. We investigated PC morphology and physiology as well as sociability, repetitive behavior, motor learning, and cognitive inflexibility of adult PC PTEN-mutant mice. Loss of PTEN in PCs results in autistic-like traits, including impaired sociability, repetitive behavior and deficits in motor learning. Mutant PCs appear hypertrophic and show structural abnormalities in dendrites and axons, decreased excitability, disrupted parallel fiber and climbing fiber synapses and late-onset cell death. Our results unveil new roles of PTEN in PC function and provide the first evidence of a link between the loss of PTEN in PCs and the genesis of ASD-like traits.

Cerebellar Expression of the Neurotrophin Receptor p75 in Naked-Ataxia Mutant Mouse

Spontaneous mutation in the lysosomal acid phosphatase 2 (Acp2) mouse (nax--naked-ataxia mutant mouse) correlates with severe cerebellar defects including ataxia, reduced size and abnormal lobulation as well as Purkinje cell (Pc) degeneration. Loss of Pcs in the nax cerebellum is compartmentalized and harmonized to the classic pattern of gene expression of the cerebellum in the wild type mouse. Usually, degeneration starts in the anterior and posterior zones and continues to the central and nodular zones of cerebellum. Studies have suggested that the p75 neurotrophin receptor (NTR) plays a role in Pc degeneration; thus, in this study, we investigated the p75NTR pattern and protein expression in the cerebellum of the nax mutant mouse. Despite massive Pc degeneration that was observed in the nax mouse cerebellum, p75NTR pattern expression was similar to the HSP25 pattern in nax mice and comparable with wild type sibling cerebellum. In addition, immunoblot analysis of p75NTR protein expression did not show any significant difference between nax and wild type sibling (p > 0.5). In comparison with wild type counterparts, p75NTR pattern expression is aligned with the fundamental cytoarchitecture organization of the cerebellum and is unchanged in the nax mouse cerebellum despite the severe neurodevelopmental disorder accompanied with Pc degeneration.

A decade from discovery to therapy: Lingo-1, the dark horse in neurological and psychiatric disorders

Leucine-rich repeat and immunoglobulin domain-containing protein (Lingo-1) is a potent negative regulator of neuron and oligodendrocyte survival, neurite extension, axon regeneration, oligodendrocyte differentiation, axonal myelination and functional recovery; all processes highly implicated in numerous brain-related functions. Although playing a major role in developmental brain functions, the potential application of Lingo-1 as a therapeutic target for the treatment of neurological disorders has so far been under-estimated. A number of preclinical studies have shown that various methods of antagonizing Lingo-1 results in neuronal and oligodendroglial survival, axonal growth and remyelination; however to date literature has only detailed applications of Lingo-1 targeted therapeutics with a focus primarily on myelination disorders such as multiple sclerosis and spinal cord injury; omitting important information regarding Lingo-1 signaling co-factors. Here, we provide for the first time a complete and thorough review of the implications of Lingo-1 signaling in a wide range of neurological and psychiatric disorders, and critically examine its potential as a novel therapeutic target for these disorders.

Cell-type-specific neuroanatomy of cliques of autism-related genes in the mouse brain

Two cliques of genes identified computationally for their high co-expression in the mouse brain according to the Allen Brain Atlas, and for their enrichment in genes related to autism spectrum disorder (ASD), have recently been shown to be highly co-expressed in the cerebellar cortex, compared to what could be expected by chance. Moreover, the expression of these cliques of genes is not homogeneous across the cerebellar cortex, and it has been noted that their expression pattern seems to highlight the granular layer. However, this observation was only made by eye, and recent advances in computational neuroanatomy allow to rank cell types in the mouse brain (characterized by their transcriptome profiles) according to the similarity between their spatial density profiles and the spatial expression profiles of the cliques. We establish by Monte Carlo simulation that with probability at least 99%, the expression profiles of the two cliques are more similar to the density profile of granule cells than 99% of the expression of cliques containing the same number of genes (Purkinje cells also score above 99% in one of the cliques). Thresholding the expression profiles shows that the signal is more intense in the granular layer. Finally, we work out pairs of cell types whose combined expression profiles are more similar to the expression profiles of the cliques than any single cell type. These pairs predominantly consist of one cortical pyramidal cell and one cerebellar cell (which can be either a granule cell or a Purkinje cell).