The Dendritic Differentiation of Purkinje Neurons: Unsolved Mystery in Formation of Unique Dendrites

Olivopontocerebellar degeneration associated with 3-hydroxy-3-methylglutaric aciduria in a domestic shorthair cat

CASE SUMMARY

A rescue charity-owned 6-month-old neutered female domestic shorthair cat was presented with progressive tetraparesis, increased extensor muscle tone and signs of spinocerebellar ataxia, including hypermetria. The cat's male sibling, with similar progressive neurological signs, had been euthanased 2 months previously. An inherited metabolic disorder was suspected. Urine for determination of organic acid concentration was obtained and the cat was prescribed carnitine and taurine supplementation. The cat was euthanased 3 months later following progressive neurological signs, including ataxia, tetraparesis, tendency to fall, bilateral absent menace response and intention tremor. A selective post-mortem examination was obtained, taking samples from the brain, cervical spinal cord, tibial branch of the sciatic nerve, muscle, liver and kidneys. Organic acid analysis results received after euthanasia revealed a marked elevation of 3-hydroxy-3-methylglutaric acid (45 mmol/mol creatine [normal range 0-2]) and isovalerylglycine (27 mmol/mol creatinine [normal range 0-2]). 3-Hydroxy-3-methylglutaric acid was deemed clinically relevant as it is a metabolite of 3-hydroxy-3-methylglutaryl-CoA lyase, the enzyme involved in the final step of leucine degradation. Post-mortem examination revealed diffuse, chronic-active, severe olivoponto-(spino)-cerebellar degeneration.

RELEVANCE AND NOVEL INFORMATION

This is the first report of 3-hydroxy-3-methylglutaric aciduria in the veterinary literature and the first description of the neuropathology of this disorder in any species. 3-Hydroxy-3-methylglutaric aciduria in humans occurs rarely and is due to a deficiency in 3-hydroxy-3-methylglutaryl-coenzyme A lyase.

Expression of an alternatively spliced variant of SORL1 in neuronal dendrites is decreased in patients with Alzheimer's disease

SORL1 is strongly associated with both sporadic and familial forms of Alzheimer's disease (AD), but a lack of information about alternatively spliced transcripts currently limits our understanding of the role of SORL1 in AD. Here, we describe a SORL1 transcript (SORL1-38b) characterized by inclusion of a novel exon (E38b) that encodes a truncated protein. We identified E38b-containing transcripts in several brain regions, with the highest expression in the cerebellum and showed that SORL1-38b is largely located in neuronal dendrites, which is in contrast to the somatic distribution of transcripts encoding the full-length SORLA protein (SORL1-fl). SORL1-38b transcript levels were significantly reduced in AD cerebellum in three independent cohorts of postmortem brains, whereas no changes were observed for SORL1-fl. A trend of lower 38b transcript level in cerebellum was found for individuals carrying the risk variant at rs2282649 (known as SNP24), although not reaching statistical significance. These findings suggest synaptic functions for SORL1-38b in the brain, uncovering novel aspects of SORL1 that can be further explored in AD research.

Refinement of Cerebellar Network Organization by Extracellular Signaling During Development

The cerebellum forms regular neural network structures consisting of a few major types of neurons, such as Purkinje cells, granule cells, and molecular layer interneurons, and receives two major inputs from climbing fibers and mossy fibers. Its regular structures consist of three well-defined layers, with each type of neuron designated to a specific location and forming specific synaptic connections. During the first few weeks of postnatal development in rodents, the cerebellum goes through dynamic changes via proliferation, migration, differentiation, synaptogenesis, and maturation, to create such a network structure. The development of this organized network structure presumably relies on the communication between developing elements in the network, including not only individual neurons, but also their dendrites, axons, and synapses. Therefore, it is reasonable that extracellular signaling via synaptic transmission, secreted molecules, and cell adhesion molecules, plays important roles in cerebellar network development. Although it is not yet clear as to how overall cerebellar development is orchestrated, there is indeed accumulating lines of evidence that extracellular signaling acts toward the development of individual elements in the cerebellar networks. In this article, we introduce what we have learned from many studies regarding the extracellular signaling required for cerebellar network development, including our recent study suggesting the importance of unbiased synaptic inputs from parallel fibers.

Embryology

With the growing recognition of the extent and prevalence of human cerebellar disorders, an understanding of developmental programs that build the mature cerebellum is necessary. In this chapter we present an overview of the basic epochs and key molecular regulators of the developmental programs of cerebellar development. These include early patterning of the cerebellar territory, the genesis of cerebellar cells from multiple spatially distinct germinal zones, and the extensive migration and coordinated cellular rearrangements that result in the formation of the exquisitely foliated and laminated mature cerebellum. This knowledge base is founded on extensive analysis of animal models, particularly mice, due in large part to the ease of genetic manipulation of this important model organism. Since cerebellar structure and function are largely conserved across species, mouse cerebellar development is highly relevant to humans and has led to important insights into the developmental pathogenesis of human cerebellar disorders. Human fetal cerebellar development remains largely undescribed; however, several human-specific developmental features are known which are relevant to human disease and underline the importance of ongoing human fetal research.

Expression of the GluA2 subunit of glutamate receptors is required for the normal dendritic differentiation of cerebellar Purkinje cells

Cerebellar Purkinje cells differentiate the most elaborate dendritic trees among neurons in the brain and constitute the principal part of cerebellar neuronal circuitry. In the present study, we examined the role of the GluA2 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors in the dendritic differentiation of Purkinje cells. Since mature Purkinje cells express the GluA2 subunit, AMPA receptors on them exhibit a low Ca²⁺ permeability. Does this expression of GluA2, leading to the loss of Ca²⁺ permeability of AMPA receptors, have a positive significance in the dendritic differentiation of Purkinje cells? To answer this question, we introduced GluA2 siRNA into immature Purkinje cells in cerebellar cell cultures using a single-cell electroporation technique. The dendritic elongation and branching, as well as spine formation, were inhibited by GluA2 knockdown in Purkinje cells. GluA2 knockdown augmented the elevation of intracellular Ca²⁺ concentrations and a higher incidence of oscillation of intracellular Ca²⁺ concentrations in response to glutamate. These findings suggest that excessive elevation of intracellular Ca²⁺ concentrations has a negative effect on the dendritic differentiation of Purkinje cells and that the expression of GluA2 inhibits this negative effect in the development of Purkinje cells.

Consensus Paper: Cerebellar Development

The development of the mammalian cerebellum is orchestrated by both cell-autonomous programs and inductive environmental influences. Here, we describe the main processes of cerebellar ontogenesis, highlighting the neurogenic strategies used by developing progenitors, the genetic programs involved in cell fate specification, the progressive changes of structural organization, and some of the better-known abnormalities associated with developmental disorders of the cerebellum.

Synaptotagmin 7 and SYNCRIP proteins are ubiquitously expressed in the rat brain and co-localize in Purkinje neurons

Synaptotagmin 7 (SYT7) is ubiquitously expressed calcium sensor, involved in neuronal membrane trafficking. Immunoprecipitation experiments demonstrated that SYT7 interacts with Synaptotagmin-binding, cytoplasmic RNA-interacting protein (SYNCRIP). SYNCRIP is a component of mRNA granules, which are transported to dendrites and are prerequisite for synaptic plasticity. Given the potential significance of SYT7 regulation in processes of neurodegeneration, which are characterized by high level of synaptic vulnerability, we aimed to analyse and compare the distribution of SYT7 and SYNCRIP proteins in the adult rat striatum, hippocampus, cerebral and cerebellar cortex. We investigated the degree of SYT7-SYNCRIP co-localization in order to examine possible functional interaction of these two proteins. We found that SYT7 is abundantly distributed in neuropil of all examined anatomical areas of the brain, most prominently in axons. On the contrary, SYNCRIP had cytoplasmic somatodendritic pattern of expression, which was most prominent in the hippocampus and cerebellum. In the striatum, hippocampus and cerebral cortex SYT7 and SYNCRIP immunofluorescent signals were mutually excluded, thus diminishing the probability for their physiological interaction. In somata of Purkinje neurons in the cerebellar cortex, both SYT7 and SYNCRIP were expressed and partially co-localized suggesting possible functional connection between SYT7 and SYNCRIP proteins in Purkinje neurons.

Developmental Injury to the Cerebellar Cortex Following Hydroxyurea Treatment in Early Postnatal Life: An Immunohistochemical and Electron Microscopic Study

Postnatal development of the cerebellar cortex was studied in rats administered with a single dose (2 mg/g) of the cytotoxic agent hydroxyurea (HU) on postnatal day (P) 9 and collected at appropriate times ranging from 6 h to 45 days. Quantification of several parameters such as the density of pyknotic, mitotic, BrdU-positive, and vimentin-stained cells revealed that HU compromises the survival of the external granular layer (EGL) cells. Moreover, vimentin immunocytochemistry revealed overexpression and thicker immunoreactive glial processes in HU-treated rats. On the other hand, we also show that HU leads to the activation of apoptotic cellular events, resulting in a substantial number of dying EGL cells, as revealed by TUNEL staining and at the electron microscope level. Additionally, we quantified several features of the cerebellar cortex of rats exposed to HU in early postnatal life and collected in adulthood. Data analysis indicated that the analyzed parameters were less pronounced in rats administered with this agent. Moreover, we observed several alterations in the cerebellar cortex cytoarchitecture of rats injected with HU. Anomalies included ectopic placement of Purkinje cells and abnormities in the dendritic arbor of these macroneurons. Ectopic granule cells were also found in the molecular layer. These findings provide a clue for investigating the mechanisms of HU-induced toxicity during the development of the central nervous system. Our results also suggest that it is essential to avoid underestimating the adverse effects of this hydroxylated analog of urea when administered during early postnatal life.

Bergmann Glia, Long-Term Depression, and Autism Spectrum Disorder

Bergmann glia (BG), a specific type of radial astrocytes in the cerebellum, play a variety of vital functions in the development of this structure. However, the possible role of BG in the development of abnormalities observed in individuals with autism spectrum disorder (ASD) seems to be underestimated. One of the most consistent findings observed in ASD patients is loss of Purkinje cells (PCs). Such a defect may be caused by dysregulation of glutamate homeostasis, which is maintained mainly by BG. Moreover, these glial cells are involved in long-term depression (LTD), a form of plasticity which can additionally subserve neuroprotective functions. The aim of presented review is to summarize the current knowledge about interactions which occur between PC and BG, with special emphasis on those which are relevant to the survival and proper functioning of cerebellar neurons.