Serotonin fiber innervation of cerebellar cell suspensions intraparenchymally grafted to the cerebellum of pcd mutant mice

The Discovery of the Monoaminergic Innervation of the Cerebellum: Convergence of Divergent and Point-to-Point Systems

This Cerebellar Classic highlights the landmark discovery of the innervation of the cerebellar cortex and cerebellar nuclei by noradrenergic and serotoninergic axons emanating, respectively, from the locus coeruleus and the raphé nuclei. Since then, modulation of the activity of cerebellar neurons by the monoamine systems has been studied extensively, as well as their reorganization and modifications during development, plasticity, and disease. The discovery of noradrenergic and serotoninergic innervation of the cerebellum has been a crucial step in understanding the neurochemical relationships between brainstem nuclei and the cerebellum, and the attempts to treat cerebellar ataxias pharmacologically. The large neurochemical repertoire of the cerebellum represents one of the complexities and challenges in the modern appraisal of cerebellar disorders.

The AGTPBP1 gene in neurobiology

The function of the Agtpbp1 gene has mainly been delineated by studying Agtpbp1pcd (pcd) mutant mice, characterized by losses in cerebellar Purkinje and granule cells along with degeneration of retinal photoreceptors, mitral cells of the olfactory bulb, thalamic neurons, and alpha-motoneurons. As a result of cerebellar degeneration, cerebellar GABA and glutamate concentrations in Agtpbp1pcd mutants decreased while monoamine concentrations increased. The salient behavioral phenotypes include cerebellar ataxia, a loss in motor coordination, and cognitive deficits. Similar neuropathogical and behavioral profiles have been described in childhood-onset human subjects with biallelic variants of AGTPBP1, including cerebellar ataxia and hypotonia.

The Childhood-Onset Neurodegeneration with Cerebellar Atrophy (CONDCA) Disease Caused by AGTPBP1 Gene Mutations: The Purkinje Cell Degeneration Mouse as an Animal Model for the Study of this Human Disease

Recent reports have identified rare, biallelic damaging variants of the AGTPBP1 gene that cause a novel and documented human disease known as childhood-onset neurodegeneration with cerebellar atrophy (CONDCA), linking loss of function of the AGTPBP1 protein to human neurodegenerative diseases. CONDCA patients exhibit progressive cognitive decline, ataxia, hypotonia or muscle weakness among other clinical features that may be fatal. Loss of AGTPBP1 in humans recapitulates the neurodegenerative course reported in a well-characterised murine animal model harbouring loss-of-function mutations in the AGTPBP1 gene. In particular, in the Purkinje cell degeneration (pcd) mouse model, mutations in AGTPBP1 lead to early cerebellar ataxia, which correlates with the massive loss of cerebellar Purkinje cells. In addition, neurodegeneration in the olfactory bulb, retina, thalamus and spinal cord were also reported. In addition to neurodegeneration, pcd mice show behavioural deficits such as cognitive decline. Here, we provide an overview of what is currently known about the structure and functional role of AGTPBP1 and discuss the various alterations in AGTPBP1 that cause neurodegeneration in the pcd mutant mouse and humans with CONDCA. The sequence of neuropathological events that occur in pcd mice and the mechanisms governing these neurodegenerative processes are also reported. Finally, we describe the therapeutic strategies that were applied in pcd mice and focus on the potential usefulness of pcd mice as a promising model for the development of new therapeutic strategies for clinical trials in humans, which may offer potential beneficial options for patients with AGTPBP1 mutation-related CONDCA.

Organization and Histochemical Phenotype of Human Fetal Cerebellar Cells following Transplantation into the Cerebellum of Nude Mice

Previous rodent studies have demonstrated the capacity of cerebellar transplants to organize into trilaminar cell layers typically observed in the normal cerebellum. In Purkinje Cell (PC)-deficient animals, PCs will migrate into the host and form synaptic connections. Recently, fetal cerebellar grafts transplanted into the Purkinje cell degeneration (pcd) mutant mouse were shown to result in an improvement of motor behaviors. These studies indicate the potential therapeutic use of neural transplantation in patients with cerebellar degeneration. In the present study, human fetal cerebellar tissue (8.5 wk postconception) was dissociated and transplanted into the normal cerebellum of nude mice. Six months following transplantation, histological analysis revealed donor cells in recipient mice. Immunostaining for the 28 kDa calcium-binding protein (calbindin) revealed the presence of donor PCs that were organized in discrete cellular layers within the transplant neuropil. In most cases the dendritic processes were oriented in a planar fashion perpendicular to the transplant cell layer. Human neurofilament immunostaining revealed bundles of donor fibers within the core of the transplant and/or at the periphery. These bundles were found to be calbindin positive (PC fibers). Three animals provided evidence of donor PC axon growth ventrally into host white matter, and in one case, this ventral migration reached the deep cerebellar nuclei. Most notable was the development of a pronounced folia-like organization by the implanted cell suspensions. Glial processes within the grafts were aligned perpendicular to the long axis of the transplant folia. These results demonstrate the capacity of human fetal cerebellar cell suspension to reorganize into cell layers typical of the normal cerebellum following transplantation into the rodent cerebellum, and develop an organotypic folia-like organization.

Amelioration of the Behavioral Phenotype in Genetically Ataxic Mice through Bilateral Intracerebellar Grafting of Fetal Purkinje Cells

We have previously applied neural grafting to “Purkinje cell degeneration” mutant mice (gene symbol pcd, mouse chromosome 13), a model of recessively inherited cerebello-olivary atrophy, to create appropriate interactions between wild-type and mutant cells in elucidating gene effects on the involved neuron populations and to address issues of the structural integration of donor Purkinje cells into the disrupted cerebellar loop. Behaviorally, pcd homozygotes manifest ataxic signs beginning at 3-4 wk of age. The functional effects of cerebellar transplants on motor performance have long remained an open question. The aim of the present study was to determine the recovery of motor responses in pcd mutants in a battery of behavioral tasks after bilateral transplantation of cerebellar cell suspensions (prepared from wild-type mice) into the parenchyma of the deep cerebellar nuclei of the hosts, according to a protocol that emphasizes the reconstruction of the missing inhibitory cortico-nuclear projection. With this approach, the denervated deep nuclei of the host receive a new Purkinje axonal innervation; further, most transplanted Purkinje cells end up occupying cortical localities anyway and display a correct dendritic tree orientation toward the pia. Motor coordination and fatigue resistance were assessed in a rotarod treadmill apparatus, a behavioral paradigm useful in studying various brain abiotrophies and treatments, including developmental perturbations of the cerebellar cytoarchitecture. Locomotor activity was quantified by the number of squares mice crossed as they moved about in an open-field matrix. Grafted pcd mice performed significantly better than sham-operated mutants in both of these tasks. Moreover, graft-recipient mice were able to sustain their abdomen above the floor on their limbs during movement, contrasting to the typical lowered, widened stance of sham-operated pcd mutants. These findings clearly demonstrate that bilateral transplants of fetal Purkinje cells have functional effects on motor performance in the pcd model of hereditary cerebellar ataxia.

Association and colocalization of G protein alpha subunits and Purkinje cell protein 2 (Pcp2) in mammalian cerebellum

Previously, we have demonstrated a novel interaction between Galpha(o) protein and Purkinje cell protein-2 (Pcp2, also known as L7) in vitro and in transfected cells (Luo and Denker [1999] J. Biol. Chem. 274:10685-10688). Pcp2 is uniquely expressed in cerebellar Purkinje cells and in retinal bipolar neurons, and it may function as a cell-type specific modulator for G protein-mediated cell signaling. This interaction has been further evaluated in the present studies. Coimmunoprecipitation experiments reveal that Pcp2 associates with Galpha(o) in vivo in mouse cerebellum and eye extract. Pcp2 also associate with Galpha(i2) in the cerebellum. No detectable associations of Pcp2 with Galpha(z) and Galpha(q) subunits are observed. The association of Galpha(o) and Pcp2 is detected at postnatal day 1 (P1), and the association remains stable from day 3 (P3) until adulthood. Further, immunofluorescent double labeling and confocal microscopy suggest that Pcp2 and Galpha(o) are colocalized in the distal processes of cerebellar Purkinje cells including axonal endings and dendritic spines. Taken together, these findings indicate colocalization and association of Galpha(o) and Pcp2 in cerebellum and suggest a functional role in regions of synaptic activity.

Cerebellar Grafts Partially Reverse Amino Acid Receptor Changes Observed in the Cerebellum of Mice with Hereditary Ataxia: Quantitative Autoradiographic Studies

We used quantitative autoradiography of [3H]CNQX (200 nM), [3H]muscimol (13 nM), and [3H]flunitrazepam (10 nM) binding to study the distribution of non-NMDA and GABAA receptors in the cerebellum of pcd mutant mice with unilateral cerebellar grafts. Nonspecific binding was determined by incubation with 1 mM Glu, 200 μM GABA, or 1 μM clonazepam, respectively. Saturation parameters were defined in wild-type and mutant cerebella. In mutants, non-NMDA receptors were reduced by 38% in the molecular layer and by 47% in the granule cell layer. The reduction of non-NMDA receptors in the pcd cerebellar cortex supports their localization on Purkinje cells. [3H] CNQX binding sites were visualized at higher density in grafts that had migrated to the cerebellar cortex of the hosts (4.1 and 11.0 pmol/mg protein, respectively, at 23 and 37 days after grafting) than in grafts arrested intraparen-chymally (2.6 and 6.2 pmol/mg protein, respectively, at 23 and 37 days after grafting). The pattern of expression of non-NMDA receptors in cortical vs. parenchymal grafts suggests a possible regulation of their levels by transacting elements from host parallel fibers. GABAA binding levels in the grafts for both ligands used were similar to normal molecular layer. Binding was increased in the deep cerebellar nuclei of pcd mutants: the increase in [3H]muscimol binding over normal was 215% and the increase in [3H]flunitrazepam binding was 89%. Such increases in the pcd deep cerebellar nuclei may reflect a denervation-induced supersensitivity subsequent to the loss of Purkinje axon terminal innervation. In the deep nuclei of pcd mutants with unilateral cerebellar grafts, [3H]muscimol binding was 31% lower in the grafted side than in the contralateral nongrafted side at 37 days after transplantation; [3H]fluni-trazepam binding was also lower in the grafted side by 15% compared to the nongrafted side. Such changes in GABAA receptors suggest a significant, albeit partial, normalizing trend of cerebellar grafts on the state of postsynaptic supersensitive receptors in the host cerebellar nuclei.

Intrinsic organization and monoaminergic innervation of the suprachiasmatic nucleus transplanted to adult rats. A light- and electron-microscopic study

Light- and electron-microscopic immunocytochemistry was used to investigate grafts of foetal hypothalamic tissue implanted close to the site of the suprachiasmatic nucleus in adult rats with bilateral surgical ablation of this nucleus. The transplants contained vasoactive intestinal peptide and vasopressin cell clusters, which have previously been shown to characterize functional suprachiasmatic nucleus grafts. Vasoactive intestinal peptide and vasopressin neurons presented synaptic features that have not been described in the native suprachiasmatic nucleus. More specifically, their terminals within the graft were involved in 'double' synapses with separate unlabelled dendrites. Moreover, in dually stained sections, an unexpected synaptic investment of vasoactive intestinal peptide neurons by vasopressin endings was detected, which revealed reversed vasoactive intestinal peptide/vasopressin interactions compared to those described in the native nucleus. These observations could reflect some immature features of the grafted neurons. Ultrastructural relationships of monoaminergic fibres arising from host and/or intragraft neurons were also examined. Within the engrafted suprachiasmatic nucleus, tyrosine hydroxylase-labelled fibres, which probably belonged to cografted dopaminergic neurons, showed normal patterns of distribution and synaptic connections, with no preferential relationships with vasoactive intestinal peptide or vasopressin neurons. Serotoninergic axons arborized within transplants but, in agreement with previous data showing an inhibitory influence of the suprachiasmatic nucleus on ingrowing serotoninergic fibres, they had no predilection for the area corresponding to that nucleus. In spite of their relative scarcity, serotoninergic fibres within the engrafted suprachiasmatic nucleus showed an almost normal synaptic incidence, but synapses were not predominantly shared with the vasoactive intestinal peptide neurons, known to be their major targets in the native nucleus. This may contribute not only to the failure of functional grafts to synchronize with environmental conditions, but also to the inability of transplants to restore hormonal rhythms such as estrous cyclicity.

Amelioration of the behavioral phenotype in genetically ataxic mice through bilateral intracerebellar grafting of fetal Purkinje cells

We have previously applied neural grafting to "Purkinje cell degeneration" mutant mice (gene symbol pcd, mouse chromosome 13), a model of recessively inherited cerebello-olivary atrophy, to create appropriate interactions between wild-type and mutant cells in elucidating gene effects on the involved neuron populations and to address issues of the structural integration of donor Purkinje cells into the disrupted cerebellar loop. Behaviorally, pcd homozygotes manifest ataxic signs beginning at 3-4 wk of age. The functional effects of cerebellar transplants on motor performance have long remained an open question. The aim of the present study was to determine the recovery of motor responses in pcd mutants in a battery of behavioral tasks after bilateral transplantation of cerebellar cell suspensions (prepared from wild-type mice) into the parenchyma of the deep cerebellar nuclei of the hosts, according to a protocol that emphasizes the reconstruction of the missing inhibitory cortico-nuclear projection. With this approach, the denervated deep nuclei of the host receive a new Purkinje axonal innervation; further, most transplanted Purkinje cells end up occupying cortical localities anyway and display a correct dendritic tree orientation toward the pia. Motor coordination and fatigue resistance were assessed in a rotarod treadmill apparatus, a behavioral paradigm useful in studying various brain abiotrophies and treatments, including developmental perturbations of the cerebellar cytoarchitecture. Locomotor activity was quantified by the number of squares mice crossed as they moved about in an open-field matrix. Grafted pcd mice performed significantly better than sham-operated mutants in both of these tasks. Moreover, graft-recipient mice were able to sustain their abdomen above the floor on their limbs during movement, contrasting to the typical lowered, widened stance of sham-operated pcd mutants. These findings clearly demonstrate that bilateral transplants of fetal Purkinje cells have functional effects on motor performance in the pcd model of hereditary cerebellar ataxia.

The cerebellar model of neural grafting: structural integration and functional recovery

A synopsis is presented of the recent history of cerebellar tissue transplantation over the past 25 years. The properties of growth and differentiation of cerebellar grafts placed intraocularly or intracranially are reviewed, as well as the interaction of heterotopic and orthotopic grafts with the host brain. Particular emphasis is placed on the use of ataxic mouse mutants as recipients of donor cerebellar tissue for the correction of their structural deficits and the functional recovery of behavioural responses.

Grafted cerebellar cells in a mouse model of hereditary ataxia express IGF-I system genes and partially restore behavioral function

Fetal grafts of normal cerebellar tissue were implanted into the cerebellum of Purkinje cell degeneration mutant mice (pcd/pcd), a model of adult-onset recessively inherited cerebello-olivary atrophy, in an attempt at correcting their cellular and motor impairment. Donor cerebellar cells engrafted in the appropriate sites, as evidenced by the pattern of expression of insulin-like growth factor-I (IGF-I) system genes. Bilateral cerebellar grafts led to an improvement of motor behaviors in balance rod tests and in the open field, providing evidence for functional integration into the atrophic mouse cerebellum and underscoring the potential of neural transplantation for counteracting the human cerebellar ataxias.

Host serotonin axons innervate intrastriatal ventral mesencephalic grafts after implantation in newborn rats

This study investigated the potential of immature and adult serotonin (5-HT) neurons for axonal growth into intrastriatal grafts of ventral mesencephalic tissue. Implantation of dissociated fetal (embryonic days 14-15) ventral mesencephalic tissue was carried out in immature [postnatal days (P) 5-14] and adult rat neostriatum. The brains were processed 2-6 months later for dopamine and 5-HT immunocytochemistry. A few grafts implanted into adult and P7 recipients contained small numbers of cotransplanted 5-HT cell bodies. These also displayed a rich network of 5-HT axons, even in adult rats prelesioned with 5,7-dihydroxytryptamine, indicating the graft origin of these axons. All other grafts were totally devoid of 5-HT cell bodies. After implantation in adults, such grafts contained rare 5-HT axons. In contrast, in P5-P7 recipients, they displayed many 5-HT fibres, which were uniformly distributed. Such was no longer the case after implantation in P14 recipients, which showed minimal 5-HT innervation, as in adult recipients. Processing of naïve rat brain at different ages for 5-HT immunocytochemistry showed that 5-HT axons were still clearly less numerous in the neostriatum at P21 than in adults, whereas in the substantia nigra the 5-HT innervation developed more rapidly and was comparable, at P21, to that of adults. It was concluded that 5-HT axons are able to grow into ventral mesencephalic grafts, but mainly at the fetal stage and with decreasing capacity after birth.(ABSTRACT TRUNCATED AT 250 WORDS)