Cerebellin is a postsynaptic neuropeptide

Conformational Plasticity in the Transsynaptic Neurexin-Cerebellin-Glutamate Receptor Adhesion Complex

Synaptic specificity is a defining property of neural networks. In the cerebellum, synapses between parallel fiber neurons and Purkinje cells are specified by the simultaneous interactions of secreted protein cerebellin with pre-synaptic neurexin and post-synaptic delta-type glutamate receptors (GluD). Here, we determined the crystal structures of the trimeric C1q-like domain of rat cerebellin-1, and the first complete ectodomain of a GluD, rat GluD2. Cerebellin binds to the LNS6 domain of α- and β-neurexin-1 through a high-affinity interaction that involves its highly flexible N-terminal domain. In contrast, we show that the interaction of cerebellin with isolated GluD2 ectodomain is low affinity, which is not simply an outcome of lost avidity when compared with binding with a tetrameric full-length receptor. Rather, high-affinity capture of cerebellin by post-synaptic terminals is likely controlled by long-distance regulation within this transsynaptic complex. Altogether, our results suggest unusual conformational flexibility within all components of the complex.

All the way from the cortex: a review of auditory corticosubcollicular pathways

Enrico Mugnaini has devoted part of his long and fruitful neuroscientific career to investigating the structural similarities between the cerebellar cortex and one of the first relay stations of the mammalian auditory pathway: the dorsal cochlear nucleus. The hypothesis of the cerebellar-like nature of the superficial layers of the dorsal cochlear nucleus received definitive support with the discovery and extensive characterization in his laboratory of unipolar brush cells, a neuron type unique to certain regions of the cerebellar cortex and to the granule cell domains of the cochlear nuclei. Paradoxically, a different line of research carried out in his laboratory revealed that, unlike the mammalian cerebellar cortex, the dorsal cochlear nucleus receives direct projections from the cerebral cortex, a fact that constitutes one of the main differences between the cerebellum and the dorsal cochlear nucleus. In an article published in 1995, Mugnaini's group described in detail the novel direct projections from the rat auditory neocortex to various subcollicular auditory centers, including the nucleus sagulum, the paralemniscal regions, the superior olivary complex, and the cochlear nuclei (Feliciano et al., Auditory Neuroscience 1995; 1:287-308). This review gives Enrico Mugnaini credit for his seminal contribution to the knowledge of auditory corticosubcollicular projections and summarizes how this growing field has evolved in the last 20 years.

Selective degeneration of synapses in the dorsal cochlear nucleus of chinchilla following acoustic trauma and effects of antioxidant treatment

The purpose of this study was to reveal synaptic plasticity within the dorsal cochlear nucleus (DCN) as a result of noise trauma and to determine whether effective antioxidant protection to the cochlea can also impact plasticity changes in the DCN. Expression of synapse activity markers (synaptophysin and precerebellin) and ultrastructure of synapses were examined in the DCN of chinchilla 10 days after a 105 dB SPL octave-band noise (centered at 4 kHz, 6 h) exposure. One group of chinchilla was treated with a combination of antioxidants (4-hydroxy phenyl N-tert-butylnitrone, N-acetyl-l-cysteine and acetyl-l-carnitine) beginning 4 h after noise exposure. Down-regulated synaptophysin and precerebellin expression, as well as selective degeneration of nerve terminals surrounding cartwheel cells and their primary dendrites were found in the fusiform soma layer in the middle region of the DCN of the noise exposure group. Antioxidant treatment significantly reduced synaptic plasticity changes surrounding cartwheel cells. Results of this study provide further evidence of acoustic trauma-induced neural plasticity in the DCN and suggest that loss of input to cartwheel cells may be an important factor contributing to the emergence of hyperactivity in the DCN after noise exposure. Results further suggest that early antioxidant treatment for acoustic trauma not only rescues cochlear hair cells, but also has impact on central auditory structures.

Synapse formation and maintenance by C1q family proteins: a new class of secreted synapse organizers

Several C1q family members, especially the Cbln and C1q-like subfamilies, are highly and predominantly expressed in the central nervous system. Cbln1, a member of the Cbln subfamily, plays two unique roles at parallel fiber (PF)-Purkinje cell synapses in the cerebellum: the formation and stabilization of synaptic contact, and the control of functional synaptic plasticity by regulating the postsynaptic endocytotic pathway. The delta2 glutamate receptor (GluD2), which is predominantly expressed in Purkinje cells, plays similar critical roles in the cerebellum. In addition, viral expression of GluD2 or the application of recombinant Cbln1 induces PF-Purkinje cell synaptogenesis in vitro and in vivo. Antigen-unmasking methods were necessary to reveal the immunoreactivities for endogenous Cbln1 and GluD2 at the synaptic junction of PF synapses. We propose that Cbln1 and GluD2 are located at the synaptic cleft, where various proteins undergo intricate molecular interactions with each other, and serve as a bidirectional synaptic organizer.

Cbln1 accumulates and colocalizes with Cbln3 and GluRdelta2 at parallel fiber-Purkinje cell synapses in the mouse cerebellum

Cbln1 (a.k.a. precerebellin) is secreted from cerebellar granule cells as homohexamer or in heteromeric complexes with Cbln3. Cbln1 plays crucial roles in regulating morphological integrity of parallel fiber (PF)-Purkinje cell (PC) synapses and synaptic plasticity. Cbln1-knockout mice display severe cerebellar phenotypes that are essentially indistinguishable from those in glutamate receptor GluRdelta2-null mice, and include severe reduction in the number of PF-PC synapses and loss of long-term depression of synaptic transmission. To understand better the relationship between Cbln1, Cbln3 and GluRdelta2, we performed light and electron microscopic immunohistochemical analyses using highly specific antibodies and antigen-exposing methods, i.e. pepsin pretreatment for light microscopy and postembedding immunogold for electron microscopy. In conventional immunohistochemistry, Cbln1 was preferentially associated with non-terminal portions of PF axons in the molecular layer but rarely overlapped with Cbln3. In contrast, antigen-exposing methods not only greatly intensified Cbln1 immunoreactivity in the molecular layer, but also revealed its high accumulation in the synaptic cleft of PF-PC synapses. No such synaptic accumulation was evident at other PC synapses. Furthermore, Cbln1 now came to overlap almost completely with Cbln3 and GluRdelta2 at PF-PC synapses. Therefore, the convergence of all three molecules provides the anatomical basis for a common signaling pathway regulating circuit development and synaptic plasticity in the cerebellum.

Characterization of trans-neuronal trafficking of Cbln1

Cbln1, a glycoprotein secreted from granule cells and GluRdelta2 in the postsynaptic densities of Purkinje cells are components of an incompletely understood pathway essential for integrity and plasticity of parallel fiber-Purkinje cell synapses. We show that Cbln1 undergoes anterograde transport from granule cells to Purkinje cells and Bergmann glia, and enters the endolysosomal trafficking system, raising the possibility that Cbln1 exerts its activity on or within Purkinje cells and Bergmann glia. Cbln1 is absent in Purkinje cells and Bergmann glia of GluRdelta2-null mice, suggesting a mechanistic convergence on Cbln1 trafficking. Ectopic expression of Cbln1 in Purkinje cells of L7-cbln1 transgenic mice reveals Cbln1 undergoes anterograde and retrograde trans-neuronal trafficking even across synapses that lack GluRDelta2, indicating that it is not universally essential for Cbln1 transport. The L7-cbln1 transgene also ameliorates the locomotor deficits of cbln1-null mice, indicating that the presence and/or release of Cbln1 from the postsynaptic neuron has functional consequences.

New (but old) molecules regulating synapse integrity and plasticity: Cbln1 and the delta2 glutamate receptor

The delta2 glutamate receptor (GluRdelta2) is predominantly expressed in cerebellar Purkinje cells and plays crucial roles in cerebellar functions: GluRdelta2-null mice display ataxia and impaired motor learning. Interestingly, the contact state of synapses between parallel fibers (PFs) and Purkinje cells is specifically and severely affected, and the number of normal PF synapses is markedly reduced in GluRdelta2-null Purkinje cells. Furthermore, long-term depression at PF-Purkinje cell synapses is abrogated. Cbln1, a member of the C1q/tumor necrosis factor (TNF) superfamily, is predominantly expressed and released from cerebellar granule cells. Unexpectedly, the behavioral, physiological and anatomical phenotypes of cbln1-null mice precisely mimic those of GluRdelta2-null mice. Thus, we propose that Cbln1, which is released from granule cells, and GluRdelta2, which is predominantly expressed in Purkinje cells, are involved in a common signaling pathway crucial for synapse formation/maintenance and plasticity in the cerebellum. Since molecules related to Cbln1 are expressed in various brain regions other than the cerebellum, other C1q/TNF superfamily proteins may also regulate various aspects of synapses in the CNS. Therefore, an understanding of the signaling mechanisms underlying Cbln1 and GluRdelta2 in the cerebellum will provide new insights into the roles of C1q/TNF superfamily proteins as new cytokines that regulate normal and abnormal brain functions.

A microarray analysis of the XX Wnt4 mutant gonad targeted at the identification of genes involved in testis vascular differentiation

One of the earliest morphological changes during testicular differentiation is the establishment of an XY specific vasculature. The testis vascular system is derived from mesonephric endothelial cells that migrate into the gonad. In the XX gonad, mesonephric cell migration and testis vascular development are inhibited by WNT4 signaling. In Wnt4 mutant XX gonads, endothelial cells migrate from the mesonephros and form a male-like coelomic vessel. Interestingly, this process occurs in the absence of other obvious features of testis differentiation, suggesting that Wnt4 specifically inhibits XY vascular development. Consequently, the XX Wnt4 mutant mice presented an opportunity to focus a gene expression screen on the processes of mesonephric cell migration and testicular vascular development. We compared differences in gene expression between XY Wnt4+/+ and XX Wnt4+/+ gonads and between XX Wnt4-/- and XX Wnt4+/+ gonads to identify sets of genes similarly upregulated in wildtype XY gonads and XX mutant gonads or upregulated in XX gonads as compared to XY gonads and XX mutant gonads. We show that several genes identified in the first set are expressed in vascular domains, and have predicted functions related to cell migration or vascular development. However, the expression patterns and known functions of other genes are not consistent with roles in these processes. This screen has identified candidates for regulation of sex specific vascular development, and has implicated a role for WNT4 signaling in the development of Sertoli and germ cell lineages not immediately obvious from previous phenotypic analyses.

Characterization of a transneuronal cytokine family Cbln − regulation of secretion by heteromeric assembly

Cbln1, a member of the C1q and tumor necrosis factor superfamily, plays crucial roles as a cerebellar granule cell-derived transneuronal regulator of synapse integrity and plasticity in Purkinje cells. Although other Cbln family members, Cbln2-Cbln4, have distinct spatial and temporal patterns of expression throughout the CNS, their biochemical and biological properties have remained largely uncharacterized. Here, we demonstrated that in mammalian heterologous cells, Cbln2 and Cbln4 were secreted as N-linked glycoproteins, like Cbln1. In contrast, despite the presence of a functional signal sequence, Cbln3 was not secreted when expressed alone but was retained in the endoplasmic reticulum (ER) or cis-Golgi because of its N-terminal domain. All members of the Cbln family formed not only homomeric but also heteromeric complexes with each other in vitro. Accordingly, when Cbln1 and Cbln3 were co-expressed in heterologous cells, a proportion of the Cbln1 proteins was retained in the ER or cis-Golgi; conversely, some Cbln3 proteins were secreted together with Cbln1. Similarly, in wild-type granule cells expressing Cbln1 and Cbln3, Cbln3 proteins were partially secreted and reached postsynaptic sites on Purkinje cell dendrites, while Cbln3 was almost completely degraded in cbln1-null granule cells. These results indicate that like Cbln1, Cbln2 and Cbln4 may also serve as transneuronal regulators of synaptic functions in various brain regions. Furthermore, heteromer formation between Cbln1 and Cbln3 in cerebellar granule cells may modulate each other's trafficking and signaling pathways; similarly, heteromerization of other Cbln family proteins may also have biological significance in other neurons.

Distinct expression of Cbln family mRNAs in developing and adult mouse brains

Cbln1 belongs to the C1q and tumour necrosis factor superfamily, and plays crucial roles as a cerebellar granule cell-derived transneuronal regulator for synapse integrity and plasticity in Purkinje cells. Although Cbln2-Cbln4 are also expressed in the brain and could form heteromeric complexes with Cbln1, their precise expressions remain unclear. Here, we investigated gene expression of the Cbln family in developing and adult C57BL mouse brains by reverse transcriptase-polymerase chain reaction (RT-PCR), Northern blot, and high-resolution in situ hybridization (ISH) analyses. In the adult brain, spatial patterns of mRNA expression were highly differential depending on Cbln subtypes. Notably, particularly high levels of Cbln mRNAs were expressed in some nuclei and neurons, whereas their postsynaptic targets often lacked or were low for any Cbln mRNAs, as seen for cerebellar granule cells/Purkinje cells, entorhinal cortex/hippocampus, intralaminar group of thalamic nuclei/caudate-putamen, and dorsal nucleus of the lateral lemniscus/central nucleus of the inferior colliculus. In the developing brain, Cbln1, 2, and 4 mRNAs appeared as early as embryonic day 10-13, and exhibited transient up-regulation during the late embryonic and neonatal periods. For example, Cbln2 mRNA was expressed in the cortical plate of the developing neocortex, displaying a high rostromedial to low caudolateral gradient. In contrast, Cbln3 mRNA was selective to cerebellar granule cells throughout development, and its onset was as late as postnatal day 7-10. These results will provide a molecular-anatomical basis for future studies that characterize roles played by the Cbln family.

A microarray analysis of the XX Wnt4 mutant gonad targeted at the identification of genes involved in testis vascular differentiation

One of the earliest morphological changes during testicular differentiation is the establishment of an XY specific vasculature. The testis vascular system is derived from mesonephric endothelial cells that migrate into the gonad. In the XX gonad, mesonephric cell migration and testis vascular development are inhibited by WNT4 signaling. In Wnt4 mutant XX gonads, endothelial cells migrate from the mesonephros and form a male-like coelomic vessel. Interestingly, this process occurs in the absence of other obvious features of testis differentiation, suggesting that Wnt4 specifically inhibits XY vascular development. Consequently, the XX Wnt4 mutant mice presented an opportunity to focus a gene expression screen on the processes of mesonephric cell migration and testicular vascular development. We compared differences in gene expression between XY Wnt4+/+ and XX Wnt4+/+ gonads and between XX Wnt4+/+ and XX Wnt4+/+ gonads to identify sets of genes similarly upregulated in wildtype XY gonads and XX mutant gonads or upregulated in XX gonads as compared to XY gonads and XX mutant gonads. We show that several genes identified in the first set are expressed in vascular domains, and have predicted functions related to cell migration or vascular development. However, the expression patterns and known functions of other genes are not consistent with roles in these processes. This screen has identified candidates for regulation of sex specific vascular development, and has implicated a role for WNT4 signaling in the development of Sertoli and germ cell lineages not immediately obvious from previous phenotypic analyses.

Cbln1 is essential for synaptic integrity and plasticity in the cerebellum

Cbln1 is a cerebellum-specific protein of previously unknown function that is structurally related to the C1q and tumor necrosis factor families of proteins. We show that Cbln1 is a glycoprotein secreted from cerebellar granule cells that is essential for three processes in cerebellar Purkinje cells: the matching and maintenance of pre- and postsynaptic elements at parallel fiber-Purkinje cell synapses, the establishment of the proper pattern of climbing fiber-Purkinje cell innervation, and induction of long-term depression at parallel fiber-Purkinje cell synapses. Notably, the phenotype of cbln1-null mice mimics loss-of-function mutations in the orphan glutamate receptor, GluR delta2, a gene selectively expressed in Purkinje neurons. Therefore, Cbln1 secreted from presynaptic granule cells may be a component of a transneuronal signaling pathway that controls synaptic structure and plasticity.

The structure and proteolytic processing of Cbln1 complexes

The hexadecapeptide cerebellin is present in the brains of many vertebrate species and is derived from a larger protein, Cbln1 (cerebellin 1 precursor protein). Although cerebellin has features of a neuropeptide, Cbln1 belongs to the C1q/tumor necrosis factor superfamily of secreted proteins, suggesting that it is the biologically active molecule and the proteolytic events that generate cerebellin serve another function. Therefore, we assessed whether Cbln1 undergoes proteolytic processing and determined what consequences the cleavage events necessary to produce cerebellin have on the structure of Cbln1. Substantial degradation of Cbln1 was evident in the synaptic compartment of cerebellum and lysates of cultured cerebellar neurons and cells transfected with Cbln1 expression vectors. However, only uncleaved Cbln1 containing the cerebellin motif was released and assembled into hexameric complexes. Using yeast two hybrid and mammalian expression systems we show that the cleavages required to produce cerebellin influence the subunit stoichiometry of Cbln1 complexes. Cleavage at the N-terminus of the cerebellin sequence in Cbln1 yields trimeric complexes by separating the trimer-mediating C-terminal C1q domain from conserved N-terminal cysteine residues that mediate higher order oligomerization. Cleavage at the C-terminus of the cerebellin motif disrupts the C1q domain and abolishes subunit interactions. Functional implications of these data are discussed.

Subtractive hybridisation screen identifies sexually dimorphic gene expression in the embryonic mouse gonad

The sex of most mammals is determined by the action of SRY. Its presence initiates testis formation resulting in male differentiation, its absence results in ovary formation and female differentiation. We have used suppression subtraction hybridisation between 12.0-12.5 days postcoitum (dpc) mouse testes and ovaries to identify genes that potentially lie within the Sry pathway. Normalised urogenital ridge libraries comprising 8,352 clones were differentially screened with subtracted probes. A total of 272 candidate cDNAs were tested for qualitative differential expression and localisation by whole mount in situ hybridisation; germ cell-dependent or -independent expression was further resolved using busulfan. Fifty-four genes were identified that showed higher expression in the testis than the ovary. One novel gene may be a candidate for interactions with WT1, based on its localisation to Sertoli cells and map position (16q24.3).

Prolonged cerebellin administration inhibits the growth, but enhances steroidogenic capacity of rat adrenal cortex

Cerebellin is a 16-amino acid peptide, that has been previously found to acutely stimulate steroid secretion from rat adrenal cortex in vivo and in vitro. We have investigated the effects of a prolonged cerebellin treatment (daily injections of 15 nmoles/kg for 6 consecutive days) on the growth and secretion of rat adrenal cortex. Cerebellin lowered adrenal weight, and morphometry showed that this was due to the decrease in the volume of each adrenocortical zone exclusively ensuing from the reduction in the number of its parenchymal cells. Cerebellin did not alter plasma concentration of ACTH, but it raised the levels of circulating aldosterone and corticosterone. The conclusion is drawn that cerebellin chronic administration evokes a marked hypoplastic atrophy of rat adrenocortical cells, that is coupled with an enhanced ACTH-independent steroidogenic capacity of the remaining parenchymal cells.

Cerebellin stimulates the secretory activity of the rat adrenal gland: in vitro and in vivo studies

Cerebellin is a 16-aminoacid peptide widely distributed in the central nervous system, where it exerts neuromodulatory functions. Cerebellin is contained in human adrenal medulla, and it has been recently demonstrated that cerebellin elicits catecholamine release by human adrenal in vitro. Aim of the present study was to ascertain whether cerebellin affects adrenal function in the rat. Cerebellin concentration-dependently (from 10(-9)to 10(-7)M) increased norepinephrine (but not epinephrine) and cyclic-AMP production by adrenomedullary tissue in vitro. The norepinephrine response to 10(-7)M cerebellin was blocked by the protein kinase (PK) A inhibitor H-89, but not by the phospholipase C inhibitor U-73122 or the PKC inhibitor calphostin-C. Cerebellin did not affect aldosterone and corticosterone secretion of dispersed zona glomerulosa and zona fasciculata-reticularis adrenocortical cells. Cerebellin concentration-dependently (from 10(-8)to 10(-7)M) enhanced norepinephrine release by in situ perfused rat adrenals. Cerebellin (10(-7)M) also elicited a significant rise in aldosterone and corticosterone output, and this effect was annulled by either the beta1-adrenoceptor antagonist l -alprenolol or H-89. Collectively, the present findings allow us to conclude that cerebellin 1) directly stimulates norepinephrine release via the adenylate cyclase/PKA-dependent signaling pathway; and 2) indirectly enhances adrenocortical secretion in vivo, through a paracrine mechanism involving medullary catecholamine release.

The cerebellar-hypothalamic axis: basic circuits and clinical observations

Experimental studies on a variety of mammals, including primates, have revealed direct and reciprocal connections between the hypothalamus and the cerebellum. Although widespread areas of the hypothalamus project to cerebellum, axons arise primarily from cells in the lateral, posterior, and dorsal hypothalamic areas; the supramammillary, tuberomammillary, and lateral mammillary nuclei; the dorsomedial and ventromedial nuclei; and the periventricular zone. Available evidence suggests that hypothalamocerebellar cortical fibers may terminate in relation to neurons in all layers of the cerebellar cortex. Cerebellohypothalamic axons arise from neurons of all four cerebellar nuclei, pass through the superior cerebellar peduncle, cross in its decussation, and enter the hypothalamus. Some axons recross the midline in caudal areas of the hypothalamus. These fibers terminate primarily in lateral, posterior, and dorsal hypothalamic areas and in the dorsomedial and paraventricular nuclei. Evidence of a cerebellar influence on the visceromotor system is presented in two patients with vascular lesions: one with a small defect in the medial cerebellar nucleus and the other with a larger area of damage involving primarily the globose and emboliform nuclei. Both patients exhibited an abnormal visceromotor response. The second, especially, showed abnormal visceromotor activity concurrent with tremor induced by voluntary movement. These experimental and clinical data suggest that the cerebellum is actively involved in the regulation of visceromotor functions.

Genomic structure and mapping of precerebellin and a precerebellin-related gene

The cerebellum-specific hexadecapeptide, cerebellin, is derived from a larger precursor, precerebellin, that has sequence homology to the complement component C1qB. We report the cloning of the murine homolog of precerebellin, Cbln1, and a closely related gene, Cbln2. Amino acid comparison of Cbln1 with Cbln2 revealed that Cbln2 is 88% identical to the carboxy terminal region of Cbln1. That these are independent genes was confirmed by Southern analysis and genome mapping. Cbln1 was positioned to the central region of mouse chromosome 8, 2.3 cM distal of JunB and 6.0 cM proximal of Mt1, while Cbln2 mapped to the distal end of mouse chromosome 18, 1.7 cM telomeric of Mbp.

Characteristics of labeling of the cerebellar Purkinje neuron by L7 antiserum

Previously, it has been shown by light microscopy that antiserum to the L7 protein labels cerebellar Purkinje neurons. Herein we show by light and electron microscopic immunocytochemistry that all cerebellar Purkinje cells express L7 and that the gene product is distributed to all neuronal compartments, including the nucleus. Possible functional roles for L7, based on its subcellular localization, are discussed. L7 is proposed as an excellent marker molecule for future studies of normal and aberrant cerebellar development.

Cerebellar Purkinje cell markers are expressed in retinal bipolar neurons

Previous studies have been directed at the elucidation of neuron-specific gene expression in the mammalian central nervous system. In particular, we have identified a series of marker molecules that are expressed in cerebellar Purkinje cells with varying degrees of specificity. Here, we show by light microscopic immunocytochemistry and Northern transfer and hybridization that two of these markers, namely, L7 and PEP19, are expressed in the retina of mouse and rabbit, while a third marker, cerebellin, is absent. Light and electron microscopic immunocytochemistry proves that L7-like immunoreactivity is restricted to rod bipolar cells, while PEP 19-like immunoreactivity is distributed in both rod and cone bipolars. PEP19 is also expressed by subsets of amacrine and ganglion cells. The density of PEP19-positive bipolar cells is greater than that of L7-positive bipolar cells, although the density of each is approximately equal in central and peripheral portions of the retina. An antiserum to a fourth Purkinje cell marker, vitamin D-dependent calcium-binding protein-28 kD (CaBP), reveals primarily axonless horizontal cells, but also subsets of rod bipolar, amacrine, and, in the mouse but not in the rabbit, ganglion cells. The processes of immunoreactive cell bodies form discrete bands in the internal plexiform layer, and mixtures of the antisera help distinguish their identity. Thus, these Purkinje cell markers can be used at the electron microscopic level to unravel the extremely complex neuropil of this retinal layer. Furthermore, knowledge of the retinal distribution of this panel of molecules is of general value for future studies of retinal neuronal typology and can serve to map the densities of subsets of bipolar cells throughout the retina. The expression of L7 and PEP19 in bipolar cells and in Purkinje cells suggests a biochemical relationship between these two spatially distant neuronal populations.

Distribution and targets of the cartwheel cell axon in the dorsal cochlear nucleus of the guinea pig

This investigation attempted to determine the mode of distribution and synaptic targets of the cartwheel cell axon in the guinea pig dorsal cochlear nucleus (DCoN). Antiserum against PEP-19, a putative calcium-binding neuropeptide, was employed at the light and electron microscopic levels. We show that in the hind-brain of the guinea pig, cerebellar Purkinje cells and DCoN cartwheel cells are the most densely immunoreactive neurons. The PEP-19 immunoreaction product is localized to all neuronal compartments of these cells. Primary targets of cartwheel cell axons are the DCoN pyramidal cells, the large efferent neurons of layer 2. These neurons receive numerous immunoreactive synaptic boutons on their cell bodies and apical and basal dendritic arbors. A PEP-19-immunoreactive axonal plexus, largely formed by cartwheel cell axons, highlights layer 3, co-extensively with the basal arbors of pyramidal cells. This plexus is oriented predominantly in the transstrial plane of the DCoN, in parallel with the sheet-like basal dendritic arbor of pyramidal neurons and with the isofrequency bands of primary cochlear nerve fibers. PEP-19-positive boutons contain pleomorphic synaptic vesicles and form symmetric synaptic junctions, indicative of inhibitory innervation. In addition, immunoreactive boutons, similar to those synapsing on pyramidal neurons, were observed on the cell bodies and main dendritic trunks of cartwheel neurons, indicating a system of recurrent collaterals. Furthermore, a small number of PEP-19-positive axons of unknown origin reach the caudal rim of the posteroventral cochlear nucleus. Within the territory of distribution of the cartwheel cell axon are the dendrites of at least two other types of DCoN neuron, the vertical cells of Lorente de Nó and the giant cells. These neurons may represent additional targets of the cartwheel cell axon, but this remains to be ascertained with specific methods. Our data demonstrate that the cartwheel neurons modulate the activity of pyramidal neurons and, therefore, play a key role in shaping the output of the DCoN superficial layers.

Precerebellin is a cerebellum-specific protein with similarity to the globular domain of complement C1q B chain

The cerebellum contains a hexadecapeptide, termed cerebellin, that is conserved in sequence from human to chicken. Three independent, overlapping cDNA clones have been isolated from a human cerebellum cDNA library that encode the cerebellin sequence. The longest clone codes for a protein of 193 amino acids that we term precerebellin. This protein has a significant similarity (31.3% identity, 52.2% similarity) to the globular (non-collagen-like) region of the B chain of human complement component C1q. The region of relatedness extends over approximately 145 amino acids located in the carboxyl terminus of both proteins. Unlike C1q B chain, no collagen-like motifs are present in the amino-terminal regions of precerebellin. The amino terminus of precerebellin contains three possible N-linked glycosylation sites. Although hydrophobic amino acids are clustered at the amino terminus, they do not conform to the classical signal-peptide motif, and no other obvious membrane-spanning domains are predicted from the cDNA sequence. The cDNA predicts that the cerebellin peptide is flanked by Val-Arg and Glu-Pro residues. Therefore, cerebellin is not liberated from precerebellin by the classical dibasic amino acid proteolytic-cleavage mechanism seen in many neuropeptide precursors. In Northern (RNA) blots, precerebellin transcripts, with four distinct sizes (1.8, 2.3, 2.7, and 3.0 kilobases), are abundant in cerebellum. These transcripts are present at either very low or undetectable levels in other brain areas and extraneural structures. A similar pattern of cerebellin precursor transcripts are seen in rat, mouse, and human cerebellum. Furthermore, a partial genomic fragment from mouse shows the same bands in Northern blots as the human cDNA clone. During rat development, precerebellin transcripts mirror the level of cerebellin peptide. Low levels of precerebellin mRNA are seen at birth. Levels increase modestly from postpartum day 1 to 8, then increase more dramatically between day 5 and 15, and eventually reach peak values between day 21 and 56. Because cerebellin-like immunoreactivity is associated with Purkinje cell postsynaptic structures, these data raise interesting possibilities concerning the function of the cerebellin precursor in synaptic physiology.

Molecular cloning of rat cerebellin-like protein cDNA which encodes a novel membrane-associated glycoprotein

Rat cerebellin and an apparent metabolite des-Ser1-cerebellin have been shown to be located in cerebellar Purkinje cells and the dorsal cochlear nucleus. A cDNA clone was isolated by screening a rat brain cDNA library using an oligonucleotide corresponding to rat cerebellins. The clone encodes 224 amino acid residues of a glycoprotein a part of whose sequence is virtually the same as that of cerebellins and which contains one putative transmembrane spanning domain. Neither an N-terminal cleavable signal peptide nor dibasic pair specific endopeptidase directly precedes or follows the cerebellin-like sequence. Expression of the cerebellin-like protein gene is developmentally regulated in rat cerebellum and shows tissue specific alternative splicing in the brain, adrenal gland and spleen of the rat. The results of Southern blot indicated the gene to possibly be a member of some gene family. Rat cerebellin-like protein, possibly a precursor of cerebellins, is a novel membrane-associated glycoprotein expressed in nervous, adrenal and immune systems and appears to synaptic functions in the central nervous system.

Developmental expression of polypeptide PEP-19 in cerebellar cell suspensions transplanted into the cerebellum of pcd mutant mice

Cerebellar cell suspensions were prepared from normal mouse embryos and implanted into the cerebellum of Purkinje cell degeneration (pcd) mutant mice, which are characterized by a virtually complete degeneration of Purkinje cells between postnatal day (P) 17 and P45. The expression of immunoreactivity for PEP-19, a developmentally-regulated brain-specific polypeptide, was analyzed in normal mouse cerebellum, as well as in pcd mutants with or without grafts. In the normal cerebellum, PEP-19 immunoreactivity was present in Purkinje cells. In unoperated mutants, 45 days of age or older, Purkinje cells were absent. In grafted pcd mice, numerous PEP-19 immunoreactive, neuroblast-like cells were seen in the graft at 5 days after transplantation. By 9 days, large PEP-19 immunoreactive neurons were found in the host molecular layer; by 17 days after transplantation, such neurons displayed an extensive dendritic tree and resembled differentiated Purkinje cells. The vast majority of PEP-19 immunoreactive cells was located in the molecular layer of the host at 9 days after transplantation and beyond; nonetheless, the same cells extended axonal processes toward the graft, indicating an affinity for co-grafted (possibly deep nuclei) neurons. These results point to the ability of donor Purkinje cells for survival, migration into the host brain and morphological and chemical differentiation following transplantation to the degenerated cerebellar cortex of the recipient mutants.

Effects of the murine mutation 'nervous' on neurons in cerebellum and dorsal cochlear nucleus

'Nervous' mutant mice are presently available on two different genetic background strains which are derived from out-breeding of the original BALB/cGr mutant stock. Light and electron microscopic studies of these mutants demonstrate that cerebellar Purkinje cells and cartwheel neurons of the dorsal cochlear nucleus (DCoN) show similar, albeit not identical, cytoplasmic and mitochondrial alterations in both background strains. In the cerebellar cortex, all Purkinje cell perikarya developed a varying number of enlarged and rounded mitochondria, as previously described. Extensive changes were observed in various components of the mitochondrial matrix. As cellular degeneration proceeded, reduction, fragmentation and dilation of cisterns of endoplasmic reticulum and the Golgi apparatus were evident. Some of the mitochondria underwent a peculiar type of degeneration, i.e. the outer membrane partially or completely dissolved, occasionally accompanied by focal interruptions of the inner membrane. In older adult mutants only 10% of cerebellar Purkinje cells rehained. The few surviving cells displayed varying states, ranging from essentially normal ultrastructure to electron-dense condensation. Many of these cells, in both strains, continued to display greatly enlarged, rounded mitochondrial profiles, indicating a change in the expression of the gene defect resulting from genetic contamination. Criteria for the identification of neuronal cell classes in layers 1 and 2 of murine DCoN were established. Cartwheel neurons in the mutant DCoN presented alterations similar to those observed in cerebellar Purkinje cells. The characteristic mitochondrial anomaly developed and proceeded in cartwheel neurons within a comparable time frame. The vast majority of affected cartwheel cells did not undergo degeneration, however, but continued to possess altered mitochondria into adulthood. The differences between normal and mutant mitochondria in Purkinje and cartwheel were quantified by morphometric analyses. Our findings lend support to the notion of a homology between cerebellar Purkinje cells and DCoN cartwheel cells. These cells represent major elements in two similar spatially related circuits, and share several genetic, structural and neurochemical properties. It is therefore proposed that these two cell populations are derived from closely related precursor cells.

Cerebellin and related postsynaptic peptides in the brain of normal and neurodevelopmentally mutant vertebrates

The rat cerebellum was previously shown to contain two polypeptides, a hexadecapeptide termed cerebellin and an apparent metabolite des-Ser1-cerebellin. The cerebellins have a high degree of sequence homology with residues 625-641 of the polyimmunoglobulin (polyIg)-receptor adjacent to its membrane-spanning domain. Since the cerebellins are localized in Purkinje cells and enriched in synaptosomes, this might indicate that cerebellin is a specific proteolytic cleavage fragment of a synaptic protein involved in the transcytosis of an unknown ligand. Using a specific cerebellin radioimmunoassay described here combined with high-performance liquid chromatography, cerebellin immunoreactivity could be demonstrated in the cerebella of all vertebrates examined from man to chicken. Cerebellin immunoreactivity is localized to Purkinje cells in the rat, mouse, and chicken. Furthermore, cerebellin expression is under developmental regulation in both the chicken and mouse. In addition, neurodevelopmental mutations of mice that eliminate granule cells cause a large deficit in cerebellin levels, suggesting some form of transneuronal regulation.