Cbln1 is essential for interaction-dependent secretion of Cbln3

Cerebellin-neurexin complexes instructing synapse properties

Cerebellins (Cbln1-4) are secreted adaptor proteins that connect presynaptic neurexins (Nrxn1-3) to postsynaptic ligands (GluD1/2 for Cbln1-3 vs. DCC and Neogenin-1 for Cbln4). Classical studies demonstrated that neurexin-Cbln1-GluD2 complexes organize cerebellar parallel-fiber synapses, but the role of cerebellins outside of the cerebellum has only recently been clarified. In synapses of the hippocampal subiculum and prefrontal cortex, Nrxn1-Cbln2-GluD1 complexes strikingly upregulate postsynaptic NMDA-receptors, whereas Nrxn3-Cbln2-GluD1 complexes conversely downregulate postsynaptic AMPA-receptors. At perforant-path synapses in the dentate gyrus, in contrast, neurexin/Cbln4/Neogenin-1 complexes are essential for LTP without affecting basal synaptic transmission or NMDA- or AMPA-receptors. None of these signaling pathways are required for synapse formation. Thus, outside of the cerebellum neurexin/cerebellin complexes regulate synapse properties by activating specific downstream receptors.

Cbln1 Directs Axon Targeting by Corticospinal Neurons Specifically toward Thoraco-Lumbar Spinal Cord

Corticospinal neurons (CSN) are centrally required for skilled voluntary movement, which necessitates that they establish precise subcerebral connectivity with the brainstem and spinal cord. However, molecular controls regulating specificity of this projection targeting remain largely unknown. We previously identified that developing CSN subpopulations exhibit striking axon targeting specificity in the spinal white matter. These CSN subpopulations with segmentally distinct spinal projections are also molecularly distinct; a subset of differentially expressed genes between these distinct CSN subpopulations regulate differential axon projection targeting. Rostrolateral CSN extend axons exclusively to bulbar-cervical segments (CSNBC-lat), while caudomedial CSN (CSNmedial) are more heterogeneous, with distinct, intermingled subpopulations extending axons to either bulbar-cervical or thoraco-lumbar segments. Here, we report, in male and female mice, that Cerebellin 1 (Cbln1) is expressed specifically by CSN in medial, but not lateral, sensorimotor cortex. Cbln1 shows highly dynamic temporal expression, with Cbln1 levels in CSN highest during the period of peak axon extension toward thoraco-lumbar segments. Using gain-of-function experiments, we identify that Cbln1 is sufficient to direct thoraco-lumbar axon extension by CSN. Misexpression of Cbln1 in CSNBC-lat either by in utero electroporation, or by postmitotic AAV-mediated gene delivery, redirects these axons past their normal bulbar-cervical targets toward thoracic segments. Further, Cbln1 overexpression in postmitotic CSNBC-lat increases the number of CSNmedial axons that extend past cervical segments into the thoracic cord. Collectively, these results identify that Cbln1 functions as a potent molecular control over thoraco-lumbar CSN axon extension, part of an integrated network of controls over segmentally-specific CSN axon projection targeting.SIGNIFICANCE STATEMENT Corticospinal neurons (CSN) exhibit remarkable diversity and precision of axonal projections to targets in the brainstem and distinct spinal segments; the molecular basis for this targeting diversity is largely unknown. CSN subpopulations projecting to distinct targets are also molecularly distinguishable. Distinct subpopulations degenerate in specific motor neuron diseases, further suggesting that intrinsic molecular differences might underlie differential vulnerability to disease. Here, we identify a novel molecular control, Cbln1, expressed by CSN extending axons to thoraco-lumbar spinal segments. Cbln1 is sufficient, but not required, for CSN axon extension toward distal spinal segments, and Cbln1 expression is controlled by recently identified, CSN-intrinsic regulators of axon extension. Our results identify that Cbln1, together with other regulators, coordinates segmentally precise CSN axon targeting.

Extracellular matrix and synapse formation

The extracellular matrix (ECM) is a complex molecular network distributed throughout the extracellular space of different tissues as well as the neuronal system. Previous studies have identified various ECM components that play important roles in neuronal maturation and signal transduction. ECM components are reported to be involved in neurogenesis, neuronal migration, and axonal growth by interacting or binding to specific receptors. In addition, the ECM is found to regulate synapse formation, the stability of the synaptic structure, and synaptic plasticity. Here, we mainly reviewed the effects of various ECM components on synapse formation and briefly described the related diseases caused by the abnormality of several ECM components.

Spatially aware dimension reduction for spatial transcriptomics

Spatial transcriptomics are a collection of genomic technologies that have enabled transcriptomic profiling on tissues with spatial localization information. Analyzing spatial transcriptomic data is computationally challenging, as the data collected from various spatial transcriptomic technologies are often noisy and display substantial spatial correlation across tissue locations. Here, we develop a spatially-aware dimension reduction method, SpatialPCA, that can extract a low dimensional representation of the spatial transcriptomics data with biological signal and preserved spatial correlation structure, thus unlocking many existing computational tools previously developed in single-cell RNAseq studies for tailored analysis of spatial transcriptomics. We illustrate the benefits of SpatialPCA for spatial domain detection and explores its utility for trajectory inference on the tissue and for high-resolution spatial map construction. In the real data applications, SpatialPCA identifies key molecular and immunological signatures in a detected tumor surrounding microenvironment, including a tertiary lymphoid structure that shapes the gradual transcriptomic transition during tumorigenesis and metastasis. In addition, SpatialPCA detects the past neuronal developmental history that underlies the current transcriptomic landscape across tissue locations in the cortex.

Cbln1 regulates axon growth and guidance in multiple neural regions

The accurate construction of neural circuits requires the precise control of axon growth and guidance, which is regulated by multiple growth and guidance cues during early nervous system development. It is generally thought that the growth and guidance cues that control the major steps of axon development have been defined. Here, we describe cerebellin-1 (Cbln1) as a novel cue that controls diverse aspects of axon growth and guidance throughout the central nervous system (CNS) by experiments using mouse and chick embryos. Cbln1 has previously been shown to function in late neural development to influence synapse organization. Here, we find that Cbln1 has an essential role in early neural development. Cbln1 is expressed on the axons and growth cones of developing commissural neurons and functions in an autocrine manner to promote axon growth. Cbln1 is also expressed in intermediate target tissues and functions as an attractive guidance cue. We find that these functions of Cbln1 are mediated by neurexin-2 (Nrxn2), which functions as the Cbln1 receptor for axon growth and guidance. In addition to the developing spinal cord, we further show that Cbln1 functions in diverse parts of the CNS with major roles in cerebellar parallel fiber growth and retinal ganglion cell axon guidance. Despite the prevailing role of Cbln1 as a synaptic organizer, our study discovers a new and unexpected function for Cbln1 as a general axon growth and guidance cue throughout the nervous system.

Cerebellin-1 leads the way

The cerebellin family of proteins influences synapse formation and function. In this issue of PLOS Biology, Han and colleagues identify a new role for Cerebellin-1 in axon growth and guidance.

Distinct neurexin-cerebellin complexes control AMPA- and NMDA-receptor responses in a circuit-dependent manner

At CA1→subiculum synapses, alternatively spliced neurexin-1 (Nrxn1SS4+) and neurexin-3 (Nrxn3SS4+) enhance NMDA-receptors and suppress AMPA-receptors, respectively, without affecting synapse formation. Nrxn1SS4+ and Nrxn3SS4+ act by binding to secreted cerebellin-2 (Cbln2) that in turn activates postsynaptic GluD1 receptors. Whether neurexin-Cbln2-GluD1 signaling has additional functions besides regulating NMDA- and AMPA-receptors, and whether such signaling performs similar roles at other synapses, however, remains unknown. Here, we demonstrate using constitutive Cbln2 deletions in mice that at CA1→subiculum synapses, Cbln2 performs no additional developmental roles besides regulating AMPA- and NMDA-receptors. Moreover, low-level expression of functionally redundant Cbln1 did not compensate for a possible synapse-formation function of Cbln2 at CA1→subiculum synapses. In exploring the generality of these findings, we examined the prefrontal cortex where Cbln2 was recently implicated in spinogenesis, and the cerebellum where Cbln1 is known to regulate parallel-fiber synapses. In the prefrontal cortex, Nrxn1SS4+-Cbln2 signaling selectively controlled NMDA-receptors without affecting spine or synapse numbers, whereas Nrxn3SS4+-Cbln2 signaling had no apparent role. In the cerebellum, conversely, Nrxn3SS4+-Cbln1 signaling regulated AMPA-receptors, whereas now Nrxn1SS4+-Cbln1 signaling had no manifest effect. Thus, Nrxn1SS4+- and Nrxn3SS4+-Cbln1/2 signaling complexes differentially control NMDA- and AMPA-receptors in different synapses in diverse neural circuits without regulating synapse or spine formation.

The porcine cerebellin gene family

Cerebellins (CBLN1-4), together with C1qTNF proteins, belong to the CBLN subfamily of C1q proteins. Cerebellin-1 (CBLN1) is active in synapse formation and functions at the parallel fiber-Purkinje cell synapses. Cerebellins form tripartite complexes with neurexins and the glutamate-receptor-related proteins GluD1 and GluD2, playing a role as trans-synaptic cell-adhesion molecules that critically contribute to both synapse formation and functioning and brain development. In this study, I present a molecular characterization of the four porcine CBLN genes. Experimental data and in silico analyses collectively describes the gene structure, chromosomal localization, and expression of CBLN1-4. Two cDNAs encoding the cerebellins CBLN1 and CBLN3 were RT-PCR cloned and sequenced. The nucleotide sequence of the CBLN1 clone contains an open reading frame of 582 nucleotides and encodes a protein of 193 amino acids. The deduced amino acid of the porcine CBLN1 protein was 99% identical to both mouse CBLN1 and to human CBLN1. The deduced CBLN1 protein contains a putative signal sequence of 21 residues, two conserved cysteine residues, and C1q domain. The nucleotide sequence of the CBLN3 cDNA clone comprises an open reading frame of 618 nucleotides and encodes a protein of 205 amino acids. The deduced amino acid sequence of the porcine CBLN3 protein was 88% identical to mouse CBLN3 and 94% identical to human CBLN3. The amino terminal ends of both the CBLN1 and CBLN3 proteins contain three possible N-linked glycosylation sites. The genomic organization of both porcine CBLN1 and CBLN3 is very similar to those of their human counterparts. The expression analyses demonstrated that CBLN1 and CBLN3 transcripts are predominantly expressed in the cerebellum. The sequences of the porcine precerebellin genes and cDNAs were submitted to DDBJ/EMBL/GenBank under the following accession numbers: CBLN1 gene (GenBank ID: FJ621565), CBLN1 cDNA (GenBank ID: EF577504), CBLN3 gene (GenBank ID: FJ621566), CBLN3 cDNA (GenBank ID: EF577505) and CBLN4 cDNA (GenBank ID: FJ196070).

GNG13 Is a Potential Marker of the State of Health of Alzheimer's Disease Patients' Cerebellum

Brain regions such as the cerebellum (CB) have been neglected for a long time in the study of Alzheimer's disease (AD) pathogenesis. In reference to a new emerging hypothesis according to which there is an altered cerebellar synaptic processing in AD, we verified the possible role played by new biomarkers in the CB of AD patients compared with not-demented healthy control subjects (NDHS). Using a bioinformatics approach, we have collected several microarray datasets and obtained 626 cerebella sample biopsies belonging to subjects who did not die from causes related to neurological diseases and 199 cerebella belonging to AD. The analysis of logical relations between the transcriptome dataset highlighted guanine nucleotide-binding protein (G protein) gamma 13 (GNG13) as a potential new biomarker for Purkinje cells (PCs). We have correlated GNG13 expression levels with already widely existing bibliography of PC marker genes, such as Purkinje cell protein 2 (PCP2), Purkinje cell protein 4 (PCP4), and cerebellin 3 (CBLN3). We showed that expression levels of GNG13 and PCP2, PCP4, and CBLN3 were significantly correlated with each other in NDHS and in AD and significantly reduced in AD patients compared with NDHS subjects. In addition, we highlighted a negative correlation between the expression levels of PC biomarkers and age. From the outcome of our investigation, it is possible to conclude that the identification of GNG13 as a potentially biomarker in PCs represents also a state of health of CB, in association with the expression of PCP2, PCP4, and CBLN3.

Mouse models and strain-dependency of Chédiak-Higashi syndrome-associated neurologic dysfunction

Chédiak-Higashi syndrome (CHS) is a lethal disorder caused by mutations in the LYST gene that involves progressive neurologic dysfunction. Lyst-mutant mice exhibit neurologic phenotypes that are sensitive to genetic background. On the DBA/2J-, but not on the C57BL/6J-background, Lyst-mutant mice exhibit overt tremor phenotypes associated with loss of cerebellar Purkinje cells. Here, we tested whether assays for ataxia could measure this observed strain-dependency, and if so, establish parameters for empowering phenotype- and candidate-driven approaches to identify genetic modifier(s). A composite phenotypic scoring system distinguished phenotypes in Lyst-mutants and uncovered a previously unrecognized background difference between wild-type C57BL/6J and DBA/2J mice. Accelerating rotarod performance also distinguished phenotypes in Lyst-mutants, but at more advanced ages. These results establish that genetic background, Lyst genotype, and age significantly influence the severity of CHS-associated neurologic deficits. Purkinje cell quantifications likewise distinguished phenotypes of Lyst-mutant mice, as well as background differences between wild-type C57BL/6J and DBA/2J mice. To aid identification of potential genetic modifier genes causing these effects, we searched public datasets for cerebellar-expressed genes that are differentially expressed and/or contain potentially detrimental genetic variants. From these approaches, Nos1, Prdx2, Cbln3, Gnb1, Pttg1 were confirmed to be differentially expressed and leading candidates.

Cbln2 and Cbln4 are expressed in distinct medial habenula-interpeduncular projections and contribute to different behavioral outputs

Cerebellins are important neurexin ligands that remain incompletely understood. Two critical questions in particular remain unanswered: do different cerebellins perform distinct functions, and do these functions act in the initial establishment of synapses or in rendering nascent synapses capable of normal synaptic transmission? Here we show that in mice, Cbln2 and Cbln4 are expressed in the medial habenula (MHb) nucleus in different types of neurons that project to distinct target neurons in the interpeduncular nucleus. Conditional genetic deletion of Cbln2 in the MHb impaired synaptic transmission at Cbln2⁺ synapses in the interpeduncular neurons within 3 wk, but decreased synapse numbers only after 3 mo, suggesting a functional, but not a structural, requirement for Cbln2 in synapses formed by Cbln2-expressing neurons. In contrast, genetic deletions of Cbln4 in the MHb had no major effect on synaptic transmission or synapse numbers in interpeduncular target neurons. Nevertheless, MHb ablation of both Cbln2 and Cbln4 significantly impaired behavioral responses in mice, but affected different types of behaviors. Specifically, Cbln2 MHb deletions decreased spatial learning, as measured in the water T-maze, whereas Cbln4 MHb deletions increased anxiety levels, as monitored in the open field test and elevated plus maze. Thus, Cbln2 and Cbln4 are expressed in distinct MHb neurons that contribute to different behaviors.

Spinal injection of newly identified cerebellin-1 and cerebellin-2 peptides induce mechanical hypersensitivity in mice

By screening for neuropeptides in the mouse spinal cord using mass spectrometry (MS), we have previously demonstrated that one of the 78 peptides that is expressed predominantly (> 6-fold) in the dorsal horn compared to the ventral spinal cord is the atypical peptide desCER [des-Ser1]-cerebellin, which originates from the precursor protein cerebellin 1 (CBLN1). Furthermore, we found that intrathecal injection of desCER induces mechanical hypersensitivity in a dose dependent manner. The current study was designed to further investigate the relative expression of other CBLN derived peptides in the spinal cord and to examine whether they share similar nociceptive properties. In addition to the peptides cerebellin (CER) and desCER we identified and relatively quantified nine novel peptides originating from cerebellin precursor proteins CBLN1 (two peptides), CBLN2 (three peptides) and CBLN4 (four peptides). Ten out of eleven peptides displayed statistically significantly (p < 0.05) higher expression levels (200-350%) in the dorsal horn compared to the ventral horn. Intrathecal injection of three of the four CBLN1 and two of the three CBLN2 derived peptides induced mechanical hypersensitivity in response to von Frey filament testing in mice during the first 6 h post-injection compared to saline injected mice, while none of the four CBLN4 derived peptides altered withdrawal thresholds. This study demonstrates that high performance MS is an effective tool for detecting novel neuropeptides in CNS tissues. We show the presence of nine novel atypical peptides originating from CBLN1, CBLN2 and CBLN4 precursor proteins in the mouse dorsal horn, whereof five peptides induce pain-like behavior upon intrathecal injection. Further studies are required to investigate the mechanisms by which CBLN1 and CBLN2 derived peptides facilitate nociceptive signal transmission.

Glycosylation of Cblns attenuates their receptor binding

Cbln1 is the prototype of a family (Cbln1-Cbln4) of secreted glycoproteins and is essential for normal synapse structure and function in cerebellum by bridging presynaptic Nrxn to postsynaptic Grid2. Here we report the effects of glycosylation on the in vitro receptor binding properties of Cblns. Cbln1, 2 and 4 harbor two N-linked glycosylation sites, one at the N-terminus is in a region implicated in Nrxn binding and the second is in the C1q domain, a region involved in Grid2 binding. Mutation (asparagine to glutamine) of the N-terminal site, increased neurexin binding whereas mutation of the C1q site markedly increased Grid2 binding. These mutations did not influence subunit composition of Cbln trimeric complexes (mediated through the C1q domain) nor their assembly into hexamers (mediated by the N-terminal region). Therefore, glycosylation likely masks the receptor binding interfaces of Cblns. As Cbln4 has undetectable Grid2 binding in vitro we assessed whether transgenic expression of wild type Cbln4 or its glycosylation mutants rescued the Cbln1-null phenotype in vivo. Cbln4 partially rescued and both glycosylation mutants completely rescued ataxia in cbln1-null mice. Thus Cbln4 has intrinsic Grid2 binding that is attenuated by glycosylation, and glycosylation mutants exhibit gain of function in vivo.

Genetic Ablation of All Cerebellins Reveals Synapse Organizer Functions in Multiple Regions Throughout the Brain

Cerebellins are synaptic organizer molecules that bind to presynaptic neurexins and postsynaptic receptors. They are well studied in the cerebellum, but three of the four cerebellins (Cbln1, Cbln2, and Cbln4) are also broadly expressed outside of the cerebellum, suggesting that they perform general functions throughout the brain. Here, we generated male and female constitutive single (KO), double KO (dKO), and triple KO (tKO) mice of Cbln1, Cbln2, and Cbln4. We found that all constitutive cerebellin-deficient mice were viable and fertile, suggesting that cerebellins are not essential for survival. Cbln1/2 dKO mice exhibited salience-induced seizures that were aggravated in Cbln1/2/4 tKO mice, suggesting that all cerebellins contribute to brain function. As described previously, Cbln1 KO mice displayed major motor impairments that were aggravated by additional KO of Cbln2. Strikingly, the Cbln1/2 dKO did not cause alterations in synapse density in the hippocampus of young adult (1- and 2-month-old) mice, but produced a selective ∼50% decrease in hippocampal synapse density in the stratum lacunosum moleculare of the CA1 region and in the dentate gyrus of aging, 6-month-old mice. A similar decrease in excitatory synapse density was observed in the striatum and retrosplenial cortex. Behaviorally, the Cbln1 KO produced dramatic changes in motor behaviors that were partly aggravated by additional deletion of Cbln2 and/or Cbln4. Our results show that cerebellins are not essential for survival and do not contribute to initial synapse formation, but perform multiple functions throughout the brain; as a consequence, their ablation results in a delayed loss of synapses and in behavioral impairments.SIGNIFICANCE STATEMENT Cerebellins (Cbln1-4) are trans-synaptic cell adhesion molecules. In the cerebellum, Cbln1 functions as a bidirectional organizer of parallel fiber-Purkinje cell synapses by binding to presynaptic neurexins and postsynaptic GluRδ2. Little is known about the function of cerebellins outside of the cerebellum; therefore, the present study used single, double, and triple constitutive KO mice of Cbln1, Cbln2, and Cbln4 to analyze the overall function of cerebellins. We show that cerebellins act as important synaptic organizers in specific subsets of neurons and likely contribute to many different brain functions. We also show that cerebellins are not initially required for synapse formation, but rather for specification and long-term synapse maintenance and demonstrate that all cerebellins, not just Cbln1, contribute to brain function.

Cbln1 and Cbln4 Are Structurally Similar but Differ in GluD2 Binding Interactions

Unlike cerebellin 1 (Cbln1), which bridges neurexin (Nrxn) receptors and δ-type glutamate receptors in a trans-synaptic triad, Cbln4 was reported to have no or weak binding for the receptors despite sharing ∼70% sequence identity with Cbln1. Here, we report crystal structures of the homotrimers of the C1q domain of Cbln1 and Cbln4 at 2.2 and 2.3 Å resolution, respectively. Comparison of the structures suggests that the difference between Cbln1 and Cbln4 in GluD2 binding might be because of their sequence and structural divergence in loop CD. Surprisingly, we show that Cbln4 binds to Nrxn1β and forms a stable complex with the laminin, nectin, sex-hormone binding globulin (LNS) domain of Nrxn1β. Furthermore, the negative-stain electron microscopy reconstruction of hexameric full-length Cbln1 at 13 Å resolution and that of the Cbln4/Nrxn1β complex at 19 Å resolution suggest that Nrxn1β binds to the N-terminal region of Cbln4, probably through strand β10 of the S4 insert.

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

Synapses are specialized junctions between neurons in brain that transmit and compute information, thereby connecting neurons into millions of overlapping and interdigitated neural circuits. Here, we posit that the establishment, properties, and dynamics of synapses are governed by a molecular logic that is controlled by diverse trans-synaptic signaling molecules. Neurexins, expressed in thousands of alternatively spliced isoforms, are central components of this dynamic code. Presynaptic neurexins regulate synapse properties via differential binding to multifarious postsynaptic ligands, such as neuroligins, cerebellin/GluD complexes, and latrophilins, thereby shaping the input/output relations of their resident neural circuits. Mutations in genes encoding neurexins and their ligands are associated with diverse neuropsychiatric disorders, especially schizophrenia, autism, and Tourette syndrome. Thus, neurexins nucleate an overall trans-synaptic signaling network that controls synapse properties, which thereby determines the precise responses of synapses to spike patterns in a neuron and circuit and which is vulnerable to impairments in neuropsychiatric disorders.

Two Classes of Secreted Synaptic Organizers in the Central Nervous System

Research in the last two decades has identified many synaptic organizers in the central nervous system that directly regulate the assembly of pre- and/or postsynaptic molecules, such as synaptic vesicles, active zone proteins, and neurotransmitter receptors. They are classified into secreted factors and cell adhesion molecules, such as neurexins and neuroligins. Certain secreted factors are termed extracellular scaffolding proteins (ESPs) because they are components of the synaptic extracellular matrix and serve as a scaffold at the synaptic cleft. These include Lgi1, Cbln1, neuronal pentraxins, Hevin, thrombospondins, and glypicans. Diffusible secreted factors, such as Wnts, fibroblast growth factors, and semaphorins, tend to act from a distance. In contrast, ESPs remain at the synaptic cleft and often help synaptic adhesion and/or accumulation of postsynaptic receptors. Many fundamental questions remain about when, how, and why various synaptic organizers establish and modify the vast numbers of connections during development and throughout life.

Cerebellins are differentially expressed in selective subsets of neurons throughout the brain

Cerebellins are secreted hexameric proteins that form tripartite complexes with the presynaptic cell-adhesion molecules neurexins or 'deleted-in-colorectal-cancer', and the postsynaptic glutamate-receptor-related proteins GluD1 and GluD2. These tripartite complexes are thought to regulate synapses. However, cerebellins are expressed in multiple isoforms whose relative distributions and overall functions are not understood. Three of the four cerebellins, Cbln1, Cbln2, and Cbln4, autonomously assemble into homohexamers, whereas the Cbln3 requires Cbln1 for assembly and secretion. Here, we show that Cbln1, Cbln2, and Cbln4 are abundantly expressed in nearly all brain regions, but exhibit strikingly different expression patterns and developmental dynamics. Using newly generated knockin reporter mice for Cbln2 and Cbln4, we find that Cbln2 and Cbln4 are not universally expressed in all neurons, but only in specific subsets of neurons. For example, Cbln2 and Cbln4 are broadly expressed in largely non-overlapping subpopulations of excitatory cortical neurons, but only sparse expression was observed in excitatory hippocampal neurons of the CA1- or CA3-region. Similarly, Cbln2 and Cbln4 are selectively expressed, respectively, in inhibitory interneurons and excitatory mitral projection neurons of the main olfactory bulb; here, these two classes of neurons form dendrodendritic reciprocal synapses with each other. A few brain regions, such as the nucleus of the lateral olfactory tract, exhibit astoundingly high Cbln2 expression levels. Viewed together, our data show that cerebellins are abundantly expressed in relatively small subsets of neurons, suggesting specific roles restricted to subsets of synapses.

Autism gene Ube3a and seizures impair sociability by repressing VTA Cbln1

Maternally inherited 15q11-13 chromosomal triplications cause a frequent and highly penetrant autism linked to increased gene dosages of UBE3A, which both possesses ubiquitin-ligase and transcriptional co-regulatory functions. Here, using in vivo mouse genetics, we show that increasing UBE3A in the nucleus down-regulates glutamatergic synapse organizer cerebellin-1 (Cbln1) that is needed for sociability in mice. Epileptic seizures also repress Cbln1 and are found to expose sociability impairments in mice with asymptomatic increases of UBE3A. This Ube3a-seizure synergy maps to glutamate neurons of the midbrain ventral tegmental area (VTA) where Cbln1 deletions impair sociability and weaken glutamatergic transmission. We provide preclinical evidence that viral-vector-based chemogenetic activations of, or Cbln1 restorations in VTA glutamatergic neurons rescues sociability deficits induced by Ube3a and/or seizures. Our results suggest a gene × seizure interaction in VTA glutamatergic neurons that impairs sociability by downregulating Cbln1, a key node in the expanding protein interaction network of autism genes.

Seasonal oscillation of liver-derived hibernation protein complex in the central nervous system of non-hibernating mammals

Mammalian hibernation elicits profound changes in whole-body physiology. The liver-derived hibernation protein (HP) complex, consisting of HP-20, HP-25 and HP-27, was shown to oscillate circannually, and this oscillation in the central nervous system (CNS) was suggested to play a role in hibernation. The HP complex has been found in hibernating chipmunks but not in related non-hibernating tree squirrels, leading to the suggestion that hibernation-specific genes may underlie the origin of hibernation. Here, we show that non-hibernating mammals express and regulate the conserved homologous HP complex in a seasonal manner, independent of hibernation. Comparative analyses of cow and chipmunk HPs revealed extensive biochemical and structural conservations. These include liver-specific expression, assembly of distinct heteromeric complexes that circulate in the blood and cerebrospinal fluid, and the striking seasonal oscillation of the HP levels in the blood and CNS. Central administration of recombinant HPs affected food intake in mice, without altering body temperature, physical activity levels or energy expenditure. Our results demonstrate that HP complex is not unique to the hibernators and suggest that the HP-regulated liver-brain circuit may couple seasonal changes in the environment to alterations in physiology.

Regulation of chromatin accessibility and Zic binding at enhancers in the developing cerebellum

To identify chromatin mechanisms of neuronal differentiation, we characterized chromatin accessibility and gene expression in cerebellar granule neurons (CGNs) of the developing mouse. We used DNase-seq to map accessibility of cis-regulatory elements and RNA-seq to profile transcript abundance across postnatal stages of neuronal differentiation in vivo and in culture. We observed thousands of chromatin accessibility changes as CGNs differentiated and verified by H3K27ac ChIP-seq, reporter gene assays, and CRISPR-mediated activation that many of these regions function as neuronal enhancers. Motif discovery within differentially accessible chromatin regions suggested a novel role for the Zic family of transcription factors in CGN maturation. We confirmed the association of Zic with these elements by ChIP-seq, and demonstrated by knockdown that Zic1/2 are required to coordinate mature neuronal gene expression patterns. Together these data reveal chromatin dynamics at thousands of gene regulatory elements that facilitate gene expression patterns necessary for neuronal differentiation and function.

Parcellation of cerebellins 1, 2, and 4 among different subpopulations of dorsal horn neurons in mouse spinal cord

The cerebellins (Cblns) are a family of secreted proteins that are widely expressed throughout the nervous system, but whose functions have been studied only in the cerebellum and striatum. Two members of the family, Cbln1 and Cbln2, bind to neurexins on presynaptic terminals and to GluRδs postsynaptically, forming trans-synaptic triads that promote synapse formation. Cbln1 has a higher binding affinity for GluRδs and exhibits greater synaptogenic activity than Cbln2. In contrast, Cbln4 does not form such triads and its function is unknown. The different properties of the three Cblns suggest that each plays a distinct role in synapse formation. To begin to elucidate Cbln function in other neuronal systems, we used in situ hybridization to examine Cbln expression in the mouse spinal cord. We find that neurons expressing Cblns 1, 2, and 4 tend to occupy different laminar positions within the dorsal spinal cord, and that Cbln expression is limited almost exclusively to excitatory neurons. Combined in situ hybridization and immunofluorescent staining shows that Cblns 1, 2, and 4 are expressed by largely distinct neuronal subpopulations, defined in part by sensory input, although there is some overlap and some individual neurons coexpress two Cblns. Our results suggest that differences in connectivity between subpopulations of dorsal spinal cord neurons may be influenced by which Cbln each subpopulation contains. Competitive interactions between axon terminals may determine the number of synapses each forms in any given region, and thereby contribute to the development of precise patterns of connectivity in the dorsal gray matter.

Neural ECM and synaptogenesis

Chemical synapses allow neurons to perform complex computations and regulate other systems of the body. At a chemical synapse, pre- and postsynaptic sites are separated by a small space (the synaptic cleft) and surrounded by astrocytes. The basement membrane (BM), a sheetlike, specialized extracellular matrix (ECM), is found ubiquitously in the PNS. It has become clear that the ECMs not only play a structural role but also serve as barriers and filters in the PNS and CNS. Moreover, proteoglycans and tenascin family proteins in the ECM regulate synapse formation and synaptic plasticity. Although CNS synapses lack the BMs, recent results indicate that the BM-associated collagens are also present in the CNS synaptic cleft and affect synaptogenesis in both the CNS and the PNS. The C1q domain-containing family proteins are important components of the CNS synaptic cleft in regulating synapse formation, maintenance, and the pruning process. The ECM is regarded as a crucial component of the tetrapartite synapse, consisting of pre- and postsynaptic neurons, astrocyte, and ECM.

Cbln1 and the δ2 glutamate receptor--an orphan ligand and an orphan receptor find their partners

Cerebellin was originally discovered as a Purkinje cell-specific peptide more than two decades ago. Later, its precursor protein precerebellin (Cbln1) was found to be produced in cerebellar granule cells. It has become increasingly clear that although the cerebellin peptide may have certain functions, Cbln1 is an actual signaling molecule that belongs to the C1q family. However, the precise function of Cbln1 has been unresolved. Cbln1 is released from granule cells, and disruption of the cbln1 gene in mice causes a severe reduction in the number of synapses between Purkinje cells and parallel fibers (PFs; axons of granule cells) and results in cerebellar ataxia. The glutamate receptor δ2 (GluD2) is highly expressed on Purkinje cells' dendritic spines which make synapses with PFs. Although GluD2 was identified as a member of the ionotropic glutamate receptors more than 15 years ago, it has been referred to as an orphan receptor because its endogenous ligands are unclear. Interestingly, GluD2-null mice phenocopy cbln1-null mice precisely. Cbln1 and GluD2 have therefore been thought to participate in a common signaling pathway that is required for the formation of PF synapses. We recently established a direct ligand-receptor relationship between Cbln1 and GluD2. The Cbln1-GluD2 complex is located at the cleft of PF-Purkinje cell synapses and bidirectionally regulates both presynaptic and postsynaptic differentiation.

Comparison of Cbln1 and Cbln2 functions using transgenic and knockout mice

Cerebellin precursor protein 1 (Cbln1) is the prototype of a family of secreted neuronal glycoproteins (Cbln1-4) and its genetic elimination results in synaptic alterations in cerebellum (CB) and striatum. In CB, Cbln1 acts as a bi-functional ligand bridging pre-synaptic β-neurexins on granule cells to post-synaptic Grid2 on Purkinje neurons. Although much is known concerning the action of Cbln1, little is known of the function of its other family members. Here, we show that Cbln1 and Cbln2 have similar binding activities to β-neurexins and Grid2 and the targeted ectopic expression of Cbln2 to Purkinje cells in transgenic mice rescues the cerebellar deficits in Cbln1-null animals: suggesting that the two proteins have redundant function mediated by their common receptor binding properties. Cbln1 and Cbln2 are also co-expressed in the endolysosomal compartment of the thalamic neurons responsible for the synaptic alterations in striatum of Cbln1-null mice. Therefore, to determine whether the two family members have similar functions, we generated Cbln2-null mice. Cbln2-null mice do not show the synaptic alterations evident in striatum of Cbln1-null mice. Thus, Cbln2 can exhibit functional redundancy with Cbln1 in CB but it does not have the same properties as Cbln1 in thalamic neurons, implying one or both utilize different receptors/mechanisms in this brain region.

The Cbln family of proteins interact with multiple signaling pathways

Cerebellin precursor protein (Cbln1) is essential for synapse integrity in cerebellum through assembly into complexes that bridge pre-synaptic β-neurexins (Nrxn) to post-synaptic GluRδ2. However, GluRδ2 is largely cerebellum-specific, yet Cbln1 and its little studied family members, Cbln2 and Cbln4, are expressed throughout brain. Therefore, we investigated whether additional proteins mediate Cbln family actions. Whereas Cbln1 and Cbln2 bound to GluRδ2 and Nrxns1-3, Cbln4 bound weakly or not at all, suggesting it has distinct binding partners. In a candidate receptor-screening assay, Cbln4 (but not Cbln1 or Cbln2) bound selectively to the netrin receptor, (deleted in colorectal cancer (DCC) in a netrin-displaceable fashion. To determine whether Cbln4 had a netrin-like function, Cbln4-null mice were generated. Cbln4-null mice did not phenocopy netrin-null mice. Cbln1 and Cbln4 were likely co-localized in neurons thought to be responsible for synaptic changes in striatum of Cbln1-null mice. Furthermore, complexes containing Cbln1 and Cbln4 had greatly reduced affinity to DCC but increased affinity to Nrxns, suggesting a functional interaction. However, Cbln4-null mice lacked the striatal synaptic changes seen in Cbln null mice. Thus, Cbln family members interact with multiple receptors/signaling pathways in a subunit composition-dependent manner and have independent functions with Cbln4 potentially involved in the less well-characterized role of netrin/DCC in adult brain.

Localization of cerebellin-2 in late embryonic chicken brain: implications for a role in synapse formation and for brain evolution

Cerebellin-1 (Cbln1), the most studied member of the cerebellin family of secreted proteins, is necessary for the formation and maintenance of parallel fiber-Purkinje cell synapses. However, the roles of the other Cblns have received little attention. We previously identified the chicken homolog of Cbln2 and examined its expression in dorsal root ganglia and spinal cord (Yang et al. [2010] J Comp Neurol 518:2818-2840). Interestingly, Cbln2 is expressed by mechanoreceptive and proprioceptive neurons and in regions of the spinal cord where those afferents terminate, as well as by preganglionic sympathetic neurons and their sympathetic ganglia targets. These findings suggest that Cbln2 may demonstrate a tendency to be expressed by synaptically connected neuronal populations. To further assess this possibility, we examined Cbln2 expression in chick brain. We indeed found that Cbln2 is frequently expressed by synaptically connected neurons, although there are exceptions, and we discuss the implications of these findings for Cbln2 function. Cbln2 expression tends to be more common in primary sensory neurons and in second-order sensory regions than it is in motor areas of the brain. Moreover, we found that the level of Cbln2 expression for many regions of the chicken brain is very similar to that of the mammalian homologs, consistent with the view that the expression patterns of molecules playing fundamental roles in processes such as neuronal communication are evolutionarily conserved. There are, however, large differences in the pattern of Cbln2 expression in avian as compared to mammalian telencephalon and in other regions that show the most divergence between the two lineages.

Cerebellin1 is a novel orexigenic peptide

AIM

Cerebellin1 (Cbln1) is highly expressed in the hypothalamus, a region of the brain involved in appetite regulation. However, the effects of Cbn1 on food intake are not known. The present study aimed to investigate the effect of Cbln1 on appetite regulation in rats.

METHODS

We determined the effect of (i) intracerebroventricular (ICV) injection of Cbln1 on food intake, behaviour and plasma pituitary hormone levels in male Wistar rats; (ii) Cbln1 on the release of hypothalamic neuropeptides known to modulate food intake from hypothalamic explants and (iii) fasting on hypothalamic Cbln1 mRNA expression.

RESULTS

(i) ICV administration of Cbln1 significantly increased food intake in rats and caused no adverse behaviours. ICV administration of Cbln1 significantly reduced plasma thyroid stimulating hormone (TSH) levels 10 min postinjection in rats. (ii) Cbln1 significantly increased the release of neuropeptide Y (NPY) from hypothalamic explants. (iii) Cbln1 mRNA expression levels were increased in the ventromedial nucleus of the hypothalamus in fasted rats.

CONCLUSIONS

These data suggest that Cbln1 is a novel orexigenic peptide, which may mediate its effects via hypothalamic NPY.

Extracerebellar role for Cerebellin1: modulation of dendritic spine density and synapses in striatal medium spiny neurons

Cerebellin1 (Cbln1) is a secreted glycoprotein that was originally isolated from the cerebellum and subsequently found to regulate synaptic development and stability. Cbln1 has a heterogeneous distribution in brain, but the only site in which it has been shown to have central effects is the cerebellar cortex, where loss of Cbln1 causes a reduction in granule cell-Purkinje cell synapses. Neurons of the thalamic parafascicular nucleus (PF), which provide glutamatergic projections to the striatum, also express high levels of Cbln1. We first examined Cbln1 in thalamostriatal neurons and then determined if cbln1 knockout mice exhibit structural deficits in striatal neurons. Virtually all PF neurons express Cbln1-immunoreactivity (-ir). In contrast, only rare Cbln1-ir neurons are present in the central medial complex, the other thalamic region that projects heavily to the dorsal striatum. In the striatum Cbln1-ir processes are apposed to medium spiny neuron (MSN) dendrites; ultrastructural studies revealed that Cbln1-ir axon terminals form axodendritic synapses with MSNs. Tract-tracing studies found that all PF cells retrogradely labeled from the striatum express Cbln1-ir. We then examined the dendritic structure of Golgi-impregnated MSNs in adult cbln1 knockout mice. MSN dendritic spine density was markedly increased in cbln1(-/-) mice relative to wildtype littermates, but total dendritic length was unchanged. Ultrastructural examination revealed an increase in the density of MSN axospinous synapses in cbln1(-/-) mice, with no change in postsynaptic density length. Thus, Cbln1 determines the dendritic structure of striatal MSNs, with effects distinct from those seen in the cerebellum.

Identification of cerebellin2 in chick and its preferential expression by subsets of developing sensory neurons and their targets in the dorsal horn

The cerebellins are a family of four secreted proteins, two of which, Cbln1 and Cbln3, play an important role in the formation and maintenance of parallel fiber-Purkinje cell synapses. We have identified the chicken homologue of Cbln2 and, through the use of in situ hybridization, shown that it is expressed by specific subsets of neurons in the dorsal root ganglia (DRGs) and spinal cord starting shortly after those neurons are generated. In the developing spinal cord, Cbln2 is highly expressed by dI1, dI3, dI5, and dILB dorsal interneurons and to a lesser extent by dI2, dI4, dI6, and dILA dorsal interneurons, but not by ventral (v0-v3) interneurons. After the spinal cord has matured and neurons have migrated to their final destinations, Cbln2 is abundant in the dorsal horn. In the DRGs, Cbln2 is expressed by TrkB+ and TrkC+ sensory neurons, but not by TrkA+ sensory neurons. Interestingly, regions of the spinal cord where TrkB+ and TrkC+ afferents terminate (i.e., laminae II, III, IV, and VI) exhibit the highest levels of Cbln2 expression. Cbln2 is also expressed by preganglionic sympathetic neurons and their targets in the sympathetic chain ganglia. Thus, the results show that Cbln2 is frequently expressed by synaptically connected neuronal populations. This, in turn, raises the possibility that if Cbln2, like Cbln1, plays a role in the formation and maintenance of synapses, it may somehow mediate bi-directional communication between discrete populations of neurons and their appropriate neuronal targets.

Mammalian Otolin: a multimeric glycoprotein specific to the inner ear that interacts with otoconial matrix protein Otoconin-90 and Cerebellin-1

Background

The mammalian otoconial membrane is a dense extracellular matrix containing bio-mineralized otoconia. This structure provides the mechanical stimulus necessary for hair cells of the vestibular maculae to respond to linear accelerations and gravity. In teleosts, Otolin is required for the proper anchoring of otolith crystals to the sensory maculae. Otoconia detachment and subsequent entrapment in the semicircular canals can result in benign paroxysmal positional vertigo (BPPV), a common form of vertigo for which the molecular basis is unknown. Several cDNAs encoding protein components of the mammalian otoconia and otoconial membrane have recently been identified, and mutations in these genes result in abnormal otoconia formation and balance deficits.

Principal Findings

Here we describe the cloning and characterization of mammalian Otolin, a protein constituent of otoconia and the otoconial membrane. Otolin is a secreted glycoprotein of ∼70 kDa, with a C-terminal globular domain that is homologous to the immune complement C1q, and contains extensive posttranslational modifications including hydroxylated prolines and glycosylated lysines. Like all C1q/TNF family members, Otolin multimerizes into higher order oligomeric complexes. The expression of otolin mRNA is restricted to the inner ear, and immunohistochemical analysis identified Otolin protein in support cells of the vestibular maculae and semi-circular canal cristae. Additionally, Otolin forms protein complexes with Cerebellin-1 and Otoconin-90, two protein constituents of the otoconia, when expressed in vitro. Otolin was also found in subsets of support cells and non-sensory cells of the cochlea, suggesting that Otolin is also a component of the tectorial membrane.

Conclusion

Given the importance of Otolin in lower organisms, the molecular cloning and biochemical characterization of the mammalian Otolin protein may lead to a better understanding of otoconial development and vestibular dysfunction.

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.

CTRP8 and CTRP9B are novel proteins that hetero-oligomerize with C1q/TNF family members

C1q/TNF family comprises over thirty secreted multimeric proteins that play diverse and important roles in immune, endocrine, skeletal, neuronal, reproductive, sensory, and vascular systems. Here we describe two novel human C1q/TNF family members, designated as CTRP8 and CTRP9B. Both genes are absent in the mouse genome. CTRP8 is expressed predominantly in lung and testis. In addition to forming homotrimers, CTRP8 also forms heteromeric complexes with C1q-related factor (CRF). CRF is a secreted multimeric protein that forms heteromeric complexes with CTRP1, CTRP9, and CTRP10. Although human CTRRP9A and CTRP9B share 98% amino acid identity, they are encoded by distinct genes and are biochemically distinct. While CTRP9A is robustly secreted as a multimeric protein, CTRP9B requires physical association with CTRP9A or adiponectin for its secretion. We propose here that combinatorial association between C1q/TNF family members is a possible mechanism to generate an expanded repertoire of functionally distinct ligands with altered function and/or receptor specificity.

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.

Insulinostatic activity of cerebellin--evidence from in vivo and in vitro studies in rats

Cerebellin (CER) is a neuromodulatory hexadecapeptide that originates from the precursor protein precerebellin (Cbln1). Four highly homologous isoforms of Cbln are known (Cbln1-Cbln4), which are expressed in the central nervous system (CNS) and in peripheral tissues. CER modulates synaptic structure formation in the CNS, whereas in the peripheral tissues CER regulates catecholamine secretion. Cbln is also expressed in the pancreas; however, its function in the pancreas is unknown. Here, we demonstrate the role of CER in regulating insulin secretion in vivo and in vitro. We identified Cbln1 and Cbln3 transcripts in rat pancreatic islets and detected Cbln-immunoreactivity, predominantly located in the periphery of the rat endocrine pancreas. In vivo, CER reduced plasma insulin levels in rats after 1 and 2 h. CER decreased insulin secretion from isolated rat pancreatic islets at high (11 mM), but not at low (3.33 mM) glucose concentration. CER inhibited stimulated insulin secretion from clonal rat insulinoma (INS-1) cells, reduced forskolin-induced production of cAMP and intracellular calcium concentration. Our study demonstrates for the first time that Cbln1 and Cbln3 are expressed in the rat endocrine pancreas. Furthermore, we identify CER as an insulinostatic factor, which decreases intracellular cAMP production and calcium in INS-1 cells.

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.

Molecular, biochemical and functional characterizations of C1q/TNF family members: adipose-tissue-selective expression patterns, regulation by PPAR-gamma agonist, cysteine-mediated oligomerizations, combinatorial associations and metabolic functions

The insulin-sensitizing hormone, adiponectin, belongs to the expanding C1q/TNF (tumour necrosis factor) family of proteins. We recently identified a family of adiponectin paralogues designated as CTRP (C1q/TNF-related protein) 1-7, and in the present study describe CTRP10. In the present study, we show that CTRP1, CTRP2, CTRP3, CTRP5 and CTRP7 transcripts are expressed predominantly by adipose tissue. In contrast, placenta and eye expressed the highest levels of CTRP6 and CTRP10 transcripts respectively. Expression levels of CTRP1, CTRP2, CTRP3, CTRP6 and CTRP7 transcripts are up-regulated in 8-week-old obese (ob/ob) mice relative to lean controls. Treatment of mice with a PPAR-gamma (peroxisome-proliferator-activated receptor-gamma) agonist, rosiglitazone, increased the expression of CTRP1 and decreased CTRP6 transcript levels. All CTRPs are secreted glycoproteins when expressed in mammalian cells. CTRP1, CTRP2, CTRP3, CTRP5 and CTRP6 circulate in the blood and are potential endocrine hormones; their serum levels vary according to the sex and genetic background of mice. Importantly, serum levels of CTRP1 and CTRP6 are increased in adiponectin-null mice. Like adiponectin, all secreted CTRP proteins form trimers as their basic structural units. CTRP3, CTRP5, CTRP6 and CTRP10 trimers are further assembled into higher-order oligomeric complexes via disulfide bonding mediated by their N-terminal cysteine residues. Besides forming homo-oligomers, CTRP1/CTRP6, CTRP2/CTRP7 and adiponectin/CTRP2 are secreted as heterotrimers, thus providing a mechanism to potentially generate functionally distinct ligands. Functional characterization of one such family member, CTRP1, showed that it specifically activates Akt and p44/42-MAPK (mitogen-activated protein kinase) signalling pathways in differentiated mouse myotubes. Moreover, injection of recombinant CTRP1 into mice significantly reduced their serum glucose levels. Thus at least CTRP1 may be considered a novel adipokine. In summary, these molecular, biochemical and functional data provide an important framework to further address the physiological functions and mechanisms of the action of this family of secreted glycoproteins in normal and disease states.

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.

Identification and characterization of CTRP9, a novel secreted glycoprotein, from adipose tissue that reduces serum glucose in mice and forms heterotrimers with adiponectin

Adiponectin is a major insulin-sensitizing, multimeric hormone derived from adipose tissue that acts on muscle and liver to regulate whole-body glucose and lipid metabolism. Here, we describe a novel and highly conserved paralog of adiponectin designated as C1q/TNF-related protein (CTRP) 9. Of all the CTRP paralogs, CTRP9 shows the highest degree of amino acid identity to adiponectin in its globular C1q domain. CTRP9 is expressed predominantly in adipose tissue and females expresses higher levels of the transcript than males. Moreover, its expression levels in ob/ob mice changed in an age-dependent manner, with significant up-regulation in younger mice. CTRP9 is a secreted glycoprotein with multiple post-translational modifications in its collagen domain that include hydroxylated prolines and hydroxylated and glycosylated lysines. It is secreted as multimers (predominantly trimers) from transfected cells and circulates in the mouse serum with levels varying according to sex and metabolic state of mice. Furthermore, CTRP9 and adiponectin can be secreted as heterooligomers when cotransfected into mammalian cells, and in vivo, adiponectin/CTRP9 complexes can be reciprocally coimmunoprecipitated from the serum of adiponectin and CTRP9 transgenic mice. Biochemical analysis demonstrates that adiponectin and CTRP9 associate via their globular C1q domain, and this interaction does not require their conserved N-terminal cysteines or their collagen domains. Furthermore, we show that adiponectin and CTRP9 form heterotrimers. In cultured myotubes, CTRP9 specifically activates AMPK, Akt, and p44/42 MAPK signaling pathways. Adenovirus-mediated overexpression of CTRP9 in obese (ob/ob) mice significantly lowered serum glucose levels. Collectively, these results suggest that CTRP9 is a novel adipokine, and further study of CTRP9 will yield novel mechanistic insights into its physiological and metabolic function.

Cbln1 regulates rapid formation and maintenance of excitatory synapses in mature cerebellar Purkinje cells in vitro and in vivo

Although many synapse-organizing molecules have been identified in vitro, their functions in mature neurons in vivo have been mostly unexplored. Cbln1, which belongs to the C1q/tumor necrosis factor superfamily, is the most recently identified protein involved in synapse formation in the mammalian CNS. In the cerebellum, Cbln1 is predominantly produced and secreted from granule cells; cbln1-null mice show ataxia and a severe reduction in the number of synapses between Purkinje cells and parallel fibers (PFs), the axon bundle of granule cells. Here, we show that application of recombinant Cbln1 specifically and reversibly induced PF synapse formation in dissociated cbln1-null Purkinje cells in culture. Cbln1 also rapidly induced electrophysiologically functional and ultrastructurally normal PF synapses in acutely prepared cbln1-null cerebellar slices. Furthermore, a single injection of recombinant Cbln1 rescued severe ataxia in adult cbln1-null mice in vivo by completely, but transiently, restoring PF synapses. Therefore, Cbln1 is a unique synapse organizer that is required not only for the normal development of PF-Purkinje cell synapses but also for their maintenance in the mature cerebellum both in vitro and in vivo. Furthermore, our results indicate that Cbln1 can also rapidly organize new synapses in adult cerebellum, implying its therapeutic potential for cerebellar ataxic disorders.

Mapping of Cbln1-like immunoreactivity in adult and developing mouse brain and its localization to the endolysosomal compartment of neurons

Cbln1 is a secreted glycoprotein essential for synapse structure and function in cerebellum that is also expressed in extracerebellar structures where its function is unknown. Furthermore, Cbln1 assembles into homomeric complexes and heteromeric complexes with three family members (Cbln2-Cbln4), thereby influencing each other's degradation and secretion. Therefore, to understand its function, it is essential to establish the location of Cbln1 relative to other family members. The localization of Cbln1 in brain was determined using immunohistochemistry and cbln1-lacZ transgenic mice. Cbln1-like immunoreactivity (CLI) was always punctate and localized to the cytoplasm of neurons. The punctate CLI colocalized with cathepsin D, a lysosomal marker, but not with markers of endoplasmic reticulum or Golgi, indicating that Cbln1 is present in neuronal endosomes/lysosomes. This may represent the cellular mechanism underlying the regulated degradation of Cbln1 observed in vivo. Outside the cerebellum, CLI mapped to multiple brain regions that were frequently synaptically interconnected, warranting their analysis in cbln1-null mice. Furthermore, whereas CLI increased dramatically in the cerebellum of cbln3-null mice it was unchanged in extracerebellar neurons. This opens the possibility that other family members that are coexpressed in these areas control Cbln1 levels, potentially by modulating processing in the endolysosomal pathway. During development of cbln1-lacZ mice, beta-galactosidase staining was first observed in proliferating granule cell precursors prior to synaptogenesis and thereafter in maturing and adult granule cells. As cbln3 is only expressed in post-mitotic, post-migratory granule cells, Cbln1 homomeric complexes in precursors and Cbln1-Cbln3 heteromeric complexes in mature granule cells may have distinct functions and turnover.