Proteolytic degradation and potential role of onconeural protein cdr2 in neurodegeneration

Cerebellar degeneration-related protein 2 (cdr2) is expressed in the central nervous system, and its ectopic expression in tumor cells of patients with gynecological malignancies elicits immune responses by cdr2-specific autoantibodies and T lymphocytes, leading to neurological symptoms. However, little is known about the regulation and function of cdr2 in neurodegenerative diseases. Because we found that cdr2 is highly expressed in the midbrain, we investigated the role of cdr2 in experimental models of Parkinson's disease (PD). We found that cdr2 levels were significantly reduced after stereotaxic injection of 1-methyl-4-phenylpyridinium (MPP(+)) into the striatum. cdr2 levels were also decreased in the brains of post-mortem PD patients. Using primary cultures of mesencephalic neurons and MN9D cells, we confirmed that MPP(+) reduces cdr2 in tyrosine hydroxylase-positive dopaminergic neuronal cells. The MPP(+)-induced decrease of cdr2 was primarily caused by calpain- and ubiquitin proteasome system-mediated degradation, and cotreatment with pharmacological inhibitors of these enzymes or overexpression of calcium-binding protein rendered cells less vulnerable to MPP(+)-mediated cytotoxicity. Consequently, overexpression of cdr2 rescued cells from MPP(+)-induced cytotoxicity, whereas knockdown of cdr2 accelerated toxicity. Collectively, our findings provide insights into the novel regulatory mechanism and potentially protective role of onconeural protein during dopaminergic neurodegeneration.

Developing global insight into RNA regulation

Systematic dissection of the activity of RNA-binding proteins (RBPs) has begun to yield global insight into how they work. The paradigm we have used has been the study of Nova, a neuron-specific RBP targeted in an autoimmune neurologic disorder associated with cancer. We have developed a combination of biochemical, genetic, and bioinformatic methods to generate a global understanding of Nova's role as a splicing regulator. Genome-wide identification and validation of Nova target RNAs have yielded unexpected insights into the protein's mechanism of action, its role in neurobiology, and the unique roles RBPs have in the biology of the neuronal synapse. These studies provide us with a paradigm for understanding the role of RBPs in neurons and in disease and, more generally, with the hope that it will be feasible to develop a comprehensive understanding of posttranscriptional regulation.

FMRP RNA targets: identification and validation

The Fragile X Syndrome is caused by the loss of function of the FMR1 gene (Pieretti et al. 1991. Cell 66, 817-822; O'Donnell & Warren 2002. Annu Rev Neurosci 25, 315-338]. Identification of the RNA targets to which FMRP binds is a key step in understanding the function of the protein and the cellular defects caused by its absence (Darnell et al. 2004 Ment Retard Dev Disabil Res Rev 10, 49-52). Here we discuss the current understanding of FMRP as an RNA-binding protein, the different approaches that have been taken to identify FMRP RNA targets and the relevance of some of these approaches to FMRP biology. In addition, we present evidence that point mutations in the K-homology (KH)1 or KH2 domains of FMRP abrogate its polyribosome association in transfected neuroblastoma cells but that the deletion of the RGG box does not. This suggests that RNA binding by the RGG box of FMRP may mediate other aspects of cellular mRNA metabolism such as mRNA localization or that it may have a role downstream of polyribosome association.

Effective antigen cross-presentation by prostate cancer patients' dendritic cells: implications for prostate cancer immunotherapy

Despite the potency with which dendritic cells (DCs) are able to utilize the exogenous MHC I antigen cross-presentation pathway to cross-present antigen for the activation of killer T cells in model systems, concern about defects in immune function in cancer patients has led to uncertainty regarding whether immune cells derived from patients can effectively be used to generate tumor vaccines. We have undertaken a careful analysis of the potency of using DCs obtained from prostate cancer patients to cross-present antigen derived from human prostate tumor cells for the activation of antigen-specific T cells. Such DCs can be matured ex vivo into functionally active cells and are capable of cross-presenting influenza antigen derived from internalized apoptotic prostate tumor cells. Importantly, we demonstrate effective stimulation of both CD4+ and CD8+ T cells, as evident by production of IFN-gamma, and the ability of CD8+ T cells to differentiate into effector CTLs. These results, defining conditions in which prostate cancer patient DCs can efficiently utilize the cross-presentation pathway and in which apoptotic tumor can serve as a source of antigen for DCs to activate T cells, demonstrate that this system warrants clinical study as a potential immunotherapy.

Microarray identification of FMRP-associated brain mRNAs and altered mRNA translational profiles in fragile X syndrome

Fragile X syndrome results from the absence of the RNA binding FMR protein. Here, mRNA was coimmunoprecipitated with the FMRP ribonucleoprotein complex and used to interrogate microarrays. We identified 432 associated mRNAs from mouse brain. Quantitative RT-PCR confirmed some to be >60-fold enriched in the immunoprecipitant. In parallel studies, mRNAs from polyribosomes of fragile X cells were used to probe microarrays. Despite equivalent cytoplasmic abundance, 251 mRNAs had an abnormal polyribosome profile in the absence of FMRP. Although this represents <2% of the total messages, 50% of the coimmunoprecipitated mRNAs with expressed human orthologs were found in this group. Nearly 70% of those transcripts found in both studies contain a G quartet structure, demonstrated as an in vitro FMRP target. We conclude that translational dysregulation of mRNAs normally associated with FMRP may be the proximal cause of fragile X syndrome, and we identify candidate genes relevant to this phenotype.

Fragile X mental retardation protein targets G quartet mRNAs important for neuronal function

Loss of fragile X mental retardation protein (FMRP) function causes the fragile X mental retardation syndrome. FMRP harbors three RNA binding domains, associates with polysomes, and is thought to regulate mRNA translation and/or localization, but the RNAs to which it binds are unknown. We have used RNA selection to demonstrate that the FMRP RGG box binds intramolecular G quartets. This data allowed us to identify mRNAs encoding proteins involved in synaptic or developmental neurobiology that harbor FMRP binding elements. The majority of these mRNAs have an altered polysome association in fragile X patient cells. These data demonstrate that G quartets serve as physiologically relevant targets for FMRP and identify mRNAs whose dysregulation may underlie human mental retardation.

Dendritic cell maturation is required for the cross-tolerization of CD8+ T cells

In vivo models have shown that tissue-restricted antigen may be captured by bone marrow-derived cells and cross-presented for the tolerization of CD8+ T cells. Although these studies have shown peripheral tolerization of CD8+ T cells, the mechanism of antigen transfer and the nature of the antigen-presenting cell (APC) remain undefined. We report here the establishment of an in vitro system for the study of cross-tolerance and show that dendritic cells (DCs) phagocytose apoptotic cells and tolerize antigen-specific CD8+ T cells when cognate CD4+ T helper cells are absent. Using this system, we directly tested the "two-signal" hypothesis for the regulation of priming versus tolerance. We found that the same CD83+ myeloid-derived DCs were required for both cross-priming and cross-tolerance. These data suggested that the current model for peripheral T cell tolerance, "signal 1 in the absence of signal 2", requires refinement: the critical checkpoint is not DC maturation, but instead the presence of a third signal, which is active at the DC-CD4+ T cell interface.

Paraneoplastic neurologic disease antigens: RNA-binding proteins and signaling proteins in neuronal degeneration

Studies of the disorders known as paraneoplastic neurologic degenerations exemplify the successful application of modern molecular biological techniques to diseases, yielding, even for these extremely rare disorders, wide-ranging insight into basic neurobiology, tumor immunity, and autoimmune neurologic disease. Immune responses to paraneoplastic neurologic degeneration antigens, also called onconeural antigens, have been exploited to clone and characterize a number of neuron-specific proteins, including several RNA-binding proteins and new kinds of signaling molecules. The biology and functions of these proteins are reviewed, and a model in which their functions are related to the pathogenesis of autoimmune neurologic disease is discussed.

Neuronal signaling through alternative splicing: some exons CaRRE

Alternative splicing represents a mechanism by which a single gene can be used to create proteins with different functions. Neurons use alternative splicing to produce channels with different sequences and biophysical or regulatory properties. O'Donovan and Darnell discuss a mechanism by which neurons can alter channel splicing in response to neuronal activity through a signal generated by calcium and calcium/calmodulin-dependent kinase activity.

The splice of life: alternative splicing and neurological disease

Splicing of pre-messenger RNA is regulated differently in the brain compared with other tissues. Recognition of aberrations in splicing events that are associated with neurological disease has contributed to our understanding of disease pathogenesis in some cases. Neuron-specific proteins involved in RNA splicing and metabolism are also affected in several neurological disorders. These findings have begun to bridge what we know about the mechanisms regulating neuron-specific splicing and our understanding of neural function and disease.

A brain-enriched polypyrimidine tract-binding protein antagonizes the ability of Nova to regulate neuron-specific alternative splicing

The Nova paraneoplastic antigens are neuron-specific RNA binding proteins that participate in the control of alternative splicing. We have used the yeast two-hybrid system to isolate Nova interacting proteins and identify an RNA binding protein that is closely related to the polypyrimidine tract-binding protein (PTB). The expression of this protein, brPTB, is enriched in the brain, where it is expressed in glia and neurons. brPTB interacts with Nova proteins in cell lines and colocalizes with Nova within neuronal nuclei. We previously found that Nova binds to a pyrimidine-rich RNA element present upstream of an alternatively spliced exon, E3A, in glycine receptor alpha2 (GlyRalpha2) pre-mRNA, and this binding is implicated in Nova-dependent regulation of splicing. Cotransfection assays with a GlyRalpha2 minigene demonstrate that brPTB antagonizes the action of Nova to increase utilization of GlyRalpha2 E3A. brPTB binds to a 90-nt GlyRalpha2 RNA adjacent to the Nova binding site, but with an affinity that is more than 10-fold lower than Nova. When a putative binding site for brPTB on the GlyRalpha2 RNA is mutated, binding is abolished and the inhibitory effect on Nova-dependent exon selection disappears. These results suggest that brPTB is a tissue-restricted RNA binding protein that interacts with and inhibits the ability of Nova to activate exon selection in neurons.

The tetranucleotide UCAY directs the specific recognition of RNA by the Nova K-homology 3 domain

The Nova family of proteins are target antigens in the autoimmune disorder paraneoplastic opsoclonus-myoclonus ataxia and contain K-homology (KH)-type RNA binding domains. The Nova-1 protein has recently been shown to regulate alternative splicing of the alpha2 glycine receptor subunit pre-mRNA by binding to an intronic element containing repeats of the tetranucleotide UCAU. Here, we have used selection-amplification to demonstrate that the KH3 domain of Nova recognizes a single UCAY element in the context of a 20-base hairpin RNA; the UCAY tetranucleotide is optimally presented as a loop element of the hairpin scaffold and requires protein residues C-terminal to the previously defined KH domain. These results suggest that KH domains in general recognize tetranucleotide motifs and that biological RNA targets of KH domains may use either RNA secondary structure or repeated sequence elements to achieve high affinity and specificity of protein binding.

Cdr2, a target antigen of naturally occuring human tumor immunity, is widely expressed in gynecological tumors

The paraneoplastic neurological disorders provide perhaps the best known example of naturally occurring tumor immunity in humans. For example, patients with paraneoplastic cerebellar degeneration (PCD) appear to suppress the growth of occult breast or ovarian tumors that express a neuronal antigen termed cdr2. PCD patients harbor cdr2-specific CTLs in their peripheral blood, and these cells are likely mediators of the tumor suppression. Whereas cdr2 therefore appears to be the target of an effective immune response in patients with PCD, the general relevance to cancer patients has been unclear, due in part to reports indicating that cdr2 is not expressed in tumors obtained from neurologically normal patients. We have reexamined this question, and we find that cdr2 is widely expressed in such tumors, indicating that cdr2 is in fact an important tumor antigen in the general population of breast and ovarian cancer patients.

Involvement of the TRAP220 component of the TRAP/SMCC coactivator complex in embryonic development and thyroid hormone action

The TRAP220 component of the TRAP/SMCC complex, a mammalian homologof the yeast Mediator that shows diverse coactivation functions, interacts directly with nuclear receptors. Ablation of the murine Trap220 gene revealed that null mutants die during an early gestational stage with heart failure and exhibit impaired neuronal development with extensive apoptosis. Primary embryonic fibroblasts derived from null mutants show an impaired cell cycle regulation and a prominent decrease of thyroid hormone receptor function that is restored by ectopic TRAP220 but no defect in activation by Gal4-RARalpha/RXRalpha, p53, or VP16. Moreover, haploinsufficient animals show growth retardation, pituitary hypothyroidism, and widely impaired transcription in certain organs. These results indicate that TRAP220 is essential for a wide range of physiological processes but also that it has gene- and activator-selective functions.

Sequence-specific RNA binding by a Nova KH domain: implications for paraneoplastic disease and the fragile X syndrome

The structure of a Nova protein K homology (KH) domain recognizing single-stranded RNA has been determined at 2.4 A resolution. Mammalian Nova antigens (1 and 2) constitute an important family of regulators of RNA metabolism in neurons, first identified using sera from cancer patients with the autoimmune disorder paraneoplastic opsoclonus-myoclonus ataxia (POMA). The structure of the third KH domain (KH3) of Nova-2 bound to a stem loop RNA resembles a molecular vise, with 5'-Ura-Cyt-Ade-Cyt-3' pinioned between an invariant Gly-X-X-Gly motif and the variable loop. Tetranucleotide recognition is supported by an aliphatic alpha helix/beta sheet RNA-binding platform, which mimics 5'-Ura-Gua-3' by making Watson-Crick-like hydrogen bonds with 5'-Cyt-Ade-3'. Sequence conservation suggests that fragile X mental retardation results from perturbation of RNA binding by the FMR1 protein.

Nova-1 regulates neuron-specific alternative splicing and is essential for neuronal viability

We have combined genetic and biochemical approaches to analyze the function of the RNA-binding protein Nova-1, the paraneoplastic opsoclonus-myoclonus ataxia (POMA) antigen. Nova-1 null mice die postnatally from a motor deficit associated with apoptotic death of spinal and brainstem neurons. Nova-1 null mice show specific splicing defects in two inhibitory receptor pre-mRNAs, glycine alpha2 exon 3A (GlyRalpha2 E3A) and GABA(A) exon gamma2L. Nova protein in brain extracts specifically bound to a previously identified GlyRalpha2 intronic (UCAUY)3 Nova target sequence, and Nova-1 acted directly on this element to increase E3A splicing in cotransfection assays. We conclude that Nova-1 binds RNA in a sequence-specific manner to regulate neuronal pre-mRNA alternative splicing; the defect in splicing in Nova-1 null mice provides a model for understanding the motor dysfunction in POMA.

Detection and treatment of activated T cells in the cerebrospinal fluid of patients with paraneoplastic cerebellar degeneration

Patients with paraneoplastic cerebellar degeneration (PCD) offer the opportunity to explore the mechanisms underlying tumor immunity and immune-mediated neuronal degeneration. Cytotoxic T lymphocytes (CTLs) specific for the PCD onconeural antigen cdr2 found in the blood of patients with PCD are likely to be effectors of PCD tumor immunity. Here, we suggest a role for CTLs in the autoimmune destruction of Purkinje neurons. More than 75% of the cells obtained from the cerebrospinal fluid (CSF) of PCD patients were CD3+ alphabeta T cells. In patients with active/progressive disease, 20% to 40% of CSF cells were activated T cells, and the CD4+ helper cells were Th1-type cells. Three PCD patients were given tacrolimus, a specific inhibitor of activated T cells, which markedly reduced these cells in the CSF. Tacrolimus also reduced the number of activated cdr2-specific CTLs in the peripheral blood, but did not lead to tumor recurrence. We suggest that activated cdr2-specific CTLs in the CSF contribute to Purkinje degeneration in PCD, and that tacrolimus therapy may benefit patients with paraneoplastic neurological disease and other T cell-mediated autoimmune neurological disorders.

Mammalian ELAV-like neuronal RNA-binding proteins HuB and HuC promote neuronal development in both the central and the peripheral nervous systems

Hu proteins are mammalian embryonic lethal abnormal visual system (ELAV)-like neuronal RNA-binding proteins that contain three RNA recognition motifs. Although Drosophila ELAV is required for the correct differentiation and survival of neurons, the roles played by the Hu genes in the mammalian nervous system remain largely unknown. To explore the in vivo functions of mouse Hu proteins, we overexpressed them in rat pheochromocytoma PC12 cells, where they induced neuronal phenotype in the absence of nerve growth factor. We have characterized the functions of various forms of mHuB and mHuC bearing point mutations or deletions. Mutants of mHuC that had amino acid exchanges in the RNP1 domain of the first or second RNA recognition motifs (RRMs) lost biologic activity as well as RNA-binding activity. In addition, the mutants containing only the third RRM failed to induce the neuronal phenotype in PC12 cells and inhibited the biologic activity of cotransfected wild-type mHuB and mHuC, thus acting as a dominant-negative form. However, these mutants could not suppress the nerve growth factor-induced differentiation of PC12 cells. Further, we misexpressed wild-type and dominant-negative Hu in E9.5 mouse embryos, by using electroporation into the neural tube at the level of the rhombencephalon. mHuB and mHuC induced the ectopic expression of neuronal markers, whereas the dominant-negative forms of mHuB and mHuC suppressed the differentiation of central nervous system motor neurons. From these results, we suggest that Hu proteins are required for neuronal differentiation in the mammalian nervous system.

The cytoplasmic Purkinje onconeural antigen cdr2 down-regulates c-Myc function: implications for neuronal and tumor cell survival

Paraneoplastic cerebellar degeneration (PCD) is a disorder in which breast or ovarian tumors express an onconeural antigen termed cdr2, which normally is expressed in cerebellar Purkinje neurons. This leads to an immune response to cdr2 that is associated with tumor immunity and autoimmune cerebellar degeneration. We have found that cdr2, a cytoplasmic protein harboring a helix-leucine zipper (HLZ) motif, interacts specifically with the HLZ motif of c-Myc. Both proteins colocalize in the cytoplasm of adult cerebellar Purkinje neurons, and coimmunoprecipitate from tumor cell lines and cerebellar extracts. cdr2 down-regulates c-Myc-dependent transcription in cotransfection assays, and redistributes Myc protein in the cytoplasm. Disease antisera from six of six PCD patients specifically blocked the interaction between cdr2 and c-Myc in vitro. These data indicate that cdr2 normally sequesters c-Myc in the neuronal cytoplasm, thereby down-regulating c-Myc activity, and suggest a mechanism whereby inhibition of cdr2 function by autoantibodies in PCD may contribute to Purkinje neuronal death.

Crystal structures of Nova-1 and Nova-2 K-homology RNA-binding domains

BACKGROUND

Nova-1 and Nova-2 are related neuronal proteins that were initially cloned using antisera obtained from patients with the autoimmune neurological disease paraneoplastic opsoclonus-myoclonus ataxia (POMA). Both of these disease gene products contain three RNA-binding motifs known as K-homology or KH domains, and their RNA ligands have been identified via binding-site selection experiments. The KH motif structure has been determined previously using NMR spectroscopy, but not using X-ray crystallography. Many proteins contain more than one KH domain, yet there is no published structural information regarding the behavior of such multimers.

RESULTS

We have obtained the first X-ray crystallographic structures of KH-domain-containing proteins. Structures of the third KH domains (KH3) of Nova-1 and Nova-2 were determined by multiple isomorphous replacement and molecular replacement at 2.6 A and 2.0 A, respectively. These highly similar RNA-binding motifs form a compact protease-resistant domain resembling an open-faced sandwich, consisting of a three-stranded antiparallel beta sheet topped by three alpha helices. In both Nova crystals, the lattice is composed of symmetric tetramers of KH3 domains that are created by two dimer interfaces.

CONCLUSIONS

The crystal structures of both Nova KH3 domains are similar to the previously determined NMR structures. The most significant differences among the KH domains involve changes in the positioning of one or more of the alpha helices with respect to the betasheet, particularly in the NMR structure of the KH1 domain of the Fragile X disease protein FMR-1. Loop regions in the KH domains are clearly visible in the crystal structure, unlike the NMR structures, revealing the conformation of the invariant Gly-X-X-Gly segment that is thought to participate in RNA-binding and of the variable region. The tetrameric arrangements of the Nova KH3 domains provide insights into how KH domains may interact with each other in proteins containing multiple KH motifs.

Tumor-specific killer cells in paraneoplastic cerebellar degeneration

Models for immune-mediated tumor regression in mice have defined an essential role for cytotoxic T lymphocytes (CTLs); however, naturally occurring tumor immunity in humans is poorly understood. Patients with paraneoplastic cerebellar degeneration (PCD) provide an opportunity to explore the mechanisms underlying tumor immunity to breast and ovarian cancer. Although tumor immunity and autoimmune neuronal degeneration in PCD correlates with a specific antibody response to the tumor and brain antigen cdr2, this humoral response has not been shown to be pathogenic. Here we present evidence for a specific cellular immune response in PCD patients. We have detected expanded populations of MHC class I-restricted cdr2-specific CTLs in the blood of 3/3 HLA-A2.1+ PCD patients, providing the first description, to our knowledge, of tumor-specific CTLs using primary human cells in a simple recall assay. Cross-presentation of apoptotic cells by dendritic cells also led to a potent CTL response. These results indicate a model whereby immature dendritic cells that engulf apoptotic tumor cells can mature and migrate to draining lymph organs where they could induce a CTL response to tissue-restricted antigens. In PCD, peripheral activation of cdr2-specific CTLs is likely to contribute to the subsequent development of the autoimmune neuronal degeneration.

The neuronal RNA-binding protein Nova-2 is implicated as the autoantigen targeted in POMA patients with dementia

Paraneoplastic opsoclonus myoclonus ataxia (POMA) is a neurologic disorder thought to be mediated by an autoimmune attack against onconeural disease antigens that are expressed by gynecologic or lung tumors and by neurons. One POMA disease antigen, termed Nova-1, has been identified as a neuron-specific KH-type RNA-binding protein. Nova-1 expression is restricted to specific regions of the central nervous system, primarily the hindbrain and ventral spinal cord, which correlate with the predominantly motor symptoms in POMA. However, POMA antisera recognize antigens that are widely expressed in both caudal and rostral regions of the central nervous system, and some patients develop cognitive symptoms. We have used POMA antisera to clone a cDNA encoding a second POMA disease antigen termed Nova-2. Nova-2 is closely related to Nova-1, and is expressed at high levels in neurons during development and in adulthood, and at lower levels in the adult lung. In the postnatal mouse brain, Nova-2 is expressed in a pattern that is largely reciprocal with Nova-1, including high levels of Nova-2 expression in the neocortex and hippocampus. Functional characterization of Nova-2 in RNA selection and nitrocellulose filter-binding assays reveals that Nova-2 binds RNA with high affinity and with sequence specificity that differs from Nova-1. Our results demonstrate that the immune response in POMA targets a family of highly related sequence-specific neuronal RNA-binding proteins. The expression pattern of the Nova-2 protein is likely to underlie the development of cognitive deficits in some POMA patients.

ATM binds to beta-adaptin in cytoplasmic vesicles

Inherited mutations in the ATM gene lead to a complex clinical phenotype characterized by neuronal degeneration, oculocutaneous telangiectasias, immune dysfunction, and cancer predisposition. Using the yeast two-hybrid system, we demonstrate that ataxia telangiectasia mutated (ATM) binds to beta-adaptin, one of the components of the AP-2 adaptor complex, which is involved in clathrin-mediated endocytosis of receptors. The interaction between ATM and beta-adaptin was confirmed in vitro, and coimmunoprecipitation and colocalization studies show that the proteins also associate in vivo. ATM also interacts in vitro with beta-NAP, a neuronal-specific beta-adaptin homolog that was identified as an autoantigen in a patient with cerebellar degeneration. Our data describing the association of ATM with beta-adaptin in vesicles indicate that ATM may play a role in intracellular vesicle and/or protein transport mechanisms.

Mouse chromosomal locations of nine genes encoding homologs of human paraneoplastic neurologic disorder antigens

The paraneoplastic neurologic disorders (PND) are a rare group of neurologic syndromes that arise when an immune response to systemic tumors expressing neuronal proteins ("onconeural antigens") develops into an autoimmune neuronal degeneration. The use of patient antisera to clone the genes encoding PND antigens has led to new insight into the mechanism of these autoimmune disorders. The tumor antigens can now be grouped into three classes: (1) neuron-specific RNA-binding proteins, (2) nerve terminal vesicle-associated proteins, and (3) cytoplasmic signaling proteins. To understand better the evolutionary relatedness of these genes and to evaluate them as candidates for inherited neurological disorders, we have determined the mouse chromosomal locations of nine of these genes-Hua, Hub, Huc, Hud, Nova1, Nova2, Natpb, Cdr2, and Cdr3. These data suggest that the Hua-Hud genes arose from gene duplication and dispersion, while the other genes are dispersed in the genome. We also predict the chromosomal locations of these genes in human and discuss the potential of these genes as candidates for uncloned mouse and human mutations.

The neuronal RNA binding protein Nova-1 recognizes specific RNA targets in vitro and in vivo

Nova-1, an autoantigen in paraneoplastic opsoclonus myoclonus ataxia (POMA), a disorder associated with breast cancer and motor dysfunction, is a neuron-specific nuclear RNA binding protein. We have identified in vivo Nova-1 RNA ligands by combining affinity-elution-based RNA selection with protein-RNA immunoprecipitation. Starting with a pool of approximately 10(15) random 52-mer RNAs, we identified long stem-loop RNA ligands that bind to Nova-1 with high affinity (Kd of approximately 2 nM). The loop region of these RNAs harbors a approximately 15-bp pyrimidine-rich element [UCAU(N)(0-2)]3 which is essential for Nova-1 binding. Mutagenesis studies defined the third KH domain of Nova-1 and the [UCAU(N)(0-2)]3 element as necessary for in vitro binding. Consensus [UCAU (N)(0-2)], elements were identified in two neuronal pre-mRNAs, one encoding the inhibitory glycine receptor alpha2 (GlyR alpha2) and a second encoding Nova-1 itself. Nova-1 protein binds these RNAs with high affinity and specificity in vitro, and this binding can be blocked by POMA antisera. Moreover, both Nova-1 and GlyR alpha2 pre-mRNAs specifically coimmunoprecipitated with Nova-1 protein from brain extracts. Thus, Nova-1 functions as a sequence-specific nuclear RNA binding protein in vivo; disruption of the specific interaction between Nova-1 and GlyR alpha2 pre-mRNA may underlie the motor dysfunction seen in POMA.

A hierarchy of Hu RNA binding proteins in developing and adult neurons

The Hu proteins are a group of antigens targeted in an immune-mediated neurodegenerative disorder associated with cancer. We have cloned and characterized four members of the Hu gene family from mouse. We find that the Hu genes encode a large number of alternatively spliced transcripts to produce a series of related neuron-specific RNA binding proteins. Despite this complexity, we have discerned several ordered features of Hu expression. In the embryo, specific Hu genes are expressed in a hierarchy during early neurogenesis. In the E16 developing cortex, mHuB is induced in very early postmitotic neurons exiting the ventricular zone, mHuD is expressed in migrating neurons of the intermediate zone, and mHuC is expressed in mature cortical plate neurons. Such a hierarchy suggests distinct functional roles for each gene in developing neurons. In the adult, all neurons express some set of Hu mRNA and protein. However, specific patterns are evident such that individual neuronal types in the hippocampus, cerebellum, olfactory cortex, neocortex, and elsewhere express from one to several Hu genes. The complexity of potential protein variants within a gene family and of different Hu family members within a neuron suggests a diverse array of function. Given the strong homologies among the Hu proteins, the Drosophila neurogenic gene elav, and the Drosophila splicing factor sxl, we predict that different combinations of Hu proteins determine different neuron-specific aspects of post-transcriptional RNA regulation. Our findings of specific developmental patterns of expression and the correlation between immune targeting of the Hu proteins and adult neurodegenerative disease suggest that the Hu proteins are critical in both the proper development and function of mature neurons.

A post-transcriptional regulatory mechanism restricts expression of the paraneoplastic cerebellar degeneration antigen cdr2 to immune privileged tissues

Paraneoplastic cerebellar degeneration (PCD) is believed to be an autoimmune disorder initiated by the ectopic expression of a neuron-specific protein in breast and ovarian tumors. PCD antisera was used previously to identify several cerebellar degeneration-related (cdr) genes encoding putative PCD antigens. We have found that the cdr2 gene, which encodes a cytoplasmic leucine zipper protein of unknown function, is expressed in PCD-associated tumors, whereas other cdr genes are not; thus, cdr2 encodes the PCD tumor antigen. To determine whether the expression pattern of cdr2 is consistent with its proposed role in PCD, we have isolated the mouse homolog and examined both the mRNA and protein distribution in adult tissues. We have found that cdr2 mRNA is expressed in almost all tissues, whereas the protein is expressed only in the brain and testis. Within the brain, both the cdr2 mRNA and immunoreactivity are confined primarily to neurons in the cerebellum and brainstem, the regions most affected in PCD. These results suggest first that the tissue-specific expression of cdr2 is regulated at a post-transcriptional level. Moreover, because the brain and testis are considered to be immune-privileged sites, the expression pattern of cdr2 is compatible with the autoimmune model of PCD pathogenesis.

Onconeural antigens and the paraneoplastic neurologic disorders: at the intersection of cancer, immunity, and the brain

Paraneoplastic neurologic disorders (PNDs) are believed to be autoimmune neuronal degenerations that develop in some patients with systemic cancer. A series of genes encoding previously undiscovered neuronal proteins have been cloned using antiserum from PND patients. Identification of these onconeural antigens suggests a reclassification of the disorders into four groups: those in which neuromuscular junction proteins, nerve terminal/vesicle-associated proteins, neuronal RNA binding proteins, or neuronal signal-transduction proteins serve as target antigens. This review considers insights into basic neurobiology, tumor immunology, and autoimmune neuronal degeneration offered by the characterization of the onconeural antigens.

A novel adaptor-related protein complex

Coat proteins are required for the budding of the transport vesicles that mediate membrane traffic pathways, but for many pathways such proteins pathways, but for many pathways such proteins have not yet been identified. We have raised antibodies against p47, a homologue of the medium chains of the adaptor complexes of clathrin-coated vesicles (Pevsner, J., W. Volknandt, B.R. Wong, and R.H. Scheller. 1994. Gene (Amst.). 146:279-283), to determine whether this protein might be a component of a new type of coat. p47 coimmunoprecipitates with three other proteins: two unknown proteins of 160 and 25 kD, and beta-NAP, a homologue of the beta/beta'-adaptins, indicating that it is a subunit of an adaptor-like heterotetrameric complex. However, p47 is not enriched in preparations of clathrin-coated vesicles. Recruitment of the p47-containing complex onto cell membranes is stimulated by GTP gamma S and blocked by brefeldin A, indicating that, like other coat proteins, its membrane association is regulated by an ARF. The newly recruited complex is localized to non-clathrin-coated buds and vesicles associated with the TGN. Endogenous complex in primary cultures of neuronal cells is also localized to the TGN, and in addition, some complex is associated with the plasma membrane. These results indicate that the complex is a component of a novel type of coat that facilitates the budding of vesicles from the TGN, possibly for transporting newly synthesized proteins to the plasma membrane.

The onconeural antigen Nova-1 is a neuron-specific RNA-binding protein, the activity of which is inhibited by paraneoplastic antibodies

Nova-1, a protein expressed in tumors and neurons, is a target antigen in a human paraneoplastic motor disorder [paraneoplastic opsoclonus-myoclonus ataxia (POMA)]. We evaluated the relationship between the function of Nova-1 and its role as a disease antigen. We show that Nova-1 is a neuron-specific RNA-binding protein with sequence and functional similarities to FMR-1. Nova-1 mRNA is restricted to the subcortical nervous system, and the protein binds to RNA with high affinity. Nova-1 KH domains mediate this RNA binding, and point mutations within them abrogate binding. POMA disease antisera (6/6) recognize the third KH domain but not an inactive point mutant, and affinity-purified antibody blocks Nova-1 RNA binding. Thus, a cardinal feature of POMA is the production of antibodies that inhibit Nova-1-RNA interactions, suggesting such inhibition may cause the neurological disease.

Beta-NAP, a cerebellar degeneration antigen, is a neuron-specific vesicle coat protein

We have identified a target antigen in autoimmune cerebellar degeneration, beta-NAP, that is closely related to the beta-adaptin and beta-COP coat proteins. Beta-NAP is a nonclathrin-associated phosphoprotein expressed exclusively in neurons, from E12 through adulthood. Beta-NAP is present in the neuronal soma and nerve terminal as soluble and membrane-bound pools and is associated with a discrete set of nerve-terminal vesicles. These results establish beta-NAP as a neuron-specific vesicle coat protein. We propose a model in which beta-NAP mediates vesicle transport between the soma and the axon terminus and suggest that beta-NAP may represent a general class of coat proteins that mediates apical transport in polarized cells.

Nova, the paraneoplastic Ri antigen, is homologous to an RNA-binding protein and is specifically expressed in the developing motor system

Paraneoplastic opsoclonus-ataxia, a disorder of motor control, develops in breast or lung cancer patients who harbor an antibody (Ri) that recognizes their tumors and a nuclear neuronal protein of 55 kd. We have characterized a gene, Nova, encoding an antigen recognized by the Ri antibody. Nova encodes a novel, highly conserved protein, homologous to the RNA-binding protein hnRNP K, the yeast splicing protein MER1, and a motif in several retroviral proteases. Northern blot analysis detects Nova transcripts only in brain, and several alternatively spliced forms are present in brain and tumor cells. Nova expression is restricted to the ventral brain stem and spinal cord in E18 mice. Since Nova encodes a target antigen in the motor disorder paraneoplastic opsoclonus-ataxia that is expressed in the developing subcortical motor system, it is a likely participant in both the pathogenesis of paraneoplastic opsoclonus-ataxia and the developmental biology of the motor system. The homology between Nova and hnRNP K suggests that Nova regulates RNA splicing or metabolism in a specific subset of developing neurons.

Reversible diencephalic dysfunction: episodic hyperhidrosis due to a trapped third ventricle

We present a patient with episodic hyperhidrosis (EH) who had 15 attacks per day for 3 years following placement of a lateral ventricular shunt. MRI revealed that she had an isolated third ventricle. Stereotactic shunting of the third ventricle resulted in immediate and complete resolution of her symptoms. We suggest that this patient had periodic rises in third ventricular pressure resulting in episodic diencephalic dysfunction and EH.

The neuronal nuclear antigen recognized by the human anti-Ri autoantibody is expressed in central but not peripheral nervous system neurons

Anti-Ri is a human autoantibody that recognizes a neuronal nuclear antigen (Ri). Biotinylated IgG from serum of two patients with high titers of anti-Ri antibodies was used to study the distribution of the Ri antigen in a panel of normal human tissues. the expression of the Ri antigen was evaluated by an avidin-biotin peroxidase technique and confirmed by immunoblotting. The Ri antigen was restricted to the neurons of the central nervous system (CNS) and some pituitary cells. Most neurons in dorsal root, Gasserian and sympathetic ganglia, and myenteric plexus were negative or, a few of them, very weakly positive. The functional implication of the different expression of the Ri antigen between neurons of the central and peripheral nervous system is presently unknown.

Regression of small-cell lung carcinoma in patients with paraneoplastic neuronal antibodies

We describe three patients with known or suspected small-cell lung cancer (SCLC), paraneoplastic neurological syndromes, and antineuronal antibodies who had unusually benign clinical courses. One patient survived 8 years free of disease and was positive for the anti-Hu antibody. A second patient survived 6 years after spontaneous tumour regression and had an atypical antineuronal antibody. A third patient with both the anti-Hu and atypical antineuronal antibody had spontaneous regression of a lung mass. All three patients had a subacute sensory neuropathy. Since paraneoplastic antineuronal antibodies also bind to tumour cells, these cases suggest that some (paraneoplastic) neurological syndromes without identifiable tumour may result from immune-mediated eradication of tumour cells.

A novel antineuronal antibody in stiff-man syndrome

Two-thirds of stiff-man syndrome (SMS) patients harbor an autoantibody specific for a 64-kD species of glutamic acid decarboxylase (GAD), the rate-limiting enzyme in GABA synthesis. We assayed SMS antisera from two patients with SMS for the presence of anti-GAD antibodies using Western blot, immunohistochemical, and enzymatic analyses. Both SMS antisera recognized an 80-kD antigen present in human and rat neuronal extracts, and failed to recognize the 64-kD GAD species. Immunohistochemistry demonstrated neuronal binding identical to that reported with anti-GAD antibodies. Both sera depleted GAD activity from brain extracts. Our analysis indicates that these SMS antisera differ from previously reported SMS antisera by recognizing a novel 80-kD antigen, and suggests that they contain antibodies directed against either a species of GAD different in size from the 64-kD enzyme, or a protein that co-immunoprecipitates with GAD.

Antiserum from a patient with cerebellar degeneration identifies a novel protein in Purkinje cells, cortical neurons, and neuroectodermal tumors

The serum and cerebrospinal fluid of a patient (NB) with subacute cerebellar degeneration were found to contain a novel antineuronal autoantibody (anti-Nb). Using this antibody, we have identified and characterized antigens present in a subset of neurons in the CNS and in some neuroectodermal tumor lines. Anti-Nb antibody bound to antigens of Mr 150, 120, and 65 kDa in Western blots using extracts of human cerebellar Purkinje cells or human cerebral cortical neurons. Immunohistochemistry demonstrated relatively specific binding of anti-Nb IgG to Purkinje cells in sections of human cerebellum and to some neocortical neurons, especially those in layer VI. Because of the association of cerebellar degeneration with occult malignancies, we screened a number of tumor cell lines for immunoreactivity to anti-Nb antibody; only tumor lines of neuroectodermal origin (melanoma, small-cell lung cancer, and neuroblastoma) expressed the Nb antigen. Anti-Nb antibody thus identifies neuronal and tumor cell antigens that appear to be unique in size and distribution of expression.

Differential expression of the human gonadotropin alpha gene in ectopic and eutopic cells

We have analyzed the regulation of the alpha gonadotropin gene in eutopic placental cells and ectopic tumor cells by constructing a series of plasmid vectors containing alpha genomic 5' flanking DNA placed upstream of the gene encoding the bacterial enzyme chloramphenicol acetyltransferase (CAT). These plasmid DNAs were transfected into a eutopic (JAr) and an ectopic (HeLa) cell line. Both cell types expressed the CAT gene from plasmid constructs containing as much as 1,500 base pairs (bp) and as little as 140 bp of alpha 5' flanking DNA; JAr cells were considerably more efficient than HeLa cells. Ectopic and eutopic cells differed qualitatively in their expression from these alpha-CAT constructs when cells were treated with cAMP or butyrate. Butyrate induced alpha expression in HeLa cells but not in JAr cells, while cAMP induced expression in JAr cells. These results are consistent with and extend previous observations suggesting that there are cell-specific differences in the regulation of alpha gene expression in ectopic and eutopic cells. However, by using deletion constructs of the alpha-CAT gene, we found that the basal expression and cell-specific induction of the alpha gene in ectopic and eutopic cells were dependent on the same 140 bp of alpha 5' flanking DNA. These 140 bp were sequenced and found to contain a 9-bp stretch of DNA homologous with the consensus viral enhancer sequence. Such features of alpha expression common to both ectopic and eutopic cells may be involved in the coordinate expression of the alpha gene and the tumorigenic phenotype observed in each cell type.

Independent regulation by sodium butyrate of gonadotropin alpha gene expression and cell cycle progression in HeLa cells

Sodium butyrate alters the growth and gene expression of a variety of differentiating and neoplastic cell types. For example, addition of 5 mM butyrate to HeLa cells is reported to both induce gonadotropin alpha subunit biosynthesis and block cell cycling in G1. We have studied these two actions of butyrate on HeLa cells and found that they are regulated in distinct ways. The induction of alpha subunit synthesis was due to an increase in the rate of transcription of the alpha gene. Using synchronized populations of HeLa cells, we determined that butyrate stimulates alpha transcription throughout the cell cycle. In contrast, treated cells arrest in G1 only if exposed to butyrate for a discrete period during the previous S phase. We conclude that butyrate inhibits DNA synthesis through a cell cycle-specific action that is independent from its direct action to stimulate transcription of the gonadotropin alpha gene.