The β-Secretase Substrate Seizure 6-Like Protein (SEZ6L) Controls Motor Functions in Mice

The membrane protein seizure 6-like (SEZ6L) is a neuronal substrate of the Alzheimer's disease protease BACE1, and little is known about its physiological function in the nervous system. Here, we show that SEZ6L constitutive knockout mice display motor phenotypes in adulthood, including changes in gait and decreased motor coordination. Additionally, SEZ6L knockout mice displayed increased anxiety-like behaviour, although spatial learning and memory in the Morris water maze were normal. Analysis of the gross anatomy and proteome of the adult SEZ6L knockout cerebellum did not reveal any major differences compared to wild type, indicating that lack of SEZ6L in other regions of the nervous system may contribute to the phenotypes observed. In summary, our study establishes physiological functions for SEZ6L in regulating motor coordination and curbing anxiety-related behaviour, indicating that aberrant SEZ6L function in the human nervous system may contribute to movement disorders and neuropsychiatric diseases.

Lack of Sez6 Family Proteins Impairs Motor Functions, Short-Term Memory, and Cognitive Flexibility and Alters Dendritic Spine Properties

Seizure-related gene 6 (Sez6), Sez6-Like (Sez6L), and Sez6-Like 2 (Sez6L2) comprise a family of homologous proteins widely expressed throughout the brain that have been linked to neurodevelopmental and psychiatric disorders. Here, we use Sez6 triple knockout (TKO) mice, which lack all three Sez6 family proteins, to demonstrate that Sez6 family proteins regulate dendritic spine structure and cognitive functions, motor learning, and maintenance of motor functions across the lifespan. Compared to WT controls, we found that Sez6 TKO mice had impaired motor learning and their motor coordination was negatively affected from 6 weeks old and declined more rapidly as they aged. Sez6 TKO mice had reduced spine density in the hippocampus and dendritic spines were shifted to more immature morphologies in the somatosensory cortex. Cognitive testing revealed that they had enhanced stress responsiveness, impaired working, and spatial short-term memory but intact spatial long-term memory in the Morris water maze albeit accompanied by a reversal deficit. Our study demonstrates that the lack of Sez6 family proteins results in phenotypes commonly associated with neuropsychiatric disorders making it likely that Sez6 family proteins contribute to the complex etiologies of these disorders.

The Sez6 Family Inhibits Complement by Facilitating Factor I Cleavage of C3b and Accelerating the Decay of C3 Convertases

The Sez6 family consists of Sez6, Sez6L, and Sez6L2. Its members are expressed throughout the brain and have been shown to influence synapse numbers and dendritic morphology. They are also linked to various neurological and psychiatric disorders. All Sez6 family members contain 2-3 CUB domains and 5 complement control protein (CCP) domains, suggesting that they may be involved in complement regulation. We show that Sez6 family members inhibit C3b/iC3b opsonization by the classical and alternative pathways with varying degrees of efficacy. For the classical pathway, Sez6 is a strong inhibitor, Sez6L2 is a moderate inhibitor, and Sez6L is a weak inhibitor. For the alternative pathway, the complement inhibitory activity of Sez6, Sez6L, and Sez6L2 all equaled or exceeded the activity of the known complement regulator MCP. Using Sez6L2 as the representative family member, we show that it specifically accelerates the dissociation of C3 convertases. Sez6L2 also functions as a cofactor for Factor I to facilitate the cleavage of C3b; however, Sez6L2 has no cofactor activity toward C4b. In summary, the Sez6 family are novel complement regulators that inhibit C3 convertases and promote C3b degradation.

Seizure protein 6 controls glycosylation and trafficking of kainate receptor subunits GluK2 and GluK3

Seizure protein 6 (SEZ6) is required for the development and maintenance of the nervous system, is a major substrate of the protease BACE1 and is linked to Alzheimer's disease (AD) and psychiatric disorders, but its molecular functions are not well understood. Here, we demonstrate that SEZ6 controls glycosylation and cell surface localization of kainate receptors composed of GluK2/3 subunits. Loss of SEZ6 reduced surface levels of GluK2/3 in primary neurons and reduced kainate-evoked currents in CA1 pyramidal neurons in acute hippocampal slices. Mechanistically, loss of SEZ6 in vitro and in vivo prevented modification of GluK2/3 with the human natural killer-1 (HNK-1) glycan, a modulator of GluK2/3 function. SEZ6 interacted with GluK2 through its ectodomain and promoted post-endoplasmic reticulum transport of GluK2 in the secretory pathway in heterologous cells and primary neurons. Taken together, SEZ6 acts as a new trafficking factor for GluK2/3. This novel function may help to better understand the role of SEZ6 in neurologic and psychiatric diseases.

Evidence that a defined population of neurons in lateral amygdala is directly involved in auditory fear learning and memory

Memory is thought to be encoded within networks of neurons within the brain, but the identity of the neurons involved and circuits they form have not been described for any memory. Previously, we used fos-tau-lacZ (FTL) transgenic mice to identify discrete populations of neurons in different regions of the brain which were specifically activated following fear conditioning. This suggested that these populations of neurons form nodes in a network that encodes fear memory. In particular, one population of learning activated neurons was found within a discrete region of the lateral amygdala (LA), a key nucleus required for fear conditioning. In order to provide evidence that this population is directly involved in fear conditioning, we have analysed the expression of a key molecular requirement for fear conditioning in LA, phosphorylated Extracellular Signal Regulated Kinase 1 and 2 (pERK1/2). The only neurons in LA that specifically expressed pERK1/2 following auditory fear conditioning were in the ventrolateral nucleus of the LA (LAvl), in the same discrete region where we found learning specific FTL⁺ neurons. Double labelling experiments in FTL mice showed that a substantial proportion of the learning activated neurons expressed both pERK1/2 and FTL. These experiments provide clear evidence that the learning specific neurons we identified within LAvl are directly involved in auditory fear conditioning. In addition, learning specific expression of pERK1/2 was found in a dense network of dendrites contained within the border region of the LAvl. This network of dendrites may represent an activated dendritic field involved in fear conditioning in LA.

Sez6 levels are elevated in cerebrospinal fluid of patients with inflammatory pain-associated conditions

INTRODUCTION

Seizure-related protein 6 (Sez6) contributes to chronic pain development as sez6 knockout mice show attenuated pain behaviours after peripheral nerve injury, compared with control mice. The type I transmembrane isoform of Sez6 is cleaved by the β-amyloid precursor protein cleavage enzyme 1 (BACE1), resulting in Sez6 extracellular domain shedding from the neuron surface.

OBJECTIVES

To determine whether this BACE1-shed form of Sez6 can be detected in the cerebrospinal fluid (CSF) and whether Sez6 levels in the CSF are altered in neuropathic pain or chronic inflammatory pain (IP).

METHODS

We analysed the CSF samples collected during surgery from patients with chronic neuropathic pain (n = 8) or IP (n = 33), comparing them to the CSF samples from patients with suspected subarachnoid haemorrhage that was subsequently excluded (nonsurgical group, n = 5). Western blots were used to determine the relative Sez6 levels in the CSF from the different patient and nonsurgical comparison groups.

RESULTS

The results show that BACE1-shed Sez6 can be readily detected in the CSF by Western blot and that the levels of Sez6 are significantly higher in the IP group than in the nonsurgical comparison group.

CONCLUSION

The association between elevated Sez6 levels in the CSF and IP is further evidence for persistent alterations in central nervous system activity in chronic IP conditions.

Seizure protein 6 and its homolog seizure 6-like protein are physiological substrates of BACE1 in neurons

Background

The protease BACE1 (beta-site APP cleaving enzyme) is a major drug target in Alzheimer’s disease. However, BACE1 therapeutic inhibition may cause unwanted adverse effects due to its additional functions in the nervous system, such as in myelination and neuronal connectivity. Additionally, recent proteomic studies investigating BACE1 inhibition in cell lines and cultured murine neurons identified a wider range of neuronal membrane proteins as potential BACE1 substrates, including seizure protein 6 (SEZ6) and its homolog SEZ6L.

Methods and results

We generated antibodies against SEZ6 and SEZ6L and validated these proteins as BACE1 substrates in vitro and in vivo. Levels of the soluble, BACE1-cleaved ectodomain of both proteins (sSEZ6, sSEZ6L) were strongly reduced upon BACE1 inhibition in primary neurons and also in vivo in brains of BACE1-deficient mice. BACE1 inhibition increased neuronal surface levels of SEZ6 and SEZ6L as shown by cell surface biotinylation, demonstrating that BACE1 controls surface expression of both proteins. Moreover, mass spectrometric analysis revealed that the BACE1 cleavage site in SEZ6 is located in close proximity to the membrane, similar to the corresponding cleavage site in SEZ6L. Finally, an improved method was developed for the proteomic analysis of murine cerebrospinal fluid (CSF) and was applied to CSF from BACE-deficient mice. Hereby, SEZ6 and SEZ6L were validated as BACE1 substrates in vivo by strongly reduced levels in the CSF of BACE1-deficient mice.

Conclusions

This study demonstrates that SEZ6 and SEZ6L are physiological BACE1 substrates in the murine brain and suggests that sSEZ6 and sSEZ6L levels in CSF are suitable markers to monitor BACE1 inhibition in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0134-z) contains supplementary material, which is available to authorized users.

Tracking the fear memory engram: discrete populations of neurons within amygdala, hypothalamus, and lateral septum are specifically activated by auditory fear conditioning

Memory formation is thought to occur via enhanced synaptic connectivity between populations of neurons in the brain. However, it has been difficult to localize and identify the neurons that are directly involved in the formation of any specific memory. We have previously used fos-tau-lacZ (FTL) transgenic mice to identify discrete populations of neurons in amygdala and hypothalamus, which were specifically activated by fear conditioning to a context. Here we have examined neuronal activation due to fear conditioning to a more specific auditory cue. Discrete populations of learning-specific neurons were identified in only a small number of locations in the brain, including those previously found to be activated in amygdala and hypothalamus by context fear conditioning. These populations, each containing only a relatively small number of neurons, may be directly involved in fear learning and memory.

Differential labeling of cell-surface and internalized proteins after antibody feeding of live cultured neurons

In order to demonstrate the cell-surface localization of a putative transmembrane receptor in cultured neurons, we labeled the protein on the surface of live neurons with a specific primary antibody raised against an extracellular portion of the protein. Given that receptors are trafficked to and from the surface, if cells are permeabilized after fixation then both cell-surface and internal protein will be detected by the same labeled secondary antibody. Here, we adapted a method used to study protein trafficking ("antibody feeding") to differentially label protein that had been internalized by endocytosis during the antibody incubation step and protein that either remained on the cell surface or was trafficked to the surface during this period. The ability to distinguish these two pools of protein was made possible through the incorporation of an overnight blocking step with highly-concentrated unlabeled secondary antibody after an initial incubation of unpermeabilized neurons with a fluorescently-labeled secondary antibody. After the blocking step, permeabilization of the neurons allowed detection of the internalized pool with a fluorescent secondary antibody labeled with a different fluorophore. Using this technique we were able to obtain important information about the subcellular location of this putative receptor, revealing that it was, indeed, trafficked to the cell-surface in neurons. This technique is broadly applicable to a range of cell types and cell-surface proteins, providing a suitable antibody to an extracellular epitope is available.

Ndfip1 is required for the development of pyramidal neuron dendrites and spines in the neocortex

Ubiquitin ligases of the Nedd4 family are important for axon and dendrite development, but little is known about their adaptor, Nedd4 family-interacting protein 1 (Ndfip1), that is responsible for their enzymatic activation. To study the function of Ndfip1 in cortical development, we generated a conditional knock-out (conditional KO) in neurons. The Ndfip1 conditional KO mice were viable; however, cortical neurons in the adult brain exhibited atrophic characteristics, including stunted dendritic arbors, blebbing of dendrites, and fewer dendritic spines. In electron micrographs, these neurons appeared shrunken with compacted somata and involutions of the nuclear membrane. In culture, Ndfip1 KO neurons exhibited exuberant sprouting suggesting loss of developmental control. Biochemical analysis of postsynaptic density (PSD) fractions from Ndfip1 KO cortical and hippocampal neurons showed that the postsynaptic proteins (Arc and PSD-95) were reduced compared with wild-type controls. In addition, the PI3 kinase/Akt signaling pathway was altered. These results indicate that Ndfip1, through its Nedd4 effectors, is important for the development of dendrites and dendritic spines in the cortex.

Cortical layer development and orientation is modulated by relative contributions of reelin-negative and -positive neurons in mouse chimeras

Reelin is an important protein that is indispensable for cortical lamination. In the absence of Reelin, cortical layers fail to form due to inappropriate neuron migration and positioning. The inversion of cortical layers is attributed to failure of neurons to migrate past earlier-generated neurons although how Reelin-insufficiency causes this is unclear. The issue is complicated by recent studies showing that very little Reelin is required for cortical layering. To test how variation in the number of Reelin-producing cells is linked to cortical lamination, we have employed Reelin(+/+) <--> Reelin(-/-) chimeras in which the number of Reelin-expressing neurons is adjusted. We found that the Reeler phenotype was rescued in chimeras with a large contribution of Reelin(+/+) neurons; conversely in chimeras with a weak contribution by Reelin(+/+) neurons, the mutant phenotype remained. However, increasing the number of Reelin(+/+) neurons beyond an unknown threshold resulted in partial rescue, with the formation of a correctly layered secondary cortex lying on top of an inverted mutant cortex. Therefore, the development of cortical layers in the correct order requires a minimal level of Reelin protein to be present although paradoxically, this is insufficient to prevent the simultaneous formation of inverted cortical layers in the same hemisphere.

Seizure-related gene 6 (Sez-6) in amacrine cells of the rodent retina and the consequence of gene deletion

Background

Seizure-related gene 6 (Sez-6) is expressed in neurons of the mouse brain, retina and spinal cord. In the cortex, Sez-6 plays a role in specifying dendritic branching patterns and excitatory synapse numbers during development.

Methodology/Principal Findings

The distribution pattern of Sez-6 in the retina was studied using a polyclonal antibody that detects the multiple isoforms of Sez-6. Prominent immunostaining was detected in GABAergic, but not in AII glycinergic, amacrine cell subpopulations of the rat and mouse retina. Amacrine cell somata displayed a distinct staining pattern with the Sez-6 antibody: a discrete, often roughly triangular-shaped bright spot positioned between the nucleus and the apical dendrite superimposed over weaker general cytoplasmic staining. Displaced amacrines in the ganglion cell layer were also positive for Sez-6 and weaker staining was occasionally observed in neurons with the morphology of alpha ganglion cells. Two distinct Sez-6 positive strata were present in the inner plexiform layer in addition to generalized punctate staining. Certain inner nuclear layer cells, including bipolar cells, stained more weakly and diffusely than amacrine cells, although some bipolar cells exhibited a perinuclear “bright spot” similar to amacrine cells. In order to assess the role of Sez-6 in the retina, we analyzed the morphology of the Sez-6 knockout mouse retina with immunohistochemical markers and compared ganglion cell dendritic arbor patterning in Sez-6 null retinae with controls. The functional importance of Sez-6 was assessed by dark-adapted paired-flash electroretinography (ERG).

Conclusions

In summary, we have reported the detailed expression pattern of a novel retinal marker with broad cell specificity, useful for retinal characterization in rodent experimental models. Retinal morphology, ganglion cell dendritic branching and ERG waveforms appeared normal in the Sez-6 knockout mouse suggesting that, in spite of widespread expression of Sez-6, retinal function in the absence of Sez-6 is not affected.

Automated analysis of neurite branching in cultured cortical neurons using HCA-Vision

Manual neuron tracing is a very labor-intensive task. In the drug screening context, the sheer number of images to process means that this approach is unrealistic. Moreover, the lack of reproducibility, objectivity, and auditing capability of manual tracing is limiting even in the context of smaller studies. We have developed fast, sensitive, and reliable algorithms for the purpose of detecting and analyzing neurites in cell cultures, and we have integrated them in software called HCA-Vision, suitable for the research environment. We validate the software on images of cortical neurons by comparing results obtained using HCA-Vision with those obtained using an established semi-automated tracing solution (NeuronJ). The effect of the Sez-6 deletion was characterized in detail. Sez-6 null neurons exhibited a significant increase in neurite branching, although the neurite field area was unchanged due to a reduction in mean branch length. HCA-Vision delivered considerable speed benefits and reliable traces.

Mph2 expression in germinal zones of the mouse brain

The telencephalon develops from a seemingly homogeneous population of precursor cells that generates neurons with divergent properties and cell fates. An important question concerns the genetic control of cell states belonging to different generations. Mph2 was identified by serial analysis of gene expression as a differentially expressed gene during cortical development. Belonging to the Polycomb group, Mph2 may be involved in transcriptional repression of cell states from one generation to the next. Here, we use in situ hybridization to determine the spatial localization of Mph2 expression in the developing nervous system. Mph2 expression evident in proliferating precursor cell populations in the developing mouse brain such as the ventricular zones of the cortex and ganglionic eminences as well as in mature structures such as the granule cell layer of the adult dentate gyrus and cerebellum indicates a role for Mph2 in both the developing and adult brain.

Genomic organisation and nervous system expression of radial spoke protein 3

Following their generation in the germinal zones, young neurons of the neocortex, hippocampus and cerebellum undergo long-distance migration to reach their final destinations. This locomotive activity depends on active deployment of cytoskeletal elements including the microtubule apparatus. In this study, we report the identification and expression of radial spoke protein 3 (RSP3), a member of a protein cluster responsible for anchoring and modifying dynein motor activity known to be crucial to microtubule sliding. The mouse RSP3 gene consists of eight exons and seven introns and spans over 230 kb. The genomic organisations of the human and rat RSP3 genes are similar although they span approximately 23 and 53 kb, respectively. This is in contrast to the Chlamydomonas RSP3 gene, where RSP3 was first isolated, which consists of four exons and three introns and spans approximately 2.7 kb. Despite these differences, the nucleotide and amino acid sequences upstream of, and throughout the RPII-binding domain of RSP3 are highly conserved between all the above-mentioned species. Mouse RSP3 mRNA was restricted to the developing neocortex, hippocampus and cerebellum during the stages when these structures are known to contain large numbers of migratory neurons. Gene expression studies suggest that RSP3 function is consistent with a locomotory role for this protein in migrating young neurons. In addition, expression of RSP3 mRNA in adult neurons point to additional, though still unknown functions. Our data provides the first evidence for the expression of radial spoke proteins in higher eukaryotes, and provides a biological framework for how these proteins may participate in microtubule sliding and neuronal migration in the embryonic brain.

MicroSAGE is highly representative and reproducible but reveals major differences in gene expression among samples obtained from similar tissues

BACKGROUND

Serial analysis of gene expression using small amounts of starting material (microSAGE) has not yet been conclusively shown to be representative, reproducible or accurate.

RESULTS

We show that microSAGE is highly representative, reproducible and accurate, but that pronounced differences in gene expression are seen between tissue samples taken from different individuals.

CONCLUSIONS

MicroSAGE is a reliable method of comprehensively profiling differences in gene expression among samples, but care should be taken in generalizing results obtained from libraries constructed from tissue obtained from different individuals and/or processed or stored differently.

Localized expression of the seizure-related gene SEZ-6 in developing and adult forebrains

Despite its initial identification in neurons exposed to the seizure-inducing drug pentylentetrazole (PTZ), the function of the seizure-related gene SEZ-6 remains obscure. Expression analysis indicates specific expression in the adult brain and testis, and the structure of the predicted protein suggests putative roles in cell-cell recognition and signalling. We report here that type I SEZ-6 mRNA is strongly expressed in highly specific regions in the developing forebrain. Specifically, the pattern of SEZ-6 expression is closely tied with the emergence of the neocortical layers and hippocampus, and implies a forebrain-specific role for this gene during development. In the adult hippocampus, SEZ-6 appears to be a CA1-specific regional marker.

Global analysis of gene expression patterns in developing mouse neocortex using serial analysis of gene expression

Molecular inventories of the developing mouse neocortex before and after birth were generated using the global gene expression profiling tool serial analysis of gene expression (SAGE). Libraries were generated from embryonic day 15 and postnatal day 1 mouse neocortex and more than 40,000 tags were collected (20,211 and 22,001 tags, representing 11,706 and 12,402 transcripts, respectively). Comparison of the two libraries resulted in the identification of 321 transcripts that were differentially expressed (P < 0.05). Differential expression was independently verified for selected genes by Northern blotting, and in situ hybridization revealed spatial expression patterns in the neocortex. Differentially expressed transcripts included genes known to be important in neocortical development (e.g., brain factor 1, neuroD2, and Id2), genes not previously associated with neocortical development (such as brahma-related gene 1, receptor for activated C-kinase I, hypermethylated in cancer 2, and Evi9), and genes of unknown identity or function.

Growth and migration markers of rat C6 glioma cells identified by serial analysis of gene expression

Tumors derived from rat C6 cell implants into rat brain exhibit similar morphological characteristics and degree of vascularization to human glioblastomas. To establish a molecular basis for C6 gliosarcoma malignancy, we have constructed a molecular profile of the most abundantly expressed genes, using serial analysis of gene expression (SAGE). Sequence tags (1168) representing 738 individual transcripts were collected and tag-to-gene mapping was carried out using the UniGene data set for rat. Differentially expressed C6 transcripts were identified by comparison of tags collected for C6 cells with a similar number (1002) of tags from a rat primary astrocyte library. Genes found to be expressed at increased levels in C6 cells are associated with cell surface interactions, migration, or metastasis formation and proliferation. These include the receptor for hyaluronan-mediated motility (RHAMM), S-100 related protein 42A, galectin I, preproenkephalin, osteopontin, autocrine motility factor, alpha-tubulin, ad1 antigen, and cofilin. In addition, a tag with no database match probably representing a previously uncharacterized transcript was differentially expressed in C6 cells. Transcripts showing reduced expression in C6 cells relative to astrocytes included the extracellular matrix glycoprotein osteonectin/SPARC (secreted protein, acidic, rich in cysteine), actin-binding proteins thymosins beta-4 and beta-10, the cysteine protease inhibitor cystatin C, the actin-gelling protein SM22/transgelin, and ferritin-H. SAGE results were confirmed by Northern blot for all transcripts tested, reaffirming the value of the SAGE technique for expression profiling in cancer biology.

The anti-apoptotic protein ITA is essential for NGF-mediated survival of embryonic chick neurons

The avian ITA is homologous to the baculoviral and mammalian inhibitor of apoptosis (IAP) proteins, which can prevent apoptosis by inhibition of specific caspases. We investigated the role of ITA in embryonic chick sympathetic and dorsal root ganglionic neurons, which depend on nerve growth factor (NGF) for their survival. Within 6 hours, NGF upregulated ITA protein production more than 25-fold in sensory and sympathetic neurons. Overexpression of ITA in primary neurons supported survival of these cells in the absence of NGF, and ita antisense constructs inhibited NGF-mediated survival. Thus the induction of ITA expression seems to be an essential signaling event for survival of sympathetic and dorsal root ganglionic sensory neurons in response to NGF.

Mice without a functional relaxin gene are unable to deliver milk to their pups

We have used gene targeting to generate relaxin (rlx)-deficient mice. The majority (15 of 17) of homozygous (rlx-/-) mice are fertile and produce normal litters. However their mammary development is deficient; pups are unable to suckle and die within 24 h of birth unless cross-fostered to a wild-type (rlx+/+) foster mother. The nipples of rlx-/- animals do not enlarge significantly during pregnancy, and their histology retains the appearance of the virgin state. Breast parenchyma is somewhat underdeveloped at term even though milk is produced. Mammary ducts become grossly dilated in these animals. Heterozygous (rlx+/-) mice lactate normally. The interpubic ligament does not relax during pregnancy in rlx-/- mice. Plasma osmolality during late gestation was significantly higher (P < 0.001) in rlx-/- mice than in wild-type controls.

Inactivation of the survival motor neuron gene, a candidate gene for human spinal muscular atrophy, leads to massive cell death in early mouse embryos

Proximal spinal muscular atrophy is an autosomal recessive human disease of spinal motor neurons leading to muscular weakness with onset predominantly in infancy and childhood. With an estimated heterozygote frequency of 1/40 it is the most common monogenic disorder lethal to infants; milder forms represent the second most common pediatric neuromuscular disorder. Two candidate genes-survival motor neuron (SMN) and neuronal apoptosis inhibitory protein have been identified on chromosome 5q13 by positional cloning. However, the functional impact of these genes and the mechanism leading to a degeneration of motor neurons remain to be defined. To analyze the role of the SMN gene product in vivo we generated SMN-deficient mice. In contrast to the human genome, which contains two copies, the mouse genome contains only one SMN gene. Mice with homozygous SMN disruption display massive cell death during early embryonic development, indicating that the SMN gene product is necessary for cellular survival and function.

Expression of human relaxin genes: characterization of a novel alternatively-spliced human relaxin mRNA species

Relaxin is a two-chain peptide hormone encoded by two non-allelic genes in humans and great apes, and by a single gene in all other species studied. We have characterized the expression of the human relaxin genes (H1 and H2) in placenta, decidua, prostate and ovary by reverse-transcription/polymerase chain reaction (RT/PCR). H2 relaxin mRNA was detected in the ovary, term placenta, decidua, and prostate gland. In contrast, H1 gene expression was detected only in the prostate gland. In addition to the relaxin PCR product of the predicted size (486 bp), a larger relaxin-specific product (587 bp) was detected in both H1 and H2 amplifications and in amplifications of chimpanzee relaxin from placenta and corpus luteum. Sequencing of human and chimpanzee PCR products, and human relaxin genomic clones, revealed that the larger product arises from an alternatively-spliced relaxin mRNA species incorporating an extra exon. This is the first evidence that the structure of the human and chimpanzee relaxin genes differ from that of other characterized relaxin genes, such as pig and rat. The novel peptide arising from this alternate message would be identical to prorelaxin in the B-chain and part of the C-peptide (extending to the position of the intron) but would differ from prorelaxin in the carboxy-terminal domain. Observation of a similar mRNA species in the chimpanzee suggests that this conserved relaxin-like peptide may have a significant biological role.

Characterization of human relaxin gene regulation in the relaxin-expressing human prostate adenocarcinoma cell line LNCaP.FGC

Relaxin is a peptide hormone which is produced in human reproductive tissues including the ovary and prostate gland. Little is known of the molecular events regulating relaxin gene transcription. We have studied this question using gene transfer of relaxin promoter/reporter gene constructs into a relaxin-expressing cell line. A number of human cells lines expressed relaxin as detected by reverse transcription-PCR. In one of these lines, the prostate adenocarcinoma cell line LNCaP.FGC, relaxin mRNA was also detected by Northern blot analysis. The DNA sequences of the proximal 5'-flanking regions (approximately 900 nucleotides) of the two human relaxin genes, H1 and H2, were determined. Deletion constructs containing portions of the 5'-flanking regions of H1 and H2 linked to the bacterial chloramphenicol acetyl transferase reporter gene were prepared. The expression of the reporter gene constructs was analysed in the LNCaP.FGC cell line and the results of these transient transfection assays have led to the identification of positive and negative regulatory regions within the 5'-flanking DNA. A difference in activity of the H1 and H2 gene promoters in this prostate cell line was observed, with the H2 promoter being more active. This situation may mimic that occurring in vivo since the relaxin secreted from the prostate gland into seminal fluid is the product of the H2 gene.

Expression of the relaxin gene in rat tissues

The peptide hormone relaxin is produced at high levels in the corpus luteum of the rat ovary during pregnancy. The biological effects of the hormone include remodelling of collagen in target tissues such as the cervix, and inhibition of uterine contractility. In addition, many paracrine actions for the hormone have been proposed, however sites of relaxin production outside the ovary have not been well characterized. We have investigated relaxin gene expression in a range of rat tissues using the techniques of reverse transcription/polymerase chain reaction (RT/PCR), RNase protection and immunohistochemistry. Relaxin mRNA was detected by RT/PCR in brain, uterus, prostate gland, pancreas and kidney, with other tissues giving weak signals. Relaxin gene expression in brain was detected by RNase protection, and relaxin-like immunoreactivity was observed in the arcuate nucleus of the hypothalamus of rat brain. This characterization of sites of relaxin gene expression provides further evidence for proposed paracrine actions of relaxin which may be important in non-pregnant and male rats in addition to the pregnant female.