Representation of the ipsilateral visual field by neurons in the macaque lateral intraparietal cortex depends on the forebrain commissures.

Our eyes are constantly moving, allowing us to attend to different visual objects in the environment. With each eye movement, a given object activates an entirely new set of visual neurons, yet we perceive a stable scene. One neural mechanism that may contribute to visual stability is remapping. Neurons in several brain regions respond to visual stimuli presented outside the receptive field when an eye movement brings the stimulated location into the receptive field. The stored representation of a visual stimulus is remapped, or updated, in conjunction with the saccade. Remapping depends on neurons being able to receive visual information from outside the classic receptive field. In previous studies, we asked whether remapping across hemifields depends on the forebrain commissures. We found that, when the forebrain commissures are transected, behavior dependent on accurate spatial updating is initially impaired but recovers over time. Moreover, neurons in lateral intraparietal cortex (LIP) continue to remap information across hemifields in the absence of the forebrain commissures. One possible explanation for the preserved across-hemifield remapping in split-brain animals is that neurons in a single hemisphere could represent visual information from both visual fields. In the present study, we measured receptive fields of LIP neurons in split-brain monkeys and compared them with receptive fields in intact monkeys. We found a small number of neurons with bilateral receptive fields in the intact monkeys. In contrast, we found no such neurons in the split-brain animals. We conclude that bilateral representations in area LIP following forebrain commissures transection cannot account for remapping across hemifields.

Spatial updating in monkey superior colliculus in the absence of the forebrain commissures: dissociation between superficial and intermediate layers.

In previous studies, we demonstrated that the forebrain commissures are the primary pathway for remapping from one hemifield to the other. Nonetheless, remapping in lateral intraparietal cortex (LIP) across hemifield is still present in split brain monkeys. This finding indicates that a subcortical structure must contribute to remapping. The primary goal of the current study was to characterize remapping activity in the superior colliculus in intact and split brain monkeys. We recorded neurons in both the superficial and intermediate layers of the SC. We found that across-hemifield remapping was reduced in magnitude and delayed compared with within-hemifield remapping in the intermediate layers of the SC in split brain monkeys. These results mirror our previous findings in area LIP. In contrast, we found no difference in the magnitude or latency for within- compared with across-hemifield remapping in the superficial layers. At the behavioral level, we compared the performance of the monkeys on two conditions of a double-step task. When the second target remained within a single hemifield, performance remained accurate. When the second target had to be updated across hemifields, the split brain monkeys' performance was impaired. Remapping activity in the intermediate layers was correlated with the accuracy and latency of the second saccade during the across-hemifield trials. Remapping in the superficial layers was correlated with latency of the second saccade during the within- and across-hemifield trials. The differences between the layers suggest that different circuits underlie remapping in the superficial and intermediate layers of the superior colliculus.

Repeated recruitment of LTR retrotransposons as promoters by the anti-apoptotic locus NAIP during mammalian evolution.

Neuronal apoptosis inhibitory protein (NAIP, also known as BIRC1) is a member of the conserved inhibitor of apoptosis protein (IAP) family. Lineage-specific rearrangements and expansions of this locus have yielded different copy numbers among primates and rodents, with human retaining a single functional copy and mouse possessing several copies, depending on the strain. Roles for this gene in disease have been documented, but little is known about transcriptional regulation of NAIP. We show here that NAIP has multiple promoters sharing no similarity between human and rodents. Moreover, we demonstrate that multiple, domesticated long terminal repeats (LTRs) of endogenous retroviral elements provide NAIP promoter function in human, mouse, and rat. In human, an LTR serves as a tissue-specific promoter, active primarily in testis. However, in rodents, our evidence indicates that an ancestral LTR common to all rodent genes is the major, constitutive promoter for these genes, and that a second LTR found in two of the mouse genes is a minor promoter. Thus, independently acquired LTRs have assumed regulatory roles for orthologous genes, a remarkable evolutionary scenario. We also demonstrate that 5' flanking regions of IAP family genes as a group, in both human and mouse are enriched for LTR insertions compared to average genes. We propose several potential explanations for these findings, including a hypothesis that recruitment of LTRs near NAIP or other IAP genes may represent a host-cell adaptation to modulate apoptotic responses.

Retroviral elements and their hosts: insertional mutagenesis in the mouse germ line.

The inbred mouse is an invaluable model for human biology and disease. Nevertheless, when considering genetic mechanisms of variation and disease, it is important to appreciate the significant differences in the spectra of spontaneous mutations that distinguish these species. While insertions of transposable elements are responsible for only approximately 0.1% of de novo mutations in humans, the figure is 100-fold higher in the laboratory mouse. This striking difference is largely due to the ongoing activity of mouse endogenous retroviral elements. Here we briefly review mouse endogenous retroviruses (ERVs) and their influence on gene expression, analyze mechanisms of interaction between ERVs and the host cell, and summarize the variety of mutations caused by ERV insertions. The prevalence of mouse ERV activity indicates that the genome of the laboratory mouse is presently behind in the "arms race" against invasion.

Transcription of two human genes from a bidirectional endogenous retrovirus promoter.

Eight percent of the human genome is derived from endogenous retrovirus (ERV) insertions. ERV long terminal repeats (LTRs) contain strong promoters that are known to contribute to the transcriptional regulation of certain human genes. While some LTRs are known to possess bidirectional promoter activity in vitro, only sense orientation LTR promoters have previously been shown to regulate human gene expression. Here we demonstrate that an ERV1 LTR acts as a bidirectional promoter for the human Down syndrome critical region 4 (DSCR4) and DSCR8 genes. We show that while DSCR4 and DSCR8 are essentially co-expressed, their shared LTR promoter is more active in the sense than the antisense orientation. Through deletion analysis of the LTR we have identified positive and negative regulatory elements, and defined a core region of the promoter that is required for transcriptional activity in both orientations. Finally, we show that the ERV LTR also exists in the genomes of several non-human primates, and present evidence that potential transcription factor binding sites in the core region have been maintained throughout primate evolution.

Estrogen receptor activation function 2 (AF-2) is essential for hormone-dependent transactivation and cell transformation induced by a v-Jun DNA binding domain-estrogen receptor chimera.

A chimeric protein consisting of the estrogen receptor alpha ligand binding domain (ER-alpha LBD) fused to the DNA binding domain (DBD) of the v-Jun oncoprotein, deltavJ-hER, was previously shown to elicit estradiol-dependent transcriptional activation and cell transformation. Remarkably, in the unliganded state deltavJ-hER is not inert, but rather inhibits cell proliferation. To understand the molecular basis for these opposite effects on cell growth, we investigated the effect of estradiol on deltavJ-hER function. We find that deltavJ-hER is localised to the cell nucleus and capable of binding TPA-response element (TRE) DNA recognition sites in the presence and absence of estradiol, indicating that these properties are unlikely to be the targets of hormonal regulation. In contrast, a mutant derivative of deltavJ-hER in which amino acid substitutions selectively disrupt activation function 2 (AF-2) function is unable to elicit estradiol-dependent transcription or cell transformation, even though DNA binding is not impaired. Taken together, these observations establish that estrogen receptor AF-2 activity is essential for cell transformation by deltavJ-hER.

Transcription of the human and rodent SPAM1 / PH-20 genes initiates within an ancient endogenous retrovirus.

BACKGROUND: Sperm adhesion molecule 1 (SPAM1) is the major mammalian testicular hyaluronidase and is expressed at high levels in sperm cells. SPAM1 protein is important for penetration of the cumulus cell layer surrounding the ovum, and is also involved in zona pellucida binding and sperm intracellular signalling. A previous study had identified SPAM1 as one of the many human genes that initiate within a transposable element. RESULTS: Examination of the human, mouse and rat SPAM1 loci revealed that transcripts initiate within the pol gene of an endogenous retrovirus (ERV) element. This is highly unusual, as all previously identified ERV-initiated cellular gene transcripts initiate within the viral long terminal repeat promoter. The SPAM1 locus therefore represents an example of the evolution of a promoter from protein-coding sequence. We have identified novel alternative promoter and splicing variants of human and murine SPAM1. We show that all transcript variants are expressed primarily in the testis and are predicted to encode identical proteins. CONCLUSION: The testis-specific promoters of the human and mouse SPAM1 genes are derived from sequence that was originally part of an ERV pol gene. This represents the first known example of an ERV-derived promoter acting in a gender-specific manner.

Endogenous retrovirus long terminal repeats as ready-to-use mobile promoters: the case of primate beta3GAL-T5.

Throughout the course of vertebrate evolution, germline retroviral infections have resulted in heritable provirus insertions into host DNA. These endogenous retroviruses (ERVs) contain long terminal repeat (LTR) promoters that can be adopted for use by nearby host genes. It is not known whether the transcription factor (TF) binding sites and tissue-specificities of modern LTR gene promoters have been retained since the time of ERV insertion, or if these features evolved later as the LTR became involved in host gene regulation. To address this issue, we have conducted a case study of the ERV-L LTR promoter of human beta1,3-galactosyltransferase 5 (beta3GAL-T5). We have previously shown that the human beta3GAL-T5 LTR promoter is responsible for the majority of gene transcripts in the colon. The murine beta3gal-t5 gene is also expressed primarily in the colon, despite the absence of an orthologous ERV-L LTR in the mouse genome. We therefore hypothesized that both the ERV-L LTR and the non-retroviral ancestral beta3GAL-T5 promoter were active in the colon at the time of ERV insertion. In support of this hypothesis, we have shown that the orthologous LTRs of four non-human primates are also active in a human colorectal cell line, and that the baboon LTR is active in primary baboon colon tissue. We also present evidence that the functional TF binding sites of the human beta3GAL-T5 LTR promoter were present in the original consensus sequence for this class of LTRs. Upon similar analysis of other ERV sequences, we have concluded that this evolutionary history is shared by certain other LTR gene promoters, and may be a general phenomenon.

Dynamic circuitry for updating spatial representations. III. From neurons to behavior.

Each time the eyes move, the visual system must adjust internal representations to account for the accompanying shift in the retinal image. In the lateral intraparietal cortex (LIP), neurons update the spatial representations of salient stimuli when the eyes move. In previous experiments, we found that split-brain monkeys were impaired on double-step saccade sequences that required updating across visual hemifields, as compared to within hemifield. Here we describe a subsequent experiment to characterize the relationship between behavioral performance and neural activity in LIP in the split-brain monkey. We recorded from single LIP neurons while split-brain and intact monkeys performed two conditions of the double-step saccade task: one required across-hemifield updating and the other required within-hemifield updating. We found that, despite extensive experience with the task, the split-brain monkeys were significantly more accurate for within-hemifield than for across-hemifield sequences. In parallel, we found that population activity in LIP of the split-brain monkeys was significantly stronger for the within-hemifield than for the across-hemifield condition of the double-step task. In contrast, in the normal monkey, both the average behavioral performance and population activity showed no bias toward the within-hemifield condition. Finally, we found that the difference between within-hemifield and across-hemifield performance in the split-brain monkeys was reflected at the level of single-neuron activity in LIP. These findings indicate that remapping activity in area LIP is present in the split-brain monkey for the double-step task and covaries with spatial behavior on within-hemifield compared to across-hemifield sequences.

An endogenous retroviral long terminal repeat is the dominant promoter for human beta1,3-galactosyltransferase 5 in the colon.

LTRs of endogenous retroviruses are known to affect expression of several human genes, typically as a relatively minor alternative promoter. Here, we report that an endogenous retrovirus LTR acts as one of at least two alternative promoters for the human beta1,3-galactosyltransferase 5 gene, involved in type 1 Lewis antigen synthesis, and show that the LTR promoter is most active in the gastrointestinal tract and mammary gland. Indeed, the LTR is the dominant promoter in the colon, indicating that this ancient retroviral element has a major impact on gene expression. Using colorectal cancer cell lines and electrophoretic mobility-shift assays, we found that hepatocyte nuclear factor 1 (HNF-1) binds a site within the retroviral promoter and that expression of HNF-1 and interaction with its binding site correlated with promoter activation. We conclude that HNF-1 is at least partially responsible for the tissue-specific activation of the LTR promoter of human beta 1,3-galactosyltransferase 5. We demonstrate that this tissue-specific transcription factor is implicated in the activation of an LTR gene promoter.