Neto Auxiliary Subunits Regulate Interneuron Somatodendritic and Presynaptic Kainate Receptors to Control Network Inhibition

Although Netos are considered auxiliary subunits critical for kainate receptor (KAR) function, direct evidence for their regulation of native KARs is limited. Because Neto KAR regulation is GluK subunit/Neto isoform specific, such regulation must be determined in cell-type-specific contexts. We demonstrate Neto1/2 expression in somatostatin (SOM)-, cholecystokinin/cannabinoid receptor 1 (CCK/CB1)-, and parvalbumin (PV)-containing interneurons. KAR-mediated excitation of these interneurons is contingent upon Neto1 because kainate yields comparable effects in Neto2 knockouts and wild-types but fails to excite interneurons or recruit inhibition in Neto1 knockouts. In contrast, presynaptic KARs in CCK/CB1 interneurons are dually regulated by both Neto1 and Neto2. Neto association promotes tonic presynaptic KAR activation, dampening CCK/CB1 interneuron output, and loss of this brake in Neto mutants profoundly increases CCK/CB1 interneuron-mediated inhibition. Our results confirm that Neto1 regulates endogenous somatodendritic KARs in diverse interneurons and demonstrate Neto regulation of presynaptic KARs in mature inhibitory presynaptic terminals.

Neto2-null mice have impaired GABAergic inhibition and are susceptible to seizures

Neto2 is a transmembrane protein that interacts with the neuron-specific K⁺-Cl⁻ cotransporter (KCC2) in the central nervous system (CNS). Efficient KCC2 transport is essential for setting the neuronal Cl⁻ gradient, which is required for fast GABAergic inhibition. Neto2 is required to maintain the normal abundance of KCC2 in neurons, and increases KCC2 function by binding to the active oligomeric form of this cotransporter. In the present study, we characterized GABAergic inhibition and KCC2-mediated neuronal chloride homeostasis in pyramidal neurons from adult hippocampal slices. Using gramicidin perforated patch clamp recordings we found that the reversal potential for GABA (E_GABA) was significantly depolarized. We also observed that surface levels of KCC2 and phosphorylation of KCC2 serine 940 (Ser940) were reduced in Neto2^−/− neurons compared to wild-type controls. To examine GABAergic inhibition we recorded spontaneous inhibitory postsynaptic currents (sIPSCs) and found that Neto2^−/− neurons had significant reductions in both their amplitude and frequency. Based on the critical role of Neto2 in regulating GABAergic inhibition we rationalized that Neto2-null mice would be prone to seizure activity. We found that Neto2-null mice demonstrated a decrease in the latency to pentylenetetrazole (PTZ)-induced seizures and an increase in seizure severity.

Kainate receptors coexist in a functional complex with KCC2 and regulate chloride homeostasis in hippocampal neurons

KCC2 is the neuron-specific K+-Cl(-) cotransporter required for maintaining low intracellular Cl(-), which is essential for fast inhibitory synaptic transmission in the mature CNS. Despite the requirement of KCC2 for inhibitory synaptic transmission, understanding of the cellular mechanisms that regulate KCC2 expression and function is rudimentary. We examined KCC2 in its native protein complex in vivo to identify key KCC2-interacting partners that regulate KCC2 function. Using blue native-polyacrylamide gel electrophoresis (BN-PAGE), we determined that native KCC2 exists in a macromolecular complex with kainate-type glutamate receptors (KARs). We found that KAR subunits are required for KCC2 oligomerization and surface expression. In accordance with this finding, acute and chronic genetic deletion of KARs decreased KCC2 function and weakened synaptic inhibition in hippocampal neurons. Our results reveal KARs as regulators of KCC2, significantly advancing our growing understanding of the tight interplay between excitation and inhibition.

Endothelin-2 deficiency causes growth retardation, hypothermia, and emphysema in mice

To explore the physiological functions of endothelin-2 (ET-2), we generated gene-targeted mouse models. Global Et2 knockout mice exhibited severe growth retardation and juvenile lethality. Despite normal milk intake, they suffered from internal starvation characterized by hypoglycemia, ketonemia, and increased levels of starvation-induced genes. Although ET-2 is abundantly expressed in the gastrointestinal tract, the intestine was morphologically and functionally normal. Moreover, intestinal epithelium-specific Et2 knockout mice showed no abnormalities in growth and survival. Global Et2 knockout mice were also profoundly hypothermic. Housing Et2 knockout mice in a warm environment significantly extended their median lifespan. However, neuron-specific Et2 knockout mice displayed a normal core body temperature. Low levels of Et2 mRNA were also detected in the lung, with transient increases soon after birth. The lungs of Et2 knockout mice showed emphysematous structural changes with an increase in total lung capacity, resulting in chronic hypoxemia, hypercapnia, and increased erythropoietin synthesis. Finally, systemically inducible ET-2 deficiency in neonatal and adult mice fully reproduced the phenotype previously observed in global Et2 knockout mice. Together, these findings reveal that ET-2 is critical for the growth and survival of postnatal mice and plays important roles in energy homeostasis, thermoregulation, and the maintenance of lung morphology and function.

Endothelin-2-mediated protection of mutant photoreceptors in inherited photoreceptor degeneration

Expression of the Endothelin-2 (Edn2) mRNA is greatly increased in the photoreceptors (PRs) of mouse models of inherited PR degeneration (IPD). To examine the role of Edn2 in mutant PR survival, we generated Edn2^−/− mice carrying homozygous Pde6b^rd1 alleles or the Tg(RHO P347S) transgene. In the Edn2^−/− background, PR survival increased 110% in Pde6b^rd1/rd1 mice at post-natal (PN) day 15, and 60% in Tg(RHO P347S) mice at PN40. In contrast, PR survival was not increased in retinal explants of Pde6b^rd1/rd1; Edn2^−/− mice. This finding, together with systemic abnormalities in Edn2^−/− mice, suggested that the increased survival of mutant PRs in the Edn2^−/− background resulted at least partly from the systemic EDN2 loss of function. To examine directly the role of EDN2 in mutant PRs, we used a scAAV5-Edn2 cDNA vector to restore Edn2 expression in Pde6b^rd1/rd1; Edn2^−/− PRs and observed an 18% increase in PR survival at PN14. Importantly, PR survival was also increased after injection of scAAV5-Edn2 into Pde6b^rd1/rd1 retinas, by 31% at PN15. Together, these findings suggest that increased Edn2 expression is protective to mutant PRs. To begin to elucidate Edn2-mediated mechanisms that contribute to PR survival, we used microarray analysis and identified a cohort of 20 genes with >4-fold increased expression in Tg(RHO P347S) retinas, including Fgf2. Notably, increased expression of the FGF2 protein in Tg(RHO P347S) PRs was ablated in Tg(RHO P347S); Edn2^−/− retinas. Our findings indicate that the increased expression of PR Edn2 increases PR survival, and suggest that the Edn2-dependent increase in PR expression of FGF2 may contribute to the augmented survival.

Neto2 is a KCC2 interacting protein required for neuronal Cl- regulation in hippocampal neurons

KCC2 is a neuron-specific K(+)-Cl(-) cotransporter that is essential for Cl(-) homeostasis and fast inhibitory synaptic transmission in the mature CNS. Despite the critical role of KCC2 in neurons, the mechanisms regulating its function are not understood. Here, we show that KCC2 is critically regulated by the single-pass transmembrane protein neuropilin and tolloid like-2 (Neto2). Neto2 is required to maintain the normal abundance of KCC2 and specifically associates with the active oligomeric form of the transporter. Loss of the Neto2:KCC2 interaction reduced KCC2-mediated Cl(-) extrusion, resulting in decreased synaptic inhibition in hippocampal neurons.

Neto2 interacts with the scaffolding protein GRIP and regulates synaptic abundance of kainate receptors

Kainate receptors (KARs) are a class of ionotropic glutamate receptors that are expressed throughout the central nervous system. The function and subcellular localization of KARs are tightly regulated by accessory proteins. We have previously identified the single-pass transmembrane proteins, Neto1 and Neto2, to be associated with native KARs. In the hippocampus, Neto1, but not Neto2, controls the abundance and modulates the kinetics of postsynaptic KARs. Here we evaluated whether Neto2 regulates synaptic KAR levels in the cerebellum where Neto1 expression is limited to the deep cerebellar nuclei. In the cerebellum, where Neto2 is present abundantly, we found a ∼40% decrease in GluK2-KARs at the postsynaptic density (PSD) of Neto2-null mice. No change, however, was observed in total level of GluK2-KARs, thereby suggesting a critical role of Neto2 on the synaptic localization of cerebellar KARs. The presence of a putative class II PDZ binding motif on Neto2 led us to also investigate whether it interacts with PDZ domain-containing proteins previously implicated in regulating synaptic abundance of KARs. We identified a PDZ-dependent interaction between Neto2 and the scaffolding protein GRIP. Furthermore, coexpression of Neto2 significantly increased the amount of GRIP associated with GluK2, suggesting that Neto2 may promote and/or stabilize GluK2:GRIP interactions. Our results demonstrate that Neto2, like Neto1, is an important auxiliary protein for modulating the synaptic levels of KARs. Moreover, we propose that the interactions of Neto1/2 with various scaffolding proteins is a critical mechanism by which KARs are stabilized at diverse synapses.

A transgenic mouse line expressing cre recombinase in undifferentiated postmitotic mouse retinal bipolar cell precursors

Approaches for manipulating cell type-specific gene expression during development depend on the identification of novel genetic tools. Here, we report the generation of a transgenic mouse line that utilizes Vsx2 upstream sequences to direct Cre recombinase to developing retinal bipolar cells. In contrast to the endogenous Vsx2 expression pattern, transgene expression was not detected in proliferating retinal progenitor cells and was restricted to post-mitotic bipolar cells. Cre immunolabeling was detected in rod bipolar cells and a subset of ON and OFF cone bipolar cells. Expression was first observed at postnatal day 3 and was detectable between 24 hours and 36 hours after the last S-phase of the cell cycle. The appearance of Cre-immunolabeled cells preceded the expression of bipolar cell type-specific markers such as PKCα and Cabp5 suggesting that transgene expression is initiated prior to terminal differentiation. In the presence of a constitutive conditional reporter transgene, reporter fluorescence was detected in Cre-expressing bipolar cells in the mature retina as expected, but was also observed in Cre-negative Type 2 bipolar cells and occasionally in Cre-negative photoreceptor cells. Together these findings reveal a new transgenic tool for directing gene expression to post-mitotic retinal precursors that are mostly committed to a bipolar cell fate.

The genomic, biochemical, and cellular responses of the retina in inherited photoreceptor degenerations and prospects for the treatment of these disorders

The association of more than 140 genes with human photoreceptor degenerations, together with studies of animal models of these monogenic diseases, has provided great insight into their pathogenesis. Here we review the responses of the retina to photoreceptor mutations, including mechanisms of photoreceptor death. We discuss the roles of oxidative metabolism, mitochondrial reactive oxygen species, metabolic stress, protein misfolding, and defects in ciliary proteins, as well as the responses of Müller glia, microglia, and the retinal vasculature. Finally, we report on potential pharmacologic and biologic therapies, the critical role of histopathology as a prerequisite to treatment, and the exciting promise of gene therapy in animal models and in phase 1 trials in humans.

Evidence of severe mitochondrial oxidative stress and a protective effect of low oxygen in mouse models of inherited photoreceptor degeneration

The role of oxidative stress within photoreceptors (PRs) in inherited photoreceptor degeneration (IPD) is unclear. We investigated this question using four IPD mouse models (Pde6b(rd1/rd1), Pde6b(atrd1/atrd1), Rho(-/-) and Prph2(rds/rds)) and compared the abundance of reduced glutathione (GSH) and the activity of mitochondrial NADH:ubiquinone oxidoreductase (complex I), which is oxidative stress sensitive, as indirect measures of redox status, in the retinas of wild type and IPD mice. All four IPD mutants had significantly reduced retinal complex I activities (14-29% of wild type) and two showed reduced GSH, at a stage prior to the occurrence of significant cell death, whereas mitochondrial citrate synthase, which is oxidative stress insensitive, was unchanged. We orally administered the mitochondrially targeted anti oxidant MitoQ in order to reduce oxidative stress but without any improvement in retinal complex I activity, GSH or rates of PR degeneration. One possible source of oxidative stress in IPDs is oxygen toxicity in the outer retina due to reduced consumption by PR mitochondria. We therefore asked whether a reduction in the ambient O(2) concentration might improve PR survival in Pde6b(rd1/rd1) retinal explants either directly, by reducing reactive oxygen species formation, or indirectly by a neuroprotective mechanism. Pde6b(rd1/rd1) retinal explants cultured in 6% O(2) showed 31% less PR death than normoxic explants. We conclude that (i) mitochondrial oxidative stress is a significant early feature of IPDs; (ii) the ineffectiveness of MitoQ may indicate its inability to reduce some mediators of oxidative stress, such as hydrogen peroxide; and (iii) elucidation of the mechanisms by which hypoxia protects mutant PRs may identify novel neuroprotective pathways in the retina.

Maximizing functional photoreceptor differentiation from adult human retinal stem cells

Retinal stem cells (RSCs) are present in the ciliary margin of the adult human eye and can give rise to all retinal cell types. Here we show that modulation of retinal transcription factor gene expression in human RSCs greatly enriches photoreceptor progeny, and that strong enrichment was obtained with the combined transduction of OTX2 and CRX together with the modulation of CHX10. When these genetically modified human RSC progeny are transplanted into mouse eyes, their retinal integration and differentiation is superior to unmodified RSC progeny. Moreover, electrophysiologic and behavioral tests show that these transplanted cells promote functional recovery in transducin mutant mice. This study suggests that gene modulation in human RSCs may provide a source of photoreceptor cells for the treatment of photoreceptor disease.

Loss of inhibitory interneurons in the dorsal spinal cord and elevated itch in Bhlhb5 mutant mice

Itch is the least well understood of all the somatic senses, and the neural circuits that underlie this sensation are poorly defined. Here we show that the atonal-related transcription factor Bhlhb5 is transiently expressed in the dorsal horn of the developing spinal cord and appears to play a role in the formation and regulation of pruritic (itch) circuits. Mice lacking Bhlhb5 develop self-inflicted skin lesions and show significantly enhanced scratching responses to pruritic agents. Through genetic fate-mapping and conditional ablation, we provide evidence that the pruritic phenotype in Bhlhb5 mutants is due to selective loss of a subset of inhibitory interneurons in the dorsal horn. Our findings suggest that Bhlhb5 is required for the survival of a specific population of inhibitory interneurons that regulate pruritus, and provide evidence that the loss of inhibitory synaptic input results in abnormal itch.

Eye evolution at high resolution: the neuron as a unit of homology

Based on differences in morphology, photoreceptor-type usage and lens composition it has been proposed that complex eyes have evolved independently many times. The remarkable observation that different eye types rely on a conserved network of genes (including Pax6/eyeless) for their formation has led to the revised proposal that disparate complex eye types have evolved from a shared and simpler prototype. Did this ancestral eye already contain the neural circuitry required for image processing? And what were the evolutionary events that led to the formation of complex visual systems, such as those found in vertebrates and insects? The recent identification of unexpected cell-type homologies between neurons in the vertebrate and Drosophila visual systems has led to two proposed models for the evolution of complex visual systems from a simple prototype. The first, as an extension of the finding that the neurons of the vertebrate retina share homologies with both insect (rhabdomeric) and vertebrate (ciliary) photoreceptor cell types, suggests that the vertebrate retina is a composite structure, made up of neurons that have evolved from two spatially separate ancestral photoreceptor populations. The second model, based largely on the conserved role for the Vsx homeobox genes in photoreceptor-target neuron development, suggests that the last common ancestor of vertebrates and flies already possessed a relatively sophisticated visual system that contained a mixture of rhabdomeric and ciliary photoreceptors as well as their first- and second-order target neurons. The vertebrate retina and fly visual system would have subsequently evolved by elaborating on this ancestral neural circuit. Here we present evidence for these two cell-type homology-based models and discuss their implications.

Neto1 is a novel CUB-domain NMDA receptor-interacting protein required for synaptic plasticity and learning

The N-methyl-D-aspartate receptor (NMDAR), a major excitatory ligand-gated ion channel in the central nervous system (CNS), is a principal mediator of synaptic plasticity. Here we report that neuropilin tolloid-like 1 (Neto1), a complement C1r/C1s, Uegf, Bmp1 (CUB) domain-containing transmembrane protein, is a novel component of the NMDAR complex critical for maintaining the abundance of NR2A-containing NMDARs in the postsynaptic density. Neto1-null mice have depressed long-term potentiation (LTP) at Schaffer collateral-CA1 synapses, with the subunit dependency of LTP induction switching from the normal predominance of NR2A- to NR2B-NMDARs. NMDAR-dependent spatial learning and memory is depressed in Neto1-null mice, indicating that Neto1 regulates NMDA receptor-dependent synaptic plasticity and cognition. Remarkably, we also found that the deficits in LTP, learning, and memory in Neto1-null mice were rescued by the ampakine CX546 at doses without effect in wild-type. Together, our results establish the principle that auxiliary proteins are required for the normal abundance of NMDAR subunits at synapses, and demonstrate that an inherited learning defect can be rescued pharmacologically, a finding with therapeutic implications for humans.

The fundamental unit for information processing in the brain is the synapse, a highly specialized site of communication between the brain's multitude of individual neurons. The strength of the communication at each synapse changes in response to neuronal activity—a process called synaptic plasticity—allowing networks of neurons to adapt and learn. How synaptic plasticity occurs is a major question in neurobiology. A central player in synaptic plasticity is an assembly of synaptic proteins called the NMDA receptor complex. Here, we discovered that the protein Neto1 is a component of the NMDA receptor complex. Neto1-deficient mice had a dramatic decrease in the number of NMDA receptors at synapses and consequently, synaptic plasticity and learning were impaired. By indirectly enhancing the function of the residual NMDA receptors in Neto1-deficient mice with a small molecule, we restored synaptic plasticity and learning to normal levels. Our findings establish the principle that inherited abnormalities of synaptic plasticity and learning due to NMDA receptor dysfunction can be pharmacologically corrected. Our discoveries also suggest that synaptic proteins that share a molecular signature, called the CUB domain, with Neto1 may be important components of synaptic receptors across species, because several CUB-domain proteins in worms have also been found to regulate synaptic receptors.

Spatial learning and memory depend on the N-methyl-D-aspartic acid receptor, a synaptic ion channel regulated by Neto1. Impaired cognition due to the absence of Neto1 can be rescued pharmacologically, a finding with implications for the therapy of inherited learning defects in humans.

Conserved role of the Vsx genes supports a monophyletic origin for bilaterian visual systems

BACKGROUND

Components of the genetic network specifying eye development are conserved from flies to humans, but homologies between individual neuronal cell types have been difficult to identify. In the vertebrate retina, the homeodomain-containing transcription factor Chx10 is required for both progenitor cell proliferation and the development of the bipolar interneurons, which transmit visual signals from photoreceptors to ganglion cells.

RESULTS

We show that dVsx1 and dVsx2, the two Drosophila homologs of Chx10, play a conserved role in visual-system development. DVSX1 is expressed in optic-lobe progenitor cells, and, in dVsx1 mutants, progenitor cell proliferation is defective, leading to hypocellularity. Subsequently, DVSX1 and DVSX2 are coexpressed in a subset of neurons in the medulla, including the transmedullary neurons that transmit visual information from photoreceptors to deeper layers of the visual system. In dVsx mutant adults, the optic lobe is reduced in size, and the medulla is small or absent. These results suggest that the progenitor cells and photoreceptor target neurons of the vertebrate retina and fly optic lobe are ancestrally related. Genetic and functional homology may extend to the neurons directly downstream of the bipolar and transmedullary neurons, the vertebrate ganglion cells and fly lobula projection neurons. Both cell types project to visual-processing centers in the brain, and both sequentially express the Math5/ATO and Brn3b/ACJ6 transcription factors during their development.

CONCLUSIONS

Our findings support a monophyletic origin for the bilaterian visual system in which the last common ancestor of flies and vertebrates already contained a primordial visual system with photoreceptors, interneurons, and projection neurons.

Late onset Leigh syndrome and ataxia due to a T to C mutation at bp 9,185 of mitochondrial DNA

A T-to-C missense mutation at nucleotide position 9,185 in the protein-coding ATP6 gene of the mitochondrial genome was present at high heteroplasmy in members of a Canadian family with Leigh syndrome with predominant ataxia and peripheral neuropathy. This mutation results in the substitution of a proline residue for an evolutionary-conserved leucine at position of amino acid 220 near the carboxyl terminus of the mitochondrial protein. The index patient and brother, who had an identical clinical presentation, had >90% mutant mtDNA in cultured skin fibroblasts, lymphocytes, and whole blood. Their mother and a maternal uncle, symptomatic with a peripheral neuropathy alone, had 86% and 85% heteroplasmy, respectively. Symptomatic maternal cousins with early onset revealed 90% and 91% mutant mtDNA in all tissues analyzed. Studies of lymphoblasts from the asymptomatic maternal grandmother and eldest brother of the proband were heteroplasmic for mutant mtDNA with 56% and 17%, respectively. Biochemical analysis demonstrated normal respiratory chain enzyme activity in muscle and fibroblasts, normal ATP synthesis, but reduced oligomycin-sensitive H(+)ATPase in cultured lymphoblast mitochondria. We propose that the 9,185T > C mtDNA mutation is pathogenic even though the initial phenotype is mild and the biochemical phenotype not easily detectable.

The proliferation and expansion of retinal stem cells require functional Pax6

Retinal stem cells (RSCs) exist as rare pigmented ciliary epithelial cells in adult mammalian eyes. We hypothesized that RSCs are at the top of the retinal cell lineage. Thus, genes expressed early in embryonic development to establish the retinal field in forebrain neuroectoderm may play important roles in RSCs. Pax6, a paired domain and homeodomain-containing transcription factor, is one of the earliest genes expressed in the eye field and is considered a master control gene for retinal and eye development. Here, we demonstrate that Pax6 is enriched in RSCs. Inactivation of Pax6 in vivo results in loss of competent RSCs as assayed by the failure to form clonal RSC spheres from the optic vesicles of conventional Pax6 knockout embryos and from the ciliary epithelial cells of adult Pax6 conditional knockout mice. In vitro clonal inactivation of Pax6 in adult RSCs results in a serious proliferation defect, suggesting that Pax6 is required for the proliferation and expansion of RSCs.

Deletion hotspot in the argininosuccinate lyase gene: association with topoisomerase II and DNA polymerase alpha sites

Molecular analysis of argininosuccinate lyase (ASAL) deficiency has led to the identification of a deletion hotspot in the ASL gene. Six individuals with ASAL deficiency had alleles that led to a complete absence of exon 13 from the ASL mRNA; each had a partial deletion of exon 13 in the genomic DNA. In all six patients, the deletions begin 18 bp upstream of the 3' end of exon 13. In four cases, the deletions were 13 bp in length, and ended within exon 13, whereas in two other patients the deletions were 25 bp and extended into intron 13. The sequence at which these deletions begin overlaps both a putative topoisomerase II recognition site and a DNA polymerase alpha mutation/frameshift site. Moreover, the topoisomerase II cut site is situated precisely at the beginning of the deletions, which are flanked by small (2- and 3-bp) direct repeats. We note that a similar concurrence of these two putative enzyme sites can be found in a number of other deletion sites in the human genome, most notably the DeltaF508 deletion in the CFTR gene. These findings suggest that the joint presence of these two enzyme sites represents a DNA sequence context that may favor the occurrence of small deletions.

Generalization of DNA microarray dispersion properties: microarray equivalent of t-distribution

Background

DNA microarrays are a powerful technology that can provide a wealth of gene expression data for disease studies, drug development, and a wide scope of other investigations. Because of the large volume and inherent variability of DNA microarray data, many new statistical methods have been developed for evaluating the significance of the observed differences in gene expression. However, until now little attention has been given to the characterization of dispersion of DNA microarray data.

Results

Here we examine the expression data obtained from 682 Affymetrix GeneChips^® with 22 different types and we demonstrate that the Gaussian (normal) frequency distribution is characteristic for the variability of gene expression values. However, typically 5 to 15% of the samples deviate from normality. Furthermore, it is shown that the frequency distributions of the difference of expression in subsets of ordered, consecutive pairs of genes (consecutive samples) in pair-wise comparisons of replicate experiments are also normal. We describe a consecutive sampling method, which is employed to calculate the characteristic function approximating standard deviation and show that the standard deviation derived from the consecutive samples is equivalent to the standard deviation obtained from individual genes. Finally, we determine the boundaries of probability intervals and demonstrate that the coefficients defining the intervals are independent of sample characteristics, variability of data, laboratory conditions and type of chips. These coefficients are very closely correlated with Student's t-distribution.

Conclusion

In this study we ascertained that the non-systematic variations possess Gaussian distribution, determined the probability intervals and demonstrated that the K_α coefficients defining these intervals are invariant; these coefficients offer a convenient universal measure of dispersion of data. The fact that the K_α distributions are so close to t-distribution and independent of conditions and type of arrays suggests that the quantitative data provided by Affymetrix technology give "true" representation of physical processes, involved in measurement of RNA abundance.

Reviewers

This article was reviewed by Yoav Gilad (nominated by Doron Lancet), Sach Mukherjee (nominated by Sandrine Dudoit) and Amir Niknejad and Shmuel Friedland (nominated by Neil Smalheiser).

Loss of retinal progenitor cells leads to an increase in the retinal stem cell population in vivo

Retinal stem cells [with the potential to produce either neural retinal progenitors or retinal pigment epithelial (RPE) progenitors] exist in the mammalian eye throughout life, and indeed the greatest absolute increase in the stem population occurs postnatally. The stem cells proliferate embryonically and thus may help to build the retina initially, but in postnatal mammals they clearly do not proliferate to regenerate the retina in response to injury. Using Chx10(orJ/orJ) and Mitf(mi/mi) mice, with small eye phenotypes due to the reduction of the neural retinal progenitor population and the retinal pigmented epithelial progenitor population, respectively, we now report that the retinal stem cell population, when assayed from the ciliary margin, increases 3-8-fold in both mutants. These findings suggest that the mammalian retinal stem cell population may be capable of responding to genetically induced signals from the progenitor populations.

The Iroquois homeobox gene, Irx5, is required for retinal cone bipolar cell development

In the mouse retina, at least ten distinct types of bipolar interneurons are involved in the transmission of visual signals from photoreceptors to ganglion cells. How bipolar interneuron diversity is generated during retinal development is poorly understood. Here, we show that Irx5, a member of the Iroquois homeobox gene family, is expressed in developing bipolar cells starting at postnatal day 5 and is localized to a subset of cone bipolar cells in the mature mouse retina. In Irx5-deficient mice, defects were observed in the expression of some, but not all, immunohistological markers that define mature Type 2 and Type 3 OFF cone bipolar cells, indicating a role for Irx5 in bipolar cell differentiation. The differentiation of these two bipolar cell types has previously been shown to require the homeodomain-CVC transcription factor, Vsx1. However, the defects observed in Irx5-deficient retinas do not coincide with a reduction of Vsx1 expression, and conversely, the expression of Irx5 in cone bipolar cells does not require the presence of a functional Vsx1 allele. These results indicate that there are at least two distinct genetic pathways (Irx5-dependent and Vsx1-dependent) regulating the development of Type 2 and Type 3 cone bipolar cells.

Natural antisense transcripts associated with genes involved in eye development

Natural antisense transcripts (NATs) are a class of genes whose role in controlling gene expression is becoming more and more relevant. We describe the identification of eight novel mouse NATs associated with transcription factors (Pax6, Pax2, Six3, Six6, Otx2, Crx, Rax and Vax2) that play an important role in eye development and function. These newly identified NATs overlap with the mature processed mRNAs or with the primary unprocessed transcript of their corresponding sense genes, are predicted to represent either protein coding or non-coding RNAs and undergo extensive alternative splicing. Expression studies, by both RT-PCR and RNA in situ hybridization, demonstrate that most of these NATs, similarly to their sense counterparts, display a specific or predominant expression in the retina, particularly at postnatal stages. We found a significant reduction of the expression levels of one of these NATs, Vax2OS (Vax2 opposite strand) in a mouse mutant carrying the inactivation of Vax2, the corresponding sense gene. In addition, we overexpressed another NAT, CrxOS, in mouse adult retina using adeno-associated viral vectors and we observed a significant decrease in the expression levels of the corresponding sense gene, Crx. These results suggest that these transcripts are functionally related to their sense counterparts and may play an important role in regulating the molecular mechanisms that underlie eye development and function in both physiological and pathological conditions.

Chx10 repression of Mitf is required for the maintenance of mammalian neuroretinal identity

During vertebrate eye development, the cells of the optic vesicle (OV) become either neuroretinal progenitors expressing the transcription factor Chx10, or retinal pigment epithelium (RPE) progenitors expressing the transcription factor Mitf. Chx10 mutations lead to microphthalmia and impaired neuroretinal proliferation. Mitf mutants have a dorsal RPE-to-neuroretinal phenotypic transformation, indicating that Mitf is a determinant of RPE identity. We report here that Mitf is expressed ectopically in the Chx10(or-J/or-J) neuroretina (NR), demonstrating that Chx10 normally represses the neuroretinal expression of Mitf. The ectopic expression of Mitf in the Chx10(or-J/or-J) NR deflects it towards an RPE-like identity; this phenotype results not from a failure of neuroretinal specification, but from a partial loss of neuroretinal maintenance. Using Chx10 and Mitf transgenic and mutant mice, we have identified an antagonistic interaction between Chx10 and Mitf in regulating retinal cell identity. FGF (fibroblast growth factor) exposure in a developing OV has also been shown to repress Mitf expression. We demonstrate that the repression of Mitf by FGF is Chx10 dependent, indicating that FGF, Chx10 and Mitf are components of a pathway that determines and maintains the identity of the NR.

Facile isolation and the characterization of human retinal stem cells

This study identifies and characterizes retinal stem cells (RSCs) in early postnatal to seventh-decade human eyes. Different subregions of human eyes were dissociated and cultured by using a clonal sphere-forming assay. The stem cells were derived only from the pars plicata and pars plana of the retinal ciliary margin, at a frequency of approximately 1:500. To test for long-term self-renewal, both the sphere assay and monolayer passaging were used. By using the single sphere passaging assay, primary spheres were dissociated and replated, and individual spheres demonstrated 100% self-renewal, with single spheres giving rise to one or more new spheres in each subsequent passage. The clonal retinal spheres were plated under differentiation conditions to assay the differentiation potential of their progeny. The spheres were produced all of the different retinal cell types, demonstrating multipotentiality. Therefore, the human eye contains a small population of cells (approximately equal to 10,000 cells per eye) that have retinal stem-cell characteristics (proliferation, self-renewal, and multipotentiality). To test the in vivo potential of the stem cells and their progeny, we transplanted dissociated human retinal sphere cells, containing both stem cells and progenitors, into the eyes of postnatal day 1 NOD/SCID mice and embryonic chick eyes. The progeny of the RSCs were able to survive, migrate, integrate, and differentiate into the neural retina, especially as photoreceptors. Their facile isolation, integration, and differentiation suggest that human RSCs eventually may be valuable in treating human retinal diseases.

Nuclear receptor NR2E3 gene mutations distort human retinal laminar architecture and cause an unusual degeneration

Mutations in the nuclear receptor gene, NR2E3, cause a disorder of human retinal photoreceptor development characterized by hyperfunction and excess of the minority S (short wavelength or blue) cone photoreceptor type, but near absence of function of the majority rod receptor. NR2E3 disease can also progress to blindness. How the human retina accommodates mis-specified types and numbers of neurons and advances to retinal degeneration are unknown. We studied the retinal organization in vivo of patients with NR2E3 mutations. Early human NR2E3 disease with S cone hyperfunction showed thickened retinal layers within an otherwise normally structured retina. With visual loss, however, lamination was coarse and there was a strikingly thick and bulging appearance to the retina, localized to an annulus encircling the central fovea. This pattern was not found in other retinal degenerations. The abnormal laminar retinal architecture of early NR2E3 disease may be due in part to larger cells with an S cone phenotype in place of rods that failed to differentiate. The later-stage dysplastic appearance suggests a previously unrecognized proliferative response in human retinal degeneration.

Control of late off-center cone bipolar cell differentiation and visual signaling by the homeobox gene Vsx1

Retinal bipolar cells are interneurons that transmit visual signals from photoreceptors to ganglion cells. Although the visual pathways mediated by bipolar cells have been well characterized, the genes that regulate their development and function are largely unknown. To determine the role in bipolar cell development of the homeobox gene Vsx1, whose retinal expression is restricted to a major subset of differentiating and mature cone bipolar (CB) cells, we targeted the gene in mice. Bipolar cell fate was not altered in the absence of Vsx1 function, because the pan-bipolar markers Chx10 and Ret-B1 continued to be expressed in inner nuclear layer neurons labeled by the Vsx1-targeting reporter gene, tauLacZ. The specification, number, and gross morphology of the subset of on-center and off-center (OFF)-CB cells defined by tauLacZ expression from the Vsx1 locus were also normal in Vsx1(tauLacZ)/Vsx1(tauLacZ) mice. However, the terminal differentiation of OFF-CB cells in the retina of Vsx1(tauLacZ)/Vsx1(tauLacZ) mice was incomplete, as demonstrated by a substantial reduction in the expression of at least four markers (recoverin, NK3R, Neto1, and CaB5) for these interneurons. These molecular abnormalities were associated with defects in retinal function and documented by electroretinography and in vitro ganglion cell recordings specific to cone visual signaling. In particular, there was a general reduction in the light-mediated activity of OFF, but not on-center, ganglion cells. Thus, Vsx1 is required for the late differentiation and function of OFF-CB cells and is associated with a heritable OFF visual pathway-specific retinal defect.

PROGRESSTOWARDUNDERSTANDING THEGENETIC ANDBIOCHEMICALMECHANISMS OFINHERITEDPHOTORECEPTORDEGENERATIONS

More than 80 genes associated with human photoreceptor degenerations have been identified. Attention must now turn toward defining the mechanisms that lead to photoreceptor death, which occurs years to decades after the birth of the cells. Consequently, this review focuses on topics that offer insights into such mechanisms, including the one-hit or constant risk model of photoreceptor death; topological patterns of photoreceptor degeneration; mutations in ubiquitously expressed splicing factor genes associated only with photoreceptor degeneration; disorders of the retinal pigment epithelium; modifier genes; and global gene expression analysis of the retina, which will greatly increase our understanding of the downstream events that occur in response to a mutation.

VSX1: a gene for posterior polymorphous dystrophy and keratoconus

We identified mutations in the VSX1 homeobox gene for two distinct inherited corneal dystrophies; posterior polymorphous dystrophy (PPD) and keratoconus. One of the mutation (R166W) responsible for keratoconus altered the homeodomain and impaired DNA binding. Two other sequence changes (L159M and G160D) were associated with keratoconus and PPD, respectively, and involved a region adjacent to the homeodomain. The G160D substitution, and a fourth defect affecting the highly conserved CVC domain (P247R), occurred in a child with very severe PPD who required a corneal transplant at 3 months of age. In this family, relatives with the G160D change alone had mild to moderate PPD, while P247R alone caused no corneal abnormalities. However, with either the G160D or P247R mutation, electroretinography detected abnormal function of the inner retina, where VSX1 is expressed. These data define the molecular basis of two important corneal dystrophies and reveal the importance of the CVC domain in the human retina.

Vsx1, a rapidly evolving paired-like homeobox gene expressed in cone bipolar cells

The paired-like homeodomain (HD) protein Chx10 is distinguished by the presence of the CVC domain, a conserved 56 amino acid sequence C-terminal to the HD. In mammals, Chx10 is essential both for the proliferation of retinal progenitor cells and for the formation or survival of retinal bipolar interneurons. We describe the cloning and characterization of a mouse Chx10 homologue, Vsx1; phylogenetic analysis suggests that Vsx1 and its putative vertebrate orthologues have evolved rapidly. Vsx1 expression in the adult is predominantly retinal. Whereas Chx10 is expressed both in retinal progenitors in the developing eye and apparently in all bipolar cells of the mature retina, Vsx1 expression is first detected in the eye at postnatal day 5, where it is restricted to cone bipolar cells.

Inherited neurodegenerative diseases: the one-hit model of neurodegeneration

The clinical manifestations of inherited neurodegenerative diseases are often delayed for periods from years to decades. This observation has led to the idea that, in these disorders, neurons die from cumulative damage. A critical prediction of the cumulative damage hypothesis is that the probability of neuronal death increases with age. However, we recently demonstrated, in 17 examples of neurodegeneration, that the kinetics of neuronal death appear to be exponential. These examples include both monogenic disorders, such as photoreceptor degenerations, as well as others that are partly or entirely acquired (such as the clinical phase of parkinsonism and retinal detachment). Exponential kinetics indicate that (i) the risk of death is constant, (ii) death occurs randomly in time and (iii) the death of each neuron is independent of other neurons. We use the term 'one-hit model' to refer to the single catastrophic intracellular biochemical event, analogous to radioactive decay, which leads to neuronal death in the diseases we analyzed. Here, we examine the major features and implications of the one-hit model and provide preliminary evidence that amyotrophic lateral sclerosis also appears to fit this model. We also discuss a testable biochemical hypothesis, the mutant steady-state hypothesis, that we proposed to account for the one-hit model. Finally, we explore six unresolved issues that appear to challenge this model. The one-hit model appears to capture a novel principle underlying many neurodegenerations. Our findings suggest that any consideration of the biochemical basis of neurodegeneration must include a meticulous examination of the kinetics of cell death.

Deficiency of rds/peripherin causes photoreceptor death in mouse models of digenic and dominant retinitis pigmentosa

Retinitis pigmentosa (RP) is a group of inherited blinding diseases caused by mutations in multiple genes including RDS. RDS encodes rds/peripherin (rds), a 36-kDa glycoprotein in the rims of rod and cone outer-segment (OS) discs. Rom1 is related to rds with similar membrane topology and the identical distribution in OS. In contrast to RDS, no mutations in ROM1 alone have been associated with retinal disease. However, an unusual digenic form of RP has been described. Affected individuals in several families were doubly heterozygous for a mutation in RDS causing a leucine 185 to proline substitution in rds (L185P) and a null mutation in ROM1. Neither mutation alone caused clinical abnormalities. Here, we generated transgenic/knockout mice that duplicate the amino acid substitutions and predicted levels of rds and rom1 in patients with RDS-mediated digenic and dominant RP. Photoreceptor degeneration in the mouse model of digenic RP was faster than in the wild-type and monogenic controls by histological, electroretinographic, and biochemical analysis. We observed a positive correlation between the rate of photoreceptor loss and the extent of OS disorganization in mice of several genotypes. Photoreceptor degeneration in RDS-mediated RP appears to be caused by a simple deficiency of rds and rom1. The critical threshold for the combined abundance of rds and rom1 is approximately 60% of wild type. Below this value, the extent of OS disorganization results in clinically significant photoreceptor degeneration.

Human microphthalmia associated with mutations in the retinal homeobox gene CHX10

Isolated human microphthalmia/anophthalmia, a cause of congenital blindness, is a clinically and genetically heterogeneous developmental disorder characterized by a small eye and other ocular abnormalities. Three microphthalmia/anophthalmia loci have been identified, and two others have been inferred by the co-segregation of translocations with the phenotype. We previously found that mice with ocular retardation (the or-J allele), a microphthalmia phenotype, have a null mutation in the retinal homeobox gene Chx10 (refs 7,8). We report here the mapping of a human microphthalmia locus on chromosome 14q24.3, the cloning of CHX10 at this locus and the identification of recessive CHX10 mutations in two families with non-syndromic microphthalmia (MIM 251600), cataracts and severe abnormalities of the iris. In affected individuals, a highly conserved arginine residue in the DNA-recognition helix of the homeodomain is replaced by glutamine or proline (R200Q and R200P, respectively). Identification of the CHX10 consensus DNA-binding sequence (TAATTAGC) allowed us to demonstrate that both mutations severely disrupt CHX10 function. Human CHX10 is expressed in progenitor cells of the developing neuroretina and in the inner nuclear layer of the mature retina. The strong conservation in vertebrates of the CHX10 sequence, pattern of expression and loss-of-function phenotypes demonstrates the evolutionary importance of the genetic network through which this gene regulates eye development.

A one-hit model of cell death in inherited neuronal degenerations

In genetic disorders associated with premature neuronal death, symptoms may not appear for years or decades. This delay in clinical onset is often assumed to reflect the occurrence of age-dependent cumulative damage. For example, it has been suggested that oxidative stress disrupts metabolism in neurological degenerative disorders by the cumulative damage of essential macromolecules. A prediction of the cumulative damage hypothesis is that the probability of cell death will increase over time. Here we show in contrast that the kinetics of neuronal death in 12 models of photoreceptor degeneration, hippocampal neurons undergoing excitotoxic cell death, a mouse model of cerebellar degeneration and Parkinson's and Huntington's diseases are all exponential and better explained by mathematical models in which the risk of cell death remains constant or decreases exponentially with age. These kinetics argue against the cumulative damage hypothesis; instead, the time of death of any neuron is random. Our findings are most simply accommodated by a 'one-hit' biochemical model in which mutation imposes a mutant steady state on the neuron and a single event randomly initiates cell death. This model appears to be common to many forms of neurodegeneration and has implications for therapeutic strategies.

Rom-1 is required for rod photoreceptor viability and the regulation of disk morphogenesis

The homologous membrane proteins Rom-1 and peripherin-2 are localized to the disk rims of photoreceptor outer segments (OSs), where they associate as tetramers and larger oligomers. Disk rims are thought to be critical for disk morphogenesis, OS renewal and the maintenance of OS structure, but the molecules which regulate these processes are unknown. Although peripherin-2 is known to be required for OS formation (because Prph2-/- mice do not form OSs; ref. 6), and mutations in RDS (the human homologue of Prph2) cause retinal degeneration, the relationship of Rom-1 to these processes is uncertain. Here we show that Rom1-/- mice form OSs in which peripherin-2 homotetramers are localized to the disk rims, indicating that peripherin-2 alone is sufficient for both disk and OS morphogenesis. The disks produced in Rom1-/- mice were large, rod OSs were highly disorganized (a phenotype which largely normalized with age) and rod photoreceptors died slowly by apoptosis. Furthermore, the maximal photoresponse of Rom1-/- rod photoreceptors was lower than that of controls. We conclude that Rom-1 is required for the regulation of disk morphogenesis and the viability of mammalian rod photoreceptors, and that mutations in human ROM1 may cause recessive photoreceptor degeneration.

Recent advances in the molecular basis of inherited photoreceptor degeneration

To date, 118 loci have been associated with photoreceptor degenerative disease. In this review, we will discuss recent advances in the identification of genes that cause progressive photoreceptor cell death when mutated. We will focus on 12 genes isolated within the last two years that have been shown to be photoreceptor-specific, or that have provided insight into photoreceptor biology and the mechanisms of photoreceptor cell death. To aid in understanding the biologic basis for these diseases, we also briefly review photoreceptor biology. Finally, we report on recent advances towards the treatment of these disorders.

Retinal stem cells in the adult mammalian eye

The mature mammalian retina is thought to lack regenerative capacity. Here, we report the identification of a stem cell in the adult mouse eye, which represents a possible substrate for retinal regeneration. Single pigmented ciliary margin cells clonally proliferate in vitro to form sphere colonies of cells that can differentiate into retinal-specific cell types, including rod photoreceptors, bipolar neurons, and Müller glia. Adult retinal stem cells are localized to the pigmented ciliary margin and not to the central and peripheral retinal pigmented epithelium, indicating that these cells may be homologous to those found in the eye germinal zone of other nonmammalian vertebrates.

PHR1 encodes an abundant, pleckstrin homology domain-containing integral membrane protein in the photoreceptor outer segments

We cloned human and murine cDNAs of a gene (designated PHR1), expressed preferentially in retina and brain. In both species, PHR1 utilizes two promoters and alternative splicing to produce four PHR1 transcripts, encoding isoforms of 243, 224, 208, and 189 amino acids, each with a pleckstrin homology domain at their N terminus and a transmembrane domain at their C terminus. Transcript 1 originates from a 5'-photoreceptor-specific promoter with at least three Crx elements ((C/T)TAATCC). Transcript 2 originates from the same promoter but lacks exon 7, which encodes 35 amino acids immediately C-terminal to the pleckstrin homology domain. Transcripts 3 and 4 originate from an internal promoter in intron 2 and either include or lack exon 7, respectively. In situ hybridization shows that PHR1 is highly expressed in photoreceptors, with lower expression in retinal ganglion cells. Immunohistochemistry localizes the PHR1 protein to photoreceptor outer segments where chemical extraction studies confirm it is an integral membrane protein. Using a series of PHR1 glutathione S-transferase fusion proteins to perform in vitro binding assays, we found PHR1 binds transducin betagamma subunits but not inositol phosphates. This activity and subcellular location suggests that PHR1 may function as a previously unrecognized modulator of the phototransduction pathway.

A range of clinical phenotypes associated with mutations in CRX, a photoreceptor transcription-factor gene

Mutations in the retinal-expressed gene CRX (cone-rod homeobox gene) have been associated with dominant cone-rod dystrophy and with de novo Leber congenital amaurosis. However, CRX is a transcription factor for several retinal genes, including the opsins and the gene for interphotoreceptor retinoid binding protein. Because loss of CRX function could alter the expression of a number of other retinal proteins, we screened for mutations in the CRX gene in probands with a range of degenerative retinal diseases. Of the 294 unrelated individuals screened, we identified four CRX mutations in families with clinical diagnoses of autosomal dominant cone-rod dystrophy, late-onset dominant retinitis pigmentosa, or dominant congenital Leber amaurosis (early-onset retinitis pigmentosa), and we identified four additional benign sequence variants. These findings imply that CRX mutations may be associated with a wide range of clinical phenotypes, including congenital retinal dystrophy (Leber) and progressive diseases such as cone-rod dystrophy or retinitis pigmentosa, with a wide range of onset.

Retinal degenerations with truncation mutations in the cone-rod homeobox (CRX) gene

PURPOSE

To define the phenotypes of retinal degenerations associated with mutations in the gene encoding CRX (cone-rod homeobox), a photoreceptor-specific transcription factor.

METHODS

Heterozygotes with the E168 [delta1 bp], E168 [delta2 bp], or G217 [delta1 bp] CRXgene mutation were studied clinically, with visual function tests, including rod and cone perimetry and electroretinography (ERG), and with optical coherence tomography (OCT).

RESULTS

Clinical diagnoses included autosomal dominant cone-rod dystrophy in one family (E168 [delta1 bp] mutation) and simplex Leber congenital amaurosis in two families (E168 [delta2 bp], G217 [delta1 bp] mutations). In the family with the E168 [delta1 bp] mutation, two siblings had relatively mild disease expression in the third decade of life. The central retinas of these two patients had profound loss of rod and short wavelength cone function; long/middle wavelength cone thresholds were elevated at fixation, but there were greater paracentral than central abnormalities. Peripheral retinal dysfunction was evident by psychophysics and by maximum amplitude loss for rod- and cone-isolated ERG photoreceptor responses. OCT cross-sectional reflectance images showed decreased central retinal thickness consistent with photoreceptor loss. An additional member of this family (E168 [delta1 bp] mutation) and two other patients (representing E168 [delta2 bp] and G217 [delta1 bp] mutations) had a severe phenotype with retina-wide loss of function and islands of function remaining only in the temporal periphery.

CONCLUSIONS

Truncation mutations in CRX are associated with retinopathies that share phenotypic features but vary in disease severity. The disease mechanism could involve abnormal photoreceptor development compounded by a disturbance in the maintenance of photoreceptors in the mature retina.

Association of the TLX-2 homeodomain and 14-3-3eta signaling proteins

Homeodomain proteins play important roles in various developmental processes, and their functions are modulated by polypeptide cofactors. Here we report that both in vitro and in vivo, 14-3-3eta is associated with the TLX-2 homeodomain transcription factor that is required for mouse embryogenesis. Expression of 14-3-3eta shifts the predominant localization of TLX-2 in COS cells from the cytoplasm to the nucleus. Tlx-2 and 14-3-3eta are expressed in the developing peripheral nervous system with spatially and temporally overlapping patterns, and they are also coexpressed in PC12 cells. Increased expression of either gene by transfection considerably inhibited nerve growth factor-induced neurite outgrowth of PC12 cells, and cotransfection of both genes led to a synergistic effect of suppression. These findings define 14-3-3eta as a functional modulator of the TLX-2 homeodomain transcription factor and suggest that the in vivo function of TLX-2 in neural differentiation is likely regulated by signaling mediated by 14-3-3eta.

Retinal gene therapy 1998: summary of a workshop

The 1998 Workshop on Retinal Gene Therapy evaluated the potential of gene therapy in treatment of retinal disease. Academic, industry, and private foundation representatives attended. Topics included: determing which retinal diseases are likely candidates for gene therapy, specific retinal degenerations and nonspecific neuronal survival mechanisms, design and use of viral and retroviral vectors in achieving regulated gene expression, animal models of retinal degeneration and associated therapies, human trials, and alternatives to gene therapy. The discussion of human trials explored the justification for moving from animal models to human testing, patient population concerns, lessons learned from previous human gene therapy trials, and the role of industry in support of basic and clinical research.

The Tlx-2 homeobox gene is a downstream target of BMP signalling and is required for mouse mesoderm development

TGFbeta-related factors are critical regulators of vertebrate mesoderm development. However, the signalling cascades required for their function during this developmental process are poorly defined. Tlx-2 is a homeobox gene expressed in the primitive streak of mouse embryos. Exogenous BMP-2 rapidly activates Tlx-2 expression in the epiblast of E6.5 embryos. A Tlx-2 promoter element responds to BMP-2 signals in P19 cells, and this response is mediated by BMP type I receptors and Smad1. These results suggest that Tlx-2 is a downstream target gene for BMP signalling in the primitive streak where BMP-4 and other TGFbeta-related factors are expressed. Furthermore, disruption of Tlx-2 function leads to early embryonic lethality. Similar to BMP4 and ALK3 mutants, the mutant embryos display severe defects in primitive streak and mesoderm formation. These experiments thus define a BMP/Tlx-2 signalling pathway that is required during early mammalian gastrulation.

Intragenic complementation at the argininosuccinate lyase locus: reconstruction of the active site

Intragenic complementation has been observed at the argininosuccinate lyase (ASL) locus and the ASL alleles in the ASL-deficient cell strains of two complementation phenotypes have been identified. The frequent complementers, strains that participate in the majority of the complementation events, were found to be either homozygous or heterozygous for the Q286R allele, while the high-activity complementers, those strains in which complementation is associated with a high restoration of activity, were found to be either homozygous or heterozygous for the D87G allele. Direct proof of the intragenic complementation observed at the ASL locus has been obtained with the co-expression of the D87G and Q286R alleles in COS cells. A significant increase in the ASL activity was observed when the two alleles were co-expressed relative to the expression of each mutant allele alone. The increase in activity was comparable to that observed previously in the fibroblast complementation studies. The structure determinations of ASL and the homologous eye lens protein, duck delta II crystallin, have revealed that the active site of ASL is made up of residues from three different monomers. The structural mapping of the Q286 and D87 residues shows that both are located near the active site but that, in any one active site, each is contributed by a different monomer. The molecular symmetry of the ASL protein is such that when mutant monomers combine randomly, one active site will contain both mutations and at least one active site will contain no mutations at all. It is these 'native' active sites in the hybrid Q286R/D87G proteins that give rise to the partial recovery of enzymatic activity observed during intragenic complementation.