Ординатура / Офтальмология / Английские материалы / Eye, Retina, and Visual System of the Mouse_Chalupa, Williams_2008

in spontaneous synaptic inputs to RGCs after eye opening in mouse retina (Tian and Copenhagen, 2001) and an agedependent remodeling of dendritic complexity of a class of RGCs in hamster retina (Wingate and Thompson, 1994). In dark-reared turtle retina, receptive field areas of RGCs expanded to more than twice the size of those observed in animals reared under normal conditions, suggesting that visual experience plays a role in controlling the outgrowth of turtle RGC dendrites (Sernagor and Grzywacz, 1996). In developing rat retina, the expression of brain-derived neurotrophic factor, a factor that controls RGC dendrites arborization, is also modulated by visual experience (Seki, 2003).

Possible mechanisms of activity-dependent developmental segregation of ON and OFF pathways in the retina

It is unclear how synaptic activity regulates the developmental segregation of RGC dendrites into ON and OFF pathways. Chalupa and colleagues (1998) proposed two possible general synaptic mechanisms. In the first model, they assumed that RGCs synapse functionally with only ON or OFF bipolar cells in early postnatal development, although their dendrites ramify in both the inner and outer IPL (see figure 29.5A). The asymmetrical synaptic inputs from ON or OFF bipolar cells could “instruct” those RGCs to sever uninnervated dendrites during later postnatal development. If this were the case, one would expect to find RGCs stratified in both the inner and outer IPL but responding only to the onset or offset of light stimulation in the early postnatal developing retina. Recent results obtained with simultaneous patch-clamp and morphological recordings of ferret RGCs revealed that all RGCs with dendrites ramifying in both the inner and outer IPL responded to both the onset and offset of light in both young and adult animals, indicating that RGCs with dendrites ramifying in both inner and outer IPL are innervated by both ON and OFF bipolar cells (Wang et al., 2001). Consistent with these results, intraocular injection of APB or light deprivation increased both the ON-OFF-responsive and multistratified RGCs in cat (Bodnarenko and Chalupa, 1993; Bodnarenko et al., 1995; Bisti et al., 1998) and mouse (Tian and Copenhagen, 2003) retina, respectively. Thus, it appears unlikely that an asymmetry of ON and OFF synaptic inputs is responsible for the elimination of exuberant processes in immature RGCs.

In the second model, it was assumed that synaptic transmission from bipolar cells triggers an intrinsic program in RGCs ramifying in both inner and outer IPL that leads to the retraction of one or another set of their dendritic processes (see figure 29.5B). This model relies on cell-specific intrinsic genetic programs that activate differential pruning of an individual cell’s dendrites in either sublamina a or b.

No molecular or genetic mechanisms that would mediate this selective pruning have been identified.

These two models are proposed based on the common assumption that RGCs achieve their mature stratified patterns from early diffuse ramification patterns by selective pruning of “misplaced” dendrites. Although it is well documented that the dendrites of RGCs are diffusely ramified in the IPL in early development and then gradually stratified into narrow strata during postnatal development in many species of mammals, including mice, two recent studies showed that the dendrites of most RGCs in mouse retina are narrowly stratified in the IPL, similar to that of adult animals before eye opening (Bansal et al., 2000; Diao et al., 2004), suggesting that dendritic pruning is largely completed at the time of eye opening. Insofar as the percentage of RGCs responding to both the onset and offset of light declined from 40%–76% at the time of eye opening to around 20% in adult ferrets and mice (Wang et al., 2001; Tian and Copenhagen, 2003) and the population of RGCs with dendrites ramifying in both sublaminae a and b of the IPL decreased from 53% at eye opening to 29% 2–3 weeks after eye opening (Tian and Copenhagen, 2003), these results strongly suggest that the developmental segregation of RGC dendrites into ON and OFF pathways after eye opening is unlikely to be achieved by simply removing some “misplaced” dendrites from already narrowly stratified dendritic plexus.

How could a narrowly stratified RGC dendritic plexus reorganize synaptic connections with ON and OFF bipolar cells in different strata of the IPL in developing retina? A recent study of developing zebrafish retina using in vivo time-lapse imaging demonstrated that the lamina-restricted dendritic plexus of RGCs could “migrate” from the inner border of the IPL to the outer border of the IPL in 2–3 days without diffusely elaborating their dendrites throughout the IPL (Mumm et al., 2006). This redistribution of stratified dendritic plexus involves simultaneous adding dendrites in one stratum and eliminating dendrites in another stratum of the IPL during the course of development. Unfortunately, the same technique has not been successfully applied to the study of developing mammalian retina, and therefore it has not been directly demonstrated whether mammalian RGCs could take the same developmental strategy as zebrafish RGCs to redistribute their dendrites in the IPL (Chalupa, 2006).

Recently, we quantitatively analyzed the dendritic stratification and ramification patterns of RGCs of developing mouse retinas fixed at different developmental time points (Xu and Tian, 2007). Our results showed that the majority of mouse RGCs have narrowly stratified dendrites at the time of eye opening and a large portion of them have their dendrites located near the center of the IPL (figure 29.4B). Therefore, these RGCs could synapse with both ON and OFF bipolar cells and respond to both onset and offset of

Figure 29.4 The developmental redistribution of RGC dendrites that altered the relative populations of RGCs receiving synaptic inputs from ON and OFF BC RGCs were classified into ON, OFF, monostratified ON-OFF, and bistratified ON-OFF RGCs based on their dendritic distribution patterns in the IPL. A, Upper panels, Stacked images of an ON RGC (left), an OFF RGC (center), and a monostratified ON-OFF RGC (right). Middle panels, 90° rotation views of the same three cells. Lower panels, Normalized dendritic distribution of each cell (open circles) and the Gaussian fitting of the data (line). Shaded area indicates the sublamina a. Scale bars =100 μm. B, Average histogram of the peak dendritic location of all monostratified RGCs of P12-aged mice. The histogram fitted well with a double Gauss distribution, with a major peak located at 60% of IPL thickness and a minor peak located at 35% of IPL thickness, respectively (χ2 = 0.536, r2 = 0.984). C, Average histogram of peak

light stimulation. After eye opening, the RGCs with dendrites located at the center of the IPL redistributed their dendrites close to the inner or outer border of the IPL (figure 29.4C) and therefore become ON or OFF cells, without a further reduction in the width of their dendritic stratification (figure 29.4D and G). Similar to the effects induced by longterm treatment of cats’ eyes with intraocular injection of

dendritic location of all monostratified RGCs of P33-aged mice. The histogram fitted well with a triple Gaussian distribution, with three peaks located at 25%, 50%, and 70% of IPL thickness, respectively (χ2 = 0.686, r2 = 0.989). D, Distributions of the dendritic widths of monostratified RGCs from P12and P33-aged mice and Gaussian fittings of the data. E, Average histogram of the peak dendritic distribution of monostratified RGCs of dark-reared mice and age-matched controls. The histogram of dark-reared mice fitted well with a triple Gaussian distribution (χ2 = 0.603, r2 = 0.982). F, Distribution of dendritic widths of all three groups of monostratified RGCs of dark-reared and control mice and Gaussian fittings of the data. G, Average percentages of ON, OFF, bistratified ON-OFF, and monostratified ON-OFF RGCs of P12and P33-aged mice raised in cyclic light (P33) and constant darkness (P33D). (Modified from Xu and Tian, 2007.)

APB (Deplano et al., 2004), light deprivation preferentially retarded the dendritic redistribution of mouse RGCs from the center of the IPL to the OFF layer of the IPL (figure 29.4E) without changing the width of dendritic distribution (figure 29.4F), which resulted in more RGCs having dendrites located near the center of the IPL and receiving synaptic inputs from both ON and OFF bipolar cells (figure

29.4G). These results suggest that the dendritic refinement of mouse RGCs after eye opening requires both targeted dendritic growth and selective dendritic elimination, similar to what was reported in zebrafish by Mumm et al. (2006), and probably driven by visually evoked synaptic activity. It is also worth noting that the number of RGCs ramified in both sublaminae a and b of the IPL with a single layer of dendritic plexus (figure 29.4A, right) decreased with age, while the number of RGCs ramified in both sublaminae a and b of the IPL with clear bistratified dendritic patterns (see figure 29.3E) increased with age (see figure 29.4G), suggesting that monostratified and bistratified ON-OFF RGCs are likely to be in different developmental status.

When all of these findings are considered together, it appears that the maturation of mouse RGC dendrites and the developmental segregation of ON and OFF pathways in the retina undergo a multistage process during postnatal development. In early postnatal development, RGCs restrict their dendrites from a diffusely ramified pattern into narrowly stratified patterns, primarily through dendritic pruning before eye opening (figure 29.5C, step 1). At this stage, some of the RGCs ramify their dendrites only in sublamina a or b of the IPL, but a large number of RGCs still synapse with

Figure 29.5 Schematic diagram illustrating possible mechanisms underlying RGC dendritic stratification and segregation. A, Asymmetrical afferent innervation model. RGCs have their dendrites initially bistratified in both ON and OFF sublaminae of IPL and have asymmetrical synaptic inputs. During development, dendrites that receive afferent inputs are maintained, whereas those that do not receive afferent input are eliminated. B, Intrinsic program model. Signal inputs from both ON and OFF bipolar cells trigger an intrinsic program in the bistratified RGCs, leading to elimination of the dendrites in either ON or OFF sublamina. C, Multistage model. The dendrites of mouse RGCs initially ramify diffusely throughout the IPL and then gradually are restricted to a narrowly stratified pattern before eye opening. After eye opening, RGCs further redistribute their dendrites into the ON or OFF layer of the IPL, probably guided by visually evoked synaptic activity.

both ON and OFF bipolar cells in both sublaminae a and b. This initial dendritic pruning is at least in part regulated by spontaneous retinal activity. Later during postnatal development, narrowly stratified RGCs further refine their dendritic distribution in the IPL and synaptic connections with ON and OFF bipolar cells (figure 29.5C, step 2). This later refinement further segregates the RGC dendrites into ON and OFF pathways after eye opening and is regulated by visually evoked synaptic inputs from bipolar cells.

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From the time the mouse grew into an acceptable species for brain research, it incorporated an ambitious purpose that distinguished it from other low-cost neurobiological model systems. Its small size, rapid breeding cycle, genetic accessibility, and structurally representative mammalian brain designated it as model species for breaking down the confines of brain research, with the ultimate goal of facilitating access to the genetic basis of functions and diseases of the human brain (Sidman et al., 1965). As neurobiological experimentation in its early days was insulated from cell biological and molecular biological research by a total gap in language, methods, and concepts, the mouse was instrumental in dissolving the boundaries with both these fields successively. Cell-biological neuroscience flourished in mice long before such techniques were adapted to other species, and the genetic insight from simpler molecular model species, such as Drosophila, expanded into vertebrate neuroscience by way of screens in the mouse for important invertebrate genes. The advance of transgenic techniques, including gene replacements and conditional null mutants, then made the mouse the foremost model in which to study gene function.

This unique standing of the mouse has only recently been challenged by the analysis of naturally occurring neurological mutations in humans: both in sheer number of living specimens and in sophistication of brain functions, laboratory mice cannot compete with humans. Nevertheless, with respect to higher visual functions, the mouse continues to be an indispensable animal model, even in the pursuit of a genetic understanding of higher cognitive functions that are considered characteristically human. A surprising result from the sequencing of entire genomes over recent years is the remarkable similarity in protein-coding genes between phenotypically different species such as mice and men. It is now apparent that the differences must be somehow inherent in the regulation of gene expression. Here we want to show that the compact mouse brain uniquely facilitates the discovery of large-scale organizational principles for the regulation of gene expression. In the current postgenomic era, however, the mouse cannot be appraised in isolation. Rather, neurobiological discoveries made in the mouse

must be integrated with relevant facts collected in other species.

The first recordings from single neurons in visual centers of the mouse established that its brain is not only anatomically but also functionally very similar to that of other mammals, including primates (Dräger, 1975; Dräger and Hubel, 1975). The frontier in exploration of higher visual functions progressed over the following years from recordings in anesthetized animals to studies of awake, behaving monkeys and then to integrating these findings with insights from psychophysical observations in humans. Although it would be futile for mouse vision research to attempt to replicate studies in primates, the anatomical investigations described in chapter 20 of this volume establish that the mouse cortex contains the anatomical requisites for higher visual processing originally defined in primates. In this chapter we show that the regulation of gene expression in higher visual pathways is part of a novel transcriptional parcellation of the cerebral cortex that undoubtedly exists in humans as well. Because our argument for the validity of the mouse in cognitive research is based on studies of the retinoid system, about which very little is known with respect to the brain, a brief introduction to the topic is provided.

It is well known that vitamin A is exceptionally important for vision, but available evidence points exclusively to the eye: vitamin A deficiency causes inborn eye malformations in developing embryos and night blindness in visual function. Whether vitamin A also plays a role in the development and function of the central visual pathways is not known. Because efforts to understand vitamin A actions in the brain tend to become hampered by several unexplained difficulties, a general review of retinoids in the brain is in order, with an emphasis on the functional dorsal telencephalon. We then describe how we use our own data to extract information from the large Web-based data banks that have aggregated information on the mouse and human genomes (Magdaleno et al., 2006; Lein et al., 2007). Finally, we explain how this combined information points to an integral role for vitamin A in selected higher visual and cognitive functions.

The retinoid system

The biological effects of vitamin A are mediated through its two active derivatives, retinaldehyde and retinoic acid (RA): retinaldehyde forms the visual chromophore bound to opsin, which makes vision possible, and RA regulates gene transcription. Although the high rhodopsin content of photoreceptors makes the functional retina the organ of highest vitamin A content in the adult, the developing eye is the RA-richest site in the embryo, as it contains very high levels of the retinaldehyde dehydrogenases (RALDHs), which oxidize retinaldehyde to RA (Dräger et al., 2001; Luo et al., 2006). Also expressed in the eye are the CYP26 enzymes, which catabolize RA and limit its spread. RA acts by binding to nuclear receptors that are members of the large family of ligand-regulated transcription factors (Mark and Chambon, 2003). Binding of RAs to their receptors that reside at RA response elements (RAREs) in gene promoters causes conformational changes, which allows recruitment of co-activa- tors to the transcriptional machinery. Dissociation of RA from the receptors causes recruitment of transcriptional corepressors. The RA receptors include eight major RAR-α, -β, and -γ isoforms and six major RXR-α, -β, and -γ isoforms, and RA-sensitive gene expression is regulated by RAR/RXR heterodimers (Mark and Chambon, 2003). In addition, the RXRs form heterodimers with a wide range of other nuclear receptors, including the thyroid hormone and vitamin D receptors. Owing to the large number of isoforms and the enormous number of different heterodimeric combinations, RA-binding receptors constitute by far the most elaborate subgroup of the nuclear receptor transcription factors.

Both vitamin A deficiency and excess can result in severe malformations that are created in growing embryos at specific sites and critical developmental stages, indicating that normal RA actions are distinctly localized. The determinants for this localization are apparent from observations of null mutant mice generated for practically every retinoid gene (Mark and Chambon, 2003): most of the retinoid mutants do not have dramatic phenotypes except for null mutants for the metabolic enzymes, which tend to have severe deformities at the sites that are also vulnerable to nutritional vitamin A deficiency. This suggests that the most important criteria for the localization of RA signaling are locally regulated RA levels, which are synthesized by locally expressed RALDHs and catabolized by CYP26s. A major role for the RALDHs in localizing RA actions is consistent with their widely dispersed and sparse expression: the enzymes are restricted to a few sites in the body, with much of the intervening tissue being free of RA-synthesizing enzymes, which is especially true for the brain. Other retinoid factors, in particular the many RARs that convey the immense transcriptional complexity, are expressed profusely

in overlapping ways and serve partially redundant functions, so that their role in simple localization is relatively minor.

Actions of retinoic acid in the brain

From in vitro studies on human stem cells, it is estimated that RAs can influence the mRNA levels of about 15% of all protein-coding genes by twofold or more (Cawley et al., 2004). Since the actual RA actions in vivo are determined by their cellular and developmental context, however, one cannot predict which of the many possibilities of RAregulated gene expression will occur at a particular site. What constitutes this context needs to be characterized in vivo for each condition, as has been done in detail for the RA-rich retina. RA actions in the brain are mostly studied in very early embryos, and only limited information is available for the postnatal, functioning brain. The brain ranks among the regions of lowest RA production in the body; its overall RA synthesis is more than 100-fold lower than in the embryonic retina. Most components of the retinoid system are expressed in the postnatal brain, and in many cases their expression patterns differ from those in the embryo, which points to unique RA contributions to brain function (Lane and Bailey, 2005). Compelling evidence exists for critical RA involvement in important neurobiological activities, including learning, memory, and sleep (Chiang et al., 1998; Misner et al., 2001; Maret et al., 2005). Although these functions are major fields of investigation in neuroscience, the RA evidence is rarely considered, because it is not clear how it is integrated with the neurobiological mechanisms of these activities.

We first summarize the results of several studies on the brain and emphasize some unique, puzzling features that hint at unknown mechanisms through which RA signaling is reinforced in the brain and integrated with neurobiological events. We then describe patterns of elevated RA levels that emerge during postnatal development in the cortex and delineate the dorsal visual stream and the pathway for visual attention (Wang and Burkhalter, 2004; Wagner et al., 2006). Using these RA patterns, we searched gene expression data banks, which have recently become available as a public resource for the mouse, to identify genes that are potentially regulated by RA in the context of the brain. Because the genomes of mice and humans are very similar and the basic brain organization is conserved, mouse data can provide clues to human diseases. Among the genes whose expression is differentially regulated along the RA band are many implicated in cognitive diseases and abnormalities of dorsal stream function. We suggest that the uneven distribution of RA is part of normal postnatal pattern formation in the cerebral cortex; elevated RA levels designate which cortical territories remain relatively more plastic, by boosting gene expression in response to rapid neuronal and physiological changes,

which impinge via signaling cascades onto a common combinatorial transcriptional network (Rosenfeld et al., 2006).

Retinoic acid reporter mice

Colocalization of RA signaling with sites of RALDH expression is strikingly obvious in studies of RA reporter mice that are transgenic for the RA-sensitive promoter of the RAR-β gene driving β-galactosidase (lacZ) (Rossant et al., 1991; Smith et al., 2001; Luo et al., 2004). In early embryos, when RA is exclusively synthesized by RALDH2 expressed in the trunk and the eye anlage, the RALDH sites are matched by high lacZ expression in the reporter embryo (figure 30.1A). This differential RA distribution can be independently verified by explant assays with RARE-lacZ reporter cells, which are F9 embryonic stem cells transfected with a RARβ-lacZ construct similar to that used for the transgenic RA reporter mice (Wagner et al., 1992) (see the three culture wells between the embryos in figure 30.1A). High lacZ induction in RA reporter mice serves as a general, reliable indicator for RALDH expression throughout most of the body in early

embryos except for the brain. Although the RA content of the brain is very low, several different RA reporter strains all show consistently very high RA signaling at specific brain sites (Misner et al., 2001; Luo et al., 2004). This strong RA signaling in the reporter brains is not, however, matched by locally elevated RA levels measured using several different methods, including RA reporter cell responses to supernatants of cultured brain explants (figure 30.1B), and no RALDHs are expressed in these regions.

These discrepancies indicate that the early embryonic brain responds to RA that diffuses from elsewhere, most likely from RALDHs expressed in the optic vesicle and stalk, and by convection via the cerebrospinal fluid in the ventricular lumen, which is wide open at this early stage (Luo et al., 2004). Consistent with such a route, lacZ at this stage is highest in the ventricular layer. Surprisingly, however, not all the brain ventricular zones are labeled, but in the telencephalon RA signaling is restricted to the dorsal part, and in the diand mesencephalon it exhibits distinct, changing patterns, some of which suggest a role in visual development. For example, the tectal target regions for the RALDH-rich

Figure 30.1 A and B, RA signaling in RA reporter mice (RARElacZ mice) (Rossant et al., 1991) is compared with RALDH2 expression by in situ hybridization (1A) and with RA measurement by RA reporter cells (RARE-lacZ cells) (Wagner et al., 1992). C, Ventricular layer lacZ expression in target areas of axons from dorsal (D) and ventral (V) retina is visible through the thin tectal roof of this E12.5 whole-brain preparation. D, E13.5 head hemi-

sected along the midsagittal plane to show the lacZ-marked borders between the diencephalic prosomers (P1–P3) and the mesencephalon (mes) in the ventricular layer (Luo et al., 2004). E, Coronal slices through adult RA reporter brain to illustrate RA signaling in the optic axons, the hippocampus, and the anterior thalamus. For other details, see Luo et al., 2004.

axon bundles from the dorsal and ventral retina compartments are delineated by lacZ before the axons arrive (figure 30.1C). In the diencephalon, lacZ outlines the segmental (prosomeric) boundaries (figure 30.1D), which represent borders along which the growth trajectories of the dorsal and ventral optic axon bundles are differentially deviated during the primary innervation of optic targets. This phenomenon points to a role for RA-regulated factors in the proportioning of the dorsal and ventral retina compartments to the diand mesencephalic visual projection maps (Luo et al., 2004). LacZ expression in reporter mice marks sites implicated in RA actions also in the postnatal brain, most prominently the hippocampus (figure 30.1E ). In the thalamus, lacZ marks the anteroventral and anteromedial nuclei, which project to the medial RALDH3-expressing cortex (van Groen et al., 1999). Measurements with several different techniques show that hippocampal RA levels are only slightly higher than the low background in the brain, and no RALDHs are expressed in the lacZ-positive anterior thalamic nuclei (Luo et al., 2004). The observations in RA reporter mice thus indicate that RA signaling is selectively amplified in the brain at sites where independent evidence also points to preferential RA actions.

Role of retinoic acid in dorsal telencephalon function

Part of the evidence for a role for RA in neurobiological activities comes from functional studies on RAR knockout mice. RAR-β null mutants lack long-term potentiation (LTP) in recordings from hippocampal slices; behaviorally, their long-term memory is severely impaired, and they have abnormal sleep and sleep EEG (Chiang et al., 1998; Maret et al., 2005). Curiously, however, RAR-β is expressed only in low amounts in the hippocampus, cerebral cortex, or thalamus, where these functions are generated (Lane and Bailey, 2005). RA is required for the formation of new neurons in the postnatal dentate gyrus, a process involved in normal learning and memory (Jacobs et al., 2006). Nutritional vitamin A deficiency impairs memory and hippocampal LTP in mice, and these defects can be cured by vitamin A supplementation (Misner et al., 2001). The cortex of deprived rats accumulates β-amyloid deposits and other signs that parallel those in Alzheimer’s disease (Corcoran et al., 2004), and normal memory loss in aging rats due to naturally occurring reductions in retinoid function with age can be alleviated with vitamin A supplements (Etchamendy et al., 2001). Unfortunately, these observations do not suggest a simple way to enhance memory, because dentate neurogenesis is also decreased in response to RA excess: chronic exposure of mice to elevated RA levels reduces the formation of new neurons here, and the mice exhibit impaired memory and depression-like behaviors (Crandall et al., 2004;

O’Reilly et al., 2006). Similarly, some humans treated with Accutane (13-cis RA) for acne develop symptoms of clinical depression.

Impaired dentate neurogenesis cannot account for the loss of hippocampal LTP, and cortical memory does not depend on the formation of new neurons. Changes of gene expression in learning and memory require that information about rapid physiological events be converted faithfully into specific transcriptional processes. However, RA levels do not show any rapid physiological fluctuations, nor do any other retinoid components undergo changes on a neurophysiological time scale. Without doubt, the strictly defined retinoid system does not possess the dynamic properties required for learning and memory. Instead, most information about dynamic events in the nervous system is conveyed via signaling cascades to the cAMP response element–binding (CREB) protein and its family members of transcription factors, which function as key mediators of stimulus-induced nuclear responses (Lonze and Ginty, 2002; Carlezon et al., 2005). Several arguments point to CREB and the transcriptional coregulator level, at which RA actions in the brain are integrated with neurobiological functions and amplified. On the one hand, RA was shown to activate CREB directly by causing its phosphorylation at Ser-133 via nongenomic mechanisms (Canon et al., 2004). On the other hand, the discrepancies in the RA reporter responses point to CREB. The promoter of the RAR-β gene, which drives lacZ expression in the transgenic mice, contains a functional CREB-binding site (Kruyt et al., 1992). A synergistic enhancement of RA responses by CREB has been suggested for bronchial epithelial cells (Aggarwal et al., 2006). In a similar manner, the spatially selective amplification of RA signaling in the reporter brains might be mediated via physiologically activated CREB at the same sites. CREB-stimulated RAR-β enhancement could also explain the observations that RAR-β null mutants show profound functional defects at sites in the brain, where normal RAR-β expression is very low (Chiang et al., 1998; Maret et al., 2005).

Both pCREB and RA-dependent gene expression depend on the transcriptional co-activator CREB-binding protein (CBP), which contains a domain for CREB interaction and a separate nuclear receptor–interacting domain. CBP, which has histone acetyl transferase (HAT) activity that facilitates access to the DNA, is one of many transcriptional coregulators with different enzymatic activities that are recruited by a wide range of DNA-binding transcription factors in a combinatorial transcriptional code. The signaling cascades that convey information about dynamic events to CREB are now recognized to target in addition other transcriptional components and to influence a network of sequentially exchanged coregulator complexes that execute diverse enzymatic modifications

of the transcriptional apparatus, which are required for context-specific gene expression ( Johannessen et al., 2004; Rochette-Egly, 2005; Lonard and O’Malley, 2006; Rosenfeld et al., 2006).

RALDH2

Both the hippocampus and cerebral cortex of the adult brain are derived from the dorsal telencephalic wall of the embryo. To address the question of which genes are responsive to RA in the context of the functional brain, we compared the topography of gene expression with the RA distribution inferred from RALDH expression sites, focusing on the postnatal cerebral cortex because of its distinct RALDH patterns. A large fraction of the total RA content of the brain is supplied by passive exchange with the low levels in the circulation, which perfuses the brain evenly (Kurlandsky et al., 1995). Two RALDHs expressed in the cerebral cortex, RALDH2 and RALDH3, can be assumed to impart spatial RA patterns on top of the diffuse RA distribution provided by the circulation (Wagner et al., 2002). The RALDH2 enzyme is expressed in the meninges covering the cortical surface, from where it is likely to generate an outside-in RA decline in the cortex but no marked RAlevel differences in the tangential dimension, as its expression is relatively uniform. RALDH2 is activated rather late during embryonic development; its levels increase to a maximum for about a week perinatally and then decrease. Low RALDH2 levels persist in the meninges throughout life. The changing RALDH2 expression over the life cycle is illustrated by Northern blots of isolated telencephala (Smith et al., 2001) (figure 30.2, left); histologically in coronal sections through an adult brain, the RALDH2-labeled meninges are best visible in the meningeal channels of the stratum lacunare in the hippocampus (Wagner et al., 2002) (figure 30.2, right).

RALDH3

RALDH3 begins to be expressed by selected neurons a day before birth, when a faint trace becomes detectable at the most medial edge of the cingulate cortex (figure 30.3, arrows). In newborns at postnatal day 0.5 (P0.5), the expression has intensified and expanded, and a day later it covers the entire rostrocaudal extent of the medial limbic cortex. By P3 the RALDH3 territory has expanded laterally beyond the limbic cortex into a narrow band of adjoining neocortex. Over the following days, RALDH3 expression decreases selectively in the caudal and intermediate limbic cortex and eventually disappears completely from there. Strong expression persists, however, in the rostral part of the medial limbic lobe and in a chain of neocortical regions that border the entire medial limbic cortex. Throughout the rest of postnatal life, the RALDH3 topography remains constant, but enzyme levels decrease slowly. The part of the limbic lobe from which RALDH3 disappears is the retrosplenial cortex, including all its anatomical subdivisions. The lasting RALDH3 expression in the caudal third of the neocortex is localized in the medial extrastriate region (see the color code in color plate 20). In the intermediate third, RALDH3 marks parietal association areas and motor cortex, and in the rostral third the enzyme is expressed in the limbic cingulate and prefrontal cortex and the adjoining secondary motor cortex (Wagner et al., 2002, 2006). The medial extrastriate cortex contains the dorsal visual stream (Wang and Burkhalter, 2004), which is best characterized in primates as the pathway for spatial vision and motion perception as distinct from the ventral visual stream for color and form perception (Merigan and Maunsell, 1993; Goodale and Westwood, 2004), and the entire medial RALDH3 band in the mouse corresponds to the cortical network for visual attention and spatial imagery in humans (Dolan, 2002; Knauff et al., 2002).