Dennis D. M. O’Leary

APP binds DR6 to trigger axon pruning and neuron death via distinct caspases

Naturally occurring axonal pruning and neuronal cell death help to sculpt neuronal connections during development, but their mechanistic basis remains poorly understood. Here we report that β-amyloid precursor protein (APP) and death receptor 6 (DR6, also known as TNFRSF21) activate a widespread caspase-dependent self-destruction program. DR6 is broadly expressed by developing neurons, and is required for normal cell body death and axonal pruning both in vivo and after trophic-factor deprivation in vitro. Unlike neuronal cell body apoptosis, which requires caspase 3, we show that axonal degeneration requires caspase 6, which is activated in a punctate pattern that parallels the pattern of axonal fragmentation. DR6 is activated locally by an inactive surface ligand(s) that is released in an active form after trophic-factor deprivation, and we identify APP as a DR6 ligand. Trophic-factor deprivation triggers the shedding of surface APP in a β-secretase (BACE)-dependent manner. Loss- and gain-of-function studies support a model in which a cleaved amino-terminal fragment of APP (N-APP) binds DR6 and triggers degeneration. Genetic support is provided by a common neuromuscular junction phenotype in mutant mice. Our results indicate that APP and DR6 are components of a neuronal self-destruction pathway, and suggest that an extracellular fragment of APP, acting via DR6 and caspase 6, contributes to Alzheimer’s disease.

Molecular Development of Sensory Maps: Representing Sights and Smells in the Brain

retinotectal and olfactory systems has clear distinctions, Introduction which relate in part to differences in their functional Defining the molecules and mechanisms that control requirements. In the retinotectal system, the main objecthe establishment of an orderly representation of the tive is to represent the visual world in the brain, that is, peripheral sense organs within the brain has long been to reconstruct a topographic representation of the world of interest to systems and developmental neurobiolothat projects onto the retina and is remapped in the gists. Classically, the projection from the retina to the tectum. To carry out this function requires the maintebrain has served as the model system for understanding nance of a precise spatial ordering of axonal connechow precise neural connections are formed. More retions within the tectum that reflects their origins in the cently, the molecular cloning of olfactory receptors retina. In contrast, in the olfactory system, since odors (ORs) has provided valuable insights into the functional have no relevant spatial component, there is no overridand anatomical organization of the olfactory system, ing need to maintain spatial continuity, either between including the projection of olfactory neurons (ONs) from cells expressing a given OR, and presumably respondthe olfactory epithelium to the olfactory bulb (OB). The ing to the same odors, or between glomeruli in the OB. mechanisms involved in establishing this projection, as This functional difference relates to differences in the well as its organization, are atypical and make for revealmapping strategies employed in the two systems. ing comparisons when juxtaposed to the development This article will review current knowledge of the mechof order in the visual system. anisms and molecules proposed to control mapping in Both the visual and olfactory systems represent senthe visual and olfactory systems and attempt a synthesis sory information within the brain through the use of to highlight differences and similarities in their organizasensory maps. The projection of sensory axons to the tions and the molecular mechanisms that may control brain forms these maps through the spatial segregation their development. We have focused almost exclusively and orderly termination of their axonal connections in on the projection of RGCs to the chick optic tectum, or specific target tissues. However, the visual map is funits equivalent in rodents, the superior colliculus (SC), as damentally different from the olfactory map in that it is well as the main olfactory system of mice. We have not strictly topographic: a two-dimensional sheet of retinal discussed the large bodies of excellent work on the ganglion cells (RGCs) in the retina is rerepresented in retinotectal system of fish and amphibians, the main the brain as more or less the same two-dimensional olfactory system of lower vertebrates or C. elegans, nor sheet through the orderly terminations of RGC axons. do we consider the projection from the vomeronasal In contrast, the olfactory map is formed by the converorgan to the accessory olfactory bulb (for reviews of gence of the axonal projections of a specific set of functhese topics see Roskies et al., 1995; Bargmann, 1997; tionally similar ONs that are randomly distributed in the Karlstrom et al., 1997; Ebrahimi and Chess, 1998). olfactory epithelium onto specific glomeruli, and in doing

In Vivo Expression of a Light-Activatable Potassium Channel Using Unnatural Amino Acids

Optical control of protein function provides excellent spatial-temporal resolution for studying proteins in situ. Although light-sensitive exogenous proteins and ligands have been used to manipulate neuronal activity, a method for optical control of neuronal proteins using unnatural amino acids (Uaa) in vivo is lacking. Here, we describe the genetic incorporation of a photoreactive Uaa into the pore of an inwardly rectifying potassium channel Kir2.1. The Uaa occluded the pore, rendering the channel nonconducting, and, on brief light illumination, was released to permit outward K(+) current. Expression of this photoinducible inwardly rectifying potassium (PIRK) channel in rat hippocampal neurons created a light-activatable PIRK switch for suppressing neuronal firing. We also expanded the genetic code of mammals to express PIRK channels in embryonic mouse neocortex in vivo and demonstrated a light-activated PIRK current in cortical neurons. These principles could be generally expanded to other proteins expressed in the brain to enable optical regulation.

Islet1-mediated activation of the β-catenin pathway is necessary for hindlimb initiation in mice

The transcriptional basis of vertebrate limb initiation, which is a well-studied system for the initiation of organogenesis, remains elusive. Specifically, involvement of the β-catenin pathway in limb initiation, as well as its role in hindlimb-specific transcriptional regulation, are under debate. Here, we show that the β-catenin pathway is active in the limb-forming area in mouse embryos. Furthermore, conditional inactivation of β-catenin as well as Islet1, a hindlimb-specific factor, in the lateral plate mesoderm results in a failure to induce hindlimb outgrowth. We further show that Islet1 is required for the nuclear accumulation of β-catenin and hence for activation of the β-catenin pathway, and that the β-catenin pathway maintains Islet1 expression. These two factors influence each other and function upstream of active proliferation of hindlimb progenitors in the lateral plate mesoderm and the expression of a common factor, Fgf10. Our data demonstrate that Islet1 and β-catenin regulate outgrowth and Fgf10-Fgf8 feedback loop formation during vertebrate hindlimb initiation. Our study identifies Islet1 as a hindlimb-specific transcriptional regulator of initiation, and clarifies the controversy regarding the requirement of β-catenin for limb initiation.

A transcription factor code defines nine sensory interneuron subtypes in the mechanosensory area of the spinal cord.

Interneurons in the dorsal spinal cord process and relay innocuous and nociceptive somatosensory information from cutaneous receptors that sense touch, temperature and pain. These neurons display a well-defined organization with respect to their afferent innervation. Nociceptive afferents innervate lamina I and II, while cutaneous mechanosensory afferents primarily innervate sensory interneurons that are located in lamina III–IV. In this study, we outline a combinatorial transcription factor code that defines nine different inhibitory and excitatory interneuron populations in laminae III–IV of the postnatal cord. This transcription factor code reveals a high degree of molecular diversity in the neurons that make up laminae III–IV, and it lays the foundation for systematically analyzing and manipulating these different neuronal populations to assess their function. In addition, we find that many of the transcription factors that are expressed in the dorsal spinal cord at early postnatal times continue to be expressed in the adult, raising questions about their function in mature neurons and opening the door to their genetic manipulation in adult animals.

Functional Consequences of Coincident Expression of EphA Receptors and ephrin-A Ligands

DA neurons through the dopamine transporter, interare quite similar to these obtained with adrenal medullary grafts, which survive poorly but can induce the reapferes with ATP production in the mitochondria, and leads pearance of host TH1 fibers (Bohn et al., 1987). Underto cell death. As a result, these monkeys developed standing the mechanisms by which the glomus grafts stable and severe bilateral parkinsonism, which maniameliorate PD symptoms in monkeys may enable their fested in tremor, freezing, impaired balance, poor posbetter usage and point to novel therapeutic avenues for ture, and reduction in spontaneous activity. In addition, this disorder. the monkey that was trained prior to the MPTP treatment Regardless of the mechanism, glomus cell transplants on a fine motor task suffered an increase in the number could become effective therapy for PD in humans. Beof failures and time to complete the task. fore clinical trials are attempted, more extensive and Three or seven months after the last MPTP injection, better controlled trials, as well as direct comparisons one carotid body was harvested and cut into 0.2–0.3 to fetal DA grafts, must be conducted in primates. In mm diameter aggregates, and two sites in the left putaaddition, it will be important to determine whether the men were grafted with three to five aggregates per site. carotid bodies show degenerative changes in PD paOne monkey also received a single vehicle injection in tients, which may have an impact on their use. The new the right putamen to control for the effects of surgical results will hopefully prompt additional investigators to trauma. Both monkeys showed improvement that began study the utility of carotid body grafts and to help deter2–4 weeks after transplantation and lasted throughout mine whether they can turn into a clinical reality. the duration of the study (3 or 5 months). Initially, the monkeys exhibited a reduction in tremor. Subsequently, they displayed an increase in spontaneous activity, more Derek Choi-Lundberg and Arnon Rosenthal frequent use of the contralateral limbs, and improveDepartment of Neuroscience ment in posture and balance, resulting in a 40% or 50% Genentech, Incorporated reduction in total disability score. The monkey that was 1 DNA Way trained on a fine motor task also showed partial restoraSouth San Francisco, California 94080 tion of fine motor skills with the limb contralateral to the graft. Selected Reading Analysis of the grafts post mortem revealed that only

Postmitotic regulation of sensory area patterning in the mammalian neocortex by Lhx2

Significance The mammalian neocortex is divided into specialized modality-specific areas that are responsible for the processing of sensory information. This architecture is critical, because altered area size affects normal sensory function and behavior in animals and humans. Current knowledge suggests that sensory area specification is dominated by patterning genes expressed in cortical progenitors. We show that postmitotic deletion of the transcription factor LIM homeobox 2 (Lhx2) in cortical neurons does not affect area patterning in progenitors but strongly alters sensory areas, demonstrating that specification of area identity in progenitors alone is insufficient. We suggest a novel and more comprehensive model of cortical area patterning that incorporates these revelations and define the relevance of postmitotic mechanisms in determining the functional properties of cortical areas. Current knowledge suggests that cortical sensory area identity is controlled by transcription factors (TFs) that specify area features in progenitor cells and subsequently their progeny in a one-step process. However, how neurons acquire and maintain these features is unclear. We have used conditional inactivation restricted to postmitotic cortical neurons in mice to investigate the role of the TF LIM homeobox 2 (Lhx2) in this process and report that in conditional mutant cortices area patterning is normal in progenitors but strongly affected in cortical plate (CP) neurons. We show that Lhx2 controls neocortical area patterning by regulating downstream genetic and epigenetic regulators that drive the acquisition of molecular properties in CP neurons. Our results question a strict hierarchy in which progenitors dominate area identity, suggesting a novel and more comprehensive two-step model of area patterning: In progenitors, patterning TFs prespecify sensory area blueprints. Sequentially, sustained function of alignment TFs, including Lhx2, is essential to maintain and to translate the blueprints into functional sensory area properties in cortical neurons postmitotically. Our results reemphasize critical roles for Lhx2 that acts as one of the terminal selector genes in controlling principal properties of neurons.

Formation of the Cortical Subventricular Zone Requires MDGA1-Mediated Aggregation of Basal Progenitors

The subventricular zone (SVZ) provides a specialized neurogenic microenvironment for proliferation and aggregation of basal progenitors (BPs). Our study reveals a mechanism for the aggregation of BPs within the SVZ required for their proliferation and generation of cortical layer neurons. The autism-related IgCAM, MDGA1, is locally expressed in the BP cell membrane where it co-localizes and complexes with the gap junction protein Connexin43. To address MDGA1 function, we created a floxed allele of MDGA1 and deleted it from BPs. MDGA1 deletion results in reduced BP proliferation and size of the SVZ, with an aberrant population of BPs ectopically positioned in the cortical plate. These defects are manifested in diminished production of cortical layer neurons and a significant reduction of the cortical layers. We conclude that MDGA1 functions to aggregate and maintain BPs within the SVZ providing the neurogenic niche required for their proliferation and generation of cortical layer neurons.

Emx1 Is Required for Neocortical Area Patterning

Establishing appropriate area patterning in the neocortex is a critical developmental event, and transcription factors whose expression is graded across the developing neural axes have been implicated in this process. While previous reports suggested that the transcription factor Emx1 does not contribute to neocortical area patterning, those studies were performed at perinatal ages prior to the emergence of primary areas. We therefore examined two different Emx1 deletion mouse lines once primary areas possess mature features. Following the deletion of Emx1, the frontal and motor areas were expanded while the primary visual area was reduced, and overall the areas shifted posterio-medially. This patterning phenotype was consistent between the two Emx1 deletion strategies. The present study demonstrates that Emx1 is an area patterning transcription factor and is required for the specification of the primary visual area.