Mark F. Bear - US grants

Evidence is presented which indicates that experience- independent modifications of functional circuitry in the visual cortex are regulated by extrahalamic afferent systems that use acetylcholine (ACh) and norepinephrine (NE) as transmitters. The presence of either ACh or NE appears sufficient to permit the ocular dominance (OD) changes that occur in kitten cortex after brief periods of monocular deprivation (MD) because destruction of both system is required to produced a reliable deficit in ocular dominance plasticity. Five experiments are proposed which will use anatomical, pharmacological and neurophysiological methods to characterize the individual and combined role of ACh and NE in modulating developmental plasticity in visual cortex. The first project will address the issue of whether destruction of the cholinergic and noradrenergic systems prevents ocular dominance changes after long-term MD. The second project is designed to determine whether the anatomical consequences of long-term MD, such as the shrinkage of neurons in the deprived lateral geniculate lamina and changes in the widths of OD columns in cortical layer IV, are prevented if cortical ACh and NE are depleted. In projects 3 and 4 I propose to use the minipump infusion method to establish which of the NE and ACh receptor types mediate the effects relevant to synaptic plasticity. The final project again will use the minipump infusion method in an attempt to restore some susceptibility to MD in adult animals by applying substances that will potentiate endogenously released ACh. These experiments should help to define the possible mechanisms by which activity-dependent synaptic modifications are regulated in the visual cortex.

DESCRIPTION (provided by applicant): The long-term goal of this project is to elucidate the molecular mechanisms of experience-dependent cortical plasticity, which must occur normally for the proper development of vision in mammals. The first aim is to assess the contribution of synaptic mobilization of glutamate receptors to deprivation-induced response depression in visual cortex and amblyopia. The second aim is to characterize the recently discovered phenomenon of stimulus-specific response potentiation (SRP), and test the hypothesis that SRP utilizes mechanisms that are revealed by the study of long-term synaptic potentiation (LTP). The proposed research promises to reveal the detailed molecular basis for experience-dependent bidirectional synaptic plasticity in the visual cortex. Besides the obvious relevance of this neural plasticity to the development of visual capabilities in humans and animals, it seems likely that similar processes form the basis for some forms of learning and memory, and also contribute to recovery of brain function after injury. Knowledge of the mechanisms of plasticity can be (and are being) applied to devise strategies to protect juvenile synapses from deleterious effects of environmental deprivation during development, and to promote synaptic strengthening and recovery of function.

9319373 Cooper The goal of this work is to understand how the connections between nerve cells (call synapses) in the visual cortex of the brain change during maturation from newborn to adult. Dr. Cooper has developed a theory that during the critical period in development when the brain must receive visual input to develop properly, it is a change in the neuronal circuits with the brain that is critical for proper development rather than changes in the properties of the synapses, as has been proposed by others. The experiments that will be done by this team of investigators will test this theory by doing experiments on neurons in in vitro brain slices from rat visual cortex at different times during development and from rats raised in different visual environments. If this theory is proven to be true, the work outlined in this proposal will change significantly the way in which we view the developing brain.

The aims of this collaboration are two-fold: First, we intend to investigate the conditions under which homosynaptic long-term depression (LTD) can be elicited in area CA1 and the dentate gyrus of the awake rat. Previous work in the applicant's laboratory has led to the discovery of a form of homosynaptic LTD in brain slices, but there is no information yet available on the extent to which what has been learned can be applied to the awake, functioning brain. The sponsor has a great deal of experience studying another form of 'heterosynaptic' LTD in the awake hippocampus, and therefore provides an ideal environment to investigate the question of whether homosynaptic LTD can be evoked reliably in the functioning brain and how it interacts with heterosynaptic LTD. Second, we intend to examine the hypothesis that the sign and magnitude of activity-dependent synaptic plasticity varies as a function of the recent history of postsynaptic cell activity. It is proposed that cell activity regulates synaptic modifiability via the activity-dependent, cell-wide regulation of high-affinity calcium buffering capacity. In dentate gyrus granule cells, the protein calbindin D28K is thought to function as an important calcium buffer. In the awake rat, we will test the specific hypothesis that increased dentate activity leads to the synthesis of calbindin, and that this increase in calcium buffering capacity reduces the probability and magnitude of LTP in the perforant path-dentate granule cell synapses. The significance of this project is that it is among the first to directly address the question of how synaptic plasticity, thought to be critical for learning and memory, is regulated by the brain.

GRANT=P50MH58880-01A1-0006 DESCRIPTION: (Adapted from the Application) Visual recognition memory engages the circuitry of the visual cortex, stretching from area 17 ventrally to area TE. Visual receptive fields of neurons in this pathway are increasingly complex as one moves from l7 to TE, and these responses are subject to experience-dependent modification. Robust plasticity is observed in area 17 only during a "critical period" of postnatal life, whereas receptive field plasticity is observed in area TE throughout life. Nonetheless, formal similarities in the characteristics of plasticity across cortical areas suggests common underlying mechanisms, with different patterns of developmental regulation. Theoretical studies suggest that the experience-dependent changes in receptive fields reflect synaptic modifications that, distributed over many cortical cells, store information and serve visual recognition memory. Individual Project #6 contributes to the overall goal of the Center by examining the possible neural basis for, and developmental regulation of, visual recognition memory in the posterior neocortex of the mouse. This choice of species is important, because much of the research in Individual Project #6 lays the foundation for studies in which synaptic plasticity will be genetically disabled in the neocortex, thus enabling the hypothesis that neocortical plasticity is in fact related to long-term memory to be examined with behavioral tests. It is crucial that these studies be conducted in parallel with the development of neocortex-specific genetic knockouts so that the experimental models will be fully established when the genetic mutants become available. This project will be conducted in a laboratory that has great experience in the study of cortical synaptic plasticity, both in vitro and in vivo, and thus complements (without duplicating) the expertise in other labs of the center. The project is highly collaborative, taking advantage of mutant mice generated by the Tonegawa and Heinemann labs. A central goal of Individual Project #6 is to establish models for long-term synaptic plasticity in the visual neocortex of the mouse. Considerable progress has been made in primary visual cortex, area 17 (Oc1), and we propose to extend this work to include extrastriate (area 18 or Oc2) and temporal (TE) visual cortex, utilizing coronal slice preparations of posterior neocortex. There is considerable evidence that the stream of visual information from area 17 to 18 to TE in the rodent is analogous to the "ventral stream" in the monkey, and is involved in object recognition. Long-term potentiation (LTP) and long-term depression (LTD) will be used as assays of synaptic plasticity in these cortical regions. The specific questions to be addressed are as follows. 1. Why is plasticity in primary sensory cortex restricted to a critical period of development? We will test the hypothesis that neurotrophins regulate developmental plasticity by studying LTP and LTD in animals in which the neurons of visual cortex are over-expressing brain-derived neurotrophic factor using mice supplied by Tonegawa. 2. How do LTP and LTD compare across cortical areas and across postnatal ages? We will test the hypothesis that plasticity in higher order visual cortex, believed to play an essential role in memory storage, is far less sensitive to age than plasticity in area 17. If correct, we will seek to understand the mechanisms that underlie the differences using genetic mutants supplied by Tonegawa and Heinemann. 3. Is there a protein-synthesis dependent "late-phase" of LTP or LTD in the neocortex? We will use a genetically engineered CRE reporter to see what types of gene response are caused by different types of tetanic stimulation, both in vitro and in vivo. We will test the hypothesis that CREB is required for late-phase neocortical LTP by utilizing regionally specific CREB knockouts provided by the Tonegawa laboratory. The same animals will be studied in parallel experiments for deficits in visual recognition memory (see below). 4. What is the contribution of TE cortex to visual recognition memory in the mouse? We will develop a behavioral assay of TE-cortex-dependent visual recognition memory in the mouse, and will investigate the behavioral and electrophysiological consequences of genetic lesions that disrupt synaptic plasticity in TE (e.g. NR1 knockout). 5. How do specific genetic lesions of glutamate receptors affect LTP and LTD? Available data suggest that, to some extent, the molecular mechanisms of LTP and LTD are conserved across cortical areas; therefore, we will use hippocampal area CA1 as a model system to investigate the underlying mechanism of cortical synaptic plasticity. In collaboration with the Heinemann and Tonegawa laboratories, we will study CA 1-specific alterations of AMPA receptor subunits and mGluR5 (see description of Individual Project #5). Modification of AMPA receptors, either by phosphorylation or translocation, is believed to be one mechanism for the expression of LTP and LTD. Metabotropic glutamate receptors may be one means by which plasticity is triggered. We will test these hypotheses.

The persistent modification of the brain by experience is the neural basis for learning and memory. A wealth of data suggest that an important site of experience-dependent modification is the synapse, and that the induction and persistence of synaptic modifications depend importantly upon new protein synthesis. It has also been established that synaptic activity can regulate gene transcription, the synapse-specific capturing of newly synthesized proteins, and the synapse-specific translation of existing mRNAs. However, precisely how these varied mechanisms contribute to synaptic plasticity remains unresolved. One major goal of this project is to investigate which modes of protein synthesis control are used for two different forms of synaptic plasticity, one in the hippocampus (mGluR-dependent LTD) and the other in the visual cortex (experience-dependent changes in NMDARs). A second major goal is to investigate the functional significance of one specific mechanism for the local synaptic control of mRNA translation, cytoplasmic polyadenylation. Thus, the work has both "top-down" and "bottom-up" components. The "top-down" component starts with two forms of protein-synthesis dependent synaptic plasticity, and dissects their mechanisms. The "bottom-up" component starts with a mechanism for translation regulation, and assesses its contributions to synaptic plasticity and memory.

Project summary/Abstract Fragile X syndrome (FXS) is the most common form of heritable human mental retardation and the leading identified cause of autism. FXS is caused by transcriptional silencing of the FMR1 gene that encodes the fragile X mental retardation protein (FMRP), but the pathogenesis of the disease is poorly understood. During the previous grant period we tested the proposal that many psychiatric and neurological symptoms of FXS result from unchecked activation of mGluR5, a metabotropic glutamate receptor. We generated Fmr1 mutant (KO) mice with a 50% reduction in mGluR5 expression and discovered that a wide range of phenotypes with relevance to the human disorder were brought significantly closer to normal. Our findings have significant therapeutic implications for fragile X and related developmental disorders, and have inspired human clinical trials of mGluR5 antagonists for the treatment of FXS. Our objective in the next grant period is to gain additional insights from the mouse model of the disease that can guide treatment in humans. We have two specific aims: Aim 1: Does postnatal inhibition of mGluR5 prevent emergence of fragile X phenotypes? Aim 2: Does late onset inhibition of mGluR5 reverse fragile X phenotypes?

During development, sensory experiences produce synaptic modifications that specify the capabilities and limitations of brain function in adults. In adults, very similar modifications appear to be the substrates of learning and memory. Therefore, a question of great significance is how synapses in the brain are modified by sensory experience. Mechanisms for long-term synaptic depression (LTD) and potentiation (LTP) have been identified, as has a mechanism for regulating the conditions for LTP induction. However, it remains to be determined the extent to which these mechanisms contribute to naturally occurring modifications in the brain. The mouse visual cortex provides an excellent model system for addressing this question. Visual cortex is well-known to be modified by simple manipulations of experience, such as depriving one eye of vision, and the mouse can be genetically modified to test specific hypotheses about molecular mechanisms. Moreover, while this type of plasticity changes over the course of postnatal life, it persists in adult mouse visual cortex and therefore can provide insight into how age alters the qualities of synaptic plasticity in the cerebral cortex. The aims of this proposal are to determine (1) the qualities of visual cortical plasticity across the life-span, (2) the contributions of LTD mechanisms to deprivation-induced response depression in vivo, (3) the contribution of LTP mechanisms to experience-dependent response potentiation in vivo, and (4) if changes in NMDA receptor subunit composition are permissive for experience-dependent response potentiation in vivo. These aims will be accomplished by taking advantage of the special expertise of this Center in mouse genetics, chronic recording, and the molecular bases for synaptic plasticity.

Precise models of cortical synaptic plasticity are essential for understanding how information is stored in the brain and how functional neural connection patterns get established. Understanding cortical plasticity requires coordinated investigation of both underlying cellular mechanisms and their systems-level consequences in the same model system. However, establishing connections between the cellular and system levels of description is non-trivial. A major contribution of theoretical neuroscience is that it can link different levels of description, and in doing so can direct experiments to the questions of greatest relevance.

The objective of the current project is to generate a theoretical description of experience-dependent plasticity in the rodent visual system. The advantages of rodents are, first, that knowledge of the molecular mechanisms of synaptic plasticity is relatively mature and continues to be advanced with genetic and pharmacological experiments, and second, rodents show robust receptive field plasticity in visual cortex that can be easily and inexpensively monitored with chronic recording methods.

In this project, models of synaptic plasticity are based on realistic assumptions about the activity of inputs to visual cortex from the lateral geniculate nucleus (LGN). These assumptions are based, when possible, on actually recordings of LGN activity in rodents, in different viewing conditions that induce receptive field (RF) plasticity. These data will be integrated into formal models of synaptic plasticity. In this proposal the dynamics of RF plasticity will be simulated using existing spike rate-based algorithms and compared with experimental observations. Additionally, the consequences of new biophysically-plausible plasticity algorithms will be analyzed and compared with experiments. Two simulation packages, Plasticity and P-spike, will be developed as part of the project and made available on the web to other researchers.

By improving our understanding of the basis for learning, memory and developmental plasticity, these investigations will suggest new strategies for treating learning disabilities and neurodevelopmental disorders in children, as well as learning and memory problems in the aging population.

ABSTRACT NOT AVAILABLE

DESCRIPTION (provided by applicant): The Brain and Cognitive Sciences (BCS) Graduate Program at the Massachusetts Institute of Technology requests renewal of the training program in the Neurobiology of Learning and Memory (NLM). The central objective of the NLM program is to prepare neuroscientists at the graduate (predoctoral) level for professional careers devoted to the scientific investigation of how information is stored and accessed by the nervous system and the application of this knowledge to relieve the burden of mental illness. Neurobiology of learning and memory has emerged as a major theme of research in the neurosciences at MIT. The structure of the Department of Brain and Cognitive Sciences, which has faculty working at all levels-from the most elementary mechanisms of synaptic transmission to the processes in the human brain required for cognition-puts MIT in a particularly strong position to provide the necessary training. Furthermore, a nucleus of researchers committed to this endeavor has already been established at The Picower Institute for Learning and Memory at MIT. The NLM program is an advanced track of the BCS graduate program, available to students after their first two years of coursework and laboratory rotations. The goals of the NLM program are to train students (a) to conduct research on the neurobiology of learning and memory, and (b) to be aware of the broader potential of such research to be translated into clinical medicine relevant to the mission of the NIMH. NLM program requirements are (1) that the student conducts thesis research in the laboratory of an NLM trainer, (2) that the courses taken by the student prior to graduation include one on the neurobiology of disease, and two courses from two different categories (cell/molecular, systems, cognitive, or computational) from the NLM course list, (3) that the student participate in Plastic Lunch, a series of seminars given by students on their ongoing research on the neurobiology of learning and memory, and (4) that the students participate in the annual retreat of the Picower Institute for Learning and Memory. Funds are requested for five years to support 8 predoctoral trainees per year.

DESCRIPTION (provided by applicant): Autism is a pervasive developmental disorder characterized by impairment in social interaction and communication, and restricted repetitive and stereotyped behavior or interest. Fragile X syndrome and autism have overlapping clinical presentation, thus may share common biochemical and neurophysiological features. Identifying these commonalities is vital in understanding the pathogenesis and providing effective therapies for both neurodevelopmental disorders. Human chromosome 16p11.2 microdeletion is the most common chromosome copy number variation (CNV) in autism. We plan to generate mice carrying deletion of a syntenic region to model human chr16p11.2 microdeletion phenotypes. We hypothesize that deletion at human chr16p11.2 causes synaptic pathophysiology that overlaps with Fragile X syndrome. This hypothesis will be tested in a battery of experiments that have been successfully conducted in Fragile X mice in our laboratory. PUBLIC HEALTH RELEVANCE: Autism and Fragile X syndrome are major public health concerns which share some common features in both clinical presentations and pathogenesis. Identifying and characterizing these features is vital in developing effective therapies for both diseases. This can be accomplished by comparing animal models for autism and Fragile X syndrome.

DESCRIPTION (provided by applicant): Central questions in the neurobiology of learning and memory are where and how experience modifies the brain to alter behavior. Recently we have described a physiological phenomenon in head-fixed awake mice termed stimulus-selective response potentiation (SRP), in which responsiveness of primary visual cortex is markedly enhanced as a result of repeated exposure to specific visual stimuli. This robust form of plasticity is induced and expressed in the adult primary visual cortex and displays many of the features that are characteristic of perceptual learning. The primary aims of this research are to; a) to determine the behavioral significance of SRP using two novel behavioral assays; b) determine how SRP is expressed at the cellular level within visual cortex and; c) pinpoint the synapses that are modified. Beyond the relevance of our proposed research to identifying the mechanisms underlying perceptual learning and/or visual recognition memory, they will broaden our understanding of how primary sensory areas are modified by sensory experience in order to modify behavior, which remains one of the great challenges in basic neuroscience.

DESCRIPTION (provided by applicant): Several genetically defined developmental disorders that manifest as autism and intellectual disability, such as fragile X (FX) and tuberous sclerosis complex (TSC), have in common a disruption of activity-regulated protein synthesis at synapses. Understanding how neural activity regulates synaptic protein synthesis is crucial for identifying new therapuetic approaches for these diseases. One key mechanism employs metabotropic glutamate receptor 5 (mGluR5) but it remains to be established how this receptor couples to the mRNA translation machinery. Here we test the hypothesis that a crucial link between mGluR5 and protein synthesis is provided by ß-arrestin. Our specific aims are to characterize the effect of ss-arrestin genetic reduction on (1) mGluR5 signaling and protein synthesis in the hippocampus using biochemical methods, (2) hippocampal synaptic function and protein synthesis-dependent plasticity using electrophysiological methods, and (3) on fragile X behavioral and electrophysiological phenotypes expressed in the Fmr1 knockout mouse. If our hypothesis is correct, we expect to observe an amelioration of fragile X phenotypes by reducing ß-arrestin, validating the mGluR5-ß-arrestin protein complex as a novel therapeutic target.

? DESCRIPTION (provided by applicant): Currently there are no mechanism-based therapies available for autism spectrum disorders (ASDs) and intellectual disability (ID). The main barrier has been identifying the defective cellular processes within the brain that disrupt behavior and cognition. Increasing evidence indicates that many cases of ASD and ID have a genetic etiology. However, these genetic changes are numerous, often very rare, and remarkably diverse. One way to make sense of these findings is to assume that a plethora of gene mutations may similarly disrupt a common set of physiological processes that ultimately manifest behaviorally as ID and ASD. Testing this assumption is of paramount importance, as not only does it narrow the search for mechanisms of disease pathogenesis, but it also suggests therapeutic strategies that might apply broadly to an entire class of etiologies. Work on animal models of single-gene disorders associated with ID and ASD has supported the idea that one axis of pathophysiology is metabotropic glutamate receptor 5 (mGluR5) mediated synaptic protein synthesis and plasticity. In the animal model of fragile X syndrome (FX), mGluR5-mediated protein synthesis and plasticity in the hippocampus are exaggerated. Conversely, in the animal model of tuberous sclerosis complex (TSC), protein synthesis and plasticity downstream of mGluR5 are diminished. Of particular interest, inhibition of mGluR5 corrects cognitive (and many other) deficits in the FX model, whereas positive modulation of mGluR5 corrects cognitive defects in the TSC model. In the current work, we are asking if gene copy number variation at human chromosome 16p11.2, a polygenic cause of psychiatric illness that can include ID and ASD, similarly disrupts this core synaptic mechanism. This hypothesis is suggested by the findings that many genes in the affected region are predicted to be involved in protein synthesis regulation, and preliminary data in the mouse model of 16p11.2 microdeletion showing disrupted mGluR5-mediated synaptic plasticity and cognitive function, and correction of memory deficits by chronic inhibition of mGluR5. The specific aims of our proposed research are to (a) further characterize the state of synaptic transmission and plasticity in the hippocampus of 16p11.2 CNV model mice, (b) characterize synaptic protein synthesis and the molecular signaling pathways which may be disrupted in these mice, (c) further assess the behavioral deficits in 16p11.2 CNV model mice, and (d) attempt to correct memory deficits with rational pharmacotherapies previously validated in animal models of FX and TSC.

Amblyopia is a prevalent form of visual disability that arises during infancy and early childhood when inputs to the visual cortex from the two eyes are poorly balanced (for example, by misalignment of the eyes, asymmetric refraction, or opacities and obstructions of one eye). Characteristics of amblyopia are very poor acuity in one eye, and an attendant loss of stereopsis. The need for improved treatments for amblyopia is widely acknowledged. Animal studies over the past 50 years have uncovered the pathophysiology of amblyopia. It is well documented that temporary monocular deprivation alters the strength of synapses in primary visual cortex that renders cortical neurons unresponsive to stimulation of the deprived eye. However, much less is known about the mechanisms that serve recovery from amblyopia. We recently discovered that temporarily inactivating the retinas with a local anesthetic sets in motion changes in the brain that enable complete recovery from the effect of early life monocular deprivation when the anesthetic wears off. Our objectives are to uncover the mechanism for how this recovery occurs, and to determine if this knowledge can be translated into new and better treatments for amblyopia.