Martin Smith - US grants

The ability to process sensory information and express complex behaviors depends on rapid communication between electrically excitable cells in the nervous system. Signalling between electrically excitable cells is largely accomplished by chemical synapses. Understanding the cellular and molecular organization of chemical synapses and the rules that control their development and maintenance are therefore central questions in modern neurobiology. The neuromuscular junction is a useful model system in which to address these questions. Like other chemical synapses, the neuromuscular junction consists of precisely registered pre-and postsynaptic elements, each specialized for their role in synaptic transmission. In particular, the postsynaptic muscle motor end- plate has high concentrations of receptors (AChR) for the neurotransmitter acetylcholine, and acetylcholinesterase (AChE), an enzyme responsible for its breakdown. Extracts of the synapse-rich electric organ of the electric ray induce the formation of patches on muscle fibres in vitro that contain both AChR and AChE, mimicking the accumulation of these post-synaptic components that occurs during normal development. Four structurally related proteins have been purified from the electric organ extract of which two, called agrin, are able to induce the formation of AChR/AChE aggregates. The remaining two agrin-like proteins however are not active by this assay. Molecules related or identical to agrin are present at the neuromuscular junction, suggesting that agrin proteins play a role in its development and maintenance. The presence of molecules similar to agrin in other tissues suggests that agrin proteins may have additional important functions. The goal of this proposal is to characterize agrin proteins at the molecular level and define the structural basis for their physiological properties. This will add to our understanding of the mechanism of agrin protein action and the molecular events occurring during synaptogenesis. These goals will be realized through biochemical and immunological analyses of native proteins and by complementary molecular studies of agrin genes and their products. These studies will enhance our ability to alleviate developmental anomalies in synaptogenesis and potentiate neuromuscular regeneration following trauma.

chemical carcinogenesis; preneoplastic state; hepatocellular carcinoma; oncoproteins; gene mutation; disease /disorder model; cytotoxicity; protein biosynthesis; tumor promoters; diethylnitrosamine; choline; diagnosis design /evaluation; necrosis; genetic mapping; tumor suppressor genes; mutant; neoplasm /cancer genetics; malnutrition; nutrition related tag; western blottings; polymerase chain reaction; laboratory rat; nucleic acid sequence; complementary DNA;

DESCRIPTION (provided by applicant): Agrin is an extracellular matrix protein that directs the accumulation of acetylcholine receptors in the postsynaptic apparatus of the developing neuromuscular junction. Agrin is also expressed by neurons in the central nervous system (CNS), but its function is less well defined. For example, in addition to a role as a postsynaptic organizer, agrin has also been implicated in regulating the differentiation of axon terminals as well as growth and branching of axons and dendrites. As an alternate approach to understanding agrin function in the CNS, we have focused on identifying signal pathways through which agrin might act. These studies recently discovered a neuronal receptor for agrin, concentrated at neuron-neuron synapses, and distinct from the muscle specific kinase complex that mediates agrin's action in muscle. Agrin binding to this receptor triggers a rapid increase in intracellular calcium and activates calcium/calmodulin-dependent kinase II and other kinases known to regulate synaptic function. Here we provide evidence the agrin receptor is a 100 kDa membrane tyrosine kinase. Chronic inactivity of this receptor results in decreased neuronal responses to excitatory neurotransmitters, correlated with alterations in calcium homeostasis. To learn more about agrin's function in brain, we will establish the molecular identity of the agrin receptor; characterize the cellular interactions that regulate its expression; examine the effects of suppressing agrin receptor expression and; identify mechanisms by which agrin influences neuronal responses to excitatory stimuli. The experiments outlined here are aimed at understanding the cellular mechanisms that control behavior of neural circuits in brain. Agrin, a molecule critical for neuromuscular connectivity, is required for development of normal responses to excitatory neurotransmitters. In light of our earlier demonstration that expression of agrin is activity dependent, agrin may prove to be a key modulator of neuronal activity in brain. The results of these studies, therefore, are likely to be directly relevant to the prevention and treatment of a number of human disorders, such as epilepsy, that disrupt behavior of neural circuits.

Sandy coastlines such as the U.S. Southeast and Gulf coasts are constantly shifted and reshaped as breaking waves move sand from one location to another. Research into how such coastlines evolve over spatial scales of kilometers to hundreds of kilometers and over time scales of decades and longer has just begun. Recent work has revealed surprising long-range interactions, with changes in one location directly affecting distant parts of the coastline. Human efforts to stabilize the shoreline position -- especially through artificial sand placement or "beach nourishment" -- are becoming increasingly prevalent, and these localized manipulations likely affect how entire coastlines evolve through their long-range as well as regional effects. This research project will incorporate human manipulations into an enhanced computer model of large-scale, long-term coastline change caused by wave-driven sediment transport. Addressing the behaviors of the fully coupled human-natural system will require developing a model that represents how humans respond to coastline changes, especially local shoreline erosion. Such a model will be based on (1) data that incorporates historical beach-nourishment decisions and economic variables; (2) economic theory;( 3) information gathered during a workshop involving coastal managers, engineers, policy makers, and stakeholders; and (4) scenarios of federal beach-nourishment subsidies, future expense of procuring beach-quality sand, and policy constraints arising from ecological concerns. The resulting coupled model will allow investigations of the types of coastline behaviors to be expected in coming decades and centuries in the context of likely climate change and the consequent changes in storm and wave patterns as well as accelerated sea-level-rise. The coastline of the Carolinas will be used as an initial case study to test the model.

Developing the social science component of the modeling endeavor will involve the first examination of how beach nourishment decisions are made. Experiments using the coupled human-coastline model will provide the first examination of how human-influenced coastlines evolve, and more specifically, how actions taken at one location are likely to affect other coastal communities in far-flung locations as well as nearby. The potential insights gained through this work could help coastal managers and planners avoid surprises arising from such spillover effects, which could be especially important as climate change and direct human manipulations become more important factors affecting coastline change. A dedicated website and targeted communications will facilitate dissemination of the main lessons to interested coastal planners and stakeholders. More broadly, this research will advance the embryonic science of human-landscape interactions, which are becoming ubiquitous across much of the Earth's surface. This project is supported by an award resulting from the FY 2005 special competition in Biocomplexity in the Environment focusing on the Dynamics of Coupled Natural and Human Systems.

Human activities increasingly influence landscape change in many environments, both directly through construction and agricultural activities and indirectly through changes to the natural processes that shape landscapes. In turn, the processes that shape landscapes affect humans, often posing natural hazards. In coastal environments these two-way interactions involve coastal erosion, which threatens coastal communities, and shoreline-stabilization efforts, which affect the evolution of the surrounding coastline. Previous numerical modeling has shown that localized shoreline stabilization efforts, such as nourishing beaches by adding sand, can alter shoreline erosion rates even in distant parts of a coastline. Thus, a coastal community that chooses to stabilize its shoreline inadvertently affects other communities, so that the economies and management of coastal communities are linked. This research will use numerical modeling to address the kinds of coupled environmental and economic patterns that emerge under different decision-making regimes. An economic component to the numerical modeling, based on an empirical relationship between property values and beach width, determines the beach replenishment strategy that optimizes the net benefits to an individual community. Coupling this model to a coastline-change model reveals the unexpected ways that communities unwittingly interact with one another, and the feedbacks that induce some communities to shoulder more of the shoreline stabilization effort than others. In contrast, a different economic-model approach will analyze what pattern of beach replenishment would maximize the net benefits of a stretch of coastline more holistically. This project will investigate the different patterns of coastline change and economic benefits these approaches would produce under different scenarios for: 1) sea-level rise; 2) changing storm climate; 3) coastline physical and economic attributes; and 4) diminishing common-pool sand resources and the associated increase in the price of beach replenishment.

Changes in coastal environments can no longer be understood by considering either physical or economic processes in isolation; this research provides a necessary step toward understanding the dynamics of developed coastlines?what causes the patterns of shoreline erosion and economic impacts under various possible futures (given uncertainties in climate change and economic driving factors including sand resources). The results of computer-model experiments testing how coordinated planning for shoreline stabilization could increase net wealth will not only increase basic knowledge about how coupled human/landscape systems work, but it could lead to improvements in coastal management strategies.

DESCRIPTION (provided by applicant): All coordinated movements such as locomotion, breathing, and swallowing, depend on the rapid transfer of information between motor neurons and the skeletal muscle fibers they innervate that takes place at a specialized contact called the neuromuscular junction. Disruption of this system by disease, as occurs in various myasthenic disorders, spinal muscular atrophy, and amyotrophic lateral sclerosis, toxins such as botulinum toxin, or traumatic injury, results in muscle weakness and even death. Thus, understanding the mechanisms responsible for the formation and maintenance of the neuromuscular junction has the potential for significant public health benefits by identifying new therapeutic targets for a wide range of disorders that impact the function of this important synapse. A major success in the field has been the elucidation of the role of a protein called agrin in mediating the motor neuron-induced differentiation of the postsynaptic apparatus formed by the muscle fiber. Agrin exerts its effects through a muscle fiber membrane receptor complex consisting of LRP4, a low-density lipoprotein receptor-related protein, and MuSK, a muscle-specific tyrosine kinase. Curiously, however, not all forms of agrin at the neuromuscular junction are competent to activate the LRP4/MuSK receptor and several studies have suggested these "inactive" agrin species play a role regulating growth and differentiation of motor axons and axon terminals. The receptor that might mediate these presynaptic effects is unknown but a good candidate is the a3 Na,K-ATPase (NKA), a neuron-specific isoform of the sodium-potassium pump that binds all forms of agrin. Agrin-dependent regulation of the a3 pump modulates excitability of central nervous system neurons, as well as the levels of cytoplasmic calcium and other second messengers implicated in regulating synaptic function, growth and maturation. The a3 NKA is also present on motor axon terminals, raising the possibility that the agrin/a3 NKA pathway plays a related role at the neuromuscular junction. To test this hypothesis we will examine the effect of knocking out the a3 NKA on development of the neuromuscular junction, the role of the a3 NKA in regulating growth of motor axons and dendrites, and the role of agrin/a3 NKA in modulating neuromuscular synaptic transmission. Previous studies of agrin function at the neuromuscular junction have focused on its interaction with the postsynaptic LRP4/MuSK receptor. This would be the first to examine the significance of an alternate agrin signaling pathway at this important synapse. Neuromuscular diseases such as the myasthenic disorders, spinal muscular atrophy, amyotrophic lateral sclerosis are characterized by profound muscle weakness linked to a decline in neuromuscular synaptic transmission. We believe the agrin/a3 NKA pathway will prove to be a novel therapeutic target that can be exploited for the treatment of these and related neuromuscular disorders. PUBLIC HEALTH RELEVANCE: The neuromuscular junction is the vital connection between the nerves and muscle fibers responsible for respiratory and voluntary movements. In many neuromuscular diseases, such as spinal muscular atrophy, myasthenia gravis and congenital myasthenic syndrome, the function of the neuromuscular junction is compromised resulting in profound muscle weakness and death. Here we examine a novel mechanism whereby agrin, a protein known for its role in directing the formation of the post-synaptic apparatus of the neuromuscular junction, influences growth and function of the pre-synaptic motor axon terminal. This novel signal pathway represents a new therapeutic target for the treatment of neuromuscular disease.

ABSTRACT

A non-technical description explaining the broader significance of the project

This project will analyze the ways in which coastal processes and economic decisions about land use and coastal engineering interact to determine the nature and timing of adaptation to climate risk. It addresses the interactions of natural forces, economic decisions, and public policies over long time horizons to determine how the built environment and patterns of human settlement react to rising seas and related coastline changes. These issues are of concern to a significant part of the US population, especially along the East Coast and Gulf of Mexico, that faces persistent flooding and storm damage. A fundamental aim of this research is to provide knowledge and tools to look further forward in time in responding to coastal and environmental changes. The results will advance knowledge about how beaches and coastal environments react to various storm-related scenarios. It will also provide insight into how real-estate markets react to complex changes in environmental conditions, public policies, scientific knowledge, and individual attitudes and values.

A technical description of the project

Changing climatic and geomorphological processes are likely to increase risks of living at the coast in the future and to increase the value of reducing those risks through engineering. However, the same factors will tend to elevate the cost and decrease the certainty of the effectiveness of those engineering actions. These dynamics may eventually make it too expensive to continue coastal habitation in its current forms. Coupled choices about modifications to the natural and built environment will determine not only the characteristics of coastal communities but also the nature of transitions to less inhabited or uninhabited states. Natural systems will be represented by state-of-the-art three-dimensional coastal geomorphology models to significantly improve predictions about the way coastal systems evolve over time. The economic system will be investigated through a novel specification of the property markets in two US east coast communities and will be informed by surveys and qualitative research into residents' knowledge of risks and preferences for coastal amenities and infrastructure. The project will investigate the way that public policies, including government-managed insurance, engineering projects, disaster relief, and infrastructure, will impact both economic decisions and the coastal environment. The resulting modeling structure will be a significant step forward in modeling community-environment interactions in response to climate change over long time scales, and the code and model structure will be made both accessible for additional research and policy decisions.