Shaoyu Ge - US grants

Neurogenesis was traditionally believed to occur only during embryonic development in mammals. Only recently has it become generally accepted that new neurons are indeed continuously produced and integrated in discrete regions of the adult mammalian central nervous system. Adult neurogenesis recapitulates the complete process of the neuronal development, from fate specification of neural progenitors, migration, synaptic integration and maturation of newborn dentate granule cells (DGCs). The molecular mechanisms that regulate these neurogenesis steps are largely unknown. Gamma-aminbutyric acid (GABA), one of the major inhibitory neurotransmitter in the adult central nervous system, activates GABA receptors in the tonic/phasic mechanisms. Emerging evidence suggests that GABA receptor activity also plays an essential role in regulating the adult neurogenesis. The specific contribution of tonic and/or phasic GABA activation to the proliferation and differentiation of adult neural progenitors and the synaptic integration of their progeny, however, is unknown, and will be examined in our current proposal. Specific Aim I: To determine the roles of tonic GABA receptor activation in regulating proliferation and fate specification of neural progenitors in the adult brain. Specific Aim II: To examine the specific roles of tonic and/or phasic GABA receptor activation in synapse formation and maturation of newborn DGCs in the adult brain. Specific Aim III: To determine the roles of GABA receptor activation in the formation of functional synapses by the axons of newborn DGCs in the adult brain. The approach will be to use engineered retroviruses to genetically manipulate GABAergic activity of the adult neural progenitors and their progeny. The proliferation and differentiation of these genetically-manipulated adult neural progenitors and the synaptic integration of these newborn neurons will be examined. Understanding these regulatory mechanisms for adult neurogenesis may provide clues into the etiology and pathology of these brain disorders and diseases. More importantly, these studies may shed light on novel therapy development such as functional replacement of damaged neurons in degenerative neurological disease utilizing stem cells, including both embryonic stem cells and adult neural stem cells.

DESCRIPTION (provided by applicant): Oligodendrocytes, the myelin forming cells of the CNS, develop from identified precursor cells known alternately as oligodendrocyte progenitor cells (OPCs), polydendrocytes, or NG2 cells. These cells appear about midgestation in rodents at specific locations within the developing CNS, migrate extensively, proliferate as they migrate, and then slowly differentiate into myelin-forming cells. Surprisingly, a large fraction of the OPC population fails to fully differentiate and persists throughout the adult CNS as a slowly dividing cell that can be considered an fourth glial cell type. The functions of these adult OPCs are not well understood, in part, because of their unusual mixture of glial and neuronal properties. Major questions remain regarding 1) the phenotypic plasticity of these cells in developing and adult animals and after injury and 2) the role of electrical excitability in OPC development and function. The goal of this proposal is to develop new viral-based tools for analyzing the development and properties of NG2+ OPCs. Our strategy takes advantage of the availability of cell-type specific inducible cre mouse driver lines and recently developed flip vectors in which tandem mutant loxP sites allow for inversion of selected genes from an anti-sense to sense orientation. This strategy allows for temporal control of the expression of marker antigens, functional proteins and RNAi. The tools to be developed here can be used widely to study other glial cell types within the central and peripheral nervous system.

DESCRIPTION (provided by applicant): The dentate gyrus of adult hippocampus continuously generates new granule cells. New granule cells integrate into the existing neural circuits and regulate hippocampal behaviors. In the aged animals, there is not only a substantial decrease in the generation of newborn granule cells but also increasing failure in circuit integration of these new neurons. This raises the importance of understanding the strategy for circuit integration of new neurons in the adult brain, which remains largely unknown. In one of our recent studies in collaboration with Dr. Dan's group, we found that clonally-related sister neurons in the developing cortex are initially synchronized. This suggests that new neurons may share similar properties during integration, and are co- activation during hippocampal function. This motivates us to establish a method to label and manipulate clonally-related newborn granule cells and study their neural circuit integration and activation. As an exploratory study, we will first characterize the development of clonally-related adult-born granule cells. We will perform dual patch clamp whole cell recording of clonally-related granule cells to test their neuronal and synaptic properties during development. Secondly, we will test whether clonally-related adult-born granule cells are preferentially co-activated during 2+ hippocampal activities. To achieve this goal, we will perform Ca and c-fos imaging to study the activation of clonally-related granule cells after they are fully integrated. Importantly, we will test whether the initial synchronizatio of clonally-related adult-born granule cells is required for their development and function. Additionally, using optogenetic method, we will test whether clonally-related new neurons are differentially activated by different laminar activation. Our finding will delineate whether clonaly-related newborn granule cells share a similar developmental track and preferentially recruited during hippocampal activities. We will also test whether the initial gap junction coupling is necessary to normal development and function of clonally-related newborn neurons. These studies will provide some basis for studying the generation and development of new neurons in the aged brain.

? DESCRIPTION (provided by applicant): The adult hippocampus continuously generates new dentate granule neurons from neural stem cells. A number of factors that preferentially activate hippocampal circuits, such as exercise, enriched environment, and many pathological conditions, regulate new neuron development. Neurons transmit activity from cell to cell mainly through synapses. However, synapses of new neurons do not form until at least two weeks after birth. This suggests the presence of diffusible factors from active circuits to regulate new neuron development. This speculation has motivated us to examine signaling pathways reacting to circuit activity to regulate new neuron development. Sphingolipid signaling recently caught massive attention because of its role in mediating activity across cells in the immune system. In a pilot study, we tested the existence of this pathway in the dentate gyrus and found that sphingosine 1-phosphate receptor 1 (S1PR1) is enriched in new neurons. Moreover, both sphingosine kinase 1 and SPNS2 are enriched in existing but not new dentate granule cells. We therefore speculate that the SPNS2-S1PR1 pathway transmits the existing circuit activity to regulate the integration of newborn neurons. To test this hypothesis, we propose the following experiments: First, we will genetically perturb theSPNS2-S1PR1 pathway and test optogenetic activation of dentate granule cells-induced development of new dentate granule cells. Second, in the S1PR1 over-expressing new dentate granule cells, we will use a retroviral method to manipulate the Cdc42 and Akt pathways to examine their roles in mediating the S1PR1 regulation to new neuron development. We will perform the same genetic manipulation of the Cdc42 and Akt pathways in optogenetic-induced development of new dentate granule cells. Third, we will use pilocarpine-induced seizures as a model system to test the development of new neurons after genetically manipulating the SPNS2-S1PR1 pathway. Moreover, we will monitor epileptic activity after disrupting the SPNS2-S1PR1 pathway of new neurons. Our proposal aims to reveal the role of the SPNS2-S1PR1 pathway in propagating neural circuit activity to regulate new neuron development. This study will provide insights towards understanding how existing neural circuits dictate the development of new neurons. Our results may introduce a novel therapeutic target for the treatment of epilepsy.

The hippocampus is one of the central brain regions governing our everyday lives, with its crucial role in such aspects as learning new things and navigating in different environments. The circuit wiring between the entorhinal cortex and the hippocampus provides the substrate for hippocampal function. Interestingly, the hippocampus not only has extensive plasticity, but more importantly, it retains the capability to continuously generate new neurons for the adult brain. The question that has been continuously asked is: since there are thousands of existing hippocampal neurons, why do we need new neurons? That is, do adult-born neurons exhibit any special functions? In this proposal, we attempt to address this question in two main aims: In the aim 1, we will use an in vivo imaging system we recently established to examine the activity of newborn neurons at different ages during hippocampus-based behaviors. We expect to observe newborn neurons at certain ages showing differential activation patterning, suggesting their unique role for hippocampus-based behaviors. In Aim 2, we will activate or silence a cohort of newborn neurons and analyze the activity pattern of existing neurons. We expect to identify the circuit role of newborn neurons. Together, these tests will for the first time analyze new neurons? activation and influence on existing circuits in vivo. If successful, our findings will advance our understanding of why the adult brain needs continuous hippocampal neurogenesis. The findings will also provide insights into the understanding of hippocampal circuit activity during behavior.

Every day we make behavioral decisions based on our environment, in which the transformation of sensory information to motor command through learning and experience is essential. The behavior and neuronal mechanisms underlying the establishment of association between single sensory modality and motor decision are widely studied. However, many of decisions are made based on combined information from multiple sensory modalities. A few cross-modality studies showed that multiple sensory information significantly influenced the behavioral decisions that differs from single modality both in human and animals. Therefore, examining the behavior and neuronal mechanisms underlying how we learn to associate multiple sensory information to one motor decision will substantially advance our knowledge in sensory-cued decision making. Recent works including our own study have demonstrated that corticostriatal activity drives animal?s decision in an auditory frequency-discrimination task, and the plasticity pattern of these corticostriatal synapses encodes the learned associated between auditory cue and rewarded action. In the task, the animals learned to go to corresponding ports for reward based on different frequencies of the tones in the auditory cues. Interestingly, in these studies the navigation process between the end of auditory cues and the time that animals reached reward ports is largely ignored. In addition to the learning of auditory discrimination, the proper navigation to the ports is obviously an important learning part in this task. Therefore, we modified the task with a prolonged path between initiation port and reward port for better navigation analysis, and propose to examine how processes of spatial information and auditory information are coordinated during task learning. Hippocampus serves as an essential circuit unit to process spatial information. Our preliminary results indicated that the intact activity of hippocampal tri-synaptic circuit is required for learning this auditory task. To understand the functional role of hippocampal spatial coding for animal learning the auditory task, in aim I we will examine the role of spatial encoding in the learning of an auditory discrimination task. To understand how spatial information and auditory information processes are combined and coordinated during the task learning, in aim II we will dissect the circuit mechanism underlying the spatial coding in the learning of the auditory task.

Newborn dentate granule cells (DGCs) are continuously generated in the adult brain. These cells integrate into the pre-existing circuit and participate in hippocampus-engaged behaviors. The mechanism underlying how the adult brain governs hippocampal neurogenesis remains poorly understood. In this proposal, we investigate how coupling of pre-existing neurons to the cerebrovascular system regulates hippocampal neurogenesis. Using a new in vivo imaging method in freely moving mice, we found that hippocampus-engaged behaviors such as exploration in a novel environment rapidly increased microvascular blood flow velocity in the dentate gyrus. We will examine whether blocking this exploration-elevated blood flow dampens experience-induced hippocampal neurogenesis. We next propose to examine what molecules mediate neurovascular coupling network in the dentate gyrus to regulate experience-induced neurogenesis in the adult brain. The findings will provide a novel path to understand how adult brain actively control the number of newborn dentate granule cells. It will also provide a novel approach for analyzing dynamic neurovascular coupling during behaviors and pathological conditions including Alzheimer disease and aging.

With an ever-aging population and an estimated prevalence of Alzheimer disease of 5.7 million people in the United States alone, the impetus for more targeted treatments for age-related cognitive disorders is greater now than ever. Neuroplasticity, the ability of neural networks to adapt and remodel given experience, dwindles with age, providing possible mechanistic insights into this decline. An important layer of neuroplasticity, unique to a couple of discrete areas of the adult mammalian brain, is the addition of newly-generated neurons into existing circuits, a process known as neurogenesis. While the existence and importance of adult hippocampal neurogenesis in young adults has been well-established, we know very little about hippocampal neurogenesis in aging brains. Importantly, hippocampal neurogenesis continues into old age although there is a substantial decline in the number of newborn neurons. For example, in 26-month-old rodents, ~1000 proliferating cells could be detected per day, although only half that detected in 5-month-old adult rats. A recent study demonstrated that thousands of new neurons could be detected in the aged adult human dentate gyrus, and further, that in patients with Alzheimer disease, newborn neurons were fewer in number and exhibited delayed maturation. As a starting point, we ask why neurogenesis declines in the aging brain. Based on our preliminary studies, we found that biased circuit activity may regulate hippocampal neurogenesis in the aging brain. During screening of potential molecules biasing circuit activity, we found that one sphingolipid signaling is active in interneurons and becomes less active in the aging brain. We propose to genetically intervene this signaling to study its role in regulating neurogenesis in the aging brain. Lastly, we determine how biased circuit activity regulates hippocampal neurogenesis. Our results will not only provide mechanistic insights into the understanding of neurogenesis in the aging brain it also provides a possible strategy to intervene aging circuit activity to regulate neurogenesis.

Goal of Proposed Research Alzheimer?s disease (AD) is the most common neurodegenerative disease, presenting with progressive and irreversible memory loss and dementia. Although medications and management strategies can temporarily improve symptoms, no cure so far exists. Therefore, there is a high demand for underlying the cause, pathological conditions and behavioral symptoms of AD for advancing possible preventions and treatments. The central goal of the proposed research is to determine functional neurovascular coupling of the dentate gyrus and its underlying neural circuit mechanism in a mouse model of AD.