Songhai Shi, PhD

	[Enlarge Image] Figure 1 Polarization of Par protein complex specifies axon and neuronal polarity. (A, B) Distribution of Par-3 in developing hippocampal neurons in culture. (C) Hippocampal neuronal polarity revealed by axonal marker Tau-1 (red) and dendritic marker MAP2 (green). Scale bar: 25 ƒÝm.

Neuronal Polarity and Migration

Neurons are highly polarized cells with two morphologically and functionally distinct subcellular compartments emanating from the cell body, axons and dendrites, which form the basis of the unidirectional information flow in the nervous system. Axons relay information to other neurons whereas dendrites receive information from other neurons. The vital function of a neuron critically depends on the development of the axon vs. dendrite polarity. Therefore, it is important to understand the molecular mechanisms that specify neuronal polarity. Despite the observation of neuronal polarity more than a century ago, very little is known about how initially neurons become polarized. Recently we reported a molecular pathway involving the evolutionarily conserved polarity complex mPar3/mPar6/aPKC and PI 3-kinase/GSK-3 that specifies axon and neuronal polarity in hippocampal dissociated culture neurons (Figure 1). Currently we are investigating the cellular function of this pathway in specifying neuronal polarity. Furthermore, it is also our belief that the proper development of neuronal polarity is tightly linked to the processes that generate neurons (i.e. neurogenesis) and essential for the neuronal migration that lays the foundation for the neuronal circuits formation, so the identification of this pathway allows us to test this hypothesis.

	[Enlarge Image] Figure 2 Dasm1, a novel member of immunoglobulin superfamily, controls mammalian neuronal dendrite arborization and synapse maturation. The synaptic function of Dasm1 requires its C-terminal type I PDZ domain-binding motif (-TLL), which interacts with two synaptic PDZ domain containing proteins Shank and S-SCAM.

Dendrite Development

After neurons become polarized, both axons and dendrites continue to develop and the connections (synapses) between neurons will form. The nervous system progresses from a large number of disconnected neurons to a network of neuronal circuitries capable of generating functional outputs. To form the functional circuits, not only is it necessary for the axons of presynaptic neurons to grow and navigate through often long distances to the correct region to meet their targets, but also the dendrites of postsynaptic neurons need to grow and elaborate into the right shape to receive and process synaptic inputs. While extensive studies over the past decade have identified many molecules underlying axonal outgrowth and navigation, molecular mechanisms that control dendrite development are less well understood. Given that dendrites differ from axons in many important aspects morphologically as well as functionally, it seems likely that specific mechanisms are employed for dendrite development. Recently, we identified a novel and evolutionarily conserved member of immunoglobulin (Ig) superfamily, Dendrite arborization and synapse maturation 1 (Dasm1), which specifically controls mammalian neuron dendrite, but not axon, development (Figure 2). The domain structure and the initial functional studies suggest that Dasm1 is a membrane receptor. Currently we are investigating the extracellular and the intracellular signaling pathways of Dasm1.

Synapse Development and Plasticity

As neurons develop, they connect with each other through specialized structures called synapses, which are the basic functional unit of the nervous system. Synapses are largely formed between proper pairs of the dendrite and the axon. After formation, synapses mature and undergo activity-dependent plastic changes in their strength. This synaptic plasticity is critical for functional circuits formation, and information processing and storage in the brain. Besides controlling the dendrite outgrowth, Dasm1 also plays an important role in glutamatergic excitatory synapse maturation (Figure 2). The cytoplasmic tail of Dasm1 contains a type I PDZ domain-binding motif, which specifically interacts with two synaptically enriched PDZ domain-containing proteins, Shank and S-SCAM. This interaction is essential for the function of Dasm1 in regulating synaptic AMPA-R, but not NMDA-R, mediated transmission. Currently we are investigating the mechanisms through which Dasm1 together with its interacting synaptic scaffolding molecules regulate excitatory synapse maturation and plasticity.

	[Enlarge Image] Figure 3 Activity-dependent plasticity of slow synaptic inhibition mediated by GIRK and GABAB-R. (A) Abundant localization of GIRK2 in dendritic spines. Scale bar, 10 ƒÝm. (B) Signaling cascades in spines that triggers synaptic plasticity.

In the mammalian brain, neurons communicate via both glutamatergic excitatory synapses and GABAergic inhibitory synapses. Much attention has been focused on activity dependent changes of glutamatergic synaptic efficacy, as a cellular correlate of learning and memory. While GABAergic synaptic plasticity could be equally important, virtually nothing is known about the plasticity of the slow inhibitory postsynaptic current (sIPSC) that is mediated by metabotropic GABAB receptor (GABAB-R) and G protein-activated, inwardly rectifying K+ channel (GIRK). Recently we found that synaptic activation of NMDA receptor (NMDA-R)—the glutamate receptor for coincidence detection—causes long-term potentiation (LTP) of sIPSC in hippocampal CA1 pyramidal neurons (Figure 3). Currently we are investigating the mechanisms underlying this new form of synaptic plasticity, such as channel/receptor trafficking and posttranslational modifications on channel/receptor, and physiological functions of this form of plasticity.