Research Description
The human brain contains billions of neurons to receive and integrate a host of electrical and biochemical signals. Efficiency in communication among neurons is made possible by synapses, contacts formed between two neurons. Despite significant progress in our understanding of synapse anatomy, relatively little is known about how different components/molecules are assembled together and how the disruption of this process and synapse function causes psychiatric and neurological disorders. The focus of our lab is to understanding the mechanisms underlying synapse formation and what goes wrong at synapses under pathological conditions. Our studies aim at identification of targets and development of potential therapeutic strategies for treating disorders whose pathogenesis involves abnormal synaptic structure and function. These include psychiatric disorders such as schizophrenia, anxiety, and depression and neurological disorders such as muscular dystrophy, spinal cord injury, and epilepsy.
NRG1, synaptic plasticity, schizophrenia and epilepsy
Neuregulin-1 (NRG1), a family of polypeptides shown to play an important role in nerve cell development. It has been implicated in a diverse array of maturational processes including neuronal differentiation, migration, neurite outgrowth, and formation of excitatory synapses (Li et al., 2007). Intriguingly, NRG1 and its receptor ErbB tyrosine kinases are expressed not only in the developing nervous system but also in adult brain. We found that ErbB receptors are concentrated at the postsynaptic density (PSD) of excitatory synapses in adult brains, presumably via interaction with PDZ domain-containing proteins including PSD-95 and erbin, or lipid rafts (Huang et al., 2000; Huang et al., 2001; Ma et al., 2003). Such interaction regulates NRG1 signaling (Huang et al., 2003; Ma et al., 2003; Dai et al., 2006). Moreover, the synaptic localization of ErbB proteins suggests to us that NRG1s may regulate synaptic plasticity. Indeed we discovered in 2000 that NRG1 suppresses induction of long-term potentiation (LTP) in the hippocampal CA1 region, a cellular model of learning and memory, in collaboration with Dr. Mike Salter and colleagues (Huang et al., 2000). This finding, the first piece of evidence of NRG1’s role in synaptic plasticity, is exciting because recent genetic studies have associated the NRG1 gene with schizophrenia in diverse populations. Malfunction of NRG1 signaling at synapses may be a mechanism underlying this devastating disease. NRG1 regulation of LTP requires ErbB4, another susceptibility gene of schizophrenia (Pitcher et al., 2008).
γ-Aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the mammalian forebrain. GABAergic inhibitory interneurons are essential to the proper function of the brain. GABAergic dysfunction is implicated in psychiatric disorders such as schizophrenia, anxiety, and depression and neurological disorders including Huntington’s chorea, Parkinson’s disease, and epilepsy. Recently, we show that ErbB4 is present at GABAergic terminals of the prefrontal cortex. NRG1 is able to regulate GABAergic transmission, identifying a novel function of NRG1 (Woo et al., 2007). These results suggest that NRG1 may regulate neuronal activity in the brain, providing intriguing leads to potential pathological mechanisms of schizophrenia and epilepsy. Work in our lab now investigates whether NRG1 regulates brain activity at circuitry levels including pyramidal neuron firing and gamma waves and cognitive functions such as working memory in transgenic mice.
NRG1/ErbB4 signaling is complex. NRG1 stimulation causes ErbB endocytosis that appears to be necessary for the activation of intracellular signaling (Yang et al., 2005; Liu et al., 2007). Stimulation of ErbB tyrosine kinases leads to activation of several intracellular pathways including Ras-Raf-Erk and PI3K (Si et al., 1996; Si et al., 1999). In non-canonical signaling, ErbB4 undergoes proteolytic cleavage to generate an intracellular domain that may regulate gene expression in the nucleus (Lee et al., 2002). On the other hand, NRG1 also releases its intracellular domain to regulate expression of PSD-95 (Bao et al., 2004). Molecular mechanisms underlying NRG1-regulated synaptic plasticity are currently under investigation.
Representative publications on NRG1
- J. Bao, H. Lin, Y. Ouyang, D. Lei, A. Osman, T.-W. Kim, L. Mei, P. Dai, K.K Ohlemiller, R.T. Ambron. Activity-Dependent Transcription Regulation of PSD-95 by Neuregulin-1 and Eos. Nature Neuroscience 7: 1250-1258, 2004
- P. Dai, W.C. Xiong, and Lin Mei. Erbin inhibits RAF activation by disrupting the SUR-8-RAS-RAF complex. J. Biol. Chem. 281:927, 2006
- Y.Z. Huang, S.S. Won, D.W. Ali, Q. Wang, M. Tanowitz, Q.S. Du, W.C. Xiong, K.A. Pelkey, M.W. Salter, and Lin Mei. Regulation of neuregulin signaling by PSD-95 interacting with ErbB4 at CNS synapses. Neuron 26:443-455, 2000
- Y.Z. Huang, Q. Wang, W.C. Xiong, L. Mei. Erbin is a protein concentrated at postsynaptic membranes that regulates surface expression of ErbB2. J. Biol. Chem. 276:19318-19326, 2001
- Y.Z. Huang, M. Zang, W.C. Xiong, Z. Luo, Lin Mei. Erbin suppresses the MAP kinase pathway. J. Biol. Chem. 278:1108-1114, 2003.
- H.-J. Lee, K.-M. Jung, Y.Z. Huang, L.B. Bennett, J.S. Lee, L. Mei, and T.-W. Kim. Presenilin-dependent gamma-secretase-like intramembrane cleavage of ErbB4. J. Biol. Chem. 277: 6318-6323, 2002
- B. Li, R.S. Woo, L. Mei, R. Malinow. The neuregulin1 receptor ErbB4 controls glutamatergic synapse maturation and plasticity. Neuron 54:583-597, 2007 (high-lighted in Preview by G. Fischbach, Neuron 54:495-497, 2007; in News and View by L.W. Role and D.A. Talmage Nature 448:263, 2007 and in Nature Review Neuroscience, 8:492, 2007).
- Y. Liu, Y. Tao, R.-S. Woo, W.C. Xiong, and Lin Mei. Stimulated ErbB4 internalization is necessary for neuregulin signaling in neurons. Biochem. Biophys. Rapid Comm. 354:505-510, 2007
- L. Ma,* Y.Z. Huang,* J.G. Valtschanoff, L.Y. Feng, B. Lu, W.C. Xiong, R.J. Weinberg, and L. Mei. Ligand dependent recruitment of the neuregulin-signaling complex into neuronal lipid rafts. J. Neurosci. 23:3164-3175, 2003. (highlighted in “This Week in The Journal”, J. Neurosci. 23(8):i, 2003) (*, equal contribution)
- G.M. Pitcher, S. Beggs, R.-S. Woo, Lin Mei and M.W. Salter. ErbB4 is a suppressor of long-term potentiation in the adult hippocampus. NeuroReport 19:139-143, 2008
- J. Si, Z. Luo, and Lin Mei: Induction of acetylcholine receptor gene by ARIA requires activation of MAP kinase. J. Biol. Chem. 271:19752-19759, 1996.
- J. Si, Q. Wang, and Lin Mei: Essential roles of c-JUN and c-JUN NH2-terminal kinase (JNK) in neuregulin increased expression of acetylcholine receptor epsilon-subunit. J. Neurosci. 19:8498-8508, 1999
- R.-S. Woo,* X.M. Li,* Y. Tao, E. Carpenter-Hyland, Y.Z. Huang, J. Weber, H. Neiswender, X..-p. Dong, J. Wu, M. Gassmann, C. Lai, W.-C. Xiong, T.-M. Gao,* L. Mei.* Neuregulin-1 Enhances Depolarization-Induced GABA Release. Neuron 54:599-610, 2007 (high-lighted in Preview by G. Fischbach, Neuron 54:495-497, 2007; in News and View by L.W. Role and D.A. Talmage Nature 448:263, 2007 and in Nature Review Neuroscience, 8:492, 2007). (*, equal contribution)
- X.L. Yang, Y.Z. Huang, W.C. Xiong, and Lin Mei. Neuregulin-induced Expression of the Acetylcholine Receptor Requires Endocytosis of ErbB Receptors. Mol. Cell. Neurosci. 28:335-346, 2005
Neuromuscular junction formation and muscular dystophy
Muscle contracts in response to motor nerve stimulation. This is controlled by the contact between the two, or the neuromuscular junction (NMJ), a synapse that uses acetylcholine (ACh) as neurotransmitter in mammals. The NMJ is usually located in the middle of muscle fiber, occupying less than 0.1% of fiber surface. The muscle cell membrane facing the nerve terminal is highly differentiated to receive nerve signals. For example, the ACh receptor (AChR) is concentrated at the tips of junctional folds. NMJ formation and maintenance are controlled by close interaction between motor nerves and skeletal muscle fibers. Its malformation or improper maintenance leads to motor nerve and muscle disorders including muscular dystrophy. Diagnosis and therapy for several neuromuscular diseases has been benefited from identification of proteins key for NMJ formation and maintenance. They include AChRs as a target in myasthenia gravis, anti-MuSK antibodies in patients with serum-negative muscular dystrophy, and genetic mutations of rapsyn in patients with congenital myasthenic syndrome (CMS). Our studies contribute to a better understanding of these disorders as well as synapse formation in the brain.
Agrin is believed to be a protein utilized by motoneurons to induce postsynaptic differentiation. It activates MuSK, a muscle-specific receptor tyrosine kinase. Mice lacking ether agrin or MuSK do not form the NMJ. However, mechanisms of agrin/MuSK are not well understood. We showed that agrin stimulation leads to MuSK recruitment into lipid rafts and endocytosis which appear to be necessary for AChR clustering in cultured muscle cells (Zhu et al., 2006; Zhu et al., 2007). Agrin stimulates a few intracellular enzymes including Abl, geranylgeranyltransferase I (GGT I) (Luo et al., 2003), small G-proteins of the Rho family, and PAK1 (Luo et al., 2002). Their activity was shown to be necessary for receptor clustering in muscle cells and neuruomuscular synapse formation in vitro or in vivo. Agrin/MuSK signaling could be regulated by proteins interacting with MuSK including Dok-7, Dvl (Luo et al., 2002), or SHP2, a protein tyrosine phosphatase. However, muscle-specific mutation of the SHP2 gene does not appear to alter NMJ formation and maintenance (Dong et al., 2006).
A few questions remain outstanding regarding agrin/MuSK signaling. First, although agrin can activate MuSK, there is no evidence that the two molecules physically interaction. It was thought a third protein may mediate MuSK activation. We showed that the extracellular domain of MuSK binds to an activity on muscle surface that is involved in AChR clustering (Wang et al., 2008). It may be the missing protein necessary for agrin to activate MuSK. The second question regards how exactly the signal is transduced from MuSK to cytoskeleton reorganization necessary for AChR clusters. Recent studies indicate that rapsyn, an AChR-interacting protein essential for NMJ formation, plays a multi-facet role in AChR clustering. It could bridge AChRs to the cytoskeleton via the b-catein/a-catenin complex (Zhang et al., 2007). On the other hand, it prevents the activation of Cdk5, a kinase downstream of the negative signal ACh to disperse AChR clusters (Chen et al., 2007). Moreover, NMJ formation may be regulated at the levels of MuSK expression (Kim et al., 2003; Kim et al., 2005).
Our recent studies reveal an important role of muscle b-catenin in the differentiation and/or function of motor nerves (Li et al., 2008). We showed that, in the absence of muscle b-catenin, AChR clusters were increased in size and distributed in a wider region, in agreement with in vitro studies (Zhang et al., 2007). Remarkably, primary nerve branches were mislocated while secondary or intramuscular nerve branches were elongated and reduced in number in muscle-specific b-catenin mutant mice. Both spontaneous and evoked neurotransmitter release was reduced at the mutant NMJs. Furthermore, short-term plasticity and calcium sensitivity of neurotransmitter release were compromised when b-catenin was deficient in the muscle. In contrast, the NMJ was normal in morphology and function in motoneuron-specific β-catenin deficient mice. Taken together, these observations indicate a role of muscle β-catenin for presynaptic differentiation and function, identifying a novel retrograde signaling in the synapse formation and synaptic plasticity. These observations demonstrate that muscle b-catenin play a role in the development of both pre- and post-synaptic membranes. It is plausible that β-catenin-dependent transcription is necessary for expression of a necessary retrograde signal protein.
Representative publications on NMJ formation
- F. Chen*, L. Qian*, Z.-H. Yang, Y. Huang, S.T. Ngo, N.-J. Ruan, J. Wang, C. Schneider, P.G. Noakes, Y.-Q. Ding, Lin Mei, Z.-G. Luo. Rapsyn interaction with calpain stabilizes AChR clusters at the neuromuscular junction. Neuron 55: 247-260, 2007 (*, equal contribution)
- X. P. Dong, X.-M. Li, T.-M. Gao, G.-S. Feng, W.C. Xiong, L. Mei. SHP2 is dispensable for neuromuscular junction formation and maintenance. NeuroSignals, 15:53-62, 2006
C.-H. Kim, W.C. Xiong, L. Mei. Regulation of MuSK expression by a novel signaling pathway. J. Biol. Chem. 278:38522-38527, 2003
- C.-H. Kim, W.C. Xiong, L. Mei. Inhibition of MuSK expression by CREB interacting with a CRE-like element and MyoD. Mol. Cell. Biol. 25:5329-5338, 2005
- X.-M. Li,* X.-P. Dong,* S.W. Luo, B. Zhang, D.H. Lee, A.K.L. Ting, H. Neiswender, C.H. Kim, E. Carpenter-Hyladn, T.M. Gao, W.C. Xiong, L. Mei. Retrograde regulation of motoneuron differentiation by muscle β-catenin. Nature Neurosci. 11:262-268, 2008. (highlighted in News & Views by Fu et al., Nature Neurosci. 11:244, 2008). (*, equal contribution)
- Z.G. Luo,* Q. Wang,* J.Z. Zhou, J. Wang, Z. Luo, M. Liu, X. He, A. Wynshaw-Boris, W.C. Xiong, B. Lu, Lin Mei. Regulation of AChR Clustering by Dishevelled Interacting with MuSK and PAK1. Neuron 35:489-505, 2002. (highlighted in Neurobiology Paper Alert, Curr. Opinion Neurobiol. 12:463-470, 2002). (*, equal contribution)
- Z.G. Luo, H.-S. Je, F. Yang, Z.-H. Yang, W.C. Xiong, B. Lu, L. Mei: Membrane Anchoring of Rho GTPases by Geranylgenanyltransferase is Required for Agrin-induced ACh Receptor Clustering. Neuron 40:703-717, 2003 (highlighted in “Faculty of 1000”, 2003)
- Q. Wang,* B. Zhang,* Y.E. Wang, W.C. Xiong, L. Mei. The Ig1/2 Domain of MuSK Binds to Muscle Surface and Is Involved in Acetylcholine Receptor Clustering. Neurosignals. 16:246-253, 2008. (*, equal contribution)
- B. Zhang,* S.W. Luo,* X.P. Dong,* Z. Luo, W.C. Xiong, L. Mei. beta-Catenin regulates AChR clustering in muscle cells through interaction with rapsyn. J. Neurosci. 27:3968-3973, 2007 (high-lighted in This Week in The Journal, J. Neurosci. 27:i, 2007 and in Nature Review Neuroscience, 8:324, 2007). (*, equal contribution)
- D. Zhu, W.-C. Xiong, and Lin Mei. Lipid Rafts Serve as a Signaling Platform for Nicotinic Acetylcholine Receptor Clustering. J. Neurosci. 26: 4841-4851, 2006
- D. Zhu, Z. Yang, Z. Luo, S. Luo, W.C. Xiong, Lin Mei. MuSK endocytosis in AChR clustering in response to agrin. J. Neurosci. 28:1688-1696, 2008. (highlighted in Faculty 1000)