1. By using caspase-3 knockout mice, we demonstrated that caspse-3 activated by mitochondria is required for induction of NMDA receptor-dependent LTD and AMPA receptor endocytosis (Li et al., Cell 2010). Furthermore, by combining gene knockdown and knockout approaches, we show that activation of BAD and BAX, two members of the BCL-2 family, is required for caspase-3 activation, AMPA receptor endocytosis and NMDA receptor-dependent LTD, but not for mGluR-LTD or long-term potentiation of synaptic transmission. Unlike in apoptosis, however, BAD is activated only transiently and mildly leading to modest and transient caspase-3 activation insufficient to induce cell death. Our findings reveal a core signaling cascade for LTD induction and provide evidence that mechanistic and quantitative differences in caspase-3 activation are critical for determining whether it induces LTD or apoptosis. 2. We collaborated with Dr. Richard Youle to examine the function of BAX in the maintenance of mitochondrial morphology, which is important for mitochondria to support the development and function of synapses. As we reported (Norris et al., Cell Death and Differentiation 2010), we identified a fragmented mitochondrial phenotype and mitochondrial fusion defect in BAX knockout neurons and determined the binding of a series of chimeras between BCL-XL and BAX to the mitofusins, mitofusin 1 (Mfn1) and mitofusin 2 (Mfn2). One chimera (containing helix 5 (H5) of BAX replacing H5 of BCL-XL (BCL-XL/BAX H5)) co-immunoprecipitated with Mfn1 and Mfn2 significantly better than either wild-type BAX or BCL-XL. Expression of BCL-XL/BAX H5 in cells reduced the mobility of Mfn1 and Mfn2 and colocalized with ectopic Mfn1 and Mfn2, as well as endogenous Mfn2 to a greater extent than wild-type BAX. Ultimately, BCL-XL/BAX H5 induced substantial mitochondrial fragmentation in healthy cells. Therefore, we propose that BCL-XL/BAX H5 disturbs mitochondrial morphology by binding and inhibiting Mfn1 and Mfn2 activity, supporting the hypothesis that Bcl-2 family members have the capacity to regulate mitochondrial morphology through binding to the mitofusins in healthy cells. 3. In collaboration with Dr. Huaibin Cai (Neuron 2009), we reported that in transgenic mice that overexpress LRRK2, excess LRRK2 greatly accelerated the progression of neuropathological abnormalities developed in PD-related A53T alpha-synuclein transgenic mice. Moreover, we found that LRRK2 promoted the abnormal aggregation and somatic accumulation of alpha-synuclein in A53T mice, which likely resulted from the impairment of microtubule dynamics, Golgi organization, and the ubiquitin-proteasome pathway. Conversely, genetic ablation of LRRK2 preserved the Golgi structure and suppressed the aggregation and somatic accumulation of alpha-synuclein, and thereby delayed the progression of neuropathology in A53T mice. These findings demonstrate that overexpression of LRRK2 enhances alpha-synuclein-mediated cytotoxicity and suggest inhibition of LRRK2 expression as a potential therapeutic option for ameliorating alpha-synuclein-induced neurodegeneration. 4. We collaborated with Dr. Forbes Porter (Human Molecular Genetics 2010) to investigate pathological processes underlying SmithLemliOpitz syndrome (SLOS), a malformation syndrome with neurocognitive deficits due to mutations of DHCR7 that impair the reduction of 7-dehydrocholesterol to cholesterol. We compared protein expression in Dhcr7+/+ and Dhcr7 3-5/ 3-5 brain tissue and found that cofilin-1, an actin depolymerizing factor is differentially expressed in these tissues. We further demonstrate that differential expression of cofilin-1 was caused by increased phosphorylation, which is mediated by increased activation of RhoA, Rac1 and Cdc42 in Dhcr7 3-5/ 3-5 brains. Consistent with increased activation of these Rho GTPases, we observed abnormal development of both dendrites and axons in hippocampal neurons. Developmental defects of neuronal process may contribute to the neurocognitive deficits found in SLOS and may represent a potential target for therapeutic intervention. 5. Our group reviewed the study examining how the schizophrenia susceptibility gene DISC1 (Disrupted-In-Schizophrenia 1) regulates dendritic spines through small GTPases (Cell Science Review 2010). Disrupted-In-Schizophrenia 1 (DISC1) is a schizophrenia risk gene which encodes a scaffold protein required for spine development. In a recent paper by Hayashi-Takagi et al. published in Nature Neuroscience, a mechanism by which DISC1 regulates spine development is reported. The authors show that a signalosome composed of DISC1, the postsynaptic scaffold protein PSD-95 and the guanine nucleotide exchange factor Kalirin-7 (Kal-7) is crucial to determine the activity of Rac1, a Rho family small GTPase that regulates spine morphogenesis by reorganizing the actin cytoskeleton. It is noteworthy that both Kal-7 and Rac1 show altered expression in brains of schizophrenic patients, so the DISC1-PSD95-Kal-7-Rac1 signalosome may well contribute to the spine pathology of schizophrenia.
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