[Show abstract][Hide abstract] ABSTRACT: Background Dopamine (DA) neuron-selective uptake and toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes parkinsonism in humans. Loss of DA neurons via mitochondrial damage and oxidative stress is reproduced by systemic injection of MPTP in animals, which serves as models of parkinsonism and Parkinson's disease (PD). This study aimed to test whether pan-neural supplementation of the longevity-related, pleiotropic deacetylase SIRT1, which confers partial tolerance to at least three models of stroke and neurodegeneration, could also alleviate MPTP-induced acute pathological changes in nigrostriatal DA neurons and neighboring glia. Results We employed a line of prion promoter-driven Sirt1-transgenic (Sirt1Tg) mice that chronically overexpress murine SIRT1 in the brain and spinal cord. Sirt1Tg and wild-type (WT) male littermates (3‒4 months old) were subjected to intraperitoneal injection of MPTP. Acute histopathological changes in the midbrain and striatum (caudoputamen) were assessed with serial coronal sections triply labeled for tyrosine hydroxylase (TH), glial fibrillary acidic protein (GFAP), and nuclear DNA. In the substantia nigra pars compacta (SNpc) of the midbrain, the number of TH-positive neurons and the reactive gliosis were comparable between the Sirt1Tg and WT littermates. In the striatum, the relative fluorescence intensity of TH-positive nerve terminals and the level of gliosis did not differ by the genotypes. Conclusions Sirt1Tg and WT littermate mice exhibited comparable acute histopathological reactions to the systemic injection of MPTP, loss of TH-positive neurons and reactive gliosis. Thus, the genetic supplementation of SIRT1 does not confer histologically recognizable protection on nigrostriatal DA neurons against acute toxicity of MPTP.
[Show abstract][Hide abstract] ABSTRACT: Background
Dominant mutations in superoxide dismutase 1 (SOD1) cause degeneration of motor neurons in a subset of inherited amyotrophic lateral sclerosis (ALS). The pathogenetic process mediated by misfolded and/or aggregated mutant SOD1 polypeptides is hypothesized to be suppressed by protein refolding. This genetic study is aimed to test whether mutant SOD1-mediated ALS pathology recapitulated in mice could be alleviated by overexpressing a longevity-related deacetylase SIRT1 whose substrates include a transcription factor heat shock factor 1 (HSF1), the master regulator of the chaperone system.ResultsWe established a line of transgenic mice that chronically overexpress SIRT1 in the brain and spinal cord. While inducible HSP70 (HSP70i) was upregulated in the spinal cord of SIRT1 transgenic mice (PrP-Sirt1), no neurological and behavioral alterations were detected. To test hypothetical benefits of SIRT1 overexpression, we crossbred PrP-Sirt1 mice with two lines of ALS model mice: A high expression line that exhibits a severe phenotype (SOD1G93A-H) or a low expression line with a milder phenotype (SOD1G93A-L). The Sirt1 transgene conferred longer lifespan without altering the time of symptomatic onset in SOD1G93A-L. Biochemical analysis of the spinal cord revealed that SIRT1 induced HSP70i expression through deacetylation of HSF1 and that SOD1G93A-L/PrP-Sirt1 double transgenic mice contained less insoluble SOD1 than SOD1G93A-L mice. Parallel experiments showed that Sirt1 transgene could not rescue a more severe phenotype of SOD1G93A-H transgenic mice partly because their HSP70i level had peaked out.Conclusions
The genetic supplementation of SIRT1 can ameliorate a mutant SOD1-linked ALS mouse model partly through the activation of the HSF1/HSP70i chaperone system. Future studies shall include testing potential benefits of pharmacological enhancement of the deacetylation activity of SIRT1 after the onset of the symptom.
[Show abstract][Hide abstract] ABSTRACT: Neurite growth requires two guanine nucleotide-binding protein polymers of tubulins and septins. However, whether and how those cytoskeletal systems are coordinated was unknown. Here we show that the acute knockdown or knockout of the pivotal septin subunit SEPT7 from cerebrocortical neurons impairs their interhemispheric and cerebrospinal axon projections and dendritogenesis in perinatal mice, when the microtubules are severely hyperacetylated. The resulting hyperstabilization and growth retardation of microtubules are demonstrated in vitro. The phenotypic similarity between SEPT7 depletion and the pharmacological inhibition of α-tubulin deacetylase HDAC6 reveals that HDAC6 requires SEPT7 not for its enzymatic activity, but to associate with acetylated α-tubulin. These and other findings indicate that septins provide a physical scaffold for HDAC6 to achieve efficient microtubule deacetylation, thereby negatively regulating microtubule stability to an optimal level for neuritogenesis. Our findings shed light on the mechanisms underlying the HDAC6-mediated coupling of the two ubiquitous cytoskeletal systems during neural development.
[Show abstract][Hide abstract] ABSTRACT: In autosomal recessive early-onset Parkinsonism (PARK2), the pathogenetic process from the loss of function of a ubiquitin ligase parkin to the death of dopamine neurons remains unclear. A dominant hypothesis attributes the neurotoxicity to accumulated substrates that are exempt from parkin-mediated degradation. Parkin substrates include two septins; SEPT4/CDCrel-2 which coaggregates with alpha-synuclein as Lewy bodies in Parkinson's disease, and its closest homolog SEPT5/CDCrel-1/PNUTL1 whose overload with viral vector can rapidly eliminate dopamine neurons in rats. However, chronic effects of pan-neural overload of septins have never been examined in mammals. To address this, we established a line of transgenic mice that express the largest gene product SEPT454kDa via the prion promoter in the entire brain.
Histological examination and biochemical quantification of SEPT4-associated proteins including alpha-synuclein and the dopamine transporter in the nigrostriatal dopamine neurons found no significant difference between Sept4Tg/+ and wild-type littermates. Thus, the hypothetical pathogenicity by the chronic overload of SEPT4 alone, if any, is insufficient to trigger neurodegenerative process in the mouse brain. Intriguingly, however, a systematic battery of behavioral tests revealed unexpected abnormalities in Sept4Tg/+ mice that include consistent attenuation of voluntary activities in distinct behavioral paradigms and altered social behaviors.
Together, these data indicate that septin dysregulations commonly found in postmortem human brains with Parkinson's disease, schizophrenia and bipolar disorders may be responsible for a subset of behavioral abnormalities in the patients.
[Show abstract][Hide abstract] ABSTRACT: Ca(2+) -regulated reorganization of actin cytoskeleton is one of the key cell biological events that critically regulate neuronal morphogenesis during circuit formation, spinogenesis during synapse development, and activity-dependent structural plasticity at mature synapses. However, it remains unclear as to what extent the underlying Ca(2+) signaling processes are shared or segregated. Here, we present evidence from the literature that collectively begins to suggest that distinct calmodulin-dependent protein kinase (CaMK) isoforms are differentially expressed in time and in subcellular space, and thus may be selectively activated and engaged by distinct upstream stimuli; each CaMK isoform, in turn, couples to related, but separate, cytoskeletal and transcriptional regulatory pathways, dependent on its abundance or physical proximity with either the upstream or downstream signaling complexes. These signal transduction characteristics provide the basis for better understanding the role of excitation-morphogenesis coupling via multiple CaMKs during neuronal circuit and synapse formation.
European Journal of Neuroscience 07/2010; 32(2):224-30. DOI:10.1111/j.1460-9568.2010.07353.x · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ca(2+) signaling plays important roles during both axonal and dendritic growth. Yet whether and how Ca(2+) rises may trigger and contribute to the development of long-range cortical connections remains mostly unknown. Here, we demonstrate that two separate limbs of the Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK)-CaMKI cascades, CaMKK-CaMKIalpha and CaMKK-CaMKIgamma, critically coordinate axonal and dendritic morphogenesis of cortical neurons, respectively. The axon-specific morphological phenotype required a diffuse cytoplasmic localization and a strikingly alpha-isoform-specific kinase activity of CaMKI. Unexpectedly, treatment with muscimol, a GABA(A) receptor agonist, selectively stimulated elongation of axons but not of dendrites, and the CaMKK-CaMKIalpha cascade critically mediated this axonogenic effect. Consistent with these findings, during early brain development, in vivo knockdown of CaMKIalpha significantly impaired the terminal axonal extension and thereby perturbed the refinement of the interhemispheric callosal projections into the contralateral cortices. Our findings thus indicate a novel role for the GABA-driven CaMKK-CaMKIalpha cascade as a mechanism critical for accurate cortical axon pathfinding, an essential process that may contribute to fine-tuning the formation of interhemispheric connectivity during the perinatal development of the CNS.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 10/2009; 29(43):13720-9. DOI:10.1523/JNEUROSCI.3018-09.2009 · 6.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ca(2+) signaling plays a central role in activity-dependent regulation of dendritic arborization, but key molecular mechanisms downstream of calcium elevation remain poorly understood. Here we show that the C-terminal region of the Ca(2+)/calmodulin-dependent protein kinase CLICK-III (CL3)/CaMKIgamma, a membrane-anchored CaMK, was uniquely modified by two sequential lipidification steps: prenylation followed by a kinase-activity-regulated palmitoylation. These modifications were essential for CL3 membrane anchoring and targeting into detergent-resistant lipid microdomains (or rafts) in the dendrites. We found that CL3 critically contributed to BDNF-stimulated dendritic growth. Raft insertion of CL3 specifically promoted dendritogenesis of cortical neurons by acting upstream of RacGEF STEF and Rac, both present in lipid rafts. Thus, CL3 may represent a key element in the Ca(2+)-dependent and lipid-raft-delineated switch that turns on extrinsic activity-regulated dendrite formation in developing cortical neurons.