[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. · 3.75 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.
Journal of Neuroscience 10/2009; 29(43):13720-9. · 6.91 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.