Septin 6 localizes to microtubules in neuronal dendrites

Polarity proteins: Shaping dendritic spines and memory

The morphogenesis and plasticity of dendritic spines are associated with synaptic strength, learning, and memory. Dendritic spines are highly compartmentalized structures, which makes proteins involved in cellular polarization and membrane compartmentalization likely candidates regulating their formation and maintenance. Indeed, recent studies suggest polarity proteins help form and maintain dendritic spines by compartmentalizing the spine neck and head. Here, we review emerging evidence that polarity proteins regulate dendritic spine plasticity and stability through the cytoskeleton, scaffolding molecules, and signaling molecules. We specifically analyze various polarity complexes known to contribute to different forms of cell polarization processes and examine the essential conceptual context linking these groups of polarity proteins to dendritic spine morphogenesis, plasticity, and cognitive functions.

The palmitoyl acyltransferase ZDHHC14 controls Kv1-family potassium channel clustering at the axon initial segment

The palmitoyl acyltransferase (PAT) ZDHHC14 is highly expressed in the hippocampus and is the only PAT predicted to bind Type-I PDZ domain-containing proteins. However, ZDHHC14’s neuronal roles are unknown. Here, we identify the PDZ domain-containing Membrane-associated Guanylate Kinase (MaGUK) PSD93 as a direct ZDHHC14 interactor and substrate. PSD93, but not other MaGUKs, localizes to the axon initial segment (AIS). Using lentiviral-mediated shRNA knockdown in rat hippocampal neurons, we find that ZDHHC14 controls palmitoylation and AIS clustering of PSD93 and also of Kv1 potassium channels, which directly bind PSD93. Neurodevelopmental expression of ZDHHC14 mirrors that of PSD93 and Kv1 channels and, consistent with ZDHHC14’s importance for Kv1 channel clustering, loss of ZDHHC14 decreases outward currents and increases action potential firing in hippocampal neurons. To our knowledge, these findings identify the first neuronal roles and substrates for ZDHHC14 and reveal a previously unappreciated role for palmitoylation in control of neuronal excitability.

Effect of SEPT6 on the biological behavior of hepatic stellate cells and liver fibrosis in rats and its mechanism

Hepatic stellate cells (HSCs) are key effectors during the development of liver fibrosis. Septin 6 (SEPT6) is a highly evolutionarily conserved GTP-binding protein that regulates various cell biological behaviors. The expression and function of SEPT6 in HSCs remain unknown. Here we demonstrate that SEPT6 expression is significantly elevated following the activation of primary rat HSCs, the human hepatic stellate cell line LX-2 and the rat hepatic stellate cell line HSC-T6, as well as in both human and rat fibrotic liver tissue. In vitro, the overexpression of SEPT6 promoted HSCs activation, proliferation, cell cycle progression and migration and inhibited HSCs apoptosis. In contrast, knockdown of SEPT6 exerted the opposite effects on HSCs. Mechanistically, SEPT6 exerted its pro-fibrogenic effect by promoting the expression of TGF-β1 and the phosphorylation of Smad2, Smad3, extracellular-signal-regulated kinase, c-Jun NH2-terminal kinase, stress-activated protein kinase-2, and protein kinase B. However, in HSC-T6 cells, blockade of the TGF-β1/Smad signaling pathway by SB431542 significantly decreased the expression of α-smooth muscle actin, cyclin D1, BCL2, and matrix metalloproteinase-2 and -9, which had been enhanced by SEPT6 overexpression. In vivo, adenovirus-mediated SEPT6 inhibition attenuated thioacetamide (TAA)-induced liver fibrosis in rats by decreasing the deposition of the extracellular matrix (ECM). SEPT6 inhibition decreased the proliferation capacity of HSCs and induced apoptosis of HSCs. Collectively, our results reveal that SEPT6 regulates various biological behaviors in HSCs through TGF-β1/Smad, mitogen-activated protein kinases and phosphatidylinositol-3-kinase/protein kinase B signaling pathways, thus promoting liver fibrosis.

From transcriptomic to protein level changes in TDP-43 and FUS loss-of-function cell models

The full definition of the physiological RNA targets regulated by TDP-43 and FUS RNA-binding proteins (RBPs) represents an important issue in understanding the pathogenic mechanisms associated to these two proteins in amyotrophic lateral sclerosis and frontotemporal dementia. In the last few years several high-throughput screenings have generated a plethora of data, which are difficult to compare due to the different experimental designs and models explored. In this study by using the Affymetrix Exon Arrays, we were able to assess and compare the effects of both TDP-43 and FUS loss-of-function on the whole transcriptome using the same human neuronal SK-N-BE cell model. We showed that TDP-43 and FUS depletion induces splicing and gene expression changes mainly distinct for the two RBPs, although they may regulate common pathways, including neuron differentiation and cytoskeleton organization as evidenced by functional annotation analysis. In particular, TDP-43 and FUS were found to regulate splicing and expression of genes related to neuronal (SEPT6, SULT4A1, TNIK) and RNA metabolism (DICER, ELAVL3/HuC, POLDIP3). Our extended analysis at protein level revealed that these changes have also impact on the protein isoform ratio and content, not always in a direct correlation with transcriptomic data. Contrarily to a loss-of-function mechanism, we showed that mutant TDP-43 proteins maintained their splicing activity in human ALS fibroblasts and experimental cell lines. Our findings further contribute to define the biological functions of these two RBPs in physiological and disease state, strongly encouraging the evaluation of the identified transcriptomic changes at protein level in neuronal experimental models.

Novel septin 9 repeat motifs altered in neuralgic amyotrophy bind and bundle microtubules

Novel septin 9 repeat motifs interact with the acidic C-terminal tails of β-tubulin to promote microtubule bundling and asymmetric neurite growth.

Septin 9 (SEPT9) interacts with microtubules (MTs) and is mutated in hereditary neuralgic amyotrophy (HNA), an autosomal-dominant neuropathy. The mechanism of SEPT9 interaction with MTs and the molecular basis of HNA are unknown. Here, we show that the N-terminal domain of SEPT9 contains the novel repeat motifs K/R-x-x-E/D and R/K-R-x-E, which bind and bundle MTs by interacting with the acidic C-terminal tails of β-tubulin. Alanine scanning mutagenesis revealed that the K/R-R/x-x-E/D motifs pair electrostatically with one another and the tails of β-tubulin, enabling septin–septin interactions that link MTs together. SEPT9 isoforms lacking repeat motifs or containing the HNA-linked mutation R88W, which maps to the R/K-R-x-E motif, diminished intracellular MT bundling and impaired asymmetric neurite growth in PC-12 cells. Thus, the SEPT9 repeat motifs bind and bundle MTs, and thereby promote asymmetric neurite growth. These results provide the first insight into the mechanism of septin interaction with MTs and the molecular and cellular basis of HNA.