Transcriptional profiling of depolarization-dependent phenotypic alterations in primary cultures of developing granule neurons

Expression analysis of prestin and selected transcription factors in newborn rats

Transcription factors (TFs) have a central role to play in regulating gene expression. To analyze the co-expression patterns of selected TFs with the motor protein prestin of the outer hair cells, we applied an real-time PCR approach combining several kinds of information: (i) expression changes during postnatal development, (ii) expression changes by exposure of organotypic cultures of the organ of Corti to factors which significantly affect prestin expression [thyroid hormone (T4), retinoic acid (RA), butyric acid (BA), increased KCl concentration] and (iii) changes along the apical-basal gradient. We found that the mRNA levels of the TF Brn-3c (Pou4f3), a member of the POU family, are significantly associated with the regulation of prestin during postnatal development and in cultures supplemented with T4 (0.5 μM), BA (0.5-2.0 mM), and high KCl (50 mM) concentration. The mRNA level of the constitutively active TF C/ebpb (CCAAT/enhancer binding protein beta) correlates positively with the prestin expression during postnatal development and in cultures exposed to T4 and RA (50-100 μM). The mRNA levels of the calcium-dependent TF CaRF correlates significantly with the prestin expression in cultures exposed to T4 and high KCl concentration. The observed coexpression patterns may suggest that the TFs Brn-3c, C/ebpb, and Carf contribute to regulating the expression of prestin under the investigated conditions.

Low-concentration tributyltin perturbs inhibitory synaptogenesis and induces neuronal death in immature but not mature neurons

Tributyltin (TBT) has harmful effects on invertebrates. Reports indicate that intoxication of humans with organotin compounds could be associated with neurological symptoms such as epilepsy and amnesia; however, the toxicity mechanisms in mammals are unknown. TBT acts as a Cl(-)/OH(-) antiporter, and likely affects the GABAergic system by disturbing Cl(-) homeostasis. This study aimed to elucidate neurotoxic actions of TBT on mouse neocortical neurons during development. From 4 days in vitro (4 DIV) or 14 DIV in culture, cortical neurons were exposed to TBT continuously for 3 days. TBT-induced neuronal death at 30nM during DIV 4-6, and at 50nM during DIV 14-16. To further characterize this age-dependent cytotoxicity, miniature postsynaptic currents (mPSCs) were analyzed by whole-cell patch-clamp. The frequency of mPSCs was significantly reduced by treatment with 30nM TBT during DIV 4-6, but not DIV 14-16. After TBT treatment during DIV 4-6, GABA(A) receptor-mediated reversal potentials (E(GABA)) were significantly shifted negatively. The TBT-induced E(GABA) shift and neuronal death were reversed by increment of extracellular Cl(-) concentration, suggesting that disruption of Cl(-) homeostasis underlies the disturbance of neuronal ontogeny induced by TBT. These data indicate that the TBT may affect synaptogenesis and neuronal survival, particularly in early development.

Posttranslational regulation of BCL2 levels in cerebellar granule cells: A mechanism of neuronal survival

Apoptosis can be modulated by K(+) and Ca(2+) inside the cell and/or in the extracellular milieu. In murine organotypic cultures, membrane potential-regulated Ca(2+) signaling through calcineurin phosphatase has a pivotal role in development and maturation of cerebellar granule cells (CGCs). P8 cultures were used to analyze the levels of expression of B cell lymphoma 2 (BCL2) protein, and, after particle-mediated gene transfer in CGCs, to study the posttranslational modifications of BCL2 fused to a fluorescent tag in response to a perturbation of K(+)/Ca(2+) homeostasis. There are no changes in Bcl2 mRNA after real time PCR, whereas the levels of the fusion protein (monitored by calculating the density of transfected CGCs under the fluorescence microscope) and of BCL2 (inWestern blotting) are increased. After using a series of agonists/antagonists for ion channels at the cell membrane or the endoplasmic reticulum (ER), and drugs affecting protein synthesis/degradation, accumulation of BCL2 was related to a reduction in posttranslational cleavage by macroautophagy. The ER functionally links the [K(+)](e) and [Ca(2+)](i) to the BCL2 content in CGCs along two different pathways. The first, triggered by elevated [K(+)](e) under conditions of immaturity, is independent of extracellular Ca(2+) and operates via IP3 channels. The second leads to influx of extracellular Ca(2+) following activation of ryanodine channels in the presence of physiological [K(+)](e), when CGCs are maintained in mature status. This study identifies novel mechanisms of neuroprotection in immature and mature CGCs involving the posttranslational regulation of BCL2.

Critical role of CDK5 and Polo-like kinase 2 in homeostatic synaptic plasticity during elevated activity

Homeostatic plasticity keeps neuronal spiking output within an optimal range in the face of chronically altered levels of network activity. Little is known about the underlying molecular mechanisms, particularly in response to elevated activity. We report that, in hippocampal neurons experiencing heightened activity, the activity-inducible protein kinase Polo-like kinase 2 (Plk2, also known as SNK) was required for synaptic scaling-a principal mechanism underlying homeostatic plasticity. Synaptic scaling also required CDK5, which acted as a "priming" kinase for the phospho-dependent binding of Plk2 to its substrate SPAR, a postsynaptic RapGAP and scaffolding molecule that is degraded following phosphorylation by Plk2. RNAi knockdown of SPAR weakened synapses, and overexpression of a SPAR mutant resistant to Plk2-dependent degradation prevented synaptic scaling. Thus, priming phosphorylation of the Plk2 binding site in SPAR by CDK5, followed by Plk2 recruitment and SPAR phosphorylation-degradation, constitutes a molecular pathway for neuronal homeostatic plasticity during chronically elevated activity.