The development of GABAergic synapses is associated with an excitatory to inhibitory shift of the actions of GABA because of a reduction of [Cl − ] i . This is due to a delayed postnatal expression of the K + -Cl − cotransporter KCC2, which has low levels at birth and peaks during the first few postnatal weeks. Whether the expression of the cotransporter and the excitatory to inhibitory shift have other consequences on the operation of GABA A receptors and synapses is not yet known. We have now expressed KCC2 in immature neurones at an early developmental stage and determined the consequences on the formation of GABA and glutamate synapses. We report that early expression of the cotransporter selectively enhances GABAergic synapses: there is a significant increase of the density of GABA A receptors and synapses and an increase of the frequency of GABAergic miniature postsynaptic currents. The density of glutamate synapses and frequency of AMPA miniature postsynaptic currents are not affected. We conclude that the expression of KCC2 and the reduction of [Cl − ] i play a critical role in the construction of GABAergic networks that extends beyond the excitatory to inhibitory shift of the actions of GABA.

Non-technical summary 'To be, or not to be' -thousands of neurons are facing this Shakespearean question in the brains of patients suffering from epilepsy or the consequences of a brain traumatism or stroke. The destiny of neurons in damaged brain depends on tiny equilibrium between pro-survival and pro-death signalling. Numerous studies have shown that the activity of the neuronal potassium chloride co-transporter KCC2 strongly decreases during a pathology. However, it remained unclear whether the change of the KCC2 function protects neurons or contributes to neuronal death. Here, using cultures of hippocampal neurons, we show that experimental silencing of endogenous KCC2 using an RNA interference approach or a dominant negative mutant reduces neuronal resistance to toxic insults. In contrast, the artificial gain of KCC2 function in the same neurons protects them from death. This finding highlights KCC2 as a molecule that plays a critical role in the destiny of neurons under toxic conditions and opens new avenues for the development of neuroprotective therapy.Abstract KCC2 is a neuron-specific potassium-chloride co-transporter controlling intracellular chloride homeostasis in mature and developing neurons. It is implicated in the regulation of neuronal migration, dendrites outgrowth and formation of the excitatory and inhibitory synaptic connections. The function of KCC2 is suppressed under several pathological conditions including neuronal trauma, different types of epilepsies, axotomy of motoneurons, neuronal inflammations and ischaemic insults. However, it remains unclear how down-regulation of the KCC2 contributes to neuronal survival during and after toxic stress. Here we show that in primary hippocampal neuronal cultures the suppression of the KCC2 function using two different shRNAs, dominant-negative KCC2 mutant C568A or DIOA inhibitor, increased the intracellular chloride concentration [Cl − ] i and enhanced the toxicity induced by lipofectamine-dependent oxidative stress or activation of the NMDA receptors. The rescuing of the KCC2 activity using over-expression of the active form of the KCC2, but not its non-active mutant Y1087D, effectively restored [Cl − ] i and enhanced neuronal resistance to excitotoxicity. The reparative effects of KCC2 were mimicked by over-expression of the KCC3, a homologue transporter. These data suggest an important role of KCC2-dependent potassium/chloride homeostasis under neurototoxic conditions and reveal a novel role of endogenous KCC2 as a neuroprotective molecule. Abbreviations DIV, days in vitro; shRNA, short hairpin RNA; RT, room temperature; TBSTD, tris-buffered saline, 0.1% Tween, 5% DMSO.

Efficient and long-lasting transfection of primary neurons is an essential tool for addressing many questions in current neuroscience using functional gene analysis. Neurons are sensitive to cytotoxicity and difficult to transfect with most methods. We provide a protocol for transfection of cDNA and RNA interference (short hairpin RNA (shRNA)) vectors, using magnetofection, into rat hippocampal neurons (embryonic day 18/19) cultured for several hours to 21 d in vitro. This protocol even allows double-transfection of DNA into a small subpopulation of hippocampal neurons (GABAergic interneurons), as well as achieving long-lasting expression of DNA and shRNA constructs without interfering with neuronal differentiation. This protocol, which uses inexpensive equipment and reagents, takes 1 h; utilizes mixed hippocampal cultures, a transfection reagent, CombiMag, and a magnetic plate; shows low toxicity and is suited for single-cell analysis. Modifications done by our three laboratories are detailed. INTRODUCTIONEvery gene transfer method aims to achieve high transfection efficiency, low toxicity and long-lasting expression. These goals are especially difficult to achieve for postmitotic primary neurons due to their high sensitivity to any microenvironmental change 1-5 . Many currently available transfection methods do not yield sufficiently good gene expression results to allow functional gene analysis in differentiated adherent neurons in vitro. A recent report provides a useful overview of seven different gene delivery methods, including two often-used transfection techniques (calcium-phosphate and lipofection), but excluding magnetofection 6 . Magnetofection is a technique that can be used to reliably and efficiently introduce DNA into a variety of cell types 7 . Therefore, we have invested in designing a reliable, magnetofection-based protocol to express DNA or shRNA constructs in rat hippocampal neurons (embryonic day (E)18/19) cultured for several hours to 21 d in vitro (DIV). After transfection, neurons expressing exogenous proteins can develop normally in culture for 5-10 d. We have determined the optimum parameters for transfection efficiency such as age of cells at transfection time point, expression levels during the time after transfection and parameters for doubletransfection. This protocol shows low toxicity, uses a combination of a transfection reagent, CombiMag, and magnetic force and shows a sufficiently high efficiency in neurons (ca. 5%) to allow single-cell analysis.We present a core protocol and illustrate the critical steps, modifications and applications carried out by our three laboratories (Medina, Fuhrer and Fritschy) [8][9][10] . This protocol allows single and double transfections of DNA vectors at different time points after plating. First, we present a detailed characterization of magnetofection parameters such as age of neurons, time course of protein expression after magnetofection, and transfection efficiency in pyramidal neurons and GABAergic interneurons. Three specific