Need for negativity: The role of chloride homeostasis and the GABA shift in brain development

Our brain is a fascinatingly complex organ comprised of a network of millions of brain cells (neurons), and their connections (synapses). Neurons come in two types: excitatory and inhibitory. When an excitatory neuron is active, it will send positive electrical signals through its excitatory synapses to the next neurons in the network. When an inhibitory neuron is active, it will provide subsequent neurons with negative electrical signals via its inhibitory synapses. All positive and negative signals simultaneously received by a brain cell, determine whether the cell will become active itself and will pass its electrical signals on to the next cells in the network. The development of the brain starts already in the womb. Genes provide the blueprint for the first steps in network formation. In addition, network development is directed by spontaneous neuronal activity generated by excitatory signaling. Excitatory signals allow the fetal neuronal network to grow quickly by stimulating neuronal proliferation, synapse formation and synaptic strengthening. At this time, excitation is mainly provided by a substance called gamma-aminobutyric acid or GABA. Only later on, sensory experiences start affecting the electrical activity in the network. Around the same time, GABA is changing from being excitatory, to inhibitory. Excitation is now provided by another signaling molecule, called glutamate. Inhibition enables the brain to handle an enormous amount of external stimuli in a controlled manner by filtering out irrelevant inputs and only passing on relevant stimuli. The gradual developmental change in the function of GABA from excitation to inhibition is called the GABA shift. During my PhD, I studied the GABA shift in brain slices from mice. Brain development in mice and humans is very similar, though much faster in mice. In comparison: in humans, GABA shifts during the last three months of pregnancy, in mice during the first two weeks after birth. To measure the GABA shift in murine brain slices, we tested a new sensor. We could discern the GABA shift by comparing dozens of neurons in slices of different ages. In addition, we used the sensor to investigate the effect of early life stress on the GABA shift. Stress seemed to delay the GABA shift in young mice. This study underlines the role of experience in brain development, which allows for adaptability, but also leads to vulnerability. We also investigated the consequences of accelerating and delaying the GABA shift. Previous studies have shown that the excitatory GABA promotes synapse formation. Therefore, we expected that a delayed GABA shift would result in an overproduction of synapses. However, this is not what we observed when we manipulated the GABA shift in slices from one week-old mice. We saw that a delayed GABA shift did not affect synapses, but found indirect changes in the neurons’ electrical properties. This work is important for neurodevelopmental disorders such as autism, in which the GABA shift often seems delayed. Our research shows that such a delay affects further brain development

Knock-in models related to Alzheimer’s disease: synaptic transmission, plaques and the role of microglia

Funder: Cure Alzheimer's Fund; doi: http://dx.doi.org/10.13039/100007625Funder: UK Dementia Research Institute (GB)Funder: Censejo Nacional de Ciencia Tecnilogia (MX)Funder: Alzheimerfonden; doi: http://dx.doi.org/10.13039/501100008599Funder: Dolby Family FundAbstract: Background: Microglia are active modulators of Alzheimer’s disease but their role in relation to amyloid plaques and synaptic changes due to rising amyloid beta is unclear. We add novel findings concerning these relationships and investigate which of our previously reported results from transgenic mice can be validated in knock-in mice, in which overexpression and other artefacts of transgenic technology are avoided. Methods: AppNL-F and AppNL-G-F knock-in mice expressing humanised amyloid beta with mutations in App that cause familial Alzheimer’s disease were compared to wild type mice throughout life. In vitro approaches were used to understand microglial alterations at the genetic and protein levels and synaptic function and plasticity in CA1 hippocampal neurones, each in relationship to both age and stage of amyloid beta pathology. The contribution of microglia to neuronal function was further investigated by ablating microglia with CSF1R inhibitor PLX5622. Results: Both App knock-in lines showed increased glutamate release probability prior to detection of plaques. Consistent with results in transgenic mice, this persisted throughout life in AppNL-F mice but was not evident in AppNL-G-F with sparse plaques. Unlike transgenic mice, loss of spontaneous excitatory activity only occurred at the latest stages, while no change could be detected in spontaneous inhibitory synaptic transmission or magnitude of long-term potentiation. Also, in contrast to transgenic mice, the microglial response in both App knock-in lines was delayed until a moderate plaque load developed. Surviving PLX5266-depleted microglia tended to be CD68-positive. Partial microglial ablation led to aged but not young wild type animals mimicking the increased glutamate release probability in App knock-ins and exacerbated the App knock-in phenotype. Complete ablation was less effective in altering synaptic function, while neither treatment altered plaque load. Conclusions: Increased glutamate release probability is similar across knock-in and transgenic mouse models of Alzheimer’s disease, likely reflecting acute physiological effects of soluble amyloid beta. Microglia respond later to increased amyloid beta levels by proliferating and upregulating Cd68 and Trem2. Partial depletion of microglia suggests that, in wild type mice, alteration of surviving phagocytic microglia, rather than microglial loss, drives age-dependent effects on glutamate release that become exacerbated in Alzheimer’s disease

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl−), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl− currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl−), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl− currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development