Chloride Potassium Symporter 5 - Developmental Changes in Expression

Developmental Changes in Expression

KCC2 levels are low during mammalian embryonic development, when neural networks are still being established and neurons are highly plastic (changeable). During this stage, intracellular chloride ion concentrations are high due to low KCC2 expression and high levels of a transporter known as NKCC1 (Na+/K+ chloride cotransporter 1), which moves chloride ions into cells. Thus, during embryonic development, the chloride gradient is such that stimulation of GABA_A receptors and glycine receptors at inhibitory synapses causes chloride ions to flow out of cells, making the internal neuronal environment less negative (i.e. more depolarized) than it would be at rest. At this stage, GABA_A receptors and glycine receptors act as excitatory rather than inhibitory effectors on postsynaptic neurons, resulting in depolarization and hyperexcitability of neural networks.

During postnatal development, KCC2 levels are strongly upregulated while NKCC1 levels are down regulated. This change in expression correlates to a developmental shift of the chloride ion concentration within neurons from high to low intracellular concentration. Effectively, as the chloride ion concentration is reduced, the chloride gradient across the cellular membrane is reversed such that GABA_A receptor and glycine receptor stimulation causes chloride ion influx, making the internal neuronal environment more negative (i.e. more hyperpolarized) than it would be at rest. This is the developmental shift of inhibitory synapses from the excitatory postsynaptic responses of the early neural development phase to the inhibitory postsynaptic responses observed throughout maturity.