GABAB Receptors Affect Chloride Transporter Function
Rebecca Wright, Sarah E. Newey, Andrei Ilie, Winnie Wefelmeyer, Joseph V. Raimondo, et al.
(see pages 5447–5462)
GABA produces fast inhibition via GABAA receptors (GABAARs), which are ligand-gated chloride channels. The direction and magnitude of current through these channels depends on the intracellular chloride concentration. In adult neurons, the chloride concentration is regulated by the potassium-chloride cotransporter, KCC2, which pumps chloride ions out of the cell. KCC2 expression increases during neuronal development, and its expression and function are regulated by growth factors, phosphorylation, and interactions with other proteins (Medina et al. 2014 Front Cell Neurosci 8:27).
GABABRs (red) and KCC2 (green) colocalize (yellow) in plasma membranes of cultured hippocampal neurons. See Wright, Newey et al. for details.
Wright, Newey et al. have discovered an additional, intriguing mechanism of KCC2 regulation mediated by metabotropic GABAB receptors (GABABRs). Co-immunoprecipitation experiments indicated that KCC2 associated with GABABRs in adult rat cortex and organotypic hippocampal slices. In slices, a selective GABABR agonist significantly reduced surface expression of KCC2 without affecting total expression levels. At the same time, surface expression of GABABRs was reduced, suggesting that agonist-induced internalization of GABABRs led to internalization of KCC2. This internalization depended on clathrin-mediated endocytosis.
Consistent with reduced KCC2 surface expression, the GABABR agonist increased intracellular chloride concentration in pyramidal neurons and caused a depolarizing shift in the GABAAR reversal potential (EGABAA). This effect was occluded by blocking KCC2, and it required activation of GABABR-associated g-proteins. The shift in EGABAA did not require activation of GIRK channels (which mediate slow inhibition induced by GABABRs), calcium influx, protein kinase C, protein kinase A, or protein phosphatases.
High-frequency electrical stimulation of GABAergic neurons, which leads to activation of postsynaptic GABABRs, also produced a depolarizing shift in postsynaptic EGABAA in organotypic slices. This effect was blocked by a GABABR antagonist and by a KCC2 inhibitor. Moreover, low-frequency stimulation, which does not activate GABABRs, did not affect EGABAA. Like the shift produced by GABABR agonist, the EGABAA shift produced by high-frequency stimulation occurred within 10 min and persisted more than 30 min.
These results suggest that activation of GABABRs blunts the effect of GABAARs by reducing surface expression of KCC2 and thus reducing the driving force for chloride ions. In this way, GABABRs might alter spike timing and even increase spike rate. Whether this effect occurs under normal physiological and/or pathological conditions remains to be determined.
Complement Receptor C5aR1 Regulates Progenitor Proliferation
Liam G. Coulthard, Owen A. Hawksworth, Rui Li, Anushree Balachandran, John D. Lee, et al.
(see pages 5395–5407)
The complement system is a fundamental part of the innate immune system. Signals from pathogens or damaged cells activate complement proteins, which initiate inflammatory processes, attract macrophages and other immune cells, and cause cell lysis. But the complement system also has roles unrelated to immune responses, particularly during development. For example, some complement proteins promote migration and proliferation of progenitor cells, and in the CNS, the complement system regulates synaptic pruning (Hawksworth et al. 2017 Mol Immunol 84:17).
An early step in activation of the complement cascade is cleavage of complement C5 into C5a and C5b. Coulthard, Hawksworth et al. hypothesized that C5a influences CNS development because C5a was present in mouse embryonic CSF, while its receptor, C5aR1, was expressed on the apical (ventricular) surface of neural progenitors. Furthermore, C5aR1 was expressed in the apical membrane of neural rosettes generated from human embryonic stem cells in culture.
The apical domain of neural progenitors plays an important role in self-renewal: during mitosis, daughter cells must receive a portion of this domain to retain the ability to self-renew. Coulthard, Hawksworth et al. found that C5aR1 colocalized with atypical protein kinase C zeta (PKCζ), a key component of a complex that maintains the apical domain in progenitor cells. Disrupting apical–basal polarity in vitro reduced expression of C5aR1, but exogenous C5a restored polarity, suggesting that C5aR1 and progenitor-cell polarity are interdependent.
Addition of C5a induced PKCζ-dependent phosphorylation of ERK kinase, a regulator of mitosis, and it increased progenitor proliferation in vitro. Notably, C5a also increased mitosis of mouse neural progenitors in vivo, whereas blocking C5aR1 reduced mitosis and resulted in a shift from symmetric (proliferative) to asymmetric (neurogenic) cell division. This led to alteration in mature brain structure, which was associated with behavioral changes in adult mice.
These results indicate that neural progenitor cell proliferation depends in part on apically expressed C5aR1. Because the number and timing of symmetric and asymmetric cell divisions influences the number and type of neurons generated, reduced activation of C5aR1 during embryogenesis may alter brain development and adult behavior. Future work should explore whether activation of the complement system during pregnancy contributes to infection-related changes in fetal brain development and the subsequent risk of neuropsychological conditions.
Footnotes
This Week in The Journal was written by Teresa Esch, Ph.D.
