Rapid modulation of inhibitory synaptic currents in cerebellar Purkinje cells by BDNF

Increased levels and activation of the IL-17 receptor in microglia contribute to enhanced neuroinflammation in cerebellum of hyperammonemic rats

BACKGROUND

Patients with liver cirrhosis may show minimal hepatic encephalopathy (MHE) with mild cognitive impairment and motor incoordination. Rats with chronic hyperammonemia reproduce these alterations. Motor incoordination in hyperammonemic rats is due to increased GABAergic neurotransmission in cerebellum, induced by neuroinflammation, which enhances TNFα-TNFR1-S1PR2-CCL2-BDNF-TrkB pathway activation. The initial events by which hyperammonemia triggers activation of this pathway remain unclear. MHE in cirrhotic patients is triggered by a shift in inflammation with increased IL-17. The aims of this work were: (1) assess if hyperammonemia increases IL-17 content and membrane expression of its receptor in cerebellum of hyperammonemic rats; (2) identify the cell types in which IL-17 receptor is expressed and IL-17 increases in hyperammonemia; (3) assess if blocking IL-17 signaling with anti-IL-17 ex-vivo reverses activation of glia and of the TNFα-TNFR1-S1PR2-CCL2-BDNF-TrkB pathway.

RESULTS

IL-17 levels and membrane expression of the IL-17 receptor are increased in cerebellum of rats with hyperammonemia and MHE, leading to increased activation of IL-17 receptor in microglia, which triggers activation of STAT3 and NF-kB, increasing IL-17 and TNFα levels, respectively. TNFα released from microglia activates TNFR1 in Purkinje neurons, leading to activation of NF-kB and increased IL-17 and TNFα also in these cells. Enhanced TNFR1 activation also enhances activation of the TNFR1-S1PR2-CCL2-BDNF-TrkB pathway which mediates microglia and astrocytes activation.

CONCLUSIONS

All these steps are triggered by enhanced activation of IL-17 receptor in microglia and are prevented by ex-vivo treatment with anti-IL-17. IL-17 and IL-17 receptor in microglia would be therapeutic targets to treat neurological impairment in patients with MHE.

Retrograde BDNF to TrkB signaling promotes synapse elimination in the developing cerebellum

Elimination of early-formed redundant synapses during postnatal development is essential for functional neural circuit formation. Purkinje cells (PCs) in the neonatal cerebellum are innervated by multiple climbing fibers (CFs). A single CF is strengthened whereas the other CFs are eliminated in each PC dependent on postsynaptic activity in PC, but the underlying mechanisms are largely unknown. Here, we report that brain-derived neurotrophic factor (BDNF) from PC facilitates CF synapse elimination. By PC-specific deletion of BDNF combined with knockdown of BDNF receptors in CF, we show that BDNF acts retrogradely on TrkB in CFs, and facilitates elimination of CF synapses from PC somata during the third postnatal week. We also show that BDNF shares signaling pathway with metabotropic glutamate receptor 1, a key molecule that triggers a canonical pathway for CF synapse elimination. These results indicate that unlike other synapses, BDNF mediates punishment signal for synapse elimination in the developing cerebellum.

During development, synapses are selectively strengthened or eliminated by activity-dependent competition. Here, the authors show that BDNF-TrkB retrograde signaling is a “punishment” signal that leads to elimination of climbing fiber-onto-Purkinje cell synapses in the developing cerebellum.

Brain-derived neurotrophic factor enhances the excitability of small-diameter trigeminal ganglion neurons projecting to the trigeminal nucleus interpolaris/caudalis transition zone following masseter muscle inflammation

Background

The trigeminal subnuclei interpolaris/caudalis transition zones (Vi/Vc) play an important role in orofacial deep pain, however, the role of primary afferent projections to the Vi/Vc remains to be determined. This study investigated the functional significance of hyperalgesia to the brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (trkB) signaling system in trigeminal ganglion (TRG) neurons projecting to the Vi/Vc transition zone following masseter muscle (MM) inflammation.

Results

The escape threshold from mechanical stimulation applied to skin above the inflamed MM was significantly lower than in naïve rats. Fluorogold (FG) labeling was used to identify the TRG neurons innervating the MM, while microbeads (MB) were used to label neurons projecting to the Vi/Vc region. FG/MB-labeled TRG neurons were immunoreactive (IR) for BDNF and trkB. The mean number of BDNF/trkB-IR small/medium-diameter TRG neurons was significantly higher in inflamed rats than in naïve rats. In whole-cell current-clamp experiments, the majority of dissociated small-diameter TRG neurons showed a depolarization response to BDNF that was associated with spike discharge, and the concentration of BDNF that evoked a depolarizing response was significantly lower in the inflamed rats. In addition, the relative number of BDNF-induced spikes during current injection was significantly higher in inflamed rats. The BDNF-induced changes in TRG neuron excitability was abolished by tyrosine kinase inhibitor, K252a.

Conclusion

The present study provided evidence that BDNF enhances the excitability of the small-diameter TRG neurons projecting onto the Vi/Vc following MM inflammation. These findings suggest that ganglionic BDNF-trkB signaling is a therapeutic target for the treatment of trigeminal inflammatory hyperalgesia.

Coupling between GABA-A receptor and chloride transporter underlies ionic plasticity in cerebellar Purkinje neurons

Ionic plasticity, a form of synaptic plasticity unique to inhibitory neurotransmission, can be induced in cerebellar Purkinje neurons by brain-derived neurotrophic factor (BDNF). It is expressed as a decrease in synaptic strength of GABA-A transmission onto Purkinje neurons due to reduced transmembrane chloride gradient. By making whole-cell recordings, we found that the effect of BDNF is mediated by neuronal potassium and chloride transporter KCC2 because it is blocked by inhibitors of KCC2 or by raising the intracellular chloride concentration. Under these conditions in which KCC2 activity is reduced, BDNF augments evoked GABA-A currents suggesting a direct facilitatory effect of BDNF on GABA-A receptor. We also found that the effect of BDNF is highly localized at the GABA-A synapse and is secured by physical coupling between GABA-A receptor and KCC2, as revealed by coimmunoprecipitation studies. Based on these results, we hypothesize that the interaction between KCC2 and specific subunit of GABA-A receptor represents a fundamental mechanism rendering the rapid induction of ionic plasticity in individual or input-specific GABA synapses possible. Such a mechanism may be important for the function of Purkinje neurons that are known to express GABA-A receptors with different subunit compositions.

Dual actions of brain-derived neurotrophic factor on GABAergic transmission in cerebellar Purkinje neurons

The ability to regulate inhibitory synapses is a critical feature of the nervous system and a growing body of evidence indicates that brain-derived neurotrophic factor (BDNF) acutely modulates the efficacy of GABA synaptic transmission. Although the neuronal potassium-chloride cotransporter 2 (KCC2) has been implied in this BDNF-induced ionic plasticity, the reports about actions of BDNF on GABA signaling remain conflicting. Here we show dual effects of BDNF on GABAergic synaptic transmission in Purkinje neurons in rat cerebellar slices. BDNF decreased the amplitude of evoked outward IPSCs postsynaptically. It induced a depolarizing shift in the reversal potential (E(IPSC)), which reduced the driving force for outward IPSCs. However, in the absence of KCC2 activity, BDNF directly potentiated rather than inhibited GABA(A) receptor, which was reflected by an increase in the amplitude of outward IPSCs. This action of BDNF coincided with its effect in increasing the amplitude of inward IPSCs. Furthermore, an interaction between GABA(A) receptor and KCC2 was revealed by co-immunoprecipitation. The effects of BDNF on both GABA(A) receptor and KCC2 were dependent on TrkB and also activation of cyclin-dependent kinase 5 (Cdk5). However, only the effect of BDNF on KCC2 activity was dependent on a rise of intracellular calcium. Taken together, these data highlight distinct actions of BDNF on KCC2 and GABA(A) receptor in the regulation of GABAergic synaptic transmission.

Both cell-autonomous and cell non-autonomous functions of GAP-43 are required for normal patterning of the cerebellum in vivo

Growth-associated protein 43 (GAP-43) is required for development of a functional cerebral cortex in vertebrates; however, its role in cerebellar development is not well understood. Recently, we showed that absence of GAP-43 caused defects in proliferation, differentiation, and polarization of cerebellar granule cells. In this paper, we show that absence of GAP-43 causes defects in cerebellar patterning that reflect both cell-autonomous and non-autonomous functions. Cell-autonomous effects of GAP-43 impact precursor proliferation and axon targeting: In its absence, (1) proliferation of granule cell precursors in response to sonic hedgehog and fibroblast growth factor is inhibited, (2) proliferation of neuroepithelial precursors is inhibited, and (3) targeting of climbing fibers to the central lobe is disrupted. Cell non-autonomous effects of GAP-43 impact differentiated Purkinje cells in which GAP-43 has been downregulated: In its absence, both maturation and mediolateral patterning of Purkinje cells are inhibited. Both cell-autonomous and non-autonomous functions of GAP-43 involve its phosphorylation by protein kinase C. GAP-43 is phosphorylated in granule cell precursors in response to sonic hedgehog in vitro, and phosphorylated GAP-43 is also found in proliferating neuroepithelium and climbing fibers. Phosphorylated GAP-43 is specifically enriched in the presynaptic terminals of parallel and climbing fibers that innervate Purkinje cell bodies and dendrites. The cell-autonomous and non-autonomous effects of GAP-43 converge on the central lobe. The multiple effects of GAP-43 on cerebellar development suggest that it is a critical downstream transducer of signaling mechanisms that integrate generation of cerebellar structure with functional parcellation at the central lobe.

Activity-dependent modulation of inhibition in Purkinje cells by TrkB ligands

In the cerebellum in vitro TrkB receptor ligands promote activity-dependent inhibitory synaptogenesis of Purkinje cells and also modulate inhibitory synaptic function. This mini review examines the roles of TrkB receptor activation by BDNF particularly in relation to activity-dependent synaptic plasticity on Purkinje cells and recent studies on the acute modulation of GABAergic synapses by BDNF.