Characterization of the HCN Interaction Partner TRIP8b/PEX5R in the Intracardiac Nervous System of TRIP8b-Deficient and Wild-Type Mice.

The tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b/PEX5R) is an interaction partner and auxiliary subunit of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are key for rhythm generation in the brain and in the heart. Since TRIP8b is expressed in central neurons but not in cardiomyocytes, the TRIP8b-HCN interaction has been studied intensely in the brain, but is deemed irrelevant in the cardiac conduction system. Still, to date, TRIP8b has not been studied in the intrinsic cardiac nervous system (ICNS), a neuronal network located within epicardial fat pads. In vitro electrophysiological studies revealed that TRIP8b-deficient mouse hearts exhibit increased atrial refractory and atrioventricular nodal refractory periods, compared to hearts of wild-type littermates. Meanwhile, heart rate, sino-nodal recovery time, and ventricular refractory period did not differ between genotypes. Trip8b mRNA was detected in the ICNS by quantitative polymerase chain reaction. RNAscope in situ hybridization confirmed Trip8b localization in neuronal somata and nerve fibers. Additionally, we found a very low amount of mRNAs in the sinus node and atrioventricular node, most likely attributable to the delicate fibers innervating the conduction system. In contrast, TRIP8b protein was not detectable. Our data suggest that TRIP8b in the ICNS may play a role in the modulation of atrial electrophysiology beyond HCN-mediated sino-nodal control of the heart.

Heterogeneity of the astrocytic AMPA-receptor transcriptome.

Astrocytes form the largest class of glial cells in the central nervous system. They serve plenty of diverse functions that range from supporting the formation and proper operation of synapses to controlling the blood-brain barrier. For many of them, the expression of ionotropic glutamate receptors of the AMPA subtype (AMPARs) in astrocytes is of key importance. AMPARs form as macromolecular protein complexes, whose composition of the pore-lining GluA subunits and of an extensive set of core and peripheral complex constituents defines both their trafficking and gating behavior. Although astrocytic AMPARs have been reported to exhibit heterogeneous properties, their molecular composition is largely unknown. In this study, we sought to quantify the astrocytic AMPAR transcriptome during brain development and with respect to selected brain regions. Whereas the early postnatal pattern of AMPAR mRNA expression showed minor variation over time, it did show significant heterogeneity in different brain regions. Cerebellar astrocytes express a combination of AMPAR complex constituents that is remarkably distinct from the one in neocortical or hippocampal astrocytes. Our study provides a workflow and a first reference for future investigations into the molecular and functional diversity of glial AMPARs.

Depletion of the AMPAR reserve pool impairs synaptic plasticity in a model of hepatic encephalopathy.

Hepatic encephalopathy (HE) is the most common neuropsychiatric complication of acute or chronic liver failure. Clinical symptoms include cognitive and intellectual dysfunction as well as impaired motor activity and coordination. There is general consensus that increased levels of ammonia play a central role in the pathogenesis of HE. However, it is still elusive how cognitive performance including the ability to learn and memorize information is affected by ammonia at molecular levels. In the present study, we have employed a neuroglial co-culture model, which preserves neuroglial interplay but allows for cell-type specific molecular and functional analyses, to investigate glutamatergic neurotransmission under conditions of high ammonia. Chronic exposure to ammonia significantly reduced neuronal mRNA and protein expression of AMPA-subtype glutamate receptors (AMPARs), which mediate most fast excitatory neurotransmission in the brain. Surprisingly, neurons were able to fully maintain basal glutamatergic neurotransmission as recorded by AMPAR-mediated miniature excitatory postsynaptic currents (mEPSCs) even when >50% of total AMPARs were lost. However, long-lasting, activity-dependent changes in the efficacy of synaptic communication, which model the capability of the brain to learn and store information, were severely constrained. Whereas synaptic efficacy could still be depressed, an increase in synaptic strength was abolished. We conclude that neurons retain basal glutamatergic transmission at the expense of the extrasynaptic population of AMPARs, which is revealed when the extrasynaptic reserve pool is recruited in vain for synaptic potentiation. Our findings thus offer a molecular model, which might not only explain impaired synaptic plasticity in HE but also in other neurological diseases accompanied by a decrease in extrasynaptic AMPAR expression.

Ontogeny repeats the phylogenetic recruitment of the cargo exporter cornichon into AMPA receptor signaling complexes.

Besides mediating most of the fast excitatory neurotransmission in the mammalian CNS, ionotropic glutamate receptors of the AMPA subtype (AMPARs) serve highly diverse functions in brain development controlling neuronal migration, synaptic growth, and synaptic maturation. Pioneering proteomic studies suggest that this functional diversity is met by a great molecular complexity in native AMPAR composition. Here, we have investigated the expression patterns of two recently identified AMPAR constituents, the cornichon homologues CNIH-2 and CNIH-3, and their assembly with the AMPAR core subunits GluA1-4 in developing rat brain. Unlike GluA1-4 expression, which is up-regulated during postnatal brain development, the two cornichon homologues show maximum mRNA and protein expression early after birth, which then decline towards adulthood. Despite rather reciprocal expression profiles, the overall ratio of CNIH-2/3 complexed with GluAs remains constant throughout development. Our data reveal an excess amount of AMPAR-free CNIH-2/3 early in development, which might serve the evolutionarily conserved role of cornichon as a cargo exporter. With progressing development, however, the amount of AMPAR-free CNIH-2/3 subsides, whereas the one being integrated into AMPAR complexes increases. Hence, the cornichon homologues CNIH-2/3 gain importance in their role as auxiliary subunits of native AMPARs during ontogeny, which reflects their functional evolution in phylogeny.