Early cortical oligodendrocyte precursor cells are transcriptionally distinct and lack synaptic connections.

Oligodendrocyte precursor cells (OPCs) generate oligodendrocytes, a process that may be tuned by neuronal activity, possibly via synaptic connections to OPCs. However, a developmental role of synaptic signaling to OPCs has so far not been shown unequivocally. To address this question, we comparatively analyzed functional and molecular characteristics of highly proliferative and migratory OPCs in the embryonic brain. Embryonic OPCs in mice (E18.5) shared the expression of voltage-gated ion channels and their dendritic morphology with postnatal OPCs, but almost completely lacked functional synaptic currents. Transcriptomic profiling of PDGFRα+ OPCs revealed a limited abundance of genes coding for postsynaptic signaling and synaptogenic cell adhesion molecules in the embryonic versus the postnatal period. RNA sequencing of single OPCs showed that embryonic synapse-lacking OPCs are found in clusters distinct from postnatal OPCs and with similarities to early progenitors. Furthermore, single-cell transcriptomics demonstrated that synaptic genes are transiently expressed only by postnatal OPCs until they start to differentiate. Taken together, our results indicate that embryonic OPCs represent a unique developmental stage biologically resembling postnatal OPCs but without synaptic input and a transcriptional signature in the continuum between OPCs and neural precursors.

Neurochemical Profile of BRAFV600E/AktT308D/S473D Mouse Gangliogliomas Reveals Impaired GABAergic System Inhibition.

Gangliogliomas (GGs), composed of dysmorphic neurons and neoplastic astroglia, represent the most frequent tumor entity associated with chronic recurrent epileptic seizures. So far, a systematic analysis of potential differences in neurochemical profiles of dysmorphic tumoral neurons as well as neurons of the peritumoral microenvironment (PTME) was hampered by the inability to unequivocally differentiate between the distinct neuronal components in human GG biopsies. Here, we have applied a novel GG mouse model that allows to clearly resolve the neurochemical profiles of GG-intrinsic versus PTME neurons. For this purpose, glioneuronal tumors in mice were induced by intraventricular in utero electroporation (IUE) of piggyBac-based plasmids for BRAFV600E and activated Akt (AktT308D/S473D, further referred to as AktDD) and analyzed neurochemically by immunocytochemistry against specific marker proteins. IUE of BRAFV600E/AktDD in mice resulted in tumors with the morphological features of human GGs. Our immunocytochemical analysis revealed a strong reduction of GABAARα1 immunoreactivity in the tumor compared to the PTME. In contrast, the extent of NMDAR1 immunoreactivity in the tumor appeared comparable to the PTME. Interestingly, tumor cells maintained the potential to express both receptors. Fittingly, the abundance of the presynaptic vesicular neurotransmitter transporters VGLUT1 and VGAT was also decreased in the tumor. Additionally, the fraction of parvalbumin and somatostatin nonneoplastic interneurons was reduced. In conclusion, changes in the levels of key proteins in neurotransmitter signaling suggest a loss of synapses and may thereby lead to neuronal network alterations in mouse GGs.

Circuit-selective cell-autonomous regulation of inhibition in pyramidal neurons by Ste20-like kinase.

Maintaining an appropriate balance between excitation and inhibition is critical for neuronal information processing. Cortical neurons can cell-autonomously adjust the inhibition they receive to individual levels of excitatory input, but the underlying mechanisms are unclear. We describe that Ste20-like kinase (SLK) mediates cell-autonomous regulation of excitation-inhibition balance in the thalamocortical feedforward circuit, but not in the feedback circuit. This effect is due to regulation of inhibition originating from parvalbumin-expressing interneurons, while inhibition via somatostatin-expressing interneurons is unaffected. Computational modeling shows that this mechanism promotes stable excitatory-inhibitory ratios across pyramidal cells and ensures robust and sparse coding. Patch-clamp RNA sequencing yields genes differentially regulated by SLK knockdown, as well as genes associated with excitation-inhibition balance participating in transsynaptic communication and cytoskeletal dynamics. These data identify a mechanism for cell-autonomous regulation of a specific inhibitory circuit that is critical to ensure that a majority of cortical pyramidal cells participate in information coding.

Heterogeneity and excitability of BRAFV600E-induced tumors is determined by Akt/mTOR-signaling state and Trp53-loss.

BACKGROUND: Developmental brain tumors harboring BRAFV600E somatic mutation are diverse. Here, we describe molecular factors that determine BRAFV600E-induced tumor biology and function. METHODS: Intraventricular in utero electroporation in combination with the piggyBac transposon system was utilized to generate developmental brain neoplasms, which were comprehensively analyzed with regard to growth using near-infrared in-vivo imaging, transcript signatures by RNA sequencing, and neuronal activity by multielectrode arrays. RESULTS: BRAF  V600E expression in murine neural progenitors elicits benign neoplasms composed of enlarged dysmorphic neurons and neoplastic astroglia recapitulating ganglioglioma (GG) only in concert with active Akt/mTOR-signaling. Purely glial tumors resembling aspects of polymorphous low-grade neuroepithelial tumors of the young (PLNTYs) emerge from BRAFV600E alone. Additional somatic Trp53-loss is sufficient to generate anaplastic GGs (aGGs) with glioneuronal clonality. Functionally, only BRAFV600E/pAkt tumors intrinsically generate substantial neuronal activity and show enhanced relay to adjacent tissue conferring high epilepsy propensity. In contrast, PLNTY- and aGG models lack significant spike activity, which appears in line with the glial differentiation of the former and a dysfunctional tissue structure combined with reduced neuronal transcript signatures in the latter. CONCLUSION: mTOR-signaling and Trp53-loss critically determine the biological diversity and electrical activity of BRAFV600E-induced tumors.

Ste20-like Kinase Is Critical for Inhibitory Synapse Maintenance and Its Deficiency Confers a Developmental Dendritopathy.

The size and structure of the dendritic arbor play important roles in determining how synaptic inputs of neurons are converted to action potential output. The regulatory mechanisms governing the development of dendrites, however, are insufficiently understood. The evolutionary conserved Ste20/Hippo kinase pathway has been proposed to play an important role in regulating the formation and maintenance of dendritic architecture. A key element of this pathway, Ste20-like kinase (SLK), regulates cytoskeletal dynamics in non-neuronal cells and is strongly expressed throughout neuronal development. However, its function in neurons is unknown. We show that, during development of mouse cortical neurons, SLK has a surprisingly specific role for proper elaboration of higher, ≥ third-order dendrites both in male and in female mice. Moreover, we demonstrate that SLK is required to maintain excitation-inhibition balance. Specifically, SLK knockdown caused a selective loss of inhibitory synapses and functional inhibition after postnatal day 15, whereas excitatory neurotransmission was unaffected. Finally, we show that this mechanism may be relevant for human disease, as dysmorphic neurons within human cortical malformations revealed significant loss of SLK expression. Overall, the present data identify SLK as a key regulator of both dendritic complexity during development and inhibitory synapse maintenance.SIGNIFICANCE STATEMENT We show that dysmorphic neurons of human epileptogenic brain lesions have decreased levels of the Ste20-like kinase (SLK). Decreasing SLK expression in mouse neurons revealed that SLK has essential functions in forming the neuronal dendritic tree and in maintaining inhibitory connections with neighboring neurons.