Nicolás Marichal

Inducing neurogenesis from glia could permit circuit remodeling and neuron replacement in the brain. Indirect evidence suggests that glia-to-neuron reprogramming is feasible, but the dynamics of conversion have not been directly observed, and it remains unclear whether glia-derived cortical neurons can functionally integrate into brain circuits. Here, we use two-photon imaging to visualize the reprogramming of astroglia into induced neurons in the mouse cortex in vivo . We track the emergence of spontaneous neuron-like calcium transients in astrocyte-derived induced neurons, demonstrating their functional activity in vivo . Importantly, these induced neurons exhibit orientation tuned, visually evoked calcium responses, demonstrating their functional integration into cortical circuitry - a prerequisite for their utility in circuit repair. Thus, astrocytes can be recruited to generate functional neurons in the postnatal cortex, enabling circuit remodeling in perinatal injury or neurodevelopmental disorders without relying on external cellular sources.

On 28 April 2025, the Epilepsy Research Institute Symposium at BNA2025 brought together four early career researchers to present on their findings in different areas of fundamental epilepsy research. Here, we provide a snapshot of the Symposium and highlights from the researchers’ talks.

Cellular reprogramming of mammalian glia to an induced neuronal fate holds the potential for restoring diseased brain circuits. While the proneural factor achaete-scute complex-like 1 ( Ascl1 ) is widely used for neuronal reprogramming, in the early postnatal mouse cortex, Ascl1 fails to induce the glia-to-neuron conversion, instead promoting the proliferation of oligodendrocyte progenitor cells (OPC). Since Ascl1 activity is posttranslationally regulated, here, we investigated the consequences of mutating six serine phospho-acceptor sites to alanine (Ascl1SA6) on lineage reprogramming in vivo. Ascl1SA6 exhibited increased neurogenic activity in the glia of the early postnatal mouse cortex, an effect enhanced by coexpression of B cell lymphoma 2 ( Bcl2 ). Genetic fate-mapping revealed that most induced neurons originated from astrocytes, while only a few derived from OPCs. Many Ascl1SA6/Bcl2-induced neurons expressed parvalbumin and were capable of high-frequency action potential firing. Our study demonstrates the authentic conversion of astroglia into neurons featuring subclass hallmarks of cortical interneurons, advancing our scope of engineering neuronal fates in the brain.

Cellular reprogramming of mammalian glia to an induced neuronal fate holds potential for restoring diseased brain circuits. While the proneural factor Ascl1 is widely used for neuronal reprogramming, in the early postnatal mouse cortex Ascl1 fails to induce glia-to-neuron conversion, instead promoting proliferation of oligodendrocyte progenitor cells (OPC). Since Ascl1 activity is post-translationally regulated, here we investigated the consequences of mutating six serine phospho-acceptor sites to alanine (Ascl1SA6) on lineage reprogramming in vivo . Ascl1SA6 exhibited increased neurogenic activity in glia of the early postnatal mouse cortex, an effect enhanced by co-expression of Bcl2. Genetic fate-mapping revealed that most induced neurons originated from astrocytes while only a few derived from OPCs. Intriguingly, many Ascl1SA6/Bcl2-induced neurons expressed parvalbumin and were capable of high-frequency action potential firing. Our study demonstrates authentic conversion of astroglia into neurons featuring subclass hallmarks of cortical interneurons, advancing our scope of engineering neuronal fates in the brain.

The proneural transcription factor Achaete-scute complex-like 1 (Ascl1) is a major regulator of neural progenitor fate, implicated both in neurogenesis and oligodendrogliogenesis. Ascl1 has been widely used to reprogram non-neuronal cells into induced neurons. In vitro , Ascl1 induces efficient reprogramming of proliferative astroglia from the early postnatal cerebral cortex into interneuron-like cells. Here, we examined whether Ascl1 can similarly induce neuronal reprogramming of glia undergoing proliferation in the postnatal mouse cerebral cortex in vivo . Toward this, we targeted cortical glia at the peak of proliferative expansion (i.e., postnatal day 5) by injecting a retrovirus encoding for Ascl1 into the mouse cerebral cortex. In sharp contrast to the very efficient reprogramming in vitro , Ascl1-transduced glial cells were converted into doublecortin-immunoreactive neurons only with low efficiency in vivo . Interfering with the phosphorylation of Ascl1 by mutation of six conserved proline-directed serine/threonine phosphorylation sites (Ascl1SA6) has been previously shown to increase its neurogenic activity in the early embryonic cerebral cortex. We therefore tested whether transduction of proliferative glia with a retrovirus encoding Ascl1SA6 improved their conversion into neurons. While in vitro glia-to-neuron conversion was markedly enhanced, in vivo reprogramming efficiency remained low. However, both wild-type and mutant Ascl1 reduced the relative number of cells expressing the astrocytic marker glial fibrillary acidic protein (GFAP) and increased the relative number of cells expressing the oligodendroglial marker Sox10 in vivo . Together, our results indicate that the enhanced neurogenic response of proliferative postnatal glia to Ascl1SA6 versus Ascl1 observed in vitro is not recapitulated in vivo .

Adult neurogenesis is involved in cognitive performance but studies that manipulated this process to improve brain function are scarce. Here, we characterized a genetic mouse model in which neural stem cells (NSC) of the subventricular zone (SVZ) were temporarily expanded by conditional expression of the cell cycle regulators Cdk4/cyclinD1, thus increasing neurogenesis. We found that supernumerary neurons matured and integrated in the olfactory bulb similarly to physiologically generated newborn neurons displaying a correct expression of molecular markers, morphology and electrophysiological activity. Olfactory performance upon increased neurogenesis was unchanged when mice were tested on relatively easy tasks using distinct odor stimuli. In contrast, intriguingly, increasing neurogenesis improved the discrimination ability of mice when challenged with a difficult task using mixtures of highly similar odorants. Together, our study provides a mammalian model to control the expansion of somatic stem cells that can in principle be applied to any tissue for basic research and models of therapy. By applying this to NSC of the SVZ, we highlighted the importance of adult neurogenesis to specifically improve performance in a challenging olfactory task.

The low-density lipoprotein receptor-related protein 4 (LRP4) is critical in muscle fibers for the establishment of the neuromuscular junction. Here, we show that LRP4 is also expressed by embryonic cortical and hippocampal neurons, and that downregulation of LRP4 in these neurons reduced density of synapses and number of primary dendrites. Accordingly, overexpression of LRP4 in cultured neurons had the opposite effect inducing more but shorter primary dendrites with an increased number of spines. Transsynaptic tracing mediated by rabies virus revealed a reduced number of neurons presynaptic to cortical neurons in which LRP4 was knocked down. Moreover, neuron-specific knock down of LRP4 by in utero electroporation of LRP4 miRNA in vivo also resulted in neurons with fewer primary dendrites and a lower density of spines in the developing cortex and hippocampus. Collectively, our results demonstrate an essential and novel role of neuronal LRP4 in dendritic development and synaptogenesis in the CNS.

The ependyma of the spinal cord is currently proposed as a latent neural stem cell niche. This chapter discusses recent knowledge on the developmental origin and nature of the heterogeneous population of cells that compose this stem cell microenviroment, their diverse physiological properties and regulation. The chapter also reviews relevant data on the ependymal cells as a source of plasticity for spinal cord repair.