During the development, tight regulation of the expansion of neural progenitor cells (NPCs) and their differentiation into neurons is crucial for normal cortical formation and function. In this study, we demonstrate that microRNA (miR)-128 regulates the proliferation and differentiation of NPCs by repressing pericentriolar material 1 (PCM1). Specifically, overexpression of miR-128 reduced NPC proliferation but promoted NPC differentiation into neurons both in vivo and in vitro. In contrast, the reduction of endogenous miR-128 elicited the opposite effects. Overexpression of miR-128 suppressed the translation of PCM1, and knockdown of endogenous PCM1 phenocopied the observed effects of miR-128 overexpression. Furthermore, concomitant overexpression of PCM1 and miR-128 in NPCs rescued the phenotype associated with miR-128 overexpression, enhancing neurogenesis but inhibiting proliferation, both in vitro and in utero. Taken together, these results demonstrate a novel mechanism by which miR-128 regulates the proliferation and differentiation of NPCs in the developing neocortex.DOI: http://dx.doi.org/10.7554/eLife.11324.001

The defining features of a neuron are its functional and anatomical connections with thousands of other neurons in the brain. Together, these neurons form functional networks that direct animal behavior. Current approaches that allow the interrogation of specific populations of neurons and neural circuits rely heavily on targeting their gene expression profiles or connectivity. However, these approaches are often unable to delineate specific neuronal populations. Here, we developed a novel intersectional split intein-mediated split-Cre recombinase system that can selectively label specific types of neurons based on their gene expression profiles and structural connectivity. We developed this system by splitting Cre recombinase into two fragments with evolved split inteins and subsequently expressed one fragment under the influence of a cell type-specific promoter in a transgenic animal, and delivered the other fragment via retrograde viral gene transfer. This approach results in the reconstitution of Cre recombinase in only specific population of neurons projecting from a specific brain region or in those of a specific neuronal type. Taken together, our split intein-based split-Cre system will be useful for sophisticated characterization of mammalian brain circuits.

Keywords: 23Split intein, split-cre recombinase, circuit mapping, projection neurons, retrograde virus, 24 DLX6, GABA, transgenic mouse. 25 Khoo et al. 2 Abstract 26The defining features of a neuron are its functional and anatomical connections with 27 thousands of other neurons in the brain. Together, these neurons form functional networks 28 that direct animal behaviour. Current approaches that allow the interrogation of specific 29 populations of neurons and neural circuits rely heavily on targeting their gene expression 30 profiles or connectivity. However, these approaches are often unable to delineate specific 31 neuronal populations. Here, we developed a novel intersectional split intein-mediated split-32Cre recombinase system that can selectively label specific types of neurons based on their 33 gene expression profiles and structural connectivity. We developed this system by splitting 34Cre recombinase into two fragments with evolved split inteins and subsequently expressed 35 one fragment under the influence of a cell type-specific promoter in a transgenic animal, and 36 delivered the other fragment via retrograde viral gene transfer. This approach results in the 37 reconstitution of Cre recombinase in only specific population of neurons projecting from a 38 specific brain region or in those of a specific neuronal type. Taken together, our split intein-39 based split-Cre system will be useful for sophisticated characterization of mammalian brain 40 circuits. 41 Khoo et al.

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