The generation of granule cells during the development and evolution of the cerebellum.

The cerebellum coordinates vestibular input into the hindbrain to control balance and movement, and its anatomical complexity is increasingly viewed as a high-throughput processing center for sensory and cognitive functions. Cerebellum development however is relatively simple, and arises from a specialized structure in the anterior hindbrain called the rhombic lip, which along with the ventricular zone of the rostral-most dorsal hindbrain region, give rise to the distinct cell types that constitute the cerebellum. Granule cells, being the most numerous cell types, arise from the rhombic lip and form a dense and distinct layer of the cerebellar cortex. In this short review, we describe the various strategies used by amniotes and anamniotes to generate and diversify granule cell types during cerebellar development.

Cux2 serves as a novel lineage marker of granule cell layer neurons from the rhombic lip in mouse and chick embryos.

BACKGROUND: The rhombic lip (RL), a germinal zone in the developing hindbrain, gives rise to all of the excitatory neurons of the cerebellum. It is presently unclear what factors distinguish between RL progenitor pools and play a role in differentiating the multiple cell types that arise from this region. The transcription factor Cux2 has been shown to play important roles in proliferation and differentiation of distinct neuronal populations during embryogenesis, but its role in cerebellar fate restriction is unknown. RESULTS: Through expression analysis and genetic fate mapping studies we show that Cux2 is expressed in the RL of the fetal brain and is restricted to a pool of cerebellar granule cell precursors and unipolar brush cells. This restriction was remarkably specific because regardless of the timing of Cux2 reporter gene activation in the RL, only granule cell layer derivatives were labeled. However, the overexpression of Cux2 in naïve hindbrain tissue was insufficient to force progenitor cells to adopt a granule cell fate. CONCLUSIONS: Our results suggest that Cux2 delineates the pool of cerebellar granule cell layer progenitors from other RL and ventricular zone derivatives, and plays a role in fate restricting, but not differentiating, this population. Developmental Dynamics 245:881-896, 2016. © 2016 Wiley Periodicals, Inc.

Cux2 activity defines a subpopulation of perinatal neurogenic progenitors in the hippocampus.

The hippocampus arises from the medial region of the subventricular (SVZ) within the telencephalon. It is one of two regions in the postnatal brain that harbors neural progenitors (NPs) capable of giving rise to new neurons. Neurogenesis in the hippocampus is restricted to the subgranular zone (SGZ) of the dentate gyrus (DG) where it contributes to the generation of granule cell layer (gcl) neurons. It is thought that SGZ progenitors are heterogeneous, differing in their morphology, expression profiles, and developmental potential, however it is currently unknown whether they display differences in their developmental origins and cell fate-restriction in the DG. Here we demonstrate that Cux2 is a marker for SGZ progenitors and nascent granule cell neurons in the perinatal brain. Cux2 was expressed in the presumptive hippocampal forming region of the embryonic forebrain from E14.5 onwards. At fetal stages, Cux2 was expressed in early-forming Prox1(+) granule cell neurons as well as the SVZ of the DG germinal matrix. In the postnatal brain, Cux2 was expressed in several types of progenitors in the SGZ of the DG, including Nestin/Sox2 double-positive radial glia, Sox2(+) cells that lacked a radial glial process, DCX(+) neuroblasts, and Calretinin-expressing nascent neurons. Another domain characterized by a low level of Cux2 expression emerged in Calbindin(+) neurons of the developing DG blades. We used Cux2-Cre mice in genetic fate-mapping studies and showed almost exclusive labeling of Calbindin-positive gcl neurons, but not in any progenitor cell types or astroglia. This suggests that Cux2(+) progenitors directly differentiate into gcl neurons and do not self-renew. Interestingly, developmental profiling of cell fate revealed an outside-in formation of gcl neurons in the DG, likely reflecting the activity of Cux2 in the germinative matrices during DG formation and maturation. However, DG morphogenesis proceeded largely normally in hypomorphic Cux2 mutants lacking Cux2 expression. Taken together we conclude that Cux2 expression reflects hippocampal neurogenesis and identifies non-self-renewing NPs in the SGZ.