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

Cerebellar granule cell migration and folia development require Mllt11/Af1q/Tcf7c

The organization of neurons into distinct layers, known as lamination, is a common feature of the nervous system. This process, which arises from the direct coupling of neurogenesis and neuronal migration, plays a crucial role in the development of the cerebellum, a structure exhibiting a distinct folding cytoarchitecture with cells arranged in discrete layers. Disruptions to neuronal migration can lead to various neurodevelopmental disorders, highlighting the significance of understanding the molecular regulation of lamination. We report a role Mllt11/Af1q/Tcf7c (myeloid/lymphoid or mixed-lineage leukemia; translocated to chromosome 11/All1 fused gene from chromosome 1q, also known as Mllt11 transcriptional cofactor 7; henceforth referred to Mllt11) in the migration of cerebellar granule cells (GCs). We now show that Mllt11 plays a role in both the tangential and radial migration of GCs. Loss of Mllt11 led to an accumulation of GC precursors in the rhombic lip region and a reduction in the number of GCs successfully populating developing folia. Consequently, this results in smaller folia and an overall reduction in cerebellar size. Furthermore, analysis of the anchoring centers reveals disruptions in the perinatal folia cytoarchitecture, including alterations in the Bergmann glia fiber orientation and reduced infolding of the Purkinje cell plate. Lastly, we demonstrate that Mllt11 interacts with non-muscle myosin IIB (NMIIB) and Mllt11 loss-reduced NMIIB expression. We propose that the dysregulation of NMIIB underlies altered GC migratory behavior. Taken together, the findings reported herein demonstrate a role for Mllt11 in regulating neuronal migration within the developing cerebellum, which is necessary for its proper neuroanatomical organization.

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.

Lmx1a drives Cux2 expression in the cortical hem through activation of a conserved intronic enhancer

During neocortical development, neurons are produced by a diverse pool of neural progenitors. A subset of progenitors express the Cux2 gene and are fate restricted to produce certain neuronal subtypes; however, the upstream pathways that specify these progenitor fates remain unknown. To uncover the transcriptional networks that regulate Cux2 expression in the forebrain, we characterized a conserved Cux2 enhancer that recapitulates Cux2 expression specifically in the cortical hem. Using a bioinformatic approach, we identified putative transcription factor (TF)-binding sites for cortical hem-patterning TFs. We found that the homeobox TF Lmx1a can activate the Cux2 enhancer in vitro Furthermore, we showed that Lmx1a-binding sites were required for enhancer activity in the cortical hem in vivo Mis-expression of Lmx1a in hippocampal progenitors caused an increase in Cux2 enhancer activity outside the cortical hem. Finally, we compared several human enhancers with cortical hem-restricted activity and found that recurrent Lmx1a-binding sites are a top shared feature. Uncovering the network of TFs involved in regulating Cux2 expression will increase our understanding of the mechanisms pivotal in establishing Cux2 lineage fates in the developing forebrain.

Recent advances in understanding the mechanisms of cerebellar granule cell development and function and their contribution to behavior

The cerebellum is the focus of an emergent series of debates because its circuitry is now thought to encode an unexpected level of functional diversity. The flexibility that is built into the cerebellar circuit allows it to participate not only in motor behaviors involving coordination, learning, and balance but also in non-motor behaviors such as cognition, emotion, and spatial navigation. In accordance with the cerebellum’s diverse functional roles, when these circuits are altered because of disease or injury, the behavioral outcomes range from neurological conditions such as ataxia, dystonia, and tremor to neuropsychiatric conditions, including autism spectrum disorders, schizophrenia, and attention-deficit/hyperactivity disorder. Two major questions arise: what types of cells mediate these normal and abnormal processes, and how might they accomplish these seemingly disparate functions? The tiny but numerous cerebellar granule cells may hold answers to these questions. Here, we discuss recent advances in understanding how the granule cell lineage arises in the embryo and how a stem cell niche that replenishes granule cells influences wiring when the postnatal cerebellum is injured. We discuss how precisely coordinated developmental programs, gene expression patterns, and epigenetic mechanisms determine the formation of synapses that integrate multi-modal inputs onto single granule cells. These data lead us to consider how granule cell synaptic heterogeneity promotes sensorimotor and non-sensorimotor signals in behaving animals. We discuss evidence that granule cells use ultrafast neurotransmission that can operate at kilohertz frequencies. Together, these data inspire an emerging view for how granule cells contribute to the shaping of complex animal behaviors.

The crux of Cux genes in neuronal function and plasticity

Cux1 and Cux2 are the vertebrate members of a family of homeodomain transcription factors (TF) containing Cut repeat DNA-binding sequences. Perturbation of their expression has been implicated in a wide variety of diseases and disorders, ranging from cancer to autism spectrum disorder (ASD). Within the nervous system, both genes are expressed during neurogenesis and in specific neuronal subpopulations. Their role during development and circuit specification is discussed here, with a particular focus on the cortex where their restricted expression in pyramidal neurons of the upper layers appears to be responsible for many of the specialized functions of these cells, and where their functions have been extensively investigated. Finally, we discuss how Cux TF represent a promising avenue for manipulating neuronal function and for reprogramming.