Genetic Analyses Demonstrate That Bone Morphogenetic Protein Signaling Is Required for Embryonic Cerebellar Development

Abstract

The cerebellum has been a useful model for studying many aspects of neural development because of its relatively simple cytoarchitecture and developmental program. Yet, the genetic mechanisms underlying early differentiation and patterning of the cerebellum are still poorly characterized. Cell expression studies and culture experiments have suggested the importance of bone morphogenetic proteins (BMPs) in development of specific populations of cerebellar neurons. Here, we examined mice with targeted mutations in the BMP type I receptor genes Bmpr1a and Bmpr1b, to genetically test the hypothesis that BMPs play an inductive role in the embryogenesis of cerebellar granule cells. In Bmpr1a;Bmpr1b double knock-out mice, severe cerebellar patterning defects are observed resulting in smaller cerebella that are devoid of foliation. In mutants containing either single BMP receptor gene mutation alone, cerebellar histogenesis appears normal, thereby demonstrating functional redundancy of type I BMP receptors during cerebellar development. Loss of BMP signaling in double mutant animals leads to a dramatic reduction in the number of cerebellar granule cells and ectopic location of many of those that remain. Molecular markers of granule cell specification, including Math1 and Zic1, are drastically downregulated. In addition, Purkinje cells are disorganized and ectopically located, but they appear to be correctly specified. Consistent with the interpretation that granule cells alone are affected, phosphorylated Smad1/5/8 is immunolocalized predominantly to granule cell precursors and not appreciably detected in Purkinje cell precursors. This study demonstrates that BMP signaling plays a crucial role in the specification of granule cells during cerebellar development.

TGF-β Family Signaling in Neural and Neuronal Differentiation, Development, and Function

“…Hindbrain roof plate cultures express Bmp6, Bmp7, and Gdf7 and these factors are sufficient to induce cerebellar granule neurons (Alder et al 1999;Machold et al 2007). BMP-4 and BMP-6 stimulate differentiation and promote survival of granule neurons (Angley et al 2003;BarnedaZahonero et al 2009), whereas targeted inactivation of Bmpr1a and Bmpr1b in mice results in fewer cerebellar granule neurons, causing a smaller cerebellar cortex without foliation (Qin et al 2006). Application of noggin to the developing midbrain -hindbrain border results in complete loss or dorsal shift of Phox2-positive neurons.…”

“…Cerebellar granule neurons originate in the rhombic lip and many BMPs, including BMP-6, BMP-7, and GDF-7, are expressed in the midline cells adjacent to the rhombic lip and induce cerebellar granular progenitors in vitro (Alder et al 1999). Conditional Bmpr1a 2/2 ; Bmpr1b 2/2 mutants show a dramatic repression of granule cell specification markers along with a loss in the number of granule neurons, whereas the number of Purkinje cells remains unaltered in vivo (Qin et al 2006). Purkinje and granule cells in the cerebellum express TGF-b2, which regulates the proliferation of cerebellar neurons in vitro (Constam et al 1994;Kane et al 1996).…”

Emerging roles for zic genes in early development

“…During further development, granule cells migrate from the EGL inward to the internal granule layer (IGL), where they continue to express Zic1 (Yokota et al, 1996), but Math1 is downregulated. As a result, cells expressing the differentiation marker Tuj1 also express Zic proteins in mouse cerebella (Qin et al, 2006).…”

Smad6 promotes neuronal differentiation in the intermediate zone of the dorsal neural tube by inhibition of the Wnt/β-catenin pathway

“…In parallel, bone morphogenic protein (BMP) signaling also is important for NPC proliferation and differentiation. Mutation of bone morphogenic protein receptor, type 1a (Bmpr-1a) and type 1b (Bmpr-1b) in mice results in the decreased proliferation of cerebellar progenitor cells (16). Overexpression of constitutively activated Bmpr-1a (caBmpr-1a) promotes NPC proliferation (17).…”