Common Partner Smad-Independent Canonical Bone Morphogenetic Protein Signaling in the Specification Process of the Anterior Rhombic Lip during Cerebellum Development

Abstract

Bone morphogenetic protein (BMP) signaling is critical for cerebellum development. However, the details of receptor regulated-Smad (R-Smad) and common partner Smad (Co-Smad, or Smad4) involvement are unclear. Here, we report that cerebellum-specific double conditional inactivation of Smad1 and Smad5 (Smad1/5) results in cerebellar hypoplasia, reduced granule cell numbers, and disorganized Purkinje neuron migration during embryonic development. However, single conditional inactivation of either Smad1 or Smad5 did not result in cerebellar abnormalities. Surprisingly, conditional inactivation of Smad4, which is considered to be the central mediator of canonical BMP-Smad signaling, resulted only in very mild cerebellar defects. Conditional inactivation of Smad1/5 led to developmental defects in the anterior rhombic lip (ARL), as shown by reduced cell proliferation and loss of Pax6 and Atoh1 expression. These defects subsequently caused the loss of the nuclear transitory zone and a region of the deep cerebellar nuclei. The normal maturation of the remaining granule cell precursors in the external granular layer (EGL) suggests Smad1/5 signaling is required for the specification process in ARL but not for the subsequent EGL development. Our results demonstrate functional redundancy for Smad1 and Smad5 but functional discrepancy between Smad1/5 and Smad4 during cerebellum development.

ACKNOWLEDGMENTS

We thank Richard R. Behringer (University of Texas M. D. Anderson Cancer Center, Houston, TX) for providing the Smad1/5 conditional mouse line, helpful advice, and comments on the manuscript; Elizabeth J. Robertson (University of Oxford, Oxford, United Kingdom) for providing the Smad4 conditional mutant mouse line; Jane E. Johnson (University of Texas Southwestern Medical Center, Dallas, TX) for providing the Math1 probe; and Sze-Nee Lim, Yuk-Lau Wong, and Wang-Chi Lau for technical assistance.

This research was supported by the General Research Fund of the Research Grants Council of Hong Kong Special Administrative Region, China (project no. CUHK 466708) and Focused Investments Scheme, Scheme B, of the Chinese University of Hong Kong (BL08650).