Siah2 integrates mitogenic and extracellular matrix signals linking neuronal progenitor ciliogenesis with germinal zone occupancy

Abstract

Evidence is lacking as to how developing neurons integrate mitogenic signals with microenvironment cues to control proliferation and differentiation. We determine that the Siah2 E3 ubiquitin ligase functions in a coincidence detection circuit linking responses to the Shh mitogen and the extracellular matrix to control cerebellar granule neurons (CGN) GZ occupancy. We show that Shh signaling maintains Siah2 expression in CGN progenitors (GNPs) in a Ras/Mapk-dependent manner. Siah2 supports ciliogenesis in a feed-forward fashion by restraining cilium disassembly. Efforts to identify sources of the Ras/Mapk signaling led us to discover that GNPs respond to laminin, but not vitronectin, in the GZ microenvironment via integrin β1 receptors, which engages the Ras/Mapk cascade with Shh, and that this niche interaction is essential for promoting GNP ciliogenesis. As GNPs leave the GZ, differentiation is driven by changing extracellular cues that diminish Siah2-activity leading to primary cilia shortening and attenuation of the mitogenic response.

The developmental stage of the medulloblastoma cell-of-origin restricts Hedgehog pathway usage and drug sensitivity

“…As development progresses, the number of ciliated cells in the EGL and later also IGL, increases (Figure 4B-D). We noticed that at neonatal stages, ciliated CGNPs were more abundant towards the outer EGL in agreement with earlier studies [45,46]. However, ciliated CGNPs were most frequent in the IGL, and these cilia were also significantly longer (Figure 4B-D).…”

“…In particular, we found that primary cilium expression was low across young CGNPs and patients, which was somewhat unexpected given the importance of primary cilia for Shh pathway activity [37][38][39][40][41][42][43][44][45][46]. In line, early embryonic CGNPs displayed a partial unresponsiveness to Smo-mediated (i.e., upstream) pathway stimulation, preventing increased proliferation.…”

Neuronal Polarity Pathways as Central Integrators of Cell-Extrinsic Information During Interactions of Neural Progenitors With Germinal Niches

“…GNPs have been shown definitively to be the cell type of origin for Shh-class medulloblastomas ( Goodrich et al, 1997 ; Kim et al, 2003 ; Oliver et al, 2005 ; Rohatgi et al, 2007 ; Yang et al, 2008 ). These tumors express elevated levels of Zeb1 and Siah2 and low levels of Pard complex ( Singh et al, 2016 ; Ong et al, 2020 ), consistent with the polarity trajectories described for mouse GNPs. Genetic deletion of the Patched1 Shh receptor, which stimulates Shh medulloblastoma formation in humans, creates a cohort of GNPs that do not leave the EGL GZ in mouse models of Shh medulloblastoma.…”

“…Recent studies have expanded our knowledge of the role of the Pard complex in mediating key integrative steps in the response of GNPs and CGNs to niche conditions to organize cell biological pathways responsible for differentiation. Ong et al (2020) used sophisticated imaging technologies to reveal how the Pard complex participates in a coincidence detection circuit between the pial ECM and Shh signaling at the level of GNP ciliogenesis ( Figure 1 ). Pioneering studies by Mueller and colleagues showed that Shh signaling required beta 1 integrin receptors to modulate GNP proliferation effectively; however, the mechanism by which this occurred was unclear ( Blaess et al, 2004 ).…”

“…Complex epistasis experiments involving Siah2, Ras, and integrin receptors revealed the molecular basis for this difference in ciliation. Siah2 regulates GNP Shh responsiveness in a feed-forward fashion by maintaining GNP primary cilia in an integrin-dependent and Ras-dependent manner ( Ong et al, 2020 ). Analysis of Siah2 ubiquitination targets defined a novel role for the Pard complex in promoting GNP differentiation.…”