Claudio Basilico, MD

Professor and Chair of Microbiology
M.D. 1960 University of Milan

New York University School of Medicine
550 First Avenue, MSB 256
New York, NY, 10016
Tel: (212) 263-5341

Research Theme(s): Adult stem cells, cancer stem cells
Keywords: Cell-cycle, Growth Factors, Oncogenes, Signal Transduction, Bone Development, Cancer Stem Cells

Research Summary:

Signals Controlling Cell Proliferation and differentiation.

The major interest of my laboratory is the control of proliferation in normal and cancer cells and the genes and gene-products whose interplay regulates proliferation and differentiation.

To understand how growth factor signaling promotes cell proliferation and differentiation, we are studying the mechanism of action and the regulation of expression of fibroblast growth factors (FGF). FGF represents a large family of growth factors which signal through their interaction with tyrosine kinase receptors (FGFR) which also make-up a gene family. FGF signaling plays a major role in a variety of developmental processes. Ectopic or excessive FGF expression can lead to oncogenesis. The main projects presently being carried out focus on the regulation of skeletal development by FGF signaling. Unregulated FGF signaling, due to FGFR activating mutations, causes a variety of dominant bone morphogenetic disorders in humans, including several forms of dwarfism and craniosynostosis syndromes. Excessive FGF signaling alters bone development by affecting the dynamics of growth and differentiation of chondrocytes and osteoblasts, the two major cell types involved in bone formation. We are studying the biological response of these two cell types to FGF and the key pathways involved in this process.

In chondrocytes, we found that FGF signaling inhibits proliferation and increases apoptosis both in vitro and in vivo. These effects are cell type-specific since FGFs induce proliferation in most other cell types, and provide a logical explanation of why excessive FGF signaling causes dwarfism and chondrodysplastic syndromes. We aim at identifying the key factors that direct the FGF response of chondrocytes to growth inhibitory pathways. FGFs induce in chondrocytes a complex network of signaling and transcriptional events which ultimately result in growth arrest and induction of several aspects of chondrocytes hypertrophic differentiation. We have shown that FGF-induced growth arrest requires the activity of two Retinoblastoma (Rb) family members, p107 and p130, but not Rb itself. One of the earliest distinguishing events following FGF treatment is the very rapid dephosphorylation of p107. We have shown that p107 dephosphorylation is a critical early event in the growth-inhibitory response of these cells to FGF signaling and that the PP2A phosphatase targets p107 for dephosphorylation in FGF-treated chondrocytes. Our studies are presently focused on the mechanisms which “activate” PP2A to target p107 upon FGF treatment. We have recently identified the regulatory subunit of PP2A that targets this enzyme to p107.

Immature osteoblasts respond to FGF with increased proliferation, while differentiating cells undergo apoptosis. Sustained FGF signaling inhibits differentiation. We have examined the program of gene expression in osteoblasts expressing activated FGFR mutants and detected a significant down-regulation of Wnt target gene expression. Concomitantly, we have observed a dramatic induction of expression of Sox2, a transcription factor of the HMG domain family, whose expression is a classical marker of embryonic stem cells. Sox2 is also induced by FGF treatment of normal osteoblasts and is clearly detectable in cranial osteoblasts in vivo. Wnt signaling promotes osteoblast function and high bone mass in humans and mice and thus inhibition of Wnt signaling is likely to be one important mechanism by which FGFs inhibit osteoblast differentiation. Our results showed that FGF utilizes multiple mechanisms to inhibit Wnt-induced transcription in osteoblasts and that Sox2 induction plays a major role since this protein can bind to β-catenin and inhibits its transcriptional activity. Indeed Sox2 overexpression can by itself inhibit osteoblast differentiation.

While investigating the role of Sox2 induction in the osteoblast response to FGF we have made the exciting discovery that Sox2 is required for self-renewal of the osteoblast lineage. Inactivation of Sox2 in cultured osteoblasts abolish proliferation capacity and cause these cells to enter a senescent-like phenotype. Conditional KO of Sox2 in the osteoblast lineage in mice produces animals which are small, osteopenic and have low bone density. Sox2 is highly expressed in osteospheres, thought to represent multipotent or unipotent stem cells in the osteoblast lineage. Sox2 maintains a proliferative stem-like state in osteoblasts by activating transcription of critical target “stemness” genes and by inhibiting the activity of the prodifferentiation Wnt pathway, using both transcriptional and post-transcriptional mechanisms. Importantly, Sox2 plays a similar role in osteosarcomas, where Sox2 downregulation abolishes tumorigenicity, promotes osteogenic differentiation and activate the Wnt pathway. This suggests that in these tumors, high Sox2 expression sustains a population of cancer stem cells. 

To better define the mechanisms by which FGF signaling controls osteoblast proliferation and differentiation we have also examined a mouse model of craniosynostosis induced by the Apert activating mutation in FGFR2, as well as mice transgenic for Sox2. Our studies suggest that the major determinant of FGFR2 induced craniosynostosis is the failure of osteoprogenitor cells with activated FGFR2 to respond to signals that would halt their recruitment/advancement at the sites where sutures should normally form, and that Sox2 overexpression inhibits osteoblast maturation in vivo.

Selected Publications: