In vitro studies on clonal growth of chondrocytes in thanatophoric dysplasia

From genotype to phenotype: the differential expression of FGF, FGFR, and TGFbeta genes characterizes human cranioskeletal development and reflects clinical presentation in FGFR syndromes

Mutations in the fibroblast growth factor receptor (FGFR) genes 1, 2, and 3 are causal in a number of craniofacial dysostosis syndromes featuring craniosynostosis with basicranial and midfacial deformity. Great clinical variability is displayed in the pathologic phenotypes encountered. To investigate the influence of developmental genetics on clinical diversity in these syndromes, the expression of several genes implicated in their pathology was studied at sequential stages of normal human embryo-fetal cranial base and facial ossification (n = 6). At 8 weeks of gestation, FGFR1, FGFR2, and FGFR3 are equally expressed throughout the predifferentiated mesenchyme of the cranium, the endochondral skull base, and midfacial mesenchyme. Both clinically significant isoforms of FGFR2, IgIIIa/c and IgIIIa/b, are coexpressed in maxillary and basicranial ossification. By 10 to 13 weeks, FGFR1 and FGFR2 are broadly expressed in epithelia, osteogenic, and chondrogenic cell lineages. FGFR3, however, is maximally expressed in dental epithelia and proliferating chondrocytes of the skull base, but poorly expressed in the osteogenic tissues of the midface. FGF2 and FGF4, but not FGF7, and TGFbeta1 and TGFbeta3 are expressed throughout both osteogenic and chondrogenic tissues in early human craniofacial skeletogenesis. Maximal FGFR expression in the skull base proposes a pivotal role for syndromic growth dysplasia at this site. Paucity of FGFR3 expression in human midfacial development correlates with the relatively benign human mutant FGFR3 midfacial phenotypes. The regulation of FGFR expression in human craniofacial skeletogenesis against background excess ligand and selected cofactors may therefore play a profound role in the pathologic craniofacial development of children bearing FGFR mutations.

Structural roles of acetylcholinesterase variants in biology and pathology

Apart from its catalytic function in hydrolyzing acetylcholine, acetylcholinesterase (AChE) affects cell proliferation, differentiation and responses to various insults, including stress. These responses are at least in part specific to the three C-terminal variants of AChE which are produced by alternative splicing of the single ACHE gene. 'Synaptic' AChE-S constitutes the principal multimeric enzyme in brain and muscle; soluble, monomeric 'readthrough' AChE-R appears in embryonic and tumor cells and is induced under psychological, chemical and physical stress; and glypiated dimers of erythrocytic AChE-E associate with red blood cell membranes. We postulate that the homology of AChE to the cell adhesion proteins, gliotactin, glutactin and the neurexins, which have more established functions in nervous system development, is the basis of its morphogenic functions. Competition between AChE variants and their homologs on interactions with the corresponding protein partners would inevitably modify cellular signaling. This can explain why AChE-S exerts process extension from cultured amphibian, avian and mammalian glia and neurons in a manner that is C-terminus-dependent, refractory to several active site inhibitors and, in certain cases, redundant to the function of AChE-like proteins. Structural functions of AChE variants can explain their proliferative and developmental roles in blood, bone, retinal and neuronal cells. Moreover, the association of AChE excess with amyloid plaques in the degenerating human brain and with progressive cognitive and neuromotor deficiencies observed in AChE-transgenic animal models most likely reflects the combined contributions of catalytic and structural roles.

Human osteogenesis involves differentiation-dependent increases in the morphogenically active 3' alternative splicing variant of acetylcholinesterase

The extended human acetylcholinesterase (AChE) promoter contains many binding sites for osteogenic factors, including 1,25-(OH)2 vitamin D3 and 17beta-estradiol. In differentiating osteosarcoma Saos-2 cells, both of these factors enhanced transcription of the AChE mRNA variant 3' terminated with exon 6 (E6-AChE mRNA), which encodes the catalytically and morphogenically active E6-AChE isoform. In contrast, antisense oligodeoxynucleotide suppression of E6-AChE mRNA expression increased Saos-2 proliferation in a dose- and sequence-dependent manner. The antisense mechanism of action was most likely mediated by mRNA destruction or translational arrest, as cytochemical staining revealed reduction in AChE gene expression. In vivo, we found that E6-AChE mRNA levels rose following midgestation in normally differentiating, postproliferative fetal chondrocytes but not in the osteogenically impaired chondrocytes of dwarf fetuses with thanatophoric dysplasia. Taken together, these findings suggest morphogenic involvement of E6-AChE in the proliferation-differentiation balance characteristic of human osteogenesis.