Characterization of the two zebrafish orthologues of the KAL-1 gene underlying X chromosome-linked Kallmann syndrome.

The gene underlying X chromosome-linked Kallmann syndrome, KAL-1, has been identified for several years, yet its role in development is still poorly understood. In order to take advantage of the zebrafish as a model in developmental genetics, we isolated the two KAL-1 orthologues, kal1.1 and kal1.2, in this species. Comparison of deduced protein sequences with the human one shows 75.5 and 66.5% overall homology, respectively. The most conserved domains are the whey acidic protein-like domain and the first of four fibronectin-like type III repeats. However, kal1.2 putative protein lacks the basic C-terminal domain (20 residues) found in kal1.1 and KAL-1. The expressions of kal1.1 and kal1.2 were studied in the embryo between 6 and 96 hours post fertilization using whole-mount in situ hybridization. Although a few structures express both genes, kal1.1 and kal1.2 expression patterns are largely non-overlapping. Taken together, these patterns match fairly well those previously reported for human KAL-1 and chicken kal1. As regards the olfactory system, kal1.1 is expressed, from 37 h.p.f. onward, in the presumptive olfactory bulbs, whereas kal1.2 transcript is only detected, from 48 h.p.f., in the epithelium of the nasal cavity. The relevance of the zebrafish as an animal model for studying both the function of KAL-1 in normal development and the developmental failure leading to the olfactory defect in Kallmann syndrome, is discussed.

Initial characterization of anosmin-1, a putative extracellular matrix protein synthesized by definite neuronal cell populations in the central nervous system.

The KAL gene is responsible for the X-chromosome linked form of Kallmann's syndrome in humans. Upon transfection of CHO cells with a human KAL cDNA, the corresponding encoded protein, KALc, was produced. This protein is N-glycosylated, secreted in the cell culture medium, and is localized at the cell surface. Several lines of evidence indicate that heparan-sulfate chains of proteoglycan(s) are involved in the binding of KALc to the cell membrane. Polyclonal and monoclonal antibodies to the purified KALc were generated. They allowed us to detect and characterize the protein encoded by the KAL gene in the chicken central nervous system at late stages of embryonic development. This protein is synthesized by definite neuronal cell populations including Purkinje cells in the cerebellum, mitral cells in the olfactory bulbs and several subpopulations in the optic tectum and the striatum. The protein, with an approximate molecular mass of 100 kDa, was named anosmin-1 in reference to the deficiency of the sense of smell which characterizes the human disease. Anosmin-1 is likely to be an extracellular matrix component. Since heparin treatment of cell membrane fractions from cerebellum and tectum resulted in the release of the protein, we suggest that one or several heparan-sulfate proteoglycans are involved in the binding of anosmin-1 to the membranes in vivo.