Dystroglycan (DG) is a cell surface adhesion complex, originally
isolated from rabbit skeletal muscle, representing the pivotal element
of a multimeric complex defined as dystrophin-glycoprotein complex
(DGC). In mammals DAG1 possesses an uncomplicated
exon-intron-exon structure, and its transcription and translation
generates a precursor protein that is post-translationally cleaved into
two noncovalently associated subunits: the highly glycosylated
extracellular α-DG and the transmembrane β-DG . The DG subunits are believed to establish a molecular bridge linking the extracellular matrix to the cytoskeleton . In skeletal muscle and in a wide variety of tissues α-DG binds extracellular matrix molecules, such as laminins, agrins and perlecan, and interacts non covalently with β-DG, that binds dystrophin via its cytoplasmic tail .
Several cDNA sequences, which in most cases correspond to a highly
conserved protein product 895 aa long, have been reported in different
organisms such as human, mouse, dog, amphibia and fish DGs. The degree
of sequence identity among mammals is remarkably high (> 90%), while
the recently identified cDNA sequences of X. laevis and D. rerio (zebrafish) confirm that a very high degree of similarity is found also in lower vertebrate species [4,5].
DG is believed to have an increasingly important role in human
health, being involved in pathological processes ranging from cancer
progression to infective diseases .
In particular, in human skeletal muscle DG, as well as several proteins
belonging to the DGC (like dystrophin and sarcoglycans), is involved in
severe forms of muscular diseases . On the other hand, until now there are no reports about muscular diseases directly generated by DAG1 mutations
(primary dystroglycanopathies), not surprisingly since the DG knockout
experiment in mice causes an early arrest of the embryonic development
(at day 6.5), due to the disruption of the Reichert's membrane .
However, in particular muscular diseases, known as congenital
dystrophies (Muscle-Eye-Brain disease, MEB; Fukuyama Congenital
Muscular Dystrophy, FCMD; Walker-Warburg Syndrome, WWS), mutations in
different genes encoding for glycosyltransferases are regarded to
generate an abnormal glycosylation of α-DG [9-11]. This alteration of the glycosylation pattern of α-DG
compromises its binding to extracellular matrix molecules and it is
thought to be the reason for the progressive muscle fibre degeneration;
this kind of human congenital disorders have been defined as "secondary
In the last years, a large body of knowledge originated from
comparative biochemical and physiological studies about dystroglycan
and the dystrophin glycoprotein complex in D. rerio [5,13-15], which showed that DG indeed plays a crucial role for adult skeletal muscle stability . With the aim of carrying out an expanded genetic and biochemical comparative analysis, we examined DAG1 from several fish species; besides D. labrax (sea bass) and D. rerio (zebrafish), we also analysed pufferfish characterized by compact genomes (T. nigroviridis and T. rubripes), other teleosts such as O. latipes (medaka) and G. aculeatus (stickleback), and Antarctic species (T. bernacchii and C. hamatus). So far it was generically assumed that all vertebrate species would share only one copy of DAG1, even if a whole genome duplication (WGD) event, involving a large number of genes, has been described in Actinopterygii .
Although a DG gene duplication event has not been identified in D. rerio ,
our computer mining of genomic data available for pufferfish indicates
that two different DG sequences are present. Accordingly, via the
analysis of DNAs (and cDNAs) from T. nigroviridis, we identified two functional paralogous DG sequences, hereinafter defined as DAG1a and DAG1b. Moreover, for the first time we have cloned and sequenced DAG1 in sea bass (D. labrax),
showing that it contains an additional mini-intronic sequence of about
150 bp, which is properly spliced out upon transcription.