There are six hyaluronidase-like sequences in the human genome (Csóka et al. 2001
) and in the chimpanzee, but seven in murine animals (Kim et al. 2005
). C. elegans
has only one hyaluronidase-like sequence, and that gene codes for a chondroitinase enzyme and not for a protein with hyaluronidase activity (Kaneiwa et al. 2008
). One of the hyaluronidase-like sequences in the vertebrate, HYAL4, has now been demonstrated to code for an enzyme with exclusively chondroitinase activity (Chatel et al. 2010
; Kaneiwa et al. 2010
). Working retrospectively, the following scenario can be postulated. The first hyaluronidase-like sequence was exclusively a chondroitinase, chondroitin being the only GAG present in C. elegans
, and from this chondroitinase, hyaluronidases evolved. There are striking sequence homologies between the following proteins: Human HYAL1, murine Hyal1, HYAL4 and the chondroitinase sequence from C. elegans
, as shown in Figure .
Fig. 4. Alignment of the conceptual translation of the cDNAs of human HYAL1, mouse Hyal1, the human chondroitinase, HYAL4 and the presumed primordial chondroitinase as reflected in the genome of C. elegans. The remarkable conservation of these sequences is apparent, (more ...)
An important proviso must be added, however. These proteins coded for by hyaluronidase-like sequences may have functions other than degradation of GAG polymers. Hyal2 functions additionally as a control for glycocalyx deposition and also interacts with ezrin-radixin-moesin (ERM), major cytoskeletal elements through cluster of determination 44 (CD44) binding (Duterme et al. 2009
). Hyaluronidases may have functions other than and perhaps even more important than their enzymatic activities. They may act as adhesion or as anti-adhesion proteins. Enzymes are blithely unaware and totally indifferent to what we call them. Many proteins have functions in addition to the activities that we measure or that their “names” imply. Caution is indicated, therefore, in hypothetical formulations such as those described here.
At some later time, HA evolved and duplications of the original chondroitinase sequence occurred, in order for HA catabolism to occur. To this day, other hyaluronidases, such as HYAL1, 2 and PH-20, retain residual chondroitinase activity, a reference possibly to the substrate of their ancestral enzyme.
The HYAL4 gene should be the one that resembles most closely the ancestral gene. The current authors attempted to show, using homology search engines and dendrogram analyses, that the C. elegans chondroitinase gene and human HYAL4 bear an ancestral relationship and are more closely related to each other than to any of the other sequences (A.C. and R.S., unpublished observations). However, this exercise proved unsuccessful. Subsequent evolutionary drift may be the basis of this failure.
Following the appearance of six hyaluronidase-like sequences in the zebra fish, a seventh sequence appeared in the mouse genome, with the unfortunate appellation of Hyal5
(Kim et al. 2005
). Subsequent evolutionary progress proceeded to discard this seventh sequence, since it is no longer present in the genome of the great apes or in that of the human (Figure ). This event must have occurred at some point between 65 and 6 Mya.
One of the hyaluronidase-like sequences in the human genome, HYALP1
, is transcribed but not translated. It is a pseudogene because there are at least two stop codons in the first three exons (Csóka et al. 1999
; Zhang et al. 2010
). However, it is translated in most mammals, including primates and has been termed Hyal6. It may be concluded that this is a sequence perhaps on its way to being deleted altogether in humans.
In summary, the mouse has seven hyaluronidase-like sequences and primates have six, while humans have six, with one silent sequence. The mouse sequences are provided in Figure . Alignment of the conceptual translation of the cDNA of all seven mouse hyaluronidase genes was obtained using the CLUSTAL W program (Higgins et al. 1996
; Chenna et al. 2003
). Identical amino acids are boxed, and similar amino acids are shaded. Conserved blocks, representing the regions most critical to enzymatic activity, can be seen throughout. Alignment was performed using the CLUSTAL W sequence alignment program on GenomeNet, a Japanese network of data bases (http:www.genome.jp/tools/clustalw/).
Fig. 5. Alignment of the conceptual translation of the cDNA of all the seven mouse hyaluronidase genes using the CLUSTAL W program (Higgins et al. 1996). Identical amino acids are boxed, and similar amino acids are shaded. Conserved blocks, representing the regions (more ...)
The data suggest that the hyaluronidase-like sequences are continuing to evolve. This appears to be occurring at a rapid rate on an evolutionary time scale. It would be interesting to establish what forces drive such evolution. How are evolution, survival, gene silencing, genetic adaptation and efficiency related to each other on a mechanistic level?
A phylogenetic tree with branch lengths for 34 hyaluronidase-like sequences from nine species (C. elegans, hydra, sea squirt, sea urchin, xenopus, zebra, fish, chicken, mouse and human) is shown in Figure . The C. elegans chondroitinase is at the base of the tree.
Fig. 6. Unrooted phylogenetic tree with branch lengths for 34 Hyal sequences from nine species (C. elegans, hydra, sea squirt, sea urchin, xenopus, zebrafish, chicken, mouse and human). Alignment was first performed using CLUSTALW on the GenomeNet server (see (more ...)