Glycosaminoglycans (GAGs) are complex carbohydrates that are ubiquitously present on the cell surface and in the extracellular matrix. Interactions between GAGs and pathogens represent the first line of contact between pathogen and host cell and are crucial to a pathogen's invasive potential. Their complexity and structural diversity allow GAGs to control a wide array of biological interactions influencing many physiological and pathological processes, including adhesion, cell-to-cell communication, biochemical cascades, and the immune response. In recent years, increasing evidence indicates an extraordinary role for GAGs in the pathogenesis of viruses, bacteria and parasites. Herein, we examine the interface between GAGs and different pathogens, and address the divergent biological functions of GAGs in infectious disease. We consider approaches to use this understanding to design novel therapeutic strategies addressing new challenges in the treatment of infectious diseases.

The translation initiator-tRNA plays a crucial role in the initiation of protein synthesis in both prokaryotic and eukaryotic cells, by employing specific base pairing between its anticodon triplet CAU and the general initiation codon AUG in the mRNA. Here we show that the initiator-tRNA may also act, in a manner that is independent of its role in protein translation, as a pre-mRNA splicing regulator. Specifically, we show that alternative splicing events that are induced by mutations in the translation initiation AUG codon can be suppressed by expressing initiator-tRNA constructs carrying anticodon mutations that compensate for the AUG mutations. These mutated initiator-tRNAs appeared to be uncharged with an amino acid. Our results imply that recognition of the initiation AUG sequence by the anticodon triplet of initiator-tRNA in its unloaded state plays a role in quality control of splicing in the cell nucleus by a yet unresolved mechanism. Identifying the initiatortRNA as a transacting splicing regulator suggests a novel involvement of this molecule in splicing regulation and provides a critical step toward deciphering this intriguing mechanism.5′ splice site | suppression of splicing | alternative splicing | premature termination codon | spliceosome A key step in both constitutive and alternative pre-mRNA splicing involves the recognition and selection of a consensus sequence that defines the 5′ splice site (5′SS) (1). However, a genomic survey of 446 genes revealed that sequences that comply with the 5′SS consensus, but are not used for splicing (hence termed latent 5′SSs), are highly abundant within introns (2). More than 95% of these latent 5′SSs are preceded by an in-frame stop codon, whose abundance is significantly greater than that expected by chance alone (2, 3). These findings invoked an RNA proofreading mechanism proposed to prevent the use of latent 5′ SSs that otherwise would generate transcripts with premature termination codons (PTCs) (4).The existence of such a putative stop codon-mediated suppression of splicing (SOS) mechanism was first suggested by examining the splicing patterns of transcripts expressed from the constitutive gene encoding the multifunctional protein CAD (carbamoyl-phosphate synthetase, aspartate transcarbamylase, dihydroorotase) in Syrian hamster cells subjected to heat shock stress. Under these conditions, an intronic latent 5′SS, which conformed to the 5′SS consensus better than the upstream authentic one, was activated by the heat shock treatment, whereas under normal growth conditions splicing at that site could not be detected (5). It was also noted that the sequence flanked by the latent 5′SS and the upstream authentic 5′SS contained four stop codons in the CAD protein reading frame. This finding suggested that suppression of splicing at the latent 5′SS could be attributed to the occurrence of these in-frame stop codons. An experimental test of SOS was then carried out using CAD minigene constructs in which all or part of the four in-frame stop codons that reside in ...

Sequences that conform to the 5′ splice site (5′SS) consensus are highly abundant in mammalian introns. Most of these sequences are preceded by at least one in-frame stop codon; thus, their use for splicing would result in pre-maturely terminated aberrant mRNAs. In normally grown cells, such intronic 5′SSs appear not to be selected for splicing. However, under heat shock conditions aberrant splicing involving such latent 5′SSs occurred in a number of specific gene transcripts. Using a splicing-sensitive microarray, we show here that stress-induced (e.g. heat shock) activation of latent splicing is widespread across the human transcriptome, thus highlighting the possibility that latent splicing may underlie certain diseases. Consistent with this notion, our analyses of data from the Gene Expression Omnibus (GEO) revealed widespread activation of latent splicing in cells grown under hypoxia and in certain cancers such as breast cancer and gliomas. These changes were found in thousands of transcripts representing a wide variety of functional groups; among them are genes involved in cell proliferation and differentiation. The GEO analysis also revealed a set of gene transcripts in oligodendroglioma, in which the level of activation of latent splicing increased with the severity of the disease.