Function of quaking in myelination: Regulation of alternative splicing

Abstract

Proteomic diversity is frequently achieved by alternative RNAsplicing events that can be fine-tuned in tissue-specific and developmentally regulated ways. Understanding this type of genetic regulation is compelling because of the extensive complexity of alternative splicing found in the nervous system. quaking (qk), one of the classical mouse dysmyelination mutants, is defective for the expression of myelin-associated glycoprotein (MAG), and the misregulation of MAG pre-mRNA alternative splicing is implicated as a causal factor. The qk locus encodes several RNA-binding proteins with heterogeneous nuclear ribonucleoprotein K-type homology, a characteristic of several known alternative splicing regulators. Here we test the nuclear-localized qk isoform (QKI-5) for its ability to regulate alternative splicing of MAG pre-mRNA in transient coexpression assays. QKI-5 exhibits properties of a negative regulator of MAG exon 12 alternative splicing. An intronic sequence element required for the repressive function and binding of QKI-5 is also identified. Direct evidence for irregularities in alternative splicing of MAG and other myelin protein transcripts in the qk mouse is demonstrated.A lternative splicing is a powerful way to regulate gene expression at the posttranscriptional level and to generate protein diversity (1, 2). Understanding the molecular basis of such mechanisms is an urgent challenge, because the number of protein-coding genes in humans has been estimated at barely double the number in Drosophila and Caenorhabditis elegans. Between 35% and 59% of human genes are predicted to generate alternatively spliced mRNA isoforms (3).Mouse quaking (qk), also known as quaking viable (qk v ), is a classical recessive dysmyelination mutation (4). The qk locus produces a diverse set of proteins by alternative splicing (5, 6). They all contain a single heterogeneous nuclear ribonucleoprotein K homology (KH) RNA-binding domain and belong to the evolutionarily conserved signal transduction and activator of RNA (STAR) family (7,8). The first three studied in detail are constructed with the same 311-aa body, but have different carboxyl tails. QKI-5 is the only nuclear isoform and shuttles between the nucleus and cytoplasm (9, 10). The expression of QKI isoforms is developmentally regulated, with QKI-5 being highly expressed throughout the embryogenesis and neonatal stages and decreasing gradually thereafter (7, 9). In postnatal day 14 (P14) mutant mice QKI proteins are decreased exclusively in myelin-forming cells. In addition, the QKI-5 expression level in qk v brain correlates with the severity of dysmyelinating phenotype, suggesting a function of QKI-5 in myelination (9).The relationship between the decreased QKI protein in affected mice and their myelination defects is not understood. It has been shown that several myelin-specific genes are alternatively spliced, including myelin basic protein (MBP), proteolipid protein (PLP), and MAG (11-13). Some splicing events seem to be abnormal in qk v mice. The best-documented c...

Altered expression of genes involved in ATP biosynthesis and GABAergic neurotransmission in the ventral prefrontal cortex of suicides with and without major depression

“…The QKI gene product appears to function as an RNA-binding protein, interacting through KH domains with numerous transcripts controlling oligodendrocyte fate specification and myelin component metabolism. 38 Following on a recent description of genetic association at the QKI locus, 39 two studies have gone on to document significant and widespread reductions in QKI expression (both global and transcript specific) in schizophrenia patients compared with controls and in regions overlapping with those studied here (BA44 and BA46). 40,41 This raises the intriguing possibility that a deficit in QKI common to multiple psychiatric disorders may lead to aberrant oligodendroglial development.…”

Deletion of the Parkin coregulated gene causes male sterility in the quaking viable mouse mutant

“…The qk gene, which encodes for RNA binding proteins involved in posttranscriptional mRNA regulation (6,7), is in close proximity to the proximal deletion breakpoint of qk v (8). The qk v deficiency affects the region upstream of the qk gene to reduce the expression of qk mRNAs in oligodendrocytes, resulting in the CNS myelination defect (9,10).…”

Human QKI, a potential regulator of mRNA expression of human oligodendrocyte-related genes involved in schizophrenia

“…The Qki gene contains an RNA-binding domain (KH domain) that binds directly to cellular RNA (8). Several different sequence motifs have been identified as targets for the Qki protein (9)(10)(11)(12). One of these Qki response elements (QREs) consists of a bipartite consensus sequence that has been shown to bind the Qki protein in vivo in at least 23 different mouse mRNA species (9).…”

“…One of these Qki response elements (QREs) consists of a bipartite consensus sequence that has been shown to bind the Qki protein in vivo in at least 23 different mouse mRNA species (9). This mouse protein directly regulates the expression of at least one myelin-specific gene, e.g., myelin basic protein (Mbp) (1), and controls the alternative splicing of the myelin-associated glycoprotein (Mag) (12). In addition, Qki protein regulates the cell cycle inhibitor cyclin-dependent kinase inhibitor 1B (Cdkn1b) (10), which is involved in the terminal differentiation of oligodendrocytes (13).…”