Synaptic Activity-Induced Conversion of Intronic to Exonic Sequence in Homer 1 Immediate Early Gene Expression

Abstract

Three Homer genes regulate the activity of metabotropic glutamate receptors mGluR1a and mGluR5 and their coupling to releasable intracellular Ca2+pools and ion channels. Only the Homer 1 gene evolved bimodal expression of constitutive (Homer 1b and c) and immediate early gene (IEG) products (Homer 1a and Ania 3). The IEG forms compete functionally with the constitutive Homer proteins. The complex expression of the Homer 1 gene, unique for IEGs, focused our attention on the gene organization. In contrast to most IEGs, which have genes that are <5 kb, the Homer 1 gene was found to span ∼100 kb. The constitutive Homer 1b/c forms are encoded by exons 1–10, whereas the IEG forms are encoded by exons 1–5 and parts of intron 5. RNase protection demonstrated a >10-fold activity-dependent increase in mRNA levels exclusively for the IEG forms. Moreover, fluorescentin situhybridization documented that new primary Homer 1 transcripts are induced in neuronal nuclei within a few minutes after seizure, typical of IEGs, and that Homer 1b-specific exons are excluded from the activity-induced transcripts. Thus, at the resting state of the neurons, the entire gene is constitutively transcribed at low levels to yield Homer 1b/c transcripts. Neuronal activity sharply increases the rate of transcription initiation, with most transcripts now ending within the central intron. These coordinate transcriptional events rapidly convert a constitutive gene to an IEG and regulate the expression of functionally different Homer 1 proteins.

Homers regulate drug-induced neuroplasticity: Implications for addiction

“…Examination of the 30 kb intron 5 sequence of Homer1 failed to reveal obvious contributing sequences and no conserved sequences have been reported 5′ or 3′ of the poly(A) sites for these IEG transcripts. Moreover, the human intron 5 of Homer1 did not provide any indication of highly conserved sequence islands that might function in the activitydependent termination of transcripts within this intron [110]. However, the mRNA for IEG Homer1 isoforms does contain several AUUUA repeats at their 3′ UTRs that may be responsible for destabilizing interactions with ribosomal translational machinery.…”

“…Since that time, 21 Homer mRNAs have been isolated from rat, mouse and human brain, however the proteins for some of these mRNAs have yet to be detected in mammalian brain tissue [107][108][109]. The mammalian genes encoding the Homer family of proteins have open reading frames that spread over 10 exons and can give rise to both constitutively expressed, as well as immediate early gene (IEG) products [102,104,110,111]. Exon 1 encodes the 5′ untranslated region (UTR) and contains the translational initiation codon ATG.…”

“…Alternative transcript splicing in regions downstream from exon 5 has been reported for all 3 Homer genes and can result in premature termination of transcription prior to the sequences encoding the CC and leucine zipper motifs [102,105,107,110]. This premature termination of transcription results in truncated or "short" Homer isoforms that lack the motifs necessary to multimerize.…”

“…In neurons, the most characterized and thoroughly investigated is the bimodal expression of Homer1 constitutive (Homer1b-g) and IEG (Homer1a and Ania-3) products [c.f., [145][146][147][148][149]. Within the Homer1 gene lies transcriptional stop codons in intron 5 and sequence comparison between rat cDNA for Homer1a [102,150] and the mouse Homer1 gene established that Homer1a mRNA terminates ~4.4 kb into intron 5 [110]. The intronic sequence extends exon 5 by 33 bases (11 codons), followed by a translational stop codon, and a ~4.4 kb 3′ UTR, colinear with the 5′ portion of intron 5.…”

“…The intronic sequence extends exon 5 by 33 bases (11 codons), followed by a translational stop codon, and a ~4.4 kb 3′ UTR, colinear with the 5′ portion of intron 5. Ania-3, the other known IEG Homer1 isoform [108], is generated by alternative transcript splicing from exon 5 into an intron 5 sequence that lies ~5.7 kb downstream of the poly adenylation (polyA) site for Homer1a mRNA [110]. The precise sequence contribution to the premature transcript termination for Homer1a and Ania-3 is not known.…”