Emerging complexity of the HuD/ELAVl4 gene; implications for neuronal development, function, and dysfunction

Abstract

Precise control of messenger RNA (mRNA) processing and abundance are increasingly being recognized as critical for proper spatiotemporal gene expression, particularly in neurons. These regulatory events are governed by a large number of trans-acting factors found in neurons, most notably RNA-binding proteins (RBPs) and micro-RNAs (miRs), which bind to specific cis-acting elements or structures within mRNAs. Through this binding mechanism, trans-acting factors, particularly RBPs, control all aspects of mRNA metabolism, ranging from altering the transcription rate to mediating mRNA degradation. In this context the best-characterized neuronal RBP, the Hu/ELAVl family member HuD, is emerging as a key component in multiple regulatory processes--including pre-mRNA processing, mRNA stability, and translation--governing the fate of a substantial amount of neuronal mRNAs. Through its ability to regulate mRNA metabolism of diverse groups of functionally similar genes, HuD plays important roles in neuronal development and function. Furthermore, compelling evidence indicates supplementary roles for HuD in neuronal plasticity, in particular, recovery from axonal injury, learning and memory, and multiple neurological diseases. The purpose of this review is to provide a detailed overview of the current knowledge surrounding the expression and roles of HuD in the nervous system. Additionally, we outline the present understanding of the molecular mechanisms presiding over the localization, abundance, and function of HuD in neurons.

Keywords: HuD/ELAVl4; RNA-binding protein; mRNA metabolism; post-transcriptional regulation.

FIGURE 1.

Organization of the mouse HuD gene, mRNA, and protein. (A) Mouse HuD gene organization with noncoding exons depicted as orange rectangles, coding exons as gray and green rectangles, and introns as black lines. (B) Alternative splicing/processing of HuD pre-mRNA produces different HuD isoforms. 5′/3′ UTRs and coding exons are represented by orange and gray rectangles, respectively. Green rectangles denote alternatively spliced exons near the 3′ end of the gene. Also shown are the approximate locations of the Hu/ELAVl (white star), miR-375 (black star), and putative ARE (gray stars) binding sites. (C) The three major HuD protein variants found in neurons with position of RRMs (gray) and amino acid extensions in the linker region (green) shown. “P” and “Me” indicate approximate locations of serine/threonine and arginine residues that are subjected to phosphorylation by PKC and methylation by CARM1, respectively.

FIGURE 2.

Model depicting the intracellular localizations and multiple post-transcriptional functions of HuD. (A) In the nucleus, HuD regulates alternative splicing and polyadenylation by competing for specific pre-mRNA binding sites with other trans-acting factors. (B) HuD forms part of an mRNP complex, and it likely facilitates mRNA export into the cytoplasm. (C) In the cytoplasm, HuD and destabilizing RBPs bind competitively or cooperatively to mRNAs. HuD might also prevent binding of miRs to the mRNA or antagonize their function. Through these mechanisms, HuD increases the half-lives of mRNAs. (D) HuD transports mRNAs to different compartments of neurons, most notably neurites, along microtubules. (E) At the synaptic terminal, HuD may promote or repress translation of transcripts.

FIGURE 3.

Relative levels of RBPs and their cooperative and competitive binding to mRNAs controls gene expression. The relative expression of RBPs that (A,B) promote (e.g., Hu/ELAVl proteins; green) or inhibit (e.g., AUF1; red) mRNA stability/translation influences gene expression. Functionally antagonistic RBPs can also bind (C) competitively or (D,E) cooperatively to regulate mRNA fate. Cooperative binding could potentially allow for a quicker response to extracellular cues in determining mRNA fate.

FIGURE 4.

Simplified model of the molecular events governing HuD expression, processing, and function in neurons. (A) CARM1 methylation of HuD prevents its binding to specific mRNA targets and maintains neuronal precursors in an undifferentiated state. On the other hand, an increase in unmethylated HuD protein levels, its post-translational modification (such as phosphorylation by PKC isoforms), and interaction with other proteins (e.g., AKT1) enable HuD to regulate mRNA metabolism of proneural genes and promote neuronal differentiation/neurite extension. (B) The stability and translation of mature HuD mRNA is negatively controlled by miR-375 and potentially other trans-acting factors. (C) HuD pre-mRNA undergoes alternative splicing, including inclusion of E6 mediated by Hu proteins. (D) During neuronal development transcription of the HuD gene is positively regulated by the Ngn2-E47 heterodimer binding to E-boxes upstream of E1c. Synthesis of HuD mRNA is negatively regulated by thyroid hormone (presumably bound to thyroid hormone receptor) in neurons and FoxO1 in pancreatic β cells. Green and red lines represent pathways that promote and inhibit neuronal differentiation, respectively.