Microexons are small sized (≤51 bp) exons which undergo extensive alternative splicing in neurons, microglia, embryonic stem cells, and cancer cells, giving rise to cell type specific protein isoforms. Due to their small sizes, microexons provide a unique challenge for the splicing machinery. They frequently lack exon splicer enhancers/repressors and require specialized neighboring trans-regulatory and cis-regulatory elements bound by RNA binding proteins (RBPs) for their inclusion. The functional consequences of including microexons within mRNAs have been extensively documented in the central nervous system (CNS) and aberrations in their inclusion have been observed to lead to abnormal processes. Despite the increasing evidence for microexons impacting cellular physiology within CNS, mechanistic details illustrating their functional importance in diseases of the CNS is still limited.
PTBP, known as hnRNPI, binds the polypyrimidine-rich region (U/CUCUCU) within introns and affects neuronal AS (Gil et al., 1991; Patton et al., 1991; Zheng, 2020). PTBPs has been extensively shown to be involved in AS of microexons in neurons during different contexts (Black, 1992; Chan & Black, 1995; Markovtsov et al., 2000). Neural progenitor cells abundantly express PTBP1 and during neurogenesis the expression of PTBP1 decreases, while the expression of its paralog PTBP2 increases (Chan & Black, 1997; Makeyev et al., 2007; Spellman et al., 2007). As a result, the synchronization of PTBP paralogs is critical for neuronal development and the switching of neuronal programs. The CLIP and RNA-seq data reveal that PTBP1 regulates microexon AS by binding upstream of the microexon (Y. I. Li et al., 2015). In the Neuro2A mouse neuroblastoma cell line, PTBP1-depletion caused microexon inclusion (~94%), whereas only 8% showed exclusion, inferring that PTBP1 is a repressor of microexon inclusion (Y. I. Li et al., 2015). This is in line with previous work from Black’s lab, where PTBP1 represses the N1 microexon inclusion of c-src mRNA in non-neuronal cells (Black, 1992; Chan & Black, 1997; Min et al., 1995). Likewise, PTBP1-depletion caused microexon skipping within the eIF4G transcript in neurons (Gonatopoulos-Pournatzis et al., 2020). Another study demonstrated that in the neural progenitor cell, microexon 5 of BAK1 transcript is skipped and this is promoted by the PTBP1 binding to the intronic region proximity to the 3′ splice site of the microexon. However, as neural progenitor cells differentiate to neurons, PTBP1 expression decreases, allowing the microexon to be included in the BAK1 transcript triggering the loss of BAK1 protein and enhancing neuronal survival (Lin et al., 2020). Therefore, PTBP1 is a microexon AS regulator playing crucial roles in neurons.
The physiological consequences or the causality of mis-spliced microexons has not been functionally examined. Possible ways to solve such a conundrum include performing gene editing with e.g., CRISPR/Cas9 to precisely remove individual microexons or flanking RNA elements and examine the functional outcomes (Du et al., 2020; Yuan et al., 2018). This approach will improve our comprehension of different small GTPase protein isoforms in regulating cellular physiology and CNS function. The observation of microexons AS in autism spectrum disorders is the beginning of mining these splicing events, especially of small GTPase regulators in CNS disorders at large, and determining whether microexon AS defects are a common feature of other disorders.
Source: PMID 34155820
子夜星河 