EPLIN is a cytoskeleton-associated protein that inhibits actin filament depolymerization and cross-links filaments in bundles (Maul et al., 2003).

By representational difference analysis of normal versus transformed oral keratinocytes, followed by screening a HeLa cell cDNA library, Maul and Chang (1999) cloned 2 splice variants of EPLIN, which they designated EPLIN-alpha and EPLIN-beta. EPLIN-alpha encodes a deduced 600-amino acid protein, and EPLIN-beta encodes a deduced 759-amino acid protein. Both proteins contain a centrally located LIM domain that is distantly related to the LIM domain of muscle LIM protein (600824). Northern blot analysis detected the 2 EPLIN transcripts at about 3.8 kb. Highest expression was in placenta, followed by kidney, pancreas, prostate, ovary, spleen, and heart; low expression was detected in all other tissues. A transcript of about 8 kb was also observed in some tissues. Western blot analysis detected a major EPLIN-alpha protein at an apparent molecular mass of 90 kD and a minor EPLIN-beta protein at about 110 kD in primary mammary, prostate, and oral epithelial cells. Low levels of EPLIN protein were detected in primary aortic endothelial cells and dermal fibroblasts, but not in myocardium. The presence of transcript but not protein in myocardium suggested that EPLIN expression is tightly regulated. Immunofluorescence analysis detected both EPLIN isoforms localized to filamentous actin.

In mouse, Zhang et al. (2018) found that LIMA1 is highly expressed in small intestine, including duodenum, jejunum, and ileum, and is mainly localized to the brush border membrane. It is modestly expressed in the liver and was minimally detectable in the heart, spleen, lung, brain, and pancreas.

Maul and Chang (1999) found that EPLIN-alpha was either downregulated or lost in 8 of 8 oral cancer cell lines, 4 of 4 prostate cancer cell lines, 3 of 3 xenograft tumors, and 5 of 6 breast cancer cell lines tested. Using an inducible promoter to overexpress EPLIN-alpha and EPLIN-beta in the U2-OS osteosarcoma cell line, they found that induction of either isoform altered the morphology of the U2-OS cells from round polygonal cells with a cobblestone appearance to larger fusiform cells with spindle cell features and cytoplasmic extensions. EPLIN overexpression suppressed cell proliferation and increased the time required for trypsinization, suggesting a change in the cell-matrix interaction.

Chen et al. (2000) found that endogenous transcription from the EPLIN-alpha promoter was stimulated by serum, and transcription was enhanced by activation of RHOA (165390). Transcription from the EPLIN-beta promoter was unaffected by serum, indicating that expression of the 2 isoforms can be independently regulated.

Maul et al. (2003) found that overexpression of either EPLIN isoform in a breast carcinoma cell line increased the number of actin stress fibers. Using pull-down assays, they established that EPLIN-alpha bound both actin monomers and purified actin filaments. Using truncation mutants, they determined that EPLIN-alpha contains at least 2 actin-binding sites, 1 on each side of the central LIM domain. The stoichiometry of binding showed that 2 actin molecules bind 1 molecule of EPLIN-alpha. Using fluorescence-labeled actin to monitor polymerization, they found that EPLIN-alpha did not affect polymerization, but slowed the rate of depolymerization in a concentration-dependent manner. EPLIN-alpha did not cap barbed actin ends, but inhibited branching nucleation of actin filaments by Arp2/3 (604221). Maul et al. (2003) hypothesized that EPLIN-alpha binding to the sides of actin filaments might prevent secondary activation of nucleation mediated by Arp2/3, delaying nucleation until polymerization saturates the filament-binding capacity of EPLIN.

Zhang et al. (2018) showed that LIMA1 interacts with NPC1L1 (608010) and myosin Vb (MYO5B; 606540), both required for efficient intestinal cholesterol absorption. In the mouse small intestine, Lima1 was mainly present in the brush border membrane and colocalized with Npc1l1 and myosin Vb. Zhang et al. (2018) found that LIMA1 regulates NPC1L1 trafficking by recruiting myosin Vb to NPC1L1, and that the interaction of LIMA1 and NPC1L1 is required for intestinal cholesterol absorption.

By affinity purification and mass spectrometry, Goncalves et al. (2020) showed that LUZP1 (601422) interacted with EPLIN, with the interaction mediated by the C-terminal region of LUZP1. Pull-down analysis showed that both proteins also interacted with actin. LUZP1 and EPLIN colocalized at a subset of actin filaments, but they accumulated at distinct actin structures, as EPLIN accumulated at the leading edge of the cell, localizing at actin ruffles, and LUZP1 was predominantly at the opposite end of the cell. Knockdown analysis in RPE-1 cells revealed that both LUZP1 and EPLIN were negative regulators of ciliogenesis, as knockdown of LUZP1 and EPLIN caused increased ciliation and ciliary lengthening. Like EPLIN, LUZP1 was an actin-stabilizing protein that regulated actin dynamics, at least in part, by mobilizing ARP2 to the centrosomes. Both LUZP1 and EPLIN interacted with ciliogenesis and cilia-length regulators and as such were involved in actin-dependent centrosome-to-basal body conversion.

Chen et al. (2000) determined that the EPLIN gene contains 11 exons and spans more than 100 kb. EPLIN-beta mRNA requires all 11 exons, and EPLIN-alpha mRNA requires exons 4 through 11. ATG initiation codons for EPLIN-beta and EPLIN-alpha are located in exons 2 and 4, respectively, and both transcripts share the same TGA stop codon. The EPLIN-alpha promoter contains a serum response element (SRE), but no TATA box. The EPLIN-beta promoter has a high GC content and contains several Sp1 (189906) consensus sites, but no TATA boxes.

By genomic sequence analysis, Chen et al. (2000) mapped the LIMA1 gene to chromosome 12q13.

To investigate the function of LIMA1, Zhang et al. (2018) generated mice in which Lima1 was specifically depleted from mouse intestine, in which it is highly expressed, without affecting the level of Npc1l1, a key transmembrane protein that facilitates intestinal cholesterol uptake. Intestine-specific (I-Lima1) null mice appeared normal without obvious morphologic changes in the small intestine. There was significantly lower cholesterol uptake in I-Lima1 heterozygous mice and null mice (35.5% for heterozygotes and 28.6% for null) compared with wildtype littermates (51.6%). The plasma dual-isotope ratio method showed that cholesterol absorption was reduced by about 40% in Lima1-deficient mice. I-Lima1 null mice had lower levels of cholesterol in intestinal epithelial cells after cholesterol gavage than wildtype mice; however, I-Lima1 null mice had increased fecal cholesterol compared to wildtype mice, with similar fecal triglyceride levels, demonstrating that intestinal cholesterol absorption was reduced in I-Lima1 null mice. When fed a chow diet, all mice showed similar total cholesterol levels in plasma and liver. In comparison, mice that consumed a high cholesterol diet had 1.63-fold and 4-fold increases in plasma and liver total cholesterol levels respectively. The plasma and liver total cholesterol content of intestinal Lima1-null mice were respectively 28.8% and 58.3% lower than those of wildtype mice fed the high cholesterol diet. Whole-body Lima1 heterozygous knockout mice appeared normal and had less dietary cholesterol absorption and lower plasma total cholesterol levels compared to wildtype mice, similar to human heterozygotes for the LIMA1 K306fs (608364.0001) mutation. Homozygous knockout mice also appeared normal and displayed reduced dietary cholesterol absorption. Cholesterol absorption was ablated to a lesser degree than in Npc1l1 knockout mice.

In affected members of a Chinese Kazakh family (Family 1) with inherited low levels of LDL cholesterol (LDLCQ8; 618079), Zhang et al. (2018) identified heterozygosity for an 8-bp deletion in exon 7 of the LIMA1 gene (K306fs; 608364.0001) that resulted in frameshift and premature termination. The mutation was identified by whole-exome sequencing and confirmed by Sanger sequencing and segregated with the phenotype in the family. Targeted sequencing of LIMA1 in approximately 1,000 Chinese Kazakhs revealed a missense variant in exon 2 (L25I; 608364.0002). in 3 additional families with low LDL cholesterol levels.