LAP2alpha maintains a mobile and low assembly state of A-type lamins in the nuclear interior

Abstract

Lamins form stable filaments at the nuclear periphery in metazoans. Unlike B-type lamins, lamins A and C localize also in the nuclear interior, where they interact with lamin-associated polypeptide 2 alpha (LAP2α). We show that lamin A in the nuclear interior is formed from newly expressed pre-lamin A during processing and from soluble mitotic mature lamins in a LAP2α-independent manner. Binding of LAP2α to lamins A/C in the nuclear interior during interphase inhibits formation of higher order structures of lamin A/C in vitro and in vivo, keeping lamin A/C in a mobile low assembly state independent of lamin A/C S22 phosphorylation. Loss of LAP2α causes formation of larger, less mobile and biochemically stable lamin A/C structures in the nuclear interior, which reduce the mobility of chromatin. We propose that LAP2α is essential to maintain a mobile lamin A/C pool in the nuclear interior, which is required for proper nuclear functions.

Hepatocyte-specific loss of LAP2α protects against diet-induced hepatic steatosis and steatohepatitis in male mice

“…To examine the distribution of lamin A/C in the absence of hepatocyte LAP2α, we performed immunofluorescence staining of mouse liver tissue. Consistent with prior reports (27, 29), livers of HKO mice showed stronger LMNA staining at the nuclear rim compared to WT, with particularly prominent LMNA staining in HKO mice after 8 weeks of HFD ( Figure 3A ); under chow diet conditions, LMNA staining was similar in WT and HKO mouse livers ( Supplementary Figure 2 ). Immunoblot analysis of whole livers did not show significantly different overall LMNA levels in livers from HFD-fed HKO and WT mice ( Figure 3B ).…”

“…These protein-protein interactions influence several physiological functions, including proliferation and differentiation, as well as chromatin organization and gene expression (43-45). Our data, in agreement with prior reports (27, 29), show that loss of LAP2α enhanced LMNA staining at the nuclear periphery without changing its overall expression level, which was accompanied by pro-steatotic, pro-inflammatory, and pro-fibrotic transcriptional changes that were opposite to those seen in Lmna -KO mice. In particular, whereas male mice lacking lamin A/C in hepatocytes were predisposed to steatohepatitis via a dramatic increase in transcriptional expression of pro-steatotic genes including Cidea, Cidec, Mogat1 , and Cd36 (24), in this study we observed opposite changes in the expression of all of these genes in the absence of LAP2α.…”

“…While additional LMNA-independent mechanism(s) of protection via loss of LAP2α cannot be ruled out, these data suggest that the observed protection in LAP2α-deficient mice is due to enhanced hepatoprotective LMNA-mediated regulation of gene expression in hepatocytes. This is in alignment with previous findings and supports the idea that lamin A/C has different properties at the nuclear periphery than within the nucleoplasm (27, 29, 46). Additionally, our data are supportive of a model in which there are mechanistic differences between LAP1-related steatosis (male equal to female, prominent nuclear lipid droplets) and LMNA/LAP2α-related steatosis (male > female, nuclear lipid droplets not prominent).…”

“…Given that Lmna deletion resulted in NAFLD (24) and that loss of Lap2α appeared to protect against NAFLD (Figure 2), and the known role of LAP2α in the regulation of LMNA, we asked whether LMNA distribution within the nucleus might be altered in HKO livers. It was previously reported that nucleoplasmic LAP2α binds to LMNA and affects the ratio of nuclear rim to nucleoplasmic LMNA staining by regulating the mobility and assembly state of LMNA (27, 29). To examine the distribution of lamin A/C in the absence of hepatocyte LAP2α, we performed immunofluorescence staining of mouse liver tissue.…”

Nuclear lamina strain states revealed by intermolecular force biosensor

“…Nucleoplasmic lamins have been shown to be assembled and to interact with chromatin. 30 To further examine nucleoplasmic lamins, as well as the potential for force transmission between nuclear lamins and chromatin, we developed an additional nanobody sensor, using a previously developed H2A-H2B nanobody, 31 to measure mechanical tension between histone H2A-H2B and lamin A/C (Lamin-histone-SS) (Fig. 4c).…”