Modeling autosomal recessive cutis laxa type 1C (ARCL1C) in mice reveals distinct functions of Ltbp-4 isoforms

Abstract

Recent studies have revealed an important role for LTBP-4 in elastogenesis. Its mutational inactivation in humans causes autosomal recessive cutis laxa type 1C (ARCL1C), which is a severe disorder caused by defects of the elastic fiber network. Although the human gene involved in ARCL1C has been discovered based on similar elastic fiber abnormalities exhibited by mice lacking the short Ltbp-4 isoform (Ltbp4S−/−), the murine phenotype does not replicate ARCL1C. We therefore inactivated both Ltbp-4 isoforms in the mouse germline to model ARCL1C. Comparative analysis of Ltbp4S−/− and Ltbp4-null (Ltbp4−/−) mice identified Ltbp-4L as an important factor for elastogenesis and postnatal survival, and showed that it has distinct tissue expression patterns and specific molecular functions. We identified fibulin-4 as a previously unknown interaction partner of both Ltbp-4 isoforms and demonstrated that at least Ltbp-4L expression is essential for incorporation of fibulin-4 into the extracellular matrix (ECM). Overall, our results contribute to the current understanding of elastogenesis and provide an animal model of ARCL1C.

Function of Ltbp-4L and fibulin-4 in survival and elastogenesis in mice

“…It has been demonstrated that Ltbp-4 isoforms functionally interact with fibulin-5 (Bultmann-Mellin et al, 2015; Noda et al, 2013). The fibrillar structure of fibulin-5 appeared normal in aortas of WT and Fibulin-4 R/R mice (Fig.…”

“…S3" type="url"/>). This was most likely causative for the higher postnatal mortality of Ltbp4S −/− ; Fibulin-4 R/R mice compared to Ltbp4 −/− mice (Bultmann-Mellin et al, 2015). …”

“…In humans and mice, both isoforms show distinct tissue expression patterns in various organs (Bultmann-Mellin et al, 2015; Kantola et al, 2010), which has been proposed to result from the activation of two known LTBP-4 promoters under the control of independent transcription factors (Bultmann et al, 2013; Kantola et al, 2010). …”

“…We previously generated Ltbp4 -null ( Ltbp4 −/− ) mice in which the expression of both Ltbp-4 isoforms (Ltbp-4S, NM_001113549.1; Ltbp-4L, NM_175641.2) is ablated (Bultmann-Mellin et al, 2015). In contrast to mice only expressing Ltbp-4L ( Ltbp4S −/− ) (Dabovic et al, 2009; Sterner-Kock et al, 2002), Ltbp4 −/− mice nearly replicate the features of ARCL1C syndrome, including high postnatal mortality, impaired pulmonary lobular architecture with emphysematous and atelectatic areas, and impaired elastic fiber formation, suggesting that the presence of LTBP-4L is required for elastic fiber formation, intact pulmonary architecture and survival (Bultmann-Mellin et al, 2015).…”

“…However, compared with Ltbp-4S, we measured a higher binding propensity for Ltbp-4L to fibulin-4. The observed differences in binding propensity are probably important, because incorporation of fibulin-4 into the matrix remains unaltered in the presence of Ltbp-4L in Ltbp4S −/− mice, whereas it is defective in Ltbp4 −/− mice, where both Ltbp-4 isoforms are lacking (Bultmann-Mellin et al, 2015). Ltbp4 −/− mice die between postnatal day (P)8 and P14 (Bultmann-Mellin et al, 2015), whereas Ltbp4S −/− mice survive to adulthood (Sterner-Kock et al, 2002), indicating that Ltbp-4L expression and possibly its interaction with fibulin-4 is crucial for survival of Ltbp4S −/− mice.…”