gms | German Medical Science

Z-discs define the lateral borders of the sarcomere in striated muscles and have primarily been considered to be important for mechanical stability. The recent identification of a multitude of novel Z-disc proteins, however, added additional functions such as intracellular signaling, mechanosensation and mechanotransduction. One of the Z-disc proteins involved in these processes is filamin C (FLNc). It is comprised of an actin-binding domain followed by 24 immunoglobulin-like domains the last of which is responsible for dimerization. FLNc serves as scaffold for many binding partners like e.g. Xin and Xirp2. These muscle-specific proteins are involved in myofibril assembly and repair. Mutations in theFLNCgene lead to severe muscle diseases.

To investigate its mobility in living cells, we expressed FLNc as EGFP-fusion protein in mouse embryonic cardiomyocytes and applied fluorescence recovery after photobleaching (FRAP). The half-life (t_1/2)of FLNc in mature Z-discs is biphasic and with t_{1/2 fast} = 0.9 sec and t_{1/2 slow} = 22.3 sec surprisingly short. Since 80% of FLNc is in the mobile fraction, this indicates a high turnover rate of the protein in Z-discs. In immature myofibrils its t_1/2 is more than doubled and only 50% of the protein is mobile. Dynamics significantly differ in cardiomyocytes isolated from Xin/Xirp2 double knockout mice. Half-life and mobility for both, Z-discs and pre-myofibrils, are similar and comparable to that of Z-discs in wild-type cells. These results suggest that Xin and Xirp2 stabilize FLNc in pre-myofibrils leading to a decreased mobility.

In addition, we performed laser microdamage of myofibrils to produce defined subsarcomeric lesions, resembling myofibrillar damage and subsequent repair processes in the living organism. FLNc is recruited to these lesions within a few seconds, whereas a-actinin appears only after 15 minutes. This indicates a distinct contribution of both proteins to myofibrillar repair processes.

Our data provide a first hint to understanding the role of FLNc in striated muscle development and repair, as well as, its regulation.