Spatial and temporal requirement of Mlp60A isoforms during muscle development and function in Drosophila melanogaster

The postnatal development of vertebrate cardiac and skeletal muscles is marked by the switching of several sarcomeric contractile proteins from their foetal to the respective adult isoforms [[1], [2], [3], [4], [5], [6], [7]]. The cellular mechanisms by which these isoforms are regulated during striated muscle development have not been studied in detail. Also, the functional significance of this developmental isoform switching [8], and the redundancy/non-redundancy among the different isoforms of several of these sarcomeric contractile proteins awaits a detailed study [1,[9], [10], [11]]. The mixed population of fibre types in mammalian skeletal muscles makes it difficult to study the functions of the specific isoforms of sarcomeric proteins [4,12]. On the other hand, the Drosophila dorsal longitudinal muscles (DLMs), which are a type of indirect flight muscles (IFMs-a term which will be used to refer to DLMs henceforth, in order to maintain consistency with previous literature), offer a unique model system to study muscle development, due to their structural similarity with the vertebrate skeletal muscles [13,14], functional similarity with the vertebrate cardiac muscles [15,16], and a relatively uniform composition of only ‘fibrillar’ type fibres [16,17]. Moreover, the later stages of IFM development are similar to the postnatal development of the vertebrate striated muscles, since isoform switching of sarcomeric proteins occurs in both. In Drosophila melanogaster, several sarcomeric proteins, such as Myosin Heavy Chain (MHC), Actin, Troponin subunits, Tropomyosin, Myosin Light Chain (MLC), Kettin and Zormin, etc., are known to undergo isoform switching from their embryonic/larval to their respective adult isoforms, that are expressed either specifically in the IFMs, or in both the IFMs and the jump muscle, Tergal Depressor of Trochanter (TDT) [[18], [19], [20], [21], [22], [23], [24], [25], [26]]. Hence, IFM- or IFM-TDT-specific null mutants of different sarcomeric proteins have been isolated, which facilitate the study of these stage and tissue specific isoforms [11,22,25,[27], [28], [29], [30]].

In the present study, we show that the Muscle LIM Protein at 60A (Mlp60A) undergoes isoform switching during muscle development in Drosophila melanogaster. The vertebrate ortholog of Mlp60A is Cysteine and Glycine-rich Protein 3 (CSRP3). Cell culture studies have demonstrated that this protein can promote myogenic differentiation by associating with several muscle-specific transcription factors, such as MyoD, MRF4, and Myogenin [31,32]. Moreover, through knockout studies performed in a mouse model, this protein has been shown to be necessary for the development of cardiomyocyte cytoarchitecture. CSRP3−/− mice show dilated cardiomyopathy (DCM)- and hypertrophic cardiomyopathy (HCM)-like phenotypes [33,34]. CSRP3 mutations have also been identified in human cardiomyopathy patients [[35], [36], [37], [38]]. However, the precise role of MLP in muscle differentiation, and its requirement for skeletal muscle development have not been addressed. MLP is expressed in both developing and adult skeletal musculature in mice and zebrafish, but its deficiency in either of these model organisms produces only mild skeletal muscle phenotypes [33,39,40]. Hence, it is not understood whether this protein is dispensable for skeletal muscle development, or if some alternate isoform or paralog compensates for its deficiency. Interestingly, an alternate isoform of MLP, called the “MLP-b” isoform, was reported by Vafiadaki et al. [41]. This isoform was found to be upregulated in tissue samples from skeletal muscle disease patients and appeared to be a negative regulator of myogenesis [41]. However, the distinct spatio-temporal requirements of the two MLP isoforms in skeletal muscle development, and the precise role of the MLP-b isoform have not been addressed. Our results show that there is an exclusive functional specialization of the Mlp60A isoforms in Drosophila melanogaster, with one isoform being constitutive, and essential for embryonic muscle development, and the other being adult-specific, and necessary for normal flight.

留言 (0)

沒有登入
gif