Drosophila melanogaster muscle LIM protein and alpha‐actinin function together to stabilize muscle cytoarchitecture: A potential role for Mlp84B in actin‐crosslinking

Abstract

Stabilization of tissue architecture during development and growth is essential to maintain structural integrity. Because of its contractile nature, muscle is especially susceptible to physiological stresses, and has multiple mechanisms to maintain structural integrity. The Drosophila melanogaster Muscle LIM Protein, Mlp84B, participates in muscle maintenance, yet its precise mechanism of action is still controversial. Through a candidate approach, we identified α-actinin as a protein that functions with Mlp84B to ensure muscle integrity. α-actinin RNAi animals die primarily as pupae, and Mlp84B RNAi animals are adult viable. RNAi knockdown of Mlp84B and α-actinin together produces synergistic early larval lethality and destabilization of Z-line structures. We recapitulated these phenotypes using combinations of traditional loss-of-function alleles and single gene RNAi. We observe that Mlp84B induces the formation of actin-loops in muscle cell nuclei in the absence of nuclear α-actinin, suggesting Mlp84B has intrinsic actin crosslinking activity, which may complement α-actinin crosslinking activity at sites of actin filament anchorage. These results reveal a molecular mechanism for MLP stabilization of muscle, and implicate reduced actin crosslinking as the primary destabilizing defect in MLP associated cardiomyopathies. Our data support a model in which α-actinin and Mlp84B have important and overlapping functions at sites of actin filament anchorage, to preserve muscle structure and function.

The actinin family of actin cross-linking proteins – a genetic perspective

“…These larvae are capable of some degree of movement, but have severely disrupted myofibrils, show muscular degeneration, and experience paralysis that is ultimately lethal [ 26 ]. Nevertheless, the initial stages of sarcomere assembly do not seem to require actinin, an observation confirmed in more recent studies using RNA interference targeting actinin [ 27 , 28 ]. These studies suggest that in flies, actinin functions to stabilize the forming sarcomeric Z-disk, rather than initiating its assembly.…”

Human Muscle LIM Protein Dimerizes along the Actin Cytoskeleton and Cross-Links Actin Filaments

“…At the Z disk, AFs from adjoining sarcomeres are assumed to be primarily crosslinked by homodimers of ␣-actinin, the most abundant Z-disk component (56). However, a recent study has suggested that the MLP-related Drosophila protein Mlp84B has a function similar and complementary to that of ␣-actinin in muscle stabilization through the cross-linking of AFs at the Z-disk and muscle-membrane attachment sites (35). Although the biochemical activity of Mlp84B has not been directly assessed, the forced accumulation of Mlp84 in muscle nuclei induced the formation of actin cables independently of ␣-actinin, supporting that, like human MLP, Mlp84 is an autonomous AF-bundling protein.…”

“…Although the biochemical activity of Mlp84B has not been directly assessed, the forced accumulation of Mlp84 in muscle nuclei induced the formation of actin cables independently of ␣-actinin, supporting that, like human MLP, Mlp84 is an autonomous AF-bundling protein. Interestingly, MLP and ␣-actinin physically interact (13,35), and cardiomyo-cyte-associated mutations have been shown to disrupt their interaction (19,57), pointing to a close functional link. Beside the colocalization of MLP and ␣-actinin in HL-1 cells ( Fig.…”

“…Remarkably, a truncated version of CRP1 containing the actin-bundling domain was sufficient to increase the number of filopodia and neuritic branches as efficiently as the full-length protein, supporting that the bundling activity of CRP1 is biologically relevant. Another recent study by Clark and Kadrmas (35) has concluded that the Drosophila melanogaster Mlp84B protein, a LIM protein highly related to vertebrate MLP, likely contributes to muscle integrity through direct stabilization of AFs at their sites of anchorage.…”

Characterization of an MLP Homologue from Haemaphysalis longicornis (Acari: Ixodidae) Ticks

“…Muscle performs necessary functions during blood sucking by moving the coxae of the appendages, retracting the chelicerae, and controlling pharyngeal action [11]. In addition, Drosophila Mlp84B can cooperate with D-titin to maintain muscle structural integrity [12]. Furthermore, as blood sucking leads to the stretching of muscle myofilaments [13], MLP may be essential in muscles under this increased pressure.…”