Contribution of the LIM domain and nebulin-repeats to the interaction of Lasp-2 with actin filaments and focal adhesions

The emergence of nebulin repeats and evolution of lasp family proteins

The LIM and SH3 domain protein (lasp) family, the smallest proteins in the nebulin superfamily, consists of vertebrate lasp-1 expressed in various non-muscle tissues, vertebrate lasp-2 expressed in the brain and cardiac muscle, and invertebrate lasp whose functions have been analyzed in Ascidiacea and Insecta. Gene evolution of the lasp family proteins was investigated by multiple alignments, comparison of gene structure, and synteny analyses in eukaryotes in which mRNA expression was confirmed. All invertebrates analyzed in this study belonging to the clade Filasterea, with the exception of Placozoa, have at least one lasp gene. The minimal actin-binding region (LIM domain and first nebulin repeat) and SH3 domain detected in vertebrate lasp-2 were found to be conserved among the lasp family proteins, and we showed that nematode lasp has actin-binding activity. The linker sequences vary among invertebrate lasp proteins, implying that the lasp family proteins have universal and diverse functions. Gene structures and syntenic analyses suggest that a gene fragment encoding two nebulin repeats and a linker emerged in Filasterea or Holozoa, and the first lasp gene was generated following combination of three gene fragments encoding the LIM domain, two nebulin repeats with a linker, and the SH3 domain. This article is protected by copyright. All rights reserved.

LIM and SH3 protein 1 localizes to the leading edge of protruding lamellipodia and regulates axon development

The actin cytoskeleton drives cell motility and is essential for neuronal development and function. LIM and SH3 protein 1 (LASP1) is a unique actin-binding protein that is expressed in a wide range of cells including neurons, but its roles in cellular motility and neuronal development are not well understood. We report that LASP1 is expressed in rat hippocampus early in development, and this expression is maintained through adulthood. High-resolution imaging reveals that LASP1 is selectively concentrated at the leading edge of lamellipodia in migrating cells and axonal growth cones. This local enrichment of LASP1 is dynamically associated with the protrusive activity of lamellipodia, depends on the barbed ends of actin filaments, and requires both the LIM domain and the nebulin repeats of LASP1. Knockdown of LASP1 in cultured rat hippocampal neurons results in a substantial reduction in axonal outgrowth and arborization. Finally, loss of the Drosophila homologue Lasp from a subset of commissural neurons in the developing ventral nerve cord produces defasciculated axon bundles that do not reach their targets. Together, our data support a novel role for LASP1 in actin-based lamellipodial protrusion and establish LASP1 as a positive regulator of both in vitro and in vivo axon development.

LIM-Nebulette Reinforces Podocyte Structural Integrity by Linking Actin and Vimentin Filaments

BACKGROUND

Maintenance of the intricate interdigitating morphology of podocytes is crucial for glomerular filtration. One of the key aspects of specialized podocyte morphology is the segregation and organization of distinct cytoskeletal filaments into different subcellular components, for which the exact mechanisms remain poorly understood.

METHODS

Cells from rats, mice, and humans were used to describe the cytoskeletal configuration underlying podocyte structure. Screening the time-dependent proteomic changes in the rat puromycin aminonucleoside-induced nephropathy model correlated the actin-binding protein LIM-nebulette strongly with glomerular function. Single-cell RNA sequencing and immunogold labeling were used to determine Nebl expression specificity in podocytes. Automated high-content imaging, super-resolution microscopy, atomic force microscopy (AFM), live-cell imaging of calcium, and measurement of motility and adhesion dynamics characterized the physiologic role of LIM-nebulette in podocytes.

RESULTS

Nebl knockout mice have increased susceptibility to adriamycin-induced nephropathy and display morphologic, cytoskeletal, and focal adhesion abnormalities with altered calcium dynamics, motility, and Rho GTPase activity. LIM-nebulette expression is decreased in diabetic nephropathy and FSGS patients at both the transcript and protein level. In mice, rats, and humans, LIM-nebulette expression is localized to primary, secondary, and tertiary processes of podocytes, where it colocalizes with focal adhesions as well as with vimentin fibers. LIM-nebulette shRNA knockdown in immortalized human podocytes leads to dysregulation of vimentin filament organization and reduced cellular elasticity as measured by AFM indentation.

CONCLUSIONS

LIM-nebulette is a multifunctional cytoskeletal protein that is critical in the maintenance of podocyte structural integrity through active reorganization of focal adhesions, the actin cytoskeleton, and intermediate filaments.

The Nebulin Family LIM and SH3 Proteins Regulate Postsynaptic Development and Function

Neuronal dendrites have specialized actin-rich structures called dendritic spines that receive and integrate most excitatory synaptic inputs. The stabilization of dendrites and spines during neuronal maturation is essential for proper neural circuit formation. Changes in dendritic morphology and stability are largely mediated by regulation of the actin cytoskeleton; however, the underlying mechanisms remain to be fully elucidated. Here, we present evidence that the nebulin family members LASP1 and LASP2 play an important role in the postsynaptic development of rat hippocampal neurons from both sexes. We find that both LASP1 and LASP2 are enriched in dendritic spines, and their knockdown impairs spine development and synapse formation. Furthermore, LASP2 exerts a distinct role in dendritic arbor and dendritic spine stabilization. Importantly, the actin-binding N-terminal LIM domain and nebulin repeats of LASP2 are required for spine stability and dendritic arbor complexity. These findings identify LASP1 and LASP2 as novel regulators of neuronal circuitry.SIGNIFICANCE STATEMENT Proper regulation of the actin cytoskeleton is essential for the structural stability of dendrites and dendritic spines. Consequently, the malformation of dendritic structures accompanies numerous neurologic disorders, such as schizophrenia and autism. Nebulin family members are best known for their role in regulating the stabilization and function of actin thin filaments in muscle. The two smallest family members, LASP1 and LASP2, are more structurally diverse and are expressed in a broader array of tissues. While both LASP1 and LASP2 are highly expressed in the brain, little is currently known about their function in the nervous system. In this study, we demonstrate the first evidence that LASP1 and LASP2 are involved in the formation and long-term maintenance of dendrites and dendritic spines.

New Frontiers for the Cytoskeletal Protein LASP1

In the recent two decades, LIM and SH3 protein 1 (LASP1) has been developed from a simple actin-binding structural protein to a tumor biomarker and subsequently to a complex, nuclear transcriptional regulator. Starting with a brief historical perspective, this review will mainly compare and contrast LASP1 and LASP2 from the angle of the newest data and importantly, examine their role in transcriptional regulation. We will summarize the current knowledge through pictorial models and tables including the roles of different microRNAs in the differential regulation of LASP1 levels and patient outcome rather than specify in detail all tumor entities. Finally, the novel functional roles of LASP1 in secretion of vesicles, expression of matrix metalloproteinases and transcriptional regulation as well as the activation of survival and proliferation pathways in different cancer types are described.

Structure-function studies of MICAL, the unusual multidomain flavoenzyme involved in actin cytoskeleton dynamics

MICAL (from the Molecule Interacting with CasL) indicates a family of multidomain proteins conserved from insects to humans, which are increasingly attracting attention for their participation in the control of actin cytoskeleton dynamics, and, therefore, in the several related key processes in health and disease. MICAL is unique among actin binding proteins because it catalyzes a NADPH-dependent F-actin depolymerizing reaction. This unprecedented reaction is associated with its N-terminal FAD-containing domain that is structurally related to p-hydroxybenzoate hydroxylase, the prototype of aromatic monooxygenases, but catalyzes a strong NADPH oxidase activity in the free state. This review will focus on the known structural and functional properties of MICAL forms in order to provide an overview of the arguments supporting the current hypotheses on the possible mechanism of action of MICAL in the free and F-actin bound state, on the modulating effect of the CH, LIM, and C-terminal domains that follow the catalytic flavoprotein domain on the MICAL activities, as well as that of small molecules and proteins interacting with MICAL.

Investigating lasp-2 in cell adhesion: new binding partners and roles in motility

Focal adhesions are intricate protein complexes that facilitate cell attachment, migration, and cellular communication. Lasp-2 (LIM-nebulette), a member of the nebulin family of actin-binding proteins, is a newly identified component of these complexes. To gain further insights into the functional role of lasp-2, we identified two additional binding partners of lasp-2: the integral focal adhesion proteins vinculin and paxillin. Of interest, the interaction of lasp-2 with its binding partners vinculin and paxillin is significantly reduced in the presence of lasp-1, another nebulin family member. The presence of lasp-2 appears to enhance the interaction of vinculin and paxillin with each other; however, as with the interaction of lasp-2 with vinculin or paxillin, this effect is greatly diminished in the presence of excess lasp-1. This suggests that the interplay between lasp-2 and lasp-1 could be an adhesion regulatory mechanism. Lasp-2's potential role in metastasis is revealed, as overexpression of lasp-2 in either SW620 or PC-3B1 cells-metastatic cancer cell lines-increases cell migration but impedes cell invasion, suggesting that the enhanced interaction of vinculin and paxillin may functionally destabilize focal adhesion composition. Taken together, these data suggest that lasp-2 has an important role in coordinating and regulating the composition and dynamics of focal adhesions.

Nebulin, a major player in muscle health and disease

Nebulin is a giant 600- to 900-kDa filamentous protein that is an integral component of the skeletal muscle thin filament. Its functions have remained largely nebulous because of its large size and the difficulty in extracting nebulin in a native state from muscle. Recent improvements in the field, especially the development of knockout mouse models deficient in nebulin (NEB-KO mice), indicate now that nebulin performs a surprisingly wide range of functions. In addition to a major role in thin-filament length specification, nebulin also functions in the regulation of muscle contraction, as indicated by the findings that muscle fibers deficient in nebulin have a higher tension cost, and develop less force due to reduced myofilament calcium sensitivity and altered crossbridge cycling kinetics. In addition, the function of nebulin extends to a role in calcium homeostasis. These novel functions indicate that nebulin might have evolved in vertebrate skeletal muscles to develop high levels of muscle force efficiently. Finally, the NEB-KO mouse models also highlight the role of nebulin in the assembly and alignment of the Z disks. Notably, rapid progress in understanding the roles of nebulin in vivo provides clinically important insights into how nebulin deficiency in patients with nemaline myopathy contributes to debilitating muscle weakness.