Gene name - Laminin A
Cytological map position - 65A10-11
|Symbol - LanA
Genetic map position - 3-
Classification - Laminin-A
Cellular location - Secreted
|Recent literature||Diaz de la Loza, M. C., Diaz-Torres, A., Zurita, F., Rosales-Nieves, A. E., Moeendarbary, E., Franze, K., Martin-Bermudo, M. D. and Gonzalez-Reyes, A. (2017). Laminin levels regulate tissue migration and anterior-posterior polarity during egg morphogenesis in Drosophila. Cell Rep 20(1): 211-223. PubMed ID: 28683315
Basement membranes (BMs) are specialized extracellular matrices required for tissue organization and organ formation. The role of laminin and its integrin receptor were studied in the regulation of tissue migration during Drosophila oogenesis. Egg production in Drosophila involves the collective migration of follicle cells (FCs) over the BM to shape the mature egg. Laminin content in the BM was shown to increase with time, whereas integrin amounts in FCs do not vary significantly. Manipulation of integrin and laminin levels reveals that a dynamic balance of integrin-laminin amounts determines the onset and speed of FC migration. Thus, the interplay of ligand-receptor levels regulates tissue migration in vivo. Laminin depletion also affects the ultrastructure and biophysical properties of the BM and results in anterior-posterior misorientation of developing follicles. Laminin emerges as a key player in the regulation of collective cell migration, tissue stiffness, and the organization of anterior-posterior polarity in Drosophila.
|Sessions, A. O., Kaushik, G., Parker, S., Raedschelders, K., Bodmer, R., Van Eyk, J. E. and Engler, A. J. (2017). Extracellular matrix downregulation in the Drosophila heart preserves contractile function and improves lifespan. Matrix Biol 62: 15-27. PubMed ID: 27793636
Aging is associated with extensive remodeling of the heart, including basement membrane (BM) components that surround cardiomyocytes. Remodeling is thought to impair cardiac mechanotransduction, but the contribution of specific BM components to age-related lateral communication between cardiomyocytes is unclear. Using a genetically tractable, rapidly aging model with sufficient cardiac genetic homology and morphology, e.g. Drosophila melanogaster, this study observed differential regulation of BM collagens between laboratory strains, correlating with changes in muscle physiology leading to cardiac dysfunction. Therefore, attempts were made to understand the extent to which BM proteins modulate contractile function during aging. Cardiac-restricted knockdown of ECM genes Pericardin, Laminin A, and Viking in Drosophila prevented age-associated heart tube restriction and increased contractility, even under viscous load. Most notably, reduction of Laminin A expression correlated with an overall preservation of contractile velocity with age and extension of organismal lifespan. Global heterozygous knockdown confirmed these data, which provides new evidence of a direct link between BM homeostasis, contractility, and maintenance of lifespan.
Laminin is an enormous complex extracellular, composed of three different huge polypeptide chains. The longest is Laminin A, made up of 3712 amino acids. In Drosophila laminin is involved in heart, gut, muscle, wing, and leg morphogenesis. Laminins are known to interact with a variety of proteins including integrins (See Myospheroid) and lectins. In tissue culture, Laminin is both a promoter and substratum for neurite outgrowth. In the fly, only one form of laminin has been characterized, but many proteins show homology to various domains of the laminin polypeptide. For example, Netrins, involved in axon guidance in many species, show extensive homology to common domains in each of the Laminin polypeptides. In vertebrates laminin is not a single molecular complex, but a family of at least seven different complexes using alternative subunits. In the mouse, mutation in one laminin subunit results in a form of muscular dystrophy, while mutation in another leads to defects at the neuromuscular junction (García-Alonso, 1996 and references).
Below, the involvement of laminin in the pathfinding process of ocellar axons will be described, following a short explanation of the ocellar axon pathfinding process.
Adult flys have three simple eyes (ocelli) located near the midline on the dorsal surface of the head. Left and right ocelli are derived from the left and right eye-antennal imaginal discs, while the median ocellus derives equally from both discs once they fuse together after puparium formation. In the adult, the ocellar photoreceptors (about 80 per ocellus) have short axons that synapse on the dendrites of ocellar giant interneurons (about 4 per ocellus) in the neuropil of the ocellar ganglion that lies just below the ocelli. The axons of the ocellar giant interneurons project to the brain from within the ocellar nerve. In the adult this nerve contains about 12 giant interneuron axons. The ocellar nerve, projecting from the brain to the ocelli of the adult, is in reality a follower and not a pioneer. Instead, the ocellar nerve is pioneered by about 200 axons from a transient population of ocellar pioneers that appear around the time of puparium formation. The axons from four separate transient populations of about 50 ocellar pioneer neurons (one from each lateral ocelli, and one each from right and left rudiments of the median ocellus) project from the pupal ocelli to the brain. These four populations form four fascicles or axon bundles that extend towards the brain along a non-cellular substratum. At a later pupal stage, the adult ocellar photoreceptors are born and differentiate concentrically outside the cluster of pioneers. The pioneers then die. The axons of the giant ocellar interneurons extend backward from the brain to the ocelli along the pathway laid out earlier by the pioneer neurons. By the time of adult eclosion, the 200 or so ocellar pioneer axons are gone, and only the approximately 12 giant axons of the ocellar interneurons remain in the ocellar nerve (García-Alonso, 1996).
Laminin A is involved in pathfinding of the pioneer interneurons. In some LanA mutants these axons do not grow as far as in wild type, while in others, axons do not form the characteristic pair of ocellar pioneer axon bundles, but rather form multiple axons fascicles, some of which enter the brain at abnormal positions. In the more extreme case of defects, the ocellar pioneer axons do not form the normal projection that traverses the head capsule from the epidermis to the brain, but rather extend for a short distance in the epidermis, and then stall, forming large fasciculated masses, and occasional whirls of axons. These stalled axons remain attached to the head epidermis. There are also striking defects in compound eye retinal axon pathfinding in LanA mutants, but there is also an abnormal distribution of glial cells. It is therefore not possible to know whether the pathfinding defects of compound eye axons is a primary or secondary defect. ECM containing Laminin A is not required for pathfinding by neighboring mechanosensory (bristle) axons in the head or bristle axons in the wing (García-Alonso, 1996).
Extracellular matrix containing Laminin A cannot be the entire story for the guidance of ocellar axons towards the brain, since guidance in mutants is not always defective. This suggests that in the absence of Laminin A, the pioneer axons can still read directional cues pointing them towards the brain. While laminin-rich extracellular matrix provides the appropriate growth promoting substratum, some other signal must provide directional cue (García-Alonso, 1996).
Retrograde signals induced by synaptic activities are derived from postsynaptic cells to potentiate presynaptic properties, such as cytoskeletal dynamics, gene expression, and synaptic growth. However, it is not known whether activity-dependent retrograde signals can also depotentiate synaptic properties. This study shows that laminin A (LanA) functions as a retrograde signal to suppress synapse growth at Drosophila neuromuscular junctions (NMJs). The presynaptic integrin pathway consists of the integrin subunit βν and focal adhesion kinase 56 (Fak56), both of which are required to suppress crawling activity-dependent NMJ growth. LanA protein is localized in the synaptic cleft and only muscle-derived LanA is functional in modulating NMJ growth. The LanA level at NMJs is inversely correlated with NMJ size and regulated by larval crawling activity, synapse excitability, postsynaptic response, and anterograde Wnt/Wingless signaling, all of which modulate NMJ growth through LanA and βν. These data indicate that synaptic activities down-regulate levels of the retrograde signal LanA to promote NMJ growth (Tsai, 2012).
This study proposes a plasticity mechanism by which the synapse growth (or size) can be modulated by larva crawling and synaptic activities. These activities modulate LanA-integrin signaling that functions to constrain NMJ growth. This trans-synaptic signaling functions in a retrograde manner, which requires postsynaptic muscle-derived LanA and presynaptic integrin. The model suggests various activities modulate NMJ growth by regulating the LanA level and integrin signaling (Tsai, 2012).
Regulation of LanA levels at NMJs is the major mechanism underlying this synaptic structural plasticity. The LanA levels at NMJs are tightly coupled to several synaptic activities that are involved in synaptic structural plasticity at NMJs. Wg signaling in both pre- and postsynaptic compartments are shown to modify synaptic structure at Drosophila NMJs. The channel mutations para and eag Sh alter both synaptic potential and NMJ size. Finally, manipulation of postsynaptic responses by altering the GluRIIA and GluRIIB compositions also fine tunes synapse size and pFAK levels. Activities that promote NMJ growth also down-regulated LanA levels at NMJs. In contrast, NMJ growth suppression was accompanied with LanA accumulation, establishing an inverse correlation between the LanA level and the NMJ size. Importantly, manipulation of the gene dosage of LanA (or βν) could override these synaptic activities in NMJ growth regulation. This study also showed that LanA down-regulation at NMJs preceded synaptic structural remodeling induced by larval crawling, further supporting that LanA is a major mediator of these activities to modulate NMJ growth (Tsai, 2012).
Integrin signaling activities play important roles in synapse development and plasticity. In mammalian central synapses, various integrin subunits are important to transmit postsynaptic signaling in various plasticity models may function redundantly with βν to mediate integrin signaling. This study indicates a distinct presynaptic integrin pathway that is likely composed of βν and αPS3 (encoded by Volado), as suggested by their strong genetic interaction in NMJ growth. In response to postsynapse-secreted LanA signals, activation of the presynaptic integrin is transmitted through Fak56 activation. Interestingly, the signaling activity is rather local, limited by the range of LanA distribution, and shown by muscle 6-specific rescue, although this does not exclude the involvement of signaling to the nuclei of motor neurons. The presynaptic integrin/Fak56 signaling is in turn mediated by two downstream signaling activities. The activation of NF1/cAMP signaling, which suppressed NMJ overgrowth induced by crawling activity or βν mutation. The integrin/Fak56 pathway also suppresses Ras/MAPK signaling Tsai, 2008), as shown by diphospho-ERK (dpERK) accumulation and Fas2 reduction at NMJs in high crawling condition. These pathways have been shown to regulate cell adhesion and cytoskeletal organization, leading to the stabilization of synapses. The activity-dependent depletion of the LanA laminins in the synaptic cleft would allow the remodeling of synapses and further growth of NMJs (Tsai, 2012).
The activity-dependent structural plasticity is specific to the presynaptic integrin pathway. hiw mutants that show large NMJ size still retained the structural plasticity and constant pFAK levels at NMJs. Interestingly, LanA levels were increased in hiw mutants, in contrast to other NMJ overgrown mutants. Two nonmutually exclusive mechanisms can regulate activity-dependent LanA expressions at NMJs. First, within hours of activity induction, the LanA levels can be regulated at NMJs by putative ECM regulators such as matrix metalloproteinases. Second, transcription regulation of LanA can provide long-term changes of LanA levels at NMJs. Activity-triggered presynaptic Wg secretion promotes Wg receptor DFz2 activation on both post- and presynaptic compartments. The LanA level is regulated by the anterograde Wg signaling that is transduced through nuclear entry of the DFz2 intracellular domain and its transcription activity. However, LanA is unlikely to mediate all aspects of Wg signaling activity as overexpression of LanA in postsynapses suppressed ghost bouton formation, a hallmark in disrupting Wg signaling. Postsynaptic BMP/Gbb functions as a retrograde signal to activate presynaptic BMP type II receptor Wit in response to synaptic activity. With the lack of genetic interaction between BMP/Gbb and integrin signaling components, and constant levels of phosphorylated Mothers against dpp (pMad) in different crawling activities, it is proposed that both BMP/Gbb and LanA pathways can function in parallel by retrograde mechanisms to regulate NMJ growth (Tsai, 2012).
cDNA clone length - 14155 bases
Exons - 15
Amino Acids - 3712
Laminin is a heterotrimer consisting of three chains, A, B1 and B2. B1 and B2 subunits show similarity to their vertebrate homologs in both the arrangement and sequence of their multidomain structures. Laminin forms a cruciform structure in which the N-terminal end region (domain I/II) of each of the three chains forms one of the short arms. The C-terminal ends are joined together in the long arm of the cross as coiled-coil amphipathic alpha helices linked by interchain disulfide bonds. The mature protein has 35 consensus sites for N-linked glycosylation. It lacks any Arg-Gly-Asp (RGD) sequence which has been implicated in binding of cells to mouse laminin. The overall level of amino acid similarity is 29% between fly and mouse, compared with 78% between mouse and human. The short arm of the Drosophila A chain is made up of the N-terminal signal sequence followed by the VI globular domain and two other globular domains (IVb and IVa) separated by cysteine-rich, thread-like segments called laminin repeats. Domain VI is very similar to domains VI of all chareacterized laminin chains. The globular domain IVb is homologous to domain IVb of the mouse laminin A chain. It is also similar to the domains IV of laminin B2 chains and to some globular domains of Perlecan. Between the globular domains of the short arm Laminin A chain are the thread-like domains V, IIIb and IIIa, consisting of laminin repeats. These repeats are structural motifs related to EGF repeats, consisting of 50 amino acid residues, but containing eight Cys residues rather than the six residues found in EGF repeats. They occur in all laminin chains and also in Perlecan and Agrin. The number of EGF repeats differs between the fly and the mouse. The C-terminal G domain, distinguishing Laminin A chains from B chains, is made up of G-loops. These are sequence motifs found in a variety of secreted and cell surface proteins. Mouse and fly G-loops show 26% amino acid identity, whereas the G domains of mouse Laminin A and human Merosin show 41% identity, suggesting that mouse Laminin A and human Merosin may have evolved by means of gene duplication after the evolutionary split leading to chordates and arthropods (Kusche-Gullberg, 1992 and Henchcliffe, 1993).
date revised: 10 August 97
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