Liprin phosphorylation regulates binding to LAR: evidence for liprin autophosphorylation

Liprins in oncogenic signaling and cancer cell adhesion

Liprins are a multifunctional family of scaffold proteins, identified by their involvement in several important neuronal functions related to signaling and organization of synaptic structures. More recently, the knowledge on the liprin family has expanded from neuronal functions to processes relevant to cancer progression, including cell adhesion, cell motility, cancer cell invasion, and signaling. These proteins consist of regions, which by prediction are intrinsically disordered, and may be involved in the assembly of supramolecular structures relevant for their functions. This review summarizes the current understanding of the functions of liprins in different cellular processes, with special emphasis on liprins in tumor progression. The available data indicate that liprins may be potential biomarkers for cancer progression and may have therapeutic importance.

PKC-phosphorylation of Liprin-α3 triggers phase separation and controls presynaptic active zone structure

The active zone of a presynaptic nerve terminal defines sites for neurotransmitter release. Its protein machinery may be organized through liquid-liquid phase separation, a mechanism for the formation of membrane-less subcellular compartments. Here, we show that the active zone protein Liprin-α3 rapidly and reversibly undergoes phase separation in transfected HEK293T cells. Condensate formation is triggered by Liprin-α3 PKC-phosphorylation at serine-760, and RIM and Munc13 are co-recruited into membrane-attached condensates. Phospho-specific antibodies establish phosphorylation of Liprin-α3 serine-760 in transfected cells and mouse brain tissue. In primary hippocampal neurons of newly generated Liprin-α2/α3 double knockout mice, synaptic levels of RIM and Munc13 are reduced and the pool of releasable vesicles is decreased. Re-expression of Liprin-α3 restored these presynaptic defects, while mutating the Liprin-α3 phosphorylation site to abolish phase condensation prevented this rescue. Finally, PKC activation in these neurons acutely increased RIM, Munc13 and neurotransmitter release, which depended on the presence of phosphorylatable Liprin-α3. Our findings indicate that PKC-mediated phosphorylation of Liprin-α3 triggers its phase separation and modulates active zone structure and function.

PTP-3 phosphatase promotes intramolecular folding of SYD-2 to inactivate kinesin-3 UNC-104 in neurons

UNC-104 is the Caenorhabditis elegans homolog of kinesin-3 KIF1A known for its fast shuffling of synaptic vesicle protein transport vesicles in axons. SYD-2 is the homolog of liprin-α in C. elegans known to activate UNC-104; however, signals that trigger SYD-2 binding to the motor remain unknown. Because SYD-2 is a substrate of PTP-3/LAR PTPR, we speculate a role of this phosphatase in SYD–2-mediated motor activation. Indeed, coimmunoprecipitation assays revealed increased interaction between UNC-104 and SYD-2 in ptp-3 knockout worms. Intramolecular FRET analysis in living nematodes demonstrates that SYD-2 largely exists in an open conformation state in ptp-3 mutants. These assays also revealed that nonphosphorylatable SYD-2 (Y741F) exists predominately in folded conformations, while phosphomimicking SYD-2 (Y741E) primarily exists in open conformations. Increased UNC-104 motor clustering was observed along axons likely as a result of elevated SYD-2 scaffolding function in ptp-3 mutants. Also, both motor velocities as well as cargo transport speeds were visibly increased in neurons of ptp-3 mutants. Lastly, epistatic analysis revealed that PTP-3 is upstream of SYD-2 to regulate its intramolecular folding.

Central presynaptic terminals are enriched in ATP but the majority lack mitochondria

Synaptic neurotransmission is known to be an energy demanding process. At the presynapse, ATP is required for loading neurotransmitters into synaptic vesicles, for priming synaptic vesicles before release, and as a substrate for various kinases and ATPases. Although it is assumed that presynaptic sites usually harbor local mitochondria, which may serve as energy powerhouse to generate ATP as well as a presynaptic calcium depot, a clear role of presynaptic mitochondria in biochemical functioning of the presynapse is not well-defined. Besides a few synaptic subtypes like the mossy fibers and the Calyx of Held, most central presynaptic sites are either en passant or tiny axonal terminals that have little space to accommodate a large mitochondrion. Here, we have used imaging studies to demonstrate that mitochondrial antigens poorly co-localize with the synaptic vesicle clusters and active zone marker in the cerebral cortex, hippocampus and the cerebellum. Confocal imaging analysis on neuronal cultures revealed that most neuronal mitochondria are either somatic or distributed in the proximal part of major dendrites. A large number of synapses in culture are devoid of any mitochondria. Electron micrographs from neuronal cultures further confirm our finding that the majority of presynapses may not harbor resident mitochondria. We corroborated our ultrastructural findings using serial block face scanning electron microscopy (SBFSEM) and found that more than 60% of the presynaptic terminals lacked discernible mitochondria in the wild-type mice hippocampus. Biochemical fractionation of crude synaptosomes into mitochondria and pure synaptosomes also revealed a sparse presence of mitochondrial antigen at the presynaptic boutons. Despite a low abundance of mitochondria, the synaptosomal membranes were found to be highly enriched in ATP suggesting that the presynapse may possess alternative mechanism/s for concentrating ATP for its function. The potential mechanisms including local glycolysis and the possible roles of ATP-binding synaptic proteins such as synapsins, are discussed.

Liprin-α4 is required for nickel induced receptor protein tyrosine phosphatase-leukocyte antigen related receptor F (RPTP-LAR) activity

Liprin-α4 was strongly induced following nickel (II) chloride exposure in a variety of cell types including BEAS-2B, A549, BEP2D and BL41 cells. Liprin-α4, a member of the Liprin alpha family, has seven isoforms but only three of these variants were detected in BEAS-2B cells (004, 201 and 202). The level of Liprin-α4 variants 201 and 004 were highly increased in BEAS-2B cells in response to nickel. We showed that Liprin-α4 bound directly to the cytoplasmic region of RPTP-LAR (receptor protein tyrosine phosphatase-leukocyte antigen-related receptor F). The cytoplasmic region of RPTP-LAR contains two phosphatase domains but only the first domain shows activity. The second domain interacts with other proteins. The phosphatase activity was increased both following nickel treatment and also in the presence of nickel ions in cell extracts. Liprin-α4 knock-down lines with decreased expression of Liprin-α4 variants 004 and 201 exhibited greater nickel toxicity compared to controls. The RPTP-LAR phosphatase activity was only slightly increased in a Liprin-α4 knock-down line. Liprin-α4 appeared necessary for the nickel induced tyrosine phosphatase activity. The presence of Liprin-α4 and nickel increased tyrosine phosphatase activity that reduced the global levels of tyrosine phosphorylation in the cell.

Three Drosophila liprins interact to control synapse formation

Liprin-α proteins are adaptors that interact with the receptor protein tyrosine phosphatase leukocyte common antigen-related (LAR) and other synaptic proteins to promote synaptic partner selection and active zone assembly. Liprin-β proteins bind to and share homology with Liprin-α proteins, but their functions at the synapse are unknown. The Drosophila genome encodes single Liprin-α and Liprin-β homologs, as well as a third related protein that we named Liprin-γ. We show that both Liprin-β and Liprin-γ physically interact with Liprin-α and that Liprin-γ also binds to LAR. Liprin-α mutations have been shown to disrupt synaptic target layer selection by R7 photoreceptors and to reduce the size of larval neuromuscular synapses. We have generated null mutations in Liprin-β and Liprin-γ to investigate their role in these processes. We find that, although Liprin-α mutant R7 axons terminate before reaching the correct target layer, Liprin-β mutant R7 axons grow beyond their target layer. Larval neuromuscular junction size is reduced in both Liprin-α and Liprin-β mutants, and further reduced in double mutants, suggesting independent functions for these Liprins. Genetic interactions demonstrate that both Liprin proteins act through the exchange factor Trio to promote stable target selection by R7 photoreceptor axons and growth of neuromuscular synapses. Photoreceptor and neuromuscular synapses develop normally in Liprin-γ mutants; however, removing Liprin-γ improves R7 targeting in Liprin-α mutants, and restores normal neuromuscular junction size to Liprin-β mutants, suggesting that Liprin-γ counteracts the functions of the other two Liprins. We propose that context-dependent interactions between the three Liprins modulate their functions in synapse formation.

KazrinE is a desmosome-associated liprin that colocalises with acetylated microtubules

Kazrin is a widely expressed, evolutionarily conserved cytoplasmic protein that binds the cytolinker protein periplakin. Multiple functions of kazrin have been reported, including regulation of desmosome assembly, embryonic tissue morphogenesis and epidermal differentiation. Here, we identify kazrinE as a kazrin isoform that contains a liprin-homology domain (LHD) and forms complexes with kazrinA, kazrinB and kazrinC. As predicted from the presence of the LHD, kazrinE can associate with the leukocyte common antigen-related (LAR) protein tyrosine phosphatase in a phosphorylation-dependent manner. When overexpressed in epidermal keratinocytes, kazrinE induces changes in cell shape and stimulates terminal differentiation. Like the other kazrin isoforms, kazrinE localises to the nucleus and desmosomes. However, in addition, kazrinE associates with stabilised microtubules via its LHD. During terminal differentiation, the keratinocyte microtubule network undergoes extensive reorganisation; in differentiating keratinocytes, endogenous kazrinE colocalises with microtubules, but periplakin does not. We speculate that the kazrinE-microtubule interaction contributes to the mechanism by which kazrin regulates desmosome formation and epidermal differentiation.

Synaptic scaffolding protein SYD-2 clusters and activates kinesin-3 UNC-104 in C. elegans

Kinesin-3 motor UNC-104/KIF1A is essential for transporting synaptic precursors to synapses. Although the mechanism of cargo binding is well understood, little is known how motor activity is regulated. We mapped functional interaction domains between SYD-2 and UNC-104 by using yeast 2-hybrid and pull-down assays and by using FRET/fluorescence lifetime imaging microscopy to image the binding of SYD-2 to UNC-104 in living Caenorhabditis elegans. We found that UNC-104 forms SYD-2-dependent axonal clusters (appearing during the transition from L2 to L3 larval stages), which behave in FRAP experiments as dynamic aggregates. High-resolution microscopy reveals that these clusters contain UNC-104 and synaptic precursors (synaptobrevin-1). Analysis of motor motility indicates bi-directional movement of UNC-104, whereas in syd-2 mutants, loss of SYD-2 binding reduces net anterograde movement and velocity (similar after deleting UNC-104's liprin-binding domain), switching to retrograde transport characteristics when no role of SYD-2 on dynein and conventional kinesin UNC-116 motility was found. These data present a kinesin scaffolding protein that controls both motor clustering along axons and motor motility, resulting in reduced cargo transport efficiency upon loss of interaction.

The mouse and human Liprin-alpha family of scaffolding proteins: genomic organization, expression profiling and regulation by alternative splicing

In the nervous system the Liprin-alpha protein family plays an important role in the regulation of dendrite development, the targeting of photoreceptor axons, and the formation and structure of synapses. To gain a better understanding of Liprin-alpha regulation we have comparatively analyzed the genomic organization of the human and mouse Liprin-alpha genes, characterized the alternative exon use in human and mouse, and studied their expression in adult rodent tissues and brain regions. Our results show that Liprins-alpha1-4 share multiple properties in their genomic structure, exhibit an identical modular organization, and are highly conserved within certain structural domains, indicating strong evolutionary cohesion. We demonstrate that all Liprin-alpha genes are subject to alternative splicing, which is regulated in a developmental manner. Interestingly, regulation via alternative splicing is not conserved between isoforms and across species and represents a post-transcriptional mechanism to independently diversify the properties of the individual isoforms.

Liprin-alpha proteins: scaffold molecules for synapse maturation

Synapses are specialized communication junctions between neurons whose plasticity provides the structural and functional basis for information processing and storage in the brain. Recent biochemical, genetic and imaging studies in diverse model systems are beginning to reveal the molecular mechanisms by which synaptic vesicles, ion channels, receptors and other synaptic components assemble to make a functional synapse. Recent evidence has shown that the formation and function of synapses are critically regulated by the liprin-alpha family of scaffolding proteins. The liprin-alphas have been implicated in pre- and post-synaptic development by recruiting synaptic proteins and regulating synaptic cargo transport. Here, we will summarize the diversity of liprin binding partners, highlight the factors that control the function of liprin-alphas at the synapse and discuss how liprin-alpha family proteins regulate synapse formation and synaptic transmission.

Liprinalpha1 degradation by calcium/calmodulin-dependent protein kinase II regulates LAR receptor tyrosine phosphatase distribution and dendrite development

Neural activity regulates dendrite and synapse development, but the underlying molecular mechanisms are unclear. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is an important sensor of synaptic activity, and the scaffold protein liprinalpha1 is involved in pre- and postsynaptic maturation. Here we show that synaptic activity can suppress liprinalpha1 protein level by two pathways: CaMKII-mediated degradation and the ubiquitin-proteasome system. In hippocampal neurons, liprinalpha1 mutants that are immune to CaMKII degradation impair dendrite arborization, reduce spine and synapse number, and inhibit dendritic targeting of receptor tyrosine phosphatase LAR, which is important for dendrite development. Thus, regulated degradation of liprinalpha1 is important for proper LAR receptor distribution, and could provide a mechanism for localized control of dendrite and synapse morphogenesis by activity and CaMKII.

Novel splice variants of ING4 and their possible roles in the regulation of cell growth and motility

The ING4 gene is a candidate tumor suppressor gene that functions in cell proliferation, contact inhibition, and angiogenesis. We identified three novel splice variants of ING4 with differing activities in controlling cell proliferation, cell spreading, and cell migration. ING4_v1 (the longest splice variant), originally identified as ING4, encodes an intact nuclear localization signal (NLS), whereas the other three splice variants (ING4_v2, ING4_v3, and ING4_v4) lack the full NLS, resulting in increased cytoplasmic localization of these proteins. We found that one of the three ING4 variants, ING4_v2, is expressed at the same level as the original ING4 (ING4_v1), suggesting that ING4 variants may have significant biological functions. Growth suppressive effects of the variants that have a partial NLS (ING4_v2 and ING4_v4) were attenuated by a weaker effect of the variants on p21(WAF1) promoter activation. ING4_v4 lost cell spreading and migration suppressive effects; on the other hand, ING4_v2 retained a cell migration suppressive effect but lost a cell spreading suppressive effect. Therefore, ING4_v2, which localized primarily into cytoplasm, might have an important role in the regulation of cell migration. We also found that ING4_v4 played dominant-negative roles in the induction of p21(WAF1) promoter activation and in the suppression of cell motility by ING4_v1. In addition, ING4 variants had different binding affinities to two cytoplasmic proteins, protein-tyrosine phosphatase, receptor type, f polypeptide (PTPRF), interacting protein (liprin), alpha1, and G3BP2a. Understanding the functions of the four splice variants may aid in defining their roles in human carcinogenesis.

Molecular organization of the presynaptic active zone

The exocytosis of neurotransmitter-filled synaptic vesicles is under tight temporal and spatial control in presynaptic nerve terminals. The fusion of synaptic vesicles is restricted to a specialized area of the presynaptic plasma membrane: the active zone. The protein network that constitutes the cytomatrix at the active zone (CAZ) is involved in the organization of docking and priming of synaptic vesicles and in mediating use-dependent changes in release during short-term and long-term synaptic plasticity. To date, five protein families whose members are highly enriched at active zones (Munc13s, RIMs, ELKS proteins, Piccolo and Bassoon, and the liprins-alpha), have been characterized. These multidomain proteins are instrumental for the diverse functions performed by the presynaptic active zone.

Liprin-alpha has LAR-independent functions in R7 photoreceptor axon targeting

In the Drosophila visual system, the color-sensing photoreceptors R7 and R8 project their axons to two distinct layers in the medulla. Loss of the receptor tyrosine phosphatase LAR from R7 photoreceptors causes their axons to terminate prematurely in the R8 layer. Here we identify a null mutation in the Liprin-alpha gene based on a similar R7 projection defect. Liprin-alpha physically interacts with the inactive D2 phosphatase domain of LAR, and this domain is also essential for R7 targeting. However, another LAR-dependent function, egg elongation, requires neither Liprin-alpha nor the LAR D2 domain. Although human and Caenorhabditis elegans Liprin-alpha proteins have been reported to control the localization of LAR, we find that LAR localizes to focal adhesions in Drosophila S2R+ cells and to photoreceptor growth cones in vivo independently of Liprin-alpha. In addition, Liprin-alpha overexpression or loss of function can affect R7 targeting in the complete absence of LAR. We conclude that Liprin-alpha does not simply act by regulating LAR localization but also has LAR-independent functions.