(Macro)autophagy is a fundamental degradation process for macromolecules and organelles of vital importance for cell and tissue homeostasis. Autophagy research has gained a strong momentum in recent ...years because of its relevance to cancer, neurodegenerative diseases, muscular dystrophy, lipid storage disorders, development, ageing and innate immunity. Autophagy has traditionally been thought of as a bulk degradation process that is mobilized upon nutritional starvation to replenish the cell with building blocks and keep up with the energy demand. This view has recently changed dramatically following an array of papers describing various forms of selective autophagy. A main driving force has been the discovery of specific autophagy receptors that sequester cargo into forming autophagosomes (phagophores). At the heart of this selectivity lies the LC3-interacting region (LIR) motif, which ensures the targeting of autophagy receptors to LC3 (or other ATG8 family proteins) anchored in the phagophore membrane. LIR-containing proteins include cargo receptors, members of the basal autophagy apparatus, proteins associated with vesicles and of their transport, Rab GTPase-activating proteins (GAPs) and specific signaling proteins that are degraded by selective autophagy. Here, we comment on these new insights and focus on the interactions of LIR-containing proteins with members of the ATG8 protein family.
Moesin is a member of the ezrin–radixin–moesin (ERM) family of proteins that link plasma membrane proteins to the cortical cytoskeleton and thus regulate diverse cellular processes. Mutations in the ...human moesin gene cause a primary immunodeficiency called X-linked moesin-associated immunodeficiency (X-MAID), which may be complicated by an autoimmune phenotype with kidney involvement. We previously reported that moesin-deficient mice exhibit lymphopenia similar to that of X-MAID and develop a lupus-like autoimmune phenotype with age. However, the mechanism through which moesin defects cause kidney pathology remains obscure. Here, we characterized immune cell infiltration and chemokine expression in the kidney of moesin-deficient mice. We found accumulation of CD4+ T and CD11b+ myeloid cells and high expression of CXCL13, whose upregulation was detected before the onset of overt nephritis. CD4+ T cell population contained IFN-γ–producing effectors and expressed the CXCL13 receptor CXCR5. Among myeloid cells, Ly6Clo patrolling monocytes and MHCIIlo macrophages markedly accumulated in moesin-deficient kidneys and expressed high CXCL13 levels, implicating the CXCL13–CXCR5 axis in nephritis development. Functionally, Ly6Clo monocytes from moesin-deficient mice showed reduced migration toward sphingosine 1-phosphate. These findings suggest that moesin plays a role in regulating patrolling monocyte homeostasis, and that its defects lead to nephritis associated with accumulation of CXCL13-producing monocytes and macrophages.
•Moesin-deficient mice develop lupus-like nephritis with high CXCL13 expression.•CXCL13 is produced by patrolling monocytes and MHCIIlo macrophages accumulated in the kidney.•CXCR5-expressing CD4+ T effector cells accumulate in the kidney.•Moesin plays a role in regulating patrolling monocyte homeostasis.
Two types of sequences, proline-rich domains (PRDs) and the WASP-homology 2 (WH2) domain, are found in most actin filament nucleation and elongation factors discovered thus far. PRDs serve as a ...platform for protein–protein interactions, often mediating the binding of profilin–actin. The WH2 domain is an abundant actin monomer-binding motif comprising ∼17 amino acids. It frequently occurs in tandem repeats, and functions in nucleation by recruiting actin subunits to form the polymerization nucleus. It is found in Spire, Cordon Bleu (Cobl), Leiomodin (Lmod), Arp2/3 complex activators (WASP, WHAMM, WAVE, etc.), the bacterial nucleators VopL/VopF and Sca2, and some formins. Yet, it is argued here that the WH2 domain plays only an auxiliary role in nucleation, always synergizing with other domains or proteins for this activity.
WH2-domain-related sequences have been found in the majority of actin filament nucleation and elongation factors discovered thus far, including several formins.
Tandem repeats of WH2 domains, found in proteins such as Spire, Cobl, VopL/VopF, and Sca2, are no longer considered sufficient to drive efficient nucleation. These proteins cooperate with other proteins or present other domains necessary for efficient nucleation.
It is now understood that optimal nucleation activity by several formins requires sequences C terminal to the FH2 domain that either directly recruit actin monomers via WH2-like domains or bind to other proteins that contribute actin subunits during nucleation.
Cooperation among different subfamilies of actin nucleators and dimerization are emerging as two widespread features in actin nucleation.
A dynamic actin cytoskeleton is essential for pollen germination and tube growth. However, the molecular mechanisms underlying the organization and turnover of the actin cytoskeleton in pollen remain ...poorly understood. Villin plays a key role in the formation of higher-order structures from actin filaments and in the regulation of actin dynamics in eukaryotic cells. It belongs to the villin/gelsolin/fragmin superfamily of actin binding proteins and is composed of six gelsolin-homology domains at its core and a villin headpiece domain at its C terminus. Recently, several villin family members from plants have been shown to sever, cap, and bundle actin filaments in vitro. Here, we characterized a villin isovariant, Arabidopsis thaliana VILLIN5 (VLN5), that is highly and preferentially expressed in pollen. VLN5 loss-of-function retarded pollen tube growth and sensitized actin filaments in pollen grains and tubes to latrunculin B. In vitro biochemical analyses revealed that VLN5 is a typical member of the villin family and retains a full suite of activities, including barbed-end capping, filament bundling, and calcium-dependent severing. The severing activity was confirmed with time-lapse evanescent wave microscopy of individual actin filaments in vitro. We propose that VLN5 is a major regulator of actin filament stability and turnover that functions in concert with oscillatory calcium gradients in pollen and therefore plays an integral role in pollen germination and tube growth.
α-Actinin-4 is a widely expressed protein that employs an actin-binding site with two calponin homology domains to crosslink actin filaments (F-actin) in a Ca²⁺-sensitive manner in vitro. An ...inherited, late-onset form of kidney failure is caused by point mutations in the α-actinin-4 actin-binding domain. Here we show that α-actinin-4/F-actin aggregates, observed in vivo in podocytes of humans and mice with disease, likely form as a direct result of the increased actin-binding affinity of the protein. We document that exposure of a buried actin-binding site 1 in mutant α-actinin-4 causes an increase in its actin-binding affinity, abolishes its Ca²⁺ regulation in vitro, and diverts its normal localization from actin stress fibers and focal adhesions in vivo. Inactivation of this buried actin-binding site returns the affinity of the mutant to that of the WT protein and abolishes aggregate formation in cells. In vitro, actin filaments crosslinked by the mutant α-actinin-4 exhibit profound changes of structural and biomechanical properties compared with WT α-actinin-4. On a molecular level, our findings elucidate the physiological importance of a dynamic interaction of α-actinin with F-actin in podocytes in vivo. We propose that a conformational change with full exposure of actin-binding site 1 could function as a switch mechanism to regulate the actin-binding affinity of α-actinin and possibly other calponin homology domain proteins under physiological conditions.
Major depressive disorder (MDD) has been shown to be associated with structural abnormalities in a variety of spatially diverse brain regions. However, the correlation between brain structural ...changes in MDD and gene expression is unclear. Here, we examine the link between brain-wide gene expression and morphometric changes in individuals with MDD, using neuroimaging data from two independent cohorts and a publicly available transcriptomic dataset. Morphometric similarity network (MSN) analysis shows replicable cortical structural differences in individuals with MDD compared to control subjects. Using human brain gene expression data, we observe that the expression of MDD-associated genes spatially correlates with MSN differences. Analysis of cell type-specific signature genes suggests that microglia and neuronal specific transcriptional changes account for most of the observed correlation with MDD-specific MSN differences. Collectively, our findings link molecular and structural changes relevant for MDD.
Transgelin-2 has been regarded as an actin-binding protein that induces actin gelation and regulates actin cytoskeleton. However, transgelin-2 has recently been shown to relax the myosin cytoskeleton ...of the airway smooth muscle cells by acting as a receptor for extracellular metallothionein-2. From a clinical perspective, these results support transgelin-2 as a promising therapeutic target for diseases such as cancer and asthma. The inhibition of transgelin-2 prevents actin gelation and thereby cancer cell proliferation, invasion, and metastasis. Conversely, the activation of transgelin-2 with specific agonists relaxes airway smooth muscles and reduces pulmonary resistance in asthma. Here, we review new studies on the biochemical properties of transgelin-2 and discuss their clinical implications for the treatment of immune, oncogenic, and respiratory disorders.
Actin is a critical component of the cellular cytoskeleton and interacts with a plethora of actin-binding proteins.Transgelin-2 is an important actin-binding protein and misregulation of transgelin-2 may contribute to diseases such as cancer and asthma.Ezrin has emerged as an essential regulator of transgelin-2 in biological functions other than those involving actin.Specific targeting of transgelin-2 may result in a high potential to selectively and effectively regulate various diseases.
Acidosis is a fundamental feature of the tumor microenvironment, which directly regulates tumor cell invasion by affecting immune cell function, clonal cell evolution, and drug resistance. Despite ...the important association of tumor microenvironment acidosis with tumor cell invasion, relatively little is known regarding which areas within a tumor are acidic and how acidosis influences gene expression to promote invasion. Here, we injected a labeled pH-responsive peptide to mark acidic regions within tumors. Surprisingly, acidic regions were not restricted to hypoxic areas and overlapped with highly proliferative, invasive regions at the tumor-stroma interface, which were marked by increased expression of matrix metalloproteinases and degradation of the basement membrane. RNA-seq analysis of cells exposed to low pH conditions revealed a general rewiring of the transcriptome that involved RNA splicing and enriched for targets of RNA binding proteins with specificity for AU-rich motifs. Alternative splicing of Mena and CD44, which play important isoform-specific roles in metastasis and drug resistance, respectively, was sensitive to histone acetylation status. Strikingly, this program of alternative splicing was reversed
and
through neutralization experiments that mitigated acidic conditions. These findings highlight a previously underappreciated role for localized acidification of tumor microenvironment in the expression of an alternative splicing-dependent tumor invasion program. SIGNIFICANCE: This study expands our understanding of acidosis within the tumor microenvironment and indicates that acidosis induces potentially therapeutically actionable changes to alternative splicing.
Mammalian Toll-like receptors (TLRs) play an important role in the innate recognition of pathogens by dendritic cells (DCs). Although TLRs are clearly involved in the detection of bacteria and ...viruses, relatively little is known about their function in the innate response to eukaryotic microorganisms. Here we identify a profilin-like molecule from the protozoan parasite Toxoplasma gondii that generates a potent interleukin-12 (IL-12) response in murine DCs that is dependent on myeloid differentiation factor 88. T. gondii profilin activates DCs through TLR11 and is the first chemically defined ligand for this TLR. Moreover, TLR11 is required in vivo for parasite-induced IL-12 production and optimal resistance to infection, thereby establishing a role for the receptor in host recognition of protozoan pathogens.
Purified actin filaments depolymerize slowly, and cytosolic conditions strongly favour actin assembly over disassembly, which has left our understanding of how actin filaments are rapidly turned over ...in vivo incomplete. One mechanism for driving filament disassembly is severing by factors such as Cofilin. However, even after severing, pointed-end depolymerization remains slow and unable to fully account for observed rates of actin filament turnover in vivo. Here we describe a mechanism by which Twinfilin and Cyclase-associated protein work in concert to accelerate depolymerization of actin filaments by 3-fold and 17-fold at their barbed and pointed ends, respectively. This mechanism occurs even under assembly conditions, allowing reconstitution and direct visualization of individual filaments undergoing tunable, accelerated treadmilling. Further, we use specific mutations to demonstrate that this activity is critical for Twinfilin function in vivo. These findings fill a major gap in our knowledge of cellular disassembly mechanisms, and suggest that depolymerization and severing may be deployed separately or together to control the dynamics and architecture of distinct actin networks.
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