Linear Filaments Research Articles

Optimization of textile parameters of plain woven vascular prostheses

This paper investigates and reports the effect of some textile parameters (yarn and fabric parameters) on the main performances of plain woven vascular prostheses in order to enhance their overall quality. We weaved tubular structures with different fabric densities using texturized Dacron® polyester filament yarns with different linear densities and filament counts, in warp and weft directions. The performances of the manufactured samples, such as wall porosity, water permeability and mechanical features, have been tested. By adopting a two‐level fractional factorial design of experiments, the responses were fitted to predictive models using a multiple linear regression method. The models were tested by variance analysis and validated by correlation between measured and predicted values. We determined the optimum settings of main textile parameters enhancing plain woven vascular prosthesis quality by contour plots method.

LKB1 tumor suppressor protein regulates actin filament assembly through Rho and its exchange factor Dbl independently of kinase activity

BackgroundGermline mutations in LKB1 result in Peutz-Jeghers Syndrome characterized by intestinal hamartomas and increased incidence of epithelial cancers. LKB1 encodes a serine/threonine kinase that plays an important role in regulating energy metabolism through the AMPK/mTOR signaling pathway. In addition, LKB1 is homologous to PAR-4, a polarity protein first described in C. elegans, while activation of LKB1 in mammalian epithelial cells induces the polarized assembly of actin filaments.ResultsTo explore the mechanism by which LKB1 interacts with the actin cytoskeleton, we introduced LKB1 into HeLa cells that lack endogenous LKB1. This results in activation of the small GTPase Rho and the assembly of linear actin filaments associated with focal adhesions. These effects on the actin cytoskeleton are attenuated by siRNA-mediated depletion of the guanine nucleotide exchange factor Dbl. Co-expression of the LKB1 with the adaptor protein STRAD induces actin filament puncta associated with phospho-ezrin.ConclusionsThis study reveals that LKB1 regulates the actin cytoskeleton through a Dbl/Rho pathway.

Crystal Structure of a Complex between Amino and Carboxy Terminal Fragments of mDia1: Insights into Autoinhibition of Diaphanous-Related Formins

Formin proteins direct the nucleation and assembly of linear actin filaments in a variety of cellular processes using their conserved formin homology 2 (FH2) domain. Diaphanous-related formins (DRFs) are effectors of Rho-family GTPases, and in the absence of Rho activation they are maintained in an inactive state by intramolecular interactions between their regulatory N-terminal region and a C-terminal segment referred to as the DAD domain. Although structures are available for the isolated DAD segment in complex with the interacting region in the N-terminus, it remains unclear how this leads to inhibition of actin assembly by the FH2 domain. Here we describe the crystal structure of the N-terminal regulatory region of formin mDia1 in complex with a C-terminal fragment containing both the FH2 and DAD domains. In the crystal structure and in solution, these fragments form a tetrameric complex composed of two interlocking N+C dimers. Formation of the tetramer is likely a consequence of the particular N-terminal construct employed, as we show that a nearly full-length mDia1 protein is dimeric, as are other autoinhibited N+C complexes containing longer N-terminal fragments. The structure provides the first view of the intact C-terminus of a DRF, revealing the relationship of the DAD to the FH2 domain. Delineation of alternative dimeric N+C interactions within the tetramer provides two general models for autoinhibition in intact formins. In both models, engagement of the DAD by the N-terminus is incompatible with actin filament formation on the FH2, and in one model the actin binding surfaces of the FH2 domain are directly blocked by the N-terminus.

A polarity factor takes the lead in chromosome segregation

A polarity factor takes the lead in chromosome segregation

Expression of Cytokeratin, Actin and Endocrine Hormones in Human Hatched Blastocyst Implantation Model in vitro

Objective To investigate the expression of cytokeratin, actin and hCG, E 2 and P of human hacthed blastocysts in the model. Methods Human hatched blastocysts were co-cultured with human endometrial decidualization stromal cell monolayer. The process of orientation, attachment, outgrowth and invasion in morphology were observed. Immunofluorescence staining for cytokeratin and actin, immunofluorescence measurement of hCG and radioimmunoassay measurement of E 2 and P were performed. Results Blastocysts attached to stromal cell layer after 5 h in co-culture. After 24 h in co-culture, the trophoblast protruded from two opposite poles of the blastocyst and underwent outgrowth into the stromal cell monolayer, blastocyst became bigger and invaded finally into the stromal cells. After 48 h in co-culture, cytokeratin staining was only visible in trophoblast but not stromal cells, actin staining was visible in both of trophoblast and stromal cells with distinct conformation and structure. Stromal cells had prominent linear actin filaments, aligned along the long axis of the cells. Cytokeratin staining in trophoblast cells was localized in short filaments arranged in a mesh. hCG, E 2 and P levels in the supernate of stromal cell-blastocyst co-culture were higher than the supernate from blastocyst cultured only (P Conclusion An implantation model for the reflection of the process of human blastocysts attachment, outgrowth and invasion into stromal cells has been established in vitro. Cytokeratin, actin and hCG, E 2 and P take place corresponding changes in the human implantation blastocyst cells.

Phospholipids regulate localization and activity of mDia1 formin

Diaphanous-related formins (DRFs) are large multi-domain proteins that nucleate and assemble linear actin filaments. Binding of active Rho family proteins to the GTPase-binding domain (GBD) triggers localization at the membrane and the activation of most formins if not all. In recent years GTPase regulation of formins has been extensively studied, but other molecular mechanisms that determine subcellular distribution or regulate formin activity have remained poorly understood. Here, we provide evidence that the activity and localization of mouse formin mDia1 can be regulated through interactions with phospholipids. The phospholipid-binding sites of mDia1 are clusters of positively charged residues in the N-terminal basic domain (BD) and at the C-terminal region. Upon binding to the lipid bilayer the N-terminal region of mDia1 induces strong clustering of phosphatidylinositol-4,5-bisphosphate (PIP 2) and subsequently inserts into the membrane bilayer thus anchoring mDia1 to the reconstituted plasma membrane. In addition, an interaction of phospholipids with the C-terminal region of mDia1 causes a drastic reduction of its actin filament assembly activity. Our data suggest that the N-terminal phospholipid-binding sites help to anchor formins at the plasma membrane, and the interaction with phospholipids in the C-terminus functions as a switch for transient inactivation.

Flightless-I (Fli-I) Regulates the Actin Assembly Activity of Diaphanous-related Formins (DRFs) Daam1 and mDia1 in Cooperation with Active Rho GTPase

Eukaryotic cells dynamically reorganize the actin cytoskeleton to regulate various cellular activities, such as cell shape change, cell motility, cytokinesis, and vesicular transport. Diaphanous-related formins (DRFs), such as Daam1 and mDia1, play central roles in actin dynamics through assembling linear actin filaments. It has been reported that the GTP-bound active Rho binds directly to DRFs and partially unleashes the intramolecular autoinhibition of DRFs. However, whether proteins other than Rho involve the regulation of the actin assembly activity of DRFs has been unclear. Here, we show that Flightless-I (Fli-I), a gelsolin family protein essential for early development, binds directly to Daam1 and mDia1. Fli-I enhances the intrinsic actin assembly activity of Daam1 and mDia1 in vitro and is required for Daam1-induced actin assembly in living cells. Furthermore, Fli-I promotes the GTP-bound active Rho-mediated relief of the autoinhibition of Daam1 and mDia1. Thus, Fli-I is a novel positive regulator of Rho-induced linear actin assembly mediated by DRFs.

Abstract LB-270: Regulation of the cortical actin cytoskeleton and amoeboid motility through the mDia2 formin;DIP complex

Abstract Tumor cells display a remarkable morphological plasticity that supports malignant progression. From abnormal survival/proliferation in host tissue to tumor egress during metastasis, dynamic remodeling of the actin and microtubule cytoskeletons drives membrane protrusions, trafficking, and adhesion events essential to cell migration. Central to this process are Rho family small GTP binding proteins and their downstream effectors, such as the Wasp/Scar activators of Arp2/3 branched actin assembly. While GTPase-activated Arp2/3 has key roles in regulated changes in cell morphology, only recently has it become clear that formins, which assembly linear actin filaments, have fundamental roles in generating protrusive cell structures, including filopodia and lamellipodia. The canonical mammalian Diaphanous (mDia)-related Formin-2 regulates the cortical actin cytoskeleton through association with Diaphanous-interacting protein (DIP). Upon interacting with mDia2, DIP inhibits mDia2's ability to nucleate, processively elongate and bundle non-branched F-actin. Direct interaction between the Formin homology-2 (FH2) domain and the leucine-rich DIP C-terminus (LRR)) disrupts the cortical actin cytoskeleton, triggering non-apoptotic plasma membrane blebbing on two-dimensional (2D) matrices. Membrane blebbing is a physiological process engaging elements of the contractile machinery and functions in amoeboid cell motility (ACM). ACM contrasts from mesenchymal-type cell motility, as it is a focal adhesion (FA)/integrin- and MMP-independent process utilizing Rho-, as opposed to Rac-based signaling mechanisms. ACM was described as a specialized mode of breast cancer cell migration and may play an essential role in metastasis. Here we demonstrate a tightly regulated complex between mDia2 and DIP that deregulates mDia2-dependent F-actin dynamics in vitro and in vivo and induces membrane blebbing. Both DIP and mDia2 are dynamically localized to blebs in constitutively blebbing cancer cells in both 2D and 3D, suggesting a critical role in cortical contractility fundamental to the amoeboid phenotype. Conversely, depleting DIP inhibits ACM and promotes elongated, filopodia-rich morphology in breast cancer cells predisposed to this type of 3D migration. We propose that DIP and mDia2 comprise a precise regulatory mechanism controlling cortical actin assembly and are required for driving cortical contractility and the amoeboid-type of cancer cell invasion. As few therapeutic treatments effectively treat metastatic disease, understanding the molecular basis of amoeboid cell movement will lend novel insight into mechanisms controlling and contributing to cancer cell migration, invasion, and metastasis and may provide a critically needed alternate therapeutic for metastatic disease. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-270.

Abstract 2539: Reactivating tumor suppression with small-molecule deregulators of the Rho-regulated diaphanous-related formin autoinhibitory mechanism

Abstract Loss of all or part of the long-arm of chromosome 5 is a frequent karyotypic defect in myeloproliferitive neoplasms, including myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). To date, there is no clear mechanism that accounts for how any of the 5q tumor suppressor candidates contribute to hematopoietic stem cell growth control or how defects in their expression/function contribute to malignant progression. An attractive 5q- MDS tumor suppressor gene is DIAPH1 (5q31.3)]. DIAPH1 encodes the canonical mammalian Diaphanous-related (mDia) formin mDia1. Consistent with a role in tumor suppression, mDia1 knockout (KO) mice develop an age-dependent MDS-like phenotype. mDia1 is an effector for the Rho-family small GTP-binding protein RhoB, which is known to harbor tumor suppressor activity. mDia1 has a central role in the actin and microtubule dynamics that underlie cell division and directed migration. In this study we sought to determine if we could manipulate mDia activity to ‘reactivate’ tumor suppression by exploiting our working knowledge of the Rho-controlled mDia autoregulatory mechanism. GTP-bound Rho proteins activate mDia formins by binding to and disrupting an intramolecular autoinhibitory mechanism mediated by regulatory Dia-inhibitory (DID) and Dia-autoregulatory (DAD) domains that flank the conserved formin homology-2 (FH2) domain found in all formins that is responsible for assembling linear actin filaments and modulating microtubule stability. To identify small molecule activators of mDia formins, a screen for inhibitors of DID-DAD binding was conducted in a high-throughput format. We identified two structurally-related semi-thiocarbazones that bind within the DAD binding pocket of DID. These compounds, called Intramimics (IMM)-1 and −2, possess IC50 values for in vitro inhibition of DAD binding of 99 and 140 nM, respectively; the values were comparable to a DAD-derived peptide (280 nM). Consistent with mDia activation, exposure of fibroblasts and AML-derived cells leads to stabilization of actin and microtubule dynamics. The consequence is the induction of apoptosis, which was assayed by a variety of approaches. Using IMMs as tool compounds to explore tumor suppression mechanisms, IMM treatment activated serum response factor (SRF)-regulated gene expression. SRF is a growth factor-regulated transcription factor that also senses changes in actin assembly and is a potent activator of Egr1 expression. Egr1 is another 5q tumor suppressor candidate and is also a transcription factor that appears to be a master regulator of the well-known tumor suppressors PTEN and p53. Our next goal is to test IMMs in preclinical models for their ability to modulate myeloproliferation and to inhibit tumor growth, alone and in combination with other anti-tumor agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2539.

Widespread occurrence of eosinophilic neuronal cytoplasmic inclusions in an asymptomatic adult: A novel ubiquitin-negative filamentous inclusion

We describe a 67-year-old woman without apparent neurological symptoms, in whom postmortem examination revealed widespread occurrence of eosinophilic neuronal cytoplasmic inclusions in the central and peripheral nervous systems. The inclusions were round, oval or rod-like in shape. Immunohistochemically, the inclusions were negative for ubiquitin and not labeled with any other antibodies, except for a partial and weak immunoreactivity with anti-neurofilament occurring rarely. Ultrastructurally, the inclusions revealed two different forms. The common form was entirely composed of bundles of wavy granule-coated filaments (20-30 nm in diameter). The other form consisted of a core containing linear filaments (12-15 nm in diameter) with electron-dense ribosome-like granules and an outer zone with wavy filaments as seen in the former. This inclusion seems to represent a new type of neuronal cytoplasmic inclusion.

Cutaneous amyloidoses and systemic amyloidoses with cutaneous involvement

Extracellular deposition of altered autologous protein (amyloid protein) within the dermis is the hallmark of cutaneous amyloidoses and systemic amyloidoses with cutaneous involvement. Amyloidoses may be acquired or hereditary in nature and subclassification differentiates between primary amyloidosis (no obvious predisposing disease) and secondary amyloidosis (specific underlying disease). More than 26 different proteins and peptides have been identified as amyloid precursors and these proteins are used to subclassify this heterogeneous group of diseases. The amyloid proteins show an anti-parallel beta-sheet conformation and form non-branching linear filaments of variable lengths and diameters of approximately 7.5 to 10 nm. However, the exact etiopathogenesis of amyloid formation still remains unclear. Depending on histoanatomical distribution and amount, amyloid may cause progressive and life-threatening organ dysfunction. Clinical presentation, histology, electron microscopy, and biochemical-immunological differentiation represent decisive tools for an accurate diagnosis.

Organization of F-Actin by Avian Smooth Muscle Synaptopodin 2 (Fesselin)

Fesselin is a member of the synaptopodin family of actin binding proteins and it is rich in smooth muscle tissue. Fesselin accelerates actin polymerization in a calcium-calmodulin dependent fashion and forms insoluble aggregates with actin. The polymerization of actin occurs even in the absence of salts usually required to initiate polymerization. The question is whether the properties of fesselin are indicative of a function in forming structures rich in fesselin such as dense bodies and Z-lines. Electron microscopy of actin filaments shows that fesselin initially produces long actin filaments that become organized into thick bundles. The structure of the bundles depends on the ratio of fesselin to actin. Most frequently there are dense parallel ordered bundles. In some cases, the bundles are frayed and thus display single actin filaments. The filaments within a bundle are in-register and have either common initiation or termination points. Decoration with S1 showed that the filaments of a bundle had the same orientation. Actin filaments were occasionally seen to be radiating from densely stained complexes. Solution studies support the idea that fesselin stimulates pointed end growth of actin filaments. That is, fesselin (a) increased the critical concentration for polymerization and (b) induced actin growth even in the presence of barbed end blockers. When added to actin in the absence of other factors, fesselin can nucleate networks of linear actin filaments in which growth occurs primarily at the pointed ends of actin filaments.

Cross-Bridges and Sarcomere Stiffness in Single Intact Frog Muscle Fibers

The number of cross-bridges formed in activated skeletal muscles is a key information for both energetics and mechanics of muscle contraction. In this study we determined the cross-bridge number in single fibers by measuring the tension Pcwhich forcibly detached the cross-bridge by fast stretches (Bagni et al. 2005, J. Physiol 565). Fibers, isolated from the tibialis anterior muscle of Rana esculenta, were mounted between an electromagnetic motor and a fast force transducer. Sarcomere length was measured by means of a striation follower device. Measurements were made during tetanus rise in normal Ringer and in sub-maximal tetanic contractions in Ringer-BTS (N-benzyl-p-toluene sulphonamide, 1 μM) at 5°C at sarcomere length of 2.1 μm. The results were compared with fiber stiffness, another indicator of cross-bridge number, measured with 4 kHz sinusoidal length oscillations (1 nmhs−1 p-p amplitude). The stiffness-tension relation was the same both during the tetanus rise and Ringer-BTS and showed the non-linearity expected from the myofilament compliance. However, the data could not be fitted satisfactorily with a simple model made of cross-bridge and linear filament compliances in series. A good fit was obtained by assuming that a fraction (∼14%) of attached bridges at tetanus plateau was generating no-force. Relative myofilament and cross-bridge compliance resulted 0.37 and 0.63 respectively. The stretch data showed a linear relation between Pcand tension with a slope consistent with the presence of the non-force generating bridges suggested by stiffness data. These results suggests the existence of a possible non-linearity between cross-bridge force and stiffness and show that the relation between fiber stiffness and cross-bridge number is not as simple as usually assumed.

HST/NICMOS Paschen-α Survey of the Galactic Centre: Overview

We have recently carried out the first wide-field hydrogen Paschen-alpha line imaging survey of the Galactic Center (GC), using the NICMOS instrument aboard the Hubble Space Telescope. The survey maps out a region of 2253 pc^2 around the central supermassive black hole (Sgr A*) in the 1.87 and 1.90 Micron narrow bands with a spatial resolution of 0.01 pc at a distance of 8 kpc. Here we present an overview of the observations, data reduction, preliminary results, and potential scientific implications, as well as a description of the rationale and design of the survey. We have produced mosaic maps of the Paschen-alpha line and continuum emission, giving an unprecedentedly high resolution and high sensitivity panoramic view of stars and photo-ionized gas in the nuclear environment of the Galaxy. We detect a significant number of previously undetected stars with Paschen-alpha in emission. They are most likely massive stars with strong winds, as confirmed by our initial follow-up spectroscopic observations. About half of the newly detected massive stars are found outside the known clusters (Arches, Quintuplet, and Central). Many previously known diffuse thermal features are now resolved into arrays of intriguingly fine linear filaments indicating a profound role of magnetic fields in sculpting the gas. The bright spiral-like Paschen-alpha emission around Sgr A* is seen to be well confined within the known dusty torus. In the directions roughly perpendicular to it, we further detect faint, diffuse Paschen-alpha emission features, which, like earlier radio images, suggest an outflow from the structure. In addition, we detect various compact Paschen-alpha nebulae, probably tracing the accretion and/or ejection of stars at various evolutionary stages.

Molecular Architecture of Synaptic Actin Cytoskeleton in Hippocampal Neurons Reveals a Mechanism of Dendritic Spine Morphogenesis

Excitatory synapses in the brain play key roles in learning and memory. The formation and functions of postsynaptic mushroom-shaped structures, dendritic spines, and possibly of presynaptic terminals, rely on actin cytoskeleton remodeling. However, the cytoskeletal architecture of synapses remains unknown hindering the understanding of synapse morphogenesis. Using platinum replica electron microscopy, we characterized the cytoskeletal organization and molecular composition of dendritic spines, their precursors, dendritic filopodia, and presynaptic boutons. A branched actin filament network containing Arp2/3 complex and capping protein was a dominant feature of spine heads and presynaptic boutons. Surprisingly, the spine necks and bases, as well as dendritic filopodia, also contained a network, rather than a bundle, of branched and linear actin filaments that was immunopositive for Arp2/3 complex, capping protein, and myosin II, but not fascin. Thus, a tight actin filament bundle is not necessary for structural support of elongated filopodia-like protrusions. Dynamically, dendritic filopodia emerged from densities in the dendritic shaft, which by electron microscopy contained branched actin network associated with dendritic microtubules. We propose that dendritic spine morphogenesis begins from an actin patch elongating into a dendritic filopodium, which tip subsequently expands via Arp2/3 complex-dependent nucleation and which length is modulated by myosin II-dependent contractility.

A Crescent-Shaped ALIX Dimer Targets ESCRT-III CHMP4 Filaments

ALIX recruits ESCRT-III CHMP4 and is involved in membrane remodeling during endosomal receptor sorting, budding of some enveloped viruses, and cytokinesis. We show that ALIX dimerizes via the middle domain (ALIX(-V)) in solution. Structural modeling based on small angle X-ray scattering (SAXS) data reveals an elongated crescent-shaped conformation for dimeric ALIX lacking the proline-rich domain (ALIX(BRO1-V)). Mutations at the dimerization interface prevent dimerization and induce an open elongated monomeric conformation of ALIX(-V) as determined by SAXS modeling. ALIX dimerizes in vivo and dimeric ALIX colocalizes with CHMP4B upon coexpression. We show further that ALIX dimerization affects HIV-1 budding. C-terminally truncated activated CHMP4B retaining the ALIX binding site forms linear, circular, and helical filaments in vitro, which can be bridged by ALIX. Our data suggest that dimeric ALIX represents the active form that interacts with ESCRT-III CHMP4 polymers and functions as a scaffolding protein during membrane remodeling processes.

Effects of Starch/ Polycaprolactone-based Blends for Spinal Cord Injury Regeneration in Neurons/Glial Cells Viability and Proliferation

Spinal cord injury (SCI) leads to drastic alterations on the quality of life of afflicted individuals. With the advent of Tissue Engineering and Regenerative Medicine where approaches combining biomaterials, cells and growth factors are used, one can envisage novel strategies that can adequately tackle this problem. The objective of this study was to evaluate a blend of starch with poly(ε-caprolactone) (SPCL) aimed to be used for the development of scaffolds spinal cord injury (SCI) repair. SPCL linear parallel filaments were deposited on polystyrene coverslips and assays were carried out using primary cultures of hippocampal neurons and glial cells. Light and fluorescence microscopy observations revealed that both cell populations were not negatively affected by the SPCL-based biomaterial. MTS and total protein quantification indicated that both cell viability and proliferation rates were similar to controls. Both neurons and astrocytes occasionally contacted the surface of SPCL filaments through their dendrites and cytoplasmatic processes, respectively, while microglial cells were unable to do so. Using single cell [Ca2+ ]i imaging, hippocampal neurons were observed growing within the patterned channels and were functional as assessed by the response to a 30 mM KCl stimulus. The present data demonstrated that SPCL-based blends are potentially suitable for the development of scaffolds in SCI regenerative medicine.

Regulation of the Yeast Formin Bni1p by the Actin‐Regulating Kinase Prk1p

The formin family of proteins promotes the assembly of linear actin filaments in the cells of diverse eukaryotic organisms. The predominant formins in mammalian cells are self-inhibited by an intramolecular interaction between two terminal domains and are activated by the binding of the Rho GTPases and other factors. In this study, we show that Bni1p, a formin required for the assembly of actin cables in budding yeast, is also regulated by an autoinhibitory mechanism and phosphorylation by the actin regulatory kinase Prk1p, and possibly Ark1p as well, plays a key role in unlocking the inhibition. Bni1p is phosphorylated by Prk1p at three [L/V/I]xxxxTG motifs in vitro, and the phosphorylation is sufficient to activate Bni1p by disrupting its intramolecular interaction. This finding extends the roles of Prk1p in the regulation of actin dynamics to be associated with both anterograde and retrograde transport pathways, i.e. exocytosis and endocytosis, in yeast.

Optical spatial solitons and modulation instabilities in transparent entangled polymer solutions

Fully transparent nondilute polydiene solutions exhibit optical nonlinearities when irradiated by a low-power cw laser in the visible. The formation of optical spatial solitons is imaged through phase contrast microscopy. Both (2+1)D and (1+1)D modulational instabilities are evidenced as 2D and 1D arrays of linear filaments formed by beam defocusing or by using a cylindrical lens. The origin of the nonlinearity remains elusive.

The NGC 672 and 784 galaxy groups: evidence for galaxy formation and growth along a nearby dark matter filament

We present U, B, V, R, I, Hα and NUV photometry of 14 galaxies in the very local Universe (within 10 Mpc). Most objects are dwarf irregular galaxies (dIrr) and are probably associated with the NGC 672/IC 1727 and NGC 784 galaxy groups. The galaxies are at low redshift (51 ≤ v⊙≤ 610 km s−1) and most appear projected on the sky as a 6° long linear filament. We show that the galaxy positions along this filament correlate with their radial velocity, hinting to an interpretation as a single kinematic entity. Our CCD photometry indicates that all objects qualify as ‘dwarf galaxies’ with MB≥−18 mag. We examine the star formation (SF) properties of individual objects in the context of their immediate environment. The current SF rate (SFR) is derived directly from the Hα line flux. An approximate SF history is derived by comparing the multiband photometry with results of galaxy evolution models from Bruzual & Charlot (2003a,b), assuming short SF bursts separated by long quiescence periods. Relations between the current SFR and the H i mass or the absolute B magnitude for the galaxies in these groups indicate that these objects behave like normal galaxies. A comparison of the photometric measurements with evolutionary synthesis model predictions indicates that most objects can be understood as containing at least one ‘old’ stellar population (≥1–10 Gyr) and one ‘young’ population (≤30 Myr). For both groups, the recent SF bursts appear to have occurred at similar times, a few to a few tens of Myr ago, arguing for synchronicity in SF in these objects. In an attempt to evaluate the possible role of galaxy–galaxy interaction, we investigate the trend of the SFR with an object's projected distance from the brightest and most massive galaxies of each group. We do not find a steadily decreasing SF as function of this distance; such a result could be expected if the SF would have been triggered by interactions. We propose that one possible explanation of the nearly synchronous SF in all objects is accretion of cold gas from intergalactic space on to dark matter haloes arranged along a filament threading the void where these dwarf galaxies reside. We point out this galaxy sample as an ideal target to study hierarchical clustering and galaxy formation among very nearby objects.