Saima Aijaz

Tight junctions: Molecular architecture and function

Tight junctions are the most apical component of the epithelial junctional complex and are crucial for the formation and functioning of epithelial and endothelial barriers. They regulate selective diffusion of ions and solutes along the paracellular pathway and restrict apical/basolateral intramembrane diffusion of lipids. Research over the past years provided much insight into the molecular composition of tight junctions, and we are starting to understand the mechanisms that permit selective paracellular diffusion. Moreover, a complex network of proteins has been identified at tight junctions that is based on cytoskeleton-linked adaptors that recruit and thereby often regulate different types of signaling components that regulate epithelial proliferation, differentiation, and polarization.

The Y-box factor ZONAB/DbpA associates with GEF-H1/Lfc and mediates Rho-stimulated transcription

Epithelial tight junctions recruit different types of signalling proteins that regulate cell proliferation and differentiation. Little is known about how such proteins interact functionally and biochemically with each other. Here, we focus on the Y‐box transcription factor ZONAB (zonula occludens 1‐associated nucleic‐acid‐binding protein)/DbpA (DNA‐binding protein A) and the Rho GTPase activator guanine nucleotide exchange factor (GEF)‐H1/Lbcs first cousin, which are two tight‐junction‐associated signalling proteins that regulate proliferation. Our data show that the two proteins interact and that ZONAB activity is Rho‐dependent. Overexpression of GEF‐H1 induces accumulation of ZONAB in the nucleus and activates transcription. Microtubule‐affinity regulating kinase/partition‐defective‐1, another type of GEF‐H1‐associated signalling protein, remains in the cytoplasm and partially co‐localizes with the exchange factor. GEF‐H1 and ZONAB are required for expression of endogenous cyclin D1, a crucial RhoA signalling target gene, and GEF‐H1‐stimulated cyclin D1 promoter activity requires ZONAB. Our data thus indicate that GEF‐H1 and ZONAB form a signalling module that mediates Rho‐regulated cyclin D1 promoter activation and expression.

A review of primary hereditary optic neuropathies.

Summary: The primary inherited optic neuropathies are a heterogeneous group of disorders that result in loss of retinal ganglion cells, leading to the clinical appearance of optic atrophy. They affect between 1:10 000 to 1:50 000 people. The main clinical features are a reduction in visual acuity, colour vision abnormalities, centro-caecal visual field defects and pallor of the optic nerve head. Electrophysiological testing shows a normal flash electroretinogram, absentor delayed pattern visually evoked potentials suggestive of a conduction deficit and N95 waveform reduction on the pattern electroretinogram, consistent with a primary ganglion cell pathology. The primary inherited optic neuropathies may be sporadic or familial. The mode of inheritance may be autosomal dominant, autosomal recessive, X-linked recessive or mitochondrial. Within each of these groups, the phenotypic characteristics vary in such features as the mode and age of onset, the severity of the visual loss, the colour deficit and the overall prognosis. A number of different genes (most as yet unidentified) in both nuclear and mitochondrial genomes, underlie these disorders. The elucidation of the role of the encoded proteins will improve our understanding of basic mechanisms of ganglion cell development, physiology and metabolism and further our understanding of the pathophysiology of optic nerve disease. It will also improve diagnosis, counselling and management of patients, and eventually lead to the development of new therapeutic modalities.

Regulation of tight junction assembly and epithelial morphogenesis by the heat shock protein Apg-2

BackgroundTight junctions are required for epithelial barrier formation and participate in the regulation of signalling mechanisms that control proliferation and differentiation. ZO-1 is a tight junction-associated adaptor protein that regulates gene expression, junction assembly and epithelial morphogenesis. We have previously demonstrated that the heat shock protein Apg-2 binds ZO-1 and thereby regulates its role in cell proliferation. Here, we addressed the question whether Apg-2 is also important for junction formation and epithelial morphogenesis.ResultsWe demonstrate that depletion of Apg-2 by RNAi in MDCK cells did not prevent formation of functional tight junctions. Similar to ZO-1, however, reduced expression of Apg-2 retarded de novo junction assembly if analysed in a Ca-switch model. Formation of functional junctions, as monitored by measuring transepithelial electrical resistance, and recruitment of tight and adherens junction markers were retarded. If cultured in three dimensional extracellular matrix gels, Apg-2 depleted cells, as previously shown for ZO-1 depleted cells, did not form hollow polarised cysts but poorly organised, irregular structures.ConclusionOur data indicate that Apg-2 regulates junction assembly and is required for normal epithelial morphogenesis in a three-dimensional culture system, suggesting that Apg-2 is an important regulator of epithelial differentiation. As the observed phenotypes are similar to those previously described for ZO-1 depleted cells and depletion of Apg-2 retards junctional recruitment of ZO-1, regulation of ZO-1 is likely to be an important functional role for Apg-2 during epithelial differentiation.