The laminin ␣4 chain, a component of laminin-8 and -9, is expressed in basement membranes, such as those beneath endothelia, the perineurium of peripheral nerves, and around developing muscle fibers. Laminin ␣4-null mice presented with hemorrhages during the embryonic and neonatal period and had extensive bleeding and deterioration of microvessel growth in experimental angiogenesis, as well as mild locomotion defects. Histological examination of newborn mice revealed delayed deposition of type IV collagen and nidogen into capillary basement membranes, and electron microscopy showed discontinuities in the lamina densa. The results demonstrate a central role for the laminin ␣4 chain in microvessel growth and, in the absence of other laminin ␣ chains, in the composition of endothelial basement membranes.

Precise apposition of pre- to postsynaptic specializations is required for optimal function of chemical synapses, but little is known about how it is achieved. At the skeletal neuromuscular junction, active zones (transmitter release sites) in the nerve terminal lie directly opposite junctional folds in the postsynaptic membrane. Few active zones or junctional folds form in mice lacking the laminin beta2 chain, which is normally concentrated in the synaptic cleft. beta2 and the broadly expressed gamma1 chain form heterotrimers with alpha chains, three of which, alpha2, alpha4 and alpha5, are present in the synaptic cleft. Thus, alpha2beta2gamma1, alpha4beta2gamma1 and alpha5beta2gamma1 heterotrimers are all lost in beta2 mutants. In mice lacking laminin alpha4, active zones and junctional folds form in normal numbers, but are not precisely apposed to each other. Thus, formation and localization of synaptic specializations are regulated separately, and alpha4beta2gamma1 (called laminin-9) is critical in the latter process.

The Schwann cell basal lamina (BL) is required for normal myelination. Loss or mutations of BL constituents, such as laminin-2 (␣2␤1␥1), lead to severe neuropathic diseases affecting peripheral nerves. The function of the second known laminin present in Schwann cell BL, laminin-8 (␣4␤1␥1), is so far unknown. Here we show that absence of the laminin ␣4 chain, which distinguishes laminin-8 from laminin-2, leads to a disturbance in radial sorting, impaired myelination, and signs of ataxia and proprioceptive disturbances, whereas the axonal regenerative capacity is not influenced. In vitro studies show poor axon growth of spinal motoneurons on laminin-8, whereas it is extensive on laminin-2. Schwann cells, however, extend longer processes on laminin-8 than on laminin-2, and, in contrast to the interaction with laminin-2, solely use the integrin receptor ␣6␤1 in their interaction with laminin-8. Thus, laminin-2 and laminin-8 have different critical functions in peripheral nerves, mediated by different integrin receptors.

During extravasation, neutrophils migrate through the perivascular basement membrane (BM), a specialized extracellular matrix rich in laminins. Laminins 8 (LN-8) (␣4␤1␥1) and 10 (LN-10) (␣5␤1␥1) are major components of the endothelial BM, but expression, recognition, and use of these laminin isoforms by neutrophils are poorly understood. In the present study, we provide evidence, using a panel of novel monoclonal antibodies against human laminin ␣4 (LN␣4) chain, that neutrophils contain and secrete LN-8, and that this endogenous laminin contributes to chemoattractant-induced, ␣M␤2-integrindependent neutrophil migration through albumin-coated filters. Phorbol esterstimulated neutrophils adhered to recombinant human (rh) LN-8, rhLN-10, and mouse LN-1 (mLN-1) (␣1␤1␥1) via ␣M␤2-integrin, and these laminin isoforms strongly promoted chemoattractant-induced neutrophil migration via the same integrin. However, only rhLN-8 enhanced the spontaneous migration. In addition, recruitment of neutrophils into the peritoneum following an inflammatory stimulus was impaired in LN␣4-deficient mice. rhLN-8 also protected isolated neutrophils from spontaneous apoptosis. This study is the first to identify a specific laminin isoform in neutrophils and provides evidence for the role of LN-8 in the adhesion, migration, extravasation, and survival of these cells. IntroductionNeutrophils are highly migratory phagocytes with pivotal roles in host defense against infections and numerous inflammatory disorders. 1 These leukocytes predominate in the early cellular infiltrate at inflammatory sites, and their migration from the circulation into tissues involves a sequential engagement of adhesion receptors. 2,3 Although considerable efforts have been made to understand the initial steps of neutrophil extravasation (rolling and tight adhesion), little is known about the migration of neutrophils through the vessel wall and the interaction of these cells with the underlying extracellular matrix proteins of the perivascular basement membrane, such as laminins, and the interstitium. 3,4 Laminins are a family of large heterotrimeric proteins that promote cell adhesion and migration via integrins and other cell-surface receptors. [5][6][7] They are major components of basement membranes of blood vessels and other tissue compartments, and are synthesized by numerous cell types. Through combination, 5 ␣, 3 ␤, and 3 ␥ laminin chains constitute more than 12 laminin isoforms. 6,7 Laminin isoforms, particularly their ␣ chain, are expressed in a cell-and tissue-specific manner, and are differentially recognized by integrins (INTs). 6,7 Laminin 1 (LN-1) (␣1␤1␥1), the prototype laminin originally isolated from a mouse tumor more than 20 years ago, has been extensively characterized, and its in vitro effects have been studied in numerous cell types. 5 However, expression of LN-1 in newborn and adult stages is mostly restricted to a subtype of epithelial cells, 8,9 thus making questionable the role of this laminin isoform in leukocyte extravasation.Laminin 8 (...