The lectin pathway of complement activation is an important component of the innate immune defense. The initiation complexes of the lectin pathway consist of a recognition molecule and associated serine proteases. Until now the autoactivating mannose-binding lectin-associated serine protease (MASP)-2 has been considered the autonomous initiator of the proteolytic cascade. The role of the much more abundant MASP-1 protease was controversial. Using unique, monospecific inhibitors against MASP-1 and MASP-2, we corrected the mechanism of lectin-pathway activation. In normal human serum, MASP-2 activation strictly depends on MASP-1. MASP-1 activates MASP-2 and, moreover, inhibition of MASP-1 prevents autoactivation of MASP-2. Furthermore we demonstrated that MASP-1 produces 60% of C2a responsible for C3 convertase formation.innate immunity | complement system | directed evolution | phage display | canonical inhibitor T he lectin pathway of the complement system serves as a first line of defense against microbial intruders. The innate immune system recognizes danger signals presented by the pathogens (pathogen-associated molecular patterns) or altered host cells (damage-associated molecular patterns) by means of germlineencoded cell-surface bound or soluble pattern recognition molecules (1, 2). These pattern recognition molecules have evolved against evolutionarily conserved structures of microorganisms, such as carbohydrates and acetylated compounds. The prompt action of the innate immune system provides sufficient time for the adaptive immune system to react with less conservative antigens (e.g., proteins) to build up a more specific response. In humans, five different humoral pattern recognition molecules have been identified that are able to initiate the lectin pathway: mannose-binding lectin (MBL) (3), three ficolins (M-, L-, and H-ficolin; also called ficolin-1, -2, and -3) (4), and collectin 11 (CL11 or CL-K1) (5). The pattern recognition molecules do not act alone; they are associated with other proteins, mainly serine proteases (6). These serine proteases [MBL-associated serine proteases (MASPs)] are present as proenzymes (zymogens) in the complexes and become activated to initiate the complement cascade when the recognition molecules bind to their target. Activation of the complement cascade culminates in the destruction and elimination of pathogens via opsonization or direct cell lysis. Although the lectin pathway was discovered some 20 years ago (7), the mechanism of the activation is still enigmatic. One of the most controversial issues is the role of the serine proteases. Up to now, three serine proteases have been discovered and designated as MASP-1, MASP-2, and MASP-3. In addition to the proteases, two nonenzymatic fragments of the MASPs, MAp44 (8, 9) and MAp19 (10), have also been found in the recognition complexes. MASP-1, MASP-3, and MAp44 are the alternative splice products of the MASP-1/3 gene, and MASP-2 and MAp19 are encoded by the MASP-2 gene. The only consensus point in the literature is that ...

The LC8 family members of dynein light chains (DYNLL1 and DYNLL2 in vertebrates) are highly conserved ubiquitous eukaryotic homodimer proteins that interact, besides dynein and myosin 5a motor proteins, with a large (and still incomplete) number of proteins involved in diverse biological functions. Despite an earlier suggestion that LC8 light chains function as cargo adapters of the above molecular motors, they are now recognized as regulatory hub proteins that interact with short linear motifs located in intrinsically disordered protein segments. The most prominent LC8 function is to promote dimerization of their binding partners that are often scaffold proteins of various complexes, including the intermediate chains of the dynein motor complex. Structural and functional aspects of this intriguing hub protein will be highlighted in this minireview.

Transforming growth factor beta (TGF-β) proteins are multifunctional cytokines whose neural functions are increasingly recognized. The machinery of TGF-β signaling, including the serine kinase type transmembrane receptors, is present in the central nervous system. However, the 3 mammalian TGF-β subtypes have distinct distributions in the brain suggesting different neural functions. Evidence of their involvement in the development and plasticity of the nervous system as well as their functions in peripheral organs suggested that they also exhibit neuroprotective functions. Indeed, TGF-β expression is induced following a variety of types of brain tissue injury. The neuroprotective function of TGF-βs is most established following brain ischemia. Damage in experimental animal models of global and focal ischemia was shown to be attenuated by TGF-βs. In addition, support for their neuroprotective actions following trauma, sclerosis multiplex, neurodegenerative diseases, infections, and brain tumors is also accumulating. The review will also describe the potential mechanisms of neuroprotection exerted by TGF-βs including anti-inflammatory, -apoptotic, -excitotoxic actions as well as the promotion of scar formation, angiogenesis, and neuroregeneration. The participation of these mechanisms in the neuroprotective effects of TGF-βs during different brain lesions will also be discussed.

MASP-3 was discovered 15 years ago as the third mannan-binding lectin (MBL)-associated serine protease of the complement lectin pathway. Lacking any verified substrate its role remained ambiguous. MASP-3 was shown to compete with a key lectin pathway enzyme MASP-2 for MBL binding, and was therefore considered to be a negative complement regulator. Later, knock-out mice experiments suggested that MASP-1 and/or MASP-3 play important roles in complement pro-factor D (pro-FD) maturation. However, studies on a MASP-1/MASP-3-deficient human patient produced contradicting results. In normal resting blood unperturbed by ongoing coagulation or complement activation, factor D is present predominantly in its active form, suggesting that resting blood contains at least one pro-FD activating proteinase that is not a direct initiator of coagulation or complement activation. We have recently showed that all three MASPs can activate pro-FD in vitro. In resting blood, however, using our previously evolved MASP-1 and MASP-2 inhibitors we proved that neither MASP-1 nor MASP-2 activates pro-FD. Other plasma proteinases, particularly MASP-3, remained candidates for that function. For this study we evolved a specific MASP-3 inhibitor and unambiguously proved that activated MASP-3 is the exclusive pro-FD activator in resting blood, which demonstrates a fundamental link between the lectin and alternative pathways.