Calpains: Attractive Targets for the Development of Synthetic Inhibitors

Abstract

The physiological roles of calpains are discussed, as are the associated pathological disorders that result from their over-activation. We also present practical information for establishing functional inhibition assays and an overview of X-ray crystal structures of calpain-inhibitor complexes to aid inhibitor design. These structures reveal the expected extended beta-strand conformation for the inhibitor backbone, a geometry that has been engineered into inhibitors with the introduction of either an N-terminal heterocycle or a macrocycle that links the P(1) and P(3) residues. The structure and function of all the main classes of inhibitors are reviewed, with most examples being classified according to the nature of the C-terminal reactive warhead group that reacts with the active site cysteine of calpains. These inhibitor classes include epoxysuccinate derivatives, aldehydes, aldehyde prodrugs (hemiacetals) and alpha-keto carbonyl compounds. Inhibitors derived from the endogenous inhibitor calpastatin and examples lacking a warhead, are now known and these are also discussed.

Selenoprotein K Is a Novel Target of m-Calpain, and Cleavage Is Regulated by Toll-like Receptor-induced Calpastatin in Macrophages

“…Pro dominates the region flanking the P 2 -P 1 Ј segment, particularly at P 3 Ј (35). Our data show that SelK follows the P 2 -P 1 rule, but contains a Pro at the P 2 Ј and P 4 Ј positions instead of P 3 Ј. Delineation of preferential amino acid sequences encompassing calpain cleavage sites is important for development of inhibitors, and calpain has been suggested to be an attractive target of therapeutic inhibitors for treatment of various inflammatory disorders (12,36). Development of such therapies will require a full characterization of the targets, cell types in which the calpain/calpastatin system operates, and modes by which proteolytic modulation through this system is regulated.…”

Attenuation of Blood-Brain Barrier Breakdown and Hyperpermeability by Calpain Inhibition

“…They are involved in a wide array of neurological pathologies like trauma, ischemia-reperfusion injury, spinal cord injury, and several non-neurological pathologies as well (9 -12). Intracellular calcium levels and the endogenous inhibitor of calpains, namely calpastatin, tightly regulate calpain levels endogenously (9,13 …”

Chalcones as Novel Non-peptidic μ-Calpain Inhibitors

“…2,3 Calpains consist of at least 15 isoforms encoded by an independent genes, which are well conserved, ubiquitously expressed and tissue specific. 4 Among them, calpain 1 (μ-calpain) and calpain 2 (μ-calapin) are ubiquitously distributed and the most widely studied isoforms that require micromolar and millimolar Ca 2+ ion concentrations respectively, for the activation in vitro. 5,6 Calpains have regulatory roles in the diverse cell physiology and abnormally elevated activation of calpain has been related with numbers of human diseases, 7,8 such as brain and spinal cord injury, Alzheimer disease, cancer, and cataract formation, etc.…”

“…Most inhibitors to date are peptide analogues represented by MDL-28170 (1a, Figure 1). 4,6,9 Peptidic calpain inhibitors, however, have limitation of clinical uses because of their lack of selectivity for calpain over other cysteine proteases, such as cathepsines B and L, and poor pharmacokinetic profiles in vivo. There is another peptidomimetic calpain inhibitor, E64d (1b) 10 which has been most widely employed as calpain inhibitor for in vivo study due to its enhanced cell permeability.…”