Calpaine Calpastatine system

Calpastatin, the body’s endogenous calpain inhibitor, is an intracellular protein encoded by the CAST gene that acts as a highly specific inhibitor of classical calpains (Starska K et al. 2016).

The protein has an N-terminal region and typically four inhibitory domains. Each of these domains can inhibit an activated calpain molecule. The binding is calcium-dependent. Calpastatin prevents excessive proteolysis without itself being rapidly cleaved like a typical substrate. It primarily inhibits calpain-1 and calpain-2, but not all members of the calpain family. Thus, calpastatin is by no means merely a passive protease inhibitor. It plays a key role in determining when, where, and to what extent calpains exert their biological effects.

Simplified functional diagram: Calcium activates calpains; calpastatin limits their proteolytic activity. Activation occurs through an increase in intracellular calcium concentration. The terms “μ-calpain” and “m-calpain,” historically used, refer to different calcium concentrations required for activation in vitro (Hanna RA et al. 2008).

In vivo, regulation is considerably more complex. Membrane phospholipids, phosphorylation, subcellular localization, and local calpastatin concentration influence actual activity.

An imbalance between calpain activity and calpastatin inhibition can influence both physiological adaptive processes and pathological tissue changes.

Genetic disorder: PLACK syndrome. There is a particularly notable dermatological association with PLACK syndrome, a rare autosomal recessive genodermatosis caused by biallelic loss-of-function variants in the CAST gene.

PLACK stands for:

- P – Peeling skin

- L – Leukonychia

- A – Acral punctate keratoses

- C – Cheilitis

- K – Knuckle pads

Calpastatin deficiency leads to inadequate control of calpain activity and is associated with disturbances in epidermal integrity and keratinization. PLACK syndrome strikingly demonstrates that impaired protease regulation can lead to a clinically distinctive genodermatosis.

Additional pathophysiological correlations

• Neurodegenerative diseases: Involvement in neuronal proteolysis and cell damage
• Cancer: Regulation of migration, invasion, and apoptosis
• Cardiovascular diseases: Involvement in endothelial dysfunction and tissue remodeling
• Inflammatory diseases: Modulation of intracellular inflammatory signaling pathways
• Genodermatoses: Loss of CAST function in PLACK syndrome

Most of these associations have been investigated mechanistically or experimentally. However, no generally established therapeutic indication for calpain inhibitors can yet be derived from this.

1. Hanna RA et al. (2008): Calcium-bound structure of calpain and its mechanism of inhibition by calpastatin. Nature 456:409–412.
2. Ono Y et al. (2012): Regulation and physiological roles of the calpain system in muscular disorders. Cardiovasc Res 96:11–22.
3. Starska K et al. (2016) Gene/protein expression of CAPN1/2-CAST system members is associated with ERK1/2 kinase activity as well as progression and clinical outcome in human laryngeal cancer. Tumor Biol 37:13185–13203.
4. Wendt A et al. (2004) Interaction of calpastatin with calpain: a review. Biol Chem 3185:465–472.