Kallikreins emerge as new regulators of viral infections

Abstract

Kallikrein-related peptidases (KLKs) or kallikreins have been linked to diverse (patho) physiological processes, such as the epidermal desquamation and inflammation, seminal clot liquefaction, neurodegeneration, and cancer. Recent mounting evidence suggests that KLKs also represent important regulators of viral infections. It is well-established that certain enveloped viruses, including influenza and coronaviruses, require proteolytic processing of their hemagglutinin or spike proteins, respectively, to infect host cells. Similarly, the capsid protein of the non-enveloped papillomavirus L1 should be proteolytically cleaved for viral uncoating. Consequently, extracellular or membrane-bound proteases of the host cells are instrumental for viral infections and represent potential targets for drug development. Here, we summarize how extracellular proteolysis mediated by the kallikreins is implicated in the process of influenza (and potentially coronavirus and papillomavirus) entry into host cells. Besides direct proteolytic activation of viruses, KLK5 and 12 promote viral entry indirectly through proteolytic cascade events, like the activation of thrombolytic enzymes that also can process hemagglutinin, while additional functions of KLKs in infection cannot be excluded. In the light of recent evidence, KLKs represent potential host targets for the development of new antivirals. Humanized animal models to validate their key functions in viral infections will be valuable.

Keywords: Coronavirus; Influenza virus; Kallikrein-related peptidases (KLKs); Papillomavirus (HPV); Varicella zoster virus (VZV); Viral entry.

Fig. 1

Schematic representation of influenza virus and hemagglutinin protein. Influenza virus is found in wild birds that constitute the natural reservoir for influenza viral strains, whereby it was transferred to humans via domestic poultry. All H1–H16 and N1–N9 subtypes infect wild birds. Virus transmissibility from avian species to domestic poultry is a major pathway for transferring genetic diversity. Then, from domestic poultry, infections can spread to humans, as depicted on the far left. During this transfer, the multibasic cleavage motif for viral activation found in birds evolves to a monobasic motif in humans [4, 49, 50]. Upper, the different domains of the hemagglutinin viral protein are shown, as well as the cleavage site by host proteases. F1, F2 represent the fusion domains of the HA1 fragment that are part of the fusion machinery. The HA2 fragment contains the fusogenic peptide sequence. Lower, multiple alignments of a partial hemagglutinin amino acid sequence encompassing the protease cleavage site (scissors). Identical residues among all sequences are highlighted in yellow, identities > 45% in dark green, and 20–45% in blue. Conservative changes are highlighted in grey. The multibasic motif is highlighted in green. The accession numbers of the aligned sequences are available in Table 1. Abbreviations HA: hemagglutinin; N neuramidase; RBD receptor binding domain; PA, PB1, PB2: the subunits of the RNA-dependent RNA polymerase

Fig. 2

Representation of a putative KLK-cascade leading to activation of serine proteases that cleave hemagglutinin in the lung. The KLK5 and KLK12 proenzymes are secreted and are autoactivated. The active KLK5 and KLK12 enzymes could activate other protease zymogens to enhance the overall proteolytic activities in the lung. Specifically, KLK12 activates plasminogen (PLG), and both KLK5 and KLK12 can activate urokinase (URO) and plasma prekallikrein (KLKB1) plasminogen [3]. Proteolytic activities could enhance viral infection by cleaving the HA and exposing the fusogenic HA2 domain. Exogenously administered aprotinin inhibits the HA-activating serine proteases, thus, it can be used to treat influenza. Similarly, KLK inhibitors, like for example the recently reported KLK5 inhibitors [16] could be employed in antiviral therapy

Fig. 3

The spike protein of coronaviruses. Upper, the different domains of the spike protein are schematically shown. The S1/S2 and S2’ are the sites for proteolytic cleavage. Receptor binding protein (RBD) contains the receptor-binding motif (RBM). Upon cleavage at S1/S2 position, RBD is subjected to conformation changes that exposes the RBM to cellular receptors. Transmembrane domain (TD) anchors the spike protein onto the viral membrane. Lower, alignment of partial sequences spanning the S1/S2 site (left) and the S2’ site (right) cleaved by host proteases. The cleavage site is framed. Basic R and K amino acid residues recognized for proteolytic cleavage are depicted in red. Identical residues among all sequences are highlighted in yellow, 50–75% in green, 50% in blue, and conservative changes in grey. The site recognized for cleavage of the HCoV-HKU1 spike protein by KLK13 is indicated by a red arrow. Spike primary sequences were retrieved from GenBank with the following accession numbers: MERS-CoV JX869059; SARS-CoV-2 Wuhan-Hu-1 NC_045512; HCoV-HKU1 genotype A AY597011; SARS-CoV BJ1 AY278488. Del1, Del2, Del3 and R685H variants of SARS-CoV-2 carry deletions in S1/S2, which could alter viral activation and associated infectivity (lower alignment) [27]