The capture of host cell’s resources: The role of heat shock proteins and polyamines in SARS-COV-2 (COVID-19) pathway to viral infection

The role of human HSPs

HSPs are the main cluster of evolutionarily conserved molecular chaperones that play an important role as housekeepers of the cell. HSPs are grouped into two main families centered on their molecular weight. The first group is small ATP-independent HSPs that are chaperones with a molecular mass between 8 and 28 kDa. These chaperones include ubiquitin, α-crystallins, Hspb1 (also known as Hsp25 in mice or Hsp27 in rats and humans), and many others [16]. These small HSPs have received growing research attention in the past and present moment, primarily due to their possible unprotective approaches [16]. Second, the large, ATP-dependent HSPs are chaperones with a molecular mass between 40 and 110 kDa. These include the well-known chaperones of the 40, 70, and 90 kDa families. The 40 kDa group (Hsp40s or J-proteins) plays an important role in facilitating the interaction between the substrate and 70 kDa molecular chaperones (Figure 1). They interact with Hsp70 through their J domain and serve as regulatory co-chaperones. The 70 kDa group consists of the stress-inducible Hsp70 and the constitutively expressed heat shock cognate 70 (Hsc70) [17,18].

Figure 1

Expression of HSPs in mammalian cells. The regulation of HSPs by heat shock factor 1 (HSF1) both under normal and heat shock results in the expression of small HSPs, Hsp60, Hsp70, Hsp90, and Hsp100 as protective tools inside the cellular system.

Similarly, the 90 kDa group consists of two major isoforms, namely, the inducible Hsp90α and the constitutively expressed Hsp90β. However, because these isoforms are often challenging to isolate, many studies have resorted to studying co-purified aggregates containing both Hsp90α and Hsp90β, simply referring to the whole as Hsp90 [18]. In a host cell, a diverse network of chaperones is responsible for proteostasis, whereas viruses only depend on a subset of the host chaperones. Here, we review the role of some of these HSPs in SARS-Cov-2 since the advent of COVID-19.

The small HSPs

Small HSPs are a highly coordinated group of proteins found across all species, including humans, and are vital for stress tolerance. Many sHsps show chaperone-like activity in inhibiting the aggregation of target proteins, keeping them in a folding-competent form, and refolding them by themselves or in concert with other ATP-dependent chaperones. In short, these chaperone proteins are ATP-independent as opposed to major chaperones such as Hsp70 for example. sHsps have subunit molecular masses of 12–43 kDa and are characterized by the presence of a highly conserved stretch of 80–100 amino acids in their C-terminal domain called the “α-crystallin domain” (ACD) that is flanked by the less conserved (except a few stretches) NTD and C-terminal extension. Some members of the sHsp family such as Hsp27, αA-, and αB-crystallin form large oligomeric species. sHsps are reported to take part in viral replication. In coronaviruses and other viruses, Hsp27 is reported to be rapidly upregulated, suggesting a role in early replication. Wan et al. (2020) discuss the role of Hsp27 in different viruses. However, not much is known about Hsp27 in SARS-CoV-2 save for speculation about its function in other coronaviruses. Hence, it has been proposed as a potential drug target in SARS-CoV-2 [40]

The synthesis of HSPs is regulated by the HSF

The transcriptional range of Hsps is regulated by HSF1 in invertebrates and HSFs 1–4 in vertebrates, with HSF1 being the most important. In response to proteotoxic stress conditions, heat shock factor 1 (HSF1) is subject to a multistep activation and attenuation cycle. Inactive HSF1 monomers are retained in the cytoplasm in a complex with regulatory proteins such as Hsps 40, 70, and 90, as well as the cytosolic chaperonin TCP1 ring complex (TRiC) [41,42]. Upon stress sensing, HSF1 is activated, causing the dissociation of inhibitory proteins, HSF1 oligomerization, and nuclear retention. HSF1 is modified by several activated posttranslational modifications (PTMs) that promote DNA binding and transcriptional activation of target genes in concert with cofactor recruitment [43,44]. HSF1 is then modified by different inhibitory PTMs and by p23, causing DNA dissociation (Figure 2). Taken together, viruses such as SARS-CoV-2 require HSP to maintain and keep their cellular proteins in good condition. Therefore, HSPs are important for viral infection. It is important to develop a strategy to block the virus from accessing these molecules (Table 1).

Figure 2

Transcriptional regulation of HSPs by HSF. The expression of Hsps 70, 90, and 100 helps in the stabilization of newly synthesized proteins. Hspb1 binds to HSF to promote the synthesis of HSPs.

How the production of HSPs is triggered by some viruses

The exposure of cells to foreign substances triggers the expression of protective molecules to keep the cellular system in good shape and viable. Among the molecules that act as cellular housekeepers are HSPs, whose role is to prevent proteins from forming aggregates that could otherwise become toxic to the entire cellular system and lead to cell death. Some of the substances that could trigger the excessive production of HSPs are bacteria and viruses. Figure 3 shows some of the viruses that could lead to the high expression of HSPs as a protective measure for the human host cellular system. The heat shock response is regulated at the transcriptional level by activities of the family of heat shock factor 1 (HSFs). The most studied HSF genes from humans are HSF1, HSF2, and HSF4, and these genes play important role in heat shock response [41,45]. HSF1 localizes at cytoplasmic level under normal conditions in mammalian cells in a negatively charge form and is associated with molecular chaperones such as HSP70 and 90 as well as co-chaperones [41]. Upon the appearance of unfolded proteins and release of interacting chaperones, HSF1 DNA-binding activity is de-repressed, and monomers oligomerize to a trimeric state, translocate to the nucleus, are inducibly phosphorylated and bind to specific sequence elements referred to as heat shock elements, located within the HSP gene promoters, activating transcription of heat shock genes and synthesis of HSP. Therefore, when foreign substances such as viruses enter the human system, they hijack human host HSPs to help in the folding of the viral proteins during their synthesis inside the mammalian system, thus changing the normal functional properties of the host [46]. For example, when SARS-CoV-2 enter the human host, they cannot produce proteins on their own and therefore exploit the human host to produce proteins such as spike proteins. Viruses use the human host’s ribosomes to produce their proteins, during the synthesis of proteins such as spike proteins, upon their exit from ribosomes like any other proteins they require the assistance of molecular chaperones also known as HSPs to fold properly. Hence, the human host’s molecular chaperones are then hijacked by the SARS-CoV-2 for their benefit of the viral proteins to be folded properly. It is crucial to design a drug that would stop the virus from entering the human host system and use the chaperones from the human host. In our laboratory, work is currently underway to develop innovative strategies to block the SARS-CoV-2 from its dependency on the human host.

Figure 3

Some of the viruses that activate the expression of HSPs in human cells. HSF1 activates the expression of HSPs in response to foreign substances such as viruses, adapted from ref. [46].

Polyamines from mammalian cells

Almost all living organisms have polyamines such as putrescine, spermidine, and spermine; however, their concentrations differ depending on the type of organism. At physiological pH, polyamines are positively charged and these biogenic molecules have a flexible carbon chain with amino acids. Organisms like bacteria and archaea possess various groups of polyamines, including spermidine, homospermidine, norspermidine, putrescine, cadaverine, and 1,3-diaminopropane in bacteria and agmatine, spermidine, homospermidine, norspermidine, and norspermine in archaea. The variety of polyamines present in each of these kingdoms varies among the different organisms [47,48]. The main polyamine synthesis pathway present in mammals is summarized in Figure 4. Polyamines’ vital role is defined as being molecules involved in cell growth, differentiation, and proliferation. In a normal cellular system or healthy cell, polyamines are involved in various cellular processes such as protein synthesis, RNA folding and bending, membrane interactions, protein–RNA interactions, DNA structure, and gene expression. Polyamines bind both RNA and DNA, altering the conformation and function of nucleic acids. Polyamines alter DNA structure by facilitating the conformational transition from the B form to the Z form or by bending DNA. Furthermore, up to 80% of polyamines in the cell are directly associated with RNA and spermine has also been implicated in the stabilization of tRNA structure [47,49]. Additionally, these molecules are reported to exhibit molecular chaperone activity, therefore protecting cells exposed to harsh conditions such as temperature increases. Polyamines include putrescine, spermidine, and spermine, thus are synthesized through ornithine and adomet as precursors with the help of various enzymes. For example, ornithine is converted to putrescine with the help of ODC, while adomet is converted into spermidine and spermine with the help of adomet decarboxylase enzyme [50].

Figure 4

Biosynthesis of polyamines in humans. Both ornithine and S-adenosylmethionine are precursors of polyamine synthesis such as putrescine, spermidine, and spermine. Abbreviations: AdoMetDC, S-adenosylmethionine decarboxylase; ODC, ornithine decarboxylase; MTA, methylthioadenosine; SSAT, Spermidine/spermine-N(1)-acetyltransferase; SMO, spermine oxidase; and APAO, acetylpolyamine oxidase.

The role of polyamines in humans and viruses

The polycationic nature of polyamines allows them to be involved in various activities inside the cellular system. Besides their role in cell proliferation, growth, differentiation, and chaperone activities, polyamines in viruses are involved in packaging, transcription, translation, protease activity, and cell binding. However, in humans, polyamines play a pivotal role in nucleic acid metabolism, transcription, translation, chromatin structure, and membrane fluidity (Figure 5). In general, viruses cannot synthesize their proteins, but they depend on the host system. Therefore, viruses hijack the role of polyamines from the human host and manipulate them to synthesize and replicate their proteins such as spike proteins [48].

Figure 5

Summary of polyamines in cellular and viral processes.

Viruses compete for molecule hosts

To initiate infection, a virus enters a host cell typically via receptor-dependent endocytosis. It then penetrates the subcellular membrane, reaching a destination that supports transcription, translation, and viral genome replication [51]. These steps lead to the assembly and morphogenesis of the new viral progeny. The mature virus finally exits the host cell to begin the next infection cycle. Strikingly, viruses hijack host molecular chaperones to accomplish these distinct entry steps. Viruses are acellular, parasitic entities that are not classified within any domain because they are not considered alive [52]. They have no plasma membrane, internal organelles, or metabolic processes, and they do not divide. Instead, they infect a host cell and use the host’s replication processes to produce progeny virus particles. Viruses infect all forms of organisms including bacteria, archaea, fungi, plants, and animals. Other organisms grow, metabolize, and reproduce whereas viruses do not, but are assembled in their mature form. Viruses replicate, but to do so, they are entirely dependent on their host cells. More so, viruses are diverse. They vary in their structure, their replication methods, and their target hosts or even host cells.