Topic Article
The Neuroendocrine Peptide Catestatin Is a Cutaneous Antimicrobial and Induced in the Skin after Injury
Katherine A Radek, Belen Lopez-Garcia, Melanie Hupe, Ingrid R Niesman, Peter M Elias, Laurent Taupenot, Sushil K Mahata, Daniel T O’Connor and Richard L Gallo
Journal of Investigative Dermatology (2008) 128, 1525–1534; doi:10.1038/sj.jid.5701225

Catestatin: A New Member of the Cutaneous Defense System

Lucy K. Martin ¹, Shasa Hu ¹ and Robert S. Kirsner ¹

Journal of Investigative Dermatology (2008), 128, 1350. doi:10.1038/jid.2008.130

The innate immune system is thought to be one of the first lines of defense against cutaneous pathogens. Defensins, including human β-defensin-2 (Harder et al., 1997) and -3 (Harder et al., 2001), as well as cathelicidins (Dorschner et al., 2001) are important in cutaneous defense. In addition, several members of the “granin” family (chromogranin/secretogranin) and other neuropeptides, including α-melanocyte-stimulating hormone, calcitonin gene-related peptide, and neuropeptide Y, have been reported to exhibit antimicrobial activities in vitro (Fox et al., 1997; Shimizu et al., 1998; Cutuli et al., 2000).
The granin family includes chromogranin A (CHGA), isolated from chromaffin cells of the adrenal medulla (Banks and Helle, 1965). CHGA is processed at dibasic sites, giving rise to several bioactive peptides, including catestatin (Cst). Radek and colleagues (2008) report Cst to be a functional antimicrobial peptide (AMP) present in the epidermis. The authors found that Cst, with a mechanism of action similar to that of other AMPs, exhibited antimicrobial activity against Gram-positive and Gram-negative bacteria, yeast, and fungi found in the skin. Induced after injury, CHGA was expressed in keratinocytes and then cleaved proteolytically into Cst. These findings suggest an innate immunologic role for Cst in skin.
Through the following questions, we examine this paper in greater detail.

QUESTIONS

1. What is the innate immune system?

2. What is catestatin, and how does it participate in the skin immune system?

3. Against which pathogens is catestatin effective, and by what mechanism is it thought to act?

4. Where is catestatin found in the skin, and when is it expressed?

5. What may be the clinical significance of this article?

ANSWERS

1. The innate immune system provides a first line of defense against potential pathogens and is needed to control the infective nature of common bacteria, fungi, and viruses; it produces the body’s initial response to pathogens, eliminating microbes and preventing infection. Rather than being driven by specific antigens in the classic sense of adaptive immunity, innate immunity is activated by evolutionarily conserved mechanisms of detecting classes of microbes, to which the host responds rapidly (Goodarzi et al., 2007).

The innate immune system’s many elements include: (1) leukocytes (neutrophils, eosinophils, and monocytes), (2) phagocytic cells (macrophages, dendritic cells, and other antigen-presenting cells), (3) cells that release inflammatory mediators (mast cells, basophils, and eosinophils), (4) natural killer cells, and (5) complement proteins, acute phase proteins, and cytokines.

One mechanism of the innate immune system recognizes microbial molecules (technically akin to antigens) known as pathogen-associated molecular patterns, such as lipopolysaccharides, peptidoglycan, lipotechoic acids, mannose, and bacterial and viral DNA/RNA. Pattern-recognition receptors such as Toll-like receptors identify these microbial structures and initiate the signaling transduction pathways that induce gene expression. The innate immune response prevents pathogen invasion and plays a role in antigen-specific acquired immunity (Arancibia, 2007). Indeed, antigen presentation by the cellular players of the innate immune system and consequent cytokine and chemokine production prime and activate the adaptive immune system. Although the innate immune system recognizes and destroys many common microorganisms, it does not recognize all pathogens. Unlike the adaptive immune system, the innate immune system does not provide immunological memory, vital for protecting the host in subsequent infections.

2. Catestatin (Cst), a 21-amino-acid peptide, was originally characterized as a catecholamine release inhibitory peptide that acted as an antagonist of the neuronal nicotinic cholinergic receptor (Mahata et al., 1997, 1998; Taupenot et al., 2003). However, results of the current study have shown that Cst has activity against common cutaneous microbes and thus should be considered an antimicrobial peptide (AMP) and part of the skin’s innate immune system. The investigators found that Cst had antimicrobial properties against Gram-positive and Gram-negative bacteria as well as yeast and fungi. Cst appears to be different from other previously characterized members of the skin’s innate immune system in that it is derived not from keratinocytes but from the neuroendocrine peptide chromogranin A (CHGA). The investigators suggest that this supports a direct relationship between the neural and the skin immune systems.

Other known AMPs are typically small molecular proteins produced by keratinocytes in the epidermis. Examples include the β-defensins and the cathelicidins. There are four known human β-defensins (HBD): HBD-1, found in epithelial tissue; HBD-2, which is upregulated in inflamed skin; HBD-3, first found in psoriatic skin but since detected in a variety of tissues; and HBD-4, which is inducible in human keratinocytes. Defensins are also considered important elements in the innate immune system because they inhibit bacterial, fungal, and viral colonization (Bhat and Milner, 2007). Only one cathelicidin, LL-37, is induced in keratinocytes in inflamed skin (Howell, 2007).

In contrast, Cst is derived from the neural system, specifically from chromogranin A. Part of the neuroendocrine system, chromogranin A is produced by the chromaffin cells in the adrenal medulla. It is a member of the larger granin family, which also includes chromogranin B and secretogranin II. Chga designates the gene encoding for the protein chromogranin A, which is processed at dibasic sites to give rise to several bioactive peptides, including Cst. The formation of Cst is typical of the way the entire granin family gives rise to other bioactive agents. In general, granins are found as pro-proteins and are stored in secretory granules. These secretory granules have cleavage sites for a variety of endoproteases. Once granins are released from secretory granules, newly formed bioactive peptides are released into the extracellular space.

3. Cst is effective against Gram-positive and -negative bacteria as well as fungi and yeasts. In this study, Cst demonstrated antimicrobial activity against Gram-positive bacteria such as Staphylococcus aureus and group A Streptococcus, Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa, yeast such as Candida albicans, and filamentous fungi such as Aspergillus niger, Aspergillus fumigatus, and Trichophyton rubrum. Cst is similar to other AMPs in that its ability to destroy pathogens depends on its charge and structure. Carbonate is a critical ionic factor that enhances microbial sensitivity to AMPs at physiological NaCl concentrations (Dorschner et al., 2001). Membrane permeability to Cst was affected by the presence of NaHCO3 in a manner similar to that of cathelicidin. Overall, antimicrobial peptides are cationic and interact with the anionic components of bacteria, fungi, and viruses to permeabilize the antimicrobial membrane. After the cell membrane has been permeabilized, the ion gradient is disrupted, leading to cell lysis (Boman et al., 1993).

4. Cst is found in suprabasal and granular keratinocytes in the epidermis. It is not found in the stratum corneum, and small amounts are found in the dermis. The investigators confirmed the presence of Cst in the epidermis by using a quantifying analysis of mRNA from whole human skin and cultured cells. The gene encoding for chromogranin A, chga, was found in large amounts in whole skin—especially in epidermal keratinocytes (as opposed to fibroblasts). The investigators could not detect CHGA in cultured mouse connective tissue mast cells or in human lipid-rich sebaceous cells using PCR and immunostaining.

Cst is expressed in response to injury or infection. To mimic these conditions, the researchers tape-stripped murine skin and immunostained it with a Cst antibody . They found increased expression of CHGA in the epidermis and dermis after injury. Furthermore, they found increased expression of CHGA in group A _Streptococcus_–infected skin as compared with normal skin.

5. This study highlights an emerging area in dermatologic research: the interplay between neural tissue and the skin, and specifically the antimicrobial functions of neuroendocrine-derived tissue. Animal studies have shown that mice deficient in cathelicidin have increased susceptibility to several infections such as group A Streptococcus (Nizet et al., 2001), Citrobacter growth in the colon (Iimura et al.,2005), and E. coli in urinary tract infections (Chromek et al.,2006). It is anticipated that lack of cathelicidin would lead to a similar finding in humans. In addition, Mahapatra et al.(2005) found that the absence of CHGA leads to hypertension in mice. Perhaps a lack of CHGA will be found to have an association with other common diseases in humans as well.

Merkel cells are known to express Chga, a marker for Merkel cell tumors (Koljonen et al.,2005). It is possible that Chga may be expressed on other types of cells that would also be of help in monitoring tumor activity.

Finally, one may hypothesize that Cst is effective against Gram-positive and -negative bacteria, fungi, and yeast and that a topical form of Cst could be effective against infection. However, randomized controlled trials must be performed to test this hypothesis.

Broader implications could include the possibilities that: (1) other neuroendocrine tissue might have antimicrobial function, (2) neuroendocrine diseases might affect cutaneous immunity and also might explain in part why stress leads to increased infection, and (3) the neuroendocrine system might be altered to benefit the skin.

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¹ Department of Dermatology and Cutaneous Surgery, University of Miami Miller School of Medicine, Miami, Florida, USA