Introduction
Rarely in human disease can a single host factor be identified as the primary causal
factor of a disease, syndrome, or disorder. Rather, the clinical manifestations that
culminate in a symptomatic state are usually triggered by a perturbation of multiple
factors derived from several distinct, but integrated, domains of cellular regulation.
Host immune responses, such as antimicrobial peptide (AMP) production and regulation,
are the result of interdependent dynamic interactions between host cells, extracellular
milieu, and the external environment. In order to truly elucidate disease pathogenesis,
it is imperative to consider the entire system at play from a global perspective,
which is the promise of “omics” technology. The term “omics” was originally coined
as a discipline in molecular biology that examines global sets of biological molecules
(Micheel et al., 2012), which consequently stimulated an “Omics Revolution” throughout
the scientific community. Genomics emerged following the sequencing of the human genome
in the 1990s and early 2000s. This subsequently led to proteomics, which includes
the entire complement of proteins and their related structure, modifications, and
function. Pauling et al. (1971) proposed that estimating the abundance of metabolites
in biological fluids may indicate the functional status of a specific biological system.
Consequently, metabolomics has recently surfaced as an approach to comprehensively
identify and quantify low molecular weight exogenous and endogenous metabolites and
compounds in a biological system using high-throughput methods. Using 16S rRNA gene
sequencing tools originally developed in the field of environmental microbiology,
we can now generate a site-specific microbiome profile for any given patient. The
Human Microbiome Project and numerous other projects aim to discern relationships
between human-associated microbes and health and disease (Zoetendal et al., 2006).
Such research has generated massive and complex data sets, which requires further
advancement and refinement of bioinformatics and biostatistical applications. Often,
the data is used to produce computational models to distinguish specific characteristics
of a given population. Due to the unavoidable complexities and required statistical
sophistication, few medical fields have developed clinically useful applications for
the resulting data. Thus, AMPs are an ideal set of biological molecules to assess,
since there biological role spans numerous areas of clinical research.
Due to their diverse microbicidal and immunomodulatory functions (Steinstraesser et
al., 2011), AMPs comprise a major aspect of the host innate immune response (Nakatsuji
and Gallo, 2012). Alterations in AMP production and/or localization are associated
with several cutaneous dermatoses. Patients diagnosed with atopic dermatitis and psoriasis
ultimately develop epidermal barrier defects and microbial susceptibility, or a hyperproliferative/inflammatory
epidermis, respectively (Schauber and Gallo, 2008; Schroder, 2011). The contributions
of AMPs to tissue integrity and epithelial defense was also established in the brain
(Schluesener et al., 2012; Williams et al., 2012), urinary tract (Saemann et al.,
2007; Zasloff, 2007; Ali et al., 2009), gastrointestinal tract (Lisitsyn et al., 2012),
and certain malignancies (Kesting et al., 2012; Scola et al., 2012). However, altered
AMP regulation does not occur exclusively, but instead embodies dynamic transformations
that emerge as a disturbed system. The integration of several different “omics” techniques
may provide a more complete understanding of the diverse factors contributing to a
specific disease state, responses to pharmacologic agents, or novel alternatives to
current treatment modalities.
AMPs as a Disease Marker
The appeal of “omics” research lies in the possibility of identifying an AMP profile
that distinguishes one particular patient in a clinically useful manner to categorize
them as high- or low-risk for developing a post-operative infection (e.g., pneumonia).
Several studies have demonstrated the importance of cathelicidin (Braff et al., 2007;
Kovach et al., 2012), beta-defensins (Chong et al., 2008; Scharf et al., 2012), and
several other AMPs in regulating epithelial immunity (Cole and Waring, 2002; Tecle
et al., 2010). Consequently, it is feasible that integrating these variables simultaneously
may reveal unexplored or intriguing connections, and likely identify invaluable correlations
that are clinically significant.
Antimicrobial peptides have been extensively evaluated for their role in inflammation
(Lai and Gallo, 2009), while little research has investigated their potential role
in rejection following organ transplantation. Although these patients are typically
immunosuppressed, abnormal alterations in AMPs following transplantation may contribute
to or serve as a marker of inflammation and rejection. A diagnostic tool to yield
a molecular AMP profile of a transplant patient could serve as a prognostic indicator
of organ failure. Nuclear magnetic resonance (NMR) based metabolomic technologies
have also attempted to identify other urine biomarkers as an indicator of chronic
renal failure and renal transplant function (Bell et al., 1991; Foxall et al., 1993).
Similar technologies could be employed to identify how AMPs may correlate with clinical
outcomes in transplant patients. In comparison, trauma and burn patients exhibit profound
defects in immune regulation following injury, including perturbations in AMPs (Steinstraesser
et al., 2004; Bhat and Milner, 2007). A diagnostic AMP profile may again provide invaluable
data to predict healing, immune integrity, or graft survival. The clinical utility
of such targeted profiles could undoubtedly be applied to numerous disease states
involving infection and/or inflammation, to serve as markers of prognosis.
Currently, NMR and Mass Spectrometry are two major platforms by which metabolomic
analyses are evaluated via bioinformatic tools. Several skin AMPs were recently implicated
in tumorigenesis of cutaneous squamous cell carcinoma (Scola et al., 2012). Significant
metabolic alterations usually ensue as normal cells are transformed into a malignant
phenotype. Using a limited rationale, AMPs may simply reflect alterations in the local
environment from the presence of the malignancy, or may simply be irrelevant to the
malignancy. A more sophisticated rationale suggests that alterations in AMPs may serve
as a biomarker for disease severity and/or progression, and denote a significant underlying
process that contributes to a malignancy. Interestingly, the wound repair process
and cancer progression are both associated with alterations in the inflammatory/immune
microenvironment. During wound repair, AMPs are released from epithelial and infiltrating
immune cells to stimulate re-epithelialization, new vessel formation, and extracellular
matrix (ECM) remodeling (Radek and Gallo, 2007). However, the dynamics of cancer progression
and tissue repair differ in that wound healing is a self-limiting process, while tumor
formation is characterized by a continuous, uncontrolled activation of similar pathways
that facilitate tumor growth and metastasis. One key observation is that prominent
associations exist between the cytokines, chemokines, and growth factors present in
healing wounds and wound fluid, as compared to tumors. In parallel, a striking difference
in the temporal regulation of these factors was determined by the combination of several
genomic technologies (Pedersen et al., 2003). Furthermore, proteomics and genomic
methods are now being employed through a multidisciplinary translational research
approach to improve the bioactive components in matrix therapies for non-healing wounds
to specifically modulate the temporal and local release of these micromolecules (Sweitzer
et al., 2006). Degradomics is emerging in the wound healing field as a new technology
that assimilates the current knowledge database of ECM regulation and deciphers the
complex interactions between proteases and their respective inhibitors using systems
biology as a means to improve wound integrity in chronic wounds (Hermes et al., 2011).
Since AMPs are an integral part of wound healing and inflammation, the knowledge gained
from utilizing these evolving “omics” technologies may be extrapolated to other dimensions
of data analysis that span other disease states which share similar mechanisms of
disease progression.
Antimicrobial peptide regulation can clearly modulate and be influenced by the composition
of the microbial flora of the human host. Several AMPs are induced in response to
both invasive pathogens, as well as commensal strains of bacteria to generate specific
down-stream innate or adaptive immune signaling events. For instance, the cutaneous
commensal Staphylococcus epidermidis induces human β-defensin-2 and -3 via a TLR-2
signaling dependent mechanism (Lai et al., 2010). This interaction is beneficial for
both the host and microbe by facilitating the eradication of pathogens on the skin
via AMP induction, while simultaneously allowing S. epidermidis to further proliferate
with fewer competitors for metabolic resources. Further complicating these interactions,
microbes have evolved several mechanisms to evade host AMPs via altered cell surface
charge, efflux transporters, proteases, or trapping proteins, and direct adaptations
of host cellular processes (Nizet, 2006). These dynamic interactions between the host
and the resident microbiota can significantly influence the overall homeostatic balance.
The integration of multiple “omics” disciplines is applicable to several tangible
clinical situations where infection is a risk factor. For example, identification
of patients most at risk for a urinary tract infection (UTI) would improve prophylactic
therapies for susceptible patient populations, including burn-injured, surgical, or
bedridden individuals. Recent studies deliver a long overdue confirmation that urine
is not sterile, which challenges the current dogma (Nelson et al., 2010; Dong et al.,
2011; Wolfe et al., 2012). Thus, integration of multiple “omics” technologies, such
as 16S rRNA gene sequencing and proteomics, may identify correlations between specific
AMPs and distinct genera of bacteria to identify unique patterns that could be employed
as a diagnostic tool to predict those individuals who may be at a higher risk for
a UTI. Furthermore, the development of a rapid, high-throughput assay that integrates
multiple “omics” technologies to correlate AMPs with tissue specific microbiota would
be invaluable to clinicians for prediction of UTI or other disease states.
AMPs as a Therapeutic Target
Aside from patient-centered applications, AMP-related “omics” research is also being
utilized in drug discovery, as AMPs have been targeted as a potential alternative
to conventional antibiotics by serving as adjuncts and/or replacements to traditional
antibiotics, although more research is clearly needed to develop these promising tools
(Hirsch et al., 2008; Baltzer and Brown, 2011; Ahmad et al., 2012; Hassan et al.,
2012). Infection remains a leading cause of death in the US, with influenza/pneumonia
and septicemia both ranking in the top 15 (Murphy et al., 2012). Several publicly
available databases have already been established in order to collect relevant information
related focused on AMPs (Brahmachary et al., 2004; Wang and Wang, 2004; Wang et al.,
2009; Seshadri Sundararajan et al., 2012) in order to construct models based on clustering
and analyzing AMP sequences, which allow accurate recognition of specific antimicrobial
classes (Fjell et al., 2007). Recently, a bioinformatics strategy using peptide sequences
was employed to classify synthetic and endogenous AMPs based on their physiochemical
properties to identify active peptides and assess antimicrobial potency (Kumari et
al., 2012; Torrent et al., 2012). Despite promising preliminary data, AMPs have yet
to prove their full potential in clinical trials.
Research Limitations
It is evident that less costly and efficient high-throughput technologies are undoubtedly
needed to fully explore the under-utilized and under-recognized potential of AMPs
for therapeutic applications. Although it is feasible to capture a given patient's
molecular profile, the interpretation of those findings remains a challenge. Given
the enormous data sets being generated by “omics” research, the development of useful
computational models has been limited by several factors. Primarily, analyses of high-dimensional
data are prone to overfitting of the models to the study samples, thus yielding inaccurate
results in subsequent follow-up studies. Therefore, replication and reproducible verification
become imperative when performing such research. Prospective technological advancement
merged with more robust bioinformatic tools and greater data analysis capacity will
help surmount the existing limitations to allow for the complete integration of micromolecules
with systems biology.
Concluding Remarks
While the “Omics Revolution” continues to rapidly expand and mature, it is apparent
that its maximal applicability and utility remain to be fully elucidated. “Omics”
research, either directly examining AMPs or researching AMPs in the context of related
factors such as the metabalome or the microbiome, could potentially identify significant
and crucial relationships between various molecular signatures and human disease (Figure
1). These enormous data sets may serve as the foundation for the evolution of computational
models that could predict disease, complications, or even prognosis based on a specific
AMP profile. Furthermore, modified AMPs have the potential to replace current antibiotic
therapies, as drug discovery begins incorporating the latest technology into their
pharmaceutical development pipelines. Ultimately, the new approach to personalize
healthcare can foster novel applications to refine the characterization of a disease
phenotype, identify predictive biomarkers, determine the efficacy of various therapies,
or determine the susceptibility to drug toxicity for each individual patient.
Figure 1
Data acquired from the combination of genomics, proteomics, metabolomics, microbiomics,
and other omics technologies either directly investigating AMPs or examining related
parameters may be integrated to design better diagnostic tools, therapeutic options,
or prognostic indicators for patients with cancer, wounds, infections, or transplant.