Published Time: 2024-11-20
Uncontrolled and chronic inflammatory states in the Central Nervous System (CNS) are the hallmark of neurodegenerative pathology and every injury or stroke-related insult. The key mediators of these neuroinflammatory states are glial cells known as microglia, the resident immune cell at the core of the inflammatory event, and astroglia, which encapsulate inflammatory insults in proteoglycan-rich scar tissue. Since the majority of neuroinflammation is exclusively based on the responses of said glia, their phenotypes have been identified to be on an inflammatory spectrum encompassing developmental, homeostatic, and reparative behaviors as opposed to their ability to affect devastating cell death cascades and scar tissue formation. Recently, research groups have focused on peptide discovery to identify these phenotypes, find novel mechanisms, and mediate or re-engineer their actions. Peptides retain the diverse function of proteins but significantly reduce the activity dependence on delicate 3D structures. Several peptides targeting unique phenotypes of microglia and astroglia have been identified, along with several capable of mediating deleterious behaviors or promoting beneficial outcomes in the context of neuroinflammation. A comprehensive review of the peptides unique to microglia and astroglia will be provided along with their primary discovery methodologies, including top-down approaches using known biomolecules and naïve strategies using peptide and phage libraries.
Neuroinflammation, inflammation centering in the central nervous system (CNS), is the core biological feature of every neural pathology be it insult or injury, including spinal cord or traumatic brain injury (SCI/TBI), and neurodegenerative disorder, including but not limited to Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). The key arbitrators in these inflammatory events are neuroglia, known as microglia, the singular immune cells, and astrocytes, the support cell that builds the scar tissue known as gliosis (Figure 1). Microglia are highly mobile surveyors of CNS tissues with archetypical ramified morphologies that are involved in developmental (Mehl et al., 2022; Hattori, 2023), homeostatic (Li and Barres, 2018; Mordelt and de Witte, 2023), and reparative roles (Lloyd et al., 2019). This is constrasted with their roles in cell death cascades and immunity, where they behave similarly to macrophages with M1 and M2 phenotypes, representing polar ends of the pro and anti-inflammatory spetrum (Yunna et al., 2020). Astrocytes have comparable phenotypical roles in support and maintenance (Liddelow and Barres, 2017; Liddelow et al., 2020; Garland et al., 2022), including synaptic plasticity and gliotransmission (Koyama, 2015), as well as neuro and myelin protection (Burda et al., 2016; Zhou et al., 2020). As microglia react to a potent pro-inflammatory injury/neurodegenerative event, they initiate an intense necrotic cascade and recruit astrocytes to generate a proteoglycan-rich network, or gliotic scar, effectively quarantining the cascade and blocking meaningful axonal regrowth (Gao et al., 2013). Newer phenotypes are emerging, indicative of priming due to chronic inflammation, which is considered to be associated with neuropsychiatric and neurodegenerative disorders (Perry and Holmes, 2014). This allows for the identification of microglia unique to a broad spectrum of clinical issues, such as the gut-brain axis (Huang et al., 2023), neurodegenerative reactivities (Kang et al., 2018; Prater et al., 2023), and substance abuse-related cell behavior (Lacagnina et al., 2017; Melbourne et al., 2021). Astrocytes often work in subsequent tandem with microglia being first responders, often generating shared phenotypes under inflammatory and support conditions, and as such, likely have complimentary roles in these novel phenotypes. It has become impactful to understand the unique molecular patterns of these cells under their various states toward developing disease-specific biomarkers and custom therapeutics.
Figure 1 Role of microglia and astrocytes in the cycle of neuroinflammation defining key aspects of their roles under homeostasis, injury, and neurodegenerative prgression.
Many new tools and methods are emerging to characterize and facilitate the unique changes in cellular paradigms, of which peptides are rapidly becoming popular (Wang et al., 2022). With respect to targeting and mediating microglia and astroglia in phenotypically complex neuropathologies and pathophysiologies, peptide-based approaches are valid strategies. Retaining the diverse function of their protein counterparts, small protein fragments, or peptides does not require delicate 3D structures to function accordingly. As such, they have several advantages. They are robust in production methods, yields, and storage methods. Countless chemistries are available for unique modifications technically challenging or impossible with cellular translation and post-translation, which could include lipid/glycolic additions (Kowalczyk et al., 2017; Crijns et al., 2021), branching (Tam and Zavala, 1989; Elter et al., 2024), cyclization (Chung et al., 2017), organometallic additions (Reginato and Taddei, 2002), photo-labile/sensitive groups (Glatthar and Giese, 2000; Mikkelsen et al., 2018), isotopes (Zheng et al., 2016), and non-natural amino acids (Castro et al., 2023). Several biopanning and discovery tools make use of their small sequence lengths, such as display techniques and libraries (Wu et al., 2016). Peptides typically have lower computational requirements for molecular and drug discovery simulations (Rodrigues et al., 2022). Solid phase chemistry allows precise planning and the incorporation into higher order bioengineering strategies (bioconjugates, scaffolds, and drug delivery) through lock and key chemistries, such as click (Li et al., 2013). Although small and short in sequence, peptides can still confer secondary structures, high specificity, and potent cellular responses.
Herein, we summarize and report these strategies and functional studies on peptides effective at interacting with neuroglia. Families of neuropeptides or hormonal peptides are known to have systemic and cell-specific responses across the entire spectrum of mammalian biology (Hook et al., 2008; Burbach, 2011). Endogenously secreted in the CNS, a large family of neuropeptides have been characterized to affect a significant functional response from microglia and astrocytes, including neuroinflammatory mediation and gliotransmission (Carniglia et al., 2016). Although a few peptides have been discovered in studies for glial biology, many glial-related peptides have been discovered in extracellular matrices (ECMs) (de Castro Brás and Frangogiannis, 2020) and growth factor sources (Sporn and Roberts, 1988), such as cell adhesion molecules and transforming growth factors. More recently, system-wide immune biology and bioengineering have become clinically meaningful, which has led to a focus on inflammatory mediating peptides (La Manna et al., 2018a). Peptides are being engineered and designed to regulate inflammatory cells, including but not limited to monocytes, T cells, and organ-specific macrophages. As microglia emulate many of the pathways of immune cells, these peptides could be an excellent source for the glial researcher's toolbox.
Given that many of these peptides have been discovered indirectly, they are expected to confer limited specificity to glia subtypes and phenotypes. Using naïve blindfolded biopanning techniques, such as phage display and peptide libraries, a few studies have yielded microglia or astrocyte-specific peptides. Bacteriophages can display randomized peptide sequences on their coat at a specified length, which generates libraries with millions in sequence diversity. Further, polar M1/M2 or activated/ramified specific peptides have been discovered for the first time (Terashima et al., 2018; Koss et al., 2023). Although these peptides may have produced little to no activity beyond binding, the potential for bio-imaging and cytometric assessment is excellent. This review discusses these strategies and systematically lists the peptide studies.
One of the most intuitive sources of glial-interacial peptides are those native to the CNS. Neuropeptides are the most extensive family of molecules involved in signaling within the CNS. The hallmark characteristics of neuropeptides are their involvement in the biosynthesis and expression of genes within neurons and their role in chemical communication via regulated storage and release from secretory pathways (Hook et al., 2008). Additionally, neuropeptides are directly involved in the modulation and mediation of various neural functions through their interactions with neural receptors (Burbach, 2011). In their review, Carniglia et al. (2016) comprehensively characterized a variety of neuropeptides and their functional relation to microglia. Below is a summary of these peptides, with an added emphasis on their function in astrocytes and additional novel peptides. All neuropeptides with known interactions with microglia, astrocytes, and their receptors are fully summarized in Table 1.
