Review
1 February 2022
1 Institute of Molecular and Cell Biology (IMCB), Agency for Science, Technology and Research (A*STAR), Singapore 138667, Singapore
2 Department of Psychological Medicine, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 119228, Singapore
3 Division of Neuroscience and Experimental Psychology, School of Biological Sciences, Faculty of Biology, Medicine and Health, University of Manchester, Manchester M13 9PL, UK
4 Institute for Health Innovation & Technology (iHealthtech), National University of Singapore, Singapore 117599, Singapore
Despite recent leaps in modern medicine, progress in the treatment of neurological diseases remains slow. The near impermeable blood-brain barrier (BBB) that prevents the entry of therapeutics into the brain, and the complexity of neurological processes, limits the specificity of potential therapeutics. Moreover, a lack of etiological understanding and the irreversible nature of neurological conditions have resulted in low tolerability and high failure rates towards existing small molecule-based treatments. Neuropeptides, which are small proteinaceous molecules produced by the body, either in the nervous system or the peripheral organs, modulate neurological function. Although peptide-based therapeutics originated from the treatment of metabolic diseases in the 1920s, the adoption and development of peptide drugs for neurological conditions are relatively recent. In this review, we examine the natural roles of neuropeptides in the modulation of neurological function and the development of neurological disorders. Furthermore, we highlight the potential of these proteinaceous molecules in filling gaps in current therapeutics.
The accelerated acquisition of scientific knowledge and development of modern medicine have resulted in numerous miracles over the past 20 years. For example, new treatment interventions have reduced mortality due to cardiovascular diseases by 40% [1], and targeting specific molecular pathways with small molecule drugs or biologics has turned some cancers from a death sentence into a chronic condition [2,3]. Furthermore, the breakthrough in mRNA vaccine technology has enabled the global COVID-19 pandemic situation to become more manageable [4]. However, there has been a comparative lack of progress in treatment methods and cures for neurological conditions and an exceptionally high failure rate of late-stage clinical trials for neurological and psychiatric diseases [5]. The complexity of neurological processes, irreversible nature of neurological decline [6,7], and presence of a highly selective blood-brain barrier (BBB) [8] have rendered conventional small molecule drugs woefully inadequate in resolving the root aetiology of neurological conditions, often resulting in unwanted neurological impact. It is expectable that drugs exhibiting demonstrable effects in palliative treatments offer no significant improvements for neurological diseases.
Small peptide drugs provide an alternate avenue for the development of novel therapeutics targeting neurological conditions. As intrinsic signalling molecules in normal cellular function, natural peptides or their mimetics mirror physiological modulation of organ or cellular processes [9] and provide specificity unachievable by small molecule drugs developed or repurposed for a specified function. Furthermore, the short, defined sequences of neuropeptides can be used or modified directly during drug synthesis, reducing the need for further structural optimization for functionality. Methods for peptide therapeutics originated from the synthesis of insulin and adrenocorticotropic hormone (ACTH) for treating type I diabetes and endocrine disorders in human patients during the 1980s and 1990s [10]. There are now over 80 peptide drugs available on the market, with many more undergoing development in clinical and preclinical trials [11]. Of these drugs, 65% were approved from the early 21st century, reflective of the growing peptide drug market. A summary of the current U.S. Food and Drug Administration (FDA) approved peptide-based therapeutics is presented in Table 1. The full list of peptide and protein therapeutics approved by the FDA thus far can be found in the THPdb database (https://www.frankenthalerfoundation.org last accessed on 29 November 2021 [12]).
Table 1. Representative FDA-approved peptide-based drugs.
Neuropeptides are small endogenous protein messengers synthesized and secreted by neurons through the regulated secretory route [37]. They are produced widely in the nervous system as pro-peptides, which are further cleaved into smaller fragments, commonly between 5 and 80 amino acids, through a multistep process. They are then matured via post-translational modifications [37]. Multiple peptide products can be generated from a single, larger pro-peptide, and each of these products can function as an independent neuropeptide. For example, opioids acting on classical opioid receptors are produced from one of the three pro-peptides proenkephalin, prodynorphin, or proopiomelanocortin [38]. Mature peptides exert their effects through direct autocrine signalling or indirect paracrine effects on neurons, astrocytes, and microglia nearby. Specifically, peptides that affect neuronal function, but are not produced by neurons, are not considered neuropeptides.
The differential storage and release of neuropeptides compared to neurotransmitters has been reviewed by Merighi et al. (2011) [39]. As opposed to the storage of neurotransmitters in small secretory vesicles, neuropeptides are stored in large granular vesicles (LGVs). Due to the relatively larger size of neuropeptides, their exocytosis is not likely to occur via the ‘kiss and run’ vesicle dynamics reported for neurotransmitter release and instead probably requires complete fusion of the LGV with the pre-synaptic membrane [40]. Neuropeptides can coexist with neurotransmitters in the same neuron, yet their storage in separate vesicles enables selective release, dependent on intrinsic cellular mechanisms and differential calcium release [39]. When combined, the release of neuropeptides and neurotransmitters enables both fast (milliseconds) and prolonged (seconds to minutes) modulation of brain circuits. Differences in neuropeptides and neurotransmitters indicate that both their independent function and interaction are essential for normal brain functioning. The neuropeptide family comprises a broad range of proteinaceous molecules with known pleiotropic effects in development, reproduction, physiology, and behaviour [41,42,43]. Specifically, they can act as hormones, neurotransmitters, and neuromodulators in the nervous system. Hence, they are good candidates for novel drug development for various neurological conditions. The involvement of druggable G protein-coupled receptors (GPCR) in neuropeptide signalling [44] and the small size of neuropeptides further support the suitability for direct use of native neuropeptide sequences in drug development.
In this review, we will examine the role of neuropeptides in neurological development, function, and disease. With a renewed knowledge of neuropeptides in neurophysiology, we are interested in understanding how specific neuropeptides can be employed to combat neurological disorders and how recent advancements in technology and methods of peptide synthesis may push the development of neurological-friendly peptide therapeutics towards a reality. Due to the vast amount of research available relating to neuropeptide function, it would be almost impossible to review a topic so broad to an all-encompassing extent. Alternatively, this review aims to provide a more general outlook of the diversity of the vibrant neuropeptide field. For more detailed discussions of the topics presented, we encourage readers to refer to review papers pointed to in the text.
Neurological research and the development of neurological therapeutics are dependent on the knowledge of chemical neurotransmission. The awarding of the Nobel Prize in 1936 to Sir Henry Dale and Otto Loewi for their work on acetylcholine’s role in parasympathetic nervous system neurotransmission challenged the initial notion that neurons communicate through direct electrical transmission and set the foundation for the construction of a chemical-based novel for neurological function [45]. Early studies that show the correlation between animal behaviour [46,47] and the levels of brain chemicals, alongside observations that specific classes of drugs mimic the effects of chemical neurotransmission systems, further supported this proposition [48]. The development of methods to detect chemical release from neuronal axon terminals and evidence from neurophysiological measurements eventually confirmed the role of neurotransmitters in neurological function. The story concluded with observations by Arvid Carlsson, Paul Greengard, and Eric Kandel that specific neurotransmitter systems drive critical aspects of animal physiology, behaviour, and cognition.
It is now well-recognized that the neurotransmitter system is fundamental for normal nervous system development and function. A significant number of drugs developed for treating neurological conditions work based on classical neurotransmitter modulation. For example, galantamine, a reversible cholinesterase inhibitor manufactured under the trade name of Reminyl [49], is prescribed for Alzheimer’s disease (AD) management, and fluoxetine, a selective serotonin reuptake inhibitor also known as Prozac [50], is prescribed to teens suffering from depression. However, neurotransmitters are not the only chemical signalling molecules used by the nervous system. Larger peptide molecules with a longer life span and a larger area of influence are released concurrently with small molecule neurotransmitters [37].
Brain development occurs soon after conception and persists throughout the lifetime of an organism. The neurodevelopmental process involves dynamic neuronal production, migration, and communication precisely orchestrated by the timed expression or effects of chemical factors in the neurological system. Vasoactive intestinal peptide (VIP), a neuropeptide often associated with gastrointestinal and circadian regulation, guides neuronal differentiation and glia-dependent neuronal survival during neural tube closure in early embryogenesis [51]. The inhibition of VIP activities during gestation results in premature neuronal differentiation, which leads to microcephaly [52] and various behavioural deficits reminiscent of Down syndrome [53]. Furthermore, increased VIP and VIP receptor 2 (VPAC2) levels in newborns are associated with an increased chance of developing autism spectrum disorder (ASD) [54,55,56], further supporting the role of VIP in neurodevelopment. Although initially supplied by the maternal uterine tissue [57], VIP is produced by the central nervous system to regulate neuronal and synaptic activity later in development. For example, it is implicated in the development and control of circadian rhythms and is expressed by neurons in the suprachiasmatic nucleus of the hypothalamus alongside gastrin-releasing peptide (GRP) and arginine vasopressin (AVP) [58]. VIP-expressing neurons have been reported to be important for the maturation of SCN neural networks during development [59]; one study has shown that the ablation of VIP neurons dramatically altered circadian gene expression in neonatal mice [60].
Although not essential for early nervous system development, peptide hormones such as oxytocin and vasopressin are involved in the experience-dependent maturation of the neuronal circuit. Oxytocin has time-specific effects on early postnatal development associated with multimodal sensory processing and integration [61]. This is likely due to the combinatorial effect of various neuronal signalling components and neural substrates involved in the stress response, such as the oestrogen receptor, vasopressin system, cortisone, and ACTH [62]. Similar to VIP, polymorphisms in the OXTR gene encoding the oxytocin receptor have been linked to the development of ASD [61,63,64,65], a further reflection of the neuropeptide’s role in neurodevelopmental processes. However, it is unclear why the modulation of oxytocin levels may lead to such alterations in neurodevelopment associated with social behaviours. Several studies (compiled and reviewed by Rajamani et al.) [66] have shown that oxytocin alterations impact both long-term potentiation (LTP) and long-term depression (LTD) of synapses during early development. Additional neuropeptides which have been suggested to play a role in neuronal synaptic plasticity development include neuropeptide Y (NPY), pituitary adenylate cyclase-activating polypeptide (PACAP), and TLQP-62 in complementary pathways contributing to early hippocampal neurogenesis [67], neuron differentiation, and neurite outgrowth during development [68,69].
Following the cessation of fetal and early postnatal development, the adult nervous system retains neurogenic potentials in restricted brain regions. The dentate gyrus contains a rich reservoir of NPY-producing gamma-aminobutyric acid (GABA)-ergic interneurons [70]. NPY generation is sensitive towards changes in hippocampal neuronal activity [71] and associated with a robust enhancement in granule cell neurogenesis [72], likely resulting from an increase in ERK1/2-dependent proliferation of neural stem cells [73]. Interestingly, a comparison between various neurogenic populations of the adult brain revealed the involvement of the NPY receptor Y1 (Y1R) in neuronal precursor cell proliferation and differentiation [74,75]. Although NPY or Y1R-deficient mice develop significantly lower numbers of olfactory neuron precursors [74] and present with an absence of NPY-induced dentate gyrus cellular proliferation [73], there are no overt changes in memory acquisition [76]—a phenomenon linked to enhancements in neurogenesis [77]. It is not clear if the preferential recruitment of Y1Rs during neurogenesis is due to a predominant recruitment of neural stem cells expressing the Y1R since Y1, Y2, and Y5 receptors are found throughout the nervous system [78].
