Intracellular Peptides: A New Frontier in Peptide Biology
By Professor Emer S. Ferro
For decades, peptide research has focused primarily on peptides generated from precursor proteins. These proteins are synthesized specifically to produce biologically active peptides, which are then released by endocrine and neural tissues to regulate essential physiological processes.
Well-known examples include peptide hormones and neuropeptides that influence metabolism, appetite, blood pressure, and multiple signaling pathways throughout the body, including the insulin, GLP-1, glucagon, opioid, and renin–angiotensin systems. In recent years, however, scientists have begun to recognize a second, much less explored class of naturally occurring peptides: Intracellular Peptides (InPeps).
Unlike peptides produced from dedicated precursor proteins, InPeps are generated through the proteasomal degradation of intracellular proteins-a process traditionally associated with protein turnover, cell-cycle regulation, and antigen presentation.
More Than Protein Fragments
For many years, the small peptide fragments generated during protein degradation were considered biologically inactive by-products, serving mainly as a source of amino acids for de novo protein synthesis. Research has gradually challenged this assumption. Studies now suggest that many InPeps have biological functions of their own. Some can be secreted and interact with seven-transmembrane/G-protein-coupled receptors (7TM/GPCRs). Others remain within the cell, where they can interfere with protein–protein interactions or influence gene expression through interactions with microRNAs.
InPeps also exhibit dynamic regulation, with their levels fluctuating in response to various physiological and pathological conditions, such as diet-induced obesity, non-alcoholic fatty liver disease, hypoxia, pancreatitis, brain aneurysms, and distinct cell cycle stages.These findings suggest that InPeps may participate in the body’s adaptive response to changes in metabolic and cellular conditions.
A Conserved Biological System
InPeps have been identified across multiple forms of life, including bacteria, fungi, plants, and mammals. Many of their sequences are conserved across species, suggesting that they may represent fundamental biological mechanisms preserved throughout evolution. Because InPeps are composed of naturally occurring amino acids, they can also be chemically synthesized while remaining identical to their naturally occurring counterparts. This makes it possible to investigate their biological activity and explore their potential as therapeutic molecules without altering their natural structure.
From Discovery to Potential Therapeutic Molecules
At Proteimax, the study of InPeps has evolved into a systematic platform for identifying and characterizing peptides with potential therapeutic applications.
The process begins with rational InPep selection. Following physiological or pathological challenges, such as: proteasome inhibition, diet-induced obesity, hypoxia, brain aneurysm, or pancreatitis, the levels of dozens or even hundreds of InPeps may be affected. Proteimax Brazil’s proprietary algorithm enables researchers to identify a smaller subset of promising candidates for further investigation.
The next step is target identification. Using a unique collection of conformation-specific antibodies, individual InPeps can be evaluated according to their ability to modulate specific 7TM/GPCRs as allosteric modulators, agonists, or inverse agonists. Because dysregulated GPCR signaling is well documented across human diseases—and GPCRs represent a major class of pharmaceutical targets—identifying these interactions can help guide the potential therapeutic application of individual InPeps.
Selected candidates can then move into preclinical research, which integrates molecular and cellular biology with in vivo animal studies to determine whether a specific InPep can modulate the identified biological pathway and potentially help restore healthier physiological function.
Finally, promising candidates may subsequently progress to human clinical studies. Many InPeps demonstrate oral activity, potentially enabling simple capsule formulations for clinical evaluation. Proteimax has also developed digital tools to simplify monitoring and reporting of clinical study data.
Pep19: From InPep Discovery to Human Clinical Research
Pep19 is one example of how this discovery process can translate into clinical research.
Pep19 was originally identified in mouse abdominal adipose tissue following a high-fat diet challenge. Using Proteimax Brazil’s conformation-specific antibody platform, Pep19 was subsequently shown to interact with high affinity with the cannabinoid type 1 receptor (CB1R), a 7TM/GPCR involved in metabolic regulation.
Preclinical studies demonstrated that Pep19 could prevent fat accumulation in specific adipose tissues, including mesenteric adipose tissue, through modulation of peripheral cannabinoid signaling. Subsequent human clinical studies demonstrated improvements across multiple markers of metabolic health, including fat distribution and body composition, insulin sensitivity, and glycemic control. Importantly, these effects were observed through an approach distinct from appetite suppression.
Pep19 therefore provides an important proof of concept for the broader InPep platform, demonstrating that naturally occurring intracellular peptides can be identified, characterized, and translated from biological discovery into novel approaches to human health.
A Growing Area of Research
Scientific interest in intracellular peptide signaling is increasingly growing. Recent publications, including research highlighted by the Nature Index, indicate that these molecules may represent an additional layer of cellular communication and regulation.
As researchers continue to uncover their functions, InPeps are expanding our understanding of how cells regulate metabolism, inflammation, and other physiological processes. Processes previously regarded as protein degradation may also represent a rich source of biologically active signaling molecules. Although much remains to be discovered, . InPeps are opening an exciting new chapter in peptide biology—one that may deepen our understanding of cellular communication and inspire new therapeutic approaches.
Cells may not communicate with words, yet they communicate constantly—within tissues, across biological pathways, and even over distance. InPeps may constitute a significant component of this cellular language.
