POMC Human

What is POMC and what are its primary derivative peptides in humans?

Proopiomelanocortin (POMC) is a 241-amino acid polypeptide precursor that undergoes extensive proteolytic cleavage via prohormone convertases to produce multiple bioactive neuropeptides. In humans, the primary POMC-derived peptides include α-, β-, and γ-melanocyte stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), β-endorphin (β-EP), and lipotrophins. These peptides play crucial roles in regulating diverse physiological processes including energy homeostasis, adrenal function, pain perception, and pigmentation through their interactions with specific receptors .

How are POMC-derived peptides distributed in the human brain?

Quantitative liquid chromatography tandem mass spectroscopy (LC-MS/MS) analyses of both human pluripotent stem cell (hPSC)-derived hypothalamic neurons and primary human hypothalamic tissue have demonstrated that β-MSH and desacetyl α-MSH (d-α-MSH) are produced at roughly equimolar concentrations in the human brain. Both peptides are produced in vast excess (5- to 200-fold higher) compared to acetylated α-MSH. This distribution pattern suggests that both d-α-MSH and β-MSH may have greater importance in human energy regulation than previously recognized. Additionally, β-endorphin (β-EP(1-31)) is present at approximately 2- to 3-fold higher concentrations than either d-α-MSH or β-MSH in the human median basal hypothalamus (MBH) and paraventricular hypothalamus (PVH) .

What receptors mediate the actions of POMC-derived peptides?

POMC-derived peptides exert their effects through two main receptor families: melanocortin receptors (MCRs) and opioid receptors (ORs). There are five MCR subtypes with distinct functions: MC1R regulates skin pigmentation, MC2R mediates adrenal steroidogenesis in response to ACTH, MC3R and MC4R regulate energy homeostasis and body weight, and MC5R has roles in exocrine function. The endorphins primarily activate μ-opioid receptors to mediate analgesic effects and can influence feeding behavior. These receptors have been identified in various tissues including the brain, adrenal glands, skin, and components of the osteoarticular system (synovium, cartilage, and bone) .

How is POMC processed differently across various human tissues?

The processing of POMC varies across different tissues due to the tissue-specific expression of processing enzymes, particularly prohormone convertases (PCs). In the anterior pituitary, PC1/3 cleaves POMC to produce ACTH and β-lipotrophin, while in the intermediate lobe and hypothalamus, additional processing by PC2 generates α-MSH, β-MSH, and β-endorphin. Recent LC-MS/MS analyses have confirmed that human hypothalamic tissue produces a range of POMC-derived peptides including α-MSH(1-13), d-α-MSH(1-13), β-MSH(1-18), and β-EP(1-31), along with other neuropeptides such as agouti-related peptide (AGRP). This tissue-specific processing allows for the production of peptides tailored to local physiological requirements .

What factors regulate POMC expression and processing in human hypothalamic neurons?

The expression and processing of POMC in human hypothalamic neurons are regulated by multiple factors. The hormone leptin has been shown to significantly increase the concentration of both d-α-MSH and β-MSH by approximately 20-30% in human pluripotent stem cell-derived hypothalamic neurons treated with 100 ng/ml (6.25 nM) leptin for 24 hours. Conversely, inhibition of prohormone convertases using Furin Inhibitor 1 significantly reduces the measured concentration of both d-α-MSH and β-MSH by 20-30%. At the transcriptional level, POMC expression is regulated by a combination of transcription factors, including ISL1, NKX2-1, and PRDM12, which are necessary for specifying the identity of POMC neurons in the developing arcuate nucleus of the hypothalamus .

How do transcription factors control the development of POMC neurons?

Single-cell transcriptomic analysis has revealed that hypothalamic POMC neurons, particularly those with the highest POMC expression levels (identified as cluster 1 Pomc(high)/Prdm12), are characterized by a unique combinatorial set of transcription factors (TFs). Key TFs include ISL1, NKX2-1, and PRDM12, which begin expression in neurons that start expressing POMC in the developing arcuate nucleus at embryonic day 10.5 (E10.5) and continue expression into adulthood. Functional studies have demonstrated that these three TFs are absolutely necessary for specifying the identity of POMC neurons, and adult mice lacking any of these TFs exclusively from POMC cells display greatly reduced hypothalamic POMC mRNA levels along with increased adiposity and body weight. Additional TFs identified in high-POMC-expressing neurons include SIX3, SIX6, SOX1/2/3, and SOX14, though their specific roles require further investigation .

What is the evidence for β-MSH existence and significance in human energy balance?

The existence of β-MSH in the human brain has been controversial, but recent LC-MS/MS analyses of both human pluripotent stem cell-derived hypothalamic neurons and primary human hypothalamic tissue have definitively confirmed its presence. β-MSH is produced at concentrations comparable to d-α-MSH in the adult human median basal hypothalamus (MBH) and paraventricular hypothalamus (PVH), suggesting it plays a significant role in energy regulation. The importance of β-MSH in human energy balance is further supported by the association between mutations in the β-MSH region of the POMC gene and human obesity. When injected into mice, β-MSH inhibits feeding as potently as α-MSH by activating MC4R, indicating its potential role in appetite regulation and energy homeostasis .

How do POMC-derived peptides interact with the leptin signaling pathway?

POMC-derived peptides, particularly α-MSH and β-MSH, function as downstream effectors in the leptin signaling pathway. Leptin, an adipocyte-derived hormone, binds to leptin receptors on POMC neurons in the arcuate nucleus of the hypothalamus, activating signal transduction pathways that increase POMC expression and processing. Experimental evidence using hPSC-derived hypothalamic neurons shows that leptin treatment (100 ng/ml) significantly increases the concentration of both d-α-MSH and β-MSH by 20-30%. These melanocortin peptides subsequently activate MC3R and MC4R in various hypothalamic nuclei, reducing food intake and increasing energy expenditure. This leptin-melanocortin pathway represents a critical neural circuit for body weight regulation, and disruptions at any level can lead to obesity .

What are the molecular mechanisms underlying POMC deficiency in human obesity?

POMC deficiency causing human obesity can result from various molecular mechanisms. Homozygous loss-of-function mutations in the POMC gene, such as the frameshift mutation c.206delC (p.P69Lfs*2), lead to complete POMC deficiency characterized by early-onset obesity, adrenal insufficiency, red hair, and pale skin. This mutation results in premature protein truncation, removing ACTH and other important peptides and completely disrupting POMC function. Other mutations affecting specific POMC-derived peptides, particularly in the β-MSH region, have also been associated with human obesity. These mutations can impair the ability of β-MSH to activate MC4R, reducing its anorexigenic effects. Additionally, epigenetic modifications affecting POMC gene expression and alterations in prohormone convertase activity can disrupt normal POMC processing and contribute to obesity .

What are the advantages and limitations of using human pluripotent stem cell-derived hypothalamic neurons for studying POMC biology?

Advantages:

• They provide an unprecedented opportunity to study human POMC biology in a controlled environment

• Can be produced at scale and are readily manipulated for experimental purposes

• Appropriately traffic and secrete POMC and its derivatives

• Respond to regulatory factors such as leptin, demonstrating physiological relevance

• Allow for dynamic studies of POMC processing and secretion not possible with fixed human tissue

Limitations:

• In vitro systems may not fully recapitulate the complex hypothalamic neural circuits

• The percentage of POMC-positive neurons is relatively low (approximately 6.5 ± 0.76 percent)

• May not perfectly model the developmental trajectory of native hypothalamic POMC neurons

• The potential influence of in vitro culture conditions on POMC processing must be considered

These neurons have been validated as a model system through comparative analyses showing that many peptides identified in the human brain are present with identical structures and post-translational modifications in hPSC-derived hypothalamic neurons .

How can LC-MS/MS be effectively applied to quantify POMC-derived peptides?

Liquid chromatography tandem mass spectroscopy (LC-MS/MS) offers significant advantages over traditional immunoassay-based methods for studying POMC-derived peptides. Unlike ELISAs, which only provide information about particular target peptides and often cannot discriminate between peptides with common epitopes but functionally relevant structural differences, LC-MS/MS is highly sensitive and provides comprehensive information on the precise sequence and post-translational modifications of thousands of peptides in a single experiment.

For effective application, researchers should:

• Use stable-isotope-labeled internal standards for accurate quantification

• Optimize sample preparation to preserve labile post-translational modifications

• Employ appropriate chromatographic separation to distinguish between similar peptides

• Utilize high-resolution mass analyzers for accurate mass determination

• Validate findings with multiple biological replicates

This approach has been successfully used to quantify POMC-derived peptides in both hPSC-derived hypothalamic neurons and primary human hypothalamic tissue, revealing important insights such as the presence of β-MSH and the relative abundance of various MSH peptides .

What experimental approaches can be used to study the functional effects of POMC mutations?

Researchers can employ multiple complementary approaches to study the functional effects of POMC mutations:

• Cell-based assays: Express wild-type and mutant POMC in cell lines to assess processing, trafficking, and secretion

• Receptor activation assays: Test the ability of POMC-derived peptides from mutant constructs to activate melanocortin receptors using cAMP accumulation or calcium mobilization assays

• CRISPR/Cas9 genome editing: Introduce specific mutations into hPSCs followed by differentiation into hypothalamic neurons to study the effects on POMC processing and release

• Patient-derived iPSCs: Generate induced pluripotent stem cells from patients with POMC mutations and differentiate them into hypothalamic neurons

• Mouse models: Create transgenic mice carrying human POMC mutations to study whole-body metabolic effects

• Ex vivo tissue analysis: When available, analyze hypothalamic tissue from patients with POMC mutations using LC-MS/MS to characterize peptide profiles

These approaches can provide insights into how specific mutations affect POMC processing, the production of bioactive peptides, and downstream signaling through melanocortin receptors .

What are the diagnostic criteria and clinical features of human POMC deficiency?

POMC deficiency is an extremely rare monogenic disorder characterized by a constellation of clinical features stemming from the absence of multiple POMC-derived peptides:

Key diagnostic criteria include:

• Early-onset severe obesity (typically manifesting in the first months of life)

• Adrenal insufficiency (presenting as hypoglycemia, hypotension, and electrolyte abnormalities)

• Red hair and decreased skin pigmentation

• Hyperphagia (excessive hunger)

• Normal growth and development in other aspects when adrenal insufficiency is treated

The diagnosis is confirmed by genetic testing showing biallelic mutations in the POMC gene, typically resulting in complete loss of function. In the reported case, a homozygous frameshift mutation (c.206delC, p.P69Lfs*2) was identified, which results in downstream frameshift and premature protein truncation, removing ACTH and other important peptides and most likely completely disrupting POMC function .

How does understanding POMC processing inform potential therapeutic approaches for obesity?

Understanding POMC processing provides several avenues for developing therapeutic approaches for obesity:

• Melanocortin receptor agonists: The confirmation that both β-MSH and d-α-MSH are produced at high levels in the human hypothalamus suggests that developing agonists that mimic these specific peptides might be more effective than those based solely on α-MSH.

• Prohormone convertase modulators: Since prohormone convertases (PCs) are critical for processing POMC into its bioactive derivatives, compounds that enhance PC activity could potentially increase the production of anorexigenic peptides.

• Leptin sensitizers: Given that leptin increases the concentration of both d-α-MSH and β-MSH in human hypothalamic neurons, developing compounds that enhance leptin sensitivity could boost endogenous melanocortin signaling.

• Gene therapy: For patients with genetic POMC deficiency, gene replacement strategies could potentially restore POMC function.

• Cell-based therapies: Transplantation of engineered POMC-expressing cells into the hypothalamus could provide a source of melanocortin peptides.

The quantitative differences in POMC-derived peptides observed in human samples compared to rodent models underscore the importance of human-specific approaches to obesity treatment .

What roles do POMC-derived peptides play in the human osteoarticular system?

POMC-derived peptides have significant functions in the human osteoarticular system:

• Anti-inflammatory actions: Melanocortins, particularly α-MSH, exert anti-inflammatory effects in synovial tissues by inhibiting the production of pro-inflammatory cytokines and reducing neutrophil migration.

• Chondroprotective effects: POMC peptides can protect articular cartilage by inhibiting the expression of matrix metalloproteinases (MMPs) that degrade cartilage matrix components.

• Bone metabolism regulation: Both melanocortins and β-endorphin influence bone remodeling by modulating osteoblast and osteoclast activity.

• Pain modulation: β-endorphin, acting through opioid receptors in joint tissues, contributes to endogenous pain control mechanisms.

Cells in the synovial tissue and bone have been identified to generate POMC-derived peptides, and melanocortin receptor subtypes (MC1R, MC2R, MC3R, MC4R) as well as opioid receptors have been detected in various cells of the synovium, cartilage, and bone. This local POMC system may have implications for the pathophysiology and potential treatment of osteoarthritis and rheumatoid arthritis .

What are the developmental origins of POMC neurons in the hypothalamus?

Single-cell transcriptomic analysis has provided insights into the developmental origins of hypothalamic POMC neurons:

• Transcription factor profile: The development of POMC neurons is guided by a specific combination of transcription factors, with a major cluster of POMC neurons (cluster 1. Pomc(high)/Prdm12) characterized by the expression of Isl1, Nkx2-1, and Prdm12.

• Temporal dynamics: POMC neurons begin expressing these transcription factors in the developing arcuate nucleus at embryonic day 10.5 (E10.5), with expression continuing into adulthood.

• Functional necessity: ISL1, NKX2-1, and PRDM12 are absolutely necessary for specifying POMC neuron identity, as demonstrated by genetic deletion studies.

• Additional factors: The scRNA-seq analysis revealed additional novel candidate transcription factors in high-POMC-expressing neurons, including Six3, Six6, Sox1/2/3, and Sox14, suggesting a more complex regulatory network than previously appreciated.

These findings indicate that POMC neurons arise from a distinct developmental lineage with a specific transcriptional program that establishes and maintains their unique identity and function .

What are the most pressing unanswered questions in human POMC research?

Despite significant advances, several critical questions remain in human POMC research:

• Complete peptidome characterization: What is the full range of bioactive peptides derived from POMC in humans, and how do their functions and relative abundances vary across different brain regions and peripheral tissues?

• Regulatory mechanisms: What are the complete set of transcription factors and epigenetic modifications that regulate POMC expression in different contexts, and how do these change throughout development and in response to environmental factors?

• Receptor specificity: What is the precise binding affinity and signaling profile of each POMC-derived peptide at its respective receptors, and how does this contribute to their physiological effects?

• Integration with other systems: How do POMC neurons integrate signals from multiple hormonal and neural pathways to coordinate responses to changes in energy status?

• Therapeutic targeting: Can selective targeting of specific POMC-derived peptides or their receptors provide effective treatments for obesity while minimizing side effects?

• Developmental plasticity: How plastic are POMC neurons throughout the lifespan, and can their function be restored in pathological conditions?

Addressing these questions will require innovative approaches combining advanced technologies such as spatial transcriptomics, optogenetics, and human iPSC-derived brain organoids .

How might emerging technologies advance our understanding of human POMC biology?

Emerging technologies offer exciting possibilities for advancing POMC research:

• Spatial proteomics: Technologies like imaging mass spectrometry could map the distribution of POMC-derived peptides with high spatial resolution in human brain tissue.

• Brain organoids: Advanced 3D hypothalamic organoids could model the development and function of human POMC neurons in a more physiologically relevant context than 2D cultures.

• Single-cell multi-omics: Integrating transcriptomic, proteomic, and epigenomic data at the single-cell level could reveal new insights into POMC neuron heterogeneity and regulation.

• In vivo sensors: Development of fluorescent or FRET-based sensors for POMC-derived peptides could enable real-time monitoring of peptide release and receptor activation.

• Humanized animal models: Creating mice with human POMC sequences or receptors could better model human-specific aspects of POMC biology.

• AI and machine learning: Computational approaches could help predict the effects of POMC mutations, identify novel regulatory elements, and discover patterns in large datasets that might not be apparent through traditional analysis.

These technologies could overcome current limitations in studying human POMC biology and provide new insights into its role in health and disease .