Expression of Calca gene-derived peptides in the murine taste system

This study reveals that while the *Calca* gene-derived CGRP is expressed in trigeminal and geniculate neurons, its alternative transcript preprocalcitonin is uniquely found in murine type II taste cells, suggesting a novel reciprocal regulatory mechanism involving CGRP1R-expressing progenitor and stromal cells that may influence taste signaling and regeneration.

The Gist

Imagine your tongue isn't just a taste sensor; it's a bustling, self-repairing city. In this city, taste buds are the neighborhoods, taste cells are the workers, and nerves are the delivery trucks bringing supplies and messages.

This paper is like a detective story where scientists discovered a secret communication network happening right on your tongue, involving two main characters: PCT (Procalcitonin) and CGRP.

Here is the story of what they found, explained simply:

1. The Two Messengers

Think of the Calca gene as a master factory. Usually, this factory can build two different products depending on how the instructions are read:

• Product A (CGRP): A famous, well-known messenger that usually lives in the nerves (the delivery trucks). It's like a "Help! We need to fix this!" signal that also fights bacteria.
• Product B (PCT): A less famous messenger that the scientists found living inside the taste cells themselves (the workers).

The Big Surprise:
For a long time, scientists thought the "factory" in the taste cells only made Product A (CGRP). But this study found that in the back of the tongue (the Circumvallate and Foliate papillae), the taste cells are actually making Product B (PCT) instead! They are barely making Product A.

2. The "Switch" on the Wall

Now, imagine the cells in the taste neighborhood have a special doorbell on their walls called CGRP1R.

• This doorbell is designed to ring when Product A (CGRP) from the nerves comes by.
• But here's the twist: Product B (PCT), made by the taste cells themselves, can also ring this doorbell!

3. The Dance of the Messengers

The scientists realized these two messengers are having a complex conversation:

• The Nerves (CGRP): They send messages from the brain to the tongue to say, "Hey, keep the taste cells healthy, grow new ones, and watch out for germs!"
• The Taste Cells (PCT): They make their own version of the message. It's like the workers in the factory making a "pause" button.
• The Interaction: PCT acts like a dimmer switch. It can turn the doorbell on a little bit, but it also blocks the loud, full signal from the nerves. This creates a delicate balance. It's like the nerves are shouting "GO, GO, GO!" and the taste cells are whispering "Okay, but let's slow down a bit."

4. Why Does This Matter? (The "Why Should I Care?" Part)

Why would a taste cell need to make its own messenger? The paper suggests two main reasons:

• The Germ Guard: The back of the tongue has deep trenches (like little canyons) where food gets stuck and bacteria love to hide. Since PCT is known to fight bacteria (it's used by doctors to detect serious infections in the blood), the taste cells might be making it to act as a local security guard, keeping the "taste canyons" clean and preventing infections.
• The Repair Crew: The doorbell (CGRP1R) is also found on the "construction workers" (stem cells) that build new taste cells. By balancing the signals from the nerves and their own PCT, the tongue might be fine-tuning how fast it repairs itself after you burn your tongue on hot pizza.

The Takeaway

This paper reveals that your tongue is smarter than we thought. It's not just passively waiting for signals from the brain. The taste cells themselves are active participants, making their own chemical tools (PCT) to:

1. Fight off bacteria in the deep crevices of the tongue.
2. Fine-tune the repair process of taste buds.
3. Have a conversation with the nerves to ensure the whole system works in harmony.

It's a bit like a neighborhood where the residents (taste cells) have their own security system and repair crew, but they still keep in touch with the police station (nerves) to make sure everything runs smoothly.

Technical Summary

1. Problem Statement

Taste cell regeneration, neuron pathfinding, and taste signaling are regulated by a complex network of growth factors and signaling molecules. While many factors (e.g., R-spondin 2, Sonic Hedgehog, Wnts) are known, the specific roles of peptides derived from the Calcitonin gene-related peptide (Calca) gene within the taste system remain largely uncharacterized.

The Calca gene undergoes tissue-specific alternative splicing to produce two distinct transcripts:

1. Prepro-CGRP: Processed into $\alpha$-CGRP.
2. Preprocalcitonin (PrePCT): Processed into Procalcitonin (PCT), Calcitonin, and Katacalcin.

While CGRP is a well-studied neuropeptide involved in nociception and immunity, the expression and function of PCT and its receptors in taste papillae are unknown. Furthermore, the spatial relationship between these peptides (produced by nerves vs. taste cells) and their receptors (CGRP1R and Calcitonin Receptor) in the taste microenvironment is unclear.

2. Methodology

The authors employed a multi-modal approach to map the expression of Calca transcripts, their protein products, and receptor subunits in murine taste systems:

• Animal Models: C57BL/6J mice and transgenic strains expressing GFP under specific promoters: Tas1r3-GFP (Type II taste cells), Lgr5-GFP (stem/progenitor cells), and Gad1-GFP (Type III taste cells).
• Bulk RNA Sequencing (RNASeq): Laser microdissection (LMD) was used to isolate taste buds from Circumvallate (CVP), Fungiform (FFP), and Foliate (FOP) papillae. Libraries were prepared and sequenced to analyze transcript abundance and splicing patterns.
• Single-Cell RNA Sequencing (scRNASeq): GFP-labeled cells were isolated from specific taste cell populations (Type II, Type III, and stem cells) from CVP and FFP to determine cell-type-specific expression.
• Quantitative PCR (qPCR) & End-point PCR: Used to validate transcript levels of Calca, Calcb (beta-CGRP), Calcrl, Ramp1-3, and Calcr in taste papillae and sensory ganglia (Geniculate and Nodose-Petrosal-Jugular [NPJ]).
• RNAscope Hiplex Assay: Fluorescent in situ hybridization was used to visualize mRNA co-expression of Calca and receptor subunits with specific cell markers (e.g., Tas1r3, Trpm5, Gnat3, Ddc, Sparc, Ccr6, Arg1).
• Immunohistochemistry (IHC): Double-labeling confocal microscopy was performed using antibodies against Procalcitonin (PCT), CGRP, and various taste cell markers (T1R3, LRMP, TUBB3) to confirm protein localization.

3. Key Contributions

• Discovery of Cell-Type Specific Splicing: The study definitively maps the alternative splicing of the Calca gene in taste papillae, showing that taste cells predominantly express the PrePCT transcript (leading to PCT), while the CGRP transcript is largely absent in taste cells but present in innervating nerves.
• Reciprocal Expression Pattern: The authors identified a unique "cross-talk" architecture where taste cells produce PCT, while the innervating sensory neurons produce CGRP. Both ligands target the CGRP1R (Calcrl + Ramp1) expressed in taste stem cells, fibroblasts, and immune cells within the papillae.
• Absence of Calcitonin Receptor: The study demonstrates that the Calcitonin Receptor (Calcr) is not expressed in taste papillae or ganglia, suggesting that Calcitonin and Katacalcin (products of PrePCT) are likely not biologically active in this tissue, leaving PCT as the primary functional ligand.

4. Key Results

A. Transcript Expression Patterns

• Taste Papillae (CVP & FOP):
  • PrePCT: Strongly expressed, specifically in Type II taste cells (marked by Tas1r3 and Trpm5). Expression is weak in FFP and absent in non-taste epithelium.
  • CGRP Transcript: Undetectable or very low in taste buds (likely nerve contamination).
  • Receptors: Calcrl and Ramp1 (CGRP1R subunits) are expressed in taste stem/progenitor cells (Lgr5+), Type II cells, and subsets of Type III cells. Calcrl is also found in mesenchymal fibroblasts (Sparc+) and immune cells (ILC2/ILC3).
  • Calcitonin Receptor (Calcr): Not detected.
• Sensory Ganglia (Geniculate & NPJ):
  • CGRP Transcript: Strongly expressed.
  • PrePCT Transcript: Not detected.
  • Receptors: Calcrl and Ramp1 are expressed; Calcr is absent.

B. Protein Localization

• PCT: Co-localized with T1R3 and LRMP (Type II markers) in CVP and FOP taste buds.
• CGRP: Found in neuronal fibers innervating taste buds and the surrounding mesenchyme, as well as in non-taste lingual epithelium and filiform papillae.
• CGRP1R Subunits: Detected in basal cells (stem/progenitors), mature taste cells, and adjacent fibroblasts/immune cells.

C. Splicing Analysis

• RNASeq Sashimi plots confirmed that reads in CVP align to PrePCT-specific exons (Exon 4) but not to the CGRP-specific exon (Exon 5), confirming that taste cells produce PrePCT, not prepro-CGRP.

5. Significance and Proposed Model

The study proposes a novel paracrine signaling model for taste papillae homeostasis:

1. Reciprocal Regulation: Taste cells secrete PCT, while innervating nerves secrete CGRP. Both ligands bind to CGRP1R on stem cells, fibroblasts, and immune cells.
2. Functional Antagonism: Since PCT is a partial agonist and antagonist of CGRP at the CGRP1R, the local balance between nerve-derived CGRP and taste-cell-derived PCT may fine-tune signaling.
3. Biological Roles:
   • Regeneration: CGRP1R signaling in Lgr5+ stem cells suggests a role in taste cell turnover and regeneration.
   • Immunity & Microbiome: Given the antimicrobial properties of CGRP and the sepsis-marker role of PCT, this axis may regulate the oral microbiome within the deep trenches of CVP/FOP (which are prone to bacterial biofilm) and modulate local immune responses (ILC2/ILC3).
   • Taste Signaling: The interaction may influence taste transduction, potentially explaining clinical observations of altered taste perception in migraine patients treated with CGRP antagonists.

Conclusion: This paper redefines the Calca gene's role in the gustatory system, shifting the focus from CGRP as the sole actor to a complex interplay between nerve-derived CGRP and taste-cell-derived PCT, mediated by CGRP1R, to regulate taste cell regeneration, immunity, and microbiome balance.