The question of whether GPRC6A is functional in buffalo is not just a genome-annotation puzzle. It connects directly to reproductive physiology, testosterone biosynthesis, and the emerging idea that bone-derived hormones can talk to the testis.
A 2024 paper in Veterinary Research Communications provides an important buffalo-specific piece of evidence:
“In vitro effects of uncarboxylated osteocalcin on buffalo Leydig cell steroidogenesis.”
The authors are B. S. Bharath Kumar, Smrutirekha Mallick, H. V. Manjunathachar, C. G. Shashank, Ankur Sharma, Dudekula Nagoorvali, Simson Soren, Vyankat Gangadhar Jadhav, and Sujata Pandita. The study was published in Veterinary Research Communications volume 48, pages 1423 to 1433, in 2024, with DOI 10.1007/s11259-024-10320-4.
The paper investigates whether uncarboxylated osteocalcin, abbreviated UcOCN, affects testosterone synthesis in buffalo Leydig cells, and whether the osteocalcin receptor GPRC6A is present in those cells.
The central argument is:
UcOCN → GPRC6A-positive buffalo Leydig cells → steroidogenic gene expression → testosterone production
The paper does not perform a GPRC6A knockdown or receptor-blocking experiment, so it does not prove every step with genetic causality. But it does provide a valuable buffalo-specific evidence stack showing that GPRC6A is present in the right cell type and that the proposed ligand, UcOCN, stimulates the steroidogenic program.
1. The study is directly about buffalo Leydig cells
The first strength of the study is species and cell-type relevance. The authors did not use a mouse model, human cell line, or generic testicular tissue. They isolated Leydig cells from adult Murrah buffalo testes.
The study used testes from buffaloes aged 4 to 6 years, collected from a local abattoir. Leydig cells were isolated by collagenase digestion and enriched using a discontinuous Percoll gradient. The cell band collected between the 30% and 58% Percoll phases was used for downstream characterization and culture.
This matters because a gene can be functional in one species and tissue context but not another. Here, the evidence is anchored in buffalo Leydig cells, the very cells responsible for testosterone production.
2. The authors first establish that they are working with Leydig cells
Before claiming anything about GPRC6A or osteocalcin signaling, the authors needed to show that their cultures actually contained Leydig cells. They did this in several ways.
Flow cytometry, Figure 1
The authors used CYP11A1-FITC staining and flow cytometry to estimate Leydig cell purity. CYP11A1 is a steroidogenic mitochondrial enzyme and a Leydig cell marker.
In Figure 1, panels A to D show forward scatter versus side scatter plots with the main representative cell population gated. Panels E to H show histograms for control and CYP11A1-FITC staining. Panel H separates CYP11A1-positive and CYP11A1-negative populations.
The authors report that immunophenotyping revealed Leydig cell populations ranging from 69% to 73.9% across trials.
That is important because the later testosterone and gene-expression assays are being performed on a Leydig-cell-enriched population, not an uncharacterized testicular soup.
Morphology, Figures 2 and 3
The cells were also followed morphologically during culture.
In Figure 2, the authors show the appearance of buffalo Leydig cells across different days of culture. The cells proliferate in focal colonies after 48 hours, reach 30 to 40% confluence by day 4, and about 70% confluence by days 5 to 6.
In Figure 3, higher magnification shows polygonal, triangular, spindle-shaped, and irregular Leydig cells. The figure also highlights fat droplets in the cytoplasm.
Those cytoplasmic lipid droplets are biologically relevant because Leydig cells use cholesterol and lipid stores as steroidogenic substrate. The cells are not just alive in culture. They look like steroidogenic cells.
3. CYP11A1 staining confirms Leydig cell identity
The next line of evidence comes from immunocytochemistry.
Immunostaining, Figure 4
In Figure 4, the authors stain the cultured cells with a primary antibody against CYP11A1, described in the caption as specific for Leydig cells. Hoechst stains nuclei, FITC marks antibody signal, and the merged image shows CYP11A1-positive Leydig cells.
The negative control omits the primary antibody and shows no CYP11A1 staining.
This is a key control. It reduces the chance that the FITC signal is just nonspecific glow, the kind of fluorescence goblin that haunts cell-biology papers.
The text states that nearly all proliferating cultured cells stained positive for CYP11A1, confirming their Leydig-cell identity.
The evidence so far:
- Flow cytometry shows a Leydig-enriched population.
- Morphology is consistent with Leydig cells.
- CYP11A1 immunostaining confirms steroidogenic Leydig identity.
Only after building this foundation do the authors turn to GPRC6A.
4. GPRC6A protein is detected in buffalo Leydig cells
This is the first direct evidence relevant to GPRC6A functionality.
GPRC6A immunostaining, Figure 5
In Figure 5, the authors stain buffalo Leydig cells with a primary antibody against GPRC6A. The figure includes:
- GPRC6A primary antibody staining
- A control where primary antibody was omitted
- Hoechst nuclear staining
- FITC-labeled secondary antibody images
- Merged Hoechst and FITC images
- 20× magnification
- 100 μm scale bar
The text reports that proliferated Leydig cells tested positive for GPRC6A antibody, suggesting the presence of an osteocalcin receptor.
This is a major buffalo-specific result. It shows that GPRC6A is not only annotated in the genome, but its protein product is detectable in cultured buffalo Leydig cells.
The paper’s wording is cautious but clear: immunostaining confirmed the presence of “GPRC6A receptors.”
This is necessary evidence for functionality. A receptor cannot mediate UcOCN action if it is absent from the relevant cell type.
5. UcOCN stimulates testosterone production in buffalo Leydig cells
The next question is whether GPRC6A-positive buffalo Leydig cells respond to the proposed ligand.
The authors treated cultured buffalo Leydig cells with different concentrations of UcOCN:
- 0 ng/ml
- 1 ng/ml
- 2 ng/ml
- 6 ng/ml
- 12 ng/ml
- 24 ng/ml
- 48 ng/ml
They also used 0.5 ng/ml luteinizing hormone, LH, as a positive control.
After 24 hours, testosterone in the culture medium was extracted and measured using a bovine-specific testosterone ELISA.
Testosterone assay, Figure 6
In Figure 6, testosterone production increases after UcOCN treatment.
The reported mean testosterone concentrations were:
- Control, 0 ng/ml UcOCN: 0.22 ± 0.01 ng/10⁶ cells/24 h
- 1 ng/ml UcOCN: 0.31 ± 0.03
- 2 ng/ml UcOCN: 0.86 ± 0.09
- 6 ng/ml UcOCN: 1.81 ± 0.17
- 12 ng/ml UcOCN: 1.51 ± 0.15
- 24 ng/ml UcOCN: 1.20 ± 0.29
- 48 ng/ml UcOCN: 1.39 ± 0.35
- LH positive control: 1.88 ± 0.24
The strongest UcOCN response occurs at 6 ng/ml, reaching almost the same testosterone output as LH.
The authors describe a “dose-dependent increase” in testosterone concentration with UcOCN supplementation, although the response becomes less consistent at higher doses.
This is a central functional observation. Buffalo Leydig cells that contain GPRC6A respond to UcOCN by increasing testosterone production.
6. UcOCN activates the steroidogenic gene program
Testosterone production is the final output. The authors also asked whether UcOCN activates the machinery that makes testosterone.
They measured mRNA expression of four steroidogenic enzyme genes by quantitative real-time PCR:
- CYP11A1
- CYP17A1
- HSD3β1
- HSD3β6
GAPDH was used as the housekeeping gene. Relative expression was calculated using the 2−ΔΔCT method.
qPCR assay, Figure 7
In Figure 7, the authors compare gene expression in:
- Control Leydig cells
- Cells treated with 6 ng/ml UcOCN
- Cells treated with 0.5 ng/ml LH
UcOCN significantly upregulates all four steroidogenic genes.
The authors report that:
- HSD3β1 increased by about 2.5-fold
- HSD3β6 increased by about 2.5-fold
- CYP11A1 increased by about 2.5-fold
- CYP17A1 increased by about 4-fold
This is powerful because it connects the hormone output to the transcriptional machinery that produces that output.
The logic is clean:
UcOCN treatment increases steroidogenic enzyme transcripts, and testosterone rises.
The paper also notes that the LH-treated cells show a similar gene-expression pattern, suggesting that UcOCN activates a steroidogenic program comparable in direction to a canonical Leydig-cell stimulus.
7. The GPRC6A localization result makes the UcOCN response biologically plausible
The key GPRC6A-specific figure is Figure 5, but its importance becomes clearer when paired with Figures 6 and 7.
Figure 5 says the receptor is present.
Figure 6 says UcOCN increases testosterone.
Figure 7 says UcOCN increases steroidogenic gene expression.
Together, these figures support the following model:
GPRC6A-positive buffalo Leydig cell + UcOCN → increased steroidogenic gene expression → increased testosterone production
The authors make this interpretation in the discussion, arguing that localization of GPRC6A on buffalo Leydig cells establishes osteocalcin’s mode of action.
That is the core evidence for functionality in buffalo.
8. Why the evidence supports GPRC6A functionality
A functional receptor should satisfy several expectations.
Expectation 1: It should be present in the relevant cell type
GPRC6A is detected in buffalo Leydig cells by immunostaining in Figure 5.
Expectation 2: The cell should respond to the receptor’s ligand
UcOCN stimulates testosterone production in Figure 6.
Expectation 3: The response should involve the expected biological pathway
UcOCN increases steroidogenic genes in Figure 7.
Expectation 4: The response should be physiologically meaningful
The output is testosterone, the key Leydig cell steroid hormone.
By these standards, the paper provides a credible argument that GPRC6A is functional in buffalo Leydig cells.
9. What the paper does not prove
A careful interpretation is important.
This paper does not include:
- GPRC6A siRNA knockdown
- GPRC6A knockout
- receptor antagonist treatment
- receptor rescue
- direct UcOCN-GPRC6A binding assay
- cAMP or CREB signaling assay
- comparison of UcOCN response before and after blocking GPRC6A
Therefore, the study does not prove that the testosterone response is completely GPRC6A-dependent.
The strongest safe conclusion is:
Buffalo Leydig cells express GPRC6A, and UcOCN stimulates testosterone production and steroidogenic gene expression in those cells. This supports a functional UcOCN-GPRC6A axis in buffalo, but direct receptor-dependence remains to be tested.
That caveat does not weaken the paper’s value. It simply places the evidence in the right category. The paper is not a receptor-knockdown causality study. It is a buffalo-specific receptor-localization and ligand-response study.
10. Why this matters
For buffalo biology, this paper is important because it moves GPRC6A beyond mere annotation.
It shows that in buffalo Leydig cells:
- the receptor is detectable,
- the cells respond to the receptor’s known ligand,
- steroidogenic genes are induced,
- testosterone production rises,
- the effect resembles the direction of LH stimulation.
That is not a ghost gene. That is a receptor with a plausible physiological job.
The study’s final message is that UcOCN affects testosterone biosynthesis in buffalo Leydig cells and that GPRC6A is positioned as the receptor through which osteocalcin may act.
In short:
GPRC6A in buffalo is not just a predicted GPCR sitting quietly in the genome. In buffalo Leydig cells, it appears as part of a bone-testis signaling axis that can stimulate steroidogenesis.
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