Nicotinic acid treatment improves the developmental potential of equine oocytes for cloned embryo production.

Nicotinic acid supplementation during the in vitro maturation process improves the developmental potential of equine oocytes, enhancing embryo production rates in cloned horses. This effect was observed in oocytes from both abattoir sources and live mares, indicating potential applications for optimizing equine cloning protocols.

Background and Objective

• In vitro maturation (IVM) of equine oocytes currently faces limitations that restrict the efficiency of producing viable embryos through cloning.
• Nicotinic acid (NA), a precursor to nicotinamide adenine dinucleotide (NAD), has been shown to elevate NAD levels, potentially enhancing oocyte developmental competence.
• The objective of this study was to evaluate the effect of NA supplementation during different stages of IVM and pre-IVM (holding period) on the developmental potential of equine oocytes for cloned embryo production.

Materials and Methods

• Oocyte Sources:
  • Abattoir-derived oocytes (n=694) were collected from slaughtered mares.
  • Oocytes from live mares were obtained via ovum pick-up (OPU; n=147 and 285 in separate experiments).
• Treatment Groups:
  • Abattoir-derived oocytes received either no NA (control), 50 μM NA, or 200 μM NA during the 18-hour pre-IVM holding period.
  • OPU-derived oocytes were either untreated or treated with 200 μM NA during pre-IVM.
  • Additionally, some OPU-derived oocytes treated with 200 μM NA during pre-IVM were also supplemented with 200 μM NA during the final 22-24 hours of IVM.
• Donor nuclei for cloning varied between cell lines, but within each replicate, the same donor cell line was used for all oocyte groups to maintain consistency.

Key Findings

• Blastocyst Development:
  • In abattoir-derived oocytes, NA supplementation during pre-IVM significantly increased blastocyst formation rates compared to controls:
    • Control: 19.9 ± 1.7%
    • 50 μM NA: 27.1 ± 1.4%
    • 200 μM NA: 32.9 ± 3.0% (statistically significant improvement)
  • For OPU-derived oocytes, pre-IVM NA treatment alone did not significantly affect blastocyst rates.
  • However, NA supplementation during the final IVM phase (following pre-IVM treatment) markedly improved blastocyst rates:
    • With NA during IVM: 53.4 ± 9.6%
    • Without NA during IVM: 31.3 ± 8.1% (statistically significant)
• Nuclear Maturation, Fusion, and Cleavage:
  • NA supplementation did not significantly affect rates of nuclear maturation, couplet fusion (fusion of donor nucleus and oocyte), or early embryo cleavage.

Interpretation and Implications

• The data indicate that NA treatment enhances the developmental competence of equine oocytes primarily by improving later stages of embryo development (blastocyst formation), rather than early maturation or fusion events.
• The beneficial effects vary depending on the oocyte source and timing of NA supplementation:
  • In abattoir-derived oocytes, pre-IVM NA treatment alone was effective.
  • In OPU-derived oocytes, NA during the final IVM phase was necessary for improvement.
• NA supplementation may elevate cellular NAD levels, supporting metabolic and epigenetic processes essential for embryo development.
• This approach could help overcome current limitations in equine IVM systems, increasing the efficiency of cloned embryo production for research or breeding applications.

Future Directions

• The study suggests further research is required to:
  • Clarify the molecular and metabolic mechanisms by which NA enhances oocyte developmental potential.
  • Evaluate the viability and health of embryos generated from NA-treated oocytes after transfer to recipient mares.
  • Determine optimal NA dosing and timing protocols for different clinical or laboratory settings.