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Here, we explain the function of reproductive proteins GnRH and ZP. These proteins are targets for contraceptive vaccines, investigated in New Zealand to help reduce possum numbers.

Scientists in the National Research Centre for Possum Biocontrol (NRCPB) investigated contraceptive vaccines that would make possums less fertile. This biocontrol method was under development to reduce possum numbers because possums are a major pest in New Zealand.

Possums are marsupials – their young are reared in a pouch. In common with other mammals, proteins are important for controlling the possum’s reproductive cycle and fertility. Vaccines that make a possum produce antibodies against these proteins will block fertilisation or shut down the possum’s reproductive cycle and prevent it from producing offspring.

Proteins targeted for contraceptive vaccines

The proteins being targeted were gonadotrophin-releasing hormone (GnRH), a small hormone crucial for fertility, and zona pellucida proteins, which form a protective coat around the female egg and are required for fertilisation.

Function of GnRH

GnRH is produced in an area of the brain called the hypothalamus.

GnRH stimulates the pituitary gland to produce two hormones – FSH (follicle-stimulating hormone) and LH (luteinising hormone).

Role of FSH (follicle-stimulating hormone)

In females, FSH stimulates the growth of follicles in the ovaries from which the egg develops.

In males, FSH stimulates sperm production.

Role of LH (luteinising hormone)

In females, LH triggers the release of an egg from a follicle in the ovary (ovulation). The collapsed follicle that remains after ovulation becomes the corpus luteum. The corpus luteum produces the hormone progesterone, which prepares the lining of the uterus to receive the fertilised egg.

In males, LH stimulates the production of testosterone from the testes.

Effect of a vaccine on GnRH

The aim of the vaccine was to destroy the GnRH hormone, and for insufficient FSH and LH to be produced. Therefore, no mature eggs could be produced in females and no testosterone or sperm produced in males. By targeting this hormone, the possums would become infertile, as they could not produce eggs or sperm.

Function of zona pellucida proteins

A glycoprotein coat called the zona pellucida surrounds mammalian eggs. The possum zona pellucida consists of three specific proteins – ZP1, ZP2 and ZP3. These proteins allow sperm to attach to the egg and help the sperm’s entry into the egg. After fertilisation, the zona pellucida proteins ensure early embryonic development occurs.

Effect of a vaccine on zona pellucida proteins

The aim of the vaccine was to block the binding of sperm to the zona pellucida which would prevent fertilisation.

Find out more about this in the article, Delivering biocontrol agents to possums.

An update

In a 2011 report, Dr Jan Wright, Parliamentary Commissioner for the Environment wrote that although significant research effort and resources were put into possum contraceptive vaccines and hormone toxins, funding for the projects finished in September 2010. There was concern about the length of time required to produce results, risks associated with the biocontrol agents and potential controversy over genetic engineering.

    Published 18 December 2008, Updated 17 September 2018 Referencing Hub articles