Possum vaccines and reproductive proteins
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).
Control of reproductive hormones
Reproductive hormone levels are controlled by signals from the brain. A brain region called the hypothalamus releases gonadotrophin-releasing hormone (GnRH), which stimulates the pituitary gland to produce luteinising hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH stimulate the production of sex hormones, like oestrogen or testosterone. The level of these sex hormones then acts directly on the brain in a feedback system that helps to regulate further hormone production.
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.
Fertilisation
Diagram showing the steps in fertilisation. The sperm attaches (1), binds (2) and penetrates (3) the zona pellucida. When the sperm fuses with the egg's membrane genetic material is released inside the egg. Fertilisation (4) occurs when the sperm's genetic material eventually combines with the egg's genetic material.
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.
