The Biochemistry of Tear Production
Since 1998 Zoukhri has been studying Sjögren’s syndrome, an autoimmune disorder that is the primary cause of severe dry eye and dry mouth. His past research has primarily focused on the tear-producing lacrimal glands, but he is expanding his research to include the salivary glands. He currently has a $1.5 million grant to continue his research on the molecular mechanisms of Sjögren’s syndrome.
“The cornea, which is the clear part of the eye, and the conjunctiva, which is the white part, are heavily innervated with sensory nerves,” says Zoukhri. “So every time anything hits the cornea – an object, extreme heat or cold, any insult – the sensory nerves signal the brain, which, through the sympathetic and parasympathetic system of nerves, signals the lacrimal glands to start making tears.” Zoukhri’s research group has been defining neurotransmitter messenger molecules that are released at nerve endings in lacrimal tissue, defining neurotransmitter receptors on the lacrimal cell surface, and defining intracellular molecules involved in the signaling pathway that induces tear production.
“The dogma was that, as in other autoimmune diseases, you have immune cells that penetrate the tissue and start destroying the cells that are supposed to make saliva or tears,” Zoukhri says. “But in fact, while immune cells do take over some of the lacrimal gland tissue, more than half remains intact. When I started in the field five or six years ago, our question was Why aren’t the intact lacrimal cells fulfilling the function of making tears?” Over the years, Zoukhri’s research group found that diseased lacrimal tissue had not lost its innervation and that the nerve endings still contained neurotransmitters. They also found that diseased lacrimal tissue had not lost its ability to respond to neurotransmitters. “Based on our data, we suspected that release of the neurotransmitters was being blocked. We showed that this is indeed the case. So we started looking at what was stopping the nerves from releasing their neurotransmitters.”
Zoukhri’s research group found that the inflammatory cytokines interleukin-1 (IL-1) and tumour necrosis factor (TNF) inhibited release of neurotransmitters from nerve endings in lacrimal tissue, thus inhibiting neural stimulation of lacrimal cells to secrete tears. They also found that cytokines inhibited secretion of tear proteins within lacrimal cells. Now the group is working on understanding how this inhibition occurs and on identifying agents to selectively block the actions of cytokines within the lacrimal tissue.
When IL-1 binds to its receptor in the plasma membrane, it turns on molecules that translocate into the nucleus of the cell and start transcription of genes that are involved in inflammation. “So what we’re doing right now is trying to define roles these molecules might play and the effect of IL-1 on lacrimal and salivary glands,” says Zoukhri. His research group is looking at cJun N-terminal kinase (JNK), which is one of the enzymes involved in the biochemical pathway between IL-1 receptor activation and gene transcription.
The challenge is to find an agent that works on an event that is specific to a particular pathway in a particular tissue. Although cytokines like IL-1 can trigger undesirable inflammation in autoimmune diseases, they are necessary for other beneficial actions throughout the body. So blocking a cytokine’s receptor, as some rheumatoid arthritis drugs do, can cause undesirable side effects such as increased susceptibility to infection. By intervening farther down the IL-1 biochemical pathway within a specific tissue, such as by inhibiting JNK in lacrimal tissue, investigators hope to localize the inhibitory effect to that tissue. However, if you continue down the biochemical pathway too far, such as to the transcription of genes, you once again reach molecules that are in general use throughout the body and that would not be appropriate targets for intervention.
Zoukhri’s research group is not currently doing tissue culture experiments because of the difficulty of growing functional lacrimal gland tissue in cell culture. The problem is that the lacrimal cells must develop around a central lumen in a specialized structure called an acinus in order to be functional. The cells are polarized and attach to each other by their apical areas. The basal sides of each cell have receptors for neurotransmitters, whereas the apical sides surround the lumen, which ultimately becomes a duct that brings the tears to the outer surface. Salivary glands have a similar structure. Zoukhri is looking forward to discussing this problem with Jonathan Garlick, who recently joined the dental school as professor of oral and maxillofacial pathology and director of the Division of Cancer Biology and Tissue Engineering. “Jonathan Garlick is a very well known and respected cell culture investigator who grows three-dimensional structures that mimic skin,” says Zoukhri. “So we’re hoping that once his lab is set up we can figure out how to grow lacrimal gland cells.”
Zoukhri is continuing collaborations with researchers at the Schepens Eye Research Institute and has begun collaborations with Noorjahan Panjwani of the ophthalmology department of the Tufts University School of Medicine. He is also planning a collaborative study with Michael Goldstein and Helen Wu of the department of ophthalmology at Tufts-NEMC.
While he concentrates on the molecular mechanisms of Sjögren’s syndrome, Zoukhri would like to find a collaborator in neuroscience who would be interested in delving more deeply into the neuronal stimulation of tear and saliva production.
For more information, contact Driss Zoukhri.