Dr. Sarah McFarlane
During development a region of the developing embryo is partitioned to become the nervous system. Through cell signalling and cell movements, this nervous tissue is further subdivided into brain regions in which tissue specific cell genesis and neural wiring gives each region its unique functional identity. We study the development of the eye, as a mini-brain, which is easily accessible and separate from the rest of the central nervous system. My laboratory is interested how cell movements are controlled to: 1) give the eye its unique structure, 2) allow the appropriate connections to be made between retinal neurons, and 3) maintain a regulated vascular supply to the eye. To do so, we use cutting-edge genetic and molecular approaches, combined with confocal imaging of cells in the living embryo. Our funded projects include:
1) Understanding the migration of the retinal pigment epithelium (RPE) around the forming eye (Foundation Fighting Blindness, 2015-2018)
The cells in your retina process visual inputs and send the information to the brain so that you can make sense of your visual world. The retinal pigment epithelium (RPE) is a thin layer of cells that covers the back of your retina, where it functions as a nursemaid for the photosensing cells of your retina, the photoreceptors: the RPE protects, provides structural and survival support, and keeps the light sensing pathways on-line. Because of the importance of the RPE in photoreceptor health, defects in the RPE lead to photoreceptor degeneration, and loss of sight. One approach to deal with diseases where photoreceptors degenerate, such as in Age-Related Macular Degeneration (AMD) and Retinitis Pigmentosa (RP), is to use stem cells to first generate and then transplant into the damaged retina the RPE cells that could block and/or delay photoreceptor degeneration. In this treatment paradigm, RPE cells have to migrate from the site of transplantation. Further, aberrant migration of RPE cells can occur in disease states where vision loss ensues, such as AMD. Thus, understanding what drives RPE migration is important. Yet, we know little about how the RPE cells move, and the molecules that provide the instructions for movement. Here, we use zebrafish as an experimental model, primarily because genetic mutagenesis is quick and effective, so we can study how specific proteins control RPE movement both in the embryo and after RPE or retinal injury. Further, the zebrafish model allows us to track RPE cells in real time in live embryos, by engineering RPE to express a fluorescent protein, providing unparalleled information about the consequences of gene disruption on RPE migration.
2) Investigating Semaphorins as negative regulators of blood vessel growth (Canadian Institutes of Health Research, 2017-2022; Brightfocus Foundation 2017-2019)
In disorders causing blindness, such as diabetic retinopathy and AMD, sprouting of new blood vessels (angiogenesis/neovascularization) that mainly occurs in the embryo leads to death of the nerve cells of the retina. These neovascular disorders place a substantial burden on patients and the health care system. AMD alone affects ? 100,000 Canadians over the age of 50, while diabetic retinopathy is the most common cause of blindness in people under 65. Understanding the molecular basis of these diseases is the best hope for developing new medications to tackle them.
Nerve cells in the retina require nutrients and oxygen provided by two distinct vessel beds: retinal vessels that support the optic nerve cells of the inner retina, and the choroid vessels that sustain the light-capturing photoreceptors of the outer eye. In neovascular eye disorders, the existing vessels sprout and new, leaky vessels extend within both vessels beds. The leakage leads to swelling in the eye that disrupts the nutrient and oxygen supply, and vision loss occurs as retinal cells degenerate. Current approaches to block new blood vessels from forming are not effective in many patients and they have serious side-effects. We urgently need an effective way to prevent these faulty new blood vessels from forming that only affects the pathological vessels and not the health of retinal nerve cells or the normal blood vessels.
This project addresses this gap by providing fundamental knowledge of a novel mechanism to prevent new blood vessels forming in the eye. We have identified a mechanism that normally controls eye vessel growth in the embryo that is implicated in pathological blood vessel growth in the adult eye. In this project we will define these mechanisms of normal and pathological eye blood vessel formation, informing the development of new treatments for neovascular eye disorders.
3) Identifying the molecular mechanisms underlying light-dependent regulation of neural crest cell development
The peripheral nervous system is critical for mounting and coordinating an organism’s response to changes in its internal and external environment. Most cells of the peripheral nervous system -- including most peripheral neurons and glia -- are derived from neural crest cells (NCC). The NCC progenitors are born at the dorsal edge of the forming neural tube, from which they delaminate, migrating to target sites where they differentiate to form target specific cell types. We understand only some of the steps of this fascinating process, including some of the molecules that control the induction and guidance of NCC progenitors; however, an important knowledge gap and the focus of this proposal, is how an organism’s environment can impinge on these intrinsic and external processes to impact NCC progenitor behaviour.
Pigmented skin cells (chromatophores) are involved in environmental responses related to camouflage and UV protection. Chromatophores also derive from NCC, and their development depends on many of the same molecular signals that control NCC-dependent peripheral neuron genesis. Their ready visibility in the skin of live embryos, and our recent demonstration that their genesis can be controlled by environmental light (Bertolesi et al., 2016), makes them a particularly good model for studying how environmental influences control the signalling events that regulate NCC development. We hypothesize that light perceived by the eye acts through a neural circuit to release systemic factors that control NCC progenitor proliferation and apoptosis. We are using the Xenopus laevis embryo model, readily amenable to surgical, molecular and pharmacological manipulations in vivo, to address this hypothesis.
This research program is at the forefront of understanding how the external environment can directly impact the developing nervous system, in that we have identified a natural signal, light, that controls NCC progenitor biology to adapt the organism to its milieu. We will study the neural circuitry and signalling events that control light-mediated changes in skin pigmentation using both in vitro and in vivo models. Skin pigmentation is readily assessed, and so the model is experimentally amenable, and will provide key foundational information that can be used to understand how the external environment can impact the processes that control the development of other NCC derived cell types.