Researcher reveals how temperature impacts development of embryos

In nature, animals face a broad range of temperatures, and at times the heat can become taxing. When it becomes too hot to survive, some animals can simply migrate to more favorable climates, but what if you are a mere embryo confined within an egg and cannot escape the heat?

High temperatures can cause proteins within the embryo to become denatured—an unraveling that results in loss of function, an ineffective or denatured protein. Moreover, denatured proteins can form aggregates that are toxic. Understanding this process has important implications for human health, because such protein aggregates are a hallmark of neurodegenerative diseases, such as Parkinson's and Huntington's.

But what happens when embryos face temperatures outside their optimal zone remains a puzzle. "Nobody has looked at the ecological context of thermal environment on early developing embryos," explains Dr. Brent Lockwood, a National Institute of Health Postdoctoral researcher at Indiana University.

Lockwood has been studying developing fruit fly embryos to understand the influence of temperature. Fruit flies are used because their genetics are well established, and the small flies are ideal for cellular microscopy. "You can watch cellular development in real time under the microscope - and get a sense of what is really going on," says Dr. Lockwood.

As presented at the 2014 Society for Integrative and Comparative Biology annual conference in Austin, Lockwood exposes the embryos to a range of temperatures to pinpoint when the eggs reach their threshold temperature, the point at which more than 50% of embryos die. He exposes one-hour old fly embryos to increasing amounts of heat over 45 minutes, a process called "heat-shocking." He then examines cellular processes using a microscope, providing an inside look into the cell, to discover precisely how temperature impacts development.

So what happens to embryo cells when they face heat stress? When the temperature rises, the cell's organization center, the cytoskeleton, unravels. Actin and tubulin, which are key proteins that coordinate early development, break down. Without their proper structure, embryos can no longer develop and so they perish. Under heat-shock, embryos lose their actin array, the tubulin becomes disorganized, and the cell loses its structure, effectively ending development of the embryo.

This work provides a unique, inside look at how temperature impacts development in animals beyond just fruit flies. "Because of the highly conserved role of the cytoskeleton, Dr. Lockwood's investigations into the mechanisms that maintain these cellular structures during times of environmental heat stress will have implications for the success of many species," explains Dr. Kristi Montooth, an Assistant Professor of Biology at Indiana University, who is also involved in this study.

When real environmental temperatures spike, how do fly eggs survive at all? The answer may be that mothers provide proteins that ease the effects of temperature. Mothers may inject heat-shock proteins — special proteins that effectively sequester and 'fix' damaged proteins by helping them fold properly. Maternal heat shock proteins may act as a safeguard to protect embryos from heat-shock until they can make their own proteins.

The next step for Lockwood's research is to look at effects beyond the single cell and the individual. He plans to examine whether mother flies from hotter environments impart to their offspring a better ability to withstand heat stress. If the environment is hot enough, natural selection should favor mothers that make protective proteins, or favor mothers who lay their eggs in cooler, shadier spots.