March 8, 2017

Lasers and LED control cell functions


Optogenetics inserts photosensitive proteins into living cells. The cell functions can then be controlled by laser or LED. This opens up whole new approaches for research and treatment.

Nerve activities are tiny electrical pulses. They are controlled via proteins in the cell membrane. For activation, they allow positively charged ions to flow into neurons. When they are inactive, the proteins hinder the flow of ions. Optogeneticists have succeeded in taking the reins in this process: They use genetic engineering methods to insert photosensitive proteins into the membranes of living cells. Once there, these multiply without causing any damage. As a result, scientists can control the functions of the manipulated cell with LED light or lasers.

Unlike activation with micro electrodes, the optical method reaches individual, specific cells. Harmless viruses or plasmids (DNA molecules from bacteria) transport light-sensitive proteins into the cells. It all began with channelrhodopsin-2 from green algae cell membranes. When activated with blue light, it allows ions to flow into the cells in which scientists place it. Soon after this, an off switch was discovered in nature: Halorhodopsin, which prevents the flow of ions under the influence of yellow light.

Basic research—and new hope in the battle against epilepsy and Parkinson’s disease

In the meantime, the variety of photosensitive proteins is growing; they are found mainly in algae, cereal, and bacteria. This opens up new applications for optogenetics. Scientists are investigating brain functions, the exact processes in nerve and muscle cells, and also the mechanics behind mutations of tumor cells by manipulating them with natural “light switches”. Thanks to the switching functions, they can clarify exactly and systematically how cells carry out specific functions, how malfunctions come about, and which brain cells participate in specific decision-making processes.

The link between photonics and genetics is not only revolutionizing basic research, but also giving hope to many patients. For example, blind mice got their light sensitivity back after scientists inserted photosensitive proteins into their retinas. In the future, this could also help people with impaired vision. The precise optogenetic method could become an alternative for deep brain stimulation of Parkinson’s patients, could suppress epileptic seizures and activate nerves in paralyzed body parts or even pave the way for new cancer treatment strategies.

The variety of switchable proteins needs flexible light sources

The choice and constant optimization of photosensitive proteins also increases demand for flexible light sources. In simple (stereo) microscopy, multispectral LEDs are primarily used. They can be switched quickly and aimed directly at the specific cell with the help of lenses. They also require little space, emit less heat and have a long service life.

By contrast, confocal microscopy needs lasers. Their high light density and good focusability offer important advantages here. Recent publications from Coherent describe precise, 3D resolution optogenetic experiments in deep tissue layers. They work with multiphoton stimulation using an ultra-short pulsed near infrared laser (1,040–1,150 nm wavelength). On the other hand, experts at Leica Microsystems refer to the flexibility of white light laser (WLL) technology in publications. Infrared lasers in combination with PCF (Photonic Crystal Fibers) and acousto-optical filters are able to generate any color in the visible spectrum. The selection can be changed within microseconds. Photonics also offers alternatives to the current deep brain stimulation with needle-like electrodes: LED or laser-fed fibers could carry out this task in combination with photosensitive proteins much more gently and accurately.