Lessons from fruit flies: A new assay to study Parkinson’s disease

Susheela Srinivas

Complex neurodegenerative disorders present a challenge to researchers who attempt to study them using model systems in the lab. Precisely measuring the behavioural defects which are hallmarks of such diseases in model organisms like the fruitfly can be difficult. Now, a team of researchers have come up with a new assay which can help assess movement-related impairments in fly models of Parkinson’s disease.

Collage Fruitflies
Right: The researchers who carried out the study - from top to bottom: Aman Aggarwal, (late) Heinrich Reichert, K VjiayRaghavan; Left: Drosophila melanogaster byAfroBrazilian (CC BY-SA)

About two decades ago, biologists discovered that fruit flies (Drosophila melanogaster) have a high degree of genetic similarity with humans. As a result, flies can act as good genetic models in laboratory experiments to study human diseases, including neurodegenerative disorders. In this regard, a test called the Drosophila climbing assay is commonly used in assessing movement defects in fly models of Parkinson’s disease.

A recent study by a team of international researchers led by K VijayRaghavan from the National Centre for Biological Sciences (NCBS), Bengaluru, proposes a modified fly assay. This new assembly is significantly more sensitive and can pick up even subtle movement flaws in the fly models. The novel method overcomes several limitations of the conventional test and opens exciting avenues for quantifying the degree of movement defects. Moreover, it reveals hitherto unknown aspects of the genetic players involved in Parkinson’s disease.

Parkinson’s disease is a disorder affecting the central nervous system, with symptoms such as uncontrollable tremors, trouble controlling the movement of arms and legs, rigidity of the limbs, and impaired body balance. Early diagnosis of Parkinson’s is a challenge as the symptoms surface several years after the disease has set in; by the time the condition is detected, the degeneration has progressed to a state beyond rescue. 

There is no known cure for Parkinson’s disease. Scientists are still deciphering the molecular mechanisms involved in the disease and actively seeking in-depth information that can detect disease severity and progression.

The conventional Drosophila climbing behavioural assay is a laboratory tool commonly used to study movement disorders. It takes advantage of the natural response of the flies to move against gravity (negative geotaxis) when placed in closed spaces such as a jar. For testing, the diseased flies are placed inside a glass jar which is then vigorously tapped to settle the flies at the bottom. As the flies try to escape, they move up the jar and a scientist or technician manually records their climbing pattern and locomotor ability.

Although this method provides valuable information, it gives only a yes-no’ answer to movement disorders. However, locomotion is a complex process in humans — subtle details like pace, coordination of limbs, or changes in orientation go unnoticed in this test. Moreover, it is a tedious method which is often plagued by errors.

Also, the tapping of the flies induces physical stress and trauma, thereby altering their behavioural response. This, in turn, could lead to erroneous observations,” says Aman Aggarwal, first author of the study.

VijayRaghavan’s team addresses these drawbacks by modifying the apparatus: a slowly rotating transparent chamber eliminates the trauma for the flies, and a fully automated set-up gives real-time readings of subtle changes in the fly’s movement.

For testing, a single, diseased fly is introduced into a specially marked chamber, and the fly’s climbing pattern is observed with relation to these markings. A high-resolution camera captures each movement of the fly and records intricate parameters such as distance covered, pace and path-straightness. A software coupled with the camera analyses the data and provides an overall picture of the defects.

This study will boost Parkinson’s disease research which may be of immediate interest to clinicians,” observes Phalguni Anand Alladi, Scientist F at the National Institute of Mental Health and Neurosciences (NIMHANS), Bengaluru. She was not involved in the study.

By using the new technique, the team analysed the effect of certain genes on locomotion in flies. The experiment consisted of three groups: wild type (control group), single-gene mutated flies, and flies with combined mutations. They targeted the genes PINK1, parkin and LRRK, which are known to contribute to Parkinson’s disease progression.

These experiments led to two discoveries. First, while most of the single-gene mutated flies exhibited some degree of movement deficits, the combined mutation of both LRRK and parkin led to more severe defects, revealing that the combination manifests into a high degree of disease expression. 

When extrapolated to human conditions, this nuance is particularly useful to explain the differences that are seen in individuals who are more likely to develop the disease, due to certain genetic composition. This aspect can further develop into a test parameter for patients,” says Alladi.

Second, the observed movement defects in male flies were more pronounced compared to the females in one type of mutation, despite the known facts that healthy males are more agile than females, and both genders are equally prone to the disease. Further research will establish if a gender disparity exists,” says Aggarwal.

Although in its initial stages, this research has long term implications. For example, when new drugs or gene therapy targeting specific motor aspects of Parkinson’s disease are developed, this model may be used for a quick assessment in the initial phase, says Alladi.