About the Lab

Research projects Epilepsy Lafora Disease Glucan Phosphatases Polyubiquitination Starch Tying it Together Recent Highlightes Funding

My research program is organized around three major, connected components that are involved in glucan metabolism: (1) regulation, signaling, and dynamics of the human glucan phosphatase laforin, (2) the role of malin-directed ubiquitination in glycogen metabolism, (3) exploiting plant/algal glucan phosphatases to control energy storage and impact starch-based industrial processing. These projects integrate synergistically to move forward our understanding of glucan metabolism and its role in both disease as well as biofuels research, and are all extramurally funded through federal and non-federal sources.

Epilepsy is a medical condition initiated in the brain and affects both mental and physical functions of the patient. The epileptic event, or seizure, is a result of miscommunication between clusters of nerve cells, called neurons. When the neurons fire inappropriately the patient experiences convulsions, muscle spasms, loss of consciousness, strange sensations, and/or strange emotions. Epilepsy affects nearly 3 million people in the U.S., at estimated annual costs of $15.5 billion. The inappropriate firing of neurons requires energy and multiple links have been established between epilepsy and metabolism.

Lafora disease (LD, OMIM 254780) is a rare type of neurodegeneration that results in severe epilepsy and death. Unlike most other forms of epilepsy, LD is only mildly managed by medication for a brief period of time. LD patients develop normally until they present with a single seizure when the patient is 15-20 years old; this single seizure is followed by progressive central nervous system degeneration and the disease ends with the death of the patient within ten years of the first seizure. A hallmark of LD is the accumulation of carbohydrate/glucan granules that form in most cells in the body, called Lafora bodies (LBs). LBs are similar to glycogen, the cell’s normal glucan storage molecule, but unlike glycogen LBs are water insoluble and are more closely related to plant starch than human glycogen. The frequency and severity of the patient’s epilepsy increase with age and with the accumulation and size of LBs. Thus, it is hypothesized that LBs are the causative agent of the patient’s epilepsy and the death of the patient. Since LD is an epileptic disorder driven by improper metabolism, studies about LD offer a unique glimpse into how epilepsy, neurodegeneration and metabolism are linked.

Lafora disease is the result of recessive mutations in either the gene encoding the phosphatase laforin or the E3 ubiquitin ligase malin. The focus of our lab is to determine how laforin and malin regulate glycogen metabolism and inhibit Lafora disease, and then apply these findings to other neurodegenerative diseases and/or epilepsies.

Laforin was previously thought to only be conserved in vertebrates; however, we identified laforin orthologs in five unicellular eukaryotes (i.e. protists). Surprisingly, the biochemical composition of LBs closely resembles that of floridean starch; an insoluble glucan synthesized by the same protists that have laforin. We demonstrated a direct correlation between the presence of laforin and synthesis of insoluble glucans amongst protists. Additionally, we demonstrated that a plant protein called Starch EXcess4 (SEX4) is a functional equivalent of laforin. Strikingly, mutations in SEX4 result in a starch excess phenotype very reminiscent to the cellular phenotype in LD patients. These insights led us to hypothesize that laforin is the founding member of a unique group of glucan phosphatases.

We also demonstrated that malin is an E3 ubiquitin ligase that polyubiquitinates multiple proteins involved in glycogen synthesis, including laforin, protein targeting to glycogen (PTG), and glycogen debranching enzyme (AGL/GDE). Polyubiquitination is a cellular signal to degrade a protein. Thus, malin decreases glycogen accumulation by promoting the degradation of these, and other proteins.

As the major energy cache in plants and algae, starch is a central component of human and animal food and a key constituent in many manufacturing processes. Additionally, starch is both a first-generation biofuel and it is vital to future efforts focused on microalgal hydrogen and oil production. Growing starch demand has impacted the drastic rise in corn prices from $85/metric ton in 2002 to $258 in 2012. Therefore, elucidation of pathways controlling starch metabolism is needed in order to develop novel strategies that manipulate them and satisfy the growing starch demand. A key pathway regulating starch metabolism – and one that is required for starch degradation – is reversible phosphorylation of glucose residues in starch outer glucans, rendering the granule surface accessible to glucan hydrolyzing enzymes. The focus of my lab towards these efforts is to determine the molecular mechanisms of glucan phosphatases. We are defining the function, dynamics, structures, and regulation of the glucan phosphatases as well as generating and evaluating engineered glucan phosphatases. Cumulatively, these studies will provide a comprehensive profile of how substrate specificity is determined as well as how glucans influence enzyme activity, providing the needed insights for current and future biotechnological exploitation of these enzymes. This work is a new effort in the lab and was initially funded by KSEF-2268-RDE-014 with recent funding secured from NSF CAREER MCB#1252345.

Cumulatively, we propose that malin- and laforin-like activities are involved in an unstudied aspect of energy metabolism and that these functions are conserved from plants to protists to humans. Our goal is to untangle the intercalated events of metabolism, neurodegeneration, and epilepsy utilizing our insights from studying Lafora disease and plant starch metabolism.

Our work on the plant glucan phosphatase LSF2 was recently highlighted in a commentary in The Plant Cell volume 25, page 1915. Additionally, our work on SEX4 was highlighted on the cover and in a commentary in PNAS volume 107(35), pages 15312-13.

NIH/NINDS K99-NS061803

Pathway to Independence Award

“Novel model systems link floridean starch metabolism to Lafora disease”

5/1/07 – 9/10/08

 

NIH/NINDS R00-NS061803-01     

“Model systems link floridean starch metabolism to Lafora disease”

9/15/08 – 6/30/12

 

NIH/NINDS R01 PA-07-070

“Regulation, signaling, and dynamics of glucan phosphatases”

7/1/10 – 6/30/15

 

National Science Foundation CAREER Grant

Office of Science, Early Career Research Program

“Glucan phosphatases: a key to designer starches and plant energy storage”

1/1/13 – 12/30/18

 

Mizutani Foundation

Mizutani Foundation for Glycoscience

“Glucan phosphatases link neurodegeneration and plant energy metabolism”

3/1/13 – 2/29/14