Department of Transport: A Profile of Sandra Encalada

Picture a tubular liquid world strewn with zippers and zeppelins…

The zippers are railways of a sort—like cog railways. The zeppelins, vast lumpy sacs stuffed not with gas but with freight, whoosh along the zippers at mind-bending speeds, pulled by long robot legs that walk in a frenetic, cog-by-cog tiptoe.

This surreal vision is brought to you in part by cell biologist Sandra Encalada, an Arlene and Arnold Goldstein Assistant Professor of Molecular and Experimental Medicine and one of the newest faculty members on The Scripps Research Institute’s California campus. Encalada is an expert on the liquid world within the axon—a nerve cell’s (neuron’s) output stalk—and specifically on its cargo-transport system, whose smooth operation is essential for neuronal health. “An axon is thin and often very, very long, and if its tracks for moving neurotransmitters, synaptic proteins, and other key molecules are blocked, the neuron may die,” she says.

While finishing her postdoctoral work at the University of California, San Diego (UCSD) last year, Encalada was first author of an important paper in the journal Cell that significantly clarified the picture of axonal transport. Encalada and her colleagues attached fluorescent reporter-molecules to cargo proteins in cultured mouse neurons, then watched as the sac-like vesicles containing this glowing freight moved to and fro along axonal railways (called microtubules). Using genetic and other techniques, she systematically removed subunits of the vesicle-hauling, microtubule-gripping “motors” (known as kinesins and dyneins) to see whether the dynamics of transport changed—and thus which subunits had been in use.

Her data unequivocally validated a model of axonal transport in which groups of kinesins and dyneins remain stably attached to an individual vesicle. “We think that these motors are activated and deactivated depending on the direction in which the vesicle will travel, but they remain bound to their vesicles throughout their journey, and it is likely that motors disengage the microtubule when the vesicle stops, and when reactivated the motors re-engage the microtubule to move the vesicle again,” Encalada says.

Kinesins haul vesicles from the main body of a neuron toward the axon terminus; while dyneins do the reverse. Encalada’s study found evidence that some vesicles have both kinesins and dyneins, and thus can run either way. Considering the simplicity and fragility of these nano-machines, they move their cargo with astonishing speed: Encalada clocked vesicles going as fast as 2.8 microns per second—the human-scale equivalent of several hundred miles per hour.

A Less Linear Life Path

Encalada’s own path through life has been less linear, more serendipitous. She was born in Quito, Ecuador, and didn’t plan on becoming a lab-based biologist until she was in her mid-20s. She spent time in Palo Alto, California, as a child, when her father was in graduate school at Stanford University, then moved to the United States as a teenager to attend the Armand Hammer United World College of the American West —a two-year international prep school in New Mexico—with a full scholarship.

Later she attended a small college in Indiana, majoring in physics. “I was very quantitative,” she says. Her father, a journalist and founder of an environmental group, had encouraged her to do well in math. “He emphasized that good math skills would open up a lot of opportunities,” she says.

An undergraduate ecology project she did in Puerto Rico sparked an interest in biological systems, and, after getting her physics degree, she spent two years at the University of Florida, studying sea turtle population genetics. She found that she liked lab work, and for her PhD at the University of Oregon she switched to molecular genetics—and, among other things, began studying kinesins.

Function and Dysfunction

At UCSD, she worked in the laboratory of cell biologist and frequent Scripps Research collaborator Lawrence S. B. Goldstein. Like others in the Goldstein lab, Encalada tried to understand not only how the elements of axonal transport function normally, but also how their dysfunction relates to the neurodegeneration seen in Alzheimer’s and Hungtinton’s diseases and related conditions. However, in the Goldstein lab, Encalada started a new avenue of research and focused on trying to understand how axonal transport dysfunction might be involved in the initiation of prion diseases—disorders where aggregates beget more aggregates and neuronal degeneration in either an infective, genetic, or sporadic paradigm.

The potential clinical implications of her research were brought to the attention of Arnold and Arlene Goldstein (no relation to UCSD’s Lawrence Goldstein), whose philanthropy supports the study of such diseases. The couple had funded an assistant professorship in the same Scripps Research department in 2009. Last year, their generosity and vision made it possible to create Encalada’s position at Scripps Research.

In last year’s Cell study, the vesicle-hauled freight protein used by Encalada and her colleagues was one with real disease significance: the mammalian prion protein, PrP^c, which is linked to Creutzfeldt-Jakob disease, “mad cow,” and other prion diseases. The normal function of this synapse-associated protein remains a mystery—subtracting its gene from mice causes no clear problem. But scientists know that, in rare cases, single copies (monomers) of PrP^Sc, a misfolded form of PrP^c, start aggregating into small clusters or “oligomers” that are somehow highly toxic to neurons.

Worse, these toxic oligomers act as DNA-like templates for assembling new copies of themselves, so that they can, in principle, pull every free-floating monomer of PrP^c in their vicinity into the PrP^Sc pathogenic oligomer form—and thus can burn infectiously wherever PrP^c is abundant. They burn quickly, too: Prion diseases such as Creutzfeldt-Jakob and “mad cow” disease can destroy the brain in a matter of months. Amyloid beta and tau proteins in Alzheimer’s disease, alpha-synuclein protein in Parkinson’s disease, and huntingtin protein in Huntington’s disease are all now suspected of driving their diseases in a similar manner, but more slowly.

A striking feature of all these diseases is a swelling of axons in affected neurons, as if blockages of normal transport routes have led to buildups of undelivered molecules. “One hypothesis we’re considering is that the pathogenic aggregates in these cases are killing neurons by blocking axonal transport,” Encalada says. And since axonal transport normally can be used to eject unwanted protein aggregates from neurons, disruptions of axonal transport could create a vicious spiral of worsening aggregate accumulation.

“In some of these diseases, the disruption of axonal transport might even be an initiating event,” Encalada notes. “There’s already some evidence for that, and it’s something that we want to study in our cell and animal models here at Scripps in the context of numerous aggregation diseases including ALS and the transthyretin amyloid diseases.”

In one ongoing project, Encalada and her students are tracking pathogenic prion aggregates as they move along axons, apparently carried at normal speeds by vesicles. She wants to know whether these aggregates use these journeys in vesicles to corrupt fellow-traveling PrP^c, ultimately blocking axonal railways. That could end up being the first precise explanation for how prions and other disease-linked protein aggregates terminate their neuronal victims.

Encalada also wants to find out more clearly how kinesins and dyneins are regulated, and whether their efficiency declines significantly as organisms get older—which, if so, could help explain why most neurodegenerative diseases are strongly linked to advancing age. “We’re now planning a comprehensive comparison of axonal transport in early, mid, and late life, using our animal models,” Encalada says.

These animal models include the nematode worm C. elegans, which is favored by many labs for its relatively quick and easy genetic manipulability. Although it is eons removed from a brainy primate like Homo sapiens, the worm makes a surprisingly good model for some aggregate-linked diseases. Among these are the transthyretin (TTR) amyloid diseases, which originate from a liver-produced protein. Scripps Research Professor Jeffery Kelly, who chairs the Department of Molecular and Experimental Medicine, recently devised a small-molecule drug, tafamidis (now approved for use in Europe and under review by the U.S. Food and Drug Administration), that inhibits the formation of toxic TTR aggregates—but still no one really knows why these aggregates are toxic. “We’re trying to find out whether they, too, are involved in disrupting axonal transport,” Encalada says.

The work is keeping her busy. An avid runner and swimmer, Encalada is a member of the Triathlon Club of San Diego, but says she has had to cut down on races while she sets up her lab: “The difference between being a postdoc and a faculty member—oh boy!”

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