The Lloyd Lab

Welcome to the Lloyd Lab at the Johns Hopkins School of Medicine!

Welcome!

We, at the Lloyd Lab, are a basic and translational research team associated with the Department of Neurology at the Johns Hopkins School of Medicine. We study the molecular underpinnings of neuromuscular diseases, including Amyotrophic Lateral Sclerosis (ALS) and Charcot-Marie-Tooth Disease (CMT). To do so, we utilize the Drosophila model of disease to arrive at a better understanding of these diseases at the molecular level.

Lab Overview

Our laboratory is interested in understanding the fundamental molecular and cellular processes that underlie degenerative diseases of the neuromuscular system. The critical cell of the peripheral neuromuscular system is the motor neuron, a nerve cell that has its cell body and dendrites (synaptic inputs) in the spinal cord and sends long axons to the periphery to form synapses on muscle cells at the neuromuscular junction (NMJ). Perhaps due to their large size (human motor axons can have axons as long as 1 meter) and inability of cells to divide after birth, motor neurons are particularly vulnerable to degeneration with aging, and the mechanisms regulating neuron and muscle maintenance with aging are poorly understood. We are particularly interested in understanding how membrane trafficking is disrupted in these degenerative diseases, as these processes are implicated in inherited and sporadic neurodegenerative and myodegenerative diseases.

In particular, we study:

  1. Axonal transport – the process of moving various cargo such as mitochondria and vesicles to and from the cell body to the synapse. Mutations in dynactin, a critical regulator of retrograde axonal transport, cause distinct neurodegenerative diseases including an inherited motor neuropathy.
  2. Autophagy – the process by which cells degrade misfolded protein aggregates and damaged organelles via transport to the lysosome. Several ALS genes are known to regulate this process, and the mechanisms of autophagy at the neuromuscular junction are poorly understood.
  3. Nucleocytoplasmic transport – we and others have recently shown that shuttling of proteins and RNA molecules in and out of the nucleus is a critical process for neuronal maintence disrupted in neurodegenerative diseases including Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) (Zhang et al, Nature, 2015), and Hungtington’s Disease (Grima et al, Neuron 2017). Importantly, modulation of nucleocytoplasmic transport may be a therapeutic target for neurodegenerative diseases, and we are working with Karyopharm Therapeutics to test this hypothesis.

Disease Models

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegerative disease affecting primarily motor neurons, leading to paralysis and death within 3-5 years. Over the last 10 years, many new ALS genes have been identified that implicate RNA metabolism and autophagy in disease pathogenesis. We are studying these process using primarily Drosophila and human iPS (induced pluripotent stem cell) models. We collaborate extensively with the laboratory of Jeffrey Rothstein, MD/PhD, director of the Robert Packard Center for ALS Research and ANSWER ALS, allowing discoveries made in Drosophila models to be rapidly validated in human iPS cell models and patient tissue.

Charcot-Marie-Tooth disease (CMT) is an inherited, genetically heterogeneous disease causing degeneration of motor and/or sensory axons, causing slowly progressive peripheral neuropathy. Over 80 genes are now known to cause different forms of CMT, and main of these genes regulate axon transport and other membrane trafficking events within neurons. We can quickly study the effects of CMT mutations in Drosophila models to understand how mutations cause disease.

Inclusion Body Myositis (IBM) is the most common acquired muscle disease in adults over the age of 50 years old. Interestingly, there are many pathological similarities between IBM and ALS (for example, TDP-43 accumulations in muscle of IBM patients resemble the TDP-43 accumulations seen in motor neurons in ALS). Furthermore, there are some genes such as Valosin Containing Protein (VCP) that when mutated can cause IBM in some family members, and ALS or CMT in others. We use Drosophila and mouse models, and human tissues, to study the underlying cause of this degenerative muscle disease.

Research Interests

Disruption of protein and vesicular transport in C9orf72 ALS/FTD

c9

Mutations in the gene C9orf72 have been identified as the most common cause of inherited ALS and frontotemporal dementia (FTD). A hexanucleotide repeat (GGGGCC) in a noncoding intron of C9orf72 is expanded hundreds of repeats in patients suffering from ALS and/or FTD. Three different pathogenic mechanisms have been suggested:

  1. I.) loss of normal function of C9orf72
  2. II.) a toxic gain of function of expanded repeat-containing mRNA, potentially due to sequestration of RNA binding proteins
  3. III.) translation of the hexanucleotide repeat expansion into toxic dipeptide proteins via noncanonical Repeat Associated Non-ATG (RAN) translation

Using a Drosophila model of GGGGCC repeat expansion- mediated neurodegeneration and iPS cells derived from c9-ALS patients, we and our collaborators find that GGGGCC RNA repeats binds to Ran GTPase Activating Protein (RanGAP) and impair its ability to regulate nuclear transport. As a result, proteins that are normally actively transported from the cytoplasm into the nucleus are stuck in the cytoplasm. One of these proteins that gets mislocalized is TDP-43 (See Figure). Interestingly, cytoplasmic accumulation of TDP-43 is the pathologic hallmark of all ALS, and mutations in TDP-43 that result in the sequestration and aggregation of TDP-43 in the cytoplasm also cause ALS. These findings suggest that GGGGCC expansions in C9orf72 may cause ALS at least in partly by inhibiting nuclear import of key proteins such as TDP-43.

We continue to study the pathogenic mechanisms of C9ORF72 repeat expansion and are interested in answer the following question:

  1. What are the proteins in the pathogenic cascade of ALS that are directly downstream of disrupted nuclear transport?
  2. How is the expanded GGGGCC repeat mRNA transported within neurons, and what other transport events are disrupted (e.g. autophagy and axonal transport)?
  3. What role does protein quality control play in toxicity?

P150

Disrupted axonal transport has long been implicated in neurodegenerative disease. Motor neurons have the largest axons in the human body, with some reaching over 4 feet long. Proper transport of cargo to and from the synapse is essential for normal neuronal function. Anterograde axonal transport (to the synapse) relies on the function of the kinesin motor protein while retrograde transport (to the cell body) requires the dynein motor. p150 is the major subunit of dynactin, a complex that functions with dynein in minus-end-directed microtubule transport. The G59S mutation in the CAP-Gly microtubule-binding domain causes a motor neuron degenerative disease. To better understand how this mutation causes motor neuron degeneration, we introduced the corresponding mutation (G38S) into Drosophila p150 (called Glued). This mutation causes a partial loss-of-function of p150, with mutant animals displaying impaired neurotransmitter release and adult-onset locomotor dysfunction. This mutation causes impairment of retrograde transport specifically at terminal boutons, the distal- most ends of synapses (See Figure above). This disruption results in the accumulation of vesicles within swollen terminal boutons of the Neuromuscular Junction. These data indicate that the p150 CAP-Gly domain regulates dynein-mediated retrograde transport at synaptic termini, and this function of dynactin is disrupted by a mutation that causes motor neuron disease.

Our lab is interested in continuing to research the role p150 and axonal transport in neurodegenerative diseases by trying to answer the following questions:

  1. How do mutations in the CAP-GLY domain of p150 impair the initiation of retrograde transport?
  2. What are the molecular mechanisms that regulate the cell type specificity of neurodegeneration? To address this, we are comparing the G38S mutation with p150 CAP-Gly mutations that cause Perry Syndrome (a Parkinson's-like disease).
  3. What are the genes and pathways downstream of p150/Glued that regulate motor neuron health?

Molecular Mechanisms of TRPV4-mediated Neurodegeneration in Charcot-Marie-Tooth Disease

TRPV4 TRPV4 Drosophila

Three distinct neuropathies are caused by missense mutations in the gene encoding Transient Receptor Potential vanilloid, family member 4 (TRPV4), including Charcot-Marie-Tooth Disease Subtype- 2C (CMT2C). TRPV4 is a Na+ and Ca2+ permeable ion channel, and disease causing variants lead to increased TRPV4 channel activity in several in vitro systems. To investigate the molecular mechanisms underlying CMT2C pathogenesis in vivo, we have generated transgenic Drosophila lines to overexpress either wild type human TRPV4 or a CMT2C-causing TRPV4 variant, TRPV4R269C. Expression of TRPV4R269C, but not TRPV4WT, in Drosophila motor neurons results in motor deficits and disruption of wing expansion. We have utilized these phenotypes to screen for genetic modifiers of TRPV4R269C mediated neuronal dysfunction, and we have found that RNAi-mediated knockdown of CaMKII potently suppresses the TRPV4R269C mediated wing expansion phenotype. We have also investigated the effect of TRPV4R269C expression in Drosophila neurons and have found that expression of the CMT2C causing mutant, but not wild type TRPV4, results in a number of morphological abnormalities including: diminished dendritic branching, axonal swellings, and aberrant synapses.

We are actively working to address the cellular and molecular mechanisms of TRPV4R269C mediated neuropathy by:

  1. Investigating how the R269C mutation alters TRPV4 channel activity in vivo.
  2. Further characterizing the morphological and functional consequences of TRPV4R269C expression.
  3. Dissecting the genetic pathways acting either upstream or downstream of TRPV4R269C which confer cellular toxicity.

Altered synaptic homeostasis in FUS-ALS

FUS

The discovery that mutations in multiple RNA binding proteins constitute a significant portion of familial ALS cases transformed ALS research. Mutations in Fused in sarcoma (FUS) account for 5-10% of inherited ALS, and a large percentage of these mutations are clustered in the nuclear localization signal, a domain required for efficient transport of FUS into the nucleus. As a consequence of these mutations, FUS is excluded from the nucleus and accumulates in the cytoplasm, but it is unknown how this redistribution of FUS results in neurodegeneration. To model FUS-ALS in Drosophila, multiple groups have developed loss-of-function alleles of Drosophila FUS (called "cabeza" or caz) as well as transgenic lines that overexpress either mutant or wildtype FUS or caz. Some initial studies have concluded that ALS-causing mutations cause a gain of function effect in the cytoplasm, others studies have suggested that the mutations are more likely to result in the loss of normal function. At the neuromuscular junction (NMJ), overexpression of either wild-type or mutant FUS results in decreased number of presynaptic active zones and altered postsynaptic glutamate receptor subunit composition, coinciding with a reduction in synaptic transmission (See figure). However, loss of FUS from motor neurons increases synaptic transmission as a result of increased quantal size. These data demonstrate that FUS regulates NMJ development and function.

To try to understand how ALS-causing mutations in FUS cause motor neuron degeneration, we are interested in:

  1. Characterizing mutation-specific phenotypes in Drosophila models of FUS ALS.
  2. Determining the role of FUS in regulation of local transport at synapses.
  3. Investigating the role of FUS in the DNA damage response pathway.