Neurodegeneration: Fly model sheds light on brain disease
Experiments on flies suggest that a gain-of-function mechanism in a protein called CSPɑ contributes to the progressive brain disease CLN4.
- Broad Institute of MIT and Harvard, United States
- Harvard University, United States
Neuronal ceroid lipofuscinosis (NCL) refers to a group of progressive brain diseases that affect between 1 and 30 per 100,000 people and are characterized by a decline in motor abilities, seizures, dementia and premature death (Nosková et al., 2011; Haltia, 2003). Most NCLs are caused by the failure of cells to recycle various proteins inside lysosomes (Wisniewski et al., 2001; Henderson et al., 2016; Sambri et al., 2017; Warrier et al., 2013). However, one type of neuronal ceroid lipofuscinosis is different: CLN4 disease occurs when a patient inherits one mutant copy of the gene that encodes for a protein called CSPɑ, but the precise cellular dysfunction underlying CLN4 disease remains a mystery (Nosková et al., 2011; Benitez and Sands, 2017).
Mutations in this gene have been associated with both loss and gain of function. In theory, the mutations could deplete normal CSPɑ, and this loss of function could contribute to disease pathology. Alternatively, the mutations could enhance the normal activity of CSPɑ or lead to an additional, toxic function to drive disease pathology. Now, in eLife, Konrad Zinsmaier and colleagues at the University of Arizona and Yale University – including Elliot Imler as first author – report the generation of a new animal model to investigate the biological mechanisms underlying CLN4 disease (Imler et al., 2019).
Imler et al. started by expressing either the normal or mutant human forms of CSPɑ in flies and confirming that both were functional in fly cells. Next they confirmed that mutant forms of CSPɑ could mimic pathological features seen in CLN4 patients. Interestingly, they found that the severity of the disease correlated with the copy number of the mutated gene: a single copy of the mutated gene did not affect lifespan, but two copies led to more severe phenotypes and early death. To confirm that these results were not an artifact of expressing a human protein in flies, Imler et al. repeated many of their experiments using mutant versions of fly CSPɑ.
The Arizona–Yale team then looked at where mutant CSPɑ resides in neurons. Normally, one would expect to find CSPɑ at nerve terminals, but the mutations resulted in lower levels of the protein at nerve terminals and higher levels in regions of the cell that contained other proteins that had been marked for degradation. By using markers of different cellular components, the researchers demonstrated that mutant CSPɑ was accumulating on prelysosomal endosomes. An endosome is a mini-compartment within a cell that internalizes molecules from the cell membrane: the endosome then fuses with an organelle called a lysosome, and the molecules inside it are broken down and recycled. Imler et al. hypothesize that mutant CSPɑ may be re-routed from nerve terminals through the endolysosomal pathway. Consistent with this, electron microscopy revealed the formation of abnormal membrane structures in cells, which may be due to mutant CSPɑ congesting the trafficking system.
Next, Imler et al. modulated the levels of normal and mutant CSPɑ in cells. Reducing the level of normal CSPɑ reduced the mutant phenotypes, while increasing it exacerbated the mutant phenotypes. Similarly, reducing the level of a heat shock protein that normally interacts with CSPɑ attenuated mutant phenotypes, suggesting that the heat shock protein may play a role in disease biology. The Arizona–Yale team suggests that the disease-causing mutations in CSPɑ enhance its normal activity to disrupt the function of neurons. This gain-of-function mechanism is distinct from that found in the other NCLs, which typically arise from a deficiency in the function of the mutated gene.
The results of Imler et al. demonstrate the value of the fruit fly model to study CLN4 disease pathology. However, these results also paint a complex picture of CLN4, and much work remains to be done to understand how each molecular or cellular pathology contributes to the devastating cognitive and motor deficits found in patients. Future work can now focus on how these biochemical disruptions impact neuronal function over time, taking advantage of the many tools and techniques available in fly genetics in order to interrogate pathways that have additive or compensatory mechanisms that may someday yield therapeutic potential.
References
-
The neuronal ceroid-lipofuscinosesJournal of Neuropathology & Experimental Neurology 62:1–13.https://doi.org/10.1093/jnen/62.1.1
-
Mutations in DNAJC5, encoding cysteine-string protein alpha, cause autosomal-dominant adult-onset neuronal ceroid lipofuscinosisAmerican Journal of Human Genetics 89:241–252.https://doi.org/10.1016/j.ajhg.2011.07.003
-
Genetic basis and phenotypic correlations of the neuronal ceroid lipofusinosesBiochimica et Biophysica Acta (BBA) - Molecular Basis of Disease 1832:1827–1830.https://doi.org/10.1016/j.bbadis.2013.03.017
Article and author information
Author details
Publication history
Copyright
© 2019, Berryer et al.
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 770
- views
-
- 71
- downloads
-
- 0
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Neurodegeneration: Fly model sheds light on brain disease
eLife 8:e53233.
https://doi.org/10.7554/eLife.53233
Further reading
-
- Neuroscience
The infralimbic cortex (IL) is essential for flexible behavioral responses to threatening environmental events. Reactive behaviors such as freezing or flight are adaptive in some contexts, but in others a strategic avoidance behavior may be more advantageous. IL has been implicated in avoidance, but the contribution of distinct IL neural subtypes with differing molecular identities and wiring patterns is poorly understood. Here, we study IL parvalbumin (PV) interneurons in mice as they engage in active avoidance behavior, a behavior in which mice must suppress freezing in order to move to safety. We find that activity in inhibitory PV neurons increases during movement to avoid the shock in this behavioral paradigm, and that PV activity during movement emerges after mice have experienced a single shock, prior to learning avoidance. PV neural activity does not change during movement toward cued rewards or during general locomotion in the open field, behavioral paradigms where freezing does not need to be suppressed to enable movement. Optogenetic suppression of PV neurons increases the duration of freezing and delays the onset of avoidance behavior, but does not affect movement toward rewards or general locomotion. These data provide evidence that IL PV neurons support strategic avoidance behavior by suppressing freezing.
-
- Neuroscience
Millions of Americans suffering from Opioid Use Disorders face a high risk of fatal overdose due to opioid-induced respiratory depression (OIRD). Fentanyl, a powerful synthetic opioid, is a major contributor to the rising rates of overdose deaths. Reversing fentanyl overdoses has proved challenging due to its high potency and the rapid onset of OIRD. We assessed the contributions of central and peripheral mu opioid receptors (MORs) in mediating fentanyl-induced physiological responses. The peripherally restricted MOR antagonist naloxone methiodide (NLXM) both prevented and reversed OIRD to a degree comparable to that of naloxone (NLX), indicating substantial involvement of peripheral MORs to OIRD. Interestingly, NLXM-mediated OIRD reversal did not produce aversive behaviors observed after NLX. We show that neurons in the nucleus of the solitary tract (nTS), the first central synapse of peripheral afferents, exhibit a biphasic activity profile following fentanyl exposure. NLXM pretreatment attenuates this activity, suggesting that these responses are mediated by peripheral MORs. Together, these findings establish a critical role for peripheral MORs, including ascending inputs to the nTS, as sites of dysfunction during OIRD. Furthermore, selective peripheral MOR antagonism could be a promising therapeutic strategy for managing OIRD by sparing CNS-driven acute opioid-associated withdrawal and aversion observed after NLX.
