A Yeast Model for Batten Disease

Proc. Natl. Acad. Sci. USA Vol. 96, pp. 11341–11345, September 1999 Cell Biology Phenotypic reversal of the btn1 defects in yeast by chloroquine: A yeast model for Batten disease DAVID A. PEARCE*, CARRIE J. CARR,BISWADIP DAS, AND FRED SHERMAN Department of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642 Contributed by Fred Sherman, July 27, 1999 ABSTRACT BTN1 of Saccharomyces cerevisiae encodes an that encode the protein, nor does the stored protein have a ortholog of CLN3, the human Batten disease gene. We have different encoded sequence from that for normal individuals reported previously that deletion of BTN1, btn1-⌬, resulted in (11, 12). Furthermore, slower degradation of mitochondrial a pH-dependent resistance to D-(؊)-threo-2-amino-1-[p- ATP synthase subunit c was found to occur in NCL fibroblasts nitrophenyl]-1,3-propanediol (ANP). This phenotype was compared with normal cells. Although initially located in the caused by btn1-⌬ strains having an elevated ability to acidify mitochondria, mitochondrial ATP synthase subunit c accumu- growth medium through an elevated activity of the plasma lated in lysosomes of NCL cells, whereas the degradation of ؉ membrane H -ATPase, resulting from a decreased vacuolar another mitochondrial inner membrane protein, cytochrome pH during early growth. We have determined that growing oxidase subunit IV, was unaffected, with no lysosomal accu- btn1-⌬ strains in the presence of chloroquine reverses the mulation (13, 14). resistance to ANP, decreases the rate of medium acidification, Genes encoding predicted proteins with high sequence ؉ decreases the activity of plasma membrane H -ATPase, and similarity to Cln3p have been identified in mouse, dog, rabbit, elevates vacuolar pH. However, an additional effect of this Caenorhabditis elegans, and the yeast Saccharomyces cerevisiae phenotypic reversal is that activity of plasma membrane (refs. 15 and 16; see also Swiss-Prot accession no. O 29611 and ؉ H -ATPase is decreased further and vacuolar pH is increased GenBank accession nos. U 92812 and Z 49335). We previously further as btn1-⌬ strains continue to grow. This phenotypic reported that the corresponding yeast gene, BTN1, encodes a reversal of btn1-⌬ can be considered for developing a therapy nonessential protein that is 39% identical and 59% similar to for Batten disease. human Cln3p (17). Deletion of BTN1 had no effect on the degradation of mitochondrial ATP synthase subunit c.We Neuronal ceroid-lipofuscinoses (NCL) are the most common further showed that yeast strains lacking Btn1p, btn1-⌬, were group of progressive neurodegenerative diseases in children, resistant to D-(Ϫ)-threo-2-amino-1-[p-nitrophenyl]-1,3- with an incidence as high as 1 in 12,500 live births and with propanediol (ANP) and that this phenotype was comple- about 440,000 carriers in the United States (1, 2). These mented by expression of human Cln3p, indicating that yeast disorders are autosomal recessive, with similar early symptoms Btn1p and human Cln3p share the same function (18) and are and disease progression. Diagnosis is often based on visual therefore orthologs. This resistance to ANP depended on the problems, behavioral changes, and seizures. Progression is ability of btn1-⌬ yeast strains to decrease the pH of growth characterized by a decline in mental abilities, increased sever- medium because of an enhanced ability to acidify growth ity of untreatable seizures, blindness, loss of motor skills, and medium through an initial increase in the activity of plasma ϩ premature death. Traditionally, several NCL disorders have membrane H -ATPase (19, 20). This elevated activity of the ϩ been divided into subtypes based on the age of onset and plasma membrane H -ATPase is most likely a response to an pathology and are denoted by the following CLN genes imbalance in pH homeostasis within the cell, resulting from an responsible for each disease: infantile-NCL, CLN1 (Santa- abnormally acidic vacuolar pH in btn1-⌬ strains (20). As btn1-⌬ ϩ vuori–Haltia disease); late infantile-NCL, CLN2 (Jansky– strains grow, activity of the plasma membrane H -ATPase and Bielschowsky disease); juvenile-NCL, CLN3 (Batten disease); vacuolar pH are returned to normal. Examination of the adult-NCL, CLN4 (Kufs’ disease); and two variant late infan- expression of all yeast genes in btn1-⌬ strains revealed that tile forms, CLN5 and CLN6. The gene products of CLN1 and expression of HSP30 and BTN2 was increased. We speculated CLN2 have been identified as a lysosomal protein thiolesterase that altered gene expression is involved in normalizing the ϩ and a lysosomal pepstatin-insensitive protease, respectively (3, activity of plasma membrane H -ATPase and vacuolar pH 4). Recently, CLN5 was identified as a protein of unknown (20). Therefore, through coordinate gene expression, pH function (5). Although the CLN3 gene responsible for Batten homeostasis in btn1-⌬ strains is maintained. disease was positionally cloned in 1995 (6), with most individ- Chloroquine, a lysosomotropic agent, is widely used as an uals with the disease harboring a 1.02-kilobase deletion of the antimalarial agent because of its toxicity to Plasmodium fal- gene, the function of this protein and the molecular basis for ciparum trophozoites (21). Chloroquine accumulates in the this disease still remain elusive. The NCL are characterized by acidic food vacuole causing an increase in pH, which is believed the accumulation of autofluorescent hydrophobic material in to inhibit the mobilization of food reserves during this stage of the lysosomes of neurons and, to a lesser extent, other cell types the parasite’s development (22–28). It is the ability of chloro- (7, 8). Furthermore, protein sequencing and immunological quine to raise the pH of the acidic vacuolar compartment that studies have revealed that subunit c of mitochondrial ATP prompted us to investigate whether the pH of the vacuole in synthase is the major component of the lysosomal storage btn1-⌬ yeast strains could be raised. We report that growing material in CLN2, CLN3, and CLN4 but not CLN1 (9, 10). This btn1-⌬ strains in the presence of chloroquine results in the loss accumulation of mitochondrial ATP synthase subunit c is not a result of increased expression of the P1 and P2 nuclear genes Abbreviations: NCL, neuronal ceroid-lipofuscinosis; ANP, D-Ϫ)- threo-2-amino-1-[p-nitrophenyl]-1,3-propanediol; YPD, bacto-yeast ͞ ͞ The publication costs of this article were defrayed in part by page charge extract bacto-peptone glucose. *To whom reprint requests should be addressed at: Department of payment. This article must therefore be hereby marked ‘‘advertisement’’ in Biochemistry and Biophysics, Box 712, University of Rochester accordance with 18 U.S.C. §1734 solely to indicate this fact. School of Medicine and Dentistry, Rochester, NY 14642. E-mail: PNAS is available online at www.pnas.org. david࿝[email protected]. 11341 Downloaded by guest on September 30, 2021 11342 Cell Biology: Pearce et al. Proc. Natl. Acad. Sci. USA 96 (1999) of resistance to ANP. During early growth of btn1-⌬ strains in Vacuolar pH was calculated from an in vivo calibration pre- the presence of chloroquine, plasma membrane Hϩ-ATPase pared by pretreating cells with ionophores to equilibrate the in activity is decreased, and vacuolar pH is increased—changes vivo pH. that result in similarity to BTN1ϩ strains. Northern Analysis. Expression of HSP30 and BTN2 was Although it remains to be shown that a defective lysosomal measured at the time point indicated by Northern blot analysis. pH occurs in individuals with Batten disease and that altered mRNA and probes for HSP30 and BTN2 were prepared, and lysosomal pH or possibly the altered gene expression acting to measurement of yeast gene expression was performed as correct this defect is responsible for this devastating neuro- described (34). logical disease, the use of drugs that can modulate this lysosomal pH may lead the way to a potential therapy in RESULTS humans. ANP Resistance of btn1-⌬ Strains Is Reversed by Chloro- MATERIALS AND METHODS quine. Chloroquine is a lysosomotropic agent best known for use as an antimalarial agent. Chloroquine is a weak-base amine Yeast Strains and Growth. The isogenic btn1-⌬ yeast strain that, in its neutral form, enters acidic compartments, such as B-10195 (MATa btn1-⌬::HIS3 CYC1ϩ cyc7-⌬::CYH2 leu2,3- the vacuole or lysosome, and becomes protonated. The pH 112 ura3-52 his3-⌬1trp1-289; denoted btn1-⌬) was derived becomes elevated as chloroquine accumulates in the compart- from B-7553 (MATa BTN1ϩ CYC1ϩ cyc7-⌬::CYH2 leu2,3-112 ment. This well characterized ability to elevate pH in an acidic ura3-52 his3-⌬1trp1-289; denoted BTN1ϩ) by gene disruption compartment prompted us to test whether chloroquine would (17). Yeast strains were grown as indicated in YPD medium affect the decreased vacuolar pH in btn1-⌬ strains during early [1% wt͞wt bacto-yeast extract͞2% wt͞wt (vol͞vol) bacto- growth, which results in resistance to ANP. We had already peptone͞2% wt͞wt glucose]. ANP and chloroquine were added established that the resistance to ANP of btn1-⌬ strains at the indicated concentrations after autoclaving. Growth in depended on pH (18). We examined the effect of chloroquine liquid medium was measured with a Klett Summerson Pho- on growth of BTN1ϩ and btn1-⌬ strains over a range of pHs in toelectric Colorimeter (Klett Manufacturing, New York). either the presence or absence of ANP. Chloroquine at a Acidification of External Medium. Extracellular acidifica- concentration above 0.5 mM in growth medium slows the tion was measured as change in pH according to the method growth of both BTN1ϩ and btn1-⌬ strains (data not shown). of Hemenway et al. (29), except that the starting pH for each Specifically, we show that, over a pH range of 6.0–7.0, 0.1 mM experiment was not adjusted. Briefly, cells harvested at the chloroquine has no effect on yeast growth (Fig.

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