2.1. Sclsm4Δ1 mutant shows regulated cell death markers and premature ageing
We previously reported that a truncated form of the
KlLSM4 gene from the yeast
K. lactis (
Kllsm4Δ1) was able to restore viability in a
S. cerevisiae strain not expressing
LSM4. Nevertheless, cells lost viability very soon and showed the markers of regulated cell death [
6]. In order to investigate this phenotype in a homologous scenario, we constructed the corresponding
Kllsm4Δ1 mutant of the
S. cerevisiae LSM4 gene (
Sclsm4Δ1), and we expressed it in the
S. cerevisae MCY4 strain, which contained the
LSM4 gene under the control of
Gal1-10 promoter. This strain can grow in galactose, but it cannot grow when glucose is the only carbon source. The expression in such a strain of the
Sclsm4Δ1 gene from a centromeric plasmid restored growth on glucose, as also reported when present
Kllsm4Δ1. Aimed to check if the phenotypes of the
Sclsm4Δ1 mutant were similar to those described for
Kllsm4Δ1, we analysed nuclei morphology, intracellular ROS production and maintenance of viability during stationary phase, also defined as Chronological Life Span (CLS). As shown in
Figure 1, panel A and B, highly fragmented, enlarged and diffused nuclei, indicative of regulated cell death, were observed in more than 15% of exponentially growing cells and in almost 40% of cells during stationary phase. These percentages are much higher compared to the wild type, which show about 3% and 12% of cells with fragmented nuclei in exponential and stationary phase cells, respectively.
Concerning intracellular ROS accumulation, the percentage of ROS positive cells during exponential phase was about 2% and 12% for the WT and the Sclsm4Δ1 mutant cells. In the latter, the percentage of ROS positive cells during stationary phase reached 60%, about six time more than the WT (Figure1, panels C and D).
One particular phenotype of
Kllsm4Δ1 was the early loss of viability during ageing and high sensitivity to oxidative stress. As shown in
Figure 1E, also
Sclsm4Δ1 cells show a very short CLS, and high sensitivity to hydrogen peroxide treatments (
Figure 1, panel E and F).
As a control, to verify that these phenotypes were not due to the expression of
Sclsm4Δ1 gene from a centromeric plasmid, we also expressed in the MCY4 strain the full legth gene
ScLSM4. As shown in
Figure 1E and 1F, the expression of the
ScLSM4 gene restored both CLS and hydrogen oxide sensitivity at the same level of the WT strain.
Finally, as also reported for
Kllsm4Δ1,
Sclsm4Δ1 showed sensitivity to caffeine and acetic acid and lower growth on glycerol medium (
Figure 1, panel G). Altogether, these results show that
Sclsm4Δ1 mutant recapitulates all the phenotypes showed by
Kllsm4Δ1, regard to regulated cell death and premature ageing [
32].
2.2. Sclsm4Δ1 mutant is defective in autophagy induction
We previously reported that the over-expression of
NEM1, which codes for the catalytic subunit of the yeast nuclear membrane-resident protein phosphatase complex Nem1/Spo7, can suppress most of the mutant phenotypes in the
S. cerevisiae lsm4 mutant expressing
Kllsm4Δ1 [
33]. It has been reported that Nem1 is required for autophagy induction after TORC1 inactivation [
34], so we assessed if
Lsm4 was involved in the macroautophagy flux through a GFP-Atg8 processing assay [
35]. As GFP β-barrel structure is more resistant than Atg8p to vacuolar hydrolysis, the presence of free GFP on western blot indicates that the autophagic process has occurred. Autophagy can be induced under a variety of deprivation conditions, such as depletion of nitrogen and during post-diauxic shift [
36]. The autophagic flux has been evaluated upon nitrogen deprivation and during the post-diauxic growth phase, both in wild type and
Sclsm4Δ1 mutant cells.
As shown in
Figure 2A, during exponential growth autophagy was not observed in the wild type nor in the mutant (Exp, lanes 1 and 4) as only the GFP-Atg8 fusion protein was detected. Protein extracts obtained from cells in the post-diauxic phase (PD, lanes 2 and 5) and nitrogen starvation (SD-N, lanes 3 and 6) showed that free GFP production was reduced in
Sclsm4Δ1 cells, suggesting a defect in this mutant in inducing macroautophagy.
Macroautophagy is important for survival during nutrient starvation, and mutants defective in autophagy rapidly lose cell viability after nitrogen starvation [
37]. In fact, defective autophagy cells fail to maintain physiological levels of amino acids, and their inability to synthesize new proteins may explain, at least in part, most of the phenotypes associated with autophagy mutants [
38]. As from the GFP-Atg8 assay the
Sclsm4Δ1 mutant seemed to have important defects in inducing autophagy, we determined the CLS in nitrogen starvation conditions. As shown in
Figure 2C, nitrogen starvation increased CLS in a wild type strain. On the contrary,
Sclsm4Δ1 cells in SD-N showed a drop in viability already after 1 day and, completely lost viability at day 4, one day before cells maintained in SD.
Another known inducer of autophagy is the antibiotic rapamycin [
35]. We found that the
Sclsm4Δ1 mutant was highly sensitive even to low doses of rapamycin in that exponential growing cells exposure to 6 nM rapamycin reduced cell viability to 4% within 4 hours, while this is not the case for the wild-type strain, which maintained cell viability equal to untreated cells (
Figure 3A).
Similar sensitivity to this drug was found in the
lsm1Δ mutant, which is a component of the heptameric ring-shaped complex formed by Lsm1 to Lsm7 [
39]. As reported in
Figure 3B, the serial dilution assay showed the high sensitivity to 6nM rapamycin of both
Sclsm4Δ1 and
lsm1Δ cells, compared to their respective wild types CML39-11A and BMA38, suggesting that in both
lsm mutant strains autophagy is impaired.
Oxidative stress can induce autophagy both in yeast and in mammalian cells [
40] ; at the same time, it has been reported that rapamycin-induced autophagy, confers neuroprotection against aging-induced oxidative stress in old rats [
41]. We explored if low doses of rapamycin could protect cells from hydrogen peroxide induced cell death. As shown in
Figure 4, the presence of 6nM rapamycin did not protect wild type cells from oxidative stress, as the differences in viability of the treated and untreated samples after exposure to different concentration of H
2O
2 were not statistically significant. On the other hand, it was not possible to evaluate the protective action of rapamycin in the
Sclsm4Δ1 mutant due to its high toxic effect.
During autophagy, bulk cytoplasmic material is sequestered by the phagophore, a double-membrane structure which expands around the cargo forming a sealed, double-membrane vesicle known as the autophagosome (AP). The autophagosome fusion to the vacuole leads to degradation and recycling of the cargo. Autophagic flux can be monitored by the localization of GFP-Atg8, which is delivered to the vacuoles to be degraded. With the aim to have more information on the autophagic step blocked in the Sclsm4Δ1 mutant, we followed the localization of the fusion protein GFP-Atg8 by fluorescence microscopy.
During exponential phase of growth in SD around 1% and 7% of the WT and
Sclsm41 mutant cells, respectively, had a single GFP-Atg8 dot denoting the PAS localized near the vacuole membrane (Figure5A, SD exp), being most of the fluorescence uniformly distributed into the cytoplasm. During this growth phase, in the
Sclsm4Δ1 mutant there is observed a small percentage of cells showing two or more GFP-Atg8 dots per cell (
Figure 5B).
After 4 hours of nitrogen starvation (SD-N) the differences between the wildtype and the mutant increased, with a mean percentage of GFP-Atg8 dots around 3% for the wildtype and 25% for the
Sclsm4Δ1 mutant (
Figure 5A, SD-N 4h). Moreover, also the number of cells showing ≥2 GFP-Atg8 dots increased in the mutant cells to about 12%, representing half of cell population with GFP-Atg8 dots (
Figure 5B).
In the post-diauxic phase there was a little increase in autophagy (
Figure 2A) and, as expected, most of the GFP was localized inside the vacuole in the wild type (
Figure 5C, PD), with about 3% of cells showing GFP-Atg8 dots. Concerning
Sclsm4Δ1 mutant, the percentage of cells showing GFP-Atg8 dots increased up to about 20%, with very few cells showing intravacuolar fluorescence.
After 16h in nitrogen starvation (
Figure 5C, SD-N 16h) it was observed a slight increase of cells showing dots in the wild type, while about 35% of
Sclsm4Δ1 mutant cells showed cytoplasmic dots and half of them presented 2 or more dots per cell (
Figure 5D).
After three days of growth in SD more than 90% of
Sclsm4Δ1 mutant cells showed GFP-Atg8 dots, being the number of the wild type cells presenting dots about 13% of the population (
Figure 5E, quantification in 5F). This percentages increases a little bit in the wild type incubated for 3 days in SD-N medium, while in the
Sclsm4Δ1 mutant cells those presenting GFP-Atg8 dots after 3 days in SD-N medium were the same as after 16h of incubation in SD-N (about 40%,
Figure 5F). This could be due to the rapid loss of viability of the
Sclsm4Δ1 mutant in SD-N observed already at day 1 (
Figure 2C). These data altogether, indicated that
Sclsm4Δ1 mutant cells accumulated autophagy-related structures when autophagy was induced by nitrogen starvation or during ageing.
The same experiments were carried out with the
lsm1Δ1 mutant with similar results (Supplementary
Figure 1), suggesting that the observed autophagy defects are a feature of
lsm mutants.