Heat-shock genes and development

PEST sequences mediate heat shock factor 2 turnover by interacting with the Cul3 subunit of the Cul3-RING ubiquitin ligase

Cullin-RING ubiquitin ligases promote the polyubiquitination and degradation of many important cellular proteins, which previous studies indicated can be targeted for degradation via interaction with BTB domain-containing subunits of this E3 ligase complex. PEST domains are known to promote the degradation of proteins that contain them. However, the molecular mechanism by which PEST sequences promote degradation of these proteins is not understood. Here we show that the PEST sequences of a short-lived protein called HSF2 interact with Cullin3, a subunit of a Cullin-RING E3 ubiquitin ligase, and that this interaction mediates the Cul3-dependent ubiquitination and degradation of HSF2. These results indicate how, at the molecular level, PEST sequences can promote the proteolysis of proteins that contain them. They also expand understanding of the mechanisms by which substrates can be recruited to Cullin-RING E3 ubiquitin ligases to include interactions between PEST sequences and Cul3.

Heat shock protein 25 plays multiple roles during mouse skin development

Heat shock protein (Hsp) 25 is a member of the small Hsp family. High levels of Hsp25 can be detected in skin. During adult epidermis differentiation, the concentration of Hsp25 increases as the distance of keratinocytes from the basal layer increases, in parallel with the extent of keratinization. We previously showed that Hsp25, mouse keratin (MK) 5, and MK14 participated in the formation of characteristic ring-shaped aggregates during the differentiation of the PAM212 keratinocyte cell line. We suggested that Hsp25 was involved in the disorganization of the MK5-MK14 keratin network before the establishment of the MK1-MK10 keratin network at the beginning of epidermis stratification. In this study, we have investigated the distribution of Hsp25 and keratins throughout skin development. We demonstrate that the distribution of Hsp25 and MK5 in the epidermis at the beginning of stratification and before keratinization is similar to that observed in PAM212 keratinocytes. These results indicate that there is a strong correlation between the mechanism we described ex vivo and the events taking place in vivo. Moreover, we show that Hsp25 is produced in different cell types in the epidermis and in the hair follicle at different stages of their development. Thus, our results suggest that Hsp25 is involved in more than one process during skin development.

α-Actinin 4 and BAT1 interaction with the Cytochrome c promoter upon skeletal muscle differentiation

To identify common regulatory features of nuclear genes encoding mitochondrial proteins we searched for regulatory elements in the Cytochrome c promoter during skeletal muscle differentiation in cell culture. A consensus element with the sequence GCTGCCGCAC-(N4-20)-GGSCGYGGG was found in both rat Cyt c and coxIV promoters. This new sequence element with yet undescribed function, but high abundance in promoters of nuclear genes encoding mitochondrial proteins available from the databases, showed a striking change in protein binding in electromobility shift assays when myoblasts were compared to myotubes. Proteins involved in the observed protein-DNA complexes were isolated from myotubes and identified by MALDI-TOF as BAT1, a DEAD-box protein of yet unknown function, heat shock protein HSP84, and alpha-actinin 4, a non-muscle isoform of the structural protein alpha-actinin. alpha-actinin 4 was found to be preferentially localized in the nucleus upon induction of myogenesis, suggesting a signaling function during muscle differentiation. In conclusion, the analyzed sequence motif may be a new candidate for common regulatory elements specific for nuclear encoded mitochondrial genes, and alpha-actinin 4 may be involved in their regulation.

HSP25 Is Involved in Two Steps of the Differentiation of PAM212 Keratinocytes

HSP25 is a member of the small heat shock protein family. This 25-kDa protein exhibits a highly specific distribution during mouse embryonic development. Although multiple functions have been proposed for HSP25, the role it plays during differentiation is still unknown. High levels of HSP25 can be detected in embryonic and adult skin. During epidermis differentiation, the concentration of HSP25 increases with the distance of keratinocytes from the basal layer, in parallel with the extent of keratinization. We used an ex vivo cellular system, PAM212 cells, to analyze quantitatively and qualitatively the dynamics of HSP25 production and phosphorylation during the differentiation of keratinocytes. Our observations suggest that HSP25 is involved in two steps of PAM212 keratinocyte differentiation. Shortly after the induction of differentiation, a transient hyperphosphorylation of HSP25 seems to be essential for the expression of differentiation markers. Later, the chaperone-active form of HSP25 is organized progressively into characteristic aggregates involved in the dynamics of keratin filament networks.

Hsp70 and thermal pretreatment mitigate developmental damage caused by mitotic poisons in Drosophila

To assess the ability of the heat-inducible molecular chaperone heat-shock protein 70 (Hsp70) to mitigate a specific developmental lesion, we administered the antimicrotubule drugs vinblastine (VB) and colchicine (COL) to larvae of Drosophila engineered to express differing levels of Hsp70 after heat pretreatment (HP). VB and COL decreased survival during metamorphosis, disrupted development of the adult eye and other structures as well as their precursor imaginal disks, and induced chromosome nondisjunction in the wing imaginal disk as indicated by the somatic mutation and recombination test (SMART) assay. Hsp70-inducing HP reduced many of these effects. For the traits viability, adult eye morphology, eye imaginal disk morphology, cell death in the eye imaginal disks, and single and total mutant clone formation in the SMART assay, HP reduced the impact of VB to a greater extent in Drosophila with 6 hsp70 transgenes than in a sister strain from which the transgenes had been excised. Because the extra-copy strain has higher levels of Hsp70 than does the excision strain but is otherwise almost identical in genetic background to the excision strain, these outcomes are attributable to Hsp70. The hsp70 copy number had a variable interaction with HP and COL administration.

Stress proteins: nomenclature, division and functions

The heat shock response, characterized by increased expression of heat shock proteins (Hsps) is induced by exposure of cells and tissues to extreme conditions that cause acute or chronic stress. Hsps function as molecular chaperones in regulating cellular homeostasis and promoting survival. If the stress is too severe, a signal that leads to programmed cell death, apoptosis, is activated, thereby providing a finely tuned balance between survival and death. In addition to extracellular stimuli, several nonstressfull conditions induce Hsps during normal cellular growth and development. The enhanced heat shock gene expression in response to various stimuli is regulated by heat shock transcription factors.

Overexpression of murine small heat shock protein HSP25 interferes with chondrocyte differentiation and decreases cell adhesion

Although multiple functions for the small heat shock protein HSP25 have been proposed, its specific role during developmental and differentiation processes is not known. Cartilage is one of the tissues in which HSP25 is specifically and highly expressed during development. C1 cells, able to form aggregates in vitro, can be induced to differentiate into chondrocytes. In this study, we generated two stable transfected clones overexpressing HSP25 at two different levels. Cell morphology and growth rate were modified in both clones, although the actin content and distribution did not seem to be altered. Overexpressing clones had more difficulties in coalescing, leading to smaller aggregates and they did not differentiate into chondrocytes. Subsequently, these aggregates tended to dissociate into loose masses of dying cells. The strength of all these effects was directly correlated to the level of HSP25 overexpression. These data suggest that overexpressing HSP25 decreases cellular adhesion and interferes with chondrocyte differentiation.

Dynamic restricted expression of the chaperone Hsc70 in early chick development

The non-inducible chaperone heat shock cognate 70 kDa (Hsc70) is regulated during development. We now characterize its dynamic expression pattern from gastrulation to early organogenesis. Throughout this developmental period, hsc70 transcripts were largely restricted to neuroectoderm- and mesoderm-derived structures. In stage 10 embryos, Hsc70 protein was expressed in the neural tube with increasing rostrocaudal and decreasing dorsoventral gradients, and in some somite cells. This highly regulated expression of Hsc70 is likely to reflect specific developmental functions, besides its well-characterized role in protein folding.