ICC staining of Hsp47 in heat shocked HeLa cells using Anti-Hsp47 (clone: 1C4-1A6) and a FITC secondary (green: cytoplasm; blue: DAPI nuclear stain)
DNAJs/HSP40s can be expressed tissue-specifically or universally in all tissues in multicellular organisms. For instance, Northern blot analyses revealed that human brain expresses DnaJB2/Hsj1 to a much higher degree than other tissues examined, and hybridization studies with riboprobes in situ show a restricted pattern of expression of the mRNA within the brain 80. Yang and co-workers recently identified a novel member of the DNAJ/HSP40 family in rats, rDJL that is preferentially expressed in testis and is located primarily to the acrosome region of spermatozoa 180. The tissue-specific expression can be achieved by alternative splicing of the corresponding DNAJ/HSP40 mRNA. In this regard, two transcript variants encoding DnaJA1/Hdj2 have been identified for the DNAJA1 gene leading to the expression of two DnaJA1/Hdj2 isoforms differing from each other by 95 amino acids 69. While one of the alternatively spliced isoforms of DnaJA1/Hdj2 was found to reside in the nucleus, indicating that it acts as a nuclear co-chaperone, the other isoform was observed throughout the cell most likely due to the elimination of a putative nuclear localization signal sequence 69. Moreover, isoform 1 of DnaJA1/Hdj2 was found as being highly expressed in brain and other tissues while the other alternatively spliced isoform was highly abundant in testis and sperm 70.
In addition to their intracellular location, DNAJs/HSP40s have been found in the extracellular milieu. How DNAJs/HSP40s are found in the extracellular medium remains enigmatic as HSPs in general lack the consensus signal for secretion via the classical Golgi pathway. The mechanisms underlying its export involve, on the one hand, an active secretory process with the participation of exosomes. Gonzales et al. detected DnaJA1, DnaJA2, DnaJB1, DnaJC7, and DnaJC13 in urinary exosomes from normal human subjects 71. Urinary exosomes are small extracellular vesicles (<100 nm in diameter) that originate from the internal vesicles of multivesicular bodies (MVB) in renal epithelial cells, such as glomerular podocytes, renal tubule cells, and the cells lining the urinary drainage system 181. Exosome release into the urine occurs after fusion of the outer membrane of the MVB with the apical plasma membrane of epithelial cells.
On the other hand, ER-resident DnaJB11/ERdj3 could be identified most recently in the extracellular space upon activation of the unfolded protein response (UPR) which is known to impact extracellular proteostasis through transcriptional remodeling of the ER proteostasis pathways 182. Such remodeling processes downregulate the release of misfolded, aggregation-prone proteins during ER stress. Genereux and collaborators provide evidence for the UPR-induced upregulation and secretion of DnaJB11/ERdj3 into the extracellular environment thereby enhancing the extracellular proteostasis capacity 182. Remarkably, DnaJB11/ERdj3 was found as being co-secreted in a stable complex with misfolding-prone protein clients thus connecting intracellular and extracellular proteostasis capacity during ER stress 182. In this context, the non-classical molecular chaperone Hsp47, also known as serpin H1, which is essential for biosynthesis and secretion of collagen molecules has been identified in the sera of rheumatoid arthritis (RA) patients 183, 184. Elevated serum levels of Hsp47 were also present in patients with mixed connective tissue disease 185, acute exacerbation (AE) of idiopathic pulmonary fibrosis 186 or acute idiopathic interstitial pneumonias 187. Work by the group of Shigeru Kohno convincingly revealed significantly elevated levels of anti-Hsp40 autoantibodies in the periphery of patients with lung cancer 188 and those with ulcerative colitis campared to normal individuals 189. Elevated levels of anti-DnaJ, anti-DnaJA1/Hdj2, and anti-DnaJC14/Hdj3 serum antibodies could also be detected in RA patients 15, 190.
Following environmental stress, expression of DNAJs/HSP40s is dramatically upregulated in order to promote cell survival in the face of endogenous or exogenous injury. In this regard, exposure of cells to the chemicals ibuprofen, indomethacin, ZnSO4, and 8-hydroxy-quinoline induced expression of Hsp40/DnaJB1 with concomitant translocation to the nucleus 191. Moreover, the ER stress gene DNAJC10/ERDJ5 was found to be induced in neuroectodermal tumor cells by the retinoid analog fenretinide 192, which has an increasingly important profile as a cancer preventive and chemotherapeutic drug 193. Chemical stress also induces release of DNAJs/HSP40s into the extracellular milieu. As demonstrated by Saito and colleagues, treatment of cells with acrylamide caused necrotic and thus irreversible cell death accompanied by the release of several HSPs including DNAJs/HSP40s 194.