Effects of ageing on the fine distribution of the circadian CLOCK protein in reticular formation neurons

Ultrastructural immunocytochemistry shows impairment of RNA pathways in skeletal muscle nuclei of old mice: A link to sarcopenia?

During aging, skeletal muscle is affected by sarcopenia, a progressive decline in muscle mass, strength and endurance that leads to loss of function and disability. Cell nucleus dysfunction is a possible factor contributing to sarcopenia because aging-associated alterations in mRNA and rRNA transcription/maturation machinery have been shown in several cell types including muscle cells. In this study, the distribution and density of key molecular factors involved in RNA pathways namely, nuclear actin (a motor protein and regulator of RNA transcription), 5-methyl cytosine (an epigenetic regulator of gene transcription), and ribonuclease A (an RNA degrading enzyme) were compared in different nuclear compartments of late adult and old mice myonuclei by means of ultrastructural immunocytochemistry. In all nuclear compartments, an age-related decrease of nuclear actin suggested altered chromatin structuring and impaired nucleus-to-cytoplasm transport of both mRNA and ribosomal subunits, while a decrease of 5-methyl cytosine and ribonuclease A in the nucleoli of old mice indicated an age-dependent loss of rRNA genes. These findings provide novel experimental evidence that, in the aging skeletal muscle, nuclear RNA pathways undergo impairment, likely hindering protein synthesis and contributing to the onset and progression of sarcopenia.

Stabilization of heterochromatin by CLOCK promotes stem cell rejuvenation and cartilage regeneration

Accumulating evidence indicates an association between the circadian clock and the aging process. However, it remains elusive whether the deregulation of circadian clock proteins underlies stem cell aging and whether they are targetable for the alleviation of aging-associated syndromes. Here, we identified a transcription factor-independent role of CLOCK, a core component of the molecular circadian clock machinery, in counteracting human mesenchymal stem cell (hMSC) decay. CLOCK expression was decreased during hMSC aging. In addition, CLOCK deficiency accelerated hMSC senescence, whereas the overexpression of CLOCK, even as a transcriptionally inactive form, rejuvenated physiologically and pathologically aged hMSCs. Mechanistic studies revealed that CLOCK formed complexes with nuclear lamina proteins and KAP1, thus maintaining heterochromatin architecture and stabilizing repetitive genomic sequences. Finally, gene therapy with lentiviral vectors encoding CLOCK promoted cartilage regeneration and attenuated age-related articular degeneration in mice. These findings demonstrate a noncanonical role of CLOCK in stabilizing heterochromatin, promoting tissue regeneration, and mitigating aging-associated chronic diseases.

Muscleblind-like1 undergoes ectopic relocation in the nuclei of skeletal muscles in myotonic dystrophy and sarcopenia

Muscleblind-like 1 (MBNL1) is an alternative splicing factor involved in postnatal development of skeletal muscles and heart in humans and mice, and its deregulation is known to be pivotal in the onset and development of myotonic dystrophy (DM). In fact, in DM patients this protein is ectopically sequestered into intranuclear foci, thus compromising the regulation of the alternative splicing of several genes. However, despite the numerous biochemical and molecular studies, scarce attention has been paid to the intranuclear location of MBNL1 outside the foci, although previous data demonstrated that in DM patients various splicing and cleavage factors undergo an abnormal intranuclear distribution suggestive of impaired RNA processing. Interestingly, these nuclear alterations strongly remind those observed in sarcopenia i.e., the loss of muscle mass and function which physiologically occurs during ageing. On this basis, in the present investigation the ultrastructural localization of MBNL1 was analyzed in the myonuclei of skeletal muscles from healthy and DM patients as well as from adult and old (sarcopenic) mice, in the attempt to elucidate possible changes in its distribution and amount. Our data demonstrate that in both dystrophic and sarcopenic muscles MBNL1 undergoes intranuclear relocation, accumulating in its usual functional sites but also ectopically moving to domains which are usually devoid of this protein in healthy adults. This accumulation/delocalization could contribute to hamper the functionality of the whole splicing machinery, leading to a lower nuclear metabolic activity and, consequently, to a less efficient protein synthesis. Moreover, the similar nuclear alterations found in DM and sarcopenia may account for the similar muscle tissue features (myofibre atrophy, fibre size variability and centrally located nuclei), and, in general, for the aging-reminiscent phenotype observed in DM patients.

Skeletal muscle features in myotonic dystrophy and sarcopenia: do similar nuclear mechanisms lead to skeletal muscle wasting?

In the cell nucleus, the gene primary transcripts undergo molecular processing to generate mature RNAs, which are finally exported to the cytoplasm. These mRNA maturation events are chronologically and spatially ordered, and mostly occur on distinct ribonucleoprotein (RNP)-containing structures. Defects in the mRNA maturation pathways have been demonstrated in myotonic dystrophy type 1 (DM1) and type 2 (DM2) whose characteristic multisystemic features are caused by the expansion of two distinct nucleotide sequences: (CTG)n in the DMPK gene on chromosome 19q13 in DM1, and (CCTG)n in the ZNF9 gene on chromosome 3q21 in DM2. By combining biomolecular and cytochemical techniques, it has been shown that the basic mechanisms of DMs reside in the accumulation of CUG- or CCUG-containing transcripts in intranuclear foci where several RNA-binding proteins necessary for the physiological processing of pre-mRNA are sequestered. Moreover, a nucleoplasmic accumulation of splicing and cleavage factors has been found in DMs. This suggests that the dystrophic phenotype could depend on a general alteration of the pre-mRNA post-transcriptional pathway. Interestingly, the accumulation of pre-mRNA processing factors in the myonuclei of DM1 and DM2 patients is reminiscent of the nuclear alterations typical of sarcopenia, i.e., the loss of muscle mass and function which physiologically occurs during ageing. Consistently, in an in vitro study, we observed that satellite-cell-derived DM2 myoblasts show cell senescence alterations and impairment of the pre-mRNA maturation pathways earlier than the myoblasts from healthy patient. These results suggest possible common cellular mechanisms responsible for skeletal muscle wasting in sarcopenia and in myotonic dystrophy.

Physical training modulates structural and functional features of cell nuclei in type II myofibers of old mice

Aging is associated with a progressive loss of muscle mass, strength, and function, a condition known as sarcopenia, which represents an important risk factor for physical disability in elderly. The mechanisms leading to sarcopenia are still largely unknown, and no specific therapy is presently available to counteract its onset or progress. Many studies have stressed the importance of physical exercise as an effective approach to prevent/limit the age-related muscle mass loss. This study investigated the effects of physical training on pre-mRNA pathways in quadriceps and gastrocnemius muscles of old mice by ultrastructural cytochemistry: Structural and in situ molecular features of myonuclei and satellite cell nuclei of type II fibers were compared in exercised versus sedentary old mice, using adult individuals as control. Our results demonstrated that in myonuclei of old mice physical exercise stimulates pre-mRNA transcription, splicing, and export to the cytoplasm, likely increasing muscle protein turnover. In satellite cells, the effect of physical exercise seems to be limited to the reactivation of some factors involved in the transcriptional and splicing apparatus without increasing RNA production, probably making these quiescent cells more responsive to activating stimuli.

Cultured myoblasts from patients affected by myotonic dystrophy type 2 exhibit senescence-related features: ultrastructural evidence

Myotonic dystrophy type 2 (DM2) is an autosomal dominant disorder caused by the expansion of the tetranucleotidic repeat (CCTG)n in the first intron of the Zinc Finger Protein-9 gene. In DM2 tissues, the expanded mutant transcripts accumulate in nuclear focal aggregates where splicing factors are sequestered, thus affecting mRNA processing. Interestingly, the ultrastructural alterations in the splicing machinery observed in the myonuclei of DM2 skeletal muscles are reminiscent of the nuclear changes occurring in age-related muscle atrophy. Here, we investigated in vitro structural and functional features of satellite cell-derived myoblasts from biceps brachii, in the attempt to investigate cell senescence indices in DM2 patients by ultrastructural cytochemistry. We observed that in satellite cell-derived DM2 myoblasts, cell-senescence alterations such as cytoplasmic vacuolization, reduction of the proteosynthetic apparatus, accumulation of heterochromatin and impairment of the pre-mRNA maturation pathways occur earlier than in myoblasts from healthy patients. These results, together with preliminary in vitro observations on the early onset of defective structural features in DM2 myoblast derived-myotubes, suggest that the regeneration capability of DM2 satellite cells may be impaired, thus contributing to the muscular dystrophy in DM2 patients.

RNA processing is altered in skeletal muscle nuclei of patients affected by myotonic dystrophy

Myotonic dystrophies (DMs) are characterised by highly variable clinical manifestations consisting of muscle weakness and atrophy, and a wide spectrum of extramuscular manifestations. In both DM1 and DM2 forms, expanded nucleotide sequences cause the accumulation of mutant transcripts in the nucleus, thus deregulating the function of some RNA-binding proteins and providing a plausible explanation for the multifactorial phenotype of DM patients. However, at the skeletal muscle level, no mechanistic explanation for the muscle wasting has so far been proposed. We therefore performed a study in situ by immunoelectron microscopy on biceps brachii biopsies from DM1, DM2 and healthy subjects, providing the first ultrastructural evidence on the distribution of some nuclear ribonucleoprotein (RNP)-containing structures and molecular factors involved in pre-mRNA transcription and maturation in dystrophic myonuclei. Our results demonstrated an accumulation of splicing and cleavage factors in myonuclei of both DM1 and DM2 patients, suggesting an impairment of post-transcriptional pre-mRNA pathways. The transcription of the expanded sequences in DM myonuclei would therefore hamper functionality of the whole splicing machinery, slowing down the intranuclear molecular trafficking; this would reduce the capability of myonuclei to respond to anabolic stimuli thus contributing to muscle wasting.

Pre-mRNA processing is partially impaired in satellite cell nuclei from aged muscles

Satellite cells are responsible for the capacity of mature mammalian skeletal muscles to repair and maintain mass. During aging, skeletal muscle mass as well as the muscle strength and endurance progressively decrease, leading to a condition termed sarcopenia. The causes of sarcopenia are manifold and remain to be completely elucidated. One of them could be the remarkable decline in the efficiency of muscle regeneration; this has been associated with decreasing amounts of satellite cells, but also to alterations in their activation, proliferation, and/or differentiation. In this study, we investigated the satellite cell nuclei of biceps and quadriceps muscles from adult and old rats; morphometry and immunocytochemistry at light and electron microscopy have been combined to assess the organization of the nuclear RNP structural constituents involved in different steps of mRNA formation. We demonstrated that in satellite cells the RNA pathways undergo alterations during aging, possibly hampering their responsiveness to muscle damage.

Perichromatin fibrils as early markers of transcriptional alterations

Perichromatin fibrils represent the morphological expression of transcription and co-transcriptional processing of pre-mRNA. They can be considered, hence, an example of work in progress. High resolution techniques such as electron microscopy demonstrate that perichromatin fibrils play a role as early markers of transcriptional alterations. In this paper, we review some experimental and physiological conditions impairing or modulating transcription as well as their effects on perichromatin fibrils. In all the situations reported, perichromatin fibrils show modifications in their amount and/or their associated proteins. Their movements are also affected, as well as their export or their intra-nuclear storage forms. Perichromatin fibrils therefore represent highly sensitive markers not only for monitoring transcriptional and processing rate but also for identifying the maturation level of pre-mRNA/mRNA occurring in the cell nucleus and the functional correlation with the cellular metabolic state.

Recent progress in histochemistry

The progress in discerning the structure and function of cells and tissues in health and disease has been achieved to a large extent by the continued development of new reagents for histochemistry, the improvement of existing techniques and new imaging techniques. This review will highlight some advancements made in these fields.