Mitochondrial regulation in stem cells

Stem cells persist throughout the lifespan to repair and regenerate tissues due to their unique ability to self-renew and differentiate. Here we reflect on the recent discoveries in stem cells that highlight a mitochondrial metabolic checkpoint at the restriction point of the stem cell cycle. Mitochondrial activation supports stem cell proliferation and differentiation by providing energy supply and metabolites as signaling molecules. Concomitant mitochondrial stress can lead to loss of stem cell self-renewal and requires the surveillance of various mitochondrial quality control mechanisms. During aging, a mitochondrial protective program mediated by several sirtuins becomes dysregulated and can be targeted to reverse stem cell aging and tissue degeneration, giving hope for targeting the mitochondrial metabolic checkpoint for treating tissue degenerative diseases.

M1BP is an essential transcriptional activator of oxidative metabolism during Drosophila development

Oxidative metabolism is the predominant energy source for aerobic muscle contraction in adult animals. How the cellular and molecular components that support aerobic muscle physiology are put in place during development through their transcriptional regulation is not well understood. Using the Drosophila flight muscle model, we show that the formation of mitochondria cristae harbouring the respiratory chain is concomitant with a large-scale transcriptional upregulation of genes linked with oxidative phosphorylation (OXPHOS) during specific stages of flight muscle development. We further demonstrate using high-resolution imaging, transcriptomic and biochemical analyses that Motif-1-binding protein (M1BP) transcriptionally regulates the expression of genes encoding critical components for OXPHOS complex assembly and integrity. In the absence of M1BP function, the quantity of assembled mitochondrial respiratory complexes is reduced and OXPHOS proteins aggregate in the mitochondrial matrix, triggering a strong protein quality control response. This results in isolation of the aggregate from the rest of the matrix by multiple layers of the inner mitochondrial membrane, representing a previously undocumented mitochondrial stress response mechanism. Together, this study provides mechanistic insight into the transcriptional regulation of oxidative metabolism during Drosophila development and identifies M1BP as a critical player in this process.

The hepatic integrated stress response suppresses the somatotroph axis to control liver damage in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) can be ameliorated by calorie restriction, which leads to the suppressed somatotroph axis. Paradoxically, the suppressed somatotroph axis is associated with patients with NAFLD and is correlated with the severity of fibrosis. How the somatotroph axis becomes dysregulated and whether the repressed somatotroph axis impacts liver damage during the progression of NAFLD are unclear. Here, we identify a regulatory branch of the hepatic integrated stress response (ISR), which represses the somatotroph axis in hepatocytes through ATF3, resulting in enhanced cell survival and reduced cell proliferation. In mouse models of NAFLD, the ISR represses the somatotroph axis, leading to reduced apoptosis and inflammation but decreased hepatocyte proliferation and exacerbated fibrosis in the liver. NAD+ repletion reduces the ISR, rescues the dysregulated somatotroph axis, and alleviates NAFLD. These results establish that the hepatic ISR suppresses the somatotroph axis to control cell fate decisions and liver damage in NAFLD.

Human ZKSCAN3 and Drosophila M1BP are functionally homologous transcription factors in autophagy regulation

Autophagy is an essential cellular process that maintains homeostasis by recycling damaged organelles and nutrients during development and cellular stress. ZKSCAN3 is the sole identified master transcriptional repressor of autophagy in human cell lines. How ZKSCAN3 achieves autophagy repression at the mechanistic or organismal level however still remains to be elucidated. Furthermore, Zkscan3 knockout mice display no discernable autophagy-related phenotypes, suggesting that there may be substantial differences in the regulation of autophagy between normal tissues and tumor cell lines. Here, we demonstrate that vertebrate ZKSCAN3 and Drosophila M1BP are functionally homologous transcription factors in autophagy repression. Expression of ZKSCAN3 in Drosophila prevents premature autophagy onset due to loss of M1BP function and conversely, M1BP expression in human cells can prevent starvation-induced autophagy due to loss of nuclear ZKSCAN3 function. In Drosophila ZKSCAN3 binds genome-wide to sequences targeted by M1BP and transcriptionally regulates the majority of M1BP-controlled genes, demonstrating the evolutionary conservation of the transcriptional repression of autophagy. This study thus  allows the potential for transitioning the mechanisms, gene targets and plethora metabolic processes controlled by M1BP onto ZKSCAN3 and opens up Drosophila as a tool in studying the function of ZKSCAN3 in autophagy and tumourigenesis.

The Hox proteins Ubx and AbdA collaborate with the transcription pausing factor M1BP to regulate gene transcription

In metazoans, the pausing of RNA polymerase II at the promoter (paused Pol II) has emerged as a widespread and conserved mechanism in the regulation of gene transcription. While critical in recruiting Pol II to the promoter, the role transcription factors play in transitioning paused Pol II into productive Pol II is, however, little known. By studying how Drosophila Hox transcription factors control transcription, we uncovered a molecular mechanism that increases productive transcription. We found that the Hox proteins AbdA and Ubx target gene promoters previously bound by the transcription pausing factor M1BP, containing paused Pol II and enriched with promoter-proximal Polycomb Group (PcG) proteins, yet lacking the classical H3K27me3 PcG signature. We found that AbdA binding to M1BP-regulated genes results in reduction in PcG binding, the release of paused Pol II, increases in promoter H3K4me3 histone marks and increased gene transcription. Linking transcription factors, PcG proteins and paused Pol II states, these data identify a two-step mechanism of Hox-driven transcription, with M1BP binding leading to Pol II recruitment followed by AbdA targeting, which results in a change in the chromatin landscape and enhanced transcription.

Reading ability and processing in Duchenne muscular dystrophy and spinal muscular atrophy

We analysed the reading abilities and processing of 21 children with Duchenne muscular dystrophy (DMD), 11 matched children suffering from spinal muscular atrophy (SMA) and 42 children receiving normal education. The principal result observed was that the DMD children exhibited a reading age which was significantly lower than the SMA children compared with their chronological age. These learning disabilities were not related to a deficit in non-verbal performance intelligence, but psycholinguistic evaluation showed a deficit in verbal intelligence, especially in the Similarities and Arithmetic WISC-R subtests, in phonological abilities, oral word repetition, and in digit span score. The results for the DMD children were heterogeneous, and ranged from normal to greater or lesser involvement. In an attempt to clarify the nature of this reading impairment in DMD children, the three groups (DMD, SMA, and normal control children) were tested by reading aloud a list of single words and non-words. The DMD children were significantly impaired in reading non-words, suggesting reading disability similar to dysphonetic dyslexia, the most frequent subtype of developmental dyslexia. These results are discussed in the light of psychometric data available for our DMD population and in the light of previous studies. The practical consequences of diagnosis on rehabilitation are very important. The precise description of the cognitive deficits seen in DMD is of value for future clinical and genetic studies.

Spontaneous retraction pockets in chronic otitis media medical and surgical therapy

Results of a series of 228 spontaneous retraction pockets in chronic otitis media are reported and a classification is proposed. Under medical therapy, 78 retraction pockets of stage I and II (out of 95) were followed for five years. Sixteen percent deteriorated into stage III and were operated on; the others stabilized or improved towards normal tympanic membrane (23%). Different methods of surgical therapy were used in 150 retraction pockets (all the stages III and many stages II). Even with silastic sheeting and strengthening of the tympanic membrane, a recurrent retraction pocket was observed in 24 cases (16%). Functional results were obviously better when the ossicular chain was rebuilt from an intact malleus to an intact stapes and either from an intact malleus to a mobile footplate or from the tympanic membrane to an intact stapes. Surgery of retraction pockets must be used not only to prevent cholesteatoma formation but also to prevent erosion of the stapes.