The Interaction of Myc with Miz1 Defines Medulloblastoma Subgroup Identity.

Four distinct subgroups of cerebellar medulloblastomas (MBs) differ in their histopathology, molecular profiles, and prognosis. c-Myc (Myc) or MycN overexpression in granule neuron progenitors (GNPs) induces Group 3 (G3) or Sonic Hedgehog (SHH) MBs, respectively. Differences in Myc and MycN transcriptional profiles depend, in part, on their interaction with Miz1, which binds strongly to Myc but not MycN, to target sites on chromatin. Myc suppresses ciliogenesis and reprograms the transcriptome of SHH-dependent GNPs through Miz1-dependent gene repression to maintain stemness. Genetic disruption of the Myc/Miz1 interaction inhibited G3 MB development. Target genes of Myc/Miz1 are repressed in human G3 MBs but not in other subgroups. Therefore, the Myc/Miz1 interaction is a defining hallmark of G3 MB development.

Miz1 is required to maintain autophagic flux.

Miz1 is a zinc finger protein that regulates the expression of cell cycle inhibitors as part of a complex with Myc. Cell cycle-independent functions of Miz1 are poorly understood. Here we use a Nestin-Cre transgene to delete an essential domain of Miz1 in the central nervous system (Miz1(ΔPOZNes)). Miz1(ΔPOZNes) mice display cerebellar neurodegeneration characterized by the progressive loss of Purkinje cells. Chromatin immunoprecipitation sequencing and biochemical analyses show that Miz1 activates transcription upon binding to a non-palindromic sequence present in core promoters. Target genes of Miz1 encode regulators of autophagy and proteins involved in vesicular transport that are required for autophagy. Miz1(ΔPOZ) neuronal progenitors and fibroblasts show reduced autophagic flux. Consistently, polyubiquitinated proteins and p62/Sqtm1 accumulate in the cerebella of Miz1(ΔPOZNes) mice, characteristic features of defective autophagy. Our data suggest that Miz1 may link cell growth and ribosome biogenesis to the transcriptional regulation of vesicular transport and autophagy.

Miz1 is required for hair follicle structure and hair morphogenesis.

Previous work has implicated the Myc-binding transcription factor Miz1 in the control of keratinocyte proliferation and in the cellular response to TGFbeta. Miz1 is expressed in basal keratinocytes of the interfollicular epidermis and in hair follicles. Here we have conditionally knocked out the POZ/BTB transactivation domain of Miz1 in keratinocytes using a keratin 14 (K14)-Cre mouse deleter strain. K14Cre(+)/Miz1(lox/lox) mice have rough fur as a result of altered hair follicle orientation, irregular hair pigmentation and disturbed hair fiber structure. A regional thickening of the epidermis at the hair funnel orifice was accompanied by suprabasal proliferation, indicating a delayed exit of keratinocytes from the cell cycle. In addition, the catagen of the hair cycle was delayed in K14Cre(+)/Miz1(lox/lox) mice and intrafollicular keratinocyte proliferation was increased. In aged K14Cre(+)/Miz1(lox/lox) animals, the number of hair follicles remained unchanged but the number of visible hairs, especially of zigzag hairs, was reduced and a pigmentary incontinence into the dermis developed. Our data show that Miz1 is involved in controlling proliferation and differentiation in hair follicles and in hair fiber morphogenesis.

Myc regulates keratinocyte adhesion and differentiation via complex formation with Miz1.

Myc plays a key role in homeostasis of the skin. We show that Miz1, which mediates Myc repression of gene expression, is expressed in the epidermal basal layer. A large percentage of genes regulated by the Myc-Miz1 complex in keratinocytes encode proteins involved in cell adhesion, and some, including the alpha6 and beta1 integrins, are directly bound by Myc and Miz1 in vivo. Using a Myc mutant deficient in Miz1 binding (MycV394D), we show that Miz1 is required for the effects of Myc on keratinocyte responsiveness to TGF-beta. Myc, but not MycV394D, decreases keratinocyte adhesion and spreading. In reconstituted epidermis, Myc induces differentiation and loss of cell polarization in a Miz1-dependent manner. In vivo, overexpression of beta1 integrins restores basal layer polarity and prevents Myc-induced premature differentiation. Our data show that regulation of cell adhesion is a major function of the Myc-Miz1 complex and suggest that it may contribute to Myc-induced exit from the epidermal stem cell compartment.

Expression of galectin-3 in the rat pancreas during regeneration following hormone-induced pancreatitis.

Supramaximal dosage of the cholecystokinin analog caerulein leads to edematous pancreatitis with subsequent acinar cell destruction predominantly by apoptosis. We have used immunohistochemistry to reveal the expression of the anti-apoptotic protein galectin-3 in pancreatic acinar cells. Galectin-3, which occurs only in duct cells under physiological conditions, is expressed in a subset of acinar cells after the end of a 12-h caerulein infusion, giving rise to a "patchy" staining pattern. During the subsequent period of inflammation and regeneration, galectin-3 expression increases in those acinar cells that undergo apoptosis. By 48 h after the end of caerulein infusion, morphologically normal cells do not contain galectin-3 and participate in regeneration by proliferation. Tubular complexes, being transient structures from degenerative acini, accumulate galectin-3 in the remnants of the epithelium cells. Stimulation with supramaximal dosages of caerulein of the cell line AR4-2J, which is derived from rat pancreatic acinar cells, also results in a marked increase of galectin-3, confirming the in vivo results. We postulate that the high expression of the anti-apoptotic protein galectin-3 regulates the time course of the apoptotic process in pancreatic acinar cells.