Huntington disease is caused by the expansion of a CAG repeat encoding an extended glutamine tract in a protein called huntingtin. Although the mutant protein is widely expressed, the earliest and most striking neuropathological changes are observed in the striatum. Here we show dramatic mutation length increases (gains of up to 1000 CAG repeats) in human striatal cells early in the disease course, most likely before the onset of pathological cell loss. Studies of knock-in HD mouse models indicate that the size of the initial CAG repeat mutation may influence both onset and tissue-specific patterns of age-dependent, expansion-biased mutation length variability. Given that CAG repeat length strongly correlates with clinical severity, we suggest that somatic increases of mutation length may play a major role in the progressive nature and cell-selective aspects of both adult-onset and juvenile-onset HD pathogenesis and we discuss the implications of this interpretation of the data presented.

CCN2 is induced by transforming growth factor-␤ (TGF␤) in fibroblasts and is overexpressed in connective tissue disease. CCN2 has been proposed to be a downstream mediator of TGF␤ action in fibroblasts; however, the role of CCN2 in regulating this process unclear. By using embryonic fibroblasts isolated from ccn2؊/؊ mice, we showed that CCN2 is required for a subset of responses to TGF␤. Affymetrix genome-wide expression profiling revealed that 942 transcripts were induced by TGF␤ greater than 2-fold in ccn2؉/؉ fibroblasts, of which 345 were not induced in ccn2؊/؊ fibroblasts, including pro-adhesive and matrix remodeling genes. Whereas TGF␤ properly induced a generic Smad3-responsive promoter in ccn2؊/؊ fibroblasts, TGF␤-induced activation of focal adhesion kinase (FAK) and Akt was reduced in ccn2؊/؊ fibroblasts. Emphasizing the importance of FAK and Akt activation in CCN2-dependent transcriptional responses to TGF␤ in fibroblasts, CCN2-dependent transcripts were not induced by TGF␤ in fak؊/؊ fibroblasts and were reduced by wortmannin in wild-type fibroblasts. Akt1 overexpression in ccn2؊/؊ fibroblasts rescued the TGF␤-induced transcription of CCN2-dependent mRNA. Finally, induction of TGF␤-induced fibroblast adhesion to fibronectin and type I collagen was significantly diminished in ccn2؊/؊ fibroblasts. Thus in embryonic fibroblasts, CCN2 is a necessary cofactor required for TGF␤ to activate the adhesive FAK/Akt/phosphatidylinositol 3-kinase cascade, FAK/Akt-dependent genes, and adhesion to matrix.Growth factors intimately contribute to the normal wound healing process, regulating chemotaxis, cell proliferation, neovascularization, and extracellular matrix (ECM) 3 synthesis. CCN2 (connective tissue growth factor), a member of the CCN family of proteins, contains 38 conserved cysteine-rich residues and a heparin-binding domain and is chemotactic and mitogenic for connective tissue cells (1-4). However, the physiological role of CCN2 is largely unknown.As an initial approach to elucidate the physiological function of CCN2, mice deleted for the ccn2 gene were recently generated (5). Mice homozygous for a deletion of the ccn2 gene die soon after birth, displaying an inability of the rib cage to ossify properly (5). The phenotype of these mice is consistent with a role for CCN2 in matrix synthesis and remodeling as ccn2Ϫ/Ϫ embryos show reduction in the expression of bone-specific matrix proteins, such as aggrecan (5). We recently found that embryonic fibroblasts isolated from ccn2Ϫ/Ϫ mice showed reduced basal adhesive signaling, including a reduction of FAK and ERK phosphorylation and delays in ␣-smooth muscle actin (␣-SMA) stress fiber formation (6), suggesting that CCN2 plays a key role in mediating the formation of attachments between the cell and matrix at focal adhesions.Although CCN2 was discovered over a decade ago, the precise biological function of CCN2 has remained elusive. CCN2 is expressed in mesenchymal cells in development, is induced during wound healing (4,8), and is overexpressed in fibrosis (7-11)....

An unstable CAG triplet repeat expansion encoding a polyglutamine stretch within the ubiquitously expressed protein huntingtin is responsible for causing Huntington's disease (HD). By quantifying the repeat sizes of individual mutant alleles in tissues derived from an accurate genetic mouse model of HD we show that the mutation becomes very unstable in striatal tissue. The expansion-biased changes increase with age, such that some striatal cells from old HD mice contain mutations that have tripled in size. If this pattern of repeat instability is recapitulated in human striatal tissue, the concomitant increased polyglutamine load may contribute to the patterns of selective neuronal cell death in HD. Our findings also suggest that trinucleotide repeat instability can occur by mechanisms that are not replication-based.

Scarring is characterized by excessive synthesis and contraction of extracellular matrix. Here, we show that fibroblasts from scarred (lesional) areas of patients with the chronic fibrotic disorder diffuse scleroderma [diffuse systemic sclerosis (dSSc)] show an enhanced ability to adhere to and contract extracellular matrix, relative to fibroblasts from unscarred (nonlesional) areas of dSSc patients and dermal fibroblasts from normal, healthy individuals. The contractile abilities of normal and dSSc dermal fibroblasts were suppressed by blocking heparin sulfate-containing proteoglycan biosynthesis or antagonizing transforming growth factor-beta receptor type I [activin-linked kinase (ALK5)] or ras/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK). Compared with both normal and nonlesional fibroblasts, lesional dSSc fibroblasts overexpressed the heparin sulfate-containing proteoglycan syndecan 4. We also found that the procontractile signals from transforming growth factor (TGF)-beta were integrated through syndecan 4 and MEK/ERK because the ability of TGFbeta to induce contraction of dermal fibroblasts was prevented by MEK antagonism. TGFbeta could not induce a contractile phenotype or phosphorylate ERK in syndecan 4(-/-) dermal fibroblasts. These results suggest that integrating TGFbeta and ERK signals via syndecan 4 is essential for the contractile ability of dermal fibroblasts. We conclude that antagonizing MEK/ERK, TGFbeta1/ALK5, or syndecan 4 may alleviate scarring in chronic fibrotic disease.