Hox transcription factor regulation of adult bone-marrow-derived cell behaviour during tissue repair and regeneration

Mechanisms and translational applications of regeneration in limbs: From renewable animals to humans

BACKGROUND

The regenerative capacity of organisms declines throughout evolution, and mammals lack the ability to regenerate limbs after injury. Past approaches to achieving successful restoration through pharmacological intervention, tissue engineering, and cell therapies have faced significant challenges.

OBJECTIVES

This review aims to provide an overview of the current understanding of the mechanisms behind animal limb regeneration and the successful translation of these mechanisms for human tissue regeneration.

RESULTS

Particular attention was paid to the Mexican axolotl (Ambystoma mexicanum), the only adult tetrapod capable of limb regeneration. We will explore fundamental questions surrounding limb regeneration, such as how amputation initiates regeneration, how the limb knows when to stop and which parts to regenerate, and how these findings can apply to mammalian systems.

CONCLUSIONS

Given the urgent need for regenerative therapies to treat conditions like diabetic foot ulcers and trauma survivors, this review provides valuable insights and ideas for researchers, clinicians, and biomedical engineers seeking to facilitate the regeneration process or elicit full regeneration from partial regeneration events.

HOX Genes Family and Cancer: A Novel Role for Homeobox B9 in the Resistance to Anti-Angiogenic Therapies

Simple Summary

The inhibition of angiogenesis, relying on the use of drugs targeting the VEGF signaling pathway, has become one of the main strategies for cancer treatment. However, the intrinsic and acquired resistance to this type of therapy limit its efficacy. Thus, the identification of novel therapeutic targets is urgently needed. The resistance to anti-angiogenic treatment often occurs through the activation of alternative VEGF independent signaling pathways and recruitment of bone marrow-derived pro-angiogenic cells in the tumor microenvironment. HOX genes are key regulators of embryonic development, also involved in angiogenesis and in cancer progression. HOXB9 upregulation occurs in many types of cancer and it has been identified as a critical transcription factor involved in tumour resistance to anti-angiogenic drugs. Indeed, HOXB9 modulates the expression of alternative pro-angiogenic secreted factors in the tumour microenvironment leading tumor escape from the anti-angiogenic treatments. Hence, HOXB9 could serves as a novel therapeutic target to overcome the resistance to anti-angiogenic therapies.

Abstract

Angiogenesis is one of the hallmarks of cancer, and the inhibition of pro-angiogenic factors and or their receptors has become a primary strategy for cancer therapy. However, despite promising results in preclinical studies, the majority of patients either do not respond to these treatments or, after an initial period of response, they develop resistance to anti-angiogenic agents. Thus, the identification of a novel therapeutic target is urgently needed. Multiple mechanisms of resistance to anti-angiogenic therapy have been identified, including the upregulation of alternative angiogenic pathways and the recruitment of pro-angiogenic myeloid cells in the tumor microenvironment. Homeobox containing (HOX) genes are master regulators of embryonic development playing a pivotal role during both embryonic vasculogenesis and pathological angiogenesis in adults. The importance of HOX genes during cancer progression has been reported in many studies. In this review we will give a brief description of the HOX genes and their involvement in angiogenesis and cancer, with particular emphasis on HOXB9 as a possible novel target for anti-angiogenic therapy. HOXB9 upregulation has been reported in many types of cancers and it has been identified as a critical transcription factor involved in resistance to anti-angiogenic drugs.

Direct and Indirect Targeting of HOXA9 Transcription Factor in Acute Myeloid Leukemia

HOXA9 (Homeobox A9) is a homeotic transcription factor known for more than two decades to be associated with leukemia. The expression of HOXA9 homeoprotein is associated with anterior-posterior patterning during embryonic development, and its expression is then abolished in most adult cells, with the exception of hematopoietic progenitor cells. The oncogenic function of HOXA9 was first assessed in human acute myeloid leukemia (AML), particularly in the mixed-phenotype associated lineage leukemia (MPAL) subtype. HOXA9 expression in AML is associated with aggressiveness and a poor prognosis. Since then, HOXA9 has been involved in other hematopoietic malignancies and an increasing number of solid tumors. Despite this, HOXA9 was for a long time not targeted to treat cancer, mainly since, as a transcription factor, it belongs to a class of protein long considered to be an "undruggable" target; however, things have now evolved. The aim of the present review is to focus on the different aspects of HOXA9 targeting that could be achieved through multiple ways: (1) indirectly, through the inhibition of its expression, a strategy acting principally at the epigenetic level; or (2) directly, through the inhibition of its transcription factor function by acting at either the protein/protein interaction or the protein/DNA interaction interfaces.

Chronic Inflammation in Response to Injury: Retention of Myeloid Cells in Injured Tissue Is Driven by Myeloid Cell Intrinsic Factors

Chronic inflammation is a hallmark of impaired healing in a plethora of tissues, including skin, and is associated with aging and diseases such as diabetes. Diabetic chronic skin wounds are characterized by excessive myeloid cells that display an aberrant phenotype, partially mediated by stable intrinsic changes induced during hematopoietic development. However, the relative contribution of myeloid cell-intrinsic factors to chronic inflammation versus aberrant signals from the local environmental was unknown. Moreover, identification of myeloid cell intrinsic factors that contribute to chronic inflammation in diabetic wounds remained elusive. Here we show that Gr-1⁺CD11b⁺ myeloid cells are retained specifically within the presumptive granulation tissue region of the wound at a higher density in diabetic mice and associate with endothelial cells at the site of injury with a higher frequency than in nondiabetic mice. Adoptive transfer of myeloid cells demonstrated that aberrant wound retention is due to myeloid cell intrinsic factors and not the local environment. RNA sequencing of bone marrow and wound-derived myeloid cells identified Selplg as a myeloid cell intrinsic factor that is deregulated in chronic wounds. In vivo blockade of this protein significantly accelerated wound healing in diabetic mice and may be a potential therapeutic target in chronic wounds and other chronic inflammatory diseases.

Mesenchymal progenitors aging highlights a miR-196 switch targeting HOXB7 as master regulator of proliferation and osteogenesis

Human aging is associated with a decrease in tissue functions combined with a decline in stem cells frequency and activity followed by a loss of regenerative capacity. The molecular mechanisms behind this senescence remain largely obscure, precluding targeted approaches to counteract aging. Focusing on mesenchymal stromal/stem cells (MSC) as known adult progenitors, we identified a specific switch in miRNA expression during aging, revealing a miR-196a upregulation which was inversely correlated with MSC proliferation through HOXB7 targeting. A forced HOXB7 expression was associated with an improved cell growth, a reduction of senescence, and an improved osteogenesis linked to a dramatic increase of autocrine basic fibroblast growth factor secretion. These findings, along with the progressive decrease of HOXB7 levels observed during skeletal aging in mice, indicate HOXB7 as a master factor driving progenitors behavior lifetime, providing a better understanding of bone senescence and leading to an optimization of MSC performance.

A potential role of homeobox transcription factors in osteoarthritis

When a healthy joint progressively becomes osteoarthritic, the structures of the affected cartilage, bone and synovia undergo an initial phase of rearrangement. The exact molecular and cellular events occurring in this early osteoarthritic transition phase still remain elusive. Homeobox (Hox) genes encode for transcription factors that typically regulate limb morphogenesis and skeletal formation during development. More recently they were shown to be required for tissue remodelling and homeostasis in adults and to be modulated in a variety of pathologies. Here we present and discuss the hypothesis that dysregulation of specific Hox genes is associated with the onset and development of osteoarthritis (OA). Discovering mechanisms modulating Hox gene expression could not only provide important information in understanding OA pathology and its initiation, but also help to identify biomarkers reflecting the state of early OA. This knowledge would allow anticipating the time window for clinical treatment of the affected cartilage and assist in the development of innovative strategies to restore joint homeostasis, e.g., by cell or gene therapy.

Lentiviral-mediated overexpression of homeobox A4 by human umbilical cord mesenchymal stem cells repairs full-thickness skin defects

A number of types of stem cells have been shown to be effective in wound repair. In the present study the effect of homeobox A4 (HOXA4) overexpression by human umbilical cord mesenchymal stem cells (hUMSCs) on full‑thickness skin repair was evaluated. Isolated hUMSCs were transfected with a lentivirus expressing HOXA4 and cultured for 21 days. Expression of the epidermal cell‑specific markers, cytokeratins 14 and 18, was detected by immunohistochemistry and flow cytometry. Full‑thickness skin defects (1.5 cm x 1.5 cm) were made on the backs of 45 nude mice, which were randomly divided into the following three treatment groups: Collagen membrane with lenti‑HOXA4 hUMSC seed cells; collagen membrane with lentivirus expressing green fluorescent protein; and collagen membrane alone. On days 7, 14 and 21 following transplantation, tissue samples were harvested and examined by histology and western blot analysis. Flow cytometry showed that the transfection efficiency was 95.41% at a multiplicity of infection of 100, and that the lenti‑HOXA4 hUMSCs differentiated into epidermal cells, expressing cytokeratins 14 and 18. In addition, re‑epithelialization of wounds treated with lenti‑HOXA4 hUMSCs was significantly greater than that in the control groups in the first week. By week three the epidermis was significantly thicker in the lenti‑HOXA4 group than the control groups. Thus, transplantation of hUMSCs modified with Ad‑HOXA4 promoted wound healing.

Hox genes are involved in vascular wall-resident multipotent stem cell differentiation into smooth muscle cells

Human vascular wall-resident CD44+ multipotent stem cells (VW-MPSCs) within the vascular adventitia are capable to differentiate into pericytes and smooth muscle cells (SMC). This study demonstrates HOX-dependent differentiation of CD44(+) VW-MPSCs into SMC that involves epigenetic modification of transgelin as a down-stream regulated gene. First, HOXB7, HOXC6 and HOXC8 were identified to be differentially expressed in VW-MPSCs as compared to terminal differentiated human aortic SMC, endothelial cells and undifferentiated pluripotent embryonic stem cells. Silencing these HOX genes in VW-MPSCs significantly reduced their sprouting capacity and increased expression of the SMC markers transgelin and calponin and the histone gene histone H1. Furthermore, the methylation pattern of the TAGLN promoter was altered. In summary, our findings suggest a role for certain HOX genes in regulating differentiation of human VW-MPSC into SMCs that involves epigenetic mechanisms. This is critical for understanding VW-MPSC-dependent vascular disease processes such as neointima formation and tumor vascularization.

Expression of Hox genes during regeneration of nereid polychaete Alitta (Nereis) virens (Annelida, Lophotrochozoa)

BACKGROUND

Hox genes are the key determinants of different morphogenetic events in all bilaterian animals. These genes are probably responsible for the maintenance of regenerative capacities by providing positional information in the regenerating animal body. Polychaetes are well known for their ability to regenerate the posterior as well as the anterior part of the body. We have recently described the expression of 10 out of 11 Hox genes during postlarval growth of Alitta (Nereis) virens. Hox genes form gradient overlapping expression patterns, which probably do not contribute to the morphological diversity of segments along the anterior-posterior axis of the homonomously segmented worm. We suggest that this gradient expression of Hox genes establishes positional information along the body that can be used to maintain coordinated growth and regeneration.

RESULTS

We showed that most of the Hox gene expression patterns are reorganized in the central nervous system, segmental ectoderm and mesoderm. The reorganization takes place long before regeneration becomes apparent. The most rapid reorganization was observed for the genes with the largest differences in expression levels in the amputation site and the terminal structures (pygidium and growth zone). Moreover, we revealed the expression of two antisense Hox RNAs (Nvi-antiHox5 and Nvi-antiHox7) demonstrating unique expression patterns during regeneration.

CONCLUSIONS

Hox genes probably participate in the maintenance and restoration of the positional information in A. virens. During postlarval growth and regeneration, Hox genes do not alter the diversity of segments but provide the positional information along the anterior-posterior axis. The reorganization of at least some Hox gene patterns during regeneration may be regulated by their anti-sense transcripts, providing a rapid response of Hox gene transcripts to positional failure. The capacity of Hox genes to maintain the positional information in the adult body is present in different bilaterian animals (planarias, polychaetes and mammals) and might be an ancestral function inherited from the common evolutionary remote ancestor.

The dual roles of homeobox genes in vascularization and wound healing

Homeobox genes represent a family of highly conserved transcription factors originally discovered to regulate organ patterning during development. More recently, several homeobox genes were shown to affect processes in adult tissue, including angiogenesis and wound healing. Whereas a subset of members of the Hox-family of homeobox genes activate growth and migration to promote angiogenesis or wound healing, other Hox genes function to restore or maintain quiescent, differentiated tissue function. Pathological tissue remodeling is linked to differential expression of activating or stabilizing Hox genes and dysregulation of Hox expression can contribute to disease progression. Studies aimed at understanding the role and regulation of Hox genes have provided insight into how these potent morphoregulatory genes can be applied to enhance tissue engineering or limit cancer progression.